Re-Os geochronology, O isotopes and mineral geochemistry of the Neoproterozoic Songshugou ultramafic massif in the Qinling Orogenic Belt, China

摘要

The Qinling Orogenic Belt was formed by subduction and collision between the North and South China Blocks along the Shangdan suture. The Songshugou ultramafic massif located on the northern side of the Shangdan suture provides essential insights into the mantle origin and evolutionary processes during spreading and subduction of the Shangdan oceanic lithosphere. The ultramafic massif comprises harzburgite, coarse- and fine-grained dunites. The spinels from harzburgite exhibit low Cr# and high Mg# numbers, suggesting a mid-ocean ridge peridotite origin, whereas spinels from both coarse- and fine-grained dunites are indicated as resulted from melt-rock reaction due to their systematic higher Cr# and low Mg# numbers. This melt-rock reaction in the dunites is also indicated by the low TiO2 (mostly <0.4 to%) in the spinel and high Fo (90-92) in olivines. Due to its relatively homogeneous nature in the mantle, oxygen isotopic composition is a sensitive indicator for the petrogenesis and tectonic setting of the Songshugou ultramafic rocks. Based on in-situ oxygen isotope analyses of olivines from twenty-six rock samples, most harzburgites from the Songshugou ultramafic massif show low delta O-18 values of 4.54-5.30 parts per thousand, suggesting the olivines are equilibrium with N-MORB magmas and originally formed in a mid-ocean ridge setting. The coarse- and fine-grained dunites exhibit slightly higher olivine delta O-18 values of 4.69-6.00 parts per thousand and 5.00-6.11 parts per thousand, respectively, suggesting they may have been modified by subduction-related boninitic melt-rock reaction. The delta O-18 values of olivines systematically increasing from the harzburgites, to coarse-grained dunites and fine-grained dunites may suggest enhancing of melt-rock reaction. The decreasing of Os concentration, Re-187/Os-188 and Os-187/Os-188 ratios from harzburgite to dunite suggest an Os-187-enriched, subduction zone melt was responsible for creating the melt channel for mel