Petrogenesis And Geological Significance Of Lizexi Pluton In Litang,Western Sichuan

Lizexi pluton is exposed in the northern part of the Yidun arc in Litang area.The geotectonic position is located in the eastern part of the Tethyan tectonic domain,which is the combination of Eurasian plate and India plate.The magmatic rocks and mineral resources are distributed within this belt.This paper systematically studies the petrology,geochemistry and geochronology of Lizexi pluton and its diorite inclusions,And analyses the origin of diorite inclusions,and petrogenesis,magmatic process and source of the granite,discusses the tectonic setting and its formation mechanism.Liezexi pluton occurs on the east side of the middle north section of the Yidun arc and on the west side of the Ganzi-Litang ophiolitic melange as stock,and is located in the sandstone of the second member of the Tumugou formation in the Late Triassic.It exposed about 43km².The zircon U-Pb age of biotite monzogranite is 215.2±1.4 Ma;and the zircon U-Pb age of dioritic inclusions is 216.5±1.6 Ma,indicating that both of them are products of the same period,all of which were formed in the Late Triassic.The pluton is composed of biotite monzogranite and its dioritic inclusions.The research shows that biotite monzogranite is characteristics by peraluminous,high-potassium,rich-silicon,poor-iron-magnesium and titanium.The inclusions are characteristics by potassium-rich and more iron-magnesium-titanium than the host rock.Both the trace and rare earth elements have similar characteristics,showing high field strength elemental elements?HFSE?losses,while large-scale ionophile elements?LILE?are significantly enriched;and the fractionation of light and heavy rare earth elements is obvious,LREE was relatively enriched,HREE was depleted,and there was a moderate negative anomaly.Shows Lizexi pluton belongs to island arc granite.The quartz crystal structure,quartz eye-spot structure,plagioclase growth zone and acicular apatite of the inclusions show that the inclusions are magma-mixing origins.Rb/Ti and La/Ce ratios show that the inclusions are formed under the magma mixing mechanism of different layers in the same magma chamber.The A/CNK of the granite and inclusions are less than 1.1,and P₂O₅is negatively correlated with SiO₂.The appearance of characteristic mineral hornblende shows that the granite is I-type granite.The granite enriches Si,K,Na,U,and Th and depletes Ba,Sr,Nb,Ta,Zr,Hf,and Ti,suggesting that the magmatic material may originate from the crust.The distribution curve of rare earths elment is similar to that of crust-melting granites.Rb/Sr and Sm/Nd ratios show that the magma source region is partially melted from the crust;the Nb/Ta ratio and the presence of corundum in the CIWP mineral,and the succession of inclusions in the zircons show that the granite from the partial melting of the crust and mixed with a small amount of mantle source materials.The structural environment discriminates that the granite was formed in volcanic arc environment under the process of subduction-collision.Combining with previous research results,it is believed that the Lizexi pluton is formed during the peak period of Ganzi-Litang subduction-collision,and the dehydration of the oceanic subduction plate resulted in partial melting of the mantle wedge,and the underplating formed a basaltic magma.The basaltic magma underplates the crust and resulting in the partial melting of the crust and mixed with a small amount of mantle materials to form Lizexi pluton.The pluton was influenced by the uplift of the plateau since the Miocene,and was pushed along the northeast to the Litang ophiolite melange under the compressional effect of the NW-NE trending tectonic stress.