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11.
Martin Thorp Peter Glanville Stephen Stokes Richard Bailey 《Comptes Rendus Geoscience》2002,334(12):903-908
This paper presents the first results of a 14C and optical luminescence dating project on extensive alluvial silt formations in the southern Anti Atlas Mountains in southern Morocco. The silts comprise three members: basal silts, an inter-silt fluvial gravel member and a top silt member and are incised by the rivers. Top silt dates are: 11 340–11 940, 12 670–13 130 cal. yr BP, and 10.6±0.7 ka (OSL). OSL dates for the inter-silt gravels are 29.6±2.0 ka and for the bottom silts is 45.0±3.7 ka. The sediments correlate with Soltanian stratigraphies known elsewhere in Morocco. To cite this article: M. Thorp et al., C. R. Geoscience 334 (2002) 903–908. 相似文献
12.
The Sidi Flah and Ougnat inliers are located in the eastern Anti-Atlas antiform between the Anti-Atlas Major Fault (AAMF) and South Atlas Fault (SAF). They consist of many granitoid intrusions emplaced into Neoproterozoic metasedimentary rocks and surmounted by upper Neoproterozoic A-type granites. The Sidi Flah (Saghro) and Ougnat granitoids are part of the Neoproterozoic magmatic activity related to northwards subduction of an oceanic plate beneath the Saghro continental margin. They are post-orogenic I- and S-type granitoids related to the ending of the compressional deformation in this Pan-African belt. A petrographic, geochemical and zircon typology study leads us to subdivide these rocks into three magmatic groups: (1) a medium- to high-K calc-alkaline group formed by quartz diorites and amphibole granodiorites is found in both Sidi Flah and Ougnat inliers; (2) a high-K calc-alkaline group is present in Sidi Flah. These two groups have a (deeper and) hybrid mantle-crust origin; (3) a peraluminous group in Ougnat is linked to the post-collisional setting and has a shallow crustal source. On a primitive mantle-normalized trace-element diagram, almost all of these rocks show a significant Nb depletion relative to K and La, which is typical of the calc-alkaline magmatism from the subduction-zone environment. Absence of structural marks of thrusting upon the West African craton (WAC) of this arc system and the ophiolitic suite in Bou-Azzer, and the presence of Imiter muscovite-bearing granite as part of Pan-African belt do not support the localization of northern limit of WAC at the level of SAF. 相似文献
13.
Development of igneous layering during growth of pluton: The Tarçouate Laccolith (Morocco) 总被引：1，自引：0，他引：1
We discuss the significance of igneous layering with respect to pluton growth processes. The case study is the Tarçouate Laccolith (Morocco), whose core consists of modally layered hornblende granodiorites with high amount of monzodioritic enclaves, contrasting with peripheral, non-layered biotite granodiorites with low amount of enclaves. Rhythmic layering, with modal grading, cross-stratification and trough layering is associated with monzodioritic layers and wraps around mafic enclaves. Its steep dips ≥ 45° result from tilting that occurred above solidus conditions, as indicated by sub-vertical and synmagmatic granite, aplite and monzodiorite dykes cutting across the layering.The systematic association of igneous layering with mafic enclaves in calc-alkaline plutons suggests that layering originates from recurrent injection of mafic magma. Viscosity calculations suggest that the physicochemical properties of magma alone cannot account for the presence of layering in the central hornblende granodiorite and its coeval absence in the peripheral biotite granodiorite of the Tarçouate Laccolith. Intermittent pulses of hot mafic magma into crystallizing granodiorite likely produced thermal perturbations able to trigger local convection, formation of mafic enclaves and development of igneous layering through protracted crystallization. 相似文献
14.
Mustapha El Ghorfi Thomas Oberthür Frank Melcher Volker Lüders Abdelmajid El Boukhari Lhou Maacha Rachid Ziadi Hssain Baoutoul 《Mineralium Deposita》2006,41(6):549-564
The structurally controlled Au–Pd mineralization at Bleïda Far West occurs in a volcano-sedimentary rock sequence in altered amphibolites and chlorite schists of the Neoproterozoic Bou Azzer–El Graara inlier. The Au–Pd mineralization is virtually sulfide-free; instead, gold is associated with hematite, barite, quartz, and calcite. The gold grains are silver- and palladium-bearing (up to 19 wt% Ag and 6.3 wt% Pd) and are intergrown with a distinct suite of mainly Pd-dominated platinum group minerals, namely mertieite-I/isomertieite, merenskyite, keithconnite, kotulskite, palladseite, and sperrylite, defining a Au–Pd–As–Sb–Se–Te chemical signature. Stable isotope and fluid inclusion studies indicate a wide range of fluid compositions with a prominent saline component. In conjunction with the mineral association, oxidizing fluids are indicated, and Au and PGE transport and deposition likely took place by chloride complexes in the epithermal range, at elevated f O2 and/or low pH. It is still speculative whether the mineralization is late Pan-African (～600–550 Ma) in age, or connected with the Variscan orogeny (～330–300 Ma), or related to some other hydrothermal event. Common to all Au–Pd mineralizations worldwide (Brazil, Australia, UK), including Bleïda Far West, is their formation in the epithermal (<300°C) range; deposition mainly in brittle structures; sulfide-poor mineral assemblages comprising hematite, sulfate minerals, and selenides; and metal transport by, and deposition from, oxidized, chloride-rich fluids. These deposits are further characterized by noble metal abundances in the order Au>Pd>Pt and the chemical signature Au–Pd–Se–Te (±As, Sb, Bi). As such, the Au–Pd association represents a discrete style of gold mineralization distinct from other classes of gold deposits. 相似文献
15.
El Hassan Abia Hassan Nachit Christian Marignac Abderrahmane Ibhi Saïd Ait Saadi 《Journal of African Earth Sciences》2003,36(4):251-271
The Bou Madine ore deposit is located SW of Jbel Ougnat, the easternmost inlier of the Anti-Atlas Pan-African belt in Morocco. The host rocks are high-K calc-alkaline volcanic rocks, that are part of the Neoproterozoic Tamerzaga-Timrachine Formation (TTF, lower PIII). The TTF consists of ignimbrites of rhyolitic to dacitic compositions, andesite flows and hypovolcanic bodies (andesite dykes and rhyolite chonoliths) emplaced along N160°E tension gashes associated with a regional N30°E sinistral fault system. The mineralization is related to a high enthalpy geothermal system, eventually evolving into a low temperature epithermal system. A regional propylitisation (T around 260 °C) overprinted the TTF rocks prior to the emplacement of the mineralization. There were two main hydrothermal stages. During the first stage, massive veins with pyrite, arsenopyrite and minor pyrrhotite and cassiterite were formed. The veins were emplaced along N160°E-trending en echelon joints related to N120°E dextral arrays. A quartz-sericite-pyrite alteration overprinted the propylites around the veins (“bleached haloes”), at temperatures up to 300–310 °C. The second stage of mineralization was coeval with dextral re-activation of the N160°E veins, in relation with a NE-ward shift of the shortening direction. First, polymetallic sulphides (sphalerite, chalcopyrite, stannite, galena) were deposited at temperatures 260 °C. Younger quartz veinlets contain arsenopyrite and minor micrometre-size sulphides and sulpho-salts, hosting the precious metals. This was the low temperature epithermal stage (≈150 °C), in relation with invading meteoric water. 相似文献
16.
Dominique Gasquet Gilles Levresse Alain Cheilletz Moulay Rachid Azizi-Samir Abdellah Mouttaqi 《Precambrian Research》2005,140(3-4):157-182
New geochronological analyses (U–Pb SIMS zircon ages) have yielded ages of 552 ± 5 Ma for the Bou Madine rhyolitic dome (Ougnat, eastern Anti-Atlas), 543 ± 9 Ma for the Tachkakacht rhyolitic dyke (Saghro–Imiter, eastern Anti-Atlas), and 531 ± 5 Ma for the Aghbar trachytic sill (Bou Azzer, central Anti-Atlas). Inherited zircon cores from the Aghbar trachytic sill and from the Bou Madine rhyolitic dome have been shown to be of Statherian age (ca. 1600–1800 Ma) and Palæoproterozoic (>2100 Ma) age, respectively, suggesting that a significantly older protolith underlies the Pan-African rocks in the Central and Eastern Anti-Atlas. Granodiorites and rhyolites from the Saghro–Imiter area have similar low 87Sr/86Sr (0.702–0.706) and 143Nd/144Nd (0.5116–0.5119) initial ratios, suggesting a mixture of mantle and lower crust sources. This can also be inferred from the low 187Os/188Os ratios obtained on pyrite crystals from the rhyolites.A recently published lithostratigraphic framework has been combined with these new geochemical and geochronological data, and those from the literature to produce a new reconstruction of the complex orogenic front that developed at the northern edge of the Eburnian West African craton during Pan-African times. Three Neoproterozoic magmatic series can be distinguished in the Anti-Atlas belt, i.e., high-K calc-alkaline granites, high-K calc-alkaline to shoshonitic rhyolites and andesites, and alkaline-shoshonitic trachytes and syenites, which have been dated at 595–570, 570–545 and 530 Ma, respectively.The accretion of the Pan-African Anti-Atlas belt to the West African super continent (WAC) was a four-stage event, involving extension, subduction, moderate collision and extension. The calc-alkaline magmatism of the subduction stage was associated with large-scale base metal and gold mineralisation. Metallogenic activity was greatest during the final extensional stage, at the Precambrian–Cambrian boundary. It is characterised by world-class precious metal deposits, base–metal porphyry and SEDEX-type occurrences. 相似文献
17.
The Tarçouate pluton (Anti-Atlas, Morocco) is an inversely zoned laccolith emplaced 583 Ma ago into low-grade metasediments, with the following succession: leucocratic granites, biotite–granodiorites (±monzodiorites), hornblende–granodiorites (±monzodiorites) and monzodiorites syn-plutonic dykes. These rocks form two distinct, chemically coherent, units:
(1) A main unit consists of layered (57
(2) A subordinate unit consists of leucocratic, distinctly peraluminous, muscovite-bearing granites (72
Magma–host interactions are closely related to differentiation and occurred at different levels, but mainly before emplacement. Field relationships and petrogenetic modelling show that the bt–granodiorites formed at levels deeper than the level of emplacement, by fractional crystallisation (0.65 These data preclude any significant material transfer process for the emplacement of the Tarçouate pluton, but rather suggest assembly of successive pulses of variably differentiated crystal-poor magmas. These shallow level granitic plutons can be considered as an end-member of magma emplacement with minimum interactions with the country rocks. 相似文献
18.
Abdelilah Fekkak André Pouclet Hassane Ouguir Hassane Ouazzani Lakhlifi Badra Dominique Gasquet 《Geodinamica Acta》2013,26(6):373-385
Abstract The Early Cryogenian groups of Sidi Flah, Kelaat Mgouna and Boumalne (Saghro, Anti-Atlas, Morocco) are constituted by turbiditic deposits and interbedded lavas, accumulated in tectonic basins. At Sidi Flah, volcanics are transitional showing initial rift tholeiites (IRT) fingerprint and alkali basalts of oceanic island basalt (OIB) compositions. At Kelaat Mgouna, volcanics consist of low-Nb continental tholeiites. At Boumalne, basalts are of IRT composition. The volcanic and sedimentary formations belong to a nascent rift caused by thermal doming along a SW-NE axis, the Saghro rift. The continental break-up occurred in the early Neopro- terozoic and during the Rodinia supercontinent dislocation, within a continent called “Ibero-saharian Craton” which was in front of the West-African Craton located near the South Pole. The Saghro rift is contemporaneous to the opening of an oceanic domain represented by Central Anti-Atlas ophiolites and related to the extension of the Brazialiano Ocean. © 2001 Éditions scientifiques et médicales Elsevier SAS 相似文献
19.
In the Moroccan western Anti-Atlas, the combined extensive tectonic events with a long-term sea-level rise is the main factor on building vertical stacking transgressive–regressive sequences. In the Ait Abdallah-Boussafene axis, the subsidence processes, relayed by a brutal platform tilting generated an elongated NE–SW graben. This is an evidence of the persistence of the Anti-Atlasic rifting process during the last part of the Lower-Cambrian succession. 相似文献
20.
Johann Tuduri Alain Chauvet Aomar Ennaciri Luc Barbanson 《Comptes Rendus Geoscience》2006,338(4):253-261
Based on a combined geometrical and mineralogical analysis, a three-stage model of formation of the mineralized veins of the giant Imiter silver deposit (Anti-Atlas, Morocco) is herein proposed. A first episode is characterized by the development of quartz, pink dolomite and Ag-rich minerals veins formed during a dextral transpressive event. The second episode is associated with a normal left-lateral motion that re-opens previous structures, filled by pink dolomite gangue. Alteration stages contribute to a local Ag enrichment. To cite this article: J. Tuduri et al., C. R. Geoscience 338 (2005). 相似文献