Geochemistry, petrogenesis, and tectonic setting of volcanic rocks in the SW of Torud (Shahrood)

The study area is located in the south of Damghan, 160 km south of Shahrood, and 17 to 30 km south of Torud village. The area geologically, lies in the Cenozoic magmatic belt, a part of the Alpine-Himalayan belt, in the north of the structural zone of Central Iran (Aghanabati, 2004). The Cenozoic magmatic belt has been studied by many researchers (e.g., Ghorbani, 2005; Khajehzadeh, 2009; Mardani-Beldaji, 2011; Tayefi, 2014; Yousefi, 2017). The volcanic rocks in the southern part of the Torud area have not been comprehensively studied. Therefore, it requires a detailed study. So, for the purpose of this study attempt has been made to investigate and to study the nature of magma, tectonic setting, and the petrogenesis of the volcanic rocks using the geochemical data of the whole rock. Also, the results of this study have been compared with some areas belonging to the Cenozoic Era located in the north of the structural zone of Central Iran.

Regional Geology:

The area under study in the Torud-Moalleman magmatic belt belongs to the Chah-Shirin-Sabzevar-Khaf magmatic complex, located in the western part of this magmatic complex. In this magmatic belt, the Eocene volcanic rocks, including the main volume of igneous rocks are basic to acidic in composition. The predominant rocks are basaltic to intermediate rocks. The Torud-Moalleman magmatic belt is mainly composed of volcanic rocks with a lithological composition consisting of olivine-basalt, basalt, andesite, and dacite rocks and their pyroclastic equivalents, as well as plastic and limestone interlayers.

During field surveying, 50 samples of the volcanic rocks with the least alteration were collected. From these samples, 30 thin sections were prepared for microscopic studies and 11 samples were selected for ICP-MS geochemical analyses for minor elements and XRF for major elements and were sent to ACME Laboratory in Vancouver (Canada). GCDkit, Excel, and Corel Draw software were used to check the results obtained from the whole rock chemistry analyses and drawing diagrams.

Petrography:

The study rocks include volcanic rocks ranging from andesite to basalt. The basalts are dark gray to black in color with glomeroporphyritic, microlithic, sieve, and trachytic textures containing plagioclase and clinopyroxene as the main minerals. These minerals along with olivine and magnetite can also be seen in the form of microcrystals in the background of the rock, and their accumulation have created the glomeroporphyritic texture in these rocks. Secondary minerals are chlorite, iron oxide, zeolite (natrolite and analcime), calcite, and gypsum, filling the holes.The andesites are light gray to slightly dark with porphyritic and glomeroporphyritic textures and are dominated by amphibole (green and brown hornblende), plagioclase, and clinopyroxene as the main, biotite, iron oxide, sphene, and zircon. as the minor, as well as sericite, chlorite, calcite, and epidote as the secondary minerals.

Whole Rocks Chemistry :

The data obtained from the whole rock geochemical analyses display that the volcanic samples of the Torud area are classified as the andesite and basalt, placed mostly in the range of calc-alkaline series (medium potassium). LREE and LILE enrichment, HREE and HFSE depletion and Nb, Ta, and Ti negative anomalies of these rocks point to their formation in subduction zones. Also, as the tectonic diagrams display the rocks belong to the active continental margin. The rocks under study have mostly mantle origin and are derived from an enriched lithospheric mantle. The flat HREE patterns also show that melting occurred in the mantle, above the stability field of garnet. Therefore, the parent magmas were formed by the melting of spinel lherzolite at a depth of 80 to 100 km and evolved due to fractional crystallization as well as contamination caused by subducted sediments and the continental crust.

The rare elements pattern of the study rocks in spider diagrams show the cogenesis of these rocks and the role of differential crystallization as the main mechanism of their formation. Based on geochemical data, the study samples and compared volcanic rocks share similar characteristics. Therefore, the normalized REE patterns with chondrite (Nakamura, 1974) NMORB (Sun and McDonough, 1989) and MORB (Pearce, 1983), indicate the enrichment of LREEs (such as La, Ce) and LILEs (e.g., Ba, K, U, Pb, Cs) compared to HREEs and HFSEs (i.e. Nb, Ta, Ti, P) indicating that the rocks under study were formed in the active continental arc margin. The samples have no negative anomaly of Eu. Volcanic rocks with the age of late Eocene and Oligo-Miocene and basaltic to trachy-basaltic composition range from alkaline to sub-alkaline rocks and volcanic rocks with the age of middle Eocene with andesite to trachy-andesite composition have the nature of calc-alkaline.

The volcanic rocks in the south of Torud, with calc-alkaline and medium potassium nature, are mainly composed of basalt and andesite characterized by LREE enrichment, negative Nb-Ta-Ti anomaly, and the high ratio of LILE/HFSE. These characteristics point to the formation of these rocks in the subduction zones.The rocks under investigation have low SiO2 content, high amounts of Sr, no significant Eu anomaly, and Mg# content greater than 40. These geochemical features indicate a mantle source for the studied volcanic rocks. The changes of Rb/Y versus Nb/Y show the enrichment by subduction components or crustal contamination in the magmatic evolution of these rocks. Based on the geochemical investigations, the productive magma originated from a spinel lherzolitic source at a depth of about 80 to 100 km; during the ascent of magma, as a result of fractional crystallization and contamination, the magma derived from the mantle has been enriched and gave rise to lithological diversity.