GEOCHEMICAL CORRELATION OF THE IVERSEN AND MINDEGO BASALTS OF NORTHERN CALIFORNIA

Whole-rock major, trace, and rare earth element analysis of the Iversen Basalt of Mendocino County and the Mindego Basalt of the Santa Cruz Mountains reveals that the two early Miocene volcanics were extruded from a common source, even though now they are separated by ~ 200 km. Both basalts lie to the west of the San Andreas fault but are on opposite sides of the intersecting San Gregorio-Hosgri fault. Present separation can be accounted for by a combination of movement along these faults.

A structurally and stratigraphically intact section of the Iversen Basalt is well-exposed along the Mendocino County coast. The Mindego, on the other hand, lies in a geologically complex area, and its stratigraphic relationships are questionable. Both basalts consist of thick, dark-gray to reddish-purple flows, flow breccia, and tuff. In places, both exhibit pillow structures and are highly vesiculated. Five samples from each area were analyzed using ICP-MS. Most of the samples are quartz-hypersthene normative, and are classified as subalkaline; two samples are nepheline normative but plot very close to the subalkaline field on a TAS diagram. Strikingly, the Iversen and Mindego display matching bimodal trace and REE chemistries. Samples from the bottom of the Iversen are enriched in LREEs, and all REEs plot along a fairly steep slope with only minor deviations from a smooth profile. An identical level of enrichment, slope, and matching subtle deviation is seen in the REE plots for Mindego samples collected near the town of La Honda in the Santa Cruz Mountains. In contrast, the REE pattern for the top of the Iversen shows far less LREE enrichment, and displays two unusual negative anomalies, one at Nd, the other at Tb. This pattern is identical to the REE pattern for Mindego samples collected along Russian Ridge.

N-MORB-normalized trace element diagrams also reveal a bimodal chemistry that retains the same Iversen/Mindego relationships seen in the REEs. Spider diagrams cluster around two slopes but exhibit very similar patterns of enrichment. All samples display sharp positive anomalies in the LIL elements Cs, U, K, and Pb, as well as moderate enrichment in Ba and Sr, perhaps reflecting a shared crustal contamination. A well-developed negative Nb anomaly is present in all samples, as is enrichment of Ti relative to Y, and an overall depletion of Y and Yb relative to N-MORB values.