a similar metamorphic temperature range (280-. 315 "C). Combining ... values represent conditions of approximately 300°C at 3000 bars. Clayton (1989) ..... m 240. 250 260 270 m 290. 300. T"C. B0 285 290 295. 300 305 310 315 320. T"C. 5 -. 4 -. 3 - ... 165--255. 0.91-1.00. H. V087079 036. 13 N. Planar quartz vein, 8 X 5 m.
Journal of the Geologica/ Society, London, Vol. 152, 1995, pp. 511-521, 7 figs, 5 tables. Printed in Northern Ireland
Sub-greenschist facies metamorphism from the Variscides of SW Ireland an early syn-extensional peak thermal event P. A . M E E R E Department of Geology, University College, Cork, Ireland Abstrad: Illite crystallinity studies on metaclastics from the Variscides of SW Ireland range in value from 0.18 to 0.25"A28indicating a metamorphic grade in the upperanchizone-lowerepizone (c. 275-325"C).Chloritegeothermometryyields a similar metamorphic temperature range(280315 "C). Combining these data with known overburden estimates for the area (c. 5 km) implies late Palaeozoic geothermal gradients in excess of 60°C km-'. Fluid inclusion studies on quartz veins in these metaclastics reveal two vein types characterized by fluid densities of 0.7-0.93 g cm-3 (Group 1) and 0.9-1.0 g cm-3 (Group 2) respectively. This variation in fluid density is thought to be dominantly and high controlled by temperature, with Group 1 veinslinked tolatePalaeozoicextension geothermal gradients and Group 2 veins associated with subsequent Variscan deformation and lower
temperature conditions. Keywords: Ireland, Varicides, low-grade metamorphsim, crystallinity,fluid inclusions.
accompanied by transgressive marine conditions with the development of deep basinal facies in the new sub-basin and stable shallow shelf sedimentation to the north. All together, these sediments reveal complete a late Palaeozoic extensional history stretching over 50 Ma. The extensional regime was terminated at the end of the Carboniferous with theonset of Variscan compressional tectonics. Models for Variscan deformation range from that of pure thin-skinned tectonics (Cooper et al. 1984) to thick-skinned transpressional tectonics (Sanderson 1984). The structure in the west is dominated by kilometre-scale, southwesterly plunging first-orderfolds with concordant lower order folding and crustal-scale high angle reverse faulting. Price & Todd (1988) and Meere (1992) argued that the main reverse faults in the orogen are reactivated high-angle extensional structures based on mapping of the western end of the fold belt. He postulated that these faults sole out into a single Variscan decolement at mid-crustal levels. There have been few metamorphic studies carried out on the metaclastics of the Irish Variscides. Price (1986) investigated conodont colour alteration and illite crystallinity in mudstones from the Iveragh Peninsula. He obtained conodontalteration indices (CAI) of 4 to >5, indicating palaeotemperatures of between200°C and 300 "C. Illite crystallinity values of 76 to 100 mm (Kubler Index; Kubler 1967, 1968) showed little variationthroughout thearea studied and indicatedametamorphicgrade in theupper anchizone. Price (1986) proposed that the illite crystallinity values represent conditions of approximately 300°C at 3000 bars. Clayton (1989) carried out a 'vitrinite' reflectance Upper Devonian-Lower Carboniferous study on the succession in the Kinsale Harbour-Old Head of Kinsale area [W 640 510W 630 3901, southern Ireland. He found reflectance ranges from 4.06 to 5.21% (Rmln,)and from 6.21 to 7.82% (Rmax,), indicating ameta-anthraciterankin the North American coal rank classification system (ASTM), which is usually equivalent to the lower epizone in terms of illite crystallinity (Table1). The data show little variation
While the last decadehasseenimportant advancesin determining P-T conditions for low grade metamorphism in metabasites (Liou et al. 1985), attempts at establishing similar constraintsfor low grade metaclastics haveproven problematical (Kisch 1987). The interpretation of data from currently available low grade palaeogeothermalindicators, including mineral crystallinity, chlorite geothermometry, vitrinite reflectance and conodont colour alteration is somewhat restricted by the lack of adequate resolution. The finite nature of the results obtained permits little temporal control on low grade metamorphic events. A good deal of discussion has centred on the possibility of peak low grade metamorphic conditions being related to basin development and extension, i.e. predating orogenic compression (Robinson 1987; Bevins & Robinson 1988; Roberts et al. 1989). This study characterizes the thermal maturation history of Upper Palaeozoicmetaclasticrocks from SW Ireland by combining results of illite crystallinity and chlorite mineralogy and fluid inclusion studies on syn-deformational quartz veins. This multidisiplinary approach has been used to establish the timing of the main Late Palaeozoic thermal event in SW Ireland.
Geological background The geology ofSW Ireland is dominated by an Upper Palaeozoicsedimentarysequenceunconformably overlying deformed sediments of the paratectonicCaledonides. The late Palaeozoictectonicevolution of thearea commenced with Mid-Devonian crustal extension and the deposition of alluvial sheet flood sediments in a large east-west-trending half graben, the Munster Basin (Williams et al. 1989). This period of extension was marked by two discrete, basin-scale fluvial influxes resulting incompositestratigraphic thicknesses of >5 km at the western end of the basin by the end of the Devonian. Thelate Devonian saw aconsiderable slowing down of sedimentation rates and a second basin, the SouthMunster Basin, was superimposed onthesouthern part of the earlier structure. Subsidence in thesouth was 511
512
P. A .
MEERE
Table 1. Correlation of Kubler zones (Kubler 1970), Winkler zones (Winkler 1979), illite crystallinity (Kisch 1990), coal rank and vitrinite reflectance (Kubler et al. 1979)
I
DlAGENESlS 1
EPlZONE
5
6
VERY Low GRADE
LOW GRADE
4
3
2
I ANCHIZONE I
DIAGENESIS
ZONES (Kubkr -70)
ZONES ~ ' i o L,79) la
I
I
META-ANTHRACITE
ANTHRACITE
COAL
I
-1
08 I
I
1.6 I
I
3.2
2.4 I
I
I
I
I
4.0 1
S.:,
48 1
1
1
COAL
RANK VlTRINITE
1
After Meere (1992).
throughoutthearea studiedand no apparent correlation with stratigraphic position. Clayton (1989) concludedfrom this that peak metamorphic temperatures were in excess of 250°C. He also highlighted major a discontinuity in maturation level at the base of the Jurassic, both offshore in the North Celtic Sea Basin and onshore at Cloyne, where reflectance levels drop considerably to mean values (R,) of 0.30-0.35% in the Jurassic rocks (Higgs & Beese 1986). Jones (1992), in a compilation of Irish conodont alteration indices, noted that values of 6 and over have been recorded from the southwest representing temperatures in excess of 400 "C.
Illite crystallinity The crystallinity of illite (IC index) in argillaceous sedimentaryrocksprovidesa valuable indicator of the degree of diagenesis and low-grademetamorphism that these rocks have undergone (Weaver 1960; Kubler 1964; Weber 1972a, b ) . A decrease in the index value signifies an increase in the crystallinity of the illite and hence metamorphic grade. The measurement of illite crystallinity, in degrees of angular separation ("A28), is very sensitive to experimental conditions. The use of standards is therefore necessary especially if thedataaretobecompared with those of previous workers. The X-ray diffractometer used in this study was standardised using five polished slate secondary inter-laboratory illite crystallinity standards plus a mountedstripfrom a flake of muscovite. The IC indices measured on the machine used in this study were virtually identical tothestandard values (Table 2). The boundary conditions for diagenesis, anchizoneandepizonemetamorphism recommended byKisch (1990) can therefore be used without modification (Table 1).
Table 2. Comparison of IC data for inter-laboratory standards measured on the diffractometer the at University of the Negev, Israel (Kisch 1990) and measured at the University of Utrecht in this study Standard Peak width(%) Utrecht Beer-Sheva Muscovite N75-47A I1 N75-59AIII N7.5-65VIII N7410CVII
0.085 0.135 0.255 0.310 0.382
Peak width("28) 0.088 0.136 0.2.54 0.293 0.416
Mudrock samples weretaken from twelve localities along a cross-strike traverse from the south western Irish Variscides, extending from Ballycrovane in the north west to Galley Head in the southeast (Fig. 1). Samples 1, 2, 9 and 12 were taken from theCastle Slate Formation in an attempt to minimize the effect of variable lithology which in the case of Samples 1 and 12 necessitated moving along strike off the geological traversestudied (Fig. 2). Sample preparation of the50pm, (6) transfer the suspension to a 20cm high sedimentation cylinder, stir well and after 70min pipette out the upper 15 cm (this contains the
