Magmatic flow and emplacement foliations in the Early Jurassic Aishihik Batholith, southwest Yukon: Implications for northern Stikinia

Abstract

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-TananaTerranes. 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-tothe-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.

Mesozoic magmatism and timing of epigenetic Pb-Zn-Ag mineralization in the western Fortymile mining district, east-central Alaska: Zircon U-Pb geochronology, whole-rock geochemistry, and Pb isotopes

“…197 Ma intrusions and one 188 Ma intrusion (Table 5), indicating emplacement of the host plutons at mesozonal crustal depths of >15 km (Werdon et al, 2001;Day et al, 2002;. Magmatic epidote also occurs in the synkinematic and synmetamorphic Early Jurassic (186.0 ± 2.8 Ma) foliated Aishihik batholith that intrudes rocks equivalent to the YTT in southwest Yukon (Johnston and Erdmer, 1995).…”

Metamorphism at a late Mesozoic accretionary margin: a study from the Coast Belt of the North American Cordillera

“…2). To the north and northeast, the Ruby Range Batholith (RRB), a sill-like intrusion of Eocene age overlying the KMA (Erdmer & Mortensen, 1993), separates the KMA from the Aishihik metamorphic suite (AMS), a continental assemblage of the Nisling terrane (Johnston & Erdmer, 1995b;Johnston et al, 1996). The pluton is largely ®ne-to medium-grained hornblende±biotite granodiorite, with minor quartz diorite and tonalite (Mezger, 1997).…”

“…8. The geology surrounding the KMA is modi®ed after Kindle (1952), Muller (1967), Tempelman-Kluit (1974), Wheeler & McFeely (1991), Dodds & Campbell (1992) and Johnston & Erdmer (1995b). RRB (Muller, 1967;Erdmer, 1991), and the annealing of foliation and lineation in schist near the contact.…”

Large-scale coast-parallel displacements in the Cordillera: a granitic resolution to a paleomagnetic dilemma

“…The main planar fabric in the metamorphic rocks of the Yukon±Tanana terrane, consists of an Early Jurassic schistosity (Johnston and Erdmer, 1995) that dips homoclinally north to north-east throughout much of the Dawson Range (Johnston, 1995). A lineation de®ned by quartz rods, and aligned, elongate and stretched minerals pitches steeply, plunging regionally north to northeast (Tempelman-Kluit, 1974;Johnston and Erdmer, 1995).…”

“…A lineation de®ned by quartz rods, and aligned, elongate and stretched minerals pitches steeply, plunging regionally north to northeast (Tempelman-Kluit, 1974;Johnston and Erdmer, 1995). However, lineations from wallrocks adjacent to, and from screens within the batholith, trend dominantly east-southeast, parallel with the long axis of the batholith (Payne et al, 1987;Johnston, 1995).…”