Petrogenesis of the Gaojiacun mafic-ultramafic pluton at the southwestern margin of Yangtze Block: Evidence from petrology, geochemistry and geochronology.

The mineral assemblage, crystallizing sequence and ubiquitous occurrence of hornblende in the gabbro-peridotite pluton near Gaojiacun Village, Yanbian County, Sichuan Province, all indicate that the rocks formed from the hydrous magma. The obvious negative correlations between the major components Al2O3, CaO and MgO imply that plagioclase is not an early cumulate phase. With the total REE contents from 5.97 x 10(-6) to 221.32 x 10(-6), the LREE are moderately enriched and the LREE and HREE are weak to moderately differentiated. While the Eu anomaly is weak in gabbro and pyroxene-bearing diorite, but prominent in olive-gabbro and peridotite. As to the trace elements, negative anomalies of Nb, Ta, Zr, Hf and Ti, but positive anomalies of Ba and Sr can be observed in the gabbro and pyroxene-bearing diorite. Meanwhile the epsilon(Nd) (t) gives the range of 0.59 similar to 1.86, which is lower than that of the contemporaneous depleted mantle, suggesting the similar feature to that of basic magma of the island arc. In addition, the Neoproterozoic age, 822 +/- 8Ma, is obtained with SHRIMP U-Pb analysis of zircon from the pyroxene bearing diorite. Summarizingly the Gaojiacun pluton was derived from the partial melting of the depleted upper mantle which had experienced alteration by the fluids from the subduction zone.