Juvenile continental crust evolution in a modern oceanic arc setting: Petrogenesis of Cenozoic felsic plutons in Fiji, SW Pacific
Juvenile continental crust evolution in a modern oceanic arc setting: Petrogenesis of Cenozoic felsic plutons in Fiji, SW Pacific
复制标题
现代大洋弧环境中的幼年大陆地壳演化：西南太平洋斐济新生代长英质岩体的岩石成因
DOI：
10.1016/j.gca.2021.11.033
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发表时间：
2022
影响因子：
5
通讯作者：
M?nker Carsten
中科院分区：
文献类型：
--
作者：
Marien Chris S.;Drewes-Todd Elizabeth K.;Stork Allen;Todd Erin;Gill James B.;Hoffmann J. Elis;Tani Kenichiro;Allen Charlotte M.;M?nker Carsten
斐济的维提岛是在洋内环境中形成幼年大陆地壳最暴露的显生宙类似物之一。英闪长岩和长岩长岩存在于几个相邻的大型（75-150 km2）中新生代岩体中。我们报告来自五个不同岩体的主量和微量元素数据，包括稀土元素 (REE) 和高精度高场强元素 (HFSE) 成分、新的 Hf-Nd-Sr-Pb 同位素数据以及锆石 U/Pb 年龄、O-Hf 同位素和微量元素。始新世 Yavuna 岩体和中新世 Colo 岩体主要由英闪长岩和长岩岩体组成，代表前维蒂亚兹岛弧暴露的中地壳。岩体可分为三组。其中一套是贫轻稀土元素 (LREE)，某些微量元素比率低于平均正常洋中脊玄武岩 (N-MORB)。第二组具有类似于当地岛弧玄武岩的平坦稀土元素图案。这两套矿产于维提岛海岸附近，包括从辉长岩到英闪长岩的广泛成分谱，并且主要可以通过镁铁质前体熔体的分步结晶来生产。第三组的特点是轻稀土富集，具有较高的 LaN/YbN (2.3–4.9)、较高的 Zr/Y (4.3–7.1) 和较低的 Nb/Ta (9.6–12.4)。它们靠近岛屿中心，是双峰长辉长岩-辉长岩侵入体。这些特征与主要由镁铁质地壳部分熔融形成的特征一致。微量元素模型表明，在存在残余角闪石的情况下，可以通过镁铁质地壳熔化 10-20% 来产生第三组微量元素比率，从而保留中等稀土元素 (MREE) 和诊断微量元素比率，包括低 Nb/Ta 和高 Zr/Y。轻稀土元素富集组与典型的“低压”太古代闪长岩-长角闪长岩-花岗闪长岩（TTG）的地球化学相似性意味着共同的岩石成因，以及通过镁铁质地壳部分熔融和残留角闪石形成幼年太古代地壳和现代分异地壳的相似机制。在现代大洋弧中，与基因无关的长英质深成岩和火山岩共存，在这方面，斐济深成岩伴随着弧过程的重大构造破坏。
Viti Levu, Fiji, provides one of the best exposed Phanerozoic analogues for the formation of juvenile continental crust in an intra-oceanic setting. Tonalites and trondhjemites are present in several large (75–150 km2) adjacent, mid-Cenozoic plutons. We report major and trace element data including rare earth element (REE) and high-precision high field strength element (HFSE) compositions, new Hf-Nd-Sr-Pb isotope data, and zircon U/Pb-ages, O-Hf isotopes, and trace elements, from five different plutons. The Eocene Yavuna pluton and the Miocene Colo plutons are mainly composed of tonalites and trondhjemites and represent the exposed middle crust of the former Vitiaz island arc. The plutons can be divided into three suites. One suite is light REE (LREE) depleted with some trace element ratios lower than average normal mid-ocean ridge basalts (N-MORB). A second suite has flat REE patterns similar to local island arc basalts. Both suites occur near the coast of Viti Levu, include a wide compositional spectrum from gabbro to tonalite, and can be produced mostly by fractional crystallization of mafic precursor melts. The third suite is characterized by LREE enrichments with higher LaN/YbN(2.3–4.9), higher Zr/Y (4.3–7.1), and lower Nb/Ta (9.6–12.4). They occur closer to the center of the island and are bimodal trondhjemite-gabbro intrusions. These characteristics are consistent with formation mostly by partial melting of mafic crust. Trace element modeling shows that the trace element ratios of the third suite can be produced by 10–20 % melting of the mafic crust in the presence of residual amphibole, resulting in the retention of the medium REE (MREE) and diagnostic trace element ratios including low Nb/Ta and high Zr/Y. Geochemical similarities of the LREE enriched suite to typical “low”-pressure Archean tonalites-trondhjemites-granodiorites (TTGs) imply a common petrogenetic origin and similar mechanisms for the generation of juvenile Archean and modern differentiated crust by partial melting of mafic crust with residual amphibole. In modern oceanic arcs, genetically unrelated felsic plutonic as well as volcanic rocks co-exist, and in this regard, the Fijian plutons accompany major tectonic disruptions to arc processes.