Upper Mantle & Lower Crustal Xenoliths
Lower crustal and upper mantle xenolithsare currently the subject of intense examination. The study of xenoliths could produce a clear picture of the nature of the earth crust and upper mantle. The wide spread interest in ophiolites and mafic and ultramafic xenoliths from basaltic rocks is caused by the informa-tion these ophiolites and xenoliths give on the nature of the lithosphere and the generation of basaltic magma from the mantle. The ophiolites and xenoliths can offer a direct evidence and a realistic model of the lower crust and upper mantle composition. Knowledge of these composition is essential to the under- standing of many large scale geological phenomena and processes. Tertiary and quaternary basaltic lavas, the so-called Harrats, of the Arabian Plate form a voluminous and wide spread volcanic province that extends from Yemen through Saudi Arabia, Jordan, Syria and Turkey, covering an area of about 80,000 km (Coleman et al. 1983). The alkali basaltic lavas of the Arabian Plate contain a wide variety of accidental crustal and mantle xenoliths (Coleman et al., 1983, Kuno and Essene, 1986, Mcguire 1988a,b Nasir and Al fugha 1988, Nasir 1988a, b, 1989, 1991, Nasir et al. 1992, Nasir & Mahmood, 1990). These xenoliths can provide important and direct information on the nature of the lithosphere beneath the Arabian Plate. The complex history of the Arabian Plate lithosphere has not been established. Many questions regarding the evolution of the lithosphere remain to answer For the better understaning it is obvious that further petrological and geochemical investigations are urgently needed. This can be undertaken by studying the ophiolite rock sequences and the scattered mafic and ultramafic xenoliths, which are poorly documented, using a combination of mineralogy, geochemistry, petrography, and geothermobarometry. The geologic history of the Arabian lithosphere has not been established. The results of this research could give many answers for the geologic questions in this region.
Geology of the area Jordan and Syria lie on the northwestern edge of the Arabian Plate. Proterozoic basement rocks outcrop in the southern part of Jordan . They consist mainly of granite and granodiorite and metasediments of the amphibolite facies (Bender, 1974). These are overlained to the north and northeast by a thick sequence of Paleozoic, Mesozoic and Tertiary sediments, mainly sandstone and limestone. Tertiary and Quaternary basaltic lavas, in sequence up to 300 m thick, cap many peaks in the area and overlies Cretaceous, Tertiary and Quaternary sediments (Bender, 1974) Barbari et al (1980) and Nasir (1990) presented K-Ar data of 18.7 to 0.35 Ma for the volcanic rocks of Jordan. Most Quaternary lavas contain awide varieties of mafic and ultramafic xenoliths of lower and upper mantle origin (Nasir and Al-Fugha, 1988, Nasir, 1988a, b, 1990a, 1992, 1995; Nasir et al. 1992). The host magmas are mainly alkali olivine basalt and basanite (Nasir, 1990a). Several recent studies (barbari et al. 1979, Nasir 1994, Guba and Mustafa, 1988) link the volcanism in Jordan and the Arabian Plate to the two stage development of the Red sea. The volcanic activity in Jordan is localized by major, deep seated fractures, which are reactivated during rifting of the Jordan Rift Valley (Nasir, 1990a). Most of the basalts were emited along NW-SE and NNW-SSE trending lineaments formed as a result of a regional mechanical stress contemporaneous with the collision of the Arabian and Eurasian plates (Hempton,1987) and with emergence of the Afro-Arabian dome (Almond 1986 a,b). The volcanic rocks of Jordan have started along E-W trend with hawaiite and alkali olivine basalt and continue with increasing alkalinity, with time along NW-SE trending lineaments. These lineaments were formed deep enough to allow easy access of magma that had little opportunity of undergoing substanial fractionation and low pressure equilibrium (Nasir, 1990a).
Crustal and upper mantle xenoliths
Jordan:
A large number of fresh mafic and ultramafic xenoliths occur in the Pliestocene and recent alkali basalts of Jordan. The basaltic lavas occur mainly at the rims of and within the rift, mainly at Ma'in area, and along the eastern rim of the rift at Karak, Dana, and Tafila areas. The largest xenoliths occurences are found in the lavas flows and the pyroclastics of the northeastern basalt plataue of Jordan. Most of the xenoliths are rounded and range from 1 to 40 cm in diameter. Field work for the last three years resulted in the collection of about 1000 xenolith samples representing a wide lithological varieties. The xenoliths have been grouped on the ground of petrographical and textural basis into the following groups:
1- Cr-Diopside lherzolite group (Wilshire and Shervais,1975), Group I (Erey and Prinz, 1978). Xenoliths of this group are the dominant type in Jordan. Most xenoliths contain primary and secondary phlogopite and hornblende. The xenoliths are characterized by protogranular and equigranular textures. They consist of olivine, clinopyroxene, orthopyroxene, and brown spinel. Two xenoliths contain garnet beside spinel. ~ F f7
2- Al-Ti augite group (Wilshire and Shervais, 1975) Group II (Frey and Prinz, 1978). The xenoliths of this group make up 30% of the xenolith suite. The xenolith include wherlite and pyroxenite which are characterized by abundant black and green spinel and garnet.
3- Metapyroxenite: These include spinel pyroxenite, garnet webesterite, and garnet-spinel pyroxenite. They are characterized by igneous texture and by the presence of abundant spinel and or garnet exsolution blebs as well as broad exsolution lamellae of orthopyroxene in clinopyroxene.
4- Composite xenoliths: Few xenoliths consist of lherzolite and pyroxenite viens.
5- Megacryst: Cr-poor megacryst suite of augite, kaursutite, spinel and olivine are abundant.
6- Lower crustal xenoliths: Mafic two pyroxene granulite, gabbronorite and plagioclase gabbro are abundant. 7- Upper crustal xenoliths: These involve fragments of the surrounding Tertiary and Mesozoic sediments. Xenoliths from other part of the Arabian Plate. Xenolith data from the Arabian Plate are few and are still poorly documented. However, the following xenolith localities have been described: Saudi Arabia: 1- Jabal Al Abyad (Harrat Khayber): Mafic xenoliths, mainly hypersthene gabbro have been described from this area (Baker et al, 1973). 2- Harrat Al Birk: The xenoliths are dominantly plagioclase bearing websterite, garnet pyroxenite and two pyroxene gabbro (Ghent et al 1980, Mcguire 1988a). 3- Harrat Al Kishb and Harrat Hutaymah: The xenoliths in this area are predominently spinel lherzolite, spinel pyroxenite and spinel garnet pyroxenite (Kuno and Essene, 1986; Mcguire, 1988a,b). Red Sea 1- Kod Ali Island: Pyroxenite, lherzolite and gabbro xenoliths were described from this island (Forbes and Kuno, 1967; Hutchinson and Gas, 1977). 2- Zabarged: Spinel lherzolite, pyroxenite and gabbro xenoliths predominate (Bonat tie et al, 1981; Kurat et al . 1982)
South Yemen The xenolith suite of south yemen consist predominantly of kaersutite megacryst (Varne 1970).
Syria The xenolith suit of Syria includes spinel and garnet peridotite, pyroxenite as well as lower crustal granulites. The xenoliths occur mainly in the pyroclastics of Al-Nabi Saleh, Al-Nabi Matta, Al-Mhailbeh, Jawbet Al-Rassafeh, Hiker Beit Salameh, Jawbet Al Berghal and Jabal Al Arab. Diamonds were first recognized during routine heavy mineral analyses for the diatreme rocks of Jabal Masayef (Haggerty & Nagieb 1989).
Palastein and Israel Mountain Carmel: pyroxenite and garnet pyroxenite are found in the Upper Cretaceous alkali basalt of Carmel area (Mittlefehld 1986, Gottlieb, 1980). REFERENCES Abu-Aljarayesh, I., Mahmood, S. and Nasir, S. 1993. Magnetic study on lower crustal and upper mantle xenoliths from northeast Jordan. Abhath Al-Yarmouk, Pure Sciences & Engineer. 2 , 41-54. -Almond DC (1986a): The relation of Mesozoic - Cenozoic volcanism to tectonics in the Afro-Arabian dome. J. Volc. Geotherm Res. 28,225-246. ----(1986b): Geological evolution of the Afro-Arabian dome. Tectonophysics 131, 301- 332. -Baker PF, Brossel R Gass IG, Neary CR (1973) Jebel Al Abyad; A recent alkali volca- nic complex in western Saudi Arabia. Lithos, 6, 292-314. -Barberi FG, Capaldi P, Gasperini G, Marinelli R, Santacroce R, Scandon M, Teruel, Varet J (1980) Recent basaltic volcanic of Jordan and its implcations on the geodynamic evolution of the Dead Sea shear zone. In Geodynamic evolution of the Afro-Arabian rift system.Rome 667-683. -Bender F, (1974) Geology of the Arabian Peninsula, Jordan. US Geol Surv. Prof. Pap. 560-1 -Bonatti E, Hamlyn P, Ottonello G (1981): Upper mantle beneath young oceanic rift: Peridtites from the island of Zabargad (Red Sea). Geology 9, 474-479. -Coleman RB, Gregory RT, Brown GE (1983) Cenozoic volcanic rocks of Saudi Arabia. U.S. Geol. Surv. Open file Rep. OF-03-93. 82. -Forbes RB, Kuno H (1967) Peridotite inclusions and basaltic host rocks. In willie A.V. 328-337. -Frey FA, Prinz, M (1978): Ultramafic inclusions from San Carlos, Arizona: Petrolo- gic and geological data bearing on their petrogenesis. Earth Planet Sci 38, 129- 176. -Gettings MD, Blank HR, Mooney WD, Haley JH (1986) Crustal structure of southwe- stern Saudi Arabia. J geophys Res. 91, 6491-6512. -Ghent ED, Coleman RG, Hadley DG (1980): Ultramafic inclusions and host alkali olivine basalt of the southern coastal plain of the Red Sea, Saudi Arabia. Am J Sci, 280-A, 479-527. -Guba, I, Mustafa, H (1988) structural control of young basaltic fissure eruptions in the plateau basalt area of the Arabian Plate, northeastern Jordan. J Volc. Geother Res. 35, 319-334. -Gottlieb SE (1980): The petrography and mineral chemistry of a suite of ultramafic xeno- liths from the Carmel area, Israel. Un Publ. Msc thesis. Hebrew Uni. 73 pp. Haggerty, S., & Nagieb, M. (1989): Diamonds in non-kimberlitic, non-lamproitic diatremes from northwest Syria. 28th Intern. Geol. Congress. Washington D.C. 2:6-7 -Hempton M (1987) Constrains on Arabian Plate motion and extensional history of the Red Sea. Tectonics, 6, 687- 705. -Hutchinson R, Gass I (1971) Mafic and ultramafic inclusions associated with Kod Ali island, southern Red Sea, Contrib Mineral Petrol, 31, 94-101. -Kurat G, Niedermayer G, Prinz M, Brandstatter F (1982): high temperature peridotite intrusion into an evaporite sequence Zabargad, Egypt. Terra Cognita, 2, 240. -Kuo L, Essene EJ (1986): Petrology of spinel harzburgite xenoliths from the Kishb plateau, Saudi Arabia. Contrib Mineral Petrol 93, 335-346. -Mcguire AV (1986a) : The mantle beneath the Red Sea margin: xenolith from Saudi Arabia. Tectonophysics 150, 101-119. -----(1986b): Petrology of mantle xenoliths from Harrat al Kishb: The mantle beneath west- ern Saudi Arabia. J. Petrol. 29, 73-92. -Mittlefehldt DW (1986) : Petrology of the clinopyroxenite series xenolith mount Carmel, Israel. Contrib Mineral Petrol, 94, 245-252. -Nasir S (1988): Ultramafic xenoliths from NE and central Jordan. Fortschr Mineral Bhf 1, 66:110. -----(1989): Rift related Cenozoic volcanic rocks in Jordan. Symp. On the Afro- Arabian rift system. Karlsruhe, 67-68. -----(1989): Petrologic interpretation of geophysical data from Jordan. Symp. On the Afro-Arabian rift system. Karlsruhe, 69-70. -----(1990): K-Ar age determinations and volcanological evolution of the northwestern part of the Arabian Plate, Jordan. Europ. J. Mineral Bhf.1 67:108 _____(1991): The lithosphere beneath the northwestern part of the Arabian plate (Jordan): evidence from xenoliths and geophyiscs. Tectonophysics 202:253-270 ---- (1992). The lithosphere beneath the northwestern part of the Arabian plate (Jordan): evidence from xenoliths and geophysics. Tectonophysics, 201 , 357-370 ---- (1994): Geochemistry and petrogenesis of Cenozoic volcanic rocks from the northwestern part of the Arabian continental alkali basalt province, Jordan. Africa Geoscience Review 1 ,(in press) ------(1995). Geochemistry of mafic granulite from the northwestern part of the Arabian lithosphere. European Journal of Mineralogy (in press). --------, and Al-Fuqha, H. (1988). Spinel lherzolite xenoliths from the Aritain volcano, NE-Jordan. Mineralogy & Petrology 38 , 127-137. --------, and Mahmood, S. (1991). Oxidation of olivine in lherzolitic xenoliths from NE-Jordan. Mu'tah Journal for Research and Studies 6 ,171- 182. --------, Abu-Aljarayesh, I., Mahmood, S., and Lehlooh, A. (1992). Oxidation state of the upper mantle beneath the northwestern part of the Arabian lithosphere. Tectonophysics 213 , 359-366. --------, Lehlooh, A., Abu-Aljarayesh, I., and Mahmood, S., (1993). Ferric iron in upper mantle Cr-spinel: A Mossbauer spectroscopic study. Chemie der Erde 53 , 265-271.
