Geochemistry and Os–Nd–Sr isotopes of the Gaositai Alaskan-type ultramafic complex from the northern North China craton: implications for mantle–crust interaction

We report petrological, chemical and Os–Nd–Sr isotopic data for the Gaositai ultramafic complex from northern North China craton (NCC) to reveal its petrogenesis. The complex shows features of Alaskan-type intrusions, including (1) the concentric zoning from dunite core, to clinopyroxenite and hornblendite in the rim, and the common cumulative textures; (2) the abundance of olivine, clinopyroxene and hornblende, and the scarcity of orthopyroxene and plagioclase, and (3) the systematic decrease in Mg# of ferromagnesian phases from core to rim, accompanied by the Fe-enrichment trend of accessory spinel. The different rock types show highly varied, radiogenic Os isotopic ratios (0.129–5.2), and unradiogenic Nd isotopic composition (εNd(t) = −8 to −15), but are homogeneous in ISr ratios (0.7054–0.7066). The (187Os/188Os)i ratios are found to be anti-correlated with εNd(t) values and whole-rock Mg# as well. These data suggest significant crustal contamination during magma evolution. The crustal contaminants are dominantly Archean mafic rocks in the lower crust, and subordinate TTG gneisses at shallower crustal levels. The parental magma was hydrous picritic in composition, derived from an enriched lithospheric mantle source above a subduction zone. The zoned pattern of the complex formed probably through “flow differentiation” of a rapidly rising crystal mush along a fracture zone that was developed as a result of lithospheric extension in a back-arc setting in the northern margin of the NCC at ca. 280 Ma.