Parageneses of Ordovician sub-greenschist to greenschist facies metabasites from Wales, U.K.
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Abstract This study reports a new rodingite type which was derived from eclogite enclosed in the ultramafic rocks of Changawuzi ophiolites, western Tianshan, China. Based on petrographical investigations, rodingite, partial rodingitized rock and completely rodingitized rock are characterized in this paper. These rocks show a continuous variation in their bulk compositions, mineralogy and their textural properties from eclogite to rodingite. The completely rodingitized rocks can be further divided into prehnite rodingites, hydrogrossular‐diopside rodingite and vesuvianite rodingites on the basis of the mineral assemblage and textural character. The chemical potential path of μ (SiO 2 )– μ (CaO/MgO) can be used to constrain the evolution of two stages of rodingitization. The first rodingitization possibly started under conditions of 410–430 °C and 7–9 kbar at upper greenschist facies, and resulted from a secondary serpentinization during exhumation of the subducted slab. A second and pervasive rodingitization took place under conditions of 250–350 °C and 2–10 kbar from greenschist to subgreenschist facies. The P – T path presented shows a retrograde evolution from eclogite to rodingite. We conclude that the process of rodingitization may also take place under subduction zone conditions in addition to its more common occurrence under ocean‐floor metamorphic conditions.
Greenschist
Ultramafic rock
Blueschist
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Allochthon
Laurentia
Baltica
Massif
Greenschist
Baltic Shield
Plutonism
Continental Margin
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Dolomitization
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The Laojunmiao metamorphic complex, exposed on the northeastern margin of the Junggar basin,consists mainly of quartz schists of greenschist facies, with some marbles bearing pre Devonian tabulates and crinoids resting on it. 40 Ar/ 39 Ar dating of muscovite from the complex yields a plateau age of 461.5±0.2 Ma and an isochron age of 462.0±4.1 Ma, indicating that its metamorphism occurred in the late Middle Ordovician. In combination with regional geology, it is supposed that the basement of the basin is composed of the Paleozoic continental crust.
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Abstract Riebeckite granite at Sams Creek, northwest Nelson, was intruded as an 8 km long dike into slate and quartzite metasediments (Wangapeka Formation) before or during deformation under greenschist facies conditions. Lamprophyre intrusion accompanied granite emplacement. Deformation after intrusion resulted in folding of metasediments on a northerly trend, and buckling of the east‐striking granite and lamprophyre dikes. The intrusives were dismembered and now occur as a sinuous train of small (c. 100 m) subequant bodies. Metamorphogenic fluid from the metasediments interacted with the granite at about 300 ± 60°C and 100–200 MPa fluid pressure. The fluid was predominantly water with minor CO2 (saturated), and contained up to 7.5 wt% dissolved salts. The fluid/rock ratio was low, and whereas little chemical or mineralogical change occurred in the granite, its distinctive Fe3+‐rich composition localised Au‐As mineralisation. Pyritisation of magnetite caused fluid pH to rise, and Au to precipitate. Further water/rock interaction resulted in a pH decrease, sericitisation of feldspars, and dissolution of Au.
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