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
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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DOI：
10.1007/s00410-018-1496-8
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发表时间：
2018-07
影响因子：
3.5
通讯作者：
A. Secchiari;A. Montanini;D. Bosch;P. Macera;Dominique Cluzel
中科院分区：
文献类型：
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作者：
A. Secchiari;A. Montanini;D. Bosch;P. Macera;Dominique Cluzel
新喀里多尼亚蛇绿岩是在新生的弧形环境中建造的罕见的壳-地幔剖面之一，为研究俯冲背景下的弧岩浆活动的第一阶段提供了独特的机会。层序由难熔的方辉橄榄岩组成，上面覆盖着超镁铁质(英安岩和魏长岩)和镁铁质岩性(辉长岩)。辉长岩赋存于层序的上部，以分米至米大小的岩床形式存在。它们主要由富钙斜长石(高达96%)和高镁#(88-92)、贫Al_2O_3(1.5-2.4%)单斜辉石(8-20Vol%)形成(≃ 55-70Vol%)，常被间隙或斜方辉石(6-27Vol%)包围。富镁橄榄石(3-15体积%，Fo = 87-89摩尔%)以无面体、吸收晶体的形式赋存。全岩(WR)成分显示出高的Mg#(86-92)和极低的微量元素含量。它们具有轻稀土亏损模式，具有近平坦(0.82YbN/YbN≤ 1.00)和低HREE(YbN= 0.2-0.9)和正的Eu异常。单斜辉石的微量元素化学反映了WR的极度枯竭。相反，在WR和单斜辉石中观察到FME的富集物。地球化学模型表明，辉长闪长岩结晶自原始的超亏损熔体，具有流体混染作用的证据，但与世界范围内的邦尼质岩石相比，具有明显不同的地球化学特征，即LREE-MREE较低，没有Nb的亏损和Zr-Hf的富集。Nd同位素(+ 8.2 ≤ εNdi≤ + 13.1)和放射性成因的铅同位素比值支持板条流体可变地改变的DMM源的起源。我们认为，新喀里多尼亚辉长闪长岩的地球化学特征反映了在弧形形成的第一阶段，原始的、非聚集的岩浆批次在下弧前地壳中侵位。
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 (≃ 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 ≤ DyN/YbN≤ 1.00) and low HREE (YbN= 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 ≤ εNdi≤ + 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.