Subduction-unrelated magmatism of southern periphery of Paleotethys: constraints from Late Paleozoic magmatism from the south of Masuleh, western Alborz

The Alborz Mountains in north of Iran correlates with the Paleotethyan Suture so preserves valuable clues for geodynamic clarifications and paleotectonic reconstructions. During life span of Paleotethys from Early Paleozoic to Late Triassic, major parts of Alborz appear as a continental margin in southern border of the oceanic basin. To test paleotectonic setting of Alborz during Late Paleozoic and its passive or active condition, geochemical data of magmatic rocks can provide useful clues. The Masuleh area (western Alborz) involves important exposures of Late Paleozoic volcanic associations. These volcanics are poorly studied and understood, so we intend to present new geochemical data about them. Field studies characterize various lithological units in tectonic and stratigraphic contact with volcanic units including Late Paleozoic low-grade metamorphics (slate-phyllite) and calcareous units of upper Devonian, Carboniferous to Permian. The whole rock geochemical data has been obtained by XRF and ICP-MS at Ferrara University, Italy. The volcanics mainly comprise basaltic to trachy-basaltic compositions. They show moderate to high alteration reflected in their LOI content (2.2 to 7.8 wt.%). Thus, for major element we used recalculated anhydrous values. The SiO2 abundances of 45.3 to 50.7 wt.% display basic nature of the studied rocks. Other major element components such as TiO2, Al2O3, CaO and MgO are in the ranges of 1-4.4, 13.7-18.3, 5-10.7 and 2.5-14.9 wt.%, respectively. Moreover, Mg# [MgO*100/(MgO+FeO*)] varies from 19-71. Wide range of major element variations likely corresponds to different modal mineralogy and also various extent of melt evolution and fractionation processes. Total alkali element abundance (Na2O+K2O) displays elevated values (1.95-7.9 wt.%) reflecting alkaline composition of the samples. Compatible elements such as Ni (2.2-213.7 ppm) and Cr (17-739 ppm) indicate highly varied amounts, as well, suggesting nearly primitive to extremely fractionated nature. In La/Sm vs. La plot, the compositional trend is consistent with fractional crystallization process. Chondrite-normalized REE patterns and primitive mantle -normalized spider diagrams are characterized by similar patterns suggesting genetic relationships of different samples. The spider diagrams display humped-shaped patterns in which the LILEs (Rb, Ba, Sr and K) and HFSEs (e.g. Th, Ta, Nb, Zr and REEs) show enrichment with increasing incompatibility and a slightly negative Nb anomaly. These patterns are consistent with typical intraplate alkaline magmatism (OIBs). REE patterns are characterized by pronounced negative slope reflecting high LREE/HREE enrichment ((La/Yb) N= 5-17). Moreover, La and Yb represent enrichment of 47-248 and 6.5-22 times chondrite abundances, respectively. Immobile trace elements (e.g. La, Y, Zr and Nb) discrimination diagrams suggest subduction unrelated within-plate mantle origin similar to OIB source. Furthermore, the mantle source nature and partial melting degrees are inferred from modeling based on incompatible element ratios (Sm/Yb vs. La/Yb plot) suggesting that parental melt derived from garnet-bearing lherzolite and partial melting of <15%. Finally, we conclude that the area located to the southern margin of Paleotethys during Late Paleozoic was a passive margin (Gondwanian affinity) and the magmatic activity was related to thermal perturbation of mantle via hot spot/plume effects in an extensional tectonic regime.