Island-arc Assemblages Research Articles

Lithological discrimination of the Fawakhir-Atalla belt in the Central Eastern Desert of Egypt based on Landsat-9 remote sensing data, airborne gamma-ray spectrometry, field and petrographic investigations with implications on the evolution of the Arabian Nubian Shield

The Central Eastern Desert (CED) is a key territory for understanding the evolution of the Arabian Nubian Shield (ANS) in Egypt as it contains Cryogenian-Tonian ensimatic supracrustal assemblages, such as ophiolitic and arc volcanic rocks, as well as gold deposits. So, it has been the subject of extensive studies over the last two decades. However, the lithological mapping of the Fawakhir-Atalla belt, located west of the Meatiq gneiss dome (infrastructure) of the CED, is still debated, as the magmatic texture of some rocks was partially obscured owing to the shearing and alteration related to the Najd Fault System, mantle CO2-bearing fluid, and igneous activity in the final stage of the Pan-African Orogeny. In this regard, integrated studies of airborne gamma-ray spectrometry and Landsat-9 satellite imagery processing techniques, as well as field and petrographic investigations, have been used for the lithological mapping of the belt. The results led to the following sequence; ophiolitic rocks > island arc assemblages > older granitoids (syn-tectonic subduction-related) > Hammamat Group (molasse-type origin) > post-Hammamat felsite > younger granite (post-tectonic suture-related). The applied color composite images, selective band ratios, and image transformation techniques show a clear distinction between rock units. Older granitoids are tonalite to quartz diorite, while younger granites are monzo-to syenogranite. They are well-discriminated from each other using band ratios, and younger granites have the highest K, eU, and eTh values coherent with the post-Hammamat felsite. The image enhancement and image transformation techniques show a clear discrepancy between the mélange rocks and Hammamat conglomerates. Mélange rocks are composed of fragments embedded in a foliated matrix, while Hammamat conglomerates are grain-supported with pebbles of volcanic source and have higher K, eU, and eTh values. Volcanic rocks in the belt are not a part of the Dokhan series as they are metamorphosed with a composition range from meta-basalts to meta-rhyolite, and show mainly aphanitic to infrequently porphyritic textures, which are typical of island arc metavolcanics.

Geochemistry of island arc assemblage in the Eastern Desert of Egypt and the role of Pan-African magmatism in crustal growth of the Arabian–Nubian Shield: A review

Geochemistry of island arc assemblage in the Eastern Desert of Egypt and the role of Pan-African magmatism in crustal growth of the Arabian–Nubian Shield: A review

Towards better delineation of hydrothermal alterations via multi-sensor remote sensing and airborne geophysical data

Integrating various tools in targeting mineral deposits increases the chance of adequate detection and characterization of mineralization zones. Selecting a convenient dataset is a key for a precise geological and hydrothermal alteration mapping. Remote sensing and airborne geophysical data have proven their efficiency as tools for reliable mineral exploration. Advanced Spaceborne Thermal Emission and Reflection Radiometer (ASTER), Advanced land imager (ALI), Landsat 8 (L8), and Sentinel 2 data are widely-used data among various types of remote sensing images in resolving lithological and hydrothermal alteration mapping over the last two decades. ASTER is a well-established satellite in geological remote sensing with detailed Short-wave infrared (SWIR) range compared to visible and near-infrared region (VNIR) that controls iron-associated alteration detection. On contrary, ALI has excellent coverage of the VNIR area (6 bands), but does not possess the potentiality of ASTER for the SWIR and thermal regions. Landsat 8 is widely used and highly recommended for lithological and hydrothermal alteration mapping. The higher spatial (up to 10 m) resolution of Sentinel 2 MSI has preserved its role in producing accurate geological mapping. Notwithstanding the foregoing, implementing the four datasets in a single study is time-consuming. Thus, an important question when commencing an exploration project for hydrothermal alterations-related mineralization (orogenic mineral deposits in the current research) is: which dataset should be adopted to fulfill proper and adequate outputs? Here the four widely recommended datasets (ASTER, ALI, L8, and sentinel 2) have been tested by applying the widely-accepted techniques (false color combinations, band ratios, directed principal component analysis, and constrained energy minimization) for geological and hydrothermal alteration mapping of Gabal El Rukham-Gabal Mueilha district, Egypt. The study area is covered mainly by Neoproterozoic heterogeneous collection of ophiolitic components, island arc assemblage, intruded by enormous granitic rocks. Additionally, airborne magnetic and radiometric data were applied and compared with the remote sensing investigations for deciphering the structural and hydrothermal alteration patterns within the study area. The results demonstrated a different extent from one sensor to another, highlighting their varied efficacy in detecting hydrothermal alterations (mainly hydroxyl-bearing alterations and iron oxides). Moreover, the analysis of airborne magnetic and radiometric data showed hydrothermal alteration zones that are consistent with the detected alteration pattern. The coincidence between high magnetic anomalies, high values of the K/eTh ratio, and the resultant alterations confirm the real alteration anomalies. Over and above that, the remote sensing results and airborne geophysical indications were verified with fieldwork and petrographic investigations, and strongly recommend combining ASTER and Sentinel 2 results in further investigations. Based on the outputs of the current research, we expect better hydrothermal alteration delineation by adopting the current findings as they sharply narrow the zones to be further investigated via costly geophysical and geochemical methods in mineral exploration projects.

Petrogenesis and Tectonic Evolution of Kab Amiri Ophiolites and Island-Arc Assemblages, Central Eastern Desert, Egypt: Petrological and Geochemical Constraints

The Kab Amiri area in the Central Eastern Desert (CED) of Egypt comprises ophiolitic rocks, island arc metasediments, and granitic rocks. This study deals with the petrography and geochemistry of the ophiolitic and island arc rocks to understand their petrogenesis and geodynamic evolution of the CED ophiolitic belt. The Kab Amiri ophiolite is dismembered, comprising serpentinites and metabasalt (spilite). Serpentinites have low contents of Al2O3 (1.14 wt%) and CaO (0.65 wt%), suggesting they are depleted peridotite protoliths (e.g., depleted harzburgites to dunites). The high L.O.I. value (13.7 wt%) of serpentinite rocks indicates intense hydration and serpentinization during alteration processes. Petrographic and geochemical studies suggest that serpentinites were likely formed after depleted peridotites in a supra-subduction zone (SSZ) setting (e.g., a fore-arc setting). Spilitic basalt shows a tholeiitic affinity of the depleted mantle source. The arc-related metasediments are represented mainly by schists and slate. Many samples of metasediments are relatively low in alumina (Al2O3 < 15%), suggesting a low clay content and formation in an island arc setting. In contrast, protoliths of island-arc metabasalts and meta-andesites crystallized from calc-alkaline magmas in the immature oceanic arcs.

Mineralogy and geochemistry of serpentinized peridotites of the Northern Nubian Shield: The origin of compositionally banded olivine and implications for Neoproterozoic supra-subduction zone metasomatism

A sequence of Neoproterozoic ultramafic rocks — serpentinized peridotite, serpentinite, and clinopyroxenite — outcrop in Wadi El-Mireiwa in the south Eastern Desert of Egypt. They represent the mantle and lowermost crustal section of a fragmented ophiolite, emplaced by thrusting above metasedimentary rocks and an island arc assemblage and later intruded by a granite mass. In samples of the serpentinized peridotite, back-scattered electron images of olivine reveal banded zoning, defined by straight and parallel Mg-enriched (Fo ∼ 96) lamellae within typically magnesian host crystals (Fo ∼ 89). The presence of clinopyroxene, the relatively low whole-rock Mg# (∼88), the low NiO content of olivine (0.1–0.32 wt%), and the relatively high TiO2 content of fresh Cr-spinel (∼0.3 wt%) are all consistent with a cumulate origin for the serpentinized peridotite and serpentinites. On the other hand, some associated serpentinite has higher whole-rock Mg# (92) and low TiO2 in Cr-spinel (∼0.01 wt%), consistent with a residual origin by high-degree melt extraction in the fore-arc region of a supra-subduction zone (SSZ) environment. The formation of Mg-enriched bands in olivine is attributed to the enhancement of metasomatic Mg-Fe exchange with high-Mg/Fe2+ fluids along crystallographically-oriented subgrain boundaries produced by high strain-rate dislocation creep. Such high-Mg/Fe2+ fluid metasomatism also explains reverse zoning of MgO in Cr-spinel, whereas the low whole-rock concentrations of REE and other trace elements exclude a significant role for melt-rock interaction. The evidence for high-Mg/Fe2+ fluid metasomatism in this suite indicates that SSZ fluids and high fore-arc strain rates affect not only the mantle wedge but also overlying ultramafic cumulates. The oxygen fugacity [ΔlogƒO2(FMQ)] values calculated from unmetasomatized (+2.48 to +2.67) and metasomatized (+3.75 to +4.53) samples reflect the oxidized nature of Neoproterozoic SSZ magma and the even more oxidizing character of the metasomatic fluids. Banded zoning has also been recorded in olivine from Mariana fore-arc peridotite and Phanerozoic SSZ ophiolites; the Neoproterozoic Wadi El-Mireiwa case shows that rapid mantle flow has been a persistent feature of the tectonic environment where fore-arc ophiolites develop.

Origin and Heavy Metals of Plagio-Granites in Egyptian Shield Oceanic Complexes: A Case Study of Abu Dabbab Area, Central Eastern Desert, Egypt

Several outcrops of felsic melt, commonly known as oceanic plagio-granites, appear as melt pockets, irregular bosses, and dikes. Three main rock successions were exposed in the Abu Dabbab area: ophiolitic sequence, island arc assemblage, and rare metal-bearing albite granites. Plagio-granite is composed mainly of plagioclase and quartz, and chemically contains high SiO2, Al2O3, and Na2O values. The petrogenetic and geochemical studies concluded that the magmas of plagio-granites and associated felsic rocks were derived from partial melting of highly depleted mantle harzburgite and/or pre-existing oceanic crustal rocks (basalts and/or gabbros). The geochemical characteristics reveal that Abu Dabbab plagio-granites are identical to the typically low-K series of ophiolitic complexes and resemble the ocean ridge granites. Elements such as Co, Nb, Pb, and Nd in Abu Dabbab plagio-granites have high Clarke concentrations (more than 1.88). This indicates Abu Dabbab plagio-granite derived from partial melting of pre-existing oceanic crustal rocks. The average concentration values of 238 U and 232 Th are lower than those of the world’s average and the average activities in granitic rocks. The scope of estimated exercises generally contrasted as their quality in rock tests relies upon their physical, synthetic, and geo-concoction properties and the appropriate condition.

Apparent undisturbed paleoposition of mid-Cretaceous plutons emplaced across a Jurassic island arc segment in the southernmost part of the Peninsular Ranges batholith, Baja California, Mexico

The Calmallí (CA), Piedra Blanca (PB), and El Barril (EB) plutons were emplaced during the Late Cretaceous at the southern limit of the Peninsular Ranges batholith (PRB) at the present latitude of ∼28°N. These plutonic suites are juxtaposed, showing contrasting compositions, probably associated with different basement rocks. The region is separated by the high magnetic values in the western zone compared with the lowest magnetic values in the eastern zone. The western CA pluton intrudes a Jurassic island arc assemblage, whereas host rocks outcrops of the PB and EB are absent, but it is inferred that they intruded into the eastern margin of the Jurassic arc. The CA plutonic suite has a ca. 100 Ma old mafic core surrounded by ca. 102–105 Ma old tonalities. Such zonation is a typical feature of many plutons of the western PRB. The PB and EB plutons show granodioritic to granitic compositions of ca. 92 Ma. We compare the paleoposition of the CA-EB section with respect to the San José, and San Pedro Mártir plutons (∼31°N), and the Compostela pluton (∼29°N) which yield clockwise rotations of 31°–45°, and 16.4°, respectively. These northern plutons record rotations by tectonic deformation at suture zones between the Alisitos island arc and the older North America craton. Based on the paleomagnetic observations in the transition zone between western and eastern type plutons, we analyzed 15 sites from the CA, nine from the PB, and 23 from EB to evaluate their paleoposition. We obtained the characteristic remanent magnetizations (ChRM) following the detailed stepwise alternating magnetic field (AF) and thermal demagnetization methods. Each pluton was compared to the reference pole of North America at 100 Ma and 90 Ma. The paleopoles do not differ much from the cratonic references since the data indicate an apparent clockwise rotation of 8.1° ± 8.8° and a flattening of −13.9° ± 10.6° for CA pluton and an apparent clockwise rotation of 1.43° ± 8.3° and −14.2° ± 9.8° of flattening for combined paleopole of PB and EB plutons. Statistically, the paleopoles displacement are not significative, similar to five cretaceous plutons located NNW of the study area, suggesting that the mid-Cretaceous plutonic suites of the southern PRB remained stable since their emplacement in a period between 128 Ma and 90 Ma.

Nd-Sr Isotopic Study of Magmatic Rocks and 40Ar/39Ar Dating of the Mafic Dike of the Proterozoic Ulan-Sar’dag Ophiolite Mélange (Southern Siberia, East Sayan, Middle Belt, Russia)

We report the first radiogenic Nd-Sr isotope data in the magmatic rocks island-arc ophiolite assemblage from the middle branch of the East Sayan ophiolite complexes to better constrain geodynamic processes in this segment of the CAOB in southern central Siberia. The magmatic rocks belong to the following geochemical types: (1) Ensimatic island-arc boninites; (2) island-arc assemblage; (3) enriched basalts of mid-ocean ridges; and (4) oceanic island-like basalts. The boninites have a positive value εNd (T), which is generated from a depleted mantle source (N-MORB). The island-arc assemblage has negative or slightly positive εNd (T) and was formed from an enriched mantle source due to the subduction of terrigenous rocks. The source of the terrigenous material was most likely the rocks of the Archean TTG (Trondhjemite Tonalite Granodiorite) complex of the Gargan block. Isotopic ratios for E-MOR and OIB-like basalts are characterized by positive or slightly negative values of εNd (T). The mafic dike, which crosscut ophiolite rocks, corresponds to OIB-like basalts. The values of εNd (T), measured 87Sr/86Sr and I (Sr), in the mafic dike correspond to the EM I mantle source. The E-MOR and OIB-like basalts appear to be formed in late-stage asthenospheric mantle melting via the decompression melting processes. The obtained isotope geochemical data for the E-MOR and OIB-like basalts probably indicate the mixing of island-arc melts with asthenospheric melts. We undertook 40Ar/39Ar dating of the mafic dike, which crosscut the ophiolite unit. The mafic dike has a whole-rock 40Ar/39Ar weighted mean plateau age of 799 ± 11 Ma. The dating constrains the minimum age of the ophiolite and island-arc magmatism in the region.

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

AbstractOphiolitic 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 partially‐serpentinized peridotite. Most of the Cr‐spinel crystals have fresh cores followed by outer zones of ferritchromite and Cr‐magnetite, 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, Al2O3 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 ocean‐floor 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).

Petrogenesis and geodynamic setting of high-Cr chromitites in fore-arc peridotites: A case study from the Halaban ophiolite, Eastern Arabian Shield, Saudi Arabia

The present study deals with chromitite pods and the associated mantle section of the Neoproterozoic Halaban ophiolite, Eastern Arabian shield, Saudi Arabia. The NNW–SSE-trending Halaban ophiolite represents a moderately dismembered Penrose-type ophiolite section that is tectonically emplaced over island arc assemblages. It comprises a mantle section below metagabbros upward to the pelagic sedimentary cover that overlies the mafic volcanics. The Halaban mantle section includes chromitite-bearing serpentinized ultramafics, as well as metaperidotites, listvenites and talc‑carbonates along shear zones and faults with variable degrees of alteration and metamorphism. Preserved textures include bastite and mesh indicating respectively harzburgite and dunite protoliths of the studied serpentinites. The chromitite pods are scarce being restricted to the uppermost mantle section and the overlying Moho transition zone (MTZ) dunite. They are commonly massive with interstitial serpentines, carbonates and Cr-rich chlorite (kämmererite). Cr-spinel of the chromitite pods hosts inclusions of isolated sulfides and silicates (serpentine, olivine and kämmererite). The Cr-spinel of the massive chromitites has higher Cr# (0.77–0.83) and Mg# (0.60–70) than the disseminated fresh Cr-spinel cores in the serpentinites (Cr# = 0.60–0.70 and Mg# = 0.43–0.60). High-Cr# combined with low-TiO2 character (<0.12 wt%) of the disseminated Cr-spinel consistent with residual mantle rocks that experienced high degrees of partial melt extraction. The mineralogical features of the Halaban serpentinites and the associated chromitite pods are similar to the mantle section that most probably formed in forearc peridotites of Supra-subduction zone (SSZ) environments. High-Cr# in Cr-spinel of chromitite pods suggesting possible boninitic parent magma. The parent magma had undergone melt–melt and melt–rock interactions with the precipitation of high-Cr chromitite from Al- and Ti-poor, and Cr-rich boninitic melts. Notably, the Cr# and Mg# of Cr-spinel increase from the harzburgite through the dunite to the chromitite due to increase intensity of melt-rock reaction during formation of the chromitite.

Geochemical and Geotectonic Setting for Island Arc Related rocks on Um Taghir Area, Central Eastern Desert, Egypt

Um Taghir area located in theCentral Eastern Desert of Egypt a long road of Safaga-Qena. Itis represented by island arc related rocks that are intruded by late to post tectonic magmatism. The island arc belonging to oceanic terrain, divided into metavolcaniclastic and metagabrroic rocks. Metavolcanoclastic sequence is represented as the older rock unit of the study area,it is noted that metavolcanoclastic intruded by metagabbro. Island arc assemblage is classified into schist and metagabbro that give clean different of their mineralconstituents according to petrographical studies. Geochemically , the investigated rock units and most of dikes have sub alkaline nature. Tectonically , metavolcanoclastic sequence is related to back arc basin basalts, while metagabbro related to volcanic arc.

Formation of the Crystalline Rock Complexes of the Tatsaingol Block of the Late Neoproterozoic Southern Khangay Metamorphic Belt, Central Asian Foldbelt

The results of geochronological studies (LA-ICP-MS) of detrital zircons of the Tatsaingol block in the southeastern part of the Neoproterozoic Southern Khangay metamorphic belt are presented. Two sequences, metaclastic rocks and hornblende schists (metavolcanics), are distinguished in the Tatsaingol complex of this block. The metaclastic rocks are found to contain manifestations of the processes of polymetamorphism. The end of the early metamorphic episode in these rocks is defined by pegmatoid granites with the ages of 561 ± 12 and 562 ± 2 Ma and granite dikes with the age of 571 ± 9 Ma; the age of the late metamorphic episode is estimated in the range of 550–540 Ma. The lower age limit of metamorphism in the metavolcanics of the island arc assemblage is defined by gabbroids with the age of 603 ± 3 Ma, but the early episode of high pressure metamorphism is not established. The late episode (550–540 Ma) is manifested in both sequences. The results of the isotopic dating of detrital zircons from biotite gneisses indicate the predominance of Paleoproterozoic and Neoarchean rocks (1.8–2.1 and 2.4–2.9 Ga, respectively) in their sources. The rocks of the island arc assemblage of the Tatsaingol complex can be viewed as a fragment of the Bayan-Khongor zone, which was accreted to the Southern Khangay metamorphic belt in the course of the accretion–collision process. The late metamorphic episode was not manifested in the central and northwestern parts of the Southern Khangay metamorphic belt, which indicates the separateness of this part of the belt from the Tatsaingol block during this period.

Metamorphic and structural evolution of the Flin Flon – Athapapuskow Lake area, west-central Manitoba

The Flin Flon – Athapapuskow Lake area, situated in the Flin Flon Greenstone Belt, Manitoba, consists of ocean-floor and island-arc assemblages, deformed and metamorphosed during the Trans-Hudson Orogeny (∼1.86–1.69 Ga). A new map of metamorphic mineral assemblages and isograds has been compiled that reveals a largely coherent regional metamorphic sequence increasing in metamorphic grade from prehnite–pumpellyite to amphibolite facies. Regional metamorphism postdates most of the deformation within the area, with the exception of the reactivation of major block-bounding faults. The regional prograde sequence has been subdivided into 10 metamorphic zones, separated by 9 isograds, that describe the transition from prehnite–pumpellyite to greenschist to amphibolite facies. The formation of contact metamorphic aureoles, pre-dating regional metamorphism, record conditions up to amphibolite facies. Equilibrium phase diagrams for the island-arc (low-Mg) and ocean-floor (high-Mg) assemblages were calculated and allow for the evaluation of the modelling techniques and determination of pressure–temperature conditions. Discrepancies between the modelling predictions and natural observations occur due to (1) limitations in the thermodynamic models for some of the complex minerals (e.g., amphibole); and (2) metastable persistence of some minerals to higher grade due to sluggish reaction kinetics. Notwithstanding these discrepancies, the modelling suggests that metamorphosed mafic rocks in the Flin Flon – Athapapuskow Lake area reached about 430–480 °C and 3.0–4.5 kbar. Peak metamorphic conditions within contact aureoles that preceded regional metamorphism did not exceed 500 °C (at a pressure between 2.7 and 4.4 kbar). The metamorphic field gradient records a transition from 250–300 °C/1.5–2.3 kbar to 430–480 °C/3–4.5 kbar (100–150 °C/kbar), defining a geothermal gradient of approximately 25–31 °C/km.

Ore-forming mechanism and its relationship with deformational and metamorphic episodes at Haimur gold mine, Nubian Shield, Egypt

AbstractThe Haimur area represents the central part of the Wadi Allaqi region in the southern block of the Egyptian Eastern Desert near Nasser Lake and the Nile valley. It is made up of ophiolitic assemblage comprising serpentinite and talc carbonate, listwenite, metagabbro/amphibolite and metabasalt and island arc assemblages. The orogenic gold deposits in the Haimur area occur in the form of smoky/white sulphide-bearing quartz / quartz-carbonate veins or lenses cutting through the listwenite zone-related rocks. The NE-trending auriferous veins were formed due to an extensional to transtensional shearing related to NW Najd shear tectonics. Gold was observed in association with late-pyrite and chalcopyrite paragenetically formed in the gold phase (second phase), while the first phase is the sulphide phase including early-pyrite and arsenopyrite. Arsenopyrite was formed during early metamorphic recrystallization at a high-temperature range between 405 and 512 °C. However, gold was formed at lower temperature through retrograde metamorphism. Three types of fluid inclusions have been recognized: aqueous (type-I), mixed aqueous–carbonic (type-II) and hydrocarbonic (type-III). The P–T conditions of trapping were obtained from the isochore lines of intersection between T = 300–320 °C and pressure range of 60–180 MPa. The Haimur gold deposit was supposed to be derived from metamorphic fluids created by dehydration and decarbonation of ophiolitic mélange assemblages and volcano-sedimentary rocks, in which the gold was transported as Au(HS)2– complexes and precipitated in reduced environment. The strong linkage between the deformation and metamorphism triggered gold deposition in structurally favourable sites related to the Najd fault system.

Geological and petrographical studies around Um Taghir area, Сentral Eastern Desert, Egypt

Um Taghir area is located in the northern extreme boundary of Central Eastern Desert of Egypt at the west of Safaga City. Um Taghir is represented by island arc related rocks and late to post tectonic magmatism. The island arc related rocks are represented by metavolcaniclastic sequences and metagabrroic rocks. Metavolcanoclastic rocks are considered as the older rock units of the study are and intruded by the metagabbro. The late to post tectonic magmatism is represented by (dokhan volcanic, gabbro, tonalite-granodiorite, monzogranite, alkali feldspar granites and different types of dikes). Usually, the gabbroic rock is bearing ilmenite lenses or bands in the bottom of the layered; this is related to magma rich of iron oxides. Petrographically, island arc assemblage is classified in to actinolite hornblende schist and metagabbro that show quite different of their content in plagioclase, hornblende, augite, quartz and biotite. Occasionally, the late to post tectonic magmatism represented by andesite, gabbro, tonalite, granodiorite monzogranite, alkali feldspar granites and different types of dikes. Andesite consists of plagioclase, quartz, alkali feldspar and hornblende. Gabbroic rocks are represented by pyroxene hornblende gabbro and leucogabbro. They show quite different of their content in plagioclase, pyroxene and clear difference in the content of both olivine and hornblende in both of them. While tonalite and granodiorite show quite different of their content in plagioclase, quartz, hornblende, alkali felspar and biotite. On the other hand, monzogranite and alkali feldspar granite, they show plagioclase is varying from oligoclase to albite; K-feldspars, quartz and muscovite are relatively more abundant in the alkali feldspar granite. Finally, the different types of dikes classified into granite, andesite, rhyolite and basalt dikes consist of the different mineral compositions.

Petrogenetic evolution of Cryogenian Halaban ophiolite, Saudi Arabia: A fragment of fore-arc oceanic lithosphere mantle

In this study, Neoproterozoic Halaban ophiolite situated in the eastern part of the Arabian shield, Saudi Arabia, was investigated. The Halaban ophiolite represents a section of a dismembered ophiolite, tectonically emplaced over a volcano-sedimentary succession of island arc assemblages. It is variably, altered, deformed and metamorphosed to serpentinized rocks. It includes serpentinized peridotite and metagabbro with rare metabasalt, chromitite pods, listvenite and rodingite. The contact between the lower mantle unit (serpentinized peridotite) and the upper crustal unit (metagabbro) is originally magmatic, although it is disrupted by tectonism. Nevertheless, because of the high degree of alteration and metamorphism, few fresh relicts of primary minerals (chromian spinel, olivine and pyroxenes) were recorded. The mineral assemblages in the Halaban ophiolite are comparable to greenschist, to lower amphibolite facies metamorphism. The fresh olivine relics have high Fo and Ni content, while the fresh chromian spinel cores have high Cr# accompanied by a low TiO2. The preserved texture (mesh and bastite), fresh relict minerals, and the whole-rock compositions (Mg# = molar Mg/ (Mg + Fe2+) = 0.91–0.93 with low CaO and Al2O3 content) suggest that the protolith is mainly harzburgite with minor dunite. The chemical composition of Halaban metagabbro is indicative of a tholeiitic affinity. In addition, the very low modal abundance of clinopyroxene and the high Ni, Cr and Co content of the serpentinized peridotite, and high-Cr# (av. 0.66) with low-Ti (mostly ≤0.1 wt%) of chromian spinel, suggest significant partial melting. The serpentinized peridotite (harzburgite and dunite) undergone different degrees of partial melting, ranging from approximately 32% to 37% for harzburgite and approximately 37% to 42% for dunite. The variation of the degree of partial melting between harzburgite and dunite is indicative of mantle heterogeneity beneath the Arabian Shield. The general petrological characteristics of the Halaban ophiolite are substantially similar to those formed by melt-peridotites interaction and subsequent melt mixing in a fore arc setting within a supra-subduction zone system. Halaban chloritite is formed as a secondary product due to the metasomatic interaction of the ophiolitic ultramafic rocks. The content of Cr, Co and Ni of the chloritite sample was consistently close to that of the serpentinites.

Geochemical Constraints of the Ediacaran Volcano‐Sedimentary Succession of the Sa'al Metamorphic Complex at Wadi Zaghra, South Sinai, Egypt

AbstractThe volcano‐sedimentary succession around Wadi Zaghra in Sinai, at the northernmost segment of the Arabian Nubian Shield, comprises volcanic rocks interbedded with rather immature sediments. The succession is dominated by intermediate to silicic volcanics of medium‐ to high‐K calc‐alkaline affinity. It is divided into two units, the lower unit includes intermediate rocks and dacites interbedded with graywackes, semi‐pelites and pelites and topped by polymict conglomerates. This unit is subjected to folding and regional metamorphism (up to garnet zone) and is intruded by quartz diorite‐granodiorite inducing, locally, low‐pressure contact thermal metamorphism. The unmetamorphosed upper unit encompasses acid volcanics intercalated with litharenite, sublitharenite and minor arenite. The rhyolites of this unit pertain to the highly fractionated granites and are characterized by an agpaitic index (NK/A) ranging from 0.87 to 0.96. They may reflect either extensive interaction of subduction‐related magmas with the continental crust or a change in the tectonic regime. The present lithological and geochemical characteristics of the studied sediments together with available zircon ages indicate rather distal provenance of their detritus. This detritus comprises fluvial‐alluvial sediments accumulated in the intermontane basins, which are half‐grabens or tilted fault blocks. The tectonic setting of the depositional basins is active continental margin and continental island arcs. Geochemical patterns of the Zaghra volcano‐sedimentary succession indicate their correlation with the Dokhan Volcanics‐Hammamat Clastics sequence of the Eastern Desert of Egypt. Also, the Zaghra volcanics display geochemical similarities with those exposed in Sinai, at the Rutig, Ferani and Iqna Shar'a areas. The Zaghra succession is dated as Ediacaran but is not related either to the ensimatic island arc assemblage or to the rift‐related assemblage formed during the early stages of the break‐up of Rodinia as previously thought.

Petrology, geochemistry, radioactivity, and M–W type rare earth element tetrads of El Sela altered granites, south eastern desert, Egypt

The southern part of the Eastern Desert of Egypt—about 30 km southwest of Abu-Ramad city—is mainly covered by ophiolitic rocks (Sul Hamed), island arc assemblage, younger granites (muscovite granites of Qash Amer and two mica granites of El Sela), and various acidic and basic dikes. Field, petrological, and geochemical studies indicate that the El Sela shear zone has been subject to hydrothermal and supergene alteration such as kaolinization, albitization, sericitization, and hematitization. It is invaded by quartz ENE–WSW veins associated with hydrothermal alteration accompanied by radioactive mineralization. The investigated younger granitic rocks had very low contents of compatible elements, such as Cr, Ni, and Co; and high contents of incompatible elements, such as Zr, and large ion lithophiles, such as Sr, especially in the El Sela shear zone. Major oxide and trace element analyses revealed calc-alkaline affinity and peraluminous character. These highly differentiated granitic rocks’ lower Zr/Hf and higher Y/Ho than the normal ratio are consistent with hydrothermal alteration. Most samples had rare earth element (REE)-patterns with an M-type tetrad effect in the first and fourth segments and a W-type tetrad in the third segment. The average ∑REE in the studied granites was lower than the world granite average; the ratio of light to heavy REEs greater. The main radioactive, uranium-bearing, and uraniferous Fe and Mn minerals are uranothorite, autunite, uranophane and autunite as compounds, kasolite, columbite, xenotime, uranophane-bearing zircon and jarosite, silver-bearing pyrite, hematite, and autunite-bearing pyrolusite.

The late Neoproterozoic Pan-African low-grade metamorphic ophiolitic and island-arc assemblages at Gebel Zabara area, Central Eastern Desert, Egypt: Petrogenesis and remote sensing - Based geologic mapping

The late Neoproterozoic Pan-African basement rocks of the Gebel Zabara area, Central Eastern Desert of Egypt is of particular interest where the medium-grade Hafafit gneisses juxtapose the low-grade ophiolitic and island-arc assemblages along major structural contacts. The area comprises important mineral deposits including gold, base metal vein-type deposits as well as emerald and talc. In the current work, various image processing algorithms including Principal Component Analyses (PCA), Minimum Noise Fraction (MNF) and Decorrelation Stretch (DS) were proposed and applied on Landsat-8, ASTER and Sentinel-2 data to delineate and discriminate the different rock units. Several ASTER mineral indices are successfully used in the mapping process to detect the alteration zones, which are mainly confined to the NNW-SSE and N-S trending structures. The petrogenesis studies for the different rock unites in the study area indicate that the ophiolitic assemblage include serpentinite-talc carbonate, metagabbros complex and metabasalts. Serpentinite rocks represent the hydrated low temperature form of a harzburgite precursor, while the talcose serpentinites evolved via SiO2 aqueou metasomatic alteration. Geochemically, the ophiolitic rocks exhibit N-MORB tholeiitic affinity and imply a back-arc setting. The island-arc rock assemblages encompass calc-alkaline meta-andesite, metadacite and metarhyolite, which imply a normal continental island-arc setting, i.e. less mature than the proper active continental margin. Apart from serpentinites, the ophiolitic and island-arc assemblages experienced regional greenschist facies metamorphism. The ophiolitic metagabbros exhibit the metamorphic assemblage: actinolite + albite (An < 5%)/oligoclase (An 19–20%) + chlorite + epidote + quartz. The coexistence of albite and oligoclase may indicate either: (1) disequilibria association, or (2) a possible greenschist-amphibolite facies transition. The P-T metamorphic condition of the metagabbros was constrained at about 5 kbar and 430 to 452°C.

Late Cryogenian arc-related volcaniclastic metasediment successions at Wadi Hammuda, Central Eastern Desert, Egypt: geology and geochemistry

Wadi Hammuda is dominated by a variety of low grade regionally metamorphosed volcaniclastic metasediments pertaining to two different geotectonic settings and intruded by arc and late collision granitic rocks. Thus, the volcaniclastic metasediments which form extensive outcrops are considered as a member of island arc assemblages. This paper deals with the petrography, geochemistry, and tectonic setting of the island arc volcaniclastic metasediment rock units. The volcaniclastic metasediments consist of interbedded metagreywackes, metasiltstones, metamudstones, and schists as well as metapyroclastics. They are well foliated, crenulated and tightly folded, metamorphosed, and intruded by granitic rocks. Geochemical data support the petrographic classification and reveal that these volcaniclastic metasediments are generally low-K, essentially tholeiitic in character, with the exception of some metasediments and metapyroclastics which exhibits calc-alkaline and tholeiitic affinities and represent the first stage of island arc volcanism. The overthrusted oceanic lithosphere blocks with fragments of the fore arc and/or back-arc marginal basins volcaniclastic metasediments were incorporated among the island arc volcanics which supported by tectonically relationship between the different rock units in the study area. Contemporaneous with this deformation event, Wadi Hammuda was subjected to low grade regional metamorphism and the rocks document an early phase of shearing and/or foliation. Occasionally minor folds were developed particularly in the metasediments and schists. The subsequent emplacement of the syn-tectonic granites (tonalites and granodiorites) resulted in minor local thrusts. During the regional thrusting event which preceded the emplacement of the late-tectonic granites (alkali feldspar granites) and affected the whole region, low grade successions cover the study area similar to the Meatiq volcaniclastic metasediments.