Bernard Bonin

Eocene exhumation of the Tuareg Shield (Sahara Desert, Africa)

The arch-and-basin geometry that characterizes North Africa was achieved at the end of Paleozoic times. It has been subsequently reactivated during the Mesozoic-Cenozoic with, in particular, the development of large topographic anomalies. Among these, the Tuareg Shield forms a topographic high in which the Pan-African basement reaches 2400 m above sea level (Hoggar core). While Cretaceous sedimentary remnants suggest a possible stage of subsidence during the Mesozoic, currently the area forms a swell, emphasized by Cenozoic volcanic episodes since 35 Ma. In this context, we present the first apatite (U-Th)/He thermochronological data acquired across this swell, with mean ages ranging from 78 ± 22 Ma to 13 ± 3 Ma. These results demonstrate the existence of a widespread Eocene exhumation of the shield before volcanic activity began, which reflects large-scale vertical processes. In the northeastern part of the swell, Cretaceous continental sedimentary remnants unconformably lying on the basement close to our samples evidence that they were near the surface at that time. This study shows that basement rocks have undergone subsequent heating at ∼60–80 °C, suggesting a burial of more than 1 km after the Early Cretaceous. This conclusion can be possibly extended over the whole Tuareg Shield.

La province des complexes annulaires alcalins sursaturés de l'adrar des Iforas, Mali

Resume La province alcaline sursaturee des Iforas dâge cambrien est constituee dune quinzaine de massifs et comprend des complexes annulaires typiques de grandes dimensions (20–30 km de diametre). Son originalite est quelle suit de tres pres un regime de subduction bloque lors de la collision pan-africaine (600 Ma) et que chaque stade du passage calcoalcalin a alcalin peut etre relie a des elements structuraux dans un cadre geodynamique etabli. Plusieurs complexes sont decrits en detail. Le Kidal-Tibeljeljeline debute par des microsyenites en filons annulaires externes souvent polygonaux qui sont tronques par un massif central dune trentaine de kilometres compose dune douzaine de venues principales. Sa complexite est due a lindividualisation de plusieurs centres dintrusion et au fait que le niveau derosion correspond aux zones de toiture. Djounhane, Takellout et Timedjelalen (20–30 km de diametre) sont plus classiques et composes de plusieurs venues concentriques et centripetes. Les autres massif sont decrits brievement. La mise en place de tous ces massifs a faible profondeur dans un milieu rigide sous une epaisse couverture rhyolitique dorigine fissurale est contemporaine dune tectonique de coulissement avec des periodes intermittentes de distension comme en temoigent les champs filoniens spectacularies associes. Letude petrographique met en evidence la presence de deux lignees, lune hyperalcaline, lautre alumineuse, et une evolution des mineraux (pyroxenes, amphiboles et micas) et des caracteres geochimiques tres comparables a celles des provinces franchement anorogeniques du Niger et du Nigeria. Notons cependant labsence de cassiterite et la predominance des granites subsolvus dans la lignee alumineuse. La remarquable similitude entre ces provinces amence les auters a envisager une origine mantelique commune, les differences etant plutot dues a linfluence de lencaissant.

Clinopyroxene compositional trends in oversaturated and undersaturated alkaline ring complexes

Abstract Clinopyroxenes from different alkaline ring complexes, representing both silica-undersaturated and silica-oversaturated associations, have been studied in relation to their host rocks. Textural, optical and chemical evidence shows that major clinopyroxene compositional trends are related to host rock chemistry. In rocks whose agpaiitic index is less than 0.9, clinopyroxenes are Ca-rich whereas they are either Ca-rich or Na-rich in more peralkaline rocks (Na + K/Al over 0.9). The Ca levels is calcic clinopyroxenes are controlled by the silica activity in the host rock. Several compositional trends are described according to the nature of the alkaline subseries. The presence of a compositional gap is greatly increased by the silica saturation of the magma. The solid solutions between Ca-clinopyroxenes and Na-clinopyroxenes are controlled by CaMg ⇋ CaFe 2+ (diopside-hedenbergite) and CaFe 2+ ⇋ NaFe 3+ (hedenbergite-aegirine) substitutions. The Fe/Fe + Mg ratios in both clinopyroxenes and host rocks are considered: calcic clinopyroxenes are Mg-enriched with respect to their host rocks ( K D > 4.0) whereas sodic clinopyroxenes are richer in Fe than their host rocks. Crystallization of calcic clinopyroxenes is followed by the crystallization of amphiboles, and in peralkaline rocks by the subsequent crystallization of sodic clinopyroxenes. Thus, early calcic clinopyroxenes control the Fe-enrichment in the liquid whereas sodic clinopyroxenes are late and reflect the late-stage differentiation of peralkaline residual liquids.

Contrasting roles of rock-forming minerals in alkaline ring complexes

Alkaline ring complexes display numerous paradoxical features. They have a mantle origin but frequently have crustal isotopic signatures. These paradoxes can be explained if one considers that the history of an alkaline complex is written in terms of both original magmatic trends followed by hydrothermal overprinting. The systematic association of strongly differentiated acid rocks with cumulative basic rocks suggests a derivation from an intermediate liquid of monzonitic composition, which may well be a mantle-derived liquid. The differentiation trends are governed by olivine, calcic clinopyroxene, calcic plagioclase and calcic amphibole, which affect respectively iron-, alkali-and silica-enrichment. Fractionation or resorption of these critical minerals controls the different evolution trends. Late iron-rich sodic (amphibole and pyroxene) and potassic (micas) minerals are the result of magmatic differentiation and they appear often as subsolidus assemblages, sensitive to oxygen fugacity and to the water content of the vapour phase. The hydrothermal fluids can be supplied either from the vesiculation of late magmatic liquids and/or by the remobilization of interstitial ground waters in a convective-type geothermal field. In this model, it is not necessary to invoke an important component of crustal fusion.

THE AMPHIBOLE EFFECT: A POSSIBLE MECHANISM FOR TRIGGERING EXPLOSIVE ERUPTIONS

Abstract The possible effect of pressure-induced breakdown of amphibole in triggering explosive eruptions is considered. Since amphibole is a hydrous mineral, when it breaks down to an anhydrous assemblage as pressure is reduced to less than 1.5–2 kbar, the water liberated might oversaturate the coexisting melt generating the necessary overpressure to trigger an explosive eruption. Resorbed amphiboles are commonly observed in evolved lavas and pyroclastic ejecta. The amount of a volatile component, such as water that will dissolve in a melt is a function of pressure, temperature and composition, and during crystallization it is also a function of the extent of crystallization and the nature of crystallizing minerals. The relation can be expressed by the simple equation: where X r is the water content of the residual liquid, X i is the initial water content, X mOH , is the water content of hydrous minerals, f is the total extent of crystallization and f ′ is the extent of crystallization of hydrous minerals such that 0 ≤ f ′ ≤ f ≤ 1. We suggest that storage of water in hydrous minerals, such as amphibole and biotite, plays an important role in the eruptive behavior of certain types of magmas; the breakdown of these minerals liberates water to the melt at a rate governed by the kinetics of the resorption reaction. If the release of water causes the liquid fraction to exceed the solubility limit and the overpressure resulting from expansion of the gas exceeds the strength of the overlying magma and rocks in the conduit, the result can be an explosive eruption. The amphibole effect can occur at different structural levels depending on the nature of the magma and physical conditions leading to instability.

The alkaline silica-saturated ultrapotassic magmatism of the Riacho do Pontal Fold Belt, NE Brazil: an example of syenite–granite Neoproterozoic association

Abstract The Neoproterozoic of northeastern Brazil was marked by the development of collisional fold belts, mainly surrounding the Sao Francisco Craton, and an associated widespread granitic magmatism. The Casa Nova (555±10 Ma , Sri=0.7068 and Engracadinha syenites and granites, intrusive in the Riacho do Pontal Fold Belt, are related to the late stages of this collisional event. Melanocratic syenites, probably generated by magmatic-flow cumulate processes, and mesocratic and leucocratic syenites, representing magmatic liquids, are associated with granites, pegmatites, and syenite–granite dykes. Homogeneous or perthitic alkali feldspar, quartz, aegirine–augite, diopside, titanite, apatite, magnesian biotite, winchite–richterite, and magnetite are the dominant mineral phases. It is suggested that these magmas belong to an ultrapotassic silica-saturated alkaline series, defined on the basis of its alkaline, silica-saturated character and by a K2O/Na2O ratio of >3.0 — that is, intermediate. Major- and trace-element evolution is consistent with mineral fractionation processes, controlled by magmatic flow, and dominated by apatite–titanite–pyroxene in the less differentiated terms and by alkali feldspar in the more evolved. The source of primary magmas is a previously subduction-metasomatised mantle, probably with anomalous enrichment in LREE and LILE elements. Barite–ilmenite mineralisations are related to the more differentiated Engracadinha granites.

The Itiúba alkaline syenite massif, Bahia state (Brazil) : mineralogical, geochemical and petrological constraints : relation to the genesis of rapakivi magmatism

Abstract Numerous alkaline massifs occur throughout the state of Bahia (Brazil). Isotopic dates fall into two age groups: Brasiliano (0.45–0.70 Ga) and Trans-Amazon (1.8–2.1 Ga) ones. Brasiliano alkaline provinces comprise a silica-undersaturated association, with related volcanic rocks; their emplacement is always controlled by fault zones. Trans-Amazon alkaline massifs are characterized by large plutons (more than 100 km2) of K-rich syenite and granite associated with mafic cumulates and abundant dyke swarms. No associated volcanic rocks have been so far recorded. The shape of the plutons varies as a function of their location within the Sao Francisco Craton. In its northeastern and southern parts, syenite massifs are elongated, trending N-S, while in its western part, they are emplaced as circular bodies displaying contact metamorphic aureoles. The presence of older (Archaean?) alkaline rocks cannot be ruled out, as some granulitic facies display syenite compositions. The Itiuba massif provides a good example of Proterozoic alkaline syenite. Located in the northern part of the Sao Francisco Craton, this 150-km-long pluton covers 1800 km2 in area. Whole-rock Rb-Sr isotopic data yield a lower Proterozoic age. A N-S-trending foliation at the margins is gradually replaced by isotropic textures toward the core. Two sets of faults, both of Trans-Amazonian age, have been defined: N-S-trending reverse faults, accompanied by intense mylonitization, and younger NW-trending arcuate transcurrent faults related to a NE-SW compressive regime. Alkaline syenites constitute 98% of the exposures and are medium- to coarse-grained clinopyroxene-amphibole, hypersolvus leucratic rocks. Cumulates are represented by mafic layers and clinopyroxene-apatite enclaves. Dykes are composed of alkaline syenite, hypersolvus and transsolvus quartz-syenites and alkaline granites. Syenites are metaluminous and rich in K, Mg, P, Ti and Ba, and their geochemical trends are controlled by alkali feldspar, clinopyroxene, apatite and Feue5f8Ti oxide fractionation. Oxidizing conditions are reflected by the weak variation in the mg-ratio, the reverse Fe → Mg mineral zonation, the synchronous precipitation of oxides and clinopyroxene in mafic layers and the late development of uralitic amphibole and low-Ti phlogopite. Temperatures for oxide equilibration of 930-880°C are compatible with the hypersolvus feldspar mineralogy. The evolution of feldspar mineralogy is a good indicator of varying thermodynamical conditions during late-stage crystallization of the pluton. Minimum values for initial crystallization temperature have been evaluated at 950°C. Structural and petrological results indicate that the presently exposed syenite massif represents the roof of a Proterozoic magma chamber, emplaced in granulite-gneiss formations. Rapakivi magmatism may represent disrupted and floating portions of the roof of earlier magma chambers refilled by new syenite-granite melts.

Plutonic rocks from Tahiti-Nui caldera (society archipelago, French Polynesia): A petrological, geochemical and mineralogical study

Abstract A plutonic massif is exposed on a 2.1-km 2 area in TahitiNui caldera (Tahiti Island, Society Archipelago, French Polynesia). All the rock types are emplaced into the caldera-filling formations as a stratified ring-intrusion. They constitute two contrasting alkaline suites, a strongly silica-undersaturated “theralitic” suite, made up of 43% theralites, 4.8% essexites and 3.1% ne-syenites, overlying a weakly silica-undersaturated “gabbroic” suite, composed of 35% gabbros, 11.6% monzonites and 1.3% syenites. Minor cumulate pyroxenites account for 1.2%. The average composition of the plutonic massif is similar to compositions of the less evolved basaltic lava flows capping the same volcano. Mineralogical and geochemical data illustrate the influence of mafic minerals during the differentiation processes. Clinopyroxene fractionation plays a significant role at the early stages. Then, amphibole becomes the critical mineral component: its fractionation leads to weakly silica-undersaturated differentiates whereas its resorption into the residual liquids accentuates silica-undersaturation towards nephelinic end-members. The two divergent trends of evolution are obvious only in the more evolved liquids, because the amphibole effect is marked after the intermediate liquids have been produced. During emplacement and crystallization of the strongly silica-undersaturated rocks, a pervasive retromorphic process produced mineral assemblages of the albite-epidote facies by fluid convection of a geothermal system around the plutonic intrusion. During the differentiation stage, fairly low computed densities and viscosities imply that crystal-poor magmas, rich in volatiles, are extracted from the magma chamber and ascend into the core of the shield volcano. Release of volatiles during crystallization causes a sharp increase of viscosity of the magmas, resulting in their sub-surface emplacement. Thermodynamic calculations indicate late-stage reequilibration from the solidus to about 350°C, where the temperatures correspond to the albite-epidote retromorphic assemblages. In the Tahiti-Nui plutonic massif, both magmatic and subsolidus stages are controlled by fluids, either incorporated as hydrous minerals or circulating free by along convective pathlines of a geothermal system.

The Cretaceous morondava volcanic province (West Madagascar): mineralogical, petrological and geochemical aspects

Upper Cretaceous continental break-up between Madagascar and Greater India resulted in intense volcanic activity throughout the island, but mostly in the east coast. The Morondava Basin, southwest Madagascar, was capped 88 Ma ago by volcanic formations triggered by this event. Mineral and bulk rock compositions reveal two groups of suites. The high Ti-P group is composed of olivine basalts and a ferrobasalt-basalt-andesitic basalt series. The low Ti-P group comprises genetically unrelated basalts and andesitic basalts. All rocks display the (olivine + plagioclase + clinopyroxene + Fe-Ti oxides) four phase assemblage. Rock-forming minerals of the high Ti-P group are compositionally unimodal, while rocks of the low Ti-P group contain olivine and plagioclase, each yielding bimodal populations of compositions, probably due to transient low Ti-P magmas storage within a deep magma chamber emplaced at the crust-mantle boundary. In the southern area of the basin near Manamana, low Ti-P andesitic basalts were produced by partial melting of subcontinental lithospheric mantle, referred to as the Manamana end member. The nearby high Ti-P series can be explained by partial melting of a mixed source comprising the Manamana end member and deeper materials and subsequent fractionation. In the central area of the basin near Ankilizato, the lithospheric source of high Ti-P olivine basalt, the Ankilizato end member, differs from the Manamana end member. The nearby low Ti-P basalts can be explained by a complex mixture of sources.

Anorogenic alkaline granites from northeastern Brazil: major, trace, and rare earth elements in magmatic and metamorphic biotite and Na-mafic minerals

Abstract The anorogenic, alkaline silica-oversaturated Serra do Meio suite is located within the Riacho do Pontal fold belt, northeast Brazil. This suite, assumed to be Paleoproterozoic in age, encompasses metaluminous and peralkaline granites which have been deformed during the Neoproterozoic collisional event. Preserved late-magmatic to subsolidus amphiboles belong to the riebeckite–arfvedsonite and riebeckite–winchite solid solutions. Riebeckite–winchite is frequently rimmed by Ti–aegirine. Ti-aegirine cores are strongly enriched in Nb, Y, Hf, and REE, which significantly decrease in concentrations towards the rims. REE patterns of Ti-aegirine are strikingly similar to Ti-pyroxenes from the Ilimaussaq peralkaline intrusion. Recrystallisation of mineral assemblages was associated with deformation although some original grains are still preserved. Magmatic annite was converted into magnetite and biotite with lower Fe/(Fe+Mg) ratios. Recrystallised amphibole is pure riebeckite. Magmatic Ti–Na-bearing pyroxene was converted to low-Ti aegirine+titanite±astrophyllite/aenigmatite. The reaction riebeckite+quartz→aegirine+magnetite+quartz+fluid is also observed. Biotite and Na-mafic minerals recrystallised under metamorphic oxidising conditions corresponding to temperatures of 600°C between the NiNiO and HM buffers.