Geochronology and geochemistry of the Tongjige granodiorites in the Jinshajiang suture zone, SW China: Constraints on petrogenesis and tectonic evolution of the Palaeo‐Tethys Ocean

The Jinshajiang suture zone (JSZ) is one of the most significant tectonic zones in the eastern Tethys area. A large number of plutons are regarded as the result of tectono-magmatic activities associated with the evolution of the Palaeo-Tethys Ocean. The Tongjige pluton in the central part of JSZ can indicate the magmatism and its geodynamic evolution. The Tongjige pluton (225.6 Ma) consists mainly of granodiorite with abundant coeval mafic microgranular enclaves (MMEs) (226.1 Ma). The host granodiorite is calc-alkaline and typical I-type, with SiO(2)contents ranging from 65.75 to 66.91 wt%. The MMEs have relatively high MgO (4.00-4.07 wt%) and Fe2O3T(9.27-9.40 wt%) abundances and low SiO2(53.59-53.87 wt%). Both MMEs and host granodiorites are characterized by the enrichment of light rare earth elements and depletion of heavy rare earth elements. They are also negative in Eu and enriched in Th and U, which are consistent with the trace element characteristics of typical island-arc magmatite. The zircon Hf(t) values (-3.48 to -0.31) are negative, with the model ages of 1.3-1.2 Ga. These petrographic and geochemical characteristics indicated that the Tongjige pluton was derived from the melting of an ancient lower crust and underwent the process of mafic-felsic magmatic mixing in the post-collision background. The Triassic granitoids in the JSZ show a temporal-spatial relationship with the subduction zone, which constrain the evolution of the Palaeo-Tethys Ocean. The mixing mechanism of the Tongjige pluton is associated with the break-off of a subducted slab, which triggered subsequent upwelling of asthenosphere beneath accreted arc fragments and induced underplate of lithospheric mantle-derived magmas and mixed with the lower crust-derived felsic magma. In general, the Late Triassic pluton recorded significant crustal growth and continental development, marking the final stage of subduction-related magmatism in the JSZ region, after which the region entered into a stage of post-collision evolution.