The most important mineralogical feature of these veins is the occurrence and significant abundance of calcite in it, which indicates the high f CO2 in the fluid of the minerals of these veins. The gabbro containing the above veins first metamorphosed in the amphibolite facies (pressure 2.7 to 9 kb, and temperature 630 to 750 ° C) and then underwent a reversible metamorphism in the green schist facies.

First of all, what is the age of formation of your metapelites and the metamorphic grade hosting the allanite? Without that knowledge only general remarks can be given herein. You can test the allanite (orthite) with regard to it relative age between host and target mineral (=allanite). If the hosting metapelite is rather young and the allanite has been derived from a rather old source the allanite should be strongly metamictic due to the high content of U ( < 180 ppm U). Detrital allanite is very much susceptible to supergene alteration and calcite and Fe oxides should be adjacent to it in the metapelites because allanite normally decomposes easily into “limonite” and calcite. If allanite formed in the metapelites as a consequence of hydrothermal or metamorphic processes in young host rocks Neogene to Paleogene the metamictisation should not have been reached an advanced stage and you can use the common optical properties handed down in many textbooks (colorless-brown-greenish tints). Pleochroic haloes should not be pronounced even if the maximum U contents are attained. During my work in the U branch, I found that even in high-grade non-cubic minerals such as brannerite of Tertiary deposits, the metamictic isotropism has not attained full completeness. In most rocks allanite is intergrown with or forms a zonality with epidote and clinozoisite.

The “royal way” is to determine the age of the metapelite and separates of allanite. Then you can determine the absolute age of formation of host and allanite and the question whether the allanite is detrital, hydrothermal or metamorphic is easy to be answered.

HGD

thanks to Farhad Pirmohammadi Alishah and Harald G. Dill for your answers. Harald G. Dill sir, the metapelites are paleoproterozoic in age and they have suffered dominantly greenschist facies metamorphism to lower amphibolite facies at places. the hydrothermal imprints also are known in multiple episodes ranging from paleo to mesoproterozoic times known to be in association with shearing and mylonitisation. As rightly described by you, there are allanite with metamict core and prominent halo as well as non metamict ones without halo. however the metamict ones occur in clusters with pronounced epidote rim and have well developed euhedral morphology. owing to their morphology and clustering along mylonitic folaition they appear to be post tectonic hydrothermal ones however some of the published dates suggest these are prior to shearing. This creates a confusion. in this context, i need guidance in ways to differentiate these three kinds of allanites for better understanding.

My query can be summarised as follows:

If occuring in randomly oriented clusters with euhedral morphology , but overall distribution is confined paralell to mylonitic foliaiton, shouldnt they be post tectonic hydrothermal phases? In that case, how come ages older than shearing be given by such phases? What are the possibilities here?

Thank you.

regards

It is one of the 10000 $ questions where you can give a lot of answers but full of speculations. I am doing geoscientific research work (by myself) since 1976 and I learnt that a profound answer to such a question is fraught with many difficulties. All what I can do is to shed some light on some facets and / or give you a rather general overview. You cannot outsource finding out a solution because you are the only one who has a complete overview of the setting

HGD