Asthenosphere-induced melting of diverse source regions for East Carpathian post-collisional volcanism

The occurrence of post-subduction magmatism in continental collision zones is a ubiquitous feature of plate tectonics, but its relation with geodynamic processes remains enigmatic. The nature of mantle sources in these settings, and their interaction with subduction-related components, are difficult to constrain using bulk rocks when magmas are subject to mixing and assimilation within the crust. Here we examine post-collisional magma sources in space and time through the chemistry of olivine-hosted melt inclusions and early-formed minerals (spinel, olivine and clinopyroxene) in primitive volcanic rocks from the Neogene–Quaternary East Carpathian volcanic range in Călimani (calc-alkaline; 10.1–6.7 Ma), Southern Harghita (calc-alkaline to shoshonitic; 5.3–0.03 Ma) and the Perșani Mountains (alkali basaltic; 1.2–0.6 Ma). Călimani calc-alkaline parental magma compositions indicate a lithospheric mantle source metasomatised by ~ 2% sediment-derived melts, and are best reproduced by ~ 2–12% melting. Mafic K-alkaline melts in Southern Harghita originate from a melt- and fluid-metasomatised lithospheric mantle source containing amphibole (± phlogopite), by ~ 5% melting. Intraplate Na-alkaline basalts from Racoș (Perșani) reflect small-degree (1–2%) asthenosphere-derived parental melts which experienced minor interaction with metasomatic components in the lithosphere. An important feature of the East Carpathian post-collisional volcanism is that the lithospheric source regions are located in the lower plate (distal Europe-Moesia), rather than the overriding plate (Tisza-Dacia). The volcanism appears to have been caused by (1) asthenospheric uprise following slab sinking and possibly south-eastward propagating delamination and breakoff, which induced melting of the subduction-modified lithospheric mantle (Călimani to Southern Harghita); and (2) decompression melting as a consequence of minor asthenospheric upwelling (Perșani)

Mt. Etna primary melts from 600 ka to the present day characterized by geochemistry of melt inclusions

The geochemical and isotopic variability of tholeiitic/calcalkaline volcanic products in the southern region of Italy suggest the involvement of an HFSE-enriched, OIB-type mantle component. The Sicily province includes recent to active volcanoes in eastern Sicily (Etna, Iblei), Sicily Channel, Ustica and Prometeo, which are host from tholeiitic to Na-alkaline lavas. The origin of Sicily magma's diversity is debated, but the prevailing hypothesis is that it results from melting a heterogeneous mantle influenced by subducting Ionian lithosphere and interaction with an ascending plume. To address the genesis of the Sicilian magmatism as a function of time, we study olivine-hosted melt inclusions (MIs) from Etna. Etna's magmatic evolution consists of six volcanic stages, started 600 ka ago with submarine tholeiitic lavas and continued until present days eruptions of Na-alkaline products. Here we present the geochemistry of MIs from Tholeiitic (542 & 332 ka), Timpe (154 – 126 ka), AAV (102 ka) and Mongibello (1669 AC) stages. Homogenized MIs are hosted by high-Fo olivine for Tholeiitic stage (Fo 90.5-87) and Timpe stage (Fo 90.5–74), and moderate Fo for AAV and Mongibello stages (Fo 81-72). Spinel from the Tholeiitic and Timpe stages show lower Cr# (~0.5) compare to the alkaline ones (~0.8). Studied MIs demonstrate a wide compositional diversity reflecting the variation of parental melt groups for the separate Etna magmatic stages. Tholeiitic melts differ from all other stages (alkaline melts) by low K2O, P2O5, depleted trace elements and high SiO2, with more refractory spinel suggesting a primitive mantle source for this first Etna magmatic stage. Alkaline MIs from 102 ka – 1669 have similar major and trace element compositions to recent alkaline lavas and published MIs. In contrast, the alkaline MIs from the Timpe stage (K2O 1-3 wt.%) differ from alkaline lavas and MIs from all other stages by higher TiO2, Al2O3, CaO, P2O5, SO3 and low SiO2. Our results indicate that the mantle under Etna is very heterogeneous and requires the involvement of at least two different lherzolite mantle sources for magmas of Tholeiitic and Timpe stages, and a contribution of subduction-derived components for magmas for the more recent stages

Constraints on crustal recycling from boron isotopes in Italian melt inclusions

Boron represents an important tracer of crustal recycling processes in subduction zones, because it is readily mobilised from the subducted lithosphere and different components in the slab are isotopically distinct. Profiles of boron content and isotope ratio across magmatic arcs generally show that B concentrations decrease with increasing slab depth, which implies decreasing amount of slab-derived fluids. To date, however, data on continental-collision zones and post-collisional subduction settings are scarce. This study examines Plio-Quaternary Italian magmatism to quantify crustal recycling in a complex subduction setting. Magmatic products vary from (ultra)potassic along the Tyrrhenian side in the north, to calcalkaline and Na-alkaline in the south. Combined major and trace element and [B] content and δ11B values are reported in 99 Melt Inclusions (MIs), analyses from a wide range of Italian lavas. [B] vary from 4 to 298 µg/g and δ11B from -29.2 to -3.9‰. The B isotopic values are considerably lower than previously reported in arcs and other post-collisional setting magmatism. We infer a role for phengite in the source of all studied Italian magmas (with the exception of Mt. Etna lavas). This white mica is stable to high pressures in subducted sediments of altered oceanic crust and records dehydration and 11B depletion due to dehydration processes. MIs hosted in highly fosteritic olivines (Fo &gt;74; median of 89) from across Italy reveal that primary melts tap heterogeneous mantle including subducted oceanic and continental components that were introduced during the Alpine, and Adriatic and Ionian subduction phases. The combined geochemical data record the involvement of sediments that variably metasomatized the mantle wedge. We propose that slab detachment and consequent heat input from the inflow of hot asthenosphere was responsible for phengite breakdown in subducted sediments and locally produced metasomatism of the mantle wedge, imposing a characteristic B isotope signature to the overlying mantle. Continued heating due to asthenosphere inflow led to melting of the metasomatized mantle wedge and generation of the Italian magmatism. Mt. Etna represents an exception being dominated by asthenosphere upwelling through a slab window with minimal influence from active subduction.</p

A clinopyroxene record of primitive melt diversity and mantle heterogeneity beneath Italy

The young potassium-rich volcanic rocks of peninsular Italy are the products of a complex post-collisional geodynamic setting. These volcanic rocks exhibit extreme compositional variability in space and time, resulting from large variations in the subducted material in their mantle sources. The genetic relationships between distinct Italian magmatic series—shoshonitic, potassic, ultrapotassic and lamproitic, among others—that are closely related in space and time, as well as the exact nature and provenance of the metasomatic agents, are subject to active debate. The earliest crystallised silicate phases from mafic lavas—olivine and clinopyroxene—carry valuable information on the nature of mantle sources and melt extraction processes. Because Mg-rich clinopyroxene incorporates significant amounts of incompatible elements and is a ubiquitous phase in mafic Italian lavas, it potentially represents a versatile instrument for delineating the compositional complexity and regional variability of subduction-modified mantle sources in this region. Here we present the results of an extensive study of Mg-rich clinopyroxene (Mg# = 88–93 mol%) from potassium-rich mafic rocks from a chain of volcanic centres in central-southern Italy, from Tuscany down to Campania. We compare major- and trace-element data from clinopyroxenes with those from bulk rocks and olivine-hosted melt inclusions, using new estimates of trace-element partitioning between clinopyroxene and potassium-rich magmas based on cogenetic clinopyroxene-olivine crystallisation. The Mg-rich clinopyroxenes show a marked compositional diversity that reflects the nature of the (near-)primary mantle-derived melts from which they crystallised, and allow us to characterise the metasomatic agents responsible for the formation of different compositional end-members. We demonstrate that clinopyroxenes provide a detailed archive of mantle heterogeneity beneath Italy, highlighting systematic variations both regionally and beneath individual volcanic complexes

The mantle source of lamproites from Torre Alfina, Italy: Evidence from melt inclusions in olivine

The complex post-collisional subduction setting of peninsular Italy, in the central-western Mediterranean region, has given rise to an extremely diverse spectrum of potassium-rich volcanic rocks. The most primitive of these products show trace-element and radiogenic isotope signatures that point to melt derivation from upper mantle domains affected by metasomatism associated with sediment recycling. The style and extent of this metasomatism, and the metasomatic agents responsible for this modification, seem to differ significantly throughout the Italian peninsula. The lamproites of the Tuscan magmatic province, central Italy, are a peculiar and rare example of rocks that require extensive source modification that is not yet well-understood. These rocks are ultrapotassic and mafic in composition and have high compatible trace-element contents. Although bulk-rock compositions have been used to interrogate their petrogenesis, bulk lavas do not reflect the full heterogeneity of their mantle source. Here, we study the geochemistry of melt inclusions in forsterite-rich olivine, which in contrast to their host lavas are snapshots of near-primary melts that have bypassed modification on their way to the surface. The olivines (Fo88-93) from the studied lamproites of Torre Alfina host melt inclusions with major- and trace-element compositions that define two distinct groups. The first is marked by lower SiO2 (47–51 vs. 50–60 wt%) and higher K2O (11–17 vs. 8–14 wt%), CaO (3.5–6 vs. 1.5–5 wt%), TiO2 (1.8–2.4 vs. 0.3–1.8 wt%), P2O5 (1.0–1.7 vs. 0.1–0.9 wt%) and different trace-element contents. Group-1 melts are generally similar to other Tuscan lamproites, whereas group-2 melts are, in terms of trace elements, more akin to the Tuscan high-K calc-alkaline mafic rocks. We interpret these two melt types to originate from a sediment-metasomatised mantle source, which is characterised by distinct (vein) lithologies arising from superimposed metasomatic events. The Sr-Nd-Pb isotope compositions of a subset of the studied inclusions, analysed by wet chemistry and TIMS techniques, will be presented to further constrain the mantle source of these unusual and hitherto unreported primitive melt compositions, and ultimately better understand lamproite petrogenesis

The South Armenian Block: Gondwanan origin and Tethyan evolution in space and time

The geodynamic evolution of the South Armenian Block (SAB) within the Tethyan realm during the Palaeozoic to present-day is poorly constrained. Much of the SAB is covered by Cenozoic sediments so that the relationships between the SAB and the neighbouring terranes of Central Iran, the Pontides and Taurides are unclear. Here we present new geochronological, palaeomagnetic, and geochemical constraints to shed light on the Gondwanan and Cimmerian provenance of the SAB, timing of its rifting, and geodynamic evolution since the Permian. We report new 40Ar/39Ar and zircon U-Pb ages and compositional data on magmatic sills and dykes in the Late Devonian sedimentary cover, as well as metamorphic rocks that constitute part of the SAB basement. Zircon age distributions, ranging from ∼3.6 Ga to 100 Ma, firmly establish a Gondwanan origin for the SAB. Trondhjemite intrusions into the basement at ∼263 Ma are consistent with a SW-dipping active continental margin. Mafic intraplate intrusions at ∼246 Ma (OIB) and ∼234 Ma (P-MORB) in the sedimentary cover likely represent the incipient stages of breakup of the NE Gondwanan margin and opening of the Neotethys. Andesitic dykes at ∼117 Ma testify to the melting of subduction-modified lithosphere. In contrast to current interpretations, we show that the SAB should be considered separate from the Taurides, and that the Armenian ophiolite complexes formed chiefly in the Eurasian forearc. Based on the new constraints, we provide a geodynamic reconstruction of the SAB since the Permian, in which it started rifting from Gondwana alongside the Pontides, likely reached the Iranian margin in Early Jurassic times, and was subject to episodes of intraplate (∼189 Ma) and NE-dipping subduction-related (∼117 Ma) magmatism

Heterogeneous mantle sources of potassium-rich magmas in central-southern Italy: Melt inclusion evidence from Roccamonfina and Ernici (Mid Latina Valley)

We present a comprehensive set of data on compositions of melt inclusions and earliest crystallized mineral phases from mafic lavas of Roccamonfina and Ernici, situated in a central sector of the string of Pliocene-Quaternary potassic volcanic centres along the Tyrrhenian border of peninsular Italy. Studied samples of mafic lavas (4.4-7wt.% MgO) cover a wide spectrum of potassium levels, and represent magmas considered to be parental to the ultrapotassic leucite-bearing high-K series (HKS, 4-8wt.% K2O) and to shoshonitic (1.5-5%) and subalkaline (1.2) and fluid-depleted signatures suggests that subalkaline magmas originate either from the same source following the exhaustion of amphibole, or from a separate wehrlitic-pyroxenitic (±apatite±carbonate?) assemblage. Our melt inclusion data are consistent with a mixed metasomatic imprint by siliceous potassium-rich and carbonate-rich (carbonatitic) potassium-poor melts. Siliceous melt components dominated in the HKS sources, but a minor group of potassium-poor melt inclusions in HKS samples, with compositions not represented by erupted products, carries trace-element signatures pointing to metasomatism by carbonatite-like melts. The melt inclusions show a general inverse relationship between fO2 and potassium enrichment, suggesting that primitive potassium-poor magmas are the most oxidized (NNO+0.5), whereas primitive HKS magmas have the lowest oxidation state (NNO-1.6).Mineral chemistry of (near-)liquidus assemblages varies systematically according to the signature of the primary melts from which they crystallized. Calcium contents of Mg-rich olivines and clinopyroxenes are unrelated to the calcium content of the melts but are higher in HKS than in M-LKS samples. Forsterite-rich olivines contain Cr-spinel inclusions with Cr/(Cr. +. Al) ratios that tend to be higher in M-LKS (0.58-0.80) than in HKS (0.48-0.67) samples. We infer that this parameter, particularly at the more elevated values, is predominantly controlled by the metasomatic veins, and does not necessarily reflect the ('refractory') nature of the pre-metasomatic peridotite component. We surmise that primitive HKS and M-LKS melts of Roccamonfina-Ernici are derived by lower degrees of partial melting of vein lithologies with a stronger metasomatic imprint of the carbonatitic component and more subordinate involvement of pristine wall-rock peridotite than their equivalents of the northern Roman Province. The distinctive HKS and M-LKS sources point to a layered vein-permeated mantle column. Melting possibly occurred in response to a heat pulse induced by exposure to hot asthenospheric mantle, facilitated by post-collisional slab tearing and shallow break-off

Phosphorous incorporation in olivine crystallized from potassium-rich magmas

Olivine phenocrysts in basaltic rocks carry valuable mineral-chemical information on early evolution processes in mafic magmatic systems. Fast intra-crystalline diffusion and re-equilibration weakens this potential for major cation constituents of olivine, but the relative immobility of phosphorous makes this element a promising tracer of early crystallization histories. Although phosphorous zoning patterns and underlying kinetic controls have been studied in recent years, little is known about compositional controls on phosphorous incorporation into igneous olivines. We have analysed olivine phenocrysts, hosting Mg-rich melt inclusions, from a range of mafic potassium-rich lavas from Quaternary volcanic centres in Italy for phosphorous and associated trace elements by laser ablation inductively coupled mass spectrometry (LA-ICP-MS), and intra-crystal zoning by electron probe micro-analysis (EPMA) using Kα X-ray elemental maps and quantitative traverses. The studied olivines are marked by low and variable phosphorous concentrations (generally ≤ 200 ppm, but up to 435 ppm in enriched zones). In most cases, phosphorous zoning is decoupled from zoning in any other element or forsterite content. From a comprehensive database of melt inclusions and host phenocrysts, we infer that the composition of the host melt (silica and phosphorous activities) and olivine crystallization dynamics (interplay between diffusion rate of cation constituents in the melt and crystal growth rate) largely regulate phosphorous incorporation in olivines. Melt composition is likely the most important control under near-equilibrium crystallization conditions, as apparent phosphorous partition coefficients tend to increase with increasing silica activity. A negative relationship between apparent partition coefficients and X PO2.5 indicates that phosphorous partitioning into olivine may deviate from Henry's law behaviour. Melt inclusions are virtually always surrounded by phosphorous-poor zones that are also depleted in Cr and enriched in Al and Ti, suggesting that supply-limited slow growth and coupled-substitution mechanisms largely govern phosphorous uptake here. Our results demonstrate the potential versatility of phosphorous as sensitive indicator of crystal-growth histories and magmatic evolution processes in mafic systems

Phosphorous incorporation in olivine crystallized from potassium-rich magmas

Olivine phenocrysts in basaltic rocks carry valuable mineral-chemical information on early evolution processes in mafic magmatic systems. Fast intra-crystalline diffusion and re-equilibration weakens this potential for major cation constituents of olivine, but the relative immobility of phosphorous makes this element a promising tracer of early crystallization histories. Although phosphorous zoning patterns and underlying kinetic controls have been studied in recent years, little is known about compositional controls on phosphorous incorporation into igneous olivines. We have analysed olivine phenocrysts, hosting Mg-rich melt inclusions, from a range of mafic potassium-rich lavas from Quaternary volcanic centres in Italy for phosphorous and associated trace elements by laser ablation inductively coupled mass spectrometry (LA-ICP-MS), and intra-crystal zoning by electron probe micro-analysis (EPMA) using Kα X-ray elemental maps and quantitative traverses. The studied olivines are marked by low and variable phosphorous concentrations (generally ≤ 200 ppm, but up to 435 ppm in enriched zones). In most cases, phosphorous zoning is decoupled from zoning in any other element or forsterite content. From a comprehensive database of melt inclusions and host phenocrysts, we infer that the composition of the host melt (silica and phosphorous activities) and olivine crystallization dynamics (interplay between diffusion rate of cation constituents in the melt and crystal growth rate) largely regulate phosphorous incorporation in olivines. Melt composition is likely the most important control under near-equilibrium crystallization conditions, as apparent phosphorous partition coefficients tend to increase with increasing silica activity. A negative relationship between apparent partition coefficients and X PO2.5 indicates that phosphorous partitioning into olivine may deviate from Henry's law behaviour. Melt inclusions are virtually always surrounded by phosphorous-poor zones that are also depleted in Cr and enriched in Al and Ti, suggesting that supply-limited slow growth and coupled-substitution mechanisms largely govern phosphorous uptake here. Our results demonstrate the potential versatility of phosphorous as sensitive indicator of crystal-growth histories and magmatic evolution processes in mafic systems

Asthenosphere-induced melting of diverse source regions for East Carpathian post-collisional volcanism

The occurrence of post-subduction magmatism in continental collision zones is a ubiquitous feature of plate tectonics, but its relation with geodynamic processes remains enigmatic. The nature of mantle sources in these settings, and their interaction with subduction-related components, are difficult to constrain using bulk rocks when magmas are subject to mixing and assimilation within the crust. Here we examine post-collisional magma sources in space and time through the chemistry of olivine-hosted melt inclusions and early-formed minerals (spinel, olivine and clinopyroxene) in primitive volcanic rocks from the Neogene–Quaternary East Carpathian volcanic range in Călimani (calc-alkaline; 10.1–6.7 Ma), Southern Harghita (calc-alkaline to shoshonitic; 5.3–0.03 Ma) and the Perșani Mountains (alkali basaltic; 1.2–0.6 Ma). Călimani calc-alkaline parental magma compositions indicate a lithospheric mantle source metasomatised by ~ 2% sediment-derived melts, and are best reproduced by ~ 2–12% melting. Mafic K-alkaline melts in Southern Harghita originate from a melt- and fluid-metasomatised lithospheric mantle source containing amphibole (± phlogopite), by ~ 5% melting. Intraplate Na-alkaline basalts from Racoș (Perșani) reflect small-degree (1–2%) asthenosphere-derived parental melts which experienced minor interaction with metasomatic components in the lithosphere. An important feature of the East Carpathian post-collisional volcanism is that the lithospheric source regions are located in the lower plate (distal Europe-Moesia), rather than the overriding plate (Tisza-Dacia). The volcanism appears to have been caused by (1) asthenospheric uprise following slab sinking and possibly south-eastward propagating delamination and breakoff, which induced melting of the subduction-modified lithospheric mantle (Călimani to Southern Harghita); and (2) decompression melting as a consequence of minor asthenospheric upwelling (Perșani)