The introduction and analysis of volcanic facies in Tarom-Sofla district, W-Qazvin

The Tarom-Sofla volcanic terrain, situated on western Alborz zone, contains Paleogene volcanic products (equivalent to Karaj Formation). Despite the contemporaneous volcanism in this area with other parts of central Alborz and even central Iran (middle-upper Eocene), the consideration of volcanic facies indicates the differences in amount and types of volcanics. Also, the ratio of lava flows onto pyroclastics is very high. Furthermore, the results of this study, as well as similar ones, show that volcanism in the western Alborz had been formed in subaerial conditions. The volcanic facies in the studied area, had been outpoured in two stages respectively: effusive and then explosive. In the effusive stage, the stratoid trachybasaltic lavas and dacitic pyroclastic flows and in the explosive stage, fall-out pyroclastics (mostly lapilli tuff and agglomerate) and then trachyandesitic lava flows appeared. The trachybasaltic lavas as the oldest volcanic unit, are recognized by stratoid appearance (composite lavas), reddened soil horizons under the flows and lava caves and tubes as well as pyroclastic inliers. In northern parts of the area, these basic lava are overlaid by a pinkish red dacitic pyroclastic flow. The most important features of this facies are columnar joints, ramp structure, very different thickness and vitroporphyric to autaxitic fabric with broken crystals. All the characteristics show that the flows were welded in a hot condition. The second volcanic stage in the studied area is distinguished by fall-out pyroclastics that are seen throughout the area and shows massive bedding, no internal fabrics and also very different clasts in shape and size. On the other hand, these pyroclastic deposits have been hydrothermally altered due to their permeability and produced various alteration zones (including argilic, advanced argilic, alunitic and silicic). Finally, the newest volcanic products are trachytic-trachyandesitic lavas that sometimes are interlayered by pyroclastics and characterized by their massive perspectives and redden soil horizons beneath flows. Geodynamically, the major element data reveal a high-K calcalkaline affinity (shoshonitic) in a continental arc regime in relation to collision.