A stable isotope study of retrograde alteration in SW Connemara, Ireland

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Authors:Jenkin, G. R. T.; Fallick, A. E.; Leake, B. E.
Author Affiliations:Primary:
Scottish Universities Research and Reactor Centre, Glasgow, United Kingdom
University of Glasgow, United Kingdom
Volume Title:Contributions to Mineralogy and Petrology
Source:Contributions to Mineralogy and Petrology, 110(2-3), p.269-288. Publisher: Springer International, Heidelberg-New York, International. ISSN: 0010-7999
Publication Date:1992
Note:In English. 72 refs.; illus. incl. 2 tables, geol. sketch map
Summary:Dalradian metamorphic rocks, Lower Ordovician meta-igneous rocks (MGS) and Caledonian granites of the Connemara complex in SW Connemara all show intense retrograde alteration. Alteration primarily involves sericitization and saussuritization of plagioclase, the alteration of biotite and hornblende to chlorite and the formation of secondary epidote. The alteration is associated with sealed microcracks in all rocks and planes of secondary fluid inclusions in quartz where it occurs, and was the result of a phase of fluid influx into these rocks. In hand specimen K-feldspar becomes progressively reddened with increasing alteration. Mineralogical alteration in the MGS and Caledonian granites took place at T approx 275 +- 15 degrees C and in the MGS Pfluid is estimated to be <= 1.5 kbar during alteration. The deltaD values of alteration phases are -18 to -29 per mille (fluid inclusions), -47 to -61 per mille (chlorites) and -11 to 31 per mille (epidotes). Chlorite delta18O-values are +0.2 to +4.3 per mille while delta18O-values for quartz-K- feldspar pairs show both positively sloped (MGS) and highly unusual negatively sloped (Caledonian granites) arrays, diverging from the normal magmatic field on a delta-delta plot. The stable isotope data show that the fluid that caused retrogression continued to be present in most rocks until T fell to 200-140 degrees C. The retrograde fluid had deltaD approx -20 to -30 per mille in all lithologies, but the fluid delta18O varied spatially and temporally within the range -4 +7 per mille . The fO2 of the fluid that deposited the epidotes in the MGS varied, with the most depleted fluid being the most oxidizing. The deltaD values, together with low (< 0 per mille) delta18O-values for the retrograde fluid in some lithologies indicate that this fluid was of meteoric origin. [Authors' abstract] [P.Br.]
Subjects:Caledonian Orogeny; Convection; D/H; Dalradian; Fluid inclusions; Geologic thermometry; Hydrogen; Hydrothermal alteration; Hydrothermal conditions; Inclusions; Isotope ratios; Isotopes; Metagabbro; Metaigneous rocks; Metamorphic rocks; Metamorphism; Metasomatism; O-18/O-16; Oxygen; P-T conditions; Paleozoic; Retrograde metamorphism; Sericitization; Stable isotopes; Connemara; Europe; Galway Ireland; Ireland; Western Europe; Saussuritization
Coordinates:N531500 N534000 W0090000 W0102000
Abstract Numbers:92M/4462
Record ID:1993027394
Copyright Information:GeoRef, Copyright 2019 American Geosciences Institute. Reference includes data from Mineralogical Abstracts, United Kingdom, Twickenham, United Kingdom
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