Evolution of lithospheric mantle beneath the Maguan region, southwestern margin of the South China block based on mantle xenoliths in Miocene alkaline volcanic rocks

Abstract

The Maguan (MG) region in the southwestern part of the South China Block contains many Neogene potassic basaltic volcanic rocks along the Ailao Shan–Red River (ASRR) shear zone. These alkaline volcanic rocks contain abundant peridotite xenoliths which are mostly spinel (Spl)-facies lherzolite. We examined xenoliths in young, ~12 Ma, alkaline basalts. Most contain clinopyroxene (Cpx), >15 vol%, and minor xenoliths contain low Cpx, ~10 vol%. Olivine (Ol) shows forsterite components ranging from 88.6 to 90.1 in most lherzolite and from 90.4 to 90.8 in depleted lherzolite, Spl Cr# (=Cr/(Cr + Al)) between 0.08–0.11 for lherzolite and 0.17–0.22 for depleted lherzolite. Platinum group elements (PGE) show a near flat primitive mantle-normalized pattern; the ratios of IPGE/PPGE are 1.3–1.8 for lherzolite and 2.1 for depleted lherzolite. The xenoliths do not contain metasomatic minerals, but Cpx in depleted lherzolite samples is enriched in light rare earth elements (LREE; (La/Sm)N = 0.97–1.74). Two pyroxene geothermometry gives the temperatures ranging from 1029 °C to 1148 °C (average 1087 °C, n = 13). Spl–Ol geothermobarometry yields the oxidation state ranging from FMQ −1.5 to −0.25 with an average value of FMQ −0.59 (n = 13), which is comparable to those of abyssal peridotite and the asthenospheric mantle. The calculated temperatures and oxidation state are similar among samples independent of mineralogy and mineral chemistry, suggesting that partial melting and the cryptic metasomatism did not change the oxidation conditions. The data suggest that the subcontinental lithospheric mantle (SCLM) beneath the region formed from the asthenosphere mantle. Since the South China Block is generally underlain by Proterozoic SCLM, the data obtained this study suggest that the ancient lithosphere was replaced by the asthenospheric mantle in the local extension along the ASRR shear zone.