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 εHfi values. They are not statistically different from those of typical Lachlan Orogen sandstones and resemble those in Rodinia and early Gondwana orogenic belts. The εHfi values of the Cambrian zircons within the Pioneer Sandstone are similar to the Mount Read Volcanics but cannot be distinguished on εHfi 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套古生代（Paleozoic）沉积地层，以及拉克伦造山带（Lachlan Orogen）的2套地层——其中一套位于塔斯马尼亚东北部，另一套位于维多利亚州沃拉泰湾（Waratah Bay）——其所含的碎屑锆石（detrital zircons）的U-Pb年龄主要集中于2.0~1.4 Ga、1.25~0.95 Ga以及0.62~0.49 Ga三个年龄簇区间。这些锆石的Th/U比值（Th/U ratios）与稀土元素（rare earth element, REE）配分模式均表明，所有岩石序列中的碎屑锆石均为岩浆成因（magmatic origin）。二元判别图（bivariate discrimination diagrams）上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（continental orogenic settings）。最古老的锆石群具有西南劳伦大陆（southwest Laurentian）特征，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εHf同位素值（εHfi values）极为相似，统计分析表明其与典型拉克伦造山带砂岩的εHf同位素值无显著差异，且与罗迪尼亚超大陆（Rodinia）及早期冈瓦纳（Gondwana）造山带的锆石εHf同位素特征高度一致。先锋砂岩（Pioneer Sandstone）内的寒武纪（Cambrian）锆石εHf同位素值与雷德山火山岩（Mount Read Volcanics）相近，但无法通过εHf同位素值与东冈瓦纳（East Gondwana）其他物源区的锆石区分。位于沃拉泰湾迪格岛泥灰岩（Digger Island Marlstone）底部的比尔峡谷燧石层（Bear Gully Chert Bed）的碎屑锆石物源包括塔斯马尼亚西部元古宙基底、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大陆（VanDieland）在晚寒武世与东冈瓦纳陆缘拼合的假说提供了证据。

核心要点：
1. 首次报道了塔斯马尼亚州及维多利亚州南部沃拉泰湾地区早古生代沉积岩的锆石Hf同位素新数据。
2. 塔斯马尼亚西部基底是塔斯马尼亚砂岩的主要物源区，同时也是范迪兰大陆东北缘拉克伦造山带砂岩的次要物源区。
3. 比尔峡谷燧石层的混合物源特征支持范迪兰大陆于晚寒武世增生至东冈瓦纳陆缘的地质认识。
4. 沃拉泰湾比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州及维多利亚州南部沃拉泰湾地区早古生代沉积岩的锆石Hf同位素新数据。塔斯马尼亚西部基底是塔斯马尼亚砂岩的主要物源区，同时也是范迪兰大陆东北缘拉克伦造山带砂岩的次要物源区。比尔峡谷燧石层的混合物源特征支持范迪兰大陆于晚寒武世增生至东冈瓦纳陆缘的地质认识。沃拉泰湾比尔峡谷燧石层中最年轻的锆石可能来源于麦夸里弧。