Thermal Evolutional History And ⁴⁰Ar/³⁹Ar Geochronological Constraints On Central-East Himalaya

Study on thermal history of Himalaya orogen provides not only accurate chronological coordinate on the cooling history, but also basic materials in verifying models related to the uplift of Himalaya. On central-east part of Himalaya orogen, systematic geochronological researches focusing on thermal evolution was neglectable, so the puzzle of how and when did Himalaya uplift has not been integrally explained. In this paper, four sections located respectively in Yadong-ZhergerLa in central Himalaya, Pai-Zaqu reach of Yalung-Tsangpo River, west slope section of NamuLa and east slope section of DuoxiongLa in Eastern Himalaya Syntaxis were chosen, and conventional and laser ⁴⁰Ar/³⁹Ar methods were used to build the framework of thermal evolution history of central-east part of Himalaya orogen, for the purpose of revealing the secret of uplif of central-east Himalaya.In Yadong district in central Himalaya, a retro-metamorphic high-pressure granulite and its country rock were focused and conventional ⁴⁰Ar/³⁹Ar tests were done. Two retro-metamorphic events in the retro-metamorphic high-pressure granulite, the ages of which were constrained by biotite and hornblende ⁴⁰Ar/³⁹Ar geochronology, happened at 48.5 and 31.1 Ma. Thereafter, two cooling ages of 11.5 and 12.6 Ma were yield, which were constrained by biotite and K-feldspar ⁴⁰Ar/³⁹Ar ages in the country rock, a biotite K-feldspar granite. At the same time, a cluster of 17 Ma zircon U-Pb age in the retro-metamorphic high-pressure granulite and two clusters of 841 and >1800 Ma zircon U-Pb ages were already published (Ji et al., 2004), the former of which suggests the time when the high-pressure granulite was captured, while the latter of which represent when did original rocks formed. In granitic gneiss located south of Yadong, two old zircon U-Pb ages (1149 and 474 Ma) and young biotite ⁴⁰Ar/³⁹Ar ages (11.3 and 11.0 Ma) and apatite fission track (AFT) ages (7 and 8 Ma) were yield. The old zircon U-Pb ages represent the time when original rocks formed and undergone Pan- African event subsequently, while the young ⁴⁰Ar/³⁹Ar and AFT results are cooling ages. Coincident biotite ⁴⁰Ar/³⁹Ar ages yield in rocks in the south of Yadong and in the high-pressure granulite shows that on the section of Yadong-ZhergerLa, rocks were cooled to 320℃contemporarily. According to geochronological data obtained in this study, a tectonic evolutional history of rocks currently exposed on the earth surface can be made: After forming before 841 Ma, the original rocks of High Himalaya Crystalline System were reformed by Pan-African event at ⁴74 Ma, then were undergone high pressure metamorphism at ⁵5 Ma (inferred) and two retro-metamorphic events at 48.5 and 31.1 Ma respectively, thereafter, they were persisted partial molten at depth of 15²4 km during 31.1¹7 Ma, and were experienced fast extrusion event in the way of channel flow in the period of 17⁷ Ma, then were exposed to the earth surface at a lower velocity.Eastern Himalaya Syntaxis is an important area in studying climatic-tectonic reciprocity. In this paper, single-crystal-high-precision laser ⁴⁰Ar/³⁹Ar method was used to build the thermal evolutional history of Eastern Himalaya Syntaxis by tests on river sediments on the lower reach of Yalung-Tsangpo River. The contrast of age statistical results against the figure showing the changes in rate and angle of convergence during India plate has been colliding against Eurasian plate since late Mesozoic and the diagram consisting of global deep-seaδ18O andδ13C records is done, for the purpose of revealing climatic and tectonic information on rock cooling and referring the relationship between thermal evolutional history and climate and tectonics. At the same time, plenty of geochronological data on ground samples were yield by conventional and laser ⁴⁰Ar/³⁹Ar tests. It shows that Pai-Zaqu reach of Yalung-Tsangpo River and west slope section of NamuLa yield ⁴⁰Ar/³⁹Ar age clusters of 1.3 and 2.5 Ma, while biotite ⁴⁰Ar/³⁹Ar age cluster of samples from east slope section of DuoxiongLa is about 4 Ma. Firstly, ⁴⁰Ar/³⁹Ar age clusters of <3 Ma reveal the fast cooling of eastern Himalaya Syntaxis since middle Pliocene. Secondly, the difference of ⁴⁰Ar/³⁹Ar age clusters between the west and east slope section of DuoxiongLa is the result of a titling structure, which strikes NE at the west side of Namjag-Barwa– NamuLa– DuoxiongLa, and which has been functioning since 4.0².2 Ma under the drive of both climatic and tectonic powers.The difference of thermal evolutional history between central Himalaya and Eastern Himalaya Syntaxis shows that they uplifted in different tectonic style at different period of time. In central Himalaya, massif was mainly uplifted in the tectonic model of channel flow during 17⁷ Ma at a velocity of ² mm/a, then was extruded to earth surface at a much lower velocity. In eastern Himalaya Syntaxis, massif was mainly uplifted in the structural style of titling at a background of isostasy since 4.0².2 Ma. As for the drive power, both of them have been driving by climatic and tectonic power under the dominant control of N-S tectonic compression.