| Summary: | 博士 === 國立臺灣大學 === 地質學研究所 === 87 === The Dabie orogenic belt is located in the eastern portion of the Qinling-Tongbai-Hong''an-Dabie orogenic belts in central China, and was formed by a continental collision between the Sino-Korean and Yangtze cratons. This orogenic belt has attracted many earth scientists for spectacular occurrence of ultrahigh-pressure (UHP,25-40 kbar) minerals, such as coesite and probable microdiamond, in metamorphosed crustal rocks especially in the Dabie Shan and Sulu terranes. The discovery of UHP phases in an orogenic belt evidently indicates that continental crust was subducted into mantle depths (100 km), and afterwards exhumed back to the surface, while UHP minerals are surprisingly well preserved. In the past ten years, intensive studies on the Dabie orogen have been carried out. Most of these studies were devoted to the determination of pressure-temperature paths through different thermobarometers including mineral assemblages or to applying some tectonic models by tectonostratigraphic and/or paleomagnetic analyses. Despite of such a large number of intensive works, controversies about the tectonic evolution of the Dabie-Sulu orogen still exist, largely due to the lack of critical geochronological and/or structural constraints. In an attempt to better reconstruct the unroofing history of the Dabie UHP terrane, and in order to provide better constrains and help decipher the tectonic evolution of this UHP terrane, this study has performed a systematic Ar-Ar dating and geochemical analyses on gneisses, dyke rocks and volcanics across the Dabie orogen.
The measured ages of minerals separated from gneisses, such as hornblendes, phengites, biotites and K-feldspars, cluster in three periods: (a) ~243-168 Ma, (b) ~168-130 Ma, and (c) ~130-100 Ma. The ages of 243-168 Ma may represent cooling of the metamorphic rocks after peak (UHP) metamorphism, during exhumation. As a mineral with highest (argon) closure temperature, hornblende records the oldest ages (254-202 Ma) among the samples. Presumably, hornblende 40Ar/39Ar ages should be closest to the age of peak metamorphism as compared with those found in micas and K-feldspars. Nevertheless, the general agreement between the hornblende 40Ar/39Ar ages and previously reported U-Pb and Sm-Nd ages of 240-209 Ma may further confirm the Triassic age for the peak metamorphism. The age spans of 168-130 and 130-100 Ma may not be related to specific metamorphic-magmatic events, but represent the cooling ages through different argon isotopic clocks in micas and K-feldspars, during unroofing of the terrane. It is surprising that hornblendes (DB19 and DB44) from North Dabie Complex yielding late Triassic ages (220-236 Ma) since the Triassic ages have never been reported for the metamorphic rocks in North Dabie Complex. This finding would imply that the unit may have been part of the (coeval) Triassic subduction-collision sequence which was only reported to be widely recorded in South Dabie Complex. On the other hand, late Jurassic to early Cretaceous ages of the dyke rocks at ~150-100 Ma, may suggest the ages of magmatic events in the terrane. It is worthy to note that the obtained 40Ar/39Ar ages for volcanic rocks are perfectly consistent with those of granitic plutons and gabbroic intrusives in the same terrane dated by others. Thus, a regional-scale, Jurassic-Cretaceous, magmatic activity can therefore be well defined.
Based on the 40Ar/39Ar plateau ages and results of closure temperature calculations, cooling paths for the tectonic units in the Dabie Shan terrane can be reconstructed. Assuming a linear T-t correlation, three stages of cooling for the Dabie terrane are evident: (a) a "fast" cooling (5.6-17.7 0C/Ma) in the period of 241-190 Ma, (b) a relatively slow cooling (1.3-2.7 0C/Ma) at 190-140 Ma, and (c) another fast cooling (5.0-9.8 0C/Ma) during 140-100 Ma. The thermochronological data evidently imply that the Dabie Shan terrane was exhumed as a "single" tectonic block. Geochemical data reveal that these igneous rocks are shoshonitic (high-potassium) and bi-modal (felsic-mafic) in nature, with characteristic features of light rare earth element enrichment and negative Nb, Ta, and Ti anomalies in the spidergrams, which indicate possible contamination from the lithosphere mantle underneath. These geochemical features are quite similar to those of "post-orogenic" magmatic rocks in an extensional tectonic regime, resulted from partial melting of the lithosphere. Since these post-orogenic magmatism appears to occur simultaneously with the final-stage exhumation of the Dabie orogen, it is thus proposed that the Jurassic-Cretaceous extensional tectonism in the Dabie terranes may have played an important role in causing the exhumation of the ultrahigh-pressure (UHP) metamorphic rocks from the middle crustal depths to the surface.
In summary, based on the inferred three-stage cooling history and geochemical features, the exhumation of the Dabie UHPM terrane emcompass the formerly compressional and the latter extensional tectonic processes. During Triassic to early Jurassic, continnental crust was subducted to mantle depth resulting in the ultrahigh pressure metamorphism and afterward been exhumed to the middle or lower crust under the compressional tectonic regime. From early Jurassic to early Cretaceous, the UHPM terrane stopped in the middle or lower crustal depth. On late Jurassic or early Cretacous, there is a large scale magmatic activities in the oregen that resulted from partial melting of the lithosphere due to "lithospheric delamination", "removal of thickened lithospheric mantle" or "slab breakoff" during mountain collapse. Since the magmatism appears to occur simultaneously with the final-stage exhumation of the Dabie orogen, it is thus proposed that the Jurassic-Cretaceous extensional tectonism in the Dabie terranes may have played an important role in causing the exhumation of the ultrahigh-pressure (UHP) metamorphic rocks from the middle crustal depths to the surface.
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