CAN HORNBLENDE BAROMETRY RESOLVE DIFFERENTIAL UPLIFT ACROSS THE MOUNT WHITNEY REGION, EASTERN SIERRA NEVADA, CALIFORNIA?

Since about 80 Ma differential uplift has tilted the Sierra Nevada westward and exposed rocks from greater depths along the eastern range front than at comparable elevations farther west. The Mount Whitney Intrusive Suite, a composite granitic intrusion that was emplaced between about 90 and 83 Ma, crops out across an area up to 24 km west of the front and so exposes rocks emplaced over a range of depths. A reconnaissance study of emplacement pressures across the suite (EOS, 87:V31B-0582) made using the hornblende barometer of Ague (Geology, 25:563-566) suggested that the eastern part of the Sugarloaf-Lone Pine Creek pluton was emplaced at a depth about 1.8 km greater than its western part. A more extensive study has failed to support this conclusion, however, and casts doubt on the ability of the barometer to resolve differential uplift across the Mount Whitney region.

In the current study analyses of the rims of coexisting hornblende, biotite, and plagioclase crystals from fourteen samples of the Sugarloaf-Lone Pine Creek pluton and six samples of the suite’s younger intervening members were combined with measured densities to estimate emplacement pressures at a uniform modern elevation of about 2300 m. Although the highest pressures are similar to those in the earlier study (≈ 360 MPa) the new data do not show systematic differences between the eastern and western parts of the Sugarloaf-Lone Pine Creek pluton. In addition, calculated pressures decrease inwards from the pluton’s margins towards its center and are generally low in the intervening more felsic plutons. These trends suggest that there may be compositional factors (e.g., changing volatile fugacities or differential subsolidus alteration of analyzed phases) that affect the barometer’s results in granitic intrusions that have protracted histories of differentiation and cooling.

The occurrence of small mafic intrusions that may be analogous to larger mafic bodies found beneath other granitic plutons in the eastern Sierra suggest that the eastern part of the suite may indeed be deeper than its western part, but any such depth difference is not resolved by the data presently available.