The discovery of Devonian strata in Xilin Gol complex of Inner Mongolia and its geological significance

Abstract: Xilin Gol complex occurs in Xilinhot-Daqingmuchang area and is composed of metamorphic supracrustal rocks, metamorphic basic-ultrabasic rocks and granitic gneiss, with some of them being Precambrian in age and constituting Precambrian block. Recent dating of the LA-MC-ICPMS U-Pb zircons from parametamorphic rocks of Xilin Gol complex in Huremutaiaobao and Baiyintaolegai area yieled ages of detrital zircons ranging from 403Ma to 3077 Ma with a group of 206Pb/238U ages in the youngest strata between 403 and 420 Ma, probably representing the lower limited age of the sedimentary rock. Combined with 337 Ma metamorphic age of the parametamorphic rocks which were emplaced by arc intrusive rocks of Early Carboniferous-early Late Carboniferous, the authors hold that the parametamorphic rocks were formed in middle Early Devonian-early Early Carboniferous, and their protolith belonged to normal sedimentary rocks without being subjected to volcanic activity, and hence it was not characterized by forearc deposition formation. Therefore, it comprised a set of strata deposited on Precambrian Xilinhot micro-block, representing initial deposition of the southern margin of Late Paleozoic Hegenshan Ocean basin with extension after Early Paleozoic collision orogenesis.