Chlorine-rich fluid activity during retrograde metamorphism – an example from Brattnipane, Sør Rondane Mountains, East Antarctica

第3回極域科学シンポジウム/第32回極域地学シンポジウム 11月30日（金） 統計数理研究所 ３階セミナー

Timing of chlorine-rich fluid infiltration and behavior of REE-bearing minerals from Balchenfjella, Sør Rondane Mountains, East Antarctica

第3回極域科学シンポジウム/第32回極域地学シンポジウム 11月30日（金） 統計数理研究所 ３階セミナー

Constraining the fluid activity in the crust by chlorine concentration of biotite and apatite -An exaple from Balchenfjella, Sør Rondane Mountains, East Antactica

第2回極域科学シンポジウム/第31回極域地学シンポジウム 11月17日（木） 国立極地研究所　2階大会議室前フロ

高度変成岩中の主要・微量元素によるマルチスケールゾーニングから制約する大陸地殻深部における塩水流入過程

京都大学0048新制・課程博士博士(理学)甲第19511号理博第4171号新制||理||1599(附属図書館)32547京都大学大学院理学研究科地球惑星科学専攻(主査)准教授 河上 哲生, 教授 平島 崇男, 教授 山 明学位規則第4条第1項該当Doctor of ScienceKyoto UniversityDFA

Ultrahigh-temperature metamorphism and melt inclusions from the Sør Rondane Mountains, East Antarctica

This paper reports the first outcrop occurrence of an ultrahigh–temperature (UHT) metamorphic rock from the Sør Rondane Mountains (SRM), East Antarctica. A pelitic gneiss from Balchenfjella, eastern SRM, contains mesoperthite that gave UHT condition (>900 °C) by ternary feldspar thermometry. The UHT mesoperthite is present both in the matrix and as an inclusion in garnet. The garnet also has nanogranitoid inclusions next to the mesoperthite, which are interpreted to be an UHT melt. The re–integrated nanogranitoid composition is plotted in the primary phase region of quartz and classified as granite. Even crystallized nanogranitoids can provide appropriate original melt composition in the An–Ab–Or and Qz–Ab–Or spaces, whereas Mg concentration is enriched due to local retrograde Fe–Mg exchange reaction between the nanogranitoid inclusions and the host garnet. Although metamorphic rocks in the SRM are highly retrogressed, this study revealed that the microstructural evidence of UHT condition is partially preserved. Further investigation of timing and areal extent of UHT metamorphism helps us to understand the tectonic model of the SRM

Metamorphic rocks with different pressure–temperature–time paths bounded by a ductile shear zone at Oyayubi ridge, Brattnipene, Sør Rondane Mountains, East Antarctica

The Sør Rondane Mountains, East Antarctica have been thought to be situated in the collision zone between East and West Gondwana during the final stage of amalgamation of the Gondwana supercontinent. They are, therefore, recognized as a key region for understanding the geological phenomena during the collisions and for testing the proposed tectonic models. We identified metamorphic rocks with different pressure-temperature-time paths that are bounded by a ductile shear zone at Oyayubi ridge, Brattnipene, Sør Rondane Mountains. Based on field and microscopic observations, chemical analyses of minerals, and zircon U-Pb dating, the sillimanite-garnet-biotite gneisses (i.e., pelitic gneisses) from higher structural level show a peak metamorphism at ∼ 590 Ma that took place under conditions of ∼ 830-840 °C and 0.8-0.9 GPa, and these high-temperature conditions lasted until ∼ 550 Ma. These rocks underwent isothermal decompression and then retrograde hydration under lower pressure-temperature conditions than 530 °C and 0.4 GPa at ∼ 530 Ma. In contrast, the orthogneisses that consist of hornblende-biotite gneiss and garnet-clinopyroxene gneiss from lower structural levels did not undergo metamorphism at ∼ 600 Ma but underwent metamorphism at ∼ 570 Ma and reached peak conditions of 700-760 °C and 0.6-0.9 GPa at ∼ 560-550 Ma. These observations suggest thrusting of the pelitic gneiss over the orthogneiss at ∼ 570-550 Ma, causing a prograde metamorphism of the orthogneisses and a decompression of the pelitic gneisses as well as uplift and subsequent rapid denudation. The results indicate two stages of collision in the Sør Rondane Mountains and that the ductile shear zone bounding the pelitic gneiss and orthogneiss units may have been part of the continental plate collision boundary at ∼ 570-550 Ma

Multiple post-peak metamorphic fluid infiltrations in southern Perlebandet, Sør Rondane Mountains, East Antarctica.

This paper reports multiple fluid infiltration events during retrograde metamorphism in the Sør Rondane Mountains, East Antarctica. Pelitic gneisses from southern part of Perlebandet have cordierite-biotite intergrowth rimming garnet, implying that garnet breakdown occurred by fluid infiltration. Using the Raman peak of CO₂ in cordierite and Cl-bearing composition in biotite, this study revealed that the cordierite-biotite intergrowth was formed in equilibrium with one-phase CO₂-Cl-H₂O fluid. The intergrowth texture is cut by thin selvages composed of Cl-bearing biotite, suggesting Cl-bearing fluid infiltration. Since andalusite is exclusively observed in the selvage, near isobaric cooling path is presumed for the pressure-temperature (P-T) path of these post-peak fluid-related reactions. The inconsistence with counter-clockwise P-T path reported from northern Perlebandet is probably due to the granodiorite/leucocratic granite bodies beneath the studied metamorphic rocks. In order to understand the tectonic evolution at the final stage of Gondwana amalgamation, therefore, effect of hidden igneous rocks needs to be taken into consideration

Geochemical behavior of zirconium during Cl–rich fluid or melt infiltration under upper amphibolite facies metamorphism -- A case study from Brattnipene, Sør Rondane Mountains, East Antarctica

The appropriateness of Zr as an ‘immobile element’ during garnet–hornblende (Grt–Hbl) vein formation potentially caused by the Cl–rich fluid or melt infiltration under upper amphibolite facies condition is examined. The sample used is a Grt–Hbl vein from Brattnipene, Sør Rondane Mountains, East Antarctica that discordantly cuts the gneissose structure of the mafic gneiss. Modal analysis of the wall rock minerals combined with the quantitative determination of their Zr contents reveals that most of the whole–rock Zr resides in zircon whereas ~ 5% is hosted in garnet and hornblende. The Zr concentration of garnet and hornblende is constant irrespective of the distance from the vein. Zircon shows no resorption or overgrowth microstructures. Moreover, the grain size, chemical zoning (CL, Th/U ratio and REE pattern) and rim ages of zircon are also similar irrespective of the distance from the vein. LA–ICPMS U–Pb dating of zircon rims does not give younger ages than the granulite facies metamorphism reported by previous studies. All of these detailed observations on zircon support that zircon is little dissolved or overgrown, and that Zr is not added nor lost during the Grt–Hbl vein formation. Therefore, Zr can be described as an appropriate ‘immobile element’ during the Grt–Hbl vein formation. Detailed microstructural observation of zircon is thus useful in evaluating the appropriateness of Zr as an immobile element

Prograde infiltration of Cl-rich fluid into the granulitic continental crust from a collision zone in East Antarctica (Perlebandet, Sør Rondane Mountains)

Utilizing microstructures of Cl-bearing biotite in pelitic and felsic metamorphic rocks, the timing of Cl-rich fluid infiltration is correlated with the pressure-temperature-time (P-T-t) path of upper amphibolite- to granulite-facies metamorphic rocks from Perlebandet, Sør Rondane Mountains (SRM), East Antarctica. Microstructural observation indicates that the stable Al2SiO5 polymorph changed from sillimanite to kyanite + andalusite + sillimanite, and P-T estimates from geothermobarometry point to a counterclockwise P-T path characteristic of the SW terrane of the SRM. In situ laser ablation inductively coupled plasma mass spectrometry for U–Pb dating of zircon inclusions in garnet yielded ca. 580 Ma, likely representing the age of garnet-forming metamorphism at Perlebandet. Inclusion-host relationships among garnet, sillimanite, and Cl-rich biotite (Cl > 0.4 wt%) reveal that formation of Cl-rich biotite took place during prograde metamorphism in the sillimanite stability field. This process probably predated partial melting consuming biotite (Cl = 0.1–0.3 wt%). This was followed by retrograde, moderately Cl-bearing biotite (Cl = 0.1–0.3 wt%) replacing garnet. Similar timings of Cl-rich biotite formation in different samples, and similar f(H2O)/f(HCl) values of coexisting fluid estimated for each stage can be best explained by prograde Cl-rich fluid infiltration. Fluid-present partial melting at the onset of prograde metamorphism probably contributed to elevate the Cl concentration (and possibly salinity) of the fluid, and consumption of the fluid resulted in the progress of dehydration melting. The retrograde fluid was released from crystallizing Cl-bearing partial melts or derived externally. The prograde Cl-rich fluid infiltration in Perlebandet presumably took place at the uppermost part of the footwall of the collision boundary. Localized distribution of Cl-rich biotite and hornblende along large-scale shear zones and detachments in the SRM supports external input of Cl-rich fluids through tectonic boundaries during continental collision