Petrogenesis Of The Indosinian Granitic Magmatism And Tectonic Evolution Of The West Qinling Orogenic Belt

As the important part of Central Orogenic Belt of China, the Qinlin orogenic belt was formed by the joining of the North and South China Blocks during Indosinian.The West Qinling is the western part of the Qinling orogen, where a nearly600-km-long Indosinian granite belt was developed. In order to identify the basement nature and understand the growth of the crust and tectonic evolution of the West Qinling during Indosinian, this paper make a thorough investigation of the representative plutons and vocanic rocks from the West Qinling. Below are the main conlusions.(1) The Indosinian magmatism in West Qinling could be divided into two stages.The early tectonomagmatism(253~234Ma), which is plutonic-volcanic complexes ranging from basaltic to granitic variants, mainly took place along the northern margin of the central-western part of the West Qinling. The late magmatism(236~203Ma) is dominated by granite rich in Si and K, and mainly distributed in the eastern and western part of the West Qinling.(2) The Dewulu pluton(~245Ma) was a product of magma mixing. The granodiorite was derived from partial melting of Meso-Paleoproterozoic mafic lower crust. The near-primitive gabbro-dioritic MME are characterized by the geochemical similarities to HMA and were most likely originated from the reaction of the subducting sediment-derived melts and the subarc mantle.(3) The Shehaliji quartz monzonite(236~234Ma) are metaluminous and high potassic calc-alkaline, and belong to the early Indosinian high Sr/Y granitoids. They were sourced from partial melting of the garnet amphibolite in the old lower crust,and then mixed with mantle-derived mafic magmas. The texture and relatively homogeneous geochemical features of the host most likely resulted from the rapid remobilization process(unzipping).(4) The Hanban pluton(228~225Ma) belong to the late Indosinian high Sr/Y granitoids. The monzogranite were derived from partial melting of garnet amphibolite in the thickened lower crust and experienced a ~10% degree of fractionalcrystallization dominated by feldspar and biotite. The two-mica granite had a source of garnet amphibolite with minor metapelite, and experienced a ~10% degree of fractional crystallization dominated by plagioclase, biotite and muscovite.Contribution of the mantle-derived magma to the Hanban pluton could be ignored.(5) Tanchang volcanic rocks belonged to the peraluminous, shoshonitic-high potassic calc-alkaline series, and erupted in the Late Triassic limited by the zircon U-Pb dating(229~228Ma). Tanchang volcanic rocks were generated by high-temperature partial melting of the K-rich meta-mafic protolith with minor metasediment in lower crust.(6) The crystalline basement of West Qinling is dominated by the proterozoic mata-basaltic rocks, with minor metasediment and Archean materials. The basement in the central-western part of the West Qinling is totally older than that in the eastern part.Reworking of the basement by the melts from enriched SCLM is the major role for the growth of the continent in the West Qinling.(7) The continental collision between the SCB and NCB in the eastern region of West Qinling most likely initiated at ca.235 Ma.