The geology, petrology, and petrogenesis of Saba Island, Lesser Antilles

Saba is the northernmost volcano along the Lesser Antilles island-arc chain. The Lesser Antilles arc results from the west-northwest subduction of the Atlantic lithosphere beneath the Caribbean Plate. Sediment thickness along the trench decreases northward away from sediment sources on the continent of South America. We focused our attention on Saba precisely because it is the furthest away from documented geochemical effects in the southern arc volcanics of the large sediment thicknesses — normally attributed to both source or upper level contamination (i.e. assimilation).Field mapping, petrology, mineralogy, K–Ar dating, and geochemical analyses (major and trace element) indicate a complex history of magma petrogenesis including crystal fractionation, magma mixing, and, surprisingly, crustal assimilation. This is the first time assimilation has been documented in the northern section of the Lesser Antilles arc. Magma mixing shows up in the field as banded pumice and petrographically and mineralogically as complex zoning in phenocrysts (such as reverse zoning in plagioclase), disequilibrium mineral assemblages (e.g. quartz and olivine), and disequilibrium between minerals and whole-rock compositions (e.g. forsterite content of olivine). Mass-balance modeling of major and trace elements support our contention that crystal fractionation (including amphibole) played an important role in magma evolution. However, various geochemical trends can only be explained by assimilation-fractional crystallization based on the fact that the trends of various trace elements and trace-element ratios vary with increasing silica. Finally, we could find no evidence of sediment source contamination in the most mafic rocks. It may exist but is overprinted by the later assimilation effects.     

Wind forcing of the Ganga-Brahmaputra river plume

Ocean Dynamics - Response of Ganga-Brahmaputra river plume to wind forcing in the Bay of Bengal is studied using a numerical ocean circulation model. Four different wind forcing scenarios, namely,...     

Volcanic Markers of the Post-Subduction Evolution of Baja California and Sonora, Mexico: Slab Tearing Versus Lithospheric Rupture of the Gulf of California

The study of the geochemical compositions and K-Ar or Ar-Ar ages of ca. 350 Neogene and Quaternary lavas from Baja California, the Gulf of California and Sonora allows us to discuss the nature of their mantle or crustal sources, the conditions of their melting and the tectonic regime prevailing during their genesis and emplacement. Nine petrographic/geochemical groups are distinguished: ??regular?? calc-alkaline lavas; adakites; magnesian andesites and related basalts and basaltic andesites; niobium-enriched basalts; alkali basalts and trachybasalts; oceanic (MORB-type) basalts; tholeiitic/transitional basalts and basaltic andesites; peralkaline rhyolites (comendites); and icelandites. We show that the spatial and temporal distribution of these lava types provides constraints on their sources and the geodynamic setting controlling their partial melting. Three successive stages are distinguished. Between 23 and 13 Ma, calc-alkaline lavas linked to the subduction of the Pacific-Farallon plate formed the Comondú and central coast of the Sonora volcanic arc. In the extensional domain of western Sonora, lithospheric mantle-derived tholeiitic to transitional basalts and basaltic andesites were emplaced within the southern extension of the Basin and Range province. The end of the Farallon subduction was marked by the emplacement of much more complex Middle to Late Miocene volcanic associations, between 13 and 7 Ma. Calc-alkaline activity became sporadic and was replaced by unusual post-subduction magma types including adakites, niobium-enriched basalts, magnesian andesites, comendites and icelandites. The spatial and temporal distribution of these lavas is consistent with the development of a slab tear, evolving into a 200-km-wide slab window sub-parallel to the trench, and extending from the Pacific coast of Baja California to coastal Sonora. Tholeiitic, transitional and alkali basalts of subslab origin ascended through this window, and adakites derived from the partial melting of its upper lip, relatively close to the trench. Calc-alkaline lavas, magnesian andesites and niobium-enriched basalts formed from hydrous melting of the supraslab mantle triggered by the uprise of hot Pacific asthenosphere through the window. During the Plio-Quaternary, the ??no-slab?? regime following the sinking of the old part of the Farallon plate within the deep mantle allowed the emplacement of alkali and tholeiitic/transitional basalts of deep asthenospheric origin in Baja California and Sonora. The lithospheric rupture connected with the opening of the Gulf of California generated a high thermal regime associated to asthenospheric uprise and emplaced Quaternary depleted MORB-type tholeiites. This thermal regime also induced partial melting of the thinned lithospheric mantle of the Gulf area, generating calc-alkaline lavas as well as adakites derived from slivers of oceanic crust incorporated within this mantle.     

Comprehensive physicochemical study of dioctahedral palygorskite-rich clay from Marrakech High Atlas (Morocco)

This study is devoted to the physicochemical and mineralogical characterizations of palygorskite from Marrakech High Atlas, Morocco. The raw clay and its Na⁺-saturated <2 μm fraction were characterized using chemical, structural, and thermal analytical techniques. Measurements of specific surface area and porous volume are reported. The clay fraction was found to be made up of 95 % of palygorskite and 5 % of sepiolite. An original feature of this palygorskite is its deficiency in zeolitic H₂O. The half-cell structural formula of its dehydrated form was determined on the basis of 21 oxygens to be (Si_7.92Al_0.08)(Mg_2.15Al_1.4Fe_0.4Ti_0.05 $ \square_{1} $ )(Ca_0.03Na_0.08K_0.04)O₂₁, while the hydrated form could be formulated as (Si_7.97Al_0.03)(Mg_2.17Al_1.46Fe_0.40Ti_0.05)(Ca_0.03Na_0.07K_0,03)O_20.18(OH)_1.94(OH₂)_3.88·2.43 H₂O. These formulas show that the (Al³⁺+Fe³⁺)/Mg²⁺ ratio is around 0.84, revealing a pronounced dioctahedral character. Further, inside its octahedral sheet, it was determined that the inner M₁ sites are occupied by vacancies, whereas the M₂ sites are shared between 90 % of trivalent cations (78 % for Al³⁺ and 22 % for Fe³⁺), 7.5 % of Mg²⁺, and 2.5 % of Ti⁴⁺, all of them linked to 1.94 of structural hydroxyls. The two remaining Mg²⁺ by half-cell occupy edge M₃ sites and are coordinated to 3.88 molecules of OH₂. Channels of this palygorskite are deficient in zeolitic H₂O since they contain only 2.43 H₂O molecules. A correlation was found between these results and the observation of very intense and well-resolved FTIR bands arising from dioctahedral domains (mainly Al₂OH, Fe₂OH, and AlFeOH) along with very small responses from a trioctahedral domain (Mg₃OH). Accordingly, a schematic representation of the composition of the octahedral sheet was proposed. The cation exchange capacity, specific surface area, and total pore volume were also assessed to be ca. 21.2 meq/100 g, 116 m²/g, and 0.458 cm³/g, respectively.     

Comparative magma/xenolith relationships in some volcanic and plutonic rocks from the French Massif Central

A comparative study of xenoliths from lavas and granitic rocks of central France indicates that:

1. The evolution of the xenoliths is essentially the same in intermediate to acid lavas as in granites. The major factor in this evolution is the assimilation by the host magmas of the quartzofeldspathic components of the xenoliths, which then become Al, Fe, Mg-rich restites.
2. Liquid immiscibility between melted xenoliths and host magmas can only be postulated for acid xenoliths in alkali basalts.
3. The basic microgranular xenoliths in some granites do not show important compositional gaps with their host rocks, though they are regularly more basic. They can be considered as early segregations (cumulates).

     

Natural occurrence of a (Fe,Mn, Mg) tetrasilicic potassium mica

A mica whose structural formula: (K_1.76Na_0.31)(Fe_2.22Mn_1.29Mg_0.99Ti_0.28Al_0.240.98) ·(Si_7.33Al_0.67)O_20.26(F_2.16OH_1.58) closely approximates that of tetrasilicic potassium mica K₂(M ₅ ²⁺ )Si₈O₂₀(OH,F)₄ where M²⁺ represents Mg²⁺, Fe²⁺, Mn²⁺, ..., has been discovered in the matrix of a peralkaline rhyolite (comendite) of the Mont-Dore massif (France). These micas had been obtained previously by synthesis only. In the groundmass of the rock, the micaceous phase is accompanied by a manganoan arfvedsonite, pyrophanite, magnetite, apatite, sphene, zircon and fluorite. The crystallographic properties of the mica are typically that of a tetrasilicic mica, with d ₀₆₀ = 1.533Å and space group C2/m. There is a regular decrease of d ₀₆₀ (parameter b) with the ionic radius of the octahedral cation in synthetic micas containing Fe²⁺, Co²⁺, Mg²⁺, Ni²⁺. The purely Mn²⁺ end-member could not be synthesised; its instability is discussed on the basis of structural considerations. The conditions of crystallization of the micaceous phase are estimated to be 760 ° C, 800 bars with a f _{o 2}=10^–14.7 bar. This mica has crystallized from a residual liquid, with high activity of silica and low activity of alumina, whose origin is discussed. The name MONT-DORITE is proposed for this natural tetrasilicic mica having Fe/Fe+Mg >1/2 and Fe/Fe+Mn >1/2. This name is from the stratovolcano Mont-Dore.     

Growth rates of Pacific manganese nodules as deduced by uranium-series and hydration-rind dating techniques

Growth rates of four Pacific ferromanganese deposits have been determined by uranium-series and hydration-rind dating techniques. This represents the first time these two methods have been applied to the same samples. Of the four samples analyzed, two agree in both techniques and two show a wide disparity. The samples that do not agree have uranium-series rates which are low compared to those deduced by the hydration-rind techniques. The same samples also display steep²³²Th activity gradients in the outer layers of the ferromanganese material. Since the half-life of²³²Th is extremely long (10¹⁰ yr), these gradients could not be due to radioactive decay. This suggests that the distribution of²³⁰Th, at least in these cases, may also be a result of some process other than radioactive decay.One of our samples which has a steep²³²Th gradient produces a²³⁰Th “exposure age” which is indistinguishable to the calculated hydration age within the analytical uncertainty. The rate of growth calculated on the basis of the slope of the decline of²³⁰Th within the deposit, however, is low by over one order of magnitude when compared to the rate determined by the hydration-rind method. Although previous results indicate that the distribution of²³²Th is usually quite random within ferromanganese deposits, our results indicate that caution should be exercised when assessing their growth rates on the basis of uranium-series results.     

Cristallisation fractionnée et contamination crustale dans la série magmatique jurassique transitionnelle du Haut Atlas central (Maroc)Fractional crystallisation and crustal contamination in the transitional Jurassic magmatic series of Central High Atlas (Morocco)

The Middle Jurassic plutonism of the Central High Atlas (Morocco) was emplaced in N45° trending anticlinal ridges. It is characterised by various petrographic facies including mafic rocks (troctolites), intermediate rocks (diorites, monzodiorites), and evolved rocks (syenites), together with heterogeneous facies resulting from mixing between acidic and the intermediate magmas. Mineralogical and chemical data show (i) the transitional character of the Jurassic magmatic series of the Central High Atlas and (ii) the implication of continental crust as a contaminant during fractional crystallization. To cite this article: R. Zayane et al., C. R. Geoscience 334 (2002) 97–104.     

Geochemistry of Adakites from the Philippines: Constraints on Their Origins

Abstract. We have identified in the Philippine Archipelago 230 samples of Late Miocene to Quaternary intermediate and evolved magmatic rocks or glasses, the compositions of which plot within the adakitic field defined by Defant and Drum-mond (1990) using Sr/Y ratios versus Y contents. These rocks belong to four different subductions, along the Manila Trench (Batan, Northern Luzon, Central Luzon), the Negros and Sulu Trenches (Negros and Western Mindano), the Cotobato Trench (Southern Mindanao) and the Philippine Trench (Eastern Mindanao). Lavas from Central Mindanao overlie the deep remnants of the Molucca Sea Plate, and were emplaced in a post-collision setting.
All these samples show a significant depletion in Y and HREE with respect to their "normal" calc-alkaline equivalents, suggesting that garnet was either a residual phase during partial melting or a fractionating mineral during differentiation or assimilation coupled with fractional crystallisation (AFC). However, only 19 samples out of our set (i.e., 8 %) display very high Sr/Y ratios (100–250). Our preferred model for the genesis of these "typical adakites" is ca. 20 % partial melting of subducted altered oceanic metabasalts converted to eclogite. This melting process could have been triggered by water from the underlying serpentinites. Most of the samples, termed "intermediate adakites", display major and trace element chemical features intermediate between those of the former group and those of normal calc-alkaline lavas. We show that magma mixing between slab-derived adakitic magmas and mafic mantle-derived melts accounts for most of the trends linking typical and intermediate adakites, although an additional contribution of mantle is required in some cases.     

The Tethyan plume: geochemical diversity of Middle Permian basalts from the Oman rifted margin

According to palinspastic reconstructions, the Neo-Tethys opening took place during the Permian between the Cimmerian fragments in the north and the Indo-Arabian margin in the south. Igneous remnants of this opening are exposed in Oman within either the Hawasina nappes or the para-autochtonous Arabian platform exposed in the Saih Hatat tectonic window. They consist predominantly of pillowed basaltic flows among which three groups have been distinguished. Group 1 is tholeiitic and characterized by low TiO₂ and incompatible trace element contents, and a large range of Nd_i values. Group 1 basalts are associated with distal sediments and plot near the boundary of or within the MORB field in the Pb–Pb correlation diagrams and between the MORB and Bulk Silica Earth (BSE) fields in Nd_i–(²⁰⁶Pb/²⁰⁴Pb)_i diagram. Group 2 basalts are alkaline and differ from Group 1 ones by their higher TiO₂, La and Nb contents, and lower and more homogeneous Nd_i values (+3 to +5). Group 2 volcanics are similar to alkali basalts from oceanic islands and share with Group 1 similar initial Pb ratios. Group 3 consists of tholeiitic and alkali basalts which are interbedded either with carbonate-platform sediments from the Saih Hatat window or with distal sediments from the Hawasina Nappes. This group differs from Groups 1 and 2 by its low to negative Nd_i (+1.6 to −2). Group 1 likely derived from the mixing of depleted and enriched sources while Group 2 derived exclusively from an enriched source. There is no indication that continental crust was involved in the genesis of both Groups 1 and 2. In contrast, the low to negative Nd_i values of Group 3 suggest that the magmas of this group were contaminated by the Arabian continental crust during their ascent. The geochemical features of the Middle Permian plume-related basalts suggest thus that the basement of the Hawasina basin was not genuine oceanic crust but either the thinned Arabian rifted continental margin or the continent–ocean transition zone of the Neo-Tethys.