The Research Of Geochemical Characteristics, Geochronology And Geology Significance Of The Tumugou Formation Volcanic Rocks Of The Litang Area In Sichuan Province

The Ganzi-Litang area in Sichuan province is an important part of the ‘Three-River’ orogenic belt in Southwest of China,which is in the joint terrain of the Eurasia plate and India plate,located in the eastern extension of the Eastern Tethys, an important polymetallic metallogenic belt of China. By the research on petrography, petrogeochemistry and chronology of the Tumugou Formation intermediate-felsic volcanic rocks in fore-arc basin facies, it is to discusse the source characteristics、tectonic setting and geological significance.The volcanic rocks of the Tumugou Formation in the study area is mainly incanus massive alteration rhyolite and grey black middle-thin rhyolitic crystal-vitric tuff, as an intercalations in clastic rock of neritic facies. The content of SiO2 of the six samples of the intermediate-felsic volcanic rocks is between 60.84%-79.41%, to belong to middle-acid rock category, the calcium alkali volcanic rocks and weakly peraluminous rocks. To exclude late alteration ofrocks, the intermediate-acidic volcanic rocks of the Tumugou Formation is high-potassium, low-sodium, low-titanium and high-alumina, which showed that the crust material have large contribution to the magma. The rare earth content of the six samples of the intermediate-felsic volcanic rocks is high, ∑REE=118.39×10^-6-169.04×10^-6, average value is 142.81×10^-6. The rare earth elements curves for the chondrite have similar characteristics, with inclining to the rightand riching in LREE and lack of HREE. The curves of the six samples of the intermediate-felsic volcanic rocks on the PM normalized spidergrams, while the rhyolites are significantly depleted in P、Ti、Nb and Ta as well as the LILE（Rb、K） enrichment and HFSE（Th、U） depletion. The curves of the six samples of the intermediate-felsic volcanic rocks on the PM normalized spidergrams are similar characteristics with the representative island arc volcanic rock. The age of the zircon U-Pb volcanic rocks of the Tumugou Formation obtained in the study area is 219.3±1.4Ma, and the deposition time should be in the Carnian of early Late Triassic.Combined with previous data, the Litang ocean basin mostly happened westward subduction as early as middle Triassic, and sustainable to late Triassic, meanwhile, the Litang ocean basin subduction aggravation lead to the mantle wedge partial melting. In about 219 Ma, associated to the formation and continuous enlargement of the mantle wedge magma chamber, a large energy transfered to the upper-crust and cause the upper-crust melting, which formed the original magma of the Tumugou Formation intermediate-felsic volcanic rocks. The majority of magma with diapirism which rose to the recent surface formed the Yidun island arc belt, and part of original magma which increased to eruption and was deposited in fore-arc depression basin of the Yidun island arc belt, formed the upper series of the Triassic Tumugou Formation volcanic-sedimentary construction.