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40Ar/39Ar Age Constraints on Deformation and Metamorphism in the Maine Central Thrust Zone and Tibetan

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40Ar/39Ar Age Constraints on Deformation and Metamorphism in the Maine Central Thrust Zone and Tibetan TECTONICS, VOL. 8, NO. 4, PAGES 865-880, AUGUST 1989 40Ar/39Ar AGE CONSTRAINTSON DEFORMATION AND METANDRPHISM IN THE MAIN CENTRAL THRUST ZONE AND TIBETAN SLAB, EASTERN NEPAL HIMALAYA Mary S. Hubbard1 Department of Earth, Atmospheric and Planetary Sciences, Massachusetts Institute of Technology, Cambridge T. Mark Harrison Department of Geological Sciences, State University of New York at Albany Abstract. In eastern Nepal, structural 6.4+0.8 Ma (Tc=225ñ20ø C). An adjacent and petrologic observations suggest that gneiss yields a 9.1ñ0.2 Ma biotite movement on the Main Central Thrust (MCT), isochron and a K-feldspar minimum age of the development of an inverted metamorphic 3.6_+0.2 Ma (Tc=210-+50øC). In the upper sequence, and leucogranite plutonism were Tibetan Slab, samples collected >200 m genetically related. Samples from across from visible intrusives yield a biotite the MCT zone and its hanging wall, the isochron age of 20.2!0.2 Ma and a complex Tibetan Slab, were collected from four hornblende age spectrum suggestive of locations in the Everest region for excess argon. At the fourth location an 40Ar/39Ar studies: (1) the MCT zone, (2) augen gneiss, a pegmatite, and a the lower Tibetan Slab, (3) the upper tourmaline-bearing leucogranite, all in Tibetan Slab away from intrusive rocks, mutual contact, yield indistinguishable and (4) the upper Tibetan Slab adjacent to mineral ages. The average biotite and and including leucogranitic material. muscovite isochron ages for these samples Within the MCT zone, muscovite and are 17.0_+1.4 and 16.6ñ0.2 Ma, hornblende yield cooling ages of 12.0-+0.2 respectively. The minimum age for K- Ma and 20.9ñ0.2 Ma, respectively. K- feldspar from the leucogranite is 15.5ñ1.8 feldspar gives a minimum age of 8.0-+0.2 Ma Ma. As thermobarometric data from the MCT (closure t•rature (Tc) =220ñ15ø C) . In zone in this area suggest synmetamorphic the lower Tibetan Slab a sample from a deformation at t•ratures of ~500ø- sheared pegmatite yields muscovite and 550ø C, the MCT hornblende (Tc=~500ø C) biotite isochron ages of 7.7_+0.4 Ma and dates this event at ~21 Ma. Diffusion 7.5_+0.6 Ma and a K-feldspar minimum age of experiments on hornblendes from the MCT zone provide data which support a maximum duration of peak metamorphic t•ratures of <_2 Ma. Biotite ages as old as ~20 Ma from the upper Tibetan Slab near 1Nowat Geologisches Institute, ETH leucogranites indicate that leucogranite Zentrum, Z•rich, Switzerland. intrusion was essentially coeval with deformation and metamorphism in the MCT Copyright 1989 zone. Very young ages in a ductile shear by the American Geophysical Union. zone in the lower Tibetan Slab suggest that there has been deformation within the Paper number 89TC00708. Tibetan Slab that postdates major movement 0278-7407/89/89TC-00708510.00 on the MCT. 866 Hubbardand Harrison: 40Ar/39ArAges, Nepal Himalaya INTRODUCTION the Himalaya' (1) the southernmost, and possibly seismically active, Main Frontal Collision between the Indian and Thrust, (2) the Main Boundary Thrust, and Eurasian continents occurred between ~40 (3) to the north the Main Central Thrust. and 50 Ma [Molnar, 1984]. Convergence The Main Frontal thrust separates terraced between these two continents has continued alluvial deposits from the overriding to the present, creating the spectacular Early Miocene - Late Pleistocene age Himalayan range and the elevated Tibetan molassic rocks, locally known as the Plateau. The original zone of collision, Siwaliks [Gansser, 1981]. To the north the Indus-Tsangpo suture, is marked by the Siwaliks are overridden along the Main ophiolitic rocks in southern Tibet. The Boundary Thrust (MBT) by the Lesser high peaks and rugged topography of the Himalayan sequence, which consists of a Himalaya coincide with metamorphic and predominantly clastic sedimentary sequence intrusive rocks within the Indian plate. of Precambrian to late Paleozoic age, These crystalline rocks are part of the although fossil control is scarce upper plate to a major, north- dipping [St6cklin, 1980]. This sequence generally thrust known as the Main Central Thrust has been metamorphosed to a low grade, but (MCT). local areas remain unmetamorphosed . The The MCT is a major intracontinental Main Central Thrust (MCT) forms the fault zone that accomodated portions of northern boundary of the Lesser Himalaya the postcollisional convergence between (Figure 1). The upper plate to this India and Eurasia. This structure may structure is the Tibetan Slab with high- have accomodated more than 100 km of grade sillimanite-bearing gneissic rocks displacement [Gansser, 1981]. It has been which have been subjected to widespread proposed that movement on the MCT may have anatexis and intruded by leucogranitic influenced metamorphism and the material. Rocks adjacent to the MCT are development of leucogranites in the characterized by an inverted metamorphism Himalaya [LeFort, 1975, 1981]. such that metamorphic grade increases Unfortunately, these hypotheses have toward structurally higher levels remained virtually untestable because [Gansser, 1964; P•cher, 1977; Hodges et relative and absolute ages of al., 1988]. These rocks generally dip metamorphism, thrusting, and leucogranitic homoclinally to the north, thus the •mplacement are lacking. Many of the metamorphic grade increases northward. A leucogranites have been dated (see LeFort variety of ideas have been proposed that et al. [1987] for summary and Copeland et link the inverted metamorphism with al. [1988b] for a discussion of possible deformation along the MCT [LeFort, 1975; ambiguities in age assessment), but the Maruo and Kizaki, 1983; Brunel and Kienast, timing of metamorphism and deformation 1986]. In central Nepal, LeFort [1981] along the MCT has not been studied in proposed that the formation and detail. emplacement of the Manaslu leucogranite in In the Everest area of eastern Nepal the Tibetan Slab is also related to the MCT is a thrust zone in which movement on the MCT. Radiometric ages, temperatures reached 500ø-700øC during from a variety of techniques, range from thrusting [Hubbard, 1989]. Amphibolite 14.3+0.6 to 25.0!_0.5 Ma (2-sigma occurs in the lower part of the zone where uncertainties are reported herein for all calculated temperatures are close to the radiometric ages) for intrusives within closure temperature for the K-Ar system in the Tibetan Slab [SchRrer et al., 1986; hornblende (~500 ø C) . This coincidence Deniel et al., 1987], although Copeland et provides an opportunity to date thrust al. [1988a] have criticized the basis for movement and to compare this age with the assessment of many of the older ages. ages of metamorphism within the hanging Movement on the MCT is generally thought wall and with the ages of leucogranites. to predate movement on the MBT, although In this paper we report the results of a no unambiguous age constraints exist for regional 40Ar/39Arstudy of hornblende, the temtooral development of either biotite, muscovite, and potassium feldspar structure. from the MCT zone and its upper plate. Study Area _ GEOIf•IC SETTING The structural setting and tectonic Three laterally continuous, north- stratigraphy of the Everest region are dipping thrusts have been identified in described in detail by Hubbard [1988]. In Hubbardand Harrison: 40Ar/39ArAges, Nepal Himalaya 867 86o40 ' MCT zone and across the Tibetan Slab. Metamorphic isograds and lithologic contacts are roughly parallel to the dominant schistosity. The lower boundary of the MCT zone is a lithologic contact between an augen gneiss in the MCT zone and a chlorite-muscovite schist in the uppermost Lesser Himalaya. The Lesser Himalayan section is a thick sequence of fine-grained psammite, schist, and quartzite. Locally, biotite and garnet occur in the uppermost unit of chlorite-muscovite schist. The upper boundary of the MCT zone is within a biotite gneiss unit at a level that separates highly sheared rocks of the thrust zone from overlying, less-deformed but migmatizedgneissic rocks of the Tibetan Slab. Shear zones do exist north of the MCT zone, but deformation does not appear to be as widespread. Lithologies within the Tibetan Slab include biotite gneiss, calc-silicate rock, quartzite, augen gneiss, granitic gneiss and pelitic T. schist. Garnet and sillimanite are common in pelitic andgneissic rocks throughout this section. Anatectic granites and small-scale granitic intrusions occur in the lower Tibetan Slab, but the frequency and volume of graniticmaterial increases toward the north. There are several large 5km' 27030 ' leucogranitic bodies in this region, such :.'•'• GRANITE • MCTZONE as the south face of Nuptse or the Makalu massif, but the majority of the granitic I•1 TIBETANSLAB ::1• LESSERHIMALAYA material occurs in networks of dikes and sills. Some of the larger intrusive Fig. 1. Simplified geologic map of the bodies have retained xenoliths of the study area in eastern Nepal. Sample country rock. Schistosity within these locations for 40Ar/39Arstudy are xenoliths has often remained parallel to indicated by the circled place names. the country rock schistosity in the area. Samples 87H13F, 87H13E, 87H12C, and Crosscutting relationships in the dikes 87H12B are from the Main Central Thrust and sills suggest multiple generations o• (MCT) zone. Samples 87H21C, and 87H21D intrusion. are from Ghat. Samples 87H19A, and Metamormhism 87H19Bare from Na. Samples 87H18A, _ 87H18B, and 87H18C are from Ngozumba. Metamorphic assemblages across the MCT zone and into the Tibetan Slab suggest an general, the MCT in this area is a 3- to increase in metamorphic grade toward 5- km-thick, north-dipping shear zone higher structural levels. Mineral rim consisting of a variety of lithologies thermobarometric data [Hubbard, 1989] from including augen gneiss, slate, phyllite, samples across the MCT zone and lower quartzite, talc-schist, amphibolite, Tibetan Slab in eastern Nepal show an pelitic schist, calc-silicate rock, and increase up-structure in tentoeratures of banded biotite gneiss.
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