Contribution to a geodynamic reconstruction of the Anti-Atlas (Morocco) during Pan-African times with the emphasis on inversion tectonics and metallogenic activity at the Precambrian–Cambrian transition (original) (raw)

Vertical movements at the fringe of the West African Craton: First zircon fission track datings from the Anti-Atlas Precambrian basement, Morocco

Comptes Rendus Geoscience, 2009

The post-Variscan uplift of the western Anti-Atlas Precambrian core is studied by zircon fission track (ZFT) analysis of ten samples of granites and schists from the Kerdous and Ifni inliers. All samples yield Carboniferous ZFT ages ranging from 358 ± 31 Ma to 319 ± 32 Ma, with nine dates younger than 338 ± 35 Ma. The weighted mean age calculated for these nine samples is 328 ± 30 Ma. These results compare with the available K–Ar datings of white mica and biotite from the same rocks or from the overlying Ediacaran-Cambrian low-grade metasediments. The fact that different systems with distinct closure temperatures yield similar ages suggests the occurrence of a short Carboniferous thermal event followed by rapid cooling. Consistent with the regional geological framework, the thermal event is assigned to the Variscan folding, being followed by rapid exhumation and cooling related to the post-folding erosion.Dix échantillons, prélevés dans les boutonnières de Kerdous et Ifni, ont été datés par la méthode des traces de fission sur zircon (ZFT), en vue d’étudier le soulèvement postvarisque du socle précambrien de l’Anti-Atlas occidental. Tous les échantillons ont fourni un âge Carbonifère, neuf résultats sur dix étant groupés entre 338 ± 35 Ma et 319,3 ± 32 Ma, avec une moyenne pondérée de 328 ± 30 Ma. Ces résultats sont comparables aux datations K–Ar disponibles sur mica blanc et biotite pour ces mêmes roches et pour les métasédiments d’âge Édiacarien-Cambrien qui les recouvrent. Le fait que ces différents systèmes donnent des résultats équivalents suggère un événement thermique court au Carbonifère, suivi par un refroidissement rapide. En accord avec les données géologiques régionales, l’événement thermique est attribué au plissement varisque. Celui-ci est suivi d’une exhumation rapide due à l’érosion.

Quartzite crests in Paleoproterozoic granites (Anti-Atlas, Morocco); a hint to Pan-African deformation of the West African Craton margin

Journal of African Earth Sciences, 2019

South of the Pan-African suture zone of the Anti-Atlas, the Agadir Melloul inlier exhibits several crests of nearly vertical conglomeratic quartzite beds inserted in Paleoproterozoic granites. These enigmatic rocky walls have been considered as Early Neoproterozoic platform sediments pinched within the 2.03 Ga-old granites during brittle faulting events at ca. 570 Ma. The data presented here contradict this interpretation. The quartzite crests correspond to parts of tight synclines pinched along brittle-ductile, strike-slip shear zones involving the granitic basement itself. This deformation occurred in low-grade greenschist facies conditions at T=260-280°C. The detrital zircon grains from the vertical quartzite beds are mostly grouped around 2 Ga, with the youngest grains at 1.8 Ga. Thus, the studied quartzites exhibit the same detrital zircon barcode as the other siliciclastic platform formations of the southwestern Anti-Atlas (Taghdout Group) whose age has been recently

Geochemical characteristics and U–Pb zircon LA-ICPMS ages of granitoids from the Pan-African Dahomeyide orogen, West Africa

The Dahomeyide orogen in southeastern Ghana and adjoining parts of Togo and Benin records the suture of West African craton (WAC) into northwest Gondwana. The suture zone is characterized by distinctive high-pressure mafic rocks that mark the collision of WAC with exotic blocks to the east. Granitoids to the east of the suture zone, which are postulated to be juvenile crust representing the arc terrane that formed during subduction and oceanic closure, were investigated. We compiled a map of granitoid gneisses in Ghana, Togo, and Benin which allows us to distinguish between two main rock types: (1) migmatitic gneisses with biotite as the dominant mafic mineral and (2) dioritic gneiss characterized by hornblende as the main mafic mineral and with occasional garnet. These rocks types are separated by a shear zone inferred to be a splay of Kandi shear zone, a segment of the Trans-Saharan shear zone that extends for $2500 km from the Sahara to the Gulf of Guinea. The granitoids are characterized by I-type signatures, display variable amounts of LREE enrichment, Eu depletion and Sr/Y ratios that are consistent with a subduction origin. The migmatitic gneiss unit, within a zone of highly strained granitoids (straight gneiss) that forms a sliver just east of the suture zone, yielded U-Pb zircon ages ranging from 615 to 589 Ma. This age range is similar to the metamorphic crystallization age of the suture zone rocks. The zircons analyzed display concentric zoning (in CL images) and are inferred to be magmatic. On the other hand, zircon separates from the dioritic gneiss, with grains also showing igneous zoning in CL images, yielded U-Pb zircon crystallization ages ranging from 2.19 to 2.14 Ga. These are the first U-Pb zircon ages to document Paleoproterozoic rocks in the postulated juvenile terrane, west of the Kandi fault zone. Nd model ages of rocks in the suture zone are also consistent with the existence of widespread involvement of older crust in the Pan-African collision zone, similar to the isotopic data from the correlative Medio Coreau domain of the northwest Borborema province in Brazil.

The evolution of the Pan-African granitoid rocks: geochemical evidences from SW Sinai massif, Egypt

Journal of African Earth Sciences (and the Middle East), 1992

Wadi Seih area is covered by "Old Granites", porphyritic granites, mafic-ultramafic cmnulates, rhyolite porphyries, Younger Granites and Post-Granite Dykes which represent > 200 Ma of Pan African orogeny. G-I "Old Granites" are given mainly by tonalites, monzodiorites and qz-monzodiorites. They are derived from calc-alkaline metaluminous to pe~aluminous I-typo magma and were emplaced under prevailing compressionai regime which are associated with subduction and arc granitic rocks formation. Porphyritic granites are given by monzogrenites and were emplacod due to compressional forces during the late orogenic stage. "Younger Granites" are distinguished into two types: G-U and G-IT[ granites. They have distinct pellologic, chemical and tectonic setting differences which supporting two independent magma sources. G-H granites are given by monzogranites and syenogranites and were derived from magma rich in Ba and Sr and depleted in Nb, Ce, Zr, Y, Rb and alkalies and were intruded under compressional regime prevailing in the granitic arc environment, at late-to post-orogenic stage. Whereas GIll granites are given by syenogranites and were derived from magma rich in Nb, Ce, Zr, Y, Rb and alkalies and were intruded under extensional forces prevailing within the plate frame which correspond to anorogenic regime. Rhyolite porphyries axe enriched in alkalies and are related to the porphyritic and G-II granites. They were extruded under an environment ~ansifional between volcanic arc and within plate regime.

U–Pb zircon geochronology of the Paleoproterozoic Tagragra de Tata inlier and its Neoproterozoic cover, western Anti-Atlas, Morocco

Precambrian Research, 2002

New U-Pb zircon data obtained by sensitive high resolution ion microprobe (SHRIMP) from the Tagragra de Tata inlier in the western Anti-Atlas, Morocco establish Paleoproterozoic ages for the basement schists, granites, and metadolerites, and a Neoproterozoic age for an ignimbrite of the Ouarzazate Series in the cover sequence. The age of interbedded felsic metatuff in the metasedimentary and metavolcanic sequence of the basement schists is 2072 9 8 Ma. This date represents: (1) the first reliable age from the metasedimentary and metavolcanic sequence; (2) the oldest reliable age for the basement of the Anti-Atlas; (3) the first date on the timing of deposition of the sediments on the northern edge of the Paleoproterozoic West African craton; (4) a lower age limit on deformation during the Eburnean orogeny; and (5) the first date obtained from the non-granitic Paleoproterozoic basement of Morocco. Ages of 20469 7 Ma (Targant granite) and 2041 9 6 Ma (Oudad granite) support earlier interpretations of a Paleoproterozoic Eburnean igneous event in the Anti-Atlas. The granites postdate the Eburnean D1 deformation event in the Paleoproterozoic schist sequence, and place a 2046 Ma limit on short-lived Eburnean deformation in the area. Cross-cutting metadolerite is 2040 9 6 Ma; this is the first date from a metadolerite in the western Anti-Atlas. All of the dolerites in the area postdate emplacement of the two granites and the new age constrains the onset of late-or post-Eburnean extension. Ignimbrite of the Ouarzazate Series, immediately above the Paleoproterozoic basement is 5659 7 Ma. This Neoproterozoic age agrees with ages of similar volcanic rocks elsewhere from the Ouarzazate Series. The date also agrees with the ages of associated hypabyssal intrusions, and marks the second and final stage of Pan-African orogenic activity in the western Anti-Atlas.

Timing of Archean crust formation and cratonization in the Awsard-Tichla zone of the NW Reguibat Rise, West African Craton: A SHRIMP, Nd-Sr isotopes, and geochemical reconnaissance study

Precambrian Research, 2014

The Awsard-Tichla zone, in the Moroccan sector of the Reguibat Rise, comprises three lithodemic units, the TTG Aghaylas Suite, the Tichla greenstone belt, and the intrusive feldspathoidal syenites of the Awsard-Lechuaf group. The Aghaylas suite consists of Mesoarchean tonalites and trondhjemites with minor granodiorites and granites that are locally migmatised. These rocks are notably depleted in HREE, Y, Nb, Ta, and U, with elevated Th/U and K/Rb, 87 Sr/ 86 Sr (t) from 0.7003 to 0.7030, Nd (t) from +2 to +5, and Nd model ages in the range T CR = 3.03-2.92 Ga. Eight TTG gneisses yielded SHRIMP U-Pb zircon crystallisation ages between 3.04 Ga and 2.92 Ga. One TTG leucocratic gneiss from the neighbouring Oulad Dlim sector has a T CR = 3.09 and a zircon crystallisation age of 2.94 Ga but also contains a 3.11 Ga population of pre-magmatic zircons. The Tichla greenstone belt, on the other hand, was formed between 3.03 Ga and 3.01 Ga, though Nd model ages suggest the contribution of an older crust. Field evidence and the chemical and isotopic compositions of the TTGs are consistent with crustal recycling of slightly younger juvenile TTG. After 2.92 Ga there is no trace of magmatic activity until the intrusion of the Awsard feldspathoidal (kalsilite and nepheline) syenites at 2.46 Ga. This reveals that, for reasons that are not yet well understood, the Archean crust in this sector of the Reguibat Rise stabilised long before than any other surrounding cratonic area. Furthermore, the crust stayed stable until the Archean-Proterozoic transition that, here, was marked by a peculiar high-K silica-subsaturated magmatism more extreme in composition than in any other known Archean terrane.

U–Pb baddeleyite and zircon ages of 2040Ma, 1650Ma and 885Ma on dolerites in the West African Craton (Anti-Atlas inliers): Possible links to break-up of Precambrian supercontinents

Lithos, 2013

Precambrian inliers of the Anti-Atlas belt in the southern part of Morocco contain numerous dolerite dyke and sill swarms which were previously poorly dated. Four dykes and two sills dated by the U-Pb TIMS method on baddeleyite and zircon provide the first steps toward a magmatic 'barcode' for the West African Craton (WAC) and constraints on the timing of breakup of the WAC from Precambrian supercontinents. A 2040± 2 Ma (U-Pb zircon) age for a WNW dyke in the Zenaga inlier, matches the published age of a dyke in the Tagragra of Tata inlier, and also those of Eburnean granites observed in several inliers, which are collectively interpreted to represent ca. 2040 Ma bimodal magmatism due to a mantle plume. Based on the presence of matching 2040 Ma ages, the WAC may have been connected to the North Atlantic Craton at the initial stage of fragmentation of a late Archean continent. U-Pb baddeleyite ages of 1656± 9 Ma and ca. 1655 Ma from sills in the Zenaga inlier and 1654± 16 Ma from a NE-trending dyke in the Agadir Melloul inlier are similar to intraplate magmatic ages in eastern and northern Baltica, and support the SAMBA reconstruction (part of the Nuna supercontinent) of the WAC adjacent to Baltica. Approximate U-Pb ages of 885 Ma for two dykes in the Iguerda-Taïfast and Zenaga inliers date a NE trending swarm (named herein the Iguerda-Taïfast swarm) which is connected to the initial breakup of the supercontinent Rodinia, and a specific link with the São Francisco/Congo and North China craton is considered.

Vertical movements at the fringe of the West at the fringe of the West African Craton: First Zircon Fission-track datings from the Anti-Atlas PreCambrian basement, Morcco

The post-Variscan uplift of the western Anti-Atlas Precambrian core is studied by zircon fission track (ZFT) analysis of ten samples of granites and schists from the Kerdous and Ifni inliers. All samples yield Carboniferous ZFT ages ranging from 358 Ma to 319 Ma, with nine dates younger than 338 Ma. The weighted mean age calculated for these nine samples is 328 Ma. These results compare with the available K–Ar datings of white mica and biotite from the same rocks or from the overlying Ediacaran-Cambrian low-grade metasediments. The fact that different systems with distinct closure temperatures yield similar ages suggests the occurrence of a short Carboniferous thermal event followed by rapid cooling. Consistent with the regional geological framework, the thermal event is assigned to the Variscan folding, being followed by rapid exhumation and cooling related to the post-folding erosion.