Age and composition of dykes emplaced before and during the opening of the Tasman Sea—source implications

Dykes are common in the wave-cut platforms along the coast from Newcastle to Sydney. According to some authors, they may be related to the opening of the Tasman Sea that commenced <i>ca</i> 84 Ma ago. However, there are few detailed radiogenic dating and geochemical studies to evaluate this. We attempt to resolve this by K–Ar dating of plagioclase in and geochemical studies of, basaltic dykes intruding Permo-Triassic sequences on the wave-cut platforms and Carboniferous and Permo-Triassic sequences inland. The plagioclase separated from the dykes give K–Ar ages ranging from 266 to 53 Ma with the majority older than 84 Ma indicating that most dykes were emplaced before the Tasman Seafloor formation. The dykes are generally mildly alkaline, high-Ti basalts; fewer are tholeiitic and calc-alkaline, low-Ti basalts. Strongly light rare earth element (LREE)-enriched patterns typify the former and flat, LREE-depleted or slightly to moderately enriched LREE patterns, the latter. High-Ti basalts have ocean-island-basalt-like and low-Ti basalts, calc-alkaline or mid-ocean ridge basalt (MORB)-like patterns. Most high-Ti and some low-Ti basalts show plume-like characteristics, others N-type MORB and arc-like characteristics. Dykes intruding the Carboniferous sequences show a distinct contamination signature that could be crustal or due to subduction-related metasomatism of the subcontinental lithospheric mantle. The sources of the basaltic magmas vary substantially and in places changes with time. All alkali basalts are derived from enriched asthenospheric sources at varying depths (90–147 km) and most tholeiitic, low-Ti basalts have been extracted from asthenospheric and depleted asthenospheric–lithospheric sources indicating substantial compositional heterogeneity of the mantle. Further, Nd model ages varying from Neoproterozoic (940–580 Ma) to Paleozoic (460–370 Ma) suggest variation in the age of mantle sources for the basalts.

从纽卡斯尔至悉尼沿岸的海蚀平台上广泛发育岩脉。部分学者提出，这些岩脉可能与约84 Ma前启动的塔斯曼海扩张事件存在关联。然而，目前针对该假说的高精度放射成因测年与地球化学研究较为匮乏，难以对其进行有效验证。本次研究通过对海蚀平台中二叠-三叠纪地层、以及内陆石炭纪与二叠-三叠纪地层中侵入的玄武岩脉及其内部斜长石开展钾氩（K–Ar）测年与地球化学分析，以期解决这一科学争议。研究结果显示，从岩脉中分离得到的斜长石钾氩年龄介于266 Ma至53 Ma之间，其中大部分样品的年龄大于84 Ma，表明绝大多数岩脉形成于塔斯曼海海底扩张事件之前。这些岩脉的岩石类型以弱碱性高钛玄武岩为主，少数为拉斑玄武岩系列与钙碱性低钛玄武岩。其中，高钛玄武岩以轻稀土元素（light rare earth element, LREE）强烈富集的配分模式为典型特征；而低钛玄武岩则多呈现平坦型、轻稀土元素亏损或轻微-中度富集的配分模式。地球化学特征方面，高钛玄武岩具有洋岛玄武岩（ocean-island-basalt, OIB）的地球化学属性，而低钛钙碱性玄武岩则表现出洋中脊玄武岩（mid-ocean ridge basalt, MORB）的特征。多数高钛玄武岩与部分低钛玄武岩展现出类似地幔柱的地球化学信号，其余样品则呈现N型洋中脊玄武岩与弧相关的地球化学特征。侵入石炭纪地层的岩脉表现出显著的地壳混染特征，该特征可能源自地壳物质混染，或是由于大陆下岩石圈地幔受到俯冲相关交代作用改造所致。玄武质岩浆的源区存在显著差异，且部分源区随时间发生演化。所有碱性玄武岩均形成于不同深度（90~147 km）的富集型软流圈源区；多数拉斑系列低钛玄武岩则源自软流圈与亏损型软流圈-岩石圈地幔源区，指示研究区地幔存在显著的成分不均一性。此外，Nd同位素模式年龄介于新元古代（940~580 Ma）至古生代（460~370 Ma）之间，表明玄武岩源区的地幔年龄存在显著差异。