Melt extraction and enrichment processes in the New Caledonia lherzolites: Evidence from geochemical and Sr–Nd isotope data
Melt extraction and enrichment processes in the New Caledonia lherzolites: Evidence from geochemical and Sr–Nd isotope data
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DOI：
10.1016/j.lithos.2016.04.030
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发表时间：
2016-09
期刊：
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（橄榄岩推覆岩）以地幔岩性为主，由尖晶石相和斜长石相中与弧前相关的难熔方辉橄榄岩和少量二辉橄榄岩组成。在本研究中，使用全面的地球化学数据集（主量元素、微量元素和 Sr-Nd 同位素）来约束二辉橄榄岩的地幔演化及其与地幔的关系。来自波亚地体的玄武岩，波亚地体在构造上位于地幔岩石之下。大多数二辉橄榄岩是低应变斑屑构造岩。它们可能记录了软流圈起源，随后在岩石圈条件下重新平衡，如地温测量估计所支持的那样（碎斑岩和新生尖晶石相组合的 T 分别为 1100–940 °C 和 920–890 °C）。橄榄石成分 (Fo = 88.5–90.0 mol%)、尖晶石 Cr#（[摩尔 100 • Cr/(Cr + Al)]= 13–17）以及相对较高含量 (7–8 vol%) 的富含 Al2O3 和 Na2O 的单斜辉石（分别高达 0.5 和 6.5 wt.%）表明地球化学物质中度贫化尖晶石二辉橄榄岩的签名。块体岩石和单斜辉石稀土元素 (REE) 模式显示出典型的深渊型特征，即陡峭的轻稀土元素伴随着几乎平坦的重稀土元素到重稀土元素。尖晶石二辉橄榄岩的单斜辉石 REE 成分可以通过石榴石二辉橄榄岩前体 (~ 4%) 的少量分步熔化来再现，然后在尖晶石橄榄岩场中进行 4%–5% 的熔化。斜长石二辉橄榄岩表现出熔融浸渍显微结构、富含 Cr 和 Ti 的尖晶石以及块状岩石和单斜辉石中不相容的微量元素富集（REE、Ti、Y 和 Zr）。这些元素的浸渍模型表明，斜长石二辉橄榄岩起源于浅海岩石圈中高度贫化（非聚集）MORB 熔体部分的残留尖晶石二辉橄榄岩。所研究的橄榄岩的 Nd 同位素组成与来自最近经历了 MORB 消耗事件的软流圈地幔来源一致。这种演化很可能是在扩张的山脊中完成的。然而，地球化学微量元素模型和 Nd 同位素并不支持波雅地体的二辉橄榄岩和富集 MOR 型玄武岩之间存在成因地幔-地壳联系。
The New Caledonia ophiolite (Peridotite Nappe) is dominated by mantle lithologies, composed of forearc-related refractory harzburgites and minor lherzolites in both the spinel and plagioclase facies.In this study, a comprehensive geochemical data set (major, trace elements and Sr–Nd isotopes) is used to constrain the mantle evolution of the lherzolites and their relationships with the basalts from the Poya Terrane, which tectonically underlies the mantle rocks. The majority of the lherzolites are low-strain porphyroclastic tectonites. They likely record an asthenospheric origin followed by re-equilibration at lithospheric conditions, as supported by geothermometric estimates (T = 1100–940 °C and 920–890 °C for porphyroclastic and neoblastic spinel-facies assemblages, respectively). Olivine composition (Fo = 88.5–90.0 mol%), spinel Cr# ([molar 100 • Cr/(Cr + Al)] = 13–17) and relatively high amounts (7–8 vol%) of Al2O3- and Na2O-rich clinopyroxene (up to 0.5 and 6.5 wt.%, respectively) indicate a moderately depleted geochemical signature for the spinel lherzolites. Bulk rock and clinopyroxene rare earth elements (REE) patterns display a typical abyssal-type signature, i.e. steeply plunging LREE accompanied by nearly flat HREE to MREE. Clinopyroxene REE compositions of the spinel lherzolites may be reproduced by small amounts of fractional melting of a garnet lherzolite precursor (~ 4%), followed by 4%–5% melting in the spinel peridotite field. The plagioclase lherzolites show melt impregnation microstructures, Cr- and Ti-rich spinels and incompatible trace element enrichments (REE, Ti, Y, and Zr) in bulk rocks and clinopyroxenes. Impregnation modelling for these elements suggests that the plagioclase lherzolites originated from residual spinel lherzolites by entrapment of highly depleted (non-aggregated) MORB melt fractions in the shallow oceanic lithosphere. Nd isotope compositions of the investigated peridotites are consistent with derivation from an asthenospheric mantle source that experienced a recent MORB-producing depletion event. This evolution was most likely accomplished in a spreading ridge. However, geochemical trace element modelling and Nd isotopes do not support a genetic mantle–crust link between the lherzolites and enriched-MOR-type basalts from the Poya Terrane.