LATE MIOCENE XENOCRYST-BEARING LAVAS ON GREEN RIDGE, CENTRAL OREGON CASCADE RANGE: IMPLICATIONS FOR THE FORMATION OF PROTEROZOIC MASSIF ANORTHOSITE

The genesis of Proterozoic (~2.7 -0.5 Ga) massif anorthosite is a fundamental problem in understanding Earth’s evolution. Here we present evidence for the formation of late Miocene massif anorthosite beneath the Cascade arc. Three lavas erupted prior to the foundering of the Cascade graben contain sparse resorbed xenocrysts of plagioclase (An43-62; ≤5 cm), Al-rich opx (En63-76; 4-9 wt% Al₂O₃; ≤1.5 cm), MgO-rich ilmenite (Xhem7-11; MgO 2.0-4.4; ≤0.7 cm), apatite, and cumulate fragments of euhedral laminated plagioclase (An63-68). Rare, very fine grained and Al-rich gabbroic inclusions (≤1.5 cm) may surround resorbed xenocrysts. Plagioclase are oscillatory zoned, some with resorbed cores. Granular cumulate plagioclase has annealed shears. Resorbed cores of higher Al opx are mantled by lower Al opx, in turn resorbed by reaction with the host. A cumulate clot of Al-rich opx surrounds grains of An57 plagioclase. Ilmenite contains rhyolitic melt inclusions with daughter amphibole. Cumulate laminated plagioclase fragments are inclusion-rich and rapidly grown; unlike other xenocrysts they lack reaction rims and are similar to groundmass plagioclase in the host lava.

The xenocryst assemblage resembles the mineralogy of Proterozoic massif anorthosite. The host lavas are Al-rich, analogous to hyperfeldspathic dikes and sills common to massif anorthosite. The Deschutes Fm. (DF) of which they are a part comprises Fe-rich lavas and ash flow tuffs with A-type rhyolite chemistry, comparable to the Fe-rich mafic rocks and A-type granites associated with massif anorthosite. Host lavas are the most incompatible element depleted basaltic andesites in the DF, consistent with resorption of a significant volume of plagioclase. Experiments on similar Cascade North Sister basaltic andesite show plagioclase as the only liquidus phase for 75°C; experiments with hyperfeldspathic Proterozoic dikes show up to 200°C drops with plagioclase only on the liquidus. Mafic magmas appear to become hyperfeldspathic by resorption of granulite plagioclase near the base of the crust; on ascent to the mid-crust they rapidly crystallize laminated plagioclase in huge quantities. The essential ingredient for the formation of massif anorthosite is the presence of granulite in the lower crust; they are not restricted in time to the Proterozoic.