Geochronology, geochemical characteristics and tectonic implications of the amphibolite from Paxialayidang area on the southern margin of Altun terrain

Amphibolite is distributed sporadically in Paxialayidang area on the southern margin of Altun terrain. The amphibolite is ellipsoidal or lenticular in shape and is wrapped by marble of Altun terrain. In this paper, the authors studied petrology, geochronology and geochemistry of the amphibolites. LA-ICPMS zircon U-Pb dating yielded ages of (432.5±5) Ma and (857.4±7.1) Ma for metamorphic event and protolith respectively, and shows that the protolith was formed during Neoproterozoic Qingbaikou period. Amphibolite is slightly enriched in REE, and shows unnoticeable fractionation between light rare earth elements (LREE) and heavy rare earth elements (LREE) and the signatures of subduction zone basalt. The primary magma originated from an enriched mantle source, which was formed by the recirculation of subducted slab, and was modified by fluid or melt subsequently. Combined with the evolution of regional tectonic, the authors hold that the protolith of amphibolite was formed in an incipient continental rift and was a transitional type formed during the transition from continental extension to continental rift. The younger age of (432.5±5) Ma represents the time of metamorphic event, which was caused by the detachment of continental lower crust during Early Silurian. In this paper, the authors provide new geochronological and geochemical evidence about basic magmatic activity during middle-late stage of Neoproterozoicin on the southern margin of Altun terrain. The basic magmatic activity was the response to the Rodinia supercontinent breakup on the southern margin of Altun terrain.