Ordovician bimodal volcanism in SW Wales: geochemical evidence for petrogenesis of the silicic rocks

Abstract

Rhyolitic lavas, pyroclastic, and high-level intrusive rocks form an important component of a bimodal tholeiitic basalt-rhyolite association of early Ordovician (Llanvirn) age exposed in North Pembrokeshire, on Ramsey Island, in the Fishguard area, and on Mynydd Preseli. In the same region, a 400 m thick tholeiitic layered differentiated sill, the St David's Head intrusion, considered to be co-genetic with this association, comprises a series of units ranging from basic (olivine + orthopyroxene-bearing gabbros) through intermediate (granophyric gabbro) to acidic (aplite) compositions. Consideration of the major and trace element concentrations in these various intrusive units suggests that they are related largely through a crystal fractionation process. The principal fractionating phases were clinopyroxene and plagioclase feldspar, with olivine and orthopyroxene in the more mafic compositions, substantial Fe-Ti oxide and minor apatite in the intermediate compositions, and minor apatite and titanite in the most evolved compositions. A comparison between the composition of a typical aplite from the St David's Head intrusion and compositions of the various silicic rocks from the bimodal suites in North Pembrokeshire suggests these latter rocks have also been derived from a more basic tholeiitic magma via crystal fractionation. Potential parental magmas are the various basaltic lavas and related doleritic intrusions of the Fishguard Volcanic Complex. However, certain silicic lavas and intrusions from the Porth Maen Melyn area appear not to have been derived from a tholeiitic parental magma, but rather have a calc-alkaline type chemistry, similar to older lavas of the Trefgarn Volcanic Group exposed a short distance to the south of Fishguard. These findings support the contention that large volume silicic units in bimodal associations can be derived by crystal fractionation, despite the obvious geochemical discontinuity, and the apparently anomalous volume relationships commonly present.

Anorogenic Acid Volcanic rocks in the Kundal area of the Malani Igneous Suite, Northwestern India: geochemical and petrogenetic studies

“…Fractional crystallization of a basaltic magma, if considered as the mechanism and source for generation of felsic derivatives, will require a huge volume of basic rocks nearby, since the felsic differentiate would only account for a small fraction of the original basic reservoir (Hyndman, 1985). Bevins et al (1991) also states that, irrespective of tectonic setting, the generation of felsic magma in such associations is debatable since the volume of silicic magma generally exceeds that expected to be produced by differentiation of a basaltic source. However, the aerial distribution of basic and acid rocks in the Kundal area yields a ratio of approximately 1:85.…”

Provenancing the rhyolitic and dacitic components of the Stonehenge landscape bluestone lithology: new petrographical and geochemical evidence

“…Analyses of originally glassy dacitic lava samples from a 40 m thick flow exposed on the western Pen Caer coast at Porth Maenmelyn have been presented previously by Bevins (1982) and Bevins et al (1991). These lavas are characterised by the presence of a welldeveloped perlitic texture (Fig.…”

“…These Stonehenge analyses are then compared with analyses of Ordovician rhyolitic and dacitic rocks from the north Pembrokeshire area, including both new and previously published analyses. These include: five new analyses of rhyolite samples from the Pont Saeson area; six analyses (five not previously published) of dacites and rhyolites from the Sealyham Volcanic Formation from the southern flanks of Mynydd Preseli; twelve dacitic and rhyolitic rocks from the area between Pen Caer and Newport described by Bevins (1979) and Bevins et al (1989Bevins et al ( , 1991, two dacites from Foel Drygarn, and a further six analyses of rhyolitic rocks from Carn Alw, the latter two outcrops lying to the south-east of Pont Saeson. The Carn Alw and one of the Foel Drygarn samples were included in the study by Thorpe et al (1991), although only one analysis from the Carn Alw set was tabled by them.…”

“…Analyses of three flow-banded and autobrecciated rhyolitic lava samples, locally showing the development of a perlitic texture, from the northern Pen Caer area have been presented in Bevins (1982) and Bevins et al (1991) and are plotted in Fig. 2aei.…”

“…The study draws on the somewhat limited geological information available for the north Pembrokeshire area, including the findings of Parkinson (1895), Part (1922), Evans (1945), Lowman (1977), Bevins (1979), Lowman and Bloxam (1981) and Bevins et al (1989Bevins et al ( , 1991Bevins et al ( , 1992, studies which contributed to the recent geological sheet for the area (British Geological Survey, 2010). Understanding the complexity of the Ordovician volcanic rocks of north Pembrokeshire is hampered, however, by a number of factors, including: poor exposure (there is much glacial drift obscuring the bedrock geology); marked lateral facies variations of the primary volcanic products (making it difficult to trace individual horizons in the field); complex/ variable alteration, destroying primary textures, causing extensive recrystallization and element mobility (the rocks have variably suffered devitrification, low-grade metamorphism and local hydrothermal alteration); and faulting (again making it difficult to trace individual horizons laterally).…”