Fluid fluxes during low-temperature alteration: implications of multi-style alteration assemblages from the Welsh Borderland, UK
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Abstract:
Localized (domainal) low-temperature alteration may indicate variations in the chemistry of alteration fluids and/or discrete fluid flow paths during metamorphism. Occurrences of epidote- and pumpellyite-dominated domains are often used as evidence for large fluxes of Ca-rich fluids. However, comparative studies of two domainal alteration styles from basaltic to andesitic lavas and volcaniclastic rocks at Builth Wells, Wales, UK suggest that such interpretations cannot be applied universally. Here, only one set of domains can be attributed to large fluxes of Ca-rich fluids. In contrast, the second set of domains formed where the host rock supplied the necessary Ca, and fluid/rock ratios were relatively low. These domains are hosted by rocks which show a Caledonian regional metamorphic imprint, characterized by the alteration assemblage albite+chlorite+titanite±prehnite±pumpellyite±calcite±muscovite±quartz (considered to have formed at c. 200°C and ≤2.5 kbar). Type 1 domains lie along fluid channelways, such as veins. Pumpellyite is the dominant secondary mineral, but the alteration assemblage is variable with pumpellyite±prehnite±calcite±titanite±chlorite (±quartz). The domains formed at temperatures of 130–230°C during Ordovician hydrothermal activity, before the peak of regional metamorphism. In contrast, Type 2 domains are not associated with obvious fluid channelways. A central prehnite-dominated zone typically has a rim with the assemblage pumpellyite+calcite±prehnite±chlorite±titanite±K-rich phyllosilicate. These domains probably formed from pelitic xenoliths which contained a volcaniclastic component at temperatures of 130–230°C and at relatively low fluid/rock ratios. However, the timing of domain formation is uncertain. Pyrobitumen, in veins and disseminated throughout the rock, may have influenced the formation of both types of domain by stabilizing pumpellyite in preference to epidote and causing metamorphic calcite formation through the release of CO2 to the alteration fluids. © 1996 John Wiley & Sons, Ltd.Keywords:
Titanite
Muscovite
Sericite
Phengite
Muscovite
Phengite
Pelite
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Titanite
Pegmatite
Microcline
Massif
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The present study aims at delimiting the titanite occurrences in the volcaniclastic metasediments and accessing the mineral chemistry of the titanite at Gabal El-Degheimi area. The study area is a part of the Arabian Nubian shield and includes serpentinites, arc volcaniclastic metasediments, older granites, Dokhan volcanics, Hammamat group post orogenic volcanics and younger granites. The processing of Landsat 8 images, field geological verification and geochemical analyses highlighted the distribution and occurrences of the arc volcaniclastic metasediments and their included alteration zones all over the study area. These rocks are composed of titanite, chlorite, calcite, epidote, sericite, muscovite and biotite in trace amounts, about 3 wt% of TiO2 was recorded. Titanite mineral is the main carrier of titanium, calcium and silicon in the volcaniclastic metasediments.
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Sericite
Muscovite
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Phengite
Muscovite
Staurolite
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Phengite
Muscovite
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