Origin of montmorillonite in the early Jurassic shales of NW Scotland

SummaryClay mineral analysis of shales in the early Liassic Lower Broadford Beds of the Hebrides reveals the presence of abundant quantities of montmorillonite whereas the Upper Broadford Beds contain illite, kaolinite and subordinate mixed layer minerals together with chlorite. Montmorillonite enrichment of the Lower Broadford Beds as a consequence of recent weathering contamination by doleritic sills can effectively be ruled out. It is argued that the montmorillonite may have been derived from the weathering of basic igneous rocks exposed in the sediment source area during Late Triassic times. By Upper Broadford Beds times, the igneous source was either eroded away or transgressed by the early Liassic sea.

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Provenance of Jurassic sedimentary rocks of south-central Quesnellia, British Columbia: implications for paleogeography

Canadian Journal of Earth Sciences ◽

10.1139/e03-073 ◽

2004 ◽

Vol 41 (1) ◽

pp. 103-125 ◽

Cited By ~ 12

Author(s):

Nathan T Petersen ◽

Paul L Smith ◽

James K Mortensen ◽

Robert A Creaser ◽

Howard W Tipper

Keyword(s):

Detrital Zircon ◽

Middle Jurassic ◽

Sedimentary Rocks ◽

Source Area ◽

Lower Jurassic ◽

Early Jurassic ◽

Late Triassic ◽

Nd Isotopes ◽

South Central ◽

History Of

Jurassic sedimentary rocks of southern to central Quesnellia record the history of the Quesnellian magmatic arc and reflect increasing continental influence throughout the Jurassic history of the terrane. Standard petrographic point counts, geochemistry, SmNd isotopes and detrital zircon geochronology, were employed to study provenance of rocks obtained from three areas of the terrane. Lower Jurassic sedimentary rocks, classified by inferred proximity to their source areas as proximal or proximal basin are derived from an arc source area. Sandstones of this age are immature. The rocks are geochemically and isotopically primitive. Detrital zircon populations, based on a limited number of analyses, have homogeneous Late Triassic or Early Jurassic ages, reflecting local derivation from Quesnellian arc sources. Middle Jurassic proximal and proximal basin sedimentary rocks show a trend toward more evolved mature sediments and evolved geochemical characteristics. The sandstones show a change to more mature grain components when compared with Lower Jurassic sedimentary rocks. There is a decrease in εNdT values of the sedimentary rocks and Proterozoic detrital zircon grains are present. This change is probably due to a combination of two factors: (1) pre-Middle Jurassic erosion of the Late Triassic Early Jurassic arc of Quesnellia, making it a less dominant source, and (2) the increase in importance of the eastern parts of Quesnellia and the pericratonic terranes, such as Kootenay Terrane, both with characteristically more evolved isotopic values. Basin shale environments throughout the Jurassic show continental influence that is reflected in the evolved geochemistry and SmNd isotopes of the sedimentary rocks. The data suggest southern Quesnellia received material from the North American continent throughout the Jurassic but that this continental influence was diluted by proximal arc sources in the rocks of proximal derivation. The presence of continent-derived material in the distal sedimentary rocks of this study suggests that southern Quesnellia is comparable to known pericratonic terranes.

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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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Triassic olivine-normative diabase from Northumberland Strait, eastern Canada: implications for continental rifting

Canadian Journal of Earth Sciences ◽

10.1139/e86-102 ◽

1986 ◽

Vol 23 (7) ◽

pp. 1013-1021 ◽

Cited By ~ 8

Author(s):

Georgia Pe-Piper ◽

Lubomir F. Jansa

Keyword(s):

Early Jurassic ◽

Igneous Rocks ◽

Late Triassic ◽

Geochemical Evolution ◽

High Alumina ◽

Continental Rifting ◽

Eastern Canada ◽

Mafic Rocks ◽

Eu Anomaly ◽

Igneous Activity

Two intervals of mafic igneous rocks were encountered within a Silurian to Carboniferous sequence in an exploratory offshore well located in the Gulf of St. Lawrence, eastern Canada. Geochemical and radiometric analyses show that the lower mafic rocks are Early Silurian continental tholeiite lavas, with their radiogenic clock thermally reset during the Late Devonian. The upper igneous interval consists of several dikes of high-alumina diabase characterized by flat, relatively unenriched REE spectra and a positive Eu anomaly. This diabase resembles olivine tholeiites. Two K/Ar dates suggest a Late Triassic age for these intrusions. The dike composition differs from that of known Late Triassic and Early Jurassic continental tholeiites nearby in Nova Scotia, which are associated with a late stage of continental rifting. The Triassic – Early Jurassic igneous activity shows a pattern of geochemical evolution that we relate to mantle upwelling.

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Reappraisal of the Mesozoic tectonic transition from the Paleo-Tethyan to Paleo-Pacific domains in South China

Geological Society of America Bulletin ◽

10.1130/b35755.1 ◽

2021 ◽

Author(s):

Chengshi Gan ◽

Yuzhi Zhang ◽

Yuejun Wang ◽

Xin Qian ◽

Yang Wang

Keyword(s):

South China ◽

Middle Jurassic ◽

Late Jurassic ◽

Sedimentary Rocks ◽

Early Jurassic ◽

Igneous Rocks ◽

Late Triassic ◽

Geochemical Data ◽

South China Block ◽

Tectonic Transition

The southeastern (SE) South China Block was mainly influenced by the Paleo-Tethyan and Paleo-Pacific dynamic domains during the Mesozoic. The initial timing of the tectonic transition between these two domains in the SE South China Block still remains debated. The transition would affect the nature of the lithosphere and material provenance of sediments, and, therefore, igneous and sedimentary rocks in the area could record such dynamic processes. In this study, published geochronological and geochemical data of the Triassic and Jurassic igneous rocks and detrital zircon data of contemporaneous sedimentary rocks in the SE South China Block were compiled, aiming to provide constraints on the tectonic transition via tracing the spatial-temporal variations in the nature of the lithosphere and sedimentary provenance signals. The compiled results suggest that the magmatic intensity and volume decreased significantly from the Late Triassic to Early−Middle Jurassic, with an obvious magmatic quiescence between them, and increased from the Early−Middle Jurassic to Late Jurassic. The εNd(t) and zircon εHf(t) values of mafic rocks, granitoids, and shoshonitic rocks remarkably increased from the Late Triassic to Early−Middle Jurassic, indicative of variations in the lithospheric mantle and continental crust. Such variations suggest that the initial tectonic transition occurred at the earliest Early Jurassic. Based on the southward paleocurrents from Early Jurassic sandstone, E-W−trending extension of Early−Middle Jurassic mafic and shoshonitic rocks, and similar sedimentary provenances of Late Triassic and Early−Middle Jurassic sedimentary rocks, these features imply that the SE South China Block was not immediately influenced by the Paleo-Pacific domain during the Early−Middle Jurassic. However, from the Early−Middle Jurassic to Late Jurassic and Early Cretaceous, the spatial distribution, geochemical signatures, magmatic intensity, and magmatic volume of igneous rocks and provenance of sedimentary rocks exhibit obvious variations, and the regional fold hinge direction changed from E-W−trending to NE-trending, suggesting significant effects from Paleo-Pacific subduction on the SE South China Block. Thus, the Mesozoic tectonic transition from the Paleo-Tethyan to the Paleo-Pacific dynamic domain in the SE South China Block likely occurred during the Early−Middle Jurassic.

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