U-Pb and Hf isotope analysis of detrital zircons from the Banks Island assemblage (coastal British Columbia) and southern Alexander terrane (southeast Alaska)
U-Pb and Hf isotope analysis of detrital zircons from the Banks Island assemblage (coastal British Columbia) and southern Alexander terrane (southeast Alaska)
复制标题
班克斯岛组合（不列颠哥伦比亚省沿海）和亚历山大南部地体（阿拉斯加东南部）碎屑锆石的 U-Pb 和 Hf 同位素分析
作者：
Clare J. Tochilin;G. Gehrels;J. Nelson;J. Mahoney
班克斯岛组合由区域变质和变形的变质沉积岩组成，它们出现在不列颠哥伦比亚省沿海亚历山大和弗兰格利亚岩之间的狭窄地带。在努力评估潜在的相关性与邻近的火山岩，我们进行了U-Pb和Hf同位素分析的碎屑锆石富石英变质沉积岩的班克斯岛组合和古生代地层的南部亚历山大火山岩。班克斯岛组合的U-Pb数据提供了从奥陶纪到二叠纪的最大沉积年龄;补充Hf数据表明，在大多数地层的堆积过程中，靠近大陆陆块。这些年龄和Hf同位素组成是完全不同的弧型岩石的特征南部亚历山大火山岩的年轻的签名，但他们强烈类似的值以前报道的北方亚历山大火山岩。因此，我们认为班克斯岛组合是北方亚历山大海的一部分，向南偏移了1000 km，现在与亚历山大海的南部相邻。至少有一部分这种运动是适应沿着早白垩世Kitkatla剪切带，现在形成的内侧边缘的班克斯岛组合。变形和变质的班克斯岛组合发生在此之前的左旋运动，但是，作为横切脉产生的U-Pb年龄为约。156 Ma.我们的数据提供了一个环北极起源的亚历山大，许多以前的工作者建议的支持。班克斯岛组合和北方亚历山大山奥陶系和更古老的岩石的U-Pb和Hf数据与Timanides的值相似，而志留纪-下泥盆统岩石与波罗的海和格陵兰北方的加里东期岩石的值相同。班克斯岛组合地层的怀疑晚古生代年龄更年轻，也许记录运动的亚历山大从环北极到古太平洋领域。
The Banks Island assemblage consists of regionally metamorphosed and deformed metasedimentary rocks that occur in a narrow belt between the Alexander and Wrangellia terranes in coastal British Columbia. In an effort to evaluate potential correlations with adjacent terranes, we conducted U-Pb and Hf isotope analyses on detrital zircons from quartz-rich metasedimentary rocks of the Banks Island assemblage and from Paleozoic strata of the southern Alexander terrane. U-Pb data from the Banks Island assemblage provide maximum depositional ages ranging from Ordovician to Permian; complementary Hf data demonstrate proximity to a continental landmass during accumulation of most strata. These ages and Hf isotope compositions are quite different from the more juvenile signatures of arc-type rocks that characterize the southern Alexander terrane, but they strongly resemble values previously reported from the northern Alexander terrane. We accordingly suggest that the Banks Island assemblage formed as part of the northern Alexander terrane and was offset southward by ∼1000 km to now reside adjacent to the southern portion of the terrane. At least some of this motion was accommodated along the Early Cretaceous Kitkatla shear zone, which now forms the inboard margin of the Banks Island assemblage. Deformation and metamorphism of the Banks Island assemblage occurred prior to this sinistral motion, however, as crosscutting dikes yield U-Pb ages as old as ca. 156 Ma. Our data provide support for a circum-Arctic origin of the Alexander terrane, as suggested by many previous workers. U-Pb and Hf data for Ordovician and older rocks of the Banks Island assemblage and the northern Alexander terrane are similar to values from the Timanides, whereas Silurian–Lower Devonian rocks yield values shared with Caledonian rocks in Baltica and northern Greenland. Banks Island assemblage strata of suspected late Paleozoic age are more juvenile, perhaps recording motion of the Alexander terrane from the circum-Arctic into the paleo-Pacific realm.