Not all Neoproterozoic continental crust exposed in the Sulu belt was deeply subducted

Abstract

A newly identified tectonic sliver of low-grade Neoproterozoic continental crust comprising hybrid granitoids is exposed between Triassic ultrahigh-pressure (UHP) metamorphic rocks of the Sulu belt and the Cretaceous Laoshan granite. Outcrop evidence of magma mingling combined with the granitic mineral assemblages and primary igneous microstructures suggest >100 km difference in likely depth of subduction compared to the proximal UHP metamorphic rocks. The granitoids are cut by a brittle-to-ductile shear zone and extensional fractures that focussed fluid flow and allowed low-temperature fluid–rock alteration. Integrated accessory mineral geochronology from zircon, allanite and apatite records crystallization ages of c. 835–700 Ma and alteration ages of c.120–90 Ma. There is no evidence of the Triassic UHP metamorphic event recorded in the Neoproterozoic granitoids. The hybrid granitoids were likely generated during Neoproterozoic rifting of Rodinia, forming part of the northern margin of the Yangtze craton, but they did not experience deep subduction like other Neoproterozoic continental crust within the Sulu belt. The adjacent Cretaceous Laoshan granite was emplaced when all units were at shallow crustal depths, after >100 km of exhumation of the UHP eclogites. The Cretaceous ages retrieved from the hybrid granitoids date the brittle deformation and fluid alteration of these rocks in the Laoshan granite aureole. Cretaceous subduction retreat (slab roll back) of the paleo-Pacific plate caused extension and lithosphere thinning of the upper plate (eastern China), forming extensional shear zones and core complexes, associated with numerous granitoid intrusions, including the Laoshan granite. The extensional geological setting also facilitated preservation of this thin sliver of hybrid granitoids at the margin of the Laoshan granite adjacent to the UHP metamorphic terrane, and likely contributed to the final exhumation of the UHP metamorphic rocks of the Sulu belt. Our study shows that post-collisional extensional could be a common mechanism promoting final exhumation and exposure of deeply subducted terrains in orogens worldwide.