Anatetic Events Along The Malashan-Gyirong Rift, Southern Tibet And Their Implication For The Tectonic Evolution Of The Himalayan Orogenic Belt

The Himalayan orogenic belt is one of the youngest active collisional belts worldwide, and provides an excellent field laboratory to study the behavior of deep crustal materials as responses to the tectonic evolution of orogenic belts. Since the continental collision between India and Eurasia Plate, the middle to lower crustal materials along this orogenic belt have experienced intensive and episodic partial melting, leading to the formation of various types of granite. These granites have preserved records on the nature of metamorphism and crustal anatexis as well as magmatic processes to generate these granites. The Malashan-Gyirong Rift Zone, one of the branches of N-S trending Southern Tibet Rift Zone (STRZ), across the Yalung-Tsangpo suture (YTS), Southern Tibet Detachment System (STDS), and extending well into the Main Center Thrust (MCT), provides important records on deep geological processes during tectonic transition from compression to extension. Along this rift valley, there is a series of leucogranites including the Malashan two-mica granites, the Paiku composite leucogranite, syn-tectonic leucogranites within the STDS and those within the High Himalayan Crystalline Sequence (HHCS).The Malashan two-mica granite started to crystallize at～18.3Ma and lasted to～16.0Ma, which suggests that its life-time is about～2.3Ma. Zircon crystallization age spectrum displays stepwise/pulse-like behavior but does not show any spatial correlationship. The Malashan two-mica granites show similar mineral assemblages and are characterized by relatively homogeneous element and isotope (Sr-Nd-Hf) geochemistry. The other leucogranites within this rift zone also show similar age patterns. Field relations, petrographic observations, geochronological data from the wall-rocks of these granites all indicate that intermediate-grade metapelites immediately next to the granitc pluton in the Northern Himalaya Gneiss Domes (NHGD) did not experience partial melting. Metamorphism to form augen granitic gneiss within the STDS as well as a variety of rock types within the HHCS experienced metamorphism postdated the intrusion of a number of leucogranites, which indicates in many circumstances, intrusion of these leucogranitic magma did not result in the metamorphism and partial melting of wall-rocks. All these line of evidence presented above indicate the relatively large leucogranitic pluton possibly was assembled at relatively shallow levels by diking, not by diapirism of large and mobile bodies of magma. The Paiku leucogranite is a composite pluton, consist of tourmaline-bearing leucogranite, two-mica granite, and garnet-bearing leucogranite. They were derived from different source rocks combined with different melting conditions and did not result from differentiation of magma.Leucogranites along the Malashan-Gyirong Rift Zone formed at mainly37～32Ma、28Ma、21Ma, and20～16Ma. A number of leucogranites also show records suggesting that their source rocks also experinenced simultaneous metamorphism at37～32Ma、28～25Ma、23～20Ma and18～16Ma, respectively. A number of zircon grains from these leucogranites contain rims with textures consistent with metamorphic origin and yielded ages younger than28Ma. This observation implies that though young the zircons from these leucogranites were, they also could undergo metamorphism due to heating or magma influx from later recharging within a very short time of periods. Detrital zircons (metamorphic and magmatic origin) from these granites as well as from granitic gneisses yield ages from448～401Ma, which provides the first evidence that the Himalaya orogenic belt could have experienced a previously unrecognized tectonic process at Caledonian time.Leucogranites in this Rift Zone were formed through two types of partial melting reactions, dehydration melting versus fluxed melting of muscovite. Leucogranites formed prior to～20Ma were derived from dehydration melting, whereas those younger than～20Ma and older than～15Ma from fluxed melting of muscovite. These broadly different types of granites show distinct geochemistry not only in major and trace element but also in radiogenic isotope compositions. Garnets in these leucogranites are magmatic origin and contain composition of metamorphic garnets from source rocks due to melting of garnet in the crustal anatexis.Combined with data on the Cenozoic crustal anatexis and characteristics of tectonic evolution in the Himalayan Belt, we can conclude that (1) prior to35Ma, the partial melting of dominantly amphibolite occurred in the thickened crustal conditions due to continental collision between the India and Eurasia plate, which produced granites with high Na/K ratio and Sr/Y ratio. These melting processes effectively change the physical properties of deep crustal rocks and trigged the tectonic transition from compression to extension and initiated the movement of STDS;(2) after35Ma, with further extension along STDS, rapid exhumation of deep crustal material resulted in larger-scale dehydration melting of muscovite in the metapelites and the formation of granites with high Rb/Sr ratios;(3) during20～16Ma, E-W extension along the STRZ, possibly with additional heat or material from the mantle results in fluxed-melting of muscovite in metapelites and produced granites with lower Rb/Sr ratio but elevated Sr and Ba contents.