1 JMU Ireland Summer Field Course 2018 an Introduction to the Geology
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JMU Ireland Summer Field Course 2018 An Introduction to the geology of Connemara’s metamorphic and igneous rocks. By Prof. Martin Feely, NUI, Galway, Ireland. Part 1. A summary of the regional geology of Connemara The rocks of Connemara contain petrified images of buckled crust and volcanic activity that are many hundreds of millions of years old. The oldest rocks (~600-460 Ma) form the dominant central sector, which is an east-west trending corridor of deformed and metamorphosed igneous and sedimentary rocks collectively known as the Connemara Metamorphic Complex (Leake and Tanner 1994 and references therein)-see Figure 1. The Oughterard Granite (~463 Ma) occurs at the eastern end of the complex. The complex is bounded to the north by igneous and sedimentary rocks of Ordovician and Silurian age (~500-410 Ma) and, in the south by the Galway Granite (~425-380 Ma) including the Roundstone, Inish and Omey Granites. Two other lithological units of note are the metamorphosed volcanic rocks of the Delaney Dome Formation and the sedimentary and volcanic rocks exposed on the islands of South Connemara (Lettermullan and Gorumna)- both of these units are of similar age (~470 Ma). The Connemara Metamorphic Complex is part of an ancient mountain belt that stretched in NE direction from the Appalachians, through Canada, Ireland, Scotland and Scandinavia. This mountain belt owes its present fragmented distribution pattern to the birth (~200 million years ago) and ongoing growth (~2 cm/yr.) of the Atlantic Ocean. The metamorphic and igneous rocks form the solid mass of the landscape, but the actual shapes of the mountains and valleys result from sculpting by ice. The past two million years of geological history, in this part of the globe, have largely been dominated by glaciations and interglacial periods (when the ice retreated). Interglacial periods can last for 100,000 years and it seems that we are now living in an interglacial period and maybe the ice will advance south again to cover most of Ireland. The ice cover in Connemara may have been up to 1km thick at times. The birthplace of minor glaciers can be seen in the scooped-out hollows (corries or cirques) on the sides of the quartzite mountains (the Twelve Bens, Maumturks and Corcogemores) of the Connemara Metamorphic Complex. Of all the rock types to be found in Co. Galway, the Connemara marble is the most celebrated. The marble stretches for about 30 km. in an E-W direction along the 1 southern flank of the Twelve Bens. Its distinctive colour variations (shades of green, sepia and white) developed as a result of the metamorphism of impure limestones ~ 470 million years ago. It is ideal as an ornamental and decorative stone, and very popular in the manufacture of jewellery. It has been widely used for panelling, pillars, fireplaces and other interior work. The interior of Galway Cathedral offers a spectacular example of the decorative use of Connemara marble (Feely, 2002). The bedrock geology of south Connemara is dominated by granite. Many varieties are encountered in the region. The granites range in colour from pink to dark grey reflecting varying proportions of the essential minerals quartz, feldspar and mica. All of these varieties are collectively termed the Galway Granite (Feely et al., 2006). The Galway Granite is exposed over an area of approx. 600 sq. km. along the north shore of Galway Bay. It extends offshore to the west, south and east reflecting a granite mass that has a total surface area of ~ 1500 sq. km. Furthermore, it has an estimated thickness of ~ 10 km. The Galway granites were emplaced into the upper 10 km of the Earth's crust between ~425 and 380Ma (Feely et al., 2003, Feely et al., 2007 & Feely et al., 2010). The younger Carboniferous limestones (~350-320 million years old) form a boundary to the Connemara basement rocks to the east and south (Lees & Feely 2016)—see Figure 1 below. 2 Figure 1. Map of Connemara and environs showing the distribution of the main geological units. The Galway Granite is undifferentiated while the earlier plutons are the Omey (O), Inish (I) and Roundstone (R) granites. The area to be visited on the fieldtrip is outlined. The Connemara Metamorphic Complex (CMC). The Connemara metamorphic complex forms part of a regional belt of rocks that extend from the Appalachians, through Ireland and Scotland, to Norway. In Ireland, the Connemara part of this belt occurs to the south of all other similarly aged rocks and has been referred to as the Connemara allochthon i.e. block or nappe which has been relocated from its site of formation. Recently, Friedrich and Hodges (2016) and Dewey and Ryan(2016) however, argue that the complex was formed in situ and therefore is not allothonous – see also Dewey and Ryan(2016) who also support this revision. The early Ordovician Grampian Orogeny, responsible for the CMC, was the result of collision between Laurentia’s margin and an oceanic island arc. The CMC differs in position and character from all other parts of the exposed Dalradian rocks of the Grampian Orogen in lying south of the Highland Boundary Fault (see Fig 6 below). Furthermore, Dewey and Ryan (2016) state that Connemara is not a terrane, displaced with respect to the remainder of the Grampian Orogen but was overridden, northwards, by the arc (Lough Nafooey Volcanic Arc) and its fore-arc basin (South Mayo Trough) its frontal ophiolite complex (Deer Park Complex) and accretionary complex (Killadangan) the latter two exposed further north in the Clew Bay area. The CMC is composed of two interfingered E-W trending metamorphic belts (- a northern and southern belt) intruded by post tectonic granites e.g. in the east by the Oughterard Granite (~463Ma) and in the south by the Galway Granite Complex (~425- 380Ma). It is overlain along its northern margin by unconformable Silurian strata while to the east it is bounded by Carboniferous Limestones. Sediments and volcanics of late Proterozoic age (~600Ma) that were metamorphosed during the Grampian orogeny (~475-463Ma) form the northern belt see Fig. 2. Quartzites, schists, marbles and amphibolites are the lithologies that characterise the northern belt and are collectively termed the Connemara schists. 3 Figure 2. Generalised geologic map of Connemara. Metamorphic isograds are shown along with the MGGS, the Silurian unconformity, the trace of the D4 Connemara antiform and the Mannin thrust responsible for the emplacement of the Connemara Metamorphic Complex over the relatively low grade meta-rhyolites of the Delaney Dome Formation (DDF). NS crossection A-A1 is shown in Fig 4 below. After Friedrich & Hodges (2016) Distribution of the major rock units in Connemara. Dalradian lithostratigraphy reflects a continuum from the rocks of the Connemara Metamorphic Complex, through Mayo and Donegal to the Central Highlands of Scotland. They were deposited on the southeastern continental margin of Laurentia. The older Dalradian rocks of Connemara (Appin Group) were deposited in a shelf environment and the younger Argyll and Southern Highland Groups formed in a series of fault bounded basins (Fig 3). Figure 3 displays the age relations of the Dalradian linking sedimentation from Neoproterozoic to early Palaeozoic (~700- 520Ma) with deformation and metamorphism mainly associated with the Grampian Orogeny (475-463Ma). The Dalradian (called after a Celtic tribe Dal Riada) is a tectono-stratigraphic term with nomenclatures rooted in the Scottish Highlands. 4 Fig 3. Geological sequence and events in the Grampian Highlands. From: Table 1 in Stephenson, D, and Gould, D. 1995. British regional geology: Grampian Highlands. Fourth edition. Keyworth, Nottingham: British Geological Survey. The southern belt is dominantly composed of meta-igneous rocks produced during synorogenic arc magmatism that spanned a period of ~12Ma (from ~475 to ~463Ma i.e. The Grampian Orogeny). This cycle of igneous activity, produced at the roots of a magmatic arc, started with the intrusion into the Dalradian metasediments of Lower Ordovician (~475-470Ma) gabbros followed by intrusions of intermediate to acid magmas up to at least 465Ma-all were metamorphosed during this period. The rocks comprising the southern belt are collectively termed the Metagabbro-Gneiss Suite (MGGS). The Metagabbros are essentially massive amphibolites that retain in places primary igneous minerals (olivine and pyroxene) and gabbroic textures. They are the most widely distributed occurring in both the northern and southern parts of Connemara. The gneisses comprise a suite of (a) orthogneisses derived from the calc- alkaline intermediate-acid magmatism that intruded the Dalradian metasediments and the metagabbros in the south and (b) paragneisses whose protoliths are the Dalradian metasediments that display evidence of partial melting (migmatites). The Connemara terrane is considered to have been exhumed from >18km to ~ 10km between ~470 and 463 Ma. 5 Deformation The Connemara metamorphic complex displays evidence of at least three deformation events (D2 to D4) and the most obvious and regional scale structure, whose trace runs along the centre of the northern belt, is the Connemara Antiform (D4) that plunges gently (~20o ) to the east. No D1 folds are recognised and evidence for D1 comes from inclusion trails in garnet augened by a D2 schistosity S2 (see Fig. 4). The D2 event produced a pervasive schistosity (S2) in the Dalradian lithologies. The most common observable folds are tight D3 structures. These formed at the peak of metamorphism. The final major phase D4 produced broad open E-W trending upright structures most notably the Connemara Antiform. The gabbros are post-D2 (in the N they are syn D2 and immediately prior to D3) but pre- to syn-D3. The younger acid magmas appear to overlap with D3. Melting in the migmatitic metasediments also spanned the D3 events. Minor D3 folds in competent unmelted strata are sometimes distorted and disrupted by the partial melts.