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6.4.1 Major elements

The five analysed samples show a very narrow compositional range for all the major elements (table 6.1). These samples have a very high A/CNK ratio (1.30 ± 0.03), indicating a high degree of peraluminosity. These analyses also show the very low content in Fe, Mg and Mn which is in good agreement with the scarcity of ferromagnesian minerals.

Major-element Harker diagrams (fig. 6.10) show that our five samples are all within the composition range of the other Zanskar leucogranites. Only two samples show a little anomaly in K2O content. The compositional fields for the Manaslu granite encompass 97 samples, the Gumburanjun analyses totalise 17 samples, and the other samples from Zanskar amount to 19 samples. With our five samples, these Harker diagrams show the compositional range of 138 samples. The overlapping of the various compositional fields for the Zanskar samples and the Manaslu samples show that there is little variation in the major-element contents in leucogranites from two very distant intrusions. We can thus fairly assume that our leucogranites share the same characteristics with other Himalayan leucogranites. The Al2O3-SiO2 diagram however shows that our samples are amongst the most aluminium-rich Himalayan leucogranites. This is also the only diagram where a correlation between major-element concentration and silica can be observed, if all our samples are considered together.

If we consider the biotite bearing leucogranites separately, one can observe a correlation between major-element concentration and silica in all the diagrams. It can also be seen that the tourmaline bearing leucogranites seem to behave differently from the biotite bearing leucogranites. The former one are depleted in CaO and Fe2O3 and enriched in K2O and P2O5 with respect to the latter one. Both type of granite also seem to evolve along different trends. These consideration would however greatly benefit from further analyses on both type of leucogranites as our set of data is somewhat insufficient to draw definitive conclusions.

The Gumburanjun leucogranite is peraluminous and alkali-rich, its composition in major elements is virtually identical to the other Himalayan tertiary granites.



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©Pierre Dèzes