Precambrian Research 122 (2003) 297–310 Age, geochemistry, and tectonic significance of Neoproterozoic alkaline granitoids in the northwestern margin of the Gyeonggi massif, South Korea Seung Ryeol Lee a,∗ , Moonsup Cho b , Chang-Sik Cheong c , Hyeoncheol Kim c , Michael T.D. Wingate d a Geology Division, Korea Institute of Geoscience and Mineral Resources, Taejon 305-350, South Korea b School of Earth and Environmental Sciences, Seoul National University, Seoul 151-742, South Korea c Isotope Research Team, Korea Basic Science Institute, Taejon 305-333, South Korea d Department of Geology and Geophysics, Tectonics Special Research Centre, The University of Western Australia, 35 Stirling Hwy, Crawley, WA 6009, Australia Received 6 December 2001; received in revised form 25 August 2002; accepted 26 August 2002 Abstract Alkaline meta-granitoids, ranging in composition from syenite to alkali granite, occur in the northwestern Gyeonggi massif. Ion microprobe U–Pb zircon analyses indicate that the granitoids were emplaced at 742 ±13 Ma, and are corroborated by a Rb–Sr whole rock age of 770 ± 40 Ma. Major and trace element characteristics, together with Sr and Nd isotopic data, suggest that the granitoid magma was derived from ancient (T DM = 2.6–2.2 Ga) continental crust with addition of juvenile mantle-derived basaltic magma. The generation of the alkaline granitoid is attributed to crustal thinning induced by deep-seated thermal activity such as mantle upwelling or mafic magma influx. Alkaline igneous activity at 742 Ma is coeval with Neoproterozoic rift-related magmatism prevalent in South Korea and the South China Block but lacking in the North China Block. Thus, we suggest that the Gyeonggi massif is correlative with the South China Block and has experienced a rifting event during Rodinia breakup. © 2002 Elsevier Science B.V. All rights reserved. Keywords: Alkaline granitoid; Gyeonggi massif; Neoproterozoic; Ion microprobe U–Pb geochronology; Sr and Nd isotopes 1. Introduction Alkaline granites or A-type granites are distinctly different in their major and trace element and iso- tope chemistries as well as modal mineralogy from the common I- and S-type granites (Collins et al., 1982; Whalen et al., 1987). Several petrogenetic models have been proposed for A-type granites, including fraction- ∗ Corresponding author. Tel.: +82-42-868-3048; fax: +82-42-861-9714. E-mail address: leesr@kigam.re.kr (S.R. Lee). ation from mantle-derived basaltic magmas, interac- tion between these mantle-derived magmas and conti- nental crust, and in some cases anatectic, halogen-rich, melts during the remelting of a terrane from which a previous melt had been extracted, and anatexis of un- depleted I-type tonalitic crustal sources (e.g. Collins et al., 1982; Harris et al., 1986; Whalen et al., 1987; Sylvester, 1989; Eby, 1990; Turner et al., 1992; Patiño Douce, 1997; Li et al., 2002). Despite of the diver- sity of their petrogenetic models, however, there is a general consensus that A-type granites are gener- ally emplaced in extensional or non-compressional 0301-9268/02/$ – see front matter © 2002 Elsevier Science B.V. All rights reserved. PII:S0301-9268(02)00216-4