Petrogenesis And Tectonic Significance Of Early Jurassic Alkalic Pluton In Nyainrong Microcontinent, Central Tibet

Nyainrong Microcontinent located geographically between the present-dayQiangtang and Lhasa Terranes, and sandwiched by ophiolitic fragments ofBangong-Nujiang suture zone both in its north and south, comprises one of the fewexposures of metamorphosed crystalline basement in Tibet Plateau. And bounded bywhich, the north(Qiangtang) and south(Lhasa) sides of the Bangong-Nujiang suturezone show very distinct geologic features, making the Nyainrong microcontinent to bean ideal object for regional research on many geological problems, especially theEarly Mesozoic tectonomagmatic evolution of the Bangong-Nujiang Ocean.The widespread, generally undeformed early Jurassic intrusions intrude in theNeoproterozoic gneiss and the Paleozoic clasolites in the Microcontinent, withmultiple exposed mafic enclaves and host granitoid. Zircon LA-ICP-MS U-Pb datingdata accordant within the range of the sampling errors, yields two main magmaemplacement eras of ~185 Ma and ~175 Ma. Moreover, the dioritic and felsic magmaemplacement are coeval in ~185 Ma, and the mafic and the felsic are coeval in ~175Ma also.The ~185 Ma intrusions of Nyainrong pluton are metaluminous to aluminous,and high-K calc-alkaline to shoshonitic-alkaline, ranging from diorite toalkali-feldspar-granite in composition. While, the ~175 Ma intrusions form part of ashoshonitic-alkaline to peralkaline that ranges in composition from maficalkali-syenite to intermediate monzodiorite and to felsic alkali-granite, with bimodalSiO2contents from metaluminous to peraluminous. They are all significantlycharacteristic of high potassic and high alkalic contents, and are petrogenetic mainlyin I-type series. They are all enriched in LREEs and LILEs but depleted in HFSEs,along with the Eu depletion. Meanwhile, they all have the negative bulk isotopicεNd(t)and zirconεHf(t) values corresponding to the ancient Nd-Hf model ages. And ofparticular note is the variation of the zircon Hf isotopic results are all extremely large,indicating the heterogeneous magma source.Based on the magmatic sequence, petrography, mineralogy, major and traceelement geochemical, bulk rock Sr-Nd isotopic data and zircon Hf isotopiccompositions of Nyainrong pluton, the early Jurassic emplacement magmasource(both the ~185 Ma period and the ~175 Ma period) having both highertemperature and lower pressure turns out to be comagmatic, and subsequent to undergo similar evolutionary process, which is a complex stage process involvingcrustal anatexis, magma mixing, crystal fractionation, and wall rock assimilation ofthe ancient mature crustal(significant input) and enriched lithospheric mantle even asthe depleted mantle sources proposed for the petrogenesis of the early JurassicNyainrong pluton. Besides, during this tectonomagmatic process with multiplemagma sources, the involved mantle derived materials are increased or enhanced withage from ~185 Ma to ~175 Ma.The data reported here, in combination with other diversified literatures aboutcoeval metamorphism-sedimentation of the research region, illustrate that the earlyJurassic shoshonitic and alkaline intrusions in Nyainrong Microcontinent are theproducts of the distinctive post-collisional magmatism along with intensifiedcrustal-mantle interactions in a passive margin due to northward subduction ofBangong-Nujiang oceanic lithosphere and the consequent Nyainrong-Qiangtangcollision.Furthermore, the middle early Jurassic(~185 Ma) magmatism marks the end ofthe Nyainrong-Qiangtang collision and the start of the post-collision and regionalextension; while the early middle Jurassic(~175 Ma) magmatism, especially theemergence of the mafic peralkalic intrusions, is in response to the finalpost-collisional stage which should be compatible with the slab thoroughly break-offand hot asthenospheric upwelling.