Petrological Geochemical Data Research Articles

Deep subduction, melting, and fast cooling of metapelites from the Cima Lunga Unit, Central Alps

AbstractThe Cima Lunga unit in the Central Alps is dominated by quartzofeldspathic gneisses with subordinate mafic, ultramafic, and metacarbonate rocks. Only mafic and ultramafic lithologies were thought to preserve clear evidence of Alpine high‐P metamorphism. This led to the questions of whether the different rock types were subducted and exhumed as a coherent unit or underwent different pressure–temperature (P–T) histories. New petrological and geochemical data from a metapelite associated with garnet peridotite from Cima di Gagnone (Cima Lunga unit, Switzerland) were obtained using major and trace element mapping. Complex zoning patterns in garnet and white mica are observed. In particular, high Ti content in phengite and increasing P, Zr, and HREE contents in pyrope‐rich garnet indicate that this metapelite underwent high‐P and high‐T (HP–HT) metamorphism involving fluid‐fluxed partial melting. A P–T path is reconstructed by combining textural analysis with petrological–geochemical data and thermodynamic simulations. We show that the mineral record preserves an evolution from prograde to HP–HT peak conditions (2.7 ± 0.1 GPa and 800℃) followed by near‐isobaric cooling (~2.5 GPa and 700–750℃) prior to decompression (1.0 GPa and ~620℃). The reconstructed P–T path suggests that the studied metapelites were subducted to depths where the slab gets heated by proximity to asthenospheric mantle related to slab break‐off. This heating resulted in the dehydration of chlorite‐ to garnet peridotite and the liberated fluids triggered partial melting in the associated metapelites, which might have favoured the fast exhumation of the entire Cima Lunga unit. Metapelites and garnet peridotite from Cima di Gagnone underwent a common prograde to peak and retrograde P–T path without significant tectonic pressure difference between the different lithologies, and deviation from lithostatic pressure is excluded. Lastly, the peak metamorphic conditions of metapelite from Cima di Gagnone are comparable with P–T estimates of ultramafic lithologies from the southern Adula nappe and the Dascio Bellinzona zone, thus opening new scenarios for the geodynamic interpretation of the Central Alps.

Geochemical Constraints on the Cold and Hot Models of the Moon’s Interior: 2—Three-Layer Mantle

Internally consistent models of the thermal state, chemical composition and mineralogy of the three-layer mantle of the Moon are constructed based on the joint inversion of gravity, seismic and petrological-geochemical data within the Na2O-TiO2-CaO-FeO-MgO-Al2O3-SiO2 system. Geochemical constraints on the chemical composition and physical properties in three zones of the mantle are obtained in terms of the cold and hot models. Velocities of P-waves in the lower mantle (∼8 km/s) are higher than in the upper mantle (∼7.7 km/s). The behavior of velocities of S-waves is conservative, they are observed in the interval 4.40–4.45 km/s in all zones of the mantle. It was found that, independently of the temperature distribution, the most probable concentrations of FeO, ∼11–14 wt % and MgO, 28–31 wt % and the values of the magnesian number MG# 80–83 are approximately the same in the upper and the lower mantles of the Moon, but drastically differ from those in the bulk composition of the silicate Earth (Bulk Silicate Earth, BSE, FeO 8%, MG# 89). On the contrary, the estimates of Al2O3 concentration in the three-layer mantle noticeably depend on the thermal state. The results of solution of the inverse problem indicate the trend towards the gradual increase in the Al2O3 content with depth, from the upper to the lower mantle to 4–7% with the higher content of garnet. For the cold models of the lower mantle of the Moon, the bulk content of Al2O3 is ∼1 × BSE, and for the hot models it can be in the interval of 1.3 × BSE-1.7 × BSE. The abundance of SiO2 depends, to a lesser degree, on the thermal state and is 50–55% in the upper and 45–50% in the lower mantle. The high pyroxene content of the upper mantle of the Moon is the geochemical consequence of the geophysical models used with the inversion into composition and temperature relations; orthopyroxene, instead of olivine, is the dominant mineral of the upper mantle. Concentrations of SiO2 in the lower (undifferentiated) mantle showing the bulk composition of the silicate Moon (Bulk Silicate Moon, BSM), are consistent with the geochemical estimates of 45–48% of SiO2 for the BSM and close to those for the Earth’s mantle (45–47%). The composition of the mantle middle zone remains discussible, since it might be partially overlapped with compositions of the over- and underlying envelopes. The results of the model suggest that the mantle of the Moon is stratified in chemical composition. For the considered thermal state models, the mantle of the Moon is enriched in FeO and depleted in MgO in relation to the primitive Earth mantle, which indicates considerable differences between compositions of the Earth and its satellite.

Neogene–Quaternary Magmatism of the Çaldıran Plain and its Vicinity (Eastern Turkey): an Example of Post-Collisional Transition from Subduction to Intraplate Type

This paper is aimed at studying the chronological evolution of the Neogene–Quaternary volcanic activity within the Caldiran plain and its mountainous framing (Eastern Turkey). It is shown that the last pulse of continental-margin magmatism related to the subduction and closure of Neotethys oceanic basin occurred in the Middle Miocene (13.5–12.5 Ma). The post-collision volcanism proceeding simultaneously with large-scale regional tectonic rearrangement and initiation of the long-term Caldiran fault began in the Late Miocene (7–6 Ma), and reached maximum activity in the Middle Pliocene (4.7–3.6 Ma). The Quaternary period in the region evolution was marked by the abundant within-plate magmatic activity restricted to the regional SW–NE trending zone, and the formation of Eastern Turkey’s largest Tendurek shield volcano (Late Pleistocene–Holocene). Petrological-geochemical data indicate that magmas during the overall evolution of young volcanism of the Caldiran plain was generated from a single mantle reservoir, whose composition gently one-way evolved with time. Calculations show that melting occurred in the upper part of the asthenosphere (immediately near the boundary with thinned lithospheric mantle), which was metasomatized by pre-existing long-continued subduction. The chemical variations of mantle source with time (from the Middle Miocene to Quaternary) were mainly determined by a decrease of subduction component and the presence of aqueous phases, with a general trend from E-MORB to OIB-type for generated magmas. The composition of Late Quaternary basic lavas of Tendurek Volcano in terms of most petrological-geochemical characteristics corresponds to within-plate alkaline basalts. The main trend of geochemical evolution of mantle source is correlated with a systematic change of the predominant serial affinity of igneous rocks from calcalkaline through moderately alkaline to Na-alkaline varieties. Discrete character of young magmatism within the Caldiran plain, and its subsequent evolution (sulrasubduction → post-collision → within-plate) were mainly determined by periodical large-scale changes in geotectonic setting within the Eurasian–Arabian collision zone: (1) cessation of subduction, (2) break-up and deepening of oceanic slab with its subsequent break off, (3) inferred emergence of incipient rift setting under conditions of intense submeridional compression.

Adakite Volcanism at the Continental Margin and Associated Problems. Part I. Adakites from the Upper Reaches of the Valovayam River: New Age, Mineral, and Chemical Data and Petrological Modeling

The paper presents new age and petrological–geochemical data on the rocks of the Valovayam River area (Kamchatka Isthmus) and some areas of the Sredinny Range of Kamchatka (Orlov Volcano, Polovinnaya, Stolbovaya, and Matera mountains). Analysis of the obtained data shows that the spatial and temporal association of the intraplate and island-arc volcanism in the Valovayam River area is not accidental, but resulted from melting of a common source related to the mantle diapir. Additional data on the composition of the mantle xenoliths and the traces of their melting in the form of residual glasses make it possible to relate the formation of different rock series into a single process. A two-stage petrogenetic model is proposed: (1) formation of “black pyroxenites” as a result of melting of mantle lherzolites and (2) formation of the whole rock assemblage through melting of the same lherzolites and newly formed pyroxenites. The concurrent study of the rocks of similar age from the Valovayam River area and the Sredinny Range allows us to distinguish two types of adakites: high-Si adakites (HSA) in the former area and low-Si adakites (LSA) in the latter. The formation of the first type is related to the fractionation, melting, and breakdown of amphibole, while the formation of the second type was controlled by the fractionation and melting of pyroxene.

Тубинско-Гайский пояс: закономерности формирования и размещения колчеданного оруденения в свете новых петролого-геохимических данных по составу рудовмещающих комплексов

Тубинско-Гайский пояс: закономерности формирования и размещения колчеданного оруденения в свете новых петролого-геохимических данных по составу рудовмещающих комплексов

Compositional variations of several Early Permian magmatic sulfide deposits in the Kalatongke district, southern Altai, western China: With genetic and exploration implications

A Permian magmatic Ni-Cu sulfide deposit cluster occurs in the Kalatongke district in the Southern Chinese Altai Orogenic Belt, western China. These deposits are associated with the mafic units of the Y1, Y2, Y3, Y9 and G21 mafic-intermediate complexes. In this paper we report the first zircon U-Pb ages for the Y3 and G21 intrusions, which are 283.3±1.3Ma and 281.1±1.5Ma, respectively. Our new age data confirm that the sulfide-bearing mafic units of the Y1, Y2 (connected with Y1 at depth), Y3, Y9 and G21 intrusions all formed in Early Permian between ∼281 and ∼287Ma. New and existing petrological-geochemical data show some important regular variations between these deposits. The host lithologies change from olivine-bearing rocks for the Y1-Y2-Y9 deposits to olivine-free rocks such as norite for the Y3 deposit and leucogabbro for the G21 deposit. The olivine Fo contents of the Y1 deposit are up to 82mol%, which are slightly higher than those of the Y2 deposit (up to 81mol%) and the Y9 deposit (up to 79mol%). The average plagioclase An contents of the olivine-bearing Y1-Y2-Y9 deposits are higher than those of the olivine-free Y3-G21 deposits. Among the three deposits (Y1, Y2 and Y3) that occur closely along the same structural lineament, the Ni/Cu ratios of bulk sulfides decrease from the olivine-bearing deposits (Y1 and Y2) to the olivine-free deposit (Y3). The PGE tenors of these deposits (Y1, Y2 and Y3) and the nearby coeval deposits (Y9 and G21) are extremely low, indicating that their parental magmas are severely depleted in PGEs. The variations of PGE tenors within a single deposit as well as among the different deposits are mainly due to variable R factors. The host rocks of these deposits are all characterized by elevated initial 87Sr/86Sr ratios from 0.7045 to 0.7047, positive εNd values from 4.95 to 6.86, positive εHf values of zircon from 9 to 16, and elevated δ18O values of zircon from 6.15 to 6.7‰. The isotope data indicate that the parental magmas for these deposits experienced up to ∼15wt% crustal contamination. The δ34S values of the sulfide minerals from these deposits are from −3.1‰ to 0.4‰, with a peak at −2.2‰, indicating the involvement of crustal sulfur. The isotope data and mineral chemistry together indicate that both olivine fractional crystallization and addition of crustal sulfur played a role in triggering sulfide saturation in the parental magmas for these deposits. Based on higher Ni/Cu ratios of sulfide mineralization in the olivine-bearing intrusions (Y1, Y2, Y9) than in the coeval olivine-free intrusions (Y3, G21), we recommend that Ni exploration in the region focus on the olivine-bearing intrusions that were emplaced in the Early Permian.

Geochemical features of the quaternary lamproitic lavas of Gaussberg Volcano, East Antarctica: Result of the impact of the Kerguelen plume

Petrological-geochemical data were obtained on the lamproites of the Gaussberg Volcano located at the eastern Antarctic coast and compared with data on the magmatic rocks developed in the East Antarctica and Indian Ocean in relation with the Kerguelen plume. It was shown that the lamproites were derived from the ancient Gondwana lithosphere repeatedly modified at the early stages of its evolution, including significant enrichment in volatiles, lithophile elements, and radiogenic Sr and Pb isotopes.

Revisiting the last major eruptions at Stromboli volcano: inferences on the role of volatiles during magma storage and decompression

Abstract Stromboli is a unique open conduit volcano and a natural laboratory for investigating how volatiles migrate and concentrate under dynamic conditions. Fluid phases are involved in magma decompression and pressurization, modulate Strombolian activity and govern magma rise and fragmentation processes. Here, we have revisited the available data on the last two major eruptions at Stromboli volcano and concentrated our analysis on the 2007 eruption. First, we analysed petrological-geochemical data to assess equilibrium conditions by using standard thermobarometry; we then used a grid of selected reactions which involve solid-melt-fluid equilibria to better constrain the P – T regimes that adequately describe our system. Primitive hydrous basaltic melts, reported in literature and preserved as melt inclusions in olivine (with 2.3–3.8 wt% of H 2 O and 890–1590 ppm CO 2 ), are in equilibrium with forsteritic olivine and a diopsidic clinopyroxene at average pressures of 260 (±47) MPa for temperatures approaching 1170 (±17) °C and calculated (mole fraction of CO 2 within the melt) in the range 0.60–0.76. Ca-rich or ultracalcic melts are regarded as the result of decompression along a steep adiabatic and/or isothermal curve. During this process the magma will cross-cut the stability field of diopside and enter the liquidus field. The earlier crystallized diopside is destabilized and reacts with the coexisting liquid phase leading to the formation of ultracalcic melts. Ejected golden pumices (with 2–3 wt% H 2 O) are in equilibrium with Ca-pyroxene, forsteritic olivine and anorthitic plagioclase at 150–220 MPa and temperatures of 1120–1150 °C. Evolved melt inclusions (substantially degassed) in less magnesian olivine ( c. Fo 70 ) of the scorias show average equilibration pressures of 78 (±20) MPa and temperatures of 1138 (±14) °C. In summary, the higher P–T regimes associated with the origin of primitive melt inclusions are representative of the base of the chamber, where the ferromagnesian phases may crystallize and cumulate. The magma with a bulk composition typical of the pumices is stored in the middle and main part of the chamber (likely its axial sector) and these materials are erupted during paroxysmal and, more rarely, major explosions. Finally, more evolved melt inclusions found in the olivine of the scorias are indicative of crystallization within the conduit or its root zone connected to the upper part of the chamber. Pure extensional regimes and recent geophysical data suggest the existence of a prolate ellipsoidal magma chamber below Stromboli. To constrain its volume we estimated the magma volumes associated with SO 2 degassing (during the 2007 major eruption) by applying a refined petrological model that allowed us to estimate the magma fluxes in the subvolcanic region (i.e. the magma flux entering the degassing zone). The long-term trend of this magma flux follows an overall exponential decay, typical of pressurized magmatic systems, and indicates that magma rise was accompanied and followed by slow decompression. This trend was shown to be consistent with release of elastic strain accumulated either by pressurization of the rocks surrounding the magma reservoir, by pressurization of the magma itself or both. By analysing the reservoir elastic response during magma decompression, we found that the current Stromboli magma chamber volume may be adequately constrained to 1–2 km 3 .

Peculiarities of the magmatism and tectonics of the Knipovich Ridge

Petrological-geochemical data were obtained for intrusive rocks (gabbroids) recovered on the eastern flank of the Knipovich Ridge by deep-sea site 344 (DSDP, Leg 238). It was found that these rocks are similar to basalts and basaltic glasses studied in the adjacent sections of the ridge rift zone [7, 8]. This indicates that the intrusive rocks and erupted lavas are comagmatic. The gabbroids, basalts, and their quenched glasses were derived by differentiation in different-depth chambers and feeder channels. The petrochemical features of the gabbros and basalts (low Fe content, oxidized magnetic minerals) caused their weak magnetic properties. Owing to the multidirectional movements of the oceanic blocks, the bodies of the intrusive and effusive rocks their lost strict linearity and produced the mosaic anomalous magnetic field of the Knipovich Ridge.

Two types of alkaline volcanics in the southwestern Iberian margin: The causes of their diversity

The diverse geodynamic conditions of the parental magma’s melting are responsible for the compositional diversity of the alkaline volcanics near the southwestern margin of Iberia. The petrological-geochemical data show that the volcanics of the Gorringe Bank originated within the continental plate. The parental melilitite melts depleted in silica and anomalously enriched with trace elements could have been generated only in deep settings with a low degree of metasomatically enriched mantle matter melting. The volcanic melilitite-nephelinite-phonolite series is widespread in alkaline provinces of the Eurasian, African, and other continental plates. The Ampere, Josephine, and other seamounts and islands of the region are largely composed of volcanic rocks belonging to the picrobasalt-hawaiite-mugearite association. Their parental magmas were generated within the oceanic plate at shallower depths under a higher degree of moderately enriched oceanic lithospheric mantle melting. Both series of volcanics were formed under the influence of mantle plumes.