Slab-Triggered Arc Flare-up in the Cretaceous Median Batholith and the Growth of Lower Arc Crust, Fiordland, New Zealand
Slab-Triggered Arc Flare-up in the Cretaceous Median Batholith and the Growth of Lower Arc Crust, Fiordland, New Zealand
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DOI：
10.1093/petrology/egx049
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发表时间：
2017-06-01
影响因子：
3.9
通讯作者：
Kylander-Clark, A. R. C.
中科院分区：
文献类型：
--
作者：
Decker, M.;Schwartz, J. J.;Kylander-Clark, A. R. C.
新西兰菲奥德兰的中生代大陆弧记录了大约110万年与俯冲有关的幕式岩浆活动的历史，最终导致镁铁质到中等、高锶/Y、钙碱性到碱性钙质岩浆的末期涌流。在这一短暂的10-15Myr事件中，超过90%的白垩纪深成弧根被侵位；然而，这些岩石的来源以及它们在多大程度上代表了下地壳镁铁质和/或变质沉积再循环与新的下弧地壳的加入，仍然是不确定的。本文报道了位于中基岩下弧壳(30-60公里深度)的18个样品的全岩地球化学、锆石痕量元素、氧同位素和Hf同位素分析，目的是(1)评价触发白垩纪弧焰事件的过程；(2)确定白垩纪弧焰导致下弧壳净增加的程度。我们发现西菲奥德兰正长片麻岩的O-18(Zrn)值在5.2-6.3之间，误差加权平均值为5.7460.04&(2SE，95%置信限)。激光烧蚀多采集器电感耦合等离子体质谱结果得到的初始epsilon Hf(Zrn)值范围为-2.0至+11.2，误差加权平均值为+4+/-260.2。我们通过各种质量平衡混合和同化-分离结晶模型探索了O-和Hf-同位素体系的明显解耦，这些模型涉及贫化和浓缩地幔来源与地壳以上贡献的混合。我们发现，最符合我们的同位素数据的是富集型地幔物质和高达15%的俯冲变质物质之间的混合。这些结果与西菲奥德兰正片麻岩岩浆的O-18(Zrn)值的均质性、高的Sr/Y特征和镁铁质特征相结合，表明白垩纪的岩浆爆发是由俯冲洋壳和富集次大陆岩石圈地幔的部分熔融和混合所触发的。我们认为，末期岩浆涌动的驱动机制是弧下不连续板片撕裂的传播，即约136-128 Ma的山脊-海沟碰撞事件。我们从早白垩世西兰迪亚获得的结果与北美和南美洲科迪勒拉和澳大利亚塔斯马尼德山脉大多数浅到中地壳、环太平洋造山带的高通量岩浆事件的强烈地壳特征形成了鲜明对比；相反，我们的结果证明了新的下弧地壳在
The Mesozoic continental arc in Fiordland, New Zealand, records a c. 110 Myr history of episodic, subduction-related magmatism that culminated in a terminal surge of mafic to intermediate, high-Sr/Y, calc-alkalic to alkali-calcic magmas. During this brief, 10-15 Myr event, more than 90% of the Cretaceous plutonic arc root was emplaced; however, the source of these rocks and the degree to which they represent lower crustal mafic and/or metasedimentary recycling versus the addition of new lower arc crust remain uncertain. We report whole-rock geochemistry and zircon trace element, O-isotope and Hf-isotope analyses from 18 samples emplaced into lower arc crust (30-60 km depth) of the Median Batholith with the goals of (1) evaluating the processes that triggered the Cretaceous arc flare-up event and (2) determining the extent to which the Cretaceous arc flare-up resulted in net addition of lower arc crust. We find that delta O-18 (Zrn) values from the Western Fiordland Orthogneiss range from 5.2 to 6.3& and yield an error-weighted average value of 5.7460.04& (2SE, 95% confidence limit). Laser ablation multicollector inductively coupled plasma mass spectrometry results yield initial epsilon Hf (Zrn) values ranging from -2.0 to + 11.2 and an error-weighted average value of + 4 +/- 260.2. We explore the apparent decoupling of O- and Hf-isotope systems through a variety of mass-balance mixing and assimilation-fractional crystallization models involving depleted-and enriched-mantle sources mixed with supra-crustal contributions. We find that the best fit to our isotope data involves mixing between an enriched, mantle-like source and up to 15% subducted, metasedimentary material. These results together with the homogeneity of delta O-18 (Zrn) values, the high-Sr/Y signature, and the mafic character of Western Fiordland Orthogneiss magmas indicate that the Cretaceous flare-up was triggered by partial melting and hybridization of subducted oceanic crust and enriched subcontinental lithospheric mantle. We argue that the driving mechanism for the terminal magmatic surge was the propagation of a discontinuous slab tear beneath the arc, or a ridge-trench collision event, at c. 136-128Ma. Our results from the Early Cretaceous Zealandia arc contrast with the strong crustal signatures that characterize high-flux magmatic events in most shallow to mid-crustal, circum- Pacific orogenic belts in the North and South American Cordillera and the Australia Tasmanides; instead, our results document the rapid addition of new lower arc crust in