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).