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.
The Salmas area, in the northernmost part of the Sanandaj-Sirjan zone of Iran, contains a crystalline mafic-intermediate complex that intrudes into the Precambrian metamorphic basement complex and is composed of gabbroic and gabbrodiorite cumulates and fine-grained non-cumulate gabbronorites and diorites. These rocks have fine-to coarse-grained texture and are mainly composed of plagioclase, pyroxenes, and amphibole. Major element geochemistry indicates that the pluton has a low-K with tholeiitic affinity. Variations of major and trace elements on Harker diagrams, including negative correlations MgO, Fe 2 O 3 , CaO, and Co and positive correlations Na 2 O, K 2 O, Rb, Ba, and La, with increasing SiO 2 and chondrite-normalized REE patterns, suggest that fractional crystallization of gabbroic rocks could have played a significant role in the formation of evolved rocks. The chondrite-normalized REE patterns are not fractionated (La N /Lu N = 1.3-5.4) and display strong Eu anomalies (Eu/Eu * = 1.15-1.76) in cumulate rocks, which we attributed to cumulus plagioclase. Sr and Nd isotopic ratios vary from 0.704698 to 0.705866 and from 0.512548 to 0.512703, respectively. Gabbronorites with high 143 Nd/ 144 Nd ratios, low 87 Sr/ 86 Sr ratios, and high MgO, Ni, and Cr contents indicate that they were generated from relatively primitive magmas. We used petrogenetic modelling to constrain sources. Trace element ratio modelling indicates that the gabbroic rocks were generated from a spinel-peridotite source via 5-20% degrees of fractional melting at a depth of ∼52 km. Major and REE modelling shows that the diorites are the products of fractional crystallization of gabbronorites.
The interaction of mafic-intermediate and felsic rocks of the Salmas plutonic rocks produced mixed rocks (granodiorites) which contain mafic microgranular enclaves (MMEs). Enclaves ranging from a few millimeters to centimeters in size, and from ellipsoidal to rounded in shape. Based on both field observation and mineralogical compositions, MMEs are composed of quartz diorite whereas the felsic host rocks comprise mainly granodiorite. MMEs are characterized by a microporphiritic texture and revealed some types of microscopic textures, e.g., prismatic-cellular plagioclase with spike zones and rounded plagioclase megacrysts, bladeshaped biotite and acicular apatite. The host rocks show textures such as oscillatory-and reversely zoned plagioclase with spike zone. Compositions of plagioclases (An 41 to An 48 ) of MMEs are similar to those of host rocks (An 38 to An 45 ) which suggest partial to complete equilibration during maficfelsic magma interactions. The individual petrographic and microstructural textures and mineral chemistry similarities between the MMEs and their host rocks and diorites indicate that the enclaves are of mixed origin and most probably formed by interaction of lower crust magma (granitic melt) and evolved mantle-derived magmas (diorites).
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