Discovery of the Mesozoic Volcanic Rocks in Lahegen Area of the Eastern North Qaidam Tectonic Belt,and Its Geological Implications:Evidence from Geochemistry and Zircon Geochronology
The stratigraphic attribution of volcanic rocks in the eastern area of the northern margin of the Qaidam Basin is difficult to determine due to a lack of geochronological data.These rocks are located between the Wa-hongshan-Wenquan Fault and the southern margin of the Zongwulong Mountain Fault.This also leads to signifi-cant disputes over the division of geotectonic units in this area,as no Mesozoic volcanic rocks have been found here before.In this study,a set of Mesozoic volcanic rocks consisting of rhyolite,intermediate-acid brecciate lava and dacite was newly confirmed in the Lahegen area.Zircon U-Pb dating was performed on the rhyolite samples using the LA-ICP-MS technique with weighted mean age of 248.3±1.2 Ma,indicating an Early Trias-sic age.The geochemical data show that the volcanic rock is a metaluminous high-potassium calc-alkaline rock with clear fractionation of light rare earth elements,enrichment of light rare earth elements and depletion of heavy rare earth elements.The mean δEu value is 0.63,indicating a weak negative anomaly.The rocks are relatively enriched in large ion lithophile elements(e.g.,Rb and Ba)and high field strength elements(e.g.,Th,U,Pb,and Zr),and are significantly depleted in Sr,Ta,and Nb.These are typical geochemical charac-teristics of continental island arc volcanic rocks.Zircon Hf isotope compositions show εHf(t)values of～-19.62 to-3.07 and tDM2values of～1,639 Ma to 2,421 Ma,indicating that the volcanic rocks are derived from the partial melting of Paleoproterozoic crustal materials.Based on geochronology and geochemical charac-teristics,the volcanic rocks are attributed to the anatexis of crustal materials(the Darken Daban Group)caused by the northward subduction of the Anemaqen Ocean to the East Kunlun block in the Early Triassic.The vol-canic rocks of the Hongshuichuan Formation are similar to those in the magma arc of Olashan,according to the regional correlation.Therefore,these rocks are identified as Early Triassic Hongshuichuan Formation volcanic rocks.The confirmation of the studied volcanic rocks indicates that the Lahegen area has a close tectonic affini-ty with the Olashan magmatic arc of the East Kunlun orogenic belt.This study provides new evidence for the tectonic division of the eastern area of the northern margin of Qaidam Basin.
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