Mark S. O'Brien HBSc thesis abstract

The Archean mafic metavolcanics of the map area represent the northwestern extremity of the southern sedimentary-dominated belt of the Beardmore-Geraldton Region.  As such these rocks occupy a part of the Wabigoon Volcanic-Plutonic Supracrustal Belt of the Superior Province, Canadian Shield.  The rocks display many interesting primary megascopic features including: pillows; amygdules; and brecciation.  Such features were used to define map units.  The highly fractionated basalts of the map area are significant for both structural and petrologic studies.

These rocks have been weakly deformed.  This was determined by measurements of:  pillow, amygdule, and phenocryst shape; and pillow selvage thickness.  The individual contributions of synvolcanic and regional tectonic deformation processes cannot be evaluated.  However, it is evident that both have played a part in producing the strain now observed in these rocks.

Petrology, including petrography and geochemistry, of study area samples can be used to ascertain the tectonic regime of the Beardmore-Geraldton Region.  Several factors tend to disguise this effort.  Spilitization, low grade regional metamorphism, and subsequent hydrothermal alteration must be considered.  However, these effects can be stripped off by the use of relatively immobile elements for classification discriminations.  With these facts in mind, the rocks appear to be ocean floor basalts extruded in an inter-arc basin.  This determination is proof that Archean volcanic rocks can be studied fully with only a few minor diagnostic restrictions.  To this end thoughts on modern and classical plate tectonic theory as applied to greenstone belt genesis are presented; this despite the fact that neither have been conclusively confirmed nor disproved for the Archean.