Caledonian strike-slip terrane accretion in W. Ireland: insights from very low-grade metamorphism (illite–chlorite crystallinity and b₀parameter)

Abstract

Analysis of pelites with detrital white-micas in the Clew Bay-Galway Bay segment of the Irish Caledonides indicates that b 0 data from whole-rock and < 2 μm fractions generally show differences smaller than the errors of the method, irrespective of (001) illite crystallinity values, probably due to metamorphic recrystallization. Intermediate pressure metamorphism of the Ordovician-Silurian Clew Bay Group indicates slow subduction, allowing partial thermal reequilibration before exhumation. In contrast, the Croagh Patrick Group Laurentian shelf-sediments underwent high-pressure alteration, suggesting rapid subduction/exhumation, synchronous with strikeslip faulting. The Murrisk Group, which underwent high-intermediate pressure metamorphism in an Ordovician back-arc, forms a separate terrane to the Croagh Patrick Group to the north and also to the Ordovician Lough Nafooey and Tourmakeady groups and Rosroe Formation in the south, in which low-intermediate pressure alteration occurred. These, together with the Silurian North Galway Group, may have undergone heating due to movement over or deposition on the hot Gowlaun Detachment as the Connemara Dalradian was exhumed. The South Connemara Group also underwent a high-pressure alteration, consistent with its inferred subduction environment. Evidence of contact alteration, due to known or inferred buried late-to post-Caledonian granitoid plutons, has been found in the Clew Bay, Louisburg-Clare Island, Croagh Patrick, Murrisk and South Connemara groups. These show evidence of lower-pressure alteration than the surrounding country-rocks.

Atypical Gold Mineralization in an Orogenic Setting--The Bohaun Deposit, Western Irish Caledonides

“…iron contents in sphalerite- Lusk et al, 1993). However, on the basis of a gradient of ~30 to 35°C/km predicted by lattice parameter b0 data (see Padan et al, 1982, for details) from mica (Rice and Williams, 2009;Kemp and Merriman, 2009), a mean illite crystallinity value of 0.48 for pelites from the Silurian metasedimentary rocks east of the Maam Fault (Rice and Williams, 2009), which hosts the Bohaun mineralization, constrains mineralization to the late diagenetic zone of low-grade metamorphism, with a maximum mineralization depth of ~5km according to the basin maturity chart of Merriman and Frey (1999). Textures and structures described above make lithostatic fluid pressure extremely unlikely.…”

“…The Carboniferous (Dinantian) strata form the north-western margin of the economically significant 'Irish Midlands basin', named the 'North West Province' (Phillips and Sevastopulo, 1986;Johnston et al, 1996). Outside the Dalradian, metamorphic grade in this part of Irish Caledonides is generally very low, ranging from late diagenetic zone to upper anchizone (Rice and Williams, 2009).…”

A revision of the Ordovician stratigraphy for the western Ireland Caledonides and implications for tectonic models

“…The horizon in the three formations where an apparently abrupt flood of detrital muscovite entered the basin was labelled the 'm-horizon'. The new field evidence and age data presented here together with the evidence presented from previous palaeontological, metamorphic and sedimentary studies (Williams, 1984(Williams, , 2002Harper et al, 1988;Evans, 1996;Harper and Parkes, 2000;Rice and Williams, 2010) require a modification of this hypothesis. The metamorphic detritus in the so-called Maumtrasna Formation can now be seen in the context of the unroofing of the Connemara Dalradian rocks to the southwest, since it is now proposed to be part of the Mweelrea Formation which contains Dalradian lithologies derived from the southwest (Pudsey, 1984;Williams, 1984).…”

“…The most recent review of the Ordovician stratigraphy of the west of Ireland (Harper and Parkes, 2000) presented the Rosroe as an isolated unit and suggested that the 'south limb' was quite different to that of the 'northern limb'. A more recent study (Rice and Williams, 2010) (Figure 1). Similar to the Rosroe Formation which is along tectonic strike from the Derryveeny Formation, the latter is bound by older rocks of the Mweelrea Formation to the westnorthwest and by Silurian rocks to the south.…”

“…The timing, erosional response and the docking of this arc (referred to as the Lough Nafooey Arc) is based on the correlation of the northern and southern limbs of the Mweelrea-Partry Syncline, in particular, the correlation of the Rosroe Formation with the Maumtrasna and Derrylea formations. Williams (2002) and Rice and Williams (2010) have questioned this correlation and therefore the timing of the interaction of the arc and its northern continent.…”