The granite rocks are the most important and highly focused rocks in the field of geochemistry. Different tectonic settings are involved for the evolution of granite. As classified by Chap.and White alphabetic scheme S- I-A-M with reference to different tectonic settings. But i am not able to find a good answer for the evolution of syenite granite, if you have any suitable answer please share.
with best Regards
Dr.Mastoi
Dear Dr. Mastoi,
I would like to start the discussion with the strangest type of them called „episyenite”. It is a granite which has been subjected to a strong desilicification which converts a normal monzo- or syenogranite into a syenite. The igneous rock often shows higher Na contents and as newly formed alteration products calcite, dolomite, Fe carbonate and/or zeolites infilling the voids and pores left over after desilicification. They are prime targets in search of uranium. The process is sparked by solutions derived from subcrustal sources during the late or postgranitic stage.
With kind regards
H.G.Dill
Dear Dr. Mastoi:
Now some comments to the common trends of fractionation in monzo- and syenogranites. In the Central European Variscides which also gave host to the afore-mentioned secondary “episyenites”. I use the Streckeisen QAP triplot characterized by quartz alkaline feldspar and plagioclase as far as the igneous/ intrusive rocks oversaturated with regard to silica are concerned. There, the trend of differentiation starts off with of dioritic to gabbroic rocks (locally called “redwitzites after a town at the heart of the orogen) through granodiorites into monzogranites which form the lion share of the younger granites and it ends up in a small fraction of syenogranites. The above mentioned trend of differentiation has been observed in an ensialic orogen during the late Paleozoic with autochthonous and allochthonous (nappe stacking) geodynamic units forming the host rock lithology of the intrusive complexes. True syenite-alkali granite suites I personally know from the East African Rift Zone which signals a strong mantle impact similar to the “episyenites”. But there are more primary and secondary syenites in other zones like that, all of which herald a strong impact from subcrustal levels.
With kind regards
H.G.Dill
Dear Respected, Dr. Harald G. Dill, you have answered well, now we can have a good idea to generation of these rocks. Furthermore, respected sir if you have any article regarding East African Rift Zone and on “episyenites”. please do share so that i would be able to conclude some points. I have found that always your answers as well as suggestions are strongly helpful for geological society which is connected on Research Gate.
Regards
Dr. Mastoi
This question needs a very broad response, in that there are various ways that syenite+granite can occur together, either separated but in single complexes or intermingled but each part of one magmatic event. The example of East African rift intrusions is very similar to many of the known alkalic provinces globally. These are generally associated with the class of intrusions we have called, 'A' type for anorogenic. From the great diversity seen in the literature, it is obvious that no simple generic trait is shared by all the syenite-granite occurrences.
I have worked in Wisconsin, where an arch-typical complex of granite-mangerite-Si saturated syenite-alkalic syenite-anorthosite represent the 'A'-type suite. My PhD is from granitic bodies in the Eastern Desert of Egypt, and there we have individual granites with some individual bodies of Si-saturated syenite. Only later in the crustal evolution of this Pan-African belt, did the more alkalic syenites and per-alkaline granites invade.
If we just focus on the K-spar or alkali-feldspar syenites, these can bear a strong affinity to granites, of either one or two felspar varieties. As mentioned above, in unusual cases, desilicification can occur as C-O-2 rich fluids cause alteration. If the fluids are more aqueous, then instead of carbonate secondary additions, hydrous minerals and albitization can occur, +/- silicification---the opposite in essence of syenitization! I think both of these "metasomatic" examples are exceptions and not related to more major depth+source material+tectonic environment variables.
I finally suggest a good look at the vast literature to note the more typical examples of granite-syenite associations,
Dear Jeffrey Greenberg
Thank you very much for informative answer.
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