The following photos show in detail important petrographic criteria to identify magmatic amphibole in meta-granite and granitic gneiss.

The identification of different amphibole growth generations (older magmatic generation and/or younger metamorphic generation) requires microstructural analysis. The relationship between crystallization and deformation of primary magmatic and subsequent regional metamorphic amphibole growth generations must be deduced from the internal and external structure of the amphibole crystal aggregates.

Depending on the pressure-temperature and fluid-phase conditions during the subsequent metamorphic overprint, the magmatic amphiboles may suffer partially or completely replacement by metamorphic amphiboles like actinolite/ferro-actinolite series and/or ferro-hornblende/magnesio-hornblende series. In favorable cases, the magmatic form-relics are still recognizable (see description of photomicrograph-3 below).

Photomicrograph-1 shows euhedral to subhedral crystal aggregates of magmatic amphibole in an amphibole granite from Uppsala, Sweden. Thin section, width of image 2 mm.

In contrast, photomicrograph-2 shows generally lens-like riebeckite and feldspar crystal aggregates, which crystallized during ductile deformation due to strong foliation in ortho-riebeckite gneiss from Gloggnitz, Austria. Thin section, width of image 1 mm.

Photomicrograph-3 shows a zoned amphibole porphyroblast with an inclusion-rich core that is sub-vertically cut by tensional cracks (outlined by dark pigmentation) from Eastern Desert, Egypt. The core represents the old magmatic growth generation, overgrown and partially replaced by the younger inclusion-poor actinolitic amphibole generation formed during regional metamorphic overprint. The magmatic core is pre-tectonic with respect to the brittle deformation. The metamorphic rim grew post-tectonically with respect to the brittle deformation of the core. Thin section, width of image 1 mm.

Best regards,

Guenter Grundmann

Hi Aniruddha,

in between, did you find any petrographic criteria to identify magmatic amphibole in meta-granite/granite gneiss?

Furthermore, there may be an exhausting way to understand if amphiboles are re-equilibrated or not (see below):

“equilibration” => equilibrium is the condition of a system in which all competing influences are balanced.

The most important competing influences in a geological system (rock, mineralization) are:

Chemical composition of the rocks

Chemical composition of the mineral phases involved

Chemical composition of the fluid phase (CO2, H2O, etc.)

Pressure conditions (P)

Temperature conditions (T)

Time span in which the mineral reactions took place (t)

The porosity of the rocks (configuration of the inter-granular space)

Defects and / or deformations in the crystal structures

Events of brittle and / or ductile (plastic) deformation (d)

Thus, in order to be able to make statements about the re-equilibration of a particular mineral species of the Amphibole supergroup, the stability relationships and the relationship between crystallization and deformation during the temporal events of the mineral species involved must also be taken into account.

Now the Amphibole supergroup includes over 100 defined mineral species as well as countless varieties (compare "Fleischer's Glossary of Mineral Species, 2018", by Malcom E. Back, The Mineralogical Record Inc., Tucson)

The question is: which mineral species of the gigantic Amphibole supergroup should be considered specifically?

Best regards,

Guenter Grundmann

Hi,

Normally in granites, Amphiboles are falling in the category of pargasite hornblende which is cleared from Mineral chemistry but it will not sufficient to brief your answer.

Secondly you have to go through petrographic studies of amphibole - hornblende, its give an idea of deformed and preserved crystal of horblende. If it is well preserved then it represent magmatic or primary hornblende or if it is deformed then it is represent as a meta amphibole or metamorphic.