Abstract
The basaltic pillow lava and basalt at Choman-Rayat towns, Erbil city, Northeastern Iraq, were studied in terms of petrology, geochemistry, morphology and mineral chemistry adjacent to the Iraqi-Iranian borderline. The basalt of Choman-Rayat area was extruded as basaltic pillow lava and basalt within the Walash sequence in the northern Zagros suture zone. According to the field observations, the examined area was partitioned into four sections (A, B, C, and D).
Petrographically, the majority of the basaltic pillow lava and basalt samples emerged as amygdaloidal and vesicular aphanitic basalt. Amygdaloidal textures have been observed in all traverses, with vesicles exhibiting circular and irregular shapes containing secondary minerals including calcite. Identical across the four traverses, they exhibit distinct textures. The D samples typically exhibit greater aphanitic textures. In contrast, the other three traversed samples exhibit more advanced magma and are distinguished by porphyritic textures, wherein plagioclase functions as a first mixing phase of the magma. Large phenocryst of pyroxene is located inside a matrix of extended quenching plagioclase. Numerous microscopic euhedral grains of opaque minerals as ilmenite and hematite are distributed in the very fine groundmass.
Mineralogically, both basaltic pillow lava and basalts predominantly consist of plagioclase, clinopyroxene, and opaque minerals with other components as secondary minerals including amphibole, epidote, calcite, serpentine, quartz, zeolite, and chlorite.
Morphological properties of pillow lava indicate that the basaltic pillow lava samples formed as spherical, tubular, soft red colored to hard green colored isolated pillows. Others exhibit cracking with planar jointed surfaces and larger vesicles with elongated prism containing secondary minerals predominantly with calcite and quartz, while the diameters of the pillow structures within this studied area varied considerably, ranging from 10 centimeters to 6 meters.
The electron microprobe analyses (EPMA) represented that the Clinopyroxene in the pillow basalts are mainly diopside composition with uncommon augite composition. The plagioclase composition in pillow basalt samples is sodium rich, predominantly consisting of albite. The K-feldspar composition in pillow basalt samples is potassium rich, predominantly consisting of orthoclase.
Geochemically based on their magmatic affinity, the basaltic pillow lava and basalt samples are classified into tholeiitic series. Numerous tectonic discriminating diagrams shown that the majority of basaltic pillow lava and basalt rocks are linked to the magma of oceanic island arc (OIA), indicating that the pillow lavas were deposited in a shallow marine marginal within-plate basin. Consequently, the active subduction system in the Neo-Tethyan region during the Eocene-Oligocene age was crucial to the formation of an island-arc complex, distinguished by basaltic pillow lavas and related volcanic rocks.
