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With the age and thermobaric ratio of metamorphism over the past 2 b. y. We compared paleogeographic reconstructions and inferred plate kinematics. The subduction zone, the mountain belt, and the orogenic hinterland are all located near plate margins, whereas intermediate- and high-T/P metamorphism is skews toward ever greater distances from these boundaries, consistent with three distinct tectonic settings: the subduction zone, the mountain belt, and the orogenic hinterland. If the style of orogenesis has changed from hotter to colder, consistent with the sudden emergence of low-T/P metamorphism in the Cryogenian, it could be explained that the observation that bimodal metamorphism is largely a function of distance from the trench and the fact that end-member T/P types rarely occur in the same region. In the Phanerozoic, the widespread introduction of high-T/P rocks in orogenic hinterlands was followed by the discovery and efficient exhumation of low-T/P rocks in subduction channels.
Source link: https://doi.org/10.1130/g49637.1
late Ottawan crystallization at ca. Inherited zircon cores have a mean age of 1136 – 5 Ma, implying derivation of these granularites by partial melting of older igneous rocks. The granulites' ferroan, anhydrous appearance is similar to that of ca. Origins in a late-to-post-Ottawan extensional environment are consistent with 1050 Ma Lyon Mountain Granite, which are also present with origin in a late-to-post-Ottawan extensional region. We present a scheme for the construction of a late Ottawa migmatitic gneiss dome in the central Adirondacks that dragged deep crustal rocks, including the Snowy Mountain and Oregon anorthosite massifs with UHT Ledge Mountain migmatites. The discovery of deep crustal metamorphism in a migmatite gneiss dome has important implications for crustal behavior in this formerly thickened orogen.
Source link: https://doi.org/10.1130/ges02318.1
Changes include complete rock depletion of Rb, Cs, Th, and U in the granulite grade rocks as compared to amphibolite-grade gneisses, as well as reduced amphibolite-facies rocks below the fayalite-magnetite bucket, as well as regionally from highly oxidized granulite-facies rocks below the fayalite-magnetite granulite-hemate The granulite-facies charnockete's crystals range from titanite and allanite in the amphibolite-facies rocks to monazite in the vicinity of the orthopyroxene-in isograd to apatite. Zincon cores from 29 samples of dioritic, tonalitic, and granitic orthogneiss from the traverse reveals magmatic zircon cores that trace the emplacement of the granitoid protoliths from about 2580 to 2550 Ma, as well as a few older mid to late Archean tonalites from the traverse. The U-depleted metamorphic zircon growth in these samples is preceded by mantles of U-undepleted zircon, indicating a trend of metamorphic zircon growth with rising depleted compositions between 2530 and 2500 Ma. Allanite and monazite minerals were removed from the assemblage and zircon became depleted in U and Th, as the oxidation state and increasing metamorphic grade increase. The end result of subduction related processes that culminated in the collision and concatenation of island arcs and continental blocks may be subordinateding fluids and element migration during metamorphism.
Source link: https://doi.org/10.1093/petrology/egac036
All the samples are compositionally layered, so it is likely that an input bulk rock composition would not match the desired bulk composition at the site of garnet growth. If a mineral assemblage stability field in a calculated P–T pseudosection match the mineral assemblage in the rock, this was taken as an early indication of a permissible input bulk rock composition. Using a new bulk rock composition with the garnet core content removed, researchers investigated element fractionation by that garnet. The sample for the garnet cores match was not determined and observed composition isopleth, indicating that initial garnet nucleation in these Mn-rich rocks was locally controlled. The new a-x models for Mn-bearing minerals, according to these garnet chert samples, extend the range of rock types that are amenable to pseudosection modeling. Items a and b may be indicated by the common finding of a mismatch between estimated and measured garnet composition isopleths for garnet cores and a mismatch between garnet composition isopleths, as well as a mismatch between garnet composition isopleths and the correct mineral assemblage field for sample AMS01, from the Southern Alps' mylonite zone, Hari Hari. For item c: Every P–T pseudosection calculated using the new a–x models for Mn-bearing minerals revealed garnet stable to very low temperatures below 300 °C.
Source link: https://doi.org/10.26686/wgtn.17014655.v1
Detailed information about morphism, deformation, and alteration on these largely uniform host rocks, as well as finding and extracting Au from those deposits is vital for finding and discovering new greenstone-hosted Au deposits. This paper explores the effects of primary stratigraphy on Au mineralisation, the effects of metamorphism, and potential limitations on metamorphism, as well as the timing of Au mineralising events at Plutonic Gold Mine, Plutonic Well Greenstone Belt, Marymia Inlier, Western Australia. Using pXRF methods, the stratigraphy of the Au-mineralised amphibolite-facies metabasalts that make up the mine sequence at Plutonic has been investigated. According to Au assay results, Au is more common along basalt flow boundaries. The elemental concentration results point to stratigraphic control of Au mineralisation that is not immediately apparent at the macroscopic level. For two common metabasaltic rocks from the Plutonic, results of P–T pseudosection results in the NCFMASHTOS scheme are given. The majority of Au-associated sulphides at Plutonic are Proterozoic in age, at 2200 Ma, indicating that Au-mineralization may have been widespread in the inlier and adjacent cratonic zones at the time. Both U-Pb dating of zircon overgrowths in the United States and a new 206 Pb-206 Pb age from a hydrothermal sphene in the chlorite-carbonate vein of 1725 - 26 Ma is dated to a 1720 Ma.
Source link: https://doi.org/10.26686/wgtn.16992820.v1
Abstract granites of the South Delhi Fold Belt, northwestern India, have outstanding reaction textures that result from the metamorphic replacement of primary minerals. The symplectite, titanite + quartz grain symplectite developed syn-tectonically on amphibole grain boundaries/fractures, followed by post-deformational gain of euhedral garnet overprinting amphibole grains. Titanite in this rock also emerged as a post-tectonic corona around magnetite, besides growing during deformation. Exclusions of ilmenite and/or ultrafine lamellae of Ti-rich oxide are present in Parent magnetite's. Textures involving coronal titanite suggest their emergence by a magnetite + ilmenite + plagioclase reaction. The Mn 2+ for garnet that grew replacing amphibole was supplied by ilmenite as it disintegrated to form coronal titanite, according to Compositional characteristics and the estimation of gain versus loss of components during the reaction. According to Zircon results from this research, the granitic rock crystallized at 988. 8 8. 8 Ma.
Source link: https://doi.org/10.1017/s0016756821001345
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