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

GeoScienceWorld Lithosphere Volume 2020, Article ID 8854615, 26 pages https://doi.org/10.2113/2020/8854615 Research Article Fluid-Present Partial Melting of Paleoproterozoic Okbang Amphibolite in the Yeongnam Massif, Korea Yuyoung Lee 1,2 and Moonsup Cho3 1Research Center for Geochronology and Isotope Analysis, Korea Basic Science Institute, Cheongju 28119, Republic of Korea 2Geology Division, Korea Institute of Geoscience and Mineral Resources, Daejeon 34132, Republic of Korea 3Department of Earth and Environmental Sciences, Chungbuk National University, Cheongju 28644, Republic of Korea Correspondence should be addressed to Yuyoung Lee; [email protected] Received 24 April 2019; Revised 11 December 2019; Accepted 18 May 2020; Published 1 September 2020 Academic Editor: Sarah M. Roeske Copyright © 2020 Yuyoung Lee and Moonsup Cho. Exclusive Licensee GeoScienceWorld. Distributed under a Creative Commons Attribution License (CC BY 4.0). 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 fl of peritectic hornblende, suggesting synkinematic uid-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 ε ð Þ has ceased. Initial Hf t values of zircon from the amphibolite range from 4.2 to 6.0, suggesting juvenile derivation of basaltic ε ð Þ – melt from the mantle. In contrast, lower Hf t 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. 1. Introduction of the continental crust as well as substantial variation in the crustal strength (e.g., [18–21]). Many previous field- Partial melting of amphibolites at middle-lower crustal based studies have focused on the dehydration melting of depths commonly takes place in response to the dehydration amphibolites [2, 5, 22] but much less on the fluid-present of amphibole (e.g., [1–5]) or the influx of externally derived melting, although the latter is one of the key melt-forming hydrous fluid (e.g., [6–9]). This anatexis accounts for the processes in the middle to lower crust during orogenesis [1, widespread occurrence of tonalitic and trondhjemitic melts 6, 7, 9, 23, 24]. Nevertheless, fluid-present melting, resulting in orogenic belts [10–15], and the migration of these melts in a volume decrease in host rocks, is associated with plate provides the mechanism for crustal reworking to yield margin tectonic structures such as crustal-scale shear zones residual granulites or restites in the continental crust (e.g., or regional thrusting that may lead to extensional collapse [16, 17]). Thus, melt formation and migration are two and exhumation of an orogenic belt [25–27]. Thus, both principal processes responsible for chemical differentiation fluid-present and fluid-absent melting processes are Downloaded from http://pubs.geoscienceworld.org/gsa/lithosphere/article-pdf/doi/10.2113/2020/8854615/5293682/8854615.pdf by guest on 01 October 2021 2 Lithosphere important for understanding the melt formation, migration scale. On the other hand, the metasedimentary rocks are to drainage, and crustal rheology during orogenesis [27]. mainly composed of interbedded pelitic and psammopelitic The Okbang amphibolite in the Yeongnam Massif, Korea gneisses together with a lesser amount of quartzite. In particu- (Figure 1) contains abundant tonalitic leucosomes with a lar, the metapelitic rocks define high dT/dP, Buchan metamor- variety of microstructures and provides a natural laboratory phic zones progressing from cordierite through sillimanite to for investigating the processes involved in fluid-present melt- garnet zones; the latter two zones are associated with wide- ing and melt migration. Based on the whole-rock geochemis- spread anatexis typified by the occurrence of abundant (up try and multigrain zircon U-Pb age, previous workers [28, to ~20 vol.% on the outcrop) cordierite-bearing leucosomes 29] suggested that the protolith of amphibolites formed in a and leucogranites [34, 35, 41]. The country rocks hosting the rift-related setting at ~1.92 Ga, but these authors failed to rec- Okbang amphibolite are strongly deformed, particularly in ognize partial melting in the amphibolite. In order to assess the vicinity of lithologic boundaries (Figures 3(a) and 3(b)). the role of fluid-present melting, we investigated a suite of Extensional (C′) shear bands typically occur as sets of closely amphibolites, neosomes, and host gneisses (for the general spaced ductile shear zones at millimeter to centimeter scales, terminology of migmatites; we followed the recommendation accompanied by partial melting and leucosome formation. of Sawyer [8]) to determine field relationships, bulk-rock Synkinematic melting and melt segregation may be accounted geochemistry, and zircon U-Pb ages using a sensitive high- for in the context of an extensional tectonic regime prevalent resolution ion microprobe (SHRIMP). In addition, the Lu- during the Paleoproterozoic in the southern Yeongnam Massif Hf isotopic compositions of zircon were analyzed to unravel [31, 33]. Whole-rock geochemical analyses revealed the rift- the crust-mantle interaction during the formation of felsic related, enriched mid-ocean ridge basalt (E-MORB) composi- melts in the amphibolite. Our results, combined with avail- tion of amphibolite [28], and the U-Pb zircon dating based able data, provide further insight into ~1.87–1.85 Ga tectono- on multigrain thermal-ionization mass spectrometry analysis magmatism in the Korean Peninsula and its linkage to the yielded a discordant date of 1918 ± 10 Ma [29]. In addition, prolonged Paleoproterozoic orogenesis of the North China precise SHRIMP U-Pb zircon age constraints are available for Craton [30–33]. the Buncheon granitic gneiss, i.e., magmatic crystallization at 1966 ± 16 Ma and subsequent metamorphism at 1862 ± 4 Ma 2. Geological Background [35, 42]. The latter is associated with late orogenic event which is widespread in the entire Korean Peninsula, including the The Korean Peninsula consists of three major Precambrian Yeongnam Massif [30, 36]. massifs (Nangrim, Gyeonggi, and Yeongnam) adjoined by the Gyeonggi Marginal Belt ([30], Figure 1(a)). The Yeong- 3. Field Relationships nam Massif is a polymetamorphic terrain bounded to the north by the Ogcheon Belt and unconformably overlain to the south- The Okbang amphibolite occurs as elongate lenses ranging in east by thick volcanic-sedimentary sequences of the Cretaceous widths from ~20 to 400 m that are stretched and folded on a Gyeongsang Basin. This massif is primarily composed of quart- map scale (Figure 2). Various lines of evidence for partial zofeldspathic gneiss, migmatitic gneiss, porphyroblastic gneiss, melting are present on the outcrops, and metatexitic amphib- andgraniticgneisstogetherwithlesseramountsofamphibolite olites are predominant (Figures 3 and 4; [8]). Penetrative fi and calc-silicate rocks [34]. Igneous protoliths of gneisses were foliations (S1) in the amphibolite are occasionally de ned largely emplaced at ~2.0–1.9 Ga in an arc-related environment by preferred orientation of hornblende neoblasts and layer- and subsequently affected by upper amphibolite to lower gran- parallel leucosomes; their attitudes are consistent with those ulite facies metamorphism at ~1.9–1.85 Ga (Figure 1(b); [30, measured in the Buncheon and metasedimentary gneisses 31, 33, 35, 36]). The latter event is associated with widespread generally striking northeast and dipping moderately to the partial melting to produce abundant leucosomes and garnet- northwest (Figure 2). Both stretching and mineral (horn- bearing leucogranites and perhaps best exemplified by the blende) lineations mostly plunge to the north. Towards the granulite-facies aureole around 1.87–1.86 Ga anorthosite-man- boundary with the Buncheon granitic gneiss, the amphibo- gerite-charnockite-granite (AMCG) suite in the southern lites are progressively deformed to yield high-strain zones Yeongnan Massif (Figure 1(a); [32, 33]). P - T conditions where leucosomes are highly stretched

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