FIELD EVIDENCE FOR INCREMENTAL ASSEMBLY OF THE LAMARCK GRANODIORITE THROUGH DIKE INTRUSION, DUSY BASIN, KINGS CANYON NATIONAL PARK, CALIFORNIA

The hypothesis that plutons are assembled incrementally implies that “homogeneous” plutons are actually composites, and should contain internal contacts. Through conventional petrographic mapping and magnetic susceptibility analysis of the Lamarck Granodiorite in Dusy Basin, California, internal contacts are now recognized in what was thought to be a homogenous pluton.

Internal contacts within the Lamarck are viewable at nearly all scales. Across a total map area of 20 square kilometers, five 250 m - 1 km wide, laterally continuous intrusions are recognized on the basis of subtle differences in composition (e.g., color index), enclave abundance and length/width ratio, grain size and morphology, and foliation. Within each of these map units is evidence for even smaller scale intrusions (<1 m to decameter) with both sharp and gradational internal contacts. Although it is often impossible to trace contacts on these small intrusions for more than a few meters, where they are sharp, they appear to represent new batches of magma or remobilization of magma. In addition to traditional mapping, the magnetic susceptibility of the granodiorite was measured at intervals of 100 to 200 m on a transect across the short axis of the pluton, and at most locations where detailed petrographic observations were made. Bulk magnetic susceptibility measurements range from 0.026 to 0.007 SI, and are correlated with estimated color index (higher magnetic susceptibility is measured at locations with high c.i.).

The Lamarck Granodiorite crops out as an elongate NNW-striking pluton and large-scale internal contacts identified in this study strike approximately parallel to the external contacts and have steep dips. These data are consistent with assembly of the Lamarck as a dike fed pluton. We have begun geochronologic analysis to determine if assembly occurred over a short interval or if there are resolvable age differences between individual intrusions. Preservation of internal contacts, however, precludes the possibility that the Lamarck existed as a single body of dominantly liquid magma.