LAVAS OF THE NORTHERN SANANDAJ-SIRJAN ZONE, IRAN: FRACTIONATION OF MULTIPLE PARENTAL MELTS

The Sanandaj-Sirjan zone is a NW-SE trending orogenic belt located immediately north of the Zagros suture in Iran. This region preserves the history of Neotethys from its inception to its removal through subduction beneath the Eurasian continental plate. Cenozoic magmatism is associated primarily with the adjacent Urumieh-Dokhtar magmatic arc but also occurred in some parts of the Sanandaj-Sirjan zone including our study area located north of Saqqez. We studied the petrography, mineral chemistry and whole-rock composition of samples from the Saqqez complex in the northern Sanandaj-Sirjan area that were emplaced during a major Eocene magmatic flare-up ~40-35 Ma. The rock types include basalt, andesite and basaltic andesite; all samples are sparsely phyric (typically <10% phenocrysts). The rock forming minerals are amphibole, plagioclase, pyroxene and biotite. Some plagioclase phenocrysts exhibit sieved texture and oscillatory zoning. Amphibole forms euhedral crystals and shows zoning and twinning, as well as abundant inclusions of Fe-Ti oxide. Clinopyroxene and orthopyroxene were found in basaltic andesites. The amphiboles plot in the field of calcic amphiboles, defined by Ʃ (Ca + Na) on M4 ≥ 1.00 with Na < 0.50, and Ca ≥ 1.50 on M4. Their chemical composition is mainly Tsch-pargasite (Mg# = 76-86; ^VIAl ≥ Fe³) with some Mg-edenite. Hornblende and cogenetic plagioclase in porphyritic basaltic-andesite and andesite in the Saqqez volcanics crystallized at temperatures of ~813-733 °C. Data calculated with the Al-in-hornblende geobarometer indicate crystallization of Saqqez basic and intermediate rocks in the pressure range between 9.5 and 4 kbar, indicating growth of amphibole over a range of crustal depths. Geochemical data show a typical calc-alkaline affinity series. Almost all the rocks display active margin geological features. They are characterized by enrichment in large ion lithophile elements and depletion in high field strength elements. The chondrite-normalized rare earth element patterns show positive Eu anomalies in these volcanic rocks (Eu/Eu*= 0.78 to 1.05, La_N/Yb_N = 4.07 to 5.92, La_N/Sm_N = 2.32 to 3.34). The geological and geochemical data help constrain magmatic activity during the anomalous Eocene flare-up event in northern Iran.