摘要:

ABSTRACTThe 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 atP=10 kbar andTrising 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‐

ISSN:0263-4929    

DOI:10.1111/j.1525-1314.1996.00048.x

出版商:Blackwell Science Inc    

年代:1996

数据来源: WILEY

摘要:

ABSTRACTThe 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 ofc.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 TDMindicates 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, exte

ISSN:0263-4929    

DOI:10.1111/j.1525-1314.1996.00054.x

出版商:Blackwell Science Inc    

年代:1996

数据来源: WILEY

摘要:

ABSTRACTQuartz 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 clockwiseP–Ttrajectory characterized by peak conditions of at least 10 kbar and 1050°C, followed by decompression from

ISSN:0263-4929    

DOI:10.1111/j.1525-1314.1996.00049.x

出版商:Blackwell Science Inc    

年代:1996

数据来源: WILEY

摘要:

ABSTRACTThe 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

ISSN:0263-4929    

DOI:10.1111/j.1525-1314.1996.00057.x

出版商:Blackwell Science Inc    

年代:1996

数据来源: WILEY

摘要:

ABSTRACTAl‐Mg granulites, with cordierite, garnet, sapphirine, orthopyroxene, sillimanite, spinel, phlogopite, K‐feldspar, plagioclase and variable quartz from Ihouhaouene (In Ouzzal, Algeria), display a range of decompression textures involving the breakdown of orthopyroxene and sillimanite, and of garnet. The succession of parageneses suggests that theP–T–tevolution corresponds to decompression with cooling from peak conditions of about 950°C and 10 kbar. This decompression path is obtained from the paragenetic analysis in the FMAS system. However, according to current KFMASH grids, thisP–T–tevolution should take place outside the stability field of phlogopite+quartz; yet this assemblage is probably stable during most of the P‐T evolution, notably during peak metamorphism. This discrepancy is interpreted as the effect of the high content of F in phlogopite which should shift its stability limit towards higher temperature. The consequences of this shift on the phase relationships in the KFeMASH system are investigated and it is concluded that a topological inversion could exist in the F

ISSN:0263-4929    

DOI:10.1111/j.1525-1314.1996.00045.x

出版商:Blackwell Science Inc    

年代:1996

数据来源: WILEY

摘要:

ABSTRACTSome granulites from the Amessmessa area (south In Ouzzal unit, Hoggar) contain the peak assemblage gedrite+garnet+sillimanite+quartz that was used to estimate theP–Tconditions of metamorphism. The rocks developed symplectites and corona textures by the breakdown of the primary paragenesis to orthopyroxene, cordierite and spinel. The successive parageneses formed in separate microdomains according to a clockwiseP–Tpath. Geothermometry, geobarometry and phase diagram calculations indicate that the textures formed by decompression and cooling from 7–9 kbar and 850–900°C to 3.5–4.5 kbar and 700–800°C. ThisP–Tevolution is consistent with low to mediumaH2O, between 0.4 and 0.7, and is similar to the metamorphic conditions deduced in Al–Mg granulites from the n

ISSN:0263-4929    

DOI:10.1111/j.1525-1314.1996.00047.x

出版商:Blackwell Science Inc    

年代:1996

数据来源: WILEY

摘要:

ABSTRACTGranulite 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 highP–Tconditions, above 1100°C

ISSN:0263-4929    

DOI:10.1111/j.1525-1314.1996.00046.x

出版商:Blackwell Science Inc    

年代:1996

数据来源: WILEY

摘要:

ABSTRACTThe 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 (ɛNd(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 describe

ISSN:0263-4929    

DOI:10.1111/j.1525-1314.1996.00055.x

出版商:Blackwell Science Inc    

年代:1996

数据来源: WILEY

摘要:

ABSTRACTIn 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 thec.2 Ga Eburnean event) and a Middle Proterozoic block to the east (again orthogneisses and metasediments, involved in thec.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 ofc.5 km, due to mixing and coo

ISSN:0263-4929    

DOI:10.1111/j.1525-1314.1996.00056.x

出版商:Blackwell Science Inc    

年代:1996

数据来源: WILEY

ISSN:0263-4929    

DOI:10.1111/j.1525-1314.1996.0vinp.x

出版商:Blackwell Science Inc    

年代:1996

数据来源: WILEY