BH250-22
Title
Subject
Description
Major Mineral: Chloritoid
Description
BH250-22 is a fine example of a highly foliated, medium-grade metamorphic rock that records a complex history of deformation and metamorphism. Below is an interpretation of the processes that led to its formation, based on mineralogical, textural, and structural features observed in thin section.
Major Minerals: Chloritoid – A key index mineral occurring in both pre-kinematic and syn-kinematic forms, indicating multiple stages of growth during progressive deformation.
Garnet – Present as porphyroblasts, suggesting growth during metamorphism. Garnets show inclusion trails that record internal foliation.
Quartz – A common matrix mineral, often exhibiting undulatory extinction and pressure shadows around porphyroblasts, indicative of deformation.
Graphite – Forms part of the opaque groundmass, typically aligned along cleavage planes and foliation surfaces.
Muscovite – Dominantly defines the foliation, contributing to the rock’s micaceous sheen and fabric.
Mineralogy Comments: BH250-22 displays the strongest foliation among related thin sections, defined by the alignment of muscovite and graphite.
The presence of internal foliation within chloritoid suggests that these crystals overgrew an earlier fabric, pointing to a multistage metamorphic evolution.
Texture and Microstructures: Brittle Mica: Micas, particularly chloritoid, show evidence of brittle deformation, likely resulting from late-stage stress at lower metamorphic temperatures.
Foliation: A well-developed planar fabric is defined by aligned muscovite and graphite. Quartz may be locally segregated into compositional bands.
Porphyroblasts: Garnet and chloritoid are the dominant porphyroblasts.
Chloritoid porphyroblasts exhibit both pre- and syn-kinematic growth, and many contain internal foliation, further supporting the interpretation of multistage mineral growth.
The coexistence of pre- and syn-kinematic garnet and chloritoid, along with their internal structures, indicates that metamorphism and deformation occurred in overlapping pulses, rather than as a single, synchronous event.
Interpretation and Geological Significance: BH250-22 represents a rock that experienced progressive metamorphism coupled with deformation, likely within the greenschist to lower amphibolite facies. The mineral assemblage of chloritoid + garnet is consistent with intermediate pressures and moderate temperatures, typical of regional metamorphism.
The complex textural evolution—from early porphyroblast formation to later foliation development and brittle deformation—reflects a dynamic tectonic environment, probably related to continental collision and orogenic processes.
Metamorphic Rocks of Eritrea: The metamorphic rocks of Eritrea are primarily Neoproterozoic in age, formed during the East African Orogeny (ca. 850–550 Ma). This major event led to the assembly of the Arabian-Nubian Shield, which includes much of Eritrea’s crystalline basement. The region’s metamorphic rocks—such as schists, gneisses, and amphibolites—were affected by high-grade metamorphism and deformation during this time.
Pan-African Orogeny (~650–550 Ma): The most significant tectono-metamorphic event in Eritrea. Many schists and gneisses formed or were reworked during this orogeny, associated with the final assembly of the Gondwana supercontinent.
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