New chronometers for the metamorphism of ophiolitic rocks: 40Ar/39Ar neptunite and 232Th/208Pb joaquinite

Abstract:

Both primary magmatism and metamorphism of ophiolite sequences are difficult to date, due in large part to the fact that the majority of lithologies present tend to contain very low concentrations of radioactive elements. As a result, researchers are often forced to process large amounts of material to search for accessory phases like zircons in gabbro, or to employ geochronometers that often yield multiply interpretable results (e.g., ⁴⁰Ar/³⁹Ar glaucophane or phengite), or to rely on indirect evidence for inferring ages. Here, we introduce two new options for chronometery of metamorphosed ophiolites: ⁴⁰Ar/³⁹Ar neptunite and ²³²Th/²⁰⁸Pb joaquinite. The best known locality for these rare minerals is the New Idria serpentinite diapir, found within the southern Diablo Range of the Coast Range Province of California. Here, both the joaquinite and neptunite chronometers record indistinguishable dates that we interpret to be the crystallization age of the phases during diapir ascent, based on the demonstrated low temperature history of the diapir as whole and the agreement of dates from chromonometers that almost certainly have different closure temperatures. This age is generally inferred to be coincident with the timing of the passage of the Mendocino Triple Junction and associated initiation of the San Andreas fault in this area. We propose that the mean⁴⁰Ar/³⁹Ar neptunite plateau date of 12.375 ± 0.082 Ma and corroborating ²³²Th/²⁰⁸Pb joaquinite date (12.08 ± 0.59 Ma) may represent a high-precision constraint on the timing of this event. We also report a second application of these chronometers to samples from the Yarlung suture, which formed at the time of initial India-Eurasia collision in southern Tibet. Here, both chronometers record indistinguishable dates of ca. 52 Ma, which we also interpret as the crystallization age. This age is consistent with most previously published estimates for the timing of the India-Eurasia collision.