It depends on the andradite content in the s.s.s. Grossular is of widespread occurrence in Ca- and CO2 - enriched protoliths undergoing medium- to high-grade metamorphic alteration with little effect on the Fe in the parent material. In metasomatized rocks, however, it may rise so as to bring about andradite.

Some examples:

Uvarovite as a member of the ugrandite s.s.s. (grossularite and andradite

garnets) develops in a metamorphic environment in serpentinites

with chromite and in metamorphosed limestones.

Calcic Fe skarns also differ from magnesian Fe

skarns by their alteration minerals evolving in the exocontact with

diopside–hedenbergite and grossularite–andradite garnet being the

most widespread skarn minerals.

Another variety known as schorlomite (black titanium-rich

andradite) is reported from Magnet Cove Arkansas, USA, which is

better known for its Ti–V–Nb mineralization and alkali metasomatism

(Flohr, 1994). Another titaniferous garnet melanite is also known

from the Kaiserstuhl alkaline complex, Germany. Garnet in both sites

is part of the magmatic differentiation process.

My suggestion is to look a bit closer at the associated trace elements such Sn, W, and anomalously high Al contents (often with jadeite) to constrain the existing environment and then trace it back to the potential parent material. The Al-alkaline element relationship seems to be the clue in context with the above trace elements. Sometimes a detour gets you faster to your goal than the direct route

HGD

These garnets can be late magmatic garnets or hydrothermal. What’s the size and occurence of garnets?

Dear Harald G. Dill Sir,

Thank you for your insightful explanation of the activities involved in the formation of garnets. I thought that metasomatism might be a suitable process as the EPMA data of these garnets revealed almost equal proportions of endmember grossular and andradite 54:46 after normalization (average of 16-point analyses). Garnets also have inclusions of sodalite, calcite, nepheline, and plagioclase.

Dear Voznyak A.A sir, Thank you for the response,

Garnets can be identified in hand specimens as tiny aggregates exhibiting various shades of brown/red and orange tinge and are distributed parallel to the defined foliation constituted by mafic and felsic mineral bands in gneisses. Under the microscope, they exhibit a subhedral form.

Even if the close-up view of the specimen only gives a small insight, one can state that the mineral orientation is very vague or almost absent, a fact which does not speak in favor of a dynamometamorphic process, rather in favor of a contact metasomatic or contact metamorphic process. My guess is a metasomatic process . Are there larger syenite plutons nearby? The Fe may have been introduced, but the sodalite and nepheline are replacement products of the albite component of plagioclase s.s.s.

Magmatic garnet must not be excluded. Is it Ti-rich? Sodalite and cancrinite could be alteration products, but they may also be primary phases.

Dear, Harald G. Dill sir,

There are no contiguous syenite rocks that surround this nepheline syenite pluton. These nepheline syenite gneisses (sub solvus variety) present at the periphery with a core consisting of hypersolvus nepheline syenite, intruded into the country rock of granite gneisses, and gabbros with streaks of amphibolites.

Dear Gavril Săbău Sir,

These garnets are Ti-poor (TiO2-0.31-1.00)

Dear Sridhar,

it is worth investigating further, pure grandite garnet in alkali syenite doesn't appear compatible, so indeed metasomatism (one way or the other) is the most probable culprit. You mentioned cancrinite, is it in any kind of relationship with calcite (or nepheline, for that matter)?

Dear Gavril Săbău sir,

Cancrinite has an association with nepheline and calcite. I have attached the microphotograph for your reference.

Cancrinite is one of the most common replacement/alteration products in Ne-bearing magmatic and metamorphic rocks under hydrothermal conditions