Post-Eocene volcanics of the Abazar district, Qazvin, Iran: Mineralogical and geochemical evidence for a complex magmatic evolution

Abstract

The style of volcanism of post-Eocene volcanism in the Alborz zone of northern Iran is different to that of Eocene volcanism (Karaj Formation). Indeed, the volcanic succession of the Abazar district, located in a narrow volcanic strip within the Alborz magmatic assemblage, is characterized by distinct mineralogical and chemical compositions linked to a complex magmatic evolution. The succession was produced by explosive eruptions followed by effusive eruptions. Two main volcanic events are recognized: (1) a thin rhyolitic ignimbritic sheet underlain by a thicker lithic breccia, and (2) lava flows including shoshonite, latite, and andesite that overlie the first event across a reddish soil horizon.

Plagioclase in shoshonite (An_48–92) shows normal zoning, whereas plagioclase in latite and andesite (An_48–75) has a similar composition but shows reverse and oscillatory zoning. QUILF temperature calculations for shoshonites and andesites yield temperatures of 1035 °C and 1029 °C, respectively. The geothermometers proposed by Ridolfi et al. (2010) and Holland and Blundy (1994) yield temperatures of 960 °C and 944 °C for latitic lava, respectively.

The samples of volcanic rock show a typical geochemical signature of the continental arc regime, but the andesites clearly differ from the shoshonites, the latites and the rhyolites. The mineralogical and chemical characteristics of these rocks are explained by the following petrogenesis: (1) intrusion of a hot, mantle-depth mafic (shoshonitic) magma, which differentiated in the magma chamber to produce a latitic and then a rhyolitic liquid; (2) rhyolitic ignimbritic eruptions from the top of the magma chamber, following by shoshonitic and then latitic extrusions; (3) magma mingling between the latitic and andesitic magmas, as indicated by the occurrence of andesite clasts within the latite; and (4) andesitic effusions.

The youngest volcanic events in the Alborz zone show a close chemical relationship with continental arc rocks, indicating that they formed in a continental collision setting.

Introduction

The volcano-plutonic complex of the Alborz Ranges in North Iran has been interpreted to represent the subduction of Neo-Tethyan oceanic lithosphere beneath the Central Iranian continental microplate and the subsequent continental collision of the Arabian and Iranian microplates in the Late Cretaceous-early Cenozoic (e.g., Alavi, 1994, Berberian and Berberian, 1981, Berberian et al., 1982, Golonka, 2004). Although magmatism in the two dominant volcanic zones in Iran (the Urumieh-Dokhtar in Southwest Iran and the Alborz in North Iran) started in the Early Cretaceous, the most voluminous magmatism occurred during the Eocene (Alavi, 1994). Younger (post-Eocene) magmatism, which has continued until the present, has a different style of volcanism but a similar geochemical signature compared with the Eocene magmatism. The Eocene volcanic succession in the Alborz magmatic assemblage has been studied previously (e.g., Dedual, 1967, Annells et al., 1975, Ebrahimi, 2000, Asiabanha et al., 2009), whereas the characteristics of the post-Eocene volcanics are poorly known because of sparse outcrop and uncertainties regarding age relations. Therefore, the aim of this study is to describe the petrological occurrence of post-Eocene magmatic events of the Alborz zone in the framework of regional tectonics and to compare these rocks with Eocene lavas, in order to obtain a better understanding of the complex geological evolution of the Alborz zone.

Asiabanha et al. (2009) divided the Eocene volcanic complex at Alborz (the Karaj Formation) into two main facies: (1) An earlier volcano-sedimentary facies composed of pyroclastic and epiclastic deposits formed by subaqueous eruptions in a shallow sedimentary basin during the Early–Middle Eocene. The basin was then uplifted and overprinted by orogenic movements that produced tight folds and thrust faults. Evaporites were deposited in the uppermost part of the volcaniclastic series. (2) Subsequently, mafic–felsic subaerial lava flows were produced with potassic calcalkaline to shoshonitic affinities, related to a continental arc regime.

Sporadic subaerial volcanism continued after the Paleogene (Berberian and Berberian, 1981), either as central volcanoes (e.g., the Damavand, Sahand, and Sabalan mountains) or as fissure eruptions. The appearance of these younger volcanic rocks is different to that of Eocene volcanism. An understanding of the volcanic styles and geochemical signatures of these rocks would be important in characterizing the geotectonic setting after continental collision in Iran.

The Abazar district, which is the focus of this study, is situated in the western part of the Alborz zone. It is a post-Eocene volcanic province, associated with Eocene volcanic rocks, which appears to have been produced by fissure eruptions, and is distinct from coexisting rocks. Two different groups of volcanic facies (Eocene and post-Eocene) are juxtaposed along a thrust fault (the North Qazvin Fault). The rocks of the facies are generally fresh and unaltered. This study aims to understand the volcanic and petrogenetic evolution of post-Eocene volcanic rocks in relation to the tectonic setting, in comparison with the older Eocene volcanic succession, based on facies analyses, mineral chemistry, and major and trace element geochemistry. A tentative magmatic model is proposed to explain the coexistence of several distinct lava types in a small geographic area.