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Volcanic stratigraphy and petrology of the mid-cretaceous Spence Bridge Group near Kingsvale, southwestern British Columbia Thorkelson, Derek John


Mid-Cretaceous volcanic rocks called the Spences Bridge and Kingsvale groups by previous authors lie in the Nicoamen Structural Depression which extends from near the town of Princeton for 215 km north-northwestward to the settlement of Pavilion where it is cut by the Fraser Fault System. The lower part of the sequence, comprising various volcaniclastic rocks and lavas, was formally named the Spences Bridge Group after a type locality near the town of Spences Bridge. The rocks subsequently defined as Kingsvale Group are now recognized as being part of the succession originally called Spences Bridge Group; the term Kingsvale Group is abandoned. A distinctive sequence of mostly amygdaloidal andesite, overlying the Spences Bridge rocks between Kingsvale and Spences Bridge, was correlated by previous workers with the Kingsvale Group; those strata are herein formally named Spius Formation. The Spences Bridge Group is expanded to include Spius Formation as its upper part; the composite sequence previously known as the Spences Bridg Group is now called the lower Spences Bridge Group. Recent studies of palynomorphs and plant megafossils indicate an upper Albian age for both the lower Spences Bridge Group and Spius Formation. In the study area, block faulting, probably related to transtensional tectonics in Eocene time, was the main cause of deformation of the Spences Bridge Group. A north-trending complex graben with vertical displacements of at least 3 km, herein called the Fig Fault Zone, was apparently filled as it formed by Eocene river sand and gravel. Folding, some of which may have been contemporaneous with volcanism, was mainly restricted to the vicinity of Shovelnose Mountain which displays an overall anticlinal geometry. The lower Spences Bridge Group in the study area was deposited on a surface of moderate relief underlain by Late Triassic and Early Jurassic volcanic and plutonic rocks. It consists of roughly equal volumes of volcaniclastic rocks and lava which are intercalated throughout the 2.4 km-thick succession. Andesite flows and clastic rocks are ubiquitous; rhyolite is laterally restricted, probably to near-vent areas. Regionally, this group was probably deposited as a set of contiguous terrestrial stratovolcanoes. Fractionation and assimilation can account for petrographic and geochemical observations. Patterns of trace elements and a calcalkaline to weakly tholeiitic trend suggest a subduction-related origin. Spius Formation in the study area, with a minimum thickness of 600 m, shares a gradational contact with the lower Spences Bridge Group. To the northwest, the stratigraphic relationship is unconformable indicating deformation during a local hiatus in volcanism. Spius flows were probably very fluid, accumulating as a shield volcano atop lower Spences Bridge and adjacent basement rocks. Petrographically, two Spius Formation lava types are recognized. The first, lower Spences Bridge-type, is very similar to andesite of the lower Spences Bridge Group. The second, Spius-type, was apparently hotter and more hydrated. Trace element geochemistry supports petrography and suggests that Spius Formation lavas are the product of variable magma mixing between melts belonging to the lower Spences Bridge Group and an inferred parental Spius-type magma. The composition of that parental magma suggests that primary Spius-type melts were produced by "within plate" fusion. The change in tectonic affinity may have been caused by disrupted and/or terminated subduction induced by mid-Cretaceous accretion of Terrane II to the North American continental margin.

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