Evidence against subduction-related magmatism for the Jiaoziyan Gabbro, northern Dabie Shan, China

The largest gabbroic intrusion, Jiaoziyan, in the Northern Orthogneiss of t he Dabie Shan was investigated to test whether such intrusions represent ar e-related or collision-related magma, as postulated in recent regional tect onic models. The Jiaoziyan intrusions comprise a gabbro-monzogabbro suite a nd are locally characterized by cumulus textured anhydrous silicates, Petro logic indications against are magmatism include (1) coexisting olivine-plag ioclase compositions beyond the documented range of are gabbros, and (2) a compositional trend of Al/Ti ratios in the gabbroic clinopyroxenes that is significantly different from subduction-related gabbros worldwide. In contr ast, the Jiaoziyan rocks are mineralogically similar to rift gabbros, Geoch ronologic and geometric arguments against collision-related magmatism (i.e. , as a consequence of slab breakoff) include the following: (1) crystalliza tion of the Jiaoziyan mafic intrusion postdated the Triassic (245-240 Ma) s ubduction and peak orogenic metamorphism by similar to 100 m.y. and (2) the rocks of the Dabie Cretaceous magmatic suite, including Jiaoziyan, are on the footwall plate, rather than on the hanging wall, of the Triassic subduc tion-zone framework and do not show a linear distribution. These observatio ns argue that the voluminous Early Cretaceous magmatism in the Northern Ort hogneiss is geodynamically unrelated to the Qinling-Dabie orogeny, but was caused by extensional tectonism related to regional magmatic underplating b eneath the thinned Dabie crust.