Neoproterozoic Volcanic Rocks in the Southern Quruqtagh of Northwest China: Geochemistry, Zircon Geochronology and Tectonic Implications

Abstract

AbstractThe Early Neoproterozoic Beiyixi Formation volcanic rocks of the southern Quruqtagh comprise mainly of a suite of tholeiitic basalts, alkaline andesites, and calc‐alkaline rhyolites. The rhyolites are characterized by variably fractionated enrichment in light rare earth elements (LREE) and flat in heavy rare earth elements (HREE), and strongly negative Eu anomalies. Compared to the rhyolites, the andesites also exhibit enrichment in LREE and flat HREE (chondrite‐normalized values of La/Yb, and La/Sm are 13.30–41.09, 3.18–6.89 respectively). Their rare earth element patterns display minor negative Eu anomalies. Both of them exhibit coherent patterns with strongly to moderately negative anomalies of Nb, Zr, Ti, and Hf on spider diagrams. Two rhyolite and one andesite magmatic zircons with defined oscillatory zoning yielded weighted mean 206Pb/238U ages of 743 ± 7 Ma, 741±2 Ma, and 727±4 Ma. These ages are interpreted to represent the riming of volcanic eruptions. According to geochemistry and rock type, these volcanic rocks formed within a continental island‐arc environment following subduction of the oceanic crust during the Early Neoproterozoic period.