Title: Concurrent island are and intraplate volcanism, Eshtehard-Karaj area NW Iran
Abstract:This paper discusses the geochemical characteristics of the Paleogene volcanic rocks from Eshtehard-Karaj area, Iran. A representative set of 15 volcanic rocks from the study area were analyzed for th...This paper discusses the geochemical characteristics of the Paleogene volcanic rocks from Eshtehard-Karaj area, Iran. A representative set of 15 volcanic rocks from the study area were analyzed for their major elements, Nb, Zr, Y, Sr, Rb, Ba, Pb, U (XRF lab., UNSW, Sydney), REE, Th, Hf, and Ta (NAA, Becquerel lab., Sydney) for the first time. The study area is the northernmost part of a major tectonomagmatic unit, Urumieh-Dokhtar magmatic assemblage, interpreted to be the product of an island arc magmatism during the subduction of the Neo-Tethyan oceanic realm in late Mesozoic and Cenozoic time (Alavi, 1996). On silica-Ti diagram, data points from the volcanic rocks are clustered in two distinct groups; one with lower Ti content than the other. Thereafter, they are called LT and HT series. The latter mostly occurs in the eastern part of the study area (i.e., Karaj). Detailed investigation of trace element geochemistry of the volcanic rocks shows fundamental differences between LT and HT series. Primordial mantlenormalized trace element patterns of the LT series rocks show highly distinctive “spiked” trace element patterns at Ba, K, and Sr, negative Nb anomalies, and a mean trace element concentration of around 10 times primordial mantle. Samples from the HT series, on the other hand, show rather smooth patterns with higher trace element abundances. Compared with the LT series, samples from the HT series are enriched in the whole range of trace elements (esp., Ta and Nb); only K and Sr remain at the same level. Volcanic rocks from the LT and HT series show remarkable geochemical similarities with the calc-alkaline island arc volcanic rocks and midalkaline oceanic island arc volcanics, respectively. This study supports the earlier finding that an island arctype magma was involved in Eocene-Oligocene magmatism in the study area. Contribution of this study is, however, the introduction for the first time of a set of volcanic rocks with the geochemical characteristics of an intraplate volcanism (i.e., OIB). These oceanic island basalt-type volcanics are derived from an enriched mantle source, in contrast with the depleted asthenospheric mantle component involved in generation of the island arc-type volcanics.Read More
Publication Year: 2003
Publication Date: 2003-09-01
Language: en
Type: article
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