TEXTURAL AND MICROANALYSIS OF IGNEOUS ROCKS: TOOLS ...

4.5.0.4 Petrogenetic Insights from Plagioclase 87 Sr/ 86 SrI Ratios and the Timing
of Crustal Contamination at Elan Bank
Plagioclase crystals from Units 4 and 10 exhibit narrow ranges of 87 Sr/ 86 SrI
that overlap their respective Site 1137 upper and lower group basalts reported
by Ingle et al. [75], which indicates that crustal assimilation was not an ongoing
process during plagioclase dominated partial crystallization. Crustal rocks (e.g.,
garnet biotite gneiss clasts) recovered during drilling at Site 1137 are plausible
crustal assimilants and have distinctly radiogenic 87 Sr/ 86 SrI (> 0.784) [74]. If
these in fact were the types of crustal rocks assimilated, as favored by Ingle [75],
assimilation must have occurred prior to plagioclase dominated partial crystal-
lization. Core, intermediate zone, and rim 87 Sr/ 86 SrI variations are subtle, which
indicates that the Unit 4 and 10 plagioclase crystals grew from already contami-
nated parent magmas. This may also suggest that over time the magma chamber
walls became well armored, which minimized late stage pulses of greater crustal
assimilation.
4.5.0.5 Where Did Plagioclase Dominated Partial Crystallization Occur?
In their study of Kerguelen Archipelago lavas Damasceno et al. [36] sug-
gested that basalts that make up the Mont Crozier section passed through a
magmatic system that consisted of a complex arrangement interconnected con-
duits and chambers. They suggested that plagioclase-dominated partial crystal-
lization occurred primarily in shallow sub-volcanic magma reservoirs [36]. Elan
Bank magmas passed through a section of crust with both oceanic and continental
components (versus oceanic crust for Mont Crozier magmas) [36], and it is thus
probable Elan Bank magmas passed through similarly complex magmatic system.
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