Dear Mr. Morad,

Generally, calc-alkaline and tholeiitic melts are belong to very different geotectonic environments. Tholeiitic basalts are related to mid-oceanic ridge sub-marine volcanism whereas calc-alkaline basalts are related to subduction related volcanism and geochemically they are very different where calc-alkaline rocks characterized by LILE & LREE enriched melts whereas MOR rocks characterized by LILE depletion and HREE enrichment. Actually tholeiitic sub-marine basalts can't be more differentiate but calc-alkaline melts can be differentiated to generate basatic, andesitic, dacitic and even rhyolitc lava.

I think he knows that this basalt was of tholeiitic not calc alkalic suit after studying the tectonic history of these rocks very well and that should be aided by other petrological and geochemical studies.

For more about calc-alkaline magma series and tholeiitic mgama series this reference is recommended (M. Wilson (2007). Igneous petrogenesis a global tectonic approach, springer, 480p).

Best regards.

thanks alot Mr Ahmed :) 

Hi Ahmed,

If your rocks are 2.85 Ga, they are likely to be affected by some kind of alteration. You can use the diagrams published in Ross, P. S., and Bédard, J. H. (2009) Magmatic affinity of modern and ancient subalkaline volcanic rocks determined from trace-element discriminant diagrams. Canadian Journal of Earth Sciences 46(11):823-839.  

They use "immobile" trace elements to help differenciate between the tholeitic and calc-alkaline magmatic affinities. This is a common diagram used for ancient rocks.

This discrepency is also a puzzle to me. To my understand, the name of calc-alkaline or tholeiitic basalts are used to express the implication of what path its corresponding magma will evolve along, towards Fe-depleted ,Al-rich  or towards Fe-rich, Al-depleted. However, the evolution trend of basaltic magma is dependt on its composition, volatile, pressure and so on. if we are informed of only partial knowledge of the rocks, for example only its whole rock composiiton, it is not sufficient for us to discriminate its evolution trend. for a given composition basaltic magma, if it is low water, low pressure, it may evolve along tholeiitic trend, but if high water, high pressure, it may evole to calc alkaline trend.

Hi Antoine

If you have this pdf, please send it to me. Thanks

Hi Hassan,

The pdf of the unedited paper is here: http://espace.inrs.ca/1898/1/Ross-Bedard(magmatic-affinity)2009[1].pdf

The diagram of interest for you is page 39.

The final version is present here, but depending on your university access, you might have to pay for it: http://www.nrcresearchpress.com/doi/abs/10.1139/E09-054#.V2GVgTX2RiY

Hi Antoine

Thank you very much. I have got the pdf from Said

Dr Ahmed Tariq answer is useful. Several diagrams have been used to discriminate between the two. I like to use Miyashiro (FeO*/MgO vs. SiO2) and Irvine and Baragar (MgO--FeO--(Na2O + K2O) diagrams for distinguishing the two series.

Confusion exist as the Kohistan arc volcanics in Chitral region are both cal-alkaline and tholeitic. The latter may be related to post collision rift set

tings.

Naveed Anjum,

I cannot make comment on the Chitral volcanics without information on the space and time (i.e., location and age) relation of the two, and the source of data you are referring to. However, in an arc set up, it is not uncommon to have tholeiitic, calc-alkaline and alkaline sequence of magmatism from beginning to end, .

But the question is not concerned with the issue of spacial occurrence of tholeiitic and calc-alkaline rocks. It is asking how to distinguish the two.

Best,

Qasim

Honourable Professor,

I want yourvaluable comments on the attached file. Data was obtained at Stockholm University, Sweden using XRF (for Major oxides) and LAICPMS (Trace and REEs ) elements.

Dear Naveed,

Thank you for the info on geochem of Gawuch volcanics..

Plots of the analyses on your diagrams 2 (Y-Nb-Zr--Meschede, 1986), 5 (Th/Yb-Nb/Yb) and 6 (Ti-Zr) show rather clearly that your rocks are arc related. I would dare to say that your diagram 4 (mantle normalized spidergram) is also suggestive of arc affinity for your analyses. The analyses here have left to right (more to less incompatible elements) slope, prominent to marked troughs and spikes, and distinct negative Nb anomaly typical of subduction-related magmas. There appear two groups on the basis of normalized REE pattern, but it can be interpreted in more than one way. Having said that, let us not forget that T Khan et al. (1996-Geodynamica Acta) and SD Khan et al. (I forgot the reference!) have suggested a back-arc origin for the Chalt volcanis in the Hunza-Gilgit area.

The Gawuch Fm seems to be complicated, as far as my memory goes back 25 years; see our description in: Pusey et al., 1985, Transactions of the Royal Society of Edinburgh: Earth Sciences, vol. 76, p. 453-479. I found out that I have lost the paper of Hueberger et al., 2007, to read more about it. If you have this paper, please forward it on [email protected].

Best, Qasim

Tholeiites

Geological Setting: Mid‐ocean ridges, oceanic islands

Shallower melting and Greater % partial melting

Groundmass is Fine-grained

No olivine, Quartz, interstitial glass common

Clinopyroxene (augite, pigeonite), Ca-poor orthopyroxene (hypersthene)

No alkali feldspar

Phenocrysts: Olivine rare, unzoned or orthopyroxene rims

Orthopyroxene uncommon

Early plagioclase common

Clinopyroxene = pale brown augite

Alkali Basalts

Geological Setting: Island arcs, collision/subduction zones

Deeper melting and Lesser % partial melting

Groundmass is Coarse

Olivine common, No quartz; interstitial glass rare

Titaniferous augite (reddish), No orthopyroxene

Interstitial alkali feldspar or feldspathoid

Phenocrysts: Olivine common, zoned

Orthopyroxene absent

Plagioclase less common, appears later

Clinopyroxene = titaniferous augite with reddish rims

To find whether the Basalt in of calc alkaline or tholeiitic type one has to conduct XRF of the powdered samples, then plotting the results into geochemical diagrams for example the AFM diagram. Petrological study is also conducted to find the different minerals and conclude whether it is a Basalt or a metabasalt.