No abstract

No abstract

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.

In situ analysis of a garnet porphyroblast from a granulite facies gneiss from Sør Rondane Mountains, East Antarctica, reveals discontinuous step‐wise zoning in phosphorus and large δ18O variations from the phosphorus‐rich core to the phosphorus‐poor rim. The gradually decreasing profile of oxygen isotope from the core (δ18O = ~15‰) to the rim (δ18O = ~11‰) suggests that the 18O/16O zoning was originally step‐wise, and modified by diffusion after the garnet rim formation at ~800°C and 0.8 GPa. Fitting of the 18O/16O data to the diffusion equation constrains a duration of the high‐T event (~800°C) to c. 0.5–40 Ma after the garnet rim formation. The low δ18O value of the garnet rim, together with the previously reported low δ18O values in metacarbonates, indicates regional infiltration, probably along a detachment fault, of low δ18O fluid/melt possibly derived from meta‐mafic to ultramafic rocks.

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 UPb 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.

No abstract

No abstract

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.