I did geochemical analysis on amphibolites rock samples from my study area, I am surprise to see that the silicate content on my data (range : 66.5-71.0 ) compare to other works (range : 49.29-50.84 ) are very high . Please, what can explains these high content of silica in my own amphibolites rock samples?
Very high ratio. Well, is this really amphibolite? Try to use Winchester and Floyd 1977 diagram.
Please look the thin section of the rock under microscope before arriving at conclusions just based on chemical analysis.
The rock may be traversed with secondary quartz veins or it could be due silicification..hydrothermal activity of the rock.
chert/chalcedony may also occur as vug fillings in the rock on later time.
Petrography will solve your problem.
Why do you assume that your amphibolite is derived from a mafic rock? As Asoori advised, check carefully the thin section for the quantity of quartz and the type of feldspar. Albite, oligoclase and microcline contain
> 64 wt% of SiO2. Some amphiboles that can occur in metamorphic rocks also contain substantial silica; Anthophyllite and cummingtonite contain about 55 % of it; actinolite can contain up to 55% silica and tremolite carries 58 wt% SiO2. However, it is difficult to accurately determine the amphibole composition optically. Keeping this information in mind, if your rock is principally composed of amphibole and felsic minerals, the latter would probably have a lot of quarts. Twinning is not well developed in many metamorphic plagioclase grains and they can be mistaken for quartz.
All best for 2018.
depending on its protolith, amphibolites may containa lot of quartz (though this is not common) - hence check the thin sections as advised by Asoori! Also, check for analytical problems!
As suggested by others, there may indeed be primary or secondary quartz in the samples. However, I doubt that something with 71% SiO2 would still be an amphibolite, so either you misclassified your rocks to start with, or there is a problem with the analyses. This sections will help, but just looking at your rock in hand speciment should already give an indication too.
I agree with the Ulrich Knittel that "depending on its protolith, amphibolites may contains lot of quartz". I am currently working on such type of amphibolite rock from Himalaya, which is a banded amphibolite i.e. contains bands of the quartz-feldspar with little amphibole alternate by a amphibole rich layer.
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