ArticleDating multistage paleofluid percolations: A K-Ar and 18O/16O study of fracture illites from altered Hercynian plutonites at the basement/cover interface (Poitou High, France)
Introduction
In fractured basement rocks, precise determination of the relative and even absolute chronology of fluid flow events is of critical interest to understand and predict the hydraulic regimes at depth. The nature of fluids circulating within a basement and within its sedimentary cover, their flow paths and the driving forces of fluid migration also need to be constrained (Blyth et al 2000, Fourcade et al 2002). The introduction of elements (Cl, C, …) to aquifers and their subsequent removal over geological time controls to some extent the composition of the present-day aquifer fluids. However, dating the fluid circulation in fractures of basement rocks is difficult because fluid events do not necessarily induce the growth of newly formed mineral parageneses. Moreover, most fractures in the crystalline basement are often filled with minerals unsuitable for isotopic dating (quartz, sulfates, carbonates). An alternative method to direct isotopic dating of fluid circulation in fractures is to examine in great detail the phyllosilicates contained in host rocks in the vicinity of fractures or in the composite fracture fillings made of vein material and host rocks. It might be expected that, in some instances, phyllosilicates of detrital origin (or inherited from early alteration stages) may react with fluids to form small amounts of newly formed or recrystallized minerals, when submitted to a new fluid flow event. The main difficulty encountered concerns the separation of the minute amounts of newly formed or recrystallized particles from inherited (or partly reequilibrated) phyllosilicates. A methodology is therefore needed to extract and carefully characterize the finest authigenic clay mineral fractions. X-ray diffraction (XRD) together with scanning electron microscopy, electron microprobe, and stable (H, O) isotopic studies may be used to identify the clay minerals and to assess the related conditions of fluid circulation, while K-Ar systematics may provide information on the minerals crystallizing from fluid flow.
An attempt to date fluid flow by this method was performed on the Hercynian fractures located in the Vienne basement plutons, at the northwestern edge of the French Massif Central (the Charroux-Civray granitoids; Rolin and Colchen 1995, Cuney et al 1999). This area was investigated by ANDRA (French National Agency for Nuclear Waste Management) for a possible setting of an experimental underground laboratory devoted to long-term nuclear waste disposal. The Vienne plutons display irregularly distributed but well-developed sets of fractures (Gros and Genter, 1999). Using paragenetic and fluid-inclusion studies, at least three main fluid flow stages were identified in the Charroux-Civray granitoids, each stage being characterized by distinct P-T conditions and geodynamic contexts (Cuney et al 1999, Coulibaly 1998, Cathelineau et al 1999, Boiron et al 2002, Fourcade et al 2002): i) Hercynian retrograde metamorphism and subsequent hydrothermal circulation in fractures; ii) after denudation up to aerial conditions during the Permo-Triassic period and transgression of the Liassic sea (Sinemurian-Hettangian times), percolation of brines at an unknown age; iii) subsequent percolation of fluids until present. A few phyllosilicate-rich fractures are recognized to have experienced these recurrent fluid circulations from Hercynian to the present time. Some of them were water productive during the drilling process (Casanova et al., 2001).
The aim of the present work is to evaluate the ability of the K-Ar method to date fluid circulation events in basement rocks using the reworked clay fractions contained in the above-mentioned fractures. The analytical approach includes nondestructive extraction, XRD identification, oxygen and hydrogen isotopic characterization, and K-Ar dating of different clay size fractions sampled in fractures located in the basement at various depths beneath the basement/cover boundary.
Section snippets
Geological setting and sample description
Samples were selected in basement rocks from four deep drill holes (CIV108, CHA117, CHA212, and CHA312, Fig. 1). The latter correspond to a recent drilling survey performed over a 125-km2 area in Hercynian granitoids located between the Massif Armorican and the Massif Central in western France (Charroux-Civray area). There, the Hercynian basement (tonalites and granodiorites; Cuney et al., 1999) is unconformably overlied by a 150-m-thick sedimentary cover consisting of Mesozoic limestones and
Methods
Our approach was to compare isotopic dates and compositions obtained from different size fractions from i) a given clay sample, ii) various depths within the granitoid body. Usual crushing procedures may contaminate the finest fractions with fragments of minerals of larger size, both supposed to have crystallized at different stages and under different conditions. To circumvent this problem, the clay fractions were extracted by gentle methods based on short repetitive ultrasonic treatments and
Discussion
The fact that large variations of 18O/16O ratios and K-Ar dates correlate focusses the discussion on several questions: i) Do the observed variations correspond to mineralogical effects, i.e., variations in the nature of the analyzed clay fractions? ii) Can they be explained in terms of mixing between two clay components? iii) Is it possible to ascribe a formation age to each of these clay components, and what could be the conditions and causes of formation? In summary, what is the reliability
Concluding remarks
This study yields the following conclusions:
1) Clay-rich fractures originally formed during late Hercynian stages underwent successive periods of fluid migration, mainly the Mesozoic brine stage. These fractures were repetitively conductive as some of them produced present-day waters and ensured the connection between the basement draining network and the fluid source likely represented by paleo-aquifers located in the upper weathered part of the granite and in the overlaying infra-Toarcian
Acknowledgements
This work has been supported by GdR FORPRO-Action 98-III (contribution paper FORPRO No 2001/10A), a National Research Program between CNRS and ANDRA. ANDRA is acknowledged for the facilities and permission of sampling the drill cores. F. Martineau is acknowledged for technical assistance during the O isotopic analyses. S. Buschaert and V. Lavastre benefited from a grant from ANDRA for their Ph.D. The manuscript was greatly improved by critical comments from D. R. Pevear, J. Aronson, an
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2018, Journal of Structural Geology
Associate editor: R. C. Burruss