Dear Mr. Ravina,

the rock should  be denominated as a meta-basalt according to your information made public. This is also indicated on a macroscopic scale by the foliation (?) and folding visuable at outcrop. Basalt undergoing a mild metamorphism often shows in addition to actinolitic amphibole, Mg-Fe chlorite and Fe-enriched epidote s.s.s. The plagioclase becomes more sodic. Ilmenite may get decomposed and converted into anatase or sphene. There may be some alteration processes leading to what previously was called spilite (have a closer look at your plagioclase s.s.s.). On increasing metamorphism (low-grade stage) the rock may loose all its primary minerals like olivine and CPx and ends up as a prasinite with epidote, amphibole, and Na-enriched plagioclase. It would be wise to carefully analyzed the rock-forming minerals in more detail to see which direction alteration sweeps its way through these basic igneous rocks.

Best regards

H.G.Dill

 I understood from your question that it is primary (not metamorphosed) olivine basalt. If so the original magma was probably of water-deficent nature, i.e. tholeiitic. In this case, the anhydrous phases (An-rich plagioclase, pyroxene, olivine) crystallized first. Latter on, with increasing residual water after the crystallization of anhydrous phases, hornblende starts to crystallize at about 3% water content or more in magam. If this the case, hornblende should be primary (not secondary amphibole).

Dear Mr. Ravina,

The term 'paragenesis' that you used in your question is actually only applied for a characteristic association or occurrence of minerals or mineral assemblages in ore deposits, provided contemporaneous formation.

The primary (contemporaneous) magmatic mineral assemblage in olivine basalt includes only the major constituents plagioclase and pyroxene. Any amphibol (hornblende) mineralization in olivine basalt generally are products of secondary alteration processes with the supply of water-rich fluid phase. Furthermore your thin section images in my view, do not show any foliation or metamorphic overprint.

Best regards,

G. Grundmann

Dear colleagues,

neither the outward expression of the rock in the first image nor the emplacement and texture of the amphibole corroborate the idea of  a basalt unaffected by post-magmatic alteration. With this in mind I cast some doubt on the primary nature of the amphibole in this rock.

Best regards

H.G.Dill

Dear colleagues,

deal with the terminology of basic magmatic rocks highlighted by the technical terms: diabase, dolerite, meta-basalt... The mineral assemblage fits, but foliation is not a "conditio-sine-qua-non".

Bet regards

H.G.Dill

Dear Jeremy,

Clearly Guenter hasn't looked at many basalts or cannot identify what he sees.  The primary mineralogy of most holocrystalline olivine basalts are: olivine, calcic pyroxene, plagioclase feldspar, ilmenite or titanomagnetite, and minor apatite.  In alkalic basalts it is not uncommon to have small amount of a titaniferous biotite, especially in close association with a titanium-rich opaque oxide, and brown amphibole (note spelling).  The brown amphibole is typically kaersutite but in detail there can be a number of other amphibole varieties, including edenite, pargasite and even ferrohastingsite.  In volcanic rocks where hornblende occurs the origin of the magma originates from a tectonic environment that is very different to that which generates olivine-bearing alkali basalts.  Looking at your images it is unclear exactly where your sample came from.  The outcrop indicates some structural dislocation and the image is also more typical of basalts that have been affected by some mild metamorphism.  The thin-section images also indicate some thermal effects and minor weathering.  I do not see any convincing amphibole in your images and the "iddingsite" appears to be more hydrated iron oxide than smectite-based.

And for Guenters information the words paragenesis has two meanings.

Dear Ravina,

The outcrop is la basaltic lava flow. The photomicrographs show amphibole crystals and olivine, pyroxene and plagioclase. Such Ti-rich amphiboles (kaersutite and others)  are frequently described in alkali basalts since a century.

Dear Ravina,

Are you sure that you have a basaltic rock? Because such porphyritic texture would classify better to a boninitic rock (low titanium content)…Do you examine any spinel in that mineral association? Besides, the above sample is affected by greenschist facies hydrothermal alteration. So the hornblende composition could be secondary one

Best regards

Adonis

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The occurence of amphibole as a primary phase in basalts depends not only on H2O content (>2-3% wt.), but also TiO2 content (>2-3% wt.), and temperature (

Dear Jeremy Ravina: 

Very interesting this thin sections of alkali basalt. Here is what I could see in the pictures you annexed. First, the greensish amphibole is definitely primary, and it is an amphibole alright, not a clinopyroxene: it is has an almost euhedral shape, rhonbic cleavage and even twinning (in the lower part even seems an incipienty polysinthetic twinning). It seems to be an Fe-poor amphibole, perhaps Mg-kaersutite o Mg-hastingsite, but being this a volcanic rocks the kaersutite is more probable. Curiously it is not zoned, but it shows in the top-left part of the crystal some alteration which has an almost rectangular outline, and even has some sort of vermicular inclusions, both I can't tell what they are from a "static" image, i.e., without rotating the stage. Another thing that, apparently, nobody has noticed is that there are TWO amphiboles in the rock. The other one has been totally altered to chlorite and has oxydised borders and fractures/cleavage. This amphibole pseudomorphs don't look at all like the green, fresh one. Note that the oxydised crystals are always associated with olivine; in fact, they are in contact with it, as it can be seen in all four images. In the first two images a perfect pseudohexagonal basal section of it can be seen in the center-left of the image; this basal section is altered to chlorite, which under crossed polars has typical anomalous Berlin blue interference color. Also in the second view, another amphibole pseudomorph, also oxydised and chloritized can be seen. This lava doesn't seem to be metamorphised at all, at most the alteration of the "first amphibole" and the olivine could be hydrothermal, but not metamorphic. To identify the "fresh" amphibole you should find the most perfect crystal not showing cleavage, this particular orientation allows in most amphiboles to obtain a good intereference figure and estimation of the axial angle, 2V. A good reference text, like the David Shelley 2nd Ed. of "Optical Mineralogy" has very good tables which typify the main optical properties that allow identification of most amphiboles. I don't think there is one single crystal of this green amphibole in the whole thin section, so find the section I told you before -parallel to (100)-, and also try to find a longitudinal section with perfect prismatic cleavage and the maximun extincion angle, which also will have the highest birefringence -parallel to (010). With those two specially oriented sections it will be easy for you to pin point the amphibole species this basalt has. The other problem is the "first" amphibole, which obviously should have been Fe-rich, due to its extensive oxydation and alteration to anomalous blue Fe-rich chlorite, this could have been an original ferro-kaersutite or ferro-edenite, now totally altered, as our colleage Harald Dill stated before.