Danling Chen

Implications Based on LA-ICP-MS Zircon U-Pb Ages of Eclogite and Its Country Rock from Jianggalesayi Area, Altyn Tagh, China

Abstract On the basis of Cathodoluminescence (CL) images and LA-ICP-MS in situ trace element analyses of zircons for finding the metamorphic age of eclogite and its country rock (garnet-biotite gneiss) from the Jianggalesayi area, Altyn Tagh, China, it is found that the zircon grains in eclogite mostly show homogeneous internal texture, and that few grains have residual cores, showing the typical magmatic origin characterized by the enrichment in HREE and high Th/U ratios generally higher than 0.4. On the other hand, the zircon rims and the homogeneous grains show that the REE pattern is flat or slightly depleted in HREE, which is the typical of metamorphic zircons coexisting with garnet. In addition, the zircon grains in garnet-biotite gneiss display a core-mantle-rim zoning texture, where the cores show the features of detrital zircons, and the mantles show the metamorphic features of zircons coexisting with garnet. By the LA-ICP-MS in situ dating of zircons, the metamorphic ages of eclogite, garnet-biotite gneiss, and the protolith of eclogite are calculated as 493 ± 4.3 Ma, 499 ± 27 Ma, and 754 ± 9 Ma, respectively. The metamorphic age of eclogite is almost the same as that of its country rock paragneiss and is about 250Ma later than its protolith age, which indicates that the ultrahigh-pressure (UHP) eclogite is the product of continental crust deep subduction.

Petrology and geochronology of ultrahigh-pressure granitic gneiss from South Dulan, North Qaidam belt, NW China

abstract An integrated study including petrography, mineral chemistry, metamorphic P–T path modelling, and zircon U–Pb dating was conducted on a granitic gneiss and enclosed eclogite from South Dulan, North Qaidam UHP (ultrahigh-pressure) belt. The result shows that the granitic gneiss underwent a clockwise P–T path with a peak-P stage at 655–745°C, 30–34 kbar, and a subsequent peak-T stage at 815–870°C, 14–18 kbar, which is similar to the P–T estimates reported for coesite-bearing continental-type eclogites in this region. The enclosed eclogite resembles an olivine–pyroxene-rich cumulate in Qaidam block. It has a similar prograde P–T path with the country gneiss and experienced a peak-P stage of 682–748°C at 27–34 kbar. Zircon U–Pb dating yields an eclogite-facies metamorphic age of 447 ± 2 Ma for the granitic gneiss and 445 ± 6 Ma for the enclosed eclogite. These ages agree with metamorphic ages obtained from paragneisses (427–439 Ma), coesite-bearing continental-type eclogites (430–451 Ma), and UHPM (ultrahigh-pressure metamorphic) oceanic crust–mantle sequence (440–445 Ma) from South Dulan, as well as UHP eclogites, garnet peridotite, and gneisses from other units (460–420 Ma) within this belt reported by others. Similar metamorphic ages as well as P–T evolution documented in gneisses and intercalated eclogites imply that both rocks experienced a coeval UHP event. Summarizing all the published geochronology data, we argue that the North Qaidam UHP belt was mainly formed by continental deep subduction at ~460 to ~420 Ma. The UHPM oceanic crust-mantle sequence in South Dulan may represent oceanic lithosphere in the transition zone between oceanic and continental crust, which was dragged upward by the exhumed continental rocks after break-off of the dense oceanic crust.

Metamorphic evolution of a newly identified Mesoproterozoic oceanic slice in the Yuka terrane and its implications for a multi-cyclic orogenic history of the North Qaidam UHPM belt

Funding information National Natural Science Foundation of China, Grant/Award Number: 41430209, 41472053, 41421002; National Basic Research Program of China (973 Program), Grant/Award Number: 2015CB856103; Program for Changjiang Scholars and Innovative Research Team in University, Grant/Award Number: IRT1281; MOST Special Found from the State Key Laboratory of Continental Dynamics, Grant/Award Number: 201210133

The Felsic Vein within the Garnet Pyroxenite from Shenglikou, North Qaidam: Episodic Fluid Flow During the Exhumation of the Rock

China, and is classified as a Paleozoic HP–UHP metamorphic zone (Chen et al., 2008, 2009; Song et al., 2003, 2004, 2005; Yang et al., 2001; Zhang et al., 2008a, 2009a, b). Although the metamorphism, geochronology and subduction background of the HP–UHP rocks of the North Qaidam have already been thoroughly studied, the characteristics, the behavior and activity of fluid during the subduction and exhumation process in this zone have yet to be researched thoroughly. In the present study, felsic veins were found in the garnet (Grt) pyroxenite from the Shenglikou area, in the North Qaidam. The Grt pyroxenite near the vein is strongly amphibolized into the Grt amphibolite. Thus, a combined study of the petrology, geochemistry and geochronology of the felsic vein, as well as for the sideward Grt amphibolite and its host Grt pyroxenite is performed. The study results provide constraints on the ages of fluid flow within the Shenglikou terrane, as well as on the origin of the fluid for the vein, which may play a key in deciphering the fluid processes in subduction zones. Geochemistry and chronology data indicate that the protolith of the Grt pyroxenite was basalt from a continental setting, and formed in the Neo–proterozoic (909±6 Ma) period. Petrographical, mineral chemical and geochronological studies imply that the Grt pyroxenite experienced a peak eclogite–facies (775–810 °C and >1.8 GPa) metamorphism at 440 Ma, subsequent granulite– facies retrograded metamorphism (774–814 °C and 1.07– 1.24 GPa) at 420 Ma, and finally amphibolite–facies (619–694 ° C and 0.55–0.68 GPa) metamorphism, suggesting that the Neo–proterozoic protolith of the rock experienced continental subduction and subsequently subjected to two stages of exhumations (Fig. 1). The formation age of the felsic vein in the Grt pyroxenite is yielded at 422±2 Ma (2σ), which is identical to the granulite–facies retrograded age (420 Ma) of the host Grt pyroxenite, suggesting that the principal vein–forming corresponds to the first exhumation stage of the rock. The fact that the Hf isotope compositions between the granulite–facies (type I) zircon rims from the host Grt pyroxenite and the zircon from the felsic vein are identical suggests that the fluid for veining is either locally sourced or internally buffered. The felsic veins have high contents of SiO2, Al2O3, Na2O, CaO and Sr, indicating that there are significant amounts of Na, Si, Ca, Al and Sr in the vein–forming fluid. Therefore, the dehydration of Omp is interpreted to be the dominated mechanism for releasing the fluid leaving the low-Na Cpx and Pl1 as the residual phase in the Grt pyroxenite. In addition, the strong amphibolization of the Grt amphibolite near the felsic vein, as well as the compositional variation of the entire rock and amphiboles between the Grt pyroxenite and Grt amphibolite, the coarse–grained titanite occurring in the Grt amphibolite, the Kfs micro–veins in the Grt amphibolite and felsic vein, and the presence of biotite and muscovite in the vein, all indicate that a low flux of external pelite–derived fluid with high K, LREE, LILE and silica contents was added and transported along the vein, where it interacted with the host Grt pyroxenite. It is possible that this external fluid migration occurred before and continued until after the amphibolite–facies stage. Therefore, it is shown that episodic fluid flow occurred during the exhumation of the Grt pyroxenite, and that the primary internal fluid for the felsic veining flowed at the transformation from peak eclogite stage to granulite stage (Fig. 1 I to II), then low flux external fluid was added before the amphibolite stage, corresponding to the final stage of the fluid flow (Fig. 1 II to III).