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TitlePicrite 'intelligence' from the Middle-Late Triassic Stikine arc: composition of mantle wedge asthenosphere
AuthorMilidragovic, DORCID logo; Zagorevski, AORCID logo; Weis, D; Joyce, NORCID logo; Chapman, J BORCID logo
SourceLithos vol. 308-309, 2018 p. 446-461,
Alt SeriesNatural Resources Canada, Contribution Series 20170247
PublisherElsevier BV
Mediapaper; on-line; digital
File formatpdf; html; xlsx; docx; doc
ProvinceBritish Columbia
NTS104G/02; 104G/03; 104G/04; 104G/05; 104G/06; 104G/07; 104G/10; 104G/11; 104G/12
AreaMess Lake
Lat/Long WENS-131.6167 -130.7500 57.5833 57.1667
Subjectsgeochronology; geochemistry; tectonics; igneous and metamorphic petrology; Upper Triassic; Middle Triassic; crustal studies; mantle; provenance; bedrock geology; lithology; igneous rocks; volcanic rocks; magmas; tuffs; picrites; basalts; intrusive rocks; intrusions; dykes; radiometric dating; argon argon dating; isotope ratios; strontium strontium ratios; rubidium-strontium ratios; trace element analyses; hafnium; neodymium; lead; tectonic setting; subduction; metasomatism; mid-ocean ridges; depositional environment; marine environments; Canadian Cordillera; Stikine Terrane; Stuhini Group; Panthalassa Ocean; Cascadia Basin; Explorer Ridge; Juan de Fuca Ridge; Phanerozoic; Mesozoic; Triassic
Illustrationsgeoscientific sketch maps; photographs; photomicrographs; spectra; tables; geochemical plots; ternary diagrams
ProgramGEM2: Geo-mapping for Energy and Minerals Western Cordillera Project Management
Released2018 03 21
AbstractPrimitive, near-primary arc magmas occur as a volumetrically minor <=100m thick unit in the Canadian Cordillera of northwestern British Columbia, Canada. These primitive magmas formed an olivine-phyric, picritic tuff near the base of the Middle-Late Triassic Stuhini Group of the Stikine Terrane (Stikinia). A new 40Ar/39Ar age on hornblende from a cross-cutting basaltic dyke constrains the tuff to be older than 221 ± 2Ma. An 87Sr/86Sr isochron of texturally-unmodified tuff samples yields 212 ± 25Ma age, which is interpreted to represent syn-depositional equilibration with sea-water. Parental trace element magma composition of the picritic tuff is strongly depleted in most incompatible trace elements relative to MORB and implies a highly depleted ambient arc mantle. High precision trace element and Hf-Nd-Pb isotopic analyses indicate an origin by mixing of a melt of depleted ambient asthenosphere with <=2% of subducted sediment melt. Metasomatic addition of non-conservative incompatible elements through melting of subducted Panthalassa Ocean floor sediments accounts for the arc signature of the Stuhini Group picritic tuff, enrichment of light rare earth elements (LREE) relative to heavy rare earth elements (HREE) and high field strength elements (HFSE), and anomalous enrichment in Pb.
The inferred Panthalassan sediments are similar in composition to the Neogene-Quaternary sediments of the modern northern Cascadia Basin. The initial Hf isotopic composition of the picritic tuff closely approximates that of the ambient Middle-Late Triassic asthenosphere beneath Stikinia and is notably less radiogenic than the age-corrected Hf isotopic composition of the Depleted (MORB) Mantle reservoir (DM or DMM). This suggests that the ambient asthenospheric mantle end-member experienced melt depletion (F <= 0.05) a short time before picrite petrogenesis. The mantle end-member in the source of the Stuhini Group picritic tuff is isotopically similar to the mantle source of enriched mid-ocean ridge basalts (E-MORB) erupted today at the southern end of the Explorer Ridge in northeastern Pacific Ocean. The isotopic similarity between the Middle-Late Triassic ambient mantle under Stikinia, and mantle presently tapped at the southern Explorer Ridge suggests that enriched domains in the northeastern Pacific mantle are long-lived (>=222 million years).
Summary(Plain Language Summary, not published)
Understanding the circumstances and processes that lead to production of magmas is critical to understanding formation of copper-gold-molybdenum mining districts in Canada and world. This study investigates Middle-late Triassic magmas, erupted more than 222 million years ago onto the Stikine Terrane of northern British Columbia. These rare and relatively pristine mantle-derived melts provide insight into the processes that occur in the Earth¿s interior (mantle) prior to formation of copper-gold-molybdenum deposits. The inferred mantle source was isotopically similar to the mantle that presently underlies the southern Explorer and northern Juan de Fuca ridges off the western coast of North America. This similarity suggests that relatively uncommon enriched mantle has resided in the northeastern Pacific for more than 222 Ma. Our investigation of the Middle-Late Triassic magma also indicates that its mantle source of was modified by a small amount (~2%) of subducted sediment, compositionally similar to the modern sediments of the northern Cascadia Basin.

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