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This work describes the petrogenesis of in situ and in-source trondhjemite leucosomes generated by low-degree partial melting of the Jurassic Cheshmeh-Ghasaban hornblende-gabbros in the northern Alvand batholith (Hamedan, NW Iran). These leucosomes occur in a metatexite migmatite as patches, net-structures, veinlets, and dikes at scales ranging from a few millimetres to a few metres. They have high SiO2 (mean ≈ 78 wt%) and Na2O (4–5 wt%) with low Al2O3 (<15 wt%), K2O (<1.5 wt%), Sr (33–267 ppm), Eu (0.31–0.62 ppm) and heavy REE (<4 ppm) contents similar to typical low-Al2O3 trondhjemites. Batch-melting models indicate these leucosomes are the result of ≈1-2% partial melting of the hornblende-gabbro, whereas thermobarometric modelling constrains their petrogenesis in the field of hornblende hornfels-facies metamorphism (pressure ≈3.0–4.5 kbar and temperature ≈700-750°C). On a wider scale, our results document natural trondhjemites generated in the upper crust by amphibole-dehydration melting of a mafic source at temperature conditions close to the solidus, filling the existing gap of data in low-pressure (P < 5kbar) and low-temperature (T < 800°C) amphibole-dominated partial melting experiments. The genesis of these trondhjemites is then discussed with respect to the Mesozoic evolution of the Alvand plutonic complex. [ABSTRACT FROM AUTHOR]

The volcanic masses in Kasian area with northwest-southeast trending, lie in Zagros crushed zone, between Sanandaj-Sirjan and Zagros thrust zones. Based on the microscopic studies, the studied volcanic rocks are: andesite- basalt and andesite. The main textures exhibited in these rocks consist of porphyritic hypocrystaline, porphyritic and hialoporphyritic. The important feature in this region is the severe and intense alteration caused the great changes in the studied rocks, especially in those susceptible to alteration. Three alteration zones including: sericitic, argillitic and propylitic zones developed in this region. The geochemical data indicate that these rocks are low to medium K oceanic island arc tholeiites. Nb depletion and Ba and Sr enrichment in the primitive mantle normalized diagram point to subduction setting. These results, in accompany with LREE enrichment in the chondrite normalized trace element, represent the island arc setting for these rocks. The lack of significant Eu anomaly in REE pattern is representing oxidation condition during magma crystallization proposing volcanic arc environment for the studied rocks. The Kasian volcanic rocks lie in young and immature volcanic arcs or in the initial stages of volcanic arc formation. This situation is consistent with geological history of Sanandaj-Sirjan zone in the Jurrasic to Cretaceous age. [ABSTRACT FROM AUTHOR]

Intrusion-bordering migmatites comprise a substantial, high-grade metamorphic part of the Alvand aureole near Hamadan, western Iran. Abundant Al-rich metasedimentary rocks and various granites occur in this region. Migmatites consist of Bt + Sill + Grt + Crd + Sp ± Opx melanosomes and Grt + Pl + Kfs + Qtz leucosomes. These assemblages reflect upper pyroxene hornfels to lower sanidinite facies physical conditions. The appearance of orthopyroxene in these rocks marks the pressure–temperature transition from the pyroxene hornfels to the sanidinite facies. Field relations, mineral parageneses, and pressure–temperature estimates suggest that intrusion of granitic magma and concomitant partial melting of metasedimentary wallrock units were the main processes involved in the migmatization. Peak metamorphism took place at 650–750°C and ∼2–4 kbar; such high-temperature/low-pressure metamorphism was caused mainly by advective heat derived from the emplacement of plutons. Regional metamorphism, granitic magmatism, and contact metamorphism reflected arc construction and collision during subduction of a Neotethyan seaway and subsequent Late Cretaceous–early Tertiary oblique collision of Afro-Arabia (Gondwana) with the Iranian microcontinent. [ABSTRACT FROM AUTHOR]