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.

Zircon U–Pb ages, major and trace elements, and Hf isotope characteristics of the Tiantangzhai granites in the North Dabie orogen, Central China: tectonic implications

2013 ◽  
Vol 151 (5) ◽  
pp. 916-937 ◽  
Author(s):  
XIN DENG ◽  
KUNGUANG YANG ◽  
ALI POLAT ◽  
TIMOTHY M. KUSKY ◽  
KAIBIN WU
Keyword(s):  
Partial Melting ◽  
Eastern China ◽  
Major And Trace Elements ◽  
Central China ◽  
Dabie Orogen ◽  
Crustal Rocks ◽  
The North ◽  
Granitic Magmatism ◽  
Two Peaks ◽  
Lower Crustal

AbstractCretaceous granites are widespread in the North Dabie orogen, Central China, but their emplacement sequence and mechanism are poorly known. The Tiantangzhai Complex in the North Dabie Complex is the largest Cretaceous granitic suite consisting of six individual intrusions. In this study, zircon U–Pb ages are used to constrain the crystallization and protolith ages of these intrusions. The Shigujian granite is a syn-tectonic intrusion with an age of 141 Ma. This granite was emplaced under a compressional regime. Oscillatory rims of zircons have yielded two peaks at 137±1 Ma and 125±1 Ma. The 137±1 Ma peak represents the beginning of orogenic extension and tectonic collapse, whereas the 125±1 Ma peak represents widespread granitic magmatism. Zircon cores have yielded concordant ages between 812 and 804 Ma, which indicate a crystallization age for the protolith. The Tiantangzhai granites show relatively high Sr contents and high La/Yb and Sr/Y ratios. The Shigujian granite has positive Eu anomalies resulting from partial melting of a plagioclase-rich source in an over-thickened crust. Correspondingly, in situ Lu–Hf analyses from zircons yield high negative εHf(t) values from −24.8 to −26.6, with two-stage Hf model ages from 2748±34 to 2864±40 Ma, suggesting that the magmas were dominantly derived from partial melting of middle to lower crustal rocks. The Dabie orogen underwent pervasive NW–SE extension at the beginning of the early Cretaceous associated with subduction of the Palaeo-Pacific plate beneath eastern China.

scholarly journals Mantle rock exposures at oceanic core complexes along mid-ocean ridges

Geologos ◽  
2015 ◽  
Vol 21 (4) ◽  
pp. 207-231 ◽  
Author(s):  
Jakub Ciazela ◽  
Juergen Koepke ◽  
Henry J.B. Dick ◽  
Andrzej Muszynski
Keyword(s):  
Partial Melting ◽  
Geophysical Methods ◽  
Ex Situ ◽  
Crustal Rocks ◽  
New Class ◽  
Ocean Ridges ◽  
Common Structure ◽  
Spreading Ridges ◽  
Slow Spreading ◽  
Lower Crustal

Abstract The mantle is the most voluminous part of the Earth. However, mantle petrologists usually have to rely on indirect geophysical methods or on material found ex situ. In this review paper, we point out the in-situ existence of oceanic core complexes (OCCs), which provide large exposures of mantle and lower crustal rocks on the seafloor on detachment fault footwalls at slow-spreading ridges. OCCs are a common structure in oceanic crust architecture of slow-spreading ridges. At least 172 OCCs have been identified so far and we can expect to discover hundreds of new OCCs as more detailed mapping takes place. Thirty-two of the thirty-nine OCCs that have been sampled to date contain peridotites. Moreover, peridotites dominate in the plutonic footwall of 77% of OCCs. Massive OCC peridotites come from the very top of the melting column beneath ocean ridges. They are typically spinel harzburgites and show 11.3–18.3% partial melting, generally representing a maximum degree of melting along a segment. Another key feature is the lower frequency of plagioclase-bearing peridotites in the mantle rocks and the lower abundance of plagioclase in the plagioclase-bearing peridotites in comparison to transform peridotites. The presence of plagioclase is usually linked to impregnation with late-stage melt. Based on the above, OCC peridotites away from segment ends and transforms can be treated as a new class of abyssal peridotites that differ from transform peridotites by a higher degree of partial melting and lower interaction with subsequent transient melt.

scholarly journals Rare allanite in granulitised eclogites constrains timing of eclogite to granulite facies transition in the Bhutan Himalaya

2021 ◽  
Author(s):  
Eleni Wood ◽  
Clare Warren ◽  
Nick Roberts ◽  
Tom Argles ◽  
Barbara Kunz ◽  
...  
Keyword(s):  
Granulite Facies ◽  
Continental Collision ◽  
Eastern Himalaya ◽  
Pb Isotope ◽  
Crustal Rocks ◽  
Metamorphic Terrane ◽  
Eclogite Facies ◽  
Lower Crustal ◽  
The Himalaya

During continental collision, crustal rocks are buried, deformed, transformed and exhumed. The rates, timescales and tectonic implications of these processes are determined by linking geochemical, geochronological and microstructural data from metamorphic rock-forming and accessory minerals. Exposures of lower orogenic crust provide important insights into orogenic evolution, but are rare in young continental collision belts such as the Himalaya. In NW Bhutan, eastern Himalaya, a high-grade metamorphic terrane provides a rare glimpse into the evolution and exhumation of the deep eastern Himalayan crust and a detailed case study for deciphering the rates and timescales of deep-crustal processes in orogenic settings. We have collected U-Pb isotope and trace element data from allanite, zircon and garnet from metabasite boudins exposed in the Masang Kang valley in NW Bhutan. Our observations and data suggest that allanite cores record growth under eclogite facies conditions (>17 kbar ~650°C) at ca. 19 Ma, zircon inner rims and garnet cores record growth during decompression under eclogite facies conditions at ca 17-15.5. Ma, and symplectitic allanite rims, garnet rims and zircon outer rims record growth under granulite facies conditions at ~9-6 kbar; >750°C at ca. 15-14.5 Ma. Allanite is generally considered unstable under granulite-facies conditions and we think that this is the first recorded example of such preservation, likely facilitated by rapid exhumation. Our new observations and petrochronological data show that the transition from eclogite to granulite facies conditions occurred within 4-5 Ma in the Eastern Himalaya. Our data indicate that the exhumation of lower crustal rocks across the Himalaya was diachronous and may have been facilitated by different tectonic mechanisms.

scholarly journals Evolution of the volcanic plumbing systemof Alicudi (Aeolian Islands - Italy): evidence from fluid and melt inclusionsin quartz xenoliths

2009 ◽  
Vol 47 (4) ◽  
Author(s):  
R. Bonelli ◽  
M. L. Frezzotti ◽  
V. Zanon ◽  
A. Peccerillo
Keyword(s):  
Partial Melting ◽  
Fluid Inclusions ◽  
Type I ◽  
Type Ii ◽  
Early Type ◽  
Late Type ◽  
Crustal Rocks ◽  
Melt And Fluid Inclusions ◽  
Two Generations ◽  
Lower Crustal

Quartz-rich xenoliths in lavas (basalts to andesites; 90-30 ka) from Alicudi contain abundant melt and fluid inclusions. Two generations of CO2-rich fluid inclusions are present in quartz-rich xenolith grains: early (Type I) inclusions related to partial melting of the host xenoliths, and late Type II inclusions related to the fluid trapping during xenolith ascent. Homogenisation temperatures of fluid inclusions correspond to two density intervals: 0.93-0.68 g/cm3 (Type I) and 0.47-0.26 g/cm3 (Type II). Early Type I fluid inclusions indicate trapping pressures around 6 kbar, which are representative for the levels of partial melting of crustal rocks and xenolith formation. Late Type II fluid inclusions show lower trapping pressures, between 1.7 kbar and 0.2 kbar, indicative for shallow magma rest and accumulation during ascent to the surface. Data suggest the presence of two magma reservoirs: the first is located at lower crustal depths (about 24 km), site of fractional crystallization, mixing with source derived magma, and various degrees of crustal assimilation. The second magma reservoir is located at shallow crustal depths (about 6 km), the site where magma rested for a short time before erupting.

Local processes involved in the generation of migmatites within mafic granulites

1988 ◽  
Vol 79 (2-3) ◽  
pp. 209-222 ◽  
Author(s):  
Rhoda E. Tait ◽  
Simon L. Harley
Keyword(s):  
Partial Melting ◽  
Granulite Facies ◽  
Major And Trace Elements ◽  
Important Field ◽  
Formation And Evolution ◽  
Alkaline Magmas ◽  
A Minor ◽  
Lower Crustal ◽  
Geochemical Mass Balance

ABSTRACTProcesses involved in the formation and evolution of melts within the lower crustal mafic granulites are considered with reference to mafic migmatites from late Proterozoic (1200-1000 Ma) granulites of the Rauer Group, East Antarctica. Metaluminous dioritic and noritic leucocratic veins on scales of 1 cm to 1 m show agmatitic, stromatic and schlieren structures. These possible melts are compositionally distinct from charnockitic and enderbitic orthogneisses, which show intrusive contacts with the migmatites in areas of low strain.Important field relationships include the following:(a) Leucocratic veins contain plagioclase and rare quartz, coarse subhedral to euhedral orthopyroxene, ilmenite and apatite. Finer (2 cm) veins and layers are richer in mafic phases than larger (2-10 cm) veins.(b) Selvedges or melanosomes are developed between the larger melt areas and enclosing mafic gneisses. These melanosomes consist of garnet, orthopyroxene, plagioclase and biotite and are apatite-rich.(c) Pyroxene granulite palaeosomes typically display bleached zones (1-2 cm) adjacent to selvedges and veins, in which the modal proportion of clinopyroxene diminishes in favour of orthopyroxene.Geochemical and petrological studies demonstrate that localised or near-localised partial melting of the mafic granulites occurred during decompression from 8-9 kb to 7 kbar at a minimum temperature of 800-850°C. Geochemical mass balance calculations using measured vein, selvedge and palaeosome compositions indicate that near-closed system melting behaviour is likely for a large number of major and trace elements, but LILE behaviour is affected by the introduction of biotite probably associated with late stage fluids. Minor- and rare-earth element modelling predicts similar percentages of melting to those observed in the field, but yields reasonable results only when garnet is included as a minor residual phase. HREE concentrations in melanosomes do not show expected enrichments, probably as a result of later subsolidus changes including the breakdown of garnet during decompression.This study demonstrates that migmatites may form through the near-localised partial melting of basic lithologies within the granulite facies. The exact role of fluids in this case cannot be determined but melting is interpreted to be vapour-undersaturated. This process may be important in the production of volumetrically significant amounts of dioritic to tonalitic calc-alkaline magmas.

Multi-stage dehydration–decompression in the metagabbros from the lower crustal rocks of the Serre (southern Calabria, Italy)

2008 ◽  
Vol 145 (3) ◽  
pp. 397-411 ◽  
Author(s):  
PASQUALE ACQUAFREDDA ◽  
ANNAMARIA FORNELLI ◽  
GIUSEPPE PICCARRETA ◽  
ANNARITA PASCAZIO
Keyword(s):  
Southern Italy ◽  
Granulite Facies ◽  
Crustal Thickening ◽  
Metamorphic Evolution ◽  
Crustal Rocks ◽  
Multi Stage ◽  
Crustal Thinning ◽  
The Matrix ◽  
Lower Crustal ◽  
Peak Assemblage

AbstractPorphyroblastic garnet-bearing metagabbros from the base of the lower crust section of the Serre (southern Italy) exhibit multi-stage dehydration and decompression after the Panafrican emplacement of their protoliths. The first dehydration event produced Am–Opx–Cpx–Pl–Grt as the peak assemblage. Two decompression stages are documented by: (1) coronas of Opx–Pl and Opx–Am, and symplectites of Opx–Am–Pl around clinopyroxene within the porphyroblastic garnet as well as in the matrix and (2) symplectites of Pl–Am–Opx–Grt having different textures around the porphyroblastic garnet. During the second decompression stage, a new local, somewhat intense, dehydration occurred and produced rims of Opx+Pl around the porphyroblastic amphibole, or lenses of Pl–Opx–Am–Spl±Bt between layers of dominant amphibole. A deformation stage separates older from younger reaction textures. The porphyroblastic garnet, its inclusions and the matrix are affected by fractures, which have been overgrown by coronas and symplectites around the porphyroblastic garnet and the amphibole of the matrix. PreferredP–Testimates are: ∼900 °C and ∼1.1 GPa at the metamorphic peak; ∼850 °C and 0.8–0.9 GPa during the formation of corona around clinopyroxene; 750–650 °C and 0.7–0.8 GPa during the formation of corona around garnet. All these textures formed under granulite-facies conditions. The subsequent metamorphic evolution consists of rehydration under amphibolite-facies conditions. TheP–T–tpath agrees with the path shown by the uppermost migmatites of the Serre section, and theP–Testimates at the top and the bottom of the section are consistent with the thickness (7–8 km) of the lower crustal segment. A contractional regime, which caused a crustal thickening of about 35 km, was followed by an extensional one producing significant crustal thinning; the change of tectonic regime probably occurred about 300 Ma ago when the emplacement of voluminous granitoids and the initial stages of exhumation of the lower crustal section had taken place.

Magmatic and tectonic emplacement of the Pukaskwa batholith, Superior Province, Ontario, CanadaThis article is one of a series of papers published in this Special Issue on the theme of Geochronology in honour of Tom Krogh.

2011 ◽  
Vol 48 (2) ◽  
pp. 187-204 ◽  
Author(s):  
Gary P. Beakhouse ◽  
Shoufa Lin ◽  
Sandra L. Kamo
Keyword(s):  
Partial Melting ◽  
Abrupt Change ◽  
Superior Province ◽  
Density Inversion ◽  
Tectonic Processes ◽  
Slab Breakoff ◽  
Greenstone Belts ◽  
Lower Crustal ◽  
Basaltic Crust ◽  
Structural Dome

The Neoarchean Pukaskwa batholith consists of pre-, syn-, and post-tectonic phases emplaced over an interval of 50 million years. Pre-tectonic phases are broadly synvolcanic and have a high-Al tonalite–trondhjemite–granodiorite (TTG) affinity interpreted to reflect derivation by partial melting of basaltic crust at lower crustal or upper mantle depths. Minor syn-tectonic phases slightly post-date volcanism and have geochemical characteristics suggesting some involvement or interaction with an ultramafic (mantle) source component. Magmatic emplacement of pre- and syn-tectonic phases occurred in the midcrust at paleopressures of 550–600 MPa and these components of the batholith are thought to be representative of the midcrust underlying greenstone belts during their development. Subsequent to emplacement of the syntectonic phases, and likely at approximately 2680 Ma, the Pukaskwa batholith was uplifted as a structural dome relative to flanking greenstone belts synchronously with ongoing regional sinistral transpressive deformation. The driving force for vertical tectonism is interpreted to be density inversion (Rayleigh–Taylor-type instabilities) involving denser greenstone belts and underlying felsic plutonic crust. The trigger for initiation of this process is interpreted to be an abrupt change in the rheology of the midcrust attributed to introduction of heat from the mantle attendant with slab breakoff or lithospheric delamination following the cessation of subduction. This process also led to partial melting of the intermediate to felsic midcrust generating post-tectonic granitic phases at approximately 2667 Ma. We propose that late density inversion-driven vertical tectonics is an inevitable consequence of horizontal (plate) tectonic processes associated with greenstone belt development within the Superior Province.

scholarly journals INFLUENCE OF WASTE DUMP “CHROBRÓW” IN POLAND ON GROUND - AND SURFACE WATER

2005 ◽  
Vol 1 ◽  
pp. 179
Author(s):  
Agnieszka Gontaszewska ◽  
Andrezej Krainski
Keyword(s):  
Surface Water ◽  
Negative Impact ◽  
Surface Water Quality ◽  
Ammonia Nitrogen ◽  
Geological Structure ◽  
Sewage Water ◽  
Waste Dump ◽  
The Moment ◽  
Sodium And Potassium

In paper an influence of waste dump “Chrobrów” on groundwater and tributary of the Bóbr river was described. This waste dump was installed in former gravel excavation. For first 10 years it had no leak stopper and sewage water could freely infiltrate. Geological structure of the waste dump subsoil is unfavourable because garbage are directly stored on gravels with high filtration coefficient which make migration of pollutants easy. At the moment the waste dump has a leak stopper made from bentonite composite but there are still polluted groundsunderneath. In this paper was analyzed data about ground- and surface water quality from years 1994 – 2004. It was found that the quality of groundwater deteriorated, especially in years 1999 and 2002. The most worsening was noted in case of chlorides, ammonia nitrogen, sodium and potassium. Unfortunately there is no data before 1994 so there is no information about hydrogeochemical background. Increased values of all groundwater components infirst period of investigation are results of exploitation in years 1984 – 1994, when waste dump had no leak stopper.But later deterioration of groundwater quality can not be explained in this way. It should be drawn a conclusion that the seal of waste dump bottom does not work correctly. It was found that there is no negative impact of waste dump on surface water what is caused by absence of hydraulic contact between river and groundwater on investigated area.

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