Petrogenesis and mantle source characteristics of Triassic alkaline basaltic rocks of North Kamarbon, Northern Central Alborz, Iran

The Triassic alkaline basaltic rocks (TABR) of North Kamarbon are located in Central Alborz, which is regarded as the northern part of the Alpine-Himalayan orogenic belt. Fractional crystallization does not appear as a major process in the genesis of TABR while different degree of partial melting may be regarded as the mail process, based on petrography and geochemistry studies. The geochemical characteristics of TABR are in conflict with a substantial involvement of continental crust or crustal contamination. High (La/Yb)N and (Dy/Yb)N ratios, along with other geochemical features, in the TABR magma, suggest an asthenospheric origin, with low degree of partial melting of a garnet-bearing mantle sources. Enrichment patterns of LILE (Ba, Sr and Th), HFSE (Nb, Ta and Zr) and P, and the depletion at HREE (Yb, Lu) are similar to what observed for OIB or intraplate alkaline magmatic rocks. The Sr and Nd isotopic ratios range from 0.70448 to 0.70522 and from 0.51269 and 0.51280, respectively, suggesting time-integrated slightly depleted magma sources. The data indicate that the evolution of TABR could be related to the rifting basin, in Late Triassic, which caused their development by upwelling and decompressional melting of an asthenospheric mantle, without a major involvement of subcontinental lithospheric mantle or crustal contamination