Geochemistry and tectonomagmatism setting of gabbroic bodies in South of Ramsar, West of Mazandaran Province: Evidence for Oceanic Island magmatism

In the study area, in Guilan and Mazandaran provinces and the northern heights of the Alborz mountain range in northern Iran, several gabbroic and ultramafic assemblages of different ages have been reported. In the Western Mazandaran provinces, in the south of Ramsar city (in the north of Iran), several gabbroic plutons with tholeiitic nature are found as a part of the Alborz Magmatic Zone. Alborz Magmatic Zone, which is a region with active deformation, is located in the collision zone of two Eurasian and Arabian plates, where a Neo-Tethyan oceanic lithosphere (Southern Caspian Sea Ocean) has subducted beneath the Central Iran continental lithosphere (Salavati et al., 2013). In this paper, the authors try to discuss the origin and tectonic situation of these gabbroic plutons in the northern part of the western Alborz orogenic belt based mainly on geochemical data.

Regional Geology:

Geologically, the study area is a part of the Ramsar 1:100,000 geological map (Baharfiruzi et al., 2002) is a part of the Gorgan-Rasht zone and the subsidence of the Caspian Sea. It is composed of the Quaternary Caspian deposits and mainly the Jurassic and Cretaceous volcanic rocks. The studied gabbroic plutons in the west of Mazandaran province in the south of Ramsar city, are exposed within the Jurassic shale and sandstone (TR3j2s,sh) around Javaherdeh (Javaherdeh F.).

In this study, 55 rock samples were collected based on the field relations. According to the petrographic studies, 13 samples have been analyzed by ICP-MS and ICP-AES methods in SGS Lab in Canada. Petrography and Whole rocks chemistry Based on microscopic studies, the studied gabbroic bodies consist of plagioclase, clinopyroxene, and olivine with intergranular texture. Iron oxides (titanomagnetite), titanite, and apatite are common accessory minerals. In addition, biotite, epidote, tremolite-actinolite, chlorite, serpentine and opaque minerals are the secondary minerals. Euhedral grains of plagioclases (50 to 70 vol.%) have been altered to chlorite and epidote (Propylitic alteration). The subhedral clinopyroxene crystals (15-30 % vol.) with diopside composition are the main mafic mineral observed in almost all studied rocks. Some clinopyroxenes show poikilitic texture. Chloritization and uralitization are also observed in the clinopyroxene. Olivine is usually serpentinized. Based on the geochemical data, these studied gabbroic bodies have SiO2 of 48.2-52.4 wt%, TiO2 of 1.25- 2.5 wt%, MgO of 4.25-8.51%, and Fe2O3tot of 11.2-14.1 wt%. On the bivariate rock type discrimination diagrams, the studied rocks fall in the subalkaline basalt field. Their chondrite, N-MORB (Normal Mid-Oceanic Ridge Basalt), OIB (Oceanic Island Basalt), and primitive mantle-normalized REE (Rare Earth Elements) and trace elements’ patterns are sub-parallel and show linear and homogeneous profiles with a moderate positive slope from HREE (Heavy Rare Earth Elements) to low field strength elements (LFSE). There are clear similarities between all samples and the typical OIB pattern. On the tectonic discrimination diagrams, all rocks fall in the within-plate basalt field and the plume sources as their tectonic environment. Based on all geochemical data, the studied rocks indicate oceanic ısland tholeiite (OIT) characteristics.

The geochemical data suggest that the studied rocks have chemical characteristics similar to HIMU-OIB and provide an additional argument for their derivation from an asthenospheric mantle source. They are distinct from the lower and upper crust and show OIT (Oceanic Island Alkaline or OIA) gabbroic signature. All geochemical signatures of investigated gabbros imply that crustal contamination did not play a significant role in the magma evolution. Based on field observation, the studied mafic plutons are similar to the alkaline gabbros of the eastern Guilan region; while based on the geochemical and lithological signature, there are a lot of similarity between these two groups. For example, both of them show oceanic island basalt characteristics (OIB). Their difference is that the rocks of eastern Guilan are alkaline type (OIA), while the investigated rocks in this research show oceanic ısland tholeiite (OIT) affinity. Based on the proposed tectonomagmatic model, the studied gabbroic samples are a part of the oceanic plume, that formed in a suprasubduction setting caused by the action of the slab window. Therefore, in the center of the plume, the investigated OIT gabbroic rocks (with the tholeiitic nature), and in the margin, the gabbros and basalts with the alkaline nature of OIB (in the west of the studied area, in the eastern part of Gilan province) were formed.

All data on the petrological and geochemical features of the studied rocks draw the following conclusions Studied gabbroic plutons are outcropped as several small bodies within the Jurassic rocks in the south of Ramsar city, along the Javaherdeh road.Based on petrographic studies, the rock specimens are dark green to black, and are including plagioclase and clinopyroxene±Olivine as the main mineral with intergranular and sub-ophitic textures. Geochemically, southern Ramsar gabbroic bodies show tholeiitic nature. In the chondrite, primitive mantle, and N-MORB normalized diagrams, they indicate enrichment of light rare earth elements and show a trend similar to OIB (oceanic island basalt) trend. In the primitive mantle-normalized pattern, they show Th, Nb, Zr, Rb depletion. Studied samples are formed from 10-20% of partial melting of garnet-spinel lherzolite at 80-100 Km Depth. Based on the proposed tectonomagmatic model, the subduction of the active spreading center of the Neo-Tethys oceanic crust (Southern Caspian Sea Ocean, to the south) produced a slab window in the subducted oceanic lithosphere, allowing infiltration of asthenospheric hot melt as an oceanic plume. The studied gabbros were formed in the center of this oceanic plume.