The Heilongjiang Complex is mainly composed of high-P/T type metamorphic rocks of blueschists and pelitic schists, and is distributed along the western margin of the Jiamusi Massif in NE China. LA-ICP-MS U-Pb ages of detrital zircons in pelitic schist (LG1-3) from the Mudanjiang district give a weighted mean age of 250.6 ± 2.3 Ma (MSWD = 2.2). Pelitic schist from the Yilan district (09YL10-1) contains 193-348 Ma and 392-561 Ma detrital zircons, with minor amounts of 783-987 Ma zircons. The weighted mean ages of the youngest detrital zircon age group are 230.6 ± 3.5 Ma (MSWD = 1.2; LG1-3) and 199.1 ± 3.1 Ma (MSWD = 1.0; 09YL10-1), and these constrain the maximum depositional age of the protoliths of the Heilongjiang pelitic schists. Phengites from pelitic schists in the Yilan (422YQ-3) and Luobei districts (408HB-1) yielded 40Ar/39Ar plateau ages of 179.9 ± 0.8 Ma and 164.7 ± 0.2 Ma, respectively. A 40Ar/39Ar phengite age of 189.8 ± 0.8 Ma as total gas age (apparent age distribution from 183 Ma to 196 Ma) was also obtained for Yilan garnet-barroisite schist (423YJ-1). Reliable geochronological data suggest that a paleo-ocean located between the Jiamusi Massif and the Songnen Massif to the west was still present at least up to 199-231 Ma, and subduction-related high-P/T type metamorphism occurred during the Jurassic at 145-184 Ma.
The Heilongjiang Complex in northeastern China is composed mainly of alternating blueschists and pelitic schists which underwent high-P/T type metamorphism of the epidote blueschist facies. Phengites in the pelitic schists from two areas in the Heilongjiang Complex yielded 40Ar/39Ar plateau ages of 165 ± 0.8 Ma (Mudanjiang area) and 171 ± 0.7 Ma (Yilan area). Phengites in blueschists from the Yilan area give 40Ar/39Ar isochron ages of 145 ± 0.9 Ma and 146 ± 1.3 Ma. The previously reported peak metamorphic temperature of 350-450 °C for the Heilongjiang Complex is close to the Ar retention temperatures of white mica, suggesting that the ages reported here closely date the metamorphic culmination in this complex. These ages indicate that a widespread high-P/T metamorphism occurred in the Heilongjiang Complex at 165-171 Ma. Subsequent to this period, a similar type of metamorphism continued at least until 145-146 Ma in the Yilian area.
<p>A-type subduction is considered to occur at the final stage of continent-continent collision. In many cases, the UHP/HP metamorphic conditions are well known but data on the type of subducted continental crust is lacking. In terms of end members, the type of subducted crust is either (1) normal thick continental crust or (2) the crust from the center of a rift zone, which is influenced by strong extension, high-temperature metamorphism due to thinning of even the continental mantle lithosphere and strong magmatism. To resolve these alternative scenarios, we investigated the southernmost part of the Eclogite-Gneiss Unit (EGU) of Cretaceous metamorphic age exposed in the Pohorje Mountains in Eastern Alps. There, UHP eclogites and ultramafic mantle rocks are exposed in a matrix of paragneiss and hitherto undated granitic orthogneises (Kirst et al., 2010). This study presents, for the first time, geochronological and geochemical data from newly discovered Permian granitic orthogneisses in this area. LA-ICP-MS zircon U&#8211;Pb ages of the orthogneisses are 255&#177;2.2 Ma and 260&#177;0.81 Ma, which are interpreted as the age of zircon crystallization in a magma. In contrast, all rounded zircons from paragneissic rocks give Cretaceous ages (89.34&#177;0.69 Ma and 90.8&#177;1.2 Ma), considered as the age of UHP/HP metamorphism. These zircons overgrew older zircons of Permian and rare older ages tentatively indicating that the metasedimentary could be not older than latest Permian. Zircon &#949;Hf(t) values of the four ortho- and paragneisses with (<sup>176</sup>Hf/<sup>177</sup>Hf) initial from 0.282201 to 0.282562, T<sub>DM2</sub> are Proterozoic (1390~1970 Ma). The granitic orthogneisses show the geochemical features (high (La/Lu)<sub> N</sub> ratios (160.3&#8211;307.3), strong negative Eu anomalies) of an evolved granite molten from continental crust. This type of orthogneisses could be considered as the source magma of seemingly rootless Late Permian to Triassic pegmatites (Knoll et al., 2018) widespread within the EGU further to the north. The paragneisses are heterogeneously composed and are associated with eclogites and ultramafic cumulates of oceanic affinity (De Hoog et al., 2011). We argue that the Permian granitic orthogneisses might be derived from partial melting of lower crust in a rift zone. We consider, therefore, this segment of the EGU as part of the distal Late Permian rift zone, which finally led to the opening of the Meliata Ocean during Middle Triassic times. If true, the new data also imply that the stretched continental crust was potentially not much wider than ca. 100 km, was subducted and then rapidly exhumed during early Late Cretaceous times.</p><p>&#160;</p><p><strong>References</strong></p><p>De Hoog, J.C.M., Jan&#225;k, M., Vrabec, M., Hatton, K.H., 2011. In: Dobrzhinetskaya, L., Faryad, S.W., Wallis, S., Cuthbert, S. (Eds.), Ultrahigh-pressure Metamorphism: 25 Years After the Discovery of Coesite and Diamond. Elsevier Insights, pp. 399&#8211;439.</p><p>Jan&#225;k, M., Froitzheim, N., Yoshida, K., Sasinkov&#225;, V., Nosko, M., Kobayashi, T., Hirajima, T., Vrabec, M., 2015. Journal of Metamorphic Geology 33, 495&#8211;512.</p><p>Kirst, F., Sandmann. S., Nagel. T., et al. 2010. Geologica Carpathica 61(6), 451-461.</p><p>Knoll, T., Schuster, R., Huet, B., Mali, H., Onuk, P., Horschinegg, M., Ertl, A., Giester, G., 2018. Canadian Mineralogist 56, 489-528.</p>
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