Petrogenesis, geochemistry, and tectonic setting of a basaltic body within the Gercus Formation of northern Iraq: first record for Eocene anorogenic volcanic activity in the region


Abstract: An abnormally odd and rare occurrence of a basaltic body has been discovered in the middle of the Gercus Formation within the large, double-plunging NW-SE trending Bekhair Anticline to the north of Duhok city, northern Iraq. This is the first discovery of such a volcanic body within the widely exposed Gercus Formation in northern Iraq, southeastern Turkey, and western Iran. This volcanic body, named by the authors the Gercus Basalt, has a long tabular/lenticular shape with a total length of ~4.5 km and a thickness ranging between 1 to 16 m. It is exposed on both limbs of the anticline and has a conformable relation with the terrigenous beds of the Gercus Formation. The basalt is greenish to grayish black in color, vesicular, amygdaloidal and porphyritic in texture, and consists of feldspar (Ab85An2Or13), pyroxene (Wo49En40Fs11), olivine (Fo87.2-Fa12.6-Tp0.2), and their alteration products. Postemplacement hydrothermal fluids resulted in pervasive alteration of the Gercus Basalt and introduction of copper mineralization filling joints, fractures, and adjacent vesicles. The alteration processes are dominated by calcitization, zeolitization, serpentinization, chloritization, silicification, iddingsitization, and copper mineralization. Seventeen basalts have been analyzed for 63 major and trace elements including REEs and PGEs. The Gercus Basalt has very similar geochemical compositions to that of OIB. Excluding Cr, Co, Ni, and PGEs, it is enriched in all other analyzed major and trace elements relative to primitive mantle. The chondrite-normalized distribution of REEs shows a smooth pattern, high La/Yb(N) ratio of 23, and LREE enrichment relative to HREE, indicating alkaline magmatic origin. The studied rocks are basanites, alkaline basalts, melanephelinites, and picrites with an OIB-like anorogenic geochemical signature. The Gercus Basalt was derived from amphibole-bearing garnet-lherzolite subcontinental asthenospheric mantle, enriched with silicate metasomatism.

Keywords: Gercus Formation, petrology, geochemistry, anorogenic volcanism, basanite

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