Slab failure dioritic porphyries derived from two cratons in a continental subduction zone, Sulu orogen, China

Syn-collisional magmatism plays an important but underappreciated role in continental crustal growth and maturation. Dioritic porphyry dikes intruding granitic gneiss in the 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. 235-205 Ma) tracking exhumation of the Sulu UHP rocks from UHP peak conditions to amphibolite facies, and are thus syn-collisional. Their high Cr (330-402 ppm) and Ni (84.5-103 ppm) contents along with high Mg# values (64-66), and arc-like trace element patterns and enriched Sr-Nd istope compositions suggest derivation from slab failure magmatism, being initially generated from asthenospheric upwelling in the gap created when the oceanic slab attached to the Yangtze Craton (YC) detached underneath the North China Craton (NCC) during late Triassic collision related to Neotethyan ocean closure. This origin is supported by the porphyries' relatively high Sm/Yb, Nb/Y, La/Yb and Gd/Yb ratios, a signature of slab failure magmatism. After slab detachment, the buoyant Yangtze continental crust rebounded isostatically beneath the NCC as the magmas were generated along the detached slab edge, and as they rose they mixed, assimilated, and homogenized with the crusts of both cratons, as indicated by the negative ɛHf(t) values of -19.5 to -17.0. of magmatic zircons. The magmas became stored in the upper plate of the North China Craton as dioritic porphyry dikes, which have consistent Sr-Nd isotope compositions and space-time relationship with the Shidao gabbro-syenite-granite complex. Thus, we argue that the dioritic 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 subducted Yangtze Craton. This study sheds new light on crustal recycling vs continental growth in collisional orogens, and implies that considerable syn-collisional intermediate magmas could be generated by slab failure in continental subduction zones, representing hybrid additions to continental growth, different and more evolved than arc magmas.