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: | M J Bickle S M Wickham H J Chapman H P Taylor Jr |
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| Institution: | (1) Department of Earth Sciences, University of Cambridge, Downing Street, CB2 3EQ Cambridge, UK;(2) Division of Geological and Planetary Sciences, California Institute of Technology, 91125 Pasadena, CA, USA;(3) Present address: Department of the Geophysical Sciences, University of Chicago, 5734 South Ellis Avenue, 60637 Chicago, IL, USA |
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| Abstract: | Nd, Sr, and O isotope analyses have been made on metamorphic and igneous rocks and minerals from a 310–340 Ma Hercynian-age metamorphic terrane in the Pyrenees, France. Lower Paleozoic shales and phyllites have 87Sr/86Sr values of 0.707–0.717 at 310 Ma, but model values at 310 Ma of 0.709–0.736 (based on assumed depositional age of 450 Ma and an initial 87Sr/86Sr=0.707). On a regional scale, 87Sr/86Sr was homogenized to about 0.713 to 0.717 in the higher-grade pelitic schists during metamorphism. Much of this 87Sr/86Sr exchange occurred at very low grades (below the biotite isograd), but significant changes also accompanied the δ 18O lowering of the phyllites (+13 to +16) during their transformation to andalusite- and sillimanite-grade schists (δ 18O=+11 to +12); all of these effects are attributed to pervasive interactions with hydrothermal fluids (Wickham and Taylor 1985). The data also show that a syn-metamorphic plutonic complex, dominated by a biotite granite body, was derived by mixing of a relatively mafic magmatic end-member (87Sr/86Sr～ 0.7025–0.7050 and δ 18O～ +7.5 to +8.0) with two metasedimentary sources, both having 87Sr/86Sr～0.715 and δ 18O～ +10.0 to +12.0, but with one being more homogeneous than the other. The more homogeneous component and the (mantle-derived?) magmatic end-member dominate at low structural levels within the complex. The less homogeneous end-member that dominates at high levels is clearly derived from the local Paleozoic pelitic schists. A Rb-Sr age of 330±20 Ma was obtained on hornblende from a deep level within the complex, which fixes this age for the regional metamorphism, as well. Although a post-metamorphic granodiorite magma body at Trois Seigneurs also displays heterogeneities in δ 18O and 87Sr/86Sr (and thus does not give a clear-cut Rb-Sr isochron), the data are consistent with an emplacement age between 260 and 310 Ma, similar to ages of other late granodiorites in the Pyrenees. 143Nd/ 144Nd is very uniform within the Hercynian crust, both at Trois Seigneurs (? Nd=?3 to ?7) and elsewhere in the Pyrenees; almost all igneous lithologies have depleted-mantle, mid-Proterozoic model ages, consistent with efficient recycling of crustal material following original crustal accretion in this area at about 1600 Ma or earlier. Rb-Sr mineral ages exhibit a complex cooling history reflecting late Hercynian and Mesozoic thermal events. Our results show that profound homogenization of the 87Sr/86Sr and 18O/16O ratios of large volumes of the crust can occur during regional metamorphism and crustal anatexis, particularly in regions undergoing extensional tectonics. Such processes can significantly modify the isotopic compositions of the protoliths of granitic magmas; this may explain why many peraluminous Hercynian granitoids of Western Europe have anomalously low (87Sr/86Sr) initial values compared to their probable sedimentary parent rocks. |
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