Migmatitic Schists Research Articles

Geochemistry evolution of Schists of northwest Obudu area southeastern Nigeria

Geochemistry of schists of Obudu area was carried out using ICP-MS and ICP-ES techniques in order to determine the geochemical evolution of the area. 40 samples were analyzed for their major, trace and REE composition. Field mapping revealed that gneisses, amphibolites and schists comprising migmatitic schists (MS), quartz-mica schists (QMS), garnet-mica schists (GMS), and hornblende biotite schists (HBS), intruded by granites, granodiorites, quartzofeldspathic rocks and dolerites occur in the area. Structural studies revealed that the schists trend approximately NE–SW (5 – 30o ) indicating the Pan-African event. Modal analysis revealed that the schists have average concentration of quartz (15vol.%), plagioclase (An45-19 vol.%), biotite (15vol.%), garnet (9.0vol.%) and muscovite (6vol.%), the remaining consists of accessory minerals. Geochemistry showed that all the schists have molecular Al2O3 > CaO+K2O+Na2O, indicating they are peraluminous metasedimentary pelites. Trace and REE element results show that all the analyzed schist samples are depleted in Hg, Ag, Be, Bi, and Sb below < 1.0ppm, but relatively enriched in Ba, Sr and Zr with average concentration of 996, 675.73, 243.13 ppm respective. The HREE are depleted with ΣHREE < 10.2, but the LREE are relatively enriched with ΣLREE > 289.54. The ΣLREE/ΣHREE ratio ranges from 9.17 to 33.4, with a large positive delta V at Eu. These findings indicate that the schists of Northwest Obudu area are highly fractionated and had attained at least the uppermost amphibolite metamorphic grade. The schists had contributed to the development of the Pan-African continent.

Coeval brittle and ductile structures associated with extreme deformation partitioning in a multilayer sequence

An investigation on the effects of a strong rheological contrast in the deformation of layered anisotropic rocks is presented. The study focuses on the geometric and kinematic analysis of complex structures developed within and adjacent to a thin marble–metapsammite multilayer unit from the Cap de Creus tectonometamorphic belt (NE Spain). Zones of high ductile strain localise in the marble layers, which exhibit complex folds, whereas metapsammites show mostly brittle (boudinage) structures. These structures strongly contrast with coeval retrogressive discrete shear zones developed in the surrounding migmatitic schists. The extreme strain partitioning is due to the rheological contrast between different lithological layers. In addition, the specific orientation of this multilayer unit induces a reversal of local kinematics with regard to bulk kinematics. Consequently, caution should be exercised when interpreting regional tectonics in highly partitioned domains associated with rheological heterogeneities.

Analogue modelling of syntectonic leucosomes in migmatitic schists

Migmatites from the Cap de Creus tectonometamorphic belt display a wide variety of structures, from those formed when the leucosomes were melt-bearing, to those developed during solid-state deformation. The observed field structures have been modelled by means of analogue experiments. The materials used in the models are layered plasticine as a schist analogue, and chocolate as analogue of the crystallizing leucosome. A model for the development of syntectonic migmatites is proposed in which initial melt-bearing patches, preferentially formed within fertile pelitic layers, progressively evolve towards lens-shaped veins. Furthermore, heterogeneous deformation of anisotropic metasediments facilitates formation of extensional sites for further melt accumulation and transport. Melt crystallization implies a rapid increase in effective viscosity of leucosomes producing a reversal in competence contrast with respect to the enclosing schists. During the whole process, deformation localizes around crystallizing veins, giving rise to different and contrasting structures for melt-bearing and for solid-state stages.

Geologic constraints on middle-crustal behavior during broadly synorogenic extension in the central East Greenland Caledonides

Structural and U–Pb geochronologic data from the Forsblad Fjord area of East Greenland (72°30′N) indicate close spatial and temporal ties between orogen-parallel shear and extensional deformation during Caledonian orogenesis. This territory is composed of three tectonostratigraphic units separated by two splays of the Fjord Region Detachment System (FRDS), a principal extensional fault system of the East Greenland Caledonides that was active in Silurian time. The oldest Caledonian fabrics in Forsblad Fjord, which developed at upper amphibolite facies metamorphic conditions, indicate that there was north–south, lateral extrusion of ductile middle-crustal material synchronous with approximately east–west shortening. U–Pb dating of synkinematic granitic leucosomes in migmatitic schists and gneisses demonstrates that this process was underway at ca. 425 Ma. Subsequently, east–west extension along the two splays of the FRDS truncated the older fabrics; the structurally highest of these shear zones – the Tindern detachment – was active as early as ca. 424 Ma. This implies either that there was a rapid transition from Caledonian shortening and possible transpressional deformation to post-orogenic collapse, or our preferred interpretation that extension was synorogenic. The FRDS shares many structural characteristics with the South Tibetan Detachment System of the Cenozoic Himalayan orogen but exhibits two important differences. First, while the South Tibetan system developed in the down-going Indian plate during the India–Eurasia collision, the Fjord Region system developed in the overriding Laurentian plate during the collision of Laurentia with Baltica. Second, whereas the exposed extensional structures in the Himalayas developed in the upper crust and are only inferred to have extended to deeper levels, those in the Forsblad Fjord area were demonstrably active at middle-crustal levels. Evidence for broadly coeval extension and contraction at differen t structural levels in both mountain belts emphasizes the general importance of crustal decoupling in the collisional orogenic process, and implies that synorogenic extensional deformation is not strictly an upper crustal phenomenon.

K-Ar ages of some metamorphic rocks of Oban massif and their implications for the tectonothermal evolution of Southeastern Nigeria.

The following K-Ar ages have been obtained on mineral separates from some metamorphic rocks in the Oban massif: amphibolites (592-930 Ma), banded gneisses (492-538 Ma), granodioritic gneiss (502 Ma), schists (514-519 Ma) and charnockite (481 Ma). Comparison of these ages with previous dates obtained by the Rb-Sr whole rock method and the Pb-Pb evaporation technique on single zircon throws light on the cooling history and orogenic events in the Oban massif. The banded gneiss in the Oban massif yields a zircon age of 1932±5 Ma, indicating that it was emplaced during the early Proterozoic (Eburnean orogeny). The K-Ar age of 492-538 Ma suggests that these rocks were metamorphosed during the Pan-African orogeny. This age is similar to the Rb-Sr isochron age of 510±10 Ma earlier suggested to be the age of migmatization in the Oban massif. The amphibolite yielded Rb-Sr isochron ages of 784±31 Ma and 1313±37 Ma. The age of 1313±37 Ma fits into the Kibaran event. The K-Ar hornblende age of 930±37 Ma on the same amphibolite strongly suggests that this amphibolite belongs to an event older than Pan-African (600±150 Ma) and has not been reset perfectly by the Pan-African event. The migmatitic schists in the Oban massif yielded a Rb-Sr isochron age of 527±16 Ma which has been interpreted as the age of its migmatization. One sample of this schist, however, gave a model age of 676±24 Ma. The K-Ar ages for the Oban massif schists (514-519 Ma) are close to the Rb-Sr isochron age of ca. 527 Ma and confirm that a metamorphic event occurred in the area at this time. This was followed by final cooling at 515±10 Ma. Charnockite in the Oban area has yielded a zircon age of 584±20 Ma, which is Pan-African. The age possibly dates the time of formation of the charnockites. The K-Ar age (481 Ma) obtained in this study is approximately 100 Ma younger than the zircon age and possibly dates the subsequent cooling of the charnockite through hornblende blocking temperature.

Basement-cover relationships and regional structure in the Grandjean Fjord - Bessel Fjord region (75°–76°N), North-East Greenland

Geological mapping and isotopic investigations demonstrate that the Grandjean Fjord – Bessel Fjord region can be divided into three rock groups: (1) a Lower Proterozoic basement gneiss complex; (2) a Middle Proterozoic supracrustal cover (Smallefjord sequence); and (3) Upper Proterozoic metasediments (Eleonore Bay Supergroup). The basement gneiss complex largely comprises c. 2.0–1.7 Ga calc-alkaline granitoid orthogneisses with intercalated migmatitic supracrustal rocks. The complex is deformed by at least two sets of approximately coaxial folds which may be either Proterozoic or Caledonian in age. The Smallefjord sequence is comprised mostly of migmatitic schists and gneisses which underwent high-grade metamorphism during the late Middle Proterozoic. The dominant deformation structures within the Smallefjord sequence are associated with the development of ductile shear zones along the boundaries of all the major tectonostratigraphic units and are thought to be Caledonian in age.

RbSr age of Pan-African migmatites in the Oban massif of southeastern Nigeria

Migmatites consisting mainly of lit-par-lit gneisses form one of the most dominant metamorphosed rocks in the Oban massif of southeastern Nigeria. Banded schists and agmatites are the other mixed rocks recognized in the area. The lit-par-lit gneisses are composed of quartz, feldspar (leucocratic portion) and biotite, chlorite, garnet (melanocratic portion). They are extensively sheared, fractured and folded. The dominant fold axes trend N-S parallel to the regional foliation of the rocks. Chemical data show that the lit-par-lit gneiss is granodioritic in composition. The agmatite is restricted in occurrence. Banding is rare in the agmatite but dissection of the paleosome by quartzo-feldspathic veins is common. The agmatite is quartz-dioritic in composition. The banded schist is coarse-grained and contains index minerals: garnet, kyanite and sillimanite, indicating that the area underwent high-grade metamorphism of the Barrovian type. It is pelitic in composition. These migmatitic rocks were formed by partial melting, metamorphic segregation and injection due to high-grade metamorphism. The migmatitic gneiss yielded a RbSr isochron age of 150±11 Ma whilst the migmatitic schists gave an age of 527±16 Ma.