Research Article Early Neoproterozoic Deformation Kinematics in The

GeoScienceWorld Lithosphere VoVolume 2020, Article ID 8820919, 26 pages https://doi.org/10.2113/2020/8820919

Research Article Early Neoproterozoic Deformation Kinematics in the Chottanagpur Gneiss Complex (Eastern India): Evidence from the Curvilinear Hundru Falls Shear ZoneAnalysis

Nicole Sequeira and Abhijit Bhattacharya

Department of Geology and Geophysics, Indian Institute of Technology, Kharagpur 721 302, India

Correspondence should be addressed to Nicole Sequeira; [email protected]

Received 11 November 2019; Accepted 31 January 2020; Published 27 August 2020

Academic Editor: Damian Nance

Curvilinear steep shear zones originate in different tectonic environments. In the Chottanagpur Gneiss Complex (CGC), the steeply dipping, left-lateral and transpressive Early Neoproterozoic Hundru Falls Shear Zone (HFSZ) with predominantly north-down kinematics comprises two domains, e.g., an arcuate NW-striking (in the west) to W-striking (in the east) domain with gently plunging stretching lineation that curves into a W-striking straight-walled domain with down-dip lineation. The basement- piercing HFSZ truncates a carapace of flat-lying amphibolite facies paraschist and granitoid mylonites, and recumbently folded anatectic gneisses. The carapace—inferred to be a midcrustal regional-scale low-angle detachment zone—structurally overlies an older basement of Early Mesoproterozoic anatectic gneisses intruded by Mid-Mesoproterozoic/Early Neoproterozoic granitoids unaffected by the Early Neoproterozoic extensional tectonics. The mean kinematic vorticity values in the steep HFSZ-hosted granitoids computed using the porphyroclast aspect ratio method are 0.74–0.83 and 0.51–0.65 in domains with shallow and steep lineations, respectively. The granitoid mylonites show a chessboard subgrain microstructure, but lack evidence for suprasolidus deformation. The timing relationship between the two domains is unclear. If the two HFSZ domains were contemporaneous, the domain of steep lineations with greater coaxial strain relative to the curvilinear domain formed due to strain partitioning induced by variations in mineralogy and/or temperature of the cooling granitoid plutons. Alternately, the domain of gently plunging lineations in the HFSZ was a distinct shear zone that curved into a subsequent straight-walled shear zone with steeply plunging lineation due to a northward shift in the convergence direction during deformation contemporaneous with the Early Neoproterozoic accretion of the CGC and the Singhbhum Craton.

1. Introduction ated with the shallowly dipping fabric and the types of rocks juxtaposed along the fabric offer significant clues to the origin Regional scale shallowly dipping foliations produced in the of the foliation [22]. ductile crust are traditionally attributed to large-scale thrust- In contrast, steeply dipping foliations within ductile shear ing in compressional regimes [1–5]. More recently, however, zones are well documented and their origins are explained in extensional processes such as gravity-driven collapse [6, 7], diverse geological settings [23–29], although curvatures in channel flow [8], metamorphic core-complex formation steeply dipping shear zones pose challenges. Plate boundary ([9] and references therein), or midcrustal extension [10, geometry controls the style and kinematics of structures 11] are also considered to be likely mechanisms for nucle- developed along curved shear zones formed at plate margins ation of flat-lying fabrics. Several occurrences of foreland [26, 30–33], while curvatures in intraplate shear zones are regions in fold-and-thrust belts experiencing extensional also influenced by irregularly shaped plate margins and deformation either contemporaneously [12–17] or immedi- indenters [34–38]. Other factors that influence the curvature ately following compressional tectonics at plate margins of intraplate shear zones include the reactivation of preexist- [11, 18–21] are known. In such cases, the shear sense associ- ing rheologically controlled basement dislocation structures

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Munger

50 km Rajgir-Munger Belt New Delhi KH Rajgir

M RH Gaya Simultala

BMB

Dumka Daltonganj P

Ranchi Saltora

NSMB K

Tatanagar N

GSB DOB SC R CBT

Legend

Quarternary alluvium

Lameta Formation Mesozoic Basalt

Paleozoic Gondwana Supergroup

Bijawar sediments

Supracrustal rocks Chottanagpur Gneiss Complex (CGC) Proterozoic Older enclaves within CGC

Dalma metavolcanics

Singhbhum Group

Archean Singhbhum Craton

Figure 1: Generalized geological map of the Chottanagpur Gneiss Complex, CGC (eastern India) showing the locations of regional-scale shear zones (thick black lines). The belts of supracrustal rocks broadly coincide with the shear zones. Box delineates the area examined in this study. The following acronyms are used: BMB—Bihar Mica Belt; RH—Rajgir Hills; KH—Kharagpur Hills; P—Paresnath Hills; NSMB—North Singhbhum Mobile Belt; GSB—Gangpur Schist Belt; DOB—Dalma Ophiolite Belt; SC—Singhbhum Craton. The Rajgangpur (R)-Tamar (T)-Katra (K) shear zone demarcates the CGC-NSMB accretion zone. The Copper Belt Thrust (CBT) marks the NSMB-SC accretion. Inset map of India shows the location of the CGC (ﬁlled area).

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(a) Patratu 2 70 10 km 27 53 6 70 62 25 12 35 60 60 60 47 5 Ramgarh 22 15 64 25 15 62 25 N 20 30 8 11 22 WR 35 17 10 30 12 70 46 55 57 73 21 20 76 59 37 55 40 35 Northern supracrustal 9 15 18 80 31 16 26 belt 25 26 3 50 31 80 RR-6 Domain-II 15 11 75 9 5 29 35 17 3 12 85 4 54 21 50 30 34 13 85 82 10 60 88 85 15 25 17 52 76 80 17 78 55 Pundag 70 24 5 75 13 Domain-I WR Water reservoir 14 65 62 RR-35 85 6 10 5 25 Domain boundary 8 25 15 56 2 65 20 70 65 82 Gola 9 Granitoids lacking 62 18 80 II 18 44 17 24 50 87 30 mesoscale fabrics 65 56 65 20 82 16 64 85 30 31 20 22 Locations of monazite 63 12 68 52 85 79 5 80 79? 68 65 35 dated samples 82 50 29 70 85 78 38 16 46 50 22 85 Ormanjhi 86 35 2 52 Gondwana sediments 55 86 Hundru 80 73 5 85 I 85 84 70 Supracrustal rocks Domain-I 11 70 22 23 Falls WR 80 37 81 13 70 6577 84 75 72 80 75 74 73 80 75 76 76 Domain-I granitoids 70 82 85 78 62 80 71 78 78 84 80 50 60 HFSZ 48 85 86 Mishirhutang 31 47 78 Domain-II granitoids 57 80 31 20 53 57 RR-21 30 26 64 50 Domain-III granitoids Domain-III 30 80 60 85 64 24 85 Southern D3 L D4 L 82 3 4 RR-10 supracrustal belt Granitoids 25 Supracrustal RR-9 Jonha Falls rocks Ranchi

Domain-I Domain-II Domain-III Supracrustal unit D3 D4 (b) (c) (d) (e) (f) Poles to foliation Stretching lineation/fold axis lineation Contouring <1% 1%-2% 2%-4% 4%-8% n = 101 n = 113 n = 63 n = 189 n = 68 8%-16% n = 41 n = 56 n = 49 n = 60 n = 63

Figure 2: (a) Geological map of the Ramgarh-Ormanjhi area showing the mesoscale structures in the arcuate Hundru Falls Shear Zone (HFSZ) and adjacent areas. The lithologies were simpliﬁed after the District Resource Maps of Koderma-Hazaribagh-Chatra [183] and Ranchi-Gumla-Lohardaga [184] in 1 : 300,000 scale published by the Geological Survey of India. The SW part of the area is poorly exposed due to the Ranchi Township and extensive soil cover. (b–f) Lower hemisphere equal area stereoplots with poles to foliation planes in black squares and lineations in red squares (n = number of measurements). (b) Shallowly dipping D3 foliations (contoured) and LS3 stretching lineations in Domain-I granitoid mylonites. Mean D3 foliation plane shown. (c) D4 steep foliations and LS4A stretching lineations in Domain-II granitoid mylonites. Mean foliation plane shown as great circle. (d) D4 steep foliations and LS4B stretching lineations in Domain-III granitoid mylonites. Mean foliation plane shown as great circle. (e) D3 foliations (contoured) and fold axes in supracrustal rocks. D3 foliation pole girdle shown. (f) D4 planes and fold axis in supracrustal rocks. Mean D4 plane shown.

[39, 40], especially those formed at former continental plate [57, 58], and in the north by the Paleoproterozoic (1.7– boundaries [38, 41]; lateral rheological heterogeneities in 1.6 Ga) rocks of the Rajgir hills (Figure 1) [59, 60]. The tec- the lithosphere [42–44]; and the interaction of synchronously tonic relevance of these Paleoproterozoic rocks to the CGC active en echelon faults [45–47] as in releasing and restrain- is unknown. Based on detailed structural mapping, kine- ing bends, pull apart basins, strike slip duplexes, and splay matic vorticity analysis, and chemical age dating in mona- structures [41, 48–50]. Curvatures on steeply dipping shear zite, this work identifies for the first time two broadly zones can also be the result of reorganization of preexisting contemporaneous Early Neoproterozoic deformation events shear zones by later deformation [36, 51–54]. in the CGC that produced a regional shallowly dipping foli- Field studies of natural shear zones primarily use varia- ation and a curvilinear steeply dipping shear zone, the tions in the orientation of stretching lineations/foliations Hundru Falls Shear Zone (HFSZ) (Figure 2(a)). The HFSZ and the strain history of the rocks to understand the mode forms a part of a network of several crustal-scale E/ENE- of formation of the shear zone. Strain modeling based on striking shear zones that transect the CGC, with shorter field observations of changing orientation of stretching line- NW-striking segments curving into the E-striking shear ations along curvilinear shear zones [55, 56] essentially attri- zones (Figure 1). This study addresses the structural and butes the changes in stretching lineation orientations to kinematic significances of the shallowly dipping foliation variations in finite strain and the angle between the move- and the shear zones/lineaments in the centrally located ment direction and the strike of the shear zone. parts of the CGC and their implications in relation to the The Chottanagpur Gneiss Complex (CGC) in Eastern accretion history of the CGC with the flanking crustal block India (Figure 1) is an 80,000 km2 Meso- to Neoproterozoic comprising the Archean Singhbhum Craton and the Early terrain flanked in the south by an arcuate terrain boundary Mesoproterozoic southern part of the North Singhbhum shear zone, the North Singhbhum Mobile Belt (NSMB) Mobile Belt ([61]; Figure 1).

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2. Regional Geology inclined W/WNW-trending folds [76, 78] that overprint recumbent to gently inclined folds interfolial to a penetrative The Chottanagpur Gneiss Complex (Figure 1) is dominated schistosity. The Mid-Mesoproterozoic granitoids of the CGC by granitoids. Structurally, the granitoids vary from massive, share the early steeply dipping N-striking fabric of the gran- foliated to mylonitic, and gneissose. Granitoid emplacement ulite facies gneisses [62, 63] but lack the intrafolial isoclinal occurred in two episodes. The older Middle Mesoproterozoic folds. When present, tectonic fabrics in the Early Neoproter- (1.4–1.2 Ga) granitoids are recognized in the central and ozoic granitoids are generally E striking, i.e., subparallel with northern parts of the CGC [59, 62, 63], whereas the younger the axial planes of the upright folds in the anatectic gneisses Late Mesoproterozoic to Early Neoproterozoic (1.1–0.9 Ga) [58, 80]. granitoids are extensive across the CGC [59, 61, 64–68]. The granitoids host outcrop to regional-scale bodies of 3. Field Relations in and Neighboring the HFSZ multiply deformed anatectic quartzofeldspathic gneisses, garnet-sillimanite and calc-silicate gneisses, mafic granulites, The field relations discussed below are based on data collated charnockite-enderbites, alkaline complexes, and anorthosite from 300 field stations spanning ~1,200 sq. km (Figure 2(a)) massifs. The regional-scale bodies of granulite facies meta- in the Ramgarh-Ormanjhi area, north of the town of Ranchi morphic rocks within the granitoids are displayed in in central CGC. The area comprises extensive exposures Figure 1. These enclaves comprise the oldest lithodemic unit of Early Neoproterozoic (see later) blastoporphyritic granit- of the CGC. Age data from the Deoghar-Dumka, Saltora- oids, granite-granodiorite in composition, intrusive into the Purulia, and Ranchi areas (Figure 1) indicate that the high-grade Early Mesoproterozoic anatectic gneisses. The polyphase orthogneisses emplaced at 1.6 Ga [61, 69, 70] and Early Neoproterozoic (see later) supracrustal rocks compris- 1.4 Ga [61, 63] were subsequently intruded by Middle ing mica schists (biotite-muscovite-quartz, rare garnet), Mesoproterozoic and Early Neoproterozoic granitoids and micaceous quartzites, quartzites, metacarbonates (amphi- reworked extensively at 1.1–0.9 Ga [61, 71], coinciding with bole-plagioclase-quartz±epidote±calcite), minor amphibo- high-grade isothermal decompression [71]. The granitoids lites (amphibole-plagioclase±quartz±epidote±sphene), and and the high-grade anatectic gneisses comprise the basement ferruginous quartzites are restricted to a tapering NW- of the CGC. striking belt and an E-striking belt, structurally overlying Amphibolite facies supracrustal rocks (Figure 1) domi- the granitoid-gneiss basement rocks (Figure 2(a)). No intru- nated by mica schists (±garnet±sillimanite) and micaceous sive contact relationships between the supracrustal rocks and quartzites/quartzites, with minor proportions of carbonate the granitoids were observed in the area. rocks/marls and amphibolites, occur as east-trending belts Four deformation events (D1-D4) are recorded in the in the CGC. These supracrustal belts dominate the topo- rocks in the Ramgarh-Ormanjhi area (Figure 2(a)). The graphic highs in the CGC and are closely associated with D1-D2 deformation events are associated with the Early the regional-scale shear zones (Figure 1). Available dates in Mesoproterozoic granulite facies metamorphism and ana- the supracrustal belts [61, 64, 65] suggest that the shallow- texis in the basement gneisses of the CGC [61, 63, 69]. The platformal foreland sediments were metamorphosed D1 deformation is identified from the isoclinal fold hinges between 1.0 and 0.9 Ga. Unmetamorphosed sedimentary on D1 leucosome layers—concordant with biotite- rocks of the Late Paleozoic to Early Cretaceous Gondwana hornblende aggregates—preserved in the interfolial domains Supergroup unconformably overlie the CGC crystalline base- of the penetrative steeply dipping D2 gneissic layers. The ment (Figure 1). Early Neoproterozoic granitoids ([61, 64, 65]; this study, see Structurally, very little is known about the sequence of later) intrusive into the basement gneisses truncate the D1- fabric forming deformation events and the large-scale tec- D2 composite layering. These Early Mesoproterozoic high tonic evolution of the CGC. Fold superposition structures temperature D1-D2 fabrics are lacking in the amphibolite are reported from a few areas, but a coherent understanding facies supracrustal rocks. of the timing and relationship of the structures in the three This study focuses on the subsequent D3 and D4 defor- lithodemic units is lacking. Existing information suggests mation events that affected the CGC during the Late that both the Early Mesoproterozoic granulite facies gneisses Mesoproterozoic-Early Neoproterozoic. Based on the struc- [58, 59, 72, 73] and the Early Neoproterozoic amphibolite tures developed during these two deformation events, the facies supracrustal rocks [74–79] experienced three deforma- investigated area is divided into three domains (Figure 2(a), tion episodes, but a disparate sequence of structures is man- Table 1). The rocks in large parts of the area (Domain-I) pos- ifested within the two lithodemic units. In the anatectic sess a shallowly dipping penetrative foliation (D3; Figure 3) gneisses, steep axial planar fabrics to tight-isoclinal interfolial prominently exhibited by mylonitic granitoids with well- folds on mineralogical segregation banding exhibited by leu- developed stretching lineations (Figure 3(a)). These mylonite cosome layers form the penetrative fabric [58, 59]. Distal fabrics are coplanar with a crenulation cleavage in the overly- from the E-striking shear zones, the composite fabric is N ing supracrustal rocks, and axial planes of recumbently striking [72, 73]. Within the shear zones, the composite fab- folded D1-D2 fabric in the anatectic gneiss enclaves ric is deflected into a set of noncylindrical E-trending upright (Figure 3(b)). The expansive nature of the regional-scale folds with locally penetrative E-striking axial planes [58, 61]. shallowly dipping D3 foliation has not been addressed earlier The supracrustal rocks on the other hand record open to in the context of tectonic evolution in the CGC. This defor- tight, gentle to moderately plunging, upright to steeply mation event is restricted to the higher levels of the crust

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Table 1: Summary of the mesoscale structures in the three lithodemic units in Domain-I to Domain-III in the Ramgarh-Ormanjhi sector (see text for details).

Hundru Falls Shear Zone (HFSZ) Domain-I Domain-II Domain-III Shallow-dipping D3 carapace over basement, Steeply dipping D4 foliation, curved, Steeply dipping D4 foliation, E-trending, locally folded subhorizontal stretching direction subvertical stretching direction Top-to-NE movement Sinistral, N-down Sinistral, N-down In carapace:S>L tectonites S>L tectonites S≥L tectonites First fabric in granitoids Gently plunging LS4A stretching lineation NW/SE and E/W plunging LS3 Steeply plunging LS4B stretching lineation LS4A collinear with LS3 Granitoids stretching lineation (1.1–0.9 Ga emplaced, affected by D3 Below carapace: massive and D4 deformations) Chessboard microstructure present Chessboard microstructure absent Chessboard microstructure present Monoclinic symmetry Triclinic symmetry Wm = 0:73 – 0:83 Wm = 0:51 – 0:65 Simple shear dominated Pure shear dominated Present only in carapace: D3 foliation Axial planar D4 foliation developed to Supracrustal rocks is a crenulation cleavage; earlier upright, asymmetric folds on D3 foliation (1.1–0.9 Ga transported, affected by D3 fabric rarely preserved Very rare Subhorizontal to gentle D4 fold axes and D4 deformations) Striping intersection lineation collinear with LS3 and LS4A collinear with LS3 in granitoids In carapace: recumbently folded In carapace: open to tight upright folds with Anatectic gneisses ∗ Mesoproterozoic D1/D2 composite layering gently plunging D4 fold axis; associated (1.6–1.4 Ga emplaced, affected by D1/D2 with nonpenetrative D3 axial planar foliation. D4 axial plane fabric poorly developed Not observed deformations; 1.1–0.9 Ga reworked by Below carapace: undisturbed steeply dipping Below carapace: steeply dipping D1/D2 D3 and D4 deformations) D1/D2 composite layering layering reoriented to NW/W orientations ∗D1/D2 composite layering is defined by penetrative gneissic fabric in the rock comprised of leucocratic layers concordant to biotite/hornblende layers and hinges of isoclinal folds in the interfolial domains. The D1/D2 fabrics are steeply dipping below the decollement (see text). 5 6 Lithosphere

LS3 T

W E 5mm5 mm SW

(a) (d)

C T T

NE 2.5 mm SE

(b) (e)

LS3

(f)

SW NE

Figure 3: (a) Flat-lying granitoid mylonite in Domain-I showing E-plunging stretching lineation (arrow); hammer head points north, T indicates top direction. (b) D3 recumbent folds in anatectic gneiss in Domain-I. (c) In Domain-I, anatectic gneisses with steep-dipping D2 layers in the lower part of the Pundag road-cut are followed upwards by D3 recumbent folds. (d) Top-to-the-NE kinematics shown by ﬁ mica sh in thin-section cut perpendicular to foliation (D3) and L3 fold axis in mica schists (crossed polar image). (e) Crossed polar image of quartz-deﬁned S-C fabrics in granitoid mylonite (Domain-I) exhibiting top-to-the-NW kinematics in XZ section (normal to the foliation plane and parallel to the stretching lineation). (f) Top-to-the-NE kinematics in granitoid mylonite (D3; Domain-I) in section

normal to stretching lineation LS3 (arrow); hammer head points north. (g) D3 thrust duplex structures in para-amphibolites interleaved with basement granitoids exhibiting top-to-the-NE sense of translation.

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and constitutes a shallowly dipping carapace over the base- sections (perpendicular to foliation and parallel to lineation) ment. Below the carapace, the D1/D2 fabrics in the gneisses of granitoids display a top-to-the-NW sense of movement and the post-D2 granitoid intrusives are unaffected by the (Figure 3(e)) and YZ sections (perpendicular to foliation D3 deformation event (Figure 3(c)). and lineation) show top-to-the-NE sense of movement The deformation event (D4) produced a ~25 km wide (Figure 3(f)). Similar YZ sections in supracrustal rocks also steeply dipping sinistral shear zone transposing all earlier have top-to-NE shear sense shown by asymmetric porphyro- fabrics into verticality within the shear zone. We identify clasts, shear band cleavage, and mica fish (Figure 3(d)). the hitherto unreported curved shear zone as the Hundru The non-anatectic amphibolite facies Early Neoprotero- Falls Shear Zone (HFSZ) encompassing Domain-II and zoic supracrustal rocks do not possess the high-grade D1- Domain-III (Figure 2(a), Table 1). D2 fabrics in the anatectic gneisses, do not share an uncon- formable relationship, and are not intruded by granitoids. 3.1. The D3 Event. In Domain-I (Figure 2(a), Table 1), the We therefore interpret that supracrustal unit to be an alloch- blastoporphyritic granitoids occur as S>L tectonites thonous block. Duplex structures in para-amphibolites of the (Figure 3(a)) with a shallowly dipping monophase mylonitic supracrustal belt interleaved with slivers of shallowly dipping foliation manifested by the planar alignment of biotite and granitoid mylonites are observed in road-cut sections within flattened quartz grains that wrap around winged porphyro- Domain-I (Figure 3(g)). The top-to-the-NE translation clasts of core-mantle structured feldspar (modally microcli- inferred from the duplex structures is identical to shear sense ne>plagioclase) with polycrystalline tails. The foliation dips observed in the YZ sections of the shallowly dipping granit- gently to the north (Figure 2(b)), and the well-developed oids and supracrustal rocks. Therefore, we infer that the D3 stretching lineations (LS3; S=stretching; Figure 3(a)) on the carapace formed due to top-to-the-NE translation of the D3 mylonitic foliation are defined by quartz ribbons, flakes allochthonous supracrustal rocks over the basement rocks. of biotite, and aggregates of recrystallized feldspar tails. LS3 The boundary between the D3 carapace and the basement plunges at low angles towards NW and SE neighboring the rocks unaffected by D3 deformation is inferred to be a shal- NW arm of the HFSZ, and E and W neighboring the eastern lowly dipping decollement (Figure 3(c)). arm of the HFSZ (Figures 2(a) and 2(b)). These mylonitic granitoids are part of the shallowly dipping carapace, 3.2. The D4 Event (the Hundru Falls Shear Zone). The steeply whereas the massive granitoids (determined to be Early Neo- dipping curvilinear HFSZ (Figure 2(a)) overprints the shal- proterozoic, see later), common in the SE of Domain-I lowly dipping D3 foliations in the supracrustal rocks, granit- (Figure 2(a)), are inferred to be windows within the carapace, oids, and anatectic gneisses. Steeply dipping D4 shears either exposing the underlying basement unaffected by the D3 truncate the D3 foliation (Figures 4(a) and 4(b)) or produce deformation. open to close upright, asymmetric folds with subhorizontal The penetrative and finely laminated crenulation schis- fold axes on the D3 foliation (Figure 4(c)) at the boundary tosity in the supracrustal rocks (Figure 2(e)) is coplanar with of the HFSZ and Domain-I and in lensoidal-shaped low-D4 the shallowly dipping granitoids in the D3 carapace overlying strain domains within the shear zone (Figure 2(a)). Within the basement (Figure 2(b)). The D3 schistosity in the mica the low-D4 strain lens along the E-trending arm of the HFSZ, schists is exhibited by shape-preferred aggregates of musco- the poles to the folded D3 foliation describe a well-defined vite and biotite in the M-domains and polygonized quartz girdle with a gentle W-plunging fold axis (Figure 4(c)). The grains in the Q-domains (Figure 3(d)). The fabric in the LS3 stretching lineations on folded D3 surfaces generate a calc-silicate rocks is defined by hornblende±epidote-rich small circle girdle centered on the fold axis of the D3 foliation layers alternating with plagioclase-calcite layers. Vestiges of pole girdle (Figure 4(c); e.g., [81]). The negligibly small angle an earlier fabric are observed in microscale as curving strands between the stretching lineations and the D4 fold axis [82] in the D3 interfolial domains and oblique inclusion trails in attests to the coaxial nature of the early LS3 lineation and pre-D3 garnets. The former fabric has the same mineral the later D4 fold axis. Identical geometric relations are noted assemblage as the D3 assemblage. The intersection of the near Ormanjhi (Figure 2(a)) where the shear zone curves two foliations appears as a striping lineation (on D3 foliation) NW. Within the shear zone, the steeply dipping D4 mylonitic defined by mineral segregation banding in the mica schists foliation becomes the penetrative fabric. and a linear alignment of prismatic minerals like hornblende The granitoid mylonites within the HFSZ are S≥L tecto- in the calc-silicate rocks. The NW-trending northern supra- nites; L>S tectonites are rare. The stretching lineations on crustal belt (Figure 2(a)) is restricted within the boundaries the D4 foliation, however, differ in orientation within the of the HFSZ wherein the D3 fabric becomes folded and trans- HFSZ. Based on the differences, the HFSZ is divided into posed into the steep-dipping D4 shear zone fabric. However, two domains (Figure 2(a), Table 1): Domain-II curving from in a few locations flanking the HFSZ where the shallowly dip- NW to E-W contains gently plunging stretching lineations ping supracrustal rocks prevail, the striping lineation on the (LS4A; Figure 4(d)), and Domain-III trending E-W contains gently dipping D3 foliation surfaces plunges at shallow angles down-dip stretching lineations (LS4B; Figure 4(e)). to the NW/SE (Figure 2(e)), largely collinear with the LS3 In Domain-II, the poles to the mylonitic D4 foliation in stretching lineations of the granitoids (Figure 2(b)). lower hemisphere stereoplots (Figure 2(c)) show an arc Shear sense indicators on the D3 foliation in both the distribution demonstrating the curvilinear nature of the granitoids and the supracrustal rocks are uncommon. When domain. The gently plunging LS4A stretching lineations present, S-C fabrics and asymmetric porphyroclasts in XZ (Figure 4(d)), like the LS3 stretching lineations in granitoids

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D3 D4

(a) (b)

�

S N

(c)

LS4B

LS4A

E W EW

(d) (e)

Figure 4: (a, b) Transposition of shallowly dipping D3 foliation into D4 vertical shears (broken line divides horizontal from the vertical surfaces). (c) North-facing curved nature of D3 foliation in Domain-I granitoid mylonite. Inset with lower hemisphere stereoplot shows β W-trending LS3 stretching lineations on D3 foliation collinear with the -axis of the poles to D3 foliation (see text). (d) Steeply dipping Domain-II granitoid mylonite with gently plunging LS4A stretching lineation (arrow); pen measures 14 cm. (e) Steeply dipping granitoid mylonite in Domain-III showing LS4B down-dip stretching lineations (arrow).

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E T T

(a) N S

(b) (c)

N E

(d) (e)

Figure 5: Porphyroclast asymmetry (a–c, e) and fold vergence (d) in granite mylonite. (a) Sinistral shear sense on XZ section in Domain-II granitoid. (b) Weakly developed N-down sense of movement in YZ section in Domain-II granitoid. (c) South-down kinematics in YZ section in Domain-II granitoid at the northern boundary of HFSZ. (d) Plan view of reclined interfolial folds within rare shear lens in Domain-III granitoid. (e) Sinistral kinematics in Domain-III granitoid mylonite.

of Domain-I, plunge towards the NW/W and SE/E indicating describe a consistent sinistral (Figure 5(e)), but poorly that the stretching direction X (X>Y>Z) is collinear between defined N-down sense of movement in sections perpendicu- Domain-I and Domain-II. The hinges of the asymmetric lar to the D4 mylonitic foliation. folds on the D3 foliation in Domain-I granitoids flanking Within the HFSZ, the penetrative D3 crenulation schis- the HFSZ are also coaxial with the stretching lineations tosity of the supracrustal rocks describes large-scale, E/ESE- (Figure 4(c)). Well-preserved shear sense indicators in the trending, steeply inclined to upright asymmetric folds with form of asymmetric porphyroclasts and S-C fabrics consis- subhorizontal (dominant) to gently plunging hinge lines tently display a sinistral shear sense in XZ sections (subhorizon- (Figure 6(a)). Although at the regional scale the folds are tal plane) of the Domain-II granitoid mylonites (Figure 5(a)); a noncylindrical, the noncylindricity is not evident in outcrop north-down sense of movement is weakly developed in the scale. The orientations of D4 axial planes and fold axes YZ section (subvertical plane) (Figure 5(b)). A south-down (Figure 2(f)) follow the trends of the D4 mylonitic foliation sense of movement on S-dipping mylonite foliations is and LS4A stretching lineation in the Domain-II granitoids restricted to the northern margin of the HFSZ (Figure 5(c)). (Figure 2(c)). The D4 axial planar foliation is well developed In Domain-III at the southern flank of the HFSZ, the in the centre of the HFSZ and describes a sinistral shear sense ≥ subvertical S L granitoid tectonites possess down-dip LS4B on plan view (Figure 6(b)). North-vergent folds are restricted stretching lineations (Figures 2(d) and 4(e)). The domain is to the northern flank of the HFSZ. The supracrustal rocks are juxtaposed against the southern supracrustal belt (Figure 2(a)). poorly exposed within Domain-III of the HFSZ. Rare out- L>S granitoid mylonites were observed at a single location crops of weathered calc-silicate rocks exhibit subvertical E- near the village of Mishirhutang (Figure 2(a)); otherwise, striking penetrative foliations with very tight to isoclinal folds fi the LS4B stretching lineations occur on a well-de ned D4 foli- preserved in the intrafolial domains. ation plane. Rare instances of shear lenses with tight, reclined Anatectic gneisses are rarely exposed in the HFSZ. The interfolial folds within the granitoid mylonites were observed composite D1-D2 fabric in these gneisses is E/ESE striking, within this domain (Figure 5(d)). Shear sense indicators steeply dipping, and characterized by moderate to gently

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(a)

E (b)

(c)

Figure 6: (a) Proﬁle section of round-hinged, horizontal, and steeply inclined D4 folds in quartz-mica schists. (b) Sinistral sense of shearing (D4) exhibited by quartzite interbanded with mica schist (plan view) in Domain-II. (c) ESE-trending gently plunging steeply inclined tight D4 folds on mineral segregation layering in gneisses in Domain-II.

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100 �m

K Q

200 �m

(a) (c)

100100 � �mm K

K Q

200 �m

(b) (d)

Figure 7: Microstructures in granitoid mylonites. (a) Straight-walled ribbons of polycrystalline quartz (Q); the quartz grain boundaries are near-orthogonal to the ribbon wall (arrow). Polygonized microcline in the matrix (cross-hatched twining) are ﬁner grained proximal to the quartz ribbons; all microcline grains share high-energy boundaries. (b) Dynamic recrystallization in microcline (K) and quartz (Q) ribbon occurring as relics. Arrows show serrated boundaries among internally strained (undulatory extinction) quartz and microcline grains. (c, d) Chessboard subgrain microstructures in quartz in Domain-I (c) and Domain-III (d) granitoids.

plunging fold axes with a very weak development of D4 axial chessboard subgrain structures in quartz grains, similar to planar foliation (Figure 6(c)), indicating that the HFSZ is those described by Blumenfeld et al. [83] and Kruhl [84], are basement piercing. observed (Figures 7(c) and 7(d)). But chessboard subgrain microstructures in quartz are lacking in Domain-II granitoids. 4. Deformation Microstructures in Granitoids Alkali feldspar, as microcline, displays core-and-mantle structures. Deformation bands, subgrains, and undulatory The granitoid mylonites comprise quartz, microcline, bio- extinction are common in the cores of microcline grains tite, and plagioclase; hornblende and garnet are rare or (sub-cm sized); the polycrystalline tails of asymmetric micro- absent. Ilmenite, apatite, zircon, and monazite are accessory cline clasts are composed of aggregates of elongate (long axis: phases. Examination of a large number of thin sections cut 100–200 μm) and subequant recrystallized grains. In domains in different orientations suggests that the modal amounts where biotite flakes are closely spaced and adjacent to the of biotite in Domain-III granitoids are slightly higher rela- quartz ribbons (Figures 7(a) and 7(b)), the microcline grains tive to the Domain-II granitoids. No other systematic min- are smaller (long axis: 50–100 μm) and elongate and grain eralogical variation is evident in the granitoids across the boundaries meet the micas/quartz ribbons at 90° ([85]; three domains. Quartz dominantly occurs as cm-scale rib- Figure 7(a)). The recrystallized microcline grains (<50 μmin bons (Figure 7(a)). Quartz grains also exist as subequant diameter) are smaller and “pinned” within the biotite segrega- dynamically recrystallized grains anchored to the recrystal- tions and adjacent to the quartz ribbons. Commonly, discrete lized microcline grains in the matrix (Figure 7(b)). Undulatory biotite flakes at low angle with the shear zone fabrics limit the extinction and strongly misoriented subgrains oblique to the microcline grain boundaries, and the recrystallized microcline walls of quartz ribbons are common; the features are also grains in turn overgrow the biotite flakes leaving the flakes observed in discrete quartz grains in the matrix stranded within the feldspar (cf. [86, 87]). The grain bound- (Figure 7(b)). In Domain-I and Domain-III, weakly developed aries of the internally strained microcline grains are invariably

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serrated due to progressive misorientation of subgrains clinically symmetric shear zones, the VNS is assumed to be (Figures 7(a) and 7(b)). The microcline-quartz contacts are either parallel (XZ plane) or perpendicular (YZ plane) to serrated and share lobate-cuspate geometries; the quartz the stretching lineations, whereas in triclinic shear zones, grains commonly show bulge nucleation against the micro- the VNS is independent of the orientation of the stretching cline grains. Myrmekites are locally observed, and triple junc- lineations in the rock [105, 106]. tions are lacking in microcline and quartz. The PAR method of Wallis et al. [97] uses a linear plot of The microstructures in the three domains suggest that grain the porphyroclast aspect ratio (R=long axis/short axis) versus size modification during granitoid mylonitization was induced φ (angle between the long axes of clasts and the flow plane) primarily by dislocation creep deformation. Subgrain rotation assuming no mechanical interaction between porphyroclasts [88] accommodated fast grain boundary migration recrystalli- rotating in a homogenously deforming matrix. A critical zation was the dominant deformation mechanism; this was aspect ratio (Rc) separates continuously rotating porphyro- aided by synkinematic grain boundary sliding in the mica-rich clasts with no preferred alignment of their long axes from por- parts. Chess-board microstructures in quartz formed by simul- phyroclasts that have achieved stable orientations. The Wm taneous activation of a- and c-slips indicate that the granitoids value is computed from the Rc value using the relationship experienced subsolidus deformation in the range 650–750°C Wm = ½ðRcÞ2 − 1/½ðRcÞ2 +1 [96, 97]. In the PHD method [83, 84]. Microstructural evidence does not support supraso- [98, 107], the aspect ratio, R, is plotted against φ of the rotated lidus deformation—manifested by trains and imbrications porphyroclasts with well-developed tails on a hyperbolic net. of touching grains of euhedral plagioclase [89], growth of Wm is the cosine of the angle subtended by the two limbs of plagioclase with rationally developed faces into quartz films the hyperbola that separates back-rotated clasts from other [90], and zoned plagioclase grains [91]—in the granitoids in clasts. The graphical RGN method of Jessup et al. [99] is essen- any of the domains. Annealing recrystallization is extremely tially a derivative of the PAR method in which theoretically limited in the granitoid mylonites. computed semihyperbolic nets for forward-rotated and back- rotated σ-andδ-type porphyroclasts (Figure 8(a) in [104]) φ B∗ B∗ ðM2 − M2Þ ðM2 M2Þ M 5. Kinematic Vorticity Analyses where versus ( = x n / x + n , where x and Mn are the short and long axes of clasts) plots are used The switched stretching lineations in Domain-II and to constrain the Wm value in a sample. Domain-III of the curvilinear, steeply dipping, sinistral HFSZ Three granitoid mylonite samples from Domain-II and attest to differing strain regimes between the two domains. two from Domain-III were chosen, all possessing well- Mutually perpendicular stretching lineations due to flip- developed stretching lineations. The VNS was determined ping of X and Y axes of strain ellipsoids in shear zones from each of the oriented samples by cutting several sections are well documented [55, 56, 92–95]. Partitioning of strain perpendicular to the foliation at 10° angle intervals from the into simple shear-dominated (subhorizontal stretching lin- horizontal and determining the section with the maximum eations) and pure shear-dominated (subvertical stretching clast asymmetry as carried out in Toy et al. [108, 109]. For lineations) segments is the most cited reason for switching the samples from Domain-II, the VNS was found to be of stretching lineations. To assess the strain conditions in the approximately parallel to the subhorizontal stretching linea- two domains, a set of five samples from Domain-II and tions, indicating a monoclinic symmetry for this domain. In Domain-III in the E-trending arm of the HFSZ was chosen Domain-III samples, the VNS was determined to be oblique for kinematic vorticity estimations. The mean vorticity num- to the down-dip stretching lineations, approximately dipping ber (Wm; [96]) was calculated from three methods based on at angles between 20° and 30° towards the west, generating a rigid porphyroclast rotation patterns, i.e., the porphyroclast triclinic symmetry for this domain. Once the VNS was deter- aspect ratio (PAR) method after Passchier [96] and Wallis mined, several parallel rock slabs were cut, and measure- et al. [97], the porphyroclast hyperbolic distribution (PHD) ments were made on images of these polished rock slabs. In method after Simpson and De Paor [98], and the rigid grain a couple of locations of Domain-II granitoids, measurements net (RGN) method after Jessup et al. [99]. Wm values vary were made manually on well-exposed horizontal surfaces between zero for pure shear-dominated deformation and (Figure 8(a)); these data were coupled with those obtained one for simple shear-dominated deformation. However, from the polished rock slabs. Only feldspar clasts were cho- numerical modeling studies show that kinematic vorticity sen for the analyses. The samples were chosen with wide estimations from asymmetric rigid clasts generally underesti- ranges of size and shape of clasts; the long axis of the clasts mate Wm values and are associated with large uncertainties ranged between 3 and 53 mm, and the aspect ratios of the [100–103]. This study uses the Wm values in a relative sense, clasts ranged between 1 and 3.5, although the aspect ratios admitting that a larger component of the simple shear com- of symmetric clasts were as high as 6. Mica-dominated ultra- ponent may be present than suggested by the values obtained. mylonites and mylonites with <25% clasts were chosen to The three methods assume plane strain and steady state ensure that the clasts experienced free rotation in the deformation, and use the orientations of asymmetric por- deforming matrix. In four samples, the number of clasts phyroclasts in a ductilely deformed rock to estimate the vor- measured per sample is between 144 and 215; in RR-250, ticity in sections perpendicular to foliation containing the fewer clasts (n =66) were measured because of the coarse maximum asymmetry [104]. Theoretically, such sections size of the clasts. are presumed to be perpendicular to the vorticity vector The three methods were applied to the Domain-II sample and are labeled as vorticity normal sections (VNS). In mono- RR-350 (Figures 8(b)–8(d)). The PHD method (Figure 8(b))

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90 Rc = 2.60 60 Rc = 3.00 6 30 5 � 4 0 4.0 5.0 6.0 3 R R –30 2 +� –60 Wm = 0.74–0.80 –90 Domain-II (RR-350) n = 200 1.0 1 (c) Wm = 0.72–0.75 90 –� 0.60 0.80 0.90 0.10 0.50 0.40 0.70 0.20 0.30 70 1.00 R 50 30 10 0 0.2 0.4 0.6 0.8 –10 (1) (2) (3) (4)(5)(6) –30 –50 �

Back-rotated -type porphyroclast macroscopic foliation Forward-rotated �-type porphyroclast –70 Angle between long axis and 0.60 0.80 0.90 0.10 0.50 0.40 0.70 0.20 0.30 �-Type porphyroclast –90 ⁎ 1.00 Symmetric-type porphyroclast Shape factor (B ) Porphyroclast-type uncertain (a) (b) (d)

Domain-II Domain-II Domain-III Domain-III 90 90 90 90 Rc = 2.61 RR-230 RR-250 RR-171 RR-251 60 60 Rc = 2.82 60 60 Rc = 1.76 Rc = 2.86 Rc = 1.95 Rc = 3.24 30 30 30 Rc = 1.96 Rc = 2.19 30 3.0 4.0 5.0 4.0 R � 0 3.0 � 0 2.0 3.0 � 0 � 0 4.0 5.0 6.0 4.0 5.0 6.0 R 5.0 R R −30 −30 −30 −30 − Wm = 0.78−0.83 Wm = 0.58−0.65 Wm = 0.51−0.59 −60 Wm = 0.74 0.78 −60 −60 −60 n = 201 n = 66 n = 144 n = 215 −90 −90 −90 −90 1.0 1.0 1.0 1.0 (e) (f) (g) (h)

Figure 8: (a) Field photograph of X-Z section in Domain-II granitoid mylonite (RR-350) used in vorticity measurements the results of which are shown in (b–d). (b–d) Mean kinematic vorticity number (Wm) determinations for RR-350 using diﬀerent methods, e.g., (b) Porphyroclast Hyperbolic Distribution of Simpson and De Paor [98, 107], (c) Porphyroclast Aspect Ratio (PAR) after Wallis et al. [97], and (d) Rigid Grain Net after Jessup et al. [99]. (e–h) Results of the PAR method for Domain-II (e, f) and Domain-III (g, h) granitoids.

yielded a Wm value of 0.69, smaller than that estimated using Domain-III to be pure shear dominated (possibly Wk ≤ 0:59). the other two methods which yielded internally consistent This is consistent with the fact that simple shear-dominated values, e.g., 0.74–0.80 (PAR) (Figure 8(c)) and 0.72–0.75 deformation produces subhorizontal stretching lineations (RGN) (Figure 8(d)). This discrepancy may reflect the obser- for monoclinic shear zones [25, 56, 110]. vations of Xypolias [104] in that the PHD method overesti- mates the pure shear components relative to the other 6. Electron Microprobe Th-U-Total Pb Age methods that produce comparable results. For consistency, Determinations in Monazites we employed the PAR method to estimate Wm values for all the five samples (Figures 8(c) and 8(e)–8(h)). The results To constrain the age of deformation in the HFSZ, we of the PAR method estimate Wm values for Domain-II in the obtained chemical dates in monazites in granitoids and mica range between 0.74 and 0.83 (Figures 8(b), 8(e), and 8(f)), schists in and neighboring HFSZ. Following Schoene et al. whereas the two samples from Domain-III yield Wm = 0:51 – [111], we use the term “date” to define the Th-U-total Pb date 0:65 (Figures 8(g) and 8(h)). It follows that Domain-III calculated from the measured element abundances using accommodates a higher pure shear component relative to decay equations following Montel et al. [112], whereas the Domain-II within the HFSZ. These values lie outside the term “age” is the geologic interpretation of a date in a range of estimated Wk values in natural samples commonly tectono-metamorphic context. between 0.65 and 0.75 in rocks [103]. Stahr III and Law Age determination using the Th-U-total Pb (total) [103] suggest that for rigid grain methods applied to natural chemical age technique in monazite has several unique samples, vorticity values are overestimated for pure shear- advantages. The technique has high spatial resolution dominated flows, while lower values are obtained for simple (beam diameter~1 μm) and can be used to date texturally shear-dominated flows. For such a scenario, the difference constrained in situ monazite [113–116]. Also, monazites tend between the vorticity values we estimate for Domain-II and to grow readily, unlike zircons, by fluid-induced dissolution- Domain-III would in reality be larger, allowing for Domain- precipitation processes in a wide range of temperature, and II to be completely simple shear dominated (Wk ≥ 0:83)and especially at low temperatures [114, 117–121] well below

Table 2: Sample-wise summary description of paragenetic relations, grain characteristics, and population ages in monazites (data provided in Supplementary Material1). Mineral abbreviations: Bt—biotite; Ms—muscovite; Chl—chlorite; Qtz—quartz; Gt—garnet; Plg—plagioclase; Kfs—K-feldspar.

Sample Rock type No. of grains/spot Range of spot date (±2σ)inMa Textural setting of monazite Zoning patterns in monazites no. Fabric relations Dates analyzed Mean date ± 2σ (Ma) Mica schist (Bt+Ms+Qtz+Kfs Monazites are strongly zoned in Th; high- +Gt+Chl) Monazites are euhedral (Figure 9(c)) to Th cores share diffuse boundaries with Range: 882 ± 36 – 1004 ± 74 Penetrative D3 fabric defined anhedral; long axis~30‐60 μm; prismatic RR-6 6 grains mantles that show rim-ward decrease in Th; Mean date of chemical domain by Ms+Bt aggregate and Qtz grains aligned with D3 fabric; occasional Domain-II 24 spots high-Th rims (5 μm wide) at grain margins Core: 969 ± 34 layers. Gt postdates D3. monazite grain clusters overprint D3 are typical (not analyzed due to inadequate Mantle: 934 ± 41 Random Chl and Ms biotite aggregates width) (Figures 9(c) and 9(d)) overprint D3 Mica schist (Bt+Ms+Qtz+Kfs) The monazites are fractured, subhedral to Range: 882 ± 36 – 1004 ± 74 Tightly crenulated D3 fabric anhedral; long axis~70 μm – 100 μm; X-ray element mapping not done; back RR-35 7 grains Mean date of chemical domain defined by Ms+Bt aggregate; elongated grains aligned parallel to scatter electron images do not show Domain-II 30 spots Core: 974 ± 13 zonal development of crenulated D3 fabric in the rock; significant variations in gray shade Rim: 962 ± 46 disjunctive D4 fabric inclusions common Granitoid mylonite Monazites are anhedral; ~60 μm–120 μm Nonconcentric zoning in Th, U, and Y; (Kfs+Qtz+Bt+Ms+Plg) RR-9 4 grains (dia); larger grains are sieve textured. distinct irregular domains present; no Range: 893 ± 52 – 991 ± 58 Monophase D3 fabric defined Domain-I 24 spots Prismatic grains are aligned with D3 fabric systematic variations in dates obtained from Mean date: 937 ± 33 by alignment of Bt+Ms, Kfs defined by biotite different domains (Figure 9(b)) aggregate, and Qtz ribbons Massive granitoid (Kfs+Qtz Strong concentric zoning in Th and Y; Range: 910 ± 66 – 971 ± 64 +Plg+Bt+Ms+Opq) Monazites are subhedral to anhedral; chemically heterogeneous cores share sharp RR-10 4 grains Mean date of chemical domain Coarse-grained, magmatic ~80 μm–120 μm (dia); grains randomly boundary with mantle; mantles are poorer in Domain-I 19 spots Core: 944 ± 27 fabric described by alignment oriented; restricted to biotite aggregates Y (distinct) and Th (indistinct); the grain Mantle+rim: 939 ± 23 of euhedral Kfs porphyries margins exhibit discontinuous Y-rich rims Granitoid mylonite (Kfs+Qtz +Plg+Bt+Ms) Strong zoning in Th, Y, and U; very high Th Range: 875 ± 46 – 1010 ± 45 Monazites are euhedral to anhedral; long RR-21 Coarse-grained, D3 fabric 3 grains domains in te core (Figure 9(a)); mantles Mean date of chemical domain axis~80 μm – 120 μm; grains overgrow D3 Domain-I defined by alignment of Bt, 23 spots have high Th, low Y; rims are discontinuous, Core: 964 ± 25 fabric; invariably associated with Bt Kfs aggregate, and quartz with high Y and low Th Mantle+rim: 895 ± 19 ribbons Lithosphere Lithosphere 15

Th Y 20 Present study area, central CGC D3 foliation 15

Q 10 Number B 100 �m 100 �m Th Ma 20 kV 100 �m Y La 20 kV 5 (a)

P K (e) D3 foliation 25 Gangpur schist belt

Q � 20 B 100 m 50 �m Th Ma 20 kV 50 �m Y La 20 kV (b) 15 D4 foliation Number B 10

5 M 200 �m 50 �m Th Ma 20 kV 50 �m Y La 20 kV (c) 800 900 1000 1100 D4 foliation Age (Ma) (f)

100 �m Q 50 �m Th Ma 20 kV 50 �m Y La 20 kV (d)

Figure 9: Backscattered electron images and Th and Y zoning maps of selected monazites (in white) from granitoids (a, b) and supracrustal rocks (c, d). Spot ages with 2σ errors are keyed to the Th maps. n = number of spot analyses. Acronyms: Q—quartz; B—biotite; M—muscovite; K—K-feldspar; P—plagioclase. The fabric in each rock is indicated in the BSE image. (e, f) Histogram and probability density plots of monazite ages computed using Isoplot software after Ludwig [127] in mica schists and granitoid mylonites in (e) the present study area (sample locations in Figure 2(a); data in Supplementary Material1) and (f) Gangpur Schist Belt (compiled from Bhattacharya et al. [65] and Chowdhury and Lentz [133]). Errors in population ages are ±2σ Ma computed using the software.

the blocking temperature of intracrystalline U, Pb diffu- analysis are identical to the protocol-I of Prabhakar [126]. sion at T < 850°C [122–125]. As a consequence, the ages of The monazites are strongly zoned in Th and Y (Figures 9(a)– the deformation fabric at greenschist/amphibolite facies can 9(d); Table 2); variations in Pb and U concentrations are be readily obtained from structurally and texturally con- negligible in the analyzed monazites (Table 2). Element strained monazite grains; also, older magmatic and high-T abundances, monazite spot dates (±2σ), and error % (error metamorphic dates may be preserved especially in dry assem- %=100×{2σ error in Ma/age in Ma}; Prabhakar [126]) are blages where fluid activity is limited as in weakly strained provided in Supplementary Data1 (available here). Spot rocks, and in monazites sequestered within robust grains dates with error% <8 were considered and statistically lodged in strain shadow zones of subsequent low-T events. resolved for mean ages of populations using Isoplot 3.0 Th-U-total Pb dates in monazites were determined in the [127]. The spot dates were monitored against the standard supracrustal rocks and granitoid mylonites within and out- Moacyr monazite; the date for the Moacyr monazite is side the HFSZ (locations in Figure 2(a), Table 2). Sample- determined to be 497 ± 10 Ma (EMP age, [128]), 487 ± 0:5 wise summary of rock types and mineralogy, compositional Ma (TIMS age, [129]), and 509:3±0:5Ma (TIMS age variations in monazites, age range, and mean ± 2σ of age [128]). For the duration of analyses, the Moacyr monazite populations are summarized in Table 2. The analyses of was analyzed 12 times, and the date (±2σ) varied between monazites were performed using a 4-WDS Cameca SX-100 487 ± 29 and 504 ± 32, with a mean value of 496 ± 8 Ma. electron probe microanalyzer in the National Facility, Indian There is no existing information on metamorphic tem- Institute of Technology, Kharagpur. The crystal configura- perature in the supracrustal rocks specifically from the tions, the analytical conditions, and the standards used for Ramagarh-Ormanjhi area. But P-T path reconstructions

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from the supracrustal rocks elsewhere in the CGC such as in area, broadly contemporaneous with the nucleation of the the Rourkela-Rajgangpur [64, 65] and Gridih-Dumka-Deo- shallowly dipping D3 carapace and the HFSZ (D4): (1) The garh-Chakai areas [130] suggest that the rocks were meta- shallowly dipping Domain-I granitoids in the carapace over morphosed between 550 and 670°C at midcrustal depth. the basement are intensely mylonitized, and microcline por- The T estimates are consistent with those estimated in this phyroclasts larger than 2 mm diameter are uncommon. But study from deformation microstructures in the granitoid microcline porphyroclasts in Domain-II and Domain-III mylonite. These T values are ~200°C below the blocking tem- granitoid mylonites are profuse and are commonly greater perature of U, Pb intracrystalline diffusion in monazites than 5 mm diameter, and the largest clasts measure 5 cm or (>850°C; [122–124]). We suggest therefore that the sharply more in length. (2) The bulk of the HFSZ granitoids possess defined Th and Y zonations in monazites lineated parallel monophase steeply dipping mylonitic foliation and lack evi- to and overgrowing the metamorphic fabrics in the mica dence of an overprinted D3 deformation fabric. Thus, if all schists (Figures 9(c) and 9(d)) are of metamorphic origin. the granitoids were pre-D3, feldspar clasts in the D4 shear These monazites were produced by fluid-induced dissolution- zone should be similar sized or smaller, and the D3 fabric is precipitation processes [131, 132], and not due to intracrystal- likely to be preserved at least locally. The occurrence of line diffusion. Hence, the chemical dates retrieved from these larger-sized K-feldspar clasts in granitoids forming the bulk chemical domains correspond to deformation-metamorphic of D4 shear zones, and the lack of D3 structures, suggest that events. By the above reasoning, monazites hosted in biotite these granitoids were unaffected by the D3 deformation. defining the mylonitic fabric in granitoids are of metamor- Taken together, this implies that a broad contemporaneity phic origin. But monazites within clasts of former magmatic existed between granitoid emplacement and D3-D4 defor- feldspar grains in the granitoid mylonites and in granitoids mations. (3) Weakly developed chessboard subgrain micro- with no mesoscale fabric are likely to be of magmatic origin. structures in quartz in Domain-I and Domain-III indicate The spot dates in all rocks taken together vary between that the Early Neoproterozoic D3 and D4 deformations 874 ± 60 Ma (RR-6) and 1066 ± 64 Ma (RR-35) (Table 2). occurred at high temperature (T > 650°C; [84]). But supraso- Only 8 out of 120 spot ages are >1000 Ma, out of which one lidus deformation microstructures such as minerals with spot date 1066 ± 64 Ma (RR-35) is >1050 Ma (Table 2). This euhedral faces piercing neighboring quartz and K-feldspar data is taken to be an outlier. Seven of the >1000 Ma spot grains [91], trains and imbrications among euhedral feldspar dates are obtained in nebulously zoned cores in mica schists aggregates [89], and optically homogenous quartz grains that are mantled by chemical domains with younger dates. A impinging against feldspars [134, 135] are lacking in the mean date of 960 ± 60 Ma is obtained for all the 120 spot granitoids in the three domains. Combining the evidences, dates taken together; further subsets of population could we suggest that the HFSZ nucleation was contemporaneous not be statistically resolved using the software of Ludwig with postemplacement cooling of Early Neoproterozoic felsic [127] for all of the data taken together (Figure 9(e)) and for granitoids. Similar situations have been invoked to explain the individual samples (Table 2). Overall, the grain margins microstructural development and strain partitioning in shear yielded younger dates relative to the interiors of the nebu- zones and shear bands in a large number of terrains, e.g., the lously zoned monazites in both mica schists and the granit- Iberian Arc [136], the Hermitage Massif, France [137], the oids, but given the 2σ errors of spot dates, the difference in Papoose Flat pluton, California [138], the Central Alps dates did not translate into statistically resolvable popula- [139], and the leucogranites of the Higher Himalayan crystal- tions. No systematic age difference could be deciphered lites [140]. between the HFSZ, the shallowly dipping Domain-I mylonites, and the massive granitoid, i.e., both D3 and D4 defor- 7.1. D3 Deformation: Shallowly Dipping Foliation. This study mations are Early Neoproterozoic in age, and immediately for the first time documents the existence of expansive followed large-scale granitoid emplacement in the CGC. domains of horizontal/shallowly dipping carapaces in the The dates from the present study area (Figure 9(e)) are iden- CGC that reoriented structurally higher parts of older (Early tical to monazite chemical dates obtained by Bhattacharya Mesoproterozoic) anatectic grade D1-D2 steeply dipping et al. [65] and Chowdhury and Lentz [133] in the Gangpur deformation fabrics (Figure 3(c)) in the basement gneisses Schist Belt (Figure 9(f)) which is part of the accretionary zone [61, 63, 69, 70]. The D3 deformation is inferred to be associ- at the southern flank of the CGC (Figure 1). It is evident ated with the transport of the allochthonous amphibolite therefore that the D3 and D4 deformations in central CGC facies supracrustal unit over the basement granitoids and are contemporaneous with the accretion of the CGC with gneisses along a gently dipping midcrustal decollement the Archean Singhbhum Craton in the south. (Figure 10(a)). The NE-directed transport of the supracrustal unit produced the penetrative shallowly dipping schistosity 7. Discussion in the supracrustal rocks, the mylonitic foliation in the granitoids, and recumbent folding of the D1-D2 structures in the The age ranges and the mean ages of Early Neoproterozoic upper parts of the high-grade anatectic gneisses. The defor- metamorphic monazites in the mica schists overlap with mation is reminiscent of a basement-involved thin-skinned – the chemical ages in monazites hosted in magmatic feldspar tectonic regime [141 143]. The NW/SE-trending LS3 stretch- clasts and in biotite aggregates in the Domain-I and ing lineations on the mylonitic D3 foliation in the granitoids Domain-II granitoid mylonites (Table 2). Several lines of evi- (Figures 2(b), 3(a), and 3(f)) suggest lengthening orthogonal dence point towards episodic granitoid emplacement in the to the transport direction (cf. [144], Figure 10(a)).

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Domain-I Domain-II Domain-III N N

LS3

D3 �=19-25º D4 D4

�=30-40º

Massive Grenvillian-age D1-D2 L granitoid below S4A L decollement S4B

(a) (b) (c)

I N

II III

LS4B

Decollement LS4A

Anatectic gneisses Granitoids Supracrustal rocks (d)

Figure 10: (a–c) Cartoons showing the progression of structural development for Domain-I, Domain-II, and Domain-III, respectively. Short dashes in cartoons are stretching lineations. (d) Block diagram integrates the lithological units and their structures in the three diﬀerent structural domains with the physiography of the area.

Several tectonic explanations can be suggested for the detachment needs to be investigated further before definite formation of the shallowly dipping carapace. The broad con- conclusions can be made. Several studies have nonetheless temporaneity of the D3 structures with the accretion of the reported near-isothermal decompression paths in the high- CGC with the Singhbhum Craton (Figures 1, 9(e), and 9(f)) grade basement rocks during the Early Neoproterozoic suggests that the deformation occurred in an overall com- deformation of the CGC [63, 71, 153]. pression regime. However, the absence of older-on-younger thrust sequences, i.e., the Early/Middle Mesoproterozoic 7.2. D4 Deformation: HFSZ. Steeply dipping to vertical shear gneisses overlying the Early Neoproterozoic supracrustal zones with subhorizontal stretching lineation form due to rocks, undermines thrusting as the causal mechanism for transpression or transtension [24, 26, 154–158]. Numerical the shallowly dipping D3 carapace. Normal shear sense strain modeling [156] suggests that folds on an initially hor- observed on the gentle N-dipping D3 foliation planes izontal layer (in this case, the shallow-dipping D3 foliation) (Figures 2(a), 2(b), and 2(e)) in the granitoids and in the in a transtension regime initiate at angles greater than 45° supracrustal rocks (Figures 3(d) and 3(e)) preserved in out- to the shear zone wall, but these fold hinges do not rotate crops largely unaffected by the D4 deformation implies that into parallelism with the shear zone wall, irrespective of the D3 foliation is akin to a midcrustal regional-scale low- the amount of pure shear or simple shear acting on the angle normal fault. Such low-angle normal faults that shear zone. In contrast, folds in a transpressive regime produce shallowly dipping mylonitic fabrics are genetically originate at angles less than 45° to the shear zone wall, associated with extensional tectonics [145–152] and are and rotate into parallelism with the shear zone wall with widely reported in detachment zones of metamorphic core progressive deformation [156]. Within Domain-II in complexes ([9] and references therein). As in these detach- HFSZ, the D4 fold axes and axial planes of close, asym- ment zones, the D3 shallow-dipping carapace separates rock metric, steeply inclined to upright, gently plunging folds units of differing metamorphic grade and origin, but exhu- on the earlier shallowly dipping D3 foliation in the supra- mation of the high-grade basement gneisses along the crustal rocks (Figures 2(e) and 6(a)) and granitoid

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mylonites (Figure 4(c)) are collinear and subparallel to the shear zones [25]. The range of Wm values determined for shear zone walls along the entire length of curvilinear the monoclinic Domain-II straddles Wk = 0:81, and conse- HFSZ. This is strong evidence for the HFSZ forming D4 quently the flow regime would be intermediate between pure deformation to have developed in a transpressive regime. and simple shear dominated. For the cut-off value of Wk = Therefore, even if the shallow D3 fabrics formed due to 0:74 proposed by Law et al. [164] and the observation of extension, the HFSZ and possibly other steeply dipping underestimating vorticity values in simple shear-dominated regional-scale shear zones traversing the CGC (Figure 1) flows [101–103], Domain-II is totally simple shear domi- accommodated the compressional strain within the fore- nated. The lower range of Wm values estimated for land region during the contemporaneous accretion of the Domain-III granitoids is indicative of a larger coaxial shear CGC with the Singhbhum Craton in the south. component, but due to uncertainties associated with Wm Theoretical models provide insights into dynamics of for- estimations from rigid clasts and the triclinicity in Domain- mation of transpressional shear zones [24, 26, 105, 106, 110, III (oblique VNS), these results are only speculative. Even 154, 158–161]. Fossen and Tikoff [26] propose five transpres- so, because of the differing strike orientations and symme- sional models that encompass the entire range of possible tries in the two domains, the flipped stretching lineations strain variations in monoclinic transpressional shear zones. are unlikely to have simply formed from an increase in the Domain-II in HFSZ with horizontal stretching lineations coaxial strain component [25, 26, 161]. compares favorably with Types D and E of Fossen and Tikoff [26]. Type D exhibits a plane strain-dominated deformation 7.3. Strain Partitioning and Shear Zone Curvature. Strain with horizontal shortening normal to the vertical shear partitioning is the fundamental mechanism for producing zone wall compensated by shear zone parallel extension in variations in structures along or across shear zones. Strain the horizontal plane, and Type E has an additional compo- partitioning occurs predominantly along structural weak- nent of vertical pure shear for similar parameters as in Type nesses [165] such as rheological heterogeneities, lithological D. But, highly constrictional strains are produced in Type E contacts, and preexisting deformation structures. Anisot- conditions resulting in subhorizontal L tectonites within ropies produced during progressive deformation can further steeply dipping shear zones [26, 154]. Such L tectonites allow subsequent strain to be partitioned. Partitioned strain are lacking in Domain-II, making Type D strain conditions is also a cause for primary curvatures on transpressional (Figure 10(b)) more probable than Type E. Notably, in the shear zones [56, 162, 163, 165, 166]. Type D model, the stretching lineation is always subhori- In transpressional shear zones, rheologically stronger zontal in orientation for a vertical shear zone irrespective units accommodate the pure shear component, while the of the amount of finite strain [26]. The triclinic symmetry simple shear component is partitioned into the weaker units. of Domain-III hinders direct comparisons with theoretical The orientation of the left-lateral transpressive HFSZ implies models. A better constraint on shear zone parameters such that the far field compressional paleostress axes were ori- as obliquity angle, extrusion angle, and vorticity number is ented in the arc between NNE and ENE. For a uniform orien- required before this domain can be modeled to produce tation of the far field stress and if Domain-II and Domain-III meaningful results. were contemporaneous, the discordant kinematics and the Therefore, we suggest that Domain-II of the HFSZ shear zone curvature could have resulted from strain parti- tended towards a monoclinic symmetry, the strain was not tioning due to rheological contrasts induced by subtle varia- constrictional but accommodated a lateral stretch parallel tions in mineralogy of the plutons or variations in rheology to the shear zone walls, and an increase in the amount of and deformation behavior in minerals in cooling plutons strain would not cause a switch in the orientation of the sub- [137]. Biotite-richer plutons theoretically allow greater parti- horizontal stretching lineations into down-dip orientations. tioning of simple shear (cf. [167]); but modal amounts of bio- Domain-III experienced a higher proportion of a pure shear tite appear to be somewhat greater in the Domain-III component, possibly including small amounts of constric- granitoids than in the Domain-II granitoids of the HFSZ. tional strain (presence of L>S tectonite at Mishirhutang, Evidently, strain partitioning in the HFSZ would have had Figure 2(a)), and the symmetry was triclinic accommodating to overcome the mineralogical differences in the two a subvertical stretch component. domains to attain the present structure. Differential cooling Flipped lineations are attributed to variation in the in the granitoid plutons in the two domains is more likely magnitude of strain, strain partitioning, or change in the to have played a role in partitioning strain. angle between the shear zone wall and the orientation of The southern boundary of Domain-III is juxtaposed with the far field stress [55, 56, 93, 95, 110, 161–163]. Simple the southern supracrustal belt (Figure 2(a)) dominated by shear-dominated strain produces subhorizontal stretching amphibolites, calc-silicate gneisses, and quartzites. The lineations, while pure shear-dominated strain produces sub- domain extends westward as far as the E-trending supracrus- vertical stretching lineations on steeply dipping transpres- tal band continues. Thus, the close association of Domain-III sional shear zones [24–26, 56, 161]. Vorticity estimations with the southern supracrustal belt (Figure 2(a)) dominated (Figure 8) demonstrate that Domain-II developed with a by weak-to-shear minerals (such as biotite and muscovite) considerably higher simple shear component of the bulk strain suggests that partitioning of the bulk strain at the junction as compared to Domain-III. A cut-off value of Wk = 0:81 has of the contrasting rheology might have helped to nucleate been proposed to separate pure shear- (Wk < 0:81) from the straight-walled E-trending Domain-III with triclinic simple shear-(Wk ≥ 0:81) dominated flows in monoclinic symmetry and down-dip lineations (Table 1). The

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0.8

0.6

0.4 Domain-II

0.2

Domain-III 0

�

Figure 11: Graph of Wk vs. α after Diaz-Azpiroz et al. [181] used to compute the convergence angles for Domain-II and Domain-III (δ = dip of shear zone). Shear zones in both domains are considered to be vertical (δ =90°).

juxtaposition of these two lithologies would lead to compe- Alternatively, the two domains within the HFSZ were not tency contrasts resulting from material heterogeneity, pro- contemporaneous, but formed during progressive D4 defor- moting strain partitioning with further deformation [168]. mation as separate shear zones with distinct characteristics Nucleation of shear bands along domain boundaries due to (Table 1), i.e., the early-formed Domain-II shear zone curved strain incompatibilities and mechanical anisotropies has into the subsequent Domain-III segment (Figures 10(b) and been noted [168–170]. Such rheological contrasts could have 10(c)). The rare shear lens with tight reclined folds interfolial induced partitioning of the bulk strain into pure shear- and to the steep-dipping mylonitic foliation of Domain-III simple shear-dominated domains, a possibility for develop- (Figure 5(b)) could be a result of overprinting relationships ing the structures and symmetry observed in Domain-III between the two domain shear zones. Reorganization of (Table 1). early-formed deformation structures during progressive If strain partitioning was indeed the major factor control- deformation is widely reported [36, 51–54]. Emerging data ling the simultaneous formation of the two domains and pro- suggests that changing convergence vectors during plate ducing the curvature of the HFSZ, the two domains can be motions can play a major part in the development of new compared to a large scale S-C fabric [171–173] within a sinis- structures that reorient earlier fabrics in the rocks [176–180]. tral setting, with Domain-II and Domain-III corresponding Considering that the two domains formed sequential to the S and C fabrics, respectively (Figure 2(a)). The two shear zones, the protocol of Diaz-Azpiroz et al. [181]) was domains also have geometric settings comparable to primary used to equate the Wm values estimated for the two domains curvatures on shear zones such as a right-stepping restrain- to the vorticity number, Wk values [182], which translate to ing bend or a splay structure ([41, 48, 50]. These possibilities convergence angles (α) equal to 19–25° for Domain-II and would however require the curving segment of the shear zone 30–40° for Domain-III (Figure 11). α is the angle between (Domain-II) to accommodate a greater amount of pure shear the plate motion vector and the strike of the shear zone, producing down-dip lineations, reverse faults, positive ﬂower where α =90° represents orthogonal convergence and α =0° structures, and vertical extrusion of material [41, 50, 174, represents strike slip. The convergence direction in the E- 175] as compared to the straight segment (Domain-III) trending Domain-III was computed to lie between 50°N [56]. This is opposite to the fact that Domain-III rather and 60°N (Figure 10(c)). By contrast, the convergence direc- than Domain-II accommodated a greater pure shear com- tion for the curved Domain-II is computed to be 87°N–94°N. ponent as determined from kinematic vorticity estimations Since the vorticity measurements were conducted on the E- (Figure 8). striking segment of Domain-II (Figures 8(b)–8(f)), and the

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Domain-II shear zone would have been drawn into parallel- ing stretching lineations (Domain-III). Kinematic vorticity ism due to superposition of the Domain-III shear zone, the analyses for Domain-II (Wm = 0:73 – 0:83) and Domain- actual strike orientation of Domain-II prior to reorientation III (Wm = 0:51 – 0:65) suggest a plane strain, sinistral was possibly NW-SE (~135°N). If so, α values of 19–25° monoclinic kinematic model for Domain-II and a sinistral, translate to a convergence orientation of 110°N–116°N pure shear-dominated, triclinic deformation regime for (Figure 10(b)). Although the values are only suggestive, the Domain-III. HFSZ could have formed because of a shift in the conver- Left-lateral simple shear-dominated transpression is sug- gence vector for transpressional shortening from E/ESE gested to have nucleated the NW arm of the HFSZ (Domain-II) to NE/ENE (Domain-III). The change in the (Figure 10(b)). Continued transpression with changing con- orientations of the strongly developed subhorizontal stretch- vergence direction caused existing structures to curve into ing lineations in Domain-II to the subvertical orientation in the progressively formed E-trending HFSZ with steeply Domain-III may have resulted from a switch in the orienta- plunging stretching lineations (Figure 10(c)) producing the tion of the convergence direction, causing the earlier formed structures observed in the area (Figure 10(d)). Early Neopro- NW fabrics to sigmoidally curve into the E-striking shear terozoic metamorphic monazites in supracrustal rocks and zone of Domain-III. A similar switch in convergence vector metamorphic rims around magmatic monazite cores in gran- has been proposed to explain the structural framework in itoid mylonites suggest that the D3 and D4 deformation Costa Rica due to the change in the convergence direction events were contemporaneous with accretion of the Archean of the Cocos plate [177]. Singhbhum Craton along the southern margin of CGC. This study adds to the growing body of emerging data that sug- 8. Conclusions gests strain field instabilities induced by variations in plate convergence angle cause reorientation of mesoscale struc- In the Early Neoproterozoic, the Chottanagpur Gneiss Com- tures in a regional scale. plex experienced a major episode of felsic granitoid emplacement. This was followed by the translation of allochthonous Data Availability supracrustal rocks which formed a shallowly dipping carapace (Figure 10(a)), and finally nucleation of several E- All the data used in this study are incorporated within the striking crustal-scale steeply dipping transpressional shear article and in the supplementary material provided along zones (Figure 1). The mesoscale structures and the deforma- with the article. No other data from any other source were tion kinematics of the shallowly dipping D3 carapace and the used in the research described in the article. shear zones (D4) especially in the interior of CGC were hitherto undocumented. This study addresses these issues using Conflicts of Interest structural mapping (Figure 2), microstructural studies (Figure 7), vorticity analyses (Figure 8), and monazite geo- The authors declare that they have no conflicts of interest. chronology (Figure 9) in central CGC. The D3 deformation that produced recumbent folding Acknowledgments of older steep-dipping basement gneisses, duplex structures in the granitoids/supracrustals, and asymmetry microstruc- The work forms a part of the doctoral dissertation of NS. tures in granitoids/supracrustal rocks suggests top-to-the- Financial support for the work was provided by University NE translation of the allochthonous supracrustal unit Grants Commission (New Delhi) through a Junior Research orthogonal to NW-SE lineations in shallowly dipping mylo- Fellowship to NS. AB acknowledges the financial support for nites (Figure 10(a)). The translation of the supracrustal unit fieldwork and chemical analyses provided by the host insti- is attributed to midcrustal extension occurring along a shal- tute through the CPDA funding scheme for the block years lowly dipping decollement that extended across large parts 2017–2020. We thank C. Fernandez and M. Diaz-Azpiroz of central CGC, similar to midcrustal low-angle normal for their insightful comments on shear zone formation in fi faults, but the implications of the nding are yet to be fully an earlier draft of the manuscript. Editorial handling of the understood. manuscript by Damien Nance is greatly appreciated. A steep-dipping curvilinear D4 Hundru Falls Shear Zone (HFSZ) is SE striking in the NW and E striking in the east and exhibits sinistral kinematics and dominantly north- Supplementary Materials down sense of movement along bounding inward dipping Supplementary Material1: electron probe microanalytical margins (Figure 10(d)). The HFSZ truncates the shallowly data in wt% oxides on monazites from the present study area, dipping granitoid mylonites (Domain-I). Gently plunging chemical ages, and ±2σ errors of spot ages in Ma determined stretching lineations in the NW-striking arm of the HFSZ using the formulation of Montel et al. [112]. Sample locations (Domain-II) are collinear with the axes of E/SE-trending are shown in Figure 2(a). (Supplementary Materials) asymmetric folds on shallowly dipping mylonites, and steeply inclined/upright folds on originally shallowly dip- References ping crenulation foliation in supracrustal rocks. Along the southern flank of the E-striking arm of the HFSZ, [1] G. I. Alsop, “Reversals in the polarity of structural facing steep mylonite foliations are associated with steeply plung- across an early ductile thrust, the Central Donegal Slide,

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