In response to your query “How to map Granulite Belts”, i have the following suggestions:
1. The granulite belts indeed provide an excellent window to study the deep crustal processes as the granulites are generally formed at greater depths (30-35 km). Field mapping, such as recognising different lithounits, their contact relatioship, structural features etc, is similar to what is generally adopted in other metamorphic terrains. However in the case of high grade granulite terrains, the primary structures which help in building up the stratigraphy will invariably be obliterated due to intense deformation and metamorphism. So meticulously recording the planar and linear fabrics developed in these granulite facies rocks and working the geometry of the folds will help you in understanding the stratigrahic sequence of the granulites.
2. In metamorphic rocks – especially in granulites, the mineral assemblages in various lithounits serve as an important tool, not only to understand the grade of metamorhism, but also help in unravelling the geodynamic evolution of the granulite terrain. Hence very critical study of the petrography of the granulites, more particularly the textural relationship among the various minerals in pelitic granulites and associated rocks, and analysing the mineral chemistry of the coexisting equilibrium phases will help in working out the different events of metamorphism (M1, M2 ,M3) and in tracing the P-T-t path of the granulites. The coTmon P-T paths ,viz. Isothermal decomression (ITD) and Isobaric Cooling (IBC) paths reveal the geodynamic processes the terrain has undergone such as crustal thickening / thinning due to collisional /extensional processes as well as the exhumation history of the granulites.
3.In the granulite terrains of Eastern Ghats Mobile Belt or the Southern Granulite Terrain of South India, Charnockites / enderbites form the major constituents with interbanded sequences of Quartzite-pelite-carbonates of platformal association. In such cases, while mapping greater attention to be paid to bring out the actual field relationship of the charnockites/enderbites with the associated metasedimentary rocks. Certain field characters like intrusive relationship and presence of xenoliths of the metasedimentary rocks will indicate the magmatic nature of the Charnockites (e.g. C-Type Charnockite as documented in East Antarctica by Kilpatrick and Ellis).
4. If we are interested in understanding the evolutionary history of the granulites in space and time, we can resort to geochronological studies by isotopic dating of the suitable minerals such as zircon, monazite, huttonite, etc by SHRIMP or SIMS methods. These isotopic data help in understanding the magmatic / sedimentation history of the protoliths and the imprints of subsequent tectothermal events / metamorphism on these granulites.
The granulite terrains all over the world are receiving very great attention by the entire geoscientific community as they provide excellent clues to understand the amalgamation and break-up of supercontinents of Pangea / Rodinia and East Gondwana, in different geological times.
Hope i am able to answer your query to some extent on this very important subject of research.
Mineral Exploration Academy
In response to your query “How to map Granulite Belts”, i have the following suggestions:
1. The granulite belts indeed provide an excellent window to study the deep crustal processes as the granulites are generally formed at greater depths (30-35 km). Field mapping, such as recognising different lithounits, their contact relatioship, structural features etc, is similar to what is generally adopted in other metamorphic terrains. However in the case of high grade granulite terrains, the primary structures which help in building up the stratigraphy will invariably be obliterated due to intense deformation and metamorphism. So meticulously recording the planar and linear fabrics developed in these granulite facies rocks and working the geometry of the folds will help you in understanding the stratigrahic sequence of the granulites.
2. In metamorphic rocks – especially in granulites, the mineral assemblages in various lithounits serve as an important tool, not only to understand the grade of metamorhism, but also help in unravelling the geodynamic evolution of the granulite terrain. Hence very critical study of the petrography of the granulites, more particularly the textural relationship among the various minerals in pelitic granulites and associated rocks, and analysing the mineral chemistry of the coexisting equilibrium phases will help in working out the different events of metamorphism (M1, M2 ,M3) and in tracing the P-T-t path of the granulites. The coTmon P-T paths ,viz. Isothermal decomression (ITD) and Isobaric Cooling (IBC) paths reveal the geodynamic processes the terrain has undergone such as crustal thickening / thinning due to collisional /extensional processes as well as the exhumation history of the granulites.
3.In the granulite terrains of Eastern Ghats Mobile Belt or the Southern Granulite Terrain of South India, Charnockites / enderbites form the major constituents with interbanded sequences of Quartzite-pelite-carbonates of platformal association. In such cases, while mapping greater attention to be paid to bring out the actual field relationship of the charnockites/enderbites with the associated metasedimentary rocks. Certain field characters like intrusive relationship and presence of xenoliths of the metasedimentary rocks will indicate the magmatic nature of the Charnockites (e.g. C-Type Charnockite as documented in East Antarctica by Kilpatrick and Ellis).
4. If we are interested in understanding the evolutionary history of the granulites in space and time, we can resort to geochronological studies by isotopic dating of the suitable minerals such as zircon, monazite, huttonite, etc by SHRIMP or SIMS methods. These isotopic data help in understanding the magmatic / sedimentation history of the protoliths and the imprints of subsequent tectothermal events / metamorphism on these granulites.
The granulite terrains all over the world are receiving very great attention by the entire geoscientific community as they provide excellent clues to understand the amalgamation and break-up of supercontinents of Pangea / Rodinia and East Gondwana, in different geological times.
Hope i am able to answer your query to some extent on this very important subject of research.
Dr.N.P.Nathan
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