Chronology and Geochemical Characteristics of the Early Paleozoic Volcanic Rocks in Longchuan Area,Western Yunnan Province,and Their Significance to the Proto-Thehyan Ocean Subduction
To explore the structural evolution characteristics of the Proto-Tethyan in western Yunnan,we selected a rock combination consisting of hornblende plagioclase granulite and monzonite leptite,identified from the Gaoligongshan Group in the Longchuan area of western Yunnan,for research.The protolith of the hornblende plagioclase granulite was basaltic andesite,while that of the monzonite leptite was rhyolite.Using LA-ICP-MS zircon U-Pb dating,we determined the zircon U-Pb ages of the hornblende plagioclase granulite and monzonite leptite to be(486.8±4.5)Ma and(486.6±4.2)Ma,respectively,indicating that the rocks formed in the early Ordovician period.The hornblende plagioclase granulite exhibits characteristics of low alkali(alk=2.75%-4.58%),high potassium(Na2O/K2O=2.32-4.79),and low titanium(TiO2=0.70%-1.12%).Chondrite-normalized REE patterns of the hornblende plagioclase granulite resemble those of MORB.Trace elements show enrichment in Rb,Th,K,Nd,and light rare earth elements,and depletion in Nb,Sr,P,and Ti.Sr-Nd-Pb isotopic characteristics indicate that the rocks originated from EMⅡ mantle,suggesting that hornblende plagioclase granulite(basaltic andesite)is a product of ocean crust subduction.The La-La/Nb diagram suggests that the hornblende plagioclase granulite exhibits geochemical features of island-arc basalts.The monzonite leptite has the characteristics of medium alkali(alk=6.09%-7.59%),high natrium(Na2O/K2O=0.4-0.9),and low titanium(TiO2=0.10%-0.21%).Chondrite-normalized REE patterns are inclined to the right.Trace elements show enrichment in Rb,Th,K,Nd,and light rare earth elements,heavy depletion in Sr,P,and Ti,and low depletion in Ba,U,Nb,and Ta,similar to the characteristics of island-arc granite.The Rb/30-Hf-3Ta diagram indicates that the monzonite leptite exhibits geochemical features of island-arc granite.The monzonite leptite(rhyolite)is formed by the intrusion of basic magma into the crust,providing sufficient heat to partially melt it.Geochemical characteristics indicate that the tectonic setting of the Early Paleozoic volcanic rocks in Longchuan area was an active continental margin.Early Paleozoic volcanic rocks were products of Proto-Tethyan ocean crust subduction to the northern margin of Gondwana.
Early Paleozoic volcanic rockgeochemistrySr-Nd-Pb isotopicLongchuan areaProto-Tethyan
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