The southwesternmost Transverse Ranges region, which includes Santa Cruz Island, the Santa Monica Mountains, and the margins of the Los Angeles Basin, is underlain in part by Jurassic igneous and metamorphic rocks. These are the 162-m.y.-old mafic Willows Plutonic Complex, the >141 -m.y.-old metavolcanics of the Santa Cruz Island Schist, and the late Oxfordian to early Kimmeridgian metasediments of the Santa Monica Formation. The area is also underlain by amphibolites, greenschist-facies metabasites, and “saussurite gabbros” of uncertain affinities and unknown ages.
The major-, minor-, and trace-element geochemistry of the Willows Plutonic Complex and the Santa Cruz Island Schist indicates that they are fragments of one or more island-arc plutonic/volcanic complexes. Rare-earth element modeling suggests that plutonic and meta-volcanic rocks could have been consanguineous. Saussurite gabbros are geochemically similar to altered portions of the Willows Plutonic Complex. Metavolcanic Santa Cruz Island Schist samples and volcanogenic metasandstone from the Santa Monica Formation contain relict igneous amphiboles compositionally indistinguishable from hornblendes in the Willows Plutonic Complex.
The maximum pressures (3 kbar) and temperatures (550 °C) of Santa Monica Formation recrystallization are consistent with P/T estimates which are based on the mineral chemistry of the calc-alkaline amphibolites; both may be products of the thermal regime of a silicic pluton emplaced at shallow depths. The contact metamorphism may be related to regional chlorite- to biotite-zone metamorphism of the Santa Monica Formation; the latter, like the regional albite + chlorite + pistacitic epidote ± actinolitic amphibole-producing metamorphism of the Santa Cruz Island Schist, did not obliterate relict structures and minerals.
The Willows Plutonic Complex, Santa Cruz Island Schist, and Santa Monica Formation are probably fragments of one or more Jurassic, island-arc igneous-sedimentary terranes, parts of which have undergone low-pressure greenschist- to amphibolite-facies metamorphism. The greenschist- and amphibolite-facies basement rocks from the northern and northeastern margins of the Los Angeles Basin have petrologic and geochemical affinities with these island-arc-like rocks. The saussurite gabbros of the California Continental Borderland are indistinguishable from altered portions of the Willows Plutonic Complex; both are probably genetically related.