Crustal Anatexis During Continental Collision:Evidence From Ultrahigh-pressure Metamorphic Rocks In The Sulu Orogen

The study of continental deep subduction and ultrahigh-pressure (UHP) metamorphism has been one of the forefronts and hotspot topics in solid Earth sciences since the groundbreaking findings of coesite and diamond in metamorphic rocks of supracrustal protolith in1980’s and1990’s. The Dabie-Sulu orogenic belt in east-central China is one of the largest and best exposed UHP metamorphic zones in the world, representing an excellent natural laboratory for the study of petrological and geochemical changes during continental subduction-zone metamorphism. This PhD thesis focuses on petrology and geochemistry of UHP metamorphic rocks in the middle and southwestern parts of the Sulu orogen. The results provide evidence for crustal anatexis during continental collision and new constraints on fluid action and element mobility, the behavior of accessory minerals, and metamorphic zirconology. This has great bearing on chemical geodynamics of continental subduction zones.A combined study of petrology and zirconology was carried out for UHP granitic gneiss and quartzite from the Yangkou, Lanshan and Qinglongshan areas. The results indicate that these rocks experienced anatexis during exhumation of deeply subducted continental crust. Petrographic observations show the occurrence of elongated, highly cuspate feldspars in grain boundaries, interstitial cuspate feldspars in triple junctions, felsic veinlets mainly consisting of K-feldspar+quartz, and feldspar crystal faces against quartz. These features indicate that some feldspar and quartz grew from anatectic melts in the granitic gneiss and quartzite, with inheritance in melt microstructure. Anatectic zircon domains grown from the melts are distinguished from metamorphic zircon domains grown at subsolidus conditions based on CL images, mineral inclusions and REE patterns. Some anatectic zircon domains contain coesite inclusions and exhibit relatively low U contents, low Th/U ratios, steep REE patterns with strong negative Eu anomalies. Their U-Pb ages are221±5to226±3Ma, mostly clustering at224±2Ma. Nevertheless, the majority of anatectic zircon domains does not contain coesite inclusion and exhibits high U contents, low Th/U ratios (<0.1), steep REE patterns with strong negative Eu anomalies, and U-Pb ages of217±2to224±2Ma. These results indicate that the UHP rocks experienced incipient melting during the early exhumation but still in the UHP regime, and then extensive anatexis at lower pressures. Muscovite relicts coexist with cuspate feldspars in the granitic gneiss, suggesting that the anatectic melts originate from dehydration melting due to phengite breakdown. Zircon grains from the impure quartzite are all metamorphic growth, and they contain not only eclogite-facies mineral inclusions of coesite, jadeite, rutile but also lower pressure mineral inclusions that include multiphase solid (MS) inclusions composed of two or more phases of muscovite, quartz, K-feldspar and plagioclase. Some MS inclusions have granitic composition and show granophyric texture, representing former hydrous melt. Thus, such zircon domains would have grown from anatectic melts, consistent with their steep REE patterns and negative Eu anomalies. The anatectic zircon domains containing different mineral inclusions exhibit similar U-Pb ages of-220Ma, suggesting that the UHP rock experienced a relatively rapid exhumation. Most zircon domains have similar δ18O values of-0.6to0.1‰, with a few in the range of-5.2to-4.3‰. The all zircon domains exhibit similar U-Pb ages, trace element and Hf isotope compositions. Thus, the anatectic melts probably have two sources with different O isotope compositions. The predominate one may derive from the quartzite itself, whereas the other from the country rocks. Zircon grains from the pure quartzite contain relict cores of magmatic origin and significant metamorphic overgrowth rims. Zircon domains that contain eclogite-facies mineral inclusions exhibit flat HREE patterns, no Eu anomalies and U-Pb ages of-220Ma. Similar U-Pb ages are obtained for domains that contain lower pressure mineral inclusions and exhibit steep REE patterns and marked negative Eu anomalies. These observations indicate that zircon records subsolidus overgrowth at eclogite-facies conditions but suprasolidus growth at lower pressures. There are two types of garnet in the impure quartzite. One has high Al contents, low LREE and Nb-Zr contents, and steep REE patterns; the other exhibits low Al but high Fe contents, significantly elevated LREE and Nb-Zr contents and variable HREE patterns. The latter garnets contain mineral inclusions of K-feldspar, plagioclase, quartz and titanite, sometimes in coexistance with amphibole. Zircon enclosed by the latter garnet gave consistent U-Pb ages of-214Ma, thus such garnet is interpreted as a peritectic product of the anatectic reaction that involves felsic minerals and possibly amphibole and titanite. The REE patterns of epidote and titanite also record their multistage growth and metasomatism by anatectic melts.A combined in-situ study of zircon U-Pb ages, trace elements and O-Hf isotopes was conducted for the UHP granitic gneiss and eclogite in the Qinglongshan area of northern Jiangsu province. The results exhibit correlations between zircon818O values, U-Pb ages, Th/U ratios and REE patterns for the relict cores of magmatic origin and the newly grown rims of metamorphic origin. Generally, the relict magmatic cores with U-Pb ages of769±9Ma have positive δ18O values of0.1to10.1‰, high Th/U and176Lu/177Hf ratios, high REE contents, and steep MREE-HREE patterns with negative Eu anomalies. They are interpreted as crystallizing from positive δ18O magmas during protolith emplacement in the Neoproterozoic. In contrast, the newly grown domains have concordant U-Pb ages of204±4to252±7Ma and negative δ18O values of-10.0to-2.2%o, low Th/U and176Lu/177Hf ratios, low REE contents, and flat HREE patterns with weak to no Eu anomalies. They are interpreted as growing from negative δ18O fluids that were produced by metamorphic dehydration of the Neoproterozoic high-T glacial-hydrothermally altered rocks during the Triassic continental collision. Large O isotope heterogeneities occur in both intergrain and intragrain of zircon, indicating the limited O isotope exchange between different zircon domains in a single grain or between the relict magmatic domains and metamorphic fluids. The results suggest that protolith magmatic zircon underwent three subtypes of metamorphic recrystallization, with the extent of recrystallization depending on the accessibility to negative δ18O fluids. The solid-state recrystallized zircon domains maintained positive δ18O values and the REE and Lu-Hf isotopes of protolith zircon, but their U-Pb ages are somewhat lowered. The dissolution recrystallized zircon domains exhibit negative δ18O values similar to the metamorphic growths, almost completely reset U-Pb ages, and partially reset REE systems. The replacement recrystallized zircon domains show variably negative δ18O values, and partially reset REE, and U-Pb and Lu-Hf isotopic systems, with local dissolution recrystallization along grain boundaries and fractures. Therefore, these results place robust constraints on the origin of negative δ18O zircon in the UHP rocks from the Sulu orogen and provide a methodological framework to distinguish the different types of metamorphic zircons in continental subduction zones.Multiphase solid inclusions in both garnet and omphacite were investigated for zoisite-bearing UHP eclogite in the Qinglongshan area. The results provide petrological evidence for local anatexis of the eclogite during the continental collision. There are three types of MS inclusions, which are composed of plagioclase+quartz, plagioclase+quartz+K-feldspar, and barite+plagioclase+K-feldspar±zoisite/epidote, respectively. They generally have low trace element contents except such large ion lithophile elements (LILE) as Sr, Ba and Pb. The first and second types of MS inclusions have high contents of SiO2(75～90wt%) and Na2O (2.9to7.3wt%), very low FeO+MgO+TiO2contents and variable K2O contents (0to2.4wt%). The host minerals mostly exhibit radial fractures surrounding the MS inclusions. These features suggest different origins for the MS inclusions. The first type of MS inclusion would be primarily derived from dehydration melting of zoisite with involvement of omphacite to form Na-rich melts, whereas the second type of MS inclusion was derived from dehydration melting of both zoisite and phengite in the eclogite to form melts with variable K contents. The third type of MS inclusion is a result of interaction between the aqueous fluid of high oxygen fugacity and the host mineral. While anatexis of the zoisite-bearing eclogite is evident from the occurrence of MS inclusions, the major and trace element compositions of such MS inclusions provide insights into the origin of partial melts in the eclogite and thus into the nature of dehydration melting in the UHP metamorphic zone.An integrated study of petrology, geochronology and geochemistry was performed for UHP gneisses in the Baihushan area close to the Qinglongshan in northern Jiangsu province. The results not only highlight the polyphase growth of such metamorphic minerals as zircon, titanite and garnet in response to P-T changes and fluid/melt action, but also provide insights into the property of metamorphic fluid/melt and their effects on trace element mobility during the continental collision. A combined result from REE patterns, mineral inclusions and Ti-in-zircon temperatures suggests three stages of zircon growth. Prograde growth occurred at-237Ma primarily at eclogite-facies, retrograde growth at-222Ma mostly at eclogite-facies, and the last growth at-205Ma possibly at granulite-facies. The three stages of zircon growth are deciphered by distinct REE patterns and trace element compositions, recording the differences in the property of metamorphic fluid/melt. The episodic growth of metamorphic zircon is primarily dictated by the episodic releasing and focusing of metamorphic fluid/melt. Relict domains of magmatic titanite are distinguished from metamorphosed and metamorphic domains by their distinctive REE patterns and trace element compositions. The metamorphic titanite exhibit variably elevated Nb contents and Nb/Ta ratios, suggesting significant Nb/Ta fractionation during titanite formation. Polyphase growth of garnet from prograde amphibolite-facies to retrograde eclogite-facies is suggested by an integrated analysis of mineral inclusions and major element compositions in large garnet grains. Trace element contents vary in different zones of garnet, which is ascribed to changes in the paragenesis and composition of matrix minerals involved in garnet-forming reactions at different P-T conditions.A comprehensive study was carried out for UHP metamorphic rocks from the Donghai area in northern Jiangsu province for their whole-rock geochemistry, mineral O isotopes and zirconology. The results confirm that protoliths of the UHP rocks experienced strong high-T glacial meltwater-rock interaction in the Neoproterozoic, leading to their variable depletion of18O, with δ18O values as low as-10.8%o. The mineral O isotope composition has a large spatial heterogeneity, with δ18O values differing up to13.5%o on the kilometer scale and3.7%o on a single outcrop. This indicates the primary O isotope heterogeneity that was established during protolith emplacement but not homogenized even at hand-specimen scale during the Triassic UHP metamorphism and very limited fluid flow during the continental subduction-zone metamorphism. However, the fluid flow is locally significant during exhumation, resulting in the formation of quartz veins, symplectites, coronas, and local anatexis in UHP rocks. Geochemical transport due to fluid action is evident in whole-rock geochemistry and mineralogical composition. The UHP rocks exhibit unreasonably low87Sr/86Sr ratios at t1=750Ma but much radiogenic Sr isotopes at t2=230Ma, suggesting the mobility of water-soluble LILE due to both the hydrothermal alteration during protolith emplacement and the metamorphic dehydration during continental collision. Fluid-rock interaction during the continental collision mobilized Na, Al, Si, Ca, LREE and LILE, resulting in the formation of high-pressure veins in the UHP eclogites. The protolith zircon of magmatic origin underwent different types of metamorphic recrystallization in response to fluid-mineral interaction, leading to different extents of mobility in trace elements and O-Hf isotopes. Both positive εHf(t) values close to the depleted mantle of Neoproterozoic age and highly variable negative εHf(t) values occur in the relict magmatic zircon domains, indicating that the protoliths of UHP rocks were formed by reworking of both juvenile and ancient crustal rocks in the middle Neoproterozoic.