When continents rift to form new ocean basins, the rifting is sometimes accompanied by massive igneous activity. We show that the production of magmatically active rifted margins and the effusion of flood basalts onto the adjacent continents can be explained by a simple model of rifting above a thermal anomaly in the underlying mantle. The igneous rocks are generated by decompression melting of hot asthenospheric mantle as it rises passively beneath the stretched and thinned lithosphere. Mantle plumes generate regions beneath the lithosphere typically 2000 km in diameter with temperatures raised 100–200°C above normal. These relatively small mantle temperature increases are sufficient to cause the generation of huge quantities of melt by decompression: an increase of 100°C above normal doubles the amount of melt whilst a 200°C increase can quadruple it. In the first part of this paper we develop our model to predict the effects of melt generation for varying amounts of stretching with a range of mantle temperatures. The melt generated by decompression migrates rapidly upward, until it is either extruded as basalt flows or intruded into or beneath the crust. Addition of large quantities of new igneous rock to the crust considerably modifies the subsidence in rifted regions. Stretching by a factor of 5 above normal temperature mantle produces immediate subsidence of more than 2 km in order to maintain isostatic equilibrium. If the mantle is 150°C or more hotter than normal, the same amount of stretching results in uplift above sea level. Melt generated from abnormally hot mantle is more magnesian rich than that produced from normal temperature mantle. This causes an increase in seismic velocity of the igneous rocks emplaced in the crust, from typically 6.8 km/s for normal mantle temperatures to 7.2 km/s or higher. There is a concomitant density increase. In the second part of the paper we review volcanic continental margins and flood basalt provinces globally and show that they are always related to the thermal anomaly created by a nearby mantle plume. Our model of melt generation in passively upwelling mantle beneath rifting continental lithosphere can explain all the major rift-related igneous provinces. These include the Tertiary igneous provinces of Britain and Greenland and the associated volcanic continental margins caused by opening of the North Atlantic in the presence of the Iceland plume; the Parana and parts of the Karoo flood basalts together with volcanic continental margins generated when the South Atlantic opened; the Deccan flood basalts of India and the Seychelles-Saya da Malha volcanic province created when the Seychelles split off India above the Reunion hot spot; the Ethiopian and Yemen Traps created by rifting of the Red Sea and Gulf of Aden region above the Afar hot spot; and the oldest and probably originally the largest flood basalt province of the Karoo produced when Gondwana split apart. New continental splits do not always occur above thermal anomalies in the mantle caused by plumes, but when they do, huge quantities of igneous material are added to the continental crust. This is an important method of increasing the volume of the continental crust through geologic time.

Magmatism at rift zones: The generation of volcanic continental margins and flood basalts

The composition of chromian spinel in alpine-type peridotites has a large reciprocal range of Cr and Al, with increasing Cr# (Cr/(Cr+Al)) reflecting increasing degrees of partial melting in the mantle. Using spinel compositions, alpine-type peridotites can be divided into three groups. Type I peridotites and associated volcanic rocks contain spinels with Cr# 0.60, and Type II peridotites and volcanics are a transitional group and contain spinels spanning the full range of spinel compositions in Type I and Type II peridotites. Spinels in abyssal peridotites lie entirely within the Type I spinel field, making ophiolites with Type I alpine-type peridotites the most likely candidates for sections of ocean lithosphere formed at a midocean ridge. The only modern analogs for Type III peridotites and associated volcanic rocks are found in arc-related volcanic and intrusive rocks, continental intrusive assemblages, and oceanic plateau basalts. We infer a sub-volcanic arc petrogenesis for most Type III alpine-type peridotites. Type II alpine-type peridotites apparently reflect composite origins, such as the formation of an island-arc on ocean crust, resulting in large variations in the degree and provenance of melting over relatively short distances. The essential difference between Type I and Type III peridotites appears to be the presence or absence of diopside in the residue at the end of melting.

Chromian spinel as a petrogenetic indicator in abyssal and alpine-type peridotites and spatially associated lavas

Some of the most important, rapid, and enigmatic changes in our Earth’s environment and biota occurred during the Neoproterozoic Era (1000540 million years ago; Ma). Paramount among these changes are the rapid evolution of eukaryotes and appearance of metazoa (Knoll 1992, Conway Morris 1993), major episodes of continental glaciation that may have extended to low latitudes (Hambrey & Harland 1985), marked increases in the oxygen concentration of the atmosphere and hydrosphere (Derry et al 1992), the reappearance of sedimentary banded iron formations (BIF; James 1983), and striking temporal variations in the isotopic composition of C and Sr (Asmerom et al 1991, Derry et al 1992). Understanding the causes of and relationships between these changes is a challenging focus of interdisciplinary research, and there are compelling indications that the most important causes were tectonic (Des Marais et al 1992, Veevers 1990). For example, development of ocean basins may have been accompanied by the development of seafloor hydrothermal systems, which lowered the 87Sr/S6Sr of seawater, led to the development of BIF, and formed anoxic basins where organic carbon could be buried, thus leading to an increase in O~. Continental collision and formation of a supercontinent may have led to continental glaciation and an increase in the 87Sr/86Sr of seawater,

http://www.utdallas.edu/~rjstern/egypt/PDFs/General/SternEAO-AREPS94.pdf

ARC Assembly and Continental Collision in the Neoproterozoic East African Orogen: Implications for the Consolidation of Gondwanaland

Comparative geology suggests that the continents adjacent to northern, western, southern, and eastern Laurentia in the Late Proterozoic were Siberia, Australia-Antarctica, southern Africa, and Amazonia-Baltica, respectively. Late Proterozoic fragmentation of the supercontinent centered on Laurentia would then have been followed by rapid fan-like collapse of the (present) southern continents and eventual consolidation of East and West Gondwanaland. In this scenario, a pole of rotation near the Weddell Sea would explain the observed dominance of wrench tectonics in (present) east-west trending Pan-African mobile belts and subduction-accretion tectonics in north-south trending belts. In the process of fragmentation, rifts originating in the interior of the Late Proterozoic supercontinent became the external margins of Paleozoic Gondwanaland; exterior margins of the Late Proterozoic supercontinent became landlocked within the interior of Gondwanaland.

https://science.sciencemag.org/content/sci/252/5011/1409.full.pdf

Did the breakout of laurentia turn gondwanaland inside-out?

United Plates of America, The Birth of a Craton: Early Proterozoic Assembly and Growth of Laurentia

Geologic mapping of an intact plutonic sequence within the Samail ophiolite in the Ibra area, southeastern Oman Mountains, reveals stratigraphic, structural, and petrologic details of oceanic layer 3. Four measured stratigraphic sections, each spaced about 5 km apart across the southern flank of Jabal Dimh, define a time-transgressive progression within the ophiolite and reveal geometric and petrologic features of a spreading-ridge magma chamber. The sections show the following vertical sequence: (1) dunite (chr-ol cumulates ± harzburgite xenoliths) 0–200 m thick, grading up from a transition zone with harzburgite tectonite, (2) interlayered wehrlite-melagabbro-gabbro (cpx-ol and ol-cpx-pl cumulates) 0–100 m thick, (3) layered gabbro (chiefly ol-cpx-pl cumulates but including recurrent intervals of cumulus wehrlite and melagabbro) 2.6–5.5 km thick, (4) planar laminated nonlayered gabbro (chiefly ol-cpx-pl cumulates) 100–400 m thick, (5) hypidiomorphic (ol)-hb-cpx gabbro (high-level gabbro) 200–800 m thick, (6) small, discontinous diorite to plagiogranite bodies at or near the top of the gabbro. Cumulus textures (adcumulus > mesocumulus), planar lamination, and cumulus layering (phase, ratio, and grain size layers at mm to 10-m scale, commonly graded) within this sequence show that crystals accumulated from the base of the magma chamber upward to within a few hundred meters of the top; downward solidification from the roof was minor. Cyclicity within the cumulus sequence is represented by the recurrence of olivine-rich intervals (melagabbo and wehrlite) up to high stratigraphic levels and by hundreds of phase-graded layers (ol-rich at the bases to pl-rich at the tops), individually up to 5 m thick. Limited cryptic variation relative to closed-system layered intrusions and the limited range in solid-solution components of olivine (Fo69–90), plagioclase (An62–95) and clinopyroxene (En40–54, Fs4–16, Wo37–49) from the cumulus suite require replenishment of the magma during its crystallization history. Zig-zag normal and reverse cryptic variation differentiation trends (both in major solid-solution components and in minor element concentration) indicate that the replenishment took place in pulses followed by periods of magma mixing and crystal fractionation. Recurrent olivine-rich intervals commonly (but not always) coincide with reverse cryptic variation trends. They are considered to be the products of fresh draughts of primitive magma, with olivine and chromite as the only liquidus phases prior to extensive mixing with the more fractionated resident magma within the chamber. The sequence of crystallization is explained using the simplified tholeiitic basalt tetrahedron of Presnall et al. (1979). The plutonic sequence is roofed by sheeted dikes that are overlain by submarine basalt, indicating that the magma crystallized beneath a spreading ocean ridge. Major and trace element geochemistry of the dike complex is similar in many respects to that of mid-ocean ridge basalt (MORB) and yield Mg/Mg + Fe+2 ratios that overlap with ratios predicted for parent liquids of the cumulus suite based on crystal/liquid equilibria. Field evidence, including continuity of layering over large areas, distinctive layer sets that are mapped for several km along strike, and lack of chamber edge contacts indicates that crystallization of Jabal Dimh plutonics took place in a single, large, long-lived magma chamber rather than in small transitory chambers. Similarity of mineral compositions and tentative correlation of cryptic variation trends between stratigraphic sections favors a large, single-chamber model. The chamber shape, controlled by floor growth upward greatly exceeding roof growth downward as the chamber halves diverged from the paleo-spreading axis, must have been funnel-shaped in cross section; i.e., the floor sloped inward to the center from a sandwich horizon that developed just beneath the roof at both sides. This conclusion is based on the occurrence of cumulates up to a very high level in the stratigraphic sections and on geologic mapping which shows that cumulus layering locally approaches the base of the cumulus section at an angle. This vergence angle is <20°, well below inferred angles of repose for cumulates. Tentative correlation of cryptic variation between sections yields similar vergence angles. The scarcity of slump structures and of layer disruption within the main layered sequence verifies that the floor sloped rather gently, surely much less than 45°. Thus, the half-width of the chamber must have exceeded its vertical dimension, which averages ∼5 km. A half-width of ∼18 km is calculated from a preferred vergence angle of 15°. The cumulates crystallized within this spreading chamber from a magma body that was repeatedly replenished by primitive magma from a central feeder zone at the spreading axis. High-level gabbro and diorite through plagiogranite crystallized from stagnant residual magma at the outer edges of the chamber, beyond reach of replenishment from the center.

Samail Ophiolite plutonic suite: Field relations, phase variation, cryptic variation and layering, and a model of a spreading ridge magma chamber

https://hal-insu.archives-ouvertes.fr/insu-00723412/document

The 2010 explosive eruption of Java's Merapi volcano—A ‘100-year’ event

THE eruptive products of calc-alkaline volcanos often show evidence for the mixing of basaltic and acid magmas before eruption (see, for example, refs 1, 2). These observations have led to the suggestion3 that the injection of basaltic magma into the base of a magma chamber (or the catastrophic overturn of a stably stratified chamber containing basaltic magma at its base) might trigger an eruption. Here we report evidence for the mixing of basaltic and dacitic magmas shortly before the paroxysmal eruptions of Pinatubo volcano on 15 June 1991. Andesitic scoriae erupted on 12 June contain minerals and glass with disequilibrium compositions, and are considerably more mafic than the dacitic pumices erupted on 15 June. Differences in crystal abundance and glass composition among the pumices may arise from pre-heating of the dacite magma by the underlying basaltic liquid before mixing. Degassing of this basaltic magma may also have contributed to the climatologically important sulphur dioxide emissions that accompanied the Pinatubo eruptions.

A basalt trigger for the 1991 eruptions of Pinatubo volcano

Precambrian ophiolites of arabia: geologic settings, UPb geochronology, Pb-isotope characteristics, and implications for continental accretion

Rare-earth element (REE) analyses of 68 rock and mineral separate samples from the Samail ophiolite clearly differentiate the various units of the ophiolite suite and indicate that the crustal suite is cogenetic, produced by crystal fractionation of basaltic magma in a spreading ridge magma chamber. Mantle peridotites are residual in rare-earth character, but cannot be clearly related to the overlying mafic rocks. Chromian spinel is probably insignificant in its effect on REE distribution during partial melting and crystal fractionation, as indicated by the low REE concentrations in chromitite from the Samail. Layered gabbro REE patterns are dominated by cumulus clinopyroxene and plagioclase. Large positive Eu anomalies demonstrate plagioclase accumulation. Modal mixing (mass balance) calculations reveal that most of these adcumulus gabbros have REE patterns that are the products of the REE concentrations of their constituent cumulus phases in the observed modal proportions; hence no appreciable REE-rich mesostasis is present. Such calculations also allow the prediction of mineral REE concentrations not actually determined by mineral separate analyses. Several high-level (noncumulus?) gabbros yield patterns with positive Eu anomalies suggesting relative plagioclase accumulation, probably due to liquid fractionation (filter pressing). Dike complex REE patterns show light rare-earth element depletions and are similar to, but not necessarily diagnostic of, midocean ridge basalt. They cluster at similar abundance levels; however, the absolute variation in abundance is large, indicative of modification by crystal fractionation. Calculations using partition coefficient data indicate that many dikes represent liquids that could have existed in equilibrium with cumulus minerals of the plutonic suite. Mineral separate REE data from layered cumulus gabbro of the Khafifah stratigraphic section reveal cryptic variation trends that are correlated with major element variation, in support of a long-lived (periodically replenished) magma chamber model (Pallister and Hopson, 1978, 1979, 1981). (Cryptic variation as used herein is defined as the change in mineral composition with respect to stratigraphic position.) The REE cryptic variation shows both direct and inverse correlation to major element variation, indicating that magmatic replenishment was complicated by changes in parent melt REE abundance. A crystal fractionation origin for the small plagiogranite bodies of the Ibra area is favored by REE modeling, although larger bodies (Dasir) may also be related to magmatic inclusion of roofrock with diabase level REE distribution (Gregory and Taylor, 1979).

John S. Pallister

Papers

Samail Ophiolite plutonic suite: Field relations, phase variation, cryptic variation and layering, and a model of a spreading ridge magma chamber

The 2010 explosive eruption of Java's Merapi volcano—A ‘100-year’ event

A basalt trigger for the 1991 eruptions of Pinatubo volcano

Precambrian ophiolites of arabia: geologic settings, UPb geochronology, Pb-isotope characteristics, and implications for continental accretion

Rare‐earth element geochemistry of the Samail Ophiolite near Ibra, Oman