| Niobium (Nb) and tantalum (Ta)are classical high field strength elements with identical valence (+5) and effective radius (0.640A). Whether the mobility and fractionation of Nb and Ta occur significantly during subduction-zone metamorphism is a controversialscientific subject. The first research subject in this doctoral dissertation is to determine Nb and Ta concentrations in whole rocks and in rutile from HP veins, ecolgites close to (ECV) and away from veins (EAV) collected at Bixiling in the mid-T/UHP Central Dabie orogenic belt. Based on these data, we discuss the mechamism and effective distance of Nb-Ta mobility and fractionation during fluid/rock interaction in continental subduction-zone metamorphism.A direct comparison of the EAV eclogites with unmetamorphosed protolithic rocks-Neoproterozoic gabbro from the Yangtze Block, would qualitatively evaluate Nb-Ta mobility and fractionation during subduction without intensive fluid-rock interaction. Nband Ta concentrationsin the EAVsrange from0.42to3.97ppm and0.03to0.24ppm, respectively. The EAVs display bulk Nb/Ta ratios ranging from14.0to19.2, which fall in the range defined by the gabbros from the Yangtze Block. Their average Nb/Ta ratio of16.9±0.8(2s.e. n=12) is very similar to that of the gabbros (16.2±0.6,2s.e. n=78). These observations indicate transformation of gabbro to UHP eclogite did not lead to the fractionation of Nb-Ta. More importantly, in the log[Nb] vs. log[Ta] diagram, the slope of0.965±0.038for Nb and Ta is very close to1and suggests negligible Nb/Ta fractionation during partial melting, magma differentiation, Triassic subduction, and subsequent eclogitisation.Rutile from the EAVs has Nb/Ta ratios ranging from12.7to25.3, with Nb and Ta concentrations ranging from19.2tol79ppm and1.12to11.9ppm,respectively.It is noteworthy that Nb/Ta ratios show much larger variations among different individual rutile grains (12.7to25.3) than that in the bulk host EAVs (14.0to19.2), indicating that rutile can fractionate Nb from Ta at mineral-scale. Nevertheless, the average Nb/Ta ofrutile grains is similar to that in the corresponding bulk host EAVs, implying that rutiledominates the Nb-Ta budget of eclogite.Epidote from the vein and ECV eclogite has similar compositions, the ECV-and vein-hosted rutile shows nearly complementary Nb/Ta zoning patterns and the veins have small sizes, ranging from5to18cm wide and25to42cm long, indicating that the investigated rutile-bearing veins originated from an internal fluid source. Rutile from the ECVs shows a much largerspread inbothNb/Ta ratios(17.8-49.8)and Nb-Ta concentrations (Nb:99.5～503ppm,Ta:2.04～14.4ppm). Similarly, the vein rutile has highly variable Nb/Ta of10.9-29.2, with Nb and Ta concentrations from155to2021ppm and9.02to151ppm, respectively. In general, compared to the ECV-hosted rutile, the vein-hosted rutile has lower Nb/Ta ratios, strongly suggesting that Nb-Ta mobility and fractionation are associated with intensive fluid-rock interaction during metamorphism but only occurred locally.The vein-hosted rutile shows lower Nb/Ta ratios in the cores than in the rims,indicating that it did not form in a single stage precipitation and probably experienced a multi-stage evolution and that the Nb/Ta of the vein-forming fluids evolved from low values at the early stage of subduction to higher values at later supercritical fluids conditions with increased temperature and pressure. At the early stage of subduction, where rutile is absent, aqueous fluids released by early-stage dehydration of amphibolite should have low Nb/Ta ratios. The aqueous fluids were probably transported into and stored in fractures and/or pore spaces. Consequently, the stored fluids will at least partly be delivered to greater depths along with ongoing subduction.When the prsessure increased above the second critical point in the basalt-H2O system, the aqueous fluids could transform into supercritical fluids. Such fluids favour Nb over Ta and thus have relatively high Nb/Ta ratios. The vein rutile cores crystallized from early-stage aqueous fluids hence should have low Nb/Ta ratios, and high Nb/Ta ratios in the rims can be attributed to precipitating from supercritical fluids. Quantitative modelling was conducted to constrain the compositional evolution of metamorphic fluids during dehydration and fluid-rock interaction focusing on Nb-Ta distribution. The modelling results based on our proposed multistage fluid phase evolution path can essentially reproduce the natural observations reported in the present study.This doctoral research has also investigated an in-situ profile drilled through the UHP granitic gneiss and eclogite (retrogressed to amphibolite) contact at Zhujiachong in the low-T/UHP South Dabie orogenic belt. A combined investigation of petrology, mineral chemistry, major and trace elements, and Sr-Nd-0isotopes on them was carried out. The results improved the better understanding of the nature and action of supercritical fluids and the associated element mobility at UHP metamorphic conditions.Oxygen isotopic composition ranges from+0.59to+1.29%o (SMOW) in gneisses and from-1.11to-2.21‰in amphibolite. The gneisses show a slight decrease, while the amphibolites display a progressive increase in δ18O values towards their lithological boundary, suggesting that the contact between different lithologies is the most favorable place for fluid activity. Gechemical analyses show that directly at the contact to the gneiss, the amphibolite has higher concentrations in K, Al, LILE, REE, HFSE, Th and U, but similar SiO2, FeOt and transitional metal element (e.g., Cr and Ni) contents relative to the other amphibolites further away from the boundary.Aqueous fluids and amphibolite-facies retrogression of eclogite are known to have no significant effect on their major and trace elements. Also, Si-rich metasomatism from partially melted gneiss should increase the silica content of the amphibolite at the contact,which is not observed.Multiphase solid minnerals inclusions in garnet from the amphibolite are mainly composed of K-feldspar+quartz+calcite+zircon±hydrous minerals (i.e., amphibole and clinozoisite)±garnet±apatite. They probablycrystallized from early-trapped supercritical fluids under peak metamorphic conditions.Theoretically, supercritical fluids were likely to be present in granitic gneiss, but unlikely in eclogite from the low-T/UHP South Dabie orogenic belt. Such fluids have a very high capacity to dissolve HFSE-and HREE-enriched accessory minerals (e.g., rutile, garnet, and allanite) in granitic gneiss under peak metamorphic conditions. Metasomatism ofsuch fluids enriched in LILE, REE, HFSE, Th and U could lead to the observed geochemical variations at the contact. Mass balance calculation shows that element mobility in the inferred supercritical fluids in the South Dabie granitic gneiss decreases in the following order:Ba> K (Li, Cs, Rb, Pb)> U> Th> REE> Nb (Ta)> Zr (Hf). |
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