Bernard Bonin - Academia.edu (original) (raw)

Papers by Bernard Bonin

Special Paper of the Geological Society of America, 2005

We review the northwest African Cenozoic volcanic fields, including their regional geology. This ... more We review the northwest African Cenozoic volcanic fields, including their regional geology. This provides a basis for understanding the relations between Hoggar volcanism and the Africa-Europe collision. Volcanic alignments are related to structural features, and no spatial age trend exists. In Hoggar, a close link is established between the volcanism and Pan-African structure. During the Mesozoic rifting period, the Hoggar area was already a topographic high well before any volcanism, which began at ca. 35 Ma, just after the initiation of the Africa-Europe collision at ca. 38 Ma. Hoggar volcanism continued episodically until now, as did the collision. We describe the Hoggar volcanic province based on available field, petrological, geochemical isotopic, and geophysical data, including data on gravimetry, heatflow, and seismic tomography. The latter suggests that northwestern African volcanism is linked to mantle structure down to 150 km but not deeper, implying a shallow mantle source. In Hoggar, lithospheric structures deduced from the seismic tomographic model and from geology are compatible when their respective resolutions are taken into account.

Bulletin de la Société Géologique de France, 1977

Mineralogy and Petrology, 2016

The Guadalupian (Mid-Permian) Highiș massif (Apuseni Mountains, Romania) displays a bimodal igneo... more The Guadalupian (Mid-Permian) Highiș massif (Apuseni Mountains, Romania) displays a bimodal igneous suite of mafic (gabbro, diorite) and A-type felsic (alkali feldspar granite, albite granite, and hybrid granodiorite) rocks. Amphibole is widespread throughout the suite, and yields markedly high chlorine contents. Three groups are identified: Cl-rich potassic hastingsite (2.60-3.40 wt% Cl) within A-type felsic rocks and diorite, mildly Cl-rich pargasite to hornblende (0.80-1.90 wt% Cl) within gabbro, and low F-Cl hornblende within gabbro and hybrid granodiorite. Coexisting biotite is either Cl-rich within diorite, or F-Cl-poor to F-rich within Atype felsic rocks. Chlorine and fluorine are distributed in both mafic phases, according to the F-Fe and Cl-Mg avoidance rules. The low-Ti contents suggest subsolidus compositions. Cl-rich amphibole within diorite and A-type felsic rocks yields a restricted temperature rangefrom 575°C down to 400°C, whereas mildly Cl-rich amphibole within gabbro displays the highest rangefrom 675 to 360°C. Temperatures recorded by Cl-rich biotite within diorite range from 590 to 410°C. Biotite within A-type felsic rocks yields higher temperatures than amphibole: the highest values-from 640 to 540°Care recorded in low-F-Cl varieties, whereas the lowest values-from 535 to 500°Care displayed by F-rich varieties. All data point to halogen-rich hydrothermal fluids at upper greenschist facies conditions percolating through fractures and shear zones and pervasively permeating the whole Highiș massif, with F precipitating as interstitial fluorite and Cl incorporating into amphibole, during one, or possibly several, hydrothermal episodes that would have occurred during a~150 My-long period of time extending from the Guadalupian (Mid-Permian) to the Albian (Mid-Cretaceous).

Geological Society, London, Special Publications, 2019

Granites (sensu lato) come in many types and flavours, defining distinct magmatic series/suites/t... more Granites (sensu lato) come in many types and flavours, defining distinct magmatic series/suites/types. A good classification not only gives generally accepted and understandable names to similar rocks but also links the bulk chemical composition to the stoichiometry of the constituent minerals and, potentially, also to the likely source, magmatic evolution and tectonic setting.The ‘ideal’ granitoid classification should be based on chemical criteria amenable to an objective treatment. Statistical analysis helps to identify the most discriminant variables. The key properties are (1) acidity/maficity, (2) alkalinity (balance of Na + K v. Ca), (3) aluminosity (balance of Al v. Ca, Na and K), (4) Fe/Mg balance and (5) Na/K balance and K contents at the given SiO2 level. These are used by successful classifications, e.g. the I/S dichotomy is based mainly on aluminosity, while the Frost et al. (2001; ‘A geochemical classification for granitic rocks', Journal of Petrology, 42, 2033–204...

In SE China the Fuzhou plutonic complex is made up of two major suites:a) A calc-alkaline suite (... more In SE China the Fuzhou plutonic complex is made up of two major suites:a) A calc-alkaline suite (Danyang, Fuzhou) emplaced about 105-100 Ma ago (Rb-Sr ages). These granitic rocks show a calc-alkaline differentiation trend, with fractionation of plagioclase and hornblende. Sr and Nd isotopic evidence suggests that ancient crustal material has played an important role in the magma genesis.b) An alkaline suite intruded about 90 Ma ago (Kuiqi, Bijiashan). It is composed of hypersolvus per-alkaline granites, which show little magmatic differentiation. These rocks are characterized by high abundances of REE. Sr and Nd isotopic data indicate that crust involvement was not significant.The petrogenetic study suggests that the successive intrusions of that complex were emplaced progressively at higher crustal levels. The calc-alkaline suite is typical of those emplaced in subduction-zone environments, whereas the hyperalkaline suite is associated with major deep crustal faults.

Precambrian Research, 1991

Numerous alkaline massifs occur throughout the state of Bahia (Brazil). Isotopic dates fall into ... more 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 km 2) 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 Silo 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 Iti6ba massif provides a good example of Proterozoic alkaline syenite. Located in the northern part of the S~o Francisco Craton, this 150-km-long pluton covers 1800 km 2 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 Fe-Ti oxide fractionation. Oxidizing conditions are reflected by the weak variation in the rag-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.

Precambrian Research, 2013

Combined field observations, petrography, mineral chemistry, geochemistry, Nd isotopes and U-Pb z... more Combined field observations, petrography, mineral chemistry, geochemistry, Nd isotopes and U-Pb zircon data are presented for the ca. 2130 Ma Alto Maranhão suite. The suite, composed by allanite-bearing biotite hornblende tonalites and commingled dioritic mafic magmatic enclaves (MME), occupies an estimated area of >300 km 2 in the southern Quadrilátero Ferrífero region, Minas Gerais, Brazil. Tonalites and commingled MME are medium-K rocks, with high-Mg (Mg-number ≥ 0.46) and high-Cr (≥55 ppm) contents. They are also enriched in large-ion lithophile elements (LILE, Sr, Ba), some high-field-strength elements (HFSE, Th, U), and light rare earth elements (REE), but depleted in Nb, Ta, and heavy REE. T DM model ages for tonalites and MME are similar and range from 2.3 to 2.4 Ga. The ε Nd (t) values are grouped around chondritic values (=0 ± 1.0). Field relations and geochemical data indicate that tonalites and commingled dioritic MME crystallized as synchronous independent magma pulses, with limited mixing. Both magmas are compositionally equivalent to high-Mg andesites in modern subduction zones, implying that the suite was issued from the partial melting of a mantle wedge metasomatised to different degrees by slab-derived melts. Geobarometry (aluminium-in-hornblende) indicates crystallization at mid-crustal depths (i.e. 0.59-0.42 GPa). The igneous suite evolved by fractional crystallization of hornblende, biotite and accessory minerals (zircon, apatite, allanite and Fe-Ti oxide). The ubiquitous occurrence of hornblende and allanite in tonalites and dioritic MME suggests that this mineral pair was largely responsible for the observed changes of light REE/heavy REE and Eu/Eu* ratios within the members of the suite. The geographical extent (>1000 km 2) and Paleoproterozoic ages of the Alto Maranhão suite (ca. 2130 Ma) and associated granitoids of the Mineiro Belt (2.1-2.2 Ga) constitute a major event of juvenile addition to the continental crust in the context of the southern São Francisco craton, and the ca. 2.2-2.1 Ga Rhyacian orogenesis of the South American platform. The age and general geochemical characteristics of the Alto Maranhão suite and its emplacement after a ca. 2.35 Ga juvenile, tholeiitic-source derived high-Al TTG suite, indicate a tectonic setting akin to that of Late-Archaean high-Mg granitoids.

Journal of Petrology, 1994

ABSTRACT