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

摘要

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 Sr-87/Sr-86 ratios (0.707-0.708), low epsilon(Nd)(t) values (- 6 to - 9) and low delta(18)O values of minerals (+ 2.92 to + 4.52 parts per thousand). Garnet peridotites of both types experienced multi-stage recrystallization; some of them record prograde histories. High-P-T estimates (760-970 degrees 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 greater than or equal to 180-200 km) and experienced UHP metamorphism in a subduction zone with an extremely low geothermal gradient of <5 degrees C km(-1). [References: 67]