日本地球惑星科学連合2019年大会

講演情報

[E] ポスター発表

セッション記号 S (固体地球科学) » S-MP 岩石学・鉱物学

[S-MP29] Oceanic and Continental Subduction Processes

2019年5月28日(火) 10:45 〜 12:15 ポスター会場 (幕張メッセ国際展示場 8ホール)

コンビーナ:Hafiz Ur REHMAN(Department of Earth and Environmental Sciences, Graduate School of Science and Engineering, Kagoshima University)、辻森 樹(東北大学)、Chin-Ho Tsai(Department of Natural Resources and Environmental Studies, National Dong Hwa University)、岡本 和明(埼玉大学教育学部地学)

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

*原 智美1辻森 樹1木村 純一2 (1.東北大学、2.海洋研究開発機構)

キーワード:ローソン石、Sr-Pb 同位体、LA-MC-ICPMS、HIMU、変成堆積岩

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.