Generation and evolution of siliceous high magnesium basaltic magmas in the formation of the Permian Huangshandong intrusion (Xinjiang, NW China)

Abstract

Numerous Permian orthopyroxene-rich mafic–ultramafic intrusions in the southern margin of the Central Asian Orogenic Belt (CAOB) have been emplaced in a post-orogenic environment. The Huangshandong mafic–ultramafic intrusion is the largest in the eastern Tianshan Orogenic Belt of the CAOB and consists of a layered unit intruded by a massive unit. The layered unit consists of lherzolite, troctolite, olivine gabbro, hornblende gabbro, gabbrodiorite and diorite, whereas the massive unit is made of gabbronorite and olivine gabbronorite.Olivine from the layered unit has Fo values of 75 to 84 and contains low NiO and MnO. Rocks from this unit have initial 87Sr/86Sr ratios (0.7031–0.7043) and εNd values (+3.4–+7.1) while plagioclase has initial 87Sr/86Sr ratios ranging from 0.70010 to 0.71569. Olivine from the massive unit has relatively low Fo values of 59 to 70 and high NiO and MnO. Rocks from this unit have relatively low and uniform initial 87Sr/86Sr ratios (0.7030) and high εNd values (+6.0–+7.5) and contain plagioclase with relatively low initial 87Sr/86Sr ratios (0.70042 to 0.70520). Olivines from both units have low CaO and Cr2O3.Both units formed from siliceous high magnesium basaltic magmas were derived from a hydrous, depleted mantle source. Magma forming the layered unit experienced earlier removal of sulfides before emplacement, with the earlier sulfide-saturation caused by the addition of crustal material. The magma that formed the massive unit has undergone fractional crystallization of olivine±Cr-spinel. Siliceous high magnesium basaltic magmatism, produced by melting of a hydrous depleted mantle source, may be an important feature of orogenic belts.