Geology, Petrography, Geochemistry, and Geochronology of the Old
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GEOLOGY, PETROGRAPHY, GEOCHEMISTRY AND GEOCHRONOLOGY OF THE OLD GRANITE BATHOLITH BETWEEN QENA AND SAFAGA, EASTERN DESERT, EGYPT . MY. ATTAWIYA*, L . M . NOSSAIR*, A.I. RAGAB** AND S . A . EL DEBEIKY* • Nuclear Materials Authority, Cairo. ** Am Shams University, Cairo. EG9601817 ABSTRACT A suite of tonalite-granodiorite and monzogranite forms a huge old granite batholith between Qena and Safaga, Eastern Desert, Egypt. The batholith was originated from the fractional crystallization of a peraluminous calk-alkaline magma rich in silica and low in potassium. It is developed in an island arc tectonic setting . The rocks forming the batholith are depleted in both U & Th elements . However a gradual increase in these two elements from tonalite to monzogranite was observed . Zircon and sphene are responsible for for U & Th contents in these rocks . Rb/Sr isotopic age determination reported an age of 632.8 ± 4.6 Ma. for these old granitoids. The low initial 87Sr / 86Sr ratio in these rocks suggested there mantle origin or derivation from lower crustal materials with low Rb/Sr ratios and short residence in the crust. INTRODUCTION Along Qena-Safaga asphaltic road, Eastern Desert of Egypt, the old granitoids form a huge batholith covering an area of several hundreds of square kilometers (Fig. 1). These rocks were subjected to several geologic studies; El Gaby and Habib 0) referred to this old granitoids as syn-to late-orogenic calc-alkaline series. Ragab (2) considered these rocks as arc-granitoids from the protocrust of the magmatic arcs in the pre-collision stage. Abdel-Rahman and Martin (3) and Abdel-Rahman (4) mentioned that the old granitoids were developed from the partial melting of the subducted oceanic crust during the early stage of the Pan-African Orogeny. Finally, Takla, et. al. (5) suggested their formation in a volcanic-arc tectonic setting and derived from calc-alkaline magma. Csl cr CX> CO o CD CXI 33° 00' 30 Km Fig. (1): Geologic map of the area between Qena and Safaga, Eastern Desert, Egypt. (After El-Ramly, 3972) In this paper, the geology, petrography, geochemistry and Rb/Sr dating of the old granite batholith outcropping along Qena-Safaga road are studied in some details to throw light on the essential characters of these old granitoid rocks. GEOLOGY AND PETROGRAPHY The studied old granitoid rocks form a huge N-S elongated batholith of a relatively high topographic relief. The batholith is acrossed by the main Qena-Safaga asphaltic road as well as traversed by several sandy wadies and tributaries of various widths and lengths. 1 The old granite batholith intrudes the surrounding older gneisses, migmatites, metasediments, metavolcanics and metagabbros with sharp contacts. On the other hand, it is intruded by some younger granite peaks (Gabal Abu Furad and Gabal Urn Taghir El-Foqani), it is also invaded by several swarms of different dikes and veins, in sharp contacts. During field studies, three main rock types, tonalites, granodiorites and adamellites can be distinguished depending on their color, visible textural relations, type and size of the mafic xenoliths and on their mineralogicaJ composition. Tonalites are more eroded, occasionally foliated, sometimes enclose mafic xenoliths and of grey to whitish grey color. Granodiorites are less affected by weathering, less foliated with rounded to subrounded mafic xenoliths and of whitish grey color. Adamellites are more resistant to erosion, occasionally porphyritic with a few number of small sized xenoliths and of whitish grey to pinkish grey color. It is observed that tonalites occupy the outer zones of the studied batholith while adamellites form its center. Granodiorites form a transitional stage between tonalites and adamellites. The transition from one rock type to the other is gradual; where plagioclases and mafic contents gradually decrease from tonalites towards adamellites, while K-feldspars increase. Generally, the old granitoid rocks are medium to coarse-grained with notable mafic contents, occasionally porphyritic, moderately to deeply eroded with well-developed exfoliation and of grey, whitish grey to pinkish grey color. Microscopically, tonalites are holocrystalline with granular fabric and hypidomorphic texture. The rock is essentially formed of plagioclases, quartz and mafics with subordinate potash feldspars and accessory minerals. Plagioclases form more than 50% of the whole rock and are mainly represented by subhedral to anhedral tabular crystals of 0.4-1.0 mm. width and 1.0-4.0mm. length. The crystals have complex twinning according to the carlsbad, albite and pericline laws. They are occasionally zoned indicating alteration of the more calcic core (Fig. 2). Fine crystals of microcline, albite and quartz are enclosed in some of the plagioclase crystals. Quartz occurs as anhedral crystals and aggregates, commonly showing wavy extinction and ranging in diameter between 0.3-3.5 mm. Some quartz crystals are cracked filling with fine grains of epidote and secondary quartz, while others displayed corrosive boundaries against the other mineral phases. Mafic minerals are mainly represented by a considerable amount of biotite and few hornblende displaying a preferred orientation. This orientation is commonly attributed to differential rates of flow within the magma as the rock was crystallized. Biotite is present as subhedral flakes of 0.2-0.4 mm. width and 0.5-2.0 mm. length. It is partially chloritized and occasionally exhibits pleochroic haloes due to zircon inclusions. Hornblende occurs as few subhedral crystals of 0.2-0.3 mm. width and 0.4-0.9 mm. length;. Most of the hornblende are connected with biotite flakes and some of them are corroded by these flakes indicating their crystallization before biotite. Potash feldspars occur as subordinate subhedral microcline microperthite crystals commonly filling plagioclase interstitials. Fine albite crystals are also present but in a few number. Albite and microcline microperthite crystals are sometimes enclosed in large plagioclase crystals. Zircon, sphene, apatite, ilmenite, magnetite, epidote and chlorite are the main accessory minerals. Under the microscope, granodiorite, is holocrystalline rock with hypidomorphic texture. It is essentially composed of plagioclases and quartz with considerable amounts of potash feldspars and mafics. A number of accessory minerals are also present. Plgioclases occur as subhedral to anhedral tabular crystals of 0.3-0.8 mm. width and 0.8-3.5 mm. length. They displayed complex twinning, sometimes zoned. The peripheries of some zoned crystals are occasionally invaded by vermicular quartz indicating the probable formation by a replacement process in the presence of hot, corrosive, water-rich solutions during the later stages of cooling. Potash feldspars are present as subhedral to anhedral microcline microperthite crystals of 0.1-0.3 mm. width and 0.3-1.0 mm. length. They usually occur as small interstitial crystals or enclosed in plagioclases. Mafic minerals are mainly represented by appreciable amount of biotite and minor hornblende crystals. The former is present as yellowish brown to green flakes of 0.2-0.5 mm. width and 0.4-1.8 mm. length. Some of these flakes enclose zircon, sphene and opaques. Hornblende forms minor subhedral prisms of 0.2- 0.4 mm. width and 0.4-0.7 mm. length. It is usually associated with biotite flakes, and partially altered to chlorite and sometimes to iron opaques. Bipyramidal zircon crystals, wedge-shaped sphene crystals, tabular chlorite crystals and dispersed muscovite flakes are the main accessories. The microscopic studies of monzogranite show that the rock is occasionally porphyritic with light pink potash feldspar phenocrysts sometimes up to about 3.0 cm. Fig. (2): Alteration in the more calcic-core of plagioclase cnstal to fine mica aggregates and zoisitc Tonalitc {C.N. X 75] A 10 Fig. (3): The modal Q-A-P classification of the studied tonalites (•), granodiorites (x) and monzogranites (A). The fields: 1. tonalite; 2. granodiorite :3. monzogranite; 4,syenogranite and 5, alkali-granite are after Streckeisen (1976). Arrows show the magmatjc differentiation trends of LameyTe and Bowdcn (1982): a, tholeiitic; b. calc-alkaline-tonalitic (low K), c. calc-alkaline-granodioritic (medium K); d. calc- alkaline-monzonitic (high K) and e, alkaline The non-porphyritic variety is mainly composed of plagioclases, potash feldspars and quartz with biotite as the only mafic mineral. Plagioclases occur as subhedral to anhedral tabular crystals of 0.4-0.7 mm. width and 1.0-2.0 mm. length. They are characterized by their complex twinning and zoning. Some crystals are intergrown with vermicular quartz showing the myrmekitic texture. Potash feldspars are represented by subhedral to anhedralarge microcline and orthoclase microperthite crystals of 0.3-0.5 mm. width and 0.8-2.0 mm. length. The crystals sometimes reach 3.0 cm in length in the porphyritic variety. An intergrowth between K-feldspars and plagioclase is well presented showing a flame perthite structure. Some K-feldspars crystals are partially kaolinized or sericitized.Quartz occurs as anhedral crystals with common wavy extinction and of 0.2-2.5 mm. grain diameter. It sometimes shows corrosive boundaries against the other mineral phases. Biotite is present as subhedral plates in a flaky form of 0.1-0.3 mm. width and 0.3-1.0 mm. length. The flakes occasionally enclose zircon, sphene and iron opaques as inclusions. Zircon prisms, sphene wedge- shaped crystals and iron opaque grains are present as accessory minerals. The modal composition of the studied old granitoids (Table 1) shows decreasing in plagioclase/perthite ratio (P/A) from 15.0 in tonalites to 3.2 in granodiorites and 1.1 in monzogranites. This indicates differentiation and change in composition of the melt as it was crystallized (Fig. 3). GEOCHEMISTRY OF MAJOR ELEMENTS Eighteen samples, 6 from tonalite, 5 from granodiorite and 7 from monzogranite are chosen for the major and trace elements analysis. XRF (Philips Spectrometer model PW-1480) is used on fused and powder pellets for major and trace elements analysis respectively.