Evolution of mantle melts intruding the lowermost continental crust: constraints from the Monte Capio-Alpe Cevia mafic-ultramafic sequences (Ivrea-Verbano Zone, northern Italy)
Abstract
This study presents a new petrological-geochemical data set for the Monte Capio and Alpe Cevia mafic-ultramafic sequences, which are exposed in the deepest levels of the Ivrea-Verbano Zone. These sequences are composed of a peridotite core, with dunite in the center, mantled by minor orthopyroxene-dominated pyroxenites and subordinate hornblende gabbronorites. Amphibole is ubiquitous in the peridotites and the pyroxenites (≤ 15 vol % and 10-40 vol %, respectively), and the peridotite-pyroxenite associations are frequently crosscut by amphibole-rich (45-90 vol %) veins/dykes showing sinuous-to-sharp planar boundaries towards host rocks. The whole-rock Mg# [100 × Mg/(Mg + Fetot2+)] decreases from the peridotites to the pyroxenites and the crosscutting amphibole-rich dykes (84-81, 80-77, and 73-66, respectively), consistently with the Mg# variations shown by included orthopyroxene, clinopyroxene, and amphibole. Olivine has relatively low forsterite and NiO amounts (84-78 mol % and ≤ 0.14 wt%), and spinel is characterized by low Cr# [100 × Cr/(Cr + Al)] of 7-24. The anorthite content of plagioclase varies from 91 to 88 mol% in plagioclase-bearing pyroxenites to 91-75 mol% in amphibole-rich dykes. The chondrite-normalized REE patterns of amphibole from peridotites and pyroxenites show nearly flat MREE-HREE, no evident Eu anomaly, and LREE that are slightly depleted to slightly enriched with respect to MREE. Amphibole from the amphibole-rich veins/dykes exhibits slight LREE depletion. Whole-rock and amphibole separates show substantial variations in initial Nd-Sr isotopic compositions (e.g., whole-rock ɛNd calculated at 290 Ma ranges from - 0.3 to - 4.7), irrespective of the rock-type and of incompatible element amphibole compositions. We propose that the Monte Capio-Alpe Cevia dunites formed by cooling of magma lenses that intruded the lowermost continental crust of the Ivrea-Verbano Zone. The chemically evolved signature of the dunites documents earlier crystallization of chemically primitive dunites at lower levels, or olivine fractionation within the dunites during melt ascent. Associated pyroxene-bearing peridotites show a magmatic evolution ruled by reaction of a melt-poor crystal mush with migrating melts relatively rich in SiO2 and H2O, which developed orthopyroxene and amphibole at the expenses of olivine ± clinopyroxene. These migrating melts may be reconciled with those feeding the crosscutting amphibole-rich veins/dykes, whose compositions suggest formation by chemically evolved H2O-rich basalts with an arc-type incompatible trace-element fingerprint. Unraveling the origin of the Monte Capio-Alpe Cevia pyroxenites is hampered by the complex open-system magmatic evolution, which also included assimilation of material released by basement metasediments and/or involvement of primary melt batches with different compositions.
- Publication:
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Contributions to Mineralogy and Petrology
- Pub Date:
- January 2020
- DOI:
- 10.1007/s00410-019-1637-8
- Bibcode:
- 2020CoMP..175....2B
- Keywords:
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- Melt-peridotite reaction;
- Amphibole;
- Dunite;
- Pyroxenite;
- Incompatible trace elements;
- Nd-Sr isotopes