AGE AND ORIGIN OF LEUCOCRATIC ROCKS OF THE NEOPROTEROZOIC TRINITY OPHIOLITE, KLAMATH MOUNTAINS, CALIFORNIA

The northwest part of the Trinity Terrane has been considered to be a Neoproterozoic ophiolite, based on the presence of gabbro, a dated dike complex, and mafic volcanics (Lindsley-Griffin, 1991). At Lover’s Leap, northeast of Callahan, leucocratic rocks described as plagiogranites occur in two positions: 1) as coarse-grained felsic pods within an amphibolitized gabbro, and 2) as plagioclase-quartz-hornblende-bearing intermediate dikes. We present new U-Pb zircon ages, geochemistry, and petrographic observations of both types of rocks.

The coarse-grained felsic pods, composed of quartz and plagioclase, are developed within a foliated gabbro in which the clinopyroxene has been altered to hornblende. Since both the felsic rock and the gabbro are foliated, the felsic pods appear to have developed early. These pods have gradational contacts with the gabbro and appear to be genetically linked to the gabbros.

In contrast, the intermediate plagioclase-quartz-hornblende-bearing dikes cross-cut the gabbro in the dike complex. Locally, hornblende makes up over 10% of the modal composition, making the term tonalite preferable over plagiogranite. Preliminary major chemistry indicates andesitic to dacitic compositions, low TiO₂ (<0.52%), moderate K₂O (<1.55%), and an enrichment in LILEs relative to HFSEs. A chemically similar rock is associated with the volcanic unit and may be an extrusive equivalent of the phenocryst-rich rock.

We consider two major models that have been proposed for the formation of plagiogranites: 1) fractional crystallization of a mafic melt, and 2) partial melting of an amphibolitized gabbro, which can occur along detachment faults in oceanic core complexes. We consider both possibilities in the context of geochemistry and ages.