We have been working on the metamorphic rocks hosting the Savage River
magnetite deposits, and have found some disagreement about the interpretation
of some albite-rich cataclastic rocks in the footwall. The metamorphics are
mostly metamafics, quartzites, dolostones/magnesitestones, amphibolites and
various muscovite and chlorite schists and skarns.
The albitites have a variety of textures. Some are very fine grained
and grains are mostly simply twinned or untwinned, typical of metamorphic albite
(e.g. photo 1). Some have coarse grains in a fine matrix (eg. fig 2), which
some have interpreted as porphyritic felsic volcanics, and others as sheared
porphyroblasts or disaggregated veins. The immobile elements Ti/Zr etc suggest
an origin from sodic alteration of associated pelites and other metasediments.
But there are also some small (<1m wide) cross-cutting coarse zones,
interpreted by some as granitoids, which are more Zr-rich - these
typically contain some quartz and show myrmekitic textures (fig. 3).
Some of the coarse plagioclase in these bodies shows microcline-like
Twinning (Fig. 4), suggested by some to be plagioclase replacing microcline(?)
I have wondered if these coarse zones could be be zones of recrystallised
cataclasites or zones of partial melting? With remobilised Zr? The rocks
locally contain calcic to sodic amphiboles and diopside, with maximum
temperatures to about 600-700oC (magnetite-ilmenite).
Has anyone seen similar textures in cataclasites, or have any other thoughts on the origins
of these rocks?
regards
Ralph Bottrill
Senior Geologist, Mineralogist and Petrologist
Metallic Minerals & Geochemistry
Mineral Resources Tasmania
PO Box 56, Rosny Park TAS 7018
Phone: 61 3 6233 8359, Fax: 61 3 6233 8338
Email: [log in to unmask] or: [log in to unmask]
CONFIDENTIALITY NOTICE AND DISCLAIMER
The information in this transmission may be confidential and/or protected by legal professional privilege, and is intended only for the person or persons to whom it is addressed. If you are not such a person, you are warned that any disclosure, copying or dissemination of the information is unauthorised. If you have received the transmission in error, please immediately contact this office by telephone, fax or email, to inform us of the error and to enable arrangements to be made for the destruction of the transmission, or its return at our cost. No liability is accepted for any unauthorised use of the information contained in this transmission.
|