Mesozoic lithospheric modification and replacement beneath the Cathaysia Block: Mineral chemistry and water contents of the Daoxian peridotite xenoliths

Abstract

Water contents (a key parameter controlling the stability of a continent) of the Mesozoic sub-continental lithospheric mantle (SCLM) beneath the Cathaysia Block (South China) are still poorly constrained. Here, we report mineral chemistry and water contents of peridotite xenoliths entrained by Mesozoic basalts from the Daoxian area in the Cathaysia Block. The peridotite xenoliths are spinel-facies lherzolites with porphyroclastic textures and have olivine Mg# (Mg2+/(Mg2++Fe2+)) of 88.0–89.3, suggesting an origin from a fertile lithosphere mantle. Clinopyroxenes in the lherzolites show LREE-depleted patterns and negative anomalies in Ba, Nb, Pb, Sr, Zr and Ti. The contents of HREE and Y in clinopyroxenes suggest low degrees of melt extraction (e.g., 1–5% fractional melting) from a primitive mantle source. The weak enrichments in Th, U show later weak metasomatism by silicate melts. The water contents of clinopyroxenes and orthopyroxenes in the xenoliths are 182–417 ppm and 53–123 ppm, respectively. Calculated whole-rock water contents, based on the mineral modes and partition coefficients, are 35–82 ppm. The water contents are similar to those estimated for the MORB source mantle and the newly accreted fertile SCLM beneath the Cathaysia Block. Because the Daoxian area is located at the Ningyuan-Jianghua fault region, the translithospheric weak zone can provide a channel for the upwelling of asthenosphere and the subsequent accretion of fertile SCLM. The Mesozoic replacement of the ancient SCLM by fertile one therefore re-shaped the lithospheric architecture of the Cathaysia Block.