Geology and tectonic history of the Lohit Valley, Eastern Arunachal Pradesh, India

Journal of Asian Earth Sciences 21 (2003) 731–741 www.elsevier.com/locate/jseaes

Geology and tectonic history of the Lohit Valley, Eastern Arunachal Pradesh, India N.S. Gururajan*, B.K. Choudhuri Wadia Institute of Himalayan Geology, 3 General Mahadeo Singh Road, Dehra Dun 248001, India Received 15 December 2001; accepted 9 May 2002

Abstract The Lohit River section of eastern Arunachal Pradesh comprises four tectonic units. From SW to NE these are: the Lesser Himalayan rocks, the Mishmi Crystallines, the Tidding Suture Zone and the Lohit Plutonic Complex. The Mishmi Thrust underlies the basal Lesser Himalayan unit, while the Mishmi Crystallines are thrust over the Lesser Himalayan unit along the Main Central Thrust. The grade of metamorphism in the Mishmi Crystallines increases up the structural section from chlorite to staurolite – kyanite zones, exhibiting inverted metamorphism. The relationship between deformation and metamorphism shows that the metamorphic peak was syn- to post-tectonic in relation to the main ductile shearing event. Continued deformation, after the metamorphic peak, was accommodated along millimetre scale shear zones, developed throughout the sequence, parallel to the regional schistosity. Movement along these shear zones has resulted in inversion of the metamorphic zones. The rocks of the Tidding Suture represent an ophiolitic me´lange, thrust over the Mishmi Crystallines, which in turn are overthrust by the Lohit Plutonic Complex along the Lohit Thrust. The Lohit Plutonic Complex is subdivided into western and eastern belts separated by the Walong Thrust. The western belt consists of deformed quartz-diorite, diorite, gabbro and trondhjemite, intruded by basic and acid dykes. The eastern belt comprises garnet – sillimanite gneiss, intercalated with crystalline marble bands, followed by a complex zone of leucogranites, aplites and pegmatites, which intrude the early foliated quartz-diorite, soda-rich granite and microdiorite. The rocks of the eastern belt are the northward continuation of the Mogok Gneissic Belt of central Burma. The occurrence of intrusive rocks in the eastern belt suggests that the magmatism related to subduction extended to the east, far from the subduction zone. The peraluminous leucogranites, aplites and pegmatites are the products of crustal melting, induced by crustal thickening related to the intracontinental Walong Thrust. Subsequent to metamorphism and shearing, the whole sequence was folded into an antiform, forming the Eastern Syntaxis, and this deformation steeply tilted the earlier low angle thrusts and foliations. Later compression partitioned into rightlateral strike-slip motion, producing a superimposed sub-horizontal lineation observed mostly in the Lohit Plutonic Complex. q 2002 Elsevier Science Ltd. All rights reserved. Keywords: Lohit Valley; Tectonic history; Mishmi Crystallines

1. Introduction In the easternmost part of the Himalaya, in eastern Arunachal Pradesh, the major tectonic units show a bend in their regional strike from ENE –WSW to NW –SE and this bend is known as the Eastern Syntaxis (Fig. 1). Field studies in this region show that the Eastern Syntaxis represents a major antiformal structure (Thakur and Jain, 1975), known as Siang antiform (Singh, 1993). Recently Burg et al. (1997), in Namcha Barwa region interpreted the Eastern Syntaxis in terms of a fast growing crustal scale antiform

* Corresponding author. E-mail address: [email protected] (N.S. Gururajan).

that folds the suture into a sharp syntaxis, similar to the western Himalaya syntaxis in northern Pakistan. The study area, the Lohit River section lies on the eastern limb of the Eastern Syntaxis. Various workers have described the geology of the eastern Arunachal Pradesh (Gansser, 1964; Nandy, 1973; Karunakaran, 1974; Stoneley, 1974; Jain et al., 1974; Valdiya, 1976; Thakur, 1986; Acharyya, 1987; Kumar, 1997 and references therein). Thakur and Jain (1975) gave an account of the deformation and metamorphism of the Mishmi Hills covering only the southwestern part of the Lohit Valley. This paper presents data on the geology, deformation, metamorphism and tectonic history of the Lohit Valley, which has a bearing on understanding the relationship between the eastern Himalaya and northern Burma.

1367-9120/03/$ - see front matter q 2002 Elsevier Science Ltd. All rights reserved. PII: S 1 3 6 7 - 9 1 2 0 ( 0 2 ) 0 0 0 4 0 - 8

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Fig. 1. Generalized geological map of eastern Arunachal Pradesh (modified from Thakur, 1986; Singh and Chowdhury, 1990; Singh, 1993). The inset map shows the Western and Eastern Syntaxes of the Himalaya.

2. Regional geological setting In the Eastern Syntaxial region, the three major tectonic units of the Himalaya, the Sub-Himalaya (Siwaliks), the Lesser Himalaya (including the Gondwana Group) and the Higher Himalaya (Gansser, 1964), can be recognized, particularly in the western limb of the syntaxis. In addition, two Trans-Himalayan units known as the Tidding Suture Zone and the Lohit Plutonic Complex are also exposed but are restricted to the eastern limb of the syntaxis. The central part of the syntaxial structure is occupied by a sequence of sedimentary rocks of Eocene age, associated with basic volcanic rocks (Fig. 1) (Singh, 1993). The Upper Tertiary fluvial sediments of Siwalik Group are overthrust by the Gondwana Group of sediments, with marine and plant fossils of Permian age, along the Main Boundary Thrust (MBT). These groups of rocks do not extend to the eastern limb of the syntaxis, where they are probably overlapped or cut-off by the Mishmi Thrust. The Lesser Himalaya consists of a non-metamorphic to low-grade (chlorite grade) sedimentary sequence, thrust over the Gondwanas, and extending around to the eastern limb where the thickness is considerably reduced. The crystalline rocks of the Higher Himalaya exhibit inverted metamorphism and are divided into two units: the lower, Bomdila Group and the upper, Sela Group (Verma and Tandon, 1976). Of the two groups, only the Bomdila Group extends onto the eastern limb of the syntaxis

(Kumar, 1997), where they are known as Mishmi Crystallines (Thakur and Jain, 1975), and this name is retained in this paper for the crystalline rocks of the Lohit Valley. The Bomdila Group consists of low to medium grade metamorphic rocks and the Sela Group consists of high-grade, kyanite – sillimanite bearing gneisses, with intrusions of Tertiary leucogranite. Sinha-Roy (1982) has correlated these units, and their equivalents in Bhutan and Darjeeling –Sikkim region, with the Central Crystalline rocks of the Central Himalaya and located the Main Central Thrust (MCT) at the base of the crystalline sequence, by following Heim and Gansser (1939) who defined the MCT as the thrust which separates the Lesser Himalayan sediments from the crystalline nappes of the Higher Himalaya. However, Thakur (1986) has located the MCT at the base of the Sela Group and has correlated this group with the Tibetan Slab or the Higher Himalayan Crystallines (HHC) of Nepal (Le Fort, 1975) and the Vaikrita Crystallines (HHC) of Kumaun Himalaya (Valdiya, 1980). In the latter two areas, the MCT is regarded as a wide shear zone, known as MCT zone, occupied by the crystalline nappe of the Lesser Himalaya, and associated with inverted metamorphism (Peacher, 1989). This zone separates the Lesser Himalaya in the south from the HHC in the north. In accordance with the above definition of the MCT, the Bomdila Group, which covers extensive areas of the Lesser Himalaya and occurs below the high-grade gneisses of Sela Group, can be regarded as the Lesser Himalayan crystalline

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Fig. 2. Traverse geological map of the Lohit Valley, eastern Arunachal Pradesh. Part B is the continuation of Part A.

nappe. However, in Arunachal Pradesh, no detailed metamorphic studies have been carried out to demarcate the MCT zone. In the present study, the MCT is marked at the contact between the Lesser Himalayan sediments and the overlying crystalline rocks.

3. Geology of the Lohit Valley The Lohit Valley section comprises four major NW – SE trending lithotectonic units: the Lesser Himalayan rocks, the Mishmi Crystallines, the Tidding Suture Zone and the Lohit Plutonic Complex. A geological map along this traverse is shown in Fig. 2. On the basis of field and structural relationships, the tectonostratigraphy is described in Table 1. The petrography and structural elements are described in Tables 2 and 3, respectively. The salient geological features and the textural details are described in this section.

3.1. Lesser Himalayan unit The lowest tectonic unit is a sequence of low-grade metasediments, dominated by quartzite, interleaved with bands of chlorite –sericite phyllites and metavolcanics. This unit rests on the Mishmi Thrust and is thrust over the recent alluvial sediments of the Lohit River (Thakur and Jain, 1975). Along the Lohit road section this unit pinches out (Fig. 2), but can be traced to the NW towards the Dibang Valley, where it is well exposed. The quartzites are light greenish, foliated orthoquartzites and contain thin monomineralic shear zones. 3.2. Mishmi crystallines The Mishmi Crystalline Unit is thrust over the Lesser Himalayan rocks along the MCT, which is represented by a broad, ductile deformation zone. It is further subdivided into two units: a lower unit, dominated by mylonitic gneiss of

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Table 1 Lithotectonic subdivisions in the Lohit Valley, eastern Arunachal Pradesh Major tectonic units

Lohit Plutonic Complex

Subunits

Rock types

Eastern Belt

Leucogranite, quartz dioritic gneiss, microdiorite, soda-rich acid gneiss, garnet– sillimanite gneiss with marble bands

Walong Thrust Western Unit

Gabbro, diorite, quartz diorite, trondhjemite, dykes of basalt, microdiorite, dacite, diorite pegmatite and hornblendite

Lohit Thrust Tidding Suture Zone

Metavolcanics (tremolite-actinolite schist) marble bands, serpentinite graphitic garnet schist. Tidding Thrust Upper Unit

Mishmi Crystallines

Thrust Lower Unit

Graphitic staurolite–kyanite schist with slivers of gneissic bands amphibolite, and graphitic garnet schist Mylonitic augen gneisses with amphibolite boudins graphitic schists with marble bands and quartzite, phyllonite, platy mylonite

Main Central Thrust (MCT) Lesser Himalayan Rocks

Schistose muscovite chlorite-quartzite phyllite, metavolcanics Mishmi Thrust Alluvium

granitic composition and an upper unit of graphitic, garnet – staurolite – kyanite schist. The metamorphic grade increases upwards, exhibiting inverted metamorphism. The mylonitic augen gneiss of the lower unit is sandwiched between bands of biotite grade graphitic phyllite, with marble and quartzite. Boudins and bands of amphibolite occur throughout the unit. The development of S – C fabrics, oblique shear bands and asymmetric porphyroclasts indicate top to the SW shearing along the MCT

(Fig. 3a). The intensity of deformation is high at the base of the MCT zone, represented by platy mylonites and quartz – mica rich phyllonites, derived from fracturing and retrograde reaction of feldspars assisted by water. The presence of quartz, albite, chlorite, muscovite and epidote indicate low temperature deformation. Away from the MCT, the gneisses are coarse-grained, showing early crystal plastic deformation microstructures, such as, the development of sub-grains and recrystallised grains in quartz, and undulose

Table 2 Deformation phases and structural elements in the Lohit Valley region Deformation Phase

D1

D2

Post-D2 D3

D4

Tectonic Units Mishmi and Tidding Units

Lohit Plutonic Complex

F1 tight, small isoclinal folds, sometimes rootless and reclined occur mainly in MCT. S1 occurs as crenulation within the penetrative S2 fabric and as inclusion trails in garnet. F2 isoclinal folds, strongly asymmetric shear folds (top to SW sense of shear) S2 penetrative ductile shear fabric; S2 S –C fabric in thrust zones L2 mineral stretching lineation Well distributed low temperature mm scale shear foliation F3 regional open folds, coaxial to F2, Small upright folds associated with crenulation type folds S3 rare open crenulation

S2 ductile S –C fabric and stretching lineation (L2) in Lohit Thrust Zone. Thrust-related asymmetric isoclinal and sheath-like folds in Walong Thrust Zone (with top to SW sense of shear).

Dextral strike-slip movement reactivating the Lohit, Walong Thrusts and shear zones. Sub-horizontal SE plunging stretching lineation.

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Table 3 Petrology of tectonic units from the Lohit Valley Unit

Rock type and Mineralogy Lower Unit Mylonitic augen gneiss

Mishmi Crystallines

Tidding Suture Zone

Lohit Plutonic Complex

Upper Unit Metapelites

Metavolcanics Metapelites Marble Serpentinite Western Belt Quartz diorite Diorite Gabbro Trondhjemite Eastern Belt Pelitic gneisses Marble Quartz diorite Microdiorite Soda rich granite Leucogranite

Quartz, albite, muscovite, chlorite, epidote, oxides Quartz, plagioclase, microcline, epidote, biotite, muscovite, oxides, tourmaline Biotite, garnet Biotite, garnet, staurolite Biotite, garnet, staurolite, kyanite Biotite, garnet, kyanite (above assemblages also contain quartz, muscovite, plagioclase and graphite) Quartz, albite, actinolite, chlorite, epidote, oxides, ^ garnet Quartz, muscovite, biotite, garnet, plagioclase Calcite, muscovite, epidote, tremolite Serpentine, oxides, olivine Plagioclase, hornblende, biotite, quartz, sphene, oxides Plagioclase, hornblende ^ biotite, sphene, oxides Plagioclase, clinopyroxene, orthopyroxene, hornblende, oxides, Quartz, plagioclase, biotite, epidote Biotite, fibrolite/sillimanite, garnet, plagioclase, oxides Calcite, diopside, epidote, sphene, quartz Biotite, hornblende, plagioclase, quartz, sphene þ garnet Biotite, hornblende, plagioclase, quartz, oxides Quartz, plagioclase, biotite, epidote ^ garnet Quartz, k-feldspar, plagioclase, biotite ^ muscovite ^ garnet

extinction, deformation bands, bent twin lamellae and marginal recrystallisation in plagioclase. Late brittle deformation features have been superimposed on these early ductile deformation microstructures, such as, the intragranular fractures in feldspars with quartz infillings and transgranular fractures filled with fine quartz and chlorite. At higher structural levels the occurrence of myrmekite and the presence of garnet in the amphibolite bands within the gneisses, indicate that the deformation occurred under higher temperature conditions, since myrmekite appears under upper greenschist or lower amphibolite facies conditions (Hanmer, 1982; Vernon et al., 1983; Simpson and Wintsch, 1989). Although the deformation is concentrated at the base of the MCT zone, thin mm scale shear zones that are parallel to, and superimposed on the mylonitic foliation, occur throughout the unit. The development of these shear zones suggests, that the deformation during later stages of translation was accommodated along these distributed zones, thus producing the inverted metamorphic grade within the gneisses. This upper unit overlies the gneissic unit, and the contact is marked by a thrust, but there is no metamorphic break across the thrust. The upper unit consists of a lower horizon of graphitic garnet – mica schist followed upwards by graphitic staurolite – kyanite schist, and the latter contains foliation-parallel bands of deformed quartzo-feldspathic gneiss and thin slivers of garnet – kyanite gneiss. In this unit, the early fabric (S1) is well preserved as inclusion trails within garnet porphyroblasts enclosed in the

planar ductile shear fabric S2 (Fig. 3b). Porphyroblast – matrix relationships indicate that peak metamorphism was syn- to post-D2. The garnets show two phases of growth. The cores contain curved inclusion trails (Si) of quartz that are oblique to the matrix schistosity (S2). The rims are generally inclusion free and often with euhedral outlines, either enclosed by the external schistosity (Se), or partly overgrowing Se. These garnets are interpreted to have overgrown the different stages of the S2 fabric development (Bell and Rubenach, 1983), and imply syn- to post-D2 growth. Textural relationships suggest that both staurolite and kyanite grew in two phases. The first phase is represented by porphyroblasts in quartz-rich microlithons, with inclusion trails enclosed by matrix S2, indicating early syn-D2 growth. Anhedral staurolite crystals and kyanite blades of the second phase cross-cutting S2, suggest post-D2 growth. Chlorite generally occurs as a secondary mineral replacing biotite and garnet along S2 and as a late mineral in oblique shear bands. It also cross-cuts S2 and shows kinking, indicating post-D2 shearing along S2. Continued shearing during post-D2 period (post-peak metamorphism) is represented by a shear foliation or mm scale thin shear zones parallel to and superimposing the S2 fabric. The late foliation occurs as discontinuous almondshaped or continuous horizontal shear bands (Fig. 3c) cut by oblique shear bands, or C0 bands (Berthe et al., 1979). Muscovite (mostly replacing biotite), graphite, chlorite and iron oxides released from biotite define this foliation.

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Fig. 3. (a) Photomicrograph of mylonite showing a tourmaline crystal with asymmetric pressure shadows. The sense of shear is top to the SW (left side of photograph is SW). (b) Photomicrograph of garnet showing two stages of growth. The crenulated S1 occur at the core, while the rim is free of inclusions. Also note the rootless fold within S2 regional schistosity. The garnet is syn-D2. Sample from the upper unit of Mishmi Crystallines. (c) Photomicrograph of graphitic garnet–mica schist from the upper unit of Mishmi Crystallines. The garnets are enclosed in the late shear foliation that occurs as discontinuous almond-shaped bands. (d) Photomicrograph of garnet from the garnet–mica schist occuring within the lower marble band of Tidding Unit near the Tidding Thrust. Garnets have curved inclusion trails at the center while the rim (top) has overgrown the matrix S2. (e) Photomicrograph of deformed quartz-diorite in the Lohit Thrust Zone. Note SW dipping oblique shear bands. Sense of shear is top to the SW. Left side of photo is SW. (f) Field photograph showing stoped blocks of dioritic gneisses occurring within leucogranite.

Movement along these distributed thin shear zones has produced inverted metamorphism. 3.3. Tidding Suture Zone The Tidding Suture Zone tectonically overlies the Mishmi Crystallines along the Tidding Thrust and the thrust

zone is highly imbricated. Greenschist facies rocks of the Tidding Unit abut directly against amphibolite facies (kyanite grade) metapelitic rocks of the Mishmi Crystallines. This unit is considered to represent a zone of ophiolitic me´lange, probably a southeast continuation of the Tsangpo Suture Zone. This unit can be traced along the eastern limb of the Siang antiform (Singh and

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Malhotra, 1983) and Acharyya (1987) who named this the Tuting-Tidding Suture. The lower portion of the Tidding unit consists of 30– 40 m of garnetiferous metavolcanics, followed upwards by a band of marble that contains slivers of graphitic garnet schist. Further NE, the metabasic volcanic rocks (actinolite – tremolite schist) occur as a thick sequence flanked by another band of 50 m thick, highly folded marble. A tabular body of serpentinised peridotite (dunite) overlies the upper marble band, followed by calcareous bands and graphitic schist that is further overlain by metavolcanics. The metavolcanics contains garnet. The occurrence of veins of serpentine within the calcareous bands gives ophicalcite (Thakur and Jain, 1975). The rock types are highly foliated parallel to S2 in the Mishmi Crystallines. In the case of the metavolcanics, the deformation and metamorphic recrystallization have completely destroyed primary igneous textures. As in the Mishmi Crystalline, the metamorphism in the Tidding unit was also largely synchronous with D2 deformation. The occurrence of garnets with both curved and straight inclusion trails (Fig. 3d), in the graphite schist and actinolite, and biotite cross-cutting to the S2 fabric in the metavolcanics indicates that these minerals grew during the syn- to post-D2 period. The serpentinite body is composed mainly of relict olivine, serpentine and iron oxides. The serpentine flakes replacing the olivine crystals exhibit mesh texture (Maltman, 1978). The serpentinite is also foliated along thin shear zones, in which elongated lenses of iron oxides (sometimes folded intrafolially) and ribbon-textured serpentine flakes define the foliation. 3.4. Lohit Plutonic Complex The Lohit Plutonic Complex overlies the Tidding Suture Zone tectonically on the Lohit Thrust. The continuation of this complex towards the NW into Tibet, and towards the SE into Burma, is poorly known. Nandy (1973) has described this complex as measuring more than 250 km in length and 100 km in width from west to east. This NW – SE trending complex is deformed and foliated, dips towards NE at high angles, and is traversed by number of shear zones parallel to the regional trend. Based on our field investigations, the Lohit Plutonic Complex can be divided into western and eastern belts, separated by the Walong Thrust. The western belt consists of gabbro –diorite, quartz-diorite and trondhjemite and the eastern belt is dominated by leucogranites and aplites and pegmatites, intruding the earlier quartz-diorite, soda-rich gneisses and high-grade pelitic gneisses. The lower part of the western belt consists of quartzdiorites, which are highly deformed in the Lohit Thrust Zone. The deformation phase is considered to be D2, since the foliation is parallel to the S2 fabric of the rocks in the footwall of the Lohit Thrust. This deformation has modified the original igneous fabric and the minerals have

737

equilibrated under different metamorphic conditions. The Lohit Thrust Zone possess a mylonitic S – C fabric (S2) and oblique shear bands (Fig. 3e), which relate to movement along the Lohit Thrust. The presence of asymmetric porphyroclasts indicates deformation under a simple shear regime and the sense of shear indicators, suggest top to the SW movement. The plastic deformation features of plagioclase, recrystallisation of quartz and presence of actinolite and abundant secondary chlorite and epidote, along with dilatant cracks in plagioclase filled with quartz, indicate that the mylonites evolved at upper greenschist facies conditions and were overprinted by low temperature deformation under brittle – ductile conditions. Away from the thrust the quartz-diorites are coarse-grained, the colour varying with the modal content of hornblende. The presence of abundant secondary minerals suggests metamorphic overprinting under low temperature conditions. At approximately the middle of the sequence a gabbro– diorite is exposed. Gabbros occur as satellite-type or ovalshaped bodies, intruded by diorite, indicating successive intrusive events. The gabbros are foliated, and sub-solidus deformation has altered the initial fabric into a granoblastic texture. Both plagioclase and pyroxenes show crystalplastic deformation features and recrystallised grains show triple point fabric. Greenish hornblende occurs as a late mineral replacing the pyroxenes, particularly clinopyroxene, in varying amounts. Actinolite and saussuritised plagioclase occur in high strain zones. The above features indicate that initially deformation occurred at a high temperature followed by deformation at lower temperatures, which can be related to emplacement at shallow levels. Above the gabbro –diorite zone highly foliated metabasics are exposed, representing a broad shear zone, in which the presence of asymmetric shear folds of quartz veins and oblique shear bands suggest top to the SW-shearing. In general, the metabasics are derived from basic intrusives, while some of the highly foliated fine-grained rocks are metamorphosed basic volcanics. These metabasites also contain garnet. This zone extends up to Walong Thrust towards NE and at higher structural levels the metabasics are associated with diorites and microdiorite. A tabular body of trondhjemite is intruded in this zone just above the gabbro– diorite zone. On a normative diagram it falls in the trondhjemite field. It is characterized by , 69 wt% SiO2 and Al2O3 is above 15 wt%. It falls chemically within the high Al-type of trondhjemite of Barker and Arth (1976). This body is also deformed and foliated like the host rocks and in places the S – C fabrics indicate top to the SE shear sense, suggesting a late phase of deformation due to dextral strike-slip movement. The trondhjemite body is intruded by pegmatites and aplites that also intrude the adjacent country rocks. In the country rocks the pegmatites and aplites occur as elongated boudins due to deformation. To the west in the gabbro – diorite sequence a few light-coloured dacitic dykes have been

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observed that are the volcanic equivalents of the trondhjemites. Dykes of basalt, microdiorite, hornblenditite, and diorite pegmatite and dacite, of which the basaltic dykes are dominant, intrude this belt. Though most of the dykes are not disrupted, there are enclaves of basic rock within the diorites. Most of the basaltic dykes are converted into amphibolite. The diorite pegmatites, occur mainly towards the eastern margin of the gabbro – diorite zone, and contain amphiboles measuring more than 15 cm in length. Around the dykes, amphibolitization and hybridization are common. The diorite pegmatites suggest a brief phase of crystallization under fluctuating water pressure and probably indicate that crystallization of the gabbro –diorite sequence ended with a stage of pegmatite intrusion. The extensive basic dyke swarm intruding the diorite – gabbro complex is absent in the trondhjemite, suggesting that the trondhjemite was a later phase. It is suggested that the high Al-trondhjemite must have been produced by extensive fractionation of hornblende from the initial subduction-related gabbro– diorite magma. The occurrence of granoblastic hornblendite (þ plagioclase) dykes to the west of the trondhjemite body indicates that hornblende was fractionated during gabbro –diorite differentiation. Rocks of the Eastern Belt rest on the Walong Thrust and the rock types strike NW –SE, and dip towards the NE at high angles. At the basal level a prominent zone about 1 km thick of migmatitic biotite –garnet –sillimanite gneiss is intercalated with highly folded coarse-grained marble bands. Plutonic rocks also intrude this zone. The regional foliation (S2) is well developed parallel to that of the western belt. In the thrust zone asymmetric folds in marble bands indicate top to the SW shearing. In the pelitic gneiss biotite and needles of fibrolite that wrap around garnet porphyroblasts define the foliation. In the thrust zone fibrolitisition is intensive and the fibrolite mat also contains prismatic sillimanite. Garnets are sub-rounded to idioblastic and the zonal boundaries are dusted by graphite. Primary muscovite is absent. The marble bands are coarse-grained, diopside-bearing and exhibit granoblastic texture. Calcite grains are extensively recrystallised, showing equilibrium texture, indicating a high temperature of deformation. Above the pelitic zone, intermediate and acid intrusive rocks are exposed throughout the area. The intrusive rocks can be classified into two groups. An early group consists of an initial phase of quartz-diorite intruded by dykes of microdiorite that were subsequently intruded by soda-rich granites. These early intrusives are highly foliated and occur mostly as stoped blocks and xenoliths (Fig. 3f) of various dimensions within the late intrusive tabular and sheet-like bodies of leucogranite and associated pegmatite and aplite dykes. The leucogranites are generally deformed, however, the foliation is particularly well developed near to the Walong Thrust. Foliated xenoliths within the leucogranite sheets suggest that the granites are syn-tectonic, while

the less deformed leucogranite sheets could very well be post-tectonic in relation to top to the SW shearing. The early intrusives are metaluminous and similar to the quartzdiorites and trondhjemites of the western belt, except for an increase in potash and rubidium, however, gabbros are absent in the eastern belt. The leucogranite is potash-rich and peraluminous, probably derived from a crustal source and is absent in the western belt. The recrystallisation of plagioclase in the early intrusives and recrystallisation of feldspars in the leucogranites near to the Walong Thrust suggests, that the deformation occurred under high temperature conditions. The presence of chlorite and muscovite in the pelitic gneiss and alteration of feldspars in the intrusives suggests cooling of the rock types under greenschist facies conditions during exhumation.

4. Structure Four phases of deformation can be recognized in the Lohit Valley area. Rare Fl folds related to Dl deformation are isoclinal, and their axes plunge towards NNW; near the MCT, they are rootless and reclined in nature. The S1 schistosity occurs as crenulations or microfolds within the differentiated S2 schistosity. The majority of the folds belong to the second phase of folding related to D2. The F2 fold axes plunge towards NW or SE, with the majority plunging towards the SE. These folds are SW-verging isoclinal folds, but become sub-vertical towards the Lohit Thrust. In the MCT zone the axes of the folds are parallel to the NE-plunging stretching lineation, suggesting rotation of the folds into the shear direction. Similarly, in the Walong Thrust Zone, the lineation is parallel to the asymmetric, isoclinal and sheath-like folds. The main regional fabric S2 is a ductile shear fabric trending NW – SE and dipping towards NE. The dip increases from a shallow angle in the SW and becomes steeper towards NE. The consistent development of NEplunging lineation and sense of shear criteria, like S – C fabric, asymmetric porphyroclasts, oblique shear bands, etc. indicate SW movement of the lithotectonic units. F3 folds of the D3 deformation are broad, open folds related to the formation of the Eastern Syntaxis, a major antiformal structure. On the mesoscopic scale F3 folds occur as small upright folds associated with crenulation type drag folds, mostly observed in the Tidding metavolcanics. A NW –SE-trending sub-horizontal lineation mostly plunging towards the SE, has been observed parallel to the F3 fold axis. During the last phase of deformation (D4) a subhorizontal lineation has developed which has overprinted the earlier steep NE-plunging stretching lineation related to thrusting. This lineation can be observed in the Lohit and Walong Thrust Zones, as well as in small scale vertical shear zones developed throughout the Lohit Plutonic Complex. The sense of shear criteria based on the S – C

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Fig. 4. Regional map showing major structural features of eastern Arunachal Pradesh and northern Burma (modified after Mitchell, 1993).

fabric developed along discrete shear zones, give a SEdirected movement, suggesting reactivation of the early steep thrust faults into dextral strike-slip faults.

5. Tectonic discussion The tectonic history of the investigated area is discussed, based on field and textural studies. During Himalayan collision, the Trans-Himalayan rocks of the Lohit Plutonic Complex were thrust southwards over the Tidding Suture Zone and the Mishmi Crystalline along the Lohit Thrust. The timing of their emplacement is not constrained; presumably it would have started before the metamorphism of the footwall units. Since the regional foliation S2 both in the footwall and hanging wall of the Lohit Thrust is parallel, movement along the thrust plane was synchronous with the main shearing event D2. Deformation and metamorphism in the footwall units, occurred due to collision-related crustal thickening, caused by the internal imbrication at different crustal levels. The metamorphism was synchronous with the early phases of deformation in both the units. The peak metamorphism was syn- to post-tectonic in relation to the S2 fabric, produced during the D2 deformation. During post-peak metamorphism the continued compression uplifted and transported the nappes further SW along the MCT. Although the maximum

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strain was accommodated along the MCT, mm scale shear zones developed throughout the sequence, parallel to S2 and to the thrust planes. We refer to these shear zones as a late shear foliation, and movement along these distributed shear zones caused the metamorphism to become inverted in the Mishmi Crystallines. Post-metamorphic thrusting (Brunel and Kienast, 1986); distributed movement along C planes (Jain and Manikavasagam, 1993); and movement along distributed ductile shears (Hubbard, 1996) are some of the models already proposed to explain the reverse metamorphism in the Zanskar and Nepal Himalaya. Our present study in the Lohit Valley of eastern Arunachal Pradesh also supports the above models. The late D2 shear foliation, developed during uplift and translation has reactivated the basal thrust MCT which climbed towards shallower levels at lower reducing temperature conditions, attested by brittle deformation microstructure in the mylonites of MCT zone, superposed on the early crystal-plastic microstucture. Contemporaneously the Lesser Himalayan Unit was metamorphosed under low-grade conditions due to tectonic loading by the Mishmi Crystalline rocks. Subsequently the Mishmi Thrust carried the Lesser Himalayan rocks over the Assam alluvium or over the Siwaliks of the sub-Himalaya. Subsequent to the metamorphic and shearing event the tectonic units were folded into a large-scale antiform known as the Siang Antiform in eastern Arunachal Pradesh (Fig. 1), the southern continuation of the Eastern Syntaxis. The investigated area lies in the eastern limb of this antiform and this folding event has caused steepening of the regional foliation and thrust faults. The continued compression has resulted in reactivation of these steep thrust faults (Lohit Thrust, Walong Thrust and other shear zones in Lohit Plutonic Complex) as dextral strike-slip faults. The tectonic situation, the Tidding Suture and the Lohit Plutonic Complex is discussed further below in a regional context, in order to understand their linkage with the Tsangpo Suture and Gangdese Batholith of southern Tibet to the north and also their continuation towards Burma in the south (Fig. 4). The tracing of the Indus –Tsangpo Suture Zone around the eastern Syntaxis becomes difficult due to complicated deformation in this region. The difficulty of linking the Tidding Ophiolite with the Indus Suture is related to thrusting and thickening of the syntaxial region during the Cenozoic, which caused uplift of the region, so that the suture has been removed (Wang and Chu, 1988; Holt et al., 1991). The lithology and tectonic setting of the Tidding Suture and Lohit Plutonic Complex have been compared with that of Indus Suture Zone and Ladakh Batholith of the NW Himalaya (Singh and Chowdhury, 1990; Bhalla et al., 1990). Mitchell (1981) has shown that the Lohit Thrust was continuous with the boundary between the Indus – Tsangpo Zone and the overlying plate to the North, in Tibet region. The eastern continuation of Indus – Tsangpo Suture has been mapped along the northeastern and southeastern segments of the Eastern Syntaxis around the Namcha-Barwa area that is

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folded around the Indian-derived core migmatites (Burg et al., 1997). The above observation suggests that the Tidding Suture is the southeastern continuation of the Tsangpo Suture Zone. Tracing the Tsangpo (Tidding) Suture southwards into Burma is difficult due to the presence of two ophiolite Zones: (1) along the eastern Indo-Burma ranges and (2) in the Myitkyina Valley, north of Mandalay, roughly along the north eastern extension of the Sagaing Fault, known as Myitkyina – Mandalay Ophiolite. Sengor and Hsu (1984) and Sengor et al. (1988), suggested that the above two ophiolite belts are the southern continuation of the Tsangpo Suture and the Bangong-co-Nujiang Suture of Tibet, respectively. According to Hutchison (1989), the Myitkyina – Mandalay Ophiolite represents a remnant of back-arc basin oceanic crust that separated western Burma from mainland Southeast Asia, and the Sagaing Fault marks the boundary. However, Mitchell (1981, 1984, 1993) suggested that during the late Eocene the Tsangpo Ophiolite extended into the Indo-Burma ranges through Tidding and Mytikyina – Mandalay as a single suture, which was, offset about 450 km along the dextral Sagaing transform fault during post-early Miocene time. Cretaceous to Eocene subduction along the Indo-Burma ranges is indicated by the mid-Cretaceous tonalite intrusives in the Burma Volcanic Arc (Fig. 4.). The similarities in the early Mesozoic rocks of the Indus –Tsangpo line and eastern Indo-Burman ranges indicate that the Mesozoic subduction zone of the IndoBurma ranges extended through Assam (Tidding Suture) into the Indus – Tsangpo Suture (Mitchell, 1981). According to Stoneley (1974), the Indus – Tsangpo Suture bends to the south from the east Himalayas and continues into the Bay of Bengal to the west of the Arakan Yoma of Burma (IndoBurma ranges) and into Java Trench. Zaw (1990) has subdivided the granitoid rocks of Burma into western, central and the eastern belts. The western belt plutons are closely associated with the Burma Volcanic Arc and consist of I-type granitoids associated with diorite and gabbro. The central belt granitoids are made up of S-type granites, granodiorites, pegmatites and aplites with minor diorites that intrude metamorphic rocks of greenschist to amphibolite facies. The eastern belt is poorly known and no clear-cut boundary can be drawn between the central and eastern belts. The Sagaing Fault separates the western granitoid belt and the central granitoid belt. According to Zaw (1990), the emplacement and evolution of the granitoid rocks in the central and western granitoid belt can be explained by a westwardly migrating, east dipping subduction zone, which lay west of the present day three granitoid belts in Burma. The most probable position for this zone was at the present Indo-Burma ranges which formed as an outer arc or fore-arc. The present study in the Lohit Valley and the information from the granitoid belt of Burma (Zaw, 1990) suggests that the western belt of the Lohit Plutonic Complex can be compared with the western granitoid belt, while the eastern belt can be compared with that of

the central granitoid belt or the Mogok Gneissic Belt of Burma. Earlier Bender (1983) traced the Mogok Gneisses towards eastern Himalaya and correlated them with the northwest striking metamorphic rocks (Mishmi Crystallines) between Lohit and Mishmi Thrusts. Our observations do not support this view. We consider that the Walong Thrust is the probable northward continuation of the Sagaing Fault, delimiting the Mogok Gneissic Belt of central Burma, while the Mishmi Crystallines belong to the Himalayan part. The occurrence of amphibolite facies garnet – sillimanite gneiss in the hanging wall of the Walong Thrust suggests upthrusting of deep level metamorphic rocks along this intracontinental thrust. The timing of movement along Walong Thrust is not constrained, due to lack of geochronological data. The garnet – sillimanite gneisses and other rock types of the eastern belt contain a regional foliation that developed during top to the SW shearing and the foliation is parallel to that of the western belt which was also developed during top to the SW shearing along the Lohit Thrust. The above features indicate that the Walong Thrust must have formed around the time of India– Asia collision, some 50 m.y. ago. The late phase peraluminous leucogranite intruding the early foliated intrusives and the pelitic gneiss suggests a close relationship between thrusting and granite genesis. The widespread occurrence of subduction-related quartz-diorites and soda-rich granites in the eastern belt, that are minerologically and geochemically similar to the western belt, suggest eastward extension of plutonism far away from the subduction zone. Similar to the Lohit Plutonic Complex, in the Gangdese Plutonic Belt of the Lhasa Block of Tibet gabbros and diorites dominate in the south, while granites and gneisses dominate in the north. In the northern segment the more southerly granite at Yangbajing is dated at 50 m.y. (U/Pb method). The granite genesis is attributed to anatexis in relation to intrablock thrusting (Xu et al., 1984). The age of the Yangbajing granite can be taken as evidence indicating that the Walong Thrust initiated around this time, which also corresponds to the time of India – Asia collision.

Acknowledgments The authors are thankful to Prof. K.S. Valdiya, President, W.I.H.G and Dr V.C. Thakur, former Director, W.I.H.G, for their planning and help to initiate geological investigation in Arunachal Pradesh. The authors are thankful to the Director, Wadia Institute of Himalayan Geology for kindly permitting us to carry out this work. The authors are also thankful to Drs Mike Searle and A.J. Barber for critically reviewing the manuscript. Additional financial support by the Department of Science and Technology, Government of India, New Delhi, is kindly acknowledged.

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References Acharyya, S.K., 1987. Cenozoic plate motions creating the Eastern Himalaya and Indo–Burmese range around the northeast corner of India. In: Ghosh, M.C., Varadarajan, S. (Eds.), Ophiolites and Indian Plate Margins, Patna University, Patna, pp. 143 –160. Barker, F., Arth, J.G., 1976. Generation of trondhjemitic– tonalitic liquids and Archean bimodal trondhjemite –basalt suites. Geology 4, 596–600. Bell, T.H., Rubenach, M.J., 1983. Sequential porphyroblast growth and crenulation cleavage development during progressive deformation. Tectonophysics 92, 171 –194. Bender, F., 1983. Geology of Burma, Gebruder Borntraeger, Berlin, p. 293. Berthe, D., Choukroune, P., Jagouzo, P., 1979. Orthogneiss, mylonites and non-coaxial deformation of granites: the example of the South Armorican shear zone. Journal of Structural Geology 1, 31–42. Bhalla, J.K., Srimal, N., Bishui, P.K., 1990. Isotopic study of Mishmi complex, Arunachal Pradesh, NE Himalaya. Records of the Geological Survey of India 123 (2), 20–21. Brunel, M., Kienast, J.R., 1986. Etude petro-structurale des chevauchements ductile Himalayans sur Ia transversale de L’ Everest-Makalu (Nepal oriental). Canadian Journal of Earth Science 23, 1117–1137. Burg, J.P., Davy, P., Oberli, F., Seward, D., Diao, Z., Meir, M., 1997. Exhumation during crustal folding in the Namche–Barwa syntaxis. Terra Nova 9, 53 –56. Gansser, A., 1964. Geology of the Himalayas, Wiley, London, p. 289. Hanmer, S.K., 1982. Microstructures and geochemistry of plagioclase and microcline in naturally deformed granite. Journal of Structural Geology 4, 197 –215. Heim, A., Gansser, A., 1939. Central Himalaya: geological observations of the Swiss Expedition 1936. Denkschriften der Schwieizerishen Naturforschenden Gasellschaft, Abhandlung 1, 245. Holt, W.E., James, F., Ni., Wallace, T.C., Haines, A.J., 1991. The active tectonics of the eastern Himalayan syntaxis and surrounding regions. Journal of Geophysical Research 96, 14595– 14632. Hubbard, M.S., 1996. Ductile shear as a cause of inverted metamorphism: example from the Nepal Himalaya. Journal of Geology 104, 493–499. Hutchison, C.S., 1989. Geological evolution of South-East Asia, Oxford Science Publications, p. 368. Jain, A.K., Manikavasagam, R.M., 1993. Inverted metamorphism in the intra-continental ductile shear zone during Himalayan collision tectonics. Geology 21, 407–410. Jain, A.K., Thakur, V.C., Tandon, S.K., 1974. Stratigraphy and structure of the Siang District of Arunachal Pradesh. Himalayan Geology 4, 28–60. Karunakaran, C., 1974. Geology and mineral resources of the states of India, part IV-Arunachal Pradesh, Assam, Manipur, Meghalaya, Mizoram, Nagaland and Tripura. Geological Survey of India, Miscellaneous Publications 30, 124. Zaw, K., 1990. Geological, petrological land geochemical characterisitcs of granitoid rocks in Burma: with special reference to the associated W– Sn mineralization and their tectonic setting. Journal of Southeast Asian Earth Sciences 4, 293–335. Kumar, G., 1997. Geology of Arunachal Pradesh. Journal of the Geological Society of India, Bangalore, 217. Le Fort, P., 1975. Himalayas: the collided range. Present knowledge of the continental arc. American Journal of Science 275-A, 1–44. Maltman, A.J., 1978. Serpentinite textures in Anglesey, North Wales, United Kingdom. Geological Society of American Bulletin 89, 972–980.

741

Mitchell, A.H.G., 1981. Phanerozic plate boundaries in mainland SE Asia, the Himalaya and Tibet. Journal of the Geological Society, London 138, 109– 122. Mitchell, A.H.J., 1984. Post Permian events in the Zangpo suture zone, Tibet. Journal of the Geological Society, London 141, 129 –136. Mitchell, A.H.J., 1993. Cretaceous –Cenozoic tectonic events in the western Myanmar (Burma)–Assam region. Journal of the Geological Society, London 150, 1089–1102. Nandy, D.R., 1973. Geology and structural lineament of the Lohit Himalaya (Arunachal Pradesh) and adjoining area: a tectonic interpretation, Proceedings of the Seminar on Geodynamics of the Himalayan region, National Geophysical Research Institute, Hyderabad, India, pp. 167–172. Peacher, A., 1989. The metamorphism in the Central Himalaya. Journal of Metamorphic Geology 7, 31–43. Xu, R.-H., Scharer, U., Allegre, C.J., 1984. Magmatism and metamorphic in the Lhasa block (Tibet): a geochronological study. Journal of Geology 93, 41–57. Sengor, A.M.C., Hsu, K.J., 1984. The Cimmerides of eastern Asia: history of the Eastern end of the Palaeo-Tethys. Memoirs Geologique de France 147, 139–167. Sengor, A.M.C., Altmer, D., Cin, A., Ustaomer, T., Hsu, K.J., 1988. Origin and assembly of the Tethyside orogenic collage at the expense of Gondwanaland. Geological Society Special Publication 37, 119–181. Simpson, C., Wintsch, R.P., 1989. Evidence for deformation in induced Kfeldspar replacement by myrmekite. Journal of Metamorphic Geology 7, 261 –275. Singh, S., 1993. Geology and tectonics of the Eastern syntaxial bend, Arunachal Himalaya. Journal of Himalayan Geology 4, 149–163. Singh, S., Chowdhury, P.K., 1990. An outline of the geological framework of the Arunachal Himalaya. Journal of Himalayan Geology 1, 189–197. Singh, S., Malhotra, G., 1983. Tectonic setup of Yang Sang Chu valley, west Siang District, Arunachal Himalaya. In: Saklani, P.S., (Ed.), Himalayan Shears, Himalayan Books, New Delhi, pp. 107 –113. Sinha-Roy, S., 1982. Himalayan Main Central Thrust and its implications for Himalayan inverted metamorphism. Tectonophysics 84, 197– 224. Stoneley, R., 1974. Evolution of the continental margins bounding a former Tethys. In: Burke, C.A., Drake, C.L. (Eds.), The Geology of the Continental Margins, Springer, Berlin, p. 1009. Thakur, V.C., 1986. Tectonic zonation and tectonic framework of eastern Himalaya. Science de la Terre, Memoir 47, 347 –366. Thakur, V.C., Jain, A.K., 1975. Some observation on deformation, metamorphism and tectonic significance of the rocks of some parts of Mishmi hills, Lohit district (NEFA), Arunachal Pradesh. Himalayan Geology 5, 339–364. Valdiya, K.S., 1976. Himalayan tranverse faults and folds and their parallelism with subsurface structures of north Indian plains. Tectonophysics 32, 353 –386. Valdiya, K.S., 1980. Geology of the Kumaun Lesser Himalaya, Wadia Institute of Himalayan Geology, Dehra Dun, p. 291. Verma, P.K., Tandon, S.K., 1976. Geologic observations in a part of the Kameng District, Arunachal Pradesh (NEFA). Himalayan Geology 6, 257– 287. Vernon, R.H., Williams, V.A., D’Arcy, W.F., 1983. Grain size reduction and foliation development in a deformed granite batholith. Tectonophysics 92, 123 –146. Wang, E., Chu, J.J., 1988. Collision tectonics in the Cenozoic orogenic zone bordering China, India and Burma. Tectonophysics 147, 71 –84.