Article
Mass balance calculations for two sections of island arc crust and implications for the formation of continents
Earth and Planetary Science Letters
(1990)
Abstract
The bulk compositions of two exposed sections of intra-oceanic island arc crust (the Talkeetna volcanics and Border Ranges ultramafic-mafic complex in southeastern Alaska, and the Canyon Mountain Complex in northeastern Oregon) represent basalt (MgO = 11%) and probably basaltic andesite (MgO = 8%), respectively. Both have REE abundances approximately 10 times chondrite, with no LREE-enrichment. These compositions, determined by mass balance based on the relative thicknesses of lithologic units in the exposed sections, are reasonable parental magma compositions based on Fe/O*MgO and Ni contents of olivine in the basal crustal cumulate rocks and the lack of significant Eu anomalies. Accretion of arcs such as these is inappropriate as a sole mechanism for generating post-Archean continental crust, since the latter is significantly LREE-enriched and is generally considered to be andesitic. Thus, if these arcs represent a major ingredient in Proterozoic and younger continental crust, their bulk compositions need to be modified. A combination of processes may be invoked, including (a) delamination of basal cumulates to make the sections less mafic, (b) addition of an alkalic component to enrich the crust in LREE and other lithophile elements, and (c) partial melting of the lower crust combined with delamination of the residue to do both. The basal parts of these crustal sections represent potential mantle heterogeneities.
Keywords
- Intra-oceanic island arc crust,
- Talkeetna volcanics,
- Border Ranges complex,
- Canyon Mountain Complex
Disciplines
Publication Date
January 1, 1990
DOI
10.1016/0012-821X(90)90018-S
Publisher Statement
Published by Elsevier B.V.
Citation Information
Pearcy, L. G., S. M. DeBari, and N. H. Sleep (1990) Mass balance calculations for two sections of island arc crust, and implications for the formation of continents, Earth and Planetary Science Letters, v. 96, p. 427-442.