Slab breakoff diorite porphyries derived from two cratons in a continental subduction zone, Sulu orogen, China

Abstract

Syncollisional magmatism plays an important but underappreciated role in continental crust growth and maturation. However, the origin of syncollisional intermediate magmas in continental subduction zones is controversial, with some models suggesting they form by arc-related processes, and others indicating they form by later slab breakoff−induced melting. Diorite porphyry dikes intruding granitic gneiss in the Paleo-Tethyan Sulu ultrahigh-pressure (UHP) continental collisional orogen have inherited zircon grains with 206Pb/238U ages of ca. 749−238 Ma, and magmatic zircons with weighted mean ages of 216−215 Ma, falling within the well-constrained time range (ca. 235−208 Ma) tracking exhumation of the Sulu UHP rocks from UHP peak conditions to amphibolite facies; they are thus syncollisional. The dikes have high Cr (330−402 ppm), Ni (84.5−103 ppm), and Mg# (64−66) values, showing a mantle origin. The porphyries have relatively high Sm/Yb, Nb/Y, La/Yb, and Gd/Yb ratios, representing a classic signature of slab breakoff magmatism. Together with the arc-like trace-element patterns and enriched Sr-Nd isotope compositions, ages, and εHf(t) values (−19.5 to −17.0) of magmatic zircons and their tectonic setting, we propose a syncollisional slab breakoff model in which the melts were initially generated from asthenospheric upwelling in the gap created when the oceanic slab attached to the Yangtze craton detached underneath the North China craton during Late Triassic collision following Paleo-Tethys Ocean closure. The diorite porphyry dikes have consistent Sr-Nd isotope compositions and spatiotemporal relationships with the nearby Shidao gabbro-syenite-granite complex, for which the tectonic affinity is controversial. Thus, we argue that the diorite porphyries and Shidao complex were sourced from two cratons, including the enriched subcontinental lithospheric mantle of the North China craton, which interacted with abundant felsic melts derived from the sinking slab breaking away from the subducted crust of the Yangtze continental-ocean transitional margin. This study sheds new light on crustal recycling versus continental growth in collisional orogens and implies that considerable syncollisional intermediate magmas can be generated by slab breakoff in continental subduction zones, representing hybrid additions to continental growth that are different and more evolved than arc magmas and have a composition similar to that of the bulk continental crust.