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A 400 Ma-long Nd-Hf isotopic evolution of melt-modified garnet-pyroxenites in an ancient subcontinental lithosphere (Lanzo North ophiolite, Western Alps)
Chemical Geology ( IF 3.9 ) Pub Date : 2021-11-23 , DOI: 10.1016/j.chemgeo.2021.120643
Alessio Sanfilippo 1, 2 , Giulio Borghini 3 , Luisa Guarnieri 4 , Eizo Nakamura 5 , Giovanni B. Piccardo 4 , Riccardo Vannucci 1, 2 , Alberto Zanetti 2
Affiliation  

Pyroxenite veining is widely preserved in peridotite massifs, and used to derive information on the origin and evolution of upper mantle domains. These lithospheric mantle sections can be isolated from the convecting mantle for >1 Ga or more, suffering a long history of melting and/or melt-rock reaction processes, which modify their original chemical and isotopic compositions. Here, we show the effect of ancient process of melt-rock reaction in the chemistry of garnet pyroxenites from Lanzo North Massif, an iconic lithospheric mantle section exhumed during the opening of the Jurassic Alpine Tethys. Selected pyroxenites are more than 10 cm thick, and embedded within peridotites that have textures and chemical compositions indicative of a complex history of interaction with migrating melts. Whole rock and clinopyroxene Nd-Hf isotopes of the pyroxenites consistently indicate that the first melt-rock reaction event occurred at ~400 Ma, likely in combination with exhumation from the garnet to the spinel-facies mantle conditions. Two samples still retain textural relicts and chemical evidence of precursor garnet and have high εNd (~12) for comparatively low εHf (~10), when recalculated at 400 Ma, which suggest that they were less affected by this ancient percolation process. The chemical evidence of such a long history of melt-rock reactions was preserved from 400 Ma until present. Finally, two pyroxenites located within plagioclase peridotites show evidence for an event of re-equilibration at plagioclase-facies conditions, likely triggered by infiltration of melt in the host rock. These samples reveal the coexistence of two internal Sm-Nd isochrones at 152 ± 30 Ma and 149 ± 13 Ma, thereby providing temporal constraints to the event of melt impregnation of the host peridotites as consequence of the opening of the Ligurian Tethys ocean.



中文翻译:

古代次大陆岩石圈(Lanzo North蛇绿岩,西阿尔卑斯山)中熔体改性石榴石辉石岩的400 Ma长Nd-Hf同位素演化

辉石岩脉广泛保存在橄榄岩地块中,用于获取有关上地幔域起源和演化的信息。这些岩石圈地幔截面可以与对流地幔隔离 >1 Ga 或更多,经历了长期的熔融和/或熔岩反应过程,这改变了它们原始的化学和同位素组成。在这里,我们展示了古老的熔岩反应过程对来自 Lanzo North Massif 的石榴石辉石岩化学的影响,Lanzo North Massif 是侏罗纪高山特提斯开放期间挖掘出的标志性岩石圈地幔部分。选定的辉石岩厚度超过 10 厘米,嵌入橄榄岩中,橄榄岩的质地和化学成分表明与迁移熔体的相互作用具有复杂的历史。辉石岩的全岩和单斜辉石 Nd-Hf 同位素一致表明,第一次熔岩反应事件发生在~400 Ma,可能与从石榴石到尖晶石相地幔条件的折返相结合。两个样品仍然保留了前体石榴石的纹理残余和化学证据,并具有高 εNd (~12) 对于相对较低的 ε Hf (~10),当在 400 Ma 重新计算时,这表明它们受这种古老的渗透过程的影响较小。从 400 Ma 到现在,熔岩反应历史悠久的化学证据被保存了下来。最后,位于斜长石橄榄岩内的两个辉石岩显示了在斜长石相条件下发生再平衡事件的证据,这可能是由熔体渗入主岩引发的。这些样品揭示了 152 ± 30 Ma 和 149 ± 13 Ma 两个内部 Sm-Nd 等时线的共存,从而为由于利古里亚特提斯洋开放而导致的寄主橄榄岩熔融浸渍事件提供了时间限制。

更新日期:2021-11-25
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