Contribution to Book
Vertical Stratification of Composition, Density, and Inferred Magmatic Processes in Exposed Arc Crustal Sections
Arc-Continent Collision
(2011)
Abstract
Comparison of exposed arc crustal sections from four ancient magmatic arcs reveals a pattern of depth-specific processes that may be typical of arcs worldwide. These processes include (1) fractionation of mafic and ultramafic cumulates from a mafic parental magma in the uppermost mantle and lowermost crust, (2) subsolidus transformation of mafic plutonic rocks into dense garnet-bearing assemblages in the region of the Moho in thick arc sections (>30 km depth) by isobaric cooling and/or partial melting, (3) dehydration melting of amphibolitized basalt and gabbro (including pre-existing oceanic basement) in the deep to mid crust (15–25 km depth), (4) mingling/mixing of differentiated magmas produced from processes 1 and 3 with mafic mantle-derived magmas in the lower mid crust (15–25 km depth), and (5) increasing homogenization of magmas in mid to upper crustal levels (<20 km depth).
Individual arcs will vary given their unique tectonic settings and petrogenesis. However, these processes may provide a general framework that can be used as an overlay for interpretation of geophysical observations in active arcs and for petrogenetic studies of arc volcanic rocks. The four sections used in the study (correlated to observed processes as listed in the first paragraph) include the Jurassic Talkeetna arc exposed in Alaska (processes 1,2,4,5), the Cretaceous Kohistan arc of northern Pakistan (processes 1,2,3,4,5), the Jurassic Bonanza arc exposed on the west coast of Vancouver Island (processes 3,4,5), and Cretaceous North Cascades crystalline core (southern Coast Plutonic Complex) (processes 3,4,5). Comparable levels of each of these arcs are strikingly similar, both physically and geochemically.
The broader issues of density and geochemical stratification of arcs, as well as their bulk composition and accretion, are critical to an understanding of the growth and compositional evolution of continental crust. The magmas produced by processes (1) and (3) produce the unsubductable nucleus of continental crust. The residual materials of processes (1) and (3), cumulates and restites, produce dense materials that must be recycled back into the mantle. Geochemical evidence for crustal foundering is apparent in the accreted Kohistan arc and Talkeetna arc. In Talkeetna, the paleo-Moho is an abrupt transition between gabbroic rocks (at the garnet-in isograd) and ultramafic rocks. Delamination such as this, plus substantial internal distillation, can result in a more felsic residuum comparable to continental crust.
Keywords
- Continental crust,
- Lower crust,
- Plutonic rock,
- Middle crust,
- Gabbroic rock
Disciplines
Publication Date
June 29, 2011
Editor
Dennis Brown and Paul D. Ryan
Publisher
Springer
Series
Frontiers in Earth Sciences
DOI
10.1007/978-3-540-88558-0_5
Citation Information
S. M. DeBari and A. R. Greene. "Vertical Stratification of Composition, Density, and Inferred Magmatic Processes in Exposed Arc Crustal Sections" Arc-Continent Collision (2011) p. 121 - 144 Available at: http://works.bepress.com/susan-debari/21/