Quaternary post-collision alkaline volcanism NW of Ahar (NW Iran): geochemical constraints of fractional crystallization process

Abstract

Major and trace elements and Sr-Nd isotopic data are presented for the Quaternary alkaline volcanism NW of Ahar (NW Iran). The exposed rocks mainly consist of alkali basalts, trachybasalts, basaltic trachyandesites and trachyandesites. Alkali basalts and trachybasalts display microlithic porphyritic texture with phenocrysts of olivine, clinopyroxene, and plagioclase in microlithic groundmass. In the more evolved rocks (basaltic trachyandesites and trachyandesites), amphibole and biotite have appeared. Sr ratio and high MgO, Ni and Cr contents indicate that they were generated from relatively primitive magmas. Ba, Cr and La/Sm ratios versus Rb suggest that fractional crystallization of alkali basalts could have played a significant role in the formation of evolved rocks. Assimilation and fractional crystallization modelling, as well as Rb/Zr, Th/Yb and Ta/Yb ratios clearly indicate that crustal contamination accompanied by the fractional crystallization played an important role in petrogenesis of the trachyandesites. The small compositional differences between magma types, isotopic composition, mineralogy and nonlinear trends on Harker diagrams also indicate that magma mixing was not an essential process in the evolution of the Ahar magmas. Petrogenetic modelling has been used to constrain sources. Trace element ratio plots and REE modelling indicate that the alkali basalts were generated from a spinel-peridotite source via small degrees (~2.5%) of fractional melting.

Petrology and geochemistry of the Karaj Dam basement sill: Implications for geodynamic evolution of the Alborz magmatic belt

“…Additionally, since the source region of back-arc basin basalts (BABB) is fluxed with volumetrically fewer slab-derived components than arc-like source regions, magmas from BABB environments have intermediate Ba/Yb values that lie between N-MORB and island arc compositions (Hastie et al, 2010;Pearce and Stern, 2006). Samples from the KDBS in this study, as well as other rocks from the AMB (Asiabanha and Foden, 2012), equivalent rocks from adjacent regions believed to have been formed in a back-arc setting (Vincent et al, 2005;Aghazadeh et al, 2010;Dabiri et al, 2011;Sarjoughian et al, 2012), along with representative data from the UDMA (Omrani et al, 2008) and the southern part of the Eastern Pontide arc of Turkey (Eyuboglu et al, 2011) are plotted on Ba/Yb-Nb/Yb (Pearce and Stern, 2006;Fig. 15b and c) and Th/Yb-Ta/Yb diagrams (Pearce, 1983;Fig.…”

“…(b) and (e) Alborz back-arc versus Urumieh-Dokhtar magmatic fore-arc (data from this study and Asiabanha and Foden, 2012 along with representative data from the UDMA from Omrani et al, 2008). (c) and (f) Samples of the KDBS (this study), other rocks from the AMB (Asiabanha and Foden, 2012), and adjacent back-arcs, (Vincent et al, 2005;Aghazadeh et al, 2010;Dabiri et al, 2011;Sarjoughian et al, 2012), along with representative data from the UDMA (Omrani et al, 2008) and south of the Eastern Pontide arc of Turkey (Eyuboglu et al, 2011). during the rifting process (Pearce et al, 2005; Table 3). Together, these lines of evidence indicate that the dominant subduction signature in the AMB is not as pronounced as in typical island arc rocks and that both subduction and rifting processes contributed to the AMB in the area.…”

Zircon U–Pb ages, geochemical and Sr–Nd–Pb–Hf isotopic constraints on petrogenesis of the Tarom-Olya pluton, Alborz magmatic belt, NW Iran

“…In addition to plutonic bodies, there is a huge volume of Eocene basic and intermediate volcanic rocks in the Alborz and Azerbaijan districts which show high-K to shoshonitic affinities (Verdel et al, 2011;Asiabanha and Foden, 2012;Nabatian et al, 2014;Zamani et al, 2015). Derivation of the Tarom-Olya pluton from an enriched subcontinental mantle is furthermore supported by the whole rock Nd-Sr (ε Nd(t = 36 Ma) = 0.39 to 2.10 and 87 Sr/ 86 Sr i(t = 36 Ma) = 0.7047 to 0.7051) isotopic signatures well within the range of the shoshonitic mantle-derived mafic magmas (Asiabanha and Foden, 2012;Dabiri et al, 2011;Eyüboglu et al, 2011;Tommasini et al, 2007;Turner et al, 1996). On the other hand, if the Tarom-Olya magmas were derived from the lower continental crust, one would expect some inherited zircons in this pluton, either as individual grains or as cores.…”

Magma mixing/mingling in Salmas granodiorite, NW Iran: evidence from mafic microgranular enclaves

“…Marble, recrystallized limestone and a skarn zone have developed at the contact of mafic-intermediate rocks with the limestone which is a common phenomenon in North West of Iran (Mollaei et al 2009;Mollai et al 2014). The Quaternary basaltic rocks which occurred after Late Miocene calc-alkaline magmatism are the last phase of magmatism activity (Dabiri et al 2011). According to field observations, the granodiorites contain mafic microgranolar enclaves (MMEs) which display magma mixing/mingling evidence.…”