ORIGIN OF THE MIDDLE TRIASSIC GEJIU-KAIYUAN BASALTS IN WESTERN YANGTZE BLOCK AND THEIR GENETIC ASSOCIATION WITH THE EMEISHAN LARGE IGNEOUS PROVINCE MAGMATISM AND PALEO-TETHYS SUBDUCTION

Two important geological events occurred on the western margin of Yangzte block during Late Permain: the Emeishan mantle plume and the eastward subduction of the Paleo-Tethys oceanic plate. Investigation was needed to answer two questions: How do the plume and subduction interplay? What product will be formed?

The Gejiu-Kaiyuan basalts, located in the southern part of the intermediate zone of the Emeishan Large Igneous Provence (ELIP), were found and investigated. These basalts were conformably interbedded with the Middle Triassic Gejiu Formation. The LA-ICP-MS zircon U-Pb dating indicated the eruption age of these basalts were 248 Ma, 9 Ma younger than the termination age of the Emeishan magmatism (257 Ma). The Gejiu-Kaiyuan basalts had plume-like trace elements and Sr-Nd isotope ratios: Nb/Th (6.4 to 9.5), Zr/Nb (4.3 to 6.9), Zr/Y (6.1 to 9.7), and Nb/Y (1.0 to 1.7); The εNd(t) values ranged from -2.2 to -0.1 while the (87Sr/86Sr)i values ranged from 0.70524 to 0.70637, which overlapped with that of the Emeishan basalts and indicated the Gejiu-Kaiyuan were derived from the same source as the Emeishan basalts. The primitive mantle-normalized platinum group element (PGE) patterns of these basalts were very uniform. However, these basalts had less Pt and Pd contents than the Emeishan basalts. During low-degree partial melting, the Pt-Pd rich sulfides in mantle controlled the PGE contents in melt. Therefore, the Pt-Pd depletion of the Gejiu-Kaiyuan basalts was due to the early extraction of Pt and Pd from the same mantle when Emeishan basalt erupted. The PGE features implied these basalts were the remelted products of Emeishan plume.

A possible genetic model for these post-ELIP basalts was proposed: After the main Emeishan plume event, the Yangtze block was encountered a post-emplacement relaxation. Meanwhile, according to the paleogeographic reconstructions, the Simao mini-block collided back with the South China/Indochina block because of the subduction of Paleo-Tethys oceanic plate in the Middle to Late Triassic period. Accordingly, both post-emplacement relaxation and back-arc extension have occurred simultaneously in the Yangzte block, which triggered the Emeishan fossil plume head decompression remelted and, hence, produced these basalts at 248 Ma.