Major and trace element geochemistry and Os isotopic composition of metalliferous umbers from the Late Cretaceous Japanese accretionary complex

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 Os-187/Os-188 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.

含金属的棕土和红色页岩是日本增生杂岩——晚白垩世四万十超群中库拉 - 太平洋洋脊 - 弧前碰撞的独特产物。这些棕土与源于洋中脊玄武岩（MORB）的绿岩密切相关，并被视为与MOR型火山作用有关的热液含金属沉淀物。棕土和红色页岩沉积在海沟区域，这里同时有来自陆地的陆源碎屑和来自洋中脊的热液含金属微粒供应。除了铁和锰占优势外，棕土相对于大陆地壳丰度在磷、钒、钴、镍、锌、钇、钼、稀土元素（REEs）和锇方面表现出显著富集。棕土的X/Fe（X = 锰、磷、钒、钴、镍、锌、钇和稀土元素）比值以及相对于后太古宙澳大利亚平均页岩（PAAS）标准化的稀土元素配分模式与沉积在洋中脊侧翼的现代热液羽流沉降物非常相似。这表明棕土保留了热液含金属沉积物的原始地球化学特征，这些沉积物清除了海水衍生的元素，因此可作为晚白垩世海水的替代指标。从晚马斯特里赫特期棕土重建的海洋锇 - 187/锇 - 188比值范围从0.42到0.56，与从太平洋和大西洋远洋碳酸盐中获得的最新数据非常一致，这些数据记录了在67.0百万年到65.7百万年期间从0.55到0.4的突然下降。