| Shuanghe and Bixiling are two typical areas with abundant ultrahigh pressure metamorphic (UHPM) rock occurrence in the Dabie UHPM belt, where suffered intensively research in petrology, geochronology, geodynamics and mineralogy etc. Rutile is a common accessory mineral in media-high grade metamorphic rocks, it frequently presented in elcogite and jadeite quartzite, but rarely occurred in marble, biotite gneiss, mica schist, ultramafic rocks and areas country rock—granitic gneiss. As a stable heavy mineral phase, rutiles carry a lot of information with respect to metamorphism, e.g. metamorphic temperature, pressure, cool age and provenance properties. In this doctoral dissertation, both in-site LA-ICP-MS and EMP were performed for investigated geochemical characteristics of rutiles from different rocks in two regions above mentioned, including rock-in rutiles from eclogite, jadeite quartzite, marble and biotite gneiss and detrital rutiles from sediment. In term of purpose and content of research, it can be separated into three aspects to introduction.The first research subject is to systematic investigated to trace element mobile of metasedimentary rock during continental subduction and exhumation. The major work included study on a continuous profile from marble to eclogite in Shuanghe. The marble displayed LILE and LREE enriched relative to eclogite, in contrast to depletion in HFSE and HREE. REE patterns in marble and eclogite have shown well coupled relation, indicating fluid is internal source without external fluid ingress. Fluid-mobile elements (e.g. Rb, Cs, Ba) contents in eclogite increase toward to contact to marble, indicating degree of fluid action gradually increase to contact zone between marble and eclogite. Rutile is rarely presented in marble, and titanite is common occurred in eclogites, indicating fluid transported considerable Ti and HFSE into marble from eclogite. Ruitles in marble with high Nb/Ta (63.4) suggested the retrogression fluid have higher Nb/Ta relative to63.4. Supercritical fluids frequently can transport Ti and HFSE during subduction zone, thus Ti and HFSE transportion in eclogites suggested supercritital fluid presented in Shuanghe metasedimetary rock. Bulk rocks of eclogite have the constant Nb/Ta (14.3-15.5), and high-Al titanite in eclogites also displayed consistent Nb/Ta (14.2-16.7), indicating during rutile replaced by titantie, produced Nb-Ta fractionation is very limited. The Nb/Ta in bulk rock of marble range10.0to10.9, rutiles in marble range from14.2to63.4, indicating rutile Nb/Ta ratios can not represent the marble bulk rock. Low Nb/Ta in rutile retained the protolith characteristics, whereas high Nb/Ta in rutile maybe response for high Nb/Ta fluid resulted from rertrogression.The second research subject in this doctoral dissertation is to explore detrital rutile as tracer in provenance study throung investigation from detrital rutiles from Shuanghe and Bixiling areas in the Central Dabie region. This study explores the potential use of detrital rutile geochemistry and thermometry as a provenance tracer in rocks from the Central Dabieshan ultrahigh-pressure metamorphic (UHPM) zone that formed during the Triassic continental collision in east-central China. Trace element data of176detrital rutile grains selected from local river sediments and91rutile grains from distinct bedrocks in the Shuanghe and Bixiling areas, obtained by both electron microprobe (EMP) and in-situ LA-ICP-MS analyses, suggest that geochemical compositions and thermometry of the two types of rutiles are comparable. After establishing the Cr-Nb discrimination method suitable for the Central Dabie UHPM zone, we reveal that in the Shuanghe area29%of the detrital rutiles were derived from metamafic sources whereas in Bixiling area that is up to76%based on the new diagram. Furthermore, this proportion of detrital rutiles combined with modal abundances of rutile in metapelites and metamafic bedrocks can be used to estimate the proportion of different lithology. Based on this method the proportion of mafic source rocks was estimated to-10%at Shuanghe and>60%at Bixiling in available range, respectively, which is in consistent with the proportions of eclogite (the major rutile-bearing metamafic rock) distribution in field occurrence. Therefore, investigation of detrital rutiles is a potential way to evaluate the proportion of metamafic rocks and even prospect the metamafic body in UHPM terranes. Zr-in-rutile temperatures were calculated at diffenent pressures and the compare with temperatures derived from rock-in rutiles and garnet-clinopyroxene Fe-Mg exhance thermometers. Temperatures calculated from detrital rutiles were estimated to range from606℃to707℃and566℃to752℃in Shuanghe and Bixiling at P=3GPa. The average temperatures are633℃and629℃, respectively. Both of the variation range and the average of temperature are well resemble with rutiles from surrounding exposed rocks. Therefore, the analogy between detrital rutiles and rock-in rutiles demonstrated that source-sediment links of rutile is extremely intimate in UHPM terranes, thus allows us to conclude that rutile is an accurate tracer of source rock characterization on sedimentary provenance studies in a small scale of the immediate catchment area. In Bixiling, Nb/Ta ratios of metamafic and metapelitic detrital rutiles fall between11.0-27.3and7.7-20.5, respectively. In Shuanghe, these ratios are highly variable, ranging from10.9to71.0and7.6to87.1, respectively. However, When ignoring four rutiles with extremely high Nb/Ta, a distinct clustering of Nb/Ta ratios has shown (metapelitic rutile:7-40vs. metamafic rutile:11-25). The obviously distinct Nb/Ta ratio range between metapelitic and metamafic detrital rutiles from Shuanghe can well reflect degree of fluid-rock interaction during rutile growth due to water contents difference between corresponding source rock. The fluid-rock interaction during metapelitic rutiles growth is stronger than metamafic rutiles. But this feature do not present in Bixiling detrital rutiles. This may be account for rock abundance difference. Thus, any Nb-Ta data from rutiles should be interpreted within the framework of special petrological environment. Overall, the Nb/Ta ratio variable in detrital rutiles can accurate reflects intensity of fluid-rock interaction in source rock.The third research subject in this doctoral dissertation is to combine with rutile data from this study and published literatures, and found that there existed considerable negative correlation between V and Nb in rutiles from Dabie-Sulu elcogites. Vanadium occurs in multiple valence states in nature, whereas Nb is exclusively pentavalent. Both are compatible in rutile, but the relationship of V-Nb partitioning and dependence on oxygen fugacity has not yet been systematically investigated. We acquired trace element concentrations on rutile grains (n=86) in nine eclogitic samples from the Dabie-Sulu orogenic belt by laser ablation-inductively coupled plasma mass spectrometry and combined the results with published data in order to assess the direct and indirect effects of oxygen fugacity on the partitioning of V and Nb into rutile. After evaluating bulk rock and mineral phase influence on V behavior in rutile, we suggest there is a well-defined negative correlation between Nb (7ppm to1200ppm) and V concentrations (50ppm to3200ppm), documenting a competitive relationship in the rutile crystal. Based on the relationship of Dy and V valence with QFM (QFM is the quartz-fayalite-magnetite oxygen buffer), the priority order of V incorporation into rutile is V4+>V3+>V5+. The Nb-V competitive relationship in rutile from the Dabie-Sulu orogenic belt is well explained by decreasing oxygen fugacity (fO2) due to dehydration reactions during continental subduction:the increased proportion of V3+and accompanying reduced V incorporation into rutile leads to an increase in lattice sites available for Nb5+. A similar effect is observed under more oxidizing conditions. When V5+species increase, V partitioning into rutile decreases and Nb concentrations increase. This is documented by rutile in highly metasomatic and oxidized mantle xenoliths (Mica-Amphibole-Rutile-Ilmenite-Diopside) from the Kaapvaal craton, which also show a negative V-Nb covariation. In addition, their Nb/Ta is dependent on V concentrations:for V concentrations<1250ppm, Nb/Ta ranges between35and45, whereas for V>1250ppm, Nb/Ta is considerably lower (5to15). This relationship is mainly controlled by a change in Nb concentrations, suggesting that V concentrations have little effect on Ta partition into rutile due to lower Ta abundance as well as its smaller ionic radius relative to Nb. This indicates that the fO2-dependent DV for rutile can exert strong influence on Nb-Ta fractionation. |
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