Reconnaissance geology of the Ishash Quadrangle, sheet 26/39C, Kingdom of Saudi Arabia

The Ishash quadrangle occupies an area of about 2,725 km2 on the northern border of the Precambrian shield in north-central Saudi Arabia between lat 26?00' and 26?30' N. and long 39?00' and 39?30' E. The northern quarter is covered by Cambrian Siq Sandstone and the southeastern corner is covered by Quaternary to Tertiary basalt. The remainder of the area is underlain by trachyandesite, andesite, and metasedimentary rocks of the lower part of the Hulayfah group and
rhyolitic flows and ruffs of the upper part of the Hulayfah group. These rocks have been intruded by rocks that range in composition from gabbro to alkali-feldspar granite. Two small outcrops of Tertiary rhyolite are present in the quadrangle.

The predominant structures are folds and faults initiated by the formation of the caldera in the southwestern part of the area. The northerly trending structures were reactivated during the late Precambrian Najd orogenic event and again in the Tertiary during the opening of the Red Sea.

No mineral deposits of economic importance were found in the quadrangle.