Petrology, geochronology and geochemistry of ca. 780 Ma A-type granites in South China: Petrogenesis and implications for crustal growth during the breakup of the supercontinent Rodinia

Abstract

There are widespread Neoproterozoic (830–740 Ma) calc-alkaline intrusive rocks in the South China Block, which has been considered a part of the Precambrian supercontinent Rodinia. The tectonic setting of these rocks, however, remains in dispute. Two distinctly different interpretations, continental rift or volcanic arc settings, imply different positions for South China in the Rodinian reconstruction. It is also unclear how such a large-scale magmatic event led to significant crustal growth. This study presents data for ca. 780 Ma A-type granites in the northern Daolinshan area, South China. A relatively early granite mineral assemblage contains katophorite, ferrorichterite, zircon, titaniferous magnetite, ilmenite, fluorapatite, and/or fayalite ± arfvedsonite, and a relatively late mineral assemblage consists of perthite, ferrowinchite, riebeckite ± arfvedsonite, quartz, albite, ilmenite, rutile, magnetite, siderite, grunerite and chamosite. Fluorite and calcite occur at all stages. The granites exhibit low Al2O3, Eu, Ba and Sr contents, high Na2O + K2O, Zr + Nb + Ce + Y, 10,000 × Ga/Al and Zr values, and high FeOT/(FeOT + MgO) (0.90–0.95) ratios, which are similar to those of typical A-type granites.They also have high whole rock Nd(t) values (+3.6 to +6.2) and zircon Hf(t) (+6.7 to +17.4). Temperature and oxygen fugacity (fO2) estimates suggest that the northern Daolinshan A-type granitic magmas were generated at high temperature (960–990 °C), low fO2 (on or slightly above the fayalite-magnetite-quartz (FMQ) buffer), and F- and CO2-rich but extremely H2O-poor conditions, indicating a rifting rather than arc setting for magma formation. Based on the compositions and geochronological and Hf isotope data of Neoproterozoic and older igneous rocks in the region, we suggest that the northern Daolinshan granites were most probably produced by high-temperature melting of slightly earlier tholeiitic rocks underplated in the lower crust as a result of a 780 Ma high-temperature mantle plume. The results support the hypothesis that the rifting away of South China from Laurentia was initiated by a ca. 780 Ma mantle plume during the breakup of Rodinia. The mantle plume activities also caused widespread crustal growth in South China in the Neoproterozoic (830–740 Ma).