科研之友
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摘要
The felsic granitoids in the north of Urumieh plutonic complex are associated with abundant mafic rocks as mafic microgranular enclaves (MMEs). Typically, mafic enclaves are gabbrodiorite and monzo-gabbro/diorite in composition, and they are hosted by quartz syenite. MMEs have variable size (from a few centimeters up to 4 m) and shapes (including rounded, ellipsoidal, discoidal, lenticular, tabular, and elongated). MMEs, structurally and petrographically are characterized by irregular, gradual, and sharp contact surfaces, finer-grained enclaves and areas enriched in mafic material, feldspar phenocrysts in enclaves, plagioclase zoning with resorbed rims, acicular apatite, blade-like biotite, and mafic clots, indicating mixing and mingling of co-existing mafic and felsic magmas with liquid-liquid relation. MMEs generally contain higher values of Fe2O3, MgO, CaO, TiO2, MnO, V, Co, U, Sr, Nb, Ta, Ti, P, and heavy earth elements (HREE) and lower values of Al2O3, Na2O3, K2O, Ba, Rb, Th, K, Hf, and Zr than their host, which are in consistent with occurrence of abundant ferromagnesian minerals in the enclaves. Geochemically, the mafic enclaves and host are metaluminous with medium- to high-K calc-alkaline nature and strongly enriched in long ionic lithophile elements (LILE) and light earth elements (LREE) compared with high field strength elements (HFSE) and HREE. Some major and trace elements form linear trends on harker diagrams, having approximately identical concentration of trace elements, similar mineralogical composition, and high Mg no. in the both enclaves and host, suggesting their co-genetic origin. Our investigation shows that the enclave-forming magma may have originated from the uppermost metasomatized asthenospheric mantle and formed from a basaltic parent magma. Consequently, felsic part might have been produced by fractional crystallization from enclave-forming mafic magma in lithospheric mantle, supported by large volume of the mafic mass relative to the felsic unit. Finally, coeval mafic and felsic magmas may have mingled and mixed at two distinct, but continuous periods at different levels (before and during ascent and emplacement) as a result of combined multistage interactions (least and slight hybridization stages), which are commonly represented by basic to intermediate or hybrid MMEs.
- 出版日期2015-9
