
Major and trace element geochemistry and Os isotopic composition of metalliferous umbers from the Late Cretaceous Japanese accretionary complex
Author(s) -
Kato Yasuhiro,
Fujinaga Koichiro,
Suzuki Katsuhiko
Publication year - 2005
Publication title -
geochemistry, geophysics, geosystems
Language(s) - English
Resource type - Journals
SCImago Journal Rank - 1.928
H-Index - 136
ISSN - 1525-2027
DOI - 10.1029/2005gc000920
Subject(s) - geology , geochemistry , hydrothermal circulation , cretaceous , detritus , terrigenous sediment , forearc , basalt , seawater , rare earth element , paleontology , mineralogy , subduction , sedimentary rock , oceanography , rare earth , tectonics
Metalliferous umbers and red shales occur as unique products of the Kula‐Pacific ridge‐forearc collision in the Late Cretaceous Shimanto Supergroup, an accretionary complex in Japan. These umbers are closely associated with greenstones of mid‐ocean ridge basalt (MORB) origin and are regarded as hydrothermal metalliferous precipitates related to MOR‐type volcanism. The umbers and red shales were deposited in the trench area where both terrigenous detritus from land and hydrothermal metalliferous particulates from a MOR were supplied simultaneously. Besides a predominance of Fe and Mn, the umbers exhibit remarkable enrichments in P, V, Co, Ni, Zn, Y, Mo, rare earth elements (REEs), and Os relative to continental crustal abundances. The X/Fe (X = Mn, P, V, Co, Ni, Zn, Y, and REEs) ratios and PAAS‐normalized REE patterns of the umbers are very similar to those of modern hydrothermal plume fallout precipitates deposited on flanks of MOR. This indicates that the umbers preserve primary geochemical signatures of hydrothermal metalliferous sediments that scavenged seawater‐derived elements and thus can be used as a proxy for Late Cretaceous seawater. The marine 187 Os/ 188 Os ratios reconstructed from the late Maastrichtian umbers range from 0.42 to 0.56 and are very consistent with recent data obtained from the Pacific and Atlantic pelagic carbonates that record an abrupt decline from 0.55 to 0.4 during the period between 67.0 Ma and 65.7 Ma.