Poor Magma Research Articles

Stress dynamics associated with the Nyasa / Malawi rift: Implication for the present-day East African Rift System dynamics

The Nyasa/Malawi rift (NMR), known for its poor magma and notable seismic activity, has sparked a debate regarding its stress kinematics. It is on one hand viewed as a transform fault, while on other hand as a rift structure characterized by normal faulting. In order to address this controversy, we conducted paleostress analysis that involved collecting fault slip data along the central to southern region of the rift. We integrated our findings with published kinematic data on focal mechanisms in the rift. Our results reveal that the central part of the rift experiences radial or sub-radial extension, while the southern half is subject to oblique NNE-SSW transtensive tectonic forces. The minimum horizontal principal stress axis aligns with an orientation of 020°. As we move further south, the extension direction changes by approximately 25°, resulting in a predominantly north-south opening with a minimum horizontal stress axis direction of 175° (Shmin = 175°). The degree of structural penetration and intensity of faulting indicate that the north-south opening is more significant and pronounced in the southern region compared to the northern region. Additionally, we observed that faults dipping to the east and trending NW-SE exhibit sinistral (left-lateral) movement, while faults dipping to the southwestern side display dextral (right-lateral) movement. This suggests that, regionally, the NMR primarily experiences a normal faulting regime, albeit with a significant strike-slip component, which accounts for the oblique kinematics observed. The tectonic regimes identified through our fault slip data encompass the crust and upper mantle, spanning a lithospheric scale.

First Report on Platinum Occurrence in Ultramafic-Mafic Complex, East-Central Karbi Hills of Assam, North East India

The results of SEM-EDS and XRD, indicating the presence of PGE in stratified vanadium bearing magnetite-ilmenite ore, hosted by layered ultramafic-mafic complex in a part of east-central Karbi Anglong district, Assam (NE India) are reported here. The initial XRD data indicate presence of PGE minerals like braggite, laurite and cooperite; the SEM-EDS indicate variable wt. % of PGE and rare metals like 11.58% Re and gold. The highest weight % value is 19.55 Pt; 13.83 Ir; 7.36 Os; 0.95 Pd and 0.45 Ru. Plume model has been cited as the key factor causing high partial melting of the upper mantle in back arc rift environment, producing platinum rich but sulphur poor magma, subsequently acquiring sulphur from pelites of Shillong Group, thereby, gaining PGE.

Moho Structure of the Southwest Sub-Basin, South China Sea, from a Multichannel Seismic Reflection Profile NH973-1

Moho structure provides important clues for understanding crustal structure, isostatic state and magmatic flux from mantle to surface. Across-basin Moho structure of the South China Sea (SCS) is important for understanding crustal evolution mechanisms of both continental break-up and seafloor spreading processes. Southwest Sub-basin (SWSB) opened up the latest and has the closest continental margins, making it the best to study the across-basin structure. Multichannel seismic (MCS) reflection data of line NH973-1 that crosses SWSB in NW-SE direction were reprocessed in order to image Moho structure. In MCS data, Moho reflectors are observed in places, which were not revealed in prior researches. The Moho generally shows symmetric structure on both sides of the central rift valley (CRV) and with variations in crustal thickness. Around CRV, the Moho is 2 seconds depth in two-way travel time (TWTT) beneath the igneous basement, which corresponds to 7 km depth, indicating normal oceanic crustal accretion during the ending of seafloor spreading. Close to the continent-ocean boundary (COB), the Moho becomes shallow to 1 second depth in TWTT (3.5 km), implying strong crustal thinning towards the continent, probably because of poor magma supply at the beginning of seafloor spreading. At south COB, the Moho depth under the crust almost reaches zero, which could be explained as a result of exhumed mantle. In addition, two low-angle, deep-penetrating normal faults are observed at south COB. The faults cut across the Moho into the upper mantle, which may be attributed to lithospheric hyper-stretching at COB during continental break-up.

3D gravity inversion and thermodynamic modelling reveal properties of shallow silicic magma reservoir beneath Laguna del Maule, Chile

Active, large volume, silicic magma systems are potentially the most hazardous form of volcanism on Earth. Knowledge of the location, size, and physical properties of silicic magma reservoirs, is therefore important for providing context in which to accurately interpret monitoring data and make informed hazard assessments. Accordingly, we present the first geophysical image of the Laguna del Maule volcanic field magmatic system, using a novel 3D inversion of gravity data constrained by thermodynamic modelling. The joint analysis of gravity and thermodynamic data allows for a rich interpretation of the magma system, and highlights the importance of considering the full thermodynamic effects on melt density, when interpreting gravity models of active magmatic systems. We image a 30 km3, low density, volatile rich magma reservoir, at around 2 km depth, containing at least 85% melt, hosted within a broader 115 km3 body interpreted as wholly or partially crystallised (>70% crystal) cumulate mush. Our model suggests a magmatic system with shallow, crystal poor magma, overlying deeper, crystal rich magma. Even though a large density contrast (−600 kg/m3) with the surrounding crust exists, the lithostatic load is 50% greater than the magma buoyancy force, suggesting buoyancy alone is insufficient to trigger an eruption. The reservoir is adjacent to the inferred extension of the Troncoso fault and overlies the location of an intruding sill, driving present day deformation. The reservoir is in close proximity to the 2.0 km3 Nieblas (rln) eruption at 2–3 ka, which we calculate tapped approximately 7% of the magma reservoir. However, we suggest that the present day magma system is not large enough to have fed all post-glacial eruptions, and that the location, or size of the system may have migrated or varied over time, with each eruption tapping only a small aliquot of the available magma. The presence of a shallow reservoir of volatile rich, near liquidus magma, in close proximity to regional scale faulting, has important implications for volcano monitoring and hazard mitigation.

Thermo-mechanical numerical model of the transition from continental rifting to oceanic spreading: the case study of the Alpine Tethys

AbstractWe develop a two-dimensional thermo-mechanical numerical model in which the formation of oceanic crust and serpentinite due to the hydration of the uprising mantle peridotite has been implemented, with the aim of discussing the behaviour of the lithosphere of the Alps and Northern Apennines during the transition from continental rifting to ocean spreading of the Alpine Tethys. The predictions of the model are compared with natural data related to the Permian–Triassic high-temperature – low-pressure (HT-LP) metamorphism affecting the continental lithosphere and data from the JurassicP–Tevolution of the oceanic lithosphere from the Alps and the Northern Apennines. Our analysis indicates that a thinned continental crust, an ocean–continent transition zone and an oceanic lithosphere characterize the final structure of the system in a poor magma rift pre-Alpine configuration. We also find that mantle serpentinization starts before crustal break-up and that denudation occurs before ocean spreading. The mantle denudation starts several million years before the gabbros/basalt formation, generating an ocean–continent transition zone from the passive continental margin to the oceanic lithosphere of size 160–280 km. The comparative analysis shows that the extension of a hot and weak lithosphere, which promotes the development of hyperextended Alpine margins, better agrees with the natural data. Finally, our comparative analysis supports the hypothesis that the lithospheric extension preceding the opening of the Alpine Tethys did not start in a stable continental lithosphere, but developed by recycling part of the old Variscan collisional suture.

Patterns of magma flow in segmented silicic dikes at Summer Coon volcano, Colorado: AMS and thin section analysis

A complex pattern of magma flow is found in two silicic dikes of a radial swarm at Summer Coon, an eroded stratovolcano in southern Colorado. The two intrusions are broken into multiple segments that suggest vertical dike propagation. However, anisotropy of magnetic susceptibility (AMS) measurements and thin section observations suggest that magma flow was often subhorizontal and away from the center of the volcano. Segments that are proximal to the central intrusion are characterized by magma that flowed steeply upward at the proximal segment extremity, then laterally along the segment, and finally downward at the distal end of the segment. Magma flow in offset segment tips located far from the volcano center was subhorizontal towards the adjacent segment, implying lateral propagation of segment tips towards one another. This observation suggests relatively high driving pressure in distal dike segments, as supported by dike thickening with radial distance from the center of the volcano. The present study indicates that radial dike evolution at stratovolcanoes is dominated by lateral flow of magma and dike segmentation is a poor magma flow indicator. A horizontally propagating radial dike has the potential to cause an eruption low on the flank of a composite cone, which poses a significant yet largely unrecognized hazard to population centers and infrastructure that may surround the volcano.

Constraints on Moho depth and crustal thickness in the Liguro-Provençal basin from a 3D gravity inversion: geodynamic implications

Abstract 3D gravity modelling is combined with seismic refraction and reflection data to constrain a new Moho depth map in the Liguro-Provençal basin (Western Mediterranean Sea). At seismically controlled points, the misfit between the gravimetric solution and the seismic data is about 2 km for a range of Moho depths between 12 km (deep basin) and 30 km (mainland). The oceanic crust thickness in the deep basin (5 km) is smaller than the average oceanic crust thickness reported in open oceans (7 km), pointing to a potential mantle temperature 30–50°C below normal and/or very slow oceanic spreading rate. Oceanic crust thickness is decreasing towards the Ligurian Sea and towards the continent-ocean boundary to values as small as 2 km. Poor magma supply is a result of low potential mantle temperature at depth, lateral thermal conduction towards an unextended continental margin, and decrease of the oceanic spreading rate close to the pole of opening in the Ligurian Sea. Re-examination of magnetic data (palaeomagnetic data and magnetic lineations) indicates that opening of the Liguro-Provençal Basin may have ceased as late as Late Burdigalian time (16.5 Ma) or even later. The absence of a significant time gap between cessation of opening in the Liguro-Provençal Basin and rifting of the Tyrrhenian domain favours a continuous extension mechanism since late Oligocene time driven by the African trench retreat.

Constraints on Moho Depth and Crustal Thickness in the Liguro-Provençal Basin from a 3d Gravity Inversion : Geodynamic Implications

3D gravity modelling is combined with seismic refraction and reflection data to constrain a new Moho depth map in the Liguro-Provencal Basin (Western Mediterranean Sea). At seismically controlled points, the misfit between the gravimetric solution and the seismic data is about 2 km for a range of Moho depth between 12 km (deep basin) and 30 km (mainlands). The oceanic crust thickness in the deep basin (5 km) is smaller than the average oceanic crust thickness reported in open oceans (7 km), pointing to a potential mantle temperature 30°C to 50°C below normal and/or very slow oceanic spreading rate. Oceanic crust thickness is decreasing towards the Ligurian Sea and towards the continent-ocean boundary to values as small as 2 km. Poor magma supply is a result of low potential mantle temperature at depth, lateral thermal conduction towards unextended continental margin, and decrease of the oceanic spreading rate close to the pole of opening in the Ligurian Sea. Re-examination of magnetic data (paleomagnetic data and magnetic lineations) indicates that opening of the Liguro-Provencal Basin may have ceased as late as Late Burdigalian (16. 5 Ma) or even later. The absence of significant time gap between cessation of opening in the Liguro-Provencal Basin and rifting of the Tyrrhenian domain favours a continuous extension mechanism since Upper Oligocene driven by the African trench retreat.

Mineralogy of the Orikabe plutonic complex, Kitakami Mountains, Northeast Japan.

The Orikabe plutonic complex in the Kitakami Mountains is divided into Main and North plutons. The Main pluton is composed of three rock types; i.e., the Tokusenjo type (gabbro), Orikabe type (monzodiorite-monzogranite) and Sasamori type (granodiorite). The oxidation states of magmas around solidus are estimated from coexisting Fe-Ti oxides as follows; near the FMQ and NNO buffers in the Tokusenjo type, in the range from the FMQ to HM buffer in the Orikabe type, and intermediate between the NNO and HM buffer in the Sasamori type. The total pressure for the Orikabe and Sasamori type magmas at their solidus temperatures is estimated to be about 2-3 kb by hornblende geobarometer. Hornblende of the Sasamori type has cores rich in A1IV and Ti, whereas that of the Orikabe type is generally poor in them. It indicates that hornblende in the Orikabe type began to crystallize at lower temperature than that in the Sasamori type, which is concordant with microscopic observation of the Orikabe type rocks, suggesting crystallization of hornblende simultaneous with or later than biotite. The delay of hornblende appearance in the Orikabe type is due to lower water content of the magma than that of the Sasamori type. In gabbros of the Tokusenjo type, the coexistence of Fe-rich olivine and Ca-poor plagioclase and scarcity of hornblende are characteristic, indicating a H2O poor magma (less than 3 wt. %) of the Tokusenjo type. Chemical composition of the minerals in the Tokusenjo type rocks is continuous to that of the Orikabe type with increase in SiO2 of host rocks, suggesting continuous differentiation of magmas of the both types.

Segmentation and a nontransform ridge offset on the Reykjanes Ridge near 58°N

Multibeam bathymetry, deep‐towed side scan sonar, gravity, and magnetic data over a 50 km square area all suggest the presence of a second‐order, nontransform offset on the obliquely spreading Reykjanes Ridge (North Atlantic ocean) near 58°N latitude. It is the first such offset to be recognized on the Reykjanes Ridge. This region is characterized by a shallow median valley containing en echelon axial volcanic ridges (AVRs) similar to those found on more northerly parts of the Reykjanes Ridge. The side scan sonar shows that all the AVRs are constructional in nature. The one immediately south of the offset basin backscatters strongly, is unmarked by faulting, and so appears extremely young. Other AVRs appear older, having lower backscatter and, off axis, being cut by faults and fissures. At 57°55′N the progression of AVRs is interrupted by a 600‐m‐deep, 20 km × 10 km basin. Residual mantle Bouguer anomalies, corrected for two‐dimensional lithospheric cooling, display a high of at least 8 mGal over the basin. Two small, off axis basins occur roughly along the flow line from this basin and are also characterized by gravity highs. The basins are interpreted as regions of crustal thinning and are believed to represent the discontinuous trace of the ridge offset, which has thus been in existence for at least 2 m.y. and has been slowly propagating south. Inversion of the magnetic field shows a very low magnetization (∼3–4 A m−1) associated with the offset basin, which is interpreted as indicating a reduced magnetic layer thickness due to poor magma supply to the offset area. The Brunhes‐Matuyama reversal boundary and Anomaly 2 traces are offset about 3 km dextrally across the proposed offset trace. The AVR immediately north of the offset displays high amplitudes of magnetization which steadily increase southward toward its tip adjacent to the offset, suggesting the presence of increasingly highly fractionated basalts toward the AVR tip. Polarity transition widths across the Brunhes‐Matuyama boundary are comparable with those on other slow spreading ridges but are wider than individual AVRs, probably because AVRs overlap in the crustal accretion zone. Transition widths are also asymmetrical across the spreading center, being narrower (∼4.5 km) to the east than the west (∼8 km), suggesting a certain degree of asymmetry in the accretionary and rifting processes at the ridge axis.

Thin crust at the western Iberia Ocean‐Continent transition and ophiolites

Western Iberia is bounded by a nonvolcanic rifted continental margin made up of three apparently independent segments. The age of breakup decreases from south to north. Seismic refraction and reflection profiles, and magnetic and gravity data from each segment, show a consistent pattern of geophysical observations across the ocean‐continent transition (OCT) zone, which is a few tens of kilometers wide. We emphasize here the discovery of thin (2–4 km) oceanic crust underlain by 7.6 km s−1 material within the OCT. The available evidence favors the suggestion that the 7.6 km s−1 layer is serpentinized peridotite and that the thin oceanic crust is primarily the result of a poor magma supply for a few million years immediately after continental breakup. This thin crust may be the source of some ophiolites which exhibit thin crustal sections and continental margin affinities.

A model for the Ni-Cu-PGE ores of the Noril'sk region and its application to other areas of flood basalt

Five principal magma types characterize the 3.8-km vertical thickness of flood basalt of the Noril9sk region. From base to top these are (1) a high TiO 2 type with alkalic affiliations (Ivakinsky and Syverminsky suites), (2) a TiO 2 -rich picrite-bearing magma (Gudchichinsky suite), (3) a picrite-bearing TiO 2 -poor magma (Tuklonsky suite), (4) a crustally contaminated TiO 2 -poor magma (Lower Nadezhdinsky suite), and (5) a picrite-free magma (Mokulaevsky, Kharayelakhsky, Kumginsky, and Samoyedsky suites) similar but not identical in its chemistry to the Tuklonsky and separated from the Lower Nadezhdinsky by transitional suites (Upper Nadezhdinsky and Morongovsky).The Ni-Cu ores are associated with olivine-rich intrusions that are geochemically similar to the Mokulaevsky magma. They are associated with about 1/15 to 1/10 of their mass in PGE-rich sulfides; this requires that these sulfides equilibrated with 15 to 200 times more magma than is represented by the intrusions themselves.Following the model of Naldrett et al. (1992), it is proposed that crustal contamination of the original Tuklonsky-type magma occurred at the top of a vertically extensive, fault-controlled magma chamber and caused segregation of immiscible sulfides with a low R factor. These depleted this magma in chalcophile elements and settled deeper in the chamber. As they settled, they reacted with less contaminated magma, scavenging additional chalcophile elements. They finally came to rest near the base of the chamber to form a zone of sulfide- (and thus chalcophile element-) enriched magma. Eruption, and in some instances intrusion, of progressively stratigraphically lower, less contaminated and thus less depleted layers in the chamber gave rise to the observed sequence of basalts and intrusions. The answer to the strong enrichment in Ni, Cu, and PGE associated with the ore-bearing intrusions is that these metals came from the magma that is now represented by the basalt magma with which the sulfides have come into contact and which shows chalcophile element depletion.Key metallogenic characteristics of the Noril9sk region are (1) a hot region in the mantle giving rise to vast amounts of basalt, (2) continental-scale rifting, (3) major faults serving as controls for extrusive and intrusive magmatism, (4) crustal contamination of magma, (5) chalcophile element depletion of contaminated magma, (6) intrusion of magma that has inherited sulfide removed from the depleted magma, and (7) the presence of evaporites in the sedimentary sequence underlying the basalts.The simplest way to recognize chalcophile element depletion is to plot Ni/MgO vs. Mg number [= at. % Mg/(Mg + 0.9Fe total )]. A strong indication that basalts have undergone crustal contamination is the correlation of high La/Sm ratios with high SiO 2 contents.The Lake Superior region shows many of the key metallogenetic characteristics of the Noril9sk region, including continental rifting, a triple junction, huge thicknesses of basalt, and Cu-Ni sulfide-bearing intrusions (Duluth Complex, Crystal Lake gabbro). Some of the Keweenawan basalts of the region show Ni depletion correlated with chalcophile element depletion. The region is an example of the potential application of the Noril9sk model to exploration.

Phase equilibria and melt productivity in the pelitic system: implications for the origin of peraluminous granitoids and aluminous granulites

Peraluminous granitoid magmas are a characteristic product of ultrametamorphism leading to anatexis of aluminous metasedimentary rocks in the continental crust. The mechanisms and characteristic length-scales over which these magmas can be mobilized depend strongly on their melt fraction, because of their high viscosities. Thus, it is of fundamental importance to understand the controls exerted by pressure, temperature and bulk composition of the source material on melt productivity. We have studied experimentally the vapour-absent melting behaviour of a natural metapelitic rock and our results differ greatly from those of previous experimental and theoretical investigations of melt productivity from metamorphic rocks. Under H2O-undersaturated conditions, bulk composition of the source material is the overriding factor controlling melt fraction at temperatures on the order of 850–900° C. Granitoid melts formed in this temperature interval by the peritectic dehydration-melting reaction: $$\begin{gathered} Biotite + plagioclase + aluminosilicate + quartz \hfill \\ = melt + garnet \hfill \\ \end{gathered} $$ have a restricted compositional range. As a consequence, melt fractions will be maximized from protoliths whose modes coincide with the stoichiometry of the melting reaction. This “optimum mode” (approximately 38% biotite, 32% quartz, 22% plagioclase and 8% aluminosilicate) reflects the fact that generation of low-temperature granitoid liquids requires both fusible quartzo-feldspathic components and H2O (from hydrous minerals). Metapelitic rocks rich in mica and aluminosilicate and poor in plagioclase contain an excess of refractory material (Al2O3, FeO, MgO) with low solubility in low-temperature silicic melts, and will therefore be poor magma sources. Melt fraction varies inversely with pressure in the range 7–13 kbar, but the effect is not strong: the decrease (at constant temperature) over this pressure range is of at most 15 vol% (absolute). The liquids produced in our experiments are silicarich (68–73 wt% SiO2), strongly peraluminous (2–5 wt% normative corundum) and very felsic (MgO+FeO* +TiO2 less than 3 wt%, even at temperatures above 1000° C). The last observation suggests that peraluminous granitoids with more than 10% mafic minerals (biotite, cordierite, garnet) contain some entrained restite. Furthermore, because liquids are also remarkably constant in composition, we believe that restite separation is more important than fractional crystallization in controlling the variability within and among peraluminous granitoids. We present liquidus phase diagrams that allow us to follow the phase relationships of melting of silica-and alumina-saturated rocks at pressures corresponding to the mid- to deep-continental crust. Garnet, aluminosilicate, quartz and ilmenite are the predominant restitic phases at temperatures of about 900° C, but Ti-rich biotite or calcic plagioclase can also be present, depending on the bulk composition of the protolith. At temperatures above 950–1050° C (depending on the pressure) the restitic assemblage is: hercynitic spinel+ilmenite+quartz±aluminosilicate. Our results therefore support the concept that aluminous granulites (garnet-spinel-plagioclase-aluminosilicate-quartz) can be the refractory residuum of anatectic events.