Petrogenesis and Tectonic Implications of Late Ordovician Alkaline Gabbro in the South Altyn Orogenic Belt

Alkaline gabbro, located at south Altyn orogeny, is used to discuss petrogenesis and mantle source by comprehensive studies of detailed field work, petrography, zircon dating and whole−rock geochemical analysis, which provide the more evidence to constrain the tectonic evolution process in the southern Altyn orogeny. Zircon U−Pb dating of the gabbro sample yielded a weighted ²⁰⁶Pb/²³⁸U age of (448.1±3.3) Ma, which is interpreted as intrusive time of the gabbro. Geochemical results show that the gabbro have relative low content of SiO₂ (42.40%～44.21%), but high values of alkali (3.56%～3.66%), and the Rittman index and Alkalinity ratios are 11～33 and 1.33～1.44, respectively. The lines of evidence indicate that the gabbro is classified as the alkaline gabbro series. In addition, the alkaline gabbro samples in the southern Altyn orogeny are enriched in REE and LILE (Sr and Sc), but depleted in HFSE (Hf and Zr), and varied contents of Nb and Th elements. Based on the plotting results of tectonic discrimination, regional geological survey and published data, it is proposed that the mantle source of the alkaline gabbro metasomatized by the fluid of the oceanic subducted slab, and contaminated by the continental crust during ascending process. Therefore, occurrence of the alkaline gabbro not only confirms that the Altyn orogeny was under the framework of the post−collision tectonic setting during early Paleozoic era, but also enriches the mafic magmatism of the orogeny.