EXTENDING THE CHALLIS MAGMATIC PROVINCE ACROSS THE 0.706 ISOPLETH

The Eocene Challis magmatic province consists of a sprawling volcanic field and coeval intrusive province in central and northern Idaho. These have traditionally been thought to occur only east of the 0.706 Sr isopleth, in regions underlain by Precambrian continental crust, and both the volcanics and plutons show isotopic evidence of derivation predominantly from ancient lithosphere. Here, we report on newly recognized localities of Eocene mafic rocks in Idaho west of the 0.706 isopleth. These localities extend the Challis magmatic province further west then previously recognized, and these rocks are the most primitive yet observed in the Challis.

The first occurrence is a suite of lavas perched on Cuddy Mountain near Council, ID that were originally thought to be part of the Miocene Weiser Basalt. Ar-Ar dating reveals that these are Challis lavas with a plateau age of 47.41 +/- 0.12 Ma (2SD, 10 of 16 steps). These lavas are basanites with large amphibole (magnesio-hastingsite) phenocrysts showing spectacular disequilibrium reaction rims. Olivine crystals have Fo values up to 82. Average ⁸⁷Sr/⁸⁶Sr_(i) and ε_Nd(i) are 0.70336 and +5.2, respectively. The second occurrence is a lamprophyre dike near Riggins that was recently dated as 46.77 +/- 0.26 Ma (Buddington et al., 2021). The lamprophyre has an ⁸⁷Sr/⁸⁶Sr_(i) and ε_Nd(i) of 0.70358 and +4.5, respectively. Trace element systematics of both units are similar and lack the depletion of high field strength elements that is a typical signpost of subduction driven magmatism.

Both units were erupted through Phanerozoic accreted lithosphere that originated in an island arc environment; melts of this young lithospheric mantle would be expected to have the juvenile isotopic signatures but they most likely would display a subduction-related trace element fingerprint. The lack of this fingerprint suggests that these mafic rocks are instead small degree partial melts of upwelling asthenosphere perhaps related to slab break-up or roll-back.