Magma mixing controlling the origin of the Early Cretaceous Fangshan granitic pluton, North China Craton: In situ U-Pb age and Sr-, Nd-, Hf- and O-isotope evidence

摘要

In situ U-Pb age and Sr-, Nd-, Hf- and O- isotope analysis of accessory minerals (zircon, apatite and titanite) have been obtained for mafic microgranular enclaves and their host granites from the Early Cretaceous Fangshan pluton in the North China Craton, in order to constrain their sources and petrogenesis. The Fangshan pluton is composed of quartz monzodiorite at the margins, and granodiorite, with abundant mafic microgranular enclaves, in the core. Zircons from the marginal quartz monzodiorite record U-Pb ages of 130 to 133 Ma, identical to those of granodiorite in the Core. The enclaves have zircon U-Pb ages of 133 to 134 Ma, identical within error to those of the host granodiorite, establishing that the mafic and felsic magmas were coeval. Furthermore, apatite and titanite from the enclaves and host granodiorite have the same U-Pb ages as the zircons, indicating rapid cooling of the Fangshan pluton. The granodiorites have relatively high SiO2 and low MgO concentrations. They have variable negative ε_(Hf)(t) values of —25.1 to —18.2 and δ~(18)O values for zircon of 6.2‰> to 8.3‰. The negative ε_(Nd)(t) values of —19.7 to -14.0 for titanite and apatite, with initial ~(87)Sr/ ~(85)Sr ratios of 0.7052-0.7058 for apatite, indicate that they were derived from an ancient lower crustal source with addition of materials with higher ε_(Hf)(t) and ε_(Nd)(t) values and low δ~(18)O values. The enclaves have relatively high MgO contents (up to 15.5 wt.%) at intermediate silica concentrations, indicating that their parental magmas were derived from a mantle source. Their zircons have strongly negative Hf isotopic compositions (ε_(Hf)(t) = —22.9 to —14.9). The Nd isotopic composition of titanite and apatite is also strongly negative (ε_(Nd)(t) = -17.6 to —12.1). All these features, combined with the δ~(18)O zircon values of 4.8‰-8.2‰ for zircons and relatively low initial Sr isotopic compositions of apatite (initial ~(87)Sr/~(86)Sr = 0.7051-0.7058), indicate that they were derived from an enriched and/or depleted mantle source, but with involvement of crustal materials. The marginal quartz monzodiorites have transitional geochemical features between those of the enclaves and host granodiorites. In situ Sr, Nd, Hf, and O isotopic data of accessory minerals, coupled with field observations and whole rock geochemical data, rule-out simple crystal-liquid fractionation or wall-rock contamination as the major genetic link between the enclaves and host rocks. Instead, mixing of mantle-derived mafic magmas with crustal-derived felsic magmas, coupled with assimilation of wall rocks, is compatible with the data. Our study shows that in situ Hf, O, Nd and Sr isotopic analyses of accessory minerals are powerful geochemical tracers that provide unique petrogenetic information regarding the origin of granitoids.