We present microbeam major- and trace-element data from 14 monzodiorites collected from the Malaspina Pluton (Fiordland, New Zealand) with the goal of evaluating processes involved in the production of andesites in lower arc crust. We focus on relict igneous assemblages consisting of plagioclase and amphibole with lesser amounts of clinopyroxene, orthopyroxene, biotite and quartz. These relict igneous assemblages are heterogeneously preserved in the lower crust within sheeted intrusions that display hypersolidus fabrics defined by alignment of unstrained plagioclase and amphibole. Trace-element data from relict igneous amphiboles in these rocks reveal two distinct groups: one relatively enriched in high field strength element concentrations and one relatively depleted. The enriched amphibole group has Zr values in the range of ∼25–110 ppm, Nb values of ∼5–32 ppm, and Th values up to 2·4 ppm. The depleted group, in contrast, shows Zr values <35 ppm and Nb values <0·25 ppm, and Th is generally below the level of detection. Amphibole crystallization temperatures calculated from major elements range from ∼960 to 830 °C for all samples in the pluton; however, we do not observe significant differences in the range of crystallization temperatures between enriched (∼960–840 °C) and depleted groups (∼940–830 °C). Bulk-rock Sr and Nd isotopes are also remarkably homogeneous and show no apparent difference between enriched (εNdi = 0·1 to –0·1; 87Sr/86Sri = 0·70420–0·70413) and depleted groups (εNdi = 0·3 to –0·4; 87Sr/86Sri = 0·70424–0·70411). Calculated amphibole-equilibrium melt compositions using chemometric equations indicate that melts were highly fractionated (molar Mg# <50), andesitic to dacitic in composition, and were much more evolved than bulk lower continental crust or primitive basalts and andesites predicted to have formed from hydrous melting of mantle-wedge peridotite beneath an arc. We suggest that melts originated from a common, isotopically homogeneous source beneath the Malaspina Pluton, and differences between enriched and depleted trace-element groups reflect varying contributions from subducted sediment-derived melt and sediment-derived fluid, respectively. Our data demonstrate that andesites and dacites were the dominant melts that intruded the lower crust, and their compositions mirror middle and upper bulk-continental crust estimates. Continental crust-like geochemical signatures were acquired in the source region from interaction between hydrous mantle-wedge melts and recycled subducted sediment rather than assimilation and/or remelting of pre-existing lower continental crust.

中文翻译：

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新西兰峡湾科迪勒拉弧下地壳的弧形安山岩和英安岩的生成

我们提供了从 Malaspina Pluton（新西兰峡湾地区）采集的 14 种二长闪长岩的微束主量和微量元素数据，目的是评估下弧地壳中安山岩的生产过程。我们专注于由斜长石和闪石组成的残余火成组合物，以及少量的单斜辉石、斜方辉石、黑云母和石英。这些残留的火成岩组合被异质地保存在下地壳中的片状侵入体内，这些侵入体显示出由未应变的斜长石和闪石排列定义的超固相结构。来自这些岩石中残留的火成岩闪石的微量元素数据揭示了两个不同的组：一组相对富含高场强元素浓度，另一组相对贫乏。富集的闪石基团的 Zr 值在~25-110 ppm 的范围内，Nb 值约为 5–32 ppm，Th 值高达 2·4 ppm。相反，耗尽组显示出Zr值＜35ppm和Nb值＜0·25ppm，并且Th通常低于检测水平。对于岩体中的所有样品，从主要元素计算的闪石结晶温度范围为~960至830°C；然而，我们没有观察到富集（~960-840°C）和贫化基团（~940-830°C）之间的结晶温度范围存在显着差异。块状岩石 Sr 和 Nd 同位素也非常均匀，富集（εNdi = 0·1 到 –0·1；87Sr/86Sri = 0·70420–0·70413）和贫化组（εNdi = 0· 3 至 –0·4；87Sr/86Sri = 0·70424–0·70411)。使用化学计量方程计算的角闪石-平衡熔体成分表明，熔体高度分馏（摩尔 Mg# <50），在成分上是安山岩到英安岩，并且比预计形成于弧下地幔楔形橄榄岩的含水熔融。我们认为熔体起源于 Malaspina Pluton 下方一个常见的同位素均质源，富集和贫化微量元素组之间的差异分别反映了俯冲沉积物衍生熔体和沉积物衍生流体的不同贡献。我们的数据表明，安山岩和英安岩是侵入下地壳的主要熔体，它们的成分反映了中部和上部大陆地壳的估计。