Mafic and felsic magma interaction during the construction of high-K calc-alkaline plutons within a metacratonic passive margin: The Early Permian Guyang batholith from the northern North China Craton

摘要

Zircon U-Pb and hornblende ~(40)Ar/~(39)Ar dating document an Early Permian emplacement time for the Hercynian Guyang batholith from the northern margin of the North China Craton (NCC), with a variety of lithologies including: gabbro-gabbroic diorite-leuconorite, diorite and granodiorite, microgranular magmatic enclaves (MME), mafic dyke and monzogranite. Field relations, petrological, elemental, whole-rock Sr-Nd and zircon Hf isotopic data show that (1) the gabbros and gabbroic diorites exhibit strong enrichment of large ion lithophile element (LILE) and light rare earth element (LREE), depletion in high field strength element (HFSE), and have moderately enriched isotopic compositions with ~(87)Sr/~(86)Sri ranging from 0.7050 to 0.7053, ε_(HF)(t) from - 8.08 to -10.2 and zircon ε_(HF)(t) from - 6.7 to -12.0; while the leuconorites are characterized by low REE contents, conspicuous positive Eu anomaly and comparable Sr-Nd compositions to gabbroic rocks. These features suggest that they may represent complementary products from a parental magma derived from a subduction-related metasomatized sub-continental lithospheric mantle (SCLM) source; (2) the monzogra-nites range from 74.3 to 75.6% SiO2, with high Sr-Ba abundances and elevated Sr/Y and La/Yb ratios. Such high-silica adakitic affinity, plus their indistinguishable isotopic compositions from the mafic ones (~(87)Sr/ ~(86)Sr_i = 0.7053 to 0.7055,ε_(Nd)(t) = -10.5 to -11.3, zircon ε_(Hf)(t) = -8.5 to -11.8), indicates a felsic parental magma from partial melting of mixed protoliths composed of newly underplated mafic lower crustal and ancient mafic lower crustal materials; (3) the diorites and granodiorites contain abundant MME and display various scales of petrologic textures indicative of magma mixing. They also have intermediate geochemical compositions between gabbroic diorite and granite. Combined with their evolved isotopic compositions (~(87)Sr/ ~(86)Sr_i = 0.7050 to 0.7064, ε_(Nd)(t) = -8.36 to -13.9, zircon ε_(Hf)(t) = -7.9 to -15.5), they are inferred to be formed by mixing between SCLM-derived mafic magma and crustal-derived felsic magma; (4) the MME show igneous texture with SiO2 contents between 49.2 and 54.7%. Their trace elements and evolved isotopic character (~(87)Sr/~(86)Sr_i = 0.7049-0.7056, ε_(Nd)(t) = - 8.48 to -10.1; zircon ε_(Hf)(t) = - 8.8 to -14.7) indicate that they represent mafic magmas derived from the enriched SCLM and hybridized by diffusion and mechanical mixing with their host granitoids; and (5) the late-stage mafic dykes are gabbroic dioritic to dioritic in composition, with enrichment in LILE and LREE relative to HFSE, and more depleted isotopic signature than the gabbroic rocks (~(87)Sr/~(86)Sr = 0.7054-0.7057, ε_(Nd)(t) = -4.67 to - 9.36). These features point to a source in a hydrous and less enriched SCLM. The successive spatial-temporal association of mafic and felsic magma suites in the Guyang batholith attests to episodic crust-mantle interactions that occurred at depth, during ascent and emplacement and after emplacement, in the construction of high-K calc-alkaline plutons during the metacratonic evolution of the northern NCC, possibly in response to linear lithospheric delamination and hot asthenospheric upwelling along crustal-scale shear zones within a post-collisional transtensional regime of a passive continental margin.