Seismic velocities of granulite-facies xenoliths from Central Ireland: Implications for lower crustal composition and anisotropy

2005 ◽  
Vol 407 (1-2) ◽  
pp. 81-99 ◽  
Author(s):  
Riana van den Berg ◽  
J. Stephen Daly ◽  
Matthew H. Salisbury
Keyword(s):  
Granulite Facies ◽  
Seismic Velocities ◽  
Crustal Composition ◽  
Lower Crustal

Overview of regional geophysical studies in Manitoba and northwestern Ontario

1982 ◽  
Vol 19 (11) ◽  
pp. 2049-2059 ◽  
Author(s):  
D. H. Hall ◽  
W. C. Brisbin
Keyword(s):  
Greenstone Belt ◽  
Seismic Velocities ◽  
Crustal Layer ◽  
Northwestern Ontario ◽  
Reflection And Refraction ◽  
Gravity And Magnetic ◽  
Winnipeg River ◽  
Lower Crustal ◽  
Gravity And Magnetic Anomaly

This paper presents an overview of six geophysical projects (seismic reflection and refraction, gravity and magnetic anomaly interpretation, specific gravity and magnetic property measurements) carried out in an area in Manitoba and northwestern Ontario bounded by 93 and 96°W longitude, and 49 and 51°N latitude.The purpose of the surveys was to define crustal structure in the Kenora–Wabigoon greenstone belt, the Winnipeg River batholithic belt, the Ear Falls – Manigotagan gneiss belt, and the Uchi greenstone belt. The following conclusions emerge.In all of the belts, a major discontinuity divides the crust into the commonly found upper and lower crustal sections. At the top of the lower crust, a seismically distinct layer (the mid-crustal layer) occurs. Seismic velocities in this layer suggest either intermediate to basic igneous rocks or metamorphic rocks of the amphibolite facies.Crustal geophysical characteristics vary sufficiently among the four belts to justify the classification of all four as distinct subprovinces of the Superior Province.Cet article présente une vue générale sur six projets de géophysique (réflexion et réfraction sismique, interprétation d'anomalies de gravité et magnétiques, déterminations de densité et de propriétés magnétiques) réalisés dans une région du Manitoba et du nord-ouest de l'Ontario encadrée par les longitudes 93 et 96°O et les latitudes 49 et 51°N.

Metamorphic evolution of the Karimnagar granulite terrane, Eastern Dharwar Craton, south India

2010 ◽  
Vol 148 (1) ◽  
pp. 112-132 ◽  
Author(s):  
D. PRAKASH ◽  
I. N. SHARMA
Keyword(s):  
Granulite Facies ◽  
Dharwar Craton ◽  
Ultramafic Rocks ◽  
Chemical Compositions ◽  
Eastern Dharwar Craton ◽  
Geochronological Data ◽  
Rock Types ◽  
Wide Range ◽  
The Subject ◽  
Lower Crustal

AbstractThe Karimnagar granulite terrane is an integral part of the Eastern Dharwar Craton (EDC), India, having been the subject of much interest because of the only reported granulite facies rocks in the EDC. It shows a large variety of rock types with a wide range of mineral parageneses and chemical compositions, namely charnockites (Opx+Pl+perthite+Qtz±Bt±Grt), gneisses (Opx+Crd+Bt+Pl+Qtz+perthite±Sil±Grt±Spl; Bt+Qtz+Pl±Crd±Hbl±Spl), mafic granulites (Cpx+Pl+Qtz±Opx±Hbl), quartz-free granulites (Spr+Spl+Bt+Crd+Kfs+Crn; Bt+Crd+Kfs±Crn±Spl±Krn; And+Bt+Kfs+Chl), granites (Qtz+Pl+Kfs±Bt±Hbl), altered ultramafic rocks (Chl+Trem+Tlc), metadolerites (Cpx+Pl±Bt±Qtz±Chl), banded magnetite quartzites and quartzites. Andalusite- and chlorite-bearing assemblages presumably suggest a retrograde origin. Investigation of quartz-free granulites of the area brings out some interesting and important observations, reflecting the presence of refractory phases. These granulites are devoid of sillimanite and contain corundum instead. Reaction textures in the gneisses include breakdown of garnet to form coronas and symplectites of orthopyroxene+cordierite, formation of cordierite from garnet+sillimanite+quartz and late retrograde biotite and biotite+quartz symplectites. In the mafic granulites, inclusions of quartz and hornblende within orthopyroxene are interpreted as being a part of the prograde assemblage. At a later stage orthopyroxene is also rimmed by hornblende. The quartz-free granulites display a variety of spectacular coronas, for example, successive rims on corundum consisting of spinel+sapphirine+cordierite±orthopyroxene, rare skeletal symplectitic intergrowth of sapphirine+cordierite+potash feldspar, and late retrograde formation of chlorite, corundum, spinel and andalusite from sapphirine±cordierite. Based on chemographic relationships and petrogenetic grids, a sequence of prograde, isothermal decompressive and retrograde reactions have been inferred. Quartz-free sapphirine granulites and mafic granulites record the highest P–T conditions (~7 kbar, 850°C), whereas the gneisses were formed at lower P–T conditions (~5 kbar, 800°C). In addition, the presence of andalusite-bearing rocks suggests a pressure of around 2.5 kbar. This change in pressure from 7 kbar to around 2.5 kbar suggests a decompressive path for the evolution of granulites in the study area, which indicates an uplift for the granulite-facies rocks from lower crustal conditions. The implications for supercontinent history are also addressed in light of available geochronological data.

scholarly journals Lower crustal zircons reveal Neogene metamorphism beneath the Pannonian Basin (Hungary)

2015 ◽  
Vol 7 (1) ◽  
Author(s):  
Hilary Downes ◽  
Andrew Carter ◽  
Richard Armstrong ◽  
Gabor Dobosi ◽  
Antal Embey-Isztin
Keyword(s):  
Pannonian Basin ◽  
Granulite Facies ◽  
Blocking Temperature ◽  
Late Devonian ◽  
Alkali Basalts ◽  
Volcanic Deposits ◽  
Eruption Age ◽  
Crustal Xenoliths ◽  
Lower Crustal ◽  
Variscan Basement

AbstractNeogene alkaline intraplate volcanic deposits in the Pannonian Basin (Hungary) contain many lower crustal granulite-facies xenoliths. U-Pb ages have been determined for zircons separated from a metasedimentary xenolith, using LA-ICPMS and SHRIMP techniques. The zircons show typical metamorphic characteristics and are not related to the hostmagmatism. The oldest age recorded is late Devonian, probably related to Variscan basement lithologies. Several grains yield Mesozoic dates for their cores, which may correspond to periods of orogenic activity. Most of the zircons show young ages, with some being Palaeocene-Eocene, but the majority being younger than 30Ma. The youngest zircons are Pliocene (5.1-4.2 Ma) and coincide with the age of eruptions of the host alkali basalts. Such young zircons, so close to the eruption age, are unusual in lower crustal xenoliths, and imply that the heat flow in the base of the Pannonian Basin was sufficiently high to keep many of them close to their blocking temperature. This suggests that metamorphism is continuing in the lower crust of the region at the present day.

scholarly journals Present structure of the Near-Bug area: conditions of formation and history of development

2021 ◽  
Vol 43 (2) ◽  
pp. 96-115
Author(s):  
O.V. Usenko
Keyword(s):  
Partial Melting ◽  
Main Part ◽  
Solidus Temperature ◽  
Age Determination ◽  
Granulite Facies ◽  
Geological Structure ◽  
General Sequence ◽  
Crustal Rocks ◽  
The Moment ◽  
Lower Crustal

General sequence establishment of geological Precambrian events and associating formations, which were created in them, to the results of isotope age definition, is the task, which has no single valued solution for southwestern part of the Ukrainian Shield. Important is to create a general development model, which will describe the modern geological structure of an area, structural and textural rocks features, accounting PT-conditions in the Earth's crust during the Archean—Paleoproterozoic. Isotopic age determination demonstrates, that from the moment of protolith creation (not later than 3.75 billion years ago, up to 1.9 billion years ago), intrusion of mantle melts and partial melting of the lower crustal rocks, occurred many times over. Pobuzhie formation cannot be imagined, as a single process of accumulation, plunge, crumpling into folds and sedimentary strata metamorphism. It is necessary, to take into account, the plume (mantle) component of the general geodynamic process. In the structure of the Bug megablock and Golovanevskaya suture zone, two main structural plans are displayed. The main part of the territory displays a region of areal distribution of Archean enderbites (generated 2.8 billion years ago) and Proterozoic granites (generated 2.03 billion years ago). The paper compares the temperature distribution with depth, corresponding to the thermal model of the metamorphic temperatures found in the samples, and the solidus temperatures of the basic rocks. It is shown that at the time of the metamorphism development, 2.0 billion years ago, the rocks were at a depth of more than 20 km, and before that — at an even greater depth. During the Archean and Paleoproterozoic, the center of partial melting was repeatedly renewed here, since the temperatures were higher than the solidus temperature of gabbro. Metamorphic changes (and more often migmatization, partial melting and following crystallization in the granulite facies conditions) happened after the presence of the thermal asthenosphere on the core—mantle border, and were accompanied by bringing the substance from it. Therefore the main part of modern surface is folded by palingenic granites. In Archean and Paleoproterozoic the composition of substances were different. After 2.0 billion years ago the level of modern surface was located higher. The second structural plan is presented with vertical structures, building of which often close to concentrically zonal or linear monoclinal. They are confined to fault zones and nodes of their intersections. These structures contain rock complexes, which did not occur until 2.0 billion years ago on any craton in the world.

Study on Crustal Composition and Geodynamics Using Seismic Velocities in the Northeastern Margin of the Tibetan Plateau

2008 ◽  
Vol 51 (2) ◽  
pp. 275-297 ◽  
Author(s):  
Ming-Jun LIU ◽  
Song-Lin LI ◽  
Sheng-Ming FANG ◽  
Ji-Chang FAN ◽  
Li ZHAO
Keyword(s):  
Tibetan Plateau ◽  
The Tibetan Plateau ◽  
Seismic Velocities ◽  
Crustal Composition

scholarly journals Fracturing and crystal plastic behaviour of garnet under seismic stress in the dry lower continental crust (Musgrave Ranges, Central Australia)

Solid Earth ◽  
2019 ◽  
Vol 10 (5) ◽  
pp. 1635-1649 ◽  
Author(s):  
Friedrich Hawemann ◽  
Neil Mancktelow ◽  
Sebastian Wex ◽  
Giorgio Pennacchioni ◽  
Alfredo Camacho
Keyword(s):  
Plastic Deformation ◽  
Continental Crust ◽  
High Stress ◽  
Shear Zones ◽  
Granulite Facies ◽  
Lower Continental Crust ◽  
Granulite Facies Metamorphism ◽  
High Calcium ◽  
Central Australia ◽  
Lower Crustal

Abstract. Garnet is a high-strength mineral compared to other common minerals such as quartz and feldspar in the felsic crust. In felsic mylonites, garnet typically occurs as porphyroclasts that mostly evade crystal plastic deformation, except under relatively high-temperature conditions. The microstructure of granulite facies garnet in felsic lower-crustal rocks of the Musgrave Ranges (Central Australia) records both fracturing and crystal plastic deformation. Granulite facies metamorphism at ∼1200 Ma generally dehydrated the rocks and produced millimetre-sized garnets in peraluminous gneisses. A later ∼550 Ma overprint under sub-eclogitic conditions (600–700 ∘C, 1.1–1.3 GPa) developed mylonitic shear zones and abundant pseudotachylyte, coeval with the neocrystallization of fine-grained, high-calcium garnet. In the mylonites, granulite facies garnet porphyroclasts are enriched in calcium along rims and fractures. However, these rims are locally narrower than otherwise comparable rims along original grain boundaries, indicating the contemporaneous diffusion and fracturing of garnet. The fractured garnets exhibit internal crystal plastic deformation, which coincides with areas of enhanced diffusion, usually along zones of crystal lattice distortion and dislocation walls associated with subgrain rotation recrystallization. The fracturing of garnet under dry lower-crustal conditions, in an otherwise viscously flowing matrix, requires transient high differential stress, most likely related to seismic rupture, consistent with the coeval development of abundant pseudotachylyte. Highlights. Garnet is deformed by fracturing and crystal plasticity under dry lower-crustal conditions. Ca diffusion profiles indicate multiple generations of fracturing. Diffusion is promoted along zones of higher dislocation density. Fracturing indicates transient high-stress (seismic) events in the lower continental crust.

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