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Granulitic metamorphism in the Laouni terrane (Central Hoggar, Tuareg Shield, Algeria)
Journal of African Earth Sciences 39 (2004) 187–192 www.elsevier.com/locate/jafrearsci
Granulitic metamorphism in the Laouni terrane (Central Hoggar, Tuareg Shield, Algeria) Abderrahmane Bendaoud a,*, Amel Derridj b, Khadidja Ouzegane a, Jean-Robert Kienast c a
Faculte´ des Sciences de la Terre, de Ge´ographie et dÕAme´nagement du Territoire, U.S.T.H.B., B.P. 32, Dar el Beida, Algiers, Algeria b Faculte´ des Hydrocarbures et de la Chimie, Universite´ mÕHamed Bougara, 35000 Boumerde´s, Algeria c Laboratoire de Pe´trologie, CNRS UMR 7097 IPGP, Universite´ de Paris 7, Tour 26-O, 4 place Jussieu, 75252 Paris, France Available online 30 September 2004
Abstract In the Laouni terrane, which belongs to the polycyclic Central Hoggar domain, various areas contain outcrops of formations showing granulite-facies parageneses. This high-temperature metamorphism was accompanied by migmatization and the emplacement of two types of magmatic suite, one of continental affinity (garnet pyroxenites and granulites with orthoferrossilite–fayalite– quartz), and the other of arc affinity (layered metanorites). Paragenetic, thermobarometric and fluid-inclusion studies of the migmatitic metapelites and metabasites make it possible to reconstruct the P–T–aH2O path undergone by these formations. This path is clockwise in the three studied areas, being characterized by a major decompression (Tamanrasset: 10.5 kbar at 825 °C to 6 kbar at 700 °C; Tidjenouine: 7.5 kbar at 875 °C; to 3.5 kbar at 700 °C; Tin Begane: 13.5 kbar at 850 °C; to 5 kbar at 720 °C), followed by amphibolitization that corresponds to a fall of temperature (from 700 to 600 °C) and an increase in water activity (from 0.2–0.4 to almost 1). The main observed features are in favour of petrogenesis and exhumation related to the Eburnean orogeny. However, the lacks of good-quality dating work and a comparison with juvenile Pan-African formations having undergone high-pressure metamorphism, in some cases reaching the eclogite facies, do not rule out the possibility that high-temperature parageneses are locally due to Pan-African events. Ó 2004 Elsevier Ltd. All rights reserved. Keywords: Laouni; Hoggar; LATEA; Granulite; Eburnean Pan-African
1. Introduction The Central Hoggar is located at the core of the Tuareg Shield (Fig. 1(A)), providing an example of a Precambrian domain that has undergone a polycyclic history. Indeed, the available dating suggests that this domain consists of Eburnean crust (Paleoproterozoic 2000 Ma), including some Archaean zones, onto
*
Corresponding author. Tel: +213 21 24 76 47; fax: +213 21 24 76
47. E-mail address: [email protected] (A. Bendaoud). 0899-5362/$ - see front matter Ó 2004 Elsevier Ltd. All rights reserved. doi:10.1016/j.jafrearsci.2004.07.050
which juvenile Pan-African (850–550 Ma) material was locally accreted (Lie´geois et al., 2003; Peucat et al., 2003). Thus, the set of formations in this area corresponds to a superposition of Pan-African nappes reworking Archean–Eburnean material to a variable extent. The base of some of these nappes is picked out by relatively well-preserved eclogitic layers. During the Pan-African orogeny (850–550 Ma), to which the region owes its current structure, the Tuareg Shield was built up from the amalgamation of different terranes and was involved in two major collisions, one with the West African Craton and the other with the Saharan Craton (Black et al., 1994).
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Fig. 1. Geological maps of the studied areas, and their situation within the Hoggar: (A) Tuareg Shield (Black et al., 1994); (B) Tamanrasset area (Ouzegane et al., 2001); (C) Tidjenouine area (Bertrand et al., 1986); (D) Tin Begane area (Derridj et al., 2003).
Several authors point out the remarkable structural and compositional unity of the ortho- and para-derived amphibolitic formations (some with granulite-facies relics) making up the Laouni, Azrou, NÕFad, Tefedest and Ege´re´-Aleksod blocks that occupy the entire centralwestern part of Central Hoggar (Fig. 1(A)). According to Latouche et al. (2000) and Lie´geois et al. (2003), these four terranes made up an Archean–Eburnean microcontinent known as LATEA. During the Pan-African orogeny, it appears that LATEA formed a passive margin that was involved in two collisions with arc-type terranes: in the West, the Iskel terrane of Mesoproterozoic age stabilized towards 840 Ma (Caby, 2003) and, in the East, the Serouanout terrane of unknown age, but including a large part of apparently juvenile material (Black et al., 1994). During the post-collisional phase, this micro-continent was dismembered following horizontal movements along major shear-zones (Lie´geois et al., 2003). The present study concerns the metamorphic history of three areas (Tamanrasset, Tidjenouine and Tin Begane) where granulitic formations are exposed. The rocks show granulite- and amphibolite-facies parageneses. These three areas all are located within the Laouni
terrane that constitutes the SW part of Central Hoggar (Fig. 1A).
2. Tamanrasset The area of Tamanrasset is made up of migmatitic gneisses and metasediments with numerous intercalations of metabasites (Fig. 1B). The various rock-types are commonly retrogressed into the amphibolite facies, and even into the greenschist facies near the mylonitic zones that form belts around this area. However, remarkably preserved lenses of metabasite containing garnet (Grt)–clinopyroxene (Cpx)–orthopyroxene (Opx)– hornblende (Hbl)–plagioclase (Pl)–quartz (Qtz) (Fig. 2(A)) allow us to estimate P–T path evolution, by combining the study of phase relations with mineral equilibria calculated by the Thermocalc program of Powell and Holland (1988), conventional and automatic geothermobarometry, and fluid inclusions (Ouzegane et al., 2001). A metamorphic peak is calculated at approximately 825 ± 25 °C and 10 ± 1 kbar (Fig. 3(A)). Then, these rocks underwent a retrograde evolution leading to conditions of 700 ± 50 °C at 6.5 ± 1 kbar, with H2O activity remaining
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Fig. 2. Photomicrographs of representative metamorphic rocks from Tamanrasset, Tidjenouine and Tin Begane areas. (A) Breakdown of garnet + clinopyroxene + quartz to orthopyroxene + plagioclase + hornblende and development of fine lamellae of amphibole along clinopyroxene cleavage planes in garnet–pyroxenites of Tamanrasset. (B) Metapelite from Tamanrasset showing the sillimanite–biotite–plagioclase–quartz–Kfeldspar assemblage. (C) Metapelite from Tidjenouine showing the breakdown of garnet and quartz to orthopyroxene + cordierite. Spinel-cordierite symplectites result from the reaction: garnet + sillimanite ) spinel + cordierite. Note that in cracks of garnet it developed orthopyroxene-spinelcordierite symplectites. (D) Exsolutions of garnet with preferred orientations in aluminous orthopyroxene of sillimanite–free metapelite from Tidjenouine. (E) Inclusion of kyanite armoured by garnet in garnet–sillimanite–biotite metapelite from Tin Begane. (F) Representative assemblages of garnet pyroxenites from Tin Begane, showing orthopyroxene-plagioclase intergrowths and orthopyroxene corona around quartz, resulting from the reaction: garnet + clinopyroxene + quartz ) orthopyroxene + plagioclase (G) Retrogressed garnet pyroxenite from Tamanrasset showing the breakdown of orthophroxene + hornblende to cummingtonite + plagioclase + quartz.
around 0.2 (Ouzegane et al., 2001). This is illustrated (Fig. 2(A)) by the destabilization of the primary assemblage containing Grt (Alm57–Py17–Grs25–Sps3; XFe = 0.53)–Cpx (XFe = 0.46)–Opx1 (XFe = 0.57)–Pl1 (An50)–Hbl1 (pargasite)–Qtz into a secondary assemblage with Opx2 (XFe = 0.61)–Pl2 (An80)–Hbl2 (magnesio-hornblende). The conditions of pressure and temperature for the secondary assemblages are comparable with those obtained in the surrounding migmatitic metapelites containing a garnet–biotite (Bt)–sillimanite (Sill)–quartz–plagioclase–K-feldspar (Kfs) assemblage (Fig. 2B).
Amphibolitization in the metabasites leads to the appearance of a third generation of parageneses (Fig. 2(G)) characterized by the presence of cummingtonite (Cum), which reflects an increase in H2O activity and a fall of temperature.
3. Tidjenouine The granulitic formations of Tidjenouine are predominately composed of migmatitic gneisses (Fig. 1(C)) that are much better preserved than in the Tamanrasset area.
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Fig. 3. P–T paths evolution of Tamanrasset granulites—with inclusion isochors (solid straight line, V in cm3/mol) inferred from Th histograms (grey = retrogressed garnet and white = metapelite). V = molar volume, d = CO2 density in g/cm3 (A); Tidjenouine (B) and Tin Begane (C). The P–T conditions have been computed essentially with average P–T Thermocalc method (Powell and Holland, 1988).
Tidjenouine is the only area of the Laouni terrane where the granulite-facies metamorphism has been dated, yielding an age of 2069 ± 49 Ma (U–Pb on zircon, (Bertrand et al., 1986), recalculated following Ludwig, 1999 by Lie´geois et al., 2003). Two suites of metabasites have been described (Bendaoud, 1999; Ouzegane et al., 2000; Bendaoud et al., 2003). The continental suite (garnet pyroxenites, amphibolites and granulites with fayalite) reflects intra-plate magmatism, whereas the other suite has an arc-type affinity (layered metanorites). The migmatitic metapelites show a great paragenetic diversity in relation to the variations of the XMg and XAl ratios in these rocks. Thus, various parageneses have been distinguished based on the presence or absence of a certain number of minerals such as orthopyroxene, gedrite, sillimanite, corundum and quartz. The metapelites show a prograde evolution, during which these formations started to melt. The peak of the metamorphism was reached at 875 ± 50 °C and 7.5 ± 1 kbar (Fig. 3(B)). This was followed by decompression and a fall in temperature that brought the rocks to conditions of 700 °C and 3–4 kbar. The earliest reactions led to the appearance of garnet following the destabilization of biotite or gedrite, in relation to reactions of the type: Bt + Sill + Qtz ± Pl ) Grt +
Melt + Kfs ± Crd and Ged + Sill + Qtz ) Grt + Crd + Melt. The peak is represented by the presence of aluminous orthopyroxene which later exsolved garnet in the metapelites without sillimanite. In the magnesian metapelites with sillimanite, on the other hand, the highest pressures are indicated by the composition of garnets that reach an XMg value of 51%. During decompression from 7 to 4 kbar, cordierite appeared in coronas around garnet or biotite, as well as in symplectites with spinel (Spl) and/or orthopyroxene, according to reactions such as Grt + Sill ) Crd + Spl; Bi + Sill ) Crd + Spl + Kfs (melt or vapour); Grt + Qtz ) Opx + Crd (Fig. 2(C)) and Grt ) Spl + Crd + Opx + P1. In the metapelites without sillimanite and with primary orthopyroxene, the exsolution of garnet by an aluminous primary orthopyroxene (Al2O3 > 6.5 wt%) indicates a stage of falling temperature at relatively high pressures (Fig. 2(D)). The final stage (600 °C at 3–4 kbar) corresponds to a fall of temperature that, for example, is reflected in gedrite-bearing granulites by parageneses with anthophyllite. The same P–T path is exhibited by granulites with fayalite–quartz–ferrosilite (Bendaoud et al., 2003). The physical conditions of peak assemblage, garnet–clinopy-
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roxene–amphibole–plagioclase–quartz, correspond to pressures of 7.1 ± 1 kbar at temperatures of 880 ± 60 °C and a H2O of 0.2. During decompression, the clinopyroxene in contact with garnet and quartz initially broke down into orthopyroxene + plagioclase. The P–T conditions computed for this paragenesis are around 750 °C and 5 kbar. Then the orthopyroxene reacted with garnet to produce fayalite + plagioclase symplectites. The latest stage corresponds to the orthopyroxene–fayalite–quartz– plagioclase assemblage reflecting low pressures 3– 4 kbar at temperature of 700 °C.
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Following a petrological and geochronological study (Sm–Nd on garnet) in the south of this same area, Lie´geois et al. (2003) determined a metamorphic evolution path (Fig. 3(C)) at the base of the Pan-African nappes that passes successively through the following conditions: 17 kbar at 790 °C (eclogite 690 Ma), 12 kbar at 830 °C (garnet amphibolite 686 Ma), 8 kbar at 700 °C (kyanite–garnet gneisses) and 4 kbar at 500 °C (greenschist 530 My).
5. Discussion and conclusion 4. Tin Begane The area of Tin Begane (Fig. 1(D)), located at 110 km to the South of Tamanrasset, shows formations equivalent to those of the preceding areas. However, it is characterized by the persistence of kyanite (Ky) relics in the metapelites and the presence of eclogitic layers in the southern part of the area. The metasedimentary succession is made up of olivine-spinel marbles intercalated with garnet–sillimanite–biotite metapelites containing kyanite relics (Fig. 2(E)) armoured by garnet. In this area, there are also found iron-bearing quartzites with hercynite–almandite–faylite and lenses of metabasite both with and without garnet. Mylonitic contacts separate the metasedimentary succession from migmatitic orthogneisses. The metabasites of the northern part of Tin Begane show geochemical compositions essentially comparable with continental tholeiites (Derridj, 2000). They differ from the metabasites of the preceding areas by the presence of three successive parageneses, the latest contains spinel. The primary paragenesis is Grt–Cpx (A12O3 = 7.15 wt%; Na2O = 1.15 wt%; XMg = 0.80)–Pl (An47)–Hbl–Qtz–Rutile. This paragenesis is remarkable for the composition of the garnet, with contents of pyrope and grossularite reaching 38% and 30%, respectively (Alm32 Py38 Grs30), values that are characteristic of very-high pressure. Thermobarometric studies of this paragenesis indicate conditions of 13.5 ± 1.5 kbar and 860 ± 60 °C (Fig. 3(C)). These conditions are located at the boundary between the granulite and the eclogite facies. The secondary paragenesis, with Opx–Pl (An60) ± Grt2 (Alm47 Py31 Grs20 Sps2) ± Hbl2, results primarily from the reactions: Grt + Cpx + Qtz ) Opx + Pl (Fig. 2(F)) and Grt + Hbl1 + Qtz ) Opx + Pl + Hbl2 (symplectitic stage 1). The latest paragenesis is formed at the expense of garnet in cracks, owing to the reaction: Grt ) Opx + Spl + Pl (An95) (symplectitic stage 2). The corresponding conditions for these two parageneses are 10.7 ± 1.3 kbar at 790 ± 60 °C and 4.8 ± 1.3 kbar at 750 °C, respectively (Derridj et al., 2003). The metapelites of the country rocks and the olivine-clinopyroxene marbles yield conditions equivalent to those recorded by the two later parageneses of the metabasites.
In several areas of the Laouni terrane, observed granulitic formations are commonly associated with an important migmatitic event. Petrological, thermobarometric and fluid-inclusion studies allow us to determine a clockwise path with a decompression stage generating spectacular coronitic and symplectitic textures in both the para- and ortho-derived metamorphic units. The succession of parageneses during this decompression is a function of various chemical compositions. The metapelites or microdomains rich in silica and magnesium are characterized by the appearance of an orthopyroxene–cordierite association at the expense of garnet, quartz and biotite in the absence of sillimanite. The metapelites and microdomains rich in aluminium and iron make up assemblages with spinel-cordierite, but without orthopyroxene, following the destabilization of garnet, sillimanite and biotite. This explains the absence of orthopyroxene in Tin Begane and Tamanrasset, where the metapelitic formations are highly ironenriched and aluminous. During the prograde stage, the formation of inclusions in the core of primary garnet indicates that the kyanite field was traversed in the Tin Begane area, while only sillimanite is present in Tamanrasset and Tidjenouine. In the three studied areas, the peak of temperature is situated at around 850 ± 25 °C (Fig. 3), whereas the pressure peak is very variable since it ranges from 7.5 kbar (Tidjenouine) through 10.5 kbar (Tamanrasset) to 13.5 kbar (Tin Begane). This implies the possibility that different structural levels can be observed in the Laouni terrane. Two types of metabasites are also distinguished, one with tholeiitic intra-plate affinity and the other showing arc affinity. Although Bertrand and Jardim de Sa´ (1990) and Ouzegane et al. (2001) propose that the observations are more likely compatible with an Eburnean orogeny, one of the fundamental problems in the interpretation of this metamorphism and the nature of the metabasites involved is the lack of precise dating. Several geologists prefer to attribute a Pan-African age to the granulites, integrating the petrogensis and exhumation of these rocks into the pan-African orogeny of Central Hoggar (e.g. Caby, 2003; Barbey et al., 1989). However, the
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age of 2000 Ma obtained on the Tidjenouine formations indeed seems to refer to the age of the granulitic protolith. In a critical review of the published U–Pb ages from Hoggar, Bertrand (1998, unpublished—written communication) indicates that the zircons dated at 2069 ± 49 Ma in Tidjenouine granulites (Bertrand et al., 1986) are clearly metamorphic—with rounded spheroidal shapes—and that the lower intercept obtained here (at around 530 Ma) is an analytical artefact. In the Tin Begane area, the isotopic data (Rb–Sr and Sm–Nd on whole rocks) of various rock types with granulitic relics would appear to indicate Pan-African ages or at least an important Pan-African reworking (Lie´geois, written communication). In the same way, the pressure–temperature path determined on these granulites does not appear to contradict, within the limits of uncertainty, the path given for the metabasites containing eclogitic relics (Fig. 3(C)). Dating work in progress on garnets (Sm–Nd) from the granulitic parageneses should soon make it possible to resolve these uncertainties and provide a better understanding of the geodynamic evolution of these areas.
Acknowledgments We thank P. Ho¨ltta¨ and L. Solari, reviewers, for their constructive and very helpful remarks that have significantly improved the quality of this work. M.S.N. Carpenter is thanked for help with the English. We also thank Abdelkader Regagda for the field facilities. This work is a contribution to the projects PNR AU 19943 and to NATO EST/CLE 979766.
References Barbey, P., Bertrand, J.M., Angoua, S., Dautel, D., 1989. Petrology and U–Pb geochronology of the Telohat migmatites, Aleksod, Central Hoggar, Algeria. Contributions to Mineralogy and Petrology 101, 207–219. Bendaoud, A., 1999. Ge´ochimie et relation de phases dans les granulites de Tidjenouine (Hoggar Central, Alge´rie). Unpublished Magister, U.S.T.H.B., Alge´r, Alge´rie, p. 215.
Bendaoud, A., Ouzegane, K., Kienast, J.R., 2003. Textures and phase relationships in ferrous granulites from Tidjenouine (Hoggar, Algeria): fayalite-ferrossilite-quartz secondary assemblage. Journal of African Earth Sciences 37, 241–255. Bertrand, J.-M., Jardim de Sa´, E.F., 1990. Where are the Eburnean– Transamazonian collisional belts?. Canadian Journal of Earth Sciences 27, 1382–1393. Bertrand, J.L., Michard, A., Boullier, A.M., Dautel, D., 1986. Structure and U/Pb geochronology of Central Hoggar (Algeria): a reappraisal of its Pan-African evolution. Tectonics 5, 955–972. Black, R., Lie´geois, J.P., Latouche, L., Caby, R., Bertrand, J.M., 1994. Pan-African displaced terranes in the Tuareg Shield (Central Sahara). Journal of Geology 22, 641–644. Caby, R., 2003. Terrane assembly and geodynamic evolution of central-western Hoggar: a synthesis. Journal of African Earth Sciences 37, 133–159. Derridj, A., 2000. Relations de phases et trajectoires PressionTemperature-aH2O dans les granulites de Haute pression de Tin Begane (Hoggar Central): exemple de Me´tape´lites et de pyrigarnites. Unpublished Magister U.S.T.H.B., Alger, Alge´rie, p. 163. Derridj, A., Ouzegane, K., Kienast, J.R., Belhaı¨e, D., 2003. P–T–X evolution in garnet pyroxenites from Tin Begane (Central Hoggar, Algeria). Journal of African Earth Sciences 37, 257–268. Latouche, L., Boughrara, M., Lie´geois, J.-P., Navez, J., Guiraud, M., 2000. Pan-African thrust sheets and eclogites in the Tuareg area, central Hoggar, Algeria. Special Abstract Issue, 18th Colloquium of African Geology, Graz, 3–7 July 2000, Journal of African Earth Sciences A 30 (4), p. 52. Lie´geois, J.P., Latouche, L., Boughrara, M., Navez, J., Guiraud, M., 2003. The LATEA metacraton (Central Hoggar, Tuareg shield, Algeria): behaviour of an old passive margin during the PanAfrican orogeny. Journal of African Earth Sciences 37, 161–190. Ludwig, K.R., 1999. Using Isoplot/Ex Version 2.01, A Geochronological Toolkit for Microsoft Excel. Berkeley Geochronology Center Special Publication 1a, p. 47. Ouzegane, K., Kienast, J.R., Bendaoud, A., 2000. P–T paths and geochemical evolution in the Tamanrasset and Tidjenouine granulitic units, central Hoggar, Algeria. Special Abstract Issue, 18th Colloquium of African Geology, Graz, 3–7 July 2000. Ouzegane, K., Bendaoud, A., Kienast, J.R., Touret, J.L.R., 2001. Pressure–temperature–fluid evolution in Eburnean metabasites and metapelites from Tamanrasset Hoggar, Algeria. Journal of Geology 109, 213–230. Peucat, J.J., Drareni, A., Latouche, L., Deloule, E., Vidal, P., 2003. U– Pb zircon (TIMS and SIMS) and Sm–Nd whole-rock geochronology of the Gour Oumelalen granulitic basement, Hoggar massif, Tuareg Shield, Algeria. Journal of African Earth Sciences 37, 229– 239. Powell, R., Holland, T.J.B., 1988. An internally consistent thermodynamic dataset with uncertainties and correlations: 3. Application methods, worked examples and a computer program. Journal of Metamorphic Geology 101, 173–204.
Granulitic metamorphism in the Laouni terrane (Central Hoggar, Tuareg Shield, Algeria) Abderrahmane Bendaoud a,*, Amel Derridj b, Khadidja Ouzegane a, Jean-Robert Kienast c a
Faculte´ des Sciences de la Terre, de Ge´ographie et dÕAme´nagement du Territoire, U.S.T.H.B., B.P. 32, Dar el Beida, Algiers, Algeria b Faculte´ des Hydrocarbures et de la Chimie, Universite´ mÕHamed Bougara, 35000 Boumerde´s, Algeria c Laboratoire de Pe´trologie, CNRS UMR 7097 IPGP, Universite´ de Paris 7, Tour 26-O, 4 place Jussieu, 75252 Paris, France Available online 30 September 2004
Abstract In the Laouni terrane, which belongs to the polycyclic Central Hoggar domain, various areas contain outcrops of formations showing granulite-facies parageneses. This high-temperature metamorphism was accompanied by migmatization and the emplacement of two types of magmatic suite, one of continental affinity (garnet pyroxenites and granulites with orthoferrossilite–fayalite– quartz), and the other of arc affinity (layered metanorites). Paragenetic, thermobarometric and fluid-inclusion studies of the migmatitic metapelites and metabasites make it possible to reconstruct the P–T–aH2O path undergone by these formations. This path is clockwise in the three studied areas, being characterized by a major decompression (Tamanrasset: 10.5 kbar at 825 °C to 6 kbar at 700 °C; Tidjenouine: 7.5 kbar at 875 °C; to 3.5 kbar at 700 °C; Tin Begane: 13.5 kbar at 850 °C; to 5 kbar at 720 °C), followed by amphibolitization that corresponds to a fall of temperature (from 700 to 600 °C) and an increase in water activity (from 0.2–0.4 to almost 1). The main observed features are in favour of petrogenesis and exhumation related to the Eburnean orogeny. However, the lacks of good-quality dating work and a comparison with juvenile Pan-African formations having undergone high-pressure metamorphism, in some cases reaching the eclogite facies, do not rule out the possibility that high-temperature parageneses are locally due to Pan-African events. Ó 2004 Elsevier Ltd. All rights reserved. Keywords: Laouni; Hoggar; LATEA; Granulite; Eburnean Pan-African
1. Introduction The Central Hoggar is located at the core of the Tuareg Shield (Fig. 1(A)), providing an example of a Precambrian domain that has undergone a polycyclic history. Indeed, the available dating suggests that this domain consists of Eburnean crust (Paleoproterozoic 2000 Ma), including some Archaean zones, onto
*
Corresponding author. Tel: +213 21 24 76 47; fax: +213 21 24 76
47. E-mail address: [email protected] (A. Bendaoud). 0899-5362/$ - see front matter Ó 2004 Elsevier Ltd. All rights reserved. doi:10.1016/j.jafrearsci.2004.07.050
which juvenile Pan-African (850–550 Ma) material was locally accreted (Lie´geois et al., 2003; Peucat et al., 2003). Thus, the set of formations in this area corresponds to a superposition of Pan-African nappes reworking Archean–Eburnean material to a variable extent. The base of some of these nappes is picked out by relatively well-preserved eclogitic layers. During the Pan-African orogeny (850–550 Ma), to which the region owes its current structure, the Tuareg Shield was built up from the amalgamation of different terranes and was involved in two major collisions, one with the West African Craton and the other with the Saharan Craton (Black et al., 1994).
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Fig. 1. Geological maps of the studied areas, and their situation within the Hoggar: (A) Tuareg Shield (Black et al., 1994); (B) Tamanrasset area (Ouzegane et al., 2001); (C) Tidjenouine area (Bertrand et al., 1986); (D) Tin Begane area (Derridj et al., 2003).
Several authors point out the remarkable structural and compositional unity of the ortho- and para-derived amphibolitic formations (some with granulite-facies relics) making up the Laouni, Azrou, NÕFad, Tefedest and Ege´re´-Aleksod blocks that occupy the entire centralwestern part of Central Hoggar (Fig. 1(A)). According to Latouche et al. (2000) and Lie´geois et al. (2003), these four terranes made up an Archean–Eburnean microcontinent known as LATEA. During the Pan-African orogeny, it appears that LATEA formed a passive margin that was involved in two collisions with arc-type terranes: in the West, the Iskel terrane of Mesoproterozoic age stabilized towards 840 Ma (Caby, 2003) and, in the East, the Serouanout terrane of unknown age, but including a large part of apparently juvenile material (Black et al., 1994). During the post-collisional phase, this micro-continent was dismembered following horizontal movements along major shear-zones (Lie´geois et al., 2003). The present study concerns the metamorphic history of three areas (Tamanrasset, Tidjenouine and Tin Begane) where granulitic formations are exposed. The rocks show granulite- and amphibolite-facies parageneses. These three areas all are located within the Laouni
terrane that constitutes the SW part of Central Hoggar (Fig. 1A).
2. Tamanrasset The area of Tamanrasset is made up of migmatitic gneisses and metasediments with numerous intercalations of metabasites (Fig. 1B). The various rock-types are commonly retrogressed into the amphibolite facies, and even into the greenschist facies near the mylonitic zones that form belts around this area. However, remarkably preserved lenses of metabasite containing garnet (Grt)–clinopyroxene (Cpx)–orthopyroxene (Opx)– hornblende (Hbl)–plagioclase (Pl)–quartz (Qtz) (Fig. 2(A)) allow us to estimate P–T path evolution, by combining the study of phase relations with mineral equilibria calculated by the Thermocalc program of Powell and Holland (1988), conventional and automatic geothermobarometry, and fluid inclusions (Ouzegane et al., 2001). A metamorphic peak is calculated at approximately 825 ± 25 °C and 10 ± 1 kbar (Fig. 3(A)). Then, these rocks underwent a retrograde evolution leading to conditions of 700 ± 50 °C at 6.5 ± 1 kbar, with H2O activity remaining
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Fig. 2. Photomicrographs of representative metamorphic rocks from Tamanrasset, Tidjenouine and Tin Begane areas. (A) Breakdown of garnet + clinopyroxene + quartz to orthopyroxene + plagioclase + hornblende and development of fine lamellae of amphibole along clinopyroxene cleavage planes in garnet–pyroxenites of Tamanrasset. (B) Metapelite from Tamanrasset showing the sillimanite–biotite–plagioclase–quartz–Kfeldspar assemblage. (C) Metapelite from Tidjenouine showing the breakdown of garnet and quartz to orthopyroxene + cordierite. Spinel-cordierite symplectites result from the reaction: garnet + sillimanite ) spinel + cordierite. Note that in cracks of garnet it developed orthopyroxene-spinelcordierite symplectites. (D) Exsolutions of garnet with preferred orientations in aluminous orthopyroxene of sillimanite–free metapelite from Tidjenouine. (E) Inclusion of kyanite armoured by garnet in garnet–sillimanite–biotite metapelite from Tin Begane. (F) Representative assemblages of garnet pyroxenites from Tin Begane, showing orthopyroxene-plagioclase intergrowths and orthopyroxene corona around quartz, resulting from the reaction: garnet + clinopyroxene + quartz ) orthopyroxene + plagioclase (G) Retrogressed garnet pyroxenite from Tamanrasset showing the breakdown of orthophroxene + hornblende to cummingtonite + plagioclase + quartz.
around 0.2 (Ouzegane et al., 2001). This is illustrated (Fig. 2(A)) by the destabilization of the primary assemblage containing Grt (Alm57–Py17–Grs25–Sps3; XFe = 0.53)–Cpx (XFe = 0.46)–Opx1 (XFe = 0.57)–Pl1 (An50)–Hbl1 (pargasite)–Qtz into a secondary assemblage with Opx2 (XFe = 0.61)–Pl2 (An80)–Hbl2 (magnesio-hornblende). The conditions of pressure and temperature for the secondary assemblages are comparable with those obtained in the surrounding migmatitic metapelites containing a garnet–biotite (Bt)–sillimanite (Sill)–quartz–plagioclase–K-feldspar (Kfs) assemblage (Fig. 2B).
Amphibolitization in the metabasites leads to the appearance of a third generation of parageneses (Fig. 2(G)) characterized by the presence of cummingtonite (Cum), which reflects an increase in H2O activity and a fall of temperature.
3. Tidjenouine The granulitic formations of Tidjenouine are predominately composed of migmatitic gneisses (Fig. 1(C)) that are much better preserved than in the Tamanrasset area.
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Fig. 3. P–T paths evolution of Tamanrasset granulites—with inclusion isochors (solid straight line, V in cm3/mol) inferred from Th histograms (grey = retrogressed garnet and white = metapelite). V = molar volume, d = CO2 density in g/cm3 (A); Tidjenouine (B) and Tin Begane (C). The P–T conditions have been computed essentially with average P–T Thermocalc method (Powell and Holland, 1988).
Tidjenouine is the only area of the Laouni terrane where the granulite-facies metamorphism has been dated, yielding an age of 2069 ± 49 Ma (U–Pb on zircon, (Bertrand et al., 1986), recalculated following Ludwig, 1999 by Lie´geois et al., 2003). Two suites of metabasites have been described (Bendaoud, 1999; Ouzegane et al., 2000; Bendaoud et al., 2003). The continental suite (garnet pyroxenites, amphibolites and granulites with fayalite) reflects intra-plate magmatism, whereas the other suite has an arc-type affinity (layered metanorites). The migmatitic metapelites show a great paragenetic diversity in relation to the variations of the XMg and XAl ratios in these rocks. Thus, various parageneses have been distinguished based on the presence or absence of a certain number of minerals such as orthopyroxene, gedrite, sillimanite, corundum and quartz. The metapelites show a prograde evolution, during which these formations started to melt. The peak of the metamorphism was reached at 875 ± 50 °C and 7.5 ± 1 kbar (Fig. 3(B)). This was followed by decompression and a fall in temperature that brought the rocks to conditions of 700 °C and 3–4 kbar. The earliest reactions led to the appearance of garnet following the destabilization of biotite or gedrite, in relation to reactions of the type: Bt + Sill + Qtz ± Pl ) Grt +
Melt + Kfs ± Crd and Ged + Sill + Qtz ) Grt + Crd + Melt. The peak is represented by the presence of aluminous orthopyroxene which later exsolved garnet in the metapelites without sillimanite. In the magnesian metapelites with sillimanite, on the other hand, the highest pressures are indicated by the composition of garnets that reach an XMg value of 51%. During decompression from 7 to 4 kbar, cordierite appeared in coronas around garnet or biotite, as well as in symplectites with spinel (Spl) and/or orthopyroxene, according to reactions such as Grt + Sill ) Crd + Spl; Bi + Sill ) Crd + Spl + Kfs (melt or vapour); Grt + Qtz ) Opx + Crd (Fig. 2(C)) and Grt ) Spl + Crd + Opx + P1. In the metapelites without sillimanite and with primary orthopyroxene, the exsolution of garnet by an aluminous primary orthopyroxene (Al2O3 > 6.5 wt%) indicates a stage of falling temperature at relatively high pressures (Fig. 2(D)). The final stage (600 °C at 3–4 kbar) corresponds to a fall of temperature that, for example, is reflected in gedrite-bearing granulites by parageneses with anthophyllite. The same P–T path is exhibited by granulites with fayalite–quartz–ferrosilite (Bendaoud et al., 2003). The physical conditions of peak assemblage, garnet–clinopy-
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roxene–amphibole–plagioclase–quartz, correspond to pressures of 7.1 ± 1 kbar at temperatures of 880 ± 60 °C and a H2O of 0.2. During decompression, the clinopyroxene in contact with garnet and quartz initially broke down into orthopyroxene + plagioclase. The P–T conditions computed for this paragenesis are around 750 °C and 5 kbar. Then the orthopyroxene reacted with garnet to produce fayalite + plagioclase symplectites. The latest stage corresponds to the orthopyroxene–fayalite–quartz– plagioclase assemblage reflecting low pressures 3– 4 kbar at temperature of 700 °C.
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Following a petrological and geochronological study (Sm–Nd on garnet) in the south of this same area, Lie´geois et al. (2003) determined a metamorphic evolution path (Fig. 3(C)) at the base of the Pan-African nappes that passes successively through the following conditions: 17 kbar at 790 °C (eclogite 690 Ma), 12 kbar at 830 °C (garnet amphibolite 686 Ma), 8 kbar at 700 °C (kyanite–garnet gneisses) and 4 kbar at 500 °C (greenschist 530 My).
5. Discussion and conclusion 4. Tin Begane The area of Tin Begane (Fig. 1(D)), located at 110 km to the South of Tamanrasset, shows formations equivalent to those of the preceding areas. However, it is characterized by the persistence of kyanite (Ky) relics in the metapelites and the presence of eclogitic layers in the southern part of the area. The metasedimentary succession is made up of olivine-spinel marbles intercalated with garnet–sillimanite–biotite metapelites containing kyanite relics (Fig. 2(E)) armoured by garnet. In this area, there are also found iron-bearing quartzites with hercynite–almandite–faylite and lenses of metabasite both with and without garnet. Mylonitic contacts separate the metasedimentary succession from migmatitic orthogneisses. The metabasites of the northern part of Tin Begane show geochemical compositions essentially comparable with continental tholeiites (Derridj, 2000). They differ from the metabasites of the preceding areas by the presence of three successive parageneses, the latest contains spinel. The primary paragenesis is Grt–Cpx (A12O3 = 7.15 wt%; Na2O = 1.15 wt%; XMg = 0.80)–Pl (An47)–Hbl–Qtz–Rutile. This paragenesis is remarkable for the composition of the garnet, with contents of pyrope and grossularite reaching 38% and 30%, respectively (Alm32 Py38 Grs30), values that are characteristic of very-high pressure. Thermobarometric studies of this paragenesis indicate conditions of 13.5 ± 1.5 kbar and 860 ± 60 °C (Fig. 3(C)). These conditions are located at the boundary between the granulite and the eclogite facies. The secondary paragenesis, with Opx–Pl (An60) ± Grt2 (Alm47 Py31 Grs20 Sps2) ± Hbl2, results primarily from the reactions: Grt + Cpx + Qtz ) Opx + Pl (Fig. 2(F)) and Grt + Hbl1 + Qtz ) Opx + Pl + Hbl2 (symplectitic stage 1). The latest paragenesis is formed at the expense of garnet in cracks, owing to the reaction: Grt ) Opx + Spl + Pl (An95) (symplectitic stage 2). The corresponding conditions for these two parageneses are 10.7 ± 1.3 kbar at 790 ± 60 °C and 4.8 ± 1.3 kbar at 750 °C, respectively (Derridj et al., 2003). The metapelites of the country rocks and the olivine-clinopyroxene marbles yield conditions equivalent to those recorded by the two later parageneses of the metabasites.
In several areas of the Laouni terrane, observed granulitic formations are commonly associated with an important migmatitic event. Petrological, thermobarometric and fluid-inclusion studies allow us to determine a clockwise path with a decompression stage generating spectacular coronitic and symplectitic textures in both the para- and ortho-derived metamorphic units. The succession of parageneses during this decompression is a function of various chemical compositions. The metapelites or microdomains rich in silica and magnesium are characterized by the appearance of an orthopyroxene–cordierite association at the expense of garnet, quartz and biotite in the absence of sillimanite. The metapelites and microdomains rich in aluminium and iron make up assemblages with spinel-cordierite, but without orthopyroxene, following the destabilization of garnet, sillimanite and biotite. This explains the absence of orthopyroxene in Tin Begane and Tamanrasset, where the metapelitic formations are highly ironenriched and aluminous. During the prograde stage, the formation of inclusions in the core of primary garnet indicates that the kyanite field was traversed in the Tin Begane area, while only sillimanite is present in Tamanrasset and Tidjenouine. In the three studied areas, the peak of temperature is situated at around 850 ± 25 °C (Fig. 3), whereas the pressure peak is very variable since it ranges from 7.5 kbar (Tidjenouine) through 10.5 kbar (Tamanrasset) to 13.5 kbar (Tin Begane). This implies the possibility that different structural levels can be observed in the Laouni terrane. Two types of metabasites are also distinguished, one with tholeiitic intra-plate affinity and the other showing arc affinity. Although Bertrand and Jardim de Sa´ (1990) and Ouzegane et al. (2001) propose that the observations are more likely compatible with an Eburnean orogeny, one of the fundamental problems in the interpretation of this metamorphism and the nature of the metabasites involved is the lack of precise dating. Several geologists prefer to attribute a Pan-African age to the granulites, integrating the petrogensis and exhumation of these rocks into the pan-African orogeny of Central Hoggar (e.g. Caby, 2003; Barbey et al., 1989). However, the
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age of 2000 Ma obtained on the Tidjenouine formations indeed seems to refer to the age of the granulitic protolith. In a critical review of the published U–Pb ages from Hoggar, Bertrand (1998, unpublished—written communication) indicates that the zircons dated at 2069 ± 49 Ma in Tidjenouine granulites (Bertrand et al., 1986) are clearly metamorphic—with rounded spheroidal shapes—and that the lower intercept obtained here (at around 530 Ma) is an analytical artefact. In the Tin Begane area, the isotopic data (Rb–Sr and Sm–Nd on whole rocks) of various rock types with granulitic relics would appear to indicate Pan-African ages or at least an important Pan-African reworking (Lie´geois, written communication). In the same way, the pressure–temperature path determined on these granulites does not appear to contradict, within the limits of uncertainty, the path given for the metabasites containing eclogitic relics (Fig. 3(C)). Dating work in progress on garnets (Sm–Nd) from the granulitic parageneses should soon make it possible to resolve these uncertainties and provide a better understanding of the geodynamic evolution of these areas.
Acknowledgments We thank P. Ho¨ltta¨ and L. Solari, reviewers, for their constructive and very helpful remarks that have significantly improved the quality of this work. M.S.N. Carpenter is thanked for help with the English. We also thank Abdelkader Regagda for the field facilities. This work is a contribution to the projects PNR AU 19943 and to NATO EST/CLE 979766.
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