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Yongfei Zheng - One of the best experts on this subject based on the ideXlab platform.

  • episodic fluid action during exhumation of deeply subducted continental crust geochemical constraints from zoisite Quartz Vein and host metabasite in the dabie orogen
    Lithos, 2012
    Co-Authors: Renxu Chen, Yongfei Zheng, Zhaochu Hu
    Abstract:

    Abstract A combined study of mineral compositions and zircon U–Pb ages was carried out for a zoisite―Quartz Vein and its host retrograded eclogite in the Dabie orogen. The results reveal two episodes of fluid action during exhumation of the deeply subducted continental crust. The Vein has a mineral paragenesis of Quartz + zoisite + epidote + white mica + garnet + rutile + zircon + apatite, which is almost the same as that of the host retrograded eclogite characterized by symplectites after omphacite and/or garnet in the eclogite. Residues of magmatic zircon in the Vein are similar to those in the eclogite, suggesting local sources of the metamorphic fluid for Veining and physical transport of the tiny accessory mineral from the host rock to Veins. There are significant differences in major and trace elements between Vein-forming minerals, which are attributed to two episodes of fluid action for Veining. Metamorphic zircon from the Vein yields concordant U–Pb ages of 215 ± 4 to 218 ± 4 Ma. Trace element analysis indicates its precipitation from aqueous fluid during two distinct episodes under eclogite-facies and amphibolite-facies conditions, respectively. Ti-in-zircon and Zr-in-rutile thermometries yield variable temperatures from 537 to 683 °C. Thus the Veining postdates the UHP metamorphic event at 225–240 Ma and took place during transition from HP eclogite-facies recrystallization to amphibolite-facies retrogression during the exhumation. The host eclogite was also retrograded in this stage to form metabasites such as amphibole eclogite to garnet amphibolite. The abundant occurrence of zoisite, epidote and phengite in the Vein suggests that the metamorphic fluid is rich in Si, Al, Ca and K. The fluid composition is estimated in terms of zoisite composition and zoisite/fluid partition coefficients. The results suggest strong enrichment of LREE, Th, U, Pb, Sr, Rb and Ba, moderate enrichment of HREE, but significant depletion of HFSE. In this regard, the occurrence of metamorphic zircon and rutile in the Vein suggests very local saturation of Zr and Ti in the Zo–Qtz Vein-forming fluid. Therefore, the two episodes of fluid action would occur at the transition from eclogite to amphibolite facies during the exhumation and proceed with different fluid compositions and different flow behaviors. One was channelized under the eclogite-facies conditions, whereas the other is pervasive under the amphibolite-facies conditions. This provides robust constraints on the time, origin and property of metamorphic fluid during the exhumation of deeply subducted continental crust.

  • Episodic fluid action during exhumation of deeply subducted continental crust：Geochemical constraints from zoisite-Quartz Vein and host metabasite in the Dabie orogen
    Lithos, 2012
    Co-Authors: Renxu Chen, Yongfei Zheng, Zhaochu Hu
    Abstract:

    Abstract A combined study of mineral compositions and zircon U–Pb ages was carried out for a zoisite―Quartz Vein and its host retrograded eclogite in the Dabie orogen. The results reveal two episodes of fluid action during exhumation of the deeply subducted continental crust. The Vein has a mineral paragenesis of Quartz + zoisite + epidote + white mica + garnet + rutile + zircon + apatite, which is almost the same as that of the host retrograded eclogite characterized by symplectites after omphacite and/or garnet in the eclogite. Residues of magmatic zircon in the Vein are similar to those in the eclogite, suggesting local sources of the metamorphic fluid for Veining and physical transport of the tiny accessory mineral from the host rock to Veins. There are significant differences in major and trace elements between Vein-forming minerals, which are attributed to two episodes of fluid action for Veining. Metamorphic zircon from the Vein yields concordant U–Pb ages of 215 ± 4 to 218 ± 4 Ma. Trace element analysis indicates its precipitation from aqueous fluid during two distinct episodes under eclogite-facies and amphibolite-facies conditions, respectively. Ti-in-zircon and Zr-in-rutile thermometries yield variable temperatures from 537 to 683 °C. Thus the Veining postdates the UHP metamorphic event at 225–240 Ma and took place during transition from HP eclogite-facies recrystallization to amphibolite-facies retrogression during the exhumation. The host eclogite was also retrograded in this stage to form metabasites such as amphibole eclogite to garnet amphibolite. The abundant occurrence of zoisite, epidote and phengite in the Vein suggests that the metamorphic fluid is rich in Si, Al, Ca and K. The fluid composition is estimated in terms of zoisite composition and zoisite/fluid partition coefficients. The results suggest strong enrichment of LREE, Th, U, Pb, Sr, Rb and Ba, moderate enrichment of HREE, but significant depletion of HFSE. In this regard, the occurrence of metamorphic zircon and rutile in the Vein suggests very local saturation of Zr and Ti in the Zo–Qtz Vein-forming fluid. Therefore, the two episodes of fluid action would occur at the transition from eclogite to amphibolite facies during the exhumation and proceed with different fluid compositions and different flow behaviors. One was channelized under the eclogite-facies conditions, whereas the other is pervasive under the amphibolite-facies conditions. This provides robust constraints on the time, origin and property of metamorphic fluid during the exhumation of deeply subducted continental crust.

  • fluid flow during exhumation of deeply subducted continental crust zircon u pb age and o isotope studies of a Quartz Vein within ultrahigh pressure eclogite
    Journal of Metamorphic Geology, 2007
    Co-Authors: Yongfei Zheng, Tianshan Gao, Bing Gong, Xiaoming Liu
    Abstract:

    Quartz Veins in high-pressure to ultrahigh-pressure metamorphic rocks witness channelized fluid flow that transports both mass and heat during collisional orogenesis. This flow can occur in the direction of changing temperature/pressure during subduction or exhumation. SHRIMP U-Pb dating of zircon from a kyanite-Quartz Vein within ultrahigh-pressure eclogite in the Dabie continental collision orogen yields two age groups at 212 ± 7 and 181 ± 13 Ma, which are similar to two groups of LA-ICPMS age at 210 ± 4 and 180 ± 5 Ma for the same sample. These ages are significantly younger than zircon U-Pb ages of 224 ± 2 Ma from the host eclogite. Thus the two age groups from the Vein date two episodes of fluid flow involving zircon growth: the first due to decompression dehydration during exhumation, and the second due to heating dehydration in response to a cryptic thermal event after continental collision. Laser fluorination O-isotope analyses gave similar δ18O values for minerals from both Vein and eclogite, indicating that the Vein-forming fluid was internally derived. Synchronous cooling between the Vein and eclogite is suggested by almost the same Quartz–mineral fractionation values, with regularly decreasing temperatures that are in concordance with rates of O diffusion in the minerals. While the Quartz Veining was caused by decompression dehydration at 700–650 °C in a transition from ultrahigh-pressure to high-pressure eclogite-facies retrogression, the postcollisional fluid flow was retriggered by heating dehydration at ∼500 °C without corresponding metamorphism. In either case, the kyanite–Quartz Vein formed later than the peak ultrahigh-pressure metamorphic event at the Middle Triassic, pointing to focused fluid flow during exhumation rather than subduction. The growth of metamorphic zircon in the eclogite appears to have depended on fluid availability, so that their occurrence is a type of geohygrometer besides geochronological applicability to dating of metamorphic events in orogenic cycles.

Zhaochu Hu - One of the best experts on this subject based on the ideXlab platform.

  • episodic fluid action during exhumation of deeply subducted continental crust geochemical constraints from zoisite Quartz Vein and host metabasite in the dabie orogen
    Lithos, 2012
    Co-Authors: Renxu Chen, Yongfei Zheng, Zhaochu Hu
    Abstract:

    Abstract A combined study of mineral compositions and zircon U–Pb ages was carried out for a zoisite―Quartz Vein and its host retrograded eclogite in the Dabie orogen. The results reveal two episodes of fluid action during exhumation of the deeply subducted continental crust. The Vein has a mineral paragenesis of Quartz + zoisite + epidote + white mica + garnet + rutile + zircon + apatite, which is almost the same as that of the host retrograded eclogite characterized by symplectites after omphacite and/or garnet in the eclogite. Residues of magmatic zircon in the Vein are similar to those in the eclogite, suggesting local sources of the metamorphic fluid for Veining and physical transport of the tiny accessory mineral from the host rock to Veins. There are significant differences in major and trace elements between Vein-forming minerals, which are attributed to two episodes of fluid action for Veining. Metamorphic zircon from the Vein yields concordant U–Pb ages of 215 ± 4 to 218 ± 4 Ma. Trace element analysis indicates its precipitation from aqueous fluid during two distinct episodes under eclogite-facies and amphibolite-facies conditions, respectively. Ti-in-zircon and Zr-in-rutile thermometries yield variable temperatures from 537 to 683 °C. Thus the Veining postdates the UHP metamorphic event at 225–240 Ma and took place during transition from HP eclogite-facies recrystallization to amphibolite-facies retrogression during the exhumation. The host eclogite was also retrograded in this stage to form metabasites such as amphibole eclogite to garnet amphibolite. The abundant occurrence of zoisite, epidote and phengite in the Vein suggests that the metamorphic fluid is rich in Si, Al, Ca and K. The fluid composition is estimated in terms of zoisite composition and zoisite/fluid partition coefficients. The results suggest strong enrichment of LREE, Th, U, Pb, Sr, Rb and Ba, moderate enrichment of HREE, but significant depletion of HFSE. In this regard, the occurrence of metamorphic zircon and rutile in the Vein suggests very local saturation of Zr and Ti in the Zo–Qtz Vein-forming fluid. Therefore, the two episodes of fluid action would occur at the transition from eclogite to amphibolite facies during the exhumation and proceed with different fluid compositions and different flow behaviors. One was channelized under the eclogite-facies conditions, whereas the other is pervasive under the amphibolite-facies conditions. This provides robust constraints on the time, origin and property of metamorphic fluid during the exhumation of deeply subducted continental crust.

  • Episodic fluid action during exhumation of deeply subducted continental crust：Geochemical constraints from zoisite-Quartz Vein and host metabasite in the Dabie orogen
    Lithos, 2012
    Co-Authors: Renxu Chen, Yongfei Zheng, Zhaochu Hu
    Abstract:

    Abstract A combined study of mineral compositions and zircon U–Pb ages was carried out for a zoisite―Quartz Vein and its host retrograded eclogite in the Dabie orogen. The results reveal two episodes of fluid action during exhumation of the deeply subducted continental crust. The Vein has a mineral paragenesis of Quartz + zoisite + epidote + white mica + garnet + rutile + zircon + apatite, which is almost the same as that of the host retrograded eclogite characterized by symplectites after omphacite and/or garnet in the eclogite. Residues of magmatic zircon in the Vein are similar to those in the eclogite, suggesting local sources of the metamorphic fluid for Veining and physical transport of the tiny accessory mineral from the host rock to Veins. There are significant differences in major and trace elements between Vein-forming minerals, which are attributed to two episodes of fluid action for Veining. Metamorphic zircon from the Vein yields concordant U–Pb ages of 215 ± 4 to 218 ± 4 Ma. Trace element analysis indicates its precipitation from aqueous fluid during two distinct episodes under eclogite-facies and amphibolite-facies conditions, respectively. Ti-in-zircon and Zr-in-rutile thermometries yield variable temperatures from 537 to 683 °C. Thus the Veining postdates the UHP metamorphic event at 225–240 Ma and took place during transition from HP eclogite-facies recrystallization to amphibolite-facies retrogression during the exhumation. The host eclogite was also retrograded in this stage to form metabasites such as amphibole eclogite to garnet amphibolite. The abundant occurrence of zoisite, epidote and phengite in the Vein suggests that the metamorphic fluid is rich in Si, Al, Ca and K. The fluid composition is estimated in terms of zoisite composition and zoisite/fluid partition coefficients. The results suggest strong enrichment of LREE, Th, U, Pb, Sr, Rb and Ba, moderate enrichment of HREE, but significant depletion of HFSE. In this regard, the occurrence of metamorphic zircon and rutile in the Vein suggests very local saturation of Zr and Ti in the Zo–Qtz Vein-forming fluid. Therefore, the two episodes of fluid action would occur at the transition from eclogite to amphibolite facies during the exhumation and proceed with different fluid compositions and different flow behaviors. One was channelized under the eclogite-facies conditions, whereas the other is pervasive under the amphibolite-facies conditions. This provides robust constraints on the time, origin and property of metamorphic fluid during the exhumation of deeply subducted continental crust.

Ruicheng Zhao - One of the best experts on this subject based on the ideXlab platform.

  • ore forming mechanism of Quartz Vein type w sn deposits of the xitian district in se china implications from the trace element analysis of wolframite and investigation of fluid inclusions
    Ore Geology Reviews, 2017
    Co-Authors: Yiqu Xiong, Yongjun Shao, Haodi Zhou, Qianhong Wu, Ruicheng Zhao
    Abstract:

    Abstract The Xitian W–Sn district is located in eastern Hunan Province of SE China and comprises the Heshuxia, Goudalan, and Longshang W–Sn deposits. Our study on the occurrence, mineral assemblage, and geochemistry of the orebodies shows that Quartz-Vein orebodies are dominant mode of mineralization and can be divided into three stages: stage I, Quartz–molybdenite–wolframite (I); stage II, Quartz–wolframite (II)–polymetallic sulfides; and stage III, Quartz–pyrite–fluorite. The elements Nb, Ta, Sc, Sn, Zn, Cr, V, Mo, Cu, and Zr are abundant in both wolframite (I) and wolframite (II). Wolframite (I) contains higher concentrations of Nb (1408–9245 ppm), Ta (241–2407 ppm), and Sc (123–521 ppm) than wolframite (II) (Nb 144–1080 ppm, Ta 3.76–315 ppm, and Sc 1.86–11.44 ppm). These results indicate that wolframite (I) was formed closer to the host rock and at a greater ore-forming depth than wolframite (II). The total REE concentrations of wolframite (I) and (II) range from 34.96 to 133.36 ppm and 21.30 to 43.73 ppm, respectively. Both wolframite (I) and wolframite (II) have low LREE concentrations (0.00–0.05 and 0.08–8.18 ppm, respectively) and high HREE concentrations (34.96–133.31 and 21.23–41.87 ppm, respectively). The enrichment of HREEs in wolframite may be due to the fact that the sizes of the HREEs 3+ (0.94–1.02 A) in combination with Nb 5+ or Ta 5+ (0.72 A) are closer to that of W 6+ (0.68 A) in combination with Ca 2+ (1.08 A) or Mg 2+ (0.80 A) than that of the LREEs 3+ (1.03–1.13 A), making the coupled substitution easier. A study of fluid inclusions in the coexisting gangue minerals shows that the homogenization temperatures of the fluid inclusions decrease from stage I to stage III (stage I, 187–382 °C, average 275 °C; stage II, 122–278 °C, average 175 °C; stage III, 92–172 °C, average 139 °C), whereas the salinities of the fluid inclusions increase from stages I to II and decrease from stages II to III (stage I, 1.8–18.2 wt% NaCl eqv , average 10.3 wt% NaCl eqv ; stage II, 7.3–24.4 wt% NaCl eqv , average 14.9 wt% NaCl eqv ; stage III, 0.2–5.3 wt% NaCl eqv , average 1.5 wt% NaCl eqv ). Raman spectroscopy reveals that the fluid inclusions mainly contain H 2 O; CO 2 is minor and H 2 S, CH 4 , and N 2 are rare. We conclude that the formation of wolframite was driven by post-magmatic thermodynamic processes. The ore-forming fluid flowed through a multi-fractured low-pressure zone in the fault system in the district, and a temperature–pressure decrease led to fluid immiscibility characterized by CO 2 escaping in a low-pH and high-Eh environment in stage I, crust–mantle mixed fluid and meteoric water mixing in stage II, and natural cooling of the fluid system in stage III.

  • ore forming mechanism of Quartz Vein type w sn deposits of the xitian district in se china implications from the trace element analysis of wolframite and investigation of fluid inclusions
    Ore Geology Reviews, 2017
    Co-Authors: Yiqu Xiong, Yongjun Shao, Haodi Zhou, Qianhong Wu, Ruicheng Zhao
    Abstract:

    Abstract The Xitian W–Sn district is located in eastern Hunan Province of SE China and comprises the Heshuxia, Goudalan, and Longshang W–Sn deposits. Our study on the occurrence, mineral assemblage, and geochemistry of the orebodies shows that Quartz-Vein orebodies are dominant mode of mineralization and can be divided into three stages: stage I, Quartz–molybdenite–wolframite (I); stage II, Quartz–wolframite (II)–polymetallic sulfides; and stage III, Quartz–pyrite–fluorite. The elements Nb, Ta, Sc, Sn, Zn, Cr, V, Mo, Cu, and Zr are abundant in both wolframite (I) and wolframite (II). Wolframite (I) contains higher concentrations of Nb (1408–9245 ppm), Ta (241–2407 ppm), and Sc (123–521 ppm) than wolframite (II) (Nb 144–1080 ppm, Ta 3.76–315 ppm, and Sc 1.86–11.44 ppm). These results indicate that wolframite (I) was formed closer to the host rock and at a greater ore-forming depth than wolframite (II). The total REE concentrations of wolframite (I) and (II) range from 34.96 to 133.36 ppm and 21.30 to 43.73 ppm, respectively. Both wolframite (I) and wolframite (II) have low LREE concentrations (0.00–0.05 and 0.08–8.18 ppm, respectively) and high HREE concentrations (34.96–133.31 and 21.23–41.87 ppm, respectively). The enrichment of HREEs in wolframite may be due to the fact that the sizes of the HREEs 3+ (0.94–1.02 A) in combination with Nb 5+ or Ta 5+ (0.72 A) are closer to that of W 6+ (0.68 A) in combination with Ca 2+ (1.08 A) or Mg 2+ (0.80 A) than that of the LREEs 3+ (1.03–1.13 A), making the coupled substitution easier. A study of fluid inclusions in the coexisting gangue minerals shows that the homogenization temperatures of the fluid inclusions decrease from stage I to stage III (stage I, 187–382 °C, average 275 °C; stage II, 122–278 °C, average 175 °C; stage III, 92–172 °C, average 139 °C), whereas the salinities of the fluid inclusions increase from stages I to II and decrease from stages II to III (stage I, 1.8–18.2 wt% NaCl eqv , average 10.3 wt% NaCl eqv ; stage II, 7.3–24.4 wt% NaCl eqv , average 14.9 wt% NaCl eqv ; stage III, 0.2–5.3 wt% NaCl eqv , average 1.5 wt% NaCl eqv ). Raman spectroscopy reveals that the fluid inclusions mainly contain H 2 O; CO 2 is minor and H 2 S, CH 4 , and N 2 are rare. We conclude that the formation of wolframite was driven by post-magmatic thermodynamic processes. The ore-forming fluid flowed through a multi-fractured low-pressure zone in the fault system in the district, and a temperature–pressure decrease led to fluid immiscibility characterized by CO 2 escaping in a low-pH and high-Eh environment in stage I, crust–mantle mixed fluid and meteoric water mixing in stage II, and natural cooling of the fluid system in stage III.

Renxu Chen - One of the best experts on this subject based on the ideXlab platform.

  • episodic fluid action during exhumation of deeply subducted continental crust geochemical constraints from zoisite Quartz Vein and host metabasite in the dabie orogen
    Lithos, 2012
    Co-Authors: Renxu Chen, Yongfei Zheng, Zhaochu Hu
    Abstract:

    Abstract A combined study of mineral compositions and zircon U–Pb ages was carried out for a zoisite―Quartz Vein and its host retrograded eclogite in the Dabie orogen. The results reveal two episodes of fluid action during exhumation of the deeply subducted continental crust. The Vein has a mineral paragenesis of Quartz + zoisite + epidote + white mica + garnet + rutile + zircon + apatite, which is almost the same as that of the host retrograded eclogite characterized by symplectites after omphacite and/or garnet in the eclogite. Residues of magmatic zircon in the Vein are similar to those in the eclogite, suggesting local sources of the metamorphic fluid for Veining and physical transport of the tiny accessory mineral from the host rock to Veins. There are significant differences in major and trace elements between Vein-forming minerals, which are attributed to two episodes of fluid action for Veining. Metamorphic zircon from the Vein yields concordant U–Pb ages of 215 ± 4 to 218 ± 4 Ma. Trace element analysis indicates its precipitation from aqueous fluid during two distinct episodes under eclogite-facies and amphibolite-facies conditions, respectively. Ti-in-zircon and Zr-in-rutile thermometries yield variable temperatures from 537 to 683 °C. Thus the Veining postdates the UHP metamorphic event at 225–240 Ma and took place during transition from HP eclogite-facies recrystallization to amphibolite-facies retrogression during the exhumation. The host eclogite was also retrograded in this stage to form metabasites such as amphibole eclogite to garnet amphibolite. The abundant occurrence of zoisite, epidote and phengite in the Vein suggests that the metamorphic fluid is rich in Si, Al, Ca and K. The fluid composition is estimated in terms of zoisite composition and zoisite/fluid partition coefficients. The results suggest strong enrichment of LREE, Th, U, Pb, Sr, Rb and Ba, moderate enrichment of HREE, but significant depletion of HFSE. In this regard, the occurrence of metamorphic zircon and rutile in the Vein suggests very local saturation of Zr and Ti in the Zo–Qtz Vein-forming fluid. Therefore, the two episodes of fluid action would occur at the transition from eclogite to amphibolite facies during the exhumation and proceed with different fluid compositions and different flow behaviors. One was channelized under the eclogite-facies conditions, whereas the other is pervasive under the amphibolite-facies conditions. This provides robust constraints on the time, origin and property of metamorphic fluid during the exhumation of deeply subducted continental crust.

  • Episodic fluid action during exhumation of deeply subducted continental crust：Geochemical constraints from zoisite-Quartz Vein and host metabasite in the Dabie orogen
    Lithos, 2012
    Co-Authors: Renxu Chen, Yongfei Zheng, Zhaochu Hu
    Abstract:

    Abstract A combined study of mineral compositions and zircon U–Pb ages was carried out for a zoisite―Quartz Vein and its host retrograded eclogite in the Dabie orogen. The results reveal two episodes of fluid action during exhumation of the deeply subducted continental crust. The Vein has a mineral paragenesis of Quartz + zoisite + epidote + white mica + garnet + rutile + zircon + apatite, which is almost the same as that of the host retrograded eclogite characterized by symplectites after omphacite and/or garnet in the eclogite. Residues of magmatic zircon in the Vein are similar to those in the eclogite, suggesting local sources of the metamorphic fluid for Veining and physical transport of the tiny accessory mineral from the host rock to Veins. There are significant differences in major and trace elements between Vein-forming minerals, which are attributed to two episodes of fluid action for Veining. Metamorphic zircon from the Vein yields concordant U–Pb ages of 215 ± 4 to 218 ± 4 Ma. Trace element analysis indicates its precipitation from aqueous fluid during two distinct episodes under eclogite-facies and amphibolite-facies conditions, respectively. Ti-in-zircon and Zr-in-rutile thermometries yield variable temperatures from 537 to 683 °C. Thus the Veining postdates the UHP metamorphic event at 225–240 Ma and took place during transition from HP eclogite-facies recrystallization to amphibolite-facies retrogression during the exhumation. The host eclogite was also retrograded in this stage to form metabasites such as amphibole eclogite to garnet amphibolite. The abundant occurrence of zoisite, epidote and phengite in the Vein suggests that the metamorphic fluid is rich in Si, Al, Ca and K. The fluid composition is estimated in terms of zoisite composition and zoisite/fluid partition coefficients. The results suggest strong enrichment of LREE, Th, U, Pb, Sr, Rb and Ba, moderate enrichment of HREE, but significant depletion of HFSE. In this regard, the occurrence of metamorphic zircon and rutile in the Vein suggests very local saturation of Zr and Ti in the Zo–Qtz Vein-forming fluid. Therefore, the two episodes of fluid action would occur at the transition from eclogite to amphibolite facies during the exhumation and proceed with different fluid compositions and different flow behaviors. One was channelized under the eclogite-facies conditions, whereas the other is pervasive under the amphibolite-facies conditions. This provides robust constraints on the time, origin and property of metamorphic fluid during the exhumation of deeply subducted continental crust.

Hujun Gong - One of the best experts on this subject based on the ideXlab platform.

  • u pb age trace element and hf isotope compositions of zircon in a Quartz Vein from eclogite in the western dabie mountains constraints on fluid flow during early exhumation of ultrahigh pressure rocks
    American Mineralogist, 2009
    Co-Authors: Shan Gao, Xiaochi Liu, Saihong Yang, Wenfang Jiao, Hongfei Zhang, Yongsheng Liu, Honglin Yuan, Hujun Gong
    Abstract:

    Quartz Veins in high-pressure (HP) to ultrahigh-pressure (UHP) rocks are the products of fluid-rock interaction, and thus provide insight into fluid processes in subduction zones. In this paper, we report an integrated study of mineral inclusion, trace-element, U-Pb age, and Lu-Hf isotope compositions of hydrothermal zircon grains from a Quartz Vein within an UHP eclogite outcrop from the Hong’an area, western Dabie Mountains. These data are used to decipher the age, conditions of formation, and source of fluid for zircon formation during the exhumation of UHP rocks. Zircon grains from the Vein have perfect euhedral shape, and show sector zoning or weak zoning, indicating that they precipitated from the aqueous fluid responsible for the Vein formation. Raman spectroscopy analysis reveals that the zircon grains contain inclusions of garnet, omphacite, rutile, Quartz, and H2O, implying that they crystallized from aqueous fluid under HP eclogite-facies conditions. The zircon grains show low Th/U and Lu/Hf ratios, nearly flat HREE patterns, absent Eu anomalies and low LREE contents. These characteristics are consistent with their precipitation in the presence of garnet and epidote, and absence of feldspar, and thus suggest that trace-element concentrations in hydrothermal zircon are controlled by co-precipitation of mineral assemblages. Crystallization temperatures of 670 to 712 °C, which were calculated using the Ti content of zircon, are consistent with their formation under eclogite-facies conditions and may correspond to the temperature of the infiltrating fluid. The weighted mean 206Pb/238U age of 224.7 ± 1.3 Ma is taken as the best estimate for the age of Quartz-Vein formation and records aqueous fluid flow during the early exhumation stage of UHP rocks. The zircon grains in the Quartz-Vein have Hf compositions similar to those in the host eclogite, which demonstrates isotopic equilibrium between fluid and rocks and that the fluid-rock ratio was likely low.

  • zircon u pb age and hf isotope composition of a Quartz Vein in ultrahigh pressure eclogite from the western dabie orogen
    Computational Systems Bioinformatics, 2007
    Co-Authors: Xiaochi Liu, W U Yuanbao, Hujun Gong, Saihong Yang, Jing Wang, Min Peng, Wenfang Jiao
    Abstract:

    In this paper, we report U-Pb age and Hf isotope composition of hydrothermal zircons from a Quartz Vein within an UHP eclogite outcrop from the Hong’an area, western Dabie orogen. These data are used to decipher the age, formation conditions and source of fluid flow during the exhumation of UHP rock. Zircons from the Vein have perfect euhedral shape, oscillatory zoning or weak zoning, and very low Th/U ratios (0.03―0.07), indicating that they precipitated from the aqueous fluid responsible for the Vein formation. The weighted mean 206Pb/238U age of 224.7 ± 1.3 Ma of these zircons is taken as the best estimated age of the Quartz Vein formation, and records aqueous fluid flow during the early exhumation stage of the UHP rocks. The zircons in the Quartz Vein show low Lu/Hf ratios and have similar Hf compositions to those in the host eclogite, indicating an internal source and small-scale fluid transport for Veining. Therefore, the Hf isotope composition of hydrothermal zircon from Quartz Vein can constrain on the nature, forming condition, and origin of high-pressure metamorphic fluids.