Fluid-Present Partial Melting of Paleoproterozoic Okbang Amphibolite in the Yeongnam Massif, Korea

Abstract

The waning stage of a long-lived collisional orogeny is commonly governed by an extensional regime in association with high-temperature metamorphism, anatexis, and magmatism. Such a late-orogenic process is well-recorded in the Okbang amphibolite, Yeongnam Massif, Korea, where thin layers or irregular patches of tonalitic leucosomes are widespread particularly in association with ductile shear zones. Various microstructures including interstitial felsic phases and former melt patches indicate that leucosomes are the product of partial melting. These leucosomes are aligned en echelon and contain large (up to ~2 cm) grains of peritectic hornblende, suggesting synkinematic fluid-present anatexis. The leucosomes are enriched in Na2O and Sr contents compared to the amphibolite but depleted in rare earth and high field-strength elements. P-T conditions of the anatexis were estimated at 4.6–5.2 kbar and 650–730°C, respectively, based on hornblende-plagioclase geothermobarometry. Sensitive high-resolution ion microprobe U-Pb analyses of zircon from an amphibolite and a leucosome sample yielded weighted mean 207Pb/206Pb ages of 1866±4 Ma and 1862±2 Ma, which are interpreted as the times for magmatic crystallization and subsequent anatexis of mafic protolith, respectively. The latter is consistent with the time of partial melting determined from a migmatitic gneiss and a biotite-sillimanite gneiss at 1861±4 Ma and 1860±9 Ma, respectively. The leucosomes are transected by an undeformed pegmatitic dyke dated at 1852±3 Ma; by this time, extensional ductile shearing has ceased. Initial εHft values of zircon from the amphibolite range from 4.2 to 6.0, suggesting juvenile derivation of basaltic melt from the mantle. In contrast, lower εHft values (–0.1 to 3.5) in leucosome zircons indicate a mixing of crust-derived melt. Taken together, the Okbang amphibolite has experienced synkinematic fluid-present melting during the waning stage of Paleoproterozoic hot orogenesis prevalent in the Yeongnam Massif as well as the North China Craton.

Petrogenesis of the Palaeoproterozoic (1.87 Ga) Damyang amphibolites in the north‐western margin of the Yeongnam Massif and tectonic implications of the Korean Peninsula

“…(a) Zircon ε Hf ( t ) versus t (Ga) diagram of the 1.87 Ga amphibolite and the 87.6 Ga gabbro dyke in Damyang area and (b) Histogram of zircon ε Hf ( t ) values for the 1.87 Ga amphibolites. Data of the 1.87 Ga Muju amphibolites and 1.99–1.98 Ga Muju magmatism in north‐central region from Lee et al (2020), the 1.87 Ga Okbang amphibolites and the 1.87 Ga Imwon granites together with the 1.99–1.97 Ga Pyeonghae magmatism in the north‐east region from Lee and Cho (2020) and Kim et al (2014), and the 2.02–1.96 Ga Jangsu‐Gokseong magmatism in the south‐west region from Cho et al (2020) are cited for comparison.…”

“…(a) SiO 2 versus Zr/TiO 2 × 0.0001; (b) Zr/TiO 2 × 0.0001 versus Nb/Y; (c) FeO t /MgO versus SiO 2 ; (d) Al 2 O 3 ‐FeOt+TiO 2 ‐MgO diagrams. Data of the Muju amphibolites from Lee et al, 2020, Okbang amphibolites from Chang et al (1993), Arakawa et al (2003), and Lee and Cho (2020), Euiam amphibolites of the Gyeonggi Massif from Lee, Oh, et al (2014) are included for comparison.…”

“…Chondrite‐normalized rare earth element pattern and primitive mantle‐normalized trace element spidergrams of the 1.87 Ga Damyang amphibolites. Data of the Muju amphibolites from Lee et al, 2020, Okbang amphibolites from Chang et al (1993), Arakawa et al (2003), and Lee and Cho (2020), Euiam amphibolites of the Gyeonggi Massif from Lee, Cho, et al (2014), Lee, Oh, et al (2014) are included for comparison. Patterns of oceanic island basalt (OIB), enriched‐mid oceanic basalt (E‐MORB), and normal mid‐oceanic basalt (N‐MORB) are added for reference.…”

“…In the last 20 years, two episodes of magmatism during the Orosirian (2.02–1.80 Ga) were identified in the Yeongnam massif: One is the 2.02–1.96 Ga magmatism, including meta‐diorite and meta‐granites widely distributed in the north‐east, north‐central and south‐west parts (Cho et al, 2020; Kim et al, 2009, 2012, 2014; Lee et al, 2019; Oh et al, 2016). The other is the 1.88–1.85 Ga magmatism, including the anorthosite‐mangerite‐charnockite‐granite (AMCG) suite distributed in the south‐central part and amphibolite with leucogranite in the north‐east and north‐central parts (Kim & Cho, 2003; Kwon et al, 2003; Lee et al, 2017, 2018, 2020; Lee & Cho, 2020; Lee, Cho, et al, 2014, 2021; Oh et al, 2013). Among them, 1.87 Ga Okbang and Muju amphibolites were identified along the northern margin of the Yeongnam Massif recently, providing a significant clue to revealing the nature of the 1.87 Ga mantle‐derived magmatism (Figure 1b; Lee & Cho, 2020; Lee et al, 2020).…”

“…The other is the 1.88–1.85 Ga magmatism, including the anorthosite‐mangerite‐charnockite‐granite (AMCG) suite distributed in the south‐central part and amphibolite with leucogranite in the north‐east and north‐central parts (Kim & Cho, 2003; Kwon et al, 2003; Lee et al, 2017, 2018, 2020; Lee & Cho, 2020; Lee, Cho, et al, 2014, 2021; Oh et al, 2013). Among them, 1.87 Ga Okbang and Muju amphibolites were identified along the northern margin of the Yeongnam Massif recently, providing a significant clue to revealing the nature of the 1.87 Ga mantle‐derived magmatism (Figure 1b; Lee & Cho, 2020; Lee et al, 2020). However, the spatial distribution of these amphibolites, their internal relationship, and their geological relationship with the 2.02–1.96 Ga magmatic event are not clear, which leads to the difficulty in understanding the nature of these geological events along the northern margin of the Yeongnam Massif.…”