Source and geodynamics of dolerites of Kuhrang salt dome

Formation and rocks of Kuhrang salt dome in Zagros zone, Chaharmahal Va Bakhtiari province, are equivalent to the same units in the Hormuz Formation of Persian Gulf. Evaporate basins of Late Precambrian – Early Cambrian resulted in the deposition of Hormuz and similar series (Arian and Noroozpour, 2015; Motiei, 2001). The basins were formed due to the extensional tectonic phase, as a continental rift, in the continental crust of Iran and neighboring countries occurred during Infracambrian (Nabavi, 1976; Stöcklin, 1968; Berberian and King, 1981). Acidic and mostly alkaline magmatism is dominated in the rifts resulting from the extensional phase (Berberian and King, 1981).Magmas of the continental rifts mainly have alkaline nature and they are rich volatile material (e.g. halogens and CO2) and LILE (Lithophile large ion elements), indicating they are driven from enriched sources of the mantle (Wilson, 1989; Bailey, 1983). In the continental rift regions, the lavas mostly form during the fractional crystallization of basaltic magmas which are contaminated by the variant degree of crustal sources (Wilson, 1989).The aim of the paper is the determination of geochemical characters, tectonic setting and source of the doleritic rocks of Kuhrang salt dome by using whole-rock analyses.

After field sampling and microscopic studies, 8 fresh samples were analyzed for whole-rock geochemical determination by ICP-ES and ICP-MS methods in ACME lab (Vancouver, Canada).

The igneous rocks of Kuhrang salt dome display the including textures (intergranular and sometimes ophitic). Their mineralogy consists of plagioclase, pyroxene, amphibole, titanite and leucoxene. Compositionally, the rocks have 47.02-48.10 %wt. of SiO2. Therefore they plot in field of basic igneous rocks. The rocks display variable contents of K2O (3.34-4.28 wt.%), medium values of Na2O (1.11-2.37 wt.%), MgO (5.21-6.29 wt.%) and Fe2O3*(3.93-4.60 wt.%).In the Zr/TiO2-Nb/Y diagram (Winchester and Floyd, 1977), the studied rocks are plotted in basalt field and have sub-alkaline nature. Also, the samples display tholeiitic nature on the Ce/Yb-Ta/Yb diagram (Pearce, 1982). The chondrite normalized REE patterns show low enrichment in the LREEs relative to HREEs, which confirm E-MORB nature for their origin. Geochemical diagrams (e.g. Zr/Y-Zr (Pearce and Norry, 1979); Zr/4-Y-2 Nb (Meschede, 1986); Zr/117-Th- Nb/16 (Wood, 1980); Ti-Zr (Pearce, 1982)) show that the rocks have occurred in tectonic setting of intracontinental rift. TiO2 concentrations (2.38-2.48 wt.%) are relatively high in the rocks suggesting asthenospheric mantle (enrichment mantle) as their source. Moreover, flat patterns in HREEs (TbN/YbN= 1.58-1.74) and CaO/Al2O3 ratios (0.48-0.66) demonstrate spinel-mantle sources rather than garnet ones. Enrichment of Ba, Rb, K and K2O>Na2O and Rb/Sr-Ba/Rb diagram (Furman and Graham, 1999) reveal a mantle source involvement of phlogopite (Rosenthal et al., 2009).

Basic magma of Kuhrang salt dome doleritic rocks, show sub-alkaline nature (tholeiite) with various K2O contents. It has been formed in tectonic setting of intra- continental rift due to low partial melting of phlogopite spinel lherzolite of asthenospheric mantle.