Eocene shoshonitic mafic dykes intruding the Monashee Complex, British Columbia: a petrogenetic relationship with the Kamloops Group volcanic sequence?

Abstract

The newly named Three Valley Suite (TVS) kersantite lamprophyre to shoshonitic mafic dykes of the Monashee Complex are inferred to be hypabyssal feeder dykes to an alkaline to calc-alkaline volcanic suite related to the Kamloops Group. These dykes were emplaced in a subvertical north-trending orientation coincident with inferred Eocene crustal extension (~50.0 Ma), based on the flat 40Ar/39Ar step-heating plateau of contact-metamorphic muscovite on the margin of a TVS dyke. These weakly altered mafic dykes are fine grained with phenocrysts (0.5–2.0 mm) of phlogopite, augite, amphibole, and olivine (pseudomorphed by clays), rare labradorite, and both primary and secondary carbonates set in a fine-grained groundmass of similar mineralogy consistent with their classification as plagioclase-bearing potassic diorite to kersantite lamprophyre. The dykes are weakly silica-undersaturated and alkalic (2.8 wt.% K2O, 7.7 wt.% MgO), with high large ion lithophile element contents (~300 times primitive mantle) and elevated high-field-strength element contents, with a prominent negative Nb (Ta) anomaly, and have radiogenic Nd and Sr isotopic signatures; these geochemical attributes are consistent with a calc-alkaline shoshonitic affinity. Therefore, it is inferred that the subducting oceanic plate influenced subcrustal mantle wedge metasomatism in the region. Decompression partial melting of this metasomatised lithospheric mantle was initiated by coupled rapid unroofing, regional trans pression, slab rollback, and slab window development to the south. The TVS is similar to the mafic volcanic rocks within the nearby Eocene volcanic rocks, suggesting that these dykes represent the feeder system to a volcanic field that is now eroded, i.e., a broad-terrane association.

Brittle faulting in the Thor–Odin culmination, Monashee complex, southern Canadian Cordillera: constraints on geometry and kinematics

“…The dykes give K-Ar ages of 51 ± 1.8 and 55.3 ± 1.9 Ma, which are interpreted to represent the timing of Ar closure during uplift. Adams et al (2005) report 40 Ar/ 39 Ar analyses for lamprophyre dykes in the study area (Three Valley Gap) that give ages of 51-55 Ma, in agreement with earlier work. A countryrock muscovite from the contact of a dyke yielded an age of 48.4 ± 0.3 Ma, which they interpret as the contact metamorphic age and thus emplacement age of the dyke.…”

Deconstructing the Infrastructure: A Complex History of Diachronous Metamorphism and Progressive Deformation during the Late Cretaceous to Eocene in the Thor-Odin–Pinnacles Area of Southeastern British Columbia

“…The P-T and geochronology data do not necessarily represent data from exactly the same rocks, but they are representative of the same general areas. Data from Read and Wheeler 1976;Archibald et al 1983;Parrish and Wheeler 1983;Parrish and Armstrong 1987;Parrish et al 1988;Coleman 1990;Carr 1990Carr , 1991aCarr , 1992Carr , 1995Smith and Gehrels 1992;Colpron et al 1996;Vanderhaeghe et al 1999;Johnston et al 2000;Norlander et al 2002;Kuiper 2003;Gibson et al 2004;Adams et al 2005;Glombick 2005;Lemieux 2006; this study. grade metasedimentary and metavolcanic rocks (Carr 1991a).…”

“…This extension was characterized by magmatism, north-south striking, shallow to moderately dipping ductile-brittle extensional detachments with fault traces between 100 and 200 km long (Fig. 1); steep brittle north-south striking fault systems; and tectonic exhumation of rocks that were in the mid-crustal infrastructure in the Eocene (Parrish et al 1988;Carr 1991a;Johnson and Brown 1996;Adams et al 2005;Johnson 2006; Kruse and Williams 2007). At the latitude of the Thor-Odin dome (Fig.…”

“…A number of volumetrically significant igneous units of Mesozoic to Eocene age occur in the study area. Some of the important igneous rocks include the Jurassic Nelson suite (Parrish and Armstrong 1987), the Cretaceous Whatshan batholith (Carr 1992), the Paleocene to Eocene Ladybird granite (Carr 1992; and the Eocene Three Valley lamprophyre suite (Adams et al 2005). The Jurassic to Eocene intrusive rocks mark major periods of igneous activity in the Canadian Cordillera (Parrish et al 1988;Gabrielse et al 1991;Parrish 1995) and, taken with field relationships, they are important as strain markers and in providing geochronological constraints on timing of deformation.…”

Effects of Paleogene faults on the reconstruction of the metamorphic history of the northwestern Thor-Odin culmination of the Monashee complex, southeastern British Columbia

“…The latest deformational event (D T+2 of Spalla et al, 2011) is characterized by shear bands in and around the Monashee complex, including a zone of concentrated extension, the westerly dipping Greenbush shear zone, along the NW flank of the Thor-Odin culmination (Fig. 2;Johnston et al, 2000), and by later brittle normal and/or dextral faults (Kruse and Williams, 2005 www.gsapubs.org | Volume 7 | Number 3 | LITHOSPHERE cal Eocene pegmatite (e.g., Johnston et al, 2000) and lamprophyre dikes (e.g., Lane, 1984;Adams et al, 2005) intruded during extension.…”