Petrology and petrogenesis of a tuff layer in the Upper Red Formation, north of Tabriz (NW Iran): shoshonitic magmatism with high Ba and Sr affinity

Introduction The studied volcanic rocks as a dacitic tuff layer intercalated with the Upper Red Formation (URF, Late Miocene) and are located in the vicinity of the North Tabriz fault. During the Neogene, the red highlands north of Tabriz fault (Eynali) were a different basin from its southern part, namely the Sahand Volcanic complex. Based on the studies carried out on the volcanic rocks and pyroclastics of Sahand Volcano, the volcanic centers of Sahand have been active intermittently from the Late Miocene to the Late Pleistocene (Ghauori, 2002; Ghalamghash et al., 2019). The Upper Red Formation consists of red conglomerate alternated with sandstone, shale and marl and is associated with evaporite units (Asadian, 1993). These sediments have been deposited following uplift in a back-arc basin and within the Neotethys volcanic arc in Central Iran (Shahabpour, 2007). The aim of the present study is to investigate the relation between the tuff layer of URF and the first volcanic manifestations of Sahand volcano. Geological Background Subduction of the Neotethys under the central Iranian plate, followed by the collision of the Iranian and the Arabian plates (continental-continental collision), is responsible for the development of four structural zones in Iran. These structural zones with northwest-southeast trend include Zagros-Folded-Thrust belt, Sanandaj-Sirjan metamorphic and magmatic zone and Urmia-Dokhter magmatic arc (Alavi, 1994; Mohajjel et al., 2003). Omrani et al. (2008) have divided the volcanic rocks of Urmia-Dokhtar magmatic arc (including the studied area) into two categories: Eocene and Miocene to Plio-Quaternary. Eocene volcanic rocks consist of andesite, tuff and intermediate pyroclastics with small amounts of basalt, andesite and rhyolite. Miocene to Plio-Quaternary volcanic rocks are composed of andesite to dacititc rocks with Late Miocene to Pliocene age, which are followed by mafic volcanic rocks (Jahangiri, 2007; Omrani et al., 2008). Dacitic domes belonging to Late Miocene in the north of Tabriz fault, with adakitic composition, intruded the Upper Red Formation or Eocene volcanic units (Jahangiri, 2007). Analytical Methods Due to the lack of textural and mineralogical diversity of the studied rocks, four fresh samples were sent to the laboratory of the SGS Company located in Toronto, Canada, for analysis of major, trace and rare earth elements with ICP-MS. In order to determine the chemical composition of the rock-forming minerals, a sample of the studied rocks, after preparing a thin-polished section, was analyzed with an electron microprobe (CAMECA SX100) device at the Mineral Processing Research Center of Iran. The analytical conditions for voltage, beam current and beam diameter were set to 15 kV, 20 nA and 5μm, respectively. Discussion Petrography The studied rocks are dominated by the presence of quartz, plagioclase, alkali feldspar and biotite as phenocrysts with a glassy groundmass (Hyaloporphyry) Apatite is rare and calcite and iron oxides form the secondary minerals. Quartz as anhedral to subhedral with embayed texture accounts for about 15% of phenocrysts. Plagioclase is subhedral and forms for about 25% of the rock phenocrysts. Mineral Chemistry Plagioclase and potassium feldspar are Ab70An25Or5 and Ab32An1Or67, in composition respectively. Thermometry of the feldspars based on the Ab-An-Or diagram (Fuhrman and Lindsley, 1988; Nekvasil, 1992) shows that they are of relatively low temperature type (~700 ºC). The composition of micas varies from biotite to phlogopite in diagram of Fe/Fe+Mg vs. total Al and are classified as primary and re-equilibrated primary biotites on [(Fe*+Mn)-10*TiO2-MgO] diagram. The studied biotites belong to calc-alkaline orogenic suites originated from a crust-mantle mixed source. Whole-Rock Geochemistry The studied rocks have a distinct enrichment of LILE (i.e. Rb, Ba, Th, U, K) and LREE compared to HFSE (i.e. Ta, Nb, Ti, Zr, Hf, Y) and HREE. The rocks have high amounts of Sr (400-540 ppm) and Ba (930-1130 ppm) as well. The studied tuff indicates the features of metaluminous and high-K calc-alkaline to shoshonite magmatic suites, and has the characteristics of rare elements indicative of arc type magmatism. The Nb/Ta ratio in the studied samples varies from 14.7 to 15.8, which is higher than the predicted values for the continental (Taylor and Mclennan, 1985), but it is similar to arc volcanic rocks (Stolz et al., 1996). The above features in combination with the negative anomaly of Nb, Ta and Ti and the high ratios of Ba/La, Ba/Zr and Ba/Nb >30 (Gill, 1981) point to their similarity with magmas related to subduction. Discussion and Conclusion The lack of geological evidence in the region, indicating the existence of active subduction at the time of formation of the rocks under study; Thus, the observed geochemical features seem to be related to the origin rather than a tectonic origin. The enrichment of the studied rocks with some elements (i.e. Ba, Sr and Rb) requires extensive crystallization, crust contamination or very small partial melting. The studied rocks show non-adakitic characteristics, and therefore their genesis may be different from the types of adakitic rocks of Sahand Volcanic Complex. The bedrock of Sahand volcano is composed of Paleozoic-Mesozoic sedimentary deposits, Eocene volcanic rocks, lower Miocene deposits (Qom Formation) and Upper Red Formation (including marls, and red sandstones and gypsum belonging to the middle to late Miocene) (Abbassi et al., 2021). On the other hand, the oldest activity of Sahand volcano is attributed to the Late Miocene (Old Sahand in the division of Ghalamghash et al. (2019) with an age of ~8 Ma; and the thick pyroclastic sequence on the western slope of the volcano named Ghermeziqul Formation in the division of Moine Vaziri and Amine Sobhani (1977) with an age of 9-12 Ma). Therefore, considering the stratigraphic position of the tuff layer and its geochemical similarities with the non-adakitic eruptions of Sahand, it is likely the tuff layer was originated as the result of the first explosive activity of Sahand at the same time with the formation of Upper Red sediments (Late Miocene).

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