The Okataina Volcanic Centre (OVC), located in the Taupo Volcanic Zone, New Zealand, is a dominantly rhyolitic magmatic system in an arc setting, where eruptions are thought to be driven by mafic recharge. Here, Sr–Pb isotopes, and compositional and textural variations in plagioclase phenocrysts from 10 rhyolitic deposits (two caldera, one immediately post-caldera, four intra-caldera, and three extra-caldera) are used to investigate the OVC magmatic system and identify the sources and assimilants within this diverse mush zone. Plagioclase interiors exhibit normal and reverse zoning, and are commonly in disequilibrium with their accompanying glass, melt inclusions, and whole-rock compositions. This indicates that the crystals nucleated in melts that differed from their carrier magma. In contrast, the outermost rims of crystals exhibit normal zoning that is compositionally consistent with growth in cooling and fractionating melts just prior to eruption. At the intra-crystal scale, the total suite of 87Sr/86Sr ratios are highly variable (0·7042–0·7065 ± 0·0004 average 2SE); however, the majority (95 %) of the crystals are internally homogeneous within error. At whole-crystal scale (where better precision is obtained), 87Sr/86Sr ratios are much more homogeneous (0·70512–0·70543 ± 0·00001 average 2SE) and overlap with their host whole-rock Sr isotopic ratios. Whole-crystal Pb isotopic ratios also largely overlap with whole-rock Pb ratios. The plagioclase and whole-rock isotopic compositions indicate significant crustal assimilation (≥20 %) of Torlesse-like metasediments (local basement rock) by a depleted mid-ocean ridge mantle magma source, and Pb isotopes require variable fluid-dominant subduction flux. The new data support previous petrogenetic models for OVC magmas that require crystal growth in compositionally and thermally distinct magmas within a complex of disconnected melt-and-mush reservoirs. These reservoirs were rejuvenated by underplating basaltic magmas that serve as an eruption trigger. However, the outermost rims of the plagioclase imply that interaction between silicic melts and eruption-triggering mafic influx is largely limited to heat and volatile transfer, and results in rapid mobilization and syn-eruption mixing of rhyolitic melts. Finally, relatively uniform isotopic compositions of plagioclase indicate balanced contributions from the crust and mantle over the lifespan of the OVC magmatic system.

中文翻译：

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糊状物、熔体和变沉积物：在斜长石中捕获的新西兰奥卡塔伊纳火山中心流纹岩的历史

位于新西兰陶波火山带的奥卡塔伊纳火山中心 (OVC) 是一个弧形环境中的主要流纹岩岩浆系统，人们认为这里的喷发是由镁铁质补给驱动的。在这里，来自 10 个流纹岩矿床（两个火山口、一个火山口后立即、四个火山口内和三个火山口外）的 Sr-Pb 同位素以及斜长石斑晶的成分和结构变化用于研究 OVC 岩浆系统并确定这个多样化的糊状区域内的来源和同化物。斜长石内部表现出正向和反向分带，并且通常与其伴随的玻璃、熔体包裹体和全岩成分不平衡。这表明晶体在与其载体岩浆不同的熔体中成核。相比之下，晶体的最外缘表现出正常的分区，其成分与喷发前冷却和分馏熔体的生长一致。在晶体内尺度上，87Sr/86Sr 比率的总套件是高度可变的（0·7042–0·7065 ± 0·0004 平均 2SE）；然而，大多数 (95%) 晶体在误差范围内是内部均匀的。在全晶体尺度（获得更好的精度），87Sr/86Sr 比率更加均匀（0·70512–0·70543 ± 0·00001 平均 2SE）并与其宿主全岩 Sr 同位素比率重叠。全晶 Pb 同位素比值也很大程度上与全岩 Pb 比值重叠。斜长石和全岩同位素组成表明，枯竭的洋中脊地幔岩浆源对 Torlesse 状变质沉积物（局部基底岩）进行了显着的地壳同化（≥20%），和 Pb 同位素需要可变的以流体为主的俯冲通量。新数据支持以前的 OVC 岩浆成岩模型，该模型需要在不连贯的熔体和糊状储层的复合体中在成分和热学不同的岩浆中生长晶体。这些储层通过底镀玄武质岩浆恢复了活力，这些岩浆充当了喷发的触发因素。然而，斜长石的最外缘意味着硅质熔体与引发喷发的镁铁质流入之间的相互作用在很大程度上仅限于热量和挥发分的传递，并导致流纹岩熔体的快速流动和同步喷发混合。最后，斜长石的同位素组成相对均匀，表明在 OVC 岩浆系统的整个生命周期中，地壳和地幔的贡献是平衡的。新数据支持以前的 OVC 岩浆成岩模型，该模型需要在不连贯的熔体和糊状储层的复合体中在成分和热学不同的岩浆中生长晶体。这些储层通过底镀玄武质岩浆恢复了活力，这些岩浆充当了喷发的触发因素。然而，斜长石的最外缘意味着硅质熔体与引发喷发的镁铁质流入之间的相互作用在很大程度上仅限于热量和挥发分的传递，并导致流纹岩熔体的快速流动和同步喷发混合。最后，斜长石的同位素组成相对均匀，表明在 OVC 岩浆系统的整个生命周期中，地壳和地幔的贡献是平衡的。新数据支持以前的 OVC 岩浆成岩模型，该模型需要在不连贯的熔体和糊状储层的复合体中在成分和热学不同的岩浆中生长晶体。这些储层通过底镀玄武质岩浆恢复了活力，这些岩浆充当了喷发的触发因素。然而，斜长石的最外缘意味着硅质熔体与引发喷发的镁铁质流入之间的相互作用在很大程度上仅限于热量和挥发分的传递，并导致流纹岩熔体的快速流动和同步喷发混合。最后，斜长石的同位素组成相对均匀，表明在 OVC 岩浆系统的整个生命周期中，地壳和地幔的贡献是平衡的。