Metamorphism of the pelitic rocks of the Snyder Group in the contact aureole of the Kiglapait layered intrusion, Labrador: effects of buffering partial pressures of water

1982 ◽  
Vol 19 (10) ◽  
pp. 1888-1909 ◽  
Author(s):  
J. Alexander Speer
Keyword(s):  
Layered Intrusion ◽  
Granulite Facies ◽  
Metamorphic Grade ◽  
Contact Metamorphism ◽  
Contact Aureole ◽  
Partial Pressures ◽  
Dehydration Reactions ◽  
Mineral Assemblages ◽  
Temperature Ranges ◽  
Pelitic Rocks

The petrography and mineral chemistries of the Aphebian Snyder Group pelitic rocks in the contact aureole of the Kiglapait layered intrusion, Labrador reveal a rapid increase in metamorphic grade over 1.7 km from the greenschist facies to the granulite facies. Three zones of metamorphic grade are defined by the aluminum silicates: I, andalusite; II, andalusite + sillimanite; and III, sillimanite. In addition to the succession in the aluminum silicates, progressive metamorphic mineral assemblages, with quartz, K-feldspar, and plagioclase, evolve from chlorite + biotite + muscovite through cordierite + biotite ± muscovite and garnet + cordierite + biotite to orthopyroxene + garnet + cordierite + biotite and eventually either orthopyroxene + cordierite ± biotite or orthopyroxene + garnet + cordierite. Anatectites, believed to be derived from pelitic rocks, intrude as small stocks in zone III. They comprise biotite + cordierite ± garnet ± orthopyroxene monzogranites or granodiorites with accessory ilmenite, rutile, monazite, and dumortierite.The contact metamorphism is isobaric with pressure just above the intersection of the muscovite + quartz decomposition with the andalusite–sillimanite transition. Most published geobarometers place the estimated pressure of metamorphism at 4 ± 1 kbar (400 ± 100 MPa), but use of the lower Holdaway triple point would put it at 2.25 kbar (225 MPa). The temperature ranges from 450 °C in zone I to 900 °C or more adjacent to the Kiglapait intrusion. The range of values of [Formula: see text] is estimated to be 0.1–0.9Ptotal. Because most reactions are dehydration reactions, conditions of [Formula: see text] less than Ptotal allow the metamorphic reactions to buffer the partial pressure of water. This results in the common occurrence of low-variance assemblages and leads to an apparent overlapping of mineral assemblages and mineral chemistries with increasing metamorphic grade.

Granulite-facies regional and contact metamorphism of the Tasiuyak paragneiss, northern Labrador: textural evolution and interpretation

2007 ◽  
Vol 44 (10) ◽  
pp. 1413-1437 ◽  
Author(s):  
Tanya Tettelaar ◽  
Aphrodite Indares
Keyword(s):  
Bulk Composition ◽  
Regional Metamorphism ◽  
Granulite Facies ◽  
Contact Metamorphism ◽  
Contact Aureole ◽  
Metamorphic Overprint ◽  
Textural Evolution ◽  
Progressive Development ◽  
Nain Plutonic Suite ◽  
Pelitic Rocks

The Tasiuyak paragneiss at the western margin of the Nain Plutonic Suite has been subjected to two granulite-facies metamorphic events: (i) regional metamorphism during the Paleoproterozoic Torngat orogeny, and (ii) contact metamorphism due to emplacement of the Mesoproterozoic Nain Plutonic Suite. Regional metamorphism led to partial melting of pelitic rocks and the development of a locally well-preserved sequence of prograde and retrograde textures. These textures are partly controlled by bulk composition and formed in the pressure–temperature (P–T) field of the continuous reaction: biotite + sillimanite + plagioclase + quartz  = garnet + K-feldspar + melt, along a hairpin P–T path with peak conditions of ~8–10 kbar (0.8–1.0 GPa) and up to 870 °C in the NaKFMASH (Na2O–K2O–FeO–MgO–Al2O3–SiO2–H2O) system. These textures controlled the development of the contact metamorphic assemblages. Contact metamorphism of the pelitic rocks between the Tessiarsuyungoakh intrusion and the Makhavinekh Lake pluton led to growth of orthopyroxene-cordierite symplectite after garnet–biotite, and cordierite–spinel symplectite after garnet–sillimanite. These phase associations attest to reactions in specific microtextural settings, some of which produced a second generation of partial melt. Maximum temperatures were above ~750 °C and pressures were lower than those of the regional metamorphism. The aureole around the Makhavinekh Lake pluton is ~4 km wide and shows a progressive development of the contact metamorphic assemblages toward the pluton. In contrast, the contact metamorphic overprint is incipient around the Tessiarsuyungoakh intrusion, which developed a ~20 m wide contact aureole and is most prominent in screens of paragneiss within that intrusion.

scholarly journals U-Pb monazite ages of the Kabanga mafic-ultramafic intrusions and contact aureoles, central Africa: Geochronological and tectonic implications

2019 ◽  
Vol 131 (11-12) ◽  
pp. 1857-1870 ◽  
Author(s):  
Jian-Wei Zi ◽  
Birger Rasmussen ◽  
Janet R. Muhling ◽  
Wolfgang D. Maier ◽  
Ian R. Fletcher
Keyword(s):  
Central Africa ◽  
Layered Intrusion ◽  
Thermal Anomaly ◽  
Contact Metamorphism ◽  
Contact Aureole ◽  
East African ◽  
Metamorphic History ◽  
History Of ◽  
Rodinia Assembly ◽  
Contact Aureoles

AbstractMafic-ultramafic rocks of the Kabanga-Musongati alignment in the East African nickel belt occur as Bushveld-type layered intrusions emplaced in metasedimentary sequences. The age of the mafic-ultramafic intrusions remains poorly constrained, though they are regarded to be part of ca. 1375 Ma bimodal magmatism dominated by voluminous S-type granites. In this study, we investigated igneous monazite and zircon from a differentiated layered intrusion and metamorphic monazite from the contact aureole. The monazite shows contrasting crystal morphology, chemical composition, and U-Pb ages. Monazite that formed by contact metamorphism in response to emplacement of mafic-ultramafic melts is characterized by extremely high Th and U and yielded a weighted mean 207Pb/206Pb age of 1402 ± 9 Ma, which is in agreement with dates from the igneous monazite and zircon. The ages indicate that the intrusion of ultramafic melts was substantially earlier (by ∼25 m.y., 95% confidence) than the prevailing S-type granites, calling for a reappraisal of the previously suggested model of coeval, bimodal magmatism. Monazite in the metapelitic rocks also records two younger growth events at ca. 1375 Ma and ca. 990 Ma, coeval with metamorphism during emplacement of S-type granites and tin-bearing granites, respectively. In conjunction with available geologic evidence, we propose that the Kabanga-Musongati mafic-ultramafic intrusions likely heralded a structurally controlled thermal anomaly related to Nuna breakup, which culminated during the ca. 1375 Ma Kibaran event, manifested as extensive intracrustal melting in the adjoining Karagwe-Ankole belt, producing voluminous S-type granites. The Grenvillian-aged (ca. 990 Ma) tin-bearing granite and related Sn mineralization appear to be the far-field record of tectonothermal events associated with collision along the Irumide belt during Rodinia assembly. Since monazite is a ubiquitous trace phase in pelitic sedimentary rocks, in contact aureoles of mafic-ultramafic intrusions, and in regional metamorphic belts, our study highlights the potential of using metamorphic monazite to determine ages of mafic-ultramafic intrusions, and to reconstruct postemplacement metamorphic history of the host terranes.

‘Decompressional’ reaction textures formed by isobaric heating: an example from the thermal aureole of the Taylor Brook Gabbro Complex, western Newfoundland

2002 ◽  
Vol 66 (6) ◽  
pp. 941-951 ◽  
Author(s):  
S. J. Ings ◽  
J. V. Owen
Keyword(s):  
Long Range ◽  
Regional Metamorphism ◽  
Contact Metamorphism ◽  
Contact Aureole ◽  
Mafic Intrusion ◽  
Pressure Release ◽  
Metamorphic Mineral ◽  
Mineral Assemblages ◽  
Tonalitic Gneiss ◽  
Isothermal Decompression

Abstract Reaction textures including corona structures in granulites from the Proterozoic Long Range Inlier of western Newfoundland are spatially associated with a Silurian (0.34 Ga) mafic intrusion, the Taylor Brook Gabbro Complex. They comprise, in metabasites and tonalitic gneiss, coronal orthopyroxene and plagioclase on garnet and, in metapelites, cordierite and spinel formed at the expense of sillimanite, garnet and quartz. Although generally interpreted to indicate near-isothermal decompression (ITD) following regional metamorphism, which in the inlier occurred at ˜1.10–1.03 Ga, these features appear to be absent elsewhere. Therefore they are interpreted to be products of contact metamorphism (near-isobaric heating – IBH) within the thermal aureole of the gabbro. Thus, there is a ˜0.7 Ga difference (i.e. mid-Proterozoic vs. mid-Silurian) between the age of the regional metamorphic mineral assemblages and the contact aureole assemblages. The observation that classic ITD features occur in this aureole environment underscores the fact that P-sensitive reactions can progress during IBH as well as by pressure release.

The contact aureole of the South Mountain Batholith in Halifax, Nova Scotia: geology, mineral assemblages, and isograds

2012 ◽  
Vol 49 (11) ◽  
pp. 1280-1296 ◽  
Author(s):  
R.A. Jamieson ◽  
G.G. Hart ◽  
G.G. Chapman ◽  
N.W. Tobey
Keyword(s):  
Nova Scotia ◽  
Regional Metamorphism ◽  
Metamorphic Grade ◽  
Contact Aureole ◽  
Sulphide Mineral ◽  
The South ◽  
South Mountain Batholith ◽  
Mineral Assemblages ◽  
Modal Abundance ◽  
The City

The contact aureole at the eastern margin of the South Mountain Batholith (Halifax Pluton) underlies most of the city of Halifax, Nova Scotia. The Halifax Group in the study area includes two lithological units, the Cunard and Bluestone formations. Before intrusion, both had been affected by greenschist facies regional metamorphism and deformed into northeast–southwest-trending, regional upright folds associated with a strong slaty cleavage. Contact metamorphic isograds trend obliquely across the Halifax peninsula, at a high angle to regional structural trends. At 2.5–3 km from the intrusive contact, sparse cordierite spots mark the outer limit of the contact aureole. The biotite-in isograd is marked by the development of biotite within chlorite + muscovite stacks inherited from regional metamorphism. Pyrrhotite is the dominant sulphide mineral throughout the contact aureole. With increasing metamorphic grade, assemblages in both units are marked by increasing modal abundance of cordierite and biotite, with K-feldspar variably developed within ca. 600 m of the contact. However, there is a marked difference in the distribution and appearance of andalusite between the two units. In aluminous pelites of the Cunard formation, idioblastic chiastolite appears before biotite more than 1500 m from the contact. In the less aluminous Bluestone formation, andalusite is present only within ca. 500 m of the contact, where it forms xenoblastic, spongy crystals. In both units, the assemblage andalusite + biotite + K-feldspar ± cordierite is developed near the contact, with local fibrolite and evidence of incipient partial melting. Petrographic constraints suggest pressure–temperature conditions at the contact of ca. 2.5–3.0 kbar (1 kbar = 100 MPa) at ca. 650 °C.

scholarly journals Geology and crystallization conditions of the Särkiniemiintrusion and related nickel-copper ore, central Finland – implications for depth of emplacement of 1.88 Ga nickel-bearing intrusions

2020 ◽  
Vol 92 (2) ◽  
pp. 111-130
Author(s):  
Hannu V. Makkonen ◽  
◽  
Pekka Tuisku ◽  
Keyword(s):  
Nickel Sulfide ◽  
Nickel Content ◽  
Crustal Contamination ◽  
Granulite Facies ◽  
Metamorphic Grade ◽  
Contact Aureole ◽  
Critical Factors ◽  
Copper Ore ◽  
Nickel Copper ◽  
Crystallization Conditions

Several Ni-Cu deposits occur within the Kotalahti area, central Finland, in proximity to an Archaean gneiss dome surrounded by a Palaeoproterozoic craton-margin supracrustal sequence comprising quartzites, limestones, calc-silicate rocks, black schists and banded diopside amphibolites. The geology of the area and age of the Ni-bearing intrusions (1.88 Ga) are similar to the Thompson Ni belt in the Canadian Trans-Hudson Orogen. The small mafic-ultramafic and Ni-Cu -bearing Särkiniemi intrusion, closely associated with the Archaean basement core of the Kotalahti Dome, is composed of a western peridotite and eastern gabbro body, both of which are mineralized. The eastern gabbro has a contact aureole several meters thick, consisting of orthopyroxene +/- cordierite bearing hornfels between the intrusion and the migmatites. Geochemically, the Särkiniemi intrusion shares many features in common with other Svecofennian mafic-ultramafic intrusions, including crustal contamination and nickel depletion. The related Ni-Cu deposit has a low Ni/Co value (15) and low nickel content in the sulphide fraction (2.8 wt.%), together with a low estimated magma/sulphide ratio of around 170. Svecofennian 1.88 Ga mafic-ultramafic intrusions occur in terrains of variable metamorphic grade (from low-amphibolite to granulite facies) and are likely to represent emplacement at different crustal depths. Multi-equilibrium thermobarometry indicates that the contact aureole at Särkiniemi reached equilibrium at pressures of 4.5–6 kbar (15–20 km depth) and temperatures of 600–670 °C. Combined with the results of earlier research on the Svecofennian intrusions, this study indicates that a depth of 15–20 km crustal level was favourable, along with other critical factors, for nickel sulfide deposition at 1.88 Ga.

Determination of the crystal structure of magnesium perchlorate hydrates by X-ray powder diffraction and the charge-flipping method

2010 ◽  
Vol 66 (6) ◽  
pp. 579-584 ◽  
Author(s):  
Kevin Robertson ◽  
David Bish
Keyword(s):  
Crystal Structures ◽  
Powder Diffraction ◽  
Fundamental Parameter ◽  
Magnesium Perchlorate ◽  
Space Group ◽  
X Ray ◽  
Partial Pressures ◽  
Dehydration Reactions

X-ray powder diffraction (XRD) data were used to solve the crystal structures of phases in the magnesium perchlorate hydrate system, Mg(ClO4)2·nH2O (n = 4, 2). A heating stage and humidity generator interfaced to an environmental cell enabled in-situ XRD analyses of dehydration reactions under controlled temperatures and partial pressures of H2O (P_{{\rm H}_2{\rm O}}). The crystal structures were determined using an ab initio charge-flipping method and were refined using fundamental-parameter Rietveld methods. Dehydration of magnesium perchlorate hexahydrate to tetrahydrate (348 K) results in a decrease in symmetry (space group = C2), where isolated Mg2+ cations are equatorially coordinated by four H2O molecules with two [ClO4]− tetrahedra at the apices. Further dehydration to the dihydrate (423 K) leads to bridging of the isolated packets to form double corner-sharing chains of octahedra and polyhedra (space group = C2/m).

Metamorphism of the Settler Schist, southwest of Yale, British Columbia

1976 ◽  
Vol 13 (3) ◽  
pp. 405-421 ◽  
Author(s):  
Lee C. Pigage
Keyword(s):  
British Columbia ◽  
Linear Regression ◽  
Microprobe Analysis ◽  
Regional Metamorphism ◽  
Fluid Phase ◽  
Contact Metamorphism ◽  
Major Constituent ◽  
Minor Components ◽  
Mineral Assemblages ◽  
Regression Techniques

Pelitic metasediments immediately southwest of Yale, British Columbia contain mineral assemblages characteristic of staurolite through sillimanite zones of the Barrovian facies series. Microprobe analyses of major constituent phases in the pelites are combined with linear regression techniques to formulate probable kyanite- and sillimanite-forming reactions.A zone some 3 km wide contains the assemblage staurolite–kyanite–garnet–biotite–muscovite–quartz–plagioclase-ilmenite-rutile, which is univariant in AFM projection. Within precision limits of microprobe analysis, this assemblage is also univariant using linear regression techniques. Univariant relations are possible if [Formula: see text] with the composition of the fluid phase being buffered by the progressing reaction. This assemblage emphasizes the need for precise analyses when using the regression method, since minor components are often within permissible error limits rather than being balanced.Pelitic and calc-silicate assemblages from the metasediments restrict estimates of pressure–temperature conditions during regional metamorphism to 6–8 kbar and 550–770 °C. Pseudomorphs after andalusite indicate that contact metamorphism preceded regional upgrading of the pelites.

scholarly journals Petrology of the metamorphic rocks (gravels) from Bhajanpur area in Panchagarh District, Bangladesh

1970 ◽  
Vol 5 ◽  
pp. 91-96
Author(s):  
Md Rahat Hossain ◽  
Ismail Hossain ◽  
ASM Zahid Hossain ◽  
Prodip Kumar Biswas
Keyword(s):  
Key Words ◽  
Metamorphic Rock ◽  
Metamorphic Rocks ◽  
Mica Schist ◽  
Mineral Assemblages ◽  
Temperature Ranges ◽  
Garnet Zone ◽  
Opaque Minerals ◽  
Mineralogical Compositions ◽  
Chlorite Schist

The present study deals with petrology of the detrital gravelly rocks from Bhajanpur area, Panchagarh, Bangladesh. The results of detailed petrography of gravelly rocks indicate the presence of quartz (monocrystalline and polycrystalline quartz), K-feldspar, plagioclase, chlorite, muscovite and biotite as major mineralogical compositions. Other minor minerals are garnet, kyanite, graphite and opaque minerals. Based on definitive mineral assemblages, blueschist and greenschist facies sequences are recognized. Correspondingly, index minerals provide chlorite zone, biotite zone, garnet zone, kyanite zone, and graphite zone. The P-T conditions of the studied rocks demonstrate the possible temperature ranges 300-550°C and pressure ranges 2-10 kbar. Most common varieties of metamorphic rocks in the study area are garnet mica schist, chlorite schist, gneiss and few quartzites. Characteristics of garnet mica schist and chlorite schist are equivalent with the lesser Himalayan metamorphic rock sequence in Sikkim area, whereas gneiss from Bhajanpur area has similar precursor as Darjeeling gneiss. Therefore, the sources of detrital metamorphic rocks in Bhajanpur area obviously come from the lesser Himalayan sequence in Sikkim and Darjeeling areas, India. Key words: Petrology; metamorphic rocks; gravels; P-T conditions; Panchagarh; lesser Himalayan sequence DOI: 10.3329/jles.v5i0.7357 J. Life Earth Sci., Vol. 5: 91-96, 2010

Peraluminous sapphirine from the Aileron district, Arunta Block, central Australia

1987 ◽  
Vol 51 (361) ◽  
pp. 409-415 ◽  
Author(s):  
R. G. Warren ◽  
B. J. Hensen
Keyword(s):  
Bulk Composition ◽  
Granulite Facies ◽  
The Other ◽  
Stable Phase ◽  
Mineral Assemblages ◽  
Central Australia

AbstractSpecimens collected from a small lens of phlogopite-rich rock in the granulite-facies terrain of the Arunta Block, central Australia, have unusual bulk compositions and mineral assemblages. One sample consists of phlogopite enclosing blue spinel (mg 96) with minute granules of corundum and sapphirine at the margins; a second of phlogopite enclosing porphyroblasts of corundum and peraluminous sapphirine. In the first the sapphirine is close to the 7 : 9 : 3 composition; in the other it is markedly peraluminous, e.g. (Mg1.628Fe0.028)Al4.714Si0.636O10, intermediate between the 7 : 9 : 3 and 3 : 5 : 1 members. The texture suggests that this sapphirine is a stable phase in equilibrium with eastonitic phlogopite and corundum. The very potassic, very magnesian bulk composition of the rocks is attributed to potassium metasomatism of a protolith consisting of magnesian chlorite and quartz.

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