Petrogenesis of the Late Triassic shoshonitic Shadegai pluton from the northern North China Craton: Implications for crust-mantle interaction and post-collisional extension

【摘要】 Latest Permian to Triassic plutons are widespread in the northern North China Craton（NCC）; most of them show calc-alkaline, high-K calc-alkaline, or alkaline geochemical features. The Shadegai pluton in the Wulashan area has shoshonitic affinity and I-type character, and is composed of syenogranites containing abundant mafic microgranular enclaves（MMEs）. LA-MC-ICP-MS U-Pb data yield weighted mean 206 Pb/238 U ages of 222 ± 1 Ma and 221 ± 1 Ma for the syenogranites and MMEs, respectively, suggesting their coeval formation during the Late Triassic. The syenogranites have high SiO₂（70.42-72.30 wt%）,K₂O（4.58-5.22 wt.%） and Na₂O（4.19-4.43 wt.%） contents but lower concentrations of P₂O₅（0.073-0.096 wt.%） and TiO₂（0.27-0.37 wt.%）, and are categorized as I-type granites, rather than A-type granites, as previously thought. These syenogranites exhibit lower(⁸⁷Sr/⁸⁶Sr)i ratios（0.70532-0.70547） and strongly negative whole-rock εNd（t） values（-12.54 to-11.86） and zircon εHf（t） values（-17.81 to-10.77）,as well as old Nd（1962-2017 Ma） and Hf（2023-2092 Ma） model ages, indicating that they were derived from the lower crust.Field and petrological observations reveal that the MMEs within the pluton probably represent magmatic globules commingled with their host magmas. Geochemically, these MMEs have low SiO₂（53.46-55.91 wt.%）but high FeOt（7.27-8.79 wt.%） contents. They are enriched in light rare earth elements（LREEs） and large ion lithophile elements（LILEs）, and are depleted in heavy rare earth elements（HREEs） and high field strength elements（HFSEs）. They have whole-rock(⁸⁷Sr/⁸⁶Sr)i ratios varying from 0.70551 to 0.70564, εNd（t） values of -10.63 to -9.82, and zircon εHf（t） values of -9.89 to 0.19. Their geochemical and isotopic features indicate that they were derived from the subcontinental lithospheric mantle mainly metasomatized by slab-derived fluids, with minor involvement of melts generated from the ascending asthenospheric mantle. Petrology integrated with elemental and isotopic geochemistry suggest that the Shadegai pluton was produced by crust-mantle interactions, i.e., partial melting of the lower continental crust induced by underplating of mantle-derived mafic magmas（including the subcontinental lithospheric mantle and asthenospheric mantle）, and subsequent mixing of the mantle-and crust-derived magmas. In combination with existing geological data, it is inferred that the Shadegai pluton formed in a post-collisional extensional regime related to lithospheric delamination following the collision between the NCC and Mongolia arc terranes.更多还原

【Abstract】 Latest Permian to Triassic plutons are widespread in the northern North China Craton（NCC）; most of them show calc-alkaline, high-K calc-alkaline, or alkaline geochemical features. The Shadegai pluton in the Wulashan area has shoshonitic affinity and I-type character, and is composed of syenogranites containing abundant mafic microgranular enclaves（MMEs）. LA-MC-ICP-MS U-Pb data yield weighted mean 206 Pb/238 U ages of 222 ± 1 Ma and 221 ± 1 Ma for the syenogranites and MMEs, respectively, suggesting their coeval formation during the Late Triassic. The syenogranites have high SiO₂（70.42-72.30 wt%）,K₂O（4.58-5.22 wt.%） and Na₂O（4.19-4.43 wt.%） contents but lower concentrations of P₂O₅（0.073-0.096 wt.%） and TiO₂（0.27-0.37 wt.%）, and are categorized as I-type granites, rather than A-type granites, as previously thought. These syenogranites exhibit lower(⁸⁷Sr/⁸⁶Sr)i ratios（0.70532-0.70547） and strongly negative whole-rock εNd（t） values（-12.54 to-11.86） and zircon εHf（t） values（-17.81 to-10.77）,as well as old Nd（1962-2017 Ma） and Hf（2023-2092 Ma） model ages, indicating that they were derived from the lower crust.Field and petrological observations reveal that the MMEs within the pluton probably represent magmatic globules commingled with their host magmas. Geochemically, these MMEs have low SiO₂（53.46-55.91 wt.%）but high FeOt（7.27-8.79 wt.%） contents. They are enriched in light rare earth elements（LREEs） and large ion lithophile elements（LILEs）, and are depleted in heavy rare earth elements（HREEs） and high field strength elements（HFSEs）. They have whole-rock(⁸⁷Sr/⁸⁶Sr)i ratios varying from 0.70551 to 0.70564, εNd（t） values of -10.63 to -9.82, and zircon εHf（t） values of -9.89 to 0.19. Their geochemical and isotopic features indicate that they were derived from the subcontinental lithospheric mantle mainly metasomatized by slab-derived fluids, with minor involvement of melts generated from the ascending asthenospheric mantle. Petrology integrated with elemental and isotopic geochemistry suggest that the Shadegai pluton was produced by crust-mantle interactions, i.e., partial melting of the lower continental crust induced by underplating of mantle-derived mafic magmas（including the subcontinental lithospheric mantle and asthenospheric mantle）, and subsequent mixing of the mantle-and crust-derived magmas. In combination with existing geological data, it is inferred that the Shadegai pluton formed in a post-collisional extensional regime related to lithospheric delamination following the collision between the NCC and Mongolia arc terranes.更多还原