Crustal Metasomatism In Continental Subduction Zone:geochemical Constraints From Orogenic Peridotite In The Sulu Orogen

Orogenic peridotites commonly occur as various sizes of lenses, blocks or layers within high-pressure(HP) to ultrahigh-pressure (UHP) metamorphic rocks. Compared to UHP gneisses and eclogites, orogenic peridotites can preserve petrological records not only for the subduction and exhumation of continental crust but also for mantle processes prior to continental collision. These unique characteristics make the orogenic peridotites an excellent window to study the nature of the mantle wedge above subducting slabs,crust-mantle interaction and chemical geodynamics at convergent plate margins. This thesis reports a combined study of petrology, geochemistry and zirconology for orogenic peridotites from the Sulu orogen in eastern China. The results are used to decipher the origin and evolution of orogenic peridotites, to determine the P-T conditions, time and mechanism of crustal metasomatism, to constrain the composition,nature and origin of metasomatic agents, and further to provide insights into the crust-mantle interaction in the continental subduction channel.Whole-rock elements and Sr-Nd isotopes, mineral elements as well as whole-rock and mineral O isotopes were measured for orogenic dunite and its host gneiss at Tengjia from the Sulu orogen. The results provide geochemical evidence for multiple episodes of crustal metasomatism at the slab-mantle interface in the continental subduction channel. The peridotite is characterized by enrichment in high refractory components but depletion in fertile components, making it dunite. Olivine has high Fo values and high NiO contents. Orthopyroxene has high Mg# but low CaO and Al2O3 contents. Spinel has high Cr# but low Ti02 contents. These characteristics indicate that the dunite would originate from the highly depleted subcontinental lithosphere mantle beneath the North China Craton. The dunite contains secondary minerals such as amphibole, chlorite, serpentine, dolomite and zircon. It shows slight enrichment in LILE and LREE but depletion in HFSE, positive Pb anomalies and enriched Sr-Nd isotope compositions. Rock-forming minerals exhibit positive Pb and Sr anomalies, low Th/U ratios and ?18O values similar to or lower than normal mantle. These observations suggest that the dunite underwent metasomatism by crust-derived fluids.Based on mineral texture, major and trace elements, multiple generations of mineral growth are identified for amphibole, pyroxene and spinel. Mineral composition and P-T estimates indicate that the dunite precursor was transformed from spinel peridotite to garnet peridotite during subduction from forearc to subarc depths. During its transport from the mantle wedge into the subduction channel and further subduction, the dunite suffered the first episode of metasomatism by the subducting continental crust-derived fluids at forarc depths to form metasomatized peridotites. These metasomatized peridotites were further subducted to subarc depths for UHP metamorphism at 4.0-4.4 GPa and 773-838? in the subduction channel. The second and third episodes of metasomatisms are recorded by growth of dolomitic carbonate from carbonate melt at the initial exhumation stage and by growth of metasomatic zircon and amphibole at the late exhumation stage, respectively. The metasomatic agent was not originated from the gneiss directly hosting the peridotite, but from another part of the deeply subducted continental crust that was not in contact with the dunite on the outcrop. These multiple episodes of crustal metasomatism indicate that the mantle wedge bottom peridotite was dragged into the subduction channel during continental subduction and underwent metasomatism at different depths by different compositions of crustal fluid.Therefore, the orogenic peridotite records geochemical transfer from the subducted slab to the mantle wedge in continental subduction zones.A combined secondary ion mass spectrometer (SIMS) and laser ablation-(multicollector)-inductively coupled plasma mass spectrometer (LA-(MC)-ICPMS) study of zircon U-Pb ages, trace elements, and O and Hf isotopes was carried out for the Tengjia dunite and its host gneiss in the Sulu orogen. The results not only reveal the genesis of zircon in the orogenic peridotite but also provide insights into crust-mantle interaction in the continental subduction channel. The majority of zircons from the Tengjia dunite show core-(mantle)-rim structure in CL images. Newly grown zircon domains exhibit weak zoning or no zoning,relatively low Th/U ratios (<0.1), low heavy rare earth element (HREE) contents, steep middle rare earth element (MREE)-HREE patterns, negative Eu anomalies, and negative to low ?18O values of-11.3 to 0.9‰and U-Pb ages of 220 ± 2 to 231 ± 4 Ma. Thus, these zircons would have grown from crustal fluids during the early exhumation of deeply subducted continental crust. The infiltration of crustal fluids into the peridotite is also indicated by the occurrence of hydrous minerals such as amphibole. In contrast, relict zircon domains exhibit oscillatory or blurred oscillatory zoning, high Th/U ratios, and steep MREE-HREE patterns, which is typical for magmatic zircon. Their U-Pb ages, trace elements and Hf-O isotopes are similar to those for protolith zircons from UHP metamorphic rocks in the Dabie-Sulu orogenic belt. Thus,these relict magmatic zircons would be physically transported into the peridotite by the crustal fluids originated from the deeply subducted continental crust. Therefore, the peridotite underwent metasomatism by aqueous solutions derived from dehydration of the deeply subducted continental crust during the early exhumation. It is these crustally derived fluids that would have brought not only such chemical components as Zr and Si but also tiny zircon grains from the deeply subducted crustal rocks into the peridotite at the slab-mantle interface in continental subduction channels. As such, the orogenic peridotite records the crust-mantle interaction at the deep continental subduction zone.Mineral water content and element composition, together with whole-rock major and trace element compositions, were determined for garnet pyroxenites enclosed by UHP metamorphic gneiss at Hujialin in the Sulu orogen. The results not only place constraint on the genesis orogenic pyroxenite but also the storage capacity of water in garnet pyroxenites, but also provide insights into the crust-mantle interaction in the subduction zone. Whole-rock and mineral element and isotope studies suggest that the garnet pyroxenites were generated in the Triassic by metasomatic reaction of the mantle wedge peridotite with hydrous felsic melts derived from dehydration melting of the deeply subducted continental crust. Measured H20 contents range from 523 to 1213 ppm for clinopyroxene, and 55 to 1476 ppm for garnet. These mineral water contents are not only correlated with mineral major and trace element abundances but also relatively homogenous within single mineral grains. Such features preclude significant disturbance of the mineral water contents during pyroxenite exhumation from the mantle depth to the surface and thus indicate preservation of the primary water contents for the mantle metasomatite. The relationships between contents of mineral water and some elements suggest that the high water contents of garnet pyroxenites are primarily dictacted by the abundance of water-rich clinopyroxene. Garnet also has high water contents,suggesting that this mineral is also an important phase in hosting water at the mantle depth. Calculated whole-rock water contents and H20/Ce ratios are 424-660 ppm and 63-145, respectively. The pyroxenite water contents are higher than those for the MORB source, similar to or higher than those for the OIB sources and close to the lower limit for the arc magma source. The pyroxenite H2O/Ce ratios are higher than those for Hawaiian garnet pyroxenites and SWIR abyssal pyroxenites. These observations suggest that metasomatic pyroxene-rich lithologies have the capacity to contribute high H2O concentrations and variable H20/Ce ratios to the mantle. This lends support to the interpretation that the source of some intraplate basalts may be a heterogeneous mixture of peridotite and pyroxenite. On the other hand, the high water contents of pyroxenites suggest that the presence of pyroxenites in the mantle wedge would enhance its water storage and thus reduce the water transport into deeper mantle by subduction.