LATE PALEOZOIC EXTENSION AND TECTONOMAGMATIC EVOLUTION IN THE SINO-MONGOLIA BORDER REGION IN THE NORTHERN PART OF ALXA

Pulsating extension had occurred in the Late Paleozoic in the Sino-Mongolia border region in the northern part of Alxa, forming polyphase rifts. A post-orogenic down-faulted basin developed in the Devoniar. The Carboniferous and Permian witnessed the occurrence of two successive phases of rifting. In the early stage there occurred calc-alkaline volcanism and molasse deposition. Middle Carboniferous volcanic rocks of the alkaline and continental tholeiitic series, gabbro and Iherzotite and early Late Permian bimodal volcanic rocks and alkaline volcanic rocks represent the peaks of the two phases of rifting respectively. The Late Carboniferous and terminal Permian were respective stages of rift decline of the two phases of rifting.