Melanges and Glaucophanic Schists of the Mona Complex (Precambrian), Southeast Anglesey, North Wales
Bieler, David Bruce
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https://hdl.handle.net/2142/71127
Description
Title
Melanges and Glaucophanic Schists of the Mona Complex (Precambrian), Southeast Anglesey, North Wales
Author(s)
Bieler, David Bruce
Issue Date
1983
Department of Study
Geology
Discipline
Geology
Degree Granting Institution
University of Illinois at Urbana-Champaign
Degree Name
Ph.D.
Degree Level
Dissertation
Keyword(s)
Geology
Abstract
The Mona Complex of Anglesey, North Wales can be modeled as a fragmented arc and upper subduction complex assemblage. In southeastern Anglesey, the two rock suites are juxtaposed, and possible relationships between these two suites can be tested.
The arc suite comprises a series of weakly metamorphosed and deformed volcanic and sedimentary rocks. Sandstones and siltstones deposited by gravity driven processes are interbedded with fine grained sediments. These sedimentary units are sometimes deformed in slump folds or olistostromal slides. Volcanic rocks are dominantly pillowed basalts and associated breccias, but some shallow sills, feeder dikes, and massive flows occur. Volcanism and sedimentation were contemporaneous phenomena. Subsedimentary instrusions provided local domes off which the sedimentary material slid. The rocks were later raised to a vertical attitude and a crude northwesterly dipping cleavage developed in the finer grained sedimentary rocks. Later tectonism reactivated earlier anisotropies and produced a brittle overprint.
The metamorphic belt comprises quartzo-feldspathic and glaucophanic amphibole schists. The rocks are complexly deformed containing a single penetrative foliation that has itself been folded several times. This foliation records only the last recrystallization; earlier deformations are obscured, but the record of the earlier metamorphic events is preserved in a few amphibole megacrysts.
The metabasites all have a characteristic greenschist facies assemblage. Amphiboles show increasing magnesium contents and decreasing octahedral aluminum contents in higher temperature assemblages. Higher temperature assemblages also show continuous variation in sodium-calcium solution in octahedral site occupancies while lower temperature assemblages show discontinuities in both those parameters; the solvus between sodic and calcic amphiboles must close at higher temperatures. Individual amphibole grains are zoned from calcic cores to sodic rims; sodic amphiboles developed as replacements of calcic precursors.
Amphibole distribution and zoning patterns suggest metamorphism in a subduction complex above a northwesterly dipping subduction zone. These rocks were among the earliest accreted material and not exposed to the thermal conditions of the mature subduction complex.
All the data from the Newborough area are consistent with an arc and subduction complex model. Later events, including possible strike slip movement, telescoped the structure and made palinspastic reconstruction impossible.
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