 The mafic-ultramafic igneous rocks formed due to underplating with crustal stretching during Carboniferous to Permian.The magma underplating during Carboniferous to Permian (330-250 Ma) period heated up the lower crust and thus the crust got hotter.The following cooling crustal episode led to part of the mountain belt sink by thermal subsidence, which ended up forming the Junggar Basin.