Late Permian appinite-granite complex from northwestern Liaoning, North China Craton: Petrogenesis and tectonic implications

摘要

The appinite-granite suite commonly occurs within post-subduction regimes during the final stage of an orogeny and thus holds the key to monitoring critical geodynamic and crustal evolutionary processes. The present zircon U-Pb dating and geochemical study documents the Late Permian appinite-granite complex from northwestern Liaoning, North China Craton. The hornblende-rich appinitic rocks have an SiO2 range from 41.8 to 53.4%, moderate to high alkali contents, enrichment in large ion lithophile elements and depletion in high field strength elements, with initial ~(87)Sr/~(86)Sr ratios of 0.7066 to 0.7128, ε_(Nd)(t) of - 8.3 to -13.3 and zircon ε_(Hf)(t) from —10.1 to —18.7. These features suggest that their generation most likely involves metasomatism of mantle peridotites by sediment-derived liquids related to subduction and a later partial melting. The associated monzogranites range in SiO2 from 72.6 to 75.0% and exhibit a high-K calc-alkaline character, with high Sr-Ba abundances and elevated Sr/Y and La/Yb ratios. Such potassic adakitic signatures, plus their evolved isotopic compositions (~(87)Sr/~86)Sr_i = 0.7063 to 0.7066, ε_(Nd)(t) = -15.6 to -16.3, zircon ε_(Hf)(t) = —16.5 to —18.8), are consistent with partial melting of mixed protoliths including newly under-plated enriched mafic lower crust and minor old lower crustal materials. Such a mafic and felsic magma association not only attests to a heterogeneously enriched sub-continental lithospheric mantle beneath the northern North China Craton, but also indicates reworking within a post-orogenic transcurrent regime of lithospheric delamination. The temporal coincidence of the present appinite-granite complex with regional transcurrent fault activity leads to the further suggestion that such mafic-felsic magmatic suites could provide spatial markers for monitoring important post-orogenic structural and geodynamic processes.