Geochemical and fractal analysis of enclaves in the Dehe-Bala intrusion, (Northwestern Iran): a new concept in the interpretation of the crust-mantle interaction process

摘要

The Dehe-Bala intrusion is one of the remarkable intrusions of granodiorite rocks with I-type affinity and abundant mafic microgranular enclaves (MMEs) in the Buin Zahra area, Qazvin, Iran. The MMEs, composed of diorite and quartz-monzodiorites, are haphazardly widespread in the granodiorites. The Dehe-Bala Granodiorites (DBG) usually are characterized by high contents of SiO2 (64.2-66.9), Na2O (3-3.23), K2O (3.49-4), Mg# 4.84 and Th/Ta ratio (approximate to 7.9). In comparison to the DBG, the MMEs can be distinguished by their lower value of SiO2 (52.8-58.2), K2O (1.4-3.8) and higher Mg# (0.4-0.46). All these characteristics show a different composition of the DBG and MMEs, more importantly, can argue in favor of a magma mixing/mingling origin in the DBG. The enrichment in total REEs and HFSEs in the MMEs clearly reflects a marked diffusional process from the felsic to mafic magma that could have been achieved by chemical exchange during the magma mixing/mingling process. The fractal dimensions (D-box) of MMEs differ from 1.14 to 1.29 with the highest frequency at 1.29. The textural heterogeneity and geochemical features combined with high Dbox values in the MMEs compared with the DBG show lower degrees of mixing/mingling between mantle-derived mafic and lower crust-derived felsic magmas.