The plutonic rocks of Vancouver Island, British Colombia: their petrography, chemistry, age and emplacement
Authors: Carson D J T 
Published: April 2014

Most plutons of Vancouver Island were emplaced between mid-Early and mid -Late Jurassic (approximately 150 to 180 million years ago), The majority of these we re intruded soon after an Early(?) to Middle Jurassic interval of broad regional folding. The Jurassic
plutons are granodiorite and quartz diorite, with minor diorite, gabbro, and quartz monzonite, and rare granite. Most are upper mesozonal and are medium grained with granitic texture, but some epizonal porphyries and catazonal(?) gneisses are also present. Early to Middle Tertiary epizonal plutons
(approximately 35 to 60 million year s) are much less abundant than Jurassic plutons. They were emplaced during part of an extended interval of block faulting and are quartz diorite, dacite porphyry, and gabbro, with minor granodiorite and quartz monzonite. Limited evidence suggests that some
dioritic gneisses and quartz monzonite on the west coast may have crystallized during the Late Cretaceous. The plutonic rocks of Vancouver Island may be divided into three northwest-trending zones of differing compositions and textures. Quartz diorites and granodiorites low in K-feldspar predominate
in the structurally elevated axial area. They are flanked along the east and northeast by a zone containing granodioritic plutons, most of which are relatively homogeneous, and along the west and southwest by another zone containing a wider variety of plutonic rocks that range in composition from
granite (very minor) to peridotite (rare ), but have an average granodioritic composition similar to that of the eastern zone. Plutonic gneisses, and complex plutons with extensive border migmatite zones, are common only in the western belt, This over all northwest-trending textural - compositional
zoning is somewhat similar to zoning of the plutonic rocks of the Coast Intrusions of mainland British Columbia. Jurassic plutons occur throughout Vancouver Island but the known Tertiary plutons are restricted to the western belt and to two zones that cross the northwesterly structural trend of the
island. The chemistry of the plutons of known Jurassic age differs some what from that of the Tertiary plutons although there is some overlap and they appear to have been derived from similar parental material. Jurassic differentiation trends show similarities to those of the southern California
batholith. The known Jurassic plutons contain more Ca and Fe, and possibly K, and less Na than the Tertiary plutons. These differences may have been the results o f greater assimilation of host rocks combined with more advanced stages of differentiation by the deeper- seated Jurassic plutons. During
emplacement, many Jurassic plutons advanced passively upward in competent volcanic rocks but spread laterally and forcibly in less competent sedimentary rocks. The net effects were the strong deformation of host rocks, the accumulation of magma, and the deposition of abundant Cu-Fe skarn deposits
(Zeballos Iron, Coast Copper, etc.) at the stratigraphic interval that included the uppermost Karmutsen Formation, the Quatsino limestone and the lower Bonanza Formation. Many of the intrusive bodies, including some of batholithic proportions, assumed roughly horizontal and sheetlike forms.
High-level offshoots of these bodies intruded upward into the Bonanza Formation and many reached the surface, supplying volcanic material to the upper Bonanza Formation. Some of these offshoots have associated porphyry copper deposits (Island Copper, etc.). Most Tertiary stocks and dyke-like bodies
were forcibly emplaced in subvolcanic environments. Intrusive and explosive breccias are common, Some of the Tertiary plutons are associated with porphyry copper deposits (Catface, etc.) and copper or gold veins (Mount Washington, Privateer, etc.). The Mount Washington intrusive complex consists of
a centrally located quartz diorite stock with a peripheral diatreme and other irregularly shaped explosion breccias, and radiating sills and laccolithic tongues of dacite porphyry intruding gently-dipping Late Cretaceous sediments of the Nanaimo Group that surround the stock. Extensive dacite
porphyry laccoliths and sills were emplaced at several other localities by lateral extension of stocks and dykes that penetrated upward into the lower Nanaimo Group. Large sill-like(?) gabbro plutons cut the Eocene Metchosin volcanics in southern Vancouver Island, where as smaller trondhjemite
plutons of the same area are stocks.