Evolution of mafic lavas in Central Anatolia: Mantle source domains

We present new Sr-Nd-Pb-Hf isotopic data on mafic lavas from the Sivas, Develidağ, Erciyes, and Erkilet volcanic complexes in central Turkey and Tendürek in eastern Turkey to evaluate the mantle sources for volcanism in the context of the geodynamic evolution of the Anatolian microplate. Early Miocene through Quaternary volcanism in Western Anatolia and latest Miocene through Quaternary activity in Central Anatolia were dominated by contributions from two distinct source regions: heterogeneous metasomatized or subduction-modified lithosphere, and roughly homogeneous sublithospheric ambient upper mantle; we model the source contributions through mixing between three end members. The sublithospheric mantle source plots close to the Northern Hemisphere reference line (NHRL) with radiogenic 206Pb/204Pb of ~19.15, while the other contributions plot substantially above the NHRL in Pb isotope space. The lithospheric source is heterogeneous, resulting from variable pollution by subduction-related processes likely including direct incorporation of sediment and/or mélange; its range in radiogenic isotopes is defined by regional oceanic sediment and ultrapotassic melts of the subcontinental lithospheric mantle. The geochemical impact of this contribution is disproportionately large, given that subduction-modified lithosphere and/or ocean sediment dominates the Pb isotope signatures of mafic Anatolian lavas. Subduction of the Aegean or Tethyan seafloor, associated with marked crustal shortening, took place throughout the region until ca. 16–17 Ma, after which time broad delamination of the thickened lower crust and/or the Tethyan slab beneath Central Anatolia allowed for sediment and/or mélange and slab-derived fluids to be released into the overlying evolving modified mantle. Aggregation of melts derived from both mantle and lithospheric domains was made possible by upwelling of warm asthenospheric material moving around and through the complexly torn younger Aegean-Cyprean slab that dips steeply to the north beneath southern Anatolia.

Origin of alkali olivine basalts and hawaiites in the western Mexican arc: Evidence of rapid phenocryst growth and magma mixing during ascent along fractures

A detailed petrological study is presented to constrain the origin of a suite of alkali olivine basalt and hawaiite (>5 wt% MgO) lavas that were erupted in a rift zone within the western Mexican arc (Trans-Mexican Volcanic Belt), adjacent to the Sangangüey andesitic stratovolcano, together with more evolved lavas (mugearites and benmoreites; <5 wt% MgO). As previously documented in the literature, the Sangangüey mafic lavas are devoid of any arc geochemical signature, despite their location within an arc. In this study, a new olivine-melt thermometer/hygrometer, based on the partition­ing behavior of Ni2+ and Mg2+, was applied to the Sangangüey basalts (SB). The results show that the high-MgO (>9 wt%) SB crystallized at higher temperatures and lower melt-water contents (0–1.3 wt%) compared to high-MgO arc basalts (≤5.7 wt% H2O) erupted in the west-central Mexican arc. The Sangangüey lavas with 5–8 wt% MgO display evidence of mixing between high-MgO alkali olivine basalts and low-MgO mugearites. It is proposed that the unique composition of the mugearites (i.e., low SiO2 contents and elevated FeO and TiO2 contents) is the result of partial melting of mafic lower crust driven by the influx of high-MgO intraplate basalts under relatively hot, dry, and reduced conditions. On the basis of crystal textures and compositional zoning patterns, it is shown that both phenocryst growth and magma mixing occurred rapidly, most likely during ascent along fractures, and not slowly during prolonged storage in a crustal magma chamber.

Supplemental Material: Evolution of mafic lavas in Central Anatolia: Mantle source domains

Figure S1: The three endmember mixing model for the Anatolian lavas in (a) ε_Nd-ε_Hf and (b) ε_Nd-⁸⁷Sr/⁸⁶Sr space. Table S1: Compositions of endmembers used in modeling.<br>

Supplemental Material: Evolution of mafic lavas in Central Anatolia: Mantle source domains

Figure S1: The three endmember mixing model for the Anatolian lavas in (a) ε_Nd-ε_Hf and (b) ε_Nd-⁸⁷Sr/⁸⁶Sr space. Table S1: Compositions of endmembers used in modeling.<br>

Reconciling the location of lava domes and eruption centers in Paleocene-Eocene calderas in northern Chile

&lt;p&gt;In the Atacama Desert, at the Precordillera of northern Chile, a series of Paleocene-Eocene caldera deposits and ring-faults are exceptionally well-preserved&lt;sup&gt;1&lt;/sup&gt;. Here we aim to build on previous mapping efforts to consider the location, timing and style of pre, syn and post caldera volcanism in the region. We focus on the partially nested caldera complexes of Lomas Bayas and El Durazno&lt;sup&gt;2,3&lt;/sup&gt; where deposits record several stages of caldera evolution (pre-collapse, collapse/intra-caldera and extra-caldera, resurgence and post-collapse eruptive deposits). The pre-caldera basement is a thick sequence of early Paleocene mafic lavas&lt;sup&gt;4, 5&lt;/sup&gt;. The caldera complex formed between around 63 and 54 Ma&lt;sup&gt;4, 5&lt;/sup&gt;. Both calderas constitute subcircular structures approximately 13 km in diameter and are cut by several NNW to NNE-trending felsic dikes which are spatially related to felsic domes interpreted as resulting from post caldera formation unrest&lt;sup&gt;1,&lt;/sup&gt;&lt;sup&gt;4&lt;/sup&gt;. These calderas have been interpreted as part of the Carrizalillo megacaldera complex&lt;sup&gt;2 &lt;/sup&gt;. We combine field observations, such as the attitude of dikes, as well as information on their dimension and composition, the size, location and composition of domes and lava flows, as well as the evidence of the regional stress field operating during the caldera evolution from measurements of fault kinematics. This data will be used as the input to finite element method models to investigate the effect of nested caldera geometry, ring-faults and crustal heterogeneities on the location of domes and eruptive centers generated during caldera unrest. The results will be potentially useful for constraining models of eruption forecasting during periods of unrest in calderas and ore deposition models which have been shown to be linked to caldera structure and magma emplacement.&lt;/p&gt;&lt;p&gt;&lt;strong&gt;References&lt;/strong&gt;&lt;/p&gt;&lt;p&gt;&lt;sup&gt;1 &lt;/sup&gt;Rivera, O. and Falc&amp;#243;n, M. (2000). Calderas tipo colapso-resurgentes del Terciario inferior en la Pre-Cordillera de la Regi&amp;#243;n de Atacama: Emplazamiento de complejos volcano-plut&amp;#243;nicos en las cuencas volcano-tect&amp;#243;nicas extensionales Hornitos y Indio Muerto: IX Congreso Geol&amp;#243;gico Chileno, v. 2.&amp;#160;Soc. Geol. de Chile, Puerto Varas.&lt;/p&gt;&lt;p&gt;&lt;sup&gt;2 &lt;/sup&gt;Rivera, O., and Mpodozis, C. (1994). La megacaldera Carrizalillo y sus calderas anidadas: Volcanismo sinextensional Cret&amp;#225;cico Superior-Terciario inferior en la Precordillera de Copiap&amp;#243;, paper presented at VII Congreso Geol&amp;#243;gico Chileno. Acad. de Cienc. del Inst. Chilecol. de Geol. de Chile, Concepci&amp;#243;n.&lt;/p&gt;&lt;p&gt;&lt;sup&gt;3 &lt;/sup&gt;Rivera, O. (1992). El complejo volcano-plut&amp;#243;nico Paleoceno-Eoceno del Cerro Durazno Alto: las calderas El Durazno y Lomas Bayas, Regi&amp;#243;n de Atacama, Chile. Tesis Departamento de Geolog&amp;#237;a, Universidad de Chile, 242. (Unpublished).&lt;/p&gt;&lt;p&gt;&lt;sup&gt;4 &lt;/sup&gt;Ar&amp;#233;valo, C. (2005). Carta Los Loros, Regi&amp;#243;n de Atacama. Servicio Nacional de Geolog&amp;#237;a y Miner&amp;#237;a, Carta Geol&amp;#243;gica de Chile, 92, 1(100.000), 53 p.&lt;/p&gt;&lt;p&gt;&lt;sup&gt;5 &lt;/sup&gt;Iriarte, S., Ar&amp;#233;valo, C., Mpodozis, C. (1999). Mapa Geol&amp;#243;gico de la Hoja La Guardia, Regi&amp;#243;n de Atacama. Servicio Nacional de Geolog&amp;#237;a y Miner&amp;#237;a. Mapas Geol&amp;#243;gicos, 13, 1(100.000).&lt;/p&gt;

Gliding and overthrust nappe tectonics of the Barberton Greenstone Belt revisited: A review of deformation styles and processes

Interpretations of the structural/tectonic evolution of the Barberton Greenstone Belt (BGB) and its surrounding granitoid rocks remain controversial, with proponents for both horizontal thrust-accretion (plate tectonic) and partial convective overturn (vertical tectonic) models. Here, an area of complex folds that was used to support the operation of plate tectonic-derived gliding and overthrust nappe tectonics is re-investigated in detail and placed within the broader structural development of the BGB and surrounding granitoid domains via a re-analysis of structures, and geochronological, stratigraphic and metamorphic data across the whole of this important geological terrain. The results of detailed field mapping show that the complex folds, which occur on the northern limb of the 20 km wavelength, vertically plunging, Onverwacht Anticline, do not represent a re-folded, originally recumbent, isoclinal fold, as previously interpreted. Instead, the folds represent a moderately shallow east-plunging fold train that formed from a single episode of deformation. Fold asymmetry is consistent with formation during originally north-side-up reverse shear on bounding faults, consistent with the offset direction required to explain the fault-repeated slices of Mendon Formation + Fig Tree Group rocks that uniquely occur across the northern limb of the Onverwacht Anticline. More broadly, a review of the BGB and surrounding granitoid rocks show that formation was likely through two discrete, ~120 Ma long, episodes of mantle upwelling, or plume, magmatism, each of which led to crustal melting and partial convective overturn (PCO), a tectonic mechanism that arises from the gravity-driven interaction between dense, upper crustal greenstones and partially melted, more buoyant, granitoid-dominated middle crust. The first mantle upwelling episode, at 3 530 to 3 410 Ma, commenced with long-lived eruption of ultramafic-mafic lavas of the Sandspruit, Theespruit, Komati, and lower Hooggenoeg formations (3 530 to 3 470 Ma). Heat from this magmatic event gave rise to partial melting of the crust that, combined with fractionation of mafic magma chambers produced widespread felsic magmatism at 3 470 to 3 410 Ma (upper Hooggenoeg Formation and Buck Reef Chert), the latter parts of which were accompanied by the formation of D1 dome-and-keel structures via PCO in deeper-levels of the crust represented by the Stolzburg Domain in the far southwest part of the belt. The second mantle upwelling, or plume, episode commenced at 3 334 to 3 215 Ma with the eruption of ultramafic-mafic lavas of the Kromberg, Mendon and Weltevreden formations. Heat from this magmatic event gave rise to renewed partial melting of the crust that, combined with fractionation of mafic magma chambers, produced widespread felsic magmatism at 3 290 to 3 215 Ma. A second, longer-lived and more complex, multi-stage episode of PCO (D2-D4) accompanied deposition of the Fig Tree and Moodies groups from 3 250 to 3 215 Ma. Late D5 deformation accompanied emplacement of the Mpulizi and Piggs Peak batholiths at ca. 3.01 Ga, as previously identified. The Inyoka and Kromberg faults, which separate domains with distinct structural styles, represent neither terrane boundaries nor suture zones, but rather axial faults that separate deformed but generally inward-facing greenstone panels that sank inwards off rising granitoid domains that surround the BGB.

Geochemical study of Cenozoic mafic volcanism in the west-central Great Basin, western Nevada, and the Ancestral Cascades Arc, California

Processes linked to shallow subduction, slab rollback, and extension are recorded in the whole-rock major-, trace-element, and Sr, Nd, and Pb isotopic compositions of mafic magmatic rocks in both time and space over southwestern United States. Eocene to Mio-Pliocene volcanic rocks were sampled along a transect across the west-central Great Basin (GB) in Nevada to the Ancestral Cascade Arc (ACA) in the northern Sierra Nevada, California (∼39°–40° latitude), which are interpreted to represent a critical segment of a magmatic sweep that occurred as a result of subduction from east-northeast convergence between the Farallon and North American plates and extension related to the change from a convergent to a transform margin along the western edge of North America. Mafic volcanic rocks from the study area can be spatially divided into three broad regions: GB (5–35 Ma), eastern ACA, and western ACA (2.5–16 Ma). The volcanic products are dominantly calc-alkalic but transition to alkalic toward the east. Great Basin lavas erupted far inland from the continental margin and have higher K, P, Ti, and La/Sm as well as lower (Sr/P)pmn, Th/Rb, and Ba/Nb compared to ACA lavas. Higher Pb isotopic values, combined with lower Ce/Ce* and high Th/Nb ratios in some ACA lavas, are interpreted to come from slab sediment. Mafic lavas from the GB and ACA have overlapping 87Sr/86Sr and 143Nd/144Nd values that are consistent with mantle wedge melts mixing with a subduction-modified lithospheric mantle source. Eastern and western ACA lavas largely overlap in age and elemental and isotopic composition, with the exception of a small subset of lavas from the westernmost ACA region; these lavas show lower 87Sr/86Sr at a given 143Nd/144Nd. Results show that although extension contributes to melting in some regions (e.g., selected lavas in the GB and Pyramid Lake), chemical signatures for most mafic melts are dominated by subduction-related mantle wedge and a lithospheric mantle component.

What is the magma storage depth under Osorno Volcano (Southern Volcanic Zone, Chile)?

&lt;p&gt;The depth at which magma chamber processes take place below magmatic arcs and the parameters controlling them are highly debated. These questions are fundamental for our understanding of the global magma differentiation as well as the formation of the continental crust at convergent margins, but also for evaluating the risks associated with volcanic eruptions.&lt;/p&gt;&lt;p&gt;In the Central Southern Volcanic Zone (Central-SVZ) of the Chilean Andes, a thin continental crust (30-40 km) and the occurrence of a major fault zone (Linqui&amp;#241;e-Ofqui) likely favor rapid magma ascent. This segment of the arc is as a consequence one of the most active in Chile with several recent eruptions (e.g. Llaima 2009, Cordon Caulle 2011, Calbuco 2015, Villarrica 2015 &amp; 2019). The Central-SVZ is characterized by dominant mafic lavas (basalts, basaltic andesites), few rhyodacitic lavas, a noticeable compositional (Daly) gap in the intermediate compositions (andesites). Noteworthy, amphibole is usually absent, except in a few volcanoes (e.g. Calbuco) or only occurs as microliths in enclaves, which suggests rather low water contents. These observations contrast sharply with the Northern-SVZ where andesitic lavas are dominant and hydrous phases common.&lt;/p&gt;&lt;p&gt;We focused our research on the eruptive products of Osorno volcano (41&amp;#176;S, CSVZ) located between two volcanoes (Calbuco and Cordon Caulle) which recently showed very explosive eruptions and partly overlies an older Pleistocene eroded volcanic edifice (La Picada). A large series of samples were collected in four units spanning 200 kyr. They define a differentiation trend ranging from tholeiitic basalts to calk-alkaline dacites with a Daly Gap between 58 wt. % and 63 wt. % SiO&lt;sub&gt;2&lt;/sub&gt;. Plagioclase and olivine are dominant before the gap while plagioclase and clino- and orthopyroxene dominate afterwards.&lt;/p&gt;&lt;p&gt;The use of recent thermobarometric models revealed two main storage regions: (1) at the MOHO interface (1-1.2GPa) and (2), at the upper/lower crust interface with rather low pressures (likely &amp;#8804;0.3 Gpa). While at (1) primary magmas differentiate, (2) is interpreted as the depth of major differentiation and volatile exsolution. Thermodynamic simulations (Gualda et al., 2012; Ghiorso &amp; Gualda, 2015) support these (2) depth estimates and reproduce the main paragenesis by simple fractional crystallization at 0.1-0.2 GPa. Our results may explain the recent seismic unrest below Osorno (from 2015 to 2019) with earthquakes mostly taking place between 0.1-0.3 GPa (4-10km below the summit). We suggest that Osorno is an important target to perform a comprehensive petrological study aiming at characterizing the Central-SVZ magmatic arc and the magmatic storage depths.&lt;/p&gt;