Petrogenesis of the Sancheong-Hadong anorthosite and adjacent high-grade gneisses in the southwestern Yeongnam Massif, Korea

Abstract

Massif-type anorthosite and adjacent high-grade gneisses in the Sancheong-Hadong (SH) area, southern Yeongnam Massif, Korea, were investigated to unravel the evolution of Paleoproterozoic massif-type anorthosite and associated high-temperature, low-pressure (HTLP) metamorphism. The SH anorthositic rocks consist of anorthosite, leucogabbro, leuconorite, hornblende gabbro, oxide-bearing gabbroic dyke, and Fe-Ti ores. They are emplaced into the Precambrian high-grade gneisses, consisting primarily of garnet-orthopyroxene granulite, charnockitic gneiss, granitic gneiss, and migmatitic gneiss. Based on SHRIMP zircon ages, leuconorite and hornblende gabbro were initially emplaced at ~1873–1870 Ma, and then anorthosite and oxide-bearing gabbroic dyke at ~1862–1860 Ma, which are indicative of episodic anorthositic magmatism over a ~10 Ma interval. Initial εHf values of zircons analyzed from five anorthositic rocks range from +2.1 to –6.1, suggesting that anorthositic parent magma should be derived from a mantle source but variably affected by crustal contamination. These results, together with available Re–Os data, are compatible with ~1.9–1.86 Ga collisional orogeny prevalent in the North China Craton and the Korean Peninsula, and suggest that orogenesis was accompanied by the mantle delamination beneath the craton. In addition, ~1.87–1.86 Ga anorthositic magmatism in the Yeongnam Massif is most likely a late orogenic product of Paleoproterozoic North China Craton amalgamation tectonically linked to the assembly of the Columbia supercontinent. P–T pseudosections and SHRIMP U–Th–Pb zircon/monazite ages are combined to decipher P–T–time evolution of the SH metamorphic complex. A suite of microstructures resulting from melt production and consumption are well preserved in migmatitic gneisses containing garnet, sillimanite, cordierite, K-feldspar, plagioclase, ilmenite, and former melt. P–T analyses based on the pseudosections and microstructures of migmatites suggest a peak metamorphic condition of 800–850 °C and 5–7 kbar, followed by near-isobaric cooling with melt crystallization at 750–800 °C and ~5 kbar. SHRIMP U–Th–Pb zircon/monazite ages from six migmatitic gneisses are in the range of 1870–1855 Ma

the oldest age yielded from monazite suggests that prograde metamorphism has commenced at ~1870 Ma. On the other hand, the melt crystallization producing widespread leucosomes and anatectic granites has culminated at 1860–1855 Ma. It is thus likely that the HTLP metamorphism in the SH complex has lasted over a period of ~15 Ma. Based on field relationships, P–T conditions and geochronological data, the high thermal gradient (~120–140 °C kbar-1) attending the granulite-facies metamorphism attributed to the coeval, pulse-like emplacement of anorthositic-gabbroic magma. Taken together, prolonged HTLP metamorphism and anatexis are associated with the SH anorthositic rocks emplaced during the late stage of Paleoproterozoic (1.95–1.85 Ga) hot orogenesis in the North China Craton.