The Arabian continental alkali basalt province: Part I. Evolution of Harrat Rahat, Kingdom of Saudi Arabia 1 Directorate General of Mineral Resources, P.O. Box 345, Jiddah, 21191 Saudi Arabia M. JOHN ROOBOL ) ABSTRACT posits are common. The final events of harrat plate, the Arabian harrats1 (Fig. 1), are not re- volcanism include 11 "post-Neolithic" erup- lated to a well-developed continental rift. Al- The Harrat Rahat lava field forms a major tions (<4,500 yr old) and two historical erup- though they are poorly known and little studied, component of an extensive but poorly known tions (in 641 and 1256 A.D.). the Arabian harrats occur throughout the west- continental alkali basalt province extending Arabian harrat magmatism may have been ern Arabian Peninsula over a south-to-north dis- from Yemen in the south, through Saudi initiated by partial melting of garnet perido- tance of 3,000 km, from Yemen through Saudi Arabia and Jordan, to Syria in the north. The tite in the asthenosphere at a depth >100 km. Arabia, Jordan, and Syria. Together they com- continental intraplate volcanism which pro- Accumulation of this partial melt at the crust- prise one of the world's largest alkali volcanic duced the Arabian lava fields (harrats) was mantle boundary (37-44 km) may have re- provinces, with an approximate area of 180,000 2 contemporaneous with opening of the Red sulted in Miocene uplift of the Afro-Arabian km (Coleman and others, 1983). Sea, collision of the Arabian and Eurasian dome. Harrat Rahat volcanism began shortly The alkalic nature of harrat volcanism has plates, and uplift of the Afro-Arabian dome. after significant initial uplift with the volumi- been established in several petrologic investiga- Harrat Rahat has evolved over the past 10 nous extrusion of Shawahit OTB lava. Chem- tions (Baker and others, 1973; Coleman and m.y. It contains two prominent lateritic dis- ical and petrographic data suggest that these others, 1977, 1983; Arno and others, 1980a, conformities which separate three stratigraph- OTB magmas ascended rapidly, with little 1980b; Barberi and others, 1980; Baubron and ic units: the Shawahit (10-2.5 Ma), Hammah crustal residence time. The data support a Maury, 1980; Berthier and others, 1981); how- (2.5-1.7 Ma), and Madinah (1.7-Recent) ba- model of open-system fractionation for AOB ever, the petrogenetic and tectonic evolution of salts, comprising 68%, 19%, and 13% of the and hawaiite lavas of the younger Hammah the Arabian harrats remains largely unresolved, total harrat volume, respectively. The Shawa- and Madinah basalts. In this model, some of primarily due to a lack of regional mapping and hit basalt is composed of coarse-grained, dic- the potential Shawahit OTB was trapped in the absence of a coherent petrochemical stratigra- tytaxitic olivine transitional basalt (OTB) and crustal chambers during magma ascent. Crys- phy for any single harrat. In order to understand minor (-10%) alkali olivine basalt (AOB) tal fractionation in these chambers was ac- the over-all processes involved in harrat evolu- which erupted through scoria cones and companied by their periodic replenishment by tion, we have selected three of these lava fields shield volcanoes. In contrast, the Hammah rising OTB magma. This resulted in the mix- in western Saudi Arabia for detailed investiga- basalt is dominated by equal volumes of fine- ing of magma types and the development tions; together they comprise more than 46,000 grained, intergranular-to-intersertal AOB and of AOB and hawaiite magmas which are km2, about 25% of the Arabian harrat province. hawaiite which erupted mainly through sco- anomalously enriched in highly incompatible A 1:250,000-scale geologic map of Harrat ria cones and relatively few shield volcanoes. elements. The mugearite, benmoreite, and Rahat has recently been completed (Camp and Sparse domes and flows of mugearite and trachyte lavas are the products of advanced Roobol, 1987), and geologic maps for Harrat benmoreite first appear in the Hammah fractionation in these crustal chambers with Khaybar and Harrat Kishb are in preparation basalt. The Madinah basalt spans the full com- little or no magma replenishment. (Fig. 1). This paper is the first installment in positional range of Harrat Rahat; 103 ana- documenting the nature and evolution of these continental alkali basalt fields. In this paper, our lyzed samples comprise OTB (8%), AOB INTRODUCTION (47%), hawaiite (32%), mugearite (4%), ben- intent is first to describe the volcano-tectonic setting of the Arabian harrats in general, and moreite (8%), and trachyte (2%). Here scoria The vast (106 km2) Cenozoic lava plains of then to delineate the field and petrochemical cones dominate, whereas shield volcanoes are the East African Rift System provide a well- sparse and occur only in the lower Madinah known example of tholeiitic-to-alkalic volcan- basalt; domes and associated pyroclastic de- ism associated with the early development of a •The word "harrat" is the possessive form of the rifted continental margin (Mohr, 1983, and refer- singular Arabic noun "harra," which means "stony *Present address: 4757 Edgeware Road, San ences therein). In contrast, adjacent and contem- area, volcanic country, lava field" (Wehr, 1976); it is Diego, California 92116. poraneous lava plains of the western Arabian related to the adjective "harr," meaning "hot." Additional material for this article (Appendices) may be obtained free of charge by requesting Supplementary Data 8904 from the GSA Documents Secretary. Geological Society of America Bulletin, v. 101, p. 71-95,18 figs., 4 tables, January 1989. 71 Downloaded from http://pubs.geoscienceworld.org/gsa/gsabulletin/article-pdf/101/1/71/3380483/i0016-7606-101-1-71.pdf by guest on 27 September 2021 72 CAMP AND ROOBOL nental collision with the Eurasian plate (Fig. 1). This event appears to be complicated by a long history of Cenozoic compression (Lovelock, 1984; Hempton, 1985, 1987). The cessation of first-stage spreading of the Red Sea, however, may correspond with the main phase of collision in the mid-Miocene, when the full thickness of the Arabian-plate lithosphere entered the Eura- sian convergent zone along the Bitlis (Sengor and Kidd, 1979; Sengor and Yilmaz, 1981) and Zagros (Bird and others, 1975; Stonely, 1981) suture zones (Fig. 1). Continental Extension and Uplift of the Afro-Arabian Dome The genesis of magmatic rocks of the western 20° Arabian plate over the past 5 m.y. is compli- cated by the contemporaneity of oceanic tholei- itic volcanism (second-stage spreading) and continental alkalic volcanism (the Arabian har- rats). It was suggested by Gass (1970) that both types of volcanism are associated with uplift of the Afro-Arabian dome as a result of a large mantle plume centered beneath the Afar triple junction. In this model, tholeiitic volcanism oc- curs along mature oceanic spreading axes near 10° the center of the dome, whereas alkali volcanism occurs along the flanking continental plateaus. Gass' model has been questioned by Almond (1986a, 1986b), who considered these two volcanic series to be the expressions of two spatially and, in part, temporally distinct tectono- magmatic systems. Because the northwest-trending Red Sea axis is oblique to most (north-trending) PRECAMBRIAN ROCKS harrat linear vent systems by about 25°, we fa- vor the latter model. Almond (1986a, 1986b) suggested that crustal extension in Arabia and northeast Africa began in the Late Cretaceous TRANSFORM FAULT WITH % OCEANIC RIDGE AXIS along northwest trends, and that continued ex- tension in the Oligo-Miocene was responsible THRUST FAULT for linear northwest downwarping and the sub- sequent development of the Red Sea Basin. Almond (1986b) presented convincing chrono- Figure 1. Cenozoic lava fields and major tectonic elements of Arabia and northeast Africa. stratigraphic evidence that (1) this early phase of After Mohr (1971), Sengor and Kidd (1979), Coleman and others (1983), and Lovelock (1984). extension was related to a persistent period of subsidence, not to uplift and doming as pro- posed by Gass (1970); and (2) uplift to produce characteristics of Harrat Rahat, a major lava (Girdler and Styles, 1976, 1978), and the Dead the Afro-Arabian dome did not occur until about 10 Ma. He concluded that the northerly field in a poorly known geologic province. Sea Rift (Freund and others, 1970) have sug- gested that the western Arabian plate evolved trending vent systems for most of the Arabian harrats are the result of uplift and extension VOLCANO-TECTONIC SETTING during two stages of sea-floor spreading. The early stage commenced in the late Oligocene at along the Miocene crest of the Afro-Arabian dome, whereas the northwest-trending Red Sea The evolution of Harrat Rahat, and all other 30 Ma and lasted until 15 Ma, at the end of the Basin and associated encratonic basins are the harrats, is controlled largely by the regional early Miocene; the later stage commenced in the result of lateral crustal attenuation and subsi- stress regime of the western Arabian plate, a Pliocene at about 5 Ma and has continued to the dence that began in the early Tertiary. regime inherently related to the Cenozoic devel- present (Ross and others, 1973; Styles and Hall, opment of the Red Sea. Complementary investi- 1980). The early Tertiary to Recent extension along gations of the Red Sea (Girdler and Styles, The northern and northeastern leading edge northwest trends appears to be associated with 1974; Styles and Hall, 1980), the Gulf of Aden of the Arabian plate represents a zone of conti- fundamentally transitional (Ethiopian and Ye- Downloaded from http://pubs.geoscienceworld.org/gsa/gsabulletin/article-pdf/101/1/71/3380483/i0016-7606-101-1-71.pdf by guest on 27 September 2021 CONTINENTAL ALKALI BASALT PROVINCE, SAUDI ARABIA 73 UPLIFT OF INTRACRATONIC CRUSTAL EXTENSION, of Harrat Rahat on the Red Sea coastal plain AFRO-ARABIAN DOME SUBSIDENCE AND MARINE INCURSIONS I (Pallister, 1987).