Advances in Rock Fabric Quantification and the Reconstruction of Progressive Dike Replacement in the Coastal Batholith of Central Chile

• Main Author: Webber, Jeffrey R.
• Format: Others
• Published: ScholarWorks @ UVM 2012
• Subjects: Pluton emplacement; Strain Analysis; Coatal Batholith; Rf/Phi
• Online Access: http://scholarworks.uvm.edu/graddis/238; http://scholarworks.uvm.edu/cgi/viewcontent.cgi?article=1237&context=graddis

The Coastal Batholith of central Chile preserves structures that record the concentration, migration, transportation, and emplacement of magma during the progressive construction of a sheeted dike complex. This sheeted dike complex is divided into three main structural-geographic domains. The northwestern domain contains an abundance of deformed microgranitoid enclaves that host features that facilitated the concentration of melt during crystallization. The formation of interconnected dilational sites produced an array of lecocratic zones that may have formed larger dike networks that facilitated the transportation of melt-rich magma producing new magmatic units of similar mineralogy. The central domain is characterized by the presence of two tonalitic units that contain enclave swarms distinguished by their general packing arrangement and degree of elongation. Di erences in the fabric architecture of these enclave swarms are displayed by two separate three-dimensional fabric analyses using the Rf/ method, which indicates an abrupt transition from low-distortion oblate fabrics to more distorted prolate geometries. These changes are compared to the statistical alignment of feldspar phenocrysts that indicate general attening in both units with a higher degree of alignment within the XZ fabric plane for the younger tonalite. The third (southeastern) domain is distinguished by meter-scale, compositionally and texturally diverse sheeted dikes intercalated with biotite-rich migmatite screens of the host gneiss along the pluton margin. The need to process large quantities of fabric data from central Chile presented the opportunity to establish a comprehensive method for the quanti cation of three-dimensional rock fabrics following the Rf/ and Fry methods. In order to test the utility of this procedure, a three-dimensional synthetic model of known strain shape, magnitude, and orientation was processed. The results of this assessment indicate that the procedure accurately calculated the expected state of strain within a small margin of error. Finally, a natural example is presented to test the method's ability to quantify the fabrics of deformed rocks. This example is a \lineation much greater than foliation" (L>>S) metagranite augen gneiss from the Coastal Batholith of central Chile. This analysis resulted in calculated fabric ellipsoids from both the Rf/ and Fry methods that clearly display signi cantly prolate geometries at moderate distortions. The development of the three-dimensional rock fabric quanti cation procedure highlighted the need to teach analytical strain techniques in three-dimensions. To allow for this application, an interactive R script (FRY3D) was created speci cally to aid in the instruction and visualization of three-dimensional strain calculation at the advanced undergraduate and graduate levels. This tutorial was presented to a structural geology course of 20 students at the undergraduate level with a two part semi-quantitative concept assessment before and after the presentation. The results of this assessment indicate a positive increase in student's understanding of three-dimensional nite strain. Finally, a simple examination of analytical error associated with the Panozzo projection technique for strain analysis is presented and indicates relationships among population size, strain magnitude, and initial fabric. My results suggest that this method is most robust when applied to sections containing greater than approximately 125 lines. Moreover, the magnitude-dependent error indicates that the method may be better suited for rocks deformed at low to moderate strains. I recommend an adaption to the initial conditional assumptions for this method that lines exhibit an initial radial symmetry when recentered to a common point.