[SMP29-P04] Evidence of HIMU seamount on the Farallon Plate: Application of in-situ Sr-Pb isotope analysis to Franciscan lawsonite-bearing metagreywacke
Keywords:Lawsonite, Sr-Pb isotope, LA-MC-ICPMS, HIMU, Metasediment
Lawsonites in the Franciscan metagreywacke record a high initial Pb isotope ratios (206Pb/204Pb = 18.74–19.66, 207Pb/204Pb = 15.58–15.70 and 208Pb/204Pb = 38.41–39.34). This Pb-isotope signature suggests that a protolith of the metagreywacke have contained a debris derived from a HIMU-type basalt. Considering the maximum depositional age of the metagreywacke at ~102 Ma, the underflow of the Farallon Plate beneath continental crust of the North American Plate would have carried a HIMU seamount to a Franciscan trench.
Our study has demonstrated the potential application of 'lawsonitology' as sedimentary provenance analyses for metamorphosed accretionary complex. Moreover, the results would manifest that Pb isotopic signature of ancient oceanic basalt can be detectable from Ca-Al silicate mineral, such as lawsonite, epidote and pumpellyite, in metamorphic rocks.