Geochemical evidence for Proterozoic continental arc and continental-margin rift magmatism along the northern margin of the Yangtze Craton, South China

Abstract

Middle to Late Proterozoic volcanic-intrusive magmatism of continental island-arc and continental-margin rift nature is well developed in the Xixiang-Beiba area, Shaanxi Province, along the northern margin of the Yangtze Craton. In the Xixiang area, volcanism evolved temporally from submarine low-K arc tholeiite through calc-alkaline to subaerial alkali or shoshonitic. This was accompanied by gradational enrichment of LREE and Zr, an increasing negative Eu anomaly, as well as strong depletion of compatible elements Cr, Co, V and Sc. The associated Hannan Intrusive Complex evolved from gabbro through tonalite-trondhjemite-granodiorite to granite with the distribution of basic to felsic plutons suggesting arc polarity. There is also a systematic lateral increase in K 2 O content, K 2 O/Na 2 O ratio, and La N /Yb N ratio towards the interior of the Yangtze Craton in these rocks. This spatial framework and compositional variation resembles that found in Circum-Pacific island arcs or active continental margins. In contrast, both volcanic and intrusive rocks in the Beiba area are characterized by alkaline compositions and a bimodal habit; they are composed of tholeiite (gabbro) and alkali rhyodacite-rhyolite (alkali granite), and lack rocks of intermediate composition. They may have formed in a continental-margin rift setting. Geological and geochemical evidence further suggests that the Xixiang paleo-arc was a mature continental arc built on older sialic basement. The emplacement of voluminous tonalites and trondhjemites thickened the crust and may have marked an important continent-building event at the northern margin of the Yangtze Craton in the Middle to Late Proterozoic.