Fluid Disturbed K-Ar Mineral Ages from the Dalradian Rocks of Connemara, Western Ireland

[摘要] This thesis is concerned with explaining the cause of a wide range (about 100 Ma) of K-Ar mineral ages in the Connemara Schists, western Ireland. This range was first reported by Elias (1985) and Elias et al. (1988) who determined a total of over sixty biotite, muscovite and hornblende K-Ar ages. These authors concluded that the ages were the result of differential uplift and cooling of three, independent 'blocks' within Connemara. Examination of the age data of Elias (1985) and Elias et al. (1988) reveals, however, that at best this can only be a partial explanation because some regions within Connemara record mica K-Ar ages significantly older than amphibole K-Ar ages. This is the opposite to the order predicted from commonly accepted closure temperature estimates (Harrison 1981, Harrison et al. 1985) and the opposite to the order recorded in many other metamorphic terranes. It was apparent, therefore, that some process other than simple cooling of the rocks was responsible for, at least, some of the K-Ar mineral ages. Fourteen biotite, twenty muscovite and thirty hornblende K-Ar mineral ages determined during the course of this study exhibited an age range indistinguishable from that of Elias (1985) and Elias et al. (1988). Hydrogen isotopic ratios measured on seven biotites and six muscovites showed slight variation and averaged around -66 and -41‰, respectively. Hydrogen isotopic ratios measured on hornblendes showed a larger variation that ranged from -51.5 to -75.1‰. The variation in the hydrogen isotopic ratios from the micas and the hornblendes is attributed to variable, incomplete, exchange between the minerals and a fluid at temperatures about 300 to 350°C. Oxygen isotopic ratios were measured on thirteen of the hornblendes. These showed some variation, from +5.5 to +11‰, which reflects primary compositional variations in the hornblendes and not partial exchange with the fluid that caused the variation in hydrogen isotopic ratios. The spread in the K-Ar mineral ages is shown to be neither geographically correlated nor the result of diachronous, slow cooling of the rocks. There is a weak correlation between the hornblende K-Ar age and the Fe/(Fe+Mg) ratio that may indicate some compositional control over the Ar closure temperature. The range of K-Ar mineral ages is shown to be the result of variable, partial resetting of individual K-Ar 'clocks' that had all previously been totally rejuvenated by the intrusion, at 490+/-1 Ma (Jagger et al. 1988), of a large (>80 km x >20 km) metagabbro-gneiss complex. The subsequent, variable and partial resetting occurred at c. 400 Ma and was induced by the intrusion of a suite of Lower Devonian Granites into the Connemara rocks. The resetting was not simply a thermal event as witnessed by the lack of any spatial correlation between sample location and K-Ar mineral age. Instead, the variable and partial resetting was brought about by interaction of the minerals with the fluid identified from the hydrogen isotopic ratio measurements. This fluid was circulating in a hydrothermal system around the granites. Different mineral species interacted with this fluid in different ways. Muscovite lost Ar where it was in contact with the fluid with a temperature in excess of the muscovite Ar closure temperature, about 350°C, long enough for Ar diffusion to occur. Biotite was reset in a similar way, except that Ar loss would have been enhanced by chloritisation of the biotite. For both micas, a correlation exists between the K-Ar ages and the extent of alteration of the rock indicated by the degree of sericitisation of the plagioclase. Oldest K-Ar mica ages were yielded by samples with little or no visible alteration of the feldspar. Argon loss from hornblendes was more complicated and was primarily governed by the extent of heterogeneity in the crystal microstructure. TEM investigation revealed that sub-micron scale exsolution and phyllosilicate growth along cleavages existed in some samples. Similiar features have been shown to cause a reduction in the Ar closure temperature of hornblende to about that of biotite (Harrison & Fitz Gerald 1986). Direct evidence of a linkage between Ar loss from hornblendes and interaction with the fluid comes from a correlation between the hornblende K-Ar ages and hydrogen stable isotopic ratios. It is suggested that sometimes Ar loss in hornblende is coupled directly with stable isotopic exchange. In such situations a hornblende could have two Ar closure temperatures; one 'wet', in the presence of a fluid, and one 'dry', in the absence of a fluid. It is possible to identify samples which yield ages which were not reset and which are consequently, geologically significant as closure ages following emplacement of the metagabbro-gneiss complex.

[发布机构] University:University of Glasgow

[关键词] Geology