Mantle control on magmatic flare-ups in the southern Coast Mountains batholith, British Columbia

Abstract

The southern Coast Mountain batholith was episodically active from Jurassic to Eocene time and experienced four distinct high magmatic flux events during that period. Similar episodicity has been recognized in arcs worldwide, yet the mechanism(s) driving such punctuated magmatic behavior are debated. This study uses zircon Hf and O isotopes, with whole-rock and mineral geochemistry, to track spatiotemporal changes in southern Coast Mountains batholith melt sources and to evaluate models of flare-up behavior and crust formation in Cordilleran arc systems. Zircon Hf isotope analysis yielded consistently primitive values, with all zircon grains recording initial εHf between +6 and +16. The majority (97%) of zircons analyzed yielded δ18O values between 4.2‰ and 6.5‰, and only five grains recorded values of up to 8.3‰. These isotopic results are interpreted to reflect magmatism dominated by mantle melting during all time periods and across all areas of the southern batholith, which argues against the periodic input of more melt-fertile crustal materials as the driver of episodic arc magmatism. They also indicate that limited crustal recycling is needed to produce the large volumes of continental crust generated in the batholith. Although the isotopic character of intrusions is relatively invariant through time, magmas emplaced during flare-ups record higher Sr/Y and La/Yb(N) and lower zircon Ti and Yb concentrations, which is consistent with melting in thickened crust with garnet present as a fractionating phase. Flare-ups are also temporally associated with periods when the southern Coast Mountains batholith both widens and advances inboard. We suggest that the landward shift of the arc into more fertile lithospheric mantle domains triggers voluminous magmatism and is accompanied by magmatic and/or tectonic thickening. Overall, these results demonstrate that the magmatic growth of Cordilleran arcs can be spatially and temporally complex without requiring variability in the contributions of crust and/or mantle to the batholith.

Crust–mantle interactions in Zijinshan ore field identified by zircon SIMS U–Pb age and zircon Hf–O isotopes

“…The Zijinshan granitoids have lower zircon δ 18 O Zrn values (6.3–12 ‰; Fig. 7a) than those of typical crustal magmas (10–30 ‰; Valley et al 2005), indicating the addition of mantle-derived materials or a pre-existing source region that was not influenced by surface processes (Cecil et al 2021). The zircon oxygen isotope values (δ 18 O Zrn ) of episodes 1 and 2 were mainly in the range 6.5–12.0 ‰, averaging 8.3 ‰ and 8.0 ‰, respectively.…”

The tempo and conditions of metamorphism during magmatism, southern Coast Mountains batholith, British Columbia, Canada

“…Increasing pressures during 72‐ to 65 and 80‐Ma metamorphism documented in this study indicate crustal thickening of metasediments correlative with at least the 85‐ to 70‐Ma HFE. However, Cecil et al (2021) demonstrate that zircon ε Hf for plutons indicate dominantly mantle‐like contributions and δ 18 O values show little to no input from surface fluids. Thus, although HFE coincide with crustal thickening and contraction, partially melted lower crustal metasediments contributed very little to plutonism in the southern CMB.…”

“…Thus, although HFE coincide with crustal thickening and contraction, partially melted lower crustal metasediments contributed very little to plutonism in the southern CMB. Instead, the HFE are most likely controlled by variations in the mantle (Cecil et al, 2021).…”

“…Decker et al (2017) proposed a slab tear mechanism as a trigger for the HFE, allowing asthenospheric mantle to upwelling, delaminating and heating the lower crust to granulite facies conditions. Therefore, HFE are not necessarily controlled by interaction/triggering from partially melted host rocks and instead are controlled by processes occurring in the subducting slab, structural changes in the overriding plate, or mantle heterogeneity as proposed by Cecil et al (2018, 2021). The WFO records HFE and associated metamorphism, yet the causes for the HFE are not directly related to metamorphism.…”

“…Juvenile ε Nd values for metasediments and δ 18 O and ε Hf from zircon in the plutons provide additional constraints on the HFE. Although the possibility of partial melts from metasedimentary sources cannot be entirely dismissed, these processes would shift the δ 18 O and this has not been documented (Cecil et al, 2021). Instead, processes in the southern CMB MASH zone may be similar to those in the deeply exhumed WFO, New Zealand, where enhanced mantle melting caused HFE, supporting the hypothesis proposed by Cecil et al (Cecil et al, 2018, Cecil et al, 2021).…”