North-Central Section - 50th Annual Meeting - 2016

Paper No. 39-5
Presentation Time: 1:30 PM-5:30 PM


TAYLOR, Alexander H. and LUNDSTROM, Craig C., Geology, University of Illinois at Urbana-Champaign, 63 Computing Applications Building, Champaign, IL 61820,

How do convergent margin silicic plutons form? Geochronology indicates incremental assembly but plutons are often zoned over km scales; this observation is difficult to explain by magma differentiation in the lower crust. An upper crustal differentiation process consistent with geochronology is thermal migration zone refining, where top-down emplacement of sills creates a temperature gradient that allows for differentiation by mush zone reaction process. A vertical transect in Torres del Paine Igneous Complex (S. Chile) provides an excellent opportunity to test thermal migration zone refining, cutting vertically through the contact between two contemporaneous and possibly cogenetic igneous bodies: the 1.5 km thick Paine Granite (12.50±0.02 Ma) lying above the 300 m Paine Mafic Complex (12.587±0.009 Ma) (Leuthold et al. 2012). By analyzing the top-down chemical variations preserved in this shallow, multi-compositional convergent margin pluton at the 5-10m length scale, we can better understand the process by which the Torres del Paine Igneous Complex, as well as other large silicic plutons form.

SEM-EDS and laser ablation ICP-MS were used to collect bulk major and trace element data from glass fusions of nine samples. Compositional variations were plotted as a function of estimated elevation, in combination with data collected from a similar study across the same transect (Leuthold et al. 2013). Some systematic changes are evident, likely reflecting changes in rock type with position. The bulk data should primarily reflect changes in modal mineralogy; trace element analyses of certain phases (titanite) will be used to assess this.

The process by which magmatic differentiation and silicic pluton formation occurs is most often attributed to crustal assimilation, fractional crystallization, and fluid exsolution. While fractional crystallization involving mechanical separation of melt and solid is widely accepted as the most significant differentiation process, the complex mineral changes in Torres support thermal migration as a key player in granite production. Using the data described above, we plan on creating a model to test the validity of thermal migration as a major magmatic differentiation mechanism, specifically assessing the effect of titanite appearance on the whole rock trace element data.