Sr-Pb isotope compositions of lawsonites in a Pacheco Pass metagraywacke, Franciscan Complex, California

Lawsonite, jadeite, and glaucophane are iconic minerals within a Pacheco Pass metagraywacke of the Franciscan Complex, California. Those minerals and the associated quartz form the distinctive very low-temperature and high-pressure metamorphic lawsonite-jadeite-glaucophane assemblage, which is diagnostic of 'cold' oceanic subduction zones. In this paper, we evaluate the ability of lawsonite geochemistry to trace protoliths with in-situ trace element and Sr-Pb isotope analyses in lawsonite from the Pacheco Pass blueschist-facies metagraywacke, a classical example of trench-fill sediments in subduction zones. Initial Sr isotope ratios are relatively high (⁸⁷Sr/⁸⁶Sr = 0.7071-0.7074), and initial Pb isotope ratios are ²⁰⁶Pb/²⁰⁴Pb = 18.74-19.66, ²⁰⁷Pb/²⁰⁴Pb = 15.58-15.70, and ²⁰⁸Pb/²⁰⁴Pb = 38.41-39.34, which range from a MORB trend to a cluster on the EMII component. These geochemical signatures suggest the protolith of the metagraywacke mainly contained material derived from continental volcaniclastic rocks and quartzofeldspathic sediments. There is also a possibility that the protolith contains plume-related oceanic island basalt that reached or intruded into the fore-arc sedimentary sequence of California. Considering the maximum depositional age of the metagraywacke at ~ 102 Ma, the subduction of the Farallon Plate beneath the continental crust of the North American Plate might have carried alkali basalt with OIB-like isotopic signatures to the Franciscan trench. Our study proves the advantage of in-situ lawsonite Sr-Pb isotope analyses to characterize protoliths of metamorphic rocks. The results would manifest that the Sr-Pb isotopic signature of Ca-Al silicate minerals, such as lawsonite, and possibly epidote and pumpellyite, in various types of metamorphic/metasomatic rocks, would be an effective tool for investigating convergent margins.