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North-Central Section - 37th Annual Meeting (March 24–25, 2003)

Paper No. 21
Presentation Time: 5:00 PM-9:00 PM

PETROGRAPHY AND PETROCHEMISTRY OF CAMPTONITE DIKE AT WINOOSKI, VERMONT


BEAR, Stephanie1, LAW, Eric2 and VAN HORN, Stephen2, (1)Geology, Muskingum college, 163 Stormont St, New Concord, OH 43762, (2)Geology, Muskingum College, 163 Stormont St, New Concord, OH 43762, sbear@muskingum.edu

Lamprophyre dikes of Mesozoic age were emplaced in Lower Paleozoic rocks in Vermont. Four samples across a 0.7-meter thick dike near Winooski, Vermont were taken for petrologic analysis. The goal of this study is to address the process of magma differentiation and the mechanism of magma emplacement in the dike.

The dike rock is reported to have a camptonitic composition. It intruded into dolomitic country rock and presented a connected en echelon structure (Stephan and Van Horn, 2000). Under optic microscope, all samples are holocrystalline and show the dominance of plagioclase, amphibole, pyroxene, and opaque oxide. These crystals of 0.1- 0.2 mm in width distribute uniformly in the rock. Plagioclase has long dimension up to 1.5 mm and shows obvious preferred orientation up and down the dike. There is no phenocryst observed, but ocelli with various composition are common.

Differences are found between the two samples on the contact with the wall rock (outer sample) and the two samples at the interior of the dike (inner sample). Outer samples contain calcite as a mineral which replaced the plagioclase and as the filling mineral of some ocelli. Calcite is not seen in the two inner samples. The outer samples also have larger nepheline crystals as either the ocelli or the micro-phenocryst. Ocelli in the two inner samples are several times larger than those in the outer samples. The ocelli are filled with plagioclase or chert and show circular vermiculated structure. This feature is not observed in the outer samples.

Observed microscopic features implies that the magma flow was obviously laminar instead of turbulent, and the dike was made by a single intrusive event. The outer samples are expected to have a significantly different magmatic composition than that of the inner samples. This suggests a magma differentiation process took place either by the cooling of the dike or by the cooling of the source magma. Soon-to-be-completed chemical analysis on these samples should provide more information to the model of magma evolution.

Session No. 13--Booth# 21

T4. Showcase of Undergraduates (Posters)
Monday, 24 March 2003: 5:00 PM-9:00 PM
Geological Society of America Abstracts with Programs. Vol. 35, No. 2, February 2003, p.45
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