Academic literature on the topic 'Whiteschist'

Abstract A series of tourmaline reference materials are developed for in situ oxygen isotope analysis by secondary ion mass spectrometry (SIMS), which allow study of the tourmaline compositions found in most igneous and metamorphic rocks. The new reference material was applied to measure oxygen isotope composition of tourmaline from metagranite, meta-leucogranite, and whiteschist from the Monte Rosa nappe (Western Alps). The protolith and genesis of whiteschist are highly debated in the literature. Whiteschists occur as 10 to 50 m tube-like bodies within the Permian Monte Rosa granite. They consist of chloritoid, talc, phengite, and quartz, with local kyanite, garnet, tourmaline, and carbonates. Whiteschist tourmaline is characterized by an igneous core and a dravitic overgrowth (XMg > 0.9). The core reveals similar chemical composition and zonation as meta-leucogranitic tourmaline (XMg = 0.25, δ18O = 11.3–11.5‰), proving their common origin. Dravitic overgrowths in whiteschists have lower oxygen isotope compositions (8.9–9.5‰). Tourmaline in metagranite is an intermediate schorl-dravite with XMg of 0.50. Oxygen isotope data reveal homogeneous composition for metagranite and meta-leucogranite tourmalines of 10.4–11.3‰ and 11.0–11.9‰, respectively. Quartz inclusions in both meta-igneous rocks show the same oxygen isotopic composition as the quartz in the matrix (13.6–13.9‰). In whiteschist the oxygen isotope composition of quartz included in tourmaline cores lost their igneous signature, having the same values as quartz in the matrix (11.4–11.7‰). A network of small fractures filled with dravitic tourmaline can be observed in the igneous core and suggested to serve as a connection between included quartz and matrix, and lead to recrystallization of the inclusion. In contrast, the igneous core of the whiteschist tourmaline fully retained its magmatic oxygen isotope signature, indicating oxygen diffusion is extremely slow in tourmaline. Tourmaline included in high-pressure chloritoid shows the characteristic dravitic overgrowth, demonstrating that chloritoid grew after the metasomatism responsible for the whiteschist formation, but continued to grow during the Alpine metamorphism. Our data on tourmaline and quartz show that tourmaline-bearing white-schists originated from the related meta-leucogranites, which were locally altered by late magmatic hydrothermal fluids prior to Alpine high-pressure metamorphism.

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The Winnecke Gorge-Two Mile Bore region (Winnecke Domain) of the Arunta Block, central Australia, contains a highly condensed geological section from greenschist-facies Amadeus Basin cover sequences (Heavitree Quartzite and Bitter Springs Formation), through amphibolite-grade assemblages (Ankala gneiss), to granulite-facies mafic, felsic and calc-silicate lithologies (Strangways Metamorphic Complex). Juxtaposition of these blocks of widely varying metamorphic grade has been facilitated via the activation (and probable reactivation in many cases) of several major sub-parallel, E-W trending shear zones. Greenschist-facies shear zones to the immediate north of the Heavitree Quartzite have previously been demonstrated to have been associated with the Palaeozoic Alice Springs Orogeny. In this study, the petrological character of assemblages from across the Winnecke Domain have been investigated. Thermobarometric estimates (using THERMOCALC and other pressure-temperature estimators) have been used to constrain the change in pressure-temperature conditions across the Winnecke Domain. Pressures obtained from gneissic assemblages increase from around 4.0 kbars in the Winnecke South gneiss (the southern extent of the transect) to around 8.5 kbars in the Cadney metamorphics (the northern extent), and step significantly at major lithological boundaries (i.e. Erontonga metamorphics - Two Mile Bore shear zone contact). The range of pressures obtained from schistose assemblages is quite variable (between 3.3 and 6.8 kbars), and does not increase consistently towards the north. This possibly indicates several phases of activation of shear zones in the region, or it may reflect the presence of variable, but significant, amounts of non-KFMASH components (e.g. Mn) in phases such as garnet. Significant, but variable, potassium and iron metasomatism was typically associated with the development of schists throughout the Winnecke Domain, and was often accompanied by coarse grained biotite, muscovite and magnetite growth. The source of such large quantities of potassium in the potassium-poor granulite terrain is unknown at present. A significant occurrence of a whiteschist (kyanite/talc-bearing) assemblage, the first of its type documented from mainland Australia, is described from the southern margin of the Erontonga metamorphics. The first reported occurrence of a kyanite-bearing schist from the Cadney metamorphics (in the Marbles Bore region) is documented in this study.
Thesis (B.Sc.(Hons)) -- University of Adelaide, School of Earth and Environmental Sciences, 1996

Constraining the evolution of continents, and the tectonic plates they reside upon, enables geoscientists to understand phenomena such as mantle dynamics, mineral and energy resource distribution, faunal evolution, and climate development. Thus, there is an underlying necessity to have rigorous palaeogeographic models that constrain plate reconfiguration and interaction throughout earth history. The Congo Craton encompasses present-day central Africa, and is comprised of Archean crustal blocks and Proterozoic orogens. The southern margin of this craton acted as a plate boundary from the Palaeoproterozoic to present. However, the evolution of this margin remains largely enigmatic. This thesis interrogates the tectonic evolution Southern Irumide Belt (SIB), a predominantly Mesoproterozoic orogen located along the southern Congo margin, which serves as a vital proxy for understanding the evolution of the Congo Craton. U–Pb dating of detrital zircons from metasedimentary rocks within the Zambian terranes of the SIB identifies Palaeoproterozoic to Mesoproterozoic age populations that are equivalent to those preserved in the Muva Supergroup, found in the Irumide Belt (sensu stricto) of the Congo Craton. This depositional connection between the SIB and Congo Craton prior to the late-Mesoproterozoic is supportive of a tectonic model where the SIB formed on the southern Congo margin. Neoproterozoic sedimentary rocks identified in the Nyimba–Sinda Terrane highlight subsequent extension in the region that is likely a response to rifting. Full-plate topological models suggest that this rifting was a southern extension of the spreading that separated Australia from Laurentia during Rodinia break-up. An isotopic and geochronological investigation of intrusions throughout this region suggest that the SIB formed on an isotopically evolved Palaeoproterozoic basement. This is equivalent to basement in the neighbouring Irumide Belt, and further supports the SIB forming on the southern Congo margin. During the late stages of Gondwana amalgamation, the Congo Craton collided with the Kalahari Craton to form central Gondwana, generating tectono-metamorphic overprints that are displayed in the rocks of the Southern Irumide and Zambezi belts. These overprints record amphibolite to granulite facies mineral assemblages, and include more exotic, high-pressure ‘whiteschist’ assemblages. An activity-composition model was created for yoderite, a key mineral in whiteschists, for use with the pressure–temperature (P–T) modelling software THERMOCALC. Using this model, P–T diagrams were calculated for both a retrogressed whiteschist and metapelite from the region, to constrain the features of the Gondwana forming collision. A thermal gradient of 30–90 °C/kbar was calculated for the whiteschist, consistent with those calculated for peak metamorphism, whereas a gradient of 70–165 °C/kbar was calculated for the metapelite. The different thermal gradients relate to different aspects of the collision. Where the amphibolite facies rocks formed in a compressional setting proximal to the southern Congo margin, the whiteschists instead formed directly at the site of continental collision, marking the suture zone between the Congo and Kalahari Cratons. U–Pb apatite and 40Ar–39Ar muscovite data reveal high-temperature cooling ages spanning the late-Neoproterozoic to Cambrian, relating to cooling after Congo–Kalahari collision. Apatite fission track dating identifies periods of low-temperature cooling during the Carboniferous, Triassic, and Cretaceous, with thermal modelling identifying rapid Cenozoic cooling. These periods are interpreted to relate to periods of exhumation in central Africa, which occurred in response to the wider-scale tectonic processes of Karoo rift basin formation, Gondwana break-up, and the development of the East African Rift System (EARS). These studies provide a framework for understanding the evolution of the Southern Irumide Belt. As a proxy for the evolution of the southern Congo margin, this work serves to constrain palaeogeographic models for the Congo Craton, further elucidating its role within the wider Earth system.
Thesis (Ph.D.) -- University of Adelaide, School of Physical Sciences, 2019