The study area falls withing Alkali Younger Granite complex of northern Nigeria.
I have rock samples of riebeckite granites with phenocrysts of quartz. The Na amphibole crystals are euhedral and the core is altered in some grains. Also, what could be the cause of poikilitic inclusion of plagioclase and microcline in the quartz (see attached photomicrographs).
Jorge Costa-de-Moura
Thanks. Am just curious to know if there is a name to describe such poikilitic inclusiion. Like we can have antiperthite, mesoperthite, myrmeckite, granophyre at different instances from textural intergrowths.
Hi Kayode,
I want to start with the most recent culmination expected after many detailed explanations: this seems a magma mingling event, and relevant textures. I think you are definitely looking at a magma mingling texture, then a mixture of having two different compositions. The sodium-rich ribekite may represent a more alkaline and partly remnant of an amphibolitic solid root source that has previously had been subjected to the crystallization process. Best wishes
Dear Kayode:
What you call alteration of riebeckite looks to me as zoning of the crystal to another kind of greenish amphibole due to magma contamination. Quartz is the last mineral crystallising in the magma, its pheocrysts will engulf all the previous crystals of plagioclase and alkali feldspar, so there's nothing strange about this. What looks really strange is the huge relative size of the quartz compared to feldspars, which could be due to contamination with a precambrian, more silicic, gneissic basement in contact with the igneous body. I don't think there's any evidence of magma mingling or mixing, both the light green alteration of the Na-amphibole to a Ca-amphibole, and the huge size of the quartz phenocrysts might be due to contamination from the country rock. A Sr isotopic analysis in both country rock and the granitic intrusion would reveal this contamination. We have a case of utmost contamination in an alkaline magma in a very remote annular complex in southern Venezuela. The original magma was silica-undersaturated, and crystallized inner rings of nepheline syenite, umptekite, and alkaline pegmatite. But the outer ring came in contact with the huge platforms of quartzite of the Roraima Supergroup, and became strongly silica oversaturated alkali-feldpar granite, retaining an unusual mineralogy which included: fayalite altered to epitaxial-radial blue riebeckite, large sphene and zircon crystals, and large deep blue adfversonite prisms. The role of silica contamiation here is unquestionable. Regards, Sebastián.
Dear Prof. Sebastián Grande , I am really grateful for your detailed explanation. My study area falls within Ropp Younger Granite Complex which is one of the Alkaline Granite complex in Northern Nigeria. Like you suggested the massive igneous intrusions spans to form a ring structure and are are enclosed within a large area of Basement rocks though the contacts are largely not exposed. Large quarts crystals occur clustered together or arranged in arcuate form between K-feldspars. The quartz appears to have crystallized in two phases with initial crystals clustered and a later quartz replacing feldspars. I will carry out the isotope analysis. Kind regards
Dear Kayoda: it won’t be easy yo find xenoliths of felsic basament in a felsic igneous body, so try yo make dime isotopic analyses to unveil the problem you have. Regards, Sebastian
Dear Kayoda and Jorge: enclaves in igneous rocks can have many origins, if a purely mechanical incorporation of fragments of the country rock is responsible, then they are truly xenoliths, other enclaves could be restites, and in some cases, cumulates of the same magma (rare in felsic rocks, common in mafic rocks). A quartzite xenolith is surely that, since its origin is metasedimentary. Surely magma which has suffered mingling/mixing can also have xenoliths and enclaves. But as Jorge says, a few pictures of some grains are not enough to reach any decent conclusion. Regards, Sebastián.
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