Provenance of lower Paleozoic sedimentary rocks in Tasmania and Waratah Bay, southern Victoria: constraints from detrital zircon hafnium isotopes and trace-element geochemistry

Nine Paleozoic sedimentary formations from western Tasmania and two from the Lachlan Orogen—one in northeast Tasmania and one in Waratah Bay in Victoria—contain detrital zircons that fall into major U–Pb age clusters at 2.0–1.4, 1.25–0.95 and 0.62–0.49 Ga. The zircon Th/U ratios and rare earth element (REE) patterns suggest a magmatic origin for detrital zircons in all these rock sequences. The REE geochemical signatures on bivariate discrimination diagrams indicate that most of the zircons originated in continental orogenic settings. The oldest group of zircons have a southwest Laurentian signature previously recognised from Proterozoic metasedimentary rocks in Tasmania. The 1.25–0.5 Ga zircons from all the samples have very similar εHf_i values. They are not statistically different from those of typical Lachlan Orogen sandstones and resemble those in Rodinia and early Gondwana orogenic belts. The εHf_i values of the Cambrian zircons within the Pioneer Sandstone are similar to the Mount Read Volcanics but cannot be distinguished on εHf_i values from other sources in East Gondwana. The detrital zircon provenance of the Bear Gully Chert Bed at the base of the Digger Island Marlstone at Waratah Bay includes the west Tasmanian Proterozoic, the East Gondwana margin and the Macquarie Arc. This mixed provenance provides evidence for the late Cambrian docking of VanDieland with East Gondwana. KEY POINTSNew zircon Hf isotope data are reported for early Paleozoic sedimentary rocks in Tasmania and Waratah Bay, southern Victoria.The western Tasmania basement was the dominant source for Tasmanian sandstones and a minor source for the Lachlan Orogen sandstones on the northeast margin of VanDieland.The mixed provenance of the Bear Gully Chert supports a late Cambrian accretion of VanDieland to the East Gondwana margin.The youngest zircons in the Bear Gully Chert at Waratah Bay were potentially sourced from the Macquarie Arc. New zircon Hf isotope data are reported for early Paleozoic sedimentary rocks in Tasmania and Waratah Bay, southern Victoria. The western Tasmania basement was the dominant source for Tasmanian sandstones and a minor source for the Lachlan Orogen sandstones on the northeast margin of VanDieland. The mixed provenance of the Bear Gully Chert supports a late Cambrian accretion of VanDieland to the East Gondwana margin. The youngest zircons in the Bear Gully Chert at Waratah Bay were potentially sourced from the Macquarie Arc.

塔斯马尼亚西部的9个古生代沉积地层，以及拉克兰造山带（Lachlan Orogen）的2个地层（分别位于塔斯马尼亚东北部与维多利亚州沃塔赫湾）均产出碎屑锆石（detrital zircons），其U-Pb年龄主要集中于2.0~1.4 Ga、1.25~0.95 Ga及0.62~0.49 Ga三个区间。锆石的Th/U比值与稀土元素（REE）配分模式表明，所有上述岩石序列中的碎屑锆石均为岩浆成因。二元判别图解中的稀土元素地球化学特征显示，绝大多数锆石形成于大陆造山环境。最古老的锆石群具有西南劳伦（Laurentia）属性，该特征此前已在塔斯马尼亚的元古宙变沉积岩（Proterozoic metasedimentary rocks）中被识别。所有样品中1.25~0.5 Ga的锆石初始εHf值（εHf_i）极为相似，其统计学特征与典型拉克兰造山带砂岩的锆石初始εHf值无显著差异，且与罗迪尼亚（Rodinia）超大陆及冈瓦纳（Gondwana）早期造山带的锆石初始εHf值特征相近。先锋砂岩（Pioneer Sandstone）内寒武纪（Cambrian）锆石的初始εHf值与雷德山火山岩（Mount Read Volcanics）相似，但无法通过初始εHf值与东冈瓦纳（East Gondwana）的其他物源锆石区分。沃塔赫湾迪杰岛泥灰岩（Digger Island Marlstone）底部的贝尔峡谷燧石层（Bear Gully Chert Bed），其碎屑锆石物源包括塔斯马尼亚西部元古宙物质、东冈瓦纳陆缘以及麦夸里弧（Macquarie Arc）。这种混合物源特征为范迪兰地（VanDieland）在晚寒武世与东冈瓦纳的拼合事件提供了佐证。

KEY POINTS
首次报道了塔斯马尼亚与维多利亚州南部沃塔赫湾早古生代沉积岩的锆石Hf同位素新数据。
塔斯马尼亚基底是塔斯马尼亚砂岩的主要物源，同时也是范迪兰地东北缘拉克兰造山带砂岩的次要物源。
贝尔峡谷燧石层的混合物源特征支持范迪兰地在晚寒武世拼合至东冈瓦纳陆缘的增生事件。
沃塔赫湾贝尔峡谷燧石层中最年轻的锆石可能来源于麦夸里弧。
首次报道了塔斯马尼亚与维多利亚州南部沃塔赫湾早古生代沉积岩的锆石Hf同位素新数据。
塔斯马尼亚基底是塔斯马尼亚砂岩的主要物源，同时也是范迪兰地东北缘拉克兰造山带砂岩的次要物源。
贝尔峡谷燧石层的混合物源特征支持范迪兰地在晚寒武世拼合至东冈瓦纳陆缘的增生事件。
沃塔赫湾贝尔峡谷燧石层中最年轻的锆石可能来源于麦夸里弧。