الفهرس 
IntroductionOnly 14 pages are availabe for public view
 |  | from | 291 |  |  |
| | | from | 291 | | |
Abstract
The present study deals with the geology, petrography, mineral chemistry, geochemistry, P-T conditions, and age dating of different rock units in the area around Wadi Seih. The studied area is located in southwest Sinai and is considered as a part of the Arabian - Nubian Shield. The exposed rock units in Wadi Seih area from the older to the younger depending on the field relations and geochronology are as follows: 1- Para-gneisses and migmatites, 2- Ortho-gneisses and migmatites, 3- Syn-tectonic gabbro, 4- Late to syn-tectonic granites, 5- Volcanics, 6- Post-tectonic granites, 7- Pegmatites, 8- Phanerozoic sediments. The para-gneisses are the less predominant type of gneisses in the studied area and are exposed only in Wadi Tayeba and partly in Wadi Umm Agraf. Based on field observations and petrographic features, the paragneisses are considered as the oldest rock units in the study area. They are subdivided into several types, such as fine-grained gneisses, amphibolite, and granulite. Ortho-gneisses are the predominant type of gneisses in the studied area. They are represented by dioritic gneiss and amphibolite. The rock unit is well exposed at Wadi Seih, Wadi Sidri, Wadi Umm Maghar, and Wadi Umm Agraf. At Wadi Seih area, the exposed gneisses exhibit signs of migmatization, especially along their contacts with the late to syn-tectonic granites. The migmatites effect is manifested by the presence of Chapter Eight: segregation of light leucosomes in the form of thin lamellae parallel to the host rock foliation or folded, giving them well-developed stromatic structures. The migmatites of Wadi Seih area could be subdivided into the following types: A. Migmatites with stromatic (layered) structures B. Migmatites with folding structures C. Migmatites with ptygmatic folded structures The syn-tectonic gabbro occurs at Wadi Naba in the southwestern part of the area. It occupies an area of about 18 km2 as well as small masses at the intersection between Wadi Seih and Wadi Umm Agraf, Wadi Tayeba and Wadi Seih, and the last location in Wadi Baraq. The gabbroic outcrop at Wadi Naba is intruded by the post-tectonic granites of Gabal Ataitir El Dehami. The present gabbroic masses at Wadi Seih and Wadi Tayeba intruded into the gneisses of Wadi Umm Agraf. These rocks are coarse-grained (crystals range in size from 1 mm to 5 mm) and dense, greyish-colored rocks. Late to syn-tectonic granites are exposed at the end of Wadi Seih and other locations, which occupy a vast area in the southern part of the mapped area. They are characterized by their dark to whitish-grey color, coarse to medium grained with massive appearance. Volcanics are located in different areas as small extrusives. At the end of Wadi Baraq, the volcanics are extruded onto syn-tectonic gabbro in north Wadi Seih. At the end of Wadi Sidri, the volcanics are extruded onto Wadi Umm Maghar gneisses. These rocks are hard, massive, and grey-rose to grey-brown in color. Post-tectonic granites occur in many locations. The first location is around Wadi Baraq, intruding Wadi Tayeba gneisses and the late to synChapter Eight: tectonic granites with discordant sharp contact. The second location is at G. Ataitir El Dehami, NW of Wadi Seih, where the post-tectonic granites cut the gneisses and migmatites with sharp intrusive contact. Generally, it has high to moderate relief, medium to coarse-grained with a pinkish white color. Pegmatites are intruded into gneisses and post-tectonic granites. They are very coarse-grained igneous rocks having a granitic composition. Petrographically, para-gneisses are composed mainly of finegrained quartzo-feldspathic gneiss, granulite, and amphibolite. The finegrained quartzo-feldspathic gneiss is composed essentially of quartz, plagioclase, K-feldspar, muscovite, garnet, and biotite. Magnetite, apatite, zircon, and sphene are the accessories. The main constituents of granulite are feldspar (perthitic alkali feldspar, plagioclase) and quartz. Typical mafic minerals are garnet, pyroxene, and other minor constituents such as hornblende, ilmenite, and magnetite. Zircon and apatite are the main accessory minerals. Amphibolite is a medium to fine-grained foliated rock and is composed mainly of hornblende, actinolite, and plagioclase feldspar with a minor amount of quartz, biotite, and epidote. Magnetite and apatite are the common accessory minerals. Ortho-gneisses are essential composed of three rock units according to their mafic contents. These are hornblende biotite gneiss, hornblende gneiss, and biotite gneiss. They are composed mainly of alternating bands of quartzo-feldspathic and hornblende or biotite with a minor amount of orthoclase. In addition to iron oxides and zircon as accessory minerals, chlorite and sericite are secondary minerals. Chapter Eight: Syn-tectonic gabbro includes two essential varieties, i.e., hornblende gabbro and anorthosite. Hornblende gabbro is composed mainly of plagioclase and hornblende. Accessory minerals include iron oxides, magnetite, apatite, and sphene. Anorthosite are composed mainly of anorthite (> 90%) with a minimal amount of hornblende. Late to syn-tectonic granites, this rock unit is represented by granodiorite. It consists mainly of plagioclase (40 to 50% by volume). Quartz represents about 30%, whereas potash feldspar constitutes about 15% of the rock. The mafic minerals are represented by hornblende and biotite. The accessory minerals are sphene, apatite, and magnetite. The secondary minerals are epidote and chlorite. Volcanics are represented by dacite and rhyolite associations. They are composed mainly of plagioclase, quartz, and potash feldspar phenocryst embedded in a microcrystalline groundmass. Secondary minerals are represented by kaolinite, sericite, chlorite, and epidote. Zircon, apatite, sphene, and iron oxides are accessories. Post-tectonic granites are composed mainly of potash feldspar and quartz, with a minor amount of plagioclase and biotite. Zircon, iron oxides, sphene, and apatite are the common accessory minerals, while chlorite and sericite are secondary minerals. The geochemical studies of syn-tectonic gabbro indicate that these rocks originated from tholeiitic to calc-alkaline magma with MORB affinity. Post-tectonic granites are mainly alkali feldspar granites and syeno-granites. According to their chemical characteristics, the studied granites are derived from peraluminous-alkaline magma formed at a depth range of (20-30 km) and crystallized at a shallower level of the earth’s crust (1-1.5 km) equivalent to pressure < 0.5 kb. The data of these granites Chapter Eight: are plotted within the post-collision, within plate, A-type granites, and A2 (granites derived from the crust) fields. The mineral chemistry of the studied minerals reveals that the studied amphiboles of the para-amphibolite are calcic (actinolitehornblende and magnesio-hornblende) in composition, while the amphiboles of the granulite have a chemical composition that lies in the calcic field and Fe-Mg-Mn field and are classified as (gedrite, tschermakite, and ferro-hornblende). The amphiboles of ortho-gneisses and ortho-amphibolite are calcic (magnesio-hornblende, tschermakite hornblende, ferro-hornblende, and Fe-tschermakite hornblende) in composition, while the amphiboles of syn-tectonic gabbro are calcic amphibole (magnesio-hornblende) in composition. Biotite of fine-grained gneiss lies in the biotite field and has a siderophyllite composition, while biotite of ortho-gneiss has a composition between annite and siderophyllite. Biotite of syn-tectonic gabbro lies in the phlogopite field and has a composition between phlogopite and estonite, while biotite of granite lies in the biotite field and has a composition between annite and siderophyllite. Plagioclase of fine-grained gneiss has an oligoclase composition. Plagioclase of the granulite has a composition between labradorite and anorthite, while plagioclase of the para-amphibolite has a bytownite composition. In ortho-gneisses, the plagioclase has a composition between andesine and oligoclase. Plagioclase of syn-tectonic gabbro has andesine and bytownite composition. On the other hand, the plagioclase of post-tectonic granites has an albite composition. There are two types of pyroxenes in the studied granulite, Ca- poor pyroxenes that have a composition between pigeonite and clinoChapter Eight: ferrosillite, whereas the Ca-rich pyroxenes have a composition between diopside and hedenbergite. Chlorite of ortho-gneisses is classified as thuringite. In contrast, chlorite from post-tectonic granites is classified as pseudothuringite. Garnet of the fine-grained gneiss (SA4C, SA4F, and E94E) is characterized by low CaO and high Al2O3 and FeO, yielding an endmember of (almandine70-80-pyrope9-13-spessartine3-6) for SA4C, (almandine79-80-spessartine11-12- pyrope6) for SA4F, and (almandine67-71- spessartine12-16-grossular8-1-pyrope5-6) for E94E. While garnet of the granulite (SA4A) is characterized by low FeO and high Al2O3 and CaO, yielding an end-member formula of (grossular45-almandine41 spessartine8). Muscovite from the fine-grained gneiss samples (SA4F) is classified as primary muscovite. Different geothermobarometers indicate that para-gneisses have a temperature range from 550 to 780 °C. Higher temperature (e.g., 870 °C) was calculated from the granulites. The granulites also record retrograde assemblage, which indicates a minimum temperature (510 °C). Geothermobarometers indicate that the ortho-gneisses metamorphosed at a temperature range from 510 to 760 °C at a pressure range of (5-7 kb), while the syn-tectonic gabbro crystallized at a temperature range from 550 to 710 °C and at a pressure range of (3-4 kb). The post-tectonic granites were crystallized at a temperature range from 550 to 600 °C. The U-Pb Laser ablation inductively coupled plasma mass spectrometry was used for dating zircon grains extracted from Ataitir El Dehami granites. The zircon grains give an age range of (580 to 603 Ma) except one sample has an age of (650-800 Ma). Zircon trace element geochemistry indicates that the post-tectonic granites in the present area most probably derived from re-melting of the old arc crust (780-710 Ma) due to arc-arc collision and heat result from the plume. Ti in zircon thermometer for the present granites gives a crystallization temperature average of (718-823 °C) except one sample records an average of (1096 °C).