In many of the Dolerite samples we get hornblende as a major mineral .If we say it is magmatic, then the magma should have sufficient water content.In general the sublakaline magma dosent have sufficient water content.So what could be the petrogenesis of hornblende? Can Hornblende,Ollivine and augite simultaneously exist?
As hornblende is amphibolie facies mineral,then before saying it is metamorphic(sample as metadolerite) hornblende, there should be the presence of other minerals of this facies
Is it possible to distinguish petrographically between primary hornblende(Magmatic) and secondary hornblende(metamorphic) in dolerite section?
Hi Amiya,
How do you know that the original magma doesn't contain sufficient water? Even water contents inferred from melt inclusions can be underestiamtes (see various works by Gaetani for some examples).
Magmatic hornblende, clinopyroxene and olivine can co-exist, but they are likely to be in a reaction relationship - amphibole will be forming at the expense of olivine or amphibole (see (cawthron & O'Hara, 1976, and my 2014 paper). Thin sections will likely show resorbed and overprinted minerals, and possibly relict cores of parent minerals in amphibole. At the risk of confusing the issue, magmatic hornblende can be secondary.
To say any more on the matter, it would be useful if you could let us know where the samples are from (is it an area that has been subject to amphibolite facies grade metamorphism?) and perhaps a thin section photomicrograph. Amphibolite grade (and retrograde greenschist grade) metamorphism may be accompanied by a range of metamorphic minerals (actinolite, chlorite, garnets, micas, sodic feldspars) atypical of a calc-alkaline dolerite.
Indeed primary amphibole may occur in dolerites (and gabbros, for that matter). Sometimes amphibole may also be late-stage or deuteric, or even metamorphic, so Daniel is correct in mentioning magmatic secondary amphibole, as well as stressing the importance of an optical study for a more judicious diagnostic. Some mafic alkaline rocks as for instance lamprophyres habitually contain hydrous minerals like biotite and amphibole.
Hi Amiya,
a few hints which may be of use:
(1) you may look for any oscillatory zoning in your amphiboles (if you see such a zoning, the magmatic origin is likely);
(2) the amphibole bearing metamorphic rocks commonly display a clear schistose fabrics except some low grade rocks. However, the latters usually contain actinolitic amphibole, pale or colorless in thin sections;
(3) amphiboles produced by hydrothermal alteration (not by regional metamorphism) are also dominantly actinolitic;
(3) if any microprobe is available, check the chemistry of amphiboles. Actinolite is never magmatic, while kaersutite- and hastingsite-rich amphiboles usually crystallize from melts. At a first approximation, high Ti, Al, and Fe3+ contents are the attributes of high temperature amphiboles.
Hornblende is often found in dolerites, diorites and gabbros. It could be primary (especially in diorites) or it could be secondary after clinopyroxene, yet still magmatic (be aware that during magma cooling, even a rather small initial water content may lead to oversaturation, at which stage clinopyroxene would be replaced by hornblende, while a significant portion of the system is still liquid - so, magmatic process, not metamorphic). A metamorphic rock with hornblende belongs to the amphibolite facies, and it is extremely rare that such rather high grade metamorphism is purely thermal (contact metamorphism). You would immediately recognize a well-developed fabric, i.e., schistosity or foliation(s) in the rock, even at the scale of the hand sample.
Great answers above. I worked on dolerites that were contaminated by country rock. That is one way to get water (meteoric/formation) into the magma and could lead to the stability of amphibole as a late stage phase... but even so I would also expect to see mantles of amphibole around the pyroxene. One way to test if it is available would be to check the oxygen isotope composition to look for signs of meteoric contamination?
There are many works related to magmatic amphiboles in alkaline mafic lavas.
many works on Victoria Land (antarctica) report of alkali-basalts and hawaiites with plagioclase + Mg-Olivine + Aegirine rich- Augite + amphibole of the Kaersutite-Katophorite Series.
Ti-content in amphiboles is higher in magmatic in respect to metamorphic, but it is not a safe method of discrimination.
Also concentric zoning is not a safe method since magmatic amphiboles could suffer requilibration with surrounding melt.
Just evaluate Texture and geochemistry, here below some suggestions:
melt is reacting with amphibole phenocrst?
is amphibole present also in ground mass?
How does work the Dy/Yb ratio in the whole rock?
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