Publication Summary
Issue ID: P2016-01-06
Title: Petrology of explosive Middle-Upper Triassic ultramafic rocks in the Mess Creek area, northern Stikine terrane
Author(s): Milidragovic, D., Joyce, N.L., Zagorevski, A., Chapman, J.B.
Series Name: Paper
Publication Year: 2016
Larger Work Citation: in Geological Fieldwork 2015, B.C. Ministry of Energy and Mines, Paper 2015-01
NTS Map Sheet(s): 104G
Place Keyword(s): British Columbia, Mess Creek, Stikine Terrane, Stuhini Group
Lat/Long (NSWE): 58, 56, -132, -130
Theme Keyword(s): Triassic, pyroclastic, ultramafic, Stikinia, Stuhini Group
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Abstract:
The Mess Lake facies of the Stuhini Group (Middle to Upper Triassic) in northwestern British Columbia includes a unit of pervasively serpentinized, olivine-rich, subaqueous lapilli breccia, lapilli tuff, and ash tuff. Relatively high whole-rock MgO concentrations (29-38 wt.%; loss on ignition-free) of pyroclastic samples from this unit refl ect signifi cant accumulation of olivine. These ultramafi c pyroclastic rocks also exhibit strong enrichment in Ba and depletion in Nb+Ta relative to La, consistent with derivation from slab-metasomatized mantle asthenosphere in a volcanic arc setting. The restored parental liquid is a primitive, mantle-derived, arc magma that contained up to 16-20 wt.% MgO. The unit contains fresh volcanic glass indicating low-grade regional metamorphism, and suggesting that it postdates emplacement of the nearby Hickman pluton, which yielded 40Ar/39Ar hornblende ages of 222.2 2.8 Ma, 221.6 2.5 Ma, and 220.1 2.7 Ma. The Alaskan-type ultramafic plutons of the Polaris suite (Middle to Late Triassic) and a post-Hickman basaltic diatreme, containing picritic fragments, are spatially associated and broadly coeval with the ultramafi c pyroclastic rocks of the Mess Lake facies, but a petrogenetic link has yet to be established.