Mount Willing in the Prince Charles Mountains (East Antarctica) is part of the Fisher Volcano–plutonic complex which formed as part of the global-scale Grenvillian mobile belt system. Mount Willing is composed of four rock complexes: 1) a metamorphic sequence, 2) gabbro intrusions, 3) deformed felsic intrusives, and 4) abundant post-metamorphic dykes and veins. Three rock types constitute the metamorphic sequence: amphibole–biotite felsic plagiogneiss, mafic to intermediate biotite–amphibole schist, and biotite paragneiss. The bulk composition of the mafic schists classifies them as tholeiitic basalts, and rarely as basaltic andesites or andesites. Index mg ranges widely from 47 to 71. Concentrations of TiO 2 , P 2 O 5 , and high-field strength elements are high in some rocks. These rocks are thought to have been derived from enriched (subcontinental) mantle sources. Sm–Nd and U–Pb isotopic data indicate a series of Mesoproterozoic thermal events between 1100 and 1300 Ma. In particular, these events occurred at 1289 ± 10 Ma (volcanic activity), at 1177 ± 16 Ma (tonalite intrusion), at 1112.7 ± 2.4 and at 1009 ± 54 Ma (amphibolite facies metamorphic events). Rb–Sr systematics also indicates a thermal overprint at 636 ± 13 Ma. Mafic schists show low initial 877 Sr/ 86 Sr ratios between 0.7024 and 0.7030. Felsic rocks show higher Sr i values between 0.7037 and 0.7061. Basaltic andesite metavolcanic and plutonic rocks form a calc-alkaline evolutionary trend, and probably originated from subduction-modified mantle sources in a convergent plate margin environment. An oceanic basin may have existed in central Prince Charles Mountains about 1300 Ma ago and was closed as a result of continental collision around 1000 to 800 Ma.
Precambrian rocks at Mt Meredith underwent granulite-facies metamorphism M1. Zircon isotope dating for two orthogneisses revealed the following age signatures: 1294±3 and 957±4Ma; 1105±5 and 887±2Ma. The oldest ages could reflect the time of orthogneiss protolith crystallization and the latest age determinations date Grenvillian metamorphism. The metamorphic rocks were intruded by two-mica and garnet-biotite granites. The granites and host rocks underwent amphibolite-facies metamorphism M2. Zircon isotope analysis of the two-mica granites showed age estimation within 550-510Ma and zircon dating of the garnet-biotite granites revealed the ages of 1107±5, 953±8, and 551±4Ma. As Pan-African age signatures were obtained from only the granite samples, it is possible to suggest that the granites were formed at the time of 510-550Ma and the zircons with greater age values were captured by granites from the host rocks.
Fisher Massif is believed to represent less metamorphosed portions of an extensive Proterozoic mobile belt, and is composed of metavolcanic rocks of different compositions and numerous intrusive bodies. U-Pb dating of six zircon fractions recovered from metavolcanic rocks of intermediate to acidic compositions defines growth time at c. 1300 Ma with prominent Pb losses at 364 Ma and in recent time. Grain morphologies do not provide unequivocal genetic evidence, but an igneous origin for the grains studied is the most probable. The dates obtained probably reflect igneous activity be co-eval with mafic dyke emplacement event elsewhere in ancient East Antarctic cratonic blocks.
Fisher Massif consists of Mesoproterozoic ( c. 1300 Ma) lower amphibolite-facies metavolcanic rocks and associated metasediments, intruded by a variety of subvolcanic and plutonic bodies (gabbro to granite). It differs in both composition and metamorphic grade from the rest of the northern Prince Charles Mountains, which were metamorphosed to granulite facies about 1000 m.y. ago. The metavolcanic rocks consist mainly of basalt, but basaltic andesite, andesite, and more felsic rocks (dacite, rhyodacite, and rhyolite) are also common. Most of the basaltic rocks have compositions similar to low-K island arc tholeiites, but some are relatively Nb-rich and more akin to P-MORB. Intermediate to felsic medium to high-K volcanic rocks, which appear to postdate the basaltic succession, have calc-alkaline affinities and probably include a significant crustal component. On the present data, an active continental margin with associated island arc was the most likely tectonic setting for generation of the Fisher Massif volcanic rocks.
The Grove Mountains consist of layered grey migmatitic biotite ± hornblende gneiss, leucocratic tonalitic to granitic gneiss, quartzite, and biotite-garnet paragneiss, with rare mafic schist and calc-silicate rocks. Granite and granodiorite bodies crop out at a few localities and charnockite at Mount Harding. Two zircon size fractions from a paragneiss at Austin Nunatak and one from a felsic layer in the same sample have markedly discordant isotopic ratios. The coarser zircon fraction from the gneiss plots close to concordia at about 800-750 Ma, whereas the other fractions are much more discordant, indicating much older inherited components. Rutile from the felsic layer and titanite from a leucogneiss are slightly discordant and their model ages point to a high-grade metamorphic event at about 510- 508 Ma. Forcing the two most discordant zircon fractions through c. 500 Ma suggests derivation from Palaeoproterozoic sedimentary protoliths or, more likely, their felsic igneous source regions. Three magmatic zircon size fractions from the charnockite have nearly concordant ages of about 504 ±2 Ma, that are undistinguishable at the 2s level, and are broadly similar to the rutile and titanite ages. In spite of being much younger, the charnockite has a very similar chemical composition to some of the abundant early Neoproterozoic ( c. 980 Ma) charnockite plutons in the northern Prince Charles Mountains and Mawson Coast believed to have originated in a late- orogenic, rather than anorogenic, tectonic environment. By analogy, the Grove Mountains area was probably affected by major Pan-African (c. 500 Ma) tectonic activity. The Grove Mountains cannot be correlated with the mainly Archaean-Palaeoproterozoic Ruker Terrane of the southern Prince Charles Mountains on isotopic grounds, and neither do the new data indicate clear geochronological similarities with the widespread late Mesoproterozoic-early Neoproterozoic mobile belt. The intensity of the Pan- African event (high-grade metamorphism and late-tectonic charnockite emplacement) suggests some affinities with the Prydz Bay coast area, but it is possible that the Grove Mountains represent a distinct terrane.
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