Seismic Velocity Structure and Composition of Continental Crust in the Dabie Sulu Area
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Velocity data from five published seismic refraction profiles traversing the Dabie Sulu area are summarized. The data are combined with seismic velocity measurements of regional eclogites and associated middle and lower crustal rocks at pressures and temperatures up to 600 MPa and 600 ℃ to study the seismic velocity structure and composition of the region. The Dabie Sulu UHP metamorphic area shows a four layered crustal structure. Its crustal thickness is thinner than the average of Eastern China and global continental crust. Both the upper and middle crusts in the Dabie Sulu are dominated by felsic gneisses with eclogite 10%. Eclogite may increase in proportion with depth, making up 24 44% and 41 68% of the upper lower and lowermost crusts, respectively, in addition to felsic and mafic granulites. The amphibolite facies metamorphism and typical middle crustal velocity of the dominant felsic gneisses suggest a middle crustal exposure level for the majority of the Dabie Sulu.Keywords:
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The consistence between the first rapid cooling time (226-219 Ma) of the untrahigh pressure metamorphic (UHPM) rocks in the Dabie Mountains and the formation time (205-220 Ma) of the syncollisional granites in the Qinling and Sulu areas suggests that the first rapid cooling and uplift of the UHPM rocks may be related to breakoff of subducted plate. Therefore the second rapid cooling and uplift (180-170 Ma) of the UHPM racks needs a post-colli-sional lithosphere delamination which resulted in the granitic magmatism with an age of about 170 Ma. In addition, the rapid rising of the Dabie dome in the early Cretaceous (130-110 Ma) and the corresponding large-scale magmatism in the Dabie Mountains need another litho-sphere delamination. The geochronology of the post-collis-ional mafic-ultramafic intrusions and geological relationship between the mafic-ultramafic intrusions and granites suggest that partial melting was initiated in the mantle, and then progressively developed in the crust, suggesting a mantle u
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In the Dabie Mountains,more and more regional geological data show that the UHP continental rocks and the coesite- bearing zircons,omphicite-bearing garnets,etc.which have extensively been discovered within gneiss country rock were relics from a melted eclogitized continental slice and kept within the high-grade gneiss country rock which is composed of a suite of metamorphic and ductile deformed plutons intruded under Newprotozoic low to mid-grade metamorphic volcanic-sedimentary strata on the Yangtze northern margin.Based on these facts,we suggest that the eclogite-bearing gneiss is not an “UHP terrance” or “m lange” included eclogite fragments and its protolith should be a suite of magmatic diapirite produced by an eclogitized continental slice melting,and advance a new hypothesis for continental crust consumption and regeneration and model of the Dabie collision zone evolutiona,that is after deep-subducted continental margin underwent the HP to UHP eclogite-facies metamorphism at different depth in anthenosphere, whit the suhducted oceanic crust and continental mantle broken-off,the continental crust rapidly underplated under the obducted margin by buoyancy and took place part melt as rapidly decompression and mantle-heat input,and the melted mass replaced “former root” of the margin and diaper penetrated under the metamorphic strata,and at the same time of crystallizing,it experienced coordinating metamorphism and deformation with its depth to produce a high-grade orthrgneiss,that is “regenerated crust”.
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Structural and thermochronological data from the Sulu terrane of eastern China document the exhumation of high-pressure and ultrahigh-pressure (UHP) metamorphic rocks to upper-crustal depths by ca. 206 Ma. 40Ar/39Ar ages from K-bearing phases record recrystallization and cooling through amphibolite- and upper greenschist–facies conditions during top-to-the-NW noncoaxial shear and suggest cooling rates on the order of 55 °C/m.y. Integration of the 40Ar/39Ar data with U/Pb constraints on the timing of UHP metamorphism yields vertical exhumation rates of >6 mm yr−1 from mantle to upper-crustal depths. Qualitative thermal models of metamorphic K-feldspars suggest transient reheating...
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New U/Pb zircon and Th/Pb monazite ages are presented from the giant Sulu ultrahigh‐pressure (UHP) terrane. Combined with Sm/Nd ages, Rb/Sr ages, inclusion relationships, and geologic relationships, they help define the timing of peak recrystallization, the timing of subsequent amphibolite‐facies metamorphism, and the architecture of the Dabie‐Sulu suture zone between the collided Sino‐Korean and Yangtze cratons. The data indicate a ∼15 Myr record of UHP recrystallization, the first clearly documented for a giant UHP terrane; this requires that continental subduction in the Dabie‐Sulu orogen involved multiple UHP tectonic or recrystallization events. A 244–236 Ma “precursor” UHP event, seen only in the Dabie Shan, was followed by a second, ∼230–220 Ma “main” UHP event, which was itself terminated by a 220–205 Ma amphibolite‐facies overprint. Older eclogite‐facies events seen in the Qinling segment of this orogenic belt raise the possibility that these rocks have undergone (U)HP metamorphism three or four times, but at present, there is no geochronological evidence in the Dabie‐Sulu area to support this. The subduction of the lower, Yangtze plate did not proceed in a simple fashion: The ages of inherited zircon cores demonstrate that a ribbon continent of Yangtze affinity escaped subduction and became wedged against the Sino‐Korean plate hanging wall.
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Rare granulitized eclogites exposed in the eastern Himalaya provide insight into conditions and processes deep within the orogen. Sensitive high-resolution ion microprobe (SHRIMP) U-Pb, Ti, and rare earth element (REE) data from zircons in mafic granulitized eclogites located in the upper structural levels of the Greater Himalayan Sequence in Bhutan show that zircon was crystallized under eclogite-facies metamorphic conditions between 15.3 ± 0.3 and 14.4 ± 0.3 Ma, within a couple million years of the later granulite-facies overprint. In conjunction with pressure estimates of the eclogite- and granulite-facies stages of metamorphism, the age data suggest that initial exhumation occurred at plate-tectonic rates (cm yr–1). These extremely rapid synconvergence exhumation rates during the later stages of the India-Asia collision require a revision of theories for the transportation and exhumation of crustal materials during continental collisions. In contrast to western Himalayan examples, the eastern Himalayan eclogites cannot be tectonically related to steep subduction of India beneath Asia. Instead, they more likely represent fragments from the base of the overthickened Tibetan crust. Based on the zircon age and trace-element data, we hypothesize that the protolith of the mafic granulites was middle Miocene mafic intrusions into the lower crust of southern Tibet, linked to Miocene volcanism in the Lhasa block. We suggest that a transient tectonic event—possibly the indenting of a strong Indian crustal ramp into crust under southern Tibet that had been weakened by partial melting—may have promoted exhumation of the eclogitized lower crust under Tibet. The mafic magmatism and volcanism themselves may have been related to the convective thinning of the lithospheric mantle triggered by a reduction in the India-Eurasia convergence rate during the middle Miocene, which in turn could have facilitated the rapid extrusion of the lower crust over the earlier-exhumed middle crust.
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