A review of Archaean and Paleoproterozoic evolution of the In Ouzzal granulitic terrane (Western Hoggar, Algeria)

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.     

Granulitic metamorphism in the Laouni terrane (Central Hoggar, Tuareg Shield, Algeria)

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.     

The LATEA metacraton (Central Hoggar, Tuareg shield, Algeria): behaviour of an old passive margin during the Pan-African orogeny

Historically, the Tuareg shield is divided into three parts bordered by mega-shear zones with the centre, the Central Polycyclic Hoggar, characterized by Archaean and Palaeoproterozoic lithologies. Nearly 10 years ago, the Tuareg shield was shown to be composed of 23 displaced terranes [Geology 22 (1994) 641] whose relationships were deciphered in Aïr to the SE [Precambr. Res. 67 (1994) 59]. The Polycyclic Central Hoggar terranes were characterized by the presence of well preserved Archaean/Palaeoproterozoic and Neoproterozoic lithologies.We show here that the terranes from Central Hoggar (Laouni, Azrou-n-Fad, Tefedest, Egéré-Aleksod) belonged to a single old passive margin, to which we gave the acronym name LATEA, which behaved as a craton during the Mesoproterozoic and the Early-Middle Neoproterozoic but was partly destabilized and dissected during the Late Neoproterozoic as a consequence of its involvement as a passive margin in the Pan-African orogen.An early Pan-African phase consisted of thrust sheets including garnet-bearing lithologies (eclogite, amphibolite, gneiss) that can be mapped and correlated in three LATEA terranes. In the Tin Begane area, P–T–t paths have been established from>15 kbar––790 °C (eclogite) to 4 kbar––500 °C (greenschist retrogression) through 12 kbar––830 °C (garnet amphibolite) and 8 kbar––700 °C (garnet gneiss), corresponding to the retrograde path of a Franciscan-type loop. Sm–Nd geochronology on minerals and laser ablation ICP-MS on garnet show the mobility of REE, particularly LREE, during the retrograde greenschist facies that affects, although slightly, some of these rocks. The amphibolite-facies metamorphism has been dated at 685 ± 19 Ma and the greenschist facies at 522 ± 27 Ma. During the thrust phase, the Archaean–Palaeoproterozoic basement was only locally affected by the Pan-African tectonics. LATEA behaved as a craton. Other juvenile terranes were also thrust early onto LATEA: the Iskel island arc at ≈850 Ma to the west of LATEA, the Serouenout terrane in the 700–620 Ma age range to the east. No subduction-related magmas have intruded LATEA during this epoch, which behaved as a passive margin.During the main Pan-African phase (625–580 Ma), LATEA was dissected by mega-shear zones that induced several hundreds km of relative displacement and allowed the emplacement of high-K calc-alkaline batholiths. Smaller movements continued till 525 Ma, accompanied by the emplacement of subcircular plutons with alkaline affinity. Here is dated the Ounane granodiorite (624 ± 15 Ma; ⁸⁷Sr/⁸⁶Sr_i=0.70839 ± 0.00016; 6WR, MSWD=0.87) and the Tisselliline granite (552 ± 15 Ma; ⁸⁷Sr/⁸⁶Sr_i=0.7074 ± 0.0001; 5WR, MSWD=1.4). Nd isotopes indicate a preponderant Palaeoproterozoic crustal source for these two plutons: _Nd=−14 to −21 at 624 Ma and T_DM=1650–2320 Ma for Ounane and _Nd=−13 to −15 at 555 Ma and T_DM=1550–1720 Ma for Tisselliline. Our model links these intrusions to a linear lithospheric delamination along mega-shear zones, allowing the hot asthenosphere to rise, melt by adiabatic pressure release and inducing the melting of the Palaeoproterozoic and Archaean lower crust.The LATEA cratonic microcontinent remained however sufficiently rigid to preserve Archaean and Palaeoproterozoic lithologies as well as Middle Neoproterozoic oceanic thrust sheets. This corresponds to the notion of metacraton [J. African Earth Sci. 34 (2002) 119], i.e. a craton that has been remobilized during an orogenic event but is still recognizable dominantly through its rheological, geochronological, isotopic and sometimes petrological characteristics.     

U–Pb zircon (TIMS and SIMS) and Sm–Nd whole-rock geochronology of the Gour Oumelalen granulitic basement, Hoggar massif, Tuareg shield, Algeria

Two major granulitic units are recognized in the Gour Oumelalen area. One of the units is composed partially of Archean gneisses (Red Gneiss complex) with U–Pb zircon SIMS and TIMS ages of approximately 2.7 Ga. Although they were formed from 3.0- to 3.2-Ga-old precursors, as indicated by Nd model ages, we find no evidence of any older history (≈3.5 Ga) as suggested by previous Pb–Pb ages. The other formation (Gour Oumelalen supergroup) is a metasedimentary sequence at least partly of Paleoproterozoic age, as indicated by zircon dates of a metavolcanic rock at approximately 2.2 Ga. A later magmatic event is recorded at approximately 1.9 Ga in both units and related to coeval granulite-facies metamorphism that affected both units. Nd model ages at approximately 2.0 Ga suggest an accretion of juvenile crust formation at that time. The existence of T_DM Nd model ages intermediate between 2.5 and 2.9 Ga could result from the mixing of 3.2 and 2.0-Ga-old material or may reflect separate events.     

Geochemistry and isotope (Sr, Nd, O) study of Al–Mg granulites from the In Ouzzal Archaean block (Hoggar, Algeria)

The In Ouzzal Al–Mg granulites are found within sedimentary units deposited after 2.7 Ga, the whole association being metamorphosed under extreme temperature conditions (c. 1000 °C) at 2 Ga. The Al–Mg granulites are interlayered with other metasediments, including metapelites, quartzites and magnetite-bearing quartzites, forsterite-spinel marbles, and a few meta-igneous rocks (mainly pyroxenites). They do not occur at a specific position in the sedimentary suite, and they do not reflect any particular structural control. The major and trace element compositions of Al–Mg granulites (especially the high Cr, Ni, Co contents) show that their peculiar ‘refractory’ chemistry is more compatible with premetamorphic sedimentary characteristics rather than with metasomatic, metamorphic or partial melting processes. Sedimentary admixtures of a common mature detrital component coming from the weathering of the local acidic igneous crustal protoliths (normal pelitic component) with an extremely immature component derived from reworking of basic/ultrabasic lithologies (Al–Mg–Cr–Co–Ni–rich chloritic component) is consistent with the geochemistry of such rocks. As in other instances, the quartz-garnet oxygen isotopic thermometer here records an apparent temperature close to the peak metamorphism (c. 1000 °C). Although the persistence of pre-existing δ¹⁸O variations on a small scale during the metamorphism does not support a major pervasive fluid flow during metamorphism, it does not rule out the presence of syn- to post-metamorphic CO₂. The low δ¹⁸O (c.+ 5 to + 6‰) of the most typical Al–Mg granulites indicate that the ‘chloritic component’ in these rocks was derived from hydrothermally altered mafic/ultramafic protoliths rather than dominantly from palaeosols. It is suggested that the presence of such Al–Mg–Cr–Co–Ni–rich sediments is indirect evidence for the presence of greenstone belts in the local crust of the In Ouzzal at 2.6–2.7 Ga.     

The Anfeg post-collisional Pan-African high-K calc-alkaline batholith (Central Hoggar, Algeria), result of the LATEA microcontinent metacratonization

The Anfeg batholith (or composite laccolith) occupies a large surface (2000 km²) at the northern tip of the Laouni terrane, just south of Tamanrasset in Hoggar. It is granodioritic to granitic in composition and comprises abundant enclaves that are either mafic microgranular enclaves (MME) or gneissic xenoliths. It intruded an Eburnian (≈2 Ga) high-grade basement belonging to the LATEA metacraton at approximately 608 Ma (recalculated from the U–Pb dating of [Tectonics 5 (1986) 955]) and cooled at approximately 4 kbar, with a temperature of about 750 °C. This emplacement occurred mainly along subhorizontal thrust planes related to Pan-African subvertical mega-shear zones close to the attachment zone of a strike-slip partitioned transpression system. Although affected by some LILE mobility, the Anfeg batholith can be ascribed to a high-K calc-alkaline suite but characterized by low heavy REE contents and high LREE/HREE ratios. The MME belong to the Anfeg magmatic trend while some xenoliths belong to Neoproterozoic island arc rocks.The Anfeg batholith defines a Nd–Sr isotopic initial ratios trend (_Nd/(⁸⁷Sr/⁸⁶Sr)_i from −2.8/0.7068 to −11.8/0.7111) pointing to a mixing between a depleted mantle and an old Rb-depleted granulitic lower crust. Both sources have been identified within LATEA and elsewhere in the Tuareg shield (_Nd/⁸⁷Sr/⁸⁶Sr)_i of +6.2/0.7028 for the depleted mantle, −22/0.708 for the old lower crust.The model proposed relates the above geochemical features to a lithospheric delamination along the subvertical mega-shear zones that dissected the rigid LATEA former passive margin without major crustal thickening (metacratonization) during the general northward tectonic escape of the Tuareg terranes, a consequence of the collision with the West African craton. This delamination allowed the uprise of the asthenosphere. In turn, this induced the melting of the asthenosphere by adiabatic pressure release and of the old felsic and mafic lower crust due to the high heat flow. A gradient in the mantle/crust ratio within the source of the Pan-African magmatism is observed in LATEA from the northeast (Egéré-Aleksod terrane) where rare plutons are rooted within the Archaean/Eburnian basement to the southwest (Laouni terrane) where abundant batholiths, including Anfeg, have a mixed signature. Some mantle melts with only slight crustal contamination (Laouni troctolitic layered intrusions) are even present. This suggests that the southern boundary of LATEA microcontinent is not far south of the Tuareg shield.     

Corundum–quartz-bearing assemblage in the Ihouhaouene area (In Ouzzal, Algeria)

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.     

The “Taourirt” magmatic province, a marker of the closing stage of the Pan-African orogeny in the Tuareg Shield: review of available data and Sr–Nd isotope evidence

The Tuareg Shield, located between the Archaean to Palaeoproterozoic Saharan metacraton and the West African craton, is composed of 23 recognized terranes that welded together during the Neoproterozoic Pan-African orogeny (750–520 Ma). Final convergence occurred mainly during the 620–580 Ma period with the emplacement of high-K calc-alkaline batholiths, but continued until 520 Ma with the emplacement of alkali-calcic and alkaline high-level complexes. The last plutons emplaced in central Hoggar at 539–523 Ma are known as the “Taourirt” province. This expression is redefined and three geographical groups are identified: the Silet-, Laouni- and Tamanrasset-Taourirts. The Silet-Taourirts are cross-cutting Pan-African island arc assemblages while the two others intrude the Archaean–Palaeoproterozoic LATEA metacraton. The Taourirts are high-level subcircular often nested alkali-calcic, sometimes alkaline, complexes. They are aligned along mega-shear zones often delimiting terranes. Mainly granitic, they comprise highly differentiated varieties such as alaskite (Silet-Taourirts) and topaz–albite leucogranite (Tamanrasset-Taourirts). Different subgroups were defined on the basis of REE patterns and major and other trace elements. The Taourirt province displays a wide transition from dominant alkali-calcic to minor alkaline granite varieties. Sr isotopes indicate that these complexes were affected by fluid circulation during the Ordovician along shear zones probably contemporaneous to the beginning of the Tassilis sandstone deposition. Nd isotope systematic indicates a major interaction with the upper crust during the emplacement of highly differentiated melts, particularly in samples showing seagull wing-shaped REE patterns. On the other hand, all Taourirt plutons are strongly contaminated by the lower crust: _Nd vary from −2 to −8 and T_DM from 1200 to 1700 Ma. This implies the presence of an old crust at depth, also below the Silet-Taourirts, which are emplaced within Pan-African island arc assemblages. A model is proposed for the genesis of the Taourirt province where reworking of the mega-shear zones, which dissected the LATEA metacraton, provoked a linear delamination of the lithospheric mantle, asthenosphere uprise and partial melting of the lower crust (or strong interaction with), giving rise to a mixed source.     

The alkaline–peralkaline granitic post-collisional Tin Zebane dyke swarm (Pan-African Tuareg shield, Algeria): prevalent mantle signature and late agpaitic differentiation

The Tin Zebane dyke swarm was emplaced at the end of the Pan-African orogeny along a mega-shear zone separating two contrasting terranes of the Tuareg shield. It is located along the western boundary of the Archaean In Ouzzal rigid terrane, but inside the adjacent Tassendjanet terrane, strongly remobilized at the end of the Precambrian. The Tin Zebane swarm was emplaced during post-collisional sinistral movements along the shear zone at 592.2±5.8 Ma (19WR Rb–Sr isochron). It is a dyke-on-dyke system consisting of dykes and stocks of gabbros and dykes of metaluminous and peralkaline granites. All rock types have Sr and Nd isotopic initial ratios (Sr_i=0.7028 and _Nd=+6.2) typical of a depleted mantle source, similar to the prevalent mantle (PREMA) at that period. No crustal contamination occurred in the genesis of the Tin Zebane swarm. Even the samples showing evidence of fluid interaction (essentially alkali mobility) have the same isotopic signature. The peralkaline granites have peculiar geochemical characteristics that mimic subduction-related granites: this geochemical signature is interpreted in terms of extensive differentiation effects due to late cumulates comprising aegirine, zircon, titanite, allanite and possibly fergusonite, separated from the liquid in the swarm itself due to magmatic flow turbulence. The Tin Zebane dyke swarm is thus of paramount importance for constraining the differentiation of mantle products to generate highly evolved alkaline granites without continental crust participation, in a post-collisional setting.     

U–Pb dating of the end of the Pan-African orogeny in the Tuareg shield: the post-collisional syn-shear Tioueine pluton (Western Hoggar, Algeria)

The Tioueine pluton intrudes the Neoproterozoic series of the Iskel terrane, located in the Tuareg shield, western Hoggar. The consistency of the internal structures as well as the nature and organization of the associated microstructures demonstrate that the Tioueine pluton was emplaced syn-kinematically while N–S strike–slip shear zones were active. The syn-tectonic emplacement of the Tioueine massif implies that this pluton, although belatedly crystallized, entirely belongs to the concept of post-collisional magmatism. In order to date precisely the late Pan-African tectono-metamorphic event in the studied area, an U–Pb age of 523±1 Ma was obtained from abraded zircons of a late quartz–syenite from the Tioueine pluton. This early Cambrian age is younger than the other plutons of the Tuareg shield, which were mainly emplaced between 630 Ma and 580 Ma. This dating also shows that the Tuareg shield was not a single coherent block at 525 Ma, but rather an amalgam of active terranes moving each other along major shear zones. Finally, the Tioueine massif represents probably the final welding of the Tuareg shield assembly of terranes and consequently the end of the post-collisional orogenic episode in the whole Pan-African belt.     

Isotopic (O, Sr, Nd) and trace element geochemistry of the Laouni layered intrusions (Pan-African belt, Hoggar, Algeria): evidence for post-collisional continental tholeiitic magmas variably contaminated by continental crust

The three layered intrusions studied in the Laouni area have been emplaced within syn-kinematic Pan-African granites and older metamorphic rocks. They have crystallized at the end of the regional high-temperature metamorphism, but are free from metamorphic recrystallization, revealing a post-collisional character. The cumulate piles can be interpreted in terms of two magmatic liquid lines of descent: one is tholeiitic and marked by plagioclase–olivine–clinopyroxene cumulates (troctolites or olivine bearing gabbros), while the other is calc-alkaline and produced orthopyroxene–plagioclase rich cumulates (norites). One intrusion (WL (West Laouni)-troctolitic massif), shows a Lower Banded Zone where olivine-chromite orthocumulates are interlayered with orthopyroxene-rich and olivine–plagioclase–clinopyroxene cumulates, whereas the Upper Massive Zone consists mainly of troctolitic and gabbroic cumulates. The other two massifs are more homogeneous: the WL-noritic massif has a calc-alkaline differentiation trend whereas the EL (East Laouni)–troctolitic massif has a tholeiitic one. Separated pyroxene and plagioclase display similar incompatible trace element patterns, regardless of the cumulate type. Calculated liquids in equilibrium with the two pyroxenes for both noritic and troctolitic cumulates are characterized by negative Nb, Ta, Zr and Hf anomalies and light REE enrichment inherited from the parental magmas. Troctolitic cumulates have mantle-derived δ¹⁸O (+5 to +6‰), initial ⁸⁷Sr/⁸⁶Sr (Sr_i=0.7030 to 0.7054), _Nd (+5 to −1) values whereas noritic cumulates are variably enriched in δ¹⁸O (+7 to +9‰), show negative _Nd (−7 to −12) and slightly higher Sr_i (0.7040–0.7065). Based on field, isotopic ratios are interpreted as resulting from a depleted mantle source (Sr_i=0.7030; _Nd=+5.1; δ¹⁸O=+5.1‰) having experience short term incompatible element enrichment and variable crustal contamination. The mantle magma was slightly contaminated by an Archaean lower crust in troctolitic cumulates, more strongly and with an additional contamination by an Eburnian upper crust in noritic cumulates. Lower crust input is recorded mainly by Sr and Nd isotopes and upper crust input by O isotopes. This is probably due to the different water/rock ratios of these two crust types. Assimilation of low amounts (<10%) of quartz-bearing felsic rocks, coming from both lower and upper crust, can explain the rise of SiO₂ activity, the enrichment in ¹⁸O and ⁸⁷Sr and the lowering of _Nd in the noritic cumulates compared to troctolitic ones. The geodynamic model proposed to account for the Laouni tholeiitic magmatism involves a late Pan-African asthenospheric rise due to a rapid lithospheric thinning associated with functioning of shear zones, which allowed tholeiitic magmas to reach high crustal levels while experiencing decreasing degrees of crustal contamination with time.     

The Temaguessine Fe-cordierite orbicular granite (Central Hoggar, Algeria): U–Pb SHRIMP age, petrology, origin and geodynamical consequences for the late Pan-African magmatism of the Tuareg shield

The Temaguessine high-level subcircular pluton is intrusive into the LATEA metacraton (Central Hoggar) Eburnian (2 Ga) basement and in the Pan-African (615 Ma) granitic batholiths along a major NW–SE oriented major shear zone. It is dated here (SHRIMP U–Pb on zircon) at 582 ± 5 Ma. Composed of amphibole–biotite granite and biotite syenogranite, it comprises abundant enclaves: mafic magmatic enclaves, country-rock xenoliths and remarkable Fe-cordierite (#Fe = 0.87) orbicules. The orbicules have a core rich in cordierite (40%) and a leucocratic quartz–feldspar rim. They are interpreted as resulting from the incongruent melting of the meta-wacke xenoliths collapsed into the magma: the breakdown of the biotite + quartz assemblage produced the cordierite and a quartz–feldspar minimum melt that is expelled, forming the leucocratic rim. The orbicule generation occurred at T < 850° and P < 0.3 GPa. The Fe-rich character of the cordierite resulted from the Fe-rich protolith (wacke with 4% Fe₂O₃ for 72% SiO₂). Strongly negative ε_Nd (−9.6 to −11.2), Nd T_DM model ages between 1.64 and 1.92 Ga, inherited zircons between 1.76 and 2.04 Ga and low to moderately high I_Sr (0.704–0.710) indicate a Rb-depleted lower continental crust source for the Temaguessine pluton; regional considerations impose however also the participation of asthenospheric material. The Temaguessine pluton, together with other high-level subcircular pluton, is considered as marking the end of the Pan-African magma generation in the LATEA metacraton, resulting from the linear delamination along mega-shear zones, allowing asthenospheric uprise and melting of the lower continental crust. This implies that the younger Taourirt granitic province (535–520 Ma) should be considered as a Cambrian intraplate anorogenic event and not as a very late Pan-African event.     

Metasomatic effects related to channelled fluid streaming through deep crust: fenites and associated carbonatites (In Ouzzal Proterozoic granulites, Hoggar, Algeria)

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.     

Metallogenesis of the late Pan-African gold-bearing East Ouzzal shear zone (Hoggar, Algeria)

In the W Hoggar (Algeria), the major transcurrent N–S East Ouzzal shear zone (EOSZ) hosts several world-class gold deposits over a 100-km length. The late Pan-African EOSZ separates two contrasting Precambrian domains: the Archaean In Ouzzal block to the west (orthogneisses with subordinate metasediments, reworked and granulitized in the c. 2 Ga Eburnean event) and a Middle Proterozoic block to the east (again orthogneisses and metasediments, involved in the c. 600 Ma Pan-African event). The EOSZ is a mylonite belt, 1–3 km wide, with a 50-m-wide ultramylonite belt hosting numerous quartz veins and lenses (giant hydrothermal quartz system) associated with a quartz-sericite-pyrite-carbonate (beresite) alteration. These hydrothermal events occurred under ductile (evolving towards brittle) conditions, between 500 and 300 MPa, at 500–300°C, with aqueous-carbonic fluids derived both from underlying devolatilized metamorphic rocks and a mantle source, as recorded by stable (C, O) isotope data. No gold mineralization was associated with these typical mesothermal events. Following a pressure drop (to 130 MPa), related to the inception of extensional tectonics, the EOSZ was later percolated by a new set of hydrothermal fluids, evolved from basinal waters that deeply penetrated into the In Ouzzal basement. These fluids were Ca-bearing brines (up to 25% wt. eq. NaCl), characterized by high δD (-9 to + 18‰ range), mobilized by the thermal energy released by the late Pan-African granite magmatism (Taourirt granites). As demonstrated by Pb isotope data, the brines leached Au from the In Ouzzal granulites (which contain 3 ppb Au). Fluid inclusion studies indicate that gold was deposited from these brines in the EOSZ at a depth of c. 5 km, due to mixing and cooling with descending diluted fluids.     

碰撞造山带超高温变质作用及构造意义——以泛非造山带为例

超高温（≥900℃）变质作用发生在自太古代以来的各个地质历史时期,目前极可能也正发生在青藏高原地壳深部。同时,它也是以冈瓦纳为代表的超大陆在最终拼合时的显著标识,这一关联指示了超高温变质作用与碰撞造山带的密切关系。本文总结了东冈瓦纳内与泛非造山作用有关的典型超高温变质岩的分布、岩石学特征、峰期变质条件、P-T轨迹及形成时代,并简要介绍我们在柴达木地块西段新识别出的泛非期超高温变质作用的基本特征。结合东冈瓦纳超高温变质作用特征和造山带热模拟研究的新进展,本文获得以东冈瓦纳超高温变质作用为代表的碰撞造山带超高温变质作用的几点认识：1）东冈瓦纳麻粒岩地块中的超高温变质岩和普通麻粒岩记录了相似的变质年龄、P-T轨迹以及呈过渡变化的峰期温度,两者可能是同一构造事件的产物,共同组成一个高温-超高温变质岩单元;2）超高温变质作用在东冈瓦纳内部持续了至少超过30Myr,但未见呈大规模的同期或近同期基性岩岩浆出露,指示此处需要的长期热源不是地幔来源岩浆;3）虽然数值模拟能成功呈现加厚地壳被放射元素衰变热加热至超高温条件的情况,且加热及持续时间与东冈瓦纳超高温变质约束的结果相当,但是模拟中需要的高生热值暗示,在自然界中,完全只靠放射性元素衰变生热或许不能让碰撞造山带内达到超高温条件;4）碰撞造山带经历了长期的构造演化,这一过程中,造山带内地壳不太可能同时达到超高温变质条件,这一特征可能反映在P-T-t轨迹的差异上,对这些轨迹的系统研究有助于对超高温变质作用的构造-热过程的理解。     

An example of post-collisional mafic magmatism: the gabbro–anorthosite layered complex from the Tin Zebane area (western Hoggar, Algeria)

The Tin Zebane gabbro–anorthosite layered mafic intrusion represented by plagioclase-rich cumulates forms a set of small lenticular to round-shaped mainly undeformed bodies intruding the Pan-African high-pressure metamorphic rocks from western Hoggar (Tuareg shield, southwest Algeria). The coarse-grained anorthosites are mainly made of slightly zoned bytownite (An_86–74) with the higher anorthite content at the cores. Anorthosites are interlayered with leucogabbros and gabbros that show preserved magmatic structures and with olivine gabbros characterised by coronitic textures. The primary assemblage in gabbros includes plagioclase (An_93–70), olivine (Fo_77–70), zoned clinopyroxene (En_43–48Fs_05–13Wo_41–49 with Al₂O₃ up to 4.3 wt.%) and rare orthopyroxene (En_73–78). Pyroxenes and olivine are commonly surrounded by Ca-amphibole. The olivine–plagioclase contact is usually marked by a fine orthopyroxene–Cr-spinel–amphibole symplectite. A magnesian pigeonite (En_70–75Fs_19–20Wo_6–10) is also involved in corona. The coronitic minerals have equilibrated with the primary mineral rims at P–T–a_H2O conditions of 797 ± 42 °C for a_H2O=0.5 and 808 ± 44 °C for a_H2O=0.6 at 6.2 ± 1.4 kbar. The Tin Zebane gabbroic rocks are depleted in REE with a positive Eu anomaly, high Sr (>10 * chondrite) and Al₂O₃ concentrations (17–33%) that support plagioclase accumulation with the extreme case represented by the anorthosites. The REE patterns can be modelised using plagioclase, clinopyroxene and orthopyroxene REE signature, without any role played by accessory minerals. High MgO content points to olivine as a major cumulate phase. Anorthositic gabbros Sr and Nd isotopic initial ratios are typical of a depleted mantle source (Sr_i=0.70257–0.70278; _Nd=+5.9 to +7.8). This isotopic signature is identical to that of the 10-km wide 592 Ma old dyke complex composed of alkaline to peralkaline granites and tholeiitic gabbros and one single bimodal complex can be inferred. The source of the Tin Zebane basic rocks corresponds to the prevalent mantle (PREMA). The Tin Zebane complex was emplaced along the mega-shear zone bounding to the west the Archaean In Ouzzal metacraton. The model proposed suggests a linear lithospheric delamination along this rigid and cold terrane due to post-collisional transtensional movements. This allowed the asthenosphere to rise rapidly and to melt by adiabatic pressure release. Transtension along a rigid body allowed these mantle melts to reach the surface rapidly without any crustal contamination.     

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

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 T_DM 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.     

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

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.     

Blue amphiboles and their significance for the metamorphic history of the Pan-African Gariep belt, Namibia

The metamorphic history of mafic exotic blocks from a tectonic melange zone within an allochthonous ophiolitic terrane (Marmora Terrane) of the Pan-African Gariep orogenic belt in south-western Namibia was studied, based on mineral parageneses and amphibole composition. Glaucophane described previously from these rocks could not be verified. Instead, two types of blue amphiboles were distinguished: (i) rims of (ferro-) edenitic to pargasitic to barroisitic hornblende composition around brownish amphibole phenocrysts replacing magmatic clinopyroxene, and (ii) deep blue porphyroblasts of magnesio-riebeckite with little ferro-glaucophane component in a highly metasomatized albite-rich rock. Textural and mineralogical evidence, particularly the existence of up to three different amphibole generations in metagabbro samples, supports a multiphase metamorphic history experienced by these exotic blocks. The first metamorphic event, M1, is interpreted as very low- P hydrothermal oceanic metamorphism that affected the igneous protoliths at up to amphibolite facies temperatures. Subsequent M2 metamorphism was syntectonic and is characterized by temperatures similar to those attained during M1 but higher pressures indicating burial to 15–20 km. This event is related to a subduction process. The third metamorphic event, M3, was low grade and of regional nature. It is the only one recorded in the sedimentary envelope of the exotic blocks. The formation of magnesio-riebeckite is considered a retrograde reaction at greenschist facies during M2. The results indicate that in the Gariep belt subduction and subsequent obduction have occurred, although blueschist facies metamorphism has not been reached.     

Petrology and geodynamic significance of the post-collisional Pan-African magmatism in the Eastern Saghro area (Anti-Atlas, Morocco)

The Saghro Group consists of a thick volcanic-sedimentary sequence with intercalated basaltic lavas, the first magmatic event in eastern Saghro area. Nd isotopes of basaltic pillow lavas show T_DM model ages ranging from 640 to 580 Ma, which represent a maximum age for basalt eruption.Granitoids within the Saghro Group consist of a charnockitic suite, tonalites, granodiorites and monzogranites. They are high-K calc-alkaline (HKCA) with a post-collisional character, and were emplaced at high-levels in the crust. Their ages of emplacement are within the 580–560 Ma bracket, implying that the entire Saghro Group is slightly older than or partly coeval to granitoid emplacement and implying a common geodynamical setting. Sr–Nd isotopic compositions and Nd T_DM model ages point to a mixed origin, combining a juvenile mantle source and an Eburnean crustal component, which could be the West African Craton (WAC). The juvenile component in the Saghro granitoids could be the depleted upper mantle that has sourced the earlier basalts.Field observations, geochemical and geochronological data together support that, during the Pan-African orogeny, the Anti-Atlas was subjected to a regional transpressional to transtensional event. This event would have been responsible for the dissection of the northern margin of the WAC into several blocks, the development of deep sedimentary basins and the emplacement of HKCA magmas.