The subducted oceanic crust within continental-type UHP metamorphic belt in the North Qaidam, NW China: Evidence from petrology, geochemistry and geochronology

摘要

Three types of eclogite, together with a serpentinized harzburgite, coexist as blocks within granitic and pelitic gneisses along the Shaliuhe cross section, the eastern part of the North Qaidam continental-type ultrahigh-pressure (UHP) metamorphic belt, NW China. The olivine (Ol~1) and orthopyroxene in the harzburgite are compositionally similar to present-day abyssal peridotites. The kyanite-eclogite is derived from a troctolitic protolith, whereas the epidote-eclogite from a gabbroic protolith, both having distinct positive Eu anomalies, low TiO2, and high Al2O3 and MgO. The kyanite-eclogite shows inherited cumulate layering. The phengite-eclogite has high TiO2, low Al2O3 and MgO with incompatible trace elements resembling enriched-type MORB. Sr-Nd isotope data indicate that the protoliths of both kyanite-eclogite and epidote-eclogite ([~(87)Sr/~(86)Sr],—0.703-0.704; ε_(Nd)(T)～5.9-8.0) are of mantle origin (e.g., ocean crust signatures). On the other hand, while the lower ε_(Nd)(T) value (1.4-4.1) of phengite-eclogite is more or less consistent with an enriched MORB protolith, their high [~(87)Sr/~(86)Sr],- ratio (0.705-0.716) points to an additional enrichment in their history, probably in an subduction-zone environment. Field relations and geochemical analyses suggest that the serpentinized harzburgite and the three types of eclogite constitute the oceanic lithological section of an ophiolitic sequence from mantle peridotite, to cumulate, and to upper basaltic rocks. The presence of coesite pseudomorphs and quartz exsolution in omphacite plus thermobarometric calculations suggests that the eclogites have undergone ultrahigh pressure metamorphism (i.e., peak P≥:2.1 GPa). The harzburgite may also have experienced the same metamorphism, but the lack of garnet suggests that the pressure conditions of ≤ 3.0 GPa. Zircon U-Pb SHRIMP dating shows that the eclogites have a protolith age of 516±8 Ma and a metamorphic age of 445±7 Ma. These data indicate the presence of a Paleo-Qilian Ocean between Qaidam and Qilian blocks before the early Ordovician. The ophiolitic assemblage may be the relics of subducted oceanic crust prior to the subduction of continental materials during Ordovician-Silurian times and ultimate continent collision. These rocks, altogether, record a complete history of ocean crust subduction, to continental subduction, and to continental collision.