Journal of the Virtual Explorer

The Southern Caspian Sea ophiolite have a mainly tholeiitic nature although some rocks have alkaline nature. Available geochemical data show that the volcanic to hypabyssal rocks exhibit transitional Mid-Ocean Ridge basalt-island arc tholeiite signatures. Salavati (2000) showed that most of the volcanic–hypabyssal rocks are tholeiitic. MORB-like and alkaline basalt rocks are also recognized (Salavati, 2000). The tholeiitic (Zr/Y=3.5-5.5; Ti/V=19-35 ) and MORB-like (Zr/Y=6-8; Ti/V=76-85) volcanic rocks occur as pillow lavas and sheet flows which are megaclasts within the SCO (Salavati, 2000). The ultramafic have pyroxene with high Mg# (Mg/Mg⁺‏Fe²⁺‏) within a range of 0.8–0.9 and olivine with Fo% ranging from 72 to 77. The gabbro have plagioclase with composition An₆₅Ab₃₃ to An₇₈Ab₂₁ and pyroxene with low Mg# (Mg/Mg⁺‏ Fe²⁺‏) within a range of 0.72–0.78.

The mineral chemical features of the ultramafic cumulates of the SCO (high Mg# in clinopyroxene and low Mg# in coexisting olivine) show that these rocks formed by crystal fractionation processes from a basaltic liquid at medium to high pressures (up to c. 10 kbar) (Salavati and Samadi Soofi, 2007). The main characteristics of low pressure (1 atm) phase relationships are that large amounts of olivine fractionation with or without plagioclase prior to pyroxene crystallization depletes the residual liquids in MgO, the Mg# of coexisting clinopyroxene, olivine is generally low (~82) and orthopyroxene has an even lower Mg# (~74) (Elthon et al. 1992 and Bağci et al. 2006) during the crystallization of oceanic basalts at low pressures.

Moreover, low-pressure crystallization of MORBs would yield dunites, troctolites and olivine gabbros (Elthon et al. 1992; Pearce et al. 1984 and Parlak et al. 2002). whereas the products of high pressure crystal fractionation in oceanic basalts would be dominated by dunite, wehrlite, clinopyroxenite, websterite and lherzolite with high Mg numbers for their mafic minerals (Elthon et al. 1982, 1992 and Parlak et al. 2002). The presence of dunite, wehrlite, olivine clinopyroxenite and clinopyroxenite within the ultramafic cumulates of the SCO would not be expected with the low-pressure crystalization of MORBs (Salavati and Samadi Soofi, 2007).

Mineral composition of clinopyroxene of ultramafic and basaltic rocks shows that Southern Caspian Sea ophiolite, were formed from the basaltic magma in an island arc/suprasubduction zone tectonic setting (Salavati, 2000 and Kananian et al. 2005).

The geochemical data for the volcanic, mafic–ultramafic cumulate rocks and their REE features (Nb, Ti and Zr negative anomalies (Salavati and Samadi Soofi, 2003), combined with the geological setting of the ophiolite, suggest that the SCO was generated in a subduction-related marginal basin, such as supra-subduction zone.