Petrogenesis and tectonic setting of granite porphyry in Keyouzhongqi area,Inner Mongolia

Petrogenesis and tectonic setting of granite porphyry in Keyouzhongqi area，Inner Mongolia

The authors studied the geochemistry，zircon U-Pb ages，Hf isotope of the granite porphyry and its petrogenesis and tectonic setting in the studied area． The zircon U-Pb dating indicates the formation of the gran- ite porphyry is in Early Cretaceous (125． 1 ±1． 5 Ma)． The granite porphyry has high-SiO2and alkali-rich fea- tures，which belongs to high-K calc-alkaline series rocks (A/CNK = 0． 95%--1． 25%)． The analyses of trace elements show the characteristics of a swallow-shaped ＲEE distribution pattern with enrichment in LＲEEs and most of LILEs and HFSEs，depletion in Ba，Sr，Nb，P and Ti，and especially strong depletion in Eu，which indicates the granite porphyry belongs to the aluminous A-type granite． Their εHf(t)range from 5． 94 to 8． 80 with Hf two-stage model ages (TDM2) of 620 Ma to 803 Ma． Combining with the regional tectonic background， we conclude that the source of the rocks is the new crust materials accreted from depleted mantle in Neoprotero- zoic and is the product of partial melting of middle and lower crustal rocks，which may be suffered from the dual impact of the subduction of the Palaeo-Pacific Ocean and the closure of the Mongol-Okhotsk Ocean．