Origin of Permian gabbroic intrusions in the southern margin of the Altai Orogenic belt: A possible link to the Permian Tarim mantle plume?

Abstract

Abstract Zircon U–Pb dating and Hf isotopic compositions, whole-rock and mineral geochemistry, as well as field and petrographic observations, are reported for the Permian gabbroic intrusions in the southern margin of the Altai orogenic belt. Similar zircon ages (~ 280 Ma) of these intrusions to that of the Permian Tarim mantle plume suggest a possible link to the Tarim plume. Olivine grains from the intrusions have Fo contents ranging from 69 to 81 and NiO from 0.05 to 0.21 wt.%, indicating the crystallization from evolved mafic magmas. Clinopyroxene grains from the intrusions have 0.3 to 1.5 wt.% TiO 2 and display a rift- to arc-like affinity. The intrusions are tholeiitic and have slightly LREE-depleted to LREE-enriched chondrite-normalized rare earth element (REE) patterns. Zircons from the intrusions have e Hf (t) values higher than + 8.0, whereas the whole-rock e Nd (t) values range from + 2.7 to + 7.0. Rocks from the intrusions have Nb/La of 0.2 to 0.8, consistent with the derivation from a depleted sub-lithospheric mantle with variable degrees of crustal contamination. The presence of primary hornblende and/or biotite in the rocks indicates metasomatism by slab-derived fluids/melts in the mantle source. We propose that upwelling of asthenospheric mantle in the Permian Tarim mantle plume along the Irtish suture zone, resulted in partial melting of the sub-continental lithospheric mantle. Based on a comparison between the Ni–Cu sulfide-bearing Kalatongke complex and un-mineralized intrusions, we suggest that Ni–Cu sulfide mineralized intrusions in the Altai orogenic belt are characterized by the enrichments of LREEs and LILEs and by the presence of primary water-bearing minerals.