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

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.

Tectonic Setting of the Eastern Margin of the Sino-Korean Block in the Pennsylvanian: Constraints from Detrital Zircon Ages

To test the previous hypothesis that upper Paleozoic sediments in the eastern Sino-Korean Block were mostly derived from the paleo-orogen located to the east, we compared published and new U–Pb age data of detrital zircons from Pennsylvanian strata distributed in the Sino-Korean Block (SKB). The age distributions of detrital zircons from different localities of Pennsylvanian strata in North China reflect varying contributions from the Inner Mongolia Paleo-uplift in the north and the Central China Orogenic Belt in the south. The supply of detritus from the northern source to distant areas, however, appears to have been limited during the Pennsylvanian times. The age distributions of detrital zircons from Korean Pennsylvanian strata located in the east of the SKB are characterized by a dense cluster of 1.84–1.90 Ga and differ from those of North China. The Korean age characteristic is best explained by strong influences of the detritus derived from the Paleoproterozoic Yeongnam Massif in southeastern Korea. Along with the significant number of zircons that record syn- to near-depositional magmatic activities, this observation supports the hypothesis of the existence of an active continental margin setting in the east of the SKB.

Heterogeneous modification and reactivation of craton margin in northeast Asia: insight from teleseismic traveltime tomography of the Korean Peninsula

<p>Margins of craton lithosphere are prone to ongoing modification process. Marginal tectonism such as slab subduction, continental collision, and mantle dynamics significantly influence properties of lithosphere in various scales. Thus, constraints on the detailed properties of craton margin are essential to understand the evolution of continental lithosphere. The eastern margin of the Eurasian plate is a natural laboratory that allows us to study the strong effects from multiple episodes of continental collision and subduction of different oceanic plates since their formation. Extensive reworking and destruction of the cratonic lithosphere mainly occurred in eastern China during the Mesozoic to Cenozoic, which leaves distinct geochemical and geophysical signatures. Specifically, the Korean Peninsula (KP) is known to consist of Archean–Proterozoic massifs (e.g., Gyeonggi, Yeongnam Massif) located in the forefront in northeast Asia, where current dynamics in the upper mantle and effects due to nearby subducting slabs are the most significant.</p><p>Here we present, for the first time in detail, 3-D velocity structure of KP by teleseismic body wave traveltime tomography. Detailed P-wave and S-wave images of the crust and upper mantle were constructed by approximately 5 years of data from dense arrays of seismometers. We newly found a thick high-velocity body beneath the southwestern KP with a thickness of ~150 km, which is thought as a fragment of lithospheric root beneath the Proterozoic Yeongnam Massif. Also, we found low velocities beneath the Gyeonggi Massif, eastern KP margin, and Gyeongsang continental arc-back-arc system, showing significant velocity contrasts (dlnVp of ~4.0% and dlnVs of ~6.0%) to the high-velocity structure. These features indicate significantly modified regions. In addition, there was a clear correlation of the upper mantle low-velocity anomalies and areas characterized by Cenozoic basaltic eruptions, high heat flow, and high tomography, suggesting that there are close associations between mantle dynamics and recent tectonic reactivation.</p><p>The presence of a remnant cratonic root beneath the KP and contrasting lithospheric structures across the different Precambrian massifs suggests highly heterogeneous modification along the Sino-Korean craton margin, which includes the KP and North China Craton. A striking localization of lithosphere modification among the different Precambrian massifs within the KP suggests that the structural heterogeneity of the craton margin is likely sharp in scale and thickness within a confined area. We suggest that intense interaction of upper mantle dynamics and inherent structural heterogeneities of a craton margin played an important role in shaping the current marginal lithosphere structure in northeast Asia.</p>

Linking high-temperature metamorphism, charnockite formation, and fluid-rock interaction during the waning stage of Paleoproterozoic hot orogeny in the Yeongnam Massif, Korea

<p><strong>Incipient charnockites are orthopyroxene-bearing granitic gneisses that are commonly considered to be a product of infiltration of CO<sub>2</sub>-rich fluids during high temperature dehydration in the granulite terrane. Greenish patches of incipient charnockite are locally present and hosted by granitic gneiss in the Sancheong-Hadong anorthosite complex, southern Yeongnam Massif. Both lithologies are foliated and show a variety of field evidence for partial melting and melt crystallization. Granitic leucosomes and biotite or garnet-rich residua are ubiquitous along ductile shear bands and/or penetrative foliations in the gneiss. These melt-related features are consistent with mineral assemblages and reaction textures, characterized by biotite-breakdown melting. Based on phase equilibria modeling, P-T conditions of peak metamorphism are constrained at </strong><strong>3.5–8.5 kbar and 770–840 </strong><strong>°</strong><strong>C. S</strong><strong>ensitive high-resolution ion microprobe </strong><strong>U-Pb analyses of inherited cores and overgrowth rims of zircon from a charnockite yielded the weighted mean <sup>207</sup>Pb/<sup>206</sup>Pb ages of 1880 </strong><strong>±</strong><strong> 5 Ma and 1861 </strong><strong>±</strong><strong> 4 Ma, which are interpreted as the times for magmatic crystallization and subsequent anatexis of granitic protolith, respectively. This timeline is consistent with that determined from the host granitic gneiss. In contrast, monazite grains from the charnockite and granitic gneiss yielded the weighted mean <sup>207</sup>Pb/<sup>206</sup>Pb ages of 1842 </strong><strong>±</strong><strong> 8 Ma and 1838 </strong><strong>±</strong><strong> 18 Ma, respectively, suggesting that an influx of aqueous fluid took place ~20 m.y. after the crystallization of granitic melt. Both charnockitic and granitic gneisses underwent high-temperature metamorphism and partial melting at ~1.86 Ga, and were followed by fluid influx at ~1.84 Ga, apparently characterized by monazite recrystallization in association with the retrogression of orthopyroxene to ferromagnesian amphibole-rich aggregates in the former. Thus, the timing and conditions of high-temperature metamorphism, charnockite formation, and fluid flow suggest that the granulite-facies metamorphism and fluid-rock interaction is linked to the waning stage of Paleoproterozoic hot orogenesis in the Yeongnam Massif.</strong></p>