Here question ends. Beyond details exist

Do they occur on the same tectonic regime? For a start to distinguish Subalkali rocks froms Alkali rocks, my knowledge says that mantle is the source of Alkali rocks. Ocean Island Basalts are Hawaii, Reunion etc are all Alkali rocks. Large ignous provinces are also sourced from mantle by the mantle plumes. Such as Deccan, Siberian traps.

But in Robil Gill's ((Gill, R. (2010). Igneous rocks and processes: a practical guide. John Wiley & Sons.)) book on page 291, (chapter 9-Alkali rocks) says that Most of the large igneous provinces are subalkaline and they are much bigger and more frequent than the Alkaline Large igneous province. From this i derive that LIP's can be subalkaline.

So are there any difference between subalkali and alkali rock occuring tectonic environments ?

  • $\begingroup$ While an interesting question (as previous qiestions you posted) maybe this is too broad to answer in stack's format, at least for the title. You could maybe precise a bit around the controversy you found on the book aseveration. Also if you are refering to a work, you should quote the title of the book etc (and add the page where you found that) $\endgroup$
    – user12525
    Sep 15 '19 at 18:31

Yes, there are many differences and subdifferences and nuances.

I will try to keep it in simple terms which will satisfy your question.

Alkaline magmas form by low degree partial melting of the deep mantle. This is why you see them in hotspots. It's very hot down there, but you do not form a lot of melt. For example, the amounts of melts in Hawaii are negligible compared to what you have in large igneous provinces (think Deccan Traps) or mid ocean ridges. Another tectonic setting with common alkaline magmas are rift zones. The best example is the east African Rift (Ethiopia, Kenya, etc). Again - deep melting of the mantle leads to alkaline magmatism.

Subalkaline magmas form in shallower melting. For example, the voluminous mid ocean ridge basalts are not alkaline, and the melt forms within a few kilometres of the surface. Arc magmas (andesites etc) are also not too alkaline because they form by fluid-assisted melting of not too deep mantle above subduction zones.

Even though large igneous provinces form by mantle plumes, the melting so extensive that the melts are not alkaline any more. To get alkaline melts, it has to be low-degree partial melt.

  • $\begingroup$ After your answer, i want to formulize it as, "in shallow environment, the melt has no time to differentiate/evolve and solidify as a bulk and without fractioning. So it has no alkali enrichment. But in a deep environment occuring melt, it has time to evolve while ascending. Potassium like -large ionic radius- elements enriching by time." what do you think about this explanation regarding only depth, rather than the dimensions of the melt. $\endgroup$ Sep 19 '19 at 10:16
  • $\begingroup$ @MuharremYavuz no, this is not the reason. It doesn’t have much to do with time allowed for fractionation. Deep melting forms alkali melts because this is the composition of the melt itself. Take a rock, melt it at shallow depth and you get subalkaline melts. Take exactly the same rock, melt it at large depth, and the liquid is now more alkaline. Low-degree melts will only cause the melts to become more alkaline because of the incompatibility of sodium and potassium. $\endgroup$
    – Gimelist
    Sep 19 '19 at 10:21
  • $\begingroup$ Would you please explain what you mean by "low-degree partial melting"? $\endgroup$ Sep 19 '19 at 18:36
  • 1
    $\begingroup$ Very low melt fractions. Mostly crystals, little melt. If you have the PDF version of Gill's book, just search for "low degree" and you will find it. $\endgroup$
    – Gimelist
    Sep 20 '19 at 0:59
  • $\begingroup$ In your answer East African Rift and MORBs lies in different titles. Under the East African Rift, a mantle plume lying. This probably makes it chemistry Alkaline. .I am adding this for the possible curios audience $\endgroup$ Sep 20 '19 at 6:30

I can't put it as a comment so it'll have to be an answer this time around.

Either way, I remember doing chemical analysis and my professor at the time was very adamant that yes, there is a difference depending on tectonic setting. Alkaline magma series tend to be present in convergent configurations, however, it's not exclusively found in these. To be honest, the question is a bit too broad to be summed up here but use it as a rule of thumb, according to my professor. However, subalkaline series are present in most plate settings.

There is a lot of interaction going on with magma from the mantle reservoir during its ascent through the crust which could be a source of contamination after all.


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