Stable Isotope Studies in the Caledonides of S.W. Connemara, Ireland

[摘要] The Dalradian rocks of Connemara, western Ireland, are a sequence of deformed and metamorphosed sediments and basic intrusives and extrusives, which were deposited between the late Pre-Cambrian and the Lower Ordovician. The metamorphic grade increases towards the south, where migmatites are formed adjacent to an elongated belt of meta-igneous rocks (the metagabbro suite - MGS). The MGS was intruded synchronously with the peak of metamorphism and intrusion has been dated at ~490+/-1 Ma by Jagger et al. (1988). The MGS is composed of isolated outcrops of ultrabasic rocks with more voluminous gabbroic rocks and intermediate and acid gneisses. Subsequent to intrusion the MGS together with the Dalradian rocks were thrust southwards on a major thrust plane, the Mannin Thrust at -460 Ma and were later later intruded by Caledonian granites at ~400 Ma. The MGS is distinctive in that amphibole (mostly hornblende) is abundant throughout the suite. Some of this hornblende is apparently magmatic in origin, while some is secondary, replacing primary pyroxenes. Throughout the MGS, the Dalradian sequence, and the Caledonian granites, intense retrograde alteration of other primary minerals to secondary (mostly hydrous) minerals is commonly observed: feldspars are often sericitised and saussuritised, biotite is frequently replaced by chlorite and secondary epidote is common. In the MGS this alteration is texturally later than the hornblende formation. This study makes use of stable isotope data from rocks in S.W. Connemara to constrain a. the origin of the water in the hornblende in the MGS and b. the origin of the fluid which caused the development of the later retrograde minerals. MGS hornblendes which are thought to be of magmatic origin have delta18O values of +6 to +8‰ and deltaD values of -60 to -80‰. Hornblendes which are thought to be of metamorphic origin have a more restricted range in delta18O (+6.8 to +7.6‰) and deltaD (-65 to -73‰) values within the range for the magmatic hornblendes. Modelling of the delta18O and deltaD ratios of the MGS magma indicates that it probably originated by mixing of a MORB- or OIB-like parental magma with -20-30 wt. % of crustal material, which was probably mostly partial melt derived from the Dalradian metasediments. This being the case, nearly all the water in the magmatic hornblendes must have ultimately been derived from the Dalradian metasediments. The metamorphic hornblendes were probably formed from residual magmatic fluids from the MGS magma. The chlorite, epidote and sericite in the MGS were formed as the result of infiltration of these rocks by a high deltaD (-25‰), low delta18O (+7 to +3‰ or lower) fluid. The MGS hornblendes did not equilibrate either oxygen or hydrogen isotopes with this fluid and are believed to have largely preserved their original delta18O and deltaD values. A similar high deltaD, low delta18O fluid to that which caused the late alteration in the MGS is found to have caused the formation of chlorite, epidote and sericite throughout the whole of S.W. Connemara, including the Caledonian granites. In all of the rock types examined, the stable isotope data suggest that this high deltaD, low delta18O fluid was not present at temperatures much above ~300°C, but continued to be present in these rocks to temperatures as low as 180°C or even lower. This fluid is also inferred to be saline throughout the entire area studied and has a very uniform H2O/(H2O+CO2) ratio of 0.98-0.987. It is inferred that the formation of chlorite, epidote and sericite took place during a single event in all rock types as the result of the infiltration of the whole area by a surface derived fluid. Occasional examples occur of rocks in which secondary minerals must have equilibrated with fluids with a delta18O significantly <0‰, which together with the very uniform deltaD values of -20 to -25‰ measured or inferred for this fluid, is taken to indicate that this fluid was of meteoric origin. It is suggested that the infiltration of meteoric fluid into this area took place as the result of the development of one or more meteoric convection system(s) caused by the thermal anomalies of the Caledonian granites. Convection probably took place soon after emplacement of these granites, but might have taken place as late as 300 Ma.

[发布机构] University:University of Glasgow

[关键词] Geology