Fluid evolution during metamorphism of the Otago Schist, New Zealand: (II) Influence of detrital apatite on fluid salinity
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Abstract:
Apatite occurs in the zeolite to greenschist facies metamorphic rocks of the Otago Schist, South Island, New Zealand, as both a groundmass constituent and as a hydrothermal phase hosted in metamorphic quartz veins. Groundmass apatite from low‐grade rocks, ranging from the zeolite facies to the pumpellyite–actinolite zone, has chloride contents ranging from 0–1.4 wt%, and fluoride contents ranging from 2.2–4.2 wt%, whilst groundmass apatite from the greenschist facies (chlorite to biotite zone) is virtually pure fluorapatite. Vein apatite from all grades is also fluorapatite with little or no chloride. This difference in composition is interpreted as resulting from the preservation of the primary magmatic compositions of detrital Cl‐apatite grains, out of equilibrium with the metamorphic fluid, at low grades, whilst higher‐grade groundmass apatite and neoformed apatite in quartz veins have compositions in equilibrium with an aqueous metamorphic fluid. The presence of detrital Cl‐bearing apatite during the early stages of metamorphism may constitute a significant reservoir of Cl, given the low porosities of compacted sediments undergoing prograde metamorphism. Calculations indicate that the release of Cl from detrital apatite in the Otago Schist, as a result of re‐equilibration of apatite with the pore fluid, may have had a significant effect on the salinity of the metamorphic fluid.Keywords:
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Fluorapatite
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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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 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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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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A subcircular area of about 650 km 2 in northern California and southwestern Oregon is occupied by rocks of the greenschist metamorphic facies called the Condrey Mountain Schist. This greenschist terrane is bordered on the east and west by rocks belonging to the amphibolite metamorphic facies that structurally overlie and are thrust over the Condrey Mountain Schist. The amphibolite facies is succeeded upward by metavolcanic and metasedimentary rocks belonging to the greenschist metamorphic facies. The Condrey Mountain Schist is composed predominantly of quartz-muscovite schist and lesser amounts of actinolite-chlorite schist formed by the metamorphism of graywacke and spilitic volcanic rocks that may have belonged to the Galice Formation of Late Jurassic age. Potassium-argon age determinations of 141?4 m.y. and 155?5 m.y. obtained on these metamorphic rocks seem to be incompatible with the Late Jurassic age usually assigned the Galice. The rocks that border the amphibolite facies are part of an extensive terrane of metavolcanic and metasedimentary rocks belonging to the western Paleozoic and Triassic belt. The metavolcanic rocks include some unmetamorphosed spilite but are mostly of the greenschist metamorphic facies composed of oligoclase (An15-20) and actinolite with subordinate amounts of chlorite and clinozoisiteepidote. The interbedded sedimentary rocks are predominantly argillite and slaty argillite, less commonly siliceous argillite and chert, and a few lenticular beds of marble. On the south, high-angle faults and a tabular granitic pluton separate the greenschist metavolcanic terrane from the amphibolite facies rocks; on the east, nonfoliated amphibolite is succeeded upward, apparently conformably, by metasedimentary rocks belonging to the greenschist metavolcanic terrane. In the southern part of Condrey Mountain quadrangle, an outlier of a thrust plate composed of the Stuart Fork Formation overlies the metavolcanic and metasedimentary rocks. The Stuart Fork in this region is composed of siliceous phyllite and phyllitic quartzite and is believed to be the metamorphosed equivalent of rocks over which it is thrust. In the Yreka-Fort Jones area, potassium-argon determinations on mica from the blueschist facies in the Stuart Fork gave ages of approximately 220 m.y. (Late Triassic) for the age of metamorphism. Rocks of the amphibolite facies structurally overlie the Condrey Mountain Schist along a moderate to steeply dipping thrust fault. The amphibolite terrane is composed of amphibolite and metasedimentary rocks in approximately equal amounts accompanied by many bodies of serpentinite and a number of gabbro and dioritic plutons. Most of the amphibolite is foliated, but some is nonfoliated; the nonfoliated amphibolite has an amphibolite mineralogy and commonly a relict volcanic rock texture. The nonfoliated amphibolite occurs on the southern and eastern borders of the amphibolite terrane between the areas offoliated amphibolite and the overly ing metavolcanic and metasedimentary rocks. Hornblende and plagioclase (An30-35) are the characteristic minerals, indicating that the rocks are of the almandine-amphibolite metamorphic facies. The metasedimentary rocks interbedded with the amphibolites include siliceous schist and phyllite, minor quartzite, and subordinate amounts of marble. Potassium-argon age dates obtained on hornblende from foliated amphibolite yield ages of 146?4 and 148? 4 m.y., suggesting a Late Jurassic metamorphic episode. Mafic and ultramafic rocks are widespread in the amphibolite terrane but are almost entirely absent from the area of greenschist facies metavolcanic and metasedimentary rocks. The ultramafic rocks, predominantly serpentinite, occur as a few large bodies and many small tabular concordant bodies interleaved with the foliated rocks. The ultramafic rocks include harzburgite and d1lIlite and their serpentinized equivalents. In the Condrey Mountain quadrangle, probably more t
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Abstract The production of large volumes of fluid from metabasic rocks, particularly in greenstone terranes heated across the greenschist–amphibolite facies transition, is widely accepted yet poorly characterized. The presence of carbonate minerals in such rocks, commonly as a consequence of sea‐floor alteration, has a strong influence, via fluid‐rock buffering, on the mineral equilibria evolution and fluid composition. Mineral equilibria modelling of metabasic rocks in the system Na 2 O‐CaO‐FeO‐MgO‐Al 2 O 3 ‐SiO 2 ‐CO 2 ‐H 2 O (NCaFMASCH) is used to constrain the stability of common metabasic assemblages. Calculated buffering paths on T – X CO2 pseudosections, illustrate the evolution of greenstone terranes during heating across the greenschist‐amphibolite transition. The calculated paths constrain the volume and the composition of fluid produced by devolatilization and buffering. The calculated amount and composition of fluid produced are shown to vary depending on P – T conditions, the proportion of carbonate minerals and the X CO2 of the rocks prior to prograde metamorphism. In rocks with an initially low proportion of carbonate minerals, the greenschist to amphibolite facies transition is the primary period of fluid production, producing fluid with a low X CO2 . Rocks with greater initial proportions of carbonate minerals experience a second fluid production event at temperatures above the greenschist to amphibolite facies transition, producing a more CO 2 ‐rich fluid ( X CO2 = 0.2–0.3). Rocks may achieve these higher proportions of carbonate minerals either via more extensive seafloor alteration or via infiltration of fluids. Fluid produced via devolatilization of rocks at deeper crustal levels may infiltrate and react with overlying lower temperature rocks, resulting in external buffering of those rocks to higher X CO2 and proportions of carbonate minerals. Subsequent heating and devolatilization of these overlying rocks results in buffering paths that produce large proportions of fluid at X CO2 = 0.2–0.3. The production of fluid of this composition is of importance to models of gold transport in Archean greenstone gold deposits occurring within extensive fluid alteration haloes, as these haloes represent the influx of fluid of X CO2 = 0.2–0.3 into the upper crust.
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Amphibolite-facies Settler Schist in the southeastern Coast Mountains of British Columbia has long been correlated with Chiwaukum Schist of the Cascade metamorphic core, North Cascade Mountains, northwestern Washington. The additional correlation proposed here of Settler Schist with Darrington Phyllite and Shuksan Greenschist (and blueschist) of the Northwest Cascade System in Washington is based on along-strike near-continuity of outcrop areas, a similar protolith composition range, the same structural position relative to the Shuksan fault zone, and distinctive irregular structures in variably metamorphosed sandstone and pelite of both Darrington Phyllite and Settler Schist. If this correlation is valid, then the record of Early Cretaceous; subduction-related blueschist metamorphism of Shuksan–Darrington rocks was destroyed in Settler Schist by overprinting by early Late Cretaceous Barrovian metamorphism; only some distinctive, premetamorphic structures remain. The implication is that within the southeastern Coast Mountains, a cryptic record of subduction is overprinted by Barrovian metamorphism.
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