A really Gneiss Earthcache ! (Tonalite Gneiss) EarthCache
A really Gneiss Earthcache ! (Tonalite Gneiss)
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This small park has it’s bedrock exposed by the Braamfonteinspruit in Roosevelt Park. Please be careful when the river is running strongly (especially with children and when crossing the river).
These rocks form part of the exposed Archaean granite-greenstone basement of the Johannesburg-Pretoria dome. Archaean relates to rocks that are older than the Cambrian time period (i.e. Precambrian). They are approximately 3200 million years old and form part of a suite of gneisses which collectively represent the oldest known granitoid rocks in the stable basement platform underlying most of the old Transvaal Province (Mpumalanga; Gauteng; Limpopo & North West together). The tonalite gneisses are nearly always intimately related to the 3500 million year old “greenstone” successions. Some examples of these are exposed about a kilometer up in Montgomery Park (see a related Earthcache – to be published in May 2008). At the top of this cascade is a remnant (or xenolith) of this greenstone. See if you can spot it. These mainly mafic (Containing or relating to a group of dark-colored minerals, composed chiefly of magnesium and iron, that occur in igneous rocks) and ultramafic rocks are believed to represent altered and deformed remnants of an ancient oceanic floor that existed prior to the development of a granitic (continental) crust in this region. My how times have changed! Consequently, the tontalitic gneisses are younger than the metabasaltic and sepentinised peridotitic greenstone successions and are considered to have been formed by the partial remelting of these rocks as they were heated and depresses into lower levels of the crust. The tonalite gneisses therefore represent an initial stage in one of the most fundamental of geological processes – namely the building of the original stable continental platforms. They are consequently among the oldest rocks known to man. Another spot where these Archaean greenstones proliferate is near Barberton Mpumalanga.
The minerals that make up the tonalite gneiss are quartz (glassy), plagioclase (white), hornblende (black), biotite (micaceous or flaky black) and epidote (pale green). These minerals are aligned into a weakly defined north-south foliation (is this visible to you?), which indicates that the rock has been deformed subsequent to its cooling and solidification. Undeformed rock would have a random crystalline pattern. This deformation is possibly the result of the buoyant uprise of the tonalite body as it intruded to higher levels in the crust and pushed past the overlaying rocks.
(Acknowledgments: guidebook to Sites of Geological & Mining Interest on the Central Witwatersrand.; Geological Society of South Africa; 1986).
In order to qualify to log this cache, you need to answer the following questions and email the cache owner. Any logs not accompanied by an email will be deleted.
1)Take a photo of you and your GPSr at this spot with the granite outcrop visible and include in your log [Optional].
2)Look at the exposed granite (try find a “fresh” exposure and not a water-worn one). Describe from the “crystal” or grain size whether this rock cooled quickly (like a dolomite or obsidian with small grains) or more slowly to produce larger grains.
3)Describe what the granite looks like (including texture and colours).
Additional Hints
(Decrypt)
Erzrzore gb fraq lbhe rznvy!