A Backarc Basin Origin for the Eocene Volcanic Rocks North of Abbas Abad, East of Shahrud, Northeast Iran

The region in northeastern Iran, bordered by the Miami fault and the Doruneh fault, mainly exposes the Eocene volcanic and Tertiary sedimentary rocks and sporadic outcrops of pre- Jurassic metamorphic rocks such as gneiss and mica-schist. We have divided the volcanic and volcanic-sedimentary rocks into six main units: E1 through the youngest E6. North of Abbas Abad, the Lower Eocene is conglomerate, sandstone, and red shale with lenses of nummulite-bearing limestone at the base, and dacitic lava (E1) at the top. The nummulites give an Early Eocene age for the limestone lenses. The E2 unit includes vesicular basalt, intercalated, intraformational conglomerate, and lenses of nummulite-bearing limestone. E3 is volcanic- sedimentary, and is made of green tuff, tuffite, shale, and nummulite bearing limestone. E4 includes basalt and vesicular trachy-basalt, and E5 is mostly sedimentary, made of tan marl, sandstone, shale, and lenses of Middle Eocene nummulite-bearing limestone. The E6 unit is the most extensive, with at least three levels of nummulite-bearing limestone lenses which give a Middle to Early Eocene age. The volcanic rocks of the E6 unit include few hundred meters of epiclastic to hyaloclastic breccia, with intercalations of lava at the base. These are overlain by four horizons of aphyric olivine basalt and basalt, and phyric trachy-andesite and trachy-basalt. The volume of the aphyric lavas decreases, and that of the phyric lavas increases upsection. The Eocene volcanic sequence is covered by turbidite; the marl washings give an Eocene-Oligocene age range. Chondrite-normalized multi-element plots indicate enrichment of the Eocene Abbas Abad volcanic rocks in the LILE elements, with variable ratios of La/Yb (4.36-19.33) and La/Sm (3.10-7.91). These plots show a gentle slope, and the volcanic rocks in the E1 to E4 units are less enriched than those in the E6 unit, probably reflecting the difference in the original source for the melt. The multi-element plots of the volcanic rocks in E6 also indicate an enrichment of the phyric lavas in the upper parts of this unit than those in the aphyric lavas. The primitive mantle-normalized multi-element plots, in addition to indicating enrichment in the LILE, show clear anomalies for Nb and Ti, indicating a backarc basin-related volcanism. The Eocene Abbas Abad basaltic lavas show a variable La/Nb=2.15-4.63 ratio, which is comparable with that in the in-arc basalt. The E1-E4 basic volcanic rocks have a Th/La ratio of 0.14-0.2, which is comparable with that of oceanic basalt or bulk continent. However, the Th/La ratio in the E6 lavas ranges from a 0.18 minimum, in the lower olivine basalt, to 0.4 in the upper, phyric trachy-andesite, which may indicate the elemental contributions of sediments to these lavas and enrichment of the LILE. Our data suggest that the Eocene volcanism in the Abbas Abad area occurred in a backarc basin, a conclusion which is consistent with the model that assumes continued subduction of the Arabian plate under the Iranian plate, and partial melting of the Paleotethys ocean and supra-ophiolite sedimentary rocks in northeastern Iran. It seems that the partial melting of the ultramafic parts of the Paleotethys has led to the formation of the E1 to E4 volcanic rocks, and the partial melting of these rocks and the supra-ophiolitic sedimentary rocks formed the volcanic rocks in the E6 unit.