THE LEGACY OF JIM MATTINSON FROM HIS EARLY RESEARCH TO CA-TIMS: EXAMPLES FROM U-Pb ZIRCON GEOCHRONOLOGY OF INTRUSIVES OF THE SALINIAN BLOCK (POINT REYES, CALIFORNIA) (Invited Presentation)

The Salinian Block (SB) is exposed in the California Coast Ranges and bound by the San Andreas Fault and the Sur-Nacimiento Fault, respectively. It is comprised of metamorphic rocks as well as intrusives of early Late Cretaceous age. Jim Mattinson’s early research more than 50 years ago was instrumental in understanding the age and history of the SB. Applying the then novel technique of hydrothermal dissolution using ultra-clean reagents (1-2), that considerably reduced laboratory background contamination allowing the analysis of minute samples, yielded U-Pb zircon ages from ca. 89 to 104 Ma, many of which were analytically discordant. We applied the CA-TIMS (Chemical Abrasion Thermal Ionization Mass Spectrometry) technique developed by Jim Mattinson (3), which revolutionized U-Pb geochronology, to single zircons from a series of cross-cutting dykes related to the Salinian igneous activity at Point Reyes (N-California). We obtain emplacement ages from 91.8 Ma to 97.2 Ma, largely confirming Jim Mattinson’s earlier findings. In addition to the Cretaceous ages we find ages as young as ca 33 Ma which we interpret as a maximum age for a magmatic event, which is substantially older than the oldest known volcanic activity in the Coast Ranges reported so far. Because of the efficiency of chemical abrasion we rule out that the latter age is due to Pb loss from Salinian age crystals. If the age closely approximates the age of magmatic activity, it may signal the initial contact between the Pacific-Farallon Ridge and the western North American continental margin. A distance of ~500 km between Point Reyes and the suggested location of the initial encounter from plate reconstruction (4), we estimate an average rate of 1.5 cm/yr of the northwest displacement of the Salinian block along the dextral San Andreas Fault. While this hypothesis is evidently provocative, it offers a tenable explanation for the observed data, but will have to be substantiated by additional constraints.

1. J. M. Mattinson, Carnegie Institution Year Book 70, 266 (1971).
2. T. E. Krogh, Carnegie Institution Year Book 70, 258 (1971).
3. J. M. Mattinson, Chemical Geology 220, 47 (2005).
4. T. Atwater, J. Stock, International Geology Review 40, 375 (1998).