Brown amphibole as tracer of tectono-magmatic evolution of the Atlantis Bank Oceanic Core Complex (IODP Hole U1473A)

Brown amphibole is a minor but common mineral component in lower oceanic crust. It is generally interpreted as products of migrating SiO2 and H2O-rich fluids or melts, which can be either residual melts from advanced magmatic differentiation of Mid-Ocean Ridge Basalt (MORB), or hydrothermal fluids including a seawater component. Within the lower oceanic crust exhumed at the Atlantis Bank Oceanic Core Complex (OCC), along the ultraslow Southwest Indian Ridge, brown amphibole is ubiquitous in all lithologies from olivine- to oxide-gabbros and diorites, including both undeformed and plastically deformed varieties. We here show the results of a systematic petrological study conceived to unravel the nature of the H2O-rich component recorded in brown amphiboles and document: (i) the evolution of migrating melts during the magmatic stage and (ii) different extents of melt-bearing deformation events recorded throughout the entire crustal transect. The low Cl contents and the light over heavy Rare Earth Elements (LREE/HREE) ratios and high Ti contents in brown amphiboles, indicate they crystallized from melts with a magmatic hydrous component. Consistently, their δ18O values are in equilibrium with MORB composition, except for diorite amphiboles that possibly record the local assimilation of altered minerals. In undeformed olivine gabbros, interstitial pargasite crystallized at hypersolidus conditions (~1000°C) from the melt residual after late stages of MORB differentiation. We speculate that before the olivine gabbro crystal mush reached fully solid state, some aliquots of residual melts were extracted and accumulated within discrete intervals. There, ferrobasaltic melts differentiated through the early crystallization of Fe-Ti oxides and clinopyroxene as liquidus phases, ultimately forming the oxide gabbros. This process promoted rapid Si enrichment and depletion in Fe, Ti, V in the residual melt, later extracted to form the crosscutting diorite veins. The mylonitic olivine gabbros record high-temperature plastic deformation (~900°C ± 50°C) under hypersolidus conditions, involving melts residual from previous crystallization of the gabbroic rock. Further solid-state plastic deformation led to substantial grain-size reduction and, consequently, to an increase in porosity. This created pathways for subsequent melt focussing, which likely represent late-stage differentiated melts migrating throughout the lower crustal section. This study shows that brown amphibole in the Atlantis Bank lower oceanic crust is the crystallization product of melts residual from advanced magmatic differentiation, which are also locally involved in the plastic deformation events during crustal accretion

Evolution of recycled crust within the mantle: Constraints from the garnet pyroxenites of the External Ligurian ophiolites (northern Apennines, Italy)

The pyroxenite-peridotite sequence from the External Ligurian ophiolites offers a snapshot on the evolution of recycled mafic crust within the mantle. We present new major and trace element compositions, and Nd-Hf isotopic compositions of garnet clinopyroxenites and garnet websterites from this mantle section. The garnet clinopyroxenites display clinopyroxene and bulk rock REE patterns with distinct positive Eu anomalies, which argue for the involvement of plagioclase-rich precursors in their origin. We propose that the garnet clinopyroxenites formed by crystallization of eclogite-derived melts that experienced negligible interaction with the host peridotites. The garnet websteritesare interpreted to have been produced by reactions between the eclogite-derived melts and peridotites, thereby giving rise to hybrid, second-stage pyroxenites with a crustal geochemical fingerprint. In our petrogenetic scenario, a rifting-related event at ~220 Ma caused melting of eclogites originated from a MORB-type gabbroic sequence. These mafic protholiths underwent a long-lived evolution of recycling into the mantle (1.5-1.0 Ga), with no evidence for substantial fractionation affecting the Lu/Hf and Sm/Nd isotopic systems. We show that heterogeneity of crustal protoliths, age of recycling and interaction with the host peridotites may lead to a significant compositional and isotopic diversity of crust-derived mantle pyroxenites

Origin of the Gabbro–Peridotite Association from the Northern Apennine Ophiolites (Italy)

The Northern Apennine ophiolites are remnants of the Middle Jurassic-Early Cretaceous lithosphere from the Ligurian Tethys. New trace element and Nd-Sr isotope investigations were performer on: (1) the rare gabbros associated with the subcontinental mantle rocks from the External Liguride ophiolites; (2) the gabbro-peridotite association from the poorly known ophiolitic bodies from Cecina valley (Southern Tuscany). Clinopyroxenes from the External Liguride and Cecina valley gabbros have similar trace element compositions, which are consistent with formation from normal mid-ocean ridge basalt (N-MORB) magmas. Sm-Nd mineral isochron ages are 179 ± 9 Ma for an External Liguride gabbro and 170 ± 13 Ma and 173.5 ± 4.8 Ma for two different gabbroic bodies from the Cecina valley ophiolites. These ages are interpreted to date the igneous crystallization of the gabbros and are slightly older than the oldest pelagic sediments of the Ligurian Tethys. Initial εNd (+8.5 to +8.9)and 87Sr/86Sr of clinopyroxene are consistent with the interpretation that the studied gabbros were derived from N-MORB magmas. The least serpentinized mantle rocks from the Cecina valley ophiolites are porphyroclastic spinel lherzolites displaying a residual geochemical signature. They are similar to the least depleted residual peridotites from modern oceans. Nd and Sr isotopic ratios for separated mantle clinopyroxene are respectively higher (e.g. εNd = +11) and lower than those of clinopyroxene from associated gabbros at the time of the gabbro intrusion. The gabbro-peridotite associations from the Northern Apennine ophiolites record the progression of the rifting process that led to opening of the Ligurian Tethys

Petrology, geochemistry and U-Pb zircon geochronology of lower crust pyroxenites from northen Apennine (Italy): insights into the post-collisional Variscan evolution

Spinel pyroxenites occur locally as clasts in polygenic breccias from the Late Cretaceous sedimentary melanges of the Northern Apennine (Italy). They are of cumulus origin and formed in the deep crust by early precipitation of clinopyroxene and minor olivine and late crystallisation of orthopyroxene, spinel, Ti-pargasite and sulphides. Pyroxenites underwent high-temperature (ca. 850 °C) subsolidus re-equilibration and ductile deformation with development of mylonitic bands made of clinopyroxene, orthopyroxene, Ti-pargasite and spinel. U-Pb geochronology on zircons revealed the occurrence of inherited grains of Early Proterozoic to Late Devonian age. The inherited zircons are locally rimmed by recrystallised zircon domains. The oldest rims yield a mean concordia U-Pb age at 306 ± 8 Ma, which is considered to date the emplacement of the pyroxenites, in the framework of the post-Variscan lithospheric extension. The incompatible element compositions of calculated melts in equilibrium with clinopyroxenes from the pyroxenites are characterised by Ba, Nb, LREE and Sr enrichment relative to N-MORB. The depleted Nd isotopic signature of the pyroxenites (initial εNd values of +5.3 to +6.1) may be thus linked to primary magmas produced by low degrees of melting of asthenospheric mantle. In addition, the pyroxenites locally record the infiltration of plagioclase-saturated hydrous melts, most likely evolved through fractional crystallisation and enriched in highly incompatible elements, within the clinopyroxene-dominated crystal mush. A thermal event in Late Permian-Middle Triassic caused the partial resetting of zircon U-Pb system

Slab-melting during Alpine orogeny: evidence from mafic cumulates of the Adamello batholith (Central Alps, Italy)

The Adamello batholith is the major Tertiary calc-alkaline intrusion of the Italian Alps. It consists primarily of tonalites to granodiorites and locally contains bodies of amphibole-rich mafic to ultramafic cumulates (hornblendites to amphibole gabbros). In these cumulates, two different parageneses of igneous origin can be distinguished. The first consists of euhedral brown amphibole with olivine, spinel and clinopyroxene inclusions. The second paragenesis consists of poikilitic plagioclase containing fine-grained euhedral clinopyroxene, minor green amphibole and accessory titanite, calcite, quartz, zircon, apatite and Fe–Ti-oxide phases. Minerals of the second paragenesis have been analysed for trace elements by laser ablation ICP-MS and ion microprobe (SIMS). Trace element compositions of liquids in equilibrium with clinopyroxene and green amphibole have been calculated by applying suitable sets of solid/liquid partition coefficients. Computed liquids show a marked LREE enrichment over HREE, abrupt enrichment in U and Th (up to 300 times N-MORB) and negative Nb-Ta anomalies. These chemical features are consistent with titanite, plagioclase and calcite trace element compositions. Literature data show that the liquid in equilibrium with the first paragenesis was less LREE enriched and had lower concentrations of Be, B, Th and U. The contrasting trace element signature between the two parageneses cannot be related to fractional crystallisation. Oxygen isotope compositions of mineral separates have been determined by laser fluorination. δ18O values of brown and green amphiboles (+5.4 to +5.7‰) and of titanite (+4.0‰) indicate that crustal contamination played a negligible role in the origin of parental melts of both parageneses. The peculiar trace element signature of the liquids in equilibrium with the second paragenesis (e.g., extremely high Th/Nb and La/Yb values) has been attributed to slab-derived melts with a high sediment component. Equilibration of the ascending slab melts with the peridotitic mantle wedge would explain the observed low δ18O of amphibole and their relatively high Mg contents