Late Triassic-Late Jurassic Magmatic-Tectonic Evolution Of The Lhasa Terrane

Recently,dramatic breakthroughs have been made into the Cenozoic plateau uplift and metallogenesis of the Lhasa terrane.However,the Mesozoic(especially during Late Triassic to Late Jurassic)tectonic evolutional history of the Lhasa terrane,such as oceanic subduction,continent/arc-continent collision,are still under hot controversy,which is not only the premise to detect the Cenozoic collisional process,but also significant in probing the geodynamic background of the Mesozoic giantsupper giant deposit developed within/along the Lhasa terrane.For these purposes and on the base of representative Jurassic magmatic rocks within/along the Lhasa terrane and Yunzhug ophiolite mélange,this Phd thesis draw following conclusions:(1)The DaruTso volcanic rock(~163 Ma)is the first recognition of Jurassic magmatic rocks in the central part of the BNSZ,and show the geochemical affinity of high-Mg andesite.Additionally,they have high Th,(La/Sm)N,low Ba/Th,Sr/Y,and enriched whole-rock Sr-Nd and zircon Hf isotopes.Prominently,their ?Nd(t)values overlap with regional sediments,indicating they are partial melts of sediment that have undergone interaction with overlying mantle during ascending.Detailed geochemical analysis suggest they were formed in a low-pressure and hightemperature condition.Taking regional magmatic-tectonic situation into account,we propose that the DaruTso volcanic rocks were formed during initial northward subduction of southern branch of the BNO.(2)The sheeted dike complex of Yunzhug ophiolite in the central Lhasa terrane was firstly confirmed to be formed at ~150 Ma,indicating there was seafloor spreading within the Lhasa terrane during Late Jurassic.Geochemically,the mafic rocks(including sheeted dike complex and pillow basalt)consist of predominant EMORB-type with minor P-and N-MORB-type rocks,and are all absence of Nb-Ta negative anomalies,hinting they were formed in MOR setting.In addition,there are some orthopyroxene-rich orthopyroxenite and websterites within the mantle peridotites which are cumulates of boninite-like rocks that derived from an ultrarefractory mantle soure as mirrored from their whole-rock major and mineral geochemical makeup.The Re-Os isotopes indicate the pyroxnenites were derived from the Jurassic asthenospheric mantle,while the peridotite were derived from ancient sub-continental lithospheric mantle.Both of their source have undergone high degree-melting and they are diapired synchronously due to continental extension before final breakup.Comprehensive studies of regional geology,peridodites,sedimentary rocks,we propose that the Yunzhug ophioltie is analogous to Red Seatype ophiolite which was representative of an embryonic ocean followed by continental breakup of the Lhasa terrane instead of the traditional view of a back-arc basin.(3)The Dongga hornblendite and diorite are synchronously formed during early Jurassic(~180 Ma)instead of Early Cretaceous as traditionally regarded.The diorite are derivative of mantle-derived H2O-rich magma,while the hornblendite are hornblende-dominant cumulates of these H2O-rich magma.Given the similar Sr-Nd composition between the hornblendite,diorite and the coeval intermediate-felsic intrusives,the crystal fractionation play a significant role in the petrogenesis of these intermediate-felsic intrusives.Although the early Jurassic magmatic rocks are widespread in the southern Lhasa terrane,only intrusives are exposed to the west of Lhasa city,and they show higher H2 O contents in the source than those of coeval volcanic and intrusive rocks distributed to the east of Lhasa city,which may be resulted from the differences in the manner of northward subducted Neo-Tethyan oceanic crust,such as intra-oceanic subduction or subduction angle.(4)In combination with previous researches,our studies led us to propose a new tectonic evolutional model for the Lhasa terrane in Late Triassic to Late Jurassic,including two steps: 1)the Neo-Tethyan Ocean stared to subduct beneath the Lhasa terrane during Late Triassic to ~165Ma,and produced the Yeba and Sangri volcanic arc and coeval intrusive rocks;in the meantime,the Bangong-Nujiang main oceanic basin have already been subducted beneath the Qiangtang terrane,but no southward subduction existed.The far-field plate stress stretched the continental lithosphere of the Lhasa terrane,in a period that the Lagkor Tso and Shiquanhe Ophiolites were formed.2)In the Late Jurassic,the transfer of Neo-Tethyan oceanic lithosphere into flat subduction and sustained Bangong-Nujiang Ocean enhanced the extension and gradually breakup of the Lhasa continental lithosphere,which led the opening of the Yunzhug embryonic ocean.