Evidence of early Miocene synextensional volcanism and deposition in the northern Calico Mountains, central Mojave metamorphic core complex, southern California, USA

The spatiotemporal link between large-scale continental crustal extension and magmatic activity has been identified by numerous past studies, yet commonly the details of these associations remain unresolved. This study in the central Mojave metamorphic core complex (CMMCC) of southern California (USA) presents new geologic mapping, stratigraphic interpretations, and 40Ar/39Ar geochronology of the Jackhammer and Pickhandle Formations in the northern Calico Mountains to provide additional age constraints on the relative timing of early Miocene volcanism, deposition, and extension. The Jackham­mer Formation, the oldest Tertiary stratigraphic unit, is nonconformable with pre-Cenozoic nonmylonitic metasedimentary and plutonic basement rocks and consists primarily of alluvial deposits and primary to reworked silicic tuffs, interbedded locally with basement-derived avalanche megabreccia, lacustrine limestone, and mafic lava; in addition, the “Mammut ignimbrite”, an ~130-m-thick crystal-rich welded lapilli tuff, is exposed only in the eastern part of the study area and appears to transition laterally into thinner, nonwelded lapilli tuff ~6 km to the west. The Pickhandle Formation conformably overlies the Jackhammer Formation and consists of: (1) a lower assemblage composed of reddish monomictic debris-flow breccias with porphyritic rhyodacitic clasts and silicic block-and-ash-flow deposits of sim­ilar composition; and (2) an upper assemblage of polymictic (metaplutonic basement and rhyodacite) alluvial deposits, primary to reworked lapilli tuff, and local rhyodacitic lava and block-and-ash flows. Rhyodacitic lava domes were emplaced during the final stages of Pickhandle Formation deposition, primarily intruded along preexisting normal fault zones. Sedimentary and volcanic lithofacies suggest that the Jackhammer and Pickhandle Formations were deposited in a volcanic vent–proximal alluvial fan system that formed within a half-graben basin bounded on the east by the southwest-dipping “Amphitheatre fault”. Growth strata within the hanging-wall deposits, primarily southwest-directed paleocurrents, and interbedded alluvial debris-flow, basement-derived megabreccia, and lacustrine deposits adjacent to this fault suggest synextensional deposition in an intra-hanging-wall basin that developed during upper-plate extension in the CMMCC. New 40Ar/39Ar ages for six samples of silicic pyroclastic flows and a lava dome from the synextensional Pickhandle and Jackhammer Formations in the Calico Mountains have a mean age of 20.10 ± 0.06 Ma. This age is 3–4 m.y. younger than the maximum age of initial extension determined by previous studies in other areas of the central Mojave, suggesting that CMMCC extension was not a synchronous large-magnitude regional event. Rather, extension and contemporaneous volcanism was more localized and asynchronous across the region, occurring in many smaller extensional basins that eventually culminated in exposure of the CMMCC mylonitic footwall rocks.

Evidence of active magmatic rifting at the Ma’Alalta volcanic field (Afar, Ethiopia)

During continental rifting, strain and magmatism are believed to localize to narrow magmatic segments, while the rift margin is progressively abandoned. We integrate volcanological, geochemical, petrological and seismic data from the Ma’Alalta volcanic field (MVF) near the western margin of Afar, to show that the MVF is an active magmatic segment. Magmatism in MVF initiated with lava flows and large-volume, caldera-forming ignimbrites from a central edifice. However, the most recent magmatic activity shifted towards mafic lava fields, cinder cones and obsidian-rich silicic domes erupted from vents aligned NNW-SSE, defining a ~ 35-km-long magmatic segment. Along the same area, a NNW-SSE alignment of earthquakes was recorded by two local seismic networks (2005–2009 and 2011–2013). The geochemistry of the mafic rocks is similar to those of nearby axial volcanoes. Inferred magma storage depth from mineral geobarometry shows that a shallow, silicic chamber existed at ~ 5-km depth below the stratovolcano, while a stacked plumbing system with at least three magma storage levels between 9 and 24 km depth fed the recent basalts. We interpret the wide set of observations from the MVF as evidence that the area is an active magmatic segment, showing that localised axial extension can be heavily offset towards the rift margin.

Tracking reservoir dynamics across a complete caldera cycle at Rabaul, Papua New Guinea

<p>Caldera-forming eruptions are some of the most devastating events on Earth; however, the volcanoes that produce these eruptions frequently have much more minor activity. Knowing if a restless caldera is currently primed for a large eruption, therefore, has important implications for hazard assessment and risk management. Many calderas, including Rabaul in Papua New Guinea, show cycles of activity with multiple caldera-forming eruptions interspersed with more minor activity. We present data that spans an entire cycle, from one caldera-forming eruption to the next and estimate the storage conditions for each eruption. The last complete caldera cycle of Rabaul started at ~10.5 ka, with the eruption of the dacitic Vunabugbug Ignimbrite. Following the Vunabugbug, little volcanic activity was preserved until ~4.4 ka, suggesting either a period quiescence or destruction and burial during the subsequent caldera-forming eruptions of the region. From 4.4 ka, there is an increase in the volume and SiO<sub>2</sub> contents of volcanic deposits that are preserved, which culminated in the eruption of the dacitic Memorial Ignimbrite at ~4.1 ka. The Memorial Ignimbrite was smaller than the Vunabugbug Ignimbrite and Rabaul Pyroclastics and may not have formed a caldera; however, it does appear to have altered the plumbing system and allowed deeper, hotter basalts to reach the surface. Following the eruption of these basalts, the system gradually evolves towards more silicic magmas, until the eruption of the dacitic Rabaul Pyroclastics at ~1.4 ka. After the Rabaul Pyroclastics hotter, more mafic magmas can again reach the surface, both as more mafic lava flows and as hybrid andesites that contain crystal cargos transported from deeper in the system.</p><p>Two-pyroxene, clinopyroxene–liquid and plagioclase–liquid thermobarometers suggest that the dacites, including those erupted during the caldera-forming eruptions, were stored at pressures of ~1 kbar (~4 km depth) and at temperatures of ~930 °C. There is a tight relationship between the temperature and the SiO<sub>2</sub> content of the magmas, with the basalts erupted after the large ignimbrites recording temperatures of up to 1100 °C. Some of the more mafic magmas also record deeper storage, at pressures of 3–4 kbar (11–15 km). Plagioclase–liquid pairs suggest melt H<sub>2</sub>O contents of ~2.8 wt.% for the dacites, although some of the more mafic magmas have slightly higher melt H<sub>2</sub>O contents (3.2–4.0 wt.%)—this may be because the basalts were saturated and stored at greater pressures. Magnetite–liquid pairs record relatively constant oxygen fugacities of ~1.2 log units above the FMQ buffer.</p><p>At Rabaul it would take on the order of a few millennia to differentiate or accumulate enough dacitic magma to produce a large explosive eruption. The eruption of highly evolved, crystal-poor, cold, hydrous magmas geochemically similar to those erupted prior to the Memorial Ignimbrite and Rabaul Pyroclastics may provide a warning of an impending large explosive eruption.</p>

Syndepositional hydrothermalism selectively preserves records of one of the earliest benthic ecosystems, Moodies Group (3.22 Ga), Barberton Greenstone Belt, South Africa

The ~3.22 Ga Moodies Group, Barberton Greenstone Belt (BGB), South Africa, provides a unique window into Archaean sedimentary, magmatic and ecological processes. In the central BGB, a regional mafic complex, consisting of a genetically related major mafic sill, a peperitic dyke stockwork, and extensive basaltic lava flows affected thick quartzose sandstones of the Moodies Group. We argue that epithermal hydrothermalism associated with this magmatic event occurred, at least in part, syndepositionally and in places destroyed, in other places preserved the abundant benthic microbial mats in terrestrial- and coastal-facies sandstone of this unit. We differentiate four principal types of hydrothermal alteration: (1) Sericitization resulted from ubiquitous feldspar breakdown; (2) iron-oxide alteration replaced the original matrix by fine-grained iron oxide; (3) silicification replaced matrix and most non-silica grains by microcrystalline silica and locally preserved kerogenous microbial mats; and (4) hydraulic fracturing at shallow depth brecciated consolidated Moodies Group sandstone and created closely spaced, randomly oriented fractures and quartz-filled veins. Because stockwork intrusion locally interacted with unconsolidated water-saturated sediment and because the dykes connect the sill with the mafic lava but also follow zones of structural weakness, we suggest that hydrothermalism associated with this magmatic event occurred syndepositionally but was also – within the resolution of radiometric age data – contemporaneous with tight regional folding. We conclude that microbial organisms in Paleoarchaean coastal (tidal, estuarine) environments may have been formerly widespread, possibly even abundant, but are nearly nowhere preserved because they were easily degradable. Preservation of Early Archaean microbial mats in a thermal aureole in the central BGB was controlled by the “just right” degree of heating and very early hydrothermal silicification.

Petrogenesis and economic potential of the Obatogamau Formation, Chibougamau area, Abitibi greenstone belt

Unravelling the petrogenesis and stratigraphy of Archean mafic lava flows is essential to our comprehension of the early geodynamic evolution and economic potential of greenstone belts. This study focuses on one of the oldest and thickest sequences of lava flows observed in the Neoarchean Abitibi Subprovince (greenstone belt), i.e., the Obatogamau Formation. The undated formation extends for more than 100 km in an E-W direction and consists mostly of aphyric and feldspar megacryst-bearing basaltic-andesite lava flows. These lava flows are tholeiitic, mostly Fe-rich and have nearly homogeneous chemistry. Petrogenetic modelling carried out using MELTS software points to limited magmatic differentiation as most samples of mafic lava flows did not reach Fe-Ti-oxide saturation. Zircon U-Pb dating establishes a crystallization age of 2726.2 ± 1.6 Ma for a felsic unit located at an intermediate stratigraphic position in the sequence of lava flows. Constraints from stratigraphically overlying volcanic units suggest that the Obatogamau Formation was likely emplaced rapidly, possibly within a few million years and as a consequence of frequent replenishment of shallow magma accumulations. High eruption rates are consistent with short episodes of volcanic quiescence deduced from field observations, indicating non-optimal conditions for volcanogenic massive sulfide systems. The pressure and temperature of peak metamorphism deduced from amphibole chemistry, however, points to favorable conditions for the release of metamorphic fluids. The study area may thus be prospective for orogenic gold mineralization, provided that fluids had access to a source of gold and that structural conduits allowed for the channeling of hydrothermal fluids.

Evidence of active magmatic rifting in Ma’alalta marginal volcano (Afar, Ethiopia)

<p>Growth of rift segments and development of crustal magmatic systems in continental rifts remain debated issues. We integrate volcanological, geochemical, petrological and seismic data from the Ma’alalta stratovolcano near the western rift margin of Afar to show that active magmatic rifting occurs there. Growth of Ma’alalta started around 0.55 ± 0.05 Ma (Barberi et al. 1972) with the age of the youngest flows unknown. Ma’alalta produced lava flows but also large-volume, caldera-forming ignimbrites, as well as silicic intracaldera domes. The products are mainly trachytic and some are slightly peralkaline. The most recent magmatic activity of Ma’alalta consists of mafic lava fields, scoria cones and peralkaline obsidianaceous silicic domes produced along the ~40 km long magmatic segment and erupted from several vents aligned NNW-SSE rather than from central volcanic activity. Local seismicity (2005-2009 and 2011-2013) also shows a NNW-SSE-trending alignment of earthquakes with good correlation to where the recent magmatic products were erupted. The geochemical features of the mafic rocks (e.g., Ba/La, Rb/Ta and Zr/Ta) as well as the petrogenesis of the recent NNW-SSE-trending silicic domes are similar to the nearby on-rift Dabbahu and Durrie volcanoes. Inferred magma storage depth from mineral geobarometry show that a shallow, silicic chamber existed at ~4-5 km depth below the stratovolcano, while a stacked plumbing system with at least two magma storage levels at ~14 and ~24 km of depth fed the recent basalts. We interpret the wide set of observations from Ma’alalta as evidences that the area is an active rift segment, showing that localised axial extension can be heavily offset towards the rift margin.</p><p> </p><p> </p>

High intensity ignimbritic activity in the Central sector of the Main Ethiopian Rift

<p>The volcano-tectonic evolution of the Main Ethiopian Rift (MER) is punctuated with periods of intense silicic volcanism, characterized by large explosive caldera-forming eruptions and the production of several ignimbrite deposits. These volcanic paroxysms require large volume of evolved silicic magma accumulated in shallow chambers into the continental crust; however, the relations between magmatism and tectonics during rifting, and the influence of the distribution and timing of regional tectonics on the ascent of magma and its stalling in large magmatic reservoirs remain poorly defined.</p><p>We present new geochronological data (<sup>40</sup>Ar/<sup>39</sup>Ar dataset of 29 samples) providing new constraints on the timing, evolution and characteristics of volcanism in the Central sector of the MER, where large ignimbrite deposits and remnants of several calderas testify the recurrence of silicic flare-ups. Specifically, we investigate in detail the eastern margin of the rift, where a voluminous, widespread, crystal-rich ignimbrite (Munesa Crystal Tuff, MCT) has been described. This deposit has been correlated to a thick ignimbrite occurring at the bottom of geothermal wells in the rift, pointing to a giant eruptive event attributed to a huge caldera structure, presumably buried beneath the rift floor. At least other two widespread ignimbrite units are present along the same margin for several tens of kilometres, testifying the high volcanicity of this sector of the MER.</p><p>Our survey and analyses suggest that, at least in the eastern margin of the rift, activity was clustered in periods of large magma production and emission, resulting in the recurrence of intense volcanic phases interspersed with periods of rest of volcanism. Ignimbrites and other volcanic deposits occur in the investigated area, spanning an age interval from 3.5 Ma to as recent as 150 ka. The MCT emission, around 3.5 Ma, was followed, after a long quiescence, by an important phase with the emplacement of both mafic (lava flows and scoria cone) and evolved (ignimbrites) products between 1.9-1.6 Ma. After that, volcanism occurred more frequently, possibly with a lower amount of erupted magma and still alternating with quiescent periods, with volcanism clusters at ~ 1.3-1.2 Ma, ~ 0.8-0.7 Ma and ~ 0.3-0.2 Ma. This clustered volcanic activity will be compared with the episodic rifting of this sector of the Main Ethiopian Rift.</p>