Conceptual model
The SMB is comprised of a patchwork of sills emplaced over ca. 2 Ma
(Claiborne et al. , 2006, 2010, Miller et al. , 2011, Walker
Jr et al. , 2007). Field relations, regional geology, and the
plagioclase results presented in this study suggest that the sills that
comprise the SMB crystallized from similar parental magma compositions.
The conceptual model we invoke to explain the origins of Spirit Mountain
batholith granites is mechanical phase separation by
repacking-accommodated compaction (Bachmann & Huber, 2019, Boyeret al. , 2011, Holness et al. , 2017), a process likely
repeated over periods of as much as several million years (emplacement
of a sill like body of parental magma and subsequent crystallization and
then compaction). Numerical models of compaction often consider the
evolution of porosity or melt fraction of a continuous crystal column
with an initially uniform melt fraction distribution (Bercovici et
al. , 2001, Huber & Parmigiani, 2018, McKenzie, 1984, McKenzie, 1985,
McKenzie, 2011, Ricard et al. , 2001). Melt and crystals separate
due to their density contrast, which at melt fractions above the maximum
packing fraction (minimum melt fraction that can be obtained by particle
reorganization) is likely accommodated by a combination of hindered
settling and the rotation and translation of matrix crystals (repacking)
(Bachmann & Huber, 2019, Boyer et al. , 2011, Holness et
al. , 2017, Huber et al. , 2024). At any depth within the
compacting layer, each sample can therefore be thought of as the sum of
different proportions of melt and crystal components; proportions are
controlled by melt-crystal separation, here driven by gravity. Under
this idealized model, at depths nearest to the base within a single sill
within the overall batholith, the driving force for phase separation is
greatest and a layer depleted in trapped melt develops. At shallower
depths the driving force for separation diminishes and less melt is
lost. Meanwhile, at the top of the sill a melt rich layer develops. A
consequence of this in the context of the SMB is the enrichment of CaO,
Al2O3, Ba, and Sr, consistent with
accumulation of feldspars in samples nearest to the base of the sills
emplaced that comprise the pluton. Meanwhile, samples furthest from the
base of a given sill within the pluton are SiO2 rich and
enriched in incompatible trace elements like Rb and Th, consistent with
melt accumulation. Samples at intermediate depths, meanwhile, display
chemical compositions intermediate to the two endmembers. Visually, this
is manifested by intermediate samples plotting along mixing (or more
precisely unmixing ) tie-lines between melt and crystal endmembers
located on either side of the parental composition (Payacán et
al. , 2023). On the other hand, unmixing between melt and crystal
compositions by phase separation occurring over a range of
MC, crystallized mass, (different points along
equilibrium liquid and solid lines of descent) results in a wedge
pattern on either side of the parental magma composition in major and
trace element space and can explain the fact that not all samples
collapse onto the same straight line (Fig. 4 and 5).
The melt and crystal loss calculations using Sr modeling predicts that
the samples nearest to the base of the batholith, in general, have lost
Sr-depleted melt (i.e. accumulated feldspar) to become enriched in Sr,
while samples farthest from the base have lost crystals (i.e.
accumulated evolved melt) (Fig. 12). Because the plagioclase crystals at
all depths within a given sill have crystallized from the same magma,
plagioclase composition in the cumulate crystals is for the most part
invariant despite a broad range in major, minor, and trace element
composition. The most melt, and therefore SiO2, depleted
samples, would be more albitic than would be expected if they had
crystallized from a parental magma assumed to be compositionally
equivalent to their bulk chemistry (Cornet et al. , 2022). Fig. 6
supports this interpretation for the SMB. Finally, melt loss is
accompanied by detectable foliation in the most melt depleted samples
nearest to the base of the pluton (Fig. 7 and 8).