Early Paleozoic Magmatic "Flare-Ups" In Western Qinling Orogeny, China: New Insights Into The Convergence History Of The North And South China Blocks At The Northern Margin Of Gondwana

The geodynamic evolution of an accretionary orogeny commonly includes multiple magmatic episodes, which can be used to trace orogenic geodynamic processes using the geochronological and petrogenetic records preserved by igneous rocks. Paleomagnetic data indicate that the juxtaposed North China Craton (NCC) and South China Block (SCB) were located on the northern margin of the Gondwana supercontinent, but the geodynamic history of the two related blocks is still unclear. To address this issue, this paper integrates literature data with new geochronological, elemental, and isotopic data from early Paleozoic intermediate and silicic intrusions of the western North Qinling Terrane. We recognize three early Paleozoic flare-ups of intermediate-silicic intrusive magmatism in this area for the first time. Despite their similar lithology, the early Paleozoic igneous rocks formed during the three pulses have distinct geochemical compositions and petrogenesis, and record the evolution from subduction to continental collision. Geochronological data record: (1) An early Silurian (c. 441-434 Ma) pulse of diorite-quartz diorite-granite magmatism. The diorites and quartz diorites probably formed by fractionation of partial melts of mantle wedge modified by slab-derived hydrous fluids, while the granites formed by the partial melting of juvenile mafic lower crust. (2) A middle Silurian high-flux pulse of diorites, quartz diorites, and leucogranites (two-mica and muscovite granites) at c. 430-423 Ma. The diorites and quartz diorites were generated by crustal assimilation and fractional crystallization of basaltic partial melts of a pre-existing mantle source that had been altered by slab-derived hydrous fluids, and that was subsequently modified by slab-melts. The two-mica and muscovite granites formed by water-present melting of early-middle Silurian metamorphosed igneous rocks derived from enriched mantle and (meta-)graywackes, respectively. (3) A late Silurian (c. 415-409 Ma) magmatic pulse formed quartz diorites, A-type granites, and leucogranites (muscovite granites). The quartz diorites formed by differentiation of basaltic partial melts of enriched mantle, with a large contribution from depleted asthenospheric mantle. The A-type granites were generated by anatexis of juvenile mafic lower crust and subsequent fractional crystallization. The muscovite granites formed by biotite dehydration melting of (meta-)graywackes. The origins of these three early Paleozoic magmatic pulses were integrated with published regional paleomagnetic data and descriptions of sedimentary, metamorphic and igneous rocks, to provide a unique record of the geotectonic evolution of the Qinling Orogenic Belt that included: (1) slab rollback at c. 448-434 Ma; (2) initial continental collision at c. 433 Ma; (3) slab break-off at c. 430-423 Ma; (4) smallscale lithospheric foundering in a post-collisional setting at c. 415-409 Ma. Literature-derived geochronological records of collision-related magmatism and paleomagnetic data indicate that docking of the NCC and SCB with the northern Gondwana margin probably ended during the early Silurian (c. 433 Ma). (C) 2020 Elsevier B.V. All rights reserved.