ORBICULAR GRANODIORITE FROM TAYLOR VALLEY ANTARCTICA.

Orbicular granodiorite occurs within the Larsen. Granodiorite in the Taylor Valley, Antarctica. The orbs occur in discontinuous bands in a zone 200 to 1000 yards from the contact of the Larson with steeply dipping metasedimentary rocks.

In most places the orbs are separated by a few centimeters of matrix, but in a few places they are so closely packed that the outer rims form a continuous network. The orbs are usually spheroids but are often deformed, broken, or rarely disc-like. The cores of the orbs are biotite plagioclase hornfels, biotite schist, pyroxene, or rarely pieces of brecciated orbs. There is a complete range in the alteration of the orbs from orbs without cores to xenoliths with only a faint white rim of feldspar around them.

The orbs result from a chemical reaction between the larsen Granodiorite and the xenoliths in it. The most easily altered xenoliths are the pyroxenes, followed by the biotite schist and then the biotite plagioclase hornfels. Brecciation of sane orbs and flow features in some of the orbicular bands suggest considerable mobility in the pluton at the time of orb formation.

It is hoped that a detailed petrographic study of the rocks of the area will answer the questions: Why do only some xenoliths alter to orbs? What are the conditions of formation for the orbs that differ from other rocks carrying xenoliths? What is the mechanics of formation of the characteristic radial and ring structure of the orbs?