scholarly journals Use of Rb-Sr isotope data to constrain the time of deposition of Precambrian metasediments: an example from Hamborgerland, West Greenland

1993 ◽  
Vol 159 ◽  
pp. 95-100
Author(s):  
F Kalsbeek
Keyword(s):  
Granulite Facies ◽  
Field Investigation ◽  
High Grade ◽  
Sr Isotope ◽  
West Greenland ◽  
Relative Age ◽  
Isotopic Evolution ◽  
High Grade Metamorphism ◽  
Isotope System ◽  
Nd Model Age

In high-grade metamorphic terrains it is often not possible to determine the relative age of metasedimentary units by field investigation. However, the time of deposition of the original sediment can be constrained by consideration of the Sr-isotopic evolution of the rocks on the scale of an outcrop. An outline of the method is given, and Rb-Sr data for high-grade (granulite facies) metasediments from HamborgerIand, West Greenland, are discussed as an example. Sm-Nd model age data indicate that these rocks were derived by erosion of a 3000–3200 Ma basement. Deposition took place not long before 2700 Ma ago, and closure of the Rb-Sr isotope system after high-grade metamorphism occurred at about 2600 Ma.

scholarly journals Post-kinematic diorite intrusions in Archaean basement rocks around outer Fiskefjord, southern West Greenland

1991 ◽  
Vol 39 ◽  
pp. 167-177
Author(s):  
A. A. Garde
Keyword(s):  
Trace Element ◽  
Final Stage ◽  
Precious Metals ◽  
Granulite Facies ◽  
High Grade ◽  
West Greenland ◽  
Mafic Intrusions ◽  
Basement Rocks ◽  
High Grade Metamorphism ◽  
Crust Deformation

About twenty small dioritic intrusions around outer Fiskefjord, southern West Greenland, which are undeformed and unmigmatised, mark the final stage in middle Archaean accretion of continental crust, deformation and high-grade metamorphism in the Akia terrane. The diorites were emplaced into hot tonalitic gneisses, some of which were thoroughly retrograded from granulite facies prior to diorite intrusion. The diorites are themselves sporadically retrograded. A conventional zircon U-Pb age of 3017 + 12/-10 Ma has been obtained from one of the diorites. The diorites have SiO2 contents between ea. 52 and 58 wt. % and up to ea. 15% MgO, and some of them border on leuconorite or anorthosite, with normative plagioclase contents up to ea. 85 wt. % . Trace element compositions are characterised by elevated amounts of Zn, Co, Ni, and especially Cr, but low contents of several LIL elements, and they were probably contaminated with sialic crust. The diorite intrusions may be related to a group of a more mafic intrusions with anomalous contents of precious metals, forming the "norite belt" some 50 km north of Fiskefjord.

scholarly journals Petrography and geochemistty of amphibolites from the Nordre Strømfjord area in the central part of the Nagssugtoqidian of West Greenland

1983 ◽  
Vol 113 ◽  
pp. 1-20
Author(s):  
F.C Mengel
Keyword(s):  
Granulite Facies ◽  
High Grade ◽  
West Greenland ◽  
Grade Metamorphism ◽  
Chemical Differentiation ◽  
High Grade Metamorphism ◽  
Tectonic Settings

The microstructures and general occurrence of orthopyroxene in the investigated amphibolites indicate that the rocks have been strongly deformed under granulite facies conditions. Microstructural features also suggest that high grade metamorphism in some cases outlasted deformation. The amphibolites show chemical resemblance to tholeiites from tectonic settings around active plate margins. Since chemical differentiation trends are not obvious, the results are insufficient to show that all the Nordre Strømfjord amphibolites originated in the same magmatic environment. The Nordre Strømfjord amphibolites may be Archaean or Proterozoic in age.

scholarly journals Some observations on the structural and metamorphic chronology on Agto and surrounding islands, central West Greenland

1969 ◽  
Vol 27 ◽  
pp. 1-32
Author(s):  
K Sørensen
Keyword(s):  
Minimum Temperature ◽  
Granulite Facies ◽  
The Other ◽  
High Grade ◽  
West Greenland ◽  
Grade Metamorphism ◽  
Discordant Group ◽  
High Grade Metamorphism ◽  
Two Phases ◽  
Basic Rocks

The report concerns an area situated in granulite facies terrain in the Precambrian Nagssugtoqidian orogenic belt (K/Ar age ca. 1700 m.y.) of central and northern West Greenland. The acid rocks are biotite gneisses, biotite-hypersthene gneisses, granitic gneisses, granites and sillimanite- and garnet-bearing gneisses. The basic rocks are amphibolites, pyroxene amphibolites and their garnetbearing equivalents. The basic rocks can be divided into an old, concordant group and a younger, discordant group. In the area mapped four structural complexes characterised by different orientations of axial planes related to deformations of different times have been mapped. The youngest structures have ENE-striking axial planes - the typical Nagssugtoqidian trend. The discordant metabasites are younger than at least two phases of folding and are preserved in the structural complex characterised by the oldest structures. They cut migmatitic structures and thus they are younger than at least one period of high grade metamorphism. The discordant metabasites are older than the youngest deformation, that which resulted in folds with ENE-striking axial planes, i. e. they are older than the typical Nagssugtoqidian structures. They are older than one period of high grade metamorphism which converted the basic dykes into pyroxene amphibolites. This period of metamorphism is at least partially contemporaneous with the Nagssugtoqidian deformation. The fieId occurrence of the discordant metabasites shows that they intruded into rocks affected by tensional stresses and the author proposes that the intrusion of the basic dykes marks the interval between two orogenic episodes, one Nagssugtoqidian and the other pre-Nagssugtoqidian. If this assumption is correct, all the rocks of the area are pre-Nagssugtoqidian. The minimum temperature during the peak of metamorphism is evaluated using evidence of anatexis and the minimum pressure from the absence of cordierite.

scholarly journals The Archaean gneiss complex of northern Labrador A review of current results, ideas and problems

1991 ◽  
Vol 39 ◽  
pp. 153-166
Author(s):  
D. Bridgwater ◽  
L. Schiøtte
Keyword(s):  
Major Part ◽  
Granulite Facies ◽  
High Grade ◽  
Late Archaean ◽  
West Greenland ◽  
Metamorphic History ◽  
Gneiss Complex ◽  
History Of ◽  
Augen Gneiss ◽  
Dominant Phase

1. The early Archaean rocks in northern Labrador can be subdivided into the ea. 3.78 Ga Nulliak supracrus­tal association, the migmatitic Uivak I gneisses, the dominant phase of which was emplaced at ea. 3.73 Ga, and the Uivak II augen gneiss. Inherited low-U rounded inclusions within igneous zircons in the Uivak I gneisses have ages between 3.73 and 3.86 Ga and are more likely to have been derived from a pre-existing high-grade metamorphic gneiss complex than from the Nulliak association. In the early Archaean there were probably several rapid cycles of sedimentary deposition and volcanism followed by emplacement of major plutons. Mid Archaean gneisses are more abundant in northern Labrador than previously realised. The late Archaean metamorphic history of these gneisses is different from the history of the early Archaean gneisses. Whereas an important part of the mid Archaean suite was emplaced in granulite facies and retrogressed at the time of granitoid veining at ea. 2.99 Ga, the major part of the early Archaean rocks were reworked under granulite facies conditions in a sequence of closely spaced events between 2. 7 and 2.8 Ga. The two groups of gneisses had different metamorphic histories until ea. 2.7 Ga, but late and post-tectonic granites of 2.5- 2. 7 Ga age cut across both. It is suggested that the terrane model in southern West Greenland can be extended to Labrador and that tectonic intercalation of early and mid Archaean gneisses took place around 2.7 Ga. Correlation between the Maggo gneisses around Hopedale, mid Archaean gneisses in northernmost Labrador and gneisses from the Akia terrane in West Greenland is suggested. Like the Malene supracrustals in West Greenland the Upernavik supracrustals in Labrador are composite associations, the youngest of which are thought to have been deposited around 2. 7 Ga.

Geology of the Bunger Hills area, Antarctica: implications for Gondwana correlations

1993 ◽  
Vol 5 (1) ◽  
pp. 85-102 ◽  
Author(s):  
John W. Sheraton ◽  
Robert J. Tingey ◽  
Lance P. Black ◽  
Robin L. Oliver
Keyword(s):  
Granulite Facies ◽  
Mobile Belt ◽  
High Grade ◽  
Yilgarn Craton ◽  
Zone Formation ◽  
Zircon Age ◽  
Peak Metamorphism ◽  
High Grade Metamorphism ◽  
Subsequent Deformation ◽  
Hills Area

The Bunger Hills area of the East Antarctic Shield consists of granulite-facies felsic orthogneiss, with subordinate paragneiss and mafic granulite. The igneous precursors of granodioritic orthogneiss were emplaced 1500-1700 Ma ago, and late Archaean (2640 Ma) tonalitic orthogneiss occurs in the nearby Obruchev Hills. Peak metamorphism (M1) (at about 750-800°C and 5-6kb) occurred 1190 ±15 Ma ago (U-Pb zircon age), and was accompanied by the first of three ductile deformations (D1). Emplacement of voluminous, mainly mantle-derived plutonic rocks, ranging from gabbro, through quartz monzogabbro and quartz monzodiorite, to granite, followed between 1170 (during D3) and 1150 Ma. Intrusion of abundant dolerite dykes of four chemically distinct suites at about 1140 Ma was associated with shear zone formation, indicating at least limited uplift; all subsequent deformation was of brittle-ductile type. Alkaline mafic dykes were emplaced 500 Ma ago. Marked geochronological similarities with the Albany Mobile Belt of Western Australia suggest that high-grade metamorphism occurred during collision between the Archaean Yilgarn Craton of Australia and the East Antarctic Shield about 1200 Ma ago.

Silicate, Oxide and Sulphide Trends in Neo-Archean Rocks from the Nilgiri Block, Southern India: the Role of Fluids During High-grade Metamorphism

2019 ◽  
Vol 60 (5) ◽  
pp. 1027-1062 ◽  
Author(s):  
Vinod O Samuel ◽  
Daniel E Harlov ◽  
Sanghoon Kwon ◽  
K Sajeev
Keyword(s):  
Granulite Facies ◽  
Southern India ◽  
Sulphide Mineral ◽  
High Grade ◽  
Regional Trend ◽  
Granulite Facies Metamorphism ◽  
Facies Metamorphism ◽  
High Grade Metamorphism ◽  
Specific Oxidation

Abstract The Nilgiri Block, southern India represents an exhumed section of lower, late Archean (2500 Ma) crust. The northern highlands of the Nilgiri Block are characterized by metagabbros with pyroxenite inlayers. A two-pyroxene granulite zone acts as a transition between the metagabbros and charnockites, which are exposed in the central and southern part of the Nilgiri highlands. Thermobarometry results indicate a SW–NE regional trend both in temperature (∼650–800°C) and in pressure (700–1100 MPa) over the Nilgiri highlands. In the charnockites, composite rutile–ilmenite grains are the dominant oxide assemblage. In the two-pyroxene granulites, hemo-ilmenite–magnetite is dominant with coexisting rutile–ilmenite composite grains in a few samples in the vicinity of the boundary with the charnockites. In the metagabbros, hemo-ilmenite–magnetite is the dominant oxide assemblage. The principal sulphide mineral in the charnockite is pyrrhotite with minor pyrite–chalcopyrite exsolution lamellae or blebs. In the two-pyroxene granulites and the metagabbros, the principal sulphide assemblage consists of discrete pyrite grains with magnetite rims and pyrite–pyrrhotite–chalcocopyrite associations. From these observations, a specific oxidation trend is seen. The northern granulite-facies metagabbros and two-pyroxene granulites of the Nilgiri highlands are highly oxidized compared with the charnockites from the central and southern regions. This higher oxidation state is proposed to be the result of highly oxidizing agents (probably as SO3) in low H2O activity grain boundary NaCl saline fluids with a dissolved CaSO4 component present during granulite-facies metamorphism of the metagabbros and two-pyroxene granulites. Eventually these agents became more reducing, owing to the inherent buffering of the original tonalite–granodiorite granitoids at the graphite–CO2 buffer, such that S took the form of H2S during the granulite-facies metamorphism of the charnockites. At the same time, these saline fluids were also responsible the solid-state conversion of biotite and amphibole to orthopyroxene and clinopyroxene in the metagabbro, two-pyroxene granulite, and charnockite.

Zircon geochronology of anatectic melts and residues from a highgrade pelitic assemblage at Ihosy, southern Madagascar: evidence for Pan-African granulite metamorphism

1996 ◽  
Vol 133 (3) ◽  
pp. 311-323 ◽  
Author(s):  
A. Kröner ◽  
I. Braun ◽  
P. Jaeckel
Keyword(s):  
Source Region ◽  
Granulite Facies ◽  
Detrital Zircons ◽  
High Grade ◽  
Left Behind ◽  
Granulite Metamorphism ◽  
Mozambique Belt ◽  
High Grade Metamorphism ◽  
Depositional Age ◽  
Heterogeneous Source

AbstractWe report U—Pb and207Pb/206Pb zircon ages for a granulite facies gneiss assemblage exposed in a large quarry at Ihosy, southern Madagascar. The granulites are derived from pelitic to arkosic sediments and attained equilibrium conditions at 650–700°C and 4–5 kbar. HigherP—Tconditions of 750–800°C and 6 kbar in the presence of low water activities have led to dehydration melting processes. The formation of granitic melts, which (partly) moved away from their source region, intruded into upper parts of the metapelitic gneisses as small granitic veins and left behind granulitic garnet-cordierite-quartz bearing rocks. Detrital zircons in a sample of metapelite and a sample of quartzofeldspathic gneiss yielded ages between ˜720 and ˜1855 Ma, suggesting a chronologically heterogeneous source region and a depositional age of less than ˜720 Ma for these rocks. High-grade metamorphism and anatexis are documented by zircon ages between 526 ±34 and 557 ±2 Ma with a mean age of about 550 Ma. The broad lithologies, metamorphic grades and ages recorded in the Ihosy rocks are similar to those in the Wanni Complex of northwestern Sri Lanka and in high-grade assemblages of southernmost India and support the contention that all these terrains were part of the Mozambique belt which formed as a result of collision of East and West Gondwana in latest Precambrian time.

SHRIMP U–Pb zircon geochronology of gneisses from the Gweta borehole, northeast Botswana: implications for the Palaeoproterozoic Magondi Belt in southern Africa

2001 ◽  
Vol 138 (3) ◽  
pp. 299-308 ◽  
Author(s):  
R. B. M. MAPEO ◽  
R. A. ARMSTRONG ◽  
A. B. KAMPUNZU
Keyword(s):  
Central Zone ◽  
Granulite Facies ◽  
Eastern Africa ◽  
High Grade ◽  
Zircon Geochronology ◽  
Granulite Facies Metamorphism ◽  
Grade Metamorphism ◽  
Facies Metamorphism ◽  
High Grade Metamorphism ◽  
Limpopo Belt

This paper presents new U–Pb zircon analyses from garnet–sillimanite paragneisses from the Gweta borehole in northeast Botswana. Concordant to near-concordant analyses of zircon from these rocks reveal a billion year history from 3015 ± 21 Ma for the oldest detrital grain measured, to the age of high-grade metamorphism, 2027 ± 8 Ma. The maximum age of sedimentation in the Magondi belt is constrained by the age of the youngest concordant detrital zircon at 2125 ± 6 Ma. This contrasts with the age of sedimentation in the Central Zone of the Limpopo belt which is Archaean. The comparison of our results with U–Pb zircon data from the Magondi belt in Zimbabwe suggests that the granulite-facies metamorphism in this belt extended between c. 2027–1960 Ma. Granulite-facies rocks with U–Pb zircon ages in this interval are also known in the Ubendian belt and lend support to the correlation of these two segments of Palaeoproterozoic belts in southern and central–eastern Africa. The granulite facies metamorphism in the Magondi belt is coeval with the high-grade metamorphism and granitoids documented further south in the Central Zone of the Limpopo Belt.

scholarly journals MAPEAMENTO GEOLÓGICO DA REGIÃO DA SERRA DOS TURVOS, CARATINGA (MG), SETOR SUL DO ORÓGENO ARAÇUAÍ

2012 ◽  
Author(s):  
Gláucia Queiroga ◽  
Tiago Novo ◽  
A. C. Pedrosa-Soares
Keyword(s):  
Sedimentary Cover ◽  
Granulite Facies ◽  
Geological Mapping ◽  
Iron Formation ◽  
High Grade ◽  
Crystalline Core ◽  
High Grade Metamorphism ◽  
Iron Formations ◽  
Araçuaí Orogen ◽  
Southern Section

A área de estudo situa-se na parte sul do núcleo cristalino do Orógeno Araçuaí, próximo à fronteira com o Orógeno Ribeira. A característica fundamental da região é a abundância de rochas de alto grau metamórfico, na transição de fácies anfibolito-granulito. Uma cobertura metassedimentar neoproterozóica é a unidade dominante e está representada por paragnaisse migmatítico, bandado, com intercalações de quartzito, formação ferrífera micácea e formação ferrífera maciça. Corpos de anfibolito, pegmatito e charnockito também ocorrem na área. A principal estrutura dúctil é a foliação (Sn) regional, paralela ao bandamento composicional do granada-biotita paragnaisse. Fraturas são abundantes no quartzito e formação ferrífera maciça. As formações ferríferas são ricas em magnetita e formam corpos lenticulares com espessura decamétrica a centimétrica, concordantes com o bandamento composicional do granada-biotita paragnaisse. Preliminarmente, interpreta-se a gênese dessas formações ferríferas como sedimentar, durante a deposição dos protolitos areno-pelíticos do paragnaisse.Palavras-chave: metamorfismo de alto grau, formação ferrífera, Orógeno Araçuaí. ABSTRACT: GEOLOGICAL MAPPING OF THE SERRA DOS TURVOS REGION, CARATINGA (MG), SOUTHERN SECTION OF THE ARAÇUAÍ OROGEN. The study area is located in the southern part of the crystalline core of the Araçuaí orogen, close to the boundary with the Ribeira orogen. The main feature of the region is the abundance of high-grade metamorphic rocks of the amphibolite-granulite facies transition. A Neoproterozoic sedimentary cover is the dominant unit in the area and consists of migmatitic banded paragneiss with intercalations of quartzite, mica-bearing iron formation and massive iron formation. Amphibolite, pegmatite and charnockite bodies are also found in the area. The main ductile structure is the regional foliation (Sn) which is parallel to the compositional banding of the garnet-biotite paragneiss. Fractures are abundant in the quartzite and massive iron formation. The iron formations form lenticular bodies ranging in thickness from centimeters to decameters, which are concordant to the banding and foliation of the garnet-biotite paragneiss. Accordingly to field data, a sedimentary genesis can be suggested for the iron formations.Keywords: high grade metamorphism, iron formation, Araçuaí Orogen

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