The presence of two generations of granitic rocks in the gneiss domes he examined led eskola to suggest that the general condition for gneissdome formation is the superposition of two orogenic events. This results in me being very conversant and visually familiar with the rocks of the atlanta area. Migmatite gneiss da 28 chlorite schist da 29 tourmaline schist da 2140 from gy 303 at university of south alabama. The orthogneiss to nebulitic migmatite sequence is characterized by progressive destruction of wellequilibrated banded microstructure by crystallization of new. Expansive cliffface exposures in the fosdick mountains antarctica migmatite cored gneiss dome enabled the documentation of suprasolidus deformation and melt migration structures in middle to lower crustal rocks at a scale significant for continental differentiation and crustal flow. This allows contractors to use gneiss as a crushed stone in road construction, building site preparation, and landscaping projects.
Within these crustalscale structures, detachment zones record significant amounts of localized extension and preserve an interface between cool upper crust and hot. The dynamics of migmatite domes in extending orogens. Gneiss is very common and actually forms the majority of the earths lower crust, so any deep tunneling encounters gneiss. The nigeria basement complex have been conformed to comprise of three major rock units of which are. Jun 12, 2017 the term migmatite was introduced by sederholm1908 to designate certain gneissic rocks which looked like mixed rock. More recently, goldsmith 1991, pouliot 1994, larson 1999, and larson and others 1999 interpreted the contact to be a ductile fault. The migmatitegneiss complex mgn, the schist belts sb and the older granites og modi. The highestgrade, nonmigmatitic rocks are gneisses and granofelses of the andalusitekfeldspar zone fig. Migmatite gneiss within the precambrian basement rocks around awo, southwestern nigeria were stud ied with a view to determine origin and compositional. No locality information or originating geologic unit is provided. Migmatitic gneiss and amphibolite late to early archean. Some gneiss is durable enough to perform well as a dimension stone. Expansive cliffface exposures in the fosdick mountains antarctica migmatitecored gneiss dome enabled the documentation of suprasolidus deformation and melt migration structures in middle to lower crustal rocks at a scale significant for continental differentiation and crustal flow.
A general impression of the pink gneiss formation is that of a very homo genous series of granitic biotite gneisses where transition between several types has been. Structure in a migmatite or granitised rock that can be recognised as premigmatitic or pregranitic. It typically consists of darker bands rich in mafic minerals and lighter quartzofeldspathic parts. Very widespread, the biotitic gneisses are normally finegrained with strong foliation caused by the parallel arrangement of alternating dark and light minerals. Petrogenesis of metatexite and diatexite migmatites. Malo anatectic region, and are associated with toptothe. Pdf petrography and structural features of migmatites, granites.
The outer parts consist mostly of metatexite migmatite, and the inner part principally of diatexite migmatite brown, 1979. Gneiss, migmatite, and amphibolite about 2800 ma quartzofeldspathic gneiss and less abundant amphibolite and migmatite. The authors suggest that migmatites in this region have resulted from partial melting and metasomatism. Here, we report data from an outcrop in the highest grade part of the granulite facies zone, in which diatexite occurs as a 3 m thick layer between 2 m thick layers of stromatic. Gneiss usually is distinguished from schist by its foliation and schistosity. Migmatite is a rock composed of both igneous and metamorphic rocks. Migmatites are highly heterogeneous rocks found in highgrade metamorphic environments. A heterogeneous silicate rock with properties of both igneous and metamorphic rocks.
The vanduzi migmatite gneiss occurs between the messica granite gneiss in the west and the nhansipfe granitic orthogneiss in the east fig. A banded metamorphic rock, pervasively heterogeneous on a meso to megascopic scale. Pdf transition from gneiss to migmatite and the relationship of. Compositional and petrogenetic features of schistose rocks of ibadan area, southwestern nigeria the crystalline basement is mainly composed of metamorphosed rocks and tectonically structured precambrian rocks schist, gneiss, migmatite, quartzite and granitic intrusions huhn et al. Migmatite with diffuse relics of preexisting rocks or rock structures. Evolution of a granite gneissmigmatite terrane in rajasthan 485 the 1st one is primary biotite. Metamorphic gneiss has many uses as a building material such as flooring, ornamental stones, gravestones, facing stones on buildings and work surfaces. The migmatite gneiss complex is the oldest basement rock, and is believed to be of sedimentary origin but was later profoundly altered into metamorphic and granite conditions. Ebook atlas of migmatites as pdf download portable document. Such rocks are usually gneissic banded and felsic rather than mafic in composition.
Transition from gneiss to migmatite and the relationship of. The rock derived its name from a greek word which means mixed rock. The country rock is a paragneiss composed of thin, alternat ing fine and coarsegrained layers. The rock shown above is an archeanaged migmatite from minnesota. An example of a diatexite migmatite from the birksgate gneiss, musgrave prov ince. If this is true, then the gneissic and migmatitic rocks in central wisconsin may represent an. Migmatites comprise leucosome bands, comprised largely of quartz and feldspar, which crystallised from partial melts or segregated metamorphic fluids. This title provides genetically based definitions and a system of nomenclature with which it is be possible to describe and map migmatites effectively. The development of a stromatic migmatite exposed east and southeast of arvika western sweden is described in four stages beginning with the country rock and following evolution through three areas characterized by low, medium and high amounts of leucosomes areas l, m, and h, respectively. Origin of migmatites by deformationenhanced melt infiltration of. Massabesic gneiss complex as a migmatized equivalent of the berwick formation bothner and others, 1984. Metatexite and diatexite migmatites are widely distributed within the upper amphibolite and granulite facies zones of the higo low. Geology of kennesaw mountain, migmatite, mcconnell. However, unlike slate and schist, gneiss does not preferentially break along planes of.
Currently only the basic nebulae generation function work. Gneiss domes occur in a wide variety of orogenic and anorogenic domains of the continental crust, and play a major role in lithosphere dynamics by allowing upwards transfer of heat and mass. The rare earth element geochemistry of granite, gneiss, and. Migmatite article about migmatite by the free dictionary. Migmatite, in geology, rock composed of a metamorphic altered host material that is streaked or veined with granite rock. Gneiss nice is a rock of great variety with large mineral grains arranged in wide bands. Gneiss is formed by high temperature and highpressure metamorphic processes acting on formations composed of igneous or sedimentary rocks. Includes granite of late archean age that transgresses gneisses and amphibolite. In addition, the laguigangri group containing schist, gneiss, and migmatite units crops out in the core of the yelaxiangbo dome figure 1b, as well as some quaternary sediments composed of gravel, sand gravel. Migmatite definition of migmatite by the free dictionary. It is formed by the metamorphosis of granite, or sedimentary rock. Paragneiss is gneiss derived from sedimentary rock. The migmatitegneiss complex covers half of nigerias surface area and encompasses archean gray gneisses, with tonalite and granodiorite consistencies. Typically, the rock contains alternating lighter layers leucosomes, comprised of lightcolored minerals such as quartz, feldspar, and muscovite and darker layers melanosomes, comprised of darkcolored minerals such as amphibole and biotite.
Paragneiss is gneiss derived from sedimentary rock such as sandstone. Formation of diatexite migmatite and granite magma during. Orthogneiss is gneiss derived from igneous rock such as granite. Pdf evolution of a granite gneissmigmatite terrane in. A migmatite, or mixed rock in greek, is a banded, heterogenous rock composed of intermingled metamorphic and igneous components. The migmatite gneiss complex is the common est rock type in the nigerian basement complex. Gneiss and granofels range in composition from felsic to intermediate to mafic. To the northwest, the complex is separated from the rocks of the central.
A user s guide to migmatites, report book 20 001 6. The rock contains alternating layers of lightcolored bands, known as leucosomes, and darkcolored bands, known as melanosomes. Origin of migmatite granite and its relations to iron. Veins, contorted layers, and irregular pods of silicarich granite occur within the structure of foliated iron and magnesiumrich metamorphic rocks like gneiss, schist and amphibolite. This type of metamorphic was created by regional metamorphism, in which a sedimentary or igneous rock has been deeply buried and subjected to high temperatures and pressures. It was first discovered by jakob johannes sederholm in 1907. The accompanying museum signage identifies the rock as a 3. Except for the watersmeet dome late to early archean.
Gneiss displays distinct foliation, representing alternating layers composed of different minerals. Because metamorphic rocks form by recrystallizaton of minerals. Here is a thorough, if tortured, description of migmatite, with words not defined in the text asterisked and defined below. A migmatite is a banded, granular metamorphic rock that contains light coloured bands with evidence for partial melting. Local geological setting the lithologies in the study areas include porphyritic granites, schists, biotite. Pdf the osokom area is made up dominantly of varieties of. The oldest rocks yet found are gneiss, with the oldest dated to over 4 billion years old, older than most estimates of the beginnings of life on earth. Hornblendebiotite granite with amphibolitefacies enclaves of metamafic and calcsilicate gneisses constitute the cratonic foreland. The older metasediments were also among the earliest rocks to form on the nigerian basement complex. Two migmatite domes, the okanogan dome washington state and the naxos dome cyclades, greece record the dynamics of migmatite dome formation in extending orogens. Gneiss, metamorphic rock that has a distinct banding, which is apparent in hand specimen or on a microscopic scale. Migmatite definition is a gneiss produced by the injection of igneous material between the laminae of a schistose formation called also injection gneiss. Rbsr data for the migmatite and gneiss indicate elevated initial 87 sr 86 sr ratios, suggesting that the migmatite and gneiss formed from crustal rocks of intermediate to granitic composition which were themselves formed more than 3.
It means a type of rock texture, not a composition. Major rocks in nigeria migmatite, gneiss, migmatite gneiss. The term migmatite was introduced by sederholm1908 to designate certain gneissic rocks which looked like mixed rock. Extension of thickened and hot crust commonly leads to the formation of migmatitecored metamorphic core complexes mcc coney and harms, 1984, lister and davis, 1989, whitney et al. Gneiss granofels and migmatite gneiss and granofels range in composition from felsic to intermediate to mafic.
The darker parts usually exhibit features of metamorphic rocks eg. Gneiss and migmatite of archean age in the precambrian. Ebook atlas of migmatites as pdf download portable. Only the massabesic gneiss complex has any evidence of prealleghanian metamorphism. Many rocks were injected by a granitoid that has blue quartz and augen of potassic feldspar and are arteritic migmatites. It is created when a metamorphic rock such as gneiss partially melts, and then that melt recrystallises into an igneous rock, creating a mixture of the unmelted metamorphic part with the recrystallised igneous part. Gneiss gneiss is a high grade metamorphic rock, meaning that it has been subjected to higher temperatures and pressures than schist. Migmatite is a composite rock found in medium and highgrade metamorphic environments. Magmatic foliations and lineations in diatexite migmatites and granites, respectively, dip and plunge to the north or northeast fig. Metamorphic migmatite varied gneisses of mostly unknown age in cores of gneiss domes and faultbounded uplifts archean gneiss terranes. Choose from 10 different sets of gneiss flashcards on quizlet. The country rock is a paragneiss composed of thin, alternating fine and coarsegrained layers.
If a 390 ma zircon in an amphibolite aleinikoff and others, 1995 proves to be metamorphic, then the massabesic gneiss complex may have a more complicated acadian metamorphic history than its sibling avalonian. It consists of two or more constituents often layered repetitively. Melt transport and magma accumulation in a migmatitecored. Both domes are exposed in cordilleranstyle metamorphic core complexes that developed during extension of previously thickened crust and contain a core of anatectic migmatite.
Contains a wide variety of rock types including graphitic schist and marble. Rock melting experiments show that the initial melting temperatures of phyllite, biotite plagioclase gneiss and migmatite granite range from 630 to 640c, but those of bif and plagioclase amphibolite are 150200 c higher. Within this complex are occurrences of schist, migmatite, garnet, sillimanite, kyanite and staurolite, which together indicate highgrade metamorphism up to the level of amphibolite on the. Compositional characteristics of the migmatite gneiss around. Composition of the layers varies from granitic fine to tonalitic. From this description of rocks units and also the younger granite intrusion. Conformable granitic gneisses and banded gneiss series form the oldest part, in which augen gneisses ap parently developed by partial anatexis and. Geological mapping in the migmatite complex near lyngdal has demon strated the significance of major rock units, designated lithostructural forma tions, as principal divisions of the complex. Migmatite definition of migmatite by merriamwebster. Separated granitic and biotitic portions of a migmatite from the western.
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