U-Pb dating of metamorphic monazite of the Neoproterozoic Kang-Dian Orogenic Belt, southwestern China

Abstract

There have been hot debates about the tectonic affinity of the western margin of Yangtze Block (the N-S striking Kang-Dian region) in the Neoproterozoic, with former discussions mainly based on petrogenesis of diverse igneous rocks. This study provides new insights about the possible tectonic affinity of the Kang-Dian region in the Neoproterozoic from metamorphic perspective. The metamorphic ages of the Kang-Dian region have been determined by LA-ICP-MS U-Pb dating of monazite to be 885 ± 3 Ma, 778 ± 4 Ma and 840 ± 3 Ma, respectively, for three representative metapelitic samples collected from the Yuanmou, Yanbian and Miyi areas. Such long age duration of the metamorphic event (over 100 Ma) of the Kang-Dian region is obviously conflict to a super-plume scenario, but more likely to be the results of a long-lived orogenic process. Furthermore, the ubiquitous gneissosity and schistosity of the Neoproterozoic intrusions and metamorphic rocks of the Kang-Dian region also indicate an extensive compressional environment with prevailing deviatoric stress, i.e., most possibly being a subduction zone. The metamorphic P-T conditions of kyanite-bearing metapelite are estimated to be 7.5–9.0 kbar / 640–670 ℃, which belong to intermediate facies series and should be metamorphosed at middle to lower crustal level, this further confirms the collision process. Combined with previous data, we suggest that the Kang-Dian region was a Neoproterozoic orogenic belt at least during ∼ 880–780 Ma, and at that time the Yangtze Block was an isolated terrane or at the margin of the Rodinia supercontinent.