Petrology and geochemistry of dunites, chromitites and mineral inclusions from the Gaositai Alaskan-type complex, North China Craton: Implications for mantle source characteristics

The Gaositai complex is a typical Alaskan-type intrusion, emplaced in the northern margin of the North China Craton at about 280Ma. It is a concentrically zoned intrusion, with dunite in the core, rimmed by wehrlite, clinopyroxenite and hornblendite outwards. To understand the chemical compositions of the parent magma to the ultramafic complex, and thus that of the mantle source, we conduct petrological and geochemical studies on the dunite core and high-Mg clinopyroxene (cpx) inclusions in the chromitite from the rock unit, including the major and trace elements of cpx inclusions and whole-rock Re-Os isotope of the chromitites and chromite-rich dunites. The cpx inclusions have chemical compositions similar to the high Mg# cpx of primitive ankaramite, suggesting that the cpx inclusions could have formed from a primitive parent magma. The calculated chemical compositions of the primitive parent magma are characterized by high Mg# and CaO, with CaO/Al₂O₃ ratios >1, and enriched in Sr and other large ion lithophile elements compared to high field strength and rare earth elements. This, together with the high fO₂ (up to FMQ+2.83) of chromitite, suggests that the parent magma was a typical arc ankaramite (wet and CaO-rich picrite), originated probably from a hydrous-carbonatite fluxed sub-continental lithospheric mantle above a subduction zone that formed as a result of the subduction of the paleo-Asian oceanic slab beneath the north China craton in late Paleozoic.