ALBERT

Description: Tertiary subvolcanic basic rocks are found as sills, dykes, and stocks in the southern flanks of the Central Alborz Magmatic Belt, north of Tehran. The rocks can be divided into two subvolcanic rock groups based on their geographic locations: (1) the western Kiga group and (2) an eastern group. The eastern group range from micromonzogabbro/diorite to microgabbro, whereas the Kiga group consists of micromozodiorite to micromonzogabbro. Mineral compositions, whole-rock major and trace elements show that these rocks have calc-alkaline affinities. The eastern group extends to higher MgO (4–10wt%) than the Kiga group (MgO= 4–5 wt%). With decreasing MgO, the contents of SiO2, TiO2, Al2O3, Na2O, and P2O5 increase and the contents of CaO and compatible trace elements (e.g., Co, Ni, Cr) decrease, consistent with olivine and clinopyroxene fractionation. At a given MgO, the Kiga rocks have higher FeOt, K2O, and P2O5 and extend to higher overall highly to moderately incompatible elements (Rb, Ba, Th, U, Nb, Ta, LREE, Sr, and Zr) and lower Al2O3 and Na2O. The depletion in Nb and Ta but enrichments in Rb, Ba, Th, U, K, Pb, and Sr, compared to N-MORB as well as high Th/Yb (at a given Nb/Yb or Ta/Yb), indicates a subduction zone origin for both subvolcanic groups of rocks. The initial Sr and Nd isotopic ratios of the subvolcanic rocks vary from 0.7048 to 0.7064 and 0.5126 to 0.5128, respectively. Furthermore, εNd (50 Ma) values (+0.64 to +5.19) associated with the two-stage model ages (0.42 to 0.78 Ga) of the samples infer a contribution of Cadomian-enriched lithospheric mantle in their source for this melt. The most evolved sample from the Kiga group has the lowest 143Nd/144Nd and highest 206Pb/204Pb and 208Pb/204Pb ratios. The isotope correlations could be explained by upper crustal assimilation/contamination by the more evolved samples or reflect source differences (i.e., higher amount of subducted sediments) in the Kiga source. In conclusion, we interpret that the subvolcanic rocks have formed in an active continental margin.