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Multistage andesite genesis in the Garibaldi Lake area, southwestern British Columbia

University of British Columbia

The products of Pleistocene-Recent volcanlsm in the area just north of Mount Garibaldi are hornblende-andesite, hornblende-blotite andesite, and less voluminous two-pyroxene andesite, olivine-basalt, basaltic-andesite and hornblende-daclte. The basic lavas are predominantly alkali-basalt, hy-normative hawaiites and mugearites. Andesites and associated intermediate rocks have a calc-alkaline affinity, and show strong enrichment in Sr (700-1400 ppm) and Ba (400-800 ppm), further emphasizing the major compositional gap between these lavas and contemporaneous basalts. All lavas display depletion in Rb (4-25 ppm), low Sr⁸⁷/Sr⁸⁶ ratios (0.7028-0.7036) and high K/Rb ratios (500-1100) relative to typical orogenic volcanic series. Cheakamus Valley basalts contain phenocrysts of olivine, clinopyroxene and plagioclase. Garibaldi Lake basaltic-andesites have similar phenocrysts, but also contain amphibole and titanomagnetite. In the Sphinx Moraine basaltic-andesite, crystal clots of diopsidic augite and hornblende, and amphibole megacrysts enclosing ragged grains of olivine (Fo[sub 87-80]) suggest that amphibole and clinopyroxene co-precipitated from more mafic parental magma, with early-formed olivine in reaction relationship with the liquid. Calculated phenocrystsequilibration pressures range from 4.5 to 11 kb and average 7.5 kb (~25 km), indicating lower crustal crystallization. Estimated f[sub H₂O] for hornblende-bearing lavas suggest phenocryst equilibration under P[sub H₂O] < P[sub Total], with X[sup Liq] [sub H₂O] ~ 0.09-0.15 (3-5 wt. %). The phenocryst mineralogy of The Table (PL + AMPB + OPX + MT), Mount Price (PL + AMPB + BO + OPX + MT), and The Black Tusk (PL + CPX + OPX + MT) silicic-andesites demonstrates, that chemically distinct magma batches crystallized under different P (2-5 kb) , T (825-975°C) and P[sub H₂O] conditions. The low K/Na and Fe/Mg ratios, strong enrichment of Sr and Ba, and marked depletion in Rb and V argue against equilibrium melting of quartz eclogite within subducted oceanic crust. The andesites have Mg/(Mg + Fe²⁺) values and Ni and Cr contents too low for these lavas to represent unmodified partial melts of mantle peridotite. The results of equilibrium calculations, however, suggest that the parental magmas of the andesites could have equilibrated with peridotitic residuum at 1150-1235°C and 15-20 kb. The evolution of Garibaldi Lake andesite magmas can only be explained by a multistage model. It involves: (1) varying degrees of partial melting of wet peridotite above the Benioff zone to produce water-undersaturated tholeiite magmas; (2) high-pressure (>5 kb) fractionation of olivine ± clinopyroxene ± amphibole ± Cr-spinel from the basic melts at or near the base of the crust; and (3) low-pressure (2-5 kb) segregation of plagioclase ± amphibole ± biotite + pyroxene + magnetite from more intermediate compositions.

Text

2010-03-05

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http://hdl.handle.net/2429/21529

Geological Sciences

University of British Columbia

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