U-Pb zircon age, geochemical and isotopic characteristics of carbonatite and syenite complexes from the Shaxiongdong, China

Abstract

The Qinling is an important orogenic belt, which formed by the joining of the North China and South China blocks. The Shaxiongdong carbonatite-syenite complexes were emplaced at the southern margin of South Qinling and border the South China block. LA-ICPMS (Laser Ablation Inductively Coupled Plasma Mass Spectrometry) zircon U-Pb geochronology constrains the syenite emplacement age to be 441.8 ± 2.2 Ma, which is significantly earlier than the collision of the South China block and South Qinling along the Mianlue suture (200-240 Ma) near where the complexes reside. Trace-element abundances and C-O-Sr-Nd-Pb isotopes for the carbonatites and calcite separates indicate an igneous origin. They and associated syenites show overlaps of initial 87Sr/86Sr ratios (0.7029-0.7033, 0.7030-0.7032) and εNd (2.8 to 4, 2.5 to 6 for syenites and carbonatites, respectively), which implies that the carbonatites may derive from a carbonated alkali silicate melt. However, the rocks are composed dominantly of calcite. They have markedly lower Th, Nb, Zr and P contents compared to average calciocarbonatites worldwide. Calcites from the carbonatites are also characterized by low REE contents and relatively flat REE patterns. This indicates that the carbonatites are calcite-rich cumulates, which were produced from a residual liquid derived from an intensively fractioned carbonatite magma. In addition, the syenites are characterized by negative Pb and no Nb anomalies. They show lower Sr isotopes and higher εNd than syn/post-orogen related granites emplaced in the Qinling. This indicates that the complexes formed in a rifting environment. It is noted that Sr and Nd isotopic compositions from the carbonatites and syenites are close to HIMU mantle source values. Variations in 207Pb/206Pb (0.785-0.842) and 208Pb/206Pb (1.954-2.110) ratios from the calcites best fit a model involving mixing HIMU and EM1 components. Therefore, plume activity may play an important role in the complex generation and tectonic evolution of the South China block. Geological support for this deduction is the presence of numbers of Silurian dyke swarms. We hypothesize that the upwelling plume metasomatizing the continental lithosphere resulted in the South Qinling separating from South China block along the Mianlue suture during the early Paleozoic period.