FIELD RELATIONS OF THE STANDARD PLUTON, SIERRA NEVADA, CALIFORNIA

The Standard Pluton located in Tuolumne County, California, is a mass of primarily dioritic material, which intruded the Foothills metamorphic belt during the Jurassic. The small footprint of the pluton, approximately 30 square kilometers, indicates a limited magma source, which in turn would lead to relatively fast cooling rates. Flow lineation was quite advanced in amphibole and plagioclase which indicates that the final expansion of the pluton took place after crystallization.

Published U/Pb isotopic ages span the range of 173-162 Ma, but the metamorphic aureole of the pluton recrystallized at least one andesitic dike of the Sonora dike swarm, for which K-Ar hornblende dates span the interval 156-152 Ma. Either the age range of the Sonora dike swarm is much broader than currently assumed, or the pluton is slightly younger than the U/Pb dates suggest. The 173 Ma date is position within the pluton however is more associated with the arc of Jawbone intrusive suite.

The western portion of the pluton has a relatively regular circular outline, and flow foliation measurements suggesting that the pluton “flared” or “ballooned” above the current level of exposure. Along its eastern portion, however, there is evidence of forceful intrusion parallel to the foliation of the metamorphic belt. Flow foliation along the southwestern boundaries has a slightly steeper dip of 75 degrees to the East, as opposed to the 58 degrees West observed at the eastern boundary. This asymmetry probably results from a gentle southwest plunge in the axis of the pluton, overprinted by various localized forceful intrusions of vertical bulges or fingers.

The Standard Pluton is compositionally zoned. Field samples have a higher density, color index, and amphibole content along the perimeter of the pluton. In contrast, quartz content increases toward the inner portions of the pluton, particularly in the southern and eastern quadrants. The original magmatic zonation appears to have been modified by metasomatism; density is consistently higher (3.006 and 3.037 Mg/m3) with the proximity to mafic dikes and where the pluton interrupted the Calaveras/Shoo-Fly thrust zone.