2019 Volume 114 Issue 6 Pages 296-301
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.