Evolution of the volcanic mechanism in the central part of the Urumieh-Dokhtar magmatic arc

Cenozoic volcanic activities in the Urumieh-Dokhtar Magmatic Arc (UDMA) have occurred in three main pulses of Eocene, upper Oligocene-Pliocene, and Pio-Quaternary (Dilek et al., 2010, Sayari, 2015). Magmatic activities in the UDMA until a few years ago were marked with calc-alkaline and occasionally shoshonitic signatures. Recent studies have reported post-collisional adakites in some parts of the UDMA (e.g., Ghadami et al., 2008; Omrani et al., 2008; Sayari, 2015; Sayari and Sharifi, 2018). Since magmatic genesis of calc-alkaline, shoshoitic, and especially adakites are absolutely different, variation in the volcanism nature of Iran is a key to recognition of geodynamic evolution of Iran. This study tries to analyze the volcanic evolution in the central part of the UDMA by systematically processing of geochemical database for three main Cenozoic volcanic pulses.   

Whole rock reliable ICP-MS analysis data from scientific texts having exact location coordinates were gathered to form a geochemical geodatabase which includes 99 samples. This database spatially covers around 200 km in the central part of the UDMA from 51°15´E and 33°47´N (north of Isfahan) to 52°57´E and 32°35´N (east of Isfahan).  

Analysis of the geochemical geodatabase indicates that none of the samples belong to alkaline and tholeiitic magmatic series. About 71 percent of group 1 (volcanic pulse of Eocene) are calc-alkaline, and the remaining 29 percent are shoshonitic. About 67 percent of group 2 (volcanic pulse of Oligocene-Miocene) are shoshonitic, and the remaining 33 percent are calc-alkaline. About 88 percent of group 3 (volcanic pulse of Plio-Quaternary) are adakite, and the remaining nearly 12 percent are both calc-alkaline/shoshonitic (samples CN4, JS13, OG4 and SK1 of Khodami, 2009). Adakitic samples are situated in two areas in Joshaghan-Ghohrud and Kajan-Kahang. Sayari and Sharifi (2018) showed that there is a correlation between UDMA adakites and positive lithospheric thickness anomalies. They showed that adakites in the central part of UDMA are restricted to 4 regions exactly where lithosphere and crust are anomalously thicker than the surrounding. In the areas where adakites lie, lithosphere-asthenosphere boundary (LAB) is situated deeper than 212 km (Sayari and Sharifi, 2018). The geochemical aspect of the studied adakites which are all related to the third volcanic pulse of UDMA shows that they have been derived from the subducted slab. They do not have adakite-like or crust-derived adakites characteristics.

The results indicate that volcanic activities from Eocene to Quaternary have evolved from calc-alkaline to shoshonitic signatures and then turned into adakitic nature. Calc-alkaline and shoshonitic magmatism resulted from partial melting of the mantle wedge, while adakitic magmatism resulted from partial melting of the subducted slab. This means that the origin of the third volcanic pulse has shifted from mantle wedge to slab. According to the La/Sm versus La diagram (Aldanmaz, 2000) calc-alkaline samples have been derived from about 15% partial melting of the spinel-garnet lherzolite, and the shoshonitic samples have resulted from about 3% partial melting of the spinel-garnet lherzolite. Based on La/Yb versus Yb diagram (Bourdon et al., 2002), adakites from Kajan-Kahang have been derived from about 10% partial melting of the garnet amphibolite. Moreover, the Adakites from Joshaghan-Ghohrud have resulted from about 6% partial melting of the hornblende eclogite.