Japan Geoscience Union Meeting 2019

Presentation information

[E] Poster

S (Solid Earth Sciences ) » S-MP Mineralogy & Petrology

[S-MP29] Oceanic and Continental Subduction Processes

Tue. May 28, 2019 5:15 PM - 6:30 PM Poster Hall (International Exhibition Hall8, Makuhari Messe)

convener:REHMAN Ur Hafiz(Department of Earth and Environmental Sciences, Graduate School of Science and Engineering, Kagoshima University), Tatsuki Tsujimori(Tohoku University), Chin-Ho Tsai(Department of Natural Resources and Environmental Studies, National Dong Hwa University), Kazuaki Okamoto(Faculty of Education, Saitama University)

[SMP29-P04] Evidence of HIMU seamount on the Farallon Plate: Application of in-situ Sr-Pb isotope analysis to Franciscan lawsonite-bearing metagreywacke

*Tomomi Hara1, Tatsuki Tsujimori1, Jun-Ichi Kimura2 (1.Tohoku University, 2.Japan Agency for Marine-Earth Science and Technology)

Keywords:Lawsonite, Sr-Pb isotope, LA-MC-ICPMS, HIMU, Metasediment

Since bulk isotopic composition of oceanic basalt is susceptible to element remobilization during alteration process, there is a risk of causing misread of geochemical signatures from bulk metabasaltic rocks in subduction complex. As Hara et al. (2018) demonstrated, however, in-situ Sr-Pb isotope analysis of lawsonite in high-pressure metabasaltic rocks has a great potential for tracing the geochemical signatures reflecting the igneous protolith. In this study, we have applied the method to lawsonite in metasedimentary rocks to explore clastic components of a trench-fill sediment. A jadeite-bearing Franciscan metagreywacke from Pacheco Pass area, east-central Diablo Range, California Coast Ranges was selected for geochemical reconnaissance.

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.