Trace-element and isotopic data and their bearing on the genesis of Precambrian spilites from the Athapuscow aulacogen, Great Slave Lake, Canada

Abstract

SummaryThe nature and timing of ‘spilitization’, and the original composition of ‘spilitized’ Precambrian lavas fron the Athapuscow aulacogen, Great Slave Lake (Canada) are investigated, using geochemical and Rb/Sr isotopic methods. Relationships and abundances of trace elements of petrogenetic significance suggest that the spilites were not derived from alkaline basaltic magmas, but show petrogenetic affinity with tholeiitic basalts from continental or volcanic island environments. This new interpretation conforms well with the supposed tectonic and geologic environment in which the lavas and pyroclastics were erupted. Assuming an ‘open system’ behaviour for Rb and Sr during intense metasomatism, the isochron age of 1872 Ma obtained for the lavas is considered as the time of ‘spilitization’. This age, when compared with geochrono metric data obtained by other techniques, suggests that the time interval between initial volcanism and subsequent ‘spilitization’ event was brief, and probably related to diagenesis or burial metamorphism during rapid subsidence and sedimentation commonly associated with rifting within aulocogens. A relatively low initial 87Sr/86Sr ratio of 0.7018±0.0005 obtained for the lavas either reflects the low Sr isotope values characteristic of diagenetic pore fluid in Precambrian seas, or suggests that the original ‘primitive’ initial ratio was not adversely affected by the ‘spilitization’ process. .