Migration of the Mendocino Triple Junction and the Origin of Titanium-Rich Mineral Suites at New Idria, California

Abstract

Titanium-rich minerals in peculiar mineral suites are associated with the New Idria serpentinite, California. Previously some of these Ti-rich minerals have been described as metasomatic in origin. This paper presents new data on the compositions and parageneses of these mineral suites, and presents a petrologic model for their formation by essentially isochemical metamorphism from precursor assemblages. All of these mineral suites are manifestations of tectonic uplift, M2 metamorphism, and magmatism associated with the passage of the Mendocino Triple Junction underneath New Idria at about 12 Ma. Ti-bearing garnets associated with chlorite and diopside occur in metamorphosed pyroxenite veins that have been tectonically dismembered during emplacement of the serpentinite. These veins have been metamorphosed under greenschist or blueschist (M1) and lower greenschist (M2) conditions. Accessory minerals in these rocks include, among others, perovskite, titanite, zircon, Ti-REEvesuvianite, zirconolite, pyrophanite, and copper sulfides. Benitoite, the state gemstone of California, is formed by lower greenschist M2 metamorphism of tectonic blocks of Franciscan greenstone entrained within the serpentinite. Benitoite is found in association with neptunite, joaquinite, natrolite, actinolite and clinozoisite. Kaersutite crystals up to 30 cm long, containing up to 4 wt% TiO2 occur with albite, apatite, and other accessory minerals in the New Idria syenite, a small group of stocks associated with the serpentinite. This syenite is shown to have an intrusive origin and an age of 12.4 ± 0.8 Ma.