Evidence for hybridisation in the Tynong Province granitoids, Lachlan Fold Belt, eastern Australia

The role of mafic–felsic magma mixing in the formation of granites is controversial. Field evidence in many granite plutons undoubtedly implies interaction of mafic (basaltic–intermediate) magma with (usually) much more abundant granitic magma, but the extent of such mixing and its effect on overall chemical features of the host intrusion are unclear. Late Devonian I-type granitoids of the Tynong Province in the western Lachlan Fold Belt, southeast Australia, show typical evidence for magma mingling and mixing, such as small dioritic stocks, hybrid zones with local host granite and ubiquitous microgranitoid enclaves. The latter commonly have irregular boundaries and show textural features characteristic of hybridisation, e.g. xenocrysts of granitic quartz and K-feldspars, rapakivi and antirapakivi textures, quartz and feldspar ocelli, and acicular apatite. Linear (well defined to diffuse) compositional trends for granites, hybrid zones and enclaves have been attributed to magma mixing but could also be explained by other mechanisms. Magmatic zircons of the Tynong and Toorongo granodiorites yield U–Pb zircon ages consistent with the known ca 370 Ma age of the province and preserve relatively unevolved ϵHf (averages for three samples are +6.9, +4.3 and +3.9). The range in zircon ϵHf in two of the three analysed samples (8.8 and 10.1 ϵHf units) exceeds that expected from a single homogeneous population (∼4 units) and suggests considerable Hf isotopic heterogeneity in the melt from which the zircon formed, consistent with syn-intrusion magma mixing. Correlated whole-rock Sr–Nd isotope data for the Tynong Province granitoids show a considerable range (0.7049–0.7074, ϵNd +1.2 to –4.7), which may map the hybridisation between a mafic magma and possibly multiple crustal magmas. Major-element variations for host granite, hybrid zones and enclaves in the large Tynong granodiorite show correlations with major-element compositions of the type expected from mixing of contrasting mafic and felsic magmas. However, chemical–isotopic correlations are poorly developed for the province as a whole, especially for 87Sr/86Sr. In a magma mixing model, such complexities could be explained in terms of a dynamic mixing/mingling environment, with multiple mixing events and subsequent interactions between hybrids and superimposed fractional crystallisation. The results indicate that features plausibly attributed to mafic–felsic magma mixing exist at all scales within this granite province and suggest a major role for magma mixing/mingling in the formation of I-type granites.基性-酸性岩浆混合（mafic–felsic magma mixing）在花岗岩形成过程中的作用一直存在争议。诸多花岗岩深成岩体（granite plutons）的野外证据无疑表明，基性（玄武质-中质）岩浆与（通常占比更高的）花岗质岩浆存在相互作用，但此类混合的程度及其对寄主侵入体整体化学特征的影响尚不明确。 澳大利亚东南部拉克伦褶皱带（Lachlan Fold Belt）西部廷农省（Tynong Province）的晚泥盆世I型花岗岩类（I-type granitoids），展现出典型的岩浆混合与混杂证据，诸如小型闪长岩岩株（dioritic stocks）、与寄主花岗岩共生的局部混合带，以及广泛分布的显微花岗质包体（microgranitoid enclaves）。此类包体通常具有不规则边界，并呈现出混合作用特有的结构特征，例如花岗质石英和钾长石捕虏晶（xenocrysts）、环斑结构（rapakivi texture）与反环斑结构（antirapakivi texture）、石英及长石眼斑，以及针状磷灰石（acicular apatite）。 花岗岩、混合带与包体所呈现的线性（从界限清晰至弥散状）成分趋势，常被归因于岩浆混合，但也可通过其他机制加以解释。廷农与图龙花岗闪长岩的岩浆锆石（magmatic zircons）所测得的锆石U-Pb年龄（U–Pb zircon ages），与该区域已知的约370 Ma年龄相符，且保留了相对未分异的εHf值（三个样品的平均值分别为+6.9、+4.3与+3.9）。三个分析样品中的两个，其锆石εHf值跨度（8.8与10.1个εHf单位）超过了单一均匀锆石种群的预期跨度（约4个单位），这表明形成锆石的熔体中存在显著的Hf同位素不均一性，与同侵入期岩浆混合的结论一致。 廷农省花岗岩类的相关全岩Sr-Nd同位素数据展现出较大范围（0.7049–0.7074，εNd值+1.2至–4.7），这或许反映了基性岩浆与可能的多种地壳岩浆之间的混合作用。大型廷农花岗闪长岩中寄主花岗岩、混合带与包体的主量元素变化，与基性与酸性岩浆混合所预期的主量元素组成呈现相关性。然而，就该区域整体而言，化学-同位素相关性并不显著，尤其是对于87Sr/86Sr比值。 在岩浆混合模型中，此类复杂性可通过动态混合/混杂环境加以解释，即存在多次混合事件，且混合产物后续与叠加的分离结晶作用发生相互作用。本研究结果表明，该花岗岩省内各尺度下均存在可归因于基性-酸性岩浆混合的特征，且暗示岩浆混合/混杂在I型花岗岩的形成过程中发挥了重要作用。