Overview of the geomorphological and hydrogeological characteristics of the Eastern Desert of Egypt

Although, the Eastern Desert of Egypt forms about 22% of the surface area of the country, the area is undeveloped due to the limited availability of water. The morphologic units of the Eastern Desert consist of a number of drainage basins covering about 147,820 km² (66.5% of the total surface area of the Eastern Desert). The basins drain the occasional rainwater, either towards the Nile Valley or to the Red Sea, causing flood hazards. The availability of water from the hydrologic systems of these basins could be improved by constructing runoff controlling systems in these areas (e.g. dykes and partially effective dams), which could save and make use of a considerable amount of water. The groundwater resources in the Eastern Desert can be divided into four main water-bearing units: the fractured crystalline Pre-Cambrian aquifer, the Nubian sandstone aquifer, the fractured limestone and sandstone aquifer and the Quaternary aquifer. The most productive aquifer is the Nubian sandstone while the fractured limestone and sandstone (Miocene) are only productive along the eastern part of the desert. The Quaternary aquifer occurs along the major dry washes (wadis) and is considered of limited potential as it is recharged mainly from the occasional rainfall. Detailed assessment of these aquifers should be carried out locally for further development of the area.

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Resumen Aunque el Desierto Oriental de Egipto constituye alrededor del 22% de la superficie del país, esta área no se encuentra desarrollada por causa de la escasez de agua. Las unidades morfológicas del Desierto Oriental consisten de un número de cuencas de drenaje que cubren alrededor de 147,820 km² (66.5% del área superficial total del Desierto Oriental). Estas cuencas drenan las aguas lluvias ocasionales, bien hacia el Valle del Nilo o hacia el Mar Rojo, causando amenazas de inundación. La disponibilidad de agua a partir de los sistemas hidrológicos de estas cuencas, puede ser mejorada al construir sistemas de control de escorrentía allí (Ej. Diques y presas de efectividad parcial), las cuales podrían almacenar y permitir el uso de una cantidad considerable de agua. Los recursos de agua subterránea en el Desierto Oriental, pueden dividirse en cuatro unidades principales portadoras de agua: El acuífero fracturado cristalino Pre – Cámbrico, el acuífero de la Arenisca de Nubia, el acuífero de arenisca y caliza fracturadas y el acuífero Cuaternario. El acuífero más productivo es la arenisca de Nubia, mientras que la arenisca y caliza fracturadas (Mioceno), son productivas únicamente a lo largo de la parte oriental del desierto. El acuífero Cuaternario se encuentra a lo largo de las corrientes intermitentes mayores (Wadis) y se le considera de potencial limitado por ser recargado principalmente a partir de lluvia ocasional. Una evaluación detallada de estos acuíferos debe llevarse a cabo localmente, para el desarrollo adicional de esta área.

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Résumé Bien que le Désert Est occupe 22% de la surface de lEgypte, la région est sousdeveloppée à cause de la ressource en eau limitée. Les unités morphologiques consistent en des bassins drainant qui couvrent une surface de 147,820 km² ,représentant 66.6% de la surface du Désert Est. Les bassins drainent les plues intermittentes vers la vallée du Nil ou vers la Mer Rouge en provocant des inondations. On peut augmenter la ressource en eau dans cette région en réalisant des systèmes de contrôle de ruissellements (des digues...) qui peuvent sauver et utiliser un volume considérable deau. La ressource en eau souterraine du Désert Est est cantonnée dans quatre grandes structures: l› aquifère fracturé cristallin Précambrien, l› aquifère gréseux Nubien, l› aquifère calcaire et gréseux et l› aquifère quaternaire. Le plus productif est l› aquifère gréseux Nubien tendis que les calcaire et les grèses dage miocène sont productives seulement dans la partie est du désert. L› aquifère quaternaire se trouve au long des oueds sèches et on considéré quil a un potentiel limité, étant rechargé seulement par les plues intermittentes. Pour le futur développement de la région il est nécessaire une évaluation détaillée, à léchelle locale de ces aquifères.

     

Fluid Inclusions in the Gold—Bearing Quartz Veins at Um Rus Area,Eastern Desert,Egypt

Fluid inclusions in the gold-bearing quartz veins at the Um Rus area are of three types: H₂O, H₂O−CO₂ and CO₂ inclusions. H₂O inclusions are the most abundant, they include two phases which exhibit low and high homogenization temperatures ranging from 150 to 200°C and 175 to 250°C, respectively. The salinity of aqueous inclusions, based on ice melting, varies between 6.1 and 8 equiv. wt% NaCl. On the other hand, H₂O−CO₂ fluid inclusions include three phases. Their total homogenization temperatures range from 270 to 325°C, and their salinity, based on clathrate melting, ranges between 0.8 and 3.8 equiv. wt% NaCl. CO₂ fluid inclusions homogenize to a liquid phase and exhibit a low density range from 0.52 to 0.66 g/cm³. The partial mixing of H₂O−CO₂ and salt H₂O−NaCl fluid inclusions is the main source of fluids from which the other types of inclusions were derived. The gold-bearing quartz veins are believed to be of medium temperature hydrothermal convective origin.     

The nature of gold-bearing fluids in Atud gold deposit,Central Eastern Desert,Egypt

Electron microprobe analyses of gold and associated ore minerals as well as stable isotope analyses of sulphide and carbonate minerals were performed in order to determine the metal and fluid sources and temperature of the mineralizing systems to better understand the genesis of the Atud gold deposit hosted in the metagabbro–diorite complex of Gabal Atud (Central Eastern Desert, Egypt). The gold can be classified as electrum (63.6–74.3 wt.% Au and 24.6–26.6 wt.% Ag) and is associated with arsenopyrite and As-bearing pyrite in the main mineralization (gold-sulphides) phase within the main mineralized quartz veins and altered host rocks. Based on the arsenopyrite geothermometer, As-contents (29.3–32.7 atom%) in arsenopyrite point to deposition in the Log ?_S2 and T ranges of ?10.5 to ?5.5 and 305–450°C, respectively, during the main mineralizing phase. Based on the δ³⁴S isotopic compositions of the sulphides, they are originated from magmatic fluids in which the sulphur is either sourced directly from magma or remobilized from the magmatic rocks (gabbroic rocks). On the other hand, calcite formed from fluids having mainly magmatic mixed with variable metamorphic signatures based on its δ¹³C and δ¹⁸O values. This work concluded that the gold-bearing ores at Atud deposit have magmatic sources leaching from the country intrusive rocks during water/rock interactions then remobilized during a metamorphic event. Therefore, the Atud gold deposit is classified as an intrusion-related gold deposit, in which the gabbro–diorite host intrusion acted as the source of metals which were mobilized and deposited as a result of the effects of NW–SE shearing.     

Age and tectonic setting of granitoid gneisses in the Eastern Desert of Egypt and south-west Sinai

Strongly deformed and locally migmatized gneisses occur at several places in the southern Eastern Desert of Egypt and in Sinai and have variously been interpreted as a basement to Pan-african (900 to 600 Ma) supracrustal and intrusive assemblages. A suite of grabbroic to granitic gneisses was investigated in the Hafafit area, which constitutes an I-type calc-alkaline intrusive assemblage whose chemistry suggests emplacement along an active continental margin and whose granitoid members can be correlated with the so-called Older Granites of Egypt.²⁰⁷Pb/²⁰⁶Pb single zircon evaporation from three samples of the Hafafit gneisses yielded protolith emplacement ages between 677 ± 9 and 700 ± 12 Ma and document granitoid activity over a period of about 23 Ma. A migmatitic granitic gneiss from Wadi Bitan, south-west of Ras Banas, has a zircon age of 704 ± 8 Ma, and its protolith was apparently generated during the same intrusive event as the granitoids at Hafafit. Single zircons from a dioritic gneiss from Wadi Feiran in south-west Sinai suggest emplacement of the protolith at 796 ± 6 Ma and this is comparable with ages for granitoids in north-east Sinai and southern Israel. None of the above gneisses is derived from remelting of older continental crust, but they are interpreted as reflecting subduction-related calc-alkaline magmatism during early Pan-african magmatic arc formation.     

Petrology and Geochemistry of Some Ophiolitic Metaperidotites from the Eastern Desert of Egypt: Insights into Geodynamic Evolution and Metasomatic Processes

Ophiolitic peridotites exposed in the Eastern Desert(ED) of Egypt record multiple stages of evolution, including different degrees of partial melting and melt extraction, serpentinization, carbonatization and metamorphism. The present study deals with metaperidotites at two selected localities in the central and southern ED, namely Wadi El-Nabá and Wadi Ghadir, respectively. They represent residual mantle sections of a Neoproterozoic dismembered ophiolite that tectonically emplaced over a volcano-sedimentary succession that represents island–arc assemblages. The studied metaperidotites are serpentinized, with the development of talc-carbonate and quartz-carbonate rocks, especially along shear and fault planes. Fresh relics of primary minerals(olivine, orthopyroxene and Cr-spinel) are preserved in a few samples of partiallyserpentinized peridotite. Most of the Cr-spinel crystals have fresh cores followed by outer zones of ferritchromite and Crmagnetite, which indicates that melt extraction from the mantle protolith took place under oxidizing conditions. The protoliths of the studied metaperidotites were dominated by harzburgites, which is supported by the abundance of mesh and bastite textures in addition to some evidence from mineral and whole-rock chemical compositions. The high Cr#(0.62–0.69; Av. 0.66) and low TiO2(0.3 wt%) contents of the fresh Cr-spinels, the higher Fo(89–92; Av. 91) and NiO(0.24–0.54 wt%, Av. 0.40) contents of the primary olivine relics, together with the high Mg#(0.91–0.93; Av. 91) and low CaO, Al2 O3 and TiO2 of the orthopyroxene relics, are all comparable with depleted to highly depleted forearc harzburgite from a suprasubduction zone setting. The investigated peridotites have suffered subsequent phases of metasomatism, from oceanfloor hydrothermal alteration(serpentinization) to magmatic hydrothermal alteration. The enrichment of the studied samples in light rare earth elements(LREEs) relative to the heavy ones(HREEs) is attributed to most probably be due to the contamination of their mantle source with granitic source hydrothermal fluids after the obduction of the ophiolite assemblage onto the continental crust. The examined rocks represent mantle residue that experienced different degrees of partial melting(～10% to 25% for W. El-Nabá rocks and ～5% to 23% for W. Ghadir rocks). Variable degrees of partial melting among the two investigated areas suggest mantle heterogeneity beneath the Arabian-Nubian Shield(ANS).     

Geology and geochemistry of the granitic rocks and associated dykes, East Gabal Nuqra, South Eastern Desert, Egypt

A crescent-shape granitic stock and associated dykes is located to the East Gabal Nuqra at the extreme western part of Wadi Natash,South Eastern Desert,Egypt.The examined granites are classified as alkali-feldspar granites and mainly consist of quartz,potash feldspars,plagioclases,and aegirine-augite.Xenotime,zircon,apatite and allanite are accessories representing the source of Y,U,Th and REEs in these rocks.These granites are characterized by high K2O,Na2O and Zn contents and Rb/Sr ratio.Also,they are highly enriched in high field strength elements(HFSE),especially Zr(1529×10-6),Nb(100×10-6),Hf(91×10-6) and Y(624×10-6) and light rare-earth elements(LREE,141×10-6) concentrations and strongly depleted in Ca,Mg,Sr and Eu contents.These features suggest that they are similar to A-type granites(type-2).The rhyolite dykes and granites have similar geochemical characteristics whereas the chondrite-normalized REE patterns show a LREE enriched feature with strong negative Eu-anomaly,whereas the REE pattern of trachydacites show slightly fractionated pattern with no Eu-anomaly.It is suggested that the trachydacites were generated by small degree of partial-melting deep-seated basic source.Such liquid,when subjected to fractional crystallization involving separation of plagioclases as residue,generated the alkali-feldspar granites.And further fractional crystallization gave rise to the alkali rhyolites.The igneous rock suite originated from metaluminous to alkaline trachytic magma,and was developed in a within-plate tectonic environment.The extension caused by NW-SE right-lateral shear in area led to the emplacement of the alkali-feldspar granites.The later extrusion of the alkali rhyolite and trachydacite dykes was due to cauldron subsidence.     

Alteration Mechanisms of Chromian-Spinel during Serpentinization at Wadi Sifein Area, Eastern Desert, Egypt

Wadi Sifein podiform chromite deposits, Central Eastern Desert of Egypt, are hosted by fully serpentinized peridotite that is a part of the dismembered Pan‐African ophiolite complexes. Relics of primary minerals and the chemical characters indicate that the ophiolitic rocks were derived from depleted mantle peridotite of harzburgite and subordinate dunite compositions. The mantle rocks were initially formed at a mid‐oceanic ridge and subsequently thrust at a supra‐subduction zone. The chromite mineralization at Wadi Sifein area displays either pod‐shaped bodies with massive and lumpy chromitite appearance or dissemination of chromian‐spinel in serpentinite matrix. The podiform chromitite exhibits a very limited compositional range in terms of Cr# [Cr/(Cr + Al) atomic ratio] and Mg# [Mg/(Mg + Fe) atomic ratio]. The chromian‐spinel, however, frequently displays optical and geochemical zoning. Four zones can be identified from core to edge: inner core representing the original composition of the chromian‐spinel; narrow Cr‐rich ferritchromit zone; wide ferritchromit zone; and outer Cr‐magnetite/magnetite zone. The zonation of chromian‐spinel is interpreted to be a result of serpentinization rather than magmatic or metamorphic processes. The geochemical data obtained from the chromitite and chromian‐spinel was statistically processed using discriminant and R‐mode factor analyses. Two trends, minor and major, were achieved considering the formation of ferritchromit. The minor trend is controlled by the redistribution of trivalent cations, where Cr₂O₃ increased on the expense mainly of Al₂O₃ and to less extent Fe₂O₃ to form zone II during the peak of serpentinization. The major trend of alteration, however, is explained by the exchange between Mg‐Fe²⁺ rather than Cr, Al, and Fe³⁺ to form zone III. Kammererite formation was accompanied the formation of zones III and IV at a 314°C temperature of formation.     

Assessment of the Hydrogeochemical Characteristics of Groundwater Resources at some Wadis in the East of El Minia Governorate, Eastern Desert, Egypt

The El Minia governorate lies within the Nile Valley, surrounded by calcareous plateaus to the east and west. The present study focuses on the hydrogeochemistry of the Eocene limestone aquifer at some wadis in the east El Minia governorate, Eastern Desert, Egypt. Hydrogeologically, two main aquifers are encountered in the study area, namely the Maghagha marly limestone and the Samalut chalky limestone aquifers. The Maghagha aquifer is composed of alternating layers of marly limestone and shale with thicknesses ranging from 3.49 m to 177.05 m and a groundwater depth ranging from 8.5 m to 59.27 m which reflects low groundwater potentiality. The groundwater salinity representing this aquifer ranges from 603.5 mg/L to 978.5 mg/L, reflecting fresh water type. Samalut aquifer is made up of chalky, cavernous and fractured limestone with thickness ranging from 30 m to 205 m and groundwater depth ranging from 9 m to 86.77 m, which indicates good groundwater potential. The groundwater salinity of the concerned aquifer ranges from 349.7 mg/L to 2043.9 mg/L, reflecting fresh to possibly brackish water types. Groundwater in the study area is of meteoric water origin; recent recharge is mainly controlled through the presence of fractures and their densities. The majority of groundwater samples in the study area are suitable for drinking and irrigation purposes.     

Cryogenian ophiolite tectonics and metallogeny of the Central Eastern Desert of Egypt

The Central Eastern Desert (CED) is characterized by the widespread distribution of Neoproterozoic intra-oceanic island arc ophiolitic assemblages. The ophiolitic units have both back-arc and forearc geochemical signatures. The forearc ophiolitic units lie to the west of the back-arc related ones, indicating formation of an intra-oceanic island arc system above an east-dipping subducted slab (present coordinates). Following final accretion of the Neoproterozoic island arc into the western Saharan Metacraton, cordilleran margin magmatism started above a new W-dipping subduction zone due to a plate polarity reversal. We identify two belts in the CED representing ancient arc–forearc and arc–back-arc assemblages. The western arc–forearc belt is delineated by major serpentinite bodies running ～NNW–SSE, marking a suture zone. Ophiolitic units in the back-arc belt to the east show an increase in the subduction geochemical signature from north to south, culminating in the occurrence of bimodal volcanic rocks farther south. This progression in subduction magmatism resulted from diachronous opening of a back-arc basin from north to south, with a bimodal volcanic arc evolving farther to the south. The intra-oceanic island arc units in the CED include coeval Algoma-type banded iron formations (BIFs) and volcanogenic massive sulphide (VMS) deposits. Formation of the BIFs was related to opening of an ocean basin to the north, whereas development of the VMS was related to rifting of the island arc in the south. Gold occurs as vein-type mineral deposits, concentrated along the NNW–SSE arc–forearc belt. The formation of these vein-type gold ore bodies was controlled by the circulation of hydrothermal fluids through serpentinites that resulted in Au mobilization, as constrained by the close spatial association of auriferous quartz veins with serpentinites along the western arc–forearc belt.     

Characterization of Magnetically Zoned Pluton, Tsushima, Japan

Abstract: Systematic measurements of magnetic susceptibility were carried out at the outcrops of the Uchiyama granitic pluton, Tsushima Islands. The pluton consists of hornblende-biotite granodiorite and biotite granite. It intrudes concordantly along an anticline axis of the Taishu Group in middle Miocene (16. 10. 5 Ma), and crops out in five areas. The results show that the low magnetic susceptibility facies (LM–facies, <100A-10^-6 emu/g) always occurs in the peripheral part of the pluton, while the high magnetic susceptibility facies (HM–facies, > 250 A 10^-6 emu/g) occupies the core through the medium magnetic susceptibility zone (MM–facies). It is pointed out that the pluton forms a magnetic zoning within a single plutonic body.
Chemical compositions of the granitoids show no conspicuous differences in major elements among the LM–, MM–, and HM–facies. Biotites from the LM–, MM–, and HM-facies also indicate uniform compositions with Fe/(Fe+Mg)=0. 6.
The pluton is principally surrounded by black mudstones of the Lower Formation of the Taishu Group, which is deltaic to shallow sea sediments of the Eocene to early Miocene age, and the carbon contents in them were analyzed. The analyses show that carbon was mostly decomposed and lost in the hornfels zone, while mudstones in the non-hornfels zone usually contain 0. 5 to 0. 7 % C.
From these facts, it is concluded that the magnetically zoned pluton at Tsushima was formed by an external volatile buffer system such as CH₄–CO₂ during the solidifying magma process.     

Emplacement History of Pasupugallu Gabbro Pluton, Eastern Ghats Belt, India: a Structural Study

Structural mapping of the Pasupugallu pluton, an elliptical intrusive gabbro-anorthosite body, emplaced into the western contact zone between the Eastern Ghats Mobile Belt and the Archaean East Dharwar Craton, along the east coast of India, reveals concentric, helicoidal and inward dipping magmatic and/or tectonic foliations. We identify a <1 km-wide structural aureole characterized by pronounced deflection of regional structures into margin parallel direction, mylonitic foliations with S-C fabrics, sigmoidal clasts, moderately plunging stretching lineations, non-cylindrical intrafolial folds, and stretched elliptical mafic enclaves in the aureole rocks. Our results suggest that the pluton emplacement is syn-tectonic with respect to the regional ductile deformation associated with the terrane boundary shear zone at the western margin of the Eastern Ghats. We present a tectonic model for the emplacement of the pluton invoking shear-related ductile deformation, rotation and a minor component of lateral expansion of magma. The intrusive activity (1450-800 Ma) along the western margin of the Eastern Ghats can be correlated with the significant event of recurring mafic, alkaline and granitic magmatism throughout the global Grenvillian orogens associated with the continent-continent collision tectonics possibly related to the amalgamation and the breakup of the supercontinent Rodinia.     

Paleozoic glaciation in NE Africa: field and remote sensing-based evidence from the South Eastern Desert of Egypt

ABSTRACT

Exhumed Paleozoic glacial deposits and landforms of the North Gondwana are reported here for the first time from the South Eastern Desert (SED) of Egypt. Using field observations and remote sensing datasets (Advanced Land Observing Satellite [ALOS], Phased Array L-band Synthetic Aperture Radar [PALSAR] radar, multispectral Landsat TM datasets, and digital elevation models [DEMs]), we mapped the distribution of Paleozoic glacial features (i.e. deposits and landforms) in the SED. Two main glaciogenic facies were identified in three locations in the SED: (1) massive, poorly sorted, matrix supported, boulder-rich diamictites in Wadi El-Naam and Korbiai, and (2) moderately-sorted, occasionally bedded outwash deposits in Betan area. Inspection of radar, DEMs, and Landsat Operational Land Imager (OLI) images revealed previously unrecognized ENE-WSW trending glacial megalineations (MLs) over the peneplained Neoproterozoic basement rocks in the central sections of the SED, whose trends align along their projected extension with those of glacial features (tunnel valleys and striation trends) reported from Saudi Arabia. The glaciogenic features in the SED are believed to be largely eroded during the uplift associated with the Red Sea opening, except for those preserved as basal units beneath the Nubia Sandstone Formation or as remnant isolated deposits within paleo-depressions within the basement complex. The apparent spatial correlation of the SED glacial features with well-defined Late Ordovician deposits in North Africa and in Saudi Arabia, and the reported thermochronometric analyses and fossil records are consistent with a Late Ordovician age for the SED glaciogenic features and support models that call on the continuation of the Late Ordovician (Hirnantian) ice sheet from the Sahara into Arabia through the SED of Egypt.     

Mineralogy,M-type tetrad effect and radioactivity of altered granites at the G. Abu Garadi shear zone,central Eastern Desert,Egypt

The G. Abu Garadi area is covered mainly by metasediments, alkali feldspar granites and stream sediments. The alkali feldspar granite is traversed by a major strike-slip fault trending in an N-S direction as well as two subordinate sets of faults trending NW to WNW for the first one and NE for the second one. These faults represent the shear zones affected by magmatic (syngenetic) as well as hydrothermal (epigenetic) activities causing alteration of the granitic rocks. The most common alteration features are albitization, greisenization and koalinitization. The mass balance calculations of the studied altered samples show enrichments in Zr, Y, Ni, U, Th and Ga and depletions in Zn, Sr, Nb, Ba, Pb, Cu and V. Only the greisenized samples exhibit a significant enrichment in Nb, ∑REE budget and pronounced lanthanide tetrad effect (M-type), especially TE1,4, while weakly expressed tetrad effects are for the other albitized and koalinitized samples. Mineralogically, the common accessory minerals in the altered samples include samarskite-(Y), betafite, uranothorite, zircon, fluorite and cassiterite. The greisenized granites contain high eU and eTh than the other altered types, where they are characterized by an assemblage of the radioactive minerals; samarskite-(Y), betafite, uranothorite in addition to zircon. The inter-element relationships between U and Th and also their ratios illustrate that the radioelement distribution in these granites is mainly governed by magmatic processes, in addition to post-magmatic ones. The distribution of chemical elements and the fractionation of some isovalents within the shear zone are largely controlled by the newly formed mineral phases. With respect to uranium mobilization, uranium migrated from the host alkali feldspar granites of G. Abu Garadi, while the shear zones acted as traps for the migrated uranium. Moreover, U migrated in the shear zone during greisenization and albitization, and migrated out during koalinitization.     

Geochemical Stream Sediment Survey in the Wadi Umm Rilan Area, South Eastern Desert, Egypt: A New Occurrence for Gold Mineralization

The present paper investigates the stream sediment geochemistry, behavior of gold and associated elements, delineates Au-anomalous sites and defines related sources in the Wadi Umm Rilan area. Fifty three major and trace elements were analyzed using ICP-MS and treated applying various statistical and mapping techniques. The results showed a significant difference of mean and median Au and most chemical elements in the three portions of the area, and higher values were recorded in the western portion. Furthermore, Au-anomalous sites appeared in upper and lower parts of the Wadi Umm Rilan, along the tributaries of metavolcanic unit and near granitoid contacts. This indicates the main source of Au mineralization is related to emplacement of granitoid plutons and accompanying hydrothermal solutions. There are significant indications for the presence of more than one mineralization event forming a probable single major episode of mineralization in the area, involving Au, Pb and U mineralizations. Geology, geomorphologic aspects and weathering processes could control stream sediment geochemistry, anomalies of Au and associated elements, elemental association and their dispersion patterns. Therefore, the area is fruitful and regarded as a promising target for Au exploration, using Ag, As, Sb, Cd, Cs, and Tl as pathfinder elements.     

Evolution of the Pan-African Wadi Haimur metamorphic sole, Eastern Desert, Egypt

By comparison with the general features of metamorphic soles (e.g. vertical and lateral extension, metamorphic grade and diagnostic mineral parageneses, deformation and dominant rock types), it is inferred that the amphibolites, metagabbros and hornblendites of the Wadi Um Ghalaga–Wadi Haimur area in the southern part of the Eastern Desert of Egypt represent the metamorphic sole of the Wadi Haimur ophiolite belt. The overlying ultramafic rocks represent overthrusted mantle peridotite. Mineral compositions and thermobarometric studies indicate that the rocks of the metamorphic sole record metamorphic conditions typical of such an environment. The highest P – T conditions ( c . 700 °C and 6.5–8.5 kbar) are preserved in clinopyroxene amphibolites and garnet amphibolites from the top of the metamorphic sole, which is exposed in the southern part of the study area. The massive amphibolites and metagabbros further north (Wadi Haimur) represent the basal parts of the sole and show the lowest P – T  conditions (450–620 °C and 4.7–7.8 kbar). The sole is the product of dynamothermal metamorphism associated with the tectonic displacement of ultramafic rocks. Heat was derived mainly from the hot overlying mantle peridotites, and an inverted P – T  gradient was caused by dynamic shearing during ophiolite emplacement. Sm/Nd dating of whole-rock–metamorphic mineral pairs yields similar ages of c . 630 Ma for clinopyroxene and hornblende, which is interpreted as a lower age limit for ophiolite formation and an upper age limit for metamorphism. A younger Sm/Nd age for a garnet-bearing rock ( c . 590 Ma) is interpreted as reflecting a meaningful cooling age close to the metamorphic peak. Hornblende K/Ar ages in the range 570–550 Ma may reflect thermal events during late orogenic granite magmatism.     

铜官山岩体矿物学-矿物化学特征：岩浆结晶动力学意义

本文对皖南官山岩体开展详细地显微镜观察鉴定,利用电子探针和LA-ICP-MS技术对岩浆岩典型矿物斜长石、角闪石和榍石进行了主量和微量元素测定。显微镜鉴定表明,铜官山岩体中存在着大量的岩浆不平衡结构：如斜长石和角闪石嵌晶结构以及针状磷灰石等。这些现象的存在表明铜官山岩体在形成过程中曾发生过一次或多次岩浆混合作用。电子探针分析结果显示,斜长石的成分环带是震荡环带,而大尺度的震荡环带可能代表了大规模的岩浆混合作用;角闪石成分TiO2-Al2O3图解、CaO/NaO2-Al2O3/TiO2图解和Mg-(Fe2++Fe3+)- LiNaKCa角闪石成因矿物族三角图解指示铜官山岩体中角闪石很可能为壳-幔混合成因。LA-ICP-MS技术对主要造岩矿物的微量和稀土元素分析表明,角闪石很可能为幔源或壳幔混合源,斜长石可能为不同分异程度岩浆的混合形成。本研究比较明确地反映了铜官山岩体的形成过程中岩浆来源和结晶动力学过程,即壳幔源区的混合交代作用,与前人通过元素-同位素手段获得的信息比较吻合。     

Assimilation and Fractional Crystallization Controlled by Transport Process of Crustal Melt: Implications from an Alkali Basalt-Dacite Suite from Rishiri Volcano, Japan

Mechanisms of fractional crystallization with simultaneous crustalassimilation (AFC) are examined for the Kutsugata and Tanetomilavas, an alkali basalt–dacite suite erupted sequentiallyfrom Rishiri Volcano, northern Japan. The major element variationswithin the suite can be explained by boundary layer fractionation;that is, mixing of a magma in the main part of the magma bodywith a fractionated interstitial melt transported from the mushyboundary layer at the floor. Systematic variations in SiO₂ correlatewith variations in the Pb, Sr and Nd isotopic compositions ofthe lavas. The geochemical variations of the lavas are explainedby a constant and relatively low ratio of assimilated mass tocrystallized mass (‘r value’). In the magma chamberin which the Kutsugata and Tanetomi magmas evolved, a strongthermal gradient was present and it is suggested that the marginalpart of the reservoir was completely solidified. The assimilantwas transported by crack flow from the partially fused floorcrust to the partially crystallized floor mush zone throughfractures in the solidified margin, formed mainly by thermalstresses resulting from cooling of the solidified margin andheating of the crust. The crustal melt was then mixed with thefractionated interstitial melt in the mushy zone, and the mixedmelt was further transported by compositional convection tothe main magma, causing its geochemical evolution to be characteristicof AFC. The volume flux of the assimilant from the crust tothe magma chamber is suggested to have decreased progressivelywith time (proportional to t^–1/2), and was about 3 x 10^–2m/year at t = 10 years and 1 x 10^–2 m/year at t = 100years. It has been commonly considered that the heat balancebetween magmas and the surrounding crust controls the couplingof assimilation and fractional crystallization processes (i.e.absolute value of r). However, it is inferred from this studythat the ratio of assimilated mass to crystallized mass canbe controlled by the transport process of the assimilant fromthe crust to magma chambers. KEY WORDS: assimilation and fractional crystallization; mass balance model; magma chamber; melt transport; Pb isotope     

北天山四棵树花岗岩体锆石U-Pb年代学及地质意义

四棵树岩体位于北天山缝合带中,是侵入依连哈比尔尕岛弧带中最大的一个花岗岩体,具有"钉合岩体"的构造意义。组成该岩体的岩石类型较多,大体上可分为3个侵入期次,依次为石英闪长岩和花岗闪长岩、二长花岗岩和钾长花岗岩、花岗斑岩;其中第二侵入期次构成了该岩体的主体。岩石SiO2含量为57.85%~74.28%,K2O含量为1.95%~5.25%,Na2O含量为3.00%~4.50%,整体表现为高K、Ca碱性特征。∑REE=113.91×10-6~169.47×10-6,(La/Sm)N为1.77~3.16,(Gd/Yb)N为1.33~2.12,(La/Yb)N为3.41~5.64,δEu为0.37~0.82,表现为轻稀土元素富集的右倾型特征;岩石具有Nb、Ta、Eu的亏损以及Th、Zr、Hf相对富集。通过形成环境判别,第一期石英闪长岩和花岗闪长岩大部投在火山弧环境,板块碰撞前—后碰撞隆起阶段;第二期二长花岗岩和钾长花岗岩大部投在后碰撞阶段;第三期花岗斑岩主要投在晚造山阶段。通过LA-ICP-MS对该岩体中石英闪长岩、二长花岗岩和钾长花岗岩进行较为系统的锆石U-Pb测年,得出其加权平均年龄分别为(324.1±4.3)Ma、(314.9±4.1)Ma、(311.5±3.9)Ma。结合前人得出的巴音沟蛇绿岩中辉长岩的(344±3.4)Ma年龄,可初步得出北天山缝合带的形成时代应该为344~311.5Ma,碰撞造山作用主要发生在324~311Ma,随后进入后碰撞伸展阶段,而四棵树岩体3期侵入岩可能分别代表了北天山造山带俯冲汇聚-碰撞-陆内造山各阶段构造-岩浆作用的产物。     

Post-orogenic and anorogenic A-type fluorite-bearing granitoids, Eastern Desert, Egypt: Petrogenetic and geotectonic implications

The present study focuses on four A-type fluorite-bearing granitic plutons in the Eastern Desert of Egypt which are classified into post-orogenic subsolvus (Homrit Waggat, 535 Ma; Homer Akarem, 541 Ma and Ineigi, 571 Ma) and anorogenic hypersolvus (Gabal Gharib, 476 Ma) granites. All the granites are Si- and alkali-rich and MgCaTi poor. Whereas both granite types appear relatively homogeneous in terms of most of their major and trace elements, they differ in that the subsolvus granites are depleted in TiO₂, FeO^*, Ba, Sr and Zr and enriched in Rb and Y with respect to the hypersolvus granites. The two granite types, however, can be distinguished more easily by their rare-earth element (REE) patterns. Chondrite-normalized REE patterns of the hypersolvus granite display a gull-wing shape, characterized by a large negative Eu anomaly and moderate-to-high LREE contents. Relative to the hypersolvus granite, subsolvus granite is depleted in LREE and more enriched in HREE contents. The increase of HREE in the subsolvus granite is presumably caused by F complexing during the late stage of its evolution. This is supported by the abundance of fluorite veins cross-cutting the subsolvus granite. The negative Eu anomalies in the subsolvus granite point to the role of feldspars as residual phase in the source, and as a crystallizing phase during magmatic differentiation.Field relations, textural, mineralogical and geochemical data of the post-orogenic subsolvus granite are consistent with its derivation from a parental basic magma through crystal-liquid fractionation of alkali feldspar, plagioclase, amphibole, FeTi oxides, titanite, zircon, monazite and allanite. Crystallization occurred in a water-enriched and rather oxidizing environment, as a result of which the entire suite has a transitional character between that of a post-orogenic and an anorogenic setting. On the other hand, the most credible mechanism for the origin of the anorogenic hypersolvus granite is partial melting of I-type granodiorite-monzogranite source rocks in the study area.