PETROGRAPHY AND GENESIS OF THE EARLY CRETACEOUS DOSEMIC RHYOLITE PORPHYRY IN CHIFENG CITY, INNER MONGOLIA

The Early Cretaceous dosemic rhyolite porphyry in Shuangsheng area of Chifeng, Inner Mongolia, is characterized by vesicular structure and multi-porphyritic texture, with 70% of phenocryst content, including sanidine(30%), quartz(35%)and plagioclase(5%). The sanidine phenocryst shows subhedral wide tabular structure with comb-like reaction rim, which is composed of Na-orthoclase and contains needle-columnar mineral crystallite. The edge or interior of the quartz phenocryst is strongly corroded, where porphyroclastic texture is developed, with the characteristics of "cracked or dispersed but not separated". Matrix components are filled in between the quartz porphyroclasts. The rock contains vesicles, showing the dual structure of lava and porphyroclastic lava. The structure characteristics of the rock reflect a rapid uplift process with adiabatic-decompression of magma, probably indicating the beginning of regional extensional tectonic setting in diagenetic period.