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首页> 外文期刊>Contributions to Mineralogy and Petrology >The contrasting geochemical message from the New Caledonia gabbronorites: insights on depletion and contamination processes of the sub-arc mantle in a nascent arc setting
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The contrasting geochemical message from the New Caledonia gabbronorites: insights on depletion and contamination processes of the sub-arc mantle in a nascent arc setting

机译：新的Caledonia Gabbronorites中的地球化学信息对比：新弧形弧环境中亚弧地幔耗尽和污染过程的见解

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The New Caledonia ophiolite hosts one of the rare examples of crust-mantle sections built in a nascent arc environment, providing the unique opportunity to investigate the first stages of arc magmatism in a subduction setting. The sequence consists of refractory harzburgites, overlain by ultramafic (dunites and wehrlites) and mafic lithologies (gabbronorites). The gabbronorites occur in the upper part of the sequence as decimetre to metre-size sills. They are mainly formed (similar or equal to 55 to 70 vol%) of Ca-rich plagioclase (An up to 96 mol%) and high Mg# (88-92), Al2O3 -poor (1.5-2.4 wt%) clinopyroxene (8-20 vol%), often rimmed by interstitial or poikilitic orthopyroxene (6-27 vol%). Mg-rich olivine (3-15 vol%, Fo = 87-89 mol%) occurs as anhedral, resorbed crystals. Whole rock (WR) compositions exhibit high Mg# (86-92) and strikingly low trace element contents. They own LREE-depleted patterns, with nearly flat (0.82 = Dy-N/Yb-N = 1.00) and low HREE (Yb-N = 0.2-0.9) and positive Eu anomalies. Clinopyroxene trace element chemistry mirrors the extreme depletion of the WR. By contrast, FME enrichments are observed for WR and clinopyroxene. Geochemical models show that the gabbronorites crystallized from primitive, ultra-depleted melts bearing evidence of fluid contamination processes, but with significantly different geochemical signatures compared to boninitic rocks worldwide, i.e. lower LREE-MREE, and absence of Nb depletion and Zr-Hf enrichments. Nd isotopes (+8.2 = epsilon Nd-i = + 13.1), together with radiogenic Pb isotopic ratios, support an origin from a DMM source variably modified by slab fluids. We propose that the geochemical signature of the New Caledonia gabbronorites reflect emplacement of primitive, non-aggregated, magma batches in the lower fore-arc crust, during the first phases of arc formation.
机译：新的Caledonia Ophiolite寄出了内置弧形环境中建立的外壳地幔部分的罕见例子之一，提供了在俯冲环境中调查弧形岩浆广告的第一阶段的独特机会。该序列由耐火喧嚣，由Ultramafic（Dunites和Wehrlites）和Mafic Lithologies（Gabbronorites）覆盖。 Gabbronorites在序列的上部发生，如仪表尺寸的窗台。它们主要形成（类似或等于55至70体积％）的Ca-富含型普拉基酶（高达96摩尔％）和高Mg＃（88-92），Al2O3 -Poor（1.5-2.4wt％）Closopogroxene（ 8-20体积％），通常由间质性或波西米骨质（6-27体积％）镶嵌。富含Mg的橄榄石（3-15克拉％，FO = 87-89mol％）作为吻合，反射晶体。整体岩石（WR）组合物表现出高Mg＃（86-92）和尖锐的低痕量元素含量。它们拥有充满耗尽的图案，几乎平坦（0.82 = dy-n / Yb-n）和低HREE（YB-N = 0.2-0.9）和正欧盟异常。 Closopoyroxene痕量元素化学反映了WR的极端耗尽。相比之下，WR和Clinococexene观察到FME富集。地球化学模型表明，与原始的超耗尽熔体结晶，含有流体污染过程的证据，但与全世界的Boninitic Rocks相比，具有显着不同的地球化学签名，即降低LREE-MREE，并且没有NB耗尽和ZR-HF富集。 Nd同位素（+8.2 =εnd-i＆ = + 13.1），以及辐射性Pb同位素比率，支持由平板流体可变改性的DMM源的起源。我们建议新喀里多尼亚的地球化学签名在弧形形成的第一阶段期间反映了新的钙尼尼古布兰特的施加原始，非聚集，岩浆批次。

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