A TALE OF TWO (POSSIBLY THREE) PLUTONS REVEALED BY GEOLOGIC, GEOPHYSICAL AND GEOCHRONOLOGIC DATA: THE MERRIMAC PLUTONS, NORTHERN SIERRA NEVADA, CALIFORNIA

We integrate geologic observations with potential-field, physical property, and geochronologic data to map the Cretaceous Merrimac plutons in the northern Sierra Nevada. Originally, geologic mapping in this poorly exposed terrain showed the plutons as a single 100-km² heart-shaped body. Subsequently, mapped concentric foliation patterns and modal compositions determined from polished slabs defined at least two separate zoned plutons; however, the contacts between these plutons are not exposed and can only be inferred from projection of foliation data, resulting in non-unique interpretations. Gravity and aeromagnetic data support the division of the pluton into two separate bodies. A significant gravity gradient trends N60°W, separating a denser pluton or plutons to the NE from a less dense pluton to the SW. This gravity gradient coincides with a magnetic gradient, with more magnetic rocks to the NE. The NE pluton(s) is denser and more magnetic and therefore more likely more mafic than the SW pluton and are consistent with composition data, densities, and magnetic susceptibilities of hand samples. The location of geophysical gradients 1-2 km NE of the inferred pluton contact suggests that the boundary between the plutons dips steeply to the NE, parallel to the dominant metamorphic fabric of the country rocks. The Big Bend fault zone, located 7-8 km to the SW, also shares a steep NE dip and N60°W strike. New ²⁰⁶U/²³⁸Pb zircon dates indicate that these plutons intruded at different times, with the SW pluton yielding a weighted mean crystallization age of 133.1 ± 1.6 Ma and the NE pluton an age of 137.7 ± 1.9 Ma. The age of the SW, more felsic pluton is nearly identical to a recently published U-Pb zircon age of the Bald Rock pluton (133.4 ± 3.3 Ma), a large trondjhemite pluton that lies 1-2 km to the south and whose significant gravity low overlaps the SW Merrimac pluton and the intervening Big Bend fault zone. This suggests that these two plutons are closely related in terms of age and geophysical signature. Furthermore, the average elevation of the more felsic pluton is nearly 400 m lower than that of the more mafic pluton(s). The elevation change is located near the inferred contact and linear geophysical boundaries between the NE and SW Merrimac plutons and, we speculate, may highlight lithologic influence on weathering.