GIANT COUNTRY ROCK BLOCKS WITHIN THE SEARCHLIGHT PLUTON, SOUTHERN NEVADA

Stoping has been interpreted to be an important mechanism facilitating contamination (Barnes et al., 2004) and emplacement (Yoshinobu et al., 2003) of plutons; alternatively, many purported examples may be more appropriately interpreted as wall rock screens, and stoping may be a minor and unimportant process (Glazner et al., 2004). Searchlight pluton, a steeply tilted, 10 km thick Miocene intrusion in the Colorado River Extensional Corridor, exposes a zone with abundant, 3-300 m long blocks of Proterozoic gneiss (Bachl et al., 2001). Blocks are present within a 2 km-thick zone that extends at least 6 km, slightly oblique to the initially subhorizontal boundary between the pluton's middle unit (granite) and lower unit (qtz monzonite); blocks are abundant in the middle unit to the N, absent to the S. Rocks at this level of the pluton are interpreted to be mostly cumulates, based upon texture and geochemistry. The blocks are similar to country rock in the adjacent wall and roof (metasedimentary and granitic gneiss, mylonite) and are commonly polylithologic and well foliated. Their long and intermediate dimensions parallel both their own foliation and that of the granitic host. In contrast, foliation in adjacent N wall gneisses is folded and therefore variable. Evidence of local mush disturbance includes schlieren and possible compaction features such as enhanced feldspar foliation beneath blocks. Abundance of blocks varies from ~0-40% within the block-bearing zone. Field observations to date, though not conclusive, appear to be consistent with repeated episodes of collapse of roof/wall blocks into a crystal-rich mush.

Field evidence demonstrates at least partial, localized disaggregation of blocks. Effective disaggregation of xenoliths has been proposed as an important mechanism of contamination of magmas (Clarke et al., 1998). Preliminary results of a LA-ICPMS analysis of zircon from host granitoids suggest only minor contamination - a sample collected 15 cm from a block contained only a few % Proterozoic zircons, slightly more than from a sample collected >100 m from any exposed block (none detected) or from previous SHRIMP samples from elsewhere in the pluton (~1%). Physical experiments in progress will investigate how the shape of blocks may influence settling orientations and how blocks interact with mushes.