Reconnaissance geology of the Zarghat Quadrangle, sheet 26/40 B, Kingdom of Saudi Arabia

Open-File Report 84-794




The Zarghat quadrangle is located in the northern Precambrian shield of Saudi Arabia between lat 26°30' and 27°00' N. and long 41°00' and 41°30 ' E. The area is underlain by three Precambrian volcanosedimentary units and a range of Precambrian dioritoid and granitoid plutonic intrusive rocks. Paleozoic(?) sandstone crops out in small areas in the northwestern part of the quadrangle, and a lobe of QuaternaryC?) basalt from Harrat Ithnain penetrates the southwest corner of the quadrangle.

The Banana greenstone, the oldest unit in the quadrangle, is composed of basic to intermediate volcanic and subvolcanic rocks and minor interbedded marble that have been metamorphosed to greenschist-facies assemblages. The volcanic rocks range from basalt to andesite and dacite, and the subvolcanic rocks are diabase and diorite.

Two less-metamorphosed Precambrian units, the Hadn formation and the Zarghat formation, unconformably overlie the Banana greenstone. The Hadn formation is composed of predominantly rhyolitic to dacitic volcanic rock and minor amounts of continental sedimentary rock; it is exposed only in the east half of the quadrangle. The Zarghat formation is also rhyolitic to dacitic volcanic rock but contains much more sedimentary rock that appears to have been deposited in a shallow-marine environment. The relation between these two formations is not clear at this time.

Two groups of intrusive rocks are recognized on the basis of their ages relative to the Hadn and Zarghat formations. An older group is composed of hornblende quartz diorite, granodiorite, tonalite, and lesser amounts of granite. Tnese rocks intrude the Banana greenstone but predate the Hadn and Zarghat formations. Post-Hadn/Zarghat intrusive rocks are voluminous undivided granite, several varieties of alkali-feldspar granite, monzogranite, granophyre, hypabyssal rhyolite, aplite, diabase, and gabbro.

The structural grain within the quadrangle appears to trend north-south. The Precambrian volcanosedimentary rocks and older plutonic rocks define three major north-trending lithologic belts. The Zarghat formation makes up the westernmost belt, and the Hadn formation comprises the easternmost belt. The central belt consists of Banana greenstone and older plutonic rocks. The north-south structural grain is crosscut by large plutons that postdate the Hadn and Zarghat formations. The dominant faults in the area belong to a northeast-trending system of shears; less common fault orientations are northwesterly, northerly, and east-westerly.

Outcrops of magnesite in the southwest corner of the quadrangle constitute the only known economic potential in the area.

Study Area

Additional publication details

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USGS Numbered Series
Reconnaissance geology of the Zarghat Quadrangle, sheet 26/40 B, Kingdom of Saudi Arabia
Series title:
Open-File Report
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U.S. Geological Survey
Publisher location:
Reston, VA
ii, 40 p. :ill., maps ;28 cm.
Saudi Arabia
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