Petrology and Geochemistry of Gole Pillow Basalt in Penjween area Kurdistan Region- NE Iraq

Abstract

The petrological, morphometric and geochemical analyses of pillow lava from Gole village (Penjween town) Sulaimani city Northeastern Iraq have been undertaken. The Gole Pillow basalt (GPB) extruded, in the form of pillow and sheet flow into the Qulqula radiolarite Formation. The basaltic intrusion restricted to a small area of about 100 m² within Penjween- Walash zone in the Zagros Suture Zone (ZSZ) of Iraq. The investigated area divided into two sections GPB1 and GPB2. It is envisaged that the studied area distinct two episodes of submarine alkaline eruptions that produced pillowed lavas that differently interacted with seawater to produce different morphologies and geochemistries. The pillows of the GPB1 section well exposed all along the Shalair river near Gole village. Although the pillows of the GPB2 section altered due to low-grade metamorphism and late hydrothermal processes, their igneous textures are still preserved. GPB samples are mostly phyric in nature and show porphyritic or sub-ophitic textures.

Petrographically, most of the GPB rock samples appeared as amygdaloidal and vesicular aphanitic basalt. Large phenocryst of hypersthene with schiller structure is present within a matrix of longer quenched plagioclase. Numerous small euhedral grains of opaque minerals like ilmenite and hematite are dispersed in the fine groundmass.

Morphological features show that the GPB appeared as spheroidal and lobate to tubular individual pillows. Although some pillow extends 2 meters with a foreset distribution some others show cracked with irregularly jointed surface and larger vesicles partly filled with calcite and quartz.

Geochemical investigation of GPB exhibit high TiO₂ (3.42 – 3.84 wt.%), Fe₂O₃ (14.84–19.93 wt.%), and high Zr/Nb and Zr/Y ratios respectively (5.85 – 7.2) (7.10-11.40). The content of alkalies, with the Nb/Y ratio≥1.4, and silica, as well as many trace element discrimination diagrams, classify the GPB as alkaline basalts.

The field, stratigraphic relationships, and geochemistry of the GPB and associated clastic and carbonate sediments suggest that the pillow lavas were emplaced in a shallow marine marginal within plate basin. The overall geochemistry of GPB resembles that of alkaline basalts generated in within-plate ocean island settings (OIB-type).