Mixing and mingling of mafic and felsic magmas along the Neo-Tethys continental margin, Sanandaj-Sirjan zone, NW Iran: A case study from the Alvand pluton

摘要

The Alvand pluton in northwest Iran consists of gabbro and granite units. Along the interface between granite and gab-bro, numerous mafic enclaves (ME) are concentrated. This interface is marked by a variety of hybridization and mingling fabrics/ textures including lobate/ellipsoid ME with chilled margins, back-veining granite, feldspar megacrysts in ME, and crenulated margins in enclaves. The chemical composition of the ME (SiO_2 = 45.04-52.07 wt percent ) has more affinity towards gabbro (SO_2 = 45.01-46.00 wt percent ) than granite (SiO_2 = 71.62-75.18 wt percent ), indicating that the ME are the result of mingling of the gabbroic melt with a granitic magma. Diffusion reactions as the source of the ME are ruled out because the abundances of immobile elements such as Ni and Cr (37-144 ppm and 60-316 ppm) in the ME differ from those of the gabbro (446-518 ppm and 977-1297 ppm). Therefore, it is possible that mixing of gabbroic and granitic melts, although to a minimal extent, was also involved in the genesis of the ME. Geochemical features of gabbro such as high Ni and Cr abundances suggest that it crystallized fnem a primary mantle-derived magma. In contrast, the peraluminous character of the granite and the presence of sedimentary and micaceous enclaves reflect a crustal origin for this rock type. Because the ME are found mainly around the contact between granite and gabbro, they likely formed at the place of final emplacement when granitic magma intruded the still unsolidified gabbro. At this stage, mingling and minor mixing processes led to the formation of melts similar to those of the ME. Such melts rose rapidly through a viscous granitic magma and chilled against it to form the ME scattered in the granite.