Discovery and Geological Significance of an Indosinian (ca. 244 Ma) Metamorphic Event in the Langshan Area, Western Inner Mongolia, China: New Evidence from Zircon LA-ICP-MS U-Pb Dating.

The Langshan Group is an important constituent of the Precambrian metamorphic rocks in the Langshan area. The accurate determination of its metamorphic age is of great scientific significance for the further study of the Precambrian geological evolution in the region. Disputes remain regarding the metamorphism and deformation overprinting of the Langshan Group. This paper presents a detailed study comprising a field geological investigation, petrological observations, and zircon U-Pb aging of garnet-bearing mica quartz schists in the BangBang District. The result of detrital zircon U-Pb dating from the metamorphosed volcanic sedimentary rock series and geological investigation of the garnet-bearing mica quartz schists suggest the strata formed in the Neoproterozoic. The results from cathodoluminescence (CL) image analysis and U-Pb dating of zircons indicate a large number of metamorphic zircons exist in the garnet-bearing mica quartz schists. The metamorphic overgrowth rims of zircon from two samples were analyzed by LA-ICP-MS. The 206Pb/238U weighted average age of ca. 244 Ma of the zircon metamorphism rims represents the timing of Indosinian greenschist-amphibolite facies metamorphism in the Langshan area, which may be in response to the collision-type orogeny of the North China and Siberian plates in the Late Paleozoic. Acid-intermediate magmatic intrusive activities occurred in the Langshan area, and metamorphic events developed at the same time or at a later stage during the closure of the Paleo-Asian Ocean. [ABSTRACT FROM AUTHOR]