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United States Department of the Interior Geological UNITED STATES DEPARTMENT OF THE INTERIOR GEOLOGICAL SURVEY Reconnaissance geology of the Ghazzalah quadrangle, sheet 26/41 A, Kingdom of Saudi Arabia by James E. Quick Open-File Report 83-J3I Prepared for Ministry of Petroleum and Mineral Resources, Deputy Ministry for Mineral Resources Jiddah, Kingdom of Saudi Arabia This report is preliminary and has not been reviewed for conformity with U.S. Geological Survey editorial standards and stratigraphic nomenclature I/ U.S. Geological Survey, Menlo Park, CA CONTENTS ABSTRACT................................................ 1 INTRODUCTION............................................ 2 Geographic setting.................................. 2 Previous investigations............................. 2 Present investigation............................... 4 Acknowledgments..................................... 5 GEOLOGIC OVERVIEW....................................... 5 VOLCANIC AND SEDIMENTARY ROCKS.......................... 6 Banana greenstone................................... 6 Age and thickness............................... 8 Hadn formation...................................... 8 Undivided volcanic rock........................ 9 Rhyolite member................................ 10 Arkose member.................................. 10 Age and thickness.............................. 10 INTRUSIVE ROCKS......................................... 12 Metagabbro.......................................... 12 Hornblende quartz diorite........................... 14 Tonalite............................................ 14 Biotite-hornblende granodiorite..................... 16 Magnetite syenogranite....:......................... 17 Monzogranite........................................ 17 Pre-Hadn monzogranite.......................... 18 Post-Hadn monzogranite......................... 19 Tentative age assignments...................... 19 Gabbro.............................................. 21 Alkali-feldspar granite............................. 21 Hornblende-feldspar porphyry........................ 22 Granophyre.......................................... 24 Diorite............................................. 24 Jufayfah syenogranite............................... 25 Rumman granite...................................... 26 Arfvedsonite granite................................ 27 Ba'gham intrusive complex........................... 28 Peralkaline granite member..................... 28 Granophyre member.............................. 28 Age of the complex............................. 29 Diabase.............................................. 29 Felsic dikes........................................ 29 CENOZOIC ROCKS.......................................... 31 Basalt.............................................. 31 Undifferentiated Quaternary deposits................ 31 Alluvium............................................ 31 Playa lake deposits................................. 31 Page STRUCTURE........................... 31 METAMORPHISM........................ 33 GEOLOGICAL HISTORY AND GEOCHRONOLOGIC| L CONSTRAINTS 33 COGENETIC SUITES.................... 35 ECONOMIC GEOLOGY..................... 36 Jibal Ba'gham radiometric anomalies 36 Field relations............. 37 Chemistry................... 40 Jibal . ar Rumman pluton........... 41 Spectrographic analyses.......... 41 Wadi sediment samples............ 42 Gravel........................... 42 DATA STORAGE.... 42 REFERENCES CITED 43 ILLUSTRATIONS .[Plates are in poc cet] Plate 1 Reconnaissance geologic map of the Ghazzalah quadrangle Map of the Ghazzalah quadrangLe showing sample numbers, locati >ns, rock types, and geographic and cul :ural features Structural sketch map of the Ghazzalah quadrangle Figure 1 Index map of western Saudi Arabia showing location of the Ghazza ah quadrangle... Ternary diagrams showing the modal compositions of pre-Hadn granitic rocks Ternary diagrams showing the modal compositions 1*5 of post-Hadn monzogranite Ternary diagrams showing the modal compositions 20 of alkali-feldspar and Jufayfah granites .... 23 Sketch map of the Jibal Sa'gham complex showing sample locations and contours from helicopter-borne cintillometer survey............... 38 ii TABLES Table 1. Petrographic properties and radioactivity of intrusive rocks in the Ghazzalah quadrangle............................... 13 2. Radioactivity in counts per second and concentrations of uranium, thorium, and rare-earth elements in the Jibal Ba'gham peralkaline pluton............... 39 111 RECONNAISSANCE GEOLOGY OF THE GHAZZALAH QUADRANGLE, SHEET 26/41 A, KINGDOM OF SAUDI ARABIA by James E. Quick if ABSTRACT The Ghazzalah 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 two lithologically distinct, Precambrian volcano- sedimentary units and a wide range of dioritoid and granitoid Plutonic intrusive rocks. The only Phanerozoic rocks consist of one outcrop of Tertiary (?) basalt and widespread but thin deposits of Quaternary detritus. The Banana greenstone, the oldest rock in the quadrangle, consists of intermediate volcanic and subvolcanic rocks and minor interbedded marble, which have been metamorphosed to greenschist-facies assemblages. Volcanic rocks mainly range in composition from basalt to andesite, and subvolcanic rocks consist of diorite and diabase. The Banana greenstone is unconformably overlain by silic­ ic volcanic rocks and minor arkosic sandstone and breccia of the Hadn formation. Preservation of delicate volcanic textures suggests that the rocks have been only incipiently metamorphosed. Unpublished rubidium/strontium isotopic data for the Hadn formation suggest an age of 620 to 610 Ma. Intrusive rocks are separable according to their ages relative to the Hadn formation. Those that are unconformably overlain by the Hadn formation consist of hornblende quartz diorite and gabbro, which may be consanguineous with the Banana greenstone, and younger tonalite, biotite-hornblende granodiorite, syenogranite, and monzogranite. Plutons of monzogranite, alkali-feldspar granite, syenogranite, peralka- line granite, and hypabyssal intrusions of granophyre were probably emplaced during a period coincident with and (or) following Hadn volcanism. Uranium-lead and rubidium/strontium isotopic data for two plutons in the adjacent Al Qasr quad­ rangle suggest that plutonic activity persisted in the region until about 580 to 570 Ma. Faulting appears to postdate all of the plutonic rocks. The dominant faults belong to a northeast-trending system of right-lateral shears; a subordinant system consists of mainly north- to northwest-trending faults. I/ U.S. Geological Survey, Menlo Park, CA 1 The peralkaline-granite plutoru underlying Jibal Ba'gham and Jibal ar Rumman have the most economic potential. Wadi samples from these areas show an jmalous concentrations of tin, lead, niobium, and yttrium, ocalized, intense radio- metric anomalies in the Ba'gham intrusive complex are asso- ciated with high concentrations f thorium, uranium, and rare-earth elements. INTRODUCTIO Geographic set ting The Ghazzalah quadrangle, shee t 26/41 A, occupies 2760 km2 between lat 26°30 ! and 27°00' N. and long 41°00' and 41°30' E. in north-central Saudi Ar bia (fig. 1). Ha'il, the nearest city with an airport, is located about 100 km north- east of the center of the quadrangl The principal access to the study area is by a major high ws.y that connects Ha'il and Al Madinah. From the highway, it is possible to drive within 1 km of most outcrops via secondars paved roads, dirt roads, and desert tracks. The topography is subdued; two-thirds of the area is underlain by pediments, peneplains , and alluvial fans and plains that have an average altitud .> of approximately 1000 to 1100 m. The most rugged terrain is along the eastern and northern margins of the quac^angle, where young granites and silicic-volcanic rocks crop ut; Jibal ar Rumman has a maxi- mum altitude of 1350 m, and Lbal B 'gham and Jabal Ma'a have maximum altitudes of about 1265 m Elsewhere, most of the inselbergs and ranges of hills ar less than 100 m higher than the surrounding plains. The drainage is dominated by Wadi Sha'bah, which crosses the enter of the quadrangle toward the southeast. All streams a e intermittent and belong to the Wadi ar Rimah drainage syster Most of the permanent settleme ts in the quadrangle are located near Jibal ar Rumman or Jibal Ba'gham, where ground water is available for agriculture. Previous investit ations The only published geologic map that includes the Ghazzalah area is the 1:500,000- scale Northeastern Hijaz quadrangle by Brown and others ( 1963), which is based on field investigations conducted prior to 1963. However, a recent increase in geologic inves ;igation in the northern shield has resulted in a number of studies that are relevant to the geology of the Ghazzalah ar a (fig. 1). The adjacent quadrangles of Al Qasr (27/41 C) to the north, Qufar (27/41 D) to the northeast, and Al Awshaziyah (26/41 B) to the east 35* 36« 37' 38' 39* 40* 41* 42 ARABIAN SHIELD 16* - 16* 36 45* 46* 47* 48' Figure 1. Index map of western Saudi Arabia showing location of the Ghazzalah quadrangle (shaded) and other quadrangles referred to*in this report: A, Al Qasr (Stoeser, unpublished data); B, Qufar (Kellogg, "d8££}\ C, Al Awshaziyah (Leo, unpublished data); D, Nuqrah (Delfour, 1977); E, Northeastern Hijaz (Brown and others, 1963). 3 have been mapped at a scale of 1:1 ,000 by D. B. Stoeser, K. S. Kellogg, and G. W. Leo,
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