Rb–Sr geochronometry of the Ecstall, Kitkiata, and Quottoon plutons and their country rocks, Prince Rupert region, Coast Plutonic Complex, British Columbia

Abstract

The minimum age of the Ecstall pluton is 78–79 Ma, based on Rb–Sr mineral isochrons for two pegmatites. At that time the entire body of the pluton, zoned concentrically from quartz monzonite to diorite, was Sr rich – Rb poor, relatively homogeneous in Rb/Sr ratio (0.024–0.061), and somewhat variable in initial Sr isotopic composition (87Sr/86Sr = 0.7044 to 0.7048). Late-stage leucocratic dikes contain radiogenic Sr that is probably derived from old and Rb-rich country rocks. Pegmatite dikes from two different localities are similar to one another in Rb/Sr and 87Sr/86Sr and either have high initial ratios (0.7055 and 0.7058), like the leucocratic dikes (0.7051 and 0.7059), or the pluton and its pegmatite dikes are very much older (~190 Ma) than the 78–79 Ma pegmatite mineral isochrons. Sr isotopic data do not resolve this ambiguity. The Kitkiata pluton has an initial 87Sr/86Sr ratio of 0.7042–0.7043, depending on its age. One radiogenic Sr-rich sample gives a model date of 165 ± 11 Ma, but the possibility of anomalous initial ratio makes this result highly uncertain.The Quottoon pluton, which lies a bit farther east of the Ecstall pluton, gives a whole-rock isochron of 51 ± 2 Ma with 0.7045 ± 1 initial ratio and somewhat older, near concordant, U–Pb zircon dates of 60 Ma. The low initial ratios indicate a preponderance of mantle-derived magma of Mesozoic or early Cenozoic age in the plutons studied.Radiogenic Sr in schists near Prince Rupert suggests an age on the order of 524 Ma, and correlation with upper Precambrian and lower Paleozoic rocks of the Alexander Terrane of the Alaska Panhandle.Sr isotopic composition of the Central Gneiss Complex is compatible with late Paleozoic – early Mesozoic ages for precursor detrital and volcanic strata and local presence of marine carbonate with moderately enriched 87Sr/86Sr. The generally low radiogenic Sr content of these core gneisses rules out an origin by remobilization of greatly older rocks.