The Role of Fluids in Crustal Processes (1990) / Chapter Skim
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6. Hydrothermal Systems Associated with Regional Metamorphism and Crustal Anatexis: Example from the Pyrenees, France
6. Hydrothermal Systems Associated with Regional Metamorphism and Crustal Anatexis: Example from the Pyrenees, France
Pages 96-112
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From page 96... ...
(Taylor, 1974, 1977~. Consequently, if aqueous fluids derived from the Earth's surface interact with igneous or metamorphic rocks at elevated 96 Examples from the temperatures where the equilibrium isotopic fractionations are small, this results in a change in the rock oxygen (and hydrogen)
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From page 97... ...
deformation and recrystallization of the Hercynian metamorphic and granitic rocks were relatively minor. The Hercynian basement ranges from virtually unmetamorphosed, fossiliferous Devonian and Carboniferous rocks, some of which were actually being deposited while metamorphism was occurring at depth, through Lower Paleozoic mica schists, marbles, and migmatites, finally to amphibolite and granulite-facies "basal gneisses" (Zwart, 1979~.
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From page 98... ...
Subdivision of the Hercynian Crust into Three Structural Levels Taken together the wide range of structural levels exposed in the different tectonic blocks of the Hercynian basement of the Pyrenees can be interpreted in terms of a composite section through the upper 25 km of the continental crust. Because the oxygen isotope systematics are very distinctive at different structural levels within this section, it is convenient to divide it into three tectonostratigraphic zones: (1)
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From page 99... ...
OXYGEN ISOTOPE SYSTEMATICS Zone 1 The Paleozoic rocks from Zone 1 comprise fossiliferous shales and carbonates that have been deformed but only very weakly metamorphosed. Rocks are typically fine "rained and may contain chlorite or fine-grained white mica but no higher-grade metamorphic minerals.
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From page 100... ...
This indicates an extremely high degree of oxygen isotopic equilibration between this layer and the surrounding metasediments, in contrast to the situation prior to metamorphism when the carbonate layers would have had much higher GINO values than the surrounding rocks (as is still the case in Zone 1~. This homogeneity of carbonate and petite oxygen isotopic compositions within the Trois Seigneurs Zone 2 rocks is typical of the oxygen isotope systematics at this structural level throughout the Pyrenees.
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From page 101... ...
In order to investigate these systematics further, we made detailed isotopic profiles across some of the individual carbonate units in the Zone 3 basal gneisses. One such profile from the Agly Massif is shown in Figure 6.8.
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From page 102... ...
6~80 values in Zone 1, the ~80depleted and homogeneous isotopic compositions of both the carbonates and the silicate lithologies in Zone 2, and the wide range of 6~80 values in all rock types in Zone 3. calcites in these carbonate layers have essentially retained their original sedimentary 6~80 values, indicating a lack of exchange with the adjacent biotite gneisses (which have 6~80 values of about +121.
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From page 103... ...
Geological evidence in the Pyrenees favors the latter reservoir because suitable low-'8O rocks are notably absent from the wide range of structural levels exposed. Basal gneisses in Zone 3 contain small amounts of mafic rock with 6'8O values of +6 to +8, but are domi 103 FIGURE 6.8 Oxygen isotopic profile through two thin carbonate layers within granulite-facies quartzo-feldspathic gneiss in the Agly Massif (see Figure 2 of Wickham and Taylor, 1987, for locality)
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From page 104... ...
In these calculations the initial and final 6180 values of the metasediments are taken to be +15.0 and +11.5, respectively, so the Zone 2 rocks are assumed to have been shifted to lower values by 3.5 per milt oxygen isotope systematics are different in the different localities (see Figures 6.6 through 6.8~. At Lapege and Agly there is considerable isotopic heterogeneity between metacarbonates and adjacent biotite gneisses, and the INTO values of some of these rocks have been little modified since deposition.
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From page 105... ...
If the water were derived from a deep plutonic source or if the homogenization reflected bulk HO exchange with the deep crust during metamorphism, there should certainly not be an increase in the degree of isotopic heterogeneity at deeper structural levels, as is observed in the Pyrenees. Although the Castillon and St.
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From page 106... ...
ED value and may possibly reflect the waning effects of surface water infiltration at deeper structural levels (similar to the downward increase in oxygen isotope inhomogeneity observed in Zone 3 at Agly and Trois Seigneurs)
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From page 107... ...
regional metamorphism of the Pyrenees have profoundly modified the isotopic composition of both strontium and oxygen and hydrogen throughout huge volumes of the crust. In this respect it would be extremely misleading to take the INTO values and the calculated model 87Sr/86Sr values of low-grade Cambro-Ordovician shales from the Pyrenees and use them directly as representative protolith end-members for any of the synmetamorphic granitic litho 107 fogies magmas produced in this region, despite the fact that modified material of this type undoubtedly formed a major component of some of the Hercynian granitic magmas.
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From page 108... ...
The data from the Lapege and Agly Zone 3 rocks (basal gneisses) suggest less isotopic homogenization at deeper structural levels (at Agly the mineral assemblages imply Hercynian equilibration pressures of about 5 kbar; Vielzeuf, 1984~.
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From page 109... ...
Hence, an external water source is also required to account for petite anatexis, even though the size of the source is substantially smaller than that demanded by the oxygen isotope data. Although we cannot at this stage prove that the high-6D marine fluids that flushed the Zone 2 rocks were also responsible for the large-scale melting effects, circumstantial evidence points strongly to a link between the two, implying that the same fluid was available to enter the melting zone during petite anatexis.
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From page 110... ...
Even in relatively small hydrothermal convective systems around igneous intrusions, calculations show that there is only time for essentially one pass (see Norton and Taylor, 1979~. Our understanding of fluid transport mechanisms in rocks undergoing high-grade metamorphism is still insufficient to say whether the large-scale fluid infiltration documented in Zone 2 in the Pyrenees is likely to be FIGURE 6.12 An approximately SO-kmwide section through the Hercynian crust of the Pyrenees showing listric normal faults and a schematic rift setting for lowpressure regional metamorphism.
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From page 111... ...
(1988~. A strontium, neodynium, and oxygen isotope study of the hydrothermal metamorphism and crustal anatexis in the Trois Seigneurs Massif, Pyrenees, France, Contributions to Mineralogy and Petrology 100, 399-417.
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From page 112... ...
. Crustal anatexis and granite petro genesis during low pressure regional metamorphism; the Trois Seigneurs Massif, Pyrenees, France, Journal of Petrology 28, 127-169.
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