Geochemistry of northwestern Saveh magmatic complex (Markazi province)

Considering the wide extent of the Urumieh-Dokhtar magmatic arc and the presence of many intrusive and volcanic rocks in this belt, an important question from scientific and exploration points of view is “Why are some plutons productive whereas others are sub-productive and/or barren?” Barren and productive magmatic systems related to calc-alkaline arc magmatism are identified as normal or non-adakitic (low Sr/Y<20) and adakitic (high Sr/Y>20) magmas, respectively. Barren magmas are non-mineralized and have a low Sr/Y ratio, while high Sr/Y magmas are responsible for Cu-mineralization and are known as productive magmas that occur in all major orogenic belts worldwide (Cooke et al., 2005). Understanding their origin and petrogenesis is of critical importance to decipher their long-term growth and stabilization of the continental crust, and formation of economically valuable ore deposits (Monecke et al., 2018). Barren granitoid magmas typically form in pre-collisional subduction zone environments (Shahabpour, 1992), which is confirmed by the results obtained in this study. Magmatism in this region began in the Early Eocene and continued until the Pliocene. The volcanic and intrusive barren-type rocks (Eocene) that formed in a subduction-related tectonic setting are characterized by calc-alkaline and tholeiitic geochemical signature (Shahabpour, 2005). The northwest of Saveh magmatic complex is situated at the central part of the Urumieh-Dokhtar magmatic belt. The volcanic rocks of northwest of Saveh are crosscut by Late Eocene-Early Oligocene granitoids that are exposed over an area of about 100 km2.

Approximately 70 samples were intrusively collected from Mount Shahpasand and Neivesht volcanic rocks. Subsequently, 9 granitoid rocks and 7 volcanic rocks that showed the least amount of alteration were selected for whole-rock geochemical analysis. The main elements analysis was performed by X-ray fluorescence method using Optima 100V device and the analysis of rare earth elements was performed using Inductively Coupled Plasma Mass Spectrometry method and with ICP NeXION 300 device in the Lab West Laboratory of Australia.

The northwest of the Saveh magmatic complex is situated at the central part of the Urumieh-Dokhtar magmatic belt. The volcanic rocks of this area are crosscut by the Late Eocene-Oligocene granitoids. Whole-rock geochemistry shows that the studied igneous rocks with low to medium potassium calc-alkaline geochemical signatures have strong depletion in Nb and Ti and enrichment in LREE and LILE, which imply formation during normal arc magmatism. Sr/La and La/Yb trace element ratios show that all samples have evidence for slab fluid metasomatism and a mantle source affected by metasomatism. La/Nb and La/Ba ratios also confirms a subduction-modified lithosphere mantle source for the magmatic rocks in the northwest of Saveh. Geochemical evidence shows that these rocks are barren-type igneous rocks that have the same origin and differential crystallization is the dominant process in their petrogenesis. Barren magmatism in the northwest of Saveh is likely a result of partial melting of juvenile lower crust caused by subduction of the Neo-Tethys oceanic lithosphere, whereas productive adakitic rocks within the Urumieh-Dokhtar magmatic belt have formed by partial melting of thickened lower crust.

Granitoids have gabbrodiortite-diorite, Quartz monzonite, granodiorite and granite composition, while volcanic rocks are petrologically classified as basaltic andesite, andesite and dacite-trachydacite. Geochemical studies of whole rocks indicate that they have strong depletions in HFSE (Nb, Ti, Zr) and enrichments in light rare earth elements and large ion lithophile elements compared to N-MORB. Geochemical signature of the igneous rocks in northwest of Saveh (low Sr/Y ratio of almost <30) and their negative Eu anomalies (Eu/Eu∗ = 0.62–1.05) suggest generation in a subduction zone and pre-collisional setting. However, productive rocks elsewhere within the Urumieh-Dokhtar magmatic belt exhibit adakite-like calc-alkaline magmatic characteristics (high Sr and Sr/Y, but low Y). Signature of this magmatic complex is consistent with other barren-type magmas through the Urumieh-Dokhtar magmatic belt. The low ratios of (La/Sm)N and (Dy/Yb)N (0.50–1.18 and 0.91–1.35, respectively) are similar to those from barren-type of granitoids. Examination of the studied samples on a Y versus MnO diagram (Baldwin and Pearce, 1982) shows that the samples have characteristics of barren-type igneous rocks. Haschke and Pearce (2006) suggested that a high Y content in barren magmas may record the participation of anhydrous phases during the early stages of magma genesis and so account for lack of associated mineralization. However, it may be possible that partial melting of the source is superficial, in agreement with a moderate pre-collisional crustal thickness of 35–45 km. Low Sr/Y (< 30) ratios measured in the Eocene–Oligocene northwest of Saveh igneous rocks suggest generation via island-arc magmatism, while a Sr/Y ratio of > 56 for productive rocks implies garnet, hornblende, and clinopyroxene minerals in the source, leading to enrichment of LREE/HREE (Castillo, 2012).