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TitleH2O-driven generation of picritic melts in the Middle to Late Triassic Stuhini arc of the Stikine terrane, British Columbia, Canada
AuthorMilidragovic, D; Chapman, J B; Bichlmaier, S; Canil, D; Zagorevski, A
SourceEarth and Planetary Science Letters vol. 454, 2016 p. 65-77,
Alt SeriesEarth Sciences Sector, Contribution Series 29874
Mediapaper; on-line; digital
File formatpdf; html
ProvinceBritish Columbia
NTS104A/12; 104A/13; 104B/09; 104B/10; 104B/11; 104B/14; 104B/15; 104B/16; 104G/01; 104G/02; 104G/03; 104G/06; 104G/07; 104G/08; 104G/09; 104G/10; 104G/11; 104H/04; 104H/05; 104H/12
Lat/Long WENS-131.5000 -129.5000 57.7500 56.5000
Subjectsigneous and metamorphic petrology; geochemistry; Middle Triassic; Upper Triassic; magmatism; island arcs; bedrock geology; lithology; igneous rocks; volcanic rocks; ultramafic rocks; tuffs; breccias; volcanic ash; pyroclastics; magmas; picrites; mineralization; porphyries; metals; copper; molybdenum; gold; olivine; chromite; phenocrysts; liquidus temperatures; crystallization; models; tectonic setting; thermal history; plate tectonics; subduction zones; mantle; lithosphere; temperature; whole rock analyses; major element analyses; trace element analyses; metamorphism; alteration; oxidation; North American Cordillera; Stikinia; Stikine terrane; Stuhini arc; picritic magmas; slab dehydration; mantle wedge; water content; element mobility; thermometry; parental magma composition; Phanerozoic; Mesozoic; Triassic
Illustrationsgeological sketch maps; schematic sections; geochemical profiles; tables; geochemical plots; graphs; diagrams
ProgramWestern Cordillera, Regional porphyry transitions, GEM2: Geo-mapping for Energy and Minerals
AbstractBasaltic to andesitic compositions predominate island arc magmatism; ultramafic magmas are rare. Ultramafic (MgO =21-33 wt.%) tuff breccia, lapilli tuff, and ash tuff of the Middle to Upper Triassic Stuhini Group were erupted in the Stikine arc of the North American Cordillera shortly preceding an episode of prolific porphyry Cu-Mo(-Au) mineralization. The ultramafic tuff shows accumulation (20-65%) of olivine (Fo91) and minor chromite into a subalkaline picritic parental magma with MgO ~16wt.%. Despite the inferred high MgO content of the parental liquid, chromite phenocrysts record relatively low liquidus temperatures (<1200°C) suggesting crystallization from relatively low temperature, hydrous melts, at oxygen fugacities one to three log units above the fayalite-magnetite-quartz (FMQ) buffer. The primary picritic magmas likely contained 5-7 wt.% H2O, inferred on the basis of olivine-liquid thermometry and thermal models for subduction zones, thus alleviating the need for catastrophic thermal perturbations in the mantle wedge. Instead, efficient release of water through slab dehydration at 2.5-3.0 GPa allows generation of picritic melts at ordinary mantle wedge temperatures through moderate degrees (F=0.10-0.15) of hydrous flux melting. The volatile-rich nature of the melt and the predominant extensional regime in the overlying lithosphere of Stikinia facilitated the near-adiabatic ascent of the Stuhini Group picrites. The high H2O content of the rapidly ascending picrite melt may have played a key role in transport of metals into the crust of the Stikinia and subsequent porphyry mineralization.
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. Previous research suggested that catastrophic geological events lead to formation of deposit-forming magma. This study investigated a rare type of magma (picrite) that provides insight into the processes that occur in the Earth¿s interior prior to formation of copper-gold-molybdenum deposits. Our investigation of the magma indicates that previously proposed catastrophic event hypotheses are incorrect. Rather steady state processes combined with efficient migration of water are responsible for formation of magma. This research provides a critical base study for assessment of the mineral systems in districts with copper-gold-molybdenum potential.