Neoarchean tectonothermal imprints in the Rengali Province, eastern India and their implication on the growth of Singhbhum Craton: evidence from zircon U–Pb SHRIMP data
Abstract
A detailed zircon U–Pb (SHRIMP) geochronological study of the amphibolite to granulite-grade rocks of the Rengali Province of eastern India records the growth history of the southern margin of the Singhbhum Craton. Pelitic and mafic granulites from the gneissic belt exhibit contrasting styles of metamorphism. Zircon of the pelitic granulites from the eastern segment yields c. 3528–3064 Ma detrital ages. Charnockitic gneiss from the eastern segment has protolith age of 3058 ± 15 Ma while that from the central segment has protolith age of 2861 ± 30 Ma. The latter rock records high-grade metamorphism at 2818 ± 15 Ma. Hornblende gneiss from the central sector has a protolith age of 2828 ± 9 Ma. Deformed leucogranite in the central and undeformed granitoid in the eastern segment were emplaced at 2807 ± 13 and 2809 ± 13 Ma respectively. The protolith of felsic gneiss from the central sector was emplaced at 2776 ± 24 Ma. Most of the zircon samples contain overgrowths of c. 2500 Ma, inferred to be the age of reworking of the gneissic belt. Our data suggest that the Rengali Province evolved as an orogenic belt in the Neoarchean time (c. 2800–2500 Ma) during southward growth of the Singhbhum Craton. These tectonothermal imprints at the margin of the Singhbhum Craton are possibly related to its inclusion within the supercontinent Ur.
Introduction
Cratons are the building blocks of continents that incrementally grew by repeated accretion-collision orogenic processes at least since the Neoarchean time (Condie, 2005; Cawood et al., 2009). The exact nature of the cratonization process and time frames are best studied in the bounding orogenic belts, albeit with geological complexities. High-grade gneissic rocks of the bounding orogenic belts provide direct insights to the responses of middle and lower crust undergoing the early Earth tectonic processes. They also provide important constraints on the nature of heat flow which ultimately help in evaluating whether Archean plate tectonics was viable. For example the Northern Marginal Zone of the Limpopo Belt preserves the growth history of the bordering Zimbabwe Craton (Blenkinshop, 2011). The geological histories of such orogenic belts additionally provide important clues about the configurations of widely dispersed supercontinents that formed during Precambrian time. The earliest supercontinent ‘Ur’ is postulated to be constituted of cratonic blocks of Australia (Pilbara Craton), Africa (Kaapvaal Craton) and India (Dharwar and Singhbhum Craton) that united at c. 3000 Ma (Santosh et al., 2009). As the reconstruction of Ur is speculative due to paucity of data, the addition of new geological data from its erstwhile components will testify the viability of its reconstruction.
Peninsular India contains a mosaic of cratons with surrounding cratonized orogenic belts. Eastern India's Singhbhum Craton (Fig. 1) is a granite-greenstone terrane that records a remarkable Archean growth history (Saha, 1994; Mukhopadhyay et al., 2012). The southern margin of the craton is composed of high-grade granulites along with medium-grade metasedimentary and metavolcanic rocks collectively termed as the Rengali Province (Mahalik, 1994; Crowe et al., 2003; Mahapatro et al., 2012; Bose et al., 2015). On its southern flank, the Rengali Province is bordered by the Eastern Ghats Belt with its characteristic Proterozoic-age high-grade metamorphism (Bose et al., 2011; Das et al., 2011; Korhonen et al., 2011, 2013; Gupta, 2012; Dasgupta et al., 2013 and references therein). Limited studies from the Rengali Province identify it as a major lineament-bound terrane having contrasting metamorphic styles and isotopic signatures compared to the immediately adjacent Eastern Ghats Belt (Crowe et al., 2001, 2003). On the other hand, available geochronological data from this terrane hint towards its consanguinity with the Singhbhum Craton (Misra et al., 2000; Mahapatro et al., 2012). In this study, we unravel the geochronological evolution of the gneissic belt of the Rengali Province using zircon U–Pb (SHRIMP) data on key rock units of the central and eastern part of the province. We further explore the possible precursors of the paragneissic rocks in the context of Singhbhum Craton-Rengali Province correlation. Orthogneissic rocks, on the other hand, provide ages of important tectonothermal imprints on the gneissic belt and constrain the time frame for orogenesis within the Rengali Province.
Geological background
The Rengali Province separates the low-grade metasedimentary and metavolcanic rocks of the Singhbhum Craton from high-grade charnockite–khondalite suite of the Eastern Ghats Belt. It preserves structural, petrological and geochronological features distinct from those of the Eastern Ghats Belt (Mahalik, 1994; Crowe et al., 2001, 2003; Dobmeier & Raith, 2003). The Singhbhum Craton at its southern margin constitutes c. 3500 Ma Southern Iron Ore Group of rocks (Mukhopadhyay et al., 2008) which is an enclave suite containing metavolcanics and banded iron formation (BIF)-bearing greenstones within c. 3400–3100 Ma granitoids of tonalite–trondjhemite–granite (TTG) affinity (Saha, 1994; Mukhopadhyay, 2001; Misra, 2006; Acharyya et al., 2010; Tait et al., 2011). A thick quartzite–conglomerate unit of c. 3000 Ma depositional age lies unconformably over these rock units (Mukhopadhyay et al., 2012, 2014). All these rocks are variably deformed and metamorphosed to greenschist facies and juxtaposed in the south with rocks of the Rengali Province along a curvilinear tectonic contact termed as the Barkot Shear Zone in the west and Sukinda Thrust in the east (Fig. 1). The WNW-ESE trending Kerajang Fault separates the Rengali Province from the Eastern Ghats Belt (Fig. 1a). An intervening strip of Phanerozoic rocks locally mask this contact indicating recurrent rejuvenation of the Kerajang Fault. The dominant structural grain within the Eastern Ghats Belt is NE-SW, which swings to WNW-ESE at its contact with the Rengali Province.
The Rengali Province constitutes greenschist to granulite facies rocks occurring as distinct lithotectonic suite bounded by regional-scale fault/shear zones (Mahalik, 1994; Crowe et al., 2003; Mahapatro et al., 2012). In the central part (Fig. 1b), it comprises an amphibolite-grade gneissic belt with enclaves of granulite and overlain by greenschist-grade supracrustal rocks in both the north and south. In contrast, the eastern part of the gneissic belt (Fig. 1c) mainly constitutes amphibolite-grade gneisses with a higher proportion of granulite enclaves. The host amphibolite-grade gneiss is dominantly quartzofeldspathic with amphibolite patches. The granulites are mostly charnockitic gneiss with small components of mafic and pelitic granulite. The gneissic belt of the central part of the Rengali Province is flanked by low-grade supracrustal rocks on both sides (Fig. 1b). The low-grade supracrustal rocks are principally made up of quartzite and mica schist with minor proportion of meta-conglomerate, metabasite and calc schist. Intrusive granitoids, pegmatites and basic dykes are smaller yet important lithological components of the gneissic belt of the Rengali Province.
Petrological data from the Rengali Province are scarce. Based on textural, geothermobarometric and monazite U–Th–total Pb data from only one pelitic granulite from the eastern Rengali Province, Mahapatro et al. (2012) argued that the peak granulite facies conditions involved 0.78 GPa, 849 °C during 3057 ± 17 Ma. A reheating of retrogressed granulite is thought to have occurred during 2781 ± 16 Ma. Recently, Bose et al. (2015) published petrological data for mafic and pelitic granulites from the eastern Rengali Province. Their data suggest that peak conditions experienced by the mafic granulite involved 1.0–1.2 GPa, 860 °C. Reheating of retrogressed mafic granulite caused dehydration melting in pelitic granulite at ~0.6 GPa, 580–730 °C. The contrasting P–T paths shown by the mafic granulite (decompression followed by heating) and the pelitic granulite (heating followed by cooling) have been interpreted as the result of crustal juxtaposition within a major orogenic setting (Bose et al., 2015).
Very few high-quality geochronological data are available from the Rengali Province. Metasedimentary and metaigneous rocks from this province have 2800–2200 and 3900–2900 Ma Nd-model ages (Rickers et al., 2001). Since the latter overlaps with the emplacement ages of igneous rocks within the Singhbhum Craton, Rickers et al. (2001) argued that the crustal materials of the Rengali Province (their isotopic Domain 4) were derived mainly from the Singhbhum Craton without much addition of juvenile material. It is to be noted that sampling of Rickers et al. was very limited (two samples only) from the Rengali Province, and a more extensive work is required to characterize the protolith of entire Rengali Province. Bounding granitoid near Bhuban area at the eastern part of the province give c. 2800 Ma emplacement age (207Pb/206Pb zircon by Misra et al., 2000). Rb–Sr data from some of the charnockites yield c. 2750–2730 Ma ages (Sarkar et al., 2000) which are rather imprecise as argued by later workers (Dobmeier & Raith, 2003). Amphibolite-grade rocks give much younger age data of c. 700–420 Ma based on 40Ar/39Ar systematics on hornblende and biotite grains (Crowe et al., 2001). This is an unrealistically prolonged time frame for activity of an orogenic belt.
Structural and petrological history
Quartzofeldspathic gneiss (including migmatitic hornblende gneiss), amphibolite and granulite pockets in the gneissic belt of the Rengali Province contain an early gneissic layering (S1). The gneissic layering is defined by alternating quartz-feldspar rich leucocratic layer and garnet + biotite-rich mafic layer in the quartzofeldspathic gneiss. In hornblende gneiss, the mafic bands are characterized by garnet, hornblende and biotite. In charnockite gneiss, the gneissic bands are made up of alternating orthopyroxene, garnet, hornblende and ilmenite-rich mafic and quartz, K-feldspar and plagioclase-rich leucocratic layers. In mafic and pelitic granulites, mafic layers are made up of clinopyroxene and garnet and garnet, orthopyroxene, cordierite, sillimanite and biotite-rich layers respectively. Quartz and feldspar dominate the alternating leucocratic layers. The intensity of development of this early gneissic layering varies over the study area, depending upon the host rock type and the amount of later deformation experienced. As a result, the host lithologies preserve an incipient to prominent fabric invariably associated with a steep dominant foliation (S2) that transposes the early layering (S1). The dominant foliation contains a prominent down-dip mineral lineation. The rocks thus commonly appear as S-L tectonites (Fig. 2a) grading to mylonite near major structural boundaries and internal shear zones. The foliation is defined by strongly stretched quartz, feldspar and mafic minerals while the lineation is defined by elongated quartz, biotite and hornblende. Though the foliation is E to WNW-trending and steeply dipping at most places, its average orientation varies over the study area. In the central Rengali Province (Fig. 1c), the mean orientation is steeply southerly and northerly dipping in northern and southern parts of the gneissic belt respectively (Ghosh et al., 2016). In the middle part of the gneissic belt, the foliation shows a sub-vertical attitude, whereas the mean orientation shows an overwhelming southerly dip in the eastern part.
The gneissic layering (S2) shows symmetric to asymmetric fold development in outcrop-scale (Fig. 2b) with the dominant foliation being axial planar to the folds. However, these late gently to moderately plunging folds (F2) are interspersed with an early, isoclinal, intrafolial fold set (F1) which is mostly reclined in geometry (Fig. 2c) and indicates an earlier episode of shortening and fold development (D1) across the length of the belt. The presence of outcrop-scale superposed fold structures in gneissic layers (Fig. 2d) and cross-cutting veins within the gneisses (Fig. 2e) indicates a complex deformation history.
The metamorphic character of quartzofeldspathic and migmatitic hornblende gneisses in the gneissic belt is mostly of amphibolite-grade based on the mineral association of quartz, plagioclase, K-feldspar, hornblende and biotite. These rocks show a prominent gneissic layering with segregation of mafic and quartzofeldspathic minerals in different layers. The quartzofeldspathic gneiss is compositionally granitic with alkali feldspar augen in places. Amphibolite enclaves within the gneissic belt contain hornblende, quartz, garnet, plagioclase and titanite with or without biotite showing a massive texture. Garnet–quartz–plagioclase–hornblende assemblage in amphibolite records 524–559 °C and 0.80–0.83 GPa, deduced from the Al-hornblende barometry (Anderson & Smith, 1995) and hornblende–plagioclase thermometry (Holland & Blundy, 1994). The amphibolite-grade gneisses show a later effect of deformation (D2) manifested by stretching and shearing of fabric forming minerals and development of the pervasive foliation (S2) while the metamorphic conditions remained in the amphibolite grade.
Metamorphic assemblages in granulites are dominated by anhydrous minerals. Mafic granulite contains garnet, clinopyroxene and plagioclase-rich mineral assemblage to represent the peak granulite assemblages which reflect the peak conditions. Individual grains are extremely deformed showing stretching (Fig. 2f), flattening and boudinage texture. This assemblage is cut by a hornblende-bearing assemblage where hornblende grains partially pseudomorph clinopyroxene grains. Some of the coarse hornblende grains shows incipient breakdown to a spongy intergrowth of garnet, clinopyroxene and quartz (Fig. 2g) implying reheating of the retrogressed granulite during the second metamorphic event or dehydration due to fluid–rock interaction.
Pelitic granulite comprises peak assemblages of garnet, cordierite, quartz and K-feldspar or garnet, orthopyroxene, cordierite, K-feldspar and quartz. These former assemblages evolved through dehydration melting reaction of biotite-bearing assemblage (Bose et al., 2015). The presence of thin film of alkali feldspar over quartz and plagioclase grains, biotite-rich selvages and cuspate nature of quartz (Fig. 2h) are possible evidences of partial melting. The peak assemblage was retrogressed to biotite–sillimanite matrix assemblages. Zircon grains are present within garnet porphyroblasts as well as matrix quartz-feldspar grains. Another sample of pelitic granulite additionally contains orthopyroxene in the peak metamorphic assemblage.
In charnockite gneiss, orthopyroxene, quartz, plagioclase and K-feldspar form the early assemblage, replaced by or retrogressed to a hornblende-bearing amphibolite-grade assemblage with concomitant shearing and evidence of foliation-parallel stretching of quartz and feldspar grains.
Sample selection and description
Nine samples of diverse mineral associations were selected for geochronological study based on metamorphic character and key field relations. These samples were collected from the eastern and the central part of the gneissic belt of the Rengali Province (Fig. 1b,c). The pelitic and the mafic granulite samples represent supracrustal materials of the Rengali Province that underwent high-grade metamorphism. Salient features of these samples are described in Table 1.
| Sample name | Sample no. | GPS coordinates | Mineralogy | Textures/structures | Other features | Age data |
|---|---|---|---|---|---|---|
| Eastern Rengali Province | ||||||
| Bhuban garnet–cordierite quartz granulite | RNG 61 |
20°55ʹ12.03ʺN 85°43ʹ32.07ʺE |
Grt–Crd–Sil–Bt–Qtz–Kfs–Pl–Rt–Ilm–Zrn–Mnz | S2 gneissic banding present. Both prograde and retrograde Bt present | Evidence of Bt dehydration melting present | 3466 ± 7, 3413 ± 12, 3308 ± 24 Ma – Protolith ages |
| Bhuban garnet–orthopyroxene quartz granulite | RNG 62 |
20°55ʹ15.03ʺN 85°43ʹ11.72ʺE |
Grt–Opx–Crd–Sil–Bt–Kfs–Rt–Ilm–Zrn–Mnz | S2 gneissic banding present. Both prograde and retrograde Bt present | Evidence of Bt dehydration melting present. Peak MG2 P–T are 730 °C, 0.58 Gpa (Bose et al., 2015) | 3353 ± 19, 3295 ± 14, 3222 ± 13 Ma – Protolith ages |
| Kalinganagar granite | JK 53 |
20°57ʹ19.56ʺN 85°59ʹ25.86ʺE |
Qtz–Kfs–Pl–Chl–Zrn | Weakly deformed and metamorphosed; Chl develops on altered feldspar | Exposed near the SC contact. Xenoliths of SC rocks present | >3000 Ma – Inherited (core) ages |
| Balarampur charnockite | JK 54 |
20°51ʹ25.14ʺN 86°0ʹ28.68ʺE |
Qtz–Kfs–Pl–Opx–Ilm–Grt–Zrn | Extremely deformed and S2 fabric is often mylonitic in character. | Contains xenoliths of mafic granulite with Grt + Cpx + Pl + Qtz MG1 assemblage. Early S1 fabric is present within the mafic granulite | >3200 Ma – Inherited (core) ages |
| 3058 ± 15 Ma – Magmatic emplacement age | ||||||
| 2836 ± 4 Ma – Metamorphic age | ||||||
| Central Rengali Province | ||||||
| Budhapal leucogranite | RNG 50D |
21°12ʹ1.12ʺN 84°52ʹ49.31ʺE |
Qtz–Kfs–Pl–Grt–Mus–Zrn–Mnz | Abundant muscovite showing a weak foliation, post-kinematic Grt present. Evidence of melt crystallization | Exposed as a part of the gneissic basement within greenschist-grade supracrustal rocks | 2807 ± 13 Ma – Magmatic emplacement age |
| 2484 ± 5 Ma – Spot date, possible metamorphic age | ||||||
| Rengali charnockite | RNG 120A |
21°16ʹ31.70ʺN 85°1ʹ49.89ʺE |
Qtz–Kfs–Pl–Opx–Hbl–Zrn | S2 gneissic banding present. Deformed grains show cooling texture. Fluid inclusions are abundant in Qtz | Exposed within a linear belt in strike continuity of the hornblende gneiss | 2861 ± 30 Ma – Magmatic emplacement age |
| 2818 ± 15 Ma – Metamorphic age | ||||||
| 2489 ± 23 Ma – Spot date, possible metamorphic age | ||||||
| Rengali mafic granulite | RNG 122 |
21°15ʹ33.32ʺN 85°4ʹ4.83ʺE |
Grt–Cpx–Pl–Qtz–Hbl–Opx–Ilm–Zrn | Show effect of strong shear along S2. Gneissic banding is present with deformed Grt + Cpx forming mafic layers | Exposed as a small enclave within charnockitic gneiss | 2844 ± 7 Ma – Magmatic or metamorphic age |
| 2488 ± 19 Ma – Metamorphic age | ||||||
| Rengali hornblende gneiss | RNG 129B |
21°15ʹ14.34ʺN 85°1ʹ6.6ʺE |
Hbl–Pl–Qtz–Kfs–Ap–Ilm–Zrn | Gneissic banding is shown by deformed Hbl grains | Tonalitic rock emplaced at mid-crustal conditions. Hbl–Pl–Qtz assemblage suggests mid-crustal metamorphism | 2860 ± 5 Ma – Inherited (core) age |
| 2828 ± 9 Ma – Magmatic emplacement age | ||||||
| 2471 ± 4 Ma – Spot date, possible metamorphic age | ||||||
| Rengali felsic gneiss | RNG 54 |
21°16ʹ9.72ʺN 85°2ʹ15.18ʺE |
Pl–Qtz–Kfs–Ilm–Zrn | The rock shows S2 mylonitic fabric with Kfs porphyroclasts surrounded by finer grained tails | Granite emplaced at mid-crustal level and subsequently deformed and metamorphosed at amphibolite grade | 2776 ± 24 Ma – Magmatic emplacement age |
| 2508 ± 14 Ma – Metamorphic age | ||||||
Eastern Rengali Province
Sample RNG 61(Bhuban garnet–cordierite–quartz granulite)
This pelitic granulite occurs as an enclave within quartzofeldspathic gneiss at the eastern segment of the Rengali Province (Fig. 1b). Faint traces of gneissic banding are present with garnet-rich mafic and quartzofeldspathic leucosome (Fig. 3a). The rock is composed of garnet, cordierite, sillimanite, biotite, quartz, K-feldspar, plagioclase, rutile and ilmenite. Zircon and monazite are present as accessory minerals both within garnet porphyroblasts and matrix.
Sample RNG 62 (Bhuban garnet–orthopyroxene–quartz granulite)
This pelitic granulite occurs in close proximity to RNG 61 with similar mineral modes. It contains garnet, orthopyroxene, cordierite, sillimanite, biotite, quartz, K-feldspar, rutile with small but variable proportions of plagioclase and ilmenite (Table 1). Here, garnet, orthopyroxene, cordierite, K-feldspar and quartz form the M1 peak assemblage which is replaced by the biotite–sillimanite M1r assemblage. Zircon grains are present within garnet porphyroblast as well as matrix quartz-feldspar grains.
Sample JK 53 (Kalinganagar granite)
The granitoid occurs at the southern boundary of the Singhbhum Craton near its contact with the Rengali Province in the eastern Bhuban-Duburi sector (Fig. 1b). The rock is massive in nature with coarse grains of quartz and feldspar. It also contains many xenoliths of basic rock, chert, pink granite and BIF, presumably taken from the Singhbhum Craton (Fig. 3b). The same granitoid is found to intrude the mafic granulite at the hangingwall side of the Sukinda Thrust further south where it develops strong deformation features. This granitoid outcrops extensively in the eastern Rengali Province (Fig. 1b). It is undeformed and constituted of quartz, plagioclase, alkali feldspar and chlorite (Table 1). Most of the feldspar grains are altered and few chlorite grains have developed in the matrix. Zircon grains are abundant with subhedral to euhedral outlines.
Sample JK 54 (Balarampur charnockite)
This rock occurs within the eastern segment of the Rengali Province around Bhuban-Duburi sector, immediately south of its contact with the Singhbhum Craton marked by the Sukinda Thrust. The rock is a charnockitic gneiss (<10 modal% orthopyroxene) and is strongly deformed (Fig. 3c). It contains xenoliths of mafic granulite, amphibolite and BIF. Gneissic banding is mm thick and continuous throughout the rock. The rock is constituted of quartz, plagioclase, alkali feldspar, orthopyroxene, ilmenite and occasional garnet (Table 1). Zircon grains are abundant up to 500 μm in length. Quartz and feldspar grains show extreme grain refinement and evidences of shearing by developing a mylonitic fabric.
Central Rengali Province
Sample RNG 50D (Budhapal leucogranite)
This leucogranite occurs within the quartzofeldspathic gneiss and quartzite–muscovite schist associations in the central part of the Rengali Province, close to its southern contact (Fig. 1b). The rock is deformed and contains quartz, alkali, feldspar, plagioclase, garnet and abundant muscovite (Table 1). Garnet grains are post-kinematic in nature with elliptical to rounded outlines (Fig. 3d). Zircon is present in the matrix, but monazite is less abundant. Quartz and feldspar grains show cuspate grain boundaries implying crystallization from a melt phase.
Sample RNG 120A (Rengali charnockite)
This rock occurs at the central part of the Rengali Province close to the Rengali dam site (Fig. 1a). It contains a gneissic foliation marked by alternating dark coloured orthopyroxene–garnet–hornblende layers and leucocratic quartz–plagioclase–K-feldspar layers (Table 1). Orthopyroxene occurs as small to medium-sized grain aggregates with quartz, plagioclase and K-feldspar to from the granulitic assemblage and is replaced by hornblende grains (Fig. 3e). Evidence of deformation is characterized by foliation-parallel stretching of quartz and feldspar grains. Zircon grains are also in abundance.
Sample RNG 122 (Rengali mafic granulite)
This sample was collected from a location very close to the Rengali charnockite where it occurs as an enclave within charnockitic gneiss. The rock is distinctly foliated with few mm-thick alternate garnet–pyroxene rich layers and plagioclase–quartz rich layers (Fig. 3f). Coarse grained garnet, clinopyroxene and plagioclase form the peak assemblage and the grains are extremely deformed (Table 1). Locally, hornblende forms along the margin of clinopyroxene. Zircon grains are few in number, and are characteristically rounded in outline.
Sample RNG 129B (Rengali hornblende gneiss)
This rock is from a stone quarry where the host gneiss contains patches of charnockitic gneiss. Layers of leucogranite intrude the gneissic foliation of the host gneiss (Fig. 3g). The rock contains alternate layers of hornblende and quartzofeldspathic minerals. All minerals exhibit flattening parallel to the S2 foliation. Zircon and apatite grains occur as accessory phases.
Sample RNG 54 (Rengali felsic gneiss)
This rock intrudes both the charnockite and hornblende gneiss of central Rengali Province and shows a well-developed foliation defined by stretched quartz and feldspar (Fig. 3h). It contains quartz, alkali feldspar (perthitic) and ilmenite with minor plagioclase. The rock is strongly sheared and mylonitized in places. Zircon grains are elongated with rounded outline.
Analytical methods
Samples for zircon analysis were prepared by crushing and milling followed by water separation, sieving, heavy liquid separation and Franz magnetic barrier separation at Presidency University, Kolkata. Extreme care was taken to avoid cross contamination during sample preparation. Zircon grains were selected and handpicked for mounting at Hiroshima University. Epoxy mounts were prepared with zircon grains and the standard Duluth gabbroic anorthosite zircon (FC1) which has an age of 1099 ± 0.6 Ma (Paces & Miller, 1993). The mounts were polished and gold coated for observation under the JEOL scanning electron microscope at the Hiroshima University. Backscatter electron (BSE) and cathodoluminescence (CL) imaging was completed for each zircon grain to study the internal morphology as well as locate analyses. BSE images of selected zircon grains are shown in Fig. 5. Zircon analysis was completed at Hiroshima University using the SHRIMP IIe facility in two analytical sessions. The external zircon standard (SL3) was used to measure U contents. Raw data were processed using the CONCH program (Nelson, 2006), whereas post-analysis data reduction was done based on the correction of the standard analyses. Each of the seven scans were checked and corrected, and those with anomalous counts were rejected. Processed data for all the samples are quoted in Table 2 with 1σ uncertainty, and plotted in a Wetherill diagram to obtain group ages. The 207Pb–206Pb group age calculations (with MSWD) were made using the program ISOPLOT (Ludwig, 2008).
| Spot no. | U (ppm) | Th (ppm) | Pb (ppm) | Th/U (ppm) | 207Pb/206Pb | ±1Ϭ | 208Pb/206Pb | ±1Ϭ | 206Pb/238U | ±1Ϭ | 207Pb/235U | ±1Ϭ | % conc | 207Pb/206Pb age (Ma) | ±1Ϭ |
|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|
| Rengali garnet–cordierite–quartz granulite (RNG 61) | |||||||||||||||
| 1.1 | 265 | 181 | 259 | 0.683 | 0.291 | 0.0026 | 0.190 | 0.0048 | 0.766 | 0.023 | 30.721 | 0.978 | 107 | 3422 | 14 |
| 1.2 | 1071 | 774 | 915 | 0.723 | 0.290 | 0.0008 | 0.195 | 0.0007 | 0.668 | 0.020 | 26.735 | 0.793 | 96 | 3420 | 4 |
| 3.1 | 1658 | 187 | 1431 | 0.113 | 0.299 | 0.0006 | 0.030 | 0.0003 | 0.754 | 0.022 | 31.032 | 0.915 | 105 | 3463 | 3 |
| 3.2 | 259 | 137 | 223 | 0.529 | 0.290 | 0.0015 | 0.143 | 0.0011 | 0.700 | 0.021 | 27.994 | 0.852 | 100 | 3419 | 8 |
| 4.1 | 73 | 39 | 61 | 0.534 | 0.273 | 0.0038 | 0.135 | 0.0059 | 0.688 | 0.021 | 25.925 | 0.896 | 102 | 3324 | 22 |
| 4.2 | 142 | 104 | 115 | 0.732 | 0.267 | 0.0034 | 0.167 | 0.0059 | 0.653 | 0.019 | 24.063 | 0.81 | 99 | 3289 | 20 |
| 4.3 | 95 | 56 | 83 | 0.589 | 0.279 | 0.0039 | 0.195 | 0.0062 | 0.702 | 0.021 | 27.053 | 0.937 | 102 | 3359 | 22 |
| 6.1 | 304 | 314 | 355 | 1.033 | 0.301 | 0.0019 | 0.281 | 0.0027 | 0.863 | 0.025 | 35.8 | 1.103 | 115 | 3475 | 10 |
| 6.2 | 85 | 41 | 82 | 0.482 | 0.294 | 0.0045 | 0.129 | 0.0069 | 0.788 | 0.024 | 31.886 | 1.13 | 109 | 3437 | 24 |
| 7.1 | 987 | 764 | 1035 | 0.774 | 0.273 | 0.0009 | 0.211 | 0.0009 | 0.822 | 0.024 | 30.971 | 0.924 | 116 | 3326 | 5 |
| 8.1 | 1309 | 5 | 909 | 0.004 | 0.237 | 0.0007 | 0.001 | 0.0005 | 0.652 | 0.019 | 21.304 | 0.634 | 104 | 3100 | 5 |
| 8.2 | 24 | 10 | 19 | 0.417 | 0.273 | 0.0050 | 0.100 | 0.0031 | 0.662 | 0.021 | 24.895 | 0.954 | 99 | 3323 | 28 |
| 9.1 | 570 | 307 | 462 | 0.539 | 0.264 | 0.0010 | 0.146 | 0.0011 | 0.668 | 0.020 | 24.262 | 0.726 | 101 | 3268 | 6 |
| 9.2 | 226 | 142 | 191 | 0.628 | 0.266 | 0.0018 | 0.173 | 0.0019 | 0.685 | 0.020 | 25.123 | 0.777 | 103 | 3282 | 10 |
| 10.1 | 784 | 330 | 639 | 0.421 | 0.273 | 0.0009 | 0.120 | 0.0009 | 0.680 | 0.020 | 25.619 | 0.765 | 101 | 3326 | 5 |
| 11.1 | 449 | 386 | 391 | 0.860 | 0.285 | 0.0012 | 0.230 | 0.0012 | 0.667 | 0.020 | 26.235 | 0.792 | 97 | 3391 | 7 |
| 12.1 | 713 | 416 | 653 | 0.583 | 0.290 | 0.0010 | 0.154 | 0.0010 | 0.736 | 0.022 | 29.41 | 0.88 | 104 | 3416 | 5 |
| 14.1 | 276 | 182 | 260 | 0.659 | 0.300 | 0.0016 | 0.174 | 0.0013 | 0.740 | 0.022 | 30.589 | 0.935 | 103 | 3469 | 8 |
| 17.1 | 728 | 805 | 746 | 1.106 | 0.300 | 0.0009 | 0.301 | 0.0009 | 0.743 | 0.022 | 30.731 | 0.915 | 103 | 3470 | 5 |
| 18.1 | 599 | 319 | 583 | 0.533 | 0.311 | 0.0011 | 0.145 | 0.0007 | 0.776 | 0.023 | 33.312 | 0.995 | 105 | 3527 | 5 |
| 19.1 | 292 | 160 | 242 | 0.548 | 0.288 | 0.0017 | 0.149 | 0.0022 | 0.672 | 0.020 | 26.647 | 0.815 | 97 | 3405 | 9 |
| Rengali garnet–orthopyroxene–quartz granulite (RNG 62) | |||||||||||||||
| 1.1 | 1224 | 672 | 1154 | 0.549 | 0.300 | 0.0009 | 0.151 | 0.0007 | 0.753 | 0.020 | 31.148 | 0.832 | 104 | 3471 | 4 |
| 1.2 | 1553 | 733 | 1346 | 0.472 | 0.307 | 0.0008 | 0.136 | 0.0005 | 0.698 | 0.018 | 29.522 | 0.786 | 97 | 3505 | 4 |
| 1.3 | 1022 | 395 | 966 | 0.386 | 0.308 | 0.0007 | 0.105 | 0.0004 | 0.777 | 0.020 | 33.005 | 0.879 | 106 | 3512 | 4 |
| 2.1 | 987 | 1487 | 930 | 1.507 | 0.267 | 0.0008 | 0.430 | 0.0011 | 0.643 | 0.017 | 23.685 | 0.634 | 97 | 3289 | 5 |
| 2.2 | 162 | 104 | 126 | 0.642 | 0.270 | 0.0019 | 0.183 | 0.0027 | 0.618 | 0.016 | 22.96 | 0.646 | 94 | 3304 | 11 |
| 2.3 | 337 | 294 | 293 | 0.872 | 0.265 | 0.0015 | 0.229 | 0.0015 | 0.675 | 0.018 | 24.677 | 0.684 | 102 | 3277 | 9 |
| 3.1 | 1176 | 286 | 978 | 0.243 | 0.273 | 0.0009 | 0.068 | 0.0005 | 0.720 | 0.019 | 27.08 | 0.726 | 105 | 3323 | 5 |
| 3.2 | 408 | 172 | 316 | 0.422 | 0.273 | 0.0014 | 0.117 | 0.0010 | 0.646 | 0.017 | 24.317 | 0.668 | 97 | 3323 | 8 |
| 4.1 | 770 | 176 | 622 | 0.229 | 0.304 | 0.0010 | 0.065 | 0.0005 | 0.686 | 0.018 | 28.779 | 0.774 | 96 | 3492 | 5 |
| 4.2 | 355 | 194 | 342 | 0.546 | 0.311 | 0.0013 | 0.147 | 0.0009 | 0.768 | 0.020 | 32.977 | 0.897 | 104 | 3528 | 6 |
| 5.1 | 234 | 199 | 221 | 0.850 | 0.283 | 0.0022 | 0.241 | 0.0027 | 0.715 | 0.019 | 27.903 | 0.793 | 103 | 3379 | 12 |
| 7.1 | 444 | 368 | 384 | 0.829 | 0.270 | 0.0013 | 0.220 | 0.0013 | 0.673 | 0.018 | 25.027 | 0.685 | 100 | 3304 | 8 |
| 7.2 | 191 | 115 | 170 | 0.602 | 0.275 | 0.0020 | 0.159 | 0.0027 | 0.721 | 0.019 | 27.366 | 0.772 | 105 | 3336 | 11 |
| 8.1 | 122 | 80 | 113 | 0.656 | 0.279 | 0.0020 | 0.164 | 0.0030 | 0.736 | 0.020 | 28.348 | 0.804 | 106 | 3359 | 11 |
| 8.2 | 178 | 168 | 157 | 0.944 | 0.281 | 0.0018 | 0.255 | 0.0020 | 0.666 | 0.018 | 25.82 | 0.72 | 98 | 3369 | 10 |
| 9.2 | 658 | 42 | 478 | 0.064 | 0.257 | 0.0009 | 0.016 | 0.0002 | 0.663 | 0.017 | 23.487 | 0.631 | 102 | 3229 | 5 |
| 10.1 | 480 | 171 | 418 | 0.356 | 0.280 | 0.0012 | 0.099 | 0.0009 | 0.731 | 0.019 | 28.222 | 0.765 | 105 | 3364 | 7 |
| 11.1 | 229 | 189 | 201 | 0.825 | 0.253 | 0.0017 | 0.232 | 0.0025 | 0.686 | 0.018 | 23.929 | 0.67 | 105 | 3204 | 11 |
| 12.2 | 262 | 223 | 229 | 0.851 | 0.265 | 0.0040 | 0.246 | 0.0086 | 0.675 | 0.018 | 24.681 | 0.797 | 101 | 3277 | 24 |
| 12.3 | 214 | 118 | 174 | 0.551 | 0.266 | 0.0017 | 0.151 | 0.0019 | 0.664 | 0.018 | 24.376 | 0.68 | 100 | 3285 | 10 |
| 14.1 | 262 | 44 | 176 | 0.168 | 0.232 | 0.0045 | 0.071 | 0.0097 | 0.615 | 0.016 | 19.649 | 0.686 | 101 | 3064 | 31 |
| 14.2 | 209 | 112 | 170 | 0.536 | 0.257 | 0.0016 | 0.141 | 0.0016 | 0.669 | 0.018 | 23.705 | 0.66 | 102 | 3228 | 10 |
| 15.1 | 1031 | 275 | 916 | 0.267 | 0.280 | 0.0008 | 0.077 | 0.0005 | 0.760 | 0.020 | 29.376 | 0.783 | 108 | 3366 | 4 |
| Balarampur charnockite (JK 54) | |||||||||||||||
| 2.1 | 194 | 121 | 140 | 0.624 | 0.233 | 0.0016 | 0.175 | 0.0023 | 0.595 | 0.013 | 19.159 | 0.463 | 98 | 3075 | 11 |
| 2.2 | 1361 | 122 | 779 | 0.090 | 0.236 | 0.0005 | 0.026 | 0.0004 | 0.527 | 0.012 | 17.153 | 0.389 | 88 | 3093 | 4 |
| 2.3 | 478 | 64 | 305 | 0.134 | 0.230 | 0.0009 | 0.036 | 0.0008 | 0.583 | 0.013 | 18.477 | 0.427 | 97 | 3051 | 6 |
| 3.1 | 439 | 69 | 273 | 0.157 | 0.232 | 0.0009 | 0.042 | 0.0012 | 0.564 | 0.013 | 18.052 | 0.42 | 94 | 3066 | 6 |
| 3.2 | 189 | 123 | 127 | 0.651 | 0.229 | 0.0009 | 0.175 | 0.0012 | 0.556 | 0.013 | 17.578 | 0.413 | 94 | 3047 | 6 |
| 4.1 | 172 | 61 | 80 | 0.355 | 0.220 | 0.0014 | 0.102 | 0.0023 | 0.408 | 0.009 | 12.382 | 0.3 | 74 | 2983 | 10 |
| 4.2 | 897 | 29 | 494 | 0.032 | 0.201 | 0.0004 | 0.009 | 0.0004 | 0.529 | 0.012 | 14.662 | 0.333 | 96 | 2836 | 4 |
| 4.3 | 976 | 37 | 460 | 0.038 | 0.203 | 0.0004 | 0.011 | 0.0002 | 0.451 | 0.010 | 12.601 | 0.286 | 84 | 2847 | 3 |
| 9.1 | 744 | 288 | 518 | 0.387 | 0.258 | 0.0015 | 0.112 | 0.0026 | 0.591 | 0.013 | 20.983 | 0.501 | 93 | 3232 | 9 |
| 12.1 | 723 | 26 | 444 | 0.036 | 0.210 | 0.0009 | 0.010 | 0.0005 | 0.585 | 0.013 | 16.957 | 0.394 | 102 | 2906 | 7 |
| 15.2 | 209 | 69 | 137 | 0.330 | 0.234 | 0.0032 | 0.096 | 0.0059 | 0.577 | 0.013 | 18.623 | 0.524 | 95 | 3079 | 22 |
| 16.1 | 3915 | 149 | 2220 | 0.038 | 0.213 | 0.0004 | 0.010 | 0.0001 | 0.539 | 0.012 | 15.838 | 0.357 | 95 | 2930 | 3 |
| 16.2 | 266 | 22 | 134 | 0.083 | 0.210 | 0.0017 | 0.022 | 0.0018 | 0.476 | 0.011 | 13.776 | 0.343 | 86 | 2906 | 13 |
| Kalinganagar granite (JK 53) | |||||||||||||||
| 1.1 | 594 | 251 | 312 | 0.423 | 0.195 | 0.0009 | 0.114 | 0.0011 | 0.463 | 0.015 | 12.424 | 0.409 | 88 | 2783 | 8 |
| 2.1 | 396 | 136 | 217 | 0.343 | 0.200 | 0.0011 | 0.097 | 0.0011 | 0.488 | 0.016 | 13.427 | 0.446 | 91 | 2823 | 9 |
| 4.1 | 328 | 127 | 185 | 0.387 | 0.195 | 0.0013 | 0.104 | 0.0017 | 0.499 | 0.016 | 13.46 | 0.45 | 94 | 2789 | 10 |
| 6.1 | 480 | 169 | 273 | 0.352 | 0.201 | 0.0010 | 0.097 | 0.0010 | 0.508 | 0.016 | 14.064 | 0.464 | 93 | 2833 | 8 |
| 8.1 | 475 | 185 | 263 | 0.389 | 0.199 | 0.0010 | 0.111 | 0.0009 | 0.491 | 0.016 | 13.432 | 0.443 | 91 | 2814 | 8 |
| 9.1 | 103 | 22 | 51 | 0.214 | 0.178 | 0.0014 | 0.060 | 0.0010 | 0.468 | 0.015 | 11.503 | 0.393 | 94 | 2636 | 13 |
| 11.1 | 382 | 191 | 228 | 0.500 | 0.200 | 0.0011 | 0.143 | 0.0013 | 0.515 | 0.017 | 14.155 | 0.469 | 95 | 2822 | 9 |
| 12.1 | 489 | 161 | 270 | 0.329 | 0.196 | 0.0010 | 0.091 | 0.0011 | 0.496 | 0.016 | 13.374 | 0.442 | 93 | 2789 | 8 |
| 14.1 | 425 | 238 | 251 | 0.560 | 0.201 | 0.0010 | 0.160 | 0.0010 | 0.504 | 0.016 | 13.951 | 0.46 | 93 | 2831 | 8 |
| 16.1 | 78 | 42 | 47 | 0.538 | 0.194 | 0.0028 | 0.131 | 0.0052 | 0.507 | 0.016 | 13.584 | 0.504 | 95 | 2778 | 24 |
| 21.1 | 298 | 103 | 169 | 0.346 | 0.196 | 0.0015 | 0.087 | 0.0020 | 0.503 | 0.016 | 13.598 | 0.461 | 94 | 2795 | 12 |
| 21.2 | 80 | 48 | 47 | 0.600 | 0.189 | 0.0025 | 0.137 | 0.0046 | 0.502 | 0.016 | 13.069 | 0.477 | 96 | 2733 | 22 |
| 22.1 | 213 | 67 | 118 | 0.315 | 0.201 | 0.0016 | 0.080 | 0.0025 | 0.497 | 0.016 | 13.771 | 0.466 | 92 | 2834 | 13 |
| 28.1 | 484 | 146 | 234 | 0.302 | 0.194 | 0.0010 | 0.089 | 0.0015 | 0.433 | 0.014 | 11.564 | 0.382 | 84 | 2773 | 9 |
| 29.1 | 361 | 124 | 218 | 0.343 | 0.201 | 0.0011 | 0.099 | 0.0012 | 0.538 | 0.017 | 14.941 | 0.494 | 98 | 2837 | 9 |
| 35.1 | 733 | 343 | 448 | 0.468 | 0.197 | 0.0008 | 0.131 | 0.0011 | 0.531 | 0.017 | 14.471 | 0.472 | 98 | 2806 | 6 |
| Rengali charnockite (RNG 120A) | |||||||||||||||
| 1.1 | 312 | 118 | 207 | 0.378 | 0.210 | 0.0019 | 0.104 | 0.0023 | 0.588 | 0.012 | 17.011 | 0.396 | 103 | 2905 | 15 |
| 1.2 | 139 | 87 | 103 | 0.626 | 0.199 | 0.0039 | 0.183 | 0.0075 | 0.623 | 0.013 | 17.076 | 0.526 | 111 | 2815 | 32 |
| 2.1 | 463 | 75 | 234 | 0.162 | 0.164 | 0.0011 | 0.044 | 0.0015 | 0.487 | 0.010 | 10.997 | 0.239 | 102 | 2495 | 12 |
| 2.2 | 138 | 81 | 100 | 0.587 | 0.205 | 0.0024 | 0.156 | 0.0035 | 0.614 | 0.013 | 17.399 | 0.433 | 108 | 2870 | 19 |
| 2.3 | 618 | 193 | 408 | 0.312 | 0.201 | 0.0009 | 0.089 | 0.0008 | 0.593 | 0.012 | 16.41 | 0.34 | 106 | 2831 | 7 |
| 3.1 | 207 | 108 | 139 | 0.522 | 0.199 | 0.0016 | 0.149 | 0.0016 | 0.580 | 0.012 | 15.899 | 0.359 | 105 | 2817 | 13 |
| 3.2 | 144 | 88 | 97 | 0.611 | 0.195 | 0.0028 | 0.176 | 0.0050 | 0.568 | 0.012 | 15.318 | 0.407 | 104 | 2788 | 24 |
| 3.3 | 627 | 375 | 426 | 0.598 | 0.204 | 0.0011 | 0.172 | 0.0016 | 0.574 | 0.011 | 16.127 | 0.34 | 102 | 2856 | 9 |
| 5.1 | 581 | 53 | 298 | 0.091 | 0.160 | 0.0010 | 0.024 | 0.0011 | 0.503 | 0.010 | 11.087 | 0.238 | 107 | 2454 | 11 |
| 5.2 | 645 | 59 | 318 | 0.091 | 0.163 | 0.0009 | 0.025 | 0.0006 | 0.482 | 0.010 | 10.83 | 0.228 | 102 | 2487 | 9 |
| 5.3 | 628 | 46 | 382 | 0.073 | 0.198 | 0.0011 | 0.021 | 0.0017 | 0.579 | 0.011 | 15.828 | 0.335 | 105 | 2811 | 9 |
| 5.4 | 212 | 54 | 120 | 0.255 | 0.197 | 0.0018 | 0.078 | 0.0015 | 0.515 | 0.010 | 13.955 | 0.325 | 96 | 2798 | 15 |
| 10.2 | 413 | 102 | 223 | 0.247 | 0.159 | 0.0013 | 0.069 | 0.0019 | 0.512 | 0.010 | 11.239 | 0.252 | 109 | 2446 | 14 |
| 10.3 | 612 | 97 | 323 | 0.158 | 0.163 | 0.0008 | 0.047 | 0.0007 | 0.505 | 0.010 | 11.362 | 0.236 | 106 | 2487 | 8 |
| 10.4 | 132 | 74 | 90 | 0.561 | 0.206 | 0.0021 | 0.158 | 0.0038 | 0.584 | 0.012 | 16.592 | 0.387 | 103 | 2875 | 16 |
| Budhapal leucogranite (RNG 50D) | |||||||||||||||
| 1.1 | 434 | 291 | 290 | 0.671 | 0.196 | 0.0012 | 0.203 | 0.0013 | 0.554 | 0.009 | 14.932 | 0.26 | 102 | 2790 | 10 |
| 2.1 | 160 | 91 | 102 | 0.569 | 0.197 | 0.0013 | 0.161 | 0.0015 | 0.546 | 0.009 | 14.864 | 0.263 | 100 | 2804 | 11 |
| 2.2 | 148 | 87 | 98 | 0.588 | 0.203 | 0.0015 | 0.164 | 0.0022 | 0.560 | 0.009 | 15.653 | 0.281 | 101 | 2850 | 12 |
| 3.1 | 114 | 65 | 75 | 0.570 | 0.198 | 0.0017 | 0.151 | 0.0028 | 0.563 | 0.009 | 15.363 | 0.29 | 103 | 2808 | 14 |
| 3.2 | 109 | 58 | 71 | 0.532 | 0.197 | 0.0018 | 0.144 | 0.0028 | 0.566 | 0.009 | 15.399 | 0.292 | 103 | 2805 | 15 |
| 4.1 | 138 | 92 | 94 | 0.667 | 0.187 | 0.0014 | 0.174 | 0.0020 | 0.582 | 0.009 | 14.999 | 0.272 | 109 | 2715 | 12 |
| 4.2 | 822 | 91 | 407 | 0.111 | 0.163 | 0.0005 | 0.032 | 0.0003 | 0.482 | 0.007 | 10.818 | 0.172 | 102 | 2484 | 5 |
| 4.3 | 106 | 56 | 68 | 0.528 | 0.198 | 0.0015 | 0.144 | 0.0026 | 0.554 | 0.009 | 15.088 | 0.277 | 101 | 2806 | 13 |
| 6.1 | 1182 | 634 | 776 | 0.536 | 0.197 | 0.0006 | 0.151 | 0.0005 | 0.566 | 0.009 | 15.378 | 0.245 | 103 | 2803 | 5 |
| 7.1 | 987 | 701 | 852 | 0.710 | 0.196 | 0.0009 | 0.175 | 0.0010 | 0.731 | 0.011 | 19.732 | 0.33 | 127 | 2791 | 8 |
| 8.2 | 802 | 428 | 578 | 0.534 | 0.200 | 0.0010 | 0.149 | 0.0008 | 0.620 | 0.010 | 17.127 | 0.288 | 110 | 2829 | 8 |
| Rengali mafic granulite (RNG 122) | |||||||||||||||
| 1.1 | 136 | 113 | 81 | 0.831 | 0.190 | 0.0011 | 0.222 | 0.0021 | 0.480 | 0.010 | 12.571 | 0.283 | 92 | 2742 | 10 |
| 1.2 | 190 | 132 | 126 | 0.695 | 0.202 | 0.0011 | 0.192 | 0.0014 | 0.548 | 0.012 | 15.26 | 0.339 | 99 | 2841 | 9 |
| 1.3 | 274 | 228 | 183 | 0.832 | 0.203 | 0.0019 | 0.239 | 0.0040 | 0.538 | 0.011 | 15.09 | 0.362 | 97 | 2853 | 15 |
| 2.1 | 152 | 131 | 68 | 0.862 | 0.152 | 0.0012 | 0.247 | 0.0019 | 0.370 | 0.008 | 7.725 | 0.18 | 86 | 2364 | 14 |
| 3.1 | 84 | 25 | 40 | 0.298 | 0.160 | 0.0017 | 0.083 | 0.0026 | 0.433 | 0.009 | 9.557 | 0.239 | 94 | 2456 | 18 |
| 5.1 | 196 | 12 | 75 | 0.061 | 0.150 | 0.0008 | 0.016 | 0.0012 | 0.377 | 0.008 | 7.83 | 0.174 | 88 | 2351 | 10 |
| 5.2 | 618 | 67 | 272 | 0.108 | 0.156 | 0.0006 | 0.031 | 0.0003 | 0.431 | 0.009 | 9.235 | 0.198 | 96 | 2408 | 6 |
| 6.3 | 87 | 51 | 46 | 0.586 | 0.164 | 0.0015 | 0.165 | 0.0025 | 0.451 | 0.009 | 10.215 | 0.241 | 96 | 2499 | 15 |
| 6.4 | 74 | 44 | 37 | 0.595 | 0.149 | 0.0014 | 0.169 | 0.0027 | 0.433 | 0.009 | 8.885 | 0.214 | 100 | 2331 | 17 |
| 7.1 | 492 | 37 | 241 | 0.075 | 0.163 | 0.0006 | 0.022 | 0.0003 | 0.479 | 0.010 | 10.798 | 0.231 | 101 | 2492 | 6 |
| 8.1 | 67 | 46 | 38 | 0.687 | 0.159 | 0.0019 | 0.187 | 0.0038 | 0.481 | 0.010 | 10.537 | 0.264 | 104 | 2444 | 20 |
| 8.2 | 315 | 182 | 173 | 0.578 | 0.163 | 0.0007 | 0.165 | 0.0009 | 0.480 | 0.010 | 10.76 | 0.233 | 102 | 2483 | 8 |
| Rengali hornblednde gneiss (RNG 129B) | |||||||||||||||
| 2.2 | 104 | 54 | 53 | 0.519 | 0.185 | 0.0009 | 0.144 | 0.0013 | 0.444 | 0.026 | 11.336 | 0.679 | 88 | 2700 | 8 |
| 3.1 | 125 | 64 | 74 | 0.512 | 0.199 | 0.0007 | 0.134 | 0.0009 | 0.513 | 0.030 | 14.087 | 0.84 | 95 | 2821 | 6 |
| 4.1 | 188 | 126 | 120 | 0.670 | 0.204 | 0.0006 | 0.177 | 0.0009 | 0.533 | 0.032 | 14.985 | 0.891 | 96 | 2858 | 5 |
| 4.2 | 174 | 120 | 102 | 0.690 | 0.194 | 0.0006 | 0.187 | 0.0009 | 0.491 | 0.029 | 13.117 | 0.781 | 93 | 2774 | 5 |
| 5.1 | 141 | 81 | 80 | 0.574 | 0.198 | 0.0009 | 0.155 | 0.0013 | 0.485 | 0.029 | 13.262 | 0.794 | 91 | 2811 | 8 |
| 5.2 | 156 | 91 | 94 | 0.583 | 0.200 | 0.0009 | 0.158 | 0.0013 | 0.513 | 0.030 | 14.162 | 0.847 | 94 | 2827 | 7 |
| 6.1 | 98 | 71 | 61 | 0.724 | 0.202 | 0.0012 | 0.203 | 0.0020 | 0.509 | 0.030 | 14.158 | 0.852 | 93 | 2840 | 10 |
| 7.1 | 123 | 74 | 73 | 0.602 | 0.205 | 0.0011 | 0.163 | 0.0018 | 0.507 | 0.030 | 14.344 | 0.861 | 92 | 2869 | 9 |
| 8.1 | 119 | 88 | 70 | 0.739 | 0.193 | 0.0010 | 0.205 | 0.0016 | 0.485 | 0.029 | 12.926 | 0.775 | 92 | 2769 | 8 |
| 9.1 | 152 | 89 | 91 | 0.586 | 0.201 | 0.0008 | 0.158 | 0.0011 | 0.510 | 0.030 | 14.161 | 0.846 | 94 | 2838 | 7 |
| 10.1 | 104 | 63 | 62 | 0.606 | 0.201 | 0.0011 | 0.158 | 0.0017 | 0.506 | 0.030 | 14.052 | 0.845 | 93 | 2837 | 9 |
| 11.1 | 137 | 80 | 78 | 0.584 | 0.196 | 0.0008 | 0.165 | 0.0010 | 0.487 | 0.029 | 13.181 | 0.788 | 91 | 2797 | 7 |
| 11.2 | 157 | 92 | 93 | 0.586 | 0.203 | 0.0011 | 0.166 | 0.0019 | 0.504 | 0.030 | 14.119 | 0.847 | 92 | 2851 | 9 |
| 12.1 | 115 | 63 | 70 | 0.548 | 0.200 | 0.0013 | 0.158 | 0.0025 | 0.530 | 0.031 | 14.634 | 0.883 | 97 | 2828 | 11 |
| 13.1 | 340 | 208 | 205 | 0.612 | 0.200 | 0.0006 | 0.168 | 0.0007 | 0.512 | 0.030 | 14.077 | 0.837 | 94 | 2823 | 4 |
| 14.1 | 181 | 112 | 99 | 0.619 | 0.189 | 0.0009 | 0.167 | 0.0016 | 0.466 | 0.028 | 12.164 | 0.728 | 90 | 2736 | 8 |
| 15.1 | 875 | 63 | 383 | 0.072 | 0.161 | 0.0004 | 0.020 | 0.0002 | 0.430 | 0.025 | 9.578 | 0.569 | 93 | 2471 | 4 |
| Rengali leucogranite (RNG 54) | |||||||||||||||
| 1.1 | 682 | 70 | 241 | 0.103 | 0.158 | 0.000 | 0.027 | 0.000 | 0.347 | 0.016 | 7.567 | 0.354 | 79 | 2437 | 5 |
| 2.1 | 790 | 58 | 452 | 0.073 | 0.192 | 0.000 | 0.021 | 0.000 | 0.547 | 0.025 | 14.505 | 0.675 | 102 | 2762 | 3 |
| 2.2 | 756 | 160 | 366 | 0.212 | 0.164 | 0.000 | 0.060 | 0.000 | 0.459 | 0.021 | 10.385 | 0.484 | 97 | 2498 | 3 |
| 3.1 | 1274 | 93 | 473 | 0.073 | 0.163 | 0.000 | 0.020 | 0.000 | 0.365 | 0.017 | 8.217 | 0.382 | 81 | 2488 | 3 |
| 4.1 | 1593 | 898 | 621 | 0.564 | 0.163 | 0.000 | 0.064 | 0.000 | 0.368 | 0.017 | 8.276 | 0.385 | 81 | 2488 | 3 |
| 4.2 | 1318 | 201 | 409 | 0.153 | 0.155 | 0.000 | 0.036 | 0.000 | 0.303 | 0.014 | 6.484 | 0.302 | 71 | 2407 | 3 |
| 5.1 | 813 | 228 | 321 | 0.280 | 0.168 | 0.000 | 0.094 | 0.000 | 0.363 | 0.017 | 8.387 | 0.391 | 79 | 2535 | 4 |
| 6.1 | 1653 | 232 | 628 | 0.140 | 0.165 | 0.000 | 0.038 | 0.000 | 0.367 | 0.017 | 8.368 | 0.389 | 80 | 2509 | 3 |
| 7.1 | 1439 | 215 | 521 | 0.149 | 0.158 | 0.000 | 0.042 | 0.000 | 0.350 | 0.016 | 7.619 | 0.355 | 79 | 2434 | 3 |
| 8.1 | 1063 | 528 | 401 | 0.497 | 0.169 | 0.000 | 0.131 | 0.000 | 0.337 | 0.016 | 7.850 | 0.366 | 74 | 2545 | 4 |
| 9.1 | 558 | 100 | 209 | 0.179 | 0.184 | 0.001 | 0.052 | 0.001 | 0.351 | 0.016 | 8.904 | 0.416 | 72 | 2689 | 5 |
| 19.1 | 2077 | 177 | 1077 | 0.085 | 0.165 | 0.000 | 0.025 | 0.000 | 0.506 | 0.023 | 11.537 | 0.536 | 105 | 2511 | 2 |
| 19.2 | 1061 | 955 | 748 | 0.900 | 0.197 | 0.000 | 0.267 | 0.000 | 0.558 | 0.026 | 15.186 | 0.707 | 102 | 2804 | 3 |
| 20.1 | 1304 | 198 | 654 | 0.152 | 0.165 | 0.000 | 0.044 | 0.000 | 0.482 | 0.022 | 10.960 | 0.510 | 101 | 2506 | 3 |
| 20.2 | 1637 | 1000 | 1033 | 0.611 | 0.195 | 0.000 | 0.153 | 0.000 | 0.543 | 0.025 | 14.596 | 0.679 | 100 | 2785 | 3 |
| 23.1 | 88 | 56 | 49 | 0.636 | 0.181 | 0.001 | 0.176 | 0.001 | 0.474 | 0.022 | 11.844 | 0.558 | 94 | 2665 | 8 |
| 24.1 | 1148 | 309 | 687 | 0.269 | 0.196 | 0.000 | 0.075 | 0.000 | 0.547 | 0.025 | 14.808 | 0.689 | 101 | 2797 | 3 |
| 24.2 | 1168 | 192 | 546 | 0.164 | 0.166 | 0.000 | 0.042 | 0.000 | 0.450 | 0.021 | 10.286 | 0.479 | 95 | 2516 | 3 |
| 28.1 | 1657 | 168 | 1016 | 0.101 | 0.196 | 0.000 | 0.032 | 0.000 | 0.580 | 0.027 | 15.686 | 0.729 | 106 | 2793 | 2 |
| 28.2 | 1060 | 169 | 505 | 0.159 | 0.162 | 0.000 | 0.049 | 0.000 | 0.457 | 0.021 | 10.209 | 0.476 | 98 | 2476 | 4 |
| 31.1 | 437 | 48 | 197 | 0.110 | 0.156 | 0.000 | 0.033 | 0.000 | 0.442 | 0.020 | 9.511 | 0.445 | 98 | 2415 | 5 |
| 31.2 | 1130 | 454 | 689 | 0.402 | 0.193 | 0.000 | 0.102 | 0.000 | 0.547 | 0.025 | 14.538 | 0.676 | 102 | 2767 | 2 |
Results
Zircon data from Eastern Rengali Province
RNG 61 (Bhuban garnet–cordierite–quartz granulite)
Zircon grains in this sample are mostly subhedral with prismatic faces. Some grains have rounded outlines and oval shapes (Fig. 4a,b). The size of the grains varies 200–300 μm in length and 50–100 μm across. Almost all the grains have oscillatory zoning which occasionally shows blurring in core regions. There is no well-defined core and a thin metamorphic overgrowth is present in most grains. The core regions of some grains show disturbed oscillatory zoning, truncation and patchy zoning; all indicative of a later tectonothermal event modifying the zircon morphology. Many zircon grains contain inclusions of quartz, biotite, apatite and feldspar mostly at the core regions. Radial cracks are developed surrounding high-U zones, but irregular cracks were possibly developed during crushing. Such zones were avoided during analysis. A very thin high-U overgrowth zone is present in some grains, but the width of this zone is mostly limited up to 10 μm. All the grains are therefore considered as detrital zircon with limited metamorphic overgrowth. Th, U and Pb contents vary widely from spots even within a single grain. Th/U ratios from analysed spots vary between 0.42 and 1.1 with two spots showing very low Th/U ratios (spots 3.1 and 8.1, Table 2). The latter spots lie within the high-U zones.
A total of 21 spots was analysed from 14 grains. More than one spot was analysed in the larger grains. All the spot data are nearly concordant (96–109% concordance, while two spots give >110% concordance). The 204Pb-corrected 207Pb–206Pb data are plotted in a Wetherill diagram (Fig. 5a). Based on the CL characters, Th/U ratios and spot data of zircon, five groups of data are identified. Out of these, three groups show statistically significant pooled (95% confidence) data of 3466 ± 7 Ma (MSWD = 1.0; n = 5), 3413 ± 12 Ma (MSWD = 3.0; n = 6) and 3308 ± 24 Ma (MSWD = 12.0; n = 8), whereas others show spot data within the range c. 3527–3100 Ma (1σ confidence). The first group is characteristic of oscillatory-zoned bright-CL zircon with Th/U ratio in the range 0.11–1.11. Spot dates in this group show weak reverse discordance. The second group represents dark-CL weakly oscillatory-zoned zircon with Th/U ratios within 0.55–0.86. The third group represents bright-CL oscillatory-zoned to sector-zoned zircon with Th/U ratios in the range 0.42–0.72. Spot 18.1 shows the oldest date of 3527 ± 5 Ma (1σ) while spot 8.1 shows the youngest date of 3100 ± 5 Ma (1σ). Probability density plot of all the spot data shows sharp peaks in the range 3300–3450 Ma (Fig. 5a inset).
RNG 62 (Bhuban garnet–orthopyroxene–quartz granulite)
Zircon grains in this sample are mostly anhedral with poorly developed prismatic faces. Some grains are broken with prominent oval outlines (Fig. 4c,d). The grain size varies from 100 to 200 μm in length and 50–80 μm across. Most grains show faint traces of oscillatory zoning. There is no clear core-mantle structure, but the inner high-U regions of zircon are patchy with high-BSE and dark-CL characters. Inclusions of quartz, biotite, apatite and feldspar are present. It was not possible to analyse the thin high-U overgrowth zone (<10 μm thick), although such a zone is present in many grains. These zircon grains are thus considered all detrital with little metamorphic overgrowth. The Th, U and Pb contents vary widely among spots, and even within a single grain. Th/U ratios from analysed spots vary in the range 0.23–1.51 with two spots showing low Th/U ratios (spots 9.2 & 14.1).
A total of 23 spots was analysed from 13 grains. Multiple analyses were done on larger grains. All spot data were nearly concordant (94–108% concordance), whereas only one (15.1) showed moderately high reverse discordance. The Weatherill plot of 204Pb-corrected 207Pb–206Pb dates shows a sparingly scattered distribution (Fig. 5b). Based on the chemistry and CL characters of zircon, eight groups of data have been identified. Of these, three groups show statistically significant pool (95% confidence) data of 3353 ± 19 Ma (MSWD = 9.8; n = 7; oscillatory-zoned moderate CL with Th/U = 0.42–1.51), 3295 ± 14 Ma (MSWD = 3.5; n = 7; oscillatory- to sector-zoned bright-CL with Th/U = 0.24–0.94) and 3222 ± 13 Ma (MSWD = 2.2; n = 3; diffused oscillatory zoning with Th/U = 0.06–0.82), whereas others show spot data within the range c. 3528–3087 Ma (1σ confidence). Spot 4.2 shows the oldest date of 3528 ± 6 Ma (1σ), whereas spot 14.1 shows the youngest date of 3064 ± 31 Ma (1σ). Probability density plot of all the spot dates shows sharp peaks in the range 3250–3400 Ma (Fig. 5b inset).
JK 54 (Balarampur charnockite)
Zircon grains are mostly euhedral with well-developed prismatic faces and high aspect ratios. The size of the grains varies from 200 to 500 μm in length and 100–200 μm across, but the majority are more than 300 μm long. They usually contain a high-BSE core surrounded by oscillatory-zoned overgrowth. Radial cracks are present surrounding the high-U cores. This oscillatory-zoned domain is surrounded by a thick domain showing simple or patchy zoning that even becomes featureless in some grains (Fig. 4g). Faint traces of sector zoning are visible in a few grains. Based on shape and zoning patterns, the zircon grains look igneous in origin with negligible metamorphic overgrowth (represented by a low-BSE and high-CL thin zone). All grains are characterized by numerous small inclusions of quartz, biotite, apatite and feldspar grains. Larger inclusions of quartz are common in the cores, while small inclusions form trails within the oscillatory and simple-zoned zircon domain (Fig. 4e,f). In places, quartz inclusions occur within the exterior part of the zircon grains. Th, U and Pb contents vary widely from spots even within a single grain. Th/U ratios from analysed spots vary widely where half of the spots show values <0.1 while the other half shows values ranging 0.13–0.99 (Table 2). Some high-U and low-Th spots show even lower Th/U ratio (<0.04).
A total of 13 spots were analysed from seven grains. Multiple analyses were made in larger grains. The majority of the spot ages are nearly concordant (93–102% concordance, n = 9), with only one spot with minor reverse discordance. The rest of the spot dates are moderately discordant (79–89% discordance, n = 4). In the Weatherill diagram, the plot of 204Pb-corrected 207Pb–206Pb dates show a moderately scattered distribution (Fig. 6c). Based on the chemistry and CL characters of zircon, several groups have been identified. Of these, only one group shows a statistically significant pooled (95% confidence) age of 3058 ± 15 Ma (MSWD = 2.4; n = 5; oscillatory zoned with Th/U ratio = 0.13–0.65) while others show spot dates within the range c. 3232–2836 Ma (1σ confidence). Spot 9.1 yields the oldest date of 3232 ± 9 Ma (Table 2), whereas spot 4.2 yields the youngest date of 2836 ± 4 Ma (Table 2). Probability density plot of all the spot dates shows a sharp twin peak in the range 3100–3050 Ma, followed by two peaks at 2930 and 2846 Ma (Fig. 5c inset).
JK 53 (Kalinganagar granite)
Zircon grains mounted for this sample are subhedral to euhedral with high aspect ratios. The majority of the grains are 150–200 μm long and 50–80 μm across. Euhedral grains show faint traces of hourglass like sector zoning (Fig. 4g,h). A few zircon grains contain a xenocrystic core with faint traces of oscillatory zoning, but most other grains appear to be neoblastic. Some even show the presence of planar zoning with a low-U dark BSE interior and high-U bright BSE exterior. Sparse quartz and biotite inclusions are present. Although U and Th contents vary between high-BSE and low-BSE sectors, Th/U ratios show moderate variation compared to other samples (0.21–0.60). Fourteen grains were analysed in which two analyses were from xenocrystic cores with highly discordant U–Pb spot data with 207Pb/206Pb dates of 3137 ± 21 and 3063 ± 13 Ma respectively. The latter coincides with the inferred crystallization age of the adjacent sample JK 54. The rest of the analyses are nearly concordant and are plot close to the Wetherill Concordia curve (Fig. 5d). Spot data define a 207Pb/206Pb pooled date of 2809 ± 13 Ma (MSWD = 5.8; 95% confidence, n = 14). Probability density plot of 207Pb/206Pb spot data defines a twin peak with ages 2828 and 2784 Ma respectively (Fig. 5d inset).
Zircon data from Central Rengali Province
RNG 120A (Rengali charnockite)
Zircon grains in this sample are subhedral in outline with few grains of high aspect ratio. The size of the grains varies from 200 to 300 μm in length and 100–150 μm across. Almost all the grains exhibit core-mantle structure (Fig. 4i,j) wherein the core region shows faint traces of oscillatory zoning with local high-U patches. A few grains contain relicts of xenocrystic cores. Large rounded inclusions of quartz are present in core regions of many grains. A moderately thick (30–50 μm) high-U overgrowth zone is found in most of the grains. This overgrowth zone is featureless in most cases, but the presence of simple planar zoning is evident in some grains. The Th, U and Pb contents in the oscillatory-zoned domains are fairly uniform (13–411, 53–121 and 99–246 ppm respectively). The overgrowth domain is compositionally distinct to cores with high U (578–642 ppm), Pb (215–423 ppm) and highly variable Th (46–375 ppm) contents. The Th/U ratios from analysed spots show minor variation for the oscillatory-zoned zircon (0.25–0.65), whereas the overgrowth domain shows uniformly low values (0.08–0.16).
A total of 15 spots was analysed from five grains. Most of the grains are large and multiple spots could be analysed from different domains of a single grain. All but one spot data show reverse discordance (102–109%), even though U concentration is not very high in the analysed spots. Nine spots from the oscillatory-zoned zircon domain yield two statistical pools of data (95% confidence) of 2861 ± 30 Ma (MSWD = 0.64; n = 3) and 2818 ± 15 Ma (MSWD = 1.5; n = 6; Fig. 5e). Spots from the same grain show these two pools of data. While the apparently unaltered oscillatory-zoned domain shows a c. 2861 Ma date, the recrystallized oscillatory-zoned domain, patches and overgrowths show a c. 2818 Ma date. Spots from the high-U overgrowth yield a third pool date of 2489 ± 23 Ma (MSWD = 0.18; n = 3). The latter is possibly the age of a prominent thermal overprint. Two spot dates from the same domain define a somewhat younger pool date of 2451 ± 8 Ma (1σ). Probability density plot shows two widely spaced peaks culminating at c. 2809 and c. 2486 Ma respectively (Fig. 5e inset).
RNG 50D (Budhapal leucogranite)
Zircon grains in this sample are subidioblastic to xenoblastic in nature and mostly oval. Elongated grains are 150–200 μm long and 50–100 μm across. Grains show faint traces of oscillatory zoning in the interior (Fig. 4k,l). The exterior part, on the other hand, shows blurred zonation and presence of thin high-U overgrowths (20–30 μm thick). Although Th, U and Pb contents vary considerably between successive zones, Th/U ratios from the analysed spots vary within a narrow range (0.52–0.67). In comparison, Th/U ratio in the overgrowth domain is much lower (0.11). Seven grains were analysed for a total of 10 spots. Spot data from the simple zoned zircon form two distinct groups when plotted (Fig. 5f). The majority of the data yield a 207Pb/206Pb pooled date of 2807 ± 13 Ma (MSWD = 2.1; 95% confidence, n = 9). Only one spot could be measured from the overgrowth domain and it gives nearly concordant date of 2484 ± 5 Ma (±1σ). The leucogranite was thus possibly emplaced at c. 2807 Ma and later metamorphosed at c. 2484 Ma. A probability density plot of 207Pb/206Pb spot data defines a very strong peak at c. 2805 Ma with a very weak peak at c. 2483 Ma (Fig. 5f inset).
RNG 122 (Rengali mafic granulite)
Zircon grains in this sample are xenoblastic and mostly oval. The size of the grains varies from 150 to 200 μm across (Fig. 4m,n). All the grains show thick metamorphic overgrowths characterized by relatively bright CL domains surrounding darker cores. Zoning in the overgrowth domain is simple, with rare sector zoning. Seven zircon grains and 12 spots were analysed from this sample. Multiple spots were chosen for each grain to determine the ages of different zircon domains. The U, Th and Pb contents vary even within a single grain. Th/U ratios vary in the range from 0.06 to 0.86. Spot data are nearly concordant to moderately discordant (86–104% concordance) and define six groups (Fig. 5g). The most dominant group yields a 207Pb/206Pb date of 2488 ± 19 Ma (MSWD = 1.5; 95% confidence, n = 4). Other groups give dates of 2844 ± 7 Ma (MSWD = 0.47; 1σ; n = 2) and 2351 ± 34 Ma (MSWD = 1.12; 95% confidence; n = 3). Two other spots yielded 2408 ± 6 Ma (1σ) and 2351 ± 34 Ma (95% confidence) dates. The older age domain of c. 2844 Ma is situated at the interior of zircon and possibly reflects a crystallization age. However, moderate Th/U ratios (0.69–0.83) and faint zoning in this domain could also indicate a metamorphic origin. A limited number of data points and small geographical extent of the domain make any interpretation uncertain. The c. 2488 Ma (some spots with low Th/U ratios) is the most dominant tectonothermal event witnessed by this rock. Three groups of data younger than c. 2488 Ma are problematic. A few spot dates are nearly concordant (e.g. 6.4 in Table 2) and are positioned in the same grain adjacent to the c. 2488 Ma domain. It is noteworthy that this sample does not contain much zircon and therefore the analytical data are inconclusive. A probability density plot for the 207Pb/206Pb spot data shows a strong peak at c. 2488 Ma followed by several smaller peaks of older and younger dates (Fig. 5g inset).
RNG 129B (Rengali hornblende gneiss)
Zircon grains mostly have a high aspect ratio, although a few oval grains are also present. Most of the grains show high-CL oscillatory zoning with patches of high-U character (Fig. 4o,p). A few grains contain dark-CL cores. Seventeen spots were analysed from 14 grains. Except for spot 15.1 (Table 2), all others show uniform Th/U ratios (0.51–0.72). Out of these, nine spots with near-concordant data yield a pooled age of 2828 ± 9 Ma (95% confidence, MSWD = 2.3; Fig 5h). Two spots from the cores form a slightly older group with 2860 ± 5 Ma (±1σ) date. Five spots from blurred oscillatory-zoned domains are more discordant with 207Pb–206Pb dates younger than 2800 Ma. These spot dates possibly indicate differential Pb-loss from the oscillatory-zoned zircon due to partial destruction of zircon lattice (Mezger & Krogstad, 1997) although their U contents are not higher compared to the other spots. The spot 15.1 (Table 2) date was acquired from a planar-zoned exterior part of oscillatory-zoned zircon. The spot has a distinctly low Th/U ratio (0.07) and younger near-concordant spot date of 2471 ± 4 Ma. The 2828 ± 9 Ma age is therefore inferred to give the emplacement age of the protolith of the hornblende gneiss. All the zircon grains are variably disturbed by a later tectonothermal event. Figure 5h (inset) shows a probability density plot for 207Pb–206Pb ages where a very strong peak at c. 2836 Ma is visible.
RNG 54 (Rengali felsic gneiss)
Zircon grains in this sample are mostly oval, 100–200 μm long and 50–150 μm across. Some grains have high aspect ratios. All grains have core-rim structure in CL and BSE imaging (Fig. 4q–t). Cores show moderate to high-CL faint oscillatory zoning truncated by a thick (30–50 μm) overgrowth. The latter domain has low-CL faint planar zoning or is unzoned. Some of the cores show a patchy dark interior surrounded by a thin bright-CL planar zoned domain. Twenty two spots from 15 grains define three populations of data. Zircon cores of variable Th/U ratios (0.07–0.90) and yield a pooled age of 2776 ± 24 Ma (95% confidence, n = 6; Fig. 5i). However, the age has very large scatter (MSWD = 42) presumably due to errors in individual spot analysis. The overgrowth domain yields a pooled age of 2508 ± 14 Ma (95% confidence, n = 9; Fig. 5i), also with large scatter (MSWD = 30). Five spots from the overgrowth domain (spots 1.1, 4.2, 7.1, 28.2 and 31.1) with a narrow range of Th/U ratio (0.10–0.16; Table 2) yielded a third group with 2434 ± 34 Ma (MSWD = 51). However, the spots show chemical characters (Th/U ratio) and CL or BSE response similar to other spots having c. 2508 Ma age. The probability density plot for 207Pb/206Pb ages shows three distinct peaks (Fig. 5i inset). It appears that the protolith of the felsic gneiss was emplaced during c. 2776 Ma and later metamorphosed during c. 2508 Ma.
It is important to note that most of the samples have varying degrees of reverse discordance in U–Pb zircon dates. Reverse discordance is reported from many zircon analyses (Kelly & Harley, 2005; Kusiak et al., 2013 and references therein). This may result from local unsupported radiogenic Pb gain (Williams et al., 1984; Compston, 1999; Kusiak et al., 2013), compositional artefacts (Wiedenbeck, 1995), or by matrix sputtering effects during analysis of high-U zircon (Harrison et al., 1987; Black et al., 1991; McLaren et al., 1994). The ‘unsupported Pb-gain’ of Williams et al. (1984) is a viable mechanism, but experimental data show that radiogenic Pb is excluded from recrystallized or newly grown zircon due to the incompatibility of Pb2+ (Geisler et al., 2003). However, radiogenic Pb can be accommodated within the partially metamictized zircon domains (Mezger & Krogstad, 1997; Geisler et al., 2003). In such cases, mobilization of Pb may be facilitated either by fluids or through annealing at elevated temperatures to repair of damaged structure. Either of the processes causes movement radiogenic Pb, whereas unmetamictized part witnesses no Pb movement (Mezger & Krogstad, 1997). This is vindicated in the study of Kusiak et al. (2013) who demonstrated that concentration of radiogeneic Pb in micro-domains produces spuriously older dates. In our study, both U-rich and U-poor zircons show reverse discordance. Variable sputtering of Pb may be responsible in some analyses, as Pb counts in such cases behaved erratically well beyond scatter predicted by counting statistics, and in contrast to stable count rates for UO, U, and ThO mass stations. On the other hand, similar shifts in Pb count rates in standard FC1 zircon were not encountered and therefore analytical instability can be ruled out. We reckon that the reverse discordance in our samples could be attributed to micron-scale movement of radiogenic Pb as demonstrated earlier (Compston, 1999; McFarlane et al., 2006; Kusiak et al., 2013).
Discussion
Major tectonothermal imprints on the gneissic belt of the Rengali Province
Petrological study shows that rocks of the Rengali Province represent different levels of a crustal section juxtaposed together during a major orogenic event (Bose et al., 2015). Pelitic granulite (Samples RNG 61 and RNG 62), mafic granulite (Sample RNG 122) along with charnockite (Samples RNG 120A and JK 54) represent components of deeper crust, whereas hornblende gneiss, felsic gneiss and granitoids represent middle crust. Available data suggest that the pelitic granulite from the eastern Rengali Province witnessed partial melting and subsequent melt extraction during peak metamorphism close to 850 °C at deep crustal conditions (Mahapatro et al., 2012; Bose et al., 2015). Mafic granulite of the eastern Rengali Province, on the contrary represents a much deeper section (34–38 km corresponding to 1.0–1.2 GPa pressure) of the crust (Bose et al., 2015). Mahapatro et al. (2012) determined the age of peak granulite metamorphism of 3058 ± 17 Ma using texturally constrained monazite EPMA data for the eastern Rengali. They also estimated the age of reheating of the granulite at 2778 ± 16 Ma. Two metamorphic ages documented by Mahapatro et al. (2012) most likely reflect the timing of M1 and M2 metamorphism in the eastern Rengali Province. The amphibolite-grade metamorphism within the gneissic rocks of the central Rengali Province occurred at a P–T window which almost overlaps with that of the retrogressive stage (M2r) in the granulite suite. Mahapatro et al. (2012) argued that granulite facies rocks within the eastern part of the Rengali Province represent the thrusted deep crustal section of the Singhbhum Craton, was also supported by later study (Bose et al., 2015). This is in sharp contrast to the argument of Misra & Gupta (2014) who infer consanguinity of the Rengali Province with the Bastar Craton lying farther west.
Geochronological data in this study indicate a punctuated evolutionary history of the Rengali Province. Rocks of diverse chemical character yield complex geochronological data. Zircon 207Pb/206Pb data from all the samples yield multiple groups of data, all of which show an Archean history of the Rengali Province, was also identified by previous workers based on geochronological records in Rb–Sr, Sm–Nd, 207Pb–206Pb and U–Th–Pb systematics (Misra et al., 2000; Sarkar et al., 2000; Bhattacharya et al., 2001; Mahapatro et al., 2012). Detrital zircon from both the pelitic granulite samples of eastern Rengali Province yields an array of Mesoarchean dates (3466 ± 7, 3413 ± 12, 3353 ± 9, 3308 ± 24, 3295 ± 14 and 3222 ± 13 Ma), from which it might be possible to interpret provenance. Zircon data from the Balarampur charnockite sample are complex. Oscillatory-zoned zircon yields older dates, but the pooled age of 3058 ± 15 Ma is emplacement age of the charnockite magma. Younger dates possibly imply partial recrystallization and metamorphism. The Rengali charnockite was emplaced at 2861 ± 30 Ma, whereas recrystallization of zircon occurred during metamorphism at 2818 ± 15 Ma. Rengali mafic granulite has a 2844 ± 7 Ma protolith which represent either magmatic or metamorphic age. Protolith of the Rengali hornblende gneiss was crystallized at 2828 ± 9 Ma possibly as TTG suite. The high-grade metamorphism and granitoid emplacement during c. 2844–2828 Ma was followed by the emplacement of post-kinematic granitoids (Kalinganagar granite, Budhapal leucogranite and protolith of Rengali felsic gneiss) at 2807 ± 13, 2809 ± 13 and 2776 ± 24 Ma respectively. Zircon grains from most of the samples show a metamorphic overprinting at c. 2500 Ma (2484 ± 5 Ma for the Kalinganagar granite, 2489 ± 23 Ma for the Rengali charnockite, 2488 ± 19 Ma for the Rengali mafic granulite and 2508 ± 14 Ma for the Rengali felsic gneiss).
Summarizing zircon data, we frame the evolutionary history of the gneissic belt of the Rengali Province. Sediments of the pelitic granulites were derived from protoliths older than c. 3060 Ma which was followed by metamorphism (M2 event in granulites) and granitoid magmatism within a span of c. 2840–2780 Ma. The dominant gneissic layering (S2) developed along the axial plane of F2 folds was formed due to this tectonothermal event which affected both lower crust and middle crust. We consider this 60 million year epoch as the dominant orogenic phase in the Rengali Province. Charnockites show complex geochronological signatures in the eastern and central Rengali Province. The crystallization history of the Balarampur charnockite is different from that of Rengali charnockite. Metamorphic ages from two studied charnockites also vary within their uncertainty limits. It is difficult to conceive of them as a part of same magmatic suite owing to the vast difference in their emplacement ages (c. 3058 and c. 2861 Ma). These two spatially separated charnockites seem to represent different suites. The c. 3058 Ma charnockite emplacement within the lower crust could also indicate an early tectonometamorphic event (M1) which was responsible for deep crustal metamorphism of mafic granulite in the eastern Rengali Province (1.0–1.2 GPa, 860 °C, Bose et al., 2015). Interestingly, Mahapatro et al. (2012) estimated monazite U–Th–Pb age of 3057 ± 17 Ma from pelitic granulite of eastern Rengali Province, and they also suggested that this age could represent an early metamorphic event. Spot dates between 2800 and 2500 Ma (Table 2) possibly are mixed ones derived from two distinct age domains of zircon that are often indistinguishable. The apparent smear of data close to the concordia curve over a period of more than 300 million years is problematic. It can result from high temperature strain-induced Pb-loss (Halpin et al., 2012) or fluid-induced resetting (Geisler et al., 2007).
Crustal growth along the southern margin of the Singhbhum Craton
Available geochronological data (Sarkar et al., 1998; Misra et al., 2000; Bhattacharya et al., 2001; Mahapatro et al., 2012; this study) indicate episodic growth of the Singhbhum Craton at its southern margin during c. 2840–2780 Ma and c. 2500–2480 Ma. Protolith ages of pelitic granulites of the eastern Rengali Province have matching precursors at the Singhbhum Craton, either as clastic sedimentary rocks (c. 3120–3090 Ma; Misra, 2006) or as granitic intrusives (c. 3400–3100 Ma; Saha, 1994; Misra, 2006). The oldest zircon spot date identified in this study is 3528 ± 6 Ma, which is close to the emplacement age of dacitic lava (c. 3510 Ma U–Pb SHRIMP age) within the southern Iron Ore Group sediments of the Singhbhum Craton (Mukhopadhyay et al., 2008). The youngest spot date from detrital zircon (c. 3058 Ma) is again close to the emplacement age of the last phase of Singhbhum granite and Mayurbhanj granite (Paul et al., 1991; Misra et al., 1999). In addition, a c. 3300–3000 Ma protolith history was reported from detrital zircon of low-grade quartzite from the southern margin of the Singhbhum Craton (Mukhopadhyay et al., 2013, 2014).
Tectonometamorphic events in the Rengali Province during the c. 2840–2780 Ma have contemporaneous granitic magmatism at the Singhbhum Craton, both at the cratonic interior (c. 2810 Ma Tampercola granite; Bandyopadhyay et al., 2001) as well as its southern margin (c. 2800 Ma Bhuban leucogranite; Misra et al., 2000). The Dhanjori volcanics extruded at c. 2800 Ma (Misra & Johnson, 2005) and a dacitic tuff from the southern part of the Singhbhum Craton crystallized at 2806 ± 6 Ma (Nelson et al., 2007). All these records suggest that a significant crustal growth of the Singhbhum Craton occurred at c. 2840–2780 Ma.
Figure 6 shows the geochronological evolution of the Rengali Province and its adjacent crustal blocks. It may be noted that the tectonothermal events in the Rengali Province partially match with those of the Jeypore Province occurring adjacent to the Bastar Craton (Fig. 6). In Jeypore Province, the Nd-model ages of orthogneissic rocks (mainly charnockites) range between 3900 and 3000 Ma (Rickers et al., 2001). This province was intruded by granitoid at c. 2700 Ma followed by an event of granulite metamorphism at c. 2500 Ma (Kovach et al., 2001; Simmat & Raith, 2008). However, there is no report of c. 2800 Ma magmatism and metamorphism in the Jeypore Province. Therefore, the possible link between Singhbhum and Bastar cratons during c. 2800 Ma was not established.
Implication to the assembly and stability of Ur
It is postulated that several cratonic nuclei assembled for the first time in Earth's history to form the supercontinent Ur (Rogers, 1996; Rogers & Santosh, 2003). The major part of Ur was stabilized at c. 3000 Ma, but its components continued to grow through repeated tectonothermal processes until c. 2500 Ma when the configuration of ‘Expanded Ur’ (fig. 2 of Rogers & Santosh, 2003) was achieved. Major cratonic blocks of Ur exhibit similar growth history during the Neoarchean time and they remained entangled until the breakup of Pangea (Rogers & Santosh, 2004). The initial configuration of Ur involved the Kaapvaal, Dharwar, Singhbhum and Pilbara cratons, but geological evidence from cratonic blocks of Madagascar, Aravalli, Bundelkhand, Napier, Grunehogna, Vestfold, Gawler and Yilgarn support their inclusion within the supercontinent (Santosh et al., 2009). These cratonic blocks were believed to have grown through juvenile crustal addition, orogeneses and anorogenic plutonism at the continent margins. Five major cratonic blocks from India are believed to be included within Ur and orogenic activities in these blocks during the time span c. 3000–25,000 Ma should considered as part of the assembly of Ur. In this context, present geochronological data from the Rengali Province furnish compelling evidence for the shared Neorchean history of Ur.
Acknowledgements
We thank S.K. Bhowmik, C. Clarke and G.L. Clarke for inviting us to contribute this paper in the special volume. Grants from CSIR, Govt. of India (no. 24(0312)/10/EMR-II) is appreciated for background study of the Rengali Province. SHRIMP analysis at Hiroshima University is carried out as a part of DST-JSPS bilateral research program (DST/INT/JSPS/P-190/2014). We are thankful to T. Pradhan, S. Mallick and S. Dutta Mal for their help during sample preparation and P. Bose for improving the English during revision. We thank D. Nelson for his comments on interpretation of geochronological data from complex zircon grains. Helpful and constructive comments from an anonymous reviewer, S. Krishnan and thorough editorial comments from G.L. Clarke are appreciated.
