Zircon U-Pb Dating and Petrogenesis of Multiple Episodes of Anatexis in the North Dabie Complex Zone, Central China

Abstract

<jats:p>The North Dabie complex zone (NDZ), central China, is a high-T ultrahigh-pressure (UHP) metamorphic terrane. It underwent a complex evolution comprising of multistage metamorphism and multiple anatectic events during the Mesozoic continental collision, characterized by granulite-facies overprinting and a variety of migmatites with different generations of leucosomes. In this contribution, we carried out an integrated study including field investigation, petrographic observations, zircon U-Pb dating, and whole-rock element and Sr-Nd-Pb isotope analysis for the migmatites in the NDZ and their leucosomes and melanosomes. As a result, four groups of leucosomes have been recognized: Group 1 (garnet-bearing leucosome), strongly deformed leucosomes with coarse-grained peritectic garnet; Group 2 (amphibole-rich leucosome), weakly deformed to undeformed amphibole-rich leucosomes with coarse-grained peritectic amphibole and no garnet; Group 3 (amphibole-poor leucosome), weakly deformed to undeformed amphibole-poor leucosomes with minor fine-grained amphibole; Group 4 (K-feldspar-rich leucosome), K-feldspar-rich leucosomes mainly composed of coarse-grained quartz, plagioclase and K-feldspar. Zircon SHRIMP and LA-ICPMS U-Pb dating suggest that the Group 1 leucosomes formed at 209 ± 2 Ma whereas the rest of the leucosome groups (Groups 2–4) occurred between 145–110 Ma, in response to decompression under granulite-facies conditions during the early stage of exhumation, and to heating during post-orogenic collapse, respectively. Furthermore, the garnet-bearing leucosomes were resulted from fluid-absent anatexis related to biotite dehydration melting, while the other three groups of leucosomes were formed during large-scale fluid-present partial melting and coeval migmatization. This migmatization comes from heating from the mountain-root removal and asthenosphere upwelling, together with the influx of fluids derived from country rocks at mid-upper crustal levels. However, all the leucosomes and melanosomes display Pb-isotopic compositions similar to those observed for the NDZ UHP rocks (eclogites and granitic gneisses), suggesting a common source from the Triassic subducted Neoproterozoic lower-crustal rocks. In addition, the Cretaceous partial melting and migmatization began at 143 ± 2 Ma with three age-peaks at 133 ± 3 Ma, 124 ± 3 Ma and 114 ± 7 Ma, respectively.</jats:p>

New constraints on P–T–t path of high–T eclogites in the Dabie orogen, China

“…1). The NDZ comprises mainly tonalitic and granitic orthogneiss (Liu et al, 2000(Liu et al, , 2005(Liu et al, , 2007bXie et al, 2006;Yang et al, 2020;Zhao et al, 2004Zhao et al, , 2007, with subordinate meta-peridotite, garnet pyroxenite, garnet-bearing amphibolite, granulite and eclogite, which occur as lenses or blocks within the gneiss. Among these lithologies, the gneisses and eclogites were considered to be of lower-crustal origin, indicating (Yang et al, 2020) that their protoliths were relatively devoid of water (Liu et al, 2005(Liu et al, , 2007a(Liu et al, , 2011a(Liu et al, , 2011b(Liu et al, , 2014.…”

“…The NDZ comprises mainly tonalitic and granitic orthogneiss (Liu et al, 2000(Liu et al, , 2005(Liu et al, , 2007bXie et al, 2006;Yang et al, 2020;Zhao et al, 2004Zhao et al, , 2007, with subordinate meta-peridotite, garnet pyroxenite, garnet-bearing amphibolite, granulite and eclogite, which occur as lenses or blocks within the gneiss. Among these lithologies, the gneisses and eclogites were considered to be of lower-crustal origin, indicating (Yang et al, 2020) that their protoliths were relatively devoid of water (Liu et al, 2005(Liu et al, , 2007a(Liu et al, , 2011a(Liu et al, , 2011b(Liu et al, , 2014. Different from the CDZ and the SDZ, the NDZ experienced pervasive post-peak granulitefacies overprinting during the initial stage of exhumation (Deng et al, 2019;Groppo et al, 2015;Liu et al, 2005Liu et al, , 2007aLiu et al, , 2011aLiu et al, , 2011bYang et al, 2020), prior to amphibolite-facies retrogression.…”

“…Among these lithologies, the gneisses and eclogites were considered to be of lower-crustal origin, indicating (Yang et al, 2020) that their protoliths were relatively devoid of water (Liu et al, 2005(Liu et al, , 2007a(Liu et al, , 2011a(Liu et al, , 2011b(Liu et al, , 2014. Different from the CDZ and the SDZ, the NDZ experienced pervasive post-peak granulitefacies overprinting during the initial stage of exhumation (Deng et al, 2019;Groppo et al, 2015;Liu et al, 2005Liu et al, , 2007aLiu et al, , 2011aLiu et al, , 2011bYang et al, 2020), prior to amphibolite-facies retrogression. Fresh eclogites are generally preserved in the core of eclogite lenses, whereas they are retrogressed into garnet granulites, garnetbearing and garnet-absent amphibolites towards the margin, suggesting that external fluid from the country gneisses played a key role in the retrogression, especially the amphibolization of the eclogites.…”

“…On the contrary, the protracted high-T decompression and relatively scarcity of water have made the NDZ eclogites ideal containers of various retrograde mineral assemblages and microstructures, and thus provide an excellent opportunity to construct their retrograde P-T-t path. Later, the NDZ experienced heating, melting and widespread migmatization at Cretaceous, which are generally considered to be ascribed to the post-collisional mountain root removal (He et al, 2013;Li et al, 2013;Liu et al, 2007bLiu et al, , 2015aLiu et al, , 2019Wang et al, 2013;Yang et al, 2020). The Cretaceous migmatization has affected mainly the NDZ granitic gneisses, whereas its influence on the NDZ eclogites has not been well understood yet.…”

Editorial for Special Issue “U-Pb Dating and Chemistry of Zircon in Metamorphic, Magmatic and Sedimentary Rocks”

“…The collected papers within the theme "Metamorphic continental crust" are dedicated to case studies regarding metamorphic basements for geological reconstruction of tectonic events forming orogenic belts. The ability of zircon to monitor the metamorphic evolution of high-grade basements affected by granulite metamorphism is revealed by Yang et al [2] and Takehara et al [3]. Zircon age clusters detected in leucosomes and melanosomes, together with the definition of mineral phase equilibrium during anatexis, can constrain the tectonic evolution of orogeny and demonstrate how decompression plays a relevant role in crustal melting.…”