Geochronology, geochemistry and lithosphere extension of Kafang diabase in Gejiu area, Yunnan Province

This paper is the result of geological survey engineering.

Objictive The exploration of the petrogenesis and tectonic setting of the Kafang diabase is crucial to know about the tectonomagmatic evolution in Gejiu area.

Methods Based on LA−ICP−MS zircon U−Pb age, whole−rock geochemistry and Sr−Nd−Pb isotope analysis, the formation age, geochemical characteristics and geological significance of Kafang diabase dike are studied.

Results Zircon U−Pb dating shows that the age of Kafang diabase is 77 Ma. Inherited zircon ages (2409 Ma, 2616 Ma, 290 Ma) indicate tectono−thermal related Neoarchean and Paleoproterozoic metamorphic basement and the magmatic activity in Early Permian in Gejiu area. The Kafang diabase belongs to the shoshonite series with the characteristics of low SiO₂ content and high K₂O, TiO₂, MgO contents. In primitive mantle normalized trace elements diagram, these samples show similarities with OIB and enriched in LILEs (such as Rb, K, Sr), depleted in HFSEs (such as Nb, Ta, Zr, Hf). High initial ⁸⁷Sr/⁸⁶Sr ratios (0.70782−0.70791), positive εNd(t) values (2.07−2.29) and initial Pb isotopic compositions (²⁰⁶Pb/²⁰⁴Pb=18.286−18.465, ²⁰⁷Pb/²⁰⁴Pb=15.668−15.717, ²⁰⁸Pb/²⁰⁴Pb=37.763−38.830) indicate the enriched mantle (EM2) source.

Conclusions The petrogenesis of Kafang diabase is that the upwelling of asthenosphere in an extensional setting induced 5%−15% partial melting of garnet lherzolite at a depth of 60−120 km in the lithospheric mantle. The new−formed magma with characteristics of EM2 consists of the primary magma of the Kafang diabase. During the ascent of magma, contamination of the lower crust occurred accompanied by weak fractional crystallization, and then formed the Kafang diabase.