| Title | Geochronology and trace element mobility in rutile from a Carboniferous syenite pegmatite and the role of halogens |
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| Author | Pe-Piper, G; Nagle, J; Piper, D J W ; McFarlane, C R M |
| Source | American Mineralogist vol. 104, no. 4, 2019 p. 501-513, https://doi.org/10.2138/am-2019-6668 |
| Image |  |
| Year | 2019 |
| Alt Series | Natural Resources Canada, Contribution Series 20180271 |
| Publisher | Mineralogical Society of America |
| Document | serial |
| Lang. | English |
| Media | paper; on-line; digital |
| File format | pdf (Adobe® Reader®); html |
| Province | Nova Scotia |
| NTS | 21H/08 |
| Area | Clarke Head; Cobequid Highlands |
| Lat/Long WENS | -64.5000 -64.0000 45.5000 45.3333 |
| Subjects | geochronology; 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 |
| Illustrations | geoscientific sketch maps; photographs; photomicrographs; tables; plots; Concordia diagrams; bar graphs |
| Program | GSC Atlantic Division |
| Released | 2019 04 01 |
| Abstract | This 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. |
| GEOSCAN ID | 313031 |
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