Neoproterozoic adakitic plutons and arc magmatism along the western margin of the Yangtze Block, South China

Abstract

Neoproterozoic adakitic plutons that crop out along the western margin of the Yangtze Block (South China) from Kangding on the north to Panxi on the south provide constraints on the origin of the giant Jinningian magmatic event of South China. Representative plutons include the Xuelongbao (750 Ma), Datian (760 Ma), and Dajianshan intrusions. The latter two bodies consist mainly of granodiorite with relatively high SiO2 (51.0-73.4 wt%) and Mg#'s (0.36-0.55). They have fractionated rare earth element patterns, with (La/Yb)N ratios ranging from 2.6 to 101.8, and are characterized by high Sr (344-1018 ppm) and low Y (4.3-17.9 ppm), yielding Sr/Y ratios ranging from 27 to 111. On primitive mantle-normalized trace-element diagrams, these rocks show enrichment of large-ion lithophile elements and depletion of high-field-strength elements (Nb, Ta), with positive Zr-Hf and negative Ti anomalies, consistent with an arc-related setting. They have relatively constant initial whole-rock 87Sr/86Sr ratios (0.704308-0.705068) and εNd values (+0.66 to -0.92). From their geochemistry, these plutons are interpreted to have formed in an arc environment. The parental magmas were generated from partial melts of a subducted oceanic slab that were modified by interaction with the overlying mantle wedge. Therefore, we conclude that the western margin of the Yangtze Block was an active magmatic arc during the Neoproterozoic. © 2007 by The University of Chicago. All rights reserved.