The amphibolite-facies metamorphosed mafic rocks from the Maxianshan area, Qilian block, NW China: A record of early Neoproterozoic arc magmatism

Abstract

The amphibolite-facies metamorphosed mafic rocks exposed in the Maxianshan area, Qilian block, NW China, intruded granitic gneisses of the Maxianshan Group. In the field, they are seen to inject into and capture fragments of the surrounding granitic gneisses. Their rock types range from amphibole-rich amphibolite to plagioclase-rich amphibolite.Their whole-rock major element compositions suggest that their protoliths were subalkalic gabbroic rocks. The rare earth element patterns show slight light rare earth element enrichment with a very small europium negative anomaly. The amphibolites exhibit enrichment in large ion lithophile elements and negative anomalies of titanium, strontium, niobium and tantalum on the basis of their spider diagrams. The SHRIMP U–Pb ages of the zircon grains are 919±10Ma for the amphibole-rich amphibolite and 905±6Ma for the plagioclase-rich amphibolite, indicating the ages of protolith formation. The εNd (910Ma) values are in the range of −1.6 and −2.4, consistent with generation in a continental arc. Tectonic discriminations by whole-rock trace element compositions suggest that they originated in a continental arc environment. The gabbroic protoliths were metamorphosed under amphibolite-facies conditions.The Maxianshan amphibolite-facies metamorphosed mafic rocks could be correlated with the mafic–ultramafic rocks from marginal areas of the Yangtze block. This correlation implies that the Qilian block and marginal areas of the Yangtze block were active continental margins in the early Neoproterozoic and provides evidence for a strong tectonic affinity of the Qilian block with the South China block and even for a unified Qilian-South China block at that time. It is inferred that the Qilian block carrying Maxianshan mafic rocks underwent amphibolite-facies metamorphism after it was rifted away from the South China block in the late Neoproterozoic as a result of the breakup of the Rodinia supercontinent and collided with the Alaxa block in the early Paleozoic.