Standardized clay mineral crystallinity data from the very low-grade metamorphic facies rocks of southern New Zealand

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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A new collection of clay mineral ‘Crystallinity’ Index Standards and revised guidelines for the calibration of Kübler and Árkai indices – ERRATUM

Clay Minerals ◽

10.1180/clm.2019.12 ◽

2019 ◽

Vol 54 (1) ◽

pp. 109-109

Author(s):

Laurence N. Warr

Keyword(s):

Clay Mineral ◽

Crystallinity Index ◽

Mineral Crystallinity

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Progressive Regional Metamorphism in Southern New Zealand

Geological Magazine ◽

10.1017/s0016756800089421 ◽

1938 ◽

Vol 75 (4) ◽

pp. 160-174 ◽

Cited By ~ 31

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.

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Low-grade metamorphism of the Brook Street Volcanics, D'Urville Island, New Zealand

New Zealand Journal of Geology and Geophysics ◽

10.1080/00288306.1984.10422525 ◽

1984 ◽

Vol 27 (2) ◽

pp. 167-190 ◽

Cited By ~ 10

Author(s):

Warwick J. Sivell

Keyword(s):

New Zealand ◽

Low Grade ◽

Grade Metamorphism

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K–Ar age determinations on the fine fractions of clay mineral ‘Crystallinity Index Standards’ from the Palaeozoic mudrocks of southwest England

Clay Minerals ◽

10.1180/clm.2019.3 ◽

2019 ◽

Vol 54 (1) ◽

pp. 15-26 ◽

Cited By ~ 1

Author(s):

Anna C. Schomberg ◽

Klaus Wemmer ◽

Laurence N. Warr ◽

Georg H. Grathoff

Keyword(s):

Clay Mineral ◽

Crystallinity Index ◽

White Mica ◽

Early Jurassic ◽

Late Triassic ◽

X Ray Diffraction ◽

Middle Ordovician ◽

Late Cambrian ◽

Isotopic Analyses ◽

Mineral Crystallinity

AbstractClay mineral ‘Crystallinity Index Standards’ (CIS) composed of Palaeozoic mudrocks from southwest England were investigated systematically in five sub-fractions per sample for the first time. X-ray diffraction was used to determine mineral assemblages, calibrated 001 illite full-width-at-half-maximum (FWHM) values and illite polytype compositions, in addition to K–Ar isotopic analyses of all fine fractions. The FWHM results of the <2 µm fraction are consistent with previous studies and reflect the range of diagenetic to epizonal grades covered by the sample set SW1 to SW7 (~0.61–0.26°2θ). Diagenetic and lower anchizone samples also show significant broadening of 001 illite reflections in the finer fractions and contain mixtures of authigenic 1M + 1Md illite and detrital 2M1 white mica polytypes suitable for illite age analysis. The estimated end-member ages of the Bude (SW1-1992) and younger Crackington (SW3-2000) mudstones yield detrital ages of Late Cambrian to Middle Ordovician (493–457 Ma) and a broad range of 1M + 1Md illite ages between Middle Permian and Early Jurassic (271–190 Ma). The detrital age of the stratigraphically older Crackington Formation mudrock (SW2-1992) is Late Devonian (384–364 Ma) with 1M + 1Md illite ages between Late Triassic and Early Jurassic (219–176 Ma). The origin of Mesozoic 1M + 1Md illite ages may represent neocrystallized illite associated with Mesozoic hydrothermal events or similar events that thermally reset older authigenic illite with partial loss of radiogenic argon and no renewed crystal growth. In contrast, upper anchizonal and epizonal Devonian slates (SW3-2012, SW4-1992, SW6-1992 and SW7-2012) contain only the 2M1 polytype, with K–Ar ages younger than the stratigraphic age. The three finest fractions of SW4-1992 yield consistent Late Carboniferous ages (331–304 ± 7 Ma) that are considered to date the neocrystallized 2M1 mica. Most fractions of epizonal slate (SW6-1992, SW7-2012) yield Early Permian ages (293.6–273 Ma) corresponding to published cooling ages of the Tintagel High-Strain Zone and the intrusion of the Bodmin granite (291.4 ± 0.8 Ma). These first K–Ar age constraints for the fine fractions of the CIS should provide useful reference values for testing analytical procedures of illite age analysis.

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Coexistence of pyrophyllite, I-S, R1 and NH4+-rich illite in Silurian black shales (Sierra de Albarracín, NE Spain): metamorphic vs. hydrothermal origin

Clay Minerals ◽

10.1180/claymin.2010.045.3.383 ◽

2010 ◽

Vol 45 (3) ◽

pp. 383-392 ◽

Cited By ~ 7

Author(s):

B. Bauluz ◽

I. Subías

Keyword(s):

Organic Matter ◽

Clay Mineral ◽

Hydrothermal Alteration ◽

Black Shales ◽

Low Grade ◽

Lower Silurian ◽

Igneous Activity ◽

Upper Silurian ◽

Ne Spain ◽

Regional Tectonics

AbstractA set of Silurian black shales from Sierra de Albarracín (NE Spain) corresponding to two different sections was studied to determine the relative influence of diagenesis, igneous activity, and regional tectonics on the clay-mineral genesis. The coexistence of pyrophyllite, I-S interstratifications (R1), ammonium-rich illite, potassium illite, kaolin, and chlorite is not the result of prograde evolution during diagenesis – very low-grade metamorphism. Three different stages may be inferred: (1) sedimentation of black shales (Aeronian, Lower Silurian, to basal Ludfordian, Upper Silurian) and the subsequent diagenetic process producing the coexistence of quartz, illite, kaolinite, organic matter, etc.; (2) intrusion of andesitic sills producing hydrothermal alteration and crystallization of pyrophyllite, ammonium-rich illites, smectite, I-S R1 phases and jarosite; and (3) and folding of shales and sills and development of penetrative schistosity during the late Variscan leading to illite and paragonite recrystallization reaching the anchizone grade.

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The stilpnomelane group of minerals

Mineralogical Magazine and Journal of the Mineralogical Society ◽

10.1180/minmag.1938.025.163.02 ◽

1938 ◽

Vol 25 (163) ◽

pp. 172-206 ◽

Cited By ~ 16

Author(s):

C. Osborne Hutton

Keyword(s):

New Zealand ◽

Analytical Data ◽

Low Grade ◽

Preliminary Work ◽

Mineral Assemblages

Work which has been in progress during the last few years on the low-grade regionally metamorphosed rocks of western Otago, New Zealand, has shown that members of the stilpnomelane group, hydrous iron- (or manganese-) rich silicates, are exceedingly widespread in rocks with the most varied mineral assemblages. In many cases the mineral in question is in sufficient quantity to rank as an essential constituent and may even be the most abundant one present. Hence in view of the importance of this mineral in the rocks of this region, it was considered that exact analytical data were necessary, and preliminary work in this connexion was carried out by Dr. F. J. Turner and the writer in 1934, but at that time, owing to the difficulties of separation, the pure minerals could not be obtained for analysis.

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Clay diagenesis and low-grade metamorphism of Tithonian and Berriasian sediments in the Cameros Basin (Spain)

Clay Minerals ◽

10.1180/000985501750539427 ◽

2001 ◽

Vol 36 (3) ◽

pp. 325-333 ◽

Cited By ~ 12

Author(s):

J. F. Barrenechea ◽

M. Rodas ◽

M. Frey ◽

J. Alonso-Azcárate ◽

J. R. Mas

Keyword(s):

Clay Mineral ◽

Mixed Layer ◽

Sedimentary Facies ◽

Metamorphic Grade ◽

Metamorphic Event ◽

Low Grade ◽

Grade Metamorphism ◽

Mineral Assemblages ◽

Cameros Basin ◽

Southern Border

AbstractThe clay mineral assemblages of the Tithonian and Berriasian sediments (Tera and Oncala Groups) in the eastern part of the Cameros basin are investigated at seven localities. The lowest-grade assemblage, located on the southern border of the basin, contains calcite + quartz + hematite + kaolinite + mixed-layer illite-smectite (R = 1, 65 85% illite layers) + discrete illite (IC = 0.5 0.65Δ°2θ). Systematic increases in the illite and chlorite crystallinities suggest increasing metamorphic grade from the northwest part of the basin to the southeast. This trend does not follow the pattern previously described for the overlying late Berriasian–early Aptian sediments (Urbión and Enciso Groups), which exhibit a higher metamorphic grade. This may result from local variations in sedimentary facies, as well as the circulation of hot migratory fluids. Tertiary compression occurring long after the main metamorphic event is considered to be responsible for the enhanced illite and chlorite crystallinities measured in the SE extreme of the basin.

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