USING PETROGRAPHY, SCANNING ELECTRON MICROSCOPY, AND GIS TO EXAMINE THE CONTACT BETWEEN A SYNPLUTONIC DIKE AND ITS GRANITOID HOST

The sample we analyzed was collected from the contact of a ~50 cm wide synplutonic mafic dike that intrudes the Oligocene Grotto batholith, a Cascade arc intrusion located near Index, WA. Field evidence, including mutual intrusive relations, indicates that the host granodiorite was still partially molten at the time of intrusion. The dike, although finer-grained at the contact than in the center, is not aphanitic, which is also consistent with emplacement into hot host rock.

Using the SEM, 63 images of the sample were acquired at 40x magnification with a backscattered electron (BSE) detector. Using the petrographic microscope, 42 images were acquired at 40x magnification using direct digital image capture into a pc. Respective image sets were then merged and processed using Adobe Photoshop CS2. The two merged images were then put into ArcGIS to index them to each other. This allowed image distortion to be accounted for in the two datasets.

The color mosaic acquired using the petrographic microscope emphasizes crystal shape and species using polarized light. Crystal shape and size is also revealed using this method. Grayscale images acquired using backscattered electron methods show differences in atomic number and emphasize mineral areas (domains) rather than individual crystals.

Using these methods, the ability to discern changes in mineralogy across the contact from the host granodiorite into the synplutonic mafic dike were noted. Using GIS methods to “geo-reference” the two datasets gave us the ability to precisely correlate both physical characteristics and chemical characteristics in the sample. Using microbeam analysis (EDS), we measured chemical changes across zoned minerals identified using the petrographic microscope.

These preliminary results should lead us to further study of the whole rock chemistry as sampling progresses further into the dike and surrounding materials. Using this technique, we hope to correlate xenoliths from the granodiorite in the synplutonic mafic dike and identify and analyze the chemistry of trace microcrysts in the synplutonic mafic dike.