Paleogene continental-arc type volcanism in North Qazvin, North Iran: facies analysis and geochemistry

摘要

Paleogene volcanics in North Qazvin, which are equivalent to the Karaj Formation in the Alborz zone, may be distinguished into three main facies: 1) A volcaniclastic subaqueous facies forming two sub-facies: pyroclastic green tuffs (PGTs) in the lower parts of the succession and epiclastic variously colored tuffs (EVTs) in the upper parts. PGTs provide evidences of explosive eruptions in a shallow sedimentary basin, whereas EVTs were formed in a quiet sedimentary basin during middle-upper Eocene. 2) Lava facies including basic lava flows and dacitic dome are effusive subaerial products that extruded after uplifting and folding of the volcano-sedimentary basin due to the Pyrenean orogeny in the late Eocene. 3) Intrusive facies that are seen as dike swarms (mostly parallel dikes) and a monzogabbroic stock, intrude volcaniclastic facies. Based on the various facies, it can be stated that the volcanic style in the North Qazvin area is characterized by an initial explosive stage in the shallow sedimentary basin and continued to the effusive stage subaerially. Geochemically, the analyzed rocks belong to a high-K calcalkaline and shoshonitic series, and evolved by magmatic differentiation. Rare earth element (REE) patterns suggest a common source. Negative anomalies for high field strength elements (especially Nb and Ti), enrichment in light REEs, and positive K and Pb anomalies indicate that these rocks originated in relation to subduction beneath a continental arc.