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- Title
New insights on the origin of troctolites from the breakaway area of the Godzilla Megamullion (Parece Vela back-arc basin): The role of melt-mantle interaction on the composition of the lower crust.
- Authors
Sanfilippo, Alessio; Dick, Henry J. B.; Ohara, Yasuhiko; Tiepolo, Massimo
- Abstract
The troctolites and olivine-gabbros from the Dive 6 K-1147 represent the most primitive gabbroic rocks collected at the Godzilla Megamullion, a giant oceanic core complex formed at an extinct spreading segment of the Parece Vela back-arc basin (Philippine Sea). Previous investigations have shown that these rocks have textural and major elements mineral compositions consistent with a formation through multistage interaction between mantle-derived melts and a pre-existing ultramafic matrix. New investigations on trace element mineral compositions basically agree with this hypothesis. Clinopyroxenes and plagioclase have incompatible element signatures similar to that of typical-MORB. However, the clinopyroxenes show very high Cr contents (similar to those of mantle clinopyroxene) and rim having sharply higher Zr/REE ratios with respect to the core. These features are in contrast with an evolution constrained by fractional crystallization processes, and suggest that the clinopyroxene compositions are controlled by melt-rock interaction processes. The plagioclase anorthite versus clinopyroxene Mg#[Mg/(Mg + FeTot)] correlation of the Dive 6 K-1147 rocks shows a trend much steeper than those depicted by other oceanic gabbroic sections. Using a thermodynamic model, we show that this trend is reproducible by fractionation of melts assimilating 1 g of mantle peridotite per 1 °C of cooling. This model predicts the early crystallization of high Mg# clinopyroxene, consistent with our petrological observation. The melt-peridotite interaction process produces Na-rich melts causing the crystallization of plagioclase with low anorthite component, typically characterizing the evolved gabbros from Godzilla Megamullion.
- Subjects
PHILIPPINE Sea; PLAGIOCLASE; PERIDOTITE; CRUST-mantle model; GEOCHEMISTRY; REGOLITH
- Publication
Island Arc, 2016, Vol 25, Issue 3, p220
- ISSN
1038-4871
- Publication type
Article
- DOI
10.1111/iar.12137