Prolonged high-temperature, low-pressure metamorphism associated with similar to 1.86 Ga Sancheong-Hadong anorthosite in the Yeongnam Massif, Korea: Paleoproterozoic hot orogenesis in the North China Craton

摘要

Elevated heat flow in association with mafic magmatism in an orogenic belt commonly leads to high-temperature, low-pressure (HTLP) metamorphism and the production of granulite-facies assemblages. We studied such a HTLP complex in the vicinity of the Sancheong-Hadong anorthosite-mangerite-charnockite-granite (AMCG) suite, Yeongnam Massif, Korea, in order to constrain the P-T conditions, timing, and duration of metamorphism. This complex primarily consists of massif-type anorthositic-gabbroic bodies emplaced at similar to 1.87-1.86 Ga and a series of country rocks comprising orthopyroxene-bearing gneisses as well as anatectic granites and migmatites. Migmatitic gneisses were studied in detail because melt-related features are abundant and well preserved; for example, inclusion-rich peritectic phases such as cordierite or K-feldspar are characteristic for the prograde melt-forming stage, whereas biotite-quartz symplectites mantling garnet or orthopyroxene represent the cooling stage consuming melt. Pseudosection P-T analyses of migmatitic gneisses suggest peak metamorphic conditions of 810-840 degrees C and 5.9-6.2 kbar, followed by near-isobaric cooling and melt crystallization at similar to 780 degrees C and similar to 5.5 kbar. SHRIMP (sensitive high-resolution ion microprobe) U-Th-Pb ages of zircon and monazite from six migmatitic gneisses are in the range of 1870-1854 Ma. The oldest age, recorded only in high Y monazite, suggests that prograde metamorphism commenced at similar to 1870 Ma. In contrast, melt crystallization had culminated by 1860-1855 Ma producing widespread leucosomes and anatectic granites. Our results suggest that high thermal gradient (similar to 40 degrees C km(-1)) attending the granulite-facies metamorphism is attributable to coeval, pulse-like emplacement of anorthositic-gabbroic magmas. Moreover, the HTLP metamorphism lasted over a period of similar to 15 Ma, indicating a long-lived process corresponding to the late stage of Paleoproterozoi