Distal volcano-tectonic (dVT) seismicity typically precedes eruption at long-dormant volcanoes by days to years. Precursory dVT seismicity may reflect magma-induced fluid-pressure pulses that intersect critically stressed faults. We explored this hypothesis using an open-source magmatic-hydrothermal code that simulates multiphase fluid and heat transport over the temperature range 0 to 1200 °C. We calculated fluid-pressure changes caused by a small (0.04 km3) intrusion and explored the effects of flow geometry (channelized vs. radial flow), magma devolatilization rates (0–15 kg/s), and intrusion depths (5 and 7.5 km, above and below the brittle-ductile transition). Magma and host-rock permeabilities were key controlling parameters and we tested a wide range of permeability (k) and permeability anisotropies (kh/kv), including k constant, k(z), k(T), and k(z, T, P) distributions, examining a total of ~ 1600 realizations to explore the relevant parameter space. Propagation of potentially causal pressure changes (ΔP ≥ 0.1 bars) to the mean dVT location (6 km lateral distance, 6 km depth) was favored by channelized fluid flow, high devolatilization rates, and permeabilities similar to those found in geothermal reservoirs (k ~ 10− 16 to 10− 13 m2). For channelized flow, magma-induced thermal pressurization alone can generate cases of ∆ P ≥ 0.1 bars for all permeabilities in the range 10− 16 to 10− 13 m2, whereas in radial flow regimes thermal pressurization causes ∆ P < 0.1 bars for all permeabilities. Changes in distal fluid pressure occurred before proximal pressure changes given modest anisotropies (kh/kv ~ 10–100). Invoking k(z,T,P) and high, sustained devolatilization rates caused large dynamic fluctuations in k and P in the near-magma environment but had little effect on pressure changes at the distal dVT location. Intrusion below the brittle-ductile transition damps but does not prevent pressure transmission to the dVT site.
[1] The data and analysis presented in this paper provide an assessment of lava morphologies and the geochemistry of lavas from the Oman ophiolite. In order to provide detailed constraints on the construction of the upper oceanic crust, a continuous volcanic transect (300 m-thick) was sampled at high-frequency in the Semail ophiolite along Wadi Shaffan. The Wadi Shaffan section is composed mainly of pillow lavas interbedded with massive flows and occasional hyaloclastites. The sampling performed along Wadi Shaffan implies temporal variations in the activity of the ridge. The section is characterized by chemical compositions consistent with those of V1-Geotimes volcanism. The Wadi Shaffan transect was built through two main petrological and geochemical sequences of volcanic activity. Trace element ratios (e.g. Zr/Nb and La/Yb) allow us to distinguish two main sequences with two different parental magmas. Differences in the degree of partial melting are required to explain these trace element ratio variations. Beyond these differences in parent melt composition, variations in trace element abundances (TiO2, Zr, REE) involve differentiation processes prior to emplacement. In the lower sequence, less differentiated lavas are in the upper part of the cycle. Magma mixing is proposed to explain this reversed geochemical evolution through time. In the upper sequence, geochemical analysis suggests a different magma chamber process. This sequence consists of multiple events of magma emplacement. Variations in trace element abundance suggest four magmatic cycles. Each magmatic cycle is characterized by primitive lavas evolving to more differentiated lavas with time. The upper sequence lavas appear to be in equilibrium with clinopyroxene and lower sills from the MTZ (Mantle-Crust Transition Zone) and with lower gabbros. We propose a model in which the upper sequence lavas were directly derived from the MTZ and lower gabbro sills and then transported to the surface without interaction with higher crustal levels. G Geochemistry Geophysics Geosystems
In the West Shasta copper-zinc district, Kuroko-type massive sulfide deposits are present in Early Devonian volcanic rocks consisting of the Copley Greenstone and the overlying Balaklala Rhyolite. The Copley Greenstone is composed of pillow lava (mainly basalts with subordinate andesites) and pyroclastic deposits. High Mg andesites occur near the top of the 1,800-m-thick volcanic succession. The Balaklala Rhyolite, 1,000 m thick, consists of highly siliceous rhyolite flows, conglomerates, and tuffs. Both units were deposited in a submarine environment. The sulfide deposits are localized in the rhyolitic flows of the upper part of the middle unit of the Balaklala Rhyolite.Petrological and geochemical data show that these lavas belong to a tholeiitic suite formed in an immature island arc. At the close of Copley eruptions, the island arc underwent rifting and tectonic extension. This is suggested by the presence of boninites, a feeder dike system for the Balaklala Rhyolite, and bimodal volcanism. The extensional environment is similar to that classically observed in the domain of the Kuroko massive sulfide deposits. This rifting led subsequently to the development of a back-arc basin represented by the Trinity basalts associated with an ophiolitic sequence.
The Benue Trough is a continental-scale intraplate tectonic megastructure which is part of Mid-African Rift System. This rift, initiated in the latest Jurassic, was related to the opening of the Central and South Atlantic oceans. Mesozoic to early Cenozoic magmatism accompanied this evolution. Two principal magmatic domains are evident, the Northern and Southern Benue. In the northern domain, magmatism is characterized by transitional alkaline basalts and transitional tholeiitic basalts. Acidic magmatism of peralkaline nature is also present. In the Southern Benue, several magmatic districts exhibit alkaline or tholeiitic affinities. A detailed chronology of emplacement of this volcanism has been established using the 40 Ar/ 39 Ar radiometric method which lead to recognition of three periods of magmatic activity: (1) 147–106 Ma, well expressed in the Northern Benue represefited by transitional basaltic types; (2) 97–81 Ma, occurring only in the Southern Benue, represented exclusively by alkaline intrusive rocks; (3) 68–49 Ma, restricted also to the Southern Benue, with alkaline intrusions followed by tholeiitic subvolcanic rocks. In the light of the general geodynamic evolution, a scenario is proposed, supported by the three chronological periods. The late Jurassic to Albian magmatism occurred when the Equatorial Atlantic was still closed, contemporaneous with the NE Brazilian magmatism. Both magmatism could represent the forerunners of opening of the Equatorial Atlantic. This activity occurred under a general extensional regime during which crustal strike-slip faults gave rise to the emplacement of transitional alkaline basalts. Transitional tholeiitic basalts erupted along normal faults. The second period of activity, Cenomanian to Santonian, restricted to the Southern Benue, occurred after the opening of Atlantic Ocean during a period of decreasing extension. This period ended with a Santonian compressional episode. The last period of activity, from late Maastrichtian to Eocene, is characterized by subsidence, generated as an isostatic response to the Early Cretaceous crustal thinning and post-rift thermal relaxation of the lithosphere, expressed by Tertiary E–W extension.
In calc-alkaline orogenic plutons, the dark xenoliths and their host rocks must be considered the expression of partial mixing of magma.Three associations of this type have been investigated and are illustrated by the Bono pluton (northern Sardinia)— a composite pluton including three intrusives of different nature (tonalitic to granodioritic) and containing a very large number of basaltic xenoliths of magmatic origin. Detailed mineralogical analysis of the two end members in each association, coupled with geochemical data, has determined the major petrogenetic mechanisms intervening in the mixing process in a plutonic setting: temperature equilibration, mechanical exchanges of crystals, chemical exchanges, etc. The most important result of this article, however, is to show that each intrusion is related to a specific group of xenoliths that is characterized by constant FeO t /MgO. The latter reflects the different composition of basaltic components, and it is concluded that each intrusive event is associated with a unique mixing episode. As in volcanic settings, the mixing process may have initiated the intrusion.The extreme compositional variations in the magmatic xenoliths, recognized in several series of orogenic plutons, is explained here by different initial basaltic end members and by variation in the intensity of the interaction mechanisms. [Journal Translation]
Summary This study explored the assimilation of Time-Domain Electromagnetic (TDEM) and Electrical Resistivity Tomography (ERT) surveys in a sharp-interface model of seawater intrusion. While these models, which do no simulate mixing between freshwater and saltwater, are computationally-efficient alternatives to advective-dispersive models, guidelines for data assimilation are limited. In the Magdalen Islands (Québec, Canada), a sharp-interface seawater intrusion model was developed to support groundwater management. In addition to freshwater head observations, saltwater interface observations were defined as the elevation of the 50% seawater salinity contour and were extracted from deep, open boreholes, TDEM and ERT surveys. Parameter estimation and linear uncertainty analysis were carried out, accompanied by a data worth analysis. The contribution of the TDEM and ERT saltwater interface observations was then explored, both in constraining parameter estimation and in reducing the uncertainty of model forecasts of interest to groundwater management. While they were highly uncertain compared to all well observations, the geophysically-derived interface observations were the most useful observations to reduce predictive uncertainty.
Neogene volcanic rocks from the northwestern coast of Algeria include calc-alkaline to shoshonitic andesites and dacites (Sahel of Oran and M`Sirda areas) and alkali basalts (Tafna valley). Seventeen new {sup 40}K-{sup 40}Ar ages indicate that these volcanics were emplaced during two distinct periods, from 11.7 to 7.2 Ma and ca. 4 Ma, respectively. All the andesites and dacites were emplaced during the first period, and their trace element characteristics are typical of subduction -and/or collision- related magmas. They are enriched in large-ion lithophile elements and display negative Nb anomalies. The associated alkali basalts are chemically heterogeneous, and contain variable amounts of TiO{sub 2} and P{sub 2}O{sub 5}. Their multielement plots show either positive or slightly negative Nb anomalies. These features suggest that the Neogene volcanics studied derive from two different types of mantle sources, i.e. an enriched sub-continental mantle and an upper mantle carrying a subduction-related geochemical imprint. The latter component may have inherited its specific signature from an earlier subduction event. This ``orogenic`` imprint tends to disappear through time as the contribution of the sub-continental mantle increases. (authors). 12 refs., 4 figs., 2 tabs.
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