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TitleGeochronology and trace element mobility in rutile from a Carboniferous syenite pegmatite and the role of halogens
AuthorPe-Piper, G; Nagle, J; Piper, D J WORCID logo; McFarlane, C R M
SourceAmerican Mineralogist vol. 104, no. 4, 2019 p. 501-513,
Alt SeriesNatural Resources Canada, Contribution Series 20180271
PublisherMineralogical Society of America
Mediapaper; on-line; digital
File formatpdf (Adobe® Reader®); html
ProvinceNova Scotia
AreaClarke Head; Cobequid Highlands
Lat/Long WENS -64.5000 -64.0000 45.5000 45.3333
Subjectsgeochronology; geochemistry; igneous and metamorphic petrology; bedrock geology; lithology; igneous rocks; intrusive rocks; syenites; pegmatites; gabbros; granites; breccias; structural features; fault zones; faults, strike-slip; rutile; tectonic setting; magmatism; emplacement; hydrothermal alteration; metasomatism; intrusions; plutons; inclusions; faulting; subduction; scanning electron microscope analyses; spectroscopic analyses; trace element analyses; titanium geochemistry; zirconium geochemistry; uranium geochemistry; geochemical anomalies; europium; mass spectrometer analysis; radiometric dating; uranium lead dating; lead lead dates; titanite; magnetite; chondrites; ilmenite; halite; scapolite; morphology, crystal; mantle; partial melting; pressure-temperature conditions; calcium geochemistry; iron geochemistry; precipitation; Minas Fault Zone; Avalon Terrane; Rheic Ocean; Dissolution; Phanerozoic; Paleozoic; Carboniferous; Devonian; Precambrian; Proterozoic
Illustrationsgeoscientific sketch maps; photographs; photomicrographs; tables; plots; Concordia diagrams; bar graphs
ProgramGSC Atlantic Division
Released2019 04 01
AbstractThis study investigates Ti mobility in the presence of halogens, as shown by the hydrothermal alteration of magmatic rutile in syenite. The syenite pegmatite studied intrudes gabbro, is preserved as a tectonic block in a major strike-slip fault zone, and formed in a back-arc environment in which there was widespread A-type granite plutonism. Rutile was studied by SEM and Raman spectroscopy, trace elements were analyzed by LA-ICP-MS, and age was determined by in situ U-Pb analysis. Magmatic rutile in the syenite forms millimetric-scale crystals rimmed by magmatic titanite and magnetite and also occurs as smaller interstitial crystals. Hydrothermal alteration occurred preferentially along crystal margins and fractures by a layer-by-layer dissolution-reprecipitation process resulting in high Zr contents (~5000 ppm) in the rutile, together with enrichment in U and depletion in high field strength elements. The magmatic emplacement age of the syenite was ~360 Ma (dated rutile G) and no younger than 353.9 ± 5.7 Ma (mean Concordia age of interstitial rutile). The syenite was synchronous with the later phases of regional A-type granite plutonism. Most magmatic rutile has REE patterns either (1) with 1-50 times chondrite enrichment, LREE > HREE and a Eu anomaly, resulting from felsic melt inclusions, or (2) flat patterns with 0.1-10 times chondrite enrichment, present in ilmenite exsolution lamellae or inclusions. Later hydrothermal halogen-rich fluids, derived from dissolution of halite, produced widespread metasomatic scapolite in the syenite. These fluids also leached Ti and other HFSE, together with REE, from large fractured rutile crystals. Such fluids resulted in local dissolution-reprecipitation of Ti and Zr and resetting of the U-Pb system in the altered rutile, at 337.4 ± 3.5 Ma. Normalized REE abundances in the hydrothermal rutile show a U-shaped pattern, with the greatest depletion in the MREE. Variations in dissolution and transport of Zr and Ti by halogen-rich fluids affect the Zr in rutile geothermometer, which yields unrealistic temperatures when applied in this study. More generally, the complexities of rutile chemistry in this hydrothermal setting could be reproduced in deeper subduction settings as a result of variations in halogen content of fluids released by prograde metamorphism.
Summary(Plain Language Summary, not published)
This study has dated emplacement of a granitic body at Clarke Head near Parrsboro NS and has also dated the time of hydrothermal mineralisation along the Minas Fault Zone. It provides new insights into the conditions under which the mineral rutile precipitates.

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