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.