HIGHLY EVOLVED TIN GRANITES: A CANADIAN EXAMPLE

Abstract

The small Seagull-Thirtymile suite of mid Cretaceous tin-related granites is found in the south-central Yukon Territory of Canada, where they intrude the displaced, and possibly accreted, Dorsey terrane of uncertain affinity to the continent. These one-mica generally leucocratic granites are atypical of the calc-alkaline batholiths found in the Omineca plutonic belt of the Cordillera. They have a major- and trace-element chemical signature similar to post-orogenic and alkaline granitoids and contain small volumes of extremely fractionated, F-Li-rich leucogranites (e.g. with Rb/Sr ratios >3000) as their youngest components, as well as an alaskite lithofacies in the STQ stock. Two possible fractionation mechanisms are indicated for the evolved lithofacies: enrichment in quartz in low-F apophyses above the main batholith and enrichment in albite, where high-F conditions prevailed. Fe-Li micas from the suite are similar to those reported for Cornubia and are considered to be primary, with no indication of either the muscovite-zinnwaldite or phengite-zinnwaldite substitution series. Those micas from the less-evolved granite facies have a more Fe-rich composition than the Cornubian examples and their composition may be diagnostic of these 'quasi-A-type' granites. Calculation of relative halogen fugacities in aqueous (magmatic) fluid existing with magma indicate that a Cl-rich (relative to F) hydrothermal system evolved, which is consistent with a shallow, periodically open system that would have allowed 'scavenging' and transport of ore met als from the granite. Existence of A-type granites in the Cordillera immediately following peak plutonism, without sufficient time gap to be able to label them truly "Post-Orogenic" is unexplained by the current tectonic model that has not addressed the possibility of either a period of crustal extension beginning at nearly equal 100Ma or intrusion of basic magma perhaps to middle crustal levels.