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Constraints on the composition of the Aleutian arc lower crust from V P / V S
Author(s) -
Shillington Donna J.,
Van Avendonk Harm J. A.,
Behn Mark D.,
Kelemen Peter B.,
Jagoutz Oliver
Publication year - 2013
Publication title -
geophysical research letters
Language(s) - English
Resource type - Journals
SCImago Journal Rank - 2.007
H-Index - 273
eISSN - 1944-8007
pISSN - 0094-8276
DOI - 10.1002/grl.50375
Subject(s) - crust , geology , arc (geometry) , island arc , oceanic crust , ultramafic rock , quartz , back arc basin , continental crust , geochemistry , subduction , seismology , tectonics , geometry , paleontology , mathematics
Determining the bulk composition of island arc lower crust is essential for distinguishing between competing models for arc magmatism and assessing the stability of arc lower crust. We present new constraints on the composition of high P ‐wave velocity ( V P = 7.3–7.6 km/s) lower crust of the Aleutian arc from best‐fitting average lower crustal V P / V S ratio using sparse converted S ‐waves from an along‐arc refraction profile. We find a low V P / V S of ~1.7–1.75. Using petrologic modeling, we show that no single composition is likely to explain the combination of high V P and low V P / V S . Our preferred explanation is a combination of clinopyroxenite (~50–70%) and alpha‐quartz bearing gabbros (~30–50%). This is consistent with Aleutian xenoliths and lower crustal rocks in obducted arcs, and implies that ~30–40% of the full Aleutian crust comprises ultramafic cumulates. These results also suggest that small amounts of quartz can exert a strong influence on V P / V S in arc crust.