Basalts are the most abundant extrusive igneous rocks which have erupted on earth throughout its history. Their most extrusive forms are lava flows and pillows in which they constitute the bulk of the major lava plateau on the continents, ocean floor and significant parts of volcanic in rift valleys. Basalt as a general term is a fine-grained, mafic volcanic rock containing (44-54%) SiO2. It consists essentially of calcic plagioclase and abundant mafic minerals mainly Ca-rich clinopyroxene, but may contains quartz, orthopyroxene, olivine, feldspathoid, small proportions of alkali feldspar (<10% total feldspar), Fe-Ti oxides and apatite. Basalt rocks may give many types according to their mineralogical composition such as tholeiite contains quartz, hypersthene basalt contains hypersthene, olivine tholeiite contains hypersthene and olivine, olivine basalt contains olivine and alkali basalt which contains nepheline (Shalley, 1993).
Basalts often associate with lava flows and narrow dykes and sills. Most basalts occur as lava flows either in volcanoes or as extensive sheets building up a lava plateau which may cover hundreds of thousands of square kilometers and may be fed by numerous fissures. The surface forms of lavas are of two principal types: Smooth or Ropy (the surface looks like a rope) which is known by the Hawaiian term of pahoehoe and scoriaceous which is rough, clinker and has the Hawaiian name A`A. Another common form is pillow lava which consists of pillows or balloon-like masses of basalt-usually with a very fine-grained or glassy outer layer. They are the major constituent of the upper layer of the ocean floors (usually as pillow lava) and hot spot volcanoes such as the Hawaiian Islands. Basalt commonly forms on the continents too, usually the result of hot spot activity. Here it may also exist as intrusive dikes and sills, or extrusive cinder volcanoes (Solyom et al., 1985).
Harrat Al-Shaam is a large intra-continental basalt terrain which covers an area of about 50,000 km2, about 25% of the Arabian Harrat. It stretches over 700 km in a NW-SE direction and extent over the southern rim of the Damascus Basin in Syria. It is marked by the folded mountain of Antilebanon, southwards through North- Jordan up to the northern rim of Al-Azraq depression and then to the "Al-Nofuth" desert in the northwestern part of Saudi Arabia (Al-Malabah et al., 2002).
The basalt originated during several phases of eruptions which were closely associated in space and time with: (1) The Cainozoic evolution of the Red Sea through two-stages of spreading; the first was before 30-15 Ma and the second was initiated over the past 5 Ma-recent, (2) Collision of the Arabian and Eurasian plates and (3) The uplift of the Afro-Arabian dome (Camp and Roobol, 1989).
Previous studies on the North Arabian volcanic province suggested the derivation of this basalt from deep mantle source material and are also characterized by a low degree of partial melting of upper mantle peridotites with minor secondary differentiation (Barberi et al., 1979; Saffarini et al., 1985). The depth of the basalt source material is between 37-60 km (Ibrahim, 1996). The aim of this study to list mineralogy and texture of the basalt in Hail region.
MATERIALS AND METHODS
The study area: Volcanic activity that accompanied the opening of the Red Sea from the Miocene (25 Ma) to the present, resulted in the formation of vast fields of subaerial basaltic flows in the western part of Saudi Arabia, referred to by the Arabic term 'Harrat'. These harrats cover an area over about 90,000 km2 and extending over parts of the Proterozoic Arabian shield and adjacent Phanerozoic rocks of the Arabian Platform and Red Sea basin (Coleman and McGuire, 1988; Camp and Roobol, 1989). The principal harrats are Harrat Rahat, Harrat Uwayrid, Harrat al Hutaymah, Harrat Kishb, Harrat Khaybar and Harrat al Birk. The lava flows are commonly composed of picritic to ankaramitic basalt and may contain peridotite nodules (Fig. 1) (Thornber, 1990).
The study area is located in the southern part of Hail region. It is located between 27°13` and 27°31`N and 42°09` and 42°35`E. The principal features of the region are the two great mountain ranges of Aja (granites) and Salma (basalts) and the immense rolling sand dunes of An-Nafûd (Al-Turki and Al-Olayan, 2003).
In order to determine the mineralogical and texture characteristics of basaltic rocks of Hail region, 20 representative samples (as fresh as possible), were collected and studied under the polarizing microscope.
Methodology and techniques: Twenty rock samples have been collected from the study area. Thin sections were studied under polarizing microscope.
Thin section preparation: Thin sections were prepared as follows: A slab of basalt with 30×20 ×10 mm thick was cut, polished using (200, 400, 800, 1000) silicon carbide powder in order to get a smooth surface easy to stack with glass slide of (26×42 mm), using Canada palsam adherent. An Automated thin section machine, called Multiplate Grinder housed was used for making the slides thinner in thickness of approximately 45 microns. Then, Heavy duty polishing table and hand polishing using (1000) silicon carbide, grinded the slide down to 30 microns (Hughes, 1982).
|Fig. 1:|| Map showing the major Cenozoic lava fields of Saudi Arabia
Polarizing optical microscope: Nikon optical microscope are used particularly to identify mineral constituents of the studied basaltic rocks and to determine the mineralogical properties and textures.
Olivine: Olivine is the major mineral phase in the studied basalts, rates between 8 and 11 vol.%. It usually present as subhedral to anhedral, light grey to colorless crystals. Olivine crystals are characterized by high relief, parallel extinction crystals that exhibit two generations of size which rather variable reflecting a seriate texture, as phenocrysts and groundmass. They range in size between 0.01 and 2 mm in all samples (Fig. 2).
Olivine minerals of different flows dont exhibit twining. However, all basalt flows show corroded olivine crystals, due to resorption, the shaping of the resorption of olivine can be noted through the bands of iddingsite that forms a long their embayment rims (Fig. 3).
|Fig. 2:|| Microphotograph showing of olivine crystals under PPL (X10)
||Microphotograph showing resorption of olivine crystals, (a) Under PPL and (b) CN (X10)
Olivine minerals had been subjected to alteration processes, such as hydration and oxidation, the processes represented by a slight chlorite and much iddingsite that occur at the rims, along the fractures of the crystals core and rarely completely at the whole olivine crystal. Iddingsite appears as a dark red, slightly pleochroic brown color.
Plagioclase: Plagioclase is the most abundant mineral phase in the studied basalts which ranges from 20-44 vol.% , this variation is due to the cooling rate, as well as the thickness of the flow where the lowest flow is the thickest. Plagioclase phenocrysts occur as lath-like Clusters of phenocrysts up to 7 crystals forming a golmeroporpheritic texture. The second flow is crystallized by one generation as phenocrysts (Fig. 4), they are commonly lath-like shape.
Plagioclases phenocrysts are subhedral but some are euhedral, tabular in shape and colorless crystals exhibit two sets of cleavage intersects at 90°. Groundmass plagioclase occurs as skeletal like microlites. The crystals are slightly fractured. Polysynthetic or multiple twining is common, simple twining are also noted, the multiple twining lamellae are wide, using the MICHEAL-LEVY method (Kerr, 1977). Plagioclase laths were found to have extinction angle between 38-42°, indicates a labradorite (An59-An68) and anorthite (An100-An80) range of composition.
||Microphotograph of plagioclase crystals, (a) Under PPL and (b) CN (X10)
|Fig. 5:|| Microphotograph showing the sericitisation of plagioclase crystals under PPL (X10)
Sericitisation of plagioclase is common, that appear as pale yellow color band at the outer most rims of the crystals, some of them affect the inner most parts of the crystals (Fig. 5).
Pyroxene: Pyroxene are grayish brown in color with anhedral crystals and can be distinguished by two sets of cleavage intersects in 90°. The abundance of pyroxene is ranging between 12-20 vol.%, as one generation (phenocrysts) in all samples. Pyroxene crystals have an inclined extinction which represents clinopyroxene minerals, this inclination is measured and found to be between (36-42°), that indicates diopside mineral of composition. These crystals have subhedral to anhedral oxides inclusions of magnetite type. Pyroxene crystals also had been affected by the alteration processes which appear as a green chlorite on the fractures, crystal rims and on the cleavage sets (Fig. 6).
Nepheline: Nepheline crystals are colorless under plane polarized light and forms about 2 vol.% of the rock. The crystals are euhedral to subhedral in shape and exhibit two sets of cleavage that are intersected at 90°. They have parallel extinction and biaxial interference figure. It is noticed that they are slightly fractured and slightly altered along rims and cleavage planes (Fig. 7).
|Fig. 6:||Microphotograph showing the pyroxene crystals under PPL (X10)
|Fig. 7:|| Microphotograph showing the Nepheline crystals under PPL (X10)
Opaque minerals: Opaque minerals is abundant- as inclusions within olivine and pyroxene crystals. Generally, they are black and show homogeneous optical properties. Most of them are anhedral to subhedral (rod-like), rarely magnetite shows four sided octagonal shape (Fig. 8).
Secondary minerals: Several secondary (authogenic) minerals are formed due to the post eruption processes such as hydration and oxidation of the original pyrogenic minerals, these processes include oxidation of olivine, albitization, sericitisation of plagioclases and chloritisation of pyroxene. The microscopic investigations show that the second flow was suffered from alteration.
Sericite: Pale yellow to turbid color, along the rims and twining planes of the crystals, formed about 5% of the highly weathered and mainly distributed the original characteristics of the plagioclase, this disturbing affecting the extinction of the crystals and the original color of the mineral.
||Microphotograph showing rod like oxides shape, (a) Under PPL and (b) CN (X10)
|Fig. 9:|| Microphotograph showing be intergranular texture
Iddingsite: The studied thin sections suffered from highly to slight iddingtisation, however particularly this pseudomorphism of olivine to iddengsite is noted mostly as a dark brown to reddish brown to yellowish orange and mostly recorded along the rims and fractures of crystals and occasionally forming about (5-15 vol.%) of crystals.
Chlorite: Chloritization is seen by the occurrences of chlorite along the edges of clino pyroxene (augite) and occasionally along cleavage sets. They are of green color, in some pyroxenes are completely chloritized.
Textures description: The texture for all basalt flows is found to be intergranular texture (Fig. 9), vesicular texture (Fig. 10) and amygdaloidal textures (Fig. 11).
||Microphotograph showing vesicle net shape, (a) Under PPL, (b) CN (X10)
|Fig. 11:|| Microphotograph showing be amygdaloidal texture
Mineralogy: The basalt samples, in hand specimen, is melanocratic, holocrystalline, medium-grained and porphyritic. Vesicles have an elongated and oval shape. Moreover, the main mineral constituents are olivine, plagioclase, pyroxene and opaque minerals (mainly magnetite). The secondary minerals include iddingsite, sericite and chlorite. Common textures occurring are intergranular, vesicular and amygdaloidal textures.
Olivine: Olivine occurs as phenocrysts and in groundmass. The phenocrysts are euhedral to subhedral. They are colourless to pale greys. Olivine in the groundmass has subhedral to rounded shape. The larger crystals are fractured slightly-to-moderately. Iddingtization is common, particularly along fractures and along edges of the crystals. Some crystals are partially to completely pseudomorphed to dull brown iddingsite. Groundmass olivine is also iddingsitized.
Plagioclase: Plagioclase occurs in two generations as larger phenocrysts and as small tabular to elongated microlites in the groundmass. The phenocrysts are subhedral laths. The extinction angles on several plagioclase phenocrysts range from 28-32°, indicating a labradorite composition (An50-An70) by using the method described by Michel-Levys (Kerr, 1977).
Pyroxene: Pyroxene occurs as brownish anhedral crystals. The pyroxene crystals have an inclined extinction of between (36-42°), that indicates diopside mineral of composition. Pyroxene crystals are affected by chloritization. Green chlorite is present along fractures and along crystal rims.
Opaque minerals: Opaque minerals are mostly magnetite phenocrysts scattered throughout the rock and throughout inclusions within olivine and pyroxene crystals. Generally, magnetite is black and shows homogeneous optical properties. Most of it is anhedral to subhedral, but crystals with square outline are also presented.
Groundmass: The groundmass consists mainly of plagioclase (labradorite), olivine, pyroxene (diopside) and opaque minerals (mainly magnetite). Sericite, iddingsite and chlorite are secondary minerals.
Minerological studies of selected basalt samples from Hail region show that the basalts are generally composed of primary minerals such as olivine, plagioclase and pyroxene. Olivine is considered the most common essential mineral phase, followed by plagioclase pyroxene, Nepheline and opaque minerals. Secondary minerals are sericite, iddingsite and chlorite. Quartz is totally absent. Presence of olivine as phenocrysts as well as in the groundmass (more iron rich, similar to one described by Brown (1968), Nagao and Sakaguchi (1990) and Shoji and Natsue (1995) is a common feature of alkali olivine basalts.
The author would like to express her deep thanks to Dr. Ahmad Alshammari from Hail University for his help in providing the geological information and samples from Hail region in Saudi Arabia.