Multiple alteration events in the history of a sub-Huronian regolith at Lauzon Bay, Ontario

1992 ◽  
Vol 29 (3) ◽  
pp. 432-445 ◽  
Author(s):  
S. J. Sutton ◽  
J. B. Maynard
Keyword(s):  
Clay Minerals ◽  
Chemical Weathering ◽  
Analytical Techniques ◽  
Mineral Chemistry ◽  
White Mica ◽  
Low Grade ◽  
Plagioclase Feldspar ◽  
Fine Grained ◽  
History Of ◽  
Greenschist Facies Metamorphism

Confusion exists over the usefulness of chemical data from Precambrian weathering profiles in constraining models of atmospheric evolution. One difficulty is in correctly identifying ancient weathering effects and isolating them from numerous other processes that are likely to have affected such ancient rocks. In this study of a middle Precambrian granitic weathering profile, we have used several analytical techniques to separate weathering-related chemical and mineralogical changes from those resulting from other processes. The profile is exposed beneath the Huronian at Lauzon Bay in the Blind River area of Ontario and has a complex history of alteration events, addition of allochthonous material, and low-grade metamorphism. Much of this history can be deciphered, and changes in mineralogy and bulk and mineral chemistry can be assigned to separate alteration events. Specifically, the granite has undergone preweathering albitization, resulting in Na enrichment, followed by chemical weathering that corroded K-feldspar and nearly destroyed plagioclase feldspar and mica in the regolith. Clay minerals replaced feldspars, resulting in enrichment in Al, Ti, and Zr and depletion in Na, Ca, Sr, and K. Fe has also been leached. After weathering, a fine-grained 0.5 m layer of strongly weathered allochthonous material was deposited on the regolith, followed by deposition of the Matinenda Formation. Sometime after Matinenda deposition, K- and Rb-metasomatim affected the regolith and overlying sediments, converting some clays to illite and depositing secondary K-feldspar. Greenschist-facies metamorphism probably postdated this metasomatism and converted clay minerals to white mica and chlorite.

Chlorite-mica stacks in low-strain rocks from central Wales

1982 ◽  
Vol 119 (3) ◽  
pp. 243-256 ◽  
Author(s):  
J. Craig ◽  
W. R. Fitches ◽  
A. J. Maltman
Keyword(s):  
New Jersey ◽  
Clay Minerals ◽  
White Mica ◽  
West Germany ◽  
Low Grade ◽  
Controlled Growth ◽  
Common Occurrence ◽  
Grade Metamorphism ◽  
Lower Palaeozoic

SummaryWeakly deformed, low grade, Lower Palaeozoic metasediments from central Wales contain abundant stack-like intergrowths of chlorite and white mica that closely resemble stacks described from the Devonian Hunsruckschiefer of West Germany; the Ordovician Martinsburg Slate, New Jersey, U.S.A.; and elsewhere. Several theories have been proposed to explain the origin of such stacks, including a detrital origin; strain-controlled growth of chlorite on a detrital mica nucleus; and strain-controlled intergrowth during metamorphism. None of these satisfactorily explains the central Wales stacks. A detrital origin is precluded by the presence of many stacks with shapes too delicate to have survived transportation, and a lack of hydrodynamic equivalence between the stacks and the clastic host grains. Features inconsistent with strain-controlled growth are constant alignment parallel to bedding but non-systematic orientation with respect to tectonic cleavage, their common occurrence in undeformed rocks, and petrographic evidence that they precede the tectonic cleavage. It is proposed that the stacks formed during diagenesis and low-grade metamorphism, and before the onset of deformation, through mimetic growth on a primary bedding fabric composed of clay minerals.

Polymetamorphism in the Archean Hemlo – Heron Bay greenstone belt, Superior Province: P–T variations and implications for tectonic evolution

1993 ◽  
Vol 30 (5) ◽  
pp. 985-996 ◽  
Author(s):  
Yuanming Pan ◽  
Michael E. Fleet
Keyword(s):  
Greenstone Belt ◽  
Regional Metamorphism ◽  
Mineral Chemistry ◽  
Superior Province ◽  
Low Grade ◽  
Garnet Porphyroblasts ◽  
Metamorphic History ◽  
Tectonic Environment ◽  
History Of ◽  
Archean Greenstone Belt

The tectono-metamorphic history of the late Archean (2800–2600 Ma) Hemlo – Heron Bay greenstone belt in the Superior Province has been delineated from textural relationships, mineral chemistry, and P–T paths in metapelites, cordierite–orthoamphibole rocks, and metabasites from the White River exploration property, Hemlo area, Ontario. An early low-temperature, medium-pressure metamorphism (about 500 °C and 6–6.5 kbar (1 kbar = 100 MPa)) is indicated by the occurrence of relict kyanite and staurolite porphyroblasts and zoned garnet porphyroblasts in metapelites and the presence of zoned calcic amphiboles in metabasites. This early metamorphism appears to have been coeval with the previously documented D1 deformation that is associated with, for example, low-angle thrusts. A second regional metamorphism predominates in the Hemlo – Heron Bay greenstone belt and is generally of relatively low grade, at about 510–530 °C and 3.2–3.5 kbar, over most of the study area and increases to medium grade (550–650 °C and 4–5 kbar) towards the southern margin with the Pukaskwa Gneissic Complex and along the central axis enclosing the Hemlo Shear Zone. The second regional metamorphism was contemporaneous with the D3 deformation and was probably related to plutonism. This type of polymetamorphism in the Hemlo – Heron Bay greenstone belt may be equivalent to those in Phanerozoic subduction complexes and therefore supports the arc–arc accretion model for the development of the southern Superior Province. Although the Hemlo – Heron Bay greenstone belt most likely represents a single tectonic environment (an oceanic island arc), the restricted occurrence of the relict kyanite and staurolite indicates that the central portion of this Archean greenstone belt probably was at a deeper crustal level at the time of the first metamorphic event.

Deformation timing and strain in Neoproterozoic strata, Jebel Akhdar, northern Oman

2020 ◽  
Author(s):  
Christopher Bailey ◽  
Claire Rae
Keyword(s):  
Strain Analysis ◽  
Low Grade ◽  
Depositional History ◽  
Fine Grained ◽  
Metasedimentary Rocks ◽  
Aspect Ratios ◽  
Deformation Kinematics ◽  
History Of ◽  
Penetrative Foliation ◽  
Ar Geochronology

<p>Neoproterozoic rocks exposed in the Jebel Akhdar massif of northern Oman preserve glaciogenic deposits associated with multiple Cryogenian glaciations. Although the depositional history of these rocks is well understood, the significance of post-depositional deformation is poorly constrained. In this study, we examine low-grade metasedimentary rocks exposed in the Ghubrah Bowl, an erosional window in the Jebel Akhdar massif, in order to quantify the 3D finite strain, understand deformation kinematics, and determine the timing of deformation/metamorphism.</p><p>In the Jebel Akhdar massif, the older Ghubrah (Sturtian glaciation) and younger Fiq (Marinoan glaciation) formations comprise a >1 km thick sequence of diamictite interbedded with sandstone, siltstone, conglomerate, volcanic rock, and minor carbonate. Diamictites contain abundant clasts of siltstone and sandstone, with lesser amounts of granite and metavolcanic rock in a fine-grained quartz + sericite ± chlorite matrix. Clasts range from granules to boulders. Harder clasts tend to be subangular and poorly aligned with low aspect ratios, whereas fine-grained rock clasts are well-aligned with large aspect ratios. Bedding generally dips to the NW, but is gently folded in accord with the overall structure of the Jebel Akhdar massif. A penetrative foliation strikes E-W and dips to the S. At some locations, a prominent elongation lineation/pencil structure occurs and plunges gently to moderately to the S.</p><p>R<sub>f</sub>/phi strain analysis in the diamictites reveals a range of 3D strain geometries (apparent flattening to apparent constriction) with strain ratios up to 2.8 in XZ sections. Strain is strongly partitioned, as clasts of igneous rock have low aspect ratios and are only weakly aligned. Penetrative strain in clast-supported sandstones is negligible (XZ ratios of <1.2). Outsized clasts of granite and sandstone are mantled by distinctive symmetric pressure shadows (double-duckbill structures) that include more recrystallized minerals than elsewhere in the diamictite. <sup>40</sup>Ar/<sup>39</sup>Ar geochronology of sericite in pressure shadows yields ages as young as 90 Ma, which are interpreted as mixed ages containing an older detrital component and a younger fraction formed during growth. Deformation is associated with southward emplacement and loading by the Oman ophiolite & Hawasina Group sediments over the autochthonous sequence in the late Cretaceous.</p>

Microscopic and megascopic fabrics in the Skiddaw Group, Black Combe inlier, English Lake District

1969 ◽  
Vol 106 (6) ◽  
pp. 587-594 ◽  
Author(s):  
Douglas G. Helm
Keyword(s):  
Clay Minerals ◽  
Regional Metamorphism ◽  
Axial Plane ◽  
Low Grade ◽  
Lake District ◽  
Tectonic History ◽  
Lower Ordovician ◽  
English Lake District ◽  
History Of

SUMMARYThe rocks of the Skiddaw Group of the Black Combe inlier were subjected to low grade regional metamorphism during the D1 movementphase of an intra-Lower Ordovician orogenic episode. Mimetic recrystallisation of clay minerals parallel to the bedding was ubiquitous. Later, axial-plane cleavages were superimposed on this fabric. The nature of the cleavages and possible mechanism of their formation is discussed.Silica has been mobilised on at least three occasions during the tectonic history of the inlier. In no case was this due to metasomatism but simply to metamorphic differentiation. The D1 veins are of either quartz, or quartz and chlorite whereas veins of later generations are of quartz alone.

scholarly journals Animal-sediment interactions: the effect of ingestion and excretion by worms on mineralogy

2004 ◽  
Vol 1 (2) ◽  
pp. 113-121 ◽  
Author(s):  
S. J. Needham ◽  
R. H. Worden ◽  
D. McIlroy
Keyword(s):  
Clay Minerals ◽  
Clay Mineral ◽  
Lumbricus Terrestris ◽  
Depositional Environments ◽  
Mineral Dissolution ◽  
Combined Processes ◽  
Accelerated Weathering ◽  
Low Grade ◽  
Fine Grained ◽  
Peak Broadening

Abstract. By controlled experiments that simulate marine depositional environments, it is shown that accelerated weathering and clay mineral authigenesis occur during the combined process of ingestion, digestion and excretion of fine-grained sediment by two species of annelid worms. Previously characterized synthetic mud was created using finely ground, low-grade metamorphic slate (temperature approximately 300°C) containing highly crystalline chlorite and muscovite. This was added to experiment and control tanks along with clean, wind-blown sand. Faecal casts were collected at regular intervals from the experimental tanks and, less frequently, from the control tanks. Over a period of many months the synthetic mud (slate) proved to be unchanged in the control tanks, but was significantly different in faecal casts from the experimental tanks that contained the worms Arenicola marina and Lumbricus terrestris. Chlorite was preferentially destroyed during digestion in the gut of A. marina. Both chlorite and muscovite underwent XRD peak broadening with a skew developing towards higher lattice spacing, characteristic of smectite formation. A neoformed Fe-Mg-rich clay mineral (possibly berthierine) and as-yet undefined clay minerals with very high d-spacing were detected in both A. marina and L. terrestris cast samples. We postulate that a combination of the low pH and bacteria-rich microenvironment in the guts of annelid worms may radically accelerate mineral dissolution and clay mineral precipitation processes during digestion. These results show that macrobiotic activity significantly accelerates weathering and mineral degradation as well as mineral authigenesis. The combined processes of sediment ingestion and digestion thus lead to early diagenetic growth of clay minerals in clastic sediments.

scholarly journals Tectonothermal history of the basement rocks within the NW Dinarides: new40 Ar/39 Ar ages and synthesis

2012 ◽  
Vol 63 (6) ◽  
pp. 441-452 ◽  
Author(s):  
Šoštarić Sibila Borojević ◽  
Neubauer Franz ◽  
Handler Robert ◽  
Palinkaš Ladislav A.
Keyword(s):  
New Age ◽  
White Mica ◽  
Low Grade ◽  
Vardar Zone ◽  
Basement Rocks ◽  
Metamorphic Basement ◽  
Step Heating ◽  
History Of

Abstract Very low-grade and low-grade metamorphosed basement rocks from distinct inliers of the Africa-derived northwestern Dinarides (Medvednica Mts and Paleozoic Sana-Una Unit, respectively) have been studied with the multigrain step-heating 40Ar/39Ar technique in order to compare and reveal their tectonothermal history. 40Ar/39Ar ages from detrital white mica of the very low-grade basement rocks of the Paleozoic Sana-Una Unit gave a Variscan age of ~335 Ma. The new age is in agreement with 40Ar/39Ar ages from the very low-grade basement exposed at Petrova and Trgovska Gora of the NW Dinarides. Within low-grade metamorphic basement rocks from the Medvednica Mts, we found no Variscan ages. White mica from phyllitic basement rocks of the Medvednica Mts gives predominantly early Alpine ages ranging between 135 and 122 Ma and younger Alpine ages of ~80 Ma. The early Alpine ages of 135 and 122 Ma are interpreted as the date to the onset of ductile nappe stacking predating the formation of Gosau-type collapse basins. The late early Alpine event of ~80 Ma can be traced in the entire Cretaceous-aged orogen of the Circum- Pannonian Region and is synchronous with subsidence of the Gosau-type basins and opening and closure of the neighbouring Sava-Vardar Zone.

scholarly journals Clay templates in Ediacaran vendotaeniaceans: Implications for the taphonomy of carbonaceous fossils

2021 ◽  
Author(s):  
B. Becker-Kerber ◽  
A. Abd Elmola ◽  
A. Zhuravlev ◽  
C. Gaucher ◽  
M.G. Simões ◽  
...  
Keyword(s):  
Clay Minerals ◽  
Soft Tissues ◽  
Bedding Plane ◽  
Mato Grosso ◽  
Fine Grained ◽  
Sedimentary Deposits ◽  
Fibrous Minerals ◽  
Mean Width ◽  
History Of ◽  
Cambrian Radiation

Although rare, sedimentary deposits containing exceptionally preserved fossils (i.e., Lagerstätten) have shaped our view on the history of life at particular intervals, such as those recording the Cambrian radiation of animals. Therefore, understanding the processes that lead to the fossilization of unmineralized tissues is crucial to better interpret these fossil assemblages. A key issue on the fossilization of exceptionally preserved fossils is linked to the role of clay minerals in the high-fidelity preservation of recalcitrant and soft tissues. Here, we show for the first time, an association of unusual fibrous clays with carbonaceous fossils (Vendotaenia) in the late Ediacaran Tamengo Formation (Mato Grosso do Sul State, western Brazil). The vendotaeniaceans occur in laminated mudstones/siltstones interpreted as being deposited in outer to distal mid-ramp depositionary settings. The fossils are characterized by ribbon-shaped compressions 0.56 mm in mean width. The fibrous clays are obliquely oriented with respect to the bedding plane, and follow the orientation of tectonically deformed structures. Our mineralogical, geochemical, and petrographic data demonstrate that these clays are mainly composed of chlorite-smectite mixed layered minerals, with >50% chlorite. Altogether, our results suggest that these fibrous minerals formed in the late-diagenetic zone to lower anchizone, reinforcing the previous idea that clay minerals associated with fossils are not necessarily related to the preservation of soft tissues. Instead, the initial preservative pathway in our fossils was probably restricted to organic matter conservation in reducing fine-grained sediments, similar to other deposits with carbonaceous fossils. This newly established mechanism, which involves the formation of clays on organic templates in the late-diagenetic zone, is likely a more widespread phenomenon than previously thought.

scholarly journals Animal-sediment interactions: the effect of ingestion and excretion by worms on mineralogy

2004 ◽  
Vol 1 (1) ◽  
pp. 533-559 ◽  
Author(s):  
S. J. Needham ◽  
R. H. Worden ◽  
D. McIlroy
Keyword(s):  
Clay Minerals ◽  
Clay Mineral ◽  
Lumbricus Terrestris ◽  
Depositional Environments ◽  
Mineral Dissolution ◽  
Combined Processes ◽  
Accelerated Weathering ◽  
Low Grade ◽  
Fine Grained ◽  
Peak Broadening

Abstract. By controlled experiments that simulate marine depositional environments, it is shown that accelerated weathering and clay mineral authigenesis occur during the combined process of ingestion, digestion and excretion of fine-grained sediment by two species of annelid worms. Previously characterized synthetic mud was created using finely ground, low-grade metamorphic slate (temperature approximately 300°C) containing highly crystalline chlorite and muscovite. This was added to experiment and control tanks along with clean, wind-blown sand. Faecal casts were collected at regular intervals from the experimental tanks and, less frequently, from the control tanks. Over a period of many months the synthetic mud (slate) proved to be unchanged in the control tanks, but was significantly different in faecal casts from the experimental tanks that contained the worms Arenicola marina and Lumbricus terrestris. Chlorite was preferentially destroyed during digestion in the gut of A. marina. Both chlorite and muscovite underwent XRD peak broadening with a skew developing towards higher lattice spacing, characteristic of smectite formation. A neoformed Fe-Mg-rich clay mineral (possibly berthierine) and as-yet undefined clay minerals with very high d-spacing were detected in both A. marina and L. terrestris cast samples. We postulate that a combination of the low pH and bacteria-rich microenvironment in the guts of annelid worms may radically accelerate mineral dissolution and clay mineral precipitation processes during digestion. These results show that macrobiotic activity significantly accelerates weathering and mineral degradation as well as mineral authigenesis. The combined processes of sediment ingestion and digestion thus lead to early diagenetic growth of clay minerals in clastic sediments.

Magma chamber evolution of the Ardestan pluton, Central Iran: evidence from mineral chemistry, zircon composition and crystal size distribution

2019 ◽  
Vol 83 (6) ◽  
pp. 763-780 ◽  
Author(s):  
Shahrouz Babazadeh ◽  
Tanya Furman ◽  
John M. Cottle ◽  
Davood Raeisi ◽  
Ianna Lima
Keyword(s):  
Size Distribution ◽  
Crystal Size ◽  
Magma Mixing ◽  
Mineral Chemistry ◽  
Crystal Size Distribution ◽  
Plagioclase Feldspar ◽  
Magmatic Origin ◽  
Field Evidence ◽  
History Of ◽  
Chemical Conditions

AbstractThe Oligo–Miocene Ardestan quartz diorite to tonalite is part of widespread Cenozoic magmatism within the Urumieh–Dokhtar Magmatic Assemblage of Iran. The Ardestan pluton is composed mainly of varying proportions of plagioclase feldspar (normally zoned from bytownite to andesine), amphibole (magnesio-hornblende) and biotite. Biotite exhibits a range of Al values (~2–2.8 apfu) over very restricted Fe# ratios (0.42–0.56) which are characteristic of continental arc magmatic suites. High Ti2O contents of biotite (<6.1 wt.%) suggest a magmatic origin. Ti-in-biotite geothermometery gives a mean crystallisation temperature of 730 ± 56°C, slightly higher than calculated TZr.Ti°C (716 ± 50°C) and similar to the average TZr.sat°C (735 ± 26°C). These results are consistent with the low bulk-rock SiO2 contents, which provide minimum estimates of temperature and indicate zircon crystallised from a fractionated magma. Zircons from the Ardestan pluton have high (Sm/La)N (>10) ratios suggesting a magmatic origin. T–$f_{{\rm O}_{\rm 2}}$ calculations of oxygen fugacity between –13.6 to –16.9 indicate oxidising crystallisation conditions between the Ni–NiO (NNO) and Fe2O3–Fe3O4 (HM) buffers. Tight linear trends of log (XF/XOH), log (XCl/XOH) and log (XCl/XOH) vs. XMg represent a narrow range of $f_{{\rm H}_2O}$, fHF and fHCl, clearly indicating that physico-chemical conditions were essentially constant throughout the formation of magmatic biotite. The shape of crystal size distribution curves along with the medium Al and Mg contents in amphibole and biotite, respectively, are consistent with a history of magma mixing involving injections of basic magma into the evolving felsic chamber. Calculated residence time for Ardestan plagioclase crystals of ~630 years support field evidence that these plutons were emplaced at shallow depths.

Low-grade evolution of clay minerals and organic matter in fault zones of the Hikurangi prism (New Zealand)

Clay Minerals ◽  
2018 ◽  
Vol 53 (4) ◽  
pp. 579-602 ◽  
Author(s):  
Tatiana Maison ◽  
Sébastien Potel ◽  
Pierre Malié ◽  
Rafael Ferreiro Mählmann ◽  
Frank Chanier ◽  
...  
Keyword(s):  
Organic Matter ◽  
New Zealand ◽  
Clay Minerals ◽  
Clay Mineral ◽  
Structural Characteristics ◽  
Mineral Chemistry ◽  
Fault Zones ◽  
Low Grade ◽  
Pore Fluid Chemistry ◽  
Mineral Crystallinity

ABSTRACTClay minerals and organic matter occur frequently in fault zones. Their structural characteristics and their textural evolution are driven by several formation processes: (1) reaction by metasomatism from circulating fluids; (2)in situevolution by diagenesis; and (3) neoformation due to deformation catalysis. Clay-mineral chemistry and precipitated solid organic matter may be used as indicators of fluid circulation in fault zones and to determine the maximum temperatures in these zones. In the present study, clay-mineral and organic-matter analyses of two major fault zones – the Adams-Tinui and Whakataki faults, Wairarapa, North Island, New Zealand – were investigated. The two faults analysed correspond to the soles of large imbricated thrust sheets formed during the onset of subduction beneath the North Island of New Zealand. The mineralogy of both fault zones is composed mainly of quartz, feldspars, calcite, chabazite and clay minerals such as illite-muscovite, kaolinite, chlorite and mixed-layer minerals such as chlorite-smectite and illite-smectite. The diagenesis and very-low-grade metamorphism of the sedimentary rock is determined by gradual changes of clay mineral ‘crystallinity’ (illite, chlorite, kaolinite), the use of a chlorite geothermometer and the reflectance of organic matter. It is concluded here that: (1) the established thermal grade is diagenesis; (2) tectonic strains affect the clay mineral ‘crystallinity’ in the fault zone; (3) there is a strong correlation between temperature determined by chlorite geothermometry and organic-matter reflectance; and (4) the duration and depth of burial as well as the pore-fluid chemistry are important factors affecting clay-mineral formation.

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