Investigation on tourmaline occurrence in the metamorphic complex in the north of Golpayegan, Sanandaj-Sirjan Zone

The occurrence of tourmaline in the metamorphic complex of the north of Golpayegan is observed within the granitoid mass and schists in contact with granitoid and marbles.Speaking of abundance, tourmaline is less frequently a minor mineral found in granites and granitic pegmatites as well as in low- to high-grade metamorphic rocks (Krmícek et al., 2020). The minerals of the tourmaline group can adjust their composition to adapt to varied settings, and therefore show a remarkable stability range in terms of pressure, temperature, fluid composition, and host rock composition (Van Hinsberg et al., 2011). Since tourmaline displays a negligible intra-crystalline diffusion, it can record the physical and chemical conditions of its setting and preserve this information in geological chronicles. Therefore, tourmaline accurately presents the composition of fluids and the melts from which it crystallized (Marks et al., 2013). Although tourmaline is common in many rocks, it is not common in metamorphosed carbonates (Krmícek et al., 2020).A petrographical and geochemical investigation of tourmalines in the north of Golpayegan was undertaken to know the formation mechanism of this mineral and to determine the differences between tourmalines crystallized at the contact, in granitoid and tourmaline in marbles.Regional GeologyThe Golpayegan Metamorphic Complex is located in the Sanandaj-Sirjan Zone and the occurrence of tourmaline in this complex is observed in two locations. The first location is on the west side of Ochestan farm where tourmaline is found in three forms:1) In the granitoid mass (Gt); 2) In the amphibole schists (At) in contact with the granitoid mass, and 3) In the mica schists (Mt) in contact with the granitoid mass. The second area is in the north of Esfajerd where tourmaline occurs inside the marbles (Ct).MethodologyWith the completion of field reconnaissance and preparation of thin sections, a petrographic study was fulfilled to determine the texture and mineralogy of the minerals, and then, some samples were selected for electron microprobe analysis.PetrographyThe tourmaline in granitoid mass (Gt), appears as idiomorphic and coarse-grained without any inclusions. Micaschists (Mt) in contact with the granitoid mass are sieve-shaped or spongy. Inside the amphibole schists (At) in contact with the granitoid mass, it is idiomorphic without any inclusions.Tourmaline in marbles (Ct) is fine-grained and blue, which can be observed around biotites with corrosion marks, indicating its reaction with fluid.Tourmaline chemistryThe tourmalines in Ochestan granite-pegmatite (Gt) are of alkali tourmaline variety with schorl composition, enriched in aluminum, and points to the replacement of Al in the Y (R2) position. The substitution type of these tourmalines is Al(NaFe+2)-1 owing to the high amount of Fe versus Mg,The tourmalines in amphibole schist (At) are alkali tourmaline with schorl-dravite composition and the tourmalines in mica schist (Mt) are alkali tourmaline with dravite composition, and both types are characterized by insignificant amounts of aluminum. This demonstrates the replacement of Al in the position of Y (R2) has not occurred. Due to the change in Mg and Fe content, the At-type tourmalines benefit from both Al(NaFe+2)-1 and Al(NaMg)-1 substitution varieties. However, the substitutions of Mt-type tourmalines are mainly Al(NaMg)-1 due to the high content of Mg versus Fe. Indeed, Fe+3Al-1 substitution can be observed in both types of tourmaline.Tourmalines in marble (Ct) are of the alkaline type of dravite composition and rich in aluminum, which indicates the replacement of Al in the Y (R2) position. Their substitution is Al(NaMg)-1 due to the high content of Mg versus Fe.DiscussionThe composition of tourmaline in Gt is of schorl type (Fe/Fe+Mg= 0.89-0.91) and has an aluminum replacement in the Y position. The composition of tourmaline in Mt type is of dravite type (Fe/Fe+Mg= 0.45-0.47) and the composition of tourmaline in At type is of schorl-dravite type (Fe/Fe+Mg= 0.49-0.51), which, replacement of aluminum in the Y position does not occur in both types of tourmaline in schists. Consequently, hydrothermal tourmalines have less aluminum (i.e. At and Mt-type tourmalines), and tourmalines in the granite-pegmatite mass have much more aluminum (i.e. Gt-type tourmalines). Based on the values of Fe# (FeO/FeO+MgO), it is possible to determine the formation site of tourmalines. If the amount of Fe# in tourmaline is >0.8, it indicates the closed magmatic system, lack of fluids interference, and their contamination with Al-rich sediments. Meanwhile, if the ratio is 566°C biotite, and lower temperatures, Ct-type tourmaline is composed of biotites.AcknowledgmentThe authors are sincerely grateful to the Geology Department of Isfahan University for all their support.