Electron microprobe monazite geochronological constraints on the Late Palaeozoic tectonothermal evolution in the Chinese Tianshan

Abstract

Electron microprobe monazite data from the early Palaeozoic granitoid gneisses in the Central Tianshan Tectonic Zone provide two main ages: (1) an age of 355 ± 12 Ma obtained on the chemically complex grains, which is good agreement with the high-pressure metamorphic age obtained on eclogite and blueschist rocks from the south Tianshan Tectonic Zone; (2) an age of 323 ± 7 Ma obtained on the complex and the monogenetic grains, which is also closely consistent with the age for a major episode of cooling–recrystallization of these high-pressure rocks. Monazite from the schist of the Xingxingxia complex yields a younger Th/Pb–U/Pb centroid age of 282 ± 9 Ma. This younger age is interpreted as being linked to a magmatic episode, possibly related to underplating–extension, and/or shear activities. These data provide timing constraints on the episodes of metamorphism and high-pressure metamorphism and evidence for transition from compression to extension during the Carboniferous to early Permian, for the Chinese Tianshan region. Moreover, this study shows that electron microprobe dating of monazite, together with recent statistical treatment, can provide a reliable method to reveal a sequence of events within an orogenic cycle.