Titanite reaction textures around ilmenite in amphibolite-facies rocks: Fluid induced?
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During high-pressure, low-temperature greenschist and epidote-amphibolite facies metamorphism in Dalradian rocks of the SW Scottish Highlands, mineral assemblages in metabasites and calcareous metasediments were dominantly controlled by infiltration of hydrous fluids; consequently, mineral assemblages capable of buffering the fluid phase composition were rare. Equilibrium prograde H 2 O-CO 2 fluids usually contained less or much less than about 1–2 mol% CO 2 . Three fluid infiltration events are recognized. During prograde greenschist facies metamorphism, metabasic sills were infiltrated by large volumes of CO 2 -bearing hydrous fluid; carbon isotope studies indicate that the CO 2 was locally derived by widespread oxidation of graphite or other organic carbon in adjacent metasediments. This may have occurred under approximately lower greenschist facies conditions as a result of mixing of fluids of varying f O 2 , initiated by thermal expansion of water during heating, decompression and consequent hydraulic fracturing. In the epidote-amphibole facies (garnet zone), dehydration reactions in metabasites generated large quantities of water, which removed carbonate from metabasites on a regional scale and infiltrated calcareous metasediments to produce assemblages containing grossular, diopside, K-feldspar, amphibole, clinozoisite and sphene. A late retrograde infiltration of CO 2 ,-bearing hydrous fluid under lower greenschist facies conditions generated assemblages containing K-feldspar + chlorite + rutile ± dolomite in calcareous rocks and albite prophyroblast schists in zones of intense secondary deformation. Large-scale infiltration of fluid into greenschist-facies metadolerite sills was intimately related to, and possibly controlled by, penetrative deformation and, in the absence of a penetrative deformation, grain-boundary diffusion by itself was an ineffectual mechanism of fluid transport.
Greenschist
Tremolite
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Pyroxene
Amphibole
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Abstract Recent petrological studies on high‐pressure ( HP )–ultrahigh‐pressure ( UHP ) metamorphic rocks in the Moldanubian Zone, mainly utilizing compositional zoning and solid phase inclusions in garnet from a variety of lithologies, have established a prograde history involving subduction and subsequent granulite facies metamorphism during the Variscan Orogeny. Two temporally separate metamorphic events are developed rather than a single P–T loop for the HP – UHP metamorphism and amphibolite–granulite facies overprint in the Moldanubian Zone. Here further evidence is presented that the granulite facies metamorphism occurred after the HP – UHP rocks had been exhumed to different levels of the middle or upper crust. A medium‐temperature eclogite that is part of a series of tectonic blocks and lenses within migmatites contains a well‐preserved eclogite facies assemblage with omphacite and prograde zoned garnet. Omphacite is partly replaced by a symplectite of diopside + plagioclase + amphibole. Garnet and omphacite equilibria and pseudosection calculations indicate that the HP metamorphism occurred at relatively low temperature conditions of ~600 °C at 2.0–2.2 GPa. The striking feature of the rocks is the presence of garnet porphyroblasts with veins filled by a granulite facies assemblage of olivine, spinel and Ca‐rich plagioclase. These minerals occur as a symplectite forming symmetric zones, a central zone rich in olivine that is separated from the host garnet by two marginal zones consisting of plagioclase with small amounts of spinel. Mineral textures in the veins show that they were first filled mostly by calcic amphibole, which was later transformed into granulite facies assemblages. The olivine‐spinel equilibria and pseudosection calculations indicate temperatures of ~850–900 °C at pressure below 0.7 GP a. The preservation of eclogite facies assemblages implies that the granulite facies overprint was a short‐lived process. The new results point to a geodynamic model where HP – UHP rocks are exhumed to amphibolite facies conditions with subsequent granulite facies heating by mantle‐derived magma in the middle and upper crust.
Omphacite
Amphibole
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Hornblende
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Greenschist
Amphibole
Ultramafic rock
Pyroxene
Protolith
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Blueschist
Rutile
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ABSTRACT The high‐grade rocks (metapelite, quartzite, metagabbro) of the Hisøy‐Torungen area represent the south‐westernmost exposures of granulites in the Proterozoic Bamble sector, south Norway. The area is isoclinally folded and a metamorphic P–T–t path through four successive stages (M1‐M4) is recognized. Petrological evidence for a prograde metamorphic event (M1) is obtained from relict staurolite + chlorite + albite, staurolite + hercynite + ilmenite, cordierite + sillimanite, fine‐grained felsic material + quartz and hercynite + biotite ± sillimanite within metapelitic garnet. The phase relations are consistent with a pressure of 3.6 ± 0.5 kbar and temperatures up to 750–850°C. M1 is connected to the thermal effect of the gabbroic intrusions prior to the main (M2) Sveconorwegian granulite facies metamorphism. The main M2 granulite facies mineral assemblages (quartz+ plagioclase + K‐feldspar + garnet + biotite ± sillimanite) are best preserved in the several‐metre‐wide Al‐rich metapelites, which represent conditions of 5.9–9.1 kbar and 790–884°C. These P–T conditions are consistent with a temperature increase of 80–100°C relative to the adjacent amphibolite facies terranes. No accompanying pressure variations are recorded. Up to 1‐mm‐wide fine‐grained felsic veinlets appear in several units and represent remnants of a former melt formed by the reaction: Bt + Sil + Qtz→Grt + lq. This dehydration reaction, together with the absence of large‐scale migmatites in the area, suggests a very reduced water activity in the rocks and XH 2 O = 0.25 in the C–O–H fluid system was calculated for a metapelitic unit. A low but variable water activity can best explain the presence or absence of fine‐grained felsic material representing a former melt in the different granulitic metapelites. The strongly peraluminous composition of the felsic veinlets is due to the reaction: Grt +former melt ± Sil→Crd + Bt ± Qtz + H 2 O, which has given poorly crystalline cordierite aggregates intergrown with well‐crystalline biotite. The cordierite‐ and biotite‐producing reaction constrains a steep first‐stage retrograde (relative to M2) uplift path. Decimetre‐ to metre‐wide, strongly banded metapelites (quartz + plagioclase + biotite + garnet ± sillimanite) inter‐layered with quartzites are retrograded to (M3) amphibolite facies assemblages. A P–T estimate of 1.7–5.6 kbar, 516–581°C is obtained from geothermobarometry based on rim‐rim analyses of garnet–biotite–plagioclase–sillimanite–quartz assemblages, and can be related to the isoclinal folding of the rocks. M4 greenschist facies conditions are most extensively developed in millimetre‐wide chlorite‐rich, calcite‐bearing veins cutting the foliation.
Sillimanite
Geothermobarometry
Felsic
Staurolite
Andalusite
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ABSTRACT Calc‐silicate rocks occur as elliptical bands and boudins intimately interlayered with eclogites and high‐pressure gneisses in the Münchberg gneiss complex of NE Bavaria. Core assemblages of the boudins consist of grossular‐rich garnet, diopside, quartz, zoisite, clinozoisite, calcite, rutile and titanite. The polygonal granoblastic texture commonly displays mineral relics and reaction textures such as post kinematic grossular‐rich garnet coronas. Reactions between these mineral phases have been modelled in the CaO‐Al 2 O 3 ‐SiO 2 ‐CO 2 ‐H 2 O system with an internally consistent thermodynamic data base. High‐pressure metamorphism in the calc‐silicate rocks has been estimated at a minimum pressure of 31 kbar at a temperature of 630d̀ C with X H2 , O ≥ 0.03. Small volumes of a CO 2 ‐N 2 ‐rich fluid whose composition was buffered on a local scale were present at peak‐metamorphic conditions. The P‐T conditions for the onset of the amphibolite facies overprint are about 10 kbar at the same temperature. X Co2 of the H 2 O‐rich fluid phase is regarded to have been <0.03 during amphibolite facies conditions. These P‐T estimates are interpreted as representing different stages of recrystallization during isothermal decompression. The presence of multiple generations of mineral phases and the preservation of very high‐pressure relics in single thin sections preclude pervasive post‐peak metamorphic fluid flow as a cause of a re‐equilibration within the calc‐silicates. The preservation of eclogite facies, very high‐pressure relics as well as amphibolite facies reactions textures in the presence of a fluid phase is in agreement with fast, tectonically driven unroofing of these rocks.
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Titanite
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A rare lithology consisting of garnet-tourmaline-sillimanite-biotite-ilmenite-quartz has been found within the granulite-facies region of south-central Massachusetts. The homogeneous, Ti-rich oxy-dravitic tourmaline, XMg = Mg/(Mg + Fe) = 0.72.0.77, falls into the alkali group, and is similar in composition to lower grade tourmaline found in corresponding metapelitic rocks in Maine. Charge-balancing calculations and binary diagrams suggest that, like biotite in the region, tourmaline has undergone deprotonation by means of the exchange vectors AlOR-1(OH)-1 and TiO2R-1(OH)-2, where R represents Fe + Mg. The restriction of a concordant tourmaline-rich horizon in otherwise tourmaline-free rocks of this granulite-facies region suggests that either: (1) B, released during prograde fluid-absent dehydration reactions of muscovite and biotite, was locally available in a fluid phase or melt for later crystallization near the peak of granulite-facies metamorphism along pathways that provided a conduit for fluid migration; or (2) that this is simply a B-rich compositional horizon (tourmalinite) that survived anatexis and granulite-facies metamorphism and that records the incipient conditions of tourmaline breakdown and subsequent recrystallization near or post-peak metamorphism.
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This study constitutes the first attempt to integrate rock magnetism and metamorphic petrology in the Hercynian basement of northern Sardinia. The investigation focused on the magnetic petrology of variably retrogressed eclogites and amphibolites from two suites of basic meta‐igneous rocks, which occur along a major tectonic line (Posada‐Asinara Line), within a medium‐grade (MG) and a high‐grade (HG) metamorphic complex. Consistent with petromagnetic results, HG metabasites contain variable amounts of monoclinic pyrrhotite (intergrown with rutile) and titanomagnetite (occurring as inclusions in garnet), abundant ilmenite (associated to secondary hornblende and with sphene ± low‐Ti‐magnetite rims), and rutile both as inclusions in ilmenite and as discrete grains. In MG metabasites, pyrrhotite is restricted to amphibolites of the Posada Valley area where it occurs as rare inclusions in garnet. All samples are characterized by variable amounts of ilmenite, rutile, and sphene which show the same microstructural features described in HG rocks. Microstructural evidence and geothermobarometric data indicate that (1) pyrrhotite and titanomagnetite likely formed prior to and remained stable during the eclogite facies metamorphic peak and (2) the growth of ilmenite and sphene can be attributed to the amphibolite facies retrogression, mainly due to model reactions such as garnet + omphacite + rutile + H 2 O → hornblende + plagioclase + ilmenite and amphibole + ilmenite + O 2 → sphene + magnetite + quartz + H 2 O. The results from our combined petrological and petromagnetic study corroborate the hypothesis that significant volumes of mafic/ultramafic rocks, similar to some of the investigated outcrops, may account for the magnetic anomalies flanking the northeastern part of the Posada‐Asinara Line.
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Hornblende
Amphibole
Almandine
Titanite
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