Silurian orogenesis in the western Penobscot Bay region, Maine

1995 ◽  
Vol 32 (11) ◽  
pp. 1845-1858 ◽  
Author(s):  
David P. West Jr ◽  
Charles V. Guidotti ◽  
Daniel R. Lux
Keyword(s):  
New England ◽  
Regional Metamorphism ◽  
Thrust Fault ◽  
Low Grade ◽  
Early Devonian ◽  
Middle Paleozoic ◽  
Thermal Signature ◽  
Structural Levels ◽  
Stratigraphic Boundary ◽  
Penobscot Bay

New 40Ar/39Ar mineral ages from rocks collected west of Penobscot Bay, Maine, indicate this region was regionally deformed, metamorphosed to amphibolite facies conditions, and intruded by plutons in Silurian times rather than in the Devonian as previously assumed. Disturbed hornblende age spectra, along with the presence of some Devonian felsic plutons and extensive retrograde metamorphic textures do suggest, however, that these rocks were subsequently affected by low-grade Devonian thermal events. In sharp contrast, rocks west of the Sennebec Pond thrust fault, a major tectono-stratigraphic boundary in this region, lack a significant Silurian tectono-thermal signature, and instead record the effects of intense Devonian deformation and high-grade regional metamorphism. The data suggest the two regions experienced very different pre-Devonian histories and were most likely juxtaposed by the Sennebec Pond thrust fault in latest Silurian to Early Devonian time. Rocks now exposed east of the Sennebec Pond fault probably occupied much higher structural levels during Devonian orogenesis and were not subjected to the same intense Devonian deformation and metamorphism as those rocks now found to the west of this structure. The Silurian tectonism now recognized in this region bears striking resemblance to events of similar age recorded along the northwest margin of the Avalon composite terrane throughout much of Atlantic Canada. This greatly extends the zone of Silurian orogenesis in the northern Appalachians and requires that previous models of New England middle Paleozoic tectonism be significantly revised.

Quartz-albite rocks of ash-fall origin

1981 ◽  
Vol 118 (1) ◽  
pp. 83-88 ◽  
Author(s):  
Evan C. Leitch
Keyword(s):  
Chemical Composition ◽  
New England ◽  
Regional Metamorphism ◽  
Low Grade ◽  
Textural Features ◽  
Early Permian ◽  
Fold Belt ◽  
Ash Fall ◽  
Eastern Australia

SummaryThin beds composed mainly of quartz and albite occur interstratified with epiclastic rocks in a thick marine Early Permian sequence in the eastern part of the New England Fold Belt, eastern Australia. The sequence has suffered very low grade regional metamorphism and the quartz-albite rocks retain few primary textural features. A pyroclastic origin for these rocks is argued on the basis of inherited sedimentary characters and their distinctive mineralogical and chemical composition, and it is suggested that they accumulated as glass-rich ash-fall tuffs. The present chemical composition of the quartz-albite rocks suggests the tuffs may have initially altered to zeolitic assemblages. Similar quartz-albite rocks, perhaps misidentified as chert or siliceous siltstone, probably occur in other low-grade metamorphic sequences, the progenitors of which accumulated adjacent to active magmatic arcs.

Progressive Regional Metamorphism in Southern New Zealand

1938 ◽  
Vol 75 (4) ◽  
pp. 160-174 ◽  
Author(s):  
F. J. Turner
Keyword(s):  
New Zealand ◽  
Regional Metamorphism ◽  
Parent Rock ◽  
Chemical Compositions ◽  
Low Grade ◽  
Scottish Highlands ◽  
The Past ◽  
Structural Peculiarities ◽  
Chemical Conditions ◽  
Pelitic Rocks

During the past ten years a number of papers dealing with progressive regional metamorphism in the southern portion of New Zealand have been published. In the following pages a brief summary of the assemblages of minerals typical of the various metamorphic zones is given, but the writer's main object is to draw attention to certain mineralogical and structural peculiarities that appear to differ in some degree from what are usually regarded as the normal features of regional metamorphism in such classic areas as the Scottish Highlands and the Caledonian chain of Norway. The possibility that such departures from the normal may in some instances be connected with chemical peculiarities in the parent rock is suggested by such phenomena as the well-known general limitation of chloritoid, staurolite, and low-grade garnets to pelitic rocks of special chemical compositions. Other unusual features, especially when found to recur in widely separated regions, may well be governed by some particular combination of physical rather than chemical conditions.

Understanding pre- and syn-orogenic tectonic evolution in western Himalaya through age and petrogenesis of Palaeozoic and Cenozoic granites from upper structural levels of Bhagirathi Valley, NW India

2021 ◽  
pp. 1-27
Author(s):  
Aranya Sen ◽  
Koushik Sen ◽  
Amitava Chatterjee ◽  
Shubham Choudhary ◽  
Alosree Dey
Keyword(s):  
Inductively Coupled Plasma ◽  
Age Distribution ◽  
Normal Fault ◽  
Western Himalaya ◽  
Indian Plate ◽  
Low Grade ◽  
Inductively Coupled ◽  
The North ◽  
Plasma Mass Spectrometry ◽  
Structural Levels

Abstract The Himalaya is characterized by the presence of both pre-Himalayan Palaeozoic and syn-Himalayan Cenozoic granitic bodies, which can help unravel the pre- to syn-collisional geodynamics of this orogen. In the Bhagirathi Valley of Western Himalaya, such granites and the Tethyan Himalayan Sequence (THS) hosting them are bound to the south by the top-to-the-N extensional Jhala Normal Fault (JNF) and low-grade metapelite of the THS to its north. The THS is intruded by a set of leucocratic dykes concordant to the JNF. Zircon U–Pb laser ablation multi-collector inductively coupled plasma mass spectrometry (LA-MC-ICP-MS) geochronology of the THS and one leucocratic dyke reveals that the two rocks have a strikingly similar age distribution, with a common and most prominent age peak at ~1000 Ma. To the north of the THS lies Bhaironghati Granite, a Palaeozoic two-mica granite, which shows a crystallization age of 512.28 ± 1.58 Ma. Our geochemical analysis indicates that it is a product of pre-Himalayan Palaeozoic magmatism owing to extensional tectonics in a back-arc or rift setting following the assembly of Gondwana (500–530 Ma). The Cenozoic Gangotri Leucogranite lies to the north of Bhaironghati Granite, and U–Pb dating of zircon from this leucogranite gives a crystallization age of 21.73 ± 0.11 Ma. Our geochemical studies suggest that the Gangotri Leucogranite is a product of muscovite-dehydration melting of the lower crust owing to flexural bending in relation to steepening of the subducted Indian plate. The leucocratic dykes are highly refracted parts of the Gangotri Leucogranite that migrated and emplaced along extensional fault zones related to the JNF and scavenged zircon from the host THS during crystallization.

scholarly journals [MnO|SiO2,Al2O3,FeO,MgO] balanced log-ratio in chlorites: a tool for chemo-stratigraphic mapping and proxy for the depositional environment

Clay Minerals ◽  
2018 ◽  
Vol 53 (3) ◽  
pp. 351-375
Author(s):  
Annette Süssenberger ◽  
Solveig Pospiech ◽  
Susanne Theodora Schmidt
Keyword(s):  
Depositional Environment ◽  
Regional Metamorphism ◽  
Upper Jurassic ◽  
Contact Metamorphism ◽  
Low Grade ◽  
Rock Composition ◽  
Magallanes Basin ◽  
Global Sea Level ◽  
Rock Analysis ◽  
Log Ratio

ABSTRACTThe [MnO|SiO2,Al2O3,FeO,MgO] balanced ratio (i.e. the isometric log-ratio of the MnO concentration relative to the concentration of SiO2, Al2O3, FeO and MgO) of chlorite and of whole-rock composition is an effective discriminant between Mesozoic stratigraphic formations in the Magallanes Basin (Chile). The MnO content in chlorite is only controlled by the host rock chemistry and is dependent on the geological environment. The MnO content in chlorite remains unchanged at low-grade metamorphic conditions. Single-grain chlorite analysis (n = 1042, electron microprobe) and whole-rock analysis (n = 40, X-ray fluorescence) were used to discriminate stratigraphic formations and to decipher differences in the depositional environment in the Magallanes Basin. The samples are from one Upper Jurassic and three Cretaceous sedimentary units that were affected either by low-grade regional metamorphism or by Miocene contact metamorphism. The highest [MnO|SiO2,Al2O3,FeO,MgO] values are recorded in the upper Zapata Formation. The Punta Barrosa, Cerro Toro and Tobífera Formations show slightly lower [MnO|SiO2,Al2O3,FeO,MgO] values. Elevated [MnO|SiO2,Al2O3,FeO,MgO] values at the transition between Zapata and Punta Barrosa Formations record an oxygenated shallow marine environment that can be linked to the closure of the Rocas Verdes Basin and the onset of fold-and-thrust belt formation. Decreasing [MnO|SiO2,Al2O3,FeO,MgO] values from the Punta Barrosa towards the Cerro Toro Formation indicate gradually increasing water depths during the Upper Cretaceous that correlate well with the global sea level.

Varying Trends in the Metamorphism of Dolerites

1951 ◽  
Vol 88 (1) ◽  
pp. 25-35 ◽  
Author(s):  
John Sutton ◽  
Janet Watson
Keyword(s):  
Regional Metamorphism ◽  
Metamorphic Grade ◽  
Low Grade ◽  
Basic Rocks ◽  
High Metamorphic Grade

AbstractThe metamorphism of Pre-Cambrian dolerites in the Northwest Highlands is described and compared with that of basic rocks in the Southwest Highlands, Banffshire and other regions. The first products of metamorphism are not the same in every area. It is suggested that the trend of regional metamorphism differs according to the environment in which it took place, and that a sequence of changes established in one area cannot be used as a standard by which to judge the changes in other areas. In particular, rocks of high metamorphic grade have not invariably passed through stages characterized by low-grade minerals.

Controls of copper mineralization at Coniston, English Lake District

1977 ◽  
Vol 114 (3) ◽  
pp. 195-202 ◽  
Author(s):  
G. W. Dagger
Keyword(s):  
Regional Metamorphism ◽  
Fracture Pattern ◽  
Main Phase ◽  
Low Grade ◽  
Lake District ◽  
Granite Intrusion ◽  
Copper Mineralization ◽  
English Lake District ◽  
Textural Evidence ◽  
Volcanic Series

SummaryAnalysis of the fracture pattern in the Coniston area of the Lake District indicates that copper mineralization is localized in a series of fractures produced during the main phase of deformation affecting the Borrowdale Volcanic Series. Three events are recognized on the basis of field and textural evidence obtained from polished sections: an early haematite mineralization, correlated with the low grade regional metamorphism affecting the rocks; the main phase of sulphide mineralization, with zoning, which is correlated with a granite intrusion at depth; and a late renewed movement on the veins, with carbonate mineralization, believed to be related to uplift of the granite.

Age of metamorphism in the Lesser Himalaya and the Main Central Thrust zone, Garhwal India: results of illite crystallinity,40Ar–39Ar fusion and K–Ar studies

1995 ◽  
Vol 132 (2) ◽  
pp. 139-149 ◽  
Author(s):  
G. J. H. Oliver ◽  
M. R. W. Johnson ◽  
A. E. Fallick
Keyword(s):  
Regional Metamorphism ◽  
Low Grade ◽  
Supporting Evidence ◽  
Lesser Himalaya ◽  
Main Central Thrust ◽  
Illite Crystallinity ◽  
Lower Eocene ◽  
Thrust Zone ◽  
Lower Palaeozoic ◽  
Upper Paleocene

AbstractIllite crystallinity data from the Lesser Himalaya of Garhwal show that the upper Paleocene-lower Eocene Subathu Formation, deposited immediately prior to or early in the Himalayan collision, has not suffered significant regional metamorphism. The regional metamorphism in the upper Precambrian–lower Palaeozoic Lesser Himalaya must therefore be precollisional. Illite crystallinity results from Lesser Himalayan fossiliferous Permian strata show grades of metamorphism intermediate between upper Paleocene–lower Eocene and Proterozoic–lower Palaeozoic strata indicating a pre-Permian regional metamorphism for the latter.K–Ar whole rock cooling ages provide supporting evidence for pre-collisional regional metamorphism in the Lesser Himalaya. Slates and phyllites below the Main Central Thrust (MCT) show pre-Cenozoic whole rock ages, as old as Ordovician (486 Ma). Whilst resetting of K–Ar whole rock ages has occurred locally in pervasively cleaved Palaeozoic strata (near thrusts?), fracture cleaved Permian and upper Paleocene–lower Eocene sediments give whole rock ages compatible with diagenesis. The illite crystallinity results confirm that these sediments have not been heated above mica blocking temperatures.Muscovite40Ar–39Ar and K–Ar mineral ages within the 5 km thick MCT zone are as young as 8 Ma indicating that temperatures of above ~ 350°C were maintained in the MCT zone for over 10 Ma after high temperature (~ 550°C) shearing on the MCT. This heating did not affect the MCT footwall Lesser Himalaya to any regional extent, where pre-Permian low grade regional metamorphism has not been overprinted.

Emplacement and thermal metamorphism associated with the post-orogenic Strath Ossian Pluton, Grampian Highlands, Scotland

1993 ◽  
Vol 130 (3) ◽  
pp. 379-390 ◽  
Author(s):  
R. M. Key ◽  
E. R. Phillips ◽  
B. C. Chacksfield
Keyword(s):  
Regional Metamorphism ◽  
Wall Rock ◽  
White Mica ◽  
Dyke Swarm ◽  
Near Surface ◽  
Early Devonian ◽  
Progressive Development ◽  
Metasedimentary Rocks ◽  
Thermal Metamorphism ◽  
History Of

AbstractThe multiphase Strath Ossian Pluton was intruded into metasedimentary rocks of the Neoproterozoic Grampian and Appin groups (Grampian Highlands, Scotland) during Silurian or early Devonian times. Emplacementfollowed the main ductile tectono-thermal history of the area and took place during post-orogenic regional uplift and cooling. Early emplacement of dioritic magma in the northern part of the pluton resulted in migmatization of its immediate country rocks with the generation of new ductile structures. The main granodiorite was then emplaced with magma migrating towards the southeast where wall-rock stoping took place. Elsewhere the pluton created its own space with little stoping or veining. Thermal metamorphism caused by granodiorite emplacement resulted in the progressive development of the assemblage quartz+ plagioclase + biotite+ cordierite +andalusite ± K-feldspar in the metapelitic country rocks. Six prograde mineral assemblage zones are identified in the aureole. Final emplacement of a marginal porphyritic microgranite was accompanied by the release of alkaline fluids into the thermal aureole. This produced sillimanite (fibrolite) in association with hydrous phases such as chlorite and white mica. The development of andalusite and cordierite-bearing assemblages is estimated to have occurred at temperatures of 650±50 °C at an estimated pressure of 3.2±0.5 kbars. An approximately isobaric temperature change of 300±50 °C across the width of the main aureole is deduced. The migmatization close to the plutons margins took place at temperatures of about 700 °C. An estimated depth of emplacement of about 11 km is obtained for the Strath Ossian Pluton. This implies considerable regional uplift both prior to, andimmediately after its emplacement. Thus it has been estimated that at the peak of regional metamorphism, probably during the Ordovician Period, the country rocks were at a depth of 15 to 18.5 km, whereas the early Devonian dykes of the Etive dyke swarm, which cut the Strath Ossian Pluton, were emplaced at, or near surface.

Low-grade regional metamorphism in the Mesozoic-Cenozoic volcanic sequences of the Central Andes

1989 ◽  
Vol 7 (5) ◽  
pp. 487-495 ◽  
Author(s):  
B. LEVI ◽  
L. AGUIRRE ◽  
J. O. NYSTRÖM ◽  
H. PADILLA ◽  
M. VERGARA
Keyword(s):  
Regional Metamorphism ◽  
Central Andes ◽  
Low Grade

scholarly journals Transmission electron microscopy study of very low-grade metamorphic evolution in Neoproterozoic pelites of the Puncoviscana formation (Cordillera Oriental, NW Argentina)

Clay Minerals ◽  
2003 ◽  
Vol 38 (4) ◽  
pp. 459-481 ◽  
Author(s):  
M. Do Campo ◽  
F. Nieto
Keyword(s):  
Electron Microscopy ◽  
High Pressure ◽  
Long Range ◽  
Regional Metamorphism ◽  
Analytical Electron Microscopy ◽  
Crystallinity Index ◽  
Microscopy Study ◽  
Electron Microscopy Study ◽  
Low Grade ◽  
Wide Range

AbstractThe Puncoviscana Formation, largely cropping out in NW Argentina, is mainly composed of a pelite-greywacke turbidite sequence affected by incipient regional metamorphism and polyphase deformation. Metapelites, composed mainly of quartz, albite, dioctahedral mica and chlorite, were sampled in the Lules-Puncoviscana and Choromoro belts. Lattice-fringe images, selected area electron diffraction and analytical electron microscopy analyses, coupled with previous data from white mica crystallinity index, indicate a state of reaction progress for Puncoviscana slates consistent with medium anchizone- to epizone-grade metamorphism. The 2Mpolytype prevails in dioctahedral micas, coexisting in a few cases with the 1Mdpolytype as a consequence of lack of equilibrium. The 2Mpolytype coexists with 3Tin two slates and long-range four-layer and ten-layer stacking sequences were identified in another sample. Samples with 3Tand long-range stacking sequences presentbvalues characteristic of intermediate–high pressure metamorphism and ordered chlorites (1L, 2L, 3Land 7L) prevail.Based on the Si contents of dioctahedral micas and considering peak temperatures of ∼350 –400ºC, pressures from 5 kbar and 5 –7 kbar were derived for metapelites from the Lules- Puncoviscana and Choromoro belts, respectively. These values agree with facies series derived from thebvalues. Micas with a wide range of phengitic substitution, as evidenced by Fe + Mg and Si contents, coexist. These variations could not arise from the disturbing effect of detrital white K-mica because TEM evidence indicates that they are absent or represent <10% of the mica population. Thus, compositional variations suggest that dioctahedral micas of individual slates crystallized at different pressure conditions in response to theP-Tpath of the metamorphism. Moreover, in several biotite-free slates the illite crystallinity (IC) values lead to an underestimation of the metamorphic grade attained in these rocks.The coexistence of IC corresponding to anchizone and the occurrence of biotite in some slates and felsic metavolcanic rocks intercalated in the Puncoviscana metasediments are interpreted to be the result of a metamorphic path including a relatively high-pressure/ low-temperature (HP/LT) event, followed by a lower-pressure overprint possibly at higher temperatures than the HP/LTevent. Small micas formed during the high-pressure stage would prevail in the <2 mm fraction, producing anchizone IC.

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