Petrogenesis of Permian-Triassic intraplate gabbro-granitic rocks in the Russian Altai

Abstract

New data have been obtained from typical Permian–Triassic gabbro-granite suites from the Russian Altai. These suites were emplaced in an intraplate setting over a short period of time between 254 and 244 Ma concurrently with the formation of the Siberian and Khangai large igneous provinces (LIPs).While the sampled igneous rocks share some similar features, they also differ considerably in their mineralogy, petrography, major- and trace-element geochemistry, and Sr and Nd isotope systematics. The gabbro samples are depleted in high field strength elements (HFSEs), and their parent magma was formed by melting of the lithospheric mantle. The highest HFSE and rare earth element (REE) enrichment is observed in intermediate rocks (syenite and monzodiorite), which owe their origin to fractional crystallisation of mantle magmas with minor crustal contamination. The granitic rocks are I- and A-types. Because their compositions evolved towards a higher silica content, the HFSE and REE content decreased progressively, while the Nd isotope composition approached that of the host crustal rocks. Therefore, the granitic magma is a result of melting of the lower crust, promoted by mantle heat and fluids. The mixing of monzodiorite or syenite magmas with granitic anatectic melts appears to be the most probable mechanism for the origin of the granodiorite and syenogranite lithologies.