Geological, Geochemical Characteristics And Petrogenesis Of The Wangchong Super Unit, Darongshan Area,Guangxi Province

Presently, the granitoid intrusives in Darongshan area are generallyacknowledged as typical S-type granites formed in the Triassic period. And theresearchers inferred that the Triassic tectonic setting of this area was syn-collision orpost-collision. But so far, the Wangchong granitic super unit, which maybe formedearlier and had mafic microgranular enclaves, has not yet been researched in detail,which affect to get a complete picture of the tectono-magmatic evolution of this area.So, this paper take the granite and mafic microgranular enclaves (MME) of GuangxiWangchong super unit granite rockmass for the research subject, through petrologyand geochemistry characteristics, to discuss the genesis, the formation of rockstectonic environment and magma mixing.In Guangxi province, Wangchong super unit granite rockmass, with outcroppedtotal area of56.2km2, distributed NE and along the Lingshan-Tengxian deep faultzone. It is composed of medium-coarse grained hornblende biotite K-feldspar graniteof Tianjingchong unit, medium-fine grained porphyritic hornblende biotite K-feldspargranite of Dawanshan unit and granite porphyry of Mojiaokou unit. The zirconSHRIMP U-Pb age is254±3Ma. The granite show a large variation in their SiO2contents (SiO2content of71.22%-74.71%), alkali-rich (Na2O+K2O6.52%-7.90%),poor of Ca and Mg, higher TFeO/MgO, metaluminous to weakly peraluminous (A/CNK0.972to1.123), with high10000×Ga/Al values. The total of REE islower-middle, the pattern display right-inclined, light rare earth elements fractionationobvious but heavy are not, and obvious negative Eu anomaly (δEu=0.27-0.48). Thesuper unit are obviously enrich Rb、Ba、Th、U、Pb (LILE) and Zr、Hf、Ga (HFSE),depleted in elements Ta、Nb、Sr、Ti.Typical characteristics of magma mixing enclaves commonly appear ocelliquartz with dark-rimmed, ablation-regeneration edge of feldspar and acicular apatite. In the same denominator of multi-elements ratio-ratio diagram, granite and MMEwithin a good linear relationship; in the different molecular and different denominatorof multi-elements ratio-ratio diagram, there show a good hyperbolic relationship. TheMME have a similar relationship with granite rockmass in composition of Sr, Ndisotope,(87Sr/86Sr)iis0.71194-0.71598, εNd(t) is-9.0--10.8, T2DM1754Ma-1867Ma,ISrlower and εNd(t) higher than Shiwan mountain typical S-type granite. This factindicate that magma mainly with cruse-sourced mixed with mantle-sourced magma.The granites rockmass of Wangchong super unit belongs to aluminum A-typegranites. In diagram of tectonic discrimination, most of the samples are drop-inintraplate zone (Post-orogenic stage subordinate to intraplate tectonic environment),formed in Post-orogenic setting. Integrated geological, geochemical data as well as itsstructural features and Regional tectonic setting also suggest that formed in apost-orogenic extensional tectonic setting.According to the characteristic which tectonic-magma-metamophism event alongthe distribution of South China plate and Yangtze plate suture zone and south activeside of Yangtze plate, repute that dynamics mechanism is collapsed of mountain rootduring post-orogenic, upwelling of the asthenosphere, which cause lithosphereextension, thinning, decompression and warming, resulted of basaltic magmaunderplating formed in partial melting. The tectonism leads to high temperature-ultrahigh temperature metamorphism of crustal rock and partial melting.