Late stage oxide growth associated with hydrothermal alteration of the Western Granite, Isle of Rum, NW Scotland

Abstract

[1] The Paleogene Western Granite, Isle of Rum, NW Scotland, represents a westward tilted, shallow level, felsic intrusion associated with the early growth stages of the classic early Paleogene Rum Igneous Centre. Rock magnetic data reveal an east to west variability in magnetic properties across the intrusion that is also reflected in petrographic and geochemical observations. It is argued that the subtle intraplutonic variations in oxide grain size and composition observed in the Western Granite might be a common feature of shallow epizonal igneous intrusions. As these mineralogical differences are not easily detectable without detailed rock magnetic study, this research underscores the need for comprehensive rock magnetic studies when interpreting the postemplacement history of shallow level plutons. Paleomagnetic data indicate that the eastern part of the intrusion represents the shallower part of the exposed intrusion as further evidenced by roof pendants of Lewisian Gneiss and abundant drusy (miarolitic) cavities indicative of shallow emplacement. In contrast, the western part represents a deeper (∼900 m) part of the intrusion as evidenced by the lack of roof pendants and existence of only minor drusy cavities. Rock magnetic data indicate that the dominant magnetic phase is low-Ti titanomagnetite with some titanomaghemite. Magnetic grain sizes range from pseudosingle domain in the west and multidomain in the east with a slight increase in Ti content of the titanomagnetite phases in the east as revealed by Curie point estimates. We interpret these data to indicate that late stage (subsolidus) hydrothermal deuteric fluids associated with emplacement of the Stage 2 Rum Layered Suite produced secondary growth of Fe-Ti oxide mineral phases along silicate grain boundaries in the eastern part of the intrusion. The secondary oxide growth resulted in a slightly coarser magnetic grain size fraction and an increase in Ti content of the titanomagnetite grains. The deeper structural levels of the western part of the intrusion and its distance from the contact of the Rum Layered Suite inhibited secondary oxide growth, and thus, this part of the intrusion preserves a primary magmatic oxide phase of a smaller magnetic grain size and near pure magnetite chemistry.