Neoproterozoic continental arc system along the NW margin of Rodinia supercontinent: Constraints from geochronological and geochemical studies of Neoproterozoic granitoids in the Diancangshan Massif

Abstract

The Neoproterozoic period is a key geological time in earth history that witnessed the breakup of the Rodinia supercontinent. The Diancangshan and Ailaoshan Massifs are two major segments along the Ailaoshan tectonic zone, and the Neoproterozoic metamorphic igneous rocks in these massifs preserve key clues to elucidate the on-going dispute over the geodynamic drives for the break-up of Rodinia. Many studies have been performed on the Neoproterozoic igneous rocks along the Ailaoshan Massif, but the contemporaneous igneous rocks in the Diancangshan Massif have been poorly studied. In this study, zircon U-Pb-Hf isotopic, whole-rock elemental and SrNd isotopic data are presented for a granitoid suite from the Diancangshan Massif. Zircon UPb dating shows that all the monzodioritic and granitic samples from the inner part of the southern Diancangshan Massif yielded Neoproterozoic crystallization ages of 701–797 Ma without exception. One sample collected from the southern edge of the Diancangshan Massif gave intrusive ages of ~239 Ma. Therefore, the Neoproterozoic magmatic activities in the Diancangshan Massif have been underestimated, and the spatial distribution of the Neoproterozoic intrusions occupies at least 1/3 of the whole area of the Diancagnshan Massif. The monzodioritic samples originated from the mixing of magmas generated from a subduction-modified mantle wedge and mafic lower crust. The granitic samples can be subdivided into two groups. The Group 1 I-type granitic samples have moderate A/CNK values of 0.80–1.07, which were derived from the partial melting of mafic lower crust. The Group 2 strongly peraluminous granitic samples (A/CNK > 1.1) originated from a hybridized source of the graywacke and mafic-derived components. Geochronological, petrological and geochemical studies show that Neoproterozoic magmatism in the ATZ was generated in a prolonged (>80 Ma) continental arc system. An Andean-type orogeny formed along the NW margin of Rodinia (i.e., the Seychelles, Himalaya, Amdo and South China including the Ailaoshan tectonic zone) and induced the regional internal lithospheric extension.