UNRAVELLING ARC MANTLE USING TRACE ELEMENT AND ISOTOPIC “INTELLIGENCE” FROM THE MIDDLE-LATE TRIASSIC STIKINE ARC

Island arc volcanics record interaction between the “baseline” asthenospheric arc mantle and melts and fluids derived from subducted oceanic sediments, crust, and mantle. Contributions from sub-arc lithospheric mantle, which may be genetically unrelated to the ongoing arc process, and arc basement may add further complexity to the elemental and isotopic compositions of arc magmas. The Stikine Terrane (Stikinia) of the North American Cordillera hosts rare occurrences of cumulate, olivine-phyric, lapilli tuff. The tuff, which forms a ≤100 m thick unit near the base of the Middle-Late Triassic Stuhini Group, crystallized from a near-primary, picritic (MgO ~16 wt. %) parental arc magma. The trace element composition of the parental magma is depleted in most incompatible trace elements relative to MORB, and implies a strongly depleted “baseline” arc mantle in which the degree of relative depletion increases with element incompatibility. Addition of non-conservative elements through melting of subducted Panthalassan sediments accounts for the arc signature of the picritic tuff.

High-precision trace element and Hf-Nd-Pb isotopic analyses of the tuff are consistent with small amounts (≤2%) of sediment melt mixed with melt of “baseline” depleted asthenosphere. Lead, and to lesser degree Nd isotopes, largely record the composition of subducted sediments. The sediments were similar in composition to modern sediments of the northern Cascadia Basin and were likely sourced from Stikinia itself, or other North American pericratonic terrane(s). In contrast to Pb and Nd, the initial Hf isotopic composition of the picritic tuff closely approximates the Hf-isotopic composition of the depleted “baseline” arc asthenosphere under Stikinia. The initial Hf isotopic composition of the tuff is notably less radiogenic than the age-corrected Hf isotopic composition of the Depleted Mantle and suggests that the “baseline” mantle underwent depletion shortly before picrite petrogenesis. The “baseline” mantle is isotopically similar to the source of E-MORB presently erupted at the southern end of the Explorer Ridge. The isotopic similarity between the ambient mantle under Stikinia and mantle tapped at the Explorer Ridge suggests that enriched domains in the northeastern Pacific mantle may be long-lived (≥230 Myr).