Migmatitic Orthogneisses Research Articles

Archean crustal evolution of the West African Craton: example of the Amsaga Area (Reguibat Rise). UPb and SmNd evidence for crustal growth and recycling

New geochronological data (UPb and SmNd) are used to constrain the Archean crustal growth and evolution of the southwestern part of the Reguibat Rise (Amsaga area) in the West African craton. The results are summarized as follows. (1) UPb zircon data indicate an emplacement age of 2986 ± 8 Ma for the magmatic protolith of a charnockitic pluton, in agreement with a whole-rock SmNd isochron at 3012 ± 142 Ma. Positive ϵ Nd(3.0) values ranging between 0.89 and 1.95 and agreement between Nd model ages (between 3.23 and 3.10 Ga) and UPb age indicate a juvenile origin for the magmatic protolith and imply a crustal growth event at ca 3.0 Ga. Seven rims of the charnockitic zircons yield discordant 207Pb/ 206Pb ages ranging from 2979 ± 11 to 2737 ± 11 Ma and are interpreted as resulting from growth during the granulitic event. (2) Whole-rock SmNd data on migmatitic orthogneisses that form the central part of the area indicate an Archean age for their magmatic protolith ( T DM ranging from 3.24 to 2.87 Ga) in agreement with a previous single zircon evaporation age of 2836 ± 16 Ma (Potrel, 1994). (3) Two late-tectonic, postgranulite plutons (both granite and gabbro) dated at ca 2.73 Ga mark the end of the granulitic event. The granite provides a UPb age of 2726 ± 7 Ma and the gabbro a mineral SmNd isochron age of 2706 ± 54 Ma. The T DM ages ranging from 3.1 to 3.2 Ga indicate an origin by crustal reworking of the granite and negative ϵ Nd(2.7) value (−0.7) of the gabbro indicate crustal contamination during emplacement. These results, combined with previous work (Potrel et al., 1996), indicate that the Archean Reguibat Rise formed through a succession of crustal growth events from at least 3.50-3.45 Ga until 2.73 Ga. A major granulitic event took place at ca 2.73 Ga and was accompanied by emplacement of crustally derived magmas.

Time of metamorphism beneath the Central Metasedimentary Belt boundary thrust zone, Grenville Orogen, Ontario: accretion at 1080 Ma?

New U–Pb zircon and titanite data from the Muskoka domain, Grenville Province, Ontario, provide protolith and metamorphic ages for the southwestern Central Gneiss Belt. Discordant analyses from a migmatitic orthogneiss and its leucosome form a linear array with an upper intercept age of 1457 ± 6 Ma and a lower intercept age of 1064 ±18 Ma. U–Pb analyses on metamorphic zircon from an amphibolite yield a set of concordant analyses with an average 207Pb/206Pb age of 1079 ± 3 Ma. A weakly migmatitic granitoid rock and a transecting charnockitic vein in the immediate footwall of the Central Metasedimentary Belt boundary thrust zone yielded a discordant array of analyses wth an upper intercept age of 1394 ± 13 Ma and a lower intercept age of 1066 ± 8 Ma. The charnockitic vein yielded concordant zircon ages of 1077 ± 2 Ma. The upper intercept ages are interpreted in terms of protolith crystallization, and the concordant and lower intercept ages as Grenviilian high-grade metamorphism and associated anatexis. We have found no evidence for a ca. 1190–1160 Ma metamorphic event in these rocks, as required by some regional tectonic interpretations. We conclude that emplacement of the Central Metasedimentary Belt over the Central Gneiss Belt, which caused high-grade metamorphism in the Muskoka domain, occurred at or shortly before ca. 1080 Ma, and that this marks the time of accretion of the Central Metasedimentary Belt to the southeast margin of Laurentia.

Fluid evolution during high-pressure partial melting in the Austroalpine Ulten Zone, northern Italy

Fluid inclusions in high-grade metamorphic rocks of the Ulten Zone (northern Italy) have been investigated by microthermometry and Raman microspectrometry. Two different types of fluid inclusions can be distinguished: (1) high- to low density CO2±N2-CH4 inclusions in kyanite and quartz of coarse grained garnet-kyanite felses and (2) aqueous inclusions of high salinity in quartz of metapelites, paragneisses and amphibolites as well as migmatitic orthogneisses. The migmatites are thought to have formed on the prograde path to the eclogite-facies event (M1) under fluid oversaturation. Kyanite and quartz of the garnet-kyanite felses contain a CO2-dominated fluid, which may result from a higher solubility of H2O in the melt. A saline, H2O-rich fluid occurs as early inclusions forming small clusters in quartz of the leucosome from migmatitic orthogneisses and as late trail-bound inclusions of lower density in quartz of metasediments and amphibolites. During uplift and crystallization of the melt, this fluid was released and together with external highly saline aqueous fluid added to the rocks, re-equilibration of paragneisses, metapelites and metabasites under amphibolite-facies conditions (M2) was enabled.

Gneiss complex of the Skærfjorden region (76°-78°N), North-East Greenland

The crystalline rocks of the Skærfjorden region are dominated by grey migmatitic orthogneisses with smaller bodies of ultramafic, mafic and metasedimentary lithologies. Archaean rocks (c. 3.0 Ga) have been documented, but crustal additions were predominantly Early Proterozoic (2.0–1.75 Ga) in age, reflecting subduction zone magmatism. FolIowing Early Proterozoic orogenesis and migmatisation, granitic sheets (c. 1.7 Ga) and dolerite dykes were intruded. These intrusive rocks were subsequently deformed and metamorphosed in the Caledonian orogeny (c. 400 Ma). The Skærfjorden region is located in the Caledonian hinterland, where orogen parallel stretching and extension followed an early nappe stage.

The crystalline rocks of Germania Land, Nordmarken and adjacent areas, North-East Greenland

The crystalline rocks of Germania Land, Nordmarken and adjacent areas are dominated by grey, migmatitic quartzo-feldspathic orthogneisses with a complex history of emplacement and deformation. Ultramafites, eclogites and metasedimentary rocks are preserved as inclusions or trails of enclaves within the migmatitic orthogneisses. Later intrusive suites include metadolerite dykes, alkali feldspar metaporphyries and metagabbros. All these crystalline rocks have suffered thorough metamorphism and deformation, and three fabric events are recognised. Three systems of late mylonite zones are found: west of the study area in Hertugen af Orleans Land, along the east side of Stormlandet and Germania Land, and in northern Store Koldewey and Lille Koldewey. The region lies within the East Greenland Caledonian fold belt. Available isotopic data suggest the crystalline rocks include Archaean and Early Proterozoic suites. These have undergone variable degrees of later reworking on several occasions. It is unclear how much of the deformation history is Caledonian and how much pre-Caledonian. The latest fault displacements are post-Caledonian, as shown by local preservation of fault bounded outcrops of Carboniferous and Jurassic sedimentary rocks.

Geological and Petrographical Investigations for the Mountain Hazards Mapping Project, Khumbu Himal, Nepal

Geological and petrographical investigations of the Khumbu area, Nepal, were undertaken to provide basic data for the United Nations University mountain hazards mapping project. Six litho-tectonic units were distinguished by petrographical means: Basal Paragneisses, Migmatitic Orthogneisses, Upper Khumbu Gneiss Complex, Upper Paragneisses, Tertiary Leuco-granites, Everest Meta-Sediments. Investigations of subsurface conditions showed that the topographical relief of the area depends largely on lithology, structures, and the depositional or erosional character of the Late-Cenozoic glaciations. Mass wasting plays a major role in the accumu- lation of debris cover. The valley system has been interpreted as a deep pre-glacial primitive surface, with V-shaped troughs and elevated platforms, which have been overprinted by glaciation. Erosional activity of the river system is occurring at present although most of the landforms are relatively stable.

The Feiran-Solaf gneiss belt, southwestern Sinai, Egypt

High-grade Archaean rocks of the remote Thrym Complex of southeastern Greenland expose a lower crustal section that formed at a depth of > 30 km at temperatures reaching 1000 °C and pressures approaching 1.5 GPa. The complex is part of the North Atlantic Craton, which is bound to the north by the Trans Hudson–Nagssugtoqidian–Ammassalik Orogen and to the south by the Makkovik–Ketilidian Orogen. The rocks in the complex include orthogneiss, migmatitic orthogneiss with banded or nebulitic fabrics, migmatite with a breccia-like structure (agmatite), and narrow bands of lensoidal mafic and ultramafic rocks (granulites) with associated widespread garnet-bearing mafic granulite, and a minor proportion of garnet–cordierite–sillimanite–biotite gneiss. The latter high-grade rocks form part of the substrate for the emplacement of the protoliths for orthogneiss that was synchronous with the ca. 2790–2700 Ma Skjoldungen Orogeny.At least two suites have been recognised in the Thrym Complex based on geochemistry. The oldest is a tholeiitic suite of mafic to ultramafic granulites, and the other is a calc-alkaline suite that includes felsic to intermediate orthogneiss. Most of the orthogneiss has a granodioritic (oldest) to quartz monzodioritic (younger) composition with rare monzogranitic, tonalitic, monzonitic and quartz dioritic protoliths. Historically, the orthogneiss units have been called TTGs, although monzogranite and tonalite are rare, and no trondhjemite is observed. In addition, they could also be called adakitic on geochemical grounds. Instead of using this terminology, which has genetic connotations, the orthogneiss is referred to as predominantly granodioritic to quartz-monzodioritic.Assuming that modern tectonics operated during the Neoarchaean, at least some of the mafic and ultramafic granulites and orthogneiss exposed in the Thrym Complex represent the root zone of a magmatic-arc that might have been relaminated around 2760 Ma during the collisional Skjoldungen Orogeny.