Ages and tectonic settings of the Neoproterozoic igneous rocks in the Gyeonggi Massif of the southern Korean Peninsula and the correlation with the Neoproterozoic igneous rocks in China

The Neoproterozoic igneous rocks in the northern and southern Gyeonggi Massifs in the southern Korean Peninsula are important for the interpretation of the Neoproterozoic tectonic evolution of the Korean Peninsula and northeastern Asia. The Gonamsan and Chuncheon metabasites in the northern Gyeonggi Massif intruded in a within-plate tectonic setting at ca. 851–873 Ma and 888 Ma, respectively. The Nb/Yb ratios (7–23) of these metabasites represent enriched mid-ocean ridge basalt (E-MORB) to ocean island basalt (OIB) characteristics. Most zircons found within the Gonamsan metabasites have positive eHf(t) values and Paleoproterozoic to Mesoproterozoic isotopic model ages (TDM2). These metabasites have initial 87Sr/86Sr ratios that range between 0.7033 and 0.7058 and eNd(t) values of −0.68 to 5.02. These geochemical characteristics of these metabasites indicate that these rocks formed in a rift-related environment. The Neoproterozoic metabasites from the Dangjin area in the southwestern Gyeonggi Massif can be subdivided into arc and rift types. The Dangjin arc-type metabasites intruded at ca. 820–833 Ma. These metabasites have Nb/Yb ratios of 4–7, Nb/U ratios of 3.58–27.5 and show an increase in Th/Nb ratios with increasing La/Sm ratios. Most zircons from these metabasites have negative eHf(t) values and Paleoproterozoic-Archean TDM2 ages, except one that has positive eHf(t) values and a Mesoproterozoic TDM2 age. Most these metabasites have initial 87Sr/86Sr ratios of 0.7048–0.7105 and negative eNd(t) values from −11.67 to −5.25. These geochemical characteristics indicate that the arc type Dangjin metabasites experienced crustal contamination during differentiation in an arc tectonic environment. The rift-related Dangjin metabasites intruded at ca. 793 Ma in a similar manner to the Gonamsan and Chuncheon metabasites. The Nb/Yb ratios (8–11) of these metabasites are higher than those of the arc-type metabasites. These metabasites have lower initial 87Sr/86Sr ratios of 0.704 and higher eNd(t) values of 1.96 than the Dangjin arc-type metabasites. Together with the results of previous work, this study indicates that the tectonic setting of the Dangjin-Hongseong area was an arc during the period 820–900 Ma and then changed into a rift during the period 703–793 Ma. The Neoproterozoic igneous rocks in the northern Gyeonggi Massif can be correlated with those formed in the within-plate tectonic setting in the southern and southeastern margins of the North China Craton during the period 830–930 Ma. On the other hand, the Neoproterozoic igneous rocks in the southwestern Gyeonggi Massif can be correlated to the Neoproterozoic arc- and rift-related igneous rocks in the northern margin of South China Craton, which intruded during 850–871 and 637–820 Ma, respectively.