Contrasting Late Carboniferous and Late Permian-Middle Triassic intrusive suites from the northern margin of the North China craton: Geochronology, petrogenesis, and tectonic implications

摘要

Two contrasting intrusive suites have been identified from the northern margin of the North China craton: a Late Carboniferous dioritegranodiorite suite mainly made up of quartz diorite, diorite, granodiorite, tonalite, and hornblende gabbro, and a Late Permian–Middle Triassic suite of granitoid intrusions consisting of monzogranite, syenogranite, and quartz monzonite. Plutons from the Late Carboniferous suite exhibit variable SiO₂ contents and calc-alkaline or high-K calc-alkaline, metaluminous geochemical features. Most have low negative whole-rock _Nd(T) values (where T is the crystallization age) of –17.1 to –11.5 and zircon _Hf(T) values of –38.3 to –11.2, indicating that they were derived mainly from anatectic melting of the ancient lower crust with some involvement of mantle materials. However, an older pluton in the suite exhibits higher _Nd(T) values of –11.5 to –9.9, Nd model ages of 1.82–1.64 Ga, lower initial 87Sr/86Sr ratios of 0.7046–0.7048, and it contains some zircon grains that are characterized by high negative to positive zircon _Hf(T) values of –8.7 to 1.2, indicating strong involvement of juvenile materials derived from the lithospheric mantle. The Late Carboniferous plutons are interpreted as subduction-related and to have been emplaced in an Andean-style continental-margin arc during the southward subduction of the paleo–Asian oceanic plate beneath the North China craton. Rocks from the Late Permian–Middle Triassic intrusive suite display geochemical signatures ranging from highly fractionated I-type to A-type. They exhibit higher zircon _Hf(T) values of –14.9 to –6.7, whole-rock _Nd(T) values of –10.6 to –8.8, and younger Hf and Nd model ages than most of the Late Carboniferous plutons, indicating that they could have been produced by extreme fractional crystallization of hybrid magmas resulted from mixing of coeval mantle- and crust-derived melts. They are interpreted as postcollisional/postorogenic granitoids linked to lithospheric extension and asthenosphere upwelling due to slab break-off and subsequent sinking after final collision and suturing of the Mongolian arc terranes with the North China craton. These two contrasting intrusive suites suggest that the final closure of the paleo–Asian Ocean and collision between the Mongolian arc terranes and the North China craton occurred during the Late Permian, and these events were followed by postcollisional/postorogenic extension, large-volume magmatism, and significant continental growth. No significant syncollisional crustal thickening, high-pressure metamorphism, or S-type granitoid magmatism occurred during the collision process.