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Geochemical Journal
Geochemical Journal An open access journal for geochemistry
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U-Pb dating of granitic cobble (dropstone) recovered from inner slope of the Chile Trench (48°S): Constraint for its provenance

Yuji Orihashi, Ryo Anma, Shiki Machida, Minoru Sasaki, Kaishi Nakao, Yuichi Takaku, Natsue Abe
Geochemical Journal, Vol. 54, No. , P. 195-201, 2020


A granitic cobble was recovered with mud, semi-consolidated and consolidated siltstones to mudstones from the Dredge Site 16 (1,831 to 1,511 m below sea level) in the inner slope of Chile Trench at 47°48ʹ S during MR16-09 Leg 2 by the JAMSTEC R/V Mirai. Considering its occurrence and its sampling location at ca. 50 km far from coastline of the Chilean Patagonia province, the granitic cobble must be a dropstone. Obtained LA-ICPMS 238U-206Pb age for the essential zircon crystals was 79.7 ± 1.0 Ma (2σ) on the basis of unmix age function. According to the previous researches, such granitic plutons having ca. 80 Ma occurs limitedly in Fjord Baker area (48°S) in the northern part and along the Magellan Strait (52°S) in the southern part of South Patagonian Batholith. Our results of U-Pb age as well as the rock description provide constraint for provenance of the granitic dropstone to be transported from the Fjord Baker area into the deep sea by an iceberg when glaciers widely covered high plateaus of the Chilean Patagonia province were melted after the last glacial maximum.


dropstone, granitic cobble, U-Pb age, provenance, South Patagonia Batholith, Chile Trench

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