Abstract
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.
Highlights
Partial melting plays an important role during continental collision and crustal evolution [1,2] because it significantly influences continental crust differentiation and subducted crust exhumation [3,4,5], and even leads to orogenic collapse [6,7]
Major and trace element contents of the four groups of leucosomes, melanosomes and the two types of migmatite samples selected from the North Dabie complex zone (NDZ) are presented in Supplementary Table S1 and illustrated in Figures 5 and 6
Geochronological data reported in this study show that zircons from the different generations of leucosomes retain complex age records related to the NDZ evolution
Summary
Partial melting (anatexis) plays an important role during continental collision and crustal evolution [1,2] because it significantly influences continental crust differentiation and subducted crust exhumation [3,4,5], and even leads to orogenic collapse [6,7]. Partial melting of crustal rocks in collisional orogens occurs during the initial stages of exhumation and/or during post-orogenic collapse [1,5,18,19,20,21], mainly involving in decompression and/or pushing rock packages up the geotherm. Thereby, the identification, characterization and accurate dating of leucosomes are crucial for revealing the nature of the post-collisional evolution of an orogen involving the exhumation of deeply subducted slices, as well as for the mountain-root collapse [6,7]
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