Journal of Metamorphic Geology
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Corundum–quartz‐bearing assemblage in the Ihouhaouene area (In Ouzzal, Algeria) ABSTRACT Granulite facies quartzites from the Ihouhaouene region, in the northern part of In Ouzzal, contain the assemblage corundum+quartz+magnetite together with hercynitic spinel+quartz+magnetite, sillimanite+quartz+magnetite and almandine‐rich garnet+quartz+magnetite. Two types of corundum have been recognized: the first is primary and is found with quartz and magnetite only; the second type is found together with magnetite and chlorite rimming spinel as a fine‐grained corona. The textures show that spinel‐rich magnetite probably exsolved primary corundum, sillimanite, spinel and garnet during the cooling history. The secondary corundum formed later from the spinel already exsolved from magnetite. The secondary corundum is certainly metastable with respect to quartz. This may also apply for the primary corundum. However, given the high‐temperature setting of this rock, it cannot be excluded that the stable contacts observed between primary corundum and quartz indicate equilibrium between the two phases. Taking into account the uncertainties in the thermodynamic data, the stability of this assemblage would imply that this part of In Ouzzal has recorded very high P–T conditions, above 1100°C at 12 kbar.
Journal of Metamorphic Geology - Tập 14 Số 6 - Trang 755-761 - 1996
An example of ultrahigh‐temperature metamorphism: orthopyroxene–sillimanite–garnet, sapphirine–quartz and spinel–quartz parageneses in Al–Mg granulites from In Hihaou, In Ouzzal, Hoggar ABSTRACT Quartz Al–Mg granulites exposed at In Hihaou, In Ouzzal (NW Hoggar), preserve an unusual high‐grade mineral association stable at temperatures up to 1050°C, involving the parageneses orthopyroxene–sillimanite–garnet–quartz, sapphirine–quartz and spinel–quartz. The phase relationships within the FMAS system show that a continuum exists between the earlier prograde reaction textures and those of the later decompressive event. The following mineral reactions involving sillimanite are deduced: (1) Grt+Qtz→Opx+Sil, (2) Opx+Sil→Grt+Spr+Qtz, (3) Grt+Sil+Qtz→Crd, (4) Grt+Sil→Crd+Spr, (5) Grt+Sil+Spr→Crd+Spl, (6) Grt+Sil→Crd+Spl, (7) Grt+Crd+Sil→Spl+Qtz and (8) Grt+Sil→Spl+Qtz. Minerals in quartz Al–Mg granulites display compositional variations consistent with the observed reactions. The Mg/(Mg+Fe2+ ) range of the main minerals is as follows: cordierite (0.81–0.97), sapphirine (0.77–0.88), orthopyroxene (0.65–0.81), garnet (0.33–0.64) and spinel (0.23–0.56). The reaction textures and the evolution of the mineral assemblages in the quartz Al–Mg granulites indicate a clockwise P–T trajectory characterized by peak conditions of at least 10 kbar and 1050°C, followed by decompression from 10 to 6 kbar at a temperature of at least 900°C.
Journal of Metamorphic Geology - Tập 14 Số 6 - Trang 693-708 - 1996
<i>P–T–X</i> relationships in the Precambrian Al–Mg‐rich granulites from In Ouzzal, Hoggar, Algeria 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–Al2 O3 –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, K d , between the various ferromagnesian phases decreased significantly whereas Al2 O3 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 K d value between orthopyroxene and garnet (K d = 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.
Journal of Metamorphic Geology - Tập 10 Số 1 - Trang 17-31 - 1992
A review of the In Ouzzal granulitic terrane (Tuareg shield, Algeria): its significance within the Pan‐African Trans‐Saharan belt ABSTRACT 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.
Journal of Metamorphic Geology - Tập 14 Số 6 - Trang 659-666 - 1996
Refining the <i>P–T</i> records of UHT crustal metamorphism Abstract Ultra‐high‐temperature (UHT) metamorphism occurs when the continental crust is subjected to temperatures of greater than 900 °C at depths of 20–40 km. UHT metamorphism provides evidence that major tectonic processes may operate under thermal conditions more extreme than those generally produced in numerical models of orogenesis. Evidence for UHT metamorphism is recorded in mineral assemblages formed in magnesian pelites, supported by high‐temperature indicators including mesoperthitic feldspar, aluminous orthopyroxene and high Zr contents in rutile. Recent theoretical, experimental and thermodynamic data set constraints on metamorphic phase equilibria in FMAS, KFMASH and more complex chemical systems have greatly improved quantification of the P–T conditions and paths of UHT metamorphic belts. However, despite these advances key issues that remain to be addressed include improving experimental constraints on the thermodynamic properties of sapphirine, quantifying the effects of oxidation state on sapphirine, orthopyroxene and spinel stabilities and quantifying the effects of H2 O–CO2 in cordierite on phase equilibria and reaction texture analysis. These areas of uncertainty mean that UHT mineral assemblages must still be examined using theoretical and semi‐quantitative approaches, such as P (–T )–μ sections, and conventional thermobarometry in concert with calculated phase equilibrium methods. In the cases of UHT terranes that preserve microtextural and mineral assemblage evidence for steep or ‘near‐isothermal’ decompression P–T paths, the presence of H2 O and CO2 in cordierite is critical to estimates of the P–T path slopes, the pressures at which reaction textures have formed and the impact of fluid infiltration. Many UHT terranes have evolved from peak P–T conditions of 8–11 kbar and 900–1030 °C to lower pressure conditions of 8 to 6 kbar whilst still at temperature in the range of 950 to 800 °C. These decompressional P–T paths, with characteristic dP /dT gradients of ∼25 ± 10 bar °C−1 , are similar in broad shape to those generated in deep‐crustal channel flow models for the later stages of orogenic collapse, but lie at significantly higher temperatures for any specified pressure. This thermal gap presents a key challenge in the tectonic modelling of UHT metamorphism, with implications for the evolution of the crust, sub‐crustal lithosphere and asthenospheric mantle during the development of hot orogens.
Journal of Metamorphic Geology - Tập 26 Số 2 - Trang 125-154 - 2008
An enlarged and updated internally consistent thermodynamic dataset with uncertainties and correlations: the system K<sub>2</sub>O–Na<sub>2</sub>O–CaO–MgO–MnO–FeO–Fe<sub>2</sub>O<sub>3</sub>–Al<sub>2</sub>O<sub>3</sub>–TiO<sub>2</sub>–SiO<sub>2</sub>–C–H<sub>2</sub>–O<sub>2</sub> We present, as a progress report, a revised and much enlarged version of the thermodynamic dataset given earlier (Holland & Powell, 1985). This new set includes data for 123 mineral and fluid end‐members made consistent with over 200 P–T–X CO2 –f O2 phase equilibrium experiments. Several improvements and advances have been made, in addition to the increased coverage of mineral phases: the data are now presented in three groups ranked according to reliability; a large number of iron‐bearing phases has been included through experimental and, in some cases, natural Fe:Mg partitioning data; H2 O and CO2 contents of cordierites are accounted for with the solution model of Kurepin (1985); simple Landau theory is used to model lambda anomalies in heat capacity and the Al/Si order–disorder behaviour in some silicates, and Tschermak‐substituted end‐members have been derived for iron and magnesium end‐members of chlorite, talc, muscovite, biotite, pyroxene and amphibole. For the subset of data which overlap those of Berman (1988), it is encouraging to find both (1) very substantial agreement between the two sets of thermodynamic data and (2) that the two sets reproduce the phase equilibrium experimental brackets to a very similar degree of accuracy. The main differences in the two datasets involve size (123 as compared to 67 end‐members), the methods used in data reduction (least squares as compared to linear programming), and the provision for estimation of uncertainties with this dataset. For calculations on mineral assemblages in rocks, we aim to maximize the information available from the dataset, by combining the equilibria from all the reactions which can be written between the end‐members in the minerals. For phase diagram calculations, we calculate the compositions of complex solid solutions (together with P and T ) involved in invariant, univariant and divariant assemblages. Moreover we strongly believe in attempting to assess the probable uncertainties in calculated equilibria and hence provide a framework for performing simple error propagation in all calculations in thermocalc, the computer program we offer for an effective use of the dataset and the calculation methods we advocate.
Journal of Metamorphic Geology - Tập 8 Số 1 - Trang 89-124 - 1990
Metasomatic effects related to channelled fluid streaming through deep crust: fenites and associated carbonatites (In Ouzzal Proterozoic granulites, Hoggar, Algeria) ABSTRACT 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 (δ13 C=–0.8 to –4.2‰/PDB; δ18 O = 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 (ɛNd(T) =–6.3 to –9.9), of Sr (87 Sr/86 Sr(T) = 0.7093–0.7104), and the O isotopic composition of metasomatic clinopyroxene (δ18 O = 6.9 to 8‰), all indicate that the fluid had a strong crustal imprint. On the basis of the C isotope ratios (δ13 C =–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.
Journal of Metamorphic Geology - Tập 14 Số 6 - Trang 763-781 - 1996
Major and trace element geochemistry and isotope (Sr, Nd, Pb, O) systematics of an Archaean basement involved in a 2.0 Ga very high‐temperature (1000°C) metamorphic event: In Ouzzal Massif, Hoggar, Algeria ABSTRACT The In Ouzzal granulitic massif is composed mainly of various meta‐igneous rocks which, in spite of Rb, U, Th, Cs and some K and Sr mobility, can be dated and generally classified according to their chemical composition as follows. Basic and ultrabasic granulites interlayered with the metasediments correspond to (1) ultrabasic cumulates from dislocated tholeiitic bodies, (2) ancient komatiitic to high‐Mg tholeiitic basalts similar to the suites found in Archaean greenstone belts and (3) calcalkaline protoliths of high‐K andesitic composition. No geochronological constraints are available apart from the depositional age of some associated sediments which is younger than 2.70 Ga detrital zircons, and the Nd model age of the andesitic granulites of c. 3.4 Ga. In spite of the high‐grade metamorphism, the acidic magmatic precursors of the charnockites can be divided in three groups. (1) The most juvenile acid orthogneisses are trondhjemitic or tonalitic in composition, being similar to the TTG suites which are classically considered to be formed by partial melting of mantle‐derived protoliths. The 3.3–3.2 Ga TDM indicates a possible age of separation from the mantle reservoir while the plutons may have been emplaced between 3.3 and 2.7 Ga (U–Pb zircon & Nd ages). (2) A group of alkaline granitic gneisses, similar in composition to rift‐related‐granites, were emplaced at 2650±10 Ma (U–Pb & Rb–Sr ages) in a thick continental crust. (3) Calcalkaline granodioritic and monzogranitic suites derived from the partial melting of continental precursors (3.5–3.3 Ga), in lower to middle levels of the continental crust. They were emplaced close to 2.5 Ga during crustal thickening. The very high‐temperature metamorphism occurred at 2002±7 Ma from the age of synfoliation intrusions and was probably related to major overthrusting. Retrogressive metamorphism is dated at 1.95 Ga from garnet‐Nd ages. In spite of the very high‐temperature conditions, partial melting during granulite facies metamorphism may be restricted to scarce cordierite‐bearing monzogranitic gneisses. The 2.0 Ga VHT metamorphism could be related to overthrusting, extensional or underplating processes.
Journal of Metamorphic Geology - Tập 14 Số 6 - Trang 667-692 - 1996
Constraining the <i>P</i>–<i>T</i> path of a MORB‐type eclogite using pseudosections, garnet zoning and garnet‐clinopyroxene thermometry: an example from the Bohemian Massif Abstract A mid‐ocean ridge basalt (MORB)‐type eclogite from the Moldanubian domain in the Bohemian Massif retains evidence of its prograde path in the form of inclusions of hornblende, plagioclase, clinopyroxene, titanite, ilmenite and rutile preserved in zoned garnet. Prograde zoning involves a flat grossular core followed by a grossular spike and decrease at the rim, whereas Fe/(Fe + Mg) is also flat in the core and then decreases at the rim. In a pseudosection for H2 O‐saturated conditions, garnet with such a zoning grows along an isothermal burial path at c. 750 °C from 10 kbar in the assemblage plagioclase‐hornblende‐diopsidic clinopyroxene‐quartz, then in hornblende‐diopsidic clinopyroxene‐quartz, and ends its growth at 17–18 kbar. From this point, there is no pseudosection‐based information on further increase in pressure or temperature. Then, with garnet‐clinopyroxene thermometry, the focus is on the dependence on, and the uncertainties stemming from the unknown Fe3+ content in clinopyroxene. Assuming no Fe3+ in the clinopyroxene gives a serious and unwarranted upward bias to calculated temperatures. A Fe3+ ‐contributed uncertainty of ±40 °C combined with a calibration and other uncertainties gives a peak temperature of 760 ± 90 °C at 18 kbar, consistent with no further heating following burial to eclogite facies conditions. Further pseudosection modelling suggests that decompression to c. 12 kbar occurred essentially isothermally from the metamorphic peak under H2 O‐undersaturated conditions (c. 1.3 mol.% H2 O) that allowed the preservation of the majority of garnet with symplectitic as well as relict clinopyroxene. The modelling also shows that a MORB‐type eclogite decompressed to c. 8 kbar ends as an amphibolite if it is H2 O saturated, but if it is H2 O‐undersaturated it contains assemblages with orthopyroxene. Increasing H2 O undersaturation causes an earlier transition to SiO2 undersaturation on decompression, leading to the appearance of spinel‐bearing assemblages. Granulite facies‐looking overprints of eclogites may develop at amphibolite facies conditions.
Journal of Metamorphic Geology - Tập 23 Số 8 - Trang 725-743 - 2005
Multistage progressive evolution of rare osumilite‐bearing assemblages preserved in ultrahigh‐temperature granulites from In Ouzzal (Hoggar, Algeria) Abstract Osumilite is reported in Palaeoproterozoic Al–Mg‐rich granulites from the Khanfous area (Tekhamalt, In Ouzzal, Hoggar, Algeria). The main peak assemblages are osumilite + sapphirine + biotite + orthopyroxene + sillimanite and osumilite + orthopyroxene + sillimanite + quartz ± biotite (±K‐feldspar) in silica‐deficient and silica‐saturated granulites respectively. Osumilite coexists with F‐rich biotite (X F P–T pseudosections modelled for bulk‐rock and reaction‐microdomain compositions indicate a clockwise P–T metamorphic evolution at ultrahigh temperatures, without substantial post‐peak deformation. The peak P–T conditions recorded by the osumilite‐bearing assemblages are 8.5–9.0 kbar and 930–980 °C. During retrogression, osumilite was partially or totally replaced by fine‐grained pseudomorphs of cordierite + orthopyroxene + K‐feldspar + quartz at ~7 kbar and ~850 °C. This study confirms that osumilite can occur only in Mg‐rich metamorphic rocks that experienced ultrahigh‐temperature metamorphism under anhydrous conditions. In the presence of a hydrous fluid, it is replaced, even at high temperatures, by cordierite‐bearing assemblages. This important feature explains the rarity of osumilite in granulite facies rocks and its common replacement by cordierite + orthopyroxene + K‐feldspar + quartz pseudomorphs. The peak conditions suggest that a delamination of the lithospheric mantle underneath the In Ouzzal crust brought the asthenosphere close to the Mohorovičić discontinuity.
Journal of Metamorphic Geology - Tập 31 Số 5 - Trang 505-524 - 2013
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