Origin and HFSE enrichment potential of a Late Cretaceous basalt to peralkaline rhyolite suite in central Queensland, Australia

Peralkaline magmatic systems can host significant enrichment in high-field-strength elements (HFSE) (<i>e.g.</i> HREE, Zr, Nb, Ta), potentially yielding large-tonnage resources. Primary enrichment is interpreted to result from low-degree melting of a metasomatised mantle source followed by extensive fractional crystallisation. Secondary processes may modify resource potential, but primary magmatic enrichment remains critical, highlighting the importance of investigating these systems from source and tectonic setting through to emplacement. To understand controls on HFSE enrichment, we combined field mapping, petrography, geochemistry, isotopic analysis and geochronology to investigate an alkali basalt to peralkaline rhyolite suite in central Queensland consisting of the Alton Downs Basalt and Mount Hedlow Trachyte. The suite comprises a widespread magmatic field of basaltic lava flows, shallow mafic intrusions and felsic cryptodomes emplaced contemporaneously in the Late Cretaceous at <i>ca</i> 77 Ma. Geochemical trends and restricted isotopic compositions suggest derivation from a common mafic parent magma with minimal crustal assimilation. Fractionation progressed through plagioclase- to alkali feldspar-dominated assemblages, producing felsic endmembers evolving towards relative HFSE enrichment and agpaitic indicators (<i>e.g.</i> aenigmatite), or less HFSE enrichment and miaskitic assemblages (<i>e.g.</i> zircon, monazite). Sm–Nd isotopic compositions (_ƐNd_(77Ma) = 6.07 to 7.34) indicate a depleted mantle source distinct from younger, plume-related elements of the Eastern Australia Volcanic Province. Trace-element enrichment in the most primitive basalts relative to MORB is consistent with a continental rift setting, which may ultimately relate to northward propagation of Gondwana breakup and development of failed rifts along the Queensland margin. Felsic rocks did not reach the extreme degrees of fractionation interpreted for more strongly HFSE-enriched suites in central Queensland (<i>e.g.</i> southern Peak Range Volcanics), and this may be the main limiting factor resulting in modest HFSE content. More fractionated bodies may exist at depth and would be more prospective. The Alton Downs Basalt and Mount Hedlow Trachyte were emplaced contemporaneously as an alkali basalt to peralkaline rhyolite suite at <i>ca</i> 77 Ma. These units form a widespread magmatic field of basaltic flows, shallow mafic intrusions and felsic cryptodomes, rather than a central volcano. Magma differentiation followed two evolutionary paths, influencing accessory phase mineralogy and HFSE content that was ultimately limited by the degree of fractionation achieved. Emplacement occurred in a failed continental rift during northward propagation of Gondwana breakup.

过碱性岩浆系统（Peralkaline magmatic systems）可显著富集高场强元素（high-field-strength elements, HFSE，如重稀土元素HREE、锆Zr、铌Nb、钽Ta），具备形成大型吨位矿床的潜力。原生富集被认为源于交代富集地幔源区的低度熔融，随后经历广泛的分离结晶作用。后期作用可能改变资源潜力，但原生岩浆富集仍是关键，这凸显了从源区、构造背景直至就位机制对该类系统开展研究的重要性。

为厘清高场强元素富集的控制因素，研究团队结合野外填图、岩石学、地球化学、同位素分析及地质年代学方法，对昆士兰州中部一套从碱性玄武岩到过碱性流纹岩的岩石组合开展研究，该组合包含奥尔顿当斯玄武岩（Alton Downs Basalt）和赫德洛山粗面岩（Mount Hedlow Trachyte）。该岩套分布广泛，由同时代形成于晚白垩世（约77 Ma）的玄武岩熔岩流、浅部镁铁质侵入体以及长英质隐爆穹窿组成。

地球化学趋势与局限的同位素组成表明，该岩套源自一套同源的镁铁质母岩浆，且地壳混染程度极低。分离结晶作用以斜长石为主导，逐步过渡到碱性长石主导的矿物组合，由此形成的长英质端元分为两类：一类相对富集高场强元素，出现霓霞石（aenigmatite）等过碱性指示矿物；另一类高场强元素富集程度较低，发育锆石（zircon）、独居石（monazite）等霞石型副矿物组合。

钐-钕同位素组成（εNd(77Ma)=6.07~7.34）表明其源区为亏损地幔源区，与澳大利亚东部火山省较年轻的柱相关源区截然不同。相较于洋中脊玄武岩（mid-ocean ridge basalt, MORB），最原始的玄武岩具有微量元素富集特征，这与大陆裂谷构造背景相符，而该背景可能最终与冈瓦纳大陆裂解的北向扩展以及昆士兰陆缘夭折裂谷的发育有关。

长英质岩石未达到昆士兰州中部高场强元素更强富集的岩套（如皮克岭南部火山岩（southern Peak Range Volcanics））所经历的极端分离结晶程度，这可能是导致其高场强元素含量适中的主要限制因素。深部可能存在分离结晶程度更高的岩体，其成矿潜力也更强。

奥尔顿当斯玄武岩与赫德洛山粗面岩以约77 Ma的时代同时就位，构成一套从碱性玄武岩到过碱性流纹岩的岩石组合。这些岩体构成了分布广泛的岩浆场，包含玄武岩流、浅部镁铁质侵入体及长英质隐爆穹窿，而非单一的中心式火山。岩浆分异作用遵循两条演化路径，分别影响副矿物组成与高场强元素含量，而分异程度最终限制了元素富集水平。该岩套就位时处于冈瓦纳大陆裂解北向扩展过程中的夭折大陆裂谷环境。