Petrology, geochemistry, and zircon U-Pb isotopes of Xintian Complex in Yanbian area, Northeast China: Evidence for magma mixing and geodynamics processes

摘要

Mixing/mingling of magmas is an important mode of magmatic evolution, crust-mantle interaction, and compositional exchanges, and the mafic microgranular enclaves (MMEs) in the host granites are commonly generated by the injection of hot mafic magmas into felsic reservoir at the crustal level. However, the exhaustion of most mafic melts causes the absence of the critical mantle marks after homogenization. The Xintian Complex in Yanbian area is composed of the gabbro diorite and granitic rocks enclosing the abundant MMEs, which likely provides the significant, but rare mantle end-member information. On the basis of geochronology study, both MMEs (126.3 +/- 1.2 Ma) and granodiorite (126.0 +/- 2.0 Ma) were formed in the Early Cretaceous, which is coincident with the earlier emplacement of gabbro diorite (128.5 +/- 1.0 Ma) within the errors. Geochemically, all of these distinguished phases have arc affinities related to subduction of oceanic slabs. Nevertheless, the dominant granodiorite rocks, typical high-K calc-alkaline I-type granites, have the source of partial melting of mafic lower crust modified by subducted sediment melts, with additions of mafic melts as indicated by high Mg# values (48-56) and special textures (acicular apatite and re-equilibrium plagioclase) of minerals in the hosted MMEs. In contrast, gabbro diorites are typical of calc-alkaline affinities with deficit of the incompatible Zr and Hf, which were formed by re-melting with low degree (3-10%) of mantle wedges metasomatized by subduction fluids with the crystallization of dominant pyroxene minerals. However, the MMEs is characterized by higher SiO2 (55.71-56.02 wt%), Cr (546-569 ppm), Ni (83.7-92.8 ppm), MgO (6.03-6.63 wt%), and heavy rare earth element (HREE) (546-569 ppm) with weak enrichment in Zr and Hf and lower (La/Yb)(N) values (17.98-20.93) compared to gabbro diorite. Therefore, the MMEs and host granodiorite are the products of heterogeneous mixing of weakly evolved mafic melts and subsolid felsic melts in the shallow storages, and this crust-mantle interaction is under a back-arc extensional setting related to rollback of subducting palaeo-Pacific slab.