TEXTURAL AND MICROANALYSIS OF IGNEOUS ROCKS: TOOLS ...

Sr - low La/Ce population A parent magmas and high La/Ce - high Sr population
B parent magmas, which is consistent with magma mixing (Fig. 3.12b). A simple
binary 50-50 mixture of two compositional end-members, each shown as a boxed
in X in figure 3.12b, yields a bulk-rock composition (shown as a black star in
figure 3.12b) similar to Unit 14 bulk-rock compositions reported by Huang et al.
[72]. The Unit 14 basalt sample I examined has 20% modal abundance plagioclase,
where ∼ 73% of the plagioclase is from population A and ∼ 27% of the plagioclase
is from population B. From this observation it seems unlikely that the bulk-rock
is the a 50-50 mixture. The range of population A parent magma La/Ce indicates
that not all of the population A crystals had similarly depleted parent magmas.
If I represent population A parent magmas as a single hybrid magma with the
La/Ce and Sr characteristics depicted in figure 3.12b as a boxed in circle, 60% of
this magma mixed with 40% of the same population B end-member used previ-
ously results in a bulk-rock composition shown by the open star in figure 3.12b.
Although this may be an excessivley simple model, it does better reconcile the
population A and B modal abundance data with the compositional data.
Based upon the simple model above, I suggest that the Unit 14 basalt is a
mixture of at least two end-member magmas, one slightly more depleted than the
other. I cannot, however, exclude a role for plagioclase accumulation and partial
resorption as was suggested by Huang et al. [72]. Marsh [98] suggested accumu-
lation and partial resorption of crystalline debris is a more common process than
generally recognized and certainly may have had a role during the genesis of the
magma mixing end-members. Indeed, the rounded nature of Unit population B
crystals indicates partial resorption of plagioclase occurred (Fig.3.7). It is appar-
ent from the large compositional range of the Unit 14 population A crystals that
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