Table 1_Petrogenesis of ultramafic and mafic rock series from the North Pyrenean Castillon massif (Ariège, France): evidence of magma–crust interactions in the Ordovician?.xlsx

The Castillon massif, in the northern Pyrenees, features a complex of decimetric to metric ultramafic and mafic layers emplaced within metasedimentary series (from the bottom to the top: garnet, sillimanite, and kyanite-bearing gneisses and sillimanite + cordierite-bearing gneisses). Ultramafic and mafic layers and metasediments have been deformed and metamorphosed under granulitic facies conditions during the Hercynian orogenesis. The mineralogical, petrologic, and geochemical characteristics of the studied samples allow us to define two distinct series: 1) a pyroxene-bearing magmatic series (UM-M1) consisting of ultramafic (UM: dunites, harzburgites, and orthopyroxenites) and mafic (M1: norites, gabbro–norites, and gabbros) rocks; and 2) a pyroxene-free and hornblende-bearing series (M2; mela-, meso-, and leucogabbros). The leucogabbros exhibit some characteristics of anorthosites, including the high Al2O3 whole-rock content (31 wt%), high An content (An84–96) in plagioclase, weak rare earth element enrichment, and very positive Eu anomalies. We propose that the rocks of the ultramafic and pyroxene-bearing rock-series (UM-M1 series) are all associated with the same magmatic event and that the M2 series rocks are associated with a distinct separate event. Isotopic data suggest that these formations are Ordovician. The M2 rocks have juvenile Nd isotopic signatures (εNd(460) from +4.59 to +8.11), suggesting that they are derived from superheated alumina-rich basaltic or basaltic–andesite melts extracted from a relatively depleted mantle source. In contrast, most of the M1 rocks derived from parental basaltic melts show partially crustal contamination, with only a few clearly juvenile samples (εNd(460) from +0.45 to +6.59). We propose a geodynamic evolution for the Castillon massif involving a two-stage genesis and the emplacement of the two series. First, the emplacement of olivine-saturated basaltic melts in a deep metasedimentary crust resulted in the M1 series. The second step involves the emplacement of alumina-rich basaltic or basaltic–andesite melts to produce the M2 hornblende-bearing series (mela-, meso-, and leucogabbros) devoid of pyroxenes. The leucogabbros show strong similarities with common anorthosites although they have not been previously observed in the Variscan Pyrenees.

北比利牛斯山脉的卡斯蒂永地块（Castillon massif）发育一套出露于变沉积岩系中的分米级至米级超镁铁质（ultramafic）与镁铁质（mafic）岩层复合体，其地层自下而上依次为含石榴子石、红柱石、蓝晶石的片麻岩，以及含红柱石+堇青石的片麻岩。超镁铁质、镁铁质岩层与变沉积岩在海西造山作用（Hercynian orogenesis）期间经历了麻粒岩相条件下的变形与变质作用。本次研究样品的矿物学、岩石学与地球化学特征可将其划分为两套截然不同的岩石系列：① 含辉石岩浆系列（UM-M1），由超镁铁质岩（UM：纯橄榄岩、方辉橄榄岩、斜方辉石岩）与镁铁质岩（M1：苏长岩、辉长苏长岩、辉长岩）组成；② 不含辉石但含角闪石的系列（M2：包括暗色辉长岩、中色辉长岩、浅色辉长岩）。其中浅色辉长岩具备斜长岩（anorthosite）的若干特征：全岩Al₂O₃含量高达31 wt%，斜长石的钙长石组分（An）占比为An84–96，稀土元素弱富集，且具有极强的正铕异常。我们认为，超镁铁质岩与含辉石岩系（UM-M1系列）的所有岩石均形成于同一岩浆事件，而M2系列岩石则源自另一独立的岩浆事件。同位素数据表明，上述岩石形成于奥陶纪（Ordovician）。M2系列岩石具有新生的Nd同位素组成（εNd(460)值介于+4.59至+8.11之间），指示其源自相对亏损地幔源区析出的富铝过热玄武质或玄武安山质熔体。与之相反，由原生玄武质熔体演化而来的多数M1系列岩石均经历了不同程度的地壳混染，仅少数样品具有明确的新生同位素特征（εNd(460)值介于+0.45至+6.59之间）。我们提出卡斯蒂永地块的地球动力学演化模式包含两期岩浆成因与岩体侵位过程，分别对应两套岩石系列的形成：第一期为橄榄石饱和玄武质熔体侵位至深部变沉积地壳中，形成M1系列岩石；第二期则为富铝玄武质或玄武安山质熔体侵位，形成不含辉石的含角闪石M2系列岩石（暗色、中色及浅色辉长岩）。尽管此前在华力西期比利牛斯山脉中尚未发现此类浅色辉长岩，但其与典型斜长岩具有极高的相似性。