Petrochemical constraints for dual origin of garnet peridotites from the Dabie‐Sulu UHP terrane, eastern‐central China

Abstract

Garnet peridotites occur as lenses, blocks or layers within granulite–amphibolite facies gneiss in the Dabie‐Sulu ultra‐high‐pressure (UHP) terrane and contain coesite‐bearing eclogite. Two distinct types of garnet peridotite were identified based on mode of occurrence and petrochemical characteristics. Type A mantle‐derived peridotites originated from either: (1) the mantle wedge above a subduction zone, (2) the footwall mantle of the subducted slab, or (3) were ancient mantle fragments emplaced at crustal depths prior to UHP metamorphism, whereas type B crustal peridotite and pyroxenite are a portion of mafic–ultramafic complexes that were intruded into the continental crust as magmas prior to subduction. Most type A peridotites were derived from a depleted mantle and exhibit petrochemical characteristics of mantle rocks; however, Sr and Nd isotope compositions of some peridotites have been modified by crustal contamination during subduction and/or exhumation. Type B peridotite and pyroxenite show cumulate structure, and some have experienced crustal metasomatism and contamination documented by high 87 Sr/86 Sr ratios (0.707–0.708), low ε Nd(t) values (−6 to −9) and low δ 18 O values of minerals (+2.92 to +4.52). Garnet peridotites of both types experienced multi‐stage recrystallization; some of them record prograde histories. High‐P–T estimates (760–970 °C and 4.0–6.5±0.2 GPa) of peak metamorphism indicate that both mantle‐derived and crustal ultramafic rocks were subducted to profound depths >100 km (the deepest may be ≥180–200 km) and experienced UHP metamorphism in a subduction zone with an extremely low geothermal gradient of <5 °C km−1.