Petrological Characteristics And Ore Potentiality Of The Tumugou Formation Volcanic Rocks Of The Litang Area In Sichuan Province

Recent studies in Litang county Ganzi prefectures,Sichuan province,mainly focused on the evolution of the Garze-Litang oceanic basin,among which stands out the Garze-Litang ophiolitic melange zone as a research hotspot.Litang Region,the research area,is located on the Yidun arc,which has been systematically studied by Hou Zengqian in early 1990 s.This paper presents a systematic study on petrography,geochemistry and geochronology of the Tumugou Formation volcanic rocks,and the magma sources,forming age and environment of which are further discussed.The Tumugou Formation,with various rock associations,is divided into three members according to their lithological features.The volcanic rocks of the Member 2,mainly contain andesite,rhyolite and crystal tuff,are described in this paper.Mainly developed in the silt or slate interlayers,the volcanic rocks are extensive.The major elements show that the studied medium acidic volcanics are calc-alkaline series,while the trace elements are characterized by relative enrichment of large ion lithophile elements like Rb,Th and K,and depletion of high field strength elements like P,Ti,Nb and Ta.The distribution patterns are similar with island-arc distribution patterns,indicates a magma source affected by crust-derived material.LREE in the volcanic rocks were obviously enriched with weak Eu anomaly.Zircons from the collected sample have high Th/ U ratios smaller than 1,indicating a magmatic origin.U-Pb Dating shows that the Early Late Triassic is the main formation epoch,as the weighted mean age is 222.1±1.7Ma(MSWD = 1.09).According to zircon Hf isotope analyses,the 176Hf/177 Hf values are 0.282823~0.282662,and the ?Hf(t)values are 0.96-6.56.These characteristics suggest that the primary magma was derived from the melting of the crust,and also affected by mantle material.In combination with metallogenic geological background,the authors hold that the ophiolitic melange zone was developed from the westward subduction of the Garze-Litang oceanic basin beneath Yidun arc in late Triassic.With a basic understanding of the geological settings and the ore-body and ore characteristics of the study area,the ore-bearing potential of the Tumugou Formation volcanic rocks was evaluated,the geochemical characteristics of which suggested a poor quality.