129-9 A Test of the Eclogite Delamination Model for the Uplift of the Wallowa Mountains: Insights from Hornblende Trace Element Crustal Thickness Estimates

Abstract:

Understanding how continental crust evolves through time requires reliable estimates of crustal thickness in ancient arc systems. Bulk-rock trace element ratios, including Sr/Y and La/Yb, are widely used as indicators of crustal thickness but may be modified by crystal accumulation and magmatic differentiation which obscure primary melt signatures and introduce uncertainty into crustal thickness estimates.

Lower crustal (eclogite) delamination has been proposed as a trigger for uplift of the Wallowa Mountains in northeastern Oregon and the ensuing eruption of Columbia River flood basalts. The Wallowa batholith, which comprises most of the Wallowa Mountains, provides an ideal test of this hypothesis. Previous studies, utilizing whole rock Sr/Y and La/Yb and zircon Eu/Eu*, have suggested substantial crustal thickening beneath the Wallowa batholith during its 18-million-year magmatic history (140-122 Ma); however, the samples used have a significant cumulate component and thus do not represent melt compositions.

Melt compositions in equilibrium with hornblende were determined using hornblende trace element abundances. The chondrite-normalized La/Yb values (ranging from 4.5 and 7.1) were then used to calculate crustal thickness, according to the calibration of Chapman et al., 2015.  A best fit trend line through the data (host tonalite/granodiorite and hornblende cumulates) show that the crust underlying the Wallowa batholith experienced thickening from ~32 to 41 km (~10 to 12.5 kbar) from 140 to 125 Ma (the 122 Ma pluton does not contain hornblende). The upper pressure estimate (12.5 kbar) is near the amphibolite/eclogite facies transition; thus, formation of a garnet amphibolite to eclogite root beneath the Wallowa Mountains during Cretaceous time is permissible. Results of this study show the promise of using mineral-derived melt compositions to estimate crustal thickness.

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