P-T-t evolution of mantle and associated crustal rocks in collisional orogens: Insight from numerical experiments

During the last decades, the intrinsic association of ultrahigh -pressure crustal rocks with ultrabasic rocks has been recognised in modern and ancient collisional orogens worldwide but their tectonic origins remain intriguing and controversial. In this study, we performed a series of 2D petrological-thermomechanical numerical experiments of continent -continent collision in order to: (i) characterize the variability of orogenic mantle rocks appearing in the exhumed crust during continental collision, (ii) investigate their relation to the host rocks, (iii) distinguish indicative P-T-t paths of peridotites for different crustal exhumation mechanisms. Based on the modeling results, we have identified 18 genetically different types of peridotites which may occur in the resulting crustal orogens. Peridotites from the mantle wedge are most commonly emplaced, but additionally we found peridotites of crustal origin, asthenospheric peridotites, and also peridotites from lithospheric mantle of the subducting continental plate. While some of these peridotites were initially located close to the subduction zone, others moved over hundreds of kilometres before incorporation into the subducted crust. Our results show that vertical extrusion and trans-lithospheric diapirism are the key exhumation mechanisms by which the largest variety of different peridotite types are emplaced in a collisional orogen. The crustal material associated with the peridotites did not always record the same P-T-t evolution but showed in some cases even higher peak metamorphic pressures. The range of P-T conditions predicted by the peridotites in the experiments cover all fields of P-T conditions recorded by peridotites from natural collisional orogens. Comparing the results from this study with individual P-T-t paths from natural occurrences may allow a better understanding on their origin.