1. INTRODUCTION
The processes that influence differentiation in magma chambers and the
rate at which the associated melt-crystal phases separate have important
ramifications for volcanic-plutonic connections among silicic igneous
rocks. Related to volcanic-plutonic connections among silicic igneous
rocks is the identification of cumulate signature in plutons. The
subtlety of crystal accumulation signals in silicic igneous rocks has
led to their interpretation as representing a true melt composition and
being genetically separated from volcanic rocks (Coleman et al. ,
2004, Glazner et al. , 2004). While the petrological signature of
the cumulate nature of silicic magmas is subtle, it is discernible
nonetheless (Bachmann et al. , 2007, Deering & Bachmann, 2010,
Gelman et al. , 2014). Knowledge of how melt loss occurs at melt
fractions relevant to silicic magma chambers and the associated textural
and chemical indicators can facilitate identification of cumulates in
plutons. Furthermore, the rate at which the associated melt-crystal
phases separate have important ramifications for volcanic hazards. Here,
we investigate the Spirit Mountain Batholith (SMB) for chemical and
textural evidence of crystallization-differentiation and phase
separation by repacking-driven compaction (grain reorganizations).
The paper is organized such that we first introduce the geologic setting
of the region and of the SMB in particular and provide evidence from
previous studies supporting melt loss in the deeper parts of the SMB.
Then, results of geochemical analyses are provided, including
acquisition of major, minor, and trace elements of bulk rock SMB samples
and results from plagioclase composition analyses. Subsequently,
textural analyses of selected SMB samples are presented. We identify a
near linear unmixing trend in major, minor, and trace element
geochemistry defined by samples within a ca. 3 km transect at the base
of the exposed batholith and pooled leucogranites near the top of the
batholith. The plagioclase compositions suggest that the samples
crystallized from the same parental magma and that the magma was less
mafic than their bulk rock compositions. We then introduce a trace
element model that allows melt and crystal to be lost to estimate
relative melt loss (cumulate) or crystal loss (silicic cap) in the SMB.
The benefit of this model is that it doesn’t assume a particular
separation mechanism; however, it is limited in that it doesn’t provide
the range of crystallinities over which melt is lost and doesn’t allow
calculation of trapped melt fractions. To accomplish this, we use an
unmixing model that treats the analyzed samples as combinations between
two different endmembers: melt and crystal at a certain crystallinity.
The trapped melt fraction profile is then compared to results from a
model of mush compaction based on a crystal repacking rheology to
provide order of magnitude timescale estimates for the growth of the
silicic cap (melt accumulation layer).