U-Pb zircon age constraints on late Neoproterozoic glaciation in Tasmania
U-Pb zircon age constraints on late Neoproterozoic glaciation in Tasmania
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DOI：
10.1130/g20713.1
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发表时间：
2004-10
期刊：
影响因子：
5.8
通讯作者：
C. Calver;L. P. Black;J. Everard;D. B. Seymour
中科院分区：
文献类型：
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作者：
C. Calver;L. P. Black;J. Everard;D. B. Seymour
两个新的U-Pb年代（锆石上灵敏的高分辨率离子探针）对马里诺（埃拉蒂纳）冰期的年龄有重要影响，这是一个假定的全球年代地层标志，以前在其数值年龄方面受到了很差的限制。在国王岛的Grassy群中，中阶岩的年代为575±3 Ma，侵入了与elatina相当的辉晶岩（Cottons角砾岩）、碳酸盐岩盖层和冰后页岩。岩壁局部呈水疱状，地层限制，可能侵入于浅深度，可能紧跟在马里诺冰期结束之后。在塔斯马尼亚西北部的Togari群，一条流纹石流的年代为582±4 Ma，位于Croles Hill二晶岩之下，至少部分是冰川形成的。Croles Hill二晶岩中未发现盖层碳酸盐岩，但在其他方面，其地层背景与cotton角砾岩相似。这两个日期加在一起表明，马里诺冰期的年龄比以前的估计要年轻得多（约580 Ma），并表明与纽芬兰的Gaskiers组有关。新的数据不能排除马里诺冰期约620 Ma的可能性，正如最近来自澳大利亚以外的证据所表明的那样，但这将需要对塔斯马尼亚地层关系进行更复杂和更不可能的解释。
Two new U-Pb dates (sensitive high-resolution ion microprobe on zircon) have an important bearing on the age of the Marinoan (Elatina) glaciation, a presumed global chronostratigraphic marker that has been previously poorly constrained in terms of its numerical age. In the Grassy Group of King Island, intermediate sills dated as 575 ± 3 Ma intrude an Elatina-equivalent diamictite (the Cottons Breccia), cap carbonate, and postglacial shale. The sills are locally vesicular, stratigraphically limited, probably intruded at shallow depth, and probably closely postdate the end of Marinoan glaciation. In the Togari Group of northwest Tasmania, a rhyodacite flow dated as 582 ± 4 Ma underlies the Croles Hill Diamictite, which is at least partly glaciogenic. No cap carbonate is known from the Croles Hill Diamictite, but in other respects its stratigraphic setting is similar to the Cottons Breccia. The two dates together support a significantly younger age (ca. 580 Ma) for the Marinoan glaciation than some previous estimates, and suggest correlation with the Gaskiers Formation of Newfoundland. The new data cannot exclude the possibility of a ca. 620 Ma age for the Marinoan glaciation, as suggested by recent evidence from outside Australia, but this would require a more complex and much less probable interpretation of the Tasmanian stratigraphic relationships.