Emeishan large igneous province, SW China

Abstract

In recent years, there have been major advances in our understanding of the Emeishan large igneous province (LIP) of SW China following publication of a number of LIP-focused investigations of the terrain and associated rocks. This paper reviews the current state of knowledge. The volcanic and upper-intrusive portion of the province is relatively small (∼0.3 × 10 6 km 3), even when offset fragments, eroded sections and buried portions are included in the volume calculation. The most reliable radiometric age dates (zircon U-Pb SHRIMP from an associated layered intrusive body several kilometers in area) indicate generation at ∼259 Ma, consistent with the end-Guadalupian (end Middle Permian) stratigraphic age. In addition, several Ar-Ar dating studies have been carried out, mainly on the volcanic rocks, with a number of reported dates ∼253-251 Ma (Late Permian), but a consensus is emerging that these ages are problematic because they are in conflict with the stratigraphic data (possibly due to a monitor standard miscalibration). The Ar-Ar investigations have also yielded a large number of secondary ages, which are clustered at ∼175, ∼142, ∼98 and ∼42 Ma, and these are inferred to record sub-regional tectonic events that affected the western Yangtze Block as East Asia was assembled and later deformed by India's collision-indentation into Asia. Magnetostratigraphic data and field observations suggest that the bulk of the volcanic sequence formed within 1-2 my. The geochemistry of the volcanic rocks and bio-lithostratigraphic studies of the underlying Maokou Formation suggests a mantle plume generated the province. The basalts can be classified into low and high Ti groups with different parental magmas. The low Ti basalts are confined to the western part of the province and are overlain by the high Ti basalts. The low Ti magmas formed at shallow mantle depths in the spinel-garnet field transition zone (60-80 km), whereas the high Ti magmas formed by low degrees of partial melting within the deeper garnet stability field. This observation suggests a deepening of the melting column as the lithosphere thickened due to under-plating and a transition from peak basalt generation to the waning stage. Outstanding issues, which might focus future studies, are also outlined. © 2004 Elsevier B.V. All rights reserved.