Petrogenesis and tectonic implications of cambrian Nb-enriched I- and aluminous A-type granites in the North Qilian suture zone
Granitoids in active continental margins are probes of sub-arc magmatic processes and are crucial for the understanding of crust–mantle interaction during subduction. Here we report data for the middle to late Cambrian Tuole (TL) and Dabaishitou (DBST) granitic plutons in the western segment of the North Qilian suture zone (NQSZ). The TL-DBST plutons comprise monzonite, alkali-feldspar granite, gabbroic diorite, monzonitic porphyry and biotite monzogranite. Zircon U‒Pb ages reveal that these granitoids crystallized at ca. 509‒492 Ma, consistent with the early subduction of the North Qilian Ocean. Mineralogical, petrological, and geochemical features indicate that the TL monzonite and gabbroic diorite are both high-K calc-alkaline I-type granitoids but the latter is Nb-enriched; the TL alkali-feldspar granites and DBST granitoids belong to aluminous A-type and transitional I- and A-type granites, respectively. The I- and aluminous A-type granites show similar and comparable isotopic compositions, e.g., εHf(t) values of ‒5.6 to +2.7 versus ‒2.4 to +3.5, but different melting conditions, e.g., high temperature and low oxygen fugacity for aluminous A-type granites as indicated by zircon trace elements. Geochemical and isotope data suggest that all these granitoids were mainly derived from the sub-arc mantle metasomatized by slab-derived fluids/asthenospheric mantle-derived melts/both prior to melting with minor input of LCC-derived melts (ca. 10–20%). The HFSE enrichment feature was inherited by granites and further enhanced by protracted crystal fractionation at low oxygen fugacity conditions to produce the TL aluminous A1-type granites but weakened by fractionation at relatively high oxygen fugacity conditions to produce the DBST I-A-transitional-type granite. Partial melting of the sub-arc lithospheric mantle was induced by upwelling of the asthenospheric mantle and their interactions in response to slab rollback during subduction initiation (ca. 509–506 Ma), and by addition of slab-derived fluids again as subduction proceeded (ca. 493–492 Ma).
