A strontium, neodymium and oxygen isotope study of hydrothermal metamorphism and crustal anatexis in the Trois Seigneurs Massif, Pyrenees, France

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Authors:Bickle, M. J.; Wickham, S. M.; Chapman, H. J.; Taylor, H. P., Jr.
Author Affiliations:Primary:
Univ. Cambridge, Dep. Earth Sci., GB-Cambridge CB2 3EQ, United Kingdom
Volume Title:Contributions to Mineralogy and Petrology
Source:Contributions to Mineralogy and Petrology, 100(4), p.399-417. Publisher: Springer International, Heidelberg-New York, International. ISSN: 0010-7999
Publication Date:1988
Note:In English. 48 refs.; illus. incl. 7 tables, geol. sketch maps
Summary:Nd, Sr and O isotopic analyses were made on metamorphic and igneous rocks and minerals from a 310-340 m.y. Hercynian metamorphic terrain in the Pyrenees, France. Lower Palaeozoic shales and phyllites have 87Sr/86Sr values of 0.707- 0.717 at 310 m.y., but model values at 310 m.y. of 0.709-0.736 (based on assumed depositional age of 450 m.y. and an initial 87Sr/86Sr = 0.707). On a regional scale, 87Sr/86Sr was homogenized to approx 0.713 to 0.717 in the higher- grade pelitic schists. Much of this Sr exchange occurred at very low grades (below the biotite isograd), but significant changes also accompaned the delta18O lowering of the phyllites during their transformation to andalusite- and sillimanite-grade schists. All of these effects are attributed to pervasive interactions with hydrothermal fluids. The data also show that a syn-metamorphic plutonic complex, dominated by a biotite granite body, was derived by mixing a relatively mafic end-member with two metasedimentary sources, one being more homogeneous than the other. The more homogeneous component and the (mantle-derived?) magmatic end-member dominate at low structural levels, while the less homogeneous end-member that dominates at high levels is clearly derived from local Palaeozoic pelitic schists. An Rb-Sr age of 330 +- 20 m.y. was obtained on hornblende from a deep level within the complex, which fixes this age for the regional metamorphism as well. The results show that profound homogenization of Sr and O isotopic ratios of large volumes of crust can occur during regional metamorphism and crustal anatexis, particularly in regions undergoing extensional tectonics. Such processes may explain why many peraluminous Hercynian granitoids of W Europe have anomalously low initial Sr values compared to their probable sedimentary parent rocks. [P.Br.]
Subjects:Alkaline earth metals; Anatexis; Geochemistry; Hydrothermal conditions; Igneous rocks; Isotopes; Metals; Metamorphic rocks; Metamorphism; Nd-144/Nd-143; Neodymium; O-18/O-16; Oxygen; Petrology; Plutonic rocks; Rare earths; Rb/Sr; Sr-87/Sr-86; Stable isotopes; Strontium; Europe; France; French Pyrenees; Pyrenees; Western Europe; Evolution; Ratios; Trois Seigneurs Massif
Abstract Numbers:89M/3328
Record ID:1989067651
Copyright Information:GeoRef, Copyright 2019 American Geosciences Institute. Reference includes data from Mineralogical Abstracts, United Kingdom, Twickenham, United Kingdom, Reference includes data from Geoline, Bundesanstalt fur Geowissenschaften und Rohstoffe
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