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Unique igneous textures and shock metamorphism of the Northwest Africa 7203 angrite: Implications for crystallization processes and the evolutionary history of the angrite parent body

2022-01-01T00:00:00Z

Title: Unique igneous textures and shock metamorphism of the Northwest Africa 7203 angrite: Implications for crystallization processes and the evolutionary history of the angrite parent body Authors: Hayashi, Hideyuki; Mikouchi, Takashi; Kim, Nak Kyu; Park, Changkun; Sano, Yuji; Takenouchi, Atsushi; Yamaguchi, Akira; Kagi, Hiroyuki; Bizzarro, Martin Abstract: Northwest Africa (NWA) 7203 is a quenched angrite, showing mineralogical features typically not present in other quenched angrites. NWA 7203 exhibits textures whose grain size varies from fine grains (<10 mu m) to coarse grains (similar to 3 mm), while other quenched angrites show only single-sized textures. Fine-grained and coarse-grained lithologies have nearly the same bulk compositions. Cooling rates were estimated to be similar to 80 degrees C h(-1) for fine-grained lithologies and similar to 1 degrees C h(-1) for coarse-grained lithologies. Mg-rich olivines (similar to Fo(64)) were found only in fine-grained lithologies. Crystallization of NWA 7203 started in the fine-grained lithologies with Mg-rich olivine grains acting as seeds for crystallization. Coarse-grained lithologies were subsequently formed under conditions of slower cooling. NWA 7203 shows clear shock metamorphic textures unlike other quenched angrites except for NWA 1670. We confirm that the oxygen isotopic ratios of NWA 7203 plot on the angrite fractionation line within uncertainty. However, the obtained Pb-Pb age of NWA 7203 is 4543 +/- 19 Ma, younger than the ages of other quenched angrites, which might be a result of disturbance by shock metamorphism. The finding of shock metamorphism of NWA 7203 suggests that some angrites might be derived from asteroids that remained large (>10 km in diameter) during the late heavy bombardment.

Tectonic constraints on formation and evolution of microplates in the Indian and Pacific Oceans: reviews and statistical inferences

2021-12-01T00:00:00Z

Title: Tectonic constraints on formation and evolution of microplates in the Indian and Pacific Oceans: reviews and statistical inferences Authors: Choi, Hakkyum; Kim, Seung-Sep; Park, Sung-Hyun Abstract: Oceanic plates are growing through narrow boundaries, such as mid-ocean ridges and transform faults. However, the discovery of diffuse plate boundary suggests another type of plate boundary that accommodates difference in plate motion via internal deformation. Along the Central and Southeast Indian ridges, for example, the Capricorn and Macquarie microplates exhibit widespread diffuse boundaries and hence divide the Indo-Australian Plate further into the Indian, Australian, Capricorn, and Macquarie plates. As for microplates distributed along the East Pacific Rise and Pacific-Antarctic Ridge in the Pacific Ocean, however, the typical plate boundaries surrounding the given microplate are distinctly established. Global plate reorganization involving the changes in plate motion or in spreading direction can be accommodated by forming a microplate through ridge extinction, ridge propagation, and pseudofault formation. However, relations between these tectonic processes have not been quantitatively assessed. In particular, we aim to examine tectonic constrains on the formation processes of microplates with diffuse plate boundary. In this study, we compare plate size, plate age, full-spreading rates, thermal structures, total rotation, and rotation rate for the 9 microplates including extinct plates (i.e., Capricorn, Macquarie, and Mammerickx* microplates in the Indian and Southern Oceans; Galapagos, Easter, Juan Fernandez, Bauer*, Friday*, and Selkirk* microplates in the Pacific Ocean; extinct plates are denoted with asterisks). From this comparison, we find that the microplate formation would require certain tectonic conditions (e.g., full-spreading rates faster than 70-80 mm/yr and rotation rates faster than 5-6 degrees/m.y.) to evolve into an independent and rigid plate with respect to the neighboring plates. If the conditions are not met, the same tectonic reorganization would result in a microplate with diffuse plate boundaries.

Oxygen isotope record of magmatic evolution of alkaline volcanic rocks at The Pleiades, northern Victoria Land, Antarctica

2021-12-01T00:00:00Z

Title: Oxygen isotope record of magmatic evolution of alkaline volcanic rocks at The Pleiades, northern Victoria Land, Antarctica Authors: Kim, Nak Kyu; Lee, Mi Jung; Lee, Jong Ik; Kim, Jihyuk Abstract: Oxygen isotopes are used to examine the well-documented magmatic evolution of alkaline volcanic rocks at The Pleiades in northern Victoria Land (NVL), Antarctica. Oxygen isotopes were measured in olivine, clinopyroxene, and plagioclase phenocrysts to better understand the origin and evolution of the sodic and potassic differentiation lineages. The volcanic rocks at The Pleiades evolved from a parental basanite by fractional crystallization in crustal magma chambers. Olivine which crystallizes first in the mafic magmas provides the initial oxygen isotope composition of the magmatic lineages. The delta O-18(OL) values for the mafic lavas in the sodic lineage are lower than the potassic lineage. The primary melt derived from the metasomatized lithospheric mantle may have consumed a low-delta O-18 amphibole metasome. Subsequently, the melt would have evolved to the normal-delta O-18 potassic lineage magma by a large contribution from surrounding peridotite. In contrast, the sodic lineage magma might preserve low-delta O-18 characteristics because of insufficient reaction with the surrounding mantle peridotite. Intermediate rocks of the potassic lineage exhibit a wide variation in their oxygen isotope compositions and it deviates from the theoretical normal-delta O-18 trend. Hence, variable delta O-18(OL) values of the intermediate rocks suggest that high-delta O-18 recorded in olivine could be reconciled with an assimilation of crustal rocks in NVL, and hydrothermally altered material may have contributed to low-delta O-18 signature of the olivines.

Magmatic-Hydrothermal Processes of Vein-Type Haman-Gunbuk-Daejang Copper Deposits in the Gyeongnam Metallogenic Belt in South Korea

2021-11-24T00:00:00Z

Title: Magmatic-Hydrothermal Processes of Vein-Type Haman-Gunbuk-Daejang Copper Deposits in the Gyeongnam Metallogenic Belt in South Korea Authors: Lee, Tong Ha; Seo, Jung Hun; Yoo, Bong Chul; Lee, Bum Han; Han, Seunghee; Yang, Yun Seok; Lee, Jun Hee Abstract: Haman, Gunbuk, and Daejang deposits are neighboring vein-type hydrothermal Cu deposits located in the SE part of the Korean Peninsula. These three deposits are formed by magmatic-hydrothermal activity associated with a series of Cretaceous granodioritic intrusions of the Jindong Granitoids, which have created a series of veins and alterations in a hornfelsed shale formation. The copper deposits have common veining and alteration features: 1) a pervasive chlorite-epidote alteration, cut by 2) Cu-Pb-Zn-bearing quartz veins with a tourmaline-biotite alteration, and 3) the latest barren calcite veins. Chalcopyrite, pyrite, and pyrrhotite are common ore minerals in the three deposits. Whereas magnetite is a dominant mineral in the Haman and Gunbuk deposits, no magnetite is present, but sphalerite and galena are abundant in the Daejang deposit. Ore-bearing quartz veins have three types of fluid inclusions: 1) liquid-rich, 2) vapor-rich, and 3) brine inclusions. Hydrothermal temperatures obtained from the brine inclusion assemblages are about 340-600, 250-500, and 320-460 degrees C in the Haman, Gunbuk, and Daejang deposits, respectively. The maximum temperatures (from 460 to 600 degrees C) recorded in the fluid inclusions of the three deposits are higher than those of the Cu ore precipitating temperature of typical porphyry-like deposits (from 300 to 400 degrees C). Raman spectroscopy of vapor inclusions showed the presence of CO2 and CH4 in the three deposits, which indicates relatively reduced hydrothermal conditions as compared with typical porphyry deposits. The Rb/Sr ratios and Cs concentrations of brine inclusions suggest that the Daejang deposit was formed by a later and more fractionated magma than the Haman and Gunbuk deposits, and the Daejang deposit has lower Fe/Mn ratios in brine inclusions than the Haman and Gunbuk deposits, which indicates contrasting redox conditions in hydrothermal fluids possibly caused by an interaction with a hosting shale formation. In brines, concentrations of base metals do not change significantly with temperature, which suggests that significant ore mineralization precipitation is unlikely below current exposure levels, especially at the Haman deposit. Ore and alteration mineral petrography and fluid inclusions suggest that the Haman deposit was formed near the top of the deep intrusion center, whereas the Gunbuk deposit was formed at a shallower intrusion periphery. The Daejang deposit was formed later at a shallow depth by relatively fractionated magma.