Formation of High-Mg Diorites through Assimilation of Peridotite by Monzodiorite Magma at Crustal Depths

Abstract

The 134-130 Ma Han-Xing diorite-monzodiorite complex intrudes the central part of the North China block (NCB). The high-Mg diorites have intermediate SiO2, high MgO, Cr and Ni contents, elevated Mg-numbers (Mg#), and are characterized by high Sr and Ba, highly fractionated rare earth elements (REE), and strong Nb and Ta depletions.These geochemical signatures are similar to those of high-Mg adakite and sanukitoid magmas that form through complex crust-mantle interactions. We have investigated felsic magma-peridotite interactions in the Han-Xing diorite complex by undertaking a detailed petrological and bulk-rock and mineral trace element study.The high-Mg diorites contain olivine xenocrysts and dunite xenoliths that have been partly resorbed by the quartz-bearing matrix.These textures, coupled with the high Cr and Ni, and low Ca, Al, P, Y, Sc and Zr contents, provide evidence that the olivines were derived from a depleted peridotite. The assimilation of olivine produced diorite with highly variable MgO, Cr, Ni and Mg# values. However, all rocks have similar incompatible element patterns, indicating that they originate from a common pristine magma. The relative timing of olivine assimilation is constrained by reversely zoned clinopyroxene and amphibole crystals. The cores of the reversely zoned minerals have low Mg# values (clinopyroxene, ~75; amphibole, ~60) and REE characteristics indicating an origin by crystallization from a felsic magma compositionally similar to the monzodiorites. Inclusions of andesine, K-feldspar and biotite are present in the low-Mg# cores of reversely zoned clinopyroxenes, and are further proof of the felsic character of the pristine magma.The high-Mg (up to 90) midsections of the reversely zoned minerals have high Cr and Ni contents and lower but subparallel REE, indicating assimilation of peridotite by the monzodiorite magma. An eclogite residue for the pristine magma is precluded by the presence of plagioclase inclusions, and low Na2O and Al2O3 and unfractionated heavy REE in the low-Mg# cores of the reversely zoned clinopyroxenes.The maximum depth of the peridotite assimilation is estimated to be ~20 km, using the Al-inamphibole geobarometer for the high-Mg# midsection of a reversely zoned amphibole. This provides further evidence that the interaction of felsic magma with peridotite occurred at crustal rather than mantle depths. We suggest that the peridotite was emplaced at middle or upper crustal levels by previous orogenic processes. The original monzodiorite magma was probably produced in an extensional setting by partial melting of lower crust. Our observations indicate that the delamination model cannot be applied to the petrogenesis of the Han-Xing high-Mg diorite.The interaction of felsic melt with ultramafic rocks at crustal pressures has not been considered as a mechanism for the generation of high-Mg adakites and sanukitoids so far. This might have important implications for crustal evolution. Whereas in previous models sanukitoids have been related to either crustal growth by partial melting of enriched mantle peridotite or crust destruction through lower crust delamination, in the case reported here, they document crustal fractionation through anatexis of lower crustal rocks.