Cordilleran Section - 97th Annual Meeting, and Pacific Section, American Association of Petroleum Geologists (April 9-11, 2001)

Paper No. 0
Presentation Time: 9:10 AM


HIRT, William H., College of the Siskiyous, 800 College Ave, Weed, CA 96094-2806,

The 85 Ma Paradise pluton is one of the largest Late Cretaceous intrusions in the Sierra Nevada. Samples collected along two transects reveal that the hornblende-biotite granodiorite that makes up most of the body has a narrow range of whole-rock and mineral compositions. This uniformity has made it difficult to recognize internal contacts and to reconstruct the history of the pluton's development. Two suites of felsic dikes that are associated with the pluton, however, offer insights into its compositional and mechanical evolution that are unavailable from the body itself.

The first suite of dikes are augite-bearing granodiorite porphyries that intrusive relationships indicate are only slightly older than the main phase of the Paradise pluton. These rocks are compositionally similar to the Paradise Granodiorite, but have distinctive textures in which phenocrysts of pl + qz + kfs + bt + hb + accessories are set in fine-grained granitic groundmasses. Hornblende phenocrysts in the porphyries contain augite cores that are absent from similar crystals in the granodiorite. Together, these features suggest that the earliest Paradise magmas to reach the exposed level were crystal-rich (50 volume %) melts that had initially been hotter or had lower water activities than the more voluminous magmas that later rose to form the main body of the pluton.

The second suite of dikes are leucogranites that crop out as subhorizontal masses up to several tens of meters thick in the high, central part of the pluton. Their equigranular textures and the presence of pegmatitic segregations suggest that these dikes were formed by quenching of volatile-rich magmas. A modest reversal of whole-rock compositional zoning that is not correlated with changes in mineral chemistries occurs in the central part of the pluton where the dikes are most abundant. As in the Whitney pluton to the south, this relationship suggests that the leucogranites originated as interstitial melts that were segregated from the surrounding granodiorite during its late stage crystallization, perhaps in response to delamination of largely solidified portions of the pluton's roof zone.