Geochronological Constraints for Boundary Shear Zones between Eastern Ghats Province and Bastar Craton: Implication for the Formation of Granulites and Their Exhumation History

Shear zones in the boundary between Eastern Ghats Province (EGP) and the cratons of Singhbhum in the north and Bastar in the west provide an excel-lent opportunity to study the tectonics of shear zone development and its timing in relation to the evolutionary history of the granulite suites. Detailed structural, microfabric and quartz C-axis patterns revealed a high temperature shear zone, at the western boundary between EGP and Bastar Craton (BC) around Paikmal. Petrological studies in this shear zone indicated decompression coeval with stretching in the sheared granulites. Geochronological constraints provided here indicate rapid exhumation of deep seated granulites in this boundary shear zone; the timing also is late in relation to the long-lived thermal (granulite formation) event in the EGP. Additionally, our geochronological data demonstrated the ~1600 Ma event in the Eastern Ghats Belt (EGB) involving sedimentation, magmatism, metamorphism and crustal anatexis, as a significant world event.

stand the evolution of such contrasting crustal pairs, it is important to focus on the boundary areas [3] [4] [5].
From the boundary between Bastar Craton (BC) and Eastern Ghats Province (EGP), around Paikmal, [6] described decompressive reaction textures in the granulitic rocks of boundary shear zone that could relate to exhumation of deep crustal granulites during shearing. Although reaction textures indicative of decompression have been described from several internal segments of the Eastern Ghats Belt (EGB) [7] [8] [9] [10], the kinematics of exhumation in these segments are not well understood. From one internal segment, around Paderu, [11] provided isotopic evidence of partial exhumation (10 to 8 kbar) in about 100 million years. [12] proposed that weak marginal zones, marked by extensional faults or shear zones, should provide evidence of rapid exhumation, compared to the much more slowly uplifted rocks of the core of the thickened mountain belt.
In view of the high-temperature shear zone and reaction textures indicative of decompression during shearing, described from the northwestern margin around Paikmal, isotopic data from this domain may attest to the rapid exhumation. In this communication we present isotopic data, using multiple systematics, on rocks both from the EGP and adjoining BC. These may shed new light on the evolution of contrasting crustal pairs.

Eastern Ghats Belt
Presently it is believed that the EGB is a collage of several provinces/domains having distinctive geological histories [13]. The provinces/domains are said to be separated by structural discontinuities (Figure 1(a)) as represented by crustal scale shear zones [14] [15]. These shear zones were described as inter-domainal shear zones. However, a recent work [16] convincingly demonstrated the Mahanadi Shear Zone (part of North Boundary Shear Zone) as an outcome of intra-terranne transpression in response to far field stress generated by collision of EGB with the Singhbhum Craton (SC) and thereby challenged the efficacy of the domainal classification [15].
The EGB comprises three broad lithological groups, namely metapelitic granulites, charnockite-enderbite gneisses and associated mafic granulites and migmatitic gneisses. NE-SW regional tectonic trend represented by S 1 gneissosity [17] [18] [19], a steep axial planar foliation, and common structural repetitions, are akin to a convergent orogen that evolved under a regional NW-SE Figure 1. (a) Geological map of the northern part of Eastern Ghats Belt, after [14], with location of Paikmal inserted. (b) Geological map of the area around Paikmal, Odisha, modified after [21]. The analysed sample locations are given in the map.

Study Area
The present study area around Paikmal, at the northwestern margin of the EGB, belongs to the Western Khondalite Zone (WKZ) of [32] and EGP of [14]. The area exposes dominantly metapelitic granulites (khondalites), with minor bands of mafic granulites of the EGP and granite gneisses of the BC with some mafic enclaves [21]. The pervasive gneissosity in the metapelitic granulites and axial traces of F 2 folds describe a bend from E-W in the northeast to NNE-SSW in the southwest ( Figure 1(b)). Besides the quartzite mylonites, mylonitic fabrics are also observed in metapelitic granulites in the boundary shear zone. The granitoids of the BC have a crude gneissosity, which is at a high angle to the boundary. [6] described microstructures and quartz C-axis fabrics, attesting to a high-temperature shear zone separating the craton from EGB. Also, the rocks of BC are affected by the shear deformation.

Petrological Background
The

Analytical Procedure
The electron microprobe analyses (EPMA) were undertaken at the Geological Survey of India, Kolkata using CAMECA SX 100 machine. Operating conditions for the electron microprobe were 15 KV accelerating voltage, 0.2 nA sample current and 2 µm beam diameter.

Analytical Results
EPMA analytical data is given in Table 1. Pressure-temperature estimates for these equilibrations were derived using multi-equilibrium calculations [33]. P-T estimate for the mafic granulite outside the shear zone was reported as 9 Kbar, 850˚C [6]. In the shear zone khondalite, using garnet-sillimanite-ilmenite-rutile barometer, pressure was estimated at ~4.7 kbar, at 600˚C; and in the mafic granulite using the garnet-orthopyroxene-plagioclase-quartz barometer, the estimated pressure is ~5.1 kbar at 600˚C. Hence, a minimum of 4 kbar exhumation (from 9 to 5 kbar) could be assigned to the shearing event. This comes out to around 15 km of exhumation.

U-Pb Isotopes in Zircon and Monazite
The isotopic analyses were undertaken at the Geoscience Centre of Sao Paulo given in [35].

Whole Rock Rb-Sr
The measured ratio of 87 Sr/ 86 Sr obtained for NBS 987 standard was 0.710254 ± 22 (2σ) and the laboratory blank for the chemical procedure during the period of analysis yielded maximum value of 4 ng for Sr.

Whole Rock Sm-Nd
The measured ratio of 143 Nd/ 144 Nd obtained for La Jolla standard was 0.511857 ± 0.000046 (2σ) and the laboratory blanks for the chemical procedure during the period of analysis yielded maximum values of 0.4 ng for Nd and 0.7 ng for Sm.

K-Ar Isotopes
Analytical procedure as reported in [36].  [38]. Monazite fraction dated 541 Ma, could represent the late Neoproterozoic thermal event in East Gondwana, which is believed to have been completely assembled by around 600 Ma [39]. It is important to note that though no extensive areas or rock suites of this age are recorded in the EGP, this Pan-African imprint only sporadically appears in several areas in the EGP [40], as also in this presentation and has no significance in the context of an orogenic event.

U-Pb isotopic data for zircon and monazite in Khondalite (
Zircon U-Pb isotopic analysis by LAICPMS is presented in Table 3   The whole rock Rb-Sr and Sm-Nd isotopic data for granulites and granites of this boundary region is presented in Table 4. It is interesting to note that some mafic granulites (4MG) are observed interbanded with the khondalites (4KH), away from the shear zone, similar to that recorded from Sunki area (cf. Figure   4(a) in [41]). Rb-Sr data in the Khondalite (4KH) indicated ~1.6 Ga Sr-model date. The Sr model date for khondalites in the EGP (including 4 KH) provide some constraints on the age of sedimentation [42]. Sm-Nd isotopic data for the interbanded mafic granulite (4MG), indicated mafic crust formation around 1.6 Ga.
As for the Paleoproterozoic provenance of the precursor khondalite sediments, the Nd-model date of the khondalite, 2.5 Ga and the dominantly granitic, but including some mafic rocks of the BC, as reported by [42], are also recorded from granite in this boundary region.
Secondary amphibole, aligned in the direction of extension (Figure 2(c)) provides 580 Ma cooling age (Table 4).

Discussion
The granitoids and mafic enclaves in the adjoining BC provide information on the Archaean cratonic evolutionary array between 3.0 Ga mafic magmatism and 2.5 Ga granitic batholiths, indicated here with Nd-model (crustal derivation) ages.
In this marginal segment of the EGP, magmatism and sedimentation at ~1.6 Ga indicated by Nd-model date and Sr-model date respectively, are indicative of International Journal of Geosciences  International Journal of Geosciences U-Pb isotopic data for zircon and monazite in the khondalite of the EGB, recorded here is consistent with the long-lived thermal event reported from the Proterozoic EGP [24]. Monazite growth between 918 Ma and 865 Ma in the khondalite could represent the decompression, as recorded from the boundary shear zone rocks, khondalite and mafic granulite. Between the end of the long-lived thermal event around 918 Ma and initiation of the Rodinia break-up around 865 Ma (zircon & monazite), a time gap of about 53 Ma was recorded in this boundary shear zone. Hence, the 15 km exhumation of the granulites in the shear zone, as revealed by petrological data in this report, could have occurred in 53 Ma. This, compared with 7 km exhumation in about 100 Ma in the core of the orogen, reported from Paderu [11], could signify relatively rapid exhumation and consistent with the model of extrusion tectonics [12].
The ~ 1600 Ma event as a significant world event involving sedimentation, magmatism, metamorphism and crustal anatexis, was reported from the SC in a review article [44]. From the EGB, ~ 1600 Ma sedimentation and magmatism in this boundary area, along with metamorphism and crustal anatexis reported from other areas [45] [46], demonstrate a similar nature of the event in the EGB, India. 2) Geochronological record reveals rapid exhumation consistent with extrusion tectonics.