A Geochemical Study Of High-to Ultrahigh-pressure Metamorphic Rocks From The Qilian Orogen

Eclogites are primary targets for studying high-pressure (HP) to ultrahigh-pressure (UHP) metamorphism and pre-subduction tectonics in collisional orogens. They may form in both oceanic and continental subduction zones, and both oceanic-and continental-type eclogites have multiple origins and versatile geochemistries. As continental subduction is pulled by oceanic subduction, oceanic-type eclogites may coexist with continental-type eclogites in collisional orogens, recording the tectonic transition from oceanic subduction to continental subduction. The Qilian orogen in the northeastern Tibet is a composite orogen that contains oceanic-type HP metamorphic zone in the north (North Qilian) and continental-type UHP metamorphic zone with outcrops of oceanic eclogites in the south (North Qaidam). This PhD thesis deals with whole-rock geochemistry and zirconology of eclogites from North Qilian and North Qaidam, respectively. The results not only illustrate the diversity of eclogite protoliths but also distinguish between oceanic-and continental-type eclogites. This provides further constraints on the tectonic evolution from oceanic subduction to continental subduction in this composite orogen. In addition, a combined study of petrology, geochronology and geochemistry was carried out for felsic veins and their host rocks from the Xitieshan zone of the North Qaidam, and the results are used to decipher the microstructure, timing, mechanism and geochemistry of anatexis in continental subduction zones.An integrated study of whole-rock geochemistry, mineral O isotope geochemistry and zirconology was carried out for low-T/HP eclogites and mafic blueschists from the North Qilian zone. The results demonstrate that these oceanic-type eclogites and blueschists were produced by metamorphism of backarc basin basalts (BABB) rather than mid-ocean ridge basalts (MORB) as commonly thought. These HP metabasites show significant heterogeneity in their major and trace element compositions, varying from island arc basalts (LAB)-like, MORB-like to oceanic island basalts (OIB)-like, respectively, in trace element distribution patterns. Such variable compositions are comparable with those of BABB at different stages. Whole-rock Nd isotope analyses indicate that the compositional difference between the metabasite protoliths can be ascribed to involvement of different amounts of subducted sediment-derived melts in the backarc mantle source. Oxygen isotope fractionations between coexisting minerals are not at equilibrium; garnet O isotope analyses indicate that whole-rock δ18O values are either higher or lower than normal mantle value, which is attributable to seawater-hydrothermal alteration during seafloor spreading to open the backarc basin. Relict magmatic zircon domains show oscillatory zoning, high Th and U contents, high Th/U ratios, steep HREE patterns, and protolith U-Pb ages of 496～486 Ma. Metamorphic zircon domains exhibit lowered Th, U and HREE contents and Th/U ratios than the relict magmatic domains and contain mineral inclusions of omphacite, rutile and phengite, giving concordant U-Pb ages of 463±10 Ma for eclogite-facies metamorphism. Zircons have markedly depleted Hf isotope compositions with Hf model ages slightly or significantly older than the protolith ages, suggesting incorporation of crustal components into the magma sources of these metabasite protoliths. Most zircons have δ18O values different from normal mantle values, and the distinct zircon O isotopes in different samples are in accordance with garnet δ18O values for the same sample, pointing to localized fluid sources for the metamorphism. Therefore, the different metabasites with different compositions were metamorphosed from different basaltic rocks generated in a backarc basin in the early Paleozoic. Their occurrence in the same region indicates that the backarc basin was transformed to the oceanic subduction zone where eclogite-facies metamorphism occurred.In order to constrain the protolith nature and metamorphic evolution of UHP eclogites in the North Qaidam zone, a comprehensive study of whole-rock major-trace elements, Sr-Nd isotopes and mineral O isotopes, as well as zircon U-Pb ages, trace elements, mineral inclusions and O-Hf isotopes was carried out for the eclogites. The results are used to discriminate between oceanic-type and continental-type eclogites and to place constraints on the tectonic transition from oceanic subduction to continental collision. CL-dark zircon domains exhibit high Th/U ratios, steep HREE patterns and significantly negative Eu anomalies, indicating that they are protolith zircons of magmatic origin with different extent of metamorphic recrystallization. Relict magmatic zircon domains in one type of eclogites yield Neoproterozoic protolith ages of ～830 Ma and Hf model ages of 850-1100 Ma, whereas those in the other type of eclogites yield Cambrian protolith U-Pb ages of ～500 Ma and Hf model ages of 500～650 Ma. The first type of eclogites is dominant in volume and also has versatile trace element compositions, mostly positive εNd(t) values and normal mantle-like δ18O values that are similar to continental rift basalts originated from different compositions of mantle sources. Therefore, this type of eclogites is the continental-type that was metamorphosed from continental rift mafic igneous rocks produced during the breakup of Rodinia supercontinent. The second type of eclogites is minor in volume and has trace element compositions varying from arc-like and MORB-to OIB-like, positive εNd(t) values and lower δ18O values than normal mantle. Therefore, this type of eclogites is the oceanic-type that was metamorphosed from oceanic mafic igneous rocks produced in a backarc basin and experienced high-T seafloor alteration. Most of the CL-bright zircon domains show low Th/U ratios, flat HREE patterns and no negative Eu anomalies, and contain mineral inclusions of garnet, omphacite and rutile. indicating their growth under eclogite-facies metamorphic conditions. These metamorphic domains have consistent eclogite-facies metamorphic ages of 433～440 Ma throughout the North Qaidam zone, regardless of the eclogite types and locations. Gneisses show similar eclogite-facies metamorphic ages of 427～439 Ma, with coesite inclusions in metamorphic zircon from one gneiss. The identical ages for the oceanic-type and continental-type metamorphic rocks indicate that the exhumed oceanic-type eclogites were primarily residing in the marginal basin. Nevertheless, the coexistence of oceanic- and continental-type eclogites in the North Qaidam zone demonstrates the tectonic transition from oceanic subduction to continental collision in the early Paleozoic. In addition, the similar protolith ages and geochemistry of oceanic metabasites from the North Qilian and North Qaidam zones suggest that together with the Qilian Block the three zones may represent a composite orogen that would have evolved from oceanic subduction to continental collision.A combined study of petrology, geochronology and geochemistry was carried out for felsic veins and their host rocks from the North Qaidam zone. The results provide insights into partial melting of deeply subducted continental crust during exhumation. Partial melting is petrographically recognized in metagranite, metapelite and metabasite, respectively. Migmatized gneisses, including metagranite and metapelite, contain microstructures such as granitic aggregates with varying outlines, small dihedral angles at mineral junctions and feldspars with magmatic habits, indicating the former presence of felsic melts. Partial melts were also present in metabasite that occurs as retrograde eclogite. Felsic veins in the both eclogites and gneisses exhibit typical melt crystalline textures such as large euhedral feldspar grains with straight crystal faces, indicating vein crystallization from anatectic melts. The Sr-Nd isotope compositions of felsic veins inside gneisses suggest melt derivation from anatexis of host gneisses themselves, but those inside metabasites suggest melt origination from hybrid sources. The felsic veins inside gneisses exhibit similar lithochemical compositions to experimental melts on the An-Ab-Or triangle diagram. In the trace element distribution diagrams, they exhibit parallel patterns to their host rocks, but with lower element contents and slightly positive Eu and Sr anomalies. The geochemistry of these felsic veins is controlled by minerals that would decompose and survive, respectively, during the anatexis. The felsic veins inside metabasites are rich either in quartz or in plagioclase with low normative orthoclase. In either case, they have low trace element contents, with significantly positive Eu and Sr anomalies in plagioclase-rich veins. Combined with cumulate structures in some veins, these felsic veins are interpreted to crystallize from anatectic melts of different origins with the effect of crystal fractionation. Nevertheless, felsic veins in different lithologies exhibit roughly consistent patterns of trace element distribution, with variable enrichment of LILE and LREE but depletion of HFSE and HREE. There are also higher contents of trace elements in the veins hosted by gneisses than the veins hosted by metabasites. Anatectic zircon domains from felsic veins and migmatized gneisses exhibit consistent U-Pb ages of ～420 Ma, significantly younger than the peak UHP eclogtie-facies metamorphic event at c.450～435 Ma. Combining the petrological observations with local P-T paths and experimentally constrained melting curves, it is inferred that the anatexis of UHP gneisses was caused by muscovite breakdown while the anatexis of UHP metabasites was caused by fluid influx. These UHP metagranite, metapelite and metabasite underwent the simultaneous anatexis during the exhumation, giving rise to anatectic melts with different compositions in various elements but similar patterns in trace element distribution.