Low-grade Metavolcanics Research Articles

Basement rocks around the eastern sector of Baranis-Aswan Road, Egypt: Remote sensing data analysis and petrology

The basement rocks around Baranis-Aswan Road, South Eastern Desert of Egypt encompasses a variety of the oldest Pan-African rock units including medium-grade hornblende schist, a variety of gneisses and migmatite as well as medium- to low-grade metavolcanics. These were intruded by syn- to late-tectonic tonalite, granodiorite, quartz diorite as well as gabbros. The exposed basement rock units in the area were discriminated with the detailed lithological mapping using the processed data of ASTER, Landsat-8 and Sentinel-2. Spectral signatures of these rocks were achieved by ASD TerraSpec Halo mineral identifier device. Petrography and geochemistry data enabled to put constraints on their petrogenesis. Geochemically, the schist and gneisses imply K-poor mafic igneous precursors that were developed in a primitive juvenile oceanic environment. The migmatite pertains to stromatic type, whereas granitic magma (neosome) was injected in hornblende schist (paleosome) through cleavage planes. The syn-tectonic tonalite to quartz diorite are also low-K, non-mature calc-alkaline rocks and pertain to oceanic plagiogranite. They display a continuous fractionation evolution trend from quartz-diorite to tonalite and granodiorite.

First U–Pb SHRIMP zircon and 40Ar/39Ar ages of metarhyolites from the Afyon–Bolkardag Zone, SW Turkey: Implications for the rifting and closure of the Neo-Tethys

The Afyon–Bolkardag Zone (ABZ) is one of the most important metamorphic zones in Turkey. This zone consists of metasedimentary and metavolcanic rocks, which have undergone regional greenschist facies metamorphism. The metavolcanic rocks are dominated by rhyolite, dacite, and trachyandesite. In the Ilgin (Konya) area, the low grade metavolcanics were originally rhyolitic lavas and tuffs. We present new precise Sensitive High-Resolution Ion MicroProbe (SHRIMP) 206Pb–238U age analyses of zircons and 40Ar/39Ar age data of phengite from metarhyolites that enable a more precise reconstruction of the ABZ. SHRIMP analyses of euhedral or sub-euhedral magmatic zircons from two metarhyolite samples yield weighted means 230±2Ma and 229±2Ma. These ages are interpreted as formation age of rhyolitic volcanics, implying they were formed in the Late Triassic time (Carnian). We suggest that these Upper Triassic magmatic rocks were formed in an extensional tectonic setting on the northern active margin of Gondwana, which led to the development of the northern branch of the Neo-Tethys. 40Ar/39Ar dating of two phengite samples from metarhyolites gave plateau ages of between 63.7±0.1Ma and 62.6±0.1Ma. These dates are interpreted as the metamorphic age of these volcanic rocks, and as the age of metamorphism that affected the ABZ. Consequently, these earliest Tertiary ages also constrain the timing of the closure of the Neo-Tethys.

Neoproterozoic structural evolution of SE Sinai, Egypt: I. Re-investigation of the structures and deformation kinematics of the Um Zariq and Malhaq Formations, northern Wadi Kid area

The Wadi Kid area of the SE Sinai comprises Neoproterozoic arc metavolcanics and metasediments bordered to the N, W and S by post-kinematic granitoid plutons. The structure and tectonics of the Kid area remain controversial despite more than two decades of investigations. In this study the structural history and characteristics of the northern part of the Kid area (Um Zariq and Malhaq Formations) are described. The earliest tectonic event, D1, produced upright folds with probably originally WNW–ESE trending hinges. The F1 folding weakens to the north. The main deformation, D2, produced regionally penetrative shallow-dipping S2 foliations and semi-recumbent tight to isoclinal F2 folds. S2 cleavage is a product of pure flattening strain that involved pressure solution deformation mechanism, rather than a shear foliation that involved mylonitization as previously reported. Macroscopic F2 folds are dissected by top-to-SSE low-angle thrusts and mylonite zones that emplaced the high T Um Zariq Formation schists over the low-grade metavolcanics in the southern part of the Kid area. The SSE direction of thrusting is parallel to stretching lineations on the S2 cleavage planes. Later folds include D3 WNW–ESE to E–W trending upright gentle folds and D4 upright gentle N–S to NE–SW trending folds. In contrast to the extensional histories recently proposed this study finds the deformation events in the northern Kid area to comprise a compressional folding and thrusting history.

The Sha’it–Nugrus shear zone separating Central and South Eastern Deserts, Egypt: A post-arc collision low-angle normal ductile shear zone

The northerly dipping Sha’it–Nugrus shear zone (SNSZ) is the boundary separating the Central Eastern Desert from the South Eastern Desert of Egypt. The hangingwall of this shear zone is composed of low-grade metavolcanics and ophiolitic nappes of the Central Eastern Desert, while the footwall consists of South Eastern Desert high-grade metapsammitic gneisses (Migif-Hafafit gneissic complex). The SNSZ is about 700 m thick and represents the shear foliated lower parts of the hangingwall and upper parts of the footwall. A significant part of the SNSZ has been truncated by a later normal fault along Wadi Sha’it, however the SNSZ is well-preserved along Wadi Nugrus. Features of the SNSZ include shear-related schistosity (termed Ss), mylonite zones, sheared syn-kinematic granitoid intrusions, diverse metasomatism and metamorphic effects (higher T overprinting of hangingwall lithologies and retrogression of footwall lithologies). Shear-sense indicators clearly show top-to-N or NW displacement sense. SNSZ structures overprint arc collision related nappe structures (∼680 Ma) and are therefore post-arc collision. SNSZ syn-kinematic intrusives have been dated at ∼600 Ma. The SNSZ is deformed (regionally and locally folded and thrust dissected) during later NE–SW compressive tectonism. The SNSZ had an originally approximately E–W strike, low-angle N-dip and a normal shear sense, making this an example of a low-angle normal ductile shear (LANF) or detachment fault. The steep NE dip of Ss foliations and low-pitching slip lineations along Wadi Nugrus are due to NW–SE folding of the SNSZ, and do not indicate a sinistral strike-slip shear zone. The normal shear sense activity is responsible for juxtaposing the low-grade Central Eastern Desert lithologies against South Eastern Desert gneisses. A displacement of 15–30 km is estimated on the SNSZ, which is comparable to LANF displacements in the Basin and Range province of the western USA. Frictional resistance along this shear was probably reduced by high magmatic fluid pressure and hydrothermal fluid pressure. The vastness and diversity of the hydrothermal activity along this shear zone is a characteristic of other LANFs in the Eastern Desert, e.g. at Gabal El-Sibai, and may be Gabal Meatiq. The SNSZ formed during the Neoproterozoic extensional tectonic phase of Eastern Desert that began ∼600 Ma, and followed arc collision and NW-ward ejection of nappes.

The significance of gneissic rocks and synmagmatic extensional ductile shear zones of the Barud area for the tectonics of the North Eastern Desert, Egypt

The mainly tonalitic gneissic rocks, amphibolites, schists and rarer migmatites of the Barud area, at the southern margin of the Egyptian North Eastern Desert (NED) have previously been viewed as products of ultrametamorphism or granitization of pre-PanAfrican basement. The Qena–Safaga Line of approximately NW-striking steeply dipping faults was also regarded as marking the boundary between these NED rocks and the low grade metavolcanics and ophiolitic melange of the Central Eastern Desert (CED). Detailed investigation of the Barud area indicates that the amphibolites, schists and migmatites formed by shearing and medium grade metamorphism of similar arc metavolcanics to those of the CED in normal shear sense extensional ductile shear zones heated by numerous syn-kinematic dolerite, gabbro, diorite, granodiorite, tonalite and granite dykes. They are thus hot sheared equivalents of the CED metavolcanics and basic arc plutonites, accompanied by sheared mafic and felsic intrusive rocks, and are not deep-seated crystalline basement rocks. The shear zones are interpreted as having formed by arc-rifting, not necessarily reaching the stage of marginal basin formation. Arc-accretion structures and those produced by later orogen squeezing are also described. The somewhat gneissic Barud Tonalitic is found to be entirely magmatic. Following intrusion of the Barud Tonalite, and before or during Hammamat and Dokhan deposition, the NED experienced a rapid uplift relative to the CED (∼620–600 Ma) that was not achieved by thrusting along the Qena–Safaga Line.

Whole-rock Ar-Ar dating for low-grade metavolcanics within the Dabie orogen and its geological significance

The genetic relationship between lowgrade and ultrahigh-pressure (UHP) metamorphic units in the interior of the Dabie orogen has been controversial with respect to preservation of volcanic texture during continental subduction to mantle depths. In order to resolve this issue, whole-rock Ar-Ar dating was carried out for greenschist-facies metatuff that is in contact with UHP eclogite in Yuexi County, Anhui Province. One sample gave a plateau age of 784.4±2.0 Ma and an isochron age of 785.0±4.7 Ma, and the other sample a plateau age of 770.9±2.0 Ma and an isochron age of 769.5±3.1 Ma. It appears that the Ar isotopic system was not disturbed since the volcanic eruption at the middle Neoproterozoic. Because of the low closure temperature of Ar diffusion in volcanic rocks, retention of the Neoproterozoic ages indicates that the low-grade metavolcanics did not experience high-grade metamorphism up to eclogite-facies conditions during the Triassic subduction of continental crust. Furthermore, the Neoproterozoic Ar-Ar ages are also in agreement with protolith ages of UHP metaigneous rocks in the Dabie orogen. Therefore, the metatuff is interpreted as a tectonic relict that was scraped off during the Triassic subduction and thus was not subducted to mantle depths like the adjacent eclogite. This lends support to the model for crustal detachment between cover and basement during continental collision. A tectonic melange model is proposed to explain the occurrence of contrasting grades of metamorphic rock within the UHP metamorphic zone.

Magma emplacement and mafic–felsic magma hybridization: structural evidence from the Pan-African Negash pluton, Northern Ethiopia

The Negash pluton (50 km 2) consists of late Pan-African, high-K, calc-alkaline granitoids intruded into low-grade metavolcanics–metasediments. This almost circular massif consists of monzogranites, granodiorites, diorites–gabbrodiorites, and hybrid diorites. The anisotropy of magnetic susceptibility (AMS) method was used to determine internal structures of the pluton. The foliation trajectories are concentric and inward dipping. The lineation pattern displays an external zone characterised by horizontal concentrically oriented lineations and an internal zone with NW–SE oriented lineations. These petro-structural data clearly locate the feeder zone at the north-western tip of the pluton and indicate the subsequent expansion of the magmas towards the SE. The pluton is a result of in-situ assembly of four magma batches, which were forcefully injected into pre-existing foliated country rocks in relation to transpressional tectonic regime. Two types of mafic–felsic magma interactions are recognised: homogeneous and heterogeneous hybrid diorites at the north-western part, and mingled interfaces at the diorite–granodiorite contact zones mainly visible in the eastern and south-eastern parts. The in-situ mingling along diorite–granodiorite contacts was achieved at the level of emplacement during the injection of dioritic magma into the felsic magmas while the hybrid diorites are assumed to result from two-way conduit mixing and mingling during simultaneous rising of mafic and felsic magmas.

Crustal growth in West Africa at 2.1 Ga

Birimian (∼2.1 Ga) terranes in the West African craton are a mixture of highly metamorphosed volcanic, sedimentary and plutonic rocks and low grade metavolcanics and metasediments. The volcanic units contain thick, commonly pillowed, tholeiitic basalts overlain by pelagic sediments cherts and black shales; The sedimentary units are characterized by an abundance of clastic turbiditic sediments. Andesites and calc‐alkaline felsic volcanics occur at uppermost stratigraphic levels and as dykes. Field relationships between the volcanic and sedimentary units remain a matter of debate. Calc‐alkaline and local alkaline granites, which intruded in distinct pulses and occasionally are related to transcurrent tectonics, represent almost half of the Birimian terranes. New isotopic work on the highly metamorphosed units greatly improved the chronology for the Birimian crust. The age of the early Dabakalian event is precisely defined by a U‐Pb zircon age at 2186 ± 19 Ma, while Rb‐Sr and Sm‐Nd methods give ages of 2162 ± 19 Ma and 2141 ± 24 Ma, respectively. A Sm‐Nd garnet‐whole rock age of 2153 ± 13 Ma suggests that metamorphism culminated at about the same time. In contrast, the most precise zircon U‐Pb and Sm‐Nd data for the more widespread Birimian terranes (sensu stricto), from this study and from the literature, cluster between 2.12 and 2.07 Ga. The major evolution of the Birimian crust apparently lasted less than 50 Ma. Isotopic evidence indicates that Birimian granitoids contain a negligible component of Archean crust: εNd(2.1‐Ga) values are positive and similar to those of Birimian basalts, crustal residence times are shorter than 200 Ma, U‐Pb ages for detrital zircons from clastic sediments range from 2098 ±11 Ma to 2125 ± 17 Ma, while granite chemistry and Nd isotopic characteristics are unrelated. Only very locally in Guinea is there isotopic evidence of interaction between Birimian felsic magmas and the Archean rocks from the Man craton. In accord with Abouchami et al.'s (1990) suggestion that Birimian basalts represent oceanic plateaus, the present data argue that the protolith of much of the West African continent was created around 2.1 Ga in an environment remote from Archean crust. Intrusion of calc‐alkaline magmas into the tholeiitic units suggests that island arcs formed on top of the assumed oceanic plateaus which then collided with the Man Archean craton. Taking the Birimian formations from the Guyana shield into account, the minimum crustal growth rate at 2.1 Ga is about 1.6 km3/a, some ∼60% higher than the present growth rate. Birimian crust growth at 2.1 Ga is reminiscent of Archean processes but contrasts with 1.7 – 1.9 Ga crust formation in the North Atlantic continent which generally involved significantly more interaction with older continental crust. A comparison of the Birimian crustal growth rate with the average crustal growth rate over the Earth history implies that a large part of the Birimian crust has been recycled into the mantle or incorporated into younger orogenic segments. This apparent deficit in the crustal budget is even more dramatic for the Archean crust.

Chemistry, origin and tectonic significance of metamorphic rocks from the Puerto Rico Trench

Constraints on the evolution of the northeastern Caribbean plate boundary and the tectonic development of the Puerto Rico Trench have been derived from a study of rocks and sediments dredged from thirty-six localities within and surrounding the trench. Marble, calc-schist, mica-schist, greenschist, amphibolite, magnesian schist and serpentinite crop out below the trench—slope break (∼ 3500 m) along the steep inner-trench wall for over 400 km. At shallower depths, limestone, calcilithite, and minor amounts of chert and sedimentary rock were sampled. Potassium—argon ages of two of the metamorphic rocks are 63 ± 3 and 66 ± 5 m.y.B.P. Rocks dredged from Mona Canyon, directly behind the inner-trench wall, are primarily igneous rocks, volcanic breccia, limestone and calcilithite that have been only slightly metamorphosed. Middle Eocene to Miocene (or younger) shallow-water limestones are interbedded and overlie the volcanogenic rocks. The outer-trench wall (oceanic side) is composed of tholeiitic basalts, serpentinite and deep-sea sediments commonly recovered from the Atlantic oceanic crust. Mineral assemblages and phase relations in the metamorphic rocks are similar to those observed in many subaerially exposed mélanges believed to be subduction complexes. The conditions of metamorphism within the Puerto Rico Trench metamorphic complex are estimated to have been between 400 and 550°C and 3 to 7 kbar. Chemical analyses of the dredged samples suggest that: 1. (1) The metamorphic rocks from the inner-trench wall were primarily island-arc igneous rocks or sediments derived from them. 2. (2) Protoliths of the marbles and calc-schists were apparently biogenic carbonates and pelagic sediments with an arc-derived sedimentary component. 3. (3) Basalt and serpentinite from the outer-trench wall are geochemically similar to oceanic tholeiites (MORB) and ultramafics. 4. (4) Only two greenschists from the inner-trench wall have geochemical characteristics similar to MORB. 5. (5) Magnesian schists and serpentinite from deep along the inner wall have either been severely affected by metasomatism or represent metamorphosed ultramafic rocks from beneath the Greater Antilles. Our results indicate that, unlike a number of arc-trench systems in the Pacific, a great deal of arc-derived material has been accreted to the inner-trench wall and that the amount of oceanic crust incorporated into the subduction wedge was relatively minor. Directly west of the inner-trench wall, metamorphic rocks, marble and limestone identical to those dredged, crop out in a “blueschist belt” in northern Hispaniola. Behind the trench, to the south, low-grade metavolcanics in Puerto Rico appear to be continuous across Mona Passage to central Hispaniola. This belt parallels the higher-grade complex within the trench, and as such constitutes a paired metamorphic belt that extends for more than 500 km along the northern Caribbean plate boundary. The development of this belt is related to the inception of the Puerto Rico Trench in the middle to late Cretaceous due to the southerly subduction of the Atlantic plate beneath the Caribbean plate. Eastward movement of the Caribbean plate relative to the North American plate in the late Cretaceous and early Tertiary initiated transcurrent and tensional faulting which enhanced the uplift of the metamorphosed subduction wedge. Local tectonic readjustments which began in late Eocene time created the grabenlike trench observed at present and caused at least 3500 m of subsidence along the innertrench wall north of Puerto Rico, since Miocene time.

Structural/lithological provinces in a Saudi Arabian shield geotraverse

A N.E.-trending field traverse across the Arabian Shield (approximately 41 o 00°E/19 o 30°N to 45°30°E/23°30°N), combined with photo-interpretation, existing maps, and geophysical data, has resulted in a strip-map covering about 60. ° 100 km. This map reveals distinct structural/lithological provinces. From S.W. to N.E. these are: (1) Salibah Province. High-grade metasediments and metavolcanics, and low-grade metavolcanics; numerous granitoid plutons and some diorite-gabbro complexes.