PETROLOGY OF THE INDEX BATHOLITH, NORTH CASCADES: COMPARISONS WITH OTHER TERTIARY BATHOLITHS IN WASHINGTON AND IMPLICATIONS FOR THE EVOLUTION OF THE CASCADE ARC

The 35 Ma Index batholith is exposed over an area of about 285 km2 in the Washington Cascades and is among the earliest intrusions in the Cascade plutonic arc. This study combines fieldwork, mineralogy and geochemistry to characterize the Index and compare it with other Cascade batholiths, the goal being to identify spatial and/or temporal patterns in granitoid chemistry over the past 35 Ma. An initial data set of 23 samples collected along an east-west transect across the southern portion of the batholith range from quartz diorite to granite (63-77 wt.% SiO2). Ba contents of these rocks (375-560 ppm) are comparable to those of samples from the Snoqualmie batholith to the south (60-73 wt.% SiO2; 300-640 ppm Ba), but systematically lower than Ba contents of samples from two batholiths that lie farther to the north: the 35 to 2 Ma Chilliwack (56-78 wt.% SiO2; 410-1040 ppm Ba) and the 23 Ma Cloudy Pass (62-71 wt.% SiO2; 640-680 ppm Ba). This “higher Ba to the north” pattern appears to have persisted for at least 35 Ma and is also evident in mafic plutons within the Chilliwack and Snoqualmie batholiths. These data imply that the observed spatial variations in granitoid Ba content reflect long-lived regional chemical variations within the mantle. Similar spatial variations are not evident in abundances of other incompatible trace elements (Rb, Nb, Y). Preliminary comparisons of trace element data from intrusions of different ages reveal no systematic temporal trends in granitoid chemistry at the regional scale. Local variations in slab age, slab geometry, or stress regime may explain why temporal trends observed in the Chilliwack (e.g., increasing Nb with decreasing age) are not apparent in other batholiths. Ongoing work will expand the data set to include additional Cascade arc intrusions including the Grotto, Cascade Pass, and Silver Star and also compare the plutonic record with the volcanic record using data from the Mount Baker and Mount Rainier areas.