TEXTURAL AND MICROANALYSIS OF IGNEOUS ROCKS: TOOLS ...

nent fragments of old continental crust were stranded amongst the Indian Ocean
lithosphere including fragments within the KP crust.
One of the scientific objectives of Leg 183 drilling was to constrain the post-
melting evolution of Kerguelen magmas, of which crustal assimilation was clearly
an important process. In this study I explore the timing and nature of crustal con-
tamination of KP magmas by focusing on the compositional and isotopic record of
assimilation contained within zoned plagioclase phenocrysts in two basalts from
Leg 183 Site 1137 - Units 4 and 10. On the basis of whol-rock geochemistry, Ingle
et al. [75] placed the Unit 4 basalt in a relatively uncontaminated upper basalt
group and and the Unit 10 basalt in a relatively contaminated lower basalt group.
Examination of plagioclase phenocrysts from Units 4 and 10 provide snapshots of
the magmatic processes that were occurring when both when crustal contamina-
tion was significant and when it was not.
Hypothesis 1: Initial LIP magma emplacement in the crust was accompanied
by large amounts of crustal assimilation followed by armoring of magma chamber
walls. Crystallization occurred by solidification front growth. In this case early
formed plagioclase crystals or zones (i.e., cores) will contain the greatest signa-
ture of the crustal wall rocks, however the contamination signature in the crystal
cores may be indistinct if contamination significantly pre-dates plagioclase crys-
tallization. Late formed crystals or zones (i.e., rims) will contain less of a crustal
signature. Hypothesis 2: Crustal assimilation occurred progressively as crystals
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