INCORPORATION OF HOST ROCK BLOCKS DURING GROWTH OF THE AZTEC WASH PLUTON, ELDORADO MOUNTAINS, NEVADA

Material transfer processes that accommodate magma ascent, collection and growth of plutons are not fully understood, in part because of inadequate exposure in the vertical dimension. The Miocene Aztec Wash pluton (AWP), which is situated in the Eldorado Mountains within the Colorado River Extensional Corridor (CREC), has been moderately to steeply tilted and therefore provides a spectacular vertical cross-sectional view. Previous work on the 15.8-15.6 Ma AWP has shown that it was constructed by vertical accumulation of mafic sheets intruded into granite magma with widespread hybridization to produce intermediate rocks. Host rocks of the AWP include Early Proterozoic granitic orthogneiss, Late Cretaceous granite, and a slightly older Miocene pluton. Within the exposed AWP, host rock xenoliths make up roughly 10% of the total area of exposure (but with a large range of local variation; 0 to >50%) and they occur at all structural levels.

Mapping at 1:6000 of a block-rich area near the structurally highest levels of the AWP has shown that: (1) size, angularity, and integrity of contacts with surrounding plutonic rocks are highly variable; (2) orientation of sub-solidus fabrics in gneiss blocks within the map area show no preferred orientation in contrast to host rocks along the AWP margins; (3) relationships between the blocks and the adjacent AWP rocks show evidence of mechanical disaggregation and physical mixing to varying degrees; (4) blocks with foliations at high angles to sheet contacts and draping of sheets over large blocks are present, and in general the sheets are less continuous than in areas of the pluton lacking abundant host rock blocks.

These observations, together with the evidence for vertical growth of the pluton and the distribution of host rock xenoliths suggest episodic detachment of surrounding host rocks (possibly during eruption?) to form xenoliths. Gravity induced downward movement of xenoliths disrupted layers, and the xenoliths were progressively incorporated into the aggrading pluton.