Magmatic flow and emplacement foliations in the Early Jurassic Aishihik Batholith, Southwest Yukon; implications for northern Stikinia

Foliated intrusions of the Early Jurassic Aishihik Plutonic Suite (APS), including the Aishihik Batholith, have been included in Stikinia and interpreted as allochthonous with respect to adjacent terranes, including the Nisling and Yukon-Tanana Terranes. The Nisling Terrane was thought to lack Early Jurassic igneous rocks. However, the Aishihik Batholith, a single plutonic body that crystallized at ca. 187 Ma, forms a west-tapering lopolith or sheet-like body that intrudes deformed strata of the Nisling Terrane. The batholith displays a margin-parallel foliation, defined by primary magmatic grains including feldspar and hornblende, that is considered to be magmatic. A parallel solid-state fabric overprints the magmatic foliation and fabric in wall rocks within 100 m of the batholith along the west (lower) margin of the batholith. This fabric is defined by gneissic banding, annealed mylonite, and by discrete shear bands. Shearing occurred at high temperatures, probably close to the granite solvus, as indicated by the breakdown of hornblende to biotite, the recrystallization of plagioclase feldspar, and by associated migmatite. Shear indicators are consistent with top-to-the-west displacement. The solid-state fabric postdates peak regional deformation of the Nisling Terrane and is inferred to have developed during late stage ballooning of the intrusion. A model of intrusion of the Nisling Terrane by the Aishihik Batholith, with subsequent ballooning of the batholith, is consistent with the lopolithic shape of the batholith, the distribution of solid-state fabrics, the shear sense and near-solvus temperatures during solid-state deformation, the presence of xenoliths similar to that of the Nisling Terrane in the batholith, and the development, in the Nisling Terrane, of a hot-side-up aureole beneath the batholith. Because the Aishihik Batholith intrudes the Nisling Terrane, (1) the APS cannot be considered diagnostic of Stikinia, and (2) the Nisling Terrane cannot be considered as lacking Early Jurassic igneous rocks. The APS may represent part of an igneous overlap assemblage that links together terranes of the Intermontane belt. Alternatively, Early Jurassic intrusions may have developed in response to the subduction of oceanic crust separating some of the Intermontane terranes.