HR: 1340h
AN: V33A-1153 [Abstracts]
TI: Significance of Picritic and Tholeiitic Lavas Within Wrangellia Flood Basalts on Vancouver Island for the Melting History and Magmatic Evolution of a Major Oceanic Plateau
AU: * Greene, A
EM: agreene@eos.ubc.ca
AF: EOS, University of BC, Vancouver, BC V6T1Z4, Canada
AU: Scoates, J S
EM: jscoates@eos.ubc.ca
AF: EOS, University of BC, Vancouver, BC V6T1Z4, Canada
AU: Weis, D
EM: dweis@eos.ubc.ca
AF: EOS, University of BC, Vancouver, BC V6T1Z4, Canada
AU: Nixon, G T
EM: graham.nixon@gems8.gov.bc.ca
AF: B.C. Geological Survey, PO Box 9320, Victoria, BC V8W9N3, Canada
AB:
Geochemical studies of lavas from the accreted Wrangellia oceanic plateau forming a large part of Vancouver
Island (~20,000 km2) in the Pacific Northwest of North America offer a view of the melting history of
plume-derived magmas that does not involve continental lithosphere and where source heterogeneity does not
play a major role. The Late Triassic Wrangellia flood basalts (~229-226 Ma) are predominantly
homogeneous tholeiitic basalt. However, the lower submarine part of the 6 km-thick stratigraphy, on northern
Vancouver Island, contains picritic pillow basalts. These high-MgO (9-20 wt%) lavas are depleted in LREE
(La/YbCN= 0.5 ± 0.2), whereas the tholeiitic lavas (6-8 wt% MgO) are LREE-enriched
(La/YbCN= 2.2 ± 0.3). Both lava groups have overlapping initial εHf (+10.3 ± 2.1) and
εHf Nd (+7.7 ± 1.3), indicating a common, depleted (but not MORB) Pacific mantle source similar to
the source of basalts from the Ontong Java and Caribbean Plateaus. The presence of picritic lavas indicates
melting of anomalously hot mantle and, along with the high degree of melting and high melt production rate,
provides strong evidence for a mantle plume origin for this oceanic plateau. Estimated primary melts, using
PRIMELT1 [Herzberg et al. (2007) G3], contain 15-17 wt% MgO and ~10 wt% CaO and they represent
23-27% melting of peridotite with a potential temperature of ~1490°C. Differences in trace element
signatures between the high-MgO and tholeiitic lavas primarily reflect differences in depth of melting. The high-
MgO lavas formed without involvement of garnet, whereas the tholeiitic basalts involved melting of both garnet
and spinel lherzolite. The evolved tholeiitic basalts underwent significant fractional crystallization (>50%)
and the fractionated residues are likely represented by high-velocity rocks beneath Vancouver Island identified
from seismic reflection studies [Clowes et al. (1995) CJES].
DE: 8410 Geochemical modeling (1009, 3610)
DE: 8415 Intra-plate processes (1033, 3615)
DE: 8425 Effusive volcanism
DE: 8427 Subaqueous volcanism
SC: Volcanology, Geochemistry, Petrology [V]
MN: 2007 Fall Meeting