CHIME monazite ages of metasediments from the Altai orogen in northwestern China: Devonian and Permian ages of metamorphism and their significance

Abstract

The chemical Th-U-total Pb isochron method (CHIME) has been applied to determine the ages of monazite grains from metasediments of greenscshist-amphibolite facies in the Altai orogen, northwest China. The area of distribution of these metamorphic rocks is divided into the Permian (261-268 Ma) central-western and Devonian (377-382 Ma) eastern units on the basis of their metamorphic ages. The Devonian CHIME ages are consistent with the Pb-Pb ages of granitoid in the eastern unit, and support the idea that emplacement of the granitoids was synchronous with regional metamorphism at deep levels. The Permian metamorphic ages (the present study) and igneous ages previously reported from the central-western unit can be interpreted in terms of subduction of crustal material and oceanic plate, and rapid exhumation.

Late Cretaceous CHIME monazite ages of Sanbagawa metamorphic rocks from Nushima, Southwest Japan

“…To avoid analyses of inappropriate points, parts of monazite damaged by metamictization and electron bombardment, which are usually opaque under a polarizing microscope, were selectively exempted from the analysis points (see Zheng et al, 2007). Abbreviations for minerals are after Whitney and Evans (2010).…”

Metamorphosed Pb–Zn–(Ag) ores of the Keketale VMS deposit, NW China: Evidence from ore textures, fluid inclusions, geochronology and pyrite compositions

“…Muscovite in stage B quartz-sulfide veins yielded an Ar-Ar age of 260 Ma, implying that stage B coincides with the later stages of metamorphism during arccontinental collision as dated in the Chinese Altay by CHIME (chemical U-Th-total Pb) at 268-261 Ma (Zheng et al, 2007).…”

Geology, fluid inclusion geochemistry, and 40Ar/39Ar geochronology of the Wulasigou Cu deposit, and their implications for ore genesis, Altay, Xinjiang, China

“…At the Wulasigou deposit, zircon grains from the metamorphosed rhyolitic tuffs in the Kangbutiebao Formation yield U-Pb ages of 407-412 Ma (Chai et al, 2009). Monazite from greenschist/ amphibolite facies metasediments in the Chinese Altay yields CHIME (chemical U-Th-total Pb) ages of 268-261 Ma that were interpreted to record the subduction-related metamorphism or an arc-continent collision (Zheng et al, 2007 and coeval to the continent-continent collision orogeny (including syn-to post-collision). Therefore, the Wulasigou Cu deposit is an epigenetic deposit formed in a continental collision regime.…”