Repacking-Driven Compaction in the Spirit Mountain Batholith, Southern Nevada

1. INTRODUCTIONThe 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).