TEXTURAL AND MICROANALYSIS OF IGNEOUS ROCKS: TOOLS ...

4.5 Discussion
4.5.0.1 Origin of Major Element Zoning and Resorption Surfaces
Abrupt changes of 5 to > 10 mole % An and the presence of resorption sur-
faces amongst Unit 4 and 10 plagioclase phenocrysts indicates these crystals were
variably exposed to magmas with which they were at a state of disequilibrium
(e.g., Fig. 4.6, 4.7, Fig. 4.8, Fig. 4.9, Fig. 4.10). Injection of hot fresh magma
into a crystallizing magma chamber can lead to partial resorption and subsequent
growth of higher An zones, which is a feature that was noted for select crystals
from each Unit (e.g., Fig. 4.6a,e,f,p, Fig. 4.7). An alternative scenario is that mo-
bilization of crystals from mush piles within the magma chamber by convective
overturn, avalanches of wall and roof debris, or magmatic stirring generated these
features [83, 98]. Crystals growing in such mush layers are thermally and me-
chanically insulated from the hottest portions of the magma chamber and can be
out of equilibrium with magma in the chamber interior. Either of these processes
may have operated to generate some or all of the observed dissolution and zoning
patterns in Unit 4 and 10 plagioclase crystals. There are, however, systematic
differences in the dominant zoning patterns between the Unit 4 and 10 basalt
samples examined in this study.
The Unit 4 basalt examined in this study contains a fairly even mixture or
normal and reverse zoned crystals, which is a first order indication that crystals
from different portions of the magmatic system were entrained during or before
eruption. Deposition of plagioclase-rich crystal debris at one or more locations
(walls, floors, etc.) in the magmatic system prior to eruption of the Unit 4 basalt
is supported by the presence of plagioclase glomerocrysts, where after deposition
the crystals were partially compacted to later be mobilized as cognate clusters of
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