Petrogenesis of an Eocene syenitic intrusion from south-central British Columbia: Evidence for increasing influence of cratonic Laurentia on alkaline magmatism of western North America

Jaroslav Dostal, J. Gregory Shellnutt, B. Neil Church

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Abstract

The Allendale Lake stock is a representative intrusion of the Eocene alkaline igneous province in south-central British Columbia (Canada), which forms the northwestern edge of the Montana-Wyoming alkaline province. The shallow-seated intrusion (~53 Ma), about 4.5 km long and 2 km wide, is composed predominately of feldspar-phyric biotite-clinopyroxene-amphibole-bearing monzonitic/syenitic rocks. The feldspar forms distinct rhomb-shaped phenocrysts, which contain abundant exsolution of perthite and antiperthite. The Allendale Lake rocks are silica-undersaturated with SiO 2 ranging from 52 to 63 wt% and high contents of alkalis, particularly K 2 O (4.5–6.5 wt%), typical of shoshonitic rocks. The rocks show a distinct enrichment of large-ion-lithophile elements relative to heavy rare earth elements and high-field strength elements. Their isotopic compositions are highlighted by high negative Ɛ Nd(t) values (−4.5 to −5.5), Neoproterozoic Nd model ages (750–900 Ma) and high but relatively uniform initial 87 Sr/ 86 Sr ratios (~0.706). The rocks were generated by fractional crystallization under oxidizing conditions. The parent magma, a hydrous alkaline basaltic melt, was formed by partial melting of an amphibole-phlogopite-bearing peridotite of the subduction-modified subcontinental lithospheric mantle. The mantle source underwent Neoproterozoic and subsequently Cenozoic metasomatic enrichment events. The alkaline volcanic rocks of the Penticton Group (Marron Formation) from the nearby Eocene Challis-Kamloops volcanic belt are volcanic correlatives of these intrusions. The Eocene alkaline rocks in the south-central part of the British Columbia are related to continental arc rifting.

Original languageEnglish
Pages (from-to)67-82
Number of pages16
JournalLithos
Volume332-333
DOIs
Publication statusPublished - 2019 May 1

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Keywords

  • Alkaline igneous rocks
  • Coryell syenite
  • Eocene
  • Metasomatized mantle source
  • Rifting

ASJC Scopus subject areas

  • Geology
  • Geochemistry and Petrology

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