QUATERNARY MAFIC VOLCANIC ROCKS ALONG THE NAYBAND FAULT, LUT BLOCK, EASTERN IRAN

The Iranian plateau occurs within a part of the active Alpine-Himalayan orogenic belt. This plateau includes the micro-continental Lut block in Eastern Iran, which extends over 900 km in a north-south direction between latitudes 28^o-35^oN, and is 200 km wide between longitudes 57^o-61^oE. The Lut block which is largely covered by Neogene volcanic rocks, is separated from the rest of the Iranian plateau by two major belts of north-south right-lateral strike-slip faulting. The relatively primitive mafic volcanic rocks erupted along the deep crustal structures on the western margins of the Lut block provide a window into the subcontinental mantle below Iran, and the opportunity to determine petrochemical information about this mantle, which is an important first step in understanding all Iranian Neogene magmatism.

Samples of Quaternary volcanic rocks collected near Tabas along the northern part of Nayband fault are mainly olivine basalts (Fo = 68-84). Contents of MgO = 4.4 to 5.9 wt%, Ni = 90 to 111 ppm and Cr = 112 to 232 ppm indicate that samples crystallized from relatively primitive mantle. Contents of TiO₂ range from 1.88 to 2.65 wt% in these samples. Based on HFS elements (Nb/Y versus Zr/TiO₂), all samples are alkali basalts. Other geotectonic chemical discrimination diagram, such as those based on Th-Hf-Nb or Th-Zr-Nb, indicate that the samples from this area are within-plate alkali basalts. ⁸⁷Sr/⁸⁶Sr ratios for these samples is between 0.70531- 0.70555 and εNd = +0.93 to +1.7. Further to the south there is another outcrop of 2 Ma olivine basalt with MgO=6.65 to 7.81wt%, TiO₂ = 2.26 to 2.37wt% and 8% normative nepheline (Conrad et al. 1976, 1981). In the southern part of Nayband fault there are very young craters, which erupted lamproites composed of phenocrysts of phlogopite, clinopyroxene and olivine in a groundmass of plagioclase, phlogopite, clinopyroxene, analcime, hauyne, opaque minerals and glass. The contents of MgO in these rocks range from 5.1 to up to 23 wt%, TiO₂ from 0.9 to 2.27 wt% , K₂O = 1.4 - 9.8 wt% and Na₂O = 0.4 – 3 wt% (Shishehbor, 1993). Cognate xenoliths in these highly alkaline volcanic rocks contain up to >20 % modal calcite, which we interpret as primary igneous calcite indicating affinities with carbonatites and/or kimberlites.