Dating Intrusions in the Salinian Block Using Single Zircon U-PB CA-TIMS Analysis

Abstract:

The Salinian block (SB) consists largely of metasedimentary basement intruded by suites of compositionally heterogeneous Early to Late Cretaceous rocks. Exposed throughout Central and Northern California, it is bordered to the northwest by the Naciemiento fault, to the southwest by the Big Pine fault and to the east by the San Andreas Fault. These faults are thought to have caused a dextral offset of the SB; however, its original position is still debated. Published radioisotopic ages from various techniques on SB intrusives suggest that the initial magmatic activity of the Salinian arc began at around 100 Ma and concluded some 40 myr later, but the timing of individual magmatic pulses are yet to be fully understood because arguably most of the available geochronological data are compromised with respect to precision and accuracy. Previous studies include U-Pb zircon ages from different lithologies and areas using multi-crystal TIMS as well as ICPMS, and SIMS techniques. Whereas the former technique is now known to be prone to producing inaccurate, although precise, ages due to averaging effects from older inheritance and open system behavior, the latter are often associated with analytical uncertainty at the several percent level as well as unrecognized open system behavior, making the interpretation of age data notoriously difficult. Complementary geochemical, petrological and structural studies have been conducted, but are also difficult to interpret in absence of precise and accurate geochronological data because geochemical trends and intrusion rates cannot be reconstructed. Here, we examine a group of intrusive rocks from the Northern California SB that include a biotite-hornblende granodiorite host rock which is intruded by a complex array of igneous dikes of varying lithology ranging from ~5% to ~50% mafic components. We perform closed system single zircon U-Pb CA-TIMS analysis and obtain ²⁰⁶Pb/²³⁸U ages of ~98 Ma and ~92 Ma (with permil level resolution) for the host rock and a crosscutting intrusion, respectively. We suggest that two magmatic pulses separated by 6 myr is evidence for independent and distinct magmatic events within the studied area. Work is currently underway to determine the ages of other SB intrusives towards a more complete understanding of the timing of emplacement of the SB.