講演情報
[T4-P-3]Mechanical behavior of pegmatites during amphibolite-facies deformation in the Kasumi Rocks, Lützow-Holm Complex, East Antarctica
*Sreehari Lakshmanan1, Tatsuro Adachi2, Yuki Mori3, Tomokazu Hokada4 (1. Shimane University, 2. Kyushu University, 3. University of Hyogo, 4. NIPR)
キーワード：
Antarctica、Lower crust、Deformation、Pegmatite
The Kasumi Rocks are located approximately 150 km northeast of Syowa Station in the Lützow-Holm Complex, East Antarctica, and expose an area of about 2 km². The main lithologies consist of biotite gneiss, pink granitic gneiss, amphibolite, marble, skarn, granitic pegmatite, and minor ultramafic rocks. Granitic rocks locally crosscut amphibolite, whereas marble and ultramafic rocks commonly occur as enclaves within the granitic units. Granitic pegmatite containing megacrystic K-feldspar forms a distinctive unit and is commonly emplaced parallel to the gneissic foliation. A later generation of granitic pegmatite locally crosscuts the main lithological units and represents the youngest magmatic activity observed in the area.Previous geochronological studies reported a U–Pb zircon age of 984 ± 6.9 Ma from biotite gneiss and an Rb–Sr isochron age of 491.2 ± 23.4 Ma from granitic pegmatite. In this study, we examine the structural evolution of the Kasumi Rocks, with particular attention to the relationship between pegmatite emplacement and regional deformation.
The mapped structures indicate progressive deformation involving early compositional layering and foliation development in metasedimentary rocks, followed by the formation of tight to isoclinal folds and later upright to steeply inclined folds. Although several fold generations can be distinguished geometrically, the main foliations are commonly defined by similar mineral assemblages, and mineral lineations are generally parallel or subparallel to fold hinge lines. In addition, an older foliation is locally preserved in ultramafic enclaves, and narrow ductile shear zones with top-to-the-north shear sense indicators occur in several places.
Mesoscopic and microscopic observations suggest that the main foliations in the granitic gneiss and related rocks developed under amphibolite-facies conditions. Amphibole-bearing mineral assemblages in the basement granitic gneiss, together with amphibolite-grade mineral assemblages in associated units, indicate that deformation occurred at relatively high temperature.
A key feature of the Kasumi Rocks is the structural behavior of pegmatites during amphibolite-facies deformation. Pegmatites are abundant and commonly intrude parallel to the dominant foliations, although local crosscutting relationships are also present. Despite being involved in regional-scale folding, many pegmatites preserve solid-state fabrics and locally behave as mechanically competent layers. This is shown by necking and boudinage structures developed within the surrounding biotite gneiss and pegmatite respectively. Megacrystic K-feldspar is commonly concentrated near the margins of pegmatite bodies, close to the contact with the host rocks. Rapid crystallization of pegmatitic melt, together with early strengthening by feldspar accumulation near the contacts, may have allowed these bodies to behave as relatively strong layers during ongoing deformation. In contrast, thinner leucocratic veins derived from the pegmatites locally intruded surrounding units and deformed as mechanically weaker domains.These observations suggest that pegmatites in the Kasumi Rocks were not passive late-stage intrusions, but contributed to strain partitioning during amphibolite-facies deformation. The Kasumi Rocks therefore provide a useful natural example for examining how pegmatite emplacement, crystallization, and mechanical strengthening influence deformation in high-grade continental crust.
The mapped structures indicate progressive deformation involving early compositional layering and foliation development in metasedimentary rocks, followed by the formation of tight to isoclinal folds and later upright to steeply inclined folds. Although several fold generations can be distinguished geometrically, the main foliations are commonly defined by similar mineral assemblages, and mineral lineations are generally parallel or subparallel to fold hinge lines. In addition, an older foliation is locally preserved in ultramafic enclaves, and narrow ductile shear zones with top-to-the-north shear sense indicators occur in several places.
Mesoscopic and microscopic observations suggest that the main foliations in the granitic gneiss and related rocks developed under amphibolite-facies conditions. Amphibole-bearing mineral assemblages in the basement granitic gneiss, together with amphibolite-grade mineral assemblages in associated units, indicate that deformation occurred at relatively high temperature.
A key feature of the Kasumi Rocks is the structural behavior of pegmatites during amphibolite-facies deformation. Pegmatites are abundant and commonly intrude parallel to the dominant foliations, although local crosscutting relationships are also present. Despite being involved in regional-scale folding, many pegmatites preserve solid-state fabrics and locally behave as mechanically competent layers. This is shown by necking and boudinage structures developed within the surrounding biotite gneiss and pegmatite respectively. Megacrystic K-feldspar is commonly concentrated near the margins of pegmatite bodies, close to the contact with the host rocks. Rapid crystallization of pegmatitic melt, together with early strengthening by feldspar accumulation near the contacts, may have allowed these bodies to behave as relatively strong layers during ongoing deformation. In contrast, thinner leucocratic veins derived from the pegmatites locally intruded surrounding units and deformed as mechanically weaker domains.These observations suggest that pegmatites in the Kasumi Rocks were not passive late-stage intrusions, but contributed to strain partitioning during amphibolite-facies deformation. The Kasumi Rocks therefore provide a useful natural example for examining how pegmatite emplacement, crystallization, and mechanical strengthening influence deformation in high-grade continental crust.
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