The Southern Caspian Sea ophiolite complex
The Southern Caspian Sea ophiolite complex is located in the north part of the Iranian Guilan province (Figure 2).
Figure 2. Location of the SCO ophiolites in Iran
Location of the SCO ophiolites with respect to the main ophiolite belts in Iran (in black), and to the main geological formations of Iran. Adapted from the ‘Magmatic Map of Iran’ at 1/1,000,000, compiled by M.E. Emami, M. Mir Mohammad Sadegi and S.J. Omrani (1993, Geological Survey of Iran) (Emami et al., 1993), and from the ‘Sedimentary-structural map of Iran’ by A. Aghanabati (2004).
The SCO occurs as lense body that has NNW-SSE trend and is one of the best-preserved oceanic crustal remnants of the Mesozoic Iranian ophiolites (Figure 3).
Figure 3. The geological map of the SCO
The geological map of Southern Caspian Sea ophiolite, showing the main geological unit of SCO.
The study of different parts of SCO ophiolite was difficult because of poor accessibility and dense rain forest. The full suite of ophiolite lithologies is present only on the southern coast of the Caspian Sea in the East Guilan. Detailed maps are shown in (Figure 3).
The study of schematic stratigraphic columns of the SCO show that the SCO is almost a complete oceanic lithospheric section (Figure 4) including, from bottom to top (east to west): ultramafic cumulates (dunite, wherlite, olivineclinopyroxenite and clinopyroxenite), layered gabbros, isotropic gabbros, sheeted dike complex and basaltic pillow lavas (Figure 3) covered by Campanian-Early Maastrichtian limestone bearing fossils of Globotruncana. Pelagic sedimentary rocks contain upper Cretaceous fossils that indicate an upper Cretaceous age for the formation of the pillow basalts, the final phase of the formation of the oceanic crust.
Figure 4. Stratigraphic columns of the SCO.
Schematic reconstructed ophiolite stratigraphy of the SCO.
a) Ultramafic cumulate rocks
The cumulate ultramafic sequence in the SCO is well exposed in the eastern part of the region along the roads between Syahkalrood and Javaherdasht and between Ramsar and Javaherdeh, whereas the basaltic volcanic unit is exposed in almost all areas (Figure 3).
Unfortunately, mantle rocks (lherzolite and/or harzburgite) cannot be found in this ophiolite complex. It seems that the mantle parts of this ophiolite are not outcroped and only crustal parts of SCO ophiolite can be observed. Ultramafic rocks of the SCO comprise a wide variety of dunites, wherlite, olivine-clinopyroxenite and pyroxenites. The SCO ultramafic sequence covers only 10% of the area of the ophiolite in SCO (Figure 3), and generally, pyroxenite is more abundant than dunite. Dunite occurs as lenses or thick layers within pyroxenites. It seems that this part is the base of gabbroic units. At the top of this unit gabbroic magma is intruded as dikes.
b) Layered to massive gabbro
The gabbroic complex is composed of different types of lithology (massive gabbro, layered gabbro). The thickness of each rock type is highly variable. The layered gabbro has varied thickness in each layer ranging from a few centimeters to 80 centimeters.
c) Sheeted dike complex—sheet flows to pillow lavas
At the top of isotropic gabbroic unit, the transitional zone from upper gabbro to sheeted dike complex is very clear and it is represented only in eastern study area. The complete sequence of sheeted dike complex is not found.
In the bottom of extrusive units a transitional zone between diabase dikes and pillow lavas is observed that gradually converts to pillow lava units.
The width of the dikes ranges from 10-20 cm, with the color varying from brown through green to yellowish green. Although the rocks have undergone low-grade greenschist-facies metamorphism as a result of ocean floor metamorphism, fresh plagioclase and clinopyroxene can still be identified. The dominant texture is intersertal to intergranular. Baking and chilling is present, although a preferential direction of chilling is absent.
Volcanic rocks are the most widespread rock-type in the SCO ophiolite (Figure 3). The volcanic rocks occur as pillow lava, massive lava and pillow breccias, and cover almost 80% of the ophiolite area, typically forming rounded hills and topographic high. In volcanic units the pillow forms are dominant and are more abundant than the other forms (the ratio of pillow lava to sheet lava is 90 to 10), indicating a slow-spreading center (Juteau and Maury, 1999). Sometimes sedimentary beds were found between the lava indicating a gap in volcanism. The pillow pieces have various sizes ranging from 10cm to 15 meters in diameter. Pillows show complete zonation from surface to core generally with a clear chilled margin. Similar to what is noted in the dike complex, the pillow lavas and sheet flows have plagioclase and clinopyroxene with textures ranging from intersertal through intergranular to spherulitic.
At their top, pillows have a carbonate or hyaloclastic breccias matrix. The volcanic unit capped by Upper Cretaceous pelagic limestones contains the following Campanian-Early Maastrichtian micro faunas: Globotruncana stuari, Globotruncana elevata, Globotruncana covanata, Globotruncana conica and Globotruncana calcarata. The limestone unit has a N110E strike with 25 dip toward southeast.
Both pillow and sheet lavas have vesicular texture and quartz, chlorite, calcite and epidote fills the vesicles.
Andesite to trachy-andesite dikes or small subsurface bodies crosscut volcanic unit and limestone. In addition, plagiogranite dikes or veins crosscut pillow unit. Gabbroic dikes with highly variable thicknesses up to 4 m were found in the volcanic unit.
Ophiolite complexes are generally metamorphosed as a result of ocean floor metamorphism. Volcanic components always suffer burial metamorphism and intense hydrothermal alteration commonly attributed to ocean floor metamorphism. Most SCO rocks show alteration of primary mineral assemblages, with varying degrees of low-grade metamorphism. Secondary mineral assemblages are generally heterogeneous and patchily distributed, even on the scale of a single thin section. Secondary mineral assemblages in the SCO rocks (from basalt to gabbro/cumulate) represent metamorphic facies ranging from zeolite to greenschists. The metamorphic grade increases downward through the ophiolite succession and reaches greenschist facies in the cumulate rocks.
Lastly all rocks of SCO are capped by agglomerate that is younger than the SCO.