First Evidence of the Post-Variscan Magmatic Pulse on the Western Edge of East European Craton: U-Pb Geochronology and Geochemistry of the Dolerite in the Lublin Podlasie Basin, Eastern Poland

The U–Pb measurements of youngest, coherent group of zircons from the Mielnik IG1 dolerite at the Teisseyre-Tornquist margin (TTZ) of East European Craton (EEC) in Poland yielded age of 300 ± 4 Ma. Zircon dated an evolved portion of magma at the late stage crystallization. It is shown that this isolated dyke from the northern margin of the Lublin Podlasie basin (Podlasie Depression) and regional dyke swarms of close ages from the Swedish Scania, Bornholm and Rügen islands, Oslo rift, Norway, and the Great Whine Sill in northeastern England, were coeval. They have been controlled by the same prominent tectonic event. The Mielnik IG1 dolerite is mafic rock with Mg-number between 52 and 50 composed of the clinopyroxene, olivine-pseudomorph, plagioclase, titanite, magnetite mineral assemblage, indicating relatively evolved melt. This hypabyssal rock has been affected by postmagmatic alteration. The subalkaline basalt composition, enrichment in incompatible trace elements, progressive crustal contamination, including abundance of zircon xenocrysts determines individual characteristics of the Mielnik IG1 dolerite. The revised age of dolerite, emplaced in vicinity of TTZ provides more evidences documenting the reach of the Permo-Carboniferous extension and rifting accompanied by magmatic pulses, that were widespread across Europe including the margin of the EEC incorporated that time into the broad foreland of the Variscan orogen.

New Perspectives on the Chronostratigraphy and Magmatic Evolution of the Auckland Volcanic Field, New Zealand

<p>Understanding the eruptive history of a volcanically active region is critical in assessing the hazard and risk posed by future eruptions. In regions where surface deposits are poorly preserved, and ambiguously sourced, tephrostratigraphy is a powerful tool to assess the characteristics of past eruptions. The city of Auckland, New Zealand’s largest urban centre and home to ca. 1.4 million people, is built on top of the active Auckland Volcanic Field (AVF). The AVF is an intraplate monogenetic basaltic volcanic field, with ca. 53 eruptive centres located in an area of ca. 360 km2. Little is known however, about the evolution of the field because the numerical and relative ages of the eruptions are only loosely constrained, and therefore the precise order of many eruptions is unknown. Here I apply tephrostratigraphic and geochemical techniques to investigate the chronology and magmatic evolution of the AVF eruptions. First, I present an improved methodology for in-situ analysis of lacustrine maar cores from the AVF by employing magnetic susceptibility and X-ray density scanning on intact cores. These techniques are coupled with geochemical microanalysis of the tephra-derived glass shards to reveal details of reworking within the cores. These details not only allow assessment of the deposit relationships within cores (e.g. primary vs. reworked horizons), but also to correlate tephra horizons between cores. Through the correlation of tephra units across cores from a variety of locations across the field, an improved regional tephrostratigraphic framework for the AVF deposits has been established. Following on from this, I detail the methods developed in this study to correlate tephra horizons within the maar cores back to their eruptive source. This technique uses geochemical fingerprinting to link the glass analyses from tephra samples to whole rock compositions. Such an approach has not been previously attempted due to the complications caused by fractional crystallisation, which affects concentrations of certain key elements in whole rock analyses. My method resolves these issues by using incompatible trace elements, which are preferentially retained in melt over crystals, and therefore retain comparable concentrations and concentration ratios between these two types of sample. Because of the primitive nature of the AVF magmas, their trace element signature is largely controlled by the involvement of several distinct mantle sources. This leads to significant variability between the volcanic centres that thus can be used for individually fingerprinting, and correlating tephra to whole rocks. Nevertheless, in some cases geochemistry cannot provide an unambiguous correlation, and a multifaceted approach is required to allow the correlation of the tephra horizons to source. The other criteria used to correlate tephra deposits to their source centre include, Ar-Ar ages of the centres, modelled and calculated ages of the tephra deposits, the scale of eruption, and the deposit locations and thicknesses. The results of this research outline the methodology for assessing occurrence and characteristics of basaltic tephra horizons within lacustrine maar cores, and the methodology for correlating these horizons to their eruptive source. In doing this the relative eruption order of the AVF is accurately determined for the first time. Temporal trends suggest acceleration of eruption repose periods to 21 ka followed by deceleration to present. Although no spatial evolution is observed, coupling of some centres is seen when spatial and temporal evolution are combined. The geochemical signature of the magmas appears to evolve in a cyclic manner with time, incorporating increasing amount of a shallow source. This evolution is seen both during a single eruption sequence and throughout the lifespan of the AVF. Finally, pre-eruptive processes are assessed as part of the study of the magmatic evolution of the AVF. The effects of contamination from the crust and lithosphere through which the magma ascends are evaluated using the Re-Os isotope system. The results show there are variable inputs from crustal sources, which have previously not been identified by traditional isotope systems (e.g. Pb-Sr-Nd isotopes). Two sources of contamination are identified based on their Os systematics relating to two terranes beneath the AVF: the metasedimentary crust and the Dun Mountain Ophiolite Belt. The identification of this process suggests there is interaction of ascending melt with the crust, contrary to what previous studies have concluded. This body of research has provided a detailed reconstruction of the chronostratigraphy and magmatic evolution of the AVF to aid accurate and detailed risk assessment of the threat posed by a future eruption from the Auckland Volcanic Field.</p>

New Perspectives on the Chronostratigraphy and Magmatic Evolution of the Auckland Volcanic Field, New Zealand

<p>Understanding the eruptive history of a volcanically active region is critical in assessing the hazard and risk posed by future eruptions. In regions where surface deposits are poorly preserved, and ambiguously sourced, tephrostratigraphy is a powerful tool to assess the characteristics of past eruptions. The city of Auckland, New Zealand’s largest urban centre and home to ca. 1.4 million people, is built on top of the active Auckland Volcanic Field (AVF). The AVF is an intraplate monogenetic basaltic volcanic field, with ca. 53 eruptive centres located in an area of ca. 360 km2. Little is known however, about the evolution of the field because the numerical and relative ages of the eruptions are only loosely constrained, and therefore the precise order of many eruptions is unknown. Here I apply tephrostratigraphic and geochemical techniques to investigate the chronology and magmatic evolution of the AVF eruptions. First, I present an improved methodology for in-situ analysis of lacustrine maar cores from the AVF by employing magnetic susceptibility and X-ray density scanning on intact cores. These techniques are coupled with geochemical microanalysis of the tephra-derived glass shards to reveal details of reworking within the cores. These details not only allow assessment of the deposit relationships within cores (e.g. primary vs. reworked horizons), but also to correlate tephra horizons between cores. Through the correlation of tephra units across cores from a variety of locations across the field, an improved regional tephrostratigraphic framework for the AVF deposits has been established. Following on from this, I detail the methods developed in this study to correlate tephra horizons within the maar cores back to their eruptive source. This technique uses geochemical fingerprinting to link the glass analyses from tephra samples to whole rock compositions. Such an approach has not been previously attempted due to the complications caused by fractional crystallisation, which affects concentrations of certain key elements in whole rock analyses. My method resolves these issues by using incompatible trace elements, which are preferentially retained in melt over crystals, and therefore retain comparable concentrations and concentration ratios between these two types of sample. Because of the primitive nature of the AVF magmas, their trace element signature is largely controlled by the involvement of several distinct mantle sources. This leads to significant variability between the volcanic centres that thus can be used for individually fingerprinting, and correlating tephra to whole rocks. Nevertheless, in some cases geochemistry cannot provide an unambiguous correlation, and a multifaceted approach is required to allow the correlation of the tephra horizons to source. The other criteria used to correlate tephra deposits to their source centre include, Ar-Ar ages of the centres, modelled and calculated ages of the tephra deposits, the scale of eruption, and the deposit locations and thicknesses. The results of this research outline the methodology for assessing occurrence and characteristics of basaltic tephra horizons within lacustrine maar cores, and the methodology for correlating these horizons to their eruptive source. In doing this the relative eruption order of the AVF is accurately determined for the first time. Temporal trends suggest acceleration of eruption repose periods to 21 ka followed by deceleration to present. Although no spatial evolution is observed, coupling of some centres is seen when spatial and temporal evolution are combined. The geochemical signature of the magmas appears to evolve in a cyclic manner with time, incorporating increasing amount of a shallow source. This evolution is seen both during a single eruption sequence and throughout the lifespan of the AVF. Finally, pre-eruptive processes are assessed as part of the study of the magmatic evolution of the AVF. The effects of contamination from the crust and lithosphere through which the magma ascends are evaluated using the Re-Os isotope system. The results show there are variable inputs from crustal sources, which have previously not been identified by traditional isotope systems (e.g. Pb-Sr-Nd isotopes). Two sources of contamination are identified based on their Os systematics relating to two terranes beneath the AVF: the metasedimentary crust and the Dun Mountain Ophiolite Belt. The identification of this process suggests there is interaction of ascending melt with the crust, contrary to what previous studies have concluded. This body of research has provided a detailed reconstruction of the chronostratigraphy and magmatic evolution of the AVF to aid accurate and detailed risk assessment of the threat posed by a future eruption from the Auckland Volcanic Field.</p>

Concurrent MORB-type and ultrapotassic volcanism in an extensional basin along the Laurentian Iapetus margin: Tectonomagmatic response to Ordovician arc-continent collision and subduction polarity flip

Arc-continent collision, followed by subduction polarity flip, occurs during closure of oceanic basins and contributes to the growth of continental crust. Such a setting may lead to a highly unusual association of ultrapotassic and mid-ocean ridge basalt (MORB)-type volcanic rocks as documented here from an Ordovician succession of the Scandinavian Caledonides. Interbedded with deep-marine turbidites, pillow basalts evolve from depleted-MORB (εNdt 9.4) to enriched-MORB (εNdt 4.8) stratigraphically upward, reflecting increasingly deeper melting of asthenospheric mantle. Intercalated intermediate to felsic lava and pyroclastic units, dated at ca. 474−469 Ma, are extremely enriched in incompatible trace elements (e.g., Th) and have low εNdt (−8.0 to −6.6) and high Sri (0.7089−0.7175). These are interpreted as ultrapotassic magmas derived from lithospheric mantle domains metasomatized by late Paleoproterozoic to Neoproterozoic crust-derived material (isotopic model ages 1.7−1.3 Ga). Detrital zircon spectra reveal a composite source for the interbedded turbidites, including Archean, Paleo-, to Neoproterozoic, and Cambro-Ordovician elements; clasts of Hølonda Porphyrite provide a link to the Hølonda terrane of Laurentian affinity. The entire volcano-sedimentary succession is interpreted to have formed in a rift basin that opened along the Laurentian margin as a result of slab rollback subsequent to arc-continent collision, ophiolite obduction and subduction polarity flip. The association of MORBs and ultrapotassic rocks is apparently a unique feature along the Caledonian-Appalachian orogen. Near-analogous modern settings include northern Taiwan and the Tyrrhenian region of the Mediterranean, but other examples of strictly concurrent MORB and ultrapotassic volcanism remain to be documented.

Geochemical, petrographic, and stratigraphic analyses of the Portage Lake Volcanics of the Keweenawan CFBP: implications for the evolution of Main stage volcanism in Continental Flood Basalt Provinces

Continental Flood Basalt Provinces (CFBPs) are large igneous features formed by the extrusion of massive amounts of lavas that require significant evolution within the lithosphere. Although sequential lava flows are effective probes of magmatic systems, CFBPs are typically poorly preserved. We focus on lava flows from the well-preserved 1.1 Ga Keweenawan CFBP that erupted within the Midcontinent Rift System. We present a new geochemical, petrographic, and stratigraphic synthesis from the Main stage Portage Lake Volcanics (PLV). Flow-by-flow analysis of the PLV reveals that major element behavior is decoupled from trace element behavior; MgO exhibits limited variability, while compatible and incompatible trace elements deviate from high to low concentrations throughout the sequence. The concentrations of incompatible trace elements slightly decrease from the base of the sequence to the top. We investigate these observations by applying a recharge, evacuation, assimilation, and fractional crystallization model to geochemical and petrographic data. Our modelling demonstrates a magmatic system experiencing increased evacuation rates while fractionation and assimilation rates decrease, indicating an increase in magmatic flux. The outcome of this modelling is a progressively more efficient magma system within the PLV. This study highlights the utility of joint petrographic and geochemical interpretation in constraining CFBP magma evolution.Supplementary material at https://doi.org/10.6084/m9.figshare.c.5424758

Ophiolitic peridotites in Xigaze (Tibet): Constraints on modes of melt transport in the mantle

&lt;p&gt;The Xigaze ophiolite (Tibet), which occurs in the central segment of the Yarlung Zangbo Suture Zone, exposes a complete portion of a mantle sequence that consists essentially of fresh as well as serpentinized peridotites. We studied a sequence beneath the crustal section that exposes fresh, Cpx-bearing harzburgites and dunites that are underlain by serpentinized Cpx-bearing harzburgites and dunites. The rocks at the bottom are crosscut by dykes that have undergone different degrees of rodingitization. The modal compositions of peridotite from both fresh and serpentinized sections plot in abyssal upper mantle fields, with clinopyroxene modes less than 5 vol. %. Although harzburgites and dunites indicate that melt has been lost relative to primitive mantle compositions, the trace element patterns carry signatures of enrichment in incompatible elements, such as (i) &amp;#8220;bowl-shaped&amp;#8221; patterns of trace elements in silicate-Earth normalized spider diagrams, (ii) positive anomalies in highly incompatible trace elements such as Rb, Th, U, Ta, and (iii) enrichment of LREE in the clinopyroxenes from dunites and harzburgites. These features are indicative of complex melt transfer processes and cannot be produced by simple melt extraction. Petrographic studies reveal that harzburgite and dunite contain interstitial polyphase aggregates of olivine + Cpx + spinel + Opx and olivine + Cpx + Spinel, respectively. Experimental studies (e.g. Morgan and Liang, 2003) suggest that these aggregates represent frozen melt-rich components, indicating that fertile melt was percolating through the depleted harzburgite &amp;#8211; dunite matrix. Presence of such &amp;#8220;melt pods&amp;#8221; would explain the trace element enrichment patterns of the bulk rock, as well as features such as reverse zoning (core: Cr, Fe&lt;sup&gt;2+&lt;/sup&gt; rich, rim: Al, Mg rich) of spinels in polyphase aggregates in fresh dunites. These results show that melt extraction from the mantle is not a single stage process, and that evidence of multiple melt pulses that propagated through a rock are preserved in the petrographic features as well as in the form of chemical signatures that indicate refertilization of initially depleted rocks.&lt;/p&gt;

Modelling clinopyroxene/melt partition coefficients for higher upper mantle pressures

&lt;p&gt;When partial melt occurs in the mantle, redistribution of trace elements between the solid mantle material and partial melt takes place. Partition coefficients play an important role when determining the amount of trace elements that get redistributed into the melt. Due to a lower density compared to surrounding solid rock, partial melt that was generated in the upper mantle will rise towards the surface, leaving the upper mantle depleted in incompatible trace elements and an enriched crust. Studies investigating trace element partitioning in the mantle typically rely on constant partition coefficients throughout the mantle, even though it is known that partition coefficients depend on pressure, temperature, and composition. Between the pressures of 0-15 GPa, partition coefficients vary by two orders of magnitude along both, solidus and liquidus. Since partition coefficients exhibit a parabolic relationship in an Onuma diagram, a similar variation is expected for all trace element partition coefficients that can be derived from the sodium partition coefficients.&lt;/p&gt;&lt;p&gt;In this study, we developed a thermodynamic model for sodium in clinopyroxene after Blundy et al. (1995). With the thermodynamic model results, we were able to deduce a P-T dependent equation for sodium partitioning that is applicable up to 12 GPa between the peridotite solidus and liquidus. Because sodium is an almost strain-free element in jadeite, it can be used as a reference to model partition coefficients for other elements, including heat producing elements like K, Th, and U. This gives us the opportunity to insert P-T dependent partition coefficient calculations of any trace element into mantle melting models, which will have a big impact on the accuracy of elemental redistribution calculations and therefore, if the partitioning of the heat producing elements is taken into account, also the evolution of the mantle and crust.&lt;/p&gt;&lt;p&gt;Blundy, J. et al. (1995): Sodium partitioning between clinopyroxene and silicate melts, J. Geophys. Res., 100, 15501-15515.&lt;/p&gt;&lt;p&gt;Schmidt, J.M. and Noack, L. (2021): Parameterizing a model of clinopyroxene/melt partition coefficients for sodium to higher upper mantle pressures (to be submitted)&lt;/p&gt;

Geochemistry of Basalts from Southwest Indian Ridge 64° E: Implications for the Mantle Heterogeneity East of the Melville Transform

As one of the regional, magmatic, robust, axial ridge segments along the ultraslow-spreading Southwest Indian Ridge (SWIR), the magmatic process and mantle composition of the axial high relief at 64° E is still unclear. Here, we present major and trace elements and Sr-Nd-Pb isotope data of mid-ocean ridge basalts (MORBs) from 64° E. The basalts show higher contents of Al2O3, SiO2, and Na2O and lower contents of TiO2, CaO, and FeO for a given MgO content, and depletion in heavy rare-earth elements (HREE), enrichment in large-ion lithophile elements, and lower 87Sr/86Sr, 143Nd/144Nd and higher radiogenic Pb isotopes than the depleted MORB mantle (DMM). The high Zr/Nb (24–43) and low Ba/Nb (3.8–7.0) ratios are consistent with typical, normal MORB (N-MORB). Extensive plagioclase fractional crystallization during magma evolution was indicated, while fractionation of olivine and clinopyroxene is not significant, which is consistent with petrographic observations. Incompatible trace elements and isotopic characteristics show that the basaltic melt was formed by the lower partial melting degree of spinel lherzolite than that of segment #27 (i.e., Duanqiao Seamount, 50.5° E), Joseph Mayes Mountain (11.5° E), etc. The samples with a DMM end-member are unevenly mixed with the lower continental crust (LCC)- and the enriched mantle end-member (EM2)-like components, genetically related to the Gondwana breakup and contaminated by upper and lower continental crust (or continental mantle) components.

Metasomatism-controlled hydrogen distribution in the Spitsbergen upper mantle

Hydrogen concentrations in minerals of peridotite xenoliths in alkali basaltic rocks from Quaternary volcanoes in northwest Spitsbergen were measured using polarized Fourier transform infrared spectroscopy (FTIR) to trace the effects of geologic processes on hydrogen distribution in the continental lithospheric mantle. The mineral grains show hydrogen profiles with lower concentrations at rims suggesting diffusive hydrogen loss during the entrapment and transport of the xenoliths in magma. However, hydrogen concentrations in the centers of the grains are uniform and appear to represent hydrogen abundances in the Spitsbergen upper mantle. The olivine, orthopyroxene, and clinopyroxene contain 1–10, 130–290, and 350–560 ppm H2O, respectively. Hydrogen abundances away from metasomatic melt conduits recorded by Type 1 xenoliths are correlated with the concentrations of incompatible trace elements, indicating that hydrogen distribution is related to mantle metasomatism. By contrast, hydrogen near the melt conduits, recorded by Type 2 xenoliths, shows no regular correlations with incompatible trace elements (except Nb in clinopyroxene) and may be affected by fractional crystallization of amphibole in the conduits. Hydrogen contents decrease away from the melt conduits and are controlled by the interaction between the depleted host mantle and percolating metasomatic melts. Therefore, the metasomatic melt could have variably hydrated the Spitsbergen upper mantle via different processes. The H2O/Ce ratios of the melt in equilibrium with clinopyroxene near the metasomatic melt conduits range from 93 to 218, i.e., within the oceanic island basalt (OIB) range. This is consistent with that the metasomatic melt could have been derived from OIB-type sources evidenced by the Sr-Nd isotope compositions of the xenoliths.