Academic literature on the topic 'Yoderite'

AbstractMetamorphic rocks of undoubted crustal origin have been described in recent years, principally from Mediterranean collision zones that have been subjected to PT conditions along very low geothermal gradients (∼ 7°C/km) and have reached pressures up to 30 kbar. MgAl-rich metapelites develop particularly diagnostic high-pressure minerals and mineral assemblages that have been and are being studied experimentally in model systems involving the components K2O, MgO, Al2O3, TiO2, SiO2, P2O5, and H2O up to pressures of 50 kbar and temperatures of 1000°C.In the present review the following synthetic phases and phase assemblages are discussed, emphasizing their water-pressure-temperature stability fields (approximated in parentheses here), their reaction relationships, and their known or potential occurrences in metamorphic rocks. Sudoite (0 to ∼ 12 kbar, 150? to 380°C) occurs in very low-grade metapelites. Mg-carpholite (∼ 7 to ∼ 45 kbar, ∼ 200 to 600°C) is found in subducted metabauxites, metapelites, and related quartz veins. Mg-chloritoid (18 to 45 kbar?; 400 to 760°C) has not been found in nature as pure or nearly pure end-member; it requires silica-deficient environments. Yoderite, known in nature only from a single talc-kyanite schist occurrence, has only a small stability field (9 to 18 kbar?, 700 to 870°C?), cannot coexist with quartz, but may be stabilized by Fe3+. Pyrope (∼ 15 to at least 50 kbar, ∼ 700°C to melting), with or without relic coesite inclusions, occurs spectacularly in quartzites. Mg-staurolite (∼ 14 to some 90 kbar?, 700 to 1000°C), recently discovered as inclusions in pyrope, requires silica-deficiency. MgMgAl-pumpellyite is a new synthetic phase in which Mg totally replaces Ca of normal pumpellyite; because of its very high-pressure, low-temperature stability (∼ 37 to at least 55 kbar, < 400 to 780°C) it may not form within our globe. Ellenbergerite, the new high-pressure mineral forming inclusions in pyrope, apparently exhibits a rather composition-dependent stability with Ti-ellenbergerite, requiring higher pressures (> 20 kbar) than P-bearing, Ti-free members; a pure hydrous Mg-phosphate with ellenbergerite structure was synthesized at 10 kbar. Phengites, the widespread MgSi-substituted muscovites, require increasingly high water pressures (up to ∼ 20 kbar) for higher degrees of substitution, but the Al-celadonite end-member is not stable under any conditions; the compositions of phengites coexisting with limiting assemblages such as phlogopite, K-feldspar, and an SiO2 phase are useful geobarometers. The common assemblage Mg-chlorite + Al2SiO5 (mainly kyanite) has an extensive stability field ranging from near zero to 31 kbar at temperatures varying from some 320 to ∼ 760°C depending on pressure. The whiteschist assemblage talc + kyanite (6 to ∼ 45 kbar, 550 to 810°C) plays an important role in collision zone metamorphism as it forms from the greenschist assemblage chlorite + quartz at low grades but is also known to break down into pyrope + coesite at the highest grade observed thus far. The assemblage talc-phengite (11 to at least 35 kbar, 300? to 820°C depending on pressure), on the other hand, is well known from subducted metapelites. At pressures of 15–20 kbar and temperatures of 400–650°C a very K,Mg-rich, siliceous fluid forms as a consequence of the mutual reaction of the minerals K-feldspar and phlogopite (biotite) which are very common in crustal rocks including granites. Such fluids are bound to cause metasomatism in neighbouring mantle rocks which, upon subsequent increase of temperature, produce post-collisional ultrapotassic, lamproitic melts.

Thirty-six new combinations are provided in Oreogrammitis Copel. (Polypodiaceae: Polypodiopsida). The species have previously been included in Radiogrammitis Parris and Themelium (T.Moore) Parris. Lectotypes are chosen for Polypodium allocotum Alderw., P. ctenoideum Brause, P. flagelliforme Brause, P. loherianum Christ, P. subpinnatifidum Blume and P. yoderi var. denudatum Alderw.

Hypselosomatinae, the big-eyed minute litter bugs, are diagnosed among Schizopteridae (infraorder Dipsocoromorpha) by the large eye, 4-segmented labium, and distinct wing venation. The group was recovered as sister taxon to all remaining Schizopteridae in recent molecular phylogenetic analyses. Described species diversity of Hypselosomatinae (13 extant genera prior to this study) is greatest in the Old World and in particular the Australian region, while only five monotypic or small genera are currently described from the New World (Glyptocombus Heidemann, Ommatides Uhler, Williamsocoris Carpintero & Dellapé, Hypsohapsis Hoey-Chamberlain & Weirauch, and Hypselosomops Hoey-Chamberlain & Weirauch). Based on examination of 60 specimens of Hypselosomatinae from South America, we here synonymize Ommatides that includes one described species from the Lesser Antilles with the monotypic genus Williamsocoris Carpintero & Dellapé from Argentina, describe seven new species of Ommatides (O. duodentis sp. nov., O. nudus sp. nov., O. parvidentis sp. nov., O. pillcopata sp. nov., O. pristis sp. nov., O. tridentis sp. nov., O. yoderi sp. nov. and O. zanderij sp. nov.) from various locations in South America, and redescribe Ommatides. We provide thorough documentation of morphological features using macroimages, SEM, and line drawings for new taxa and Ommatides insignis Uhler and a distribution map for all currently known New World Hypselosomatinae.

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