Journal
LITHOS
Volume 127, Issue 1-2, Pages 282-297Publisher
ELSEVIER
DOI: 10.1016/j.lithos.2011.07.026
Keywords
Chugach Metamorphic Complex; Amphibolite; Intra-oceanic island arc; Subduction; Accretionary prism
Categories
Funding
- FWF [P19366-N10]
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A 600 km long metabasite belt is exposed at the southern border of the Chugach terrane in southern Alaska, south of the Eocene Chugach Metamorphic Complex (CMC). In this contribution, we present petrologic and geochemical results for parts of this metabasite belt. The metabasites studied are amphibolite grade and their PT conditions are evaluated with hornblende-plagioclase thermometry and the average PT method. From west to east the peak metamorphic conditions calculated are: about 730-793 degrees C for pressures between 5 and 15 kbar in the westernmost part, about 740-760 degrees C and 5 kbar in the middle locality and about 640-675 degrees C and 8 kbar in the easternmost locality. These results are comparable with the metamorphic conditions obtained on metapelite of the CMC for the westernmost and easternmost localities. In contrast, in the central part of the CMC, the metabasites experienced probably lower pressures than the metapelites to the north. Rare earth and trace element patterns of the metabasite belt are comparable with typical altered basalt patterns and reveal MORB and arc-tholeiitic geochemical characteristics. The presence of Ba and U anomalies are interpreted as a result of alteration prior to subduction, the Pb anomaly as a result of an intra-oceanic island arc signature and the Sr anomaly as a result of the interaction of sediments with the metabasites during subduction. We suggest that the association of MORB and arc tholeiitic rocks in the metabasite belt is likely derived from an intra-oceanic island arc which accreted to the Alaskan margin. (C) 2011 Elsevier B.V. All rights reserved.
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