Dr. Michael Zieg, Slippery Rock University, will be speaking at the weekly Geoscience Seminar over the lunch hour on November 12, 2010. This title of his talk is “The Beacon Sill, Antarctica: Evidence for Compaction and Reinjection, or Who Knew Diabase Sills Could Be So Much Fun?”
All seminars are in Walsh Hall, room 104, from 12:15 to 1:15 p.m.
The Beacon sill, a 150m-thick diabase intrusion, is part of the Early Jurassic Ferrar group. Ferrar magmas were emplaced ~180 million years ago during the breakup of Gondwana, extend across Antarctica to SE Australia and Tasmania, and are synchronous with the Karoo flood basalts in southern Africa. Ferrar intrusions are exposed throughout the Transantarctic Mountains, particularly in the McMurdo Dry Valleys. While the lower sills frequently contain a central layered sequence, the upper sills (including the Beacon sill) are crushingly uniform on a field and macroscopic scale. This monotonous homogeneity, which is a common attribute of diabase sills, unfortunately relegates them to second-class status among igneous intrusions.
Because of their simple geometry, however, sills can be extremely valuable natural laboratories for the study of magmatic differentiation, or chemical evolution during cooling and solidification. Vertical sections through sills often illustrate the effects of crystal settling (e.g., the Palisades sill) or the assimilation of wall rock (typically concentrated near the contacts). In the Beacon sill, there is no mineralogical or chemical evidence to suggest that either crystal settling or wall-rock assimilation played significant roles. Rather, the dominant differentiation process involved in the post emplacement evolution of the Beacon sill was compaction-driven redistribution of slightly evolved interstitial liquid. Of special interest is the segmentation of the compaction-generated geochemical profiles. This chemical segmentation, together with textural irregularities, suggests that emplacement of this sill consisted of at least four injections of magma over a time span of approximately one hundred years.
Lacking large-scale geochemical or mineralogical variations due to modal layering, the Beacon sill provides an opportunity to examine the fundamental processes that occur during magmatic intrusion, processes that may be obscured in more exotic layered bodies. Ongoing work with SRU undergraduates is attempting to clarify the extent to which multiple injection and compaction play a role in the development of other sills. In particular, we are examining a continuous drill core through a 1.1 Ga Nipigon sill in Ontario. It is hoped that this study will provide additional insight into the range of igneous processes that occur in this sadly neglected class of intrusions.