Titaniferous accessory minerals in very low-grade metamorphic rocks, Keweenaw Peninsula Michigan, USA

Hansen, Edward C.; Reimink, Jesse; Harlov, Daniel E.

doi:10.1016/j.lithos.2010.02.001

Cited by 6 publications

(2 citation statements)

References 22 publications

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“…Grains are cross‐cut by fluid inclusion trails. Originally euhedral grains of ilmenite are first altered to aggregates of anatase (type‐1 Ant)+calcite then to titanite (Figure a,b), following the reaction proposed by Hansen, Reimink, and Harlov () for low‐ T conditions. Anatase is itself replaced by titanite, in textural equilibrium with chlorite flakes (Figure a,b).…”

Section: Sample Description and Chronology Of Mineral Assemblagesmentioning

confidence: 86%

Pre‐orogenic upper crustal softening by lower greenschist facies metamorphic reactions in granites of the central Pyrenees

Airaghi

Bellahsen

Dubacq

et al. 2020

Journal Metamorphic Geology

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Pre-kinematic greenschist facies metamorphism is often observed in granites and basement units of mountain belts, but rarely dated and accounted for in orogenic cycle reconstructions. Studying pre-kinematic alteration is challenging because of its usual obliteration by subsequent syn-kinematic metamorphism often occurring at conditions typical of the brittle-ductile transition. It is, however, to be expected that pre-kinematic alteration has major implications for the rheology of the upper crust. In the 305 Ma-old Variscan basement of the Bielsa massif (located in the Axial Zone of the Pyrenees), successive fluid-rock interaction events are recorded in granites below 350°C. Combined microstructural and petrographic analysis, low-T thermobarometry and in situ U-Th/Pb dating of anatase, titanite and monazite show extensive pre-orogenic (pre-Alpine) and pre-kinematic alteration related to feldspar sericitization and chloritization of biotite and amphibole at temperatures of 270-350°C at 230-300 Ma. This event is followed by a second fluid-rock interaction stage marked by new crystallization of phyllosilicates at 200-280°C and is associated with the formation of mylonitic shear zones and fractures parallel to the shear planes. U-Pb anatase and monazite ages as well as the microtextural relationships of accessory minerals suggest an age for this event at 40-70 Ma, consistent with independent regional geology constraints. The Variscan basement was therefore softened at late to post-Variscan time, at least 150-200 Ma before the main Alpine shortening while Alpine-age compression (c. 35-50 Ma) leads to the formation of a dense net of mylonites. The associated deformation, both distributed at the scale of the Bielsa massif and localized at decametric scale in mylonitic corridors, precedes the strain localization along the major thrusts of the Axial Zone. The Bielsa massif is a good example where inherited, pre-orogenic upper crustal softening controls the deformation patterns in granitic basement units through low-grade metamorphic reactions. K E Y W O R D SAxial Zone, Bielsa, chlorite-white mica thermobarometry, mylonites, U-Th/Pb anatase-titanitemonazite dating 184 | AIRAGHI et Al.

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Section: Sample Description and Chronology Of Mineral Assemblagesmentioning

confidence: 86%

Pre‐orogenic upper crustal softening by lower greenschist facies metamorphic reactions in granites of the central Pyrenees

Airaghi

Bellahsen

Dubacq

et al. 2020

Journal Metamorphic Geology

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“…The effect of metamorphism on Fe-Ti oxide assemblages is insufficiently studied and poorly compiled and analyzed, despite the fact that ilmenite, magnetite, hematite, and rutile are common accessory minerals in both igneous and metamorphic rocks (see, e.g., [1][2][3][4][5][6][7][8][9][10][11]). Numerous studies provide experimental research as well as detailed observations of natural mineral paragenesis, textural and compositional features, internal structures, and alteration products of such Fe-Ti oxides, as all these characteristics are known to be sensitive to temperature and oxygen fugacity (see, e.g., [12][13][14][15][16][17][18][19][20][21]).…”

Section: Introductionmentioning

confidence: 99%

Fe-Ti Oxide Assemblages from the Contact-Metamorphosed Mafic-Ultramafic Rocks of the Sedova Zaimka Intrusion (Western Siberia, Russia): The Tracking of Metamorphic Transformations

2020

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Studies that track and compiled transformations of ilmenite and magnetite under conditions of different metamorphic grades are still very limited. The Sedova Zaimka intrusion in Western Siberia (Russia) is a good example to examine the effect of contact metamorphism from greenschist to low-amphibolite facies on magmatic Fe-Ti oxide minerals, as this small mafic-ultramafic body is located within contact metamorphic aureole of a large granite pluton. In the Sedova Zaimka, ilmenite with little to no magnetite is present as an accessory dissemination throughout metamorphosed rocks. Ilmenite is variably enriched in MnO (1 to 13.3 wt%) and depleted in MgO (less than 0.3 wt%), suggesting that its primary magmatic composition has been unevenly modified by diffusion re-equilibrium with coexisting metamorphic silicates. The elevated content of ZnO (up to 0.5 wt%) and NiO (up to 0.4 wt%) in ilmenite suggests that both ZnO and NiO, like MnO, must be strongly partitioned into ilmenite relatively to silicate minerals under the reducing contact metamorphic conditions, if chromite is absent. The textural observations of ilmenite–sulfide and ilmenite–titanite–sulfide relationships indicate that Ti-magnetite, in contrast to ilmenite, is an unstable phase in the presence of sulfur-rich reduced metamorphic fluids and is completely replaced by sulfides, with the development of specific “octahedral meshes” of ilmenite in sulfides.

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Micro- and nano-textural evidence of Ti(–Ca–Fe) mobility during fluid–rock interactions in carbonaceous lawsonite-bearing rocks from New Zealand

Gálvez

Beyssac

Benzerara

et al. 2012

Contrib Mineral Petrol

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Titaniferous accessory minerals in very low-grade metamorphic rocks, Keweenaw Peninsula Michigan, USA

Cited by 6 publications

References 22 publications

Pre‐orogenic upper crustal softening by lower greenschist facies metamorphic reactions in granites of the central Pyrenees

Pre‐orogenic upper crustal softening by lower greenschist facies metamorphic reactions in granites of the central Pyrenees

Fe-Ti Oxide Assemblages from the Contact-Metamorphosed Mafic-Ultramafic Rocks of the Sedova Zaimka Intrusion (Western Siberia, Russia): The Tracking of Metamorphic Transformations

Micro- and nano-textural evidence of Ti(–Ca–Fe) mobility during fluid–rock interactions in carbonaceous lawsonite-bearing rocks from New Zealand

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