Petrogenesis And Geological Significance Of Dongga Granites And Dazhuka Gabbros In The Southern Margin Of Gangdese Belt

Qinghai-Tibet Plateau is the highest altitude plateau in the world,Because of its unique uplift and evolution history,has been closely concerned by the vast number of geological researchers.The Lhasa block in the south of the Qinghai-Tibet Plateau,as the front of the collision between India and Eurasia.Its Mesozoic and Cenozoic magmatism and tectonic evolution are particularly noticeable.In the south of Lhasa block,there are thousands of kilometers of Gangdese batholith distributed from east to west,recording the tectonic evolution process of the North subduction of the Neo-Tethys ocean and the collision between India and Eurasia since Mesozoic.The current research results show that the magmatism in Gangdese arc is mainly concentrated in four periods,namely early Jurassic and late Cretaceous in Mesozoic and early Tertiary and Miocene in Cenozoic.Although the previous achievements play an important role in understanding the tectonic evolution process of Mesozoic and Cenozoic in the south of the Qinghai-Tibet Plateau,generally speaking,the previous studies are based on a certain rock body or a part,and are not considered from the overall evolution of the Gangdese arc.Based on the geochronology,geochemistry and Sr-Nd isotopic composition of Dongga Jurassic granites and dazhuka Eocene gabbros in the middle of Gangdese arc,combined with the previous research results,this paper aims to understand the temporal and spatial distribution characteristics and deep dynamic mechanism of Gangdese arc magmatism in the Jurassic and early Cenozoic in general.The main achievements of this paper are as follows:1.The Middle Jurassic?¹69.6 Ma?granitic body,located in the central part of the Gangdese arc,is dominated by granites and granodiorites,with high SiO₂?68.32⁷1.54wt.%?,Al₂O₃?15.52¹6.45 wt.%?,Na₂O?4.29⁵.09 wt.%?,low MgO?0.55⁰.66 wt.%?,Mg#?17³4?,K₂O?0.99¹.98 wt.%?;At the same time,it shows some characteristics of adakitic geochemical composition,such as low Y?8¹1 ppm?and Yb?0.9¹.6 ppm?,and high Sr?580⁶84 ppm?values,and has depleted Sr-Nd isotopic composition(I_Sr=0.70359⁰.70602;?Nd?t?=+4.51⁺5.39).The above geochemical and isotopic characteristics of the Dongga Middle Jurassic granites indicate that it originated from the newly thickened lower crust,and the source area is amphibolite facies.During the partial melting process,there may be amphibole,garnet,rutile and other melting residual facies.2.The Dazhuka Eocene gabbros in the middle of Gangdese arc is in the form of small dyke.They show low SiO₂?48.09⁵1.33 wt.%?,K₂O?0.4¹.67 wt.%?,high MgO?4.94⁶.39 wt.%?,Al₂O₃?16.95¹7.53 wt.%?,Na₂O?2.63³.45 wt.%?,although their REE distribution map is similar to that of enriched ridge basalt?E-MORB?,and has evidently depleted Sr-Nd isotopic composition?Isr=0.70420⁰.70385;?Nd?t?=+4.8⁺5.1?,but the trace element spider map shows the characteristics of high field strength element?HFSE?depletion such as Nb,Ta,Ti.The above geochemical and isotopic characteristics of the Dazhuka Eocene gabbros indicate that it is formed by partial melting of the asthenosphere mantle under decompression,and is likely to be contaminated by the lithospheric mantle during the ascending process,which has experienced fractional crystallization of mafic minerals?such as olivine,clinopyroxene?,Fe-Ti oxide,and the cumulation of a small amount of plagioclase.Most of the felsic magmatic rocks?SiO₂>69 wt.%?formed at 65⁴1 Ma in the southern margin of Gangdese belt have high zircon saturation temperature?687⁸74??,indicating that these mafic rocks and their contemporaneous intermediate-acid magmatism may be caused by slab break off.3.Jurassic magmatic rocks of Gangdese arc have the following characteristics of the spatial and temporal distribution:?1?the basic and acidic magmatic activities are developed in the Gangdese arc as a whole,but the eastern and Western segments are mainly volcanic rocks and intrusive rocks respectively;?2?in the east-west direction,the Jurassic magmatic activities to the east of 90°E have a trend of gradually becoming younger from east to west,but there is no obvious trend to the west of 90°E;in the north-south direction,the Jurassic magmatic activities mainly distributed in the south of 29.48°N;?3?In the process of arc magmatic activity in Jurassic,the main enrichment components are subduction sediments and subduction plates in the early and late stages,respectively.influenced by the ancient basement of the central Lhasa block,the Jurassic arc magmatic rocks have the characteristics of wide range change of isotopic composition in the east part.4.A large number of Jurassic magmatism in the Gangdese arc may have contributed to crustal thickening in this area.5.The geochemical composition of the Paleocene-Eocene magmatic rocks in the Gangdese arc are characterized as follows:?1?fluid contribution of mafic rocks gradually decreases from south to north;?2?felsic magmatic rocks?SiO₂>69 Wt.%?zircon saturation temperature?Tzr?increases gradually from south to north.