Two‐stage growth of porphyroblastic biotite and garnet in the Barrovian metapelites of the Imjingang belt, central Korea

Abstract

Porphyroblastic biotite and garnet in the Barrovian metapelites of the Imjingang belt, Korea, were investigated to unravel the sequence and mechanism of mineral growth. Poikiloblastic biotite contains straight inclusion trails (S i ) discontinuous to the major foliation, and develops clear zones at the grain margin. These microstructures suggest an initial growth of biotite between two contractional deformations (D n)1 and D n ) followed by an overgrowth during D n . Although garnet poikiloblasts contain variable S i patterns, their major growth is likely to have occurred during D n on the basis of compositional relationships among variable garnet types. Early poikiloblasts of both minerals were formed by chemical replacement of the matrix that consisted mainly of chlorite, muscovite and quartz. Subsequent growth of biotite was governed by a crack-filling mechanism, and was accompanied by the production of extensional cracks inside or around biotite, providing fluid pathways. The overgrowth of garnet was favoured at the biotite-garnet interface, and the consequence was a partial replacement of inclusion-poor garnet after biotite subsequent to D n . In addition, clear zones and pressure shadows as well as the matrix around biotite porphyroblasts were replaced by garnet, suggesting an inheritance of various pre-existing microstructures in the S i pattern of garnet. Further attention is thus required for any attempt to delineate the microstructural interaction between deformation and metamorphism, particularly in a sample containing early-grown porphyroblasts. Microstructural evidence for the twostage growth of biotite and garnet is present up to the kyanite zone, indicating that this growth mechanism is prevalent during progressive metamorphism of Barrovian metapelites.

P–T evolution and episodic zircon growth in barroisite eclogites of the Lanterman Range, northern Victoria Land, Antarctica

“…The M 3A stage is represented by the irregularly shaped, isolated inner sub‐domain of the outermost part enriched in grossular (Grs = 0.26–0.30; Figures and ), which may represent an inheritance from the former calcic minerals such as omphacite and amphibole due to slow diffusivity of Ca (Chernoff & Carlson, ; Kim & Cho, ). Relatively dry conditions of the eclogite subsequent to the M 1–2 dehydration might further decelerate the Ca mobility during the M 3A garnet growth (e.g.…”

“…Konrad‐Schmolke et al., ) cannot account for an abrupt increase in grossular of the outermost sub‐domain (M 3B garnet). Thus, the grossular enrichment in the M 3A garnet likely reflects the variation in local bulk compositions instead of P–T conditions (Castro & Spear, ; Kim & Cho, ); the P–T conditions for the M 3 garnet overgrowth are estimated only from the composition of the M 3B garnet. The M 3B growth stage was subject to the same reaction producing the M 2 garnet.…”

Growth, behavior, and textural sector zoning of biotite porphyroblasts during regional metamorphism and the implications for interpretation of inclusion trails: Insights from the Pequop Mountains and Wood Hills, Nevada, USA

“…This may be an issue for garnet because the garnet-forming reaction may involve replacement of biotite, and hence garnet may inherit inclusion trails and other microstructures that represent the sum of the infl uence of the constructive and destructive phases of biotite growth in the biotite zone rather than conditions in the garnet zone. Garnet that inherited biotite inclusion trails was documented in the Scottish Highlands by Barker (2002) and in the Imjingang belt in Korea by Kim and Cho (2008). In summary, the foregoing observations indicate that caution should be used when interpreting inclusion trails in biotite in general and specifi cally in higher grade index porphyroblasts that may have replaced other porphyroblast species.…”

“…These porphyroblasts are probably all syntectonic, but the microstructure refl ects variations in rheology, strain and growth rate, and cut effects. Furthermore, growing evidence suggests that some porphyroblast species that chemically replace others may inherit their inclusion trails (e.g., Rubenach and Bell, 1988;Barker, 2002;Passchier and Trouw, 2005;Kim and Cho, 2008), which could lead to erroneous interpretations, especially if replacement of biotite is involved. This may be an issue for garnet because the garnet-forming reaction may involve replacement of biotite, and hence garnet may inherit inclusion trails and other microstructures that represent the sum of the infl uence of the constructive and destructive phases of biotite growth in the biotite zone rather than conditions in the garnet zone.…”

“…This study builds on work in Australia, the Pyrenees, Japan, the Scottish Highlands, and Korea by Vernon and Flood (1979), Lister et al (1986), Miyake (1993), Barker (2002), and Kim and Cho (2008), respectively. These authors collectively showed that the growth of biotite in upper greenschist to lower amphibolite facies is a two-stage process.…”