Magmatic gap associated with stepwise arc‐continent collision during the subduction of Banggong‐Nujiang Tethys Ocean: Evidence from Late Jurassic bimodal magmas

The magmatic suites in the southern margin of South Qiangtang Terrane (SQT) provide important clues to the process of northward subduction of the Banggong‐Nujiang Tethys Ocean (BNTO) and its geodynamic evolution. Previous studies identified a magmatic gap from Late Jurassic to Early Cretaceous (ca. 150–130 Ma) in the southern margin of SQT. Although the gap is generally correlated with the process related to BNTO subduction, the previous models on this remain equivocal. In this study, we investigated a suite of bimodal magmatic rocks newly recognized in the Rigenco area of SQT, including gabbros and granodiorites which were dated at 151.7 ± 3.8 Ma and 150.0 ± 2.0 Ma respectively. Based on geochemical and isotopic analyses, it is inferred that the gabbroic magmas were derived from a sub‐arc mantle source enriched by metasomatism of voluminous slab‐derived and crustal compositions, and that the granodiorites were derived from sub‐arc juvenile lower crust. Combining the petrogenesis of gabbros and granodiorites, it is proposed that the bimodal magmas were produced under a tectonic setting of general compression with local extension. A detailed evaluation of the tectonic setting of the magmatic bimodal suite suggests crustal thickening at ca. 150 Ma and that the BNTO involved multiple arcs and sub‐basins. A new model of arc‐SQT collision is proposed to account for the magmatic gap which commenced at ca. 150 Ma. The model of arc–continent collision in this case can well explain the present distribution and occurrence of magmatic rocks of late Jurassic age in the southern margin of the SQT, especially the magmatic gap. Our discovery of Late Jurassic bimodal rocks provides a critical evidence for step‐wise arc–continent collision during BNTO subduction, and contributes to the understanding of the magmatic gap in SQT and the evolution of BNTO.