PETROLOGY OF HIGH-PRESSURE GRANULITES FROM THE EASTERN HIMALAYAN SYNTAXIS

The eastern Himalayan syntaxis, situated at the eastern terminus of th e Himalayas, is the least-known segment of the Himalayas. Recent resea rch in this area has revealed that the syntaxis consists of the Gangdi se, the Yarlung Zangbo, and the Himalayan units, each of which is boun ded by faults. The Himalayan unit, the northernmost exposed part of th e Indian plate, mainly contains amphibolite facies rocks, marked by th e assemblages staurolite + kyanite + plagioclase + biotite + muscovite +/- sillimanite and garnet + amphibole + plagioclase, in the south; t o the north, low- to medium-pressure granulite grade pelitic gneisses and marbles are present and are characterized by the assemblages garne t + sillimanite + K-feldspar + plagioclase or antiperthite + biotite quartz +/- spinel +/- cordierite +/- orthopyroxene in gneisses, and a northite + diopside +/- wollastonite and plagioclase + diopside + quar tz + phlogopite + calcite in marbles. Within this unit, the Namula thr ust system is a series of moderately north-dipping structures that dis placed the granulite facies rocks southwards over the amphibolite faci es rocks. High-pressure granulites occur as relies within these granul ite facies rocks and contain garnet-kyanite granulite and garnet clino pyroxenite. The peak assemblage of the garnet-kyanite granulite includ es garnet (core part) + kyanite + ternary feldspar + quartz + rutile. Sillimanite + garnet (rim part) + K-feldspar + oligoclase + ilmenite biotite and spinel + albite + biotite or spinel + cordierite +/- orth opyroxene, which are coronas around sillimanite and garnet, are retrog rade products of this peak assemblage. Another peak assemblage include s very-high-Ca garnet (CaO 32-34 wt%, Alm(10) +/- Grs(>80)) and diopsi de (CaO 22-24 wt%), scapolite, meionite, quartz, and accessory Al-bear ing titanite (Al2O3 4-4.5 wt%). The diopside has kink bands. Partial o r complete breakdown of Ca-rich garnet during post-peak metamorphism p roduced pseudomorphs and coronas consisting of fine-grained symplectic intergrowths of hedenbergite and anorthite. Thermobarometric estimate s in combination with reaction textures, mineral compositions, and rec ent experimental studies indicate that these peak assemblages were for med at P = c. 1.7-1.8 GPa, T = c. 890 degrees C, and the retrograde as semblages experienced near-isothermal decompression to P = 0.5 +/- 0.1 GPa, T = 850 +/- 50 degrees C. The whole-rock compositions indicate t hat marble and pelite are plausible candidates for the protoliths. The se facts suggest the following (1) sedimentary rocks were transported to upper-mantle depths and equilibrated at those conditions to form th ese high-pressure granulites, which were then emplaced into the crust quickly. During the rapid exhumation of these rocks, the earlier high- pressure assemblages were overprinted by the later low- to medium-pres sure assemblages, that is, the high-pressure granulite belt formed in the syntaxis. (2) The Namula thrust system is an important tectonic bo undary in the syntaxis, or even in the Higher Himalaya more generally.