RELEASE OF TRACE ELEMENTS THROUGH THE SUB-GREENSCHIST FACIES BREAKDOWN OF DETRITAL RUTILE TO METAMORPHIC TITANITE IN THE OTAGO SCHIST, NEW ZEALAND
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Abstract:
Detrital rutile is observed in trace amounts in the sub-greenschist facies, graywacke, and argillite lithologies of the Torlesse Terrane-derived Otago Schist, New Zealand. Trace element analyses reveal that mean compositions of detrital rutile grains are characterized by high concentrations (500–3600 ppm) of V, Cr, Fe, Zr, Nb, and W, moderate amounts (100–500 ppm) of Sn and Ta, and minor amounts (50–100 ppm) of Mn and Hf. Recrystallization of detrital rutile to metamorphic titanite is first observed in the prehnite-pumpellyite facies rocks, with recrystallization largely complete by upper chlorite greenschist facies. Dissolution, material transport, and precipitation reactions were crucial in the progression of this recrystallization reaction, with pore spaces generated allowing the transportation of Ca, Si, and O to the titanite-rutile interface. Redistribution of trace elements during the mineral transition of detrital rutile to metamorphic titanite was assessed using mass-balance techniques. These calculations indicate that V, Cr, Zr, Nb, Ta, and W were released during the prograde mineral reaction. The liberation of this suite of trace elements potentially provides a source for the W and Cr enrichment observed in the orogenic deposits of the Otago Schist.Keywords:
Titanite
Greenschist
Abstract Cretaceous (possibly older) metamorphic rock occurs mainly in the Blue Mountain inlier in eastern Jamaica. Fault‐bounded blocks reveal two styles of metamorphism, Westphalia Schist (upper amphibolite facies) and Mt. Hibernia Schist (blueschist (BS)–greenschist (GS) facies). Both Westphalia Schist and Mt. Hibernia Schist preserve detailed records of retrograde P–T paths. The paths are independent, but consistent with different parts of the type‐Sanbagawa metamorphic facies series in Japan. For each path, phase relationships and estimated P–T conditions support a two‐stage P–T history involving residence at depth, followed by rapid uplift and cooling. Conditions of residence vary depending on the level in a tectonic block. For the critical mineral reaction (isograd) in Westphalia Schist, conditions were P ∼7.5 kbars, T ∼600°C (upper amphibolite facies). Retrograde conditions in Hibernia Schist were P = 2.6–3.0 kbars, T = 219–237°C for a(H 2 O) = 0.8–1.0 (GS facies). Mt. Hibernia Schist may represent a volume of rock that was separated and uplifted at an early time from an otherwise protracted P–T path of the sort that produced the Westphalia Schist. Reset K–Ar ages for hornblende and biotite indicate only that retrograde metamorphism of Westphalia Schist took place prior to 76.5 Ma (pre‐Campanian). Uplift may have commenced with an Albian–Aptian (∼112 Ma) orogenic event. Copyright © 2008 John Wiley & Sons, Ltd.
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Blueschist
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Petrochemistry
Outwash plain
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Abstract Psammitic schist, 2 types of pelitic schist (grey and porphyroblastic), and 4 types of greenschist (metavolcanic rock — light, spotted, foliated, and epidote rich) are recognisable on outcrop scale in textural zone 4 of the Otago Schist, northwest Otago, New Zealand. Thin horizons of metachert, marble, and ultramafic rock are commonly associated with greenschist. Broad units with 1 predominant rock type (greenschist, psammite, grey pelite, or porphyroblastic pelite) are mappable on a regional scale. These units also contain most or all of the above rock types and have poorly defined boundaries. The degree of original Stratigraphic continuity within and between these broad units is unknown. The studied area can be subdivided into 2 lithologic associations, the "Aspiring association" and the eastern "Wanaka association", separated by a north-trending, poorly defined but lithologically gradational boundary. The Aspiring association is made up predominantly of pelitic rock types with considerable quantities of greenschist and minor marble, chert, and ultramafic horizons. The Wanaka association consists of psammitic schist with subordinate greenschist and pelitic schist. The proportion of greenschist decreases eastwards. These "associations" represent fundamental lithologic differences within the Otago Schist belt, on a scale equivalent to the previously defined Te Anau Assemblage. Keywords: Otago Schistlithologypelitic schistgreenschistmetachertmarblepsammitic schist
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Abstract The Sanbagawa metamorphic terrain of the study area is divided into two units, the Shirakura and Sejiri units. The metamorphic thermal structure is interpreted on the basis of the degree of graphitization (GD) of carbonaceous material in pelitic schists. The areal variations of the metamorphic grade are presented by the distribution of GD calculated using the Lc and d 002 of carbonaceous material. As a result, the two units are classified into four metamorphic zones, respectively: A 1 , A 2 , B 1 and B 2 for the Shirakura Unit; and I 1 , I 2 , II 1 and II 2 for the Sejiri Unit. The metamorphic grades of A 1 , A 2 , I 1 and I 2 are included in the chlorite zone, and that of B 1 , B 2 , II 1 and II 2 in the garnet zone of the Sanbagawa metamorphism. The degree of graphitization at the boundary between A 2 and B 1 zones is the same as that between I 2 and II 1 zones. Detailed study on the variation of GD suggests that the present‐day structure of the study area is best interpreted as a model of shuffled‐cards structure. An estimated minimum thickness of a stack of continuous cards is about 25 m. The compositions of garnet in pelitic schists and of amphibole in basic schists are different from those in the identical metamorphic range of the Shirakura and Sejiri units. It is suggested that rocks of the Shirakura Unit were metamorphosed under higher P/T conditions than those of the Sejiri Unit.
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Abstract Ductile structures in the greenschist facies rocks of the Alpine Schist that lie in the headwaters of the Whataroa, Callery, and Balfour valleys formed during Mesozoic deformation, similar to those in the Otago Schist to the south. A general westward increase in metamorphic grade is locally disrupted and repeated by several late metamorphic faults. The first appearance of biotite occurs at tectonic boundaries in the Callery and Balfour sections. Metagreywackes are juxtaposed against a greenschist‐metapelite‐metachert sequence by a synmetamorphic ductile high strain zone in the lower Balfour valley. A prominent quartz rodding lineation in the high strain zone defines a stretching direction that plunges about 30° southwest. Structures in the high strain zone are locally cut by quartz (±biotite) veins, which have been weakly deformed. Fluid inclusions in quartz veins indicate that the veins formed about 8–10 km deep and that uplift was initially nearly isothermal. Pervasive foliation that formed during Mesozoic deformation was deformed into upright, open, kilometre‐scale folds, with some foliation‐parallel boudinage, associated with the Miocene inception of the Alpine Fault. The resultant north to northeast trending folds plunge gently southwards. Deformation associated with Alpine Fault movement was only minor in the Alpine Schist greenschist facies and resulted in passive differential uplift of deeper parts of the schist pile in the west.
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Petrology and Mineralogy of Metamorphic Rocks in The Pringsewu District, Lampung Province, Indonesia
The origin of metamorphic rock is greatly influenced by the temperature and pressure changes in every tectonic setting, especially in active subduction margins. One of the wide outcrops of metamorphic rock occurs in the Pringsewu District, Lampung, and it is a part of the Palaeozoic Gunung Kasih Complex. The presence of metamorphic rocks in Pringsewu has raised several questions and debates for some time due to the lack of research and field evidence found in this area, especially, since the origin of metamorphic rock in Lampung has been rarely studied. This research aims to determine the metamorphic rock facies and the tectonic setting underlying the formation of metamorphic rocks in the study area.
Petrographic analysis on 19 thin-section samples shows that metamorphic rock in the research area can be divided into two regions i.e., western and eastern regions. The Eastern part is characterized by muscovite-epidote schist and greenschist which consist of quartz, muscovite, actinolite, epidote, and garnet as index minerals. Whereas the western part is characterized by quartzite and biotite-epidote schist that consist of quartz, biotite, and muscovite. Based on the mineral index, metamorphic rock’s protoliths are pelitic rock, mafic rock, and quartz-feldspathic sandstone. The metamorphic rock zonation shows the created temperature is from 280-550oC.
The foliation structure such as schistose and porphyroblastic texture in the whole rocks sample indicated that metamorphic rocks are created in medium-grade metamorphism and are characterized by the greenschist facies to epidote-amphibolite facies. The abundance of quartz, k-feldspar, and labradorite minerals show that metamorphic rocks came from the microcontinent which was caused by a collision between the intra-oceanic Woyla plate and West Sumatra microcontinent in the Cretaceous. Indications of tectonic activity that create the lithology in Lampung Province need more geological study, especially to determine the absolute age of the metamorphic rock.
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The Taku Schist, which is located in the north-east Peninsular Malaysia, is characterized by its North-South oriented elongated body. It forms part of the Indonesian orogenic build-up that was generated via the convergence of the Sibumasu continental unit and Sukhothai Arc. Subsequent petrography analyses of the metasedimentary rocks sourced from the Taku Schist revealed that their formation was attributable to the metamorphism of greenschist into amphibolite facies, which could be observed near the Triassic and Cretaceous intrusions of the Kemahang Granite. The evolutionary process of the rocks could be linked with the interactions occurring between contact and regional metamorphisms. The resulting chemical classification upon their assessment disclosed that the metasedimentary rocks of Taku Schist were made up of greywacke and shale, grouped into the quartzose sedimentary provenance, and belonged to the Continental Island Arc (CIA). This information is required for the tectonic setting discrimination purpose. It is a reflection of the episodic contractions underwent by the Taku Schist, wherein they would lead to the Sibumasu sedimentary cover along with both an accretionary wedge and the genetically-correlated Bentong-Raub melange to different greenschist. Otherwise associated with amphibolite facies, the conditions and depths of the facies were determined according to their position in relation to the upper plate of the Sukhothai Arc.
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Greenschist
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Isochron dating
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Greenschist
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In the Stražun forest at Pobrežje near Maribor, (Eastern Slovenia) six boreholes have been drilled from 860 to 1600m deep. The paper describes geological conditions in mentioned boreholes, as well as in wider surroundings of Maribor with stress on metamorphic rocks. Based on petrographic analysis of the rocks cuttings and well logs the upper phyllitic part ant the lower Pohorje series of the metamorphic complex could be separated. The first one includes phyllites with phyllitic quartzites and silicate marmorized limestones. The Pohorje series is represented by two-mica gneiss and schist, mainly with inclusions of amphibolite and eclogite, and subordinately retrograde chlorite-amphibole schist. The well log responses for particular lithological sequences of metamorphic complex have been distinguished on the basis of conventional electrologs and gamma ray measurements. The problem of lithological interpretation of well logs in these rocks is described. Two fields of well log responses are distinguishable, as separated by the degree of natural radioactivity. Apart from veined quartzite, all rocks from the phyllitic part of the metamorphic complex are highly radioactive. In the Pohorje series gneiss, schist and diaphtorite-phyllonite are highly radioactive, while amphibolite, eclogite and retrograde chlorite-amphibole schist have low radioactivity. Finally, typical well log responses in lithological sequences of the discussed metamorphic rocks are presented.
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