M. Cathelineau1, D. Banks2, M. Ayt Ougougdal1, M.C. Boiron1 and B. Poty1
The objective of this work was : i) to examine the spatial distribution of the P-T conditions and fluid features at the scale of a granite massif (e.g; the Mont-Blanc massif), i) to describe the fluid migration nearby major channels, and to determine if the microfissural permeability around the cleft networks has played a significant role in the fluid transfer, iii) to obtain constraints on the source of waters implicated in the strong water-rock interaction processes which affected the granites, iv) to compare the nature of the fluids and fluid migration processes with other zones of the alpine chain (reference to the Aar massif) subjected to similar geodynamic events.
Quartz, fluorite, epidote, and calcite crystals from fault infillings and oriented blocks of fresh and altered granites have been systematically sampled in 20 localities from the Mont Blanc granite (Col des Cristaux, Refuge Charlet-Straton, Pointe des Amethystes, Pointe Kurtz, Pointe Hellbronner, ...), and its surroundings (metamorphic and sedimentary series at Tres Les Eaux, Les Mottets, Bochard). Some comparisons are made to samples from the Grimsel zone (Aar granodiorite, Switzerland), and La Gardette (Bourg d'Oisans, France).
The relative chronology between fluid inclusions, the genetic relationships between a given fluid event and the quartz healing (FIP) or crystallization (new growth zones in euhedral crystals) has been studied with respect to the geometric features of their host micro to macrostructures following procedures published elsewhere (Boiron et al., 1992). The methodology used consists of studying fluid inclusions and their host microstructural domains in quartz veins with the help of observations carried out by using optical and scanning electron microscopy (back scaterred electron mode, cathodoluminescence imaging). Systematic 3D measurements of microfractures are carried out on each sample using an Image Analyser in the case of the granite samples.
Characterization of P-T-V-X features of percolating fluids is obtained through a multidisciplinary study of fluid inclusions including the determination of the temperatures of phase changes using microthermometry, the quantitative in-situ analysis of gas and molecular species contents by Raman spectroscopy and FT-IR for the inclusions displaying a low density volatile phase.
The determination of the bulk ion chemistry has been carried out using crush-leach procedures and combined micro-analytical techniques (ion chromatography for halogens, FES, ICP-AES and GF-AA for major and trace elements) of the leachates using the method from Yardley et al. (1993) on 28 representative samples.
Geometry of the fluid migration
Two extreme cases are distinguished:
- In the Mt Blanc granite, strong water rock interactions are exemplified by a complete dissolution (episyenites) of the granite quartz and redeposition of the transported silica, as euhedral quartz and silicates (albite, adularia, chlorite) in nearby fractures displaying (or not) quartz dissolution textures at their edges. This process occurs along subhorizontal extensional vein networks formed under ductile/ brittle conditions at the end of the main compressional alpine event .
Fluids migrate not only in the sealed veins, but in healed cracks of the granite (fluid inclusion planes (FIP)) attesting the existence of a significant microfissural permeability in the whole granite mass. At Pointe Kurtz, the main EW FIP are subhorizontal (0-30°ree; N) and parallel to the main extensional vein, and associated to a complex network of FIP (NW-SE , large range of dip). These FIP contain fluids displaying the same features as those observed in the main infillings of the tension gashes. Thus, it is clear that the transfer of silica which is extracted during the quartz dissolution process without any alteration of the granite mineral textures (other than the quartz) is maily linked to interconnected microcrack networks in the wall-rocks of the main fractures.
A more limited mass transfer around the cleft, especially during the first stages of the cleft mineral crystallization are observed in the Aar massif. The deformed Aar granodiorite show especially little alteration around the cleft, and microfissures are not abundant attesting to a relatively limited fluid circulation around the cleft in contrast to the Mont Blanc granite.
Fluids responsible for the above processes are aqueous with a low to moderate salinity. Tm-Th relationships show a complex trend in the Tm-Th pairs. Values are nearly constant within specific FIP networks or cleft localities. However, the whole data shows an evolution from the lowest salinities (around 3 wt.% eq. NaClC) corresponding to the lowest Th (around 150oC), to the highest salinities ( around 14 wt.% eq. NaCl).
Two main stages are identified:
-quartz-chlorite(adularia-albite) stage: No volatiles are identified in earliest fluid generations. The significant Tm-Th trends are exemplified by several degrees of mixing between two main end-members : dilute fluids found mostly in the granite environment, and a saline end-member which is typical of fluids found in quartz crystallized within the metamorphic or sedimentary series.
The main analyzed cations are Na, K, Ca, Mg, Sr, Fe, Mn. The ratio Na/K is rather constant for most early quartz generations, and ranges from 7 to 10, confirming earlier result obtained on larger samples (Poty et al., 1974). Temperatures are from 350 to 410oC using the Na-felspar/ microcline geothermeter. By comparison the values found in Aar granodiorite quartz clefts are slightly lower (in between 6 to 7) confirming that temperature was slightly higher in this massif (420
The mixing trend is confirmed by the existence of two contrasting fluid chemistries : fluids enriched in Ba, Li and metals, and with Br/ Cl ratios higher than that of sea water in sediments (3 to 4.5 10-3), and fluids with lower Br/ Cl ratio (1.2 to 2.8 10-3) , lower Li, Ba contents and higher F contents in the granite. The Aar granodiorite clefts have fluids with a very similar fluid chemistry than the Mont Blanc granite fluids indicating a smilarity in the processes (source, water-rock interaction) controlling the alpine fluid chemistry in both massifs.
The P-T conditions derived for Mont Blanc from Na/ K geothermometry and isochore reconstruction are estimated to be around 300-400oC, 1.5 to 4 kb. These conditions are significantly lower than in sampled clefts in Aar granodiorite where the P-T path is estimated to be from 3.5 -4kb ; 420-440oC to 2.5-3 kb : 370-400oC.
- amethyst/ ankerite-phengite stage: the presence of a low to high density CO2 component is common as shown by FT-IR analysis, and by Raman spectroscopy, and indicate the input of volatiles from external sources. The data confirm that the main stages of CO2 inputs are related to the crystallization of the late assemblages (amethyst quarz, and ankerite-phengite which destabilize the earlier assemblages). The presence of CO2 is in some cases attested by clathrate dissociation or by TmCO2 or Th CO2. Raman analyses show that CO2 is accompanied by small amounts of N2, and sometimes CH4 (in the case of La Gardette for instance). In that case Th fluctuations are linked to the mixing with the CO2 end-member.
The Na/K(at.) ratio is significantly different in samples displaying inclusions with detectable CO2. This is particularly the case for amethyst quartz in Mont Blanc, and La Gardette samples) with Na/ K ratios ranging from 12 to 30.
The P-T conditions are estimated to be around 300-350oC, 1.3 to 3 kb , thus similar to the first fluid stage. P fluctutations result from the probable seismic activity of the faults, where periods of P decrease has alternated with restoration of lithostatic P pressure.
The fluid chemistry data documents the interactions between the fluids and the basement rocks, and give further constains on the model of mass transfer and equilibria between percolating fluids and the granites. However, the fluid chemistry at the scale of most alpine granites is rather similar indicating that the processes governing the interaction between the percolating fluids and the granite have similar causes and produce the same effects at the scale of the alpine belt. However, the significant differences observed at the scale of the Mont Blanc massif show that in spite of intense fracturing, mixing between the two end members was heterogeneous and that no homogenization of the fluids occurred even at the kilometer scale.
Aknowledgments : This work has been supported by the programme JOULE II (No JOU-CT93-0318, the HCM network (CEE-XIIG-contract CT930198-PL922279: "Hydrothermal/ metamorphic wtare rock intercations in crystalline rocks: a multidisciplinary approach based on paleofluid analysis").and the programme DBT "Fluides dans la croûte".