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1.
Two mineralogically different rare metal granites located in two distinct terranes from the Tuareg area are compared: the Tin-Amzi granite in the north of the Laouni Terrane and the Ebelekan granite in the Assodé–Issalane Terrane.The Tin-Amzi granite is enclosed within Eburnean granulitic gneisses, and consists of albite, quartz, protolithionite, K-feldspar and topaz granite (PG). The accessory minerals include columbite tantalite, U- and Hf-rich zircon, Th-uraninite, wolframoixiolite and wolframite. This facies is characterised by a mineralogical evolution from the bottom to the top underlined by a strong resorption of K-feldspar and albite and the crystalliK-feldspar of more abundant topaz and protolithionite II which is further altered in muscovite and Mn-siderite. It is underlain by an albite, K-feldspar, F-rich topaz, quartz and muscovite granite (MG), with W–Nb–Ta oxides, wolframite, Nb-rutile, zircon and scarce uranothorite as accessories.The Ebelekan granite intrudes into a coarse-grained biotite granite enclosed within upper amphibolite-facies metasediments. It comprises a zinnwaldite, albite, topaz porphyritic granite (ZG) with “snow ball” quartz and K-feldspar. The accessories are zircon, monazite, uranothorite, Ta bearing cassiterite, columbite tantalite and wodginite. It is capped by a banded aplite-pegmatite (AP).The geochemistry of Tin-Amzi and Ebelekan granites is nearly comparable. Both are peraluminous (A/CNK=1.10–1.29; ASI=1.17–1.31), sodolithic and fluorine rich with high SiO2, Al2O3, Na2O+K2O, Rb, Ga, Li, Ta, Nb, Sn and low FeO, MgO, TiO2, Ba, Sr, Y, Zr and REE contents. These rare metal Ta bearing granites belong to the P-poor subclass, relating to their P2O5 content ( 0.03–0.15 wt.%). Nevertheless, they are distinguished by their concentration of W, Sn and Ta. The Tin-Amzi granite is W–Ta bearing with high W/Sn ratio whereas the Ebelekan granite is Ta–Sn bearing with insignificant W content.At Tin-Amzi the W–Nb–Ta minerals define a sequence formed by W-columbite tantalite followed by wolframoixiolite and finally wolframite showing the effect of hydrothermal overprinting with an extreme W enrichment of the fluids. At Ebelekan, the Sn–Nb–Ta oxides follow a Mn sequence: manganocolumbite→manganotantalite→wodginite+titanowodginite→cassiterite that represents a trend of primary crystallisation resulting from progressive substitution Fe→Mn and Nb→Ta during the magmatic fractionation. 相似文献
2.
Renaud Caby 《Journal of African Earth Sciences》2003,37(3-4):133
After a review of the rock sequences and evolution of the eastern and central terranes of Hoggar, this paper focusses on the Neoproterozoic subduction-related evolution and collision stages in the central–western part of the Tuareg shield. Rock sequences are described and compared with their counterparts identified in the western and the eastern terranes exposed in Hoggar and northern Mali. The Pharusian terrane that is described in detail, is floored in the east by the Iskel basement, a Mesoproterozoic arc-type terrane cratonized around 840 Ma and in the southeast by Late Paleoproterozoic rock sequences (1.85–1.75 Ga) similar to those from northwestern Hoggar. Unconformable Late Neoproterozoic volcanosedimentary formations that mainly encompass volcanic greywackes were deposited in troughs adjacent to subduction-related andesitic volcanic ridges during the c. 690–650 Ma period. Abundant arc-related pre-collisional calc-alkaline batholiths (650–635 Ma) intruded the volcanic and volcaniclastic units at rather shallow crustal levels prior to collisional processes. The main E–W shortening in the Pharusian arc-type crust occurred through several stages of transpression and produced overall greenschist facies regional metamorphism and upright folding, thus precluding significant crustal thickening. It was accompanied by the shallow emplacement of calc-alkaline batholiths and plutons. Ages of syn-collisional granitoids range from 620 Ma in the western terranes, to 580 Ma in the Pharusian terrane, thus indicating a severe diachronism. After infill of molassic basins unconformable above the Pan-African greenschists, renewed dextral transpression took place in longitudinal domains such as the Adrar fault. The lithology, volcanic and plutonic suites, deep greenschist facies metamorphism, structures and kinematics from the Adrar fault molassic belt previously considered as Neoproterozoic are described in detail. The younger late-kinematic plutons emplaced in the Pharusian terrane at 523 Ma [Lithos 45 (1998) 245] relate to a Cambrian tectonic pulse that post-dates molasse deposition. The new geodynamic scenario presented considers several paleosubductions. The major east-dipping subduction, corresponding to the closure of a large Pan-African oceanic domain in the west (680–620 Ma) post-dates an older west-dipping “Pharusian” subduction (690–650 Ma?) to the east of the eastern Pharusian terrane. Such a diachronism is suggested by the 690 Ma old eclogites of the western part of the LATEA terrane of central Hoggar [J. African Earth Sci. this volume (2003)] that are nearly synchronous with the building up of the Pharusian terrane, thus suggesting that the 4°50′ lithospheric fault represents a reactivated cryptic suture. 相似文献
3.
Khadidja Ouzegane Jean-Robert Kienast Abderrahmane Bendaoud Amar Drareni 《Journal of African Earth Sciences》2003,37(3-4):207
The In Ouzzal terrane (Western Hoggar) is an example of Archaean crust remobilized during a very-high-temperature metamorphism related to the Paleoproterozoic orogeny (2 Ga). Pan-African events (≈0.6 Ga) are localized and generally of low intensity. The In Ouzzal terrane is composed of two Archaean units, a lower crustal unit made up essentially of enderbites and charnockites, and a supracrustal unit of quartzites, banded iron formations, marbles, Al–Mg and Al–Fe granulites commonly associated with mafic (metanorites and garnet pyroxenites) and ultramafic (pyroxenites, lherzolites and harzburgites) lenses. Cordierite-bearing monzogranitic gneisses and anorthosites occur also in this unit. The continental crust represented by the granulitic unit of In Ouzzal was formed during various orogenic reworking events spread between 3200 and 2000 Ma. The formation of a continental crust made up of tonalites and trondhjemites took place between 3200 and 2700 Ma. Towards 2650 Ma, extension-related alkali-granites were emplaced. The deposition of the metasedimentary protoliths between 2700 and 2650 Ma, was coeval with rifting. The metasedimentary rocks such as quartzites and Al–Mg pelites anomalously rich in Cr and Ni, are interpreted as a mixture between an immature component resulting from the erosion and hydrothermal alteration of mafic to ultramafic materials, and a granitic mature component. The youngest Archaean igneous event at 2500 Ma includes calc-alkaline granites resulting from partial melting of a predominantly tonalitic continental crust. These granites were subsequently converted into charnockitic orthogneisses. This indicates crustal thickening or heating, and probably late Archaean high-grade metamorphism coeval with the development of domes and basins. The Paleoproterozoic deformation consists essentially of a re-activation of the pre-existing Archaean structures. The structural features observed at the base of the crust argue in favour of deformation under granulite-facies. These features are compatible with homogeneous horizontal shortening of overall NW–SE trend that accentuated the vertical stretching and flattening of old structures in the form of basins and domes. This shortening was accommodated by horizontal displacements along transpressive shear corridors. Reactional textures and the development of parageneses during the Paleoproterozoic suggest a clockwise P–T path characterized by prograde evolution at high pressures (800–1050 °C at 10–11 kbar), leading to the appearance of exceptional parageneses with corundum–quartz, sapphirine–quartz and sapphirine–spinel–quartz. This was followed by an isothermal decompression (9–5 kbar). Despite the high temperatures attained, the dehydrated continental crust did not undergo any significant partial melting. The P–T path followed by the granulites is compatible with a continental collision, followed by delamination of the lithosphere and uprise of the asthenosphere. During exhumation of this chain, the shear zones controlled the emplacement of carbonatites associated with fenites. 相似文献
4.
The In Ouzzal granulitic unit (IOGU) consists predominantly of felsic orthogneisses most of which correspond to granitoids emplaced during the Archaean, plus metasediments, including olivine-spinel marbles, of late Archaean age. All units were metamorphosed at granulite facies during the Eburnean (2 Ga). The stable isotope signature of the marbles (δ13C=–0.8 to –4.2‰/PDB; δ18O = 7.9 to 18.9‰/SMOW) does not record a massive streaming of C-bearing fluids during metamorphism. Most of the isotopic variation in the marbles is explained in terms of pregranulitic features. Metasomatic transformation of granulites into layered potassic syenitic rocks and emplacement of carbonate veins and breccias occurred during retrogressive granulite facies conditions. The chemistry of these rocks is comparable with that of fenites and carbonatites with high contents of (L)REEs, Th, U, F, C, Ba and Sr but, with respect to these elements, a relative depletion in Nb, Ta, Hf, Zr and Ti. The isotopic compositions of Nd (?Nd(T)=–6.3 to –9.9), of Sr (87Sr/86Sr(T)= 0.7093–0.7104), and the O isotopic composition of metasomatic clinopyroxene (δ18O = 6.9 to 8‰), all indicate that the fluid had a strong crustal imprint. On the basis of the C isotope ratios (δ13C =–3.5 to –9.7‰), the fluid responsible for the crystallization of carbonates and metasomatic alteration is thought to be derived from the mantle, presumably through degassing of mantle-derived magmas at depth. Intense interaction with the crust during the upward flow of the fluid may explain its chemical and isotopic signatures. The zones of metasomatic alteration in the In Ouzzal granulites may be the deep-seated equivalents of the zones of channelled circulation of carbonated fluids described at shallower levels in the crust. 相似文献
5.
Gedrite–garnet–sillimanite-bearing granulites from Amessmessa area, south In Ouzzal, Hoggar, Algeria
Some granulites from the Amessmessa area (south In Ouzzal unit, Hoggar) contain the peak assemblage gedrite+garnet+sillimanite+quartz that was used to estimate the P–T conditions of metamorphism. The rocks developed symplectites and corona textures by the breakdown of the primary paragenesis to orthopyroxene, cordierite and spinel. The successive parageneses formed in separate microdomains according to a clockwise P–T path. Geothermometry, geobarometry and phase diagram calculations indicate that the textures formed by decompression and cooling from 7–9 kbar and 850–900°C to 3.5–4.5 kbar and 700–800°C. This P–T evolution is consistent with low to medium aH2O, between 0.4 and 0.7, and is similar to the metamorphic conditions deduced in Al–Mg granulites from the north of In Ouzzal. 相似文献
6.
7.
The Al–Mg-rich granulites from the In Ouzzal craton, Algeria, show a great diversity of mineral reactions which correspond to continuous equilibria as predicted by phase relationships in the FeO–MgO–Al2O3–SiO2 system. The sequence of mineral reactions can be subdivided into three distinct stages: (1) a high-P stage characterized by the growth of coarse mineral assemblages involving sapphirine and the disappearance of early corundum and spinel-bearing assemblages; (2) a high-T stage characterized by the development of Sa–Qz-bearing assemblages; and (3) a later stage, in which garnet-bearing assemblages are replaced by more or less fine symplectites involving cordierite. During the course of early mineral reactions, the distribution coefficient, Kd, between the various ferromagnesian phases decreased significantly whereas Al2O3 in pyroxene increased concomitantly. These observations, when combined with topological constraints, clearly indicate that the high-P stage 1 was accompanied by a significant rise in temperature (estimated at 150 ± 50° C) under near isobaric conditions, in agreement with the reaction textures. By stage 2, pressure and temperature were extreme as evidenced by the low Kd value between orthopyroxene and garnet (Kd= 2.06–1.99), the high alumina content in pyroxene (up to 11.8%) and the high magnesium content in garnet [100 Mg/(Mg + Fe) = 60.6]. Mineral thermometry based on Fe–Mg exchange between garnet and pyroxene and on Al-solubility in pyroxene gives temperatures close to 970 ± 70° C at 10 ± 1.5 kbar. These results are in agreement with the development of Sa–Qz assemblages on a local scale. Late mineral reactions have been produced during a decompression stage from about 9 to 6 kbar. Except for local re-equilibration of Mg and Fe at grain boundaries, there is no evidence for further reactions below 700° C. We interpreted the whole set of mineral reactions as due to changes in pressure and temperature during a tectonic episode located at c. 2 Ga. Because of the lack of evidence for further uplift after the thermal relaxation which occurred at c. 6 kbar, it is possible however that the exhumation of this granulitic terrane occurred in a later tectonic event unrelated to its formation. 相似文献
8.
In the Laouni region (Central Hoggar, Algeria), retrogression of high-grade orthopyroxene–cordierite-bearing rocks led to the crystallization of orthoamphibole and garnet, and at a later stage of chlorite, from the original paragenesis. Calculated phase diagrams show that this retrogression occurred at about 3 kbar with the simplest model involving hydration at 650–700° C and at around 500° C, with the rocks experiencing a H2 O less than 1, except possibly in the last stages of chlorite crystallization. As the other rock types occurring in the same area as the orthopyroxene–cordierite rocks display similar features, it is concluded that regional hydration occurred, presumably related to the release of fluids during the crystallization of the Pan-African granitic and mafic magmas that are widespread in the Laouni area. 相似文献
9.
Hamou Djellit Hervé Bellon Aziouz Ouabadi Mohamed E.M. Derder Bernard Henry Boualem Bayou Allaoua Khaldi Kamal Baziz Mounir K. Merahi 《Comptes Rendus Geoscience》2006,338(9):624-631
Palaeozoic formations of the Tassilis Oua-n-Ahaggar (southeastern Hoggar) include magmatic rocks in the Tin Serririne syncline. Slight contact metamorphism of the overlying bed and studies of anisotropy of magnetic susceptibility of these rocks show that the latter correspond to sills and NW–SE or north–south dykes. 40K/40Ar dating of separated feldspars and whole rock for one sample and of whole rock for two other samples give a mean age of (at the 2-σ level), thus corresponding to a Lower Carboniferous (Tournaisian) age. Taking into account both the age of this magmatism and the stratigraphic and structural data for this region suggests that dolerites were emplaced within distensive zones that are related to the reactivation of Panafrican faults. To cite this article: H. Djellit et al., C. R. Geoscience 338 (2006). 相似文献
10.
A review of the In Ouzzal granulitic terrane (Tuareg shield, Algeria): its significance within the Pan-African Trans-Saharan belt 总被引：4，自引：0，他引：4
R. CABY 《Journal of Metamorphic Geology》1996,14(6):659-666
The In Ouzzal terrane (IOT) or In Ouzzal granulite unit (IOGU) is an elongated Palaeoproterozoic block within the Neoproterozoic Pan-African belt of north-west Africa. The granulites derive from Archaean protoliths that include a large volume of metasediments which were deposited on a 3.2-Ga gneissic basement. Near-peak granulite facies conditions between 2.17 and 2 Ga were estimated at P=10 kbar and T rising from 800 to 1000°C. Premetamorphic orthogneisses were intruded at 2.5 Ga, and followed by the emplacement of syn- to late-kinematic charnockites, syenites and carbonatites at around 2 Ga. Cooling of the granulites occurred till 1800 Ma. Shortly after its exhumation coeval with crustal extension and related alkaline magmatism in adjacent areas, the IOT was buried beneath late Palaeoproterozoic and Neoproterozoic cover sequences, and then behaved as a rigid block. Both margins are lithospheric faults, as evidenced by the occurrence of shear-zone-related mafic and felsic plutons. Pan-African tectonothermal events were negligible in the north, but granulites in the south were significantly reworked under lower greenschist facies conditions during the northern motion of the block with respect to both the western and the eastern Pan-African terranes. The Cambrian molasse, associated with widespread alkaline volcanism and subvolcanic granites, is horizontal in the north. The IOT, which was part of a larger continental mass including its counterpart in northern Mali, is interpreted as an exotic terrane which may have docked during Pan-African plate convergence and lateral collision. The unchanged pediplain since c. 1.7 Ga in the north suggests that the IOT is underlain by thick Palaeoproterozoic lithospheric mantle, whereas its southern part is probably allochthonous and overlies Pan-African structural units. 相似文献