δ18O OF RHYOLITES AT HIGH ROCK CALDERA COMPLEX, NW NEVADA: IMPLICATIONS FOR SILICIC MAGMA GENESIS ASSOCIATED WITH MID-MIOCENE FLOOD BASALTS

About 700 km³ of rhyolitic magma erupted at 16.5-15.5 Ma from High Rock Caldera Complex (HRCC) contemporaneous with main-stage Steens and Columbia River flood basalt volcanism. HRCC and McDermitt Volcanic Field represent the earliest silicic volcanism associated with the eastward-younging calderas of the Snake River Plain-Yellowstone (SRP-Y) trend. New oxygen isotope data on quartz phenocrysts (by laser fluorination, 2 mg samples) indicate that HRCC rhyolites have exclusively “normal” δ¹⁸O_(qtz) values (δ18O_(qtz) = 6.17-8.65‰), in contrast to the commonly low values in the eastern and central SRP [1, 2, 3, 4, 5]. AFC modeling suggests that the most peralkaline rhyolites can be derived by partial melting of a gabbro similar in composition to the upper Steens Basalt to produce trachytic magma, followed by fractional crystallization accompanied by little or no crustal assimilation (r<0.1). Metaluminous to weakly peralkaline rhyolites are best modeled as representing a larger degree of partial melting of gabbro, followed by fractional crystallization accompanied by ~30% assimilation of non-hydrothermally altered upper crust. Modeled r values are small, suggesting that the magma chambers resided in the upper crust.

We suggest that the lack of low-δ¹⁸O values at HRCC may be a result of the small size of the calderas compared to those in the eastern and central SRP, and so did not generate large hydrothermal systems. Moreover, the calderas only partially overlap, limiting opportunities for assimilating hydrothermally altered intracaldera fill. Finally, HRCC erupted through a crustal section straddling the ⁸⁷Sr/⁸⁶Sr_i=0.704 isopleth that lacks evidence of previous hydrothermal alteration.

The transition of SRP-Y magmatism across the Precambrian craton margin is marked by shifts in Nd, Sr, and Hf isotopes [6]. Our new data, combined with published values for the SRP-Y trend [1, 2, 3, 4, 5], demonstrate that this shift is also mirrored by O-isotope variations.

[1] Hildreth et al. (1984) JGR 89. [2] Boroughs et al. (2005) Geology 33. [3] Bonnichsen et al. (2008) Bull. Volcanol. 70. [4] Watts et al. (2011) J. Petrol. 52. [5] Cathey et al. (2007) Eos Trans. AGU 88 (52). [6] Nash et al. (2006) Earth Planet. Sci. Letters 247.