Atypical hotspot chains: evidence for a secondary melting zone below the Marquesas (French Polynesia)

The atypical age/distance to the hotspot relationships observed for the Marquesas linear chain, which present a considerable scatter, are best explained considering (1) a Pacific plate motion of 10.5 cm yr^?1 in the N115°E direction and (2) rejuvenation of volcanism in Ua Huka island. New K–Ar ages show that the main hotspot activity in Ua Huka emplaced successively shield olivine tholeiites and post‐shield alkali basalts between 3.11 ± 0.04 and 2.43 ± 0.04 Ma. Then, after a quiescence period of 1.28 ± 0.06 Myr, two small basanitic volcanoes were emplaced between 1.15 ± 0.02 and 0.763 ± 0.013 Ma. With respect to the main hotspot volcanics, their lavas originated from weaker partial melting rates of a deeper source. Field, petrologic and geochemical arguments are consistent with the existence of a secondary melting zone located c. 140 km downstream the Marquesas hotspot, which produced the rejuvenated volcanics.     

Composition des Sphalerites du District Minier de Hualgayoc (Perou)

Microprobe analysis of mineralisations from the district of Hualgayoc, Peru, shows a zonation of sphalerite compositions all around a rhyodacitic body. The differences in the iron content of sphalerites in veins inside the rhyodacite and in sedimentary strata may be explained by a contamination of the hydrothermal solutions by those strata.     

Partition coefficients of trace elements: Application to volcanic rocks of St. Vincent,West Indies

Island arc basaltic rocks (basalts and basic andesites with SiO₂ < 56.5%) from the Soufrière volcano. St. Vincent, West Indies (prehistoric lavas and 1902 and 1979 eruptions) underwent extensive fractional crystallization at various levels during the ascent of the magma. Although the precipitation of minerals occurring in coarse-grained cumulate inclusions dominated the derivation of basic andesites from basaltic magma, the distribution of the trace elements is not consistent with a simple fractional crystallization process. The lavas have a partially cumulate character and were probably generated from similar but separate parental magmas. The partition coefficients of transition and large ion lithophile elements are given for clinopyroxene, amphibole. olivine, plagioclase and titanomagnetite in basaltic liquid which crystallized under well-defined P-T conditions. The temperatures obtained from the geothermometers based upon the distribution of the major elements are in good agreement with the data from trace element geothermometers.     

Petrology of basaltic xenoliths in andesitic to dacitic host lavas from Martinique (lesser antilles): Evidence for magma mixing

Some recent calc-alkaline andesites and dacites from southern and central Martinique contain basic xenoliths belonging to two main petrographic types:

The most frequent one has a hyalodoleritic texture (« H type ») with hornblende + plagioclase + Fe-Ti oxides, set in an abundant glassy and vacuolar groundmass.

The other one exhibits a typical porphyritic basaltic texture (« B type ») and mineralogy (olivine + plagioclase + orthopyroxene + clinopyroxene + Fe-Ti oxides and scarce, or absent hornblende).

Gradual textural and mineralogical transitions occur between these two types (« I type ») with the progressive development of hornblende at the expense of olivine and pyroxenes. Mineralogical and chemical studies show no primary compositional correlations between the basaltic xenoliths and their host lavas, thus demonstrating that the former are not cognate inclusions; they are remnants of basaltic liquids intruded into andesitic to dacitic magma chambers. This interpretation is strengthened by the typical calc-alkaline basaltic composition of the xenoliths, whatever their petrographic type (« H », « I » or « B »). The intrusion of partly liquid, hot basaltic magma into colder water-saturated andesitic to dacitic bodies leads to drastic changes in physical conditions. The two components; the basaltic xenoliths are quenched and homogeneized with their host lavas with respect to T^o;fO₂ andpH₂O conditions. « H type » xenoliths represent original mostly liquid basalts in which such physical changes lead to the formation of hornblende and the development of a vacuolar and hyalodoleritic texture. The temperature increase of the acid magma depends on the amount of the intruding basalt and on the thermal contrast between the two components. The textural diversity which characterizes the xenoliths reflects the cooling rate of the basaltic fragments and/or their position relative to the basaltic bodies (chilled margins or inner, more crystallized, portions). In addition to physical equilibration (T, fO₂) between the magmas, mixing involves:

mechanical transfer of phenocrysts from one component to another, in both directions;

volatile transfer to the basaltic xenoliths, with chemical exchanges.

It is here demonstrated that a short period of time (some ten hours to a few days) separates the mixing event from the eruption, outlining the importance of magma mixing in the triggering of eruption. The common occurrence of basaltic xenoliths (generally of « H » type) in calc-alkaline lavas is emphasized, showing that this mechanism is of first importance in calc-alkaline magma petrogenesis.     

Geothermometry and geobarometry of a cummingtonite-bearing dacite from Martinique,Lesser Antilles

A dacite sample from a nuée ardente deposit at Pitons du Carbet, Martinique, contained cummingtonite, orthopyroxene, quartz, magnetite, and ilmenite as phenocrysts, and thus was suitable for evaluation of the conditions of crystallization of nuée ardente materials according to the method of Ewart et al. (1971). The estimations were obtained with the aid of a Fortran IV program, which permitted the operation of all the arrangements between the available microprobe analyses. The results indicated that the phenocrystic association crystallized at moderate temperatures (ca. 750° C), under high oxygen fugacities (> NNO) and total pressures (2.3–4.5 Kb, with a maximal density of points at about 3.5 Kb), and water pressures approximating total pressures. The groundmass Fe-Ti oxides equilibrated at lower temperatures and under relatively increasing oxygen fugacities, the variation of which is tentatively ascribed to the aerial emplacement of the nuée ardente materials. The phenocryst data imply that in the case studied the high fluid pressures characterizing the nuée ardente eruptions were not acquired at a shallow level because of the deep intratelluric stage of crystallization of these differentiated calcalkalic magmas.     

Partial melting below Tubuai (Austral Islands, French Polynesia)

Process identification diagrams based on trace element data show that mafic lavas from Tubuai, including alkali basalts, basanites, analcitites and nephelinites, result from different degrees of partial melting of an isotopically homogeneous mantle source. Our fractionation-corrected data are consistent with a batch melting model or a dynamic melting model involving a threshold value for melt separation close to 1% and degrees of melting ranging from 5–8% (alkali basalts) to 1.5–3% (nephelinites). The relative source concentration pattern, calculated using an inverse numerical method, shows an enrichment in highly incompatible elements. We propose that the Tubuai lava suite was derived from a two-stage partial melting process. Melting first affected the plume material located within the transition zone between garnet and spinel domains, producing alkali basalts and basanites. Then, the melting zone migrated upwards to the base of the overlying spinel-bearing lithospheric mantle, producing highly silica-undersaturated lavas. The lower lithosphere had previously been enriched by intrusion of pyroxenite veins representing plume-derived melts which percolated away from the main magma conduits. Received: 11 June 1996 / Accepted: 8 January 1997     

Représentation graphique de la cristallisation fractionnée des séries volcaniques. Application aux séries alcalines

Résumé Les modalités de la cristallisation fractionnée des séries volcaniques peuvent Être suivies en reportant sur le diagramme rectangulaire R₁=6 Ca+2 Mg+Al; R₂=4 Si–11 (Na+K)–2 (Fe+Ti), (en nombres de millications dans l'analyse) les compositions des liquides successifs (laves aphyriques) et des phases qui fractionnent (phénocristaux, cumulats). A titre d'exemple d'application sont comparées deux séries alcalines de la Polynésie. La série basalte alcalin-phonolite de l'Île de Rapa (Australes) résulte de la séparation de minéraux du plan critique (olivine-plagioclase-clinopyroxène) du tétraèdre basaltique, alors que dans l'Île de Nuku-Hiva (Marquises) la différenciation conduit à des trachytes quartzifères en raison du fractionnement de phases hydroxylées sous-saturées en silice (amphibole et mica).

---

The fractional crystallization sequence of volcanic series can be studied with the rectangular diagram R₁=6 Ca+2 Mg+Al; R₂=4 Si–11 (Na+K)–2 (Fe+Ti), (numbers of millications in the analysis) on which are reported the compositions of successive liquids (aphyric lavas) and fractionating minerals (phenocrysts, cumulates). The usefulness of this diagram is illustrated in the comparison of two alkali basalt series from Polynesia. The alkali basalt-phonolite association of Rapa island (Australs) is a result of the fractionation of minerals belonging to the critical plane (olivine — plagioclase — clinopyroxene) of the basalt tetrahedron. In the Nuku-Hiva (Marquesas) series, the origin of oversaturated trachtytes results from the fractionation of undersaturated hydrous minerals (amphibole and mica).

---

Zusammenfassung Der Vorgang einer fraktionierten Kristallisation vulkanischer Reihen kann in folgendem rechtwinkligen Diagramm (Gehalte in Millikationen ausgedrückt) gut verfolgt werden: R₁=6 Ca+2 Mg+Al; R₂=4 Si–11 (Na+K)–2 (Fe+Ti), indem die Zusammensetzung der aufeinanderfolgenden Schmelzen (aphyrische Laven) und der auskristallisierenden Phasen (Phänokristalle und Kumulate) eingetragen werden. Als Anwendungsbeispiel werden zwei Alkali-Reihen aus Polynesian verglichen. Die Reihe Alkali-Basalt — Phonolit der Insel Rapa (Südliche Inseln) entstammt der Abtrennung von Mineralien, die auf der kritischen Ebene (Olivin, Plagioklas, Klinopyroxenen) des basaltischen Tetraeders liegen. Auf der Insel von Nuku-Hiva (Marquesas Inseln) hingegen, führt die Differentiation zu Quartz-Trachyten, bedingt durch die Fraktionierung in SiO₂-untersättigte und Hydroxyl-haltige Minerale (Amphibol und Glimmer).

---

( ): jhen — — — . 2 . — — — , (-- ) . - — —. , , .

     

Melting of acid xenoliths into a basanite: An approach to the possible mechanisms of crustal contamination

Biotite gneiss xenoliths included in a basanitic flow in the Causses region (France) show several kinds of partial melting. Rhyolitic glasses appear at the expense of the quartz + feldspar assemblage; initially undersaturated latitic glasses at the expense of biotite + feldspar (+ quartz); lastly hyperaluminous products derivating from large phenoblasts of alkali feldspar. The microprobe study of these glasses, of their contacts with the host lava, as well as that of the neogenic mineral phases, show that contamination occurs through, at least, four possible processes:

1. Diffusion at the interface between two liquids of contrasted composition (rhyolitic and basanitic).
2. Mechanical mixing of two liquids of less contrasted chemistry (latitic and basanitic).
3. Progressive solution of a solid phase (quartz) into the magma.
4. Vapour phase transfers.

Qualitative magmatic contamination in alkalies (mainly K) and silica results from all these processes; quantitatively, the most important of them seems to be 4.     

An extraction,analysis, and prioritization of Asna river sub-basins,based on geomorphometric parameters using geospatial tools

The Asna river basin is located in Hingoli and Nanded districts of Marathwada region of Maharashtra. A geomorphometric analysis is an important method for the investigation and management of natural resources of watershed. The geomorphometric analysis of Asna river basin classifies three sub-basins that have been delineated using GIS and remote sensing through measurements of linear, aerial, and relief aspects. The Asna river basin comprises an area of 1187 km² with seventh-order drainage pattern. As per Strahler classification, the upper part of the basin shows dendritic to sub-dendritic and the lower part exhibits parallel to sub-parallel drainage pattern. The total numbers of stream segments are 2422 and length of streams is 2187.92 km. The bifurcation value ranges from 1.26 to 5.58 indicating that there are no structural disturbances. The form factor value (0.49) indicates that the shape of the basin is moderately circular. The high values of drainage density, stream frequency, and low infiltration number indicate the high runoff due to impermeable lithology. The slope of the basin varies from 1 to 32.2%, terrain elevation ranges from 333 to 551 m, and overall relief of the basin is 218 m amsl. River sub-basin prioritization has an immense importance in natural resource management, especially in semi-arid regions. The present study is an attempt to prioritize the sub-basins of Asna river based on geomorphometric parameters. The weightage is assigned to different morphometric parameters of sub-basins based on erosion potential. The Asna river sub-basins have been classified into three categories as high, medium, and low on the basis of priorities for soil and water conservation. It is confirmed that sub-basin I is characterized as highly vulnerable to erosion and has high sedimentation load; sub-basin II has low priority, i.e., very low erodibility; and sub-basin III is of moderate type. The morphometric analysis and prioritization methods can be applied to hydrological studies in surface as well as subsurface water, climatic studies, rainwater harvesting, groundwater recharging sites, and watershed management.