229-14 The Wallowa Batholith: 10+ Million Years of Magmatism Through Progressively Thickening Crust?

Session: Crustal Petrology (Posters)

Presenting Author:

Authors:

Abstract:

The Blue Mountains province in northeastern Oregon and westernmost Idaho comprises two island arcs (Wallowa and Olds Ferry terranes) and the intervening oceanic lithosphere (Baker terrane).  The two arcs collided in the Late Jurassic and were accreted as a superterrane to the margin of Laurentia during the Early Cretaceous.  The composite Wallowa batholith consists of four tonalitic-granodioritic intrusions (from N to S: Pole Bridge, Hurricane Divide, Craig Mountain, and Needle Point plutons) that were emplaced into the Wallowa arc terrane from 140 to ~126 Ma.  Late-stage bodies of cordierite trondhjemite were emplaced at ~123 Ma.  The entirety of the batholith was emplaced during the time span of accretion of the terrane package to Laurentia; it is thus plausible that accretion was accompanied by thickening of the terrane crust.

The method of Chapman et al. (2015) was used to estimate crustal thickness. Usable whole rock data had MgO=1-6 wt.% and SiO₂=55-70 wt.%.  Outliers of Sr/Y values were identified using the Thompson-tau statistical method and discarded.  After outliers were omitted, datasets with Rb/Sr<0.05 or >0.2 and Sr/Y standard deviations >10 were eliminated.  Based on these criteria, only rocks from the Pole Bridge (n=6; 140 Ma), Needle Point (n=6; 131 Ma), and Hurricane Divide (n=9; 130 Ma) were used in the calculations.  The median Sr/Y values for the Pole Bridge pluton yielded a crustal thickness of 34±5 km, whereas those from the Needle Point and Hurricane Divide plutons gave an estimate of 47±5 km.  These results suggest a rate of thickening of ~1.3 mm/y over a 10 million year period.  Higher Sr/Y and chondrite-normalized La/Yb values in the 123 Ma cordierite trondhjemite, in comparison to those of the other intrusions, suggests an even greater crustal thickness by the end of Wallowa magmatism.

Given that magmatic processes such as plagioclase accumulation can affect the Sr/Y value, along with studies that suggest plutonic rocks can have a substantial cumulate component (and therefore do not represent melt compositions), caution should be exercised when interpreting Sr/Y data from intrusive rocks in terms of crustal thickness.  Results of this study provide the impetus for a future study of Wallowa magmatism and crustal thickening.

Geological Society of America Abstracts with Program. Vol. 57, No. 6, 2025

doi: 10.1130/abs/2025AM-11053

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