The geology of SW Connemara, Ireland: a fold and thrust Dalradian and metagabbroic-gneiss complex

Abstract: Sillimanite grade Dalradian schists composed of pelites, impure psammites, quartzites and calcite marbles, all containing amphibolites, were intruded and hornfelsed (giving hornfels sillimanite, garnet and cordierite) by a plutonic basic to acid igneous rock series now metamorphosed and folded

Geochemically unravelling the sedimentary components of Archaean metasediments from Western Australia

Major and trace elements of metasediments of the granulite facies Narryer and the amphibolite facies Jack Hills metasediments have been used to devise plots which enable the original sedimentary components of these rocks to be unravelled, despite their metamorphic state. These plots overcome the predominant effect of quartz which ranges up to 100% and can be used to study other metasediment suites. Six components controlled the composition of the clastic metasediments (1) kaolinite, (2) illite-muscovite, (3) chlorite–montmorillonite, (4) dolomite, (5) quartz, (6) resistant heavy minerals such as rutile, zircon, chromite and monazite. Identifying the first four of these components enables the trace element composition associated with the components to be estimated. Feldspar was largely destroyed by chemical weathering but there is a suggestion that traces of it formed a minor seventh component in the Narryer rocks. There was little or no mixing of the original kaolinite-bearing and illite–muscovite-bearing samples, indicating two distinct sources, although both sets of samples contain >4 Ga zircons, suggesting a third zircon source. The old zircons may have been derived from re-worked sediment or metasediment as they occur in the matrix and in the quartzite clasts of conglomerates. Using Bhatia's criteria, a passive margin is the likely site of deposition. Calc alkaline tholeiites in the Narryer succession are intrusive and did not contribute to the sediments which were derived from extremely heavily weathered granites, komatiites, schists and reworked sandstones or quartzites

Some metasomatic calc-magnesium silicate rocks from Connemara, Western Ireland: mineralogical control of rock composition

Chemical analyses for 33 major oxides and minor elements are reported for a series of 26 associated metasediments that range from staurolite-garnet schists through similar pelites with graphite to schists with varying contents of actinolitic amphibole and diopside-rich calcmagnesian silicate rocks. The calc-magnesian silicate rocks cannot be matched with any likely original sediment and have probably formed by metasomatism of pelites by processes similar to those that produce rodingites. The presence of graphite, gradation into pettes, and the positive correlation of Ti and Niggli al-alk support a part sedimentary parentage for the calc-magnesian rocks. ca, Mg, cr, and Ni (maxima l3.7vo CaO,24.2Vo MgO,247O ppm ct atd 620 ppm Ni) were added from solutions derived from nearby ultramafic rocks serpentinized during static regional metamorphism, which caused andalusite and cordierite growth in some of the associated pelites. Although the original sedimentary composition and the composition of the metasomattzitg fluids were important in determining the chemical composition of the metasomatic rocks, the stable amphibole and pyroxene mineralogy in the calc-magnesian silicate rocks was no less important. These rocks are an example of the partial mineralogical control of the composition of metamorphic rocks. It is proposed that a characteristic feature found in many sedimentary rocks, but not in igneous rocks, is a marked positive correlation of Ti and Niggli al-alk

Mechanism of emplacement and crystallisation history of the northern margin and centre of the Galway Granite, western Ireland

The main phase (~400 Ma) emplacement of the central and northern part of the reversely zoned Galway Granite was incremental by progressive northward marginal dyke injection and stoping of the 470–467 Ma Connemara metagabbro-gneiss country rock. The space was provided by the synchronous ESE-opening, along the strike of the country rocks, of extensional fractures generated successively northward by a releasing bend in the sinistrally moving Skird Rocks Fault or an equivalent Galway Bay Fault. This fault is a prolongation of the Antrim–Galway (a splay off the Highland Boundary Fault) and Southern Upland Faults. The ESE-strike of the spalled-off rocks controlled the resultant ESE-elongated shape of the batholith. The magma pulses (~5–30 m in thickness) were progressively more fractionated towards the northern margin so that the coarse Porphyritic (or Megacrystic) Granite (GP; technically granodiorite) in the centre was followed outwards by finer grained, drier and more siliceous granite, until the movements opening the fractures ceased and the magma became too viscous to intrude. ‘Out-of-sequence’ pulses of more basic diorite-granodiorite (including the Mingling–Mixing Zone) and late main phase, more acid, coarse but Aphyric Granite, into the centre of the batholith, complicated the outward fractionation scheme. The outward expansion, caused by the intrusions into the centre, caused a foliation and flattening of cognate xenoliths within the partly crystallised northern marginal granite and in the Mingling–Mixing Zone to the south. Late phase (~380 Ma) central intrusions of the newly-discovered aphyric Shannapheasteen Finegrained Granite (technically granodiorite), the Knock, the Lurgan and the Costello Murvey Granites, all more siliceous and less dense than the GP, were emplaced by pushing up the already solid and jointed GP along marginal faults. This concentration of lighter granites plus compression shown in thrusting, caused overall fault uplift of the Central Block of the Galway batholith so that the originally deepest part of the GP is exposed where there is the most late phase granite. Chemical analyses show the main and late phase magmas, including late dykes, were very similar, with repetition of the same fractionation except that the late phase magmas were drier and more quickly cooled, giving finer grained rocks