Rb-Sr whole-rock isochron ages, δ18O values and geochemical data for the Sarn igneous complex and the Parwyd Gneisses of the Mona Complex of Llyn, N Wales

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doi: 10.1144/gsjgs.141.4.0701
Authors:Beckinsale, R. D.; Evans, J. A.; Thorpe, R. S.; Gibbons, W.; Harmon, Russell S.
Author Affiliations:Primary:
Br. Geol. Surv., Nottingham, United Kingdom
Other:
Open Univ., Dep. Earth Sci., United Kingdom
Univ. Coll., Dep. Geol., United Kingdom
South. Methodist Univ., United States
Volume Title:Journal of the Geological Society of London
Source:Journal of the Geological Society of London, 141(4), p.701-709. Publisher: Geological Society of London, London, United Kingdom. ISSN: 0016-7649
Publication Date:1984
Note:In English. 33 refs.; illus. incl. 5 tables, geol. sketch map
Summary:The Late Precambrian Mona complex exposed on the Llyn peninsula includes the Sarn complex, a suite of variably deformed plutonic rocks (originally mapped as gneisses) ranging from gabbro to granite, and an inlier of high-grade paragneisses. Rb/Sr data for samples from two geochemical groups yield significantly different ages: the older age of 549 + or - 19 m.y. with initial 87Sr/86Sr 0.7096 + or - 3, is identified as the age of intrusion in late Precambrian-Cambrian times, whereas a younger age of 458 + or - 16 m.y. may approximate to the age of a resetting event. The Parwyd paragneisses yield a Rb/Sr whole-rock isochron age of 542 + or - 17 m.y., concordant with the age of intrusion of the Sarn complex, with an initial 87Sr/86Sr of 0.7069 + or - 4. For the Sarn complex, delta 18O values range + 9.5 to + 12.0per mille (SMOW) and in the Parwyd metasediments + 9.4 to + 13.3per mille. It is concluded that the earliest magmatism and metamorphism in the Mona complex of Llyn occurred in late Precambrian to Cambrian times, broadly contemporaneous with similar events in Anglesey. The high initial 87Sr/86Sr and high delta 18O values may reflect either a crustal source-region or incorporation of a significant proportion of crustal material (such as the Parwyd gneisses) into a mantle-derived magma. [R.A.H.]
Subjects:Absolute age; Age; Alkaline earth metals; Chemical composition; Crystalline rocks; Dates; Diorites; Geochemistry; Gneisses; Igneous rocks; Intrusions; Isotopes; Metals; Metamorphic rocks; Mineral composition; O-18/O-16; Oxygen; Petrology; Plutonic rocks; Plutons; Precambrian; Rb/Sr; Sr-87/Sr-86; Stable isotopes; Strontium; Tonalite; Europe; Great Britain; United Kingdom; Wales; Western Europe; Intrusive rocks; Llyn; Mona Complex; Northern Wales; Parwyd Gneisses; Ratios; Sarn igneous complex
Abstract Numbers:84M/3643
Record ID:1986078413
Copyright Information:GeoRef, Copyright 2019 American Geosciences Institute. Reference includes data from Mineralogical Abstracts, United Kingdom, Twickenham, United Kingdom
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