TEXTURAL AND MICROANALYSIS OF IGNEOUS ROCKS: TOOLS ...

KP has been done via drilling at 11 drillsites during Ocean Drilling Program Legs
119, 120, and 183.
Prior to drilling during Leg 183 a number of workers suggested that continen-
tal crust was involved during the petrogenesis some KP basalts (e.g., [51, 92]).
Drilling at Site 1137 on Elan Bank confirmed hypotheses of continental crust
involvement when clasts of garnet-biotite gneiss were recovered from a fluvial con-
glomerate unit (Unit 6 in Fig. 4.2) [33, 74]. Nicolaysen et al. [111] and Frey
et al. [49] suggested that during the break up of the Gondwana supercontinent
fragments of old continental crust were stranded amongst the Indian Ocean litho-
sphere 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. Ingle et al. [75] placed the Unit 4 basalt in a
relatively uncontaminated upper basalt group and and the Unit 10 basalt in a rel-
atively contaminated lower basalt group. Examination of plagioclase phenocrysts
from Units 4 and 10 provide snapshots of the magmatic processes that were oc-
curring when both when crustal contamination was significant and when it was
not. From this work I suggest that Unit 4 and 10 plagioclase phenocrysts origi-
nated from the shallowest magma chambers of what was likely a complex system
of interconnected dikes and sills. I contend that crustal assimilation was not an
active process in these shallowest chambers but rather pre-dated plagioclase dom-
inated partial crystallization. Crustal assimilation occurred deeper in the crust
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