U-Pb isotopic dating of fluid infiltration and metasomatism during Dalradian regional metamorphism in Connemara, western Ireland

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Authors:Cliff, R. A.; Yardley, B. W. D.; Bussy, F.
Author Affiliations:Primary:
University of Leeds, Department of Earth Sciences, Leeds, United Kingdom
Other:
Université de Lausanne, Switzerland
Volume Title:Journal of Metamorphic Geology
Source:Journal of Metamorphic Geology, 11(2), p.185-191. Publisher: Blackwell, Oxford, United Kingdom. ISSN: 0263-4929
Publication Date:1993
Note:In English. 29 refs.; illus. incl. 2 tables, geol. sketch map
Summary:A metasomatic diopside rock occurs at the top of the dolomitic Connemara marble formation. It contains titanite and K-feldspar in addition to approx 90% diopside (XMg = 0.90-9.97). U-Pb isotopic measurements show that the titanite is both unusually U- rich and isotopically concordant, so a precise U-Pb age of 478 +- 2.5 m.y. can be determined. This is identical within error to a less precise Rb-Sr age of diopside-K-feldspar of 483 +- 6 m.y. Petrological evidence indicates that the assemblage crystallized at approx 620 degrees C, close to or below the closure T of titanite. The age thus provides a precise estimate of the time of metamorphism which is 11 +- 3 m.y. younger than the 490 m.y. age for nearby gabbroic plutons previously used to constrain the peak metamorphic age. This differenceaccords well with geological evidence that the gabbros were emplaced prior to the metamorphic peak. Analysis of minerals with high closure T from assemblages whose crystallization is unambiguously associated with a specific episode of fluid infiltration at the peak of metamorphism provides the basis for a new approach to dating metamorphism. The success of this approach is demonstrated by the results from Connemara. [R.E.S.]
Subjects:Absolute age; Dalradian; Dates; Hydrothermal alteration; Metamorphic rocks; Metamorphism; Metasomatism; Ordovician; Paleozoic; Rb/Sr; Regional metamorphism; U/Pb; Connemara; Europe; Galway Ireland; Ireland; Western Europe; Western Ireland
Coordinates:N530000 N533000 W0090000 W0100000
Abstract Numbers:93M/3786
Record ID:1993004215
Copyright Information:GeoRef, Copyright 2019 American Geosciences Institute. Reference includes data from Mineralogical Abstracts, United Kingdom, Twickenham, United Kingdom
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100 1 |a Cliff, R. A.  |e analytic author  |u University of Leeds, Department of Earth Sciences, Leeds 
245 1 0 |a U-Pb isotopic dating of fluid infiltration and metasomatism during Dalradian regional metamorphism in Connemara, western Ireland 
300 |a p. 185-191 
500 |a In English. 29 refs. 
500 |a Abstract number: 93M/3786 
500 |a Abstractor: R.E.S. 
500 |a Affiliation: University of Leeds, Department of Earth Sciences; Leeds; GBR; United Kingdom 
500 |a Affiliation: Université de Lausanne; ; CHE; Switzerland 
500 |a Key title: Journal of Metamorphic Geology 
500 |a Source note: Journal of Metamorphic Geology, 11(2), p.185-191. Publisher: Blackwell, Oxford, United Kingdom. ISSN: 0263-4929 
500 |a Publication type: journal article 
504 |b 29 refs. 
510 3 |a GeoRef, Copyright 2019 American Geosciences Institute. Reference includes data from Mineralogical Abstracts, United Kingdom, Twickenham, United Kingdom 
520 |a A metasomatic diopside rock occurs at the top of the dolomitic Connemara marble formation. It contains titanite and K-feldspar in addition to approx 90% diopside (X<Mg` = 0.90-9.97). U-Pb isotopic measurements show that the titanite is both unusually U- rich and isotopically concordant, so a precise U-Pb age of 478 +- 2.5 m.y. can be determined. This is identical within error to a less precise Rb-Sr age of diopside-K-feldspar of 483 +- 6 m.y. Petrological evidence indicates that the assemblage crystallized at approx 620 degrees C, close to or below the closure T of titanite. The age thus provides a precise estimate of the time of metamorphism which is 11 +- 3 m.y. younger than the 490 m.y. age for nearby gabbroic plutons previously used to constrain the peak metamorphic age. This differenceaccords well with geological evidence that the gabbros were emplaced prior to the metamorphic peak. Analysis of minerals with high closure T from assemblages whose crystallization is unambiguously associated with a specific episode of fluid infiltration at the peak of metamorphism provides the basis for a new approach to dating metamorphism. The success of this approach is demonstrated by the results from Connemara. 
650 7 |a Absolute age  |2 georeft 
650 7 |a Dalradian  |2 georeft 
650 7 |a Dates  |2 georeft 
650 7 |a Hydrothermal alteration  |2 georeft 
650 7 |a Metamorphic rocks  |2 georeft 
650 7 |a Metamorphism  |2 georeft 
650 7 |a Metasomatism  |2 georeft 
650 7 |a Ordovician  |2 georeft 
650 7 |a Paleozoic  |2 georeft 
650 7 |a Rb/Sr  |2 georeft 
650 7 |a Regional metamorphism  |2 georeft 
650 7 |a U/Pb  |2 georeft 
651 7 |a Connemara  |2 georeft 
651 7 |a Europe  |2 georeft 
651 7 |a Galway Ireland  |2 georeft 
651 7 |a Ireland  |2 georeft 
651 7 |a Western Europe  |2 georeft 
653 |a Western Ireland 
700 1 |a Yardley, B. W. D.,  |e analytic author 
700 1 |a Bussy, F.,  |e analytic author 
773 0 |t Journal of Metamorphic Geology  |d Oxford : Blackwell, Mar. 1993  |x 0263-4929  |n Journal of Metamorphic Geology, 11(2), p.185-191. Publisher: Blackwell, Oxford, United Kingdom. ISSN: 0263-4929 Publication type: journal article  |g Vol. 11, no. 2  |h illus. incl. 2 tables, geol. sketch map