Investigation of petrology and petrogenesis of the Gaz Boland Neogene basalts, northwest of Shahr-e-Babak

The Gaz Boland area is located in the northwest of Shahr-e-Babak city within the southern extension of the Urumieh-Dokhtar magmatic arc. The extended convergence history of the Neo-Tethys Ocean between Arabia and Eurasia (from ∼150 to 0 Ma) comprised of a long-lasting period of subduction followed by continental collision during the Tertiary (Omrani et al., 2008). Following the collision, volcanism continued dramatically in some parts of the Urumieh-Dokhtar volcanic-plutonic belt, such as Pleistocene basic volcanism in the Shahr-e-Babak area in western Kerman. Thus, Neogene basalts in the Gaz Boland area in Kerman are known as the last magmatic activity of this part of Iran.

Ten samples of volcanic rocks were selected for geochemical analyses. All samples were analyzed for major elements by X-ray fluorescent (XRF) and trace elements using Inductively Coupled Plasma Mass Spectrometry (ICP-MS), in the Kansaran Binaloud Co., Iran. The results of the analyses were evaluated using the GCDKIT software package.

Plio‐Pleistocene basaltic rocks are the youngest volcanic activity in the Gaz Boland area. The main texture of these rocks is porphyric with microlithic form and they contain major minerals of olivine, clinopyroxene, and plagioclase. Based on geochemical data, the volcanic rocks of the Gaz Boland region have been derived from a calc-alkaline magma. Moreover, examination of trace element diagrams of these lavas indicates that magma is related to the subduction zone and active continental margin. Based on various elemental ratios and diagrams, the volcanic rock-forming magma in the Gaz Boland area have been derived from the asthenospheric mantle deep in the subduction zone. The source rock composition of these basalts is garnet-bearing lherzolite, which has been slightly enriched during the subduction process by fluids originating from the subducting oceanic crust. The rock-forming magma was also contaminated by the continental crust during the ascent and has endured the AFC process.

The Gaz Boland calc-alkaline basalts show enrichment in LILE, LREE, Th, and U, but depletion in HFSE (Ta, Ti, and Hf) and HREE. These rocks show characteristics of subduction-related (active) continental margin tectonic environments. According to the Sm vs. Sm/Yb diagram (Aldanmaz et al., 2000), the Gaz Boland samples were plotted in the partial melting range of about 10 to 15% of a garnet-rich lherzolite source. Asthenospheric mantle-derived magmas have Nb/La ratios > 1 or La/Nb ≈ 0.7. A low Nb/La ratio (<0.5) indicates lithospheric mantle and high Nb/La ratio (>1) indicates asthenospheric mantle (Smith et al., 1999). On the other hand, lithospheric mantle-dependent magmas have a La/Nb ratio greater than 1, whereas, in asthenospheric mantle-derived magmas, it is about 0.7 (DePaolo and Daley, 2000). In volcanic rocks of the study area, La/Nb and Nb/La ratios are 0.2 to 0.7 and 1.5 to 4.8, respectively. In addition, volcanic arcs can be classified into highly enriched and poorly enriched categories based on Ce/Yb ratios. Enriched arcs are defined as having Ce/Yb >15 (Hawkesworth et al., 1991; Juteau and Maury 1997). The mean Ce/Yb of the Gaz Boland rocks is 9.7 which defines a poorly enriched arc signature. Certain chemical parameters can be used to assess the degree of contamination. For example, basaltic rocks affected by crustal contamination exhibit La/Ta ratios > 22 (Abdel-Rahman and Nassar, 2004) and Nb/Th ratios < 5 (Condie, 2003). The values of such elemental ratios in the Gaz Boland basalts are 18 to 64 and 3 to 5, respectively (Table 1), which suggest that the magma was subjected to crustal contamination.