Median tectonic zone : Brook Street Terrane relations in Melita Valley, northeast Fiordland

Wilson, Scott Richard, 1972-

The boundary between the Brook Street Terrane and the Median Tectonic Zone in upper Eglinton Valley ~mapped. This study ).,S: focusse'} on detailed mapping and petrographical studies of the contact between GR 195 825. a-n-d- 2-0-0- 8-4-5- -in Melita Valley. The Brook Street Terrane in the upper Eglinton area is represented by basaltic to basaltic-andesite volcanogenic metasediments and dike rocks of the Eglinton Subgroup. Stratigraphy and structure of Eglinton Subgroup rocks are described and a synclinal fold within the Eglinton rocks is recognised. These units were formerly referred to as the Plato Terrane, a component of the Alabaster Group. Williams and Harper (1978) mapped Plato rocks as having their western margin faulted against the Mistake Diorite (Triassic) by the Eglinton Fault. A single exception to this was mapped in Melita Valley where they interpreted the contact as being intrusive. Uncertainty over the nature of this contact has been heightened since the proposal of a Median Tectonic Zone (Kimbrough et al., 1993), since this contact represents the location of the original Median Tectonic Line and the eastern edge of the recently proposed MTZ. This study supports the intrusive interpretation, as fiel-d observations accurately located four sections showing an intrusive contact. tn addition, an increase in metamorphic grade, from prehnite-pumpellyite to hornblende hornfels fades, is identified in the Eglinton Subgroup as the contact of the Mistake Diorite is approached. A K-Ar date of 180 Ma was obtained from a hornblende separation from the contact zone. Dikes cutting the Eglinton Subgroup, have been correlated with the Mistake Diorite using petrographical and geochemical observations. Correlations are also made with metasediments and leucocratic intrusions in the Skippers Range, which further support an intrusive contact. Faulting along the Eglinton Fault Zone occured subsequent to intrusion and was localised at the intrusive contact due to differences in competency.- The evidence for the intrusive contact implies that the Mistake Diorite may be included within the Brook Street Terrane, and that any fundamental suture (MTZ) is either intruded by the Mistake Diorite or located further to the west.

87 leaves : ill. (some col.), maps (some col.) ; 30 cm.

1994Wilson

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