The main units in the batholith, identified in Claiborne et al.(2006) and Walker Jr et al. (2007), include the Spirit Mountain
granite (SMG), Spirit Mountain leucogranite (SMLG), the fine-grained
granite (FGG) suite, diorite sheets, dike swarms, and the Mirage pluton.
The SMG and SMLG were combined in Walker Jr et al. (2007) and
referred to simply as “SMG”. The SMG and SMLG make up the vast
majority of the pluton.
Walker Jr et al. (2007) documented a continuous range in wt %
SiO2 of ca. 61 – 79 between the SMG and SMLG (excluding
the more mafic diorite sheets). Leucogranite samples are typically fine-
to medium-grained and contain ca. 10% plagioclase, 40 – 50% alkali
feldspar, 30 – 40 % quartz, and small amounts of biotite (Walker Jret al. , 2007). The shallowest (western most) portions of the SMLG
include aplite sheets, granite porphyry, and miarolitic cavities. SMG
samples, on the otherhand, are coarse (millimeter scale) and contain 20
– 50 % to plagioclase, 30 – 50 % alkali feldspar, 5 – 30 % quartz,
and 3 – 15% biotite. The deepest (eastern most) and most quartz-poor
samples are enriched in major, minor, and trace elements consistent with
accumulation of early crystallized phases (i.e., Sr, Ba, and
Al2O3 suggesting accumulation of
feldspars). This suggests mechanical phase separation may be important
in the SMG; however, the extent and mechanism responsible for separation
remain unconstrained.
The FGG, Mirage pluton, and dike swarms are similar in texture
(uniformally fine-grained), composition (71-74 wt %
SiO2), and in general relative timing (late stage), as
they all intrude the SMG. The FGG and diorite units are found as
sill-like sheets, pods, and pillows mostly towards the base of the
exposed batholith, while the Mirage pluton intrudes the southern portion
of the SMG. Several magmatic units with similar traits (composition,
grain size, field relations) are found within other intrusive Miocene
complexes in the region, suggesting that such magmas were perhaps the
parental magmas from which different portions of the SMG differentiated
(Gualda et al. , 2023). Here, we investigate the SMB (SMG in
particular) for records of phase separation to explore the mechanisms
that lead to chemical differentiation and the formation and accumulation
of evolved melt caps.