SUBDUCTION-RELATED ORIGIN OF THE 750 MA XUELONGBAO ADAKITIC COMPLEX (SICHUAN PROVINCE, CHINA): IMPLICATIONS FOR THE TECTONIC SETTING OF THE GIANT NEOPROTEROZOIC MAGMATIC EVENT IN SOUTH CHINA

Abstract

The Xuelongbao plutonic complex in the eastern margin of the Tibetan Plateau is dated at 748 ± 7 Ma using the SHRIMP zircon U-Pb method and represents part of the Neoproterozoic igneous assemblage of South China. Rocks in the complex include tonalite and granodiorite and have SiO2 ranging from 62.0 to 74.8 wt.% and Al2O3 from 14.3 to 20.9 wt.%. Their Na2O contents range from 4.2 to 6.7 wt.% and K2O from 0.47 to 1.96 wt.%, indicating that they belong to the Na-series. These rocks show chondrite-normalized REE patterns depleted in HREE and variably enriched in LREE. They have positive Sr and negative Nb and Ti anomalies in the primitive mantle-normalized trace elemental spider diagram. Their Sr contents range from 320 to 780 ppm and Y contents are lower than 10 ppm, resulting in high Sr/Y ratios (52-320), characteristic of typical adakites. Their εNd (t) and initial Sr isotopic compositions range from + 0.36 to + 2.88 and from 0.7033 to 0.7054, respectively. The geochemical features of the Xuelongbao plutonic complex are consistent with an origin from adakitic magmas that were likely derived from partial melting of a subducted oceanic slab. Together with arc signatures of other granites and mafic intrusions in the region, the 750 Ma Xuelongbao adakitic complex provides evidence for a major, subduction-related Neoproterozoic magmatic event of South China. © 2006 Elsevier B.V. All rights reserved.