| Abstract: | The alkali-basalt and basaltic trachy-andesites volcanic rocks of south Marzanabad were erupted during Cretaceous in central Alborz, which is regarded as the northern part of the Alpine-Himalayan orogenic belt. Based on petrography and geochemistry, en route fractional crystallization of ascending magma was an important process in the evolution of the volcanic rocks. Geochemical characteristics imply that the south Marzanabad alkaline basaltic magma was originated from the asthenospheric mantle source,whereas the high ratios of(La/Yb)Nand(Dy/Yb)Nare related to the low degree of partial melting from the garnet bearing mantle source. Enrichment pattern of Nb and depletion of Rb, K and Y, are similar to the OIB pattern and intraplate alkaline magmatic rocks. The K/Nb and Zr/Nb ratios of volcanic rocks range from 62 to 588 and from 4.27 to 9 respectively, that are some higher in more evolved samples which may reflect minor crustal contamination. The isotopic ratios of Sr and Nd respectively vary from 0.70370 to0.704387 and from 0.51266 to 0.51281 that suggest the depleted mantle as a magma source. The development of south Marzanabad volcanic rocks could be related to the presence of extensional phase,upwelling and decompressional melting of asthenospheric mantle in the rift basin which made the alkaline magmatism in Cretaceous, in northern central Alborz of Iran. |
