J. Dostal

Zr/hf fractionation in intraplate basaltic rocks: Carbonate metasomatism in the mantle source

In continental and oceanic intraplate basaltic rocks, the ZrHf ratio is frequently higher than the chondritic value and varies over a large range of 38–87. The ratio increases with the LaSm ratio and, in each volcanic locality, is positively correlated with the degree of silica undersaturation of the basaltic rocks. The variation of the ZrHf ratios probably reflects a heterogeneous mantle source variably metasomatized by carbonate-rich fluids.

Carbonate metasomatism in the lithospheric mantle: peridotitic xenoliths from a melilititic district of the Sahara basin

The petrological and geochemical study of harzburgitic and dunitic xenoliths from the melilititic district of In Teria (Algerian Sahara) shows that the lighospheric mantle of this region has been affected by a multi-stage metasomatism. The first metasomatic event is related to the injection of alkali silicated (basaltic or kimberlitic) melt and was responsible for the crystallization of phlogopite at depths ranging between 80 and 100 km and the crystallization of amphibole at about 60 km. During this first event, carbonate probably precipitated in the garnet stability field. In a second stage, the spinal peridotites suffered strong mineral changes resulting in an extensive formation of high-Cr endiopside and leading to conversion of harzburgite and dunite into lherzolite and wehrlite. These changes are associated with an enrichment in the most incompatible trace elements including light REE (rare-earth elements), Ta, Th and variable values of ratios such as Th/La and Ta/La. This second event is atributed to the injection (under conditions of decarbonatation and release of CO2) of a carbonatitic melt resulting from incipient melting of the garnet peridotites, which were previously carbonated. This interpretation is corroborated by the calculation of a diffusion-percolation model which reproduces well the observed distribution of incompatible trace elements in the spinel peridotites. Given the proposed sequence of events, it appears that most of the specificities of the In Teria xenoliths can be explained by the successive geochemical modifications induced within the lithospheric mantle during reheating.

Geochronology and geochemistry of Grenvillian igneous suites in the northern Oaxacan Complex, southern Mexico: tectonic implications

Chemical and U � /Pb isotopic analyses of metaigneous rocks in the northern Oaxacan Complex in southern Mexico indicate that they form part of two granitic � /gabbroic suites intruded at � /1157 � /1130 and � /1012 Ma, which were metamorphosed under granulite facies conditions between � /1004 and 980 Ma. Although the older suite has both within-plate and arc geochemical signatures, the arc characteristics (enrichment of La and Ce relative to Nb, Ta, and Th) are inferred to result from crustal contamination, a conclusion consistent with their negative oNd signatures. The younger suite is spatially associated with anorthosites (from which we were unable to acquire a protolith age), suggesting that collectively it forms part of anorthosite � /mangerite � /charnockite � /granite (AMCG) suites. The tholeiitic nature of the mafic rocks along with the within-plate character of the felsic rocks suggests that they were intruded during extension related to either farfield backarc rifting, rifting above a slab window, or anorogenic intercontinental rifting. Potentially correlative AMCG suites are widespread in Mexico, the Grenville Province of eastern Canada and northeastern USA, and the Andean massifs of Colombia, however, Pb isotopic data most closely resemble those in South America. These data are consistent with published hypotheses that suggest Oaxaquia represents an exotic terrane derived from Amazonia. # 2002 Elsevier Science B.V. All rights reserved.

Contrasting behaviour of Nb/Ta and Zr/Hf ratios in a peraluminous granitic pluton (Nova Scotia, Canada)

Abstract Two pairs of high-field-strength elements (Nb–Ta and Zr–Hf) display contrasting behaviour during the evolution of a peraluminous granitic pluton that is composed of leucomonzogranites and subordinate greisen-hosted topaz-bearing leucogranites. Zr and Hf progressively decrease with differentiation and their ratio remains the same, close to the chondritic value (∼36). The crystallization of zircon which contains the bulk of these two elements in the rocks, does not modify the Zr/Hf ratio. With increasing differentiation, Nb and Ta show an increase accompanied by a distinct decrease of the Nb/Ta ratio from 14 in leucomonzogranites to 2 in leucogranites and greisens. Tantalum correlates positively with Rb, Li and F and is strongly enriched in the evolved rocks, implying that an addition of internally derived mineralizing fluids and fluid fractionation played a key role in the variation of the Nb/Ta ratio in rocks with K/Rb ratios

Geochemistry of the Neoproterozoic Tilemsi belt of Iforas (Mali, Sahara): a crustal section of an oceanic island arc

Abstract The pre-Pan-African Neoproterozoic Tilemsi belt of the western Iforas (Mali, Sahara) comprises a predominantly bimodal mafic and felsic volcanic assemblage associated with numerous intrusions mainly of mafic composition. The volcanics include island arc tholeiites and the calc-alkali rocks (rhyolites, dacites and andesites). The volcanic domain shows similarities to recent oceanic island arc systems as well as to Archaean greenstone belts. The Tilemsi magmatic sequence represents an example of a section through the middle and upper levels of a Neoproterozoic intra-oceanic island arc crust that is composed nearly exclusively of igneous material. The middle crustal section is mainly of mafic composition while the upper crustal level contains more silica-rich plutonic, volcanic and volcaniclastic assemblages. The mafic intrusions include a large proportion of cumulates indicating that fractional crystallization played an important role during the evolution of the oceanic arc suites. Compositionally, the arc changed through time from an immature to a relatively developed system. The rocks became more silica- and incompatible-element-rich with higher Sr isotopic ratios. This trend reflects the increasing growth and maturity of the arc.

Subducted and recycled lithosphere as the mantle source of ocean island basalts from southern Polynesia, central Pacific

Dupuy, C., Barsczus, H.G., Dostal, J., Vidal, P. and Liotard, J.-M., 1989. Subducted and recycled lithosphere as the mantle source of ocean island basalts from southern Polynesia, central Pacific. Chem. Geol., 77: 1-18. The Marquesas, Society and Austral-Cook Islands, three volcanic chains in the central Pacific Ocean (French Polynesia), are composed mainly of alkali basalts, basanites and tholeiites, which have geochemical characteristics typical of ocean island basalts. The lavas from the Marquesas and Society Islands display generally chondritic ratios of highly incompatible trace elements and have higher s7Sr/s6Sr than the basalts from the Austral-Cook Islands which have many trace-element ratios similar to those of mid-ocean ridge basalts. This grouping probably reflects differences in the composition of an ancient subducted and recycled lithosphere incorporated into the mantle source of the Polynesian basalts. Compared to Marquesas and Society Islands basalts, the mantle source of the Austral-Cook Islands basalts contains refractory oceanic lithosphere from which a larger amount of basaltic melt was extracted during subduction.

A Grenvillian arc on the margin of Amazonia: evidence from the southern Oaxacan Complex, southern Mexico

Abstract The rocks in the southern Oaxacan Complex of Mexico form part of the basement of Oaxaquia, a terrane that underlies most of eastern Mexico. They have undergone granulite facies metamorphism at ∼1 Ga, that was initially overprinted by lower amphibolite facies metamorphism accompanied by extensive hydration, and then by greenschist facies metamorphism. Geochemical data indicate that protoliths include volcanic arc lavas and sediments intruded by a rift-related granite. The metavolcanic rocks have SiO2 ranging between 55 and 72%, are typically enriched in highly incompatible elements such as Ba, Rb and light rare earth elements, and are accompanied by relative depletion of Nb and Ti characteristic of subduction-related magmas. The granite has SiO2 ranging between 67 and 70%, is enriched in strongly incompatible elements, and depleted in Nb and Ti indicative of either a crustal source or a subduction-related tectonic setting. Concordant, U-Pb analyses of single zircons from this granite yield an age of 1117±4 and 988±5 Ma interpreted as the age of intrusion and the time of peak granulite facies metamorphism, respectively. Combined with published information from other parts of Mexico, these data indicate that Oaxaquia was a juvenile arc metamorphosed during a relatively young Grenvillian metamorphism at 990 Ma. Similar characteristics may also be found in the Andean massifs of Colombia and in the Appalachian terranes (Carolina–Piedmont and Avalon) of Amazonian provenance. This suggests that Oaxaquia may also have lain adjacent to northwestern Amazonia at ∼1 Ga, a location that may allow Neoproterozoic subduction as the origin of the hydrous, lower amphibolite facies metamorphism. It would also provide a proximal source for low grade to unmetamorphosed ∼1 Ga and Neoproterozoic igneous boulders in northern Oaxaquia and 1000–900 Ma detrital zircons in Neoproterozoic and Paleozoic sedimentary rocks in Avalonia and Iberia.

Origin of topaz-bearing and related peraluminous granites of the Late Devonian Davis Lake pluton, Nova Scotia, Canada: crystal versus fluid fractionation

Abstract The Davis Lake pluton, in the Late Devonian South Mountain Batholith in southwestern Nova Scotia (Canada), is composed of peraluminous leucomonzogranites and subordinate topaz-bearing leucogranites that host greisens and a primary tin deposit. The concentrations of incompatible trace elements (e.g., Rb, Li, U, F, Ta and Sn) and element (e.g., K Rb , Nb Ta and La Sm ) and isotopic ratios (e.g., 206 Pb 204 Pb and 87 Sr 86 Sr display a spatial zonation. The granitic rocks were generated by extensive fractional crystallization of major phases (feldspars and biotite) and accessory minerals (monazite, apatite and zircon) in a compositionally zoned magma chamber. Fractional crystallization of the evolving F-rich peraluminous granitic magma was accompanied, particularly in later stages, by fluid fractionation involving fluorine complexing. Fluid fractionation was a major differentiation process during the terminal stages of the plutons evolution. It played an important role in the genesis of the evolved rocks which are characterized by low K Rb (24–150), Nb Ta (1–6), La Sm (1.5–4) and Eu Eu ∗ (0.01–0.3) ratios. These ratios as well as the enhanced concentrations of Li, Rb, U, Ta and Sn in these rocks correlate with F. Fluorine-rich fluids modified the Rb Sr and U Pb ratios which produced the distinct variations of 87 Sr 86 Sr and 206 Pb 204 Pb isotopic ratios. The parent magma was formed by dehydration melting of lower-crustal metasedimentary rocks. The melting was related to crustal thickening in association with terrane and collision along the eastern margin of the Appalachians.

Behaviour of trace elements during magmatic processes in the crust: Application to acidic volcanic rocks of Tuscany (Italy)

Abstract Plio-Quaternary acidic rock suites from three volcanic centres of Tuscany (Italy) — San Vincenzo, Roccastrada and Monte Amiata — are composed predominantly of rhyolitic and rhyodacitic ignimbrites. The rocks are characterized by distinct enrichments, particularly of K and Rb, and display considerable compositional differences among the various volcanic centres. The volcanic rocks of Roccastrada and most of S. Vincenzo were probably generated by partial melting of metapelitic rocks from the Paleozoic basement of Tuscany, leaving a residue composed of biotite, cordierite and quartz and sometimes plagioclase or garnet. The enrichment of K and related elements can be attributed to selective contamination by fluids. The other volcanic rocks including trachytes of Mt. Amiata have also been affected by interaction with an upper-mantle component, probably K-rich basaltic rocks. A similar petrogenetic model can be applied to the granodiorite pluton of Mt. Capanne (Elba) which belongs to the same magmatic cycle. Since the variation trends observed in the Tuscany volcanic rocks are similar to those reported from many acidic plutons, it appears that crustal anatexis accompanied by interaction with upper-mantle components has frequently played an important role in the genesis of the granitic rocks.

Palaeozoic within-plate volcanic rocks in Nova Scotia (Canada) reinterpreted: isotopic constraints on magmatic source and palaeocontinental reconstructions

Palaeozoic volcanism in the Avalon Terrane of northern Nova Scotia occurred during three time intervals: Cambrian-early Ordovician, late Ordovician-early Silurian and middle-late Devonian. In the Meguma Terrane of southern Nova Scotia, Palaeozoic volcanism is limited to the middle Ordovician. Geochemical data show that most of these volcanic rocks are bimodal, within-plate suites. Initial e Nd signa- tures range from +5.4 to -1.9 in the rhyolites and +6.8 to +2.7 in the basalts, a difference attributable to the absence or presence, respectively, of a significant crustal component. The data and regional tectonic settings of the Avalon and Meguma terranes suggest that the volcanism was generated in three different within-plate settings: (1) Cambrian-early Ordovician volcanism related to thermal decay of late Proterozoic arc magma- tism during transtensional deformation; (2) middle Ordovician-early Silurian volcanism during sinistral tele- scoping between Laurentia and Gondwana where extensional bends in the Appalachians produced rifting; and (3) Devonian volcanism resulting from lithospheric delamination during dextral transpression and tele- scoping. In each setting, active faults served as conduits for the magmas. Nd isotopic data indicate that the source of the Palaeozoic felsic volcanic rocks is isotopically indistinguishable beneath southern and northern Nova Scotia and did not substantially change with time. This crustal source appears to have separated from the mantle during the Proterozoic, a conclusion consistent with the hypothesis that the Palaeozoic rocks in Nova Scotia were deposited upon a late Proterozoic oceanic-cratonic volcanic arc terrane. The Nd data, when combined with published faunal, palaeomagnetic and U-Pb isotopic data, suggest that the Avalon Terrane was peripheral to Gondwana off northwestern South America during Neoproterozoic and early Palaeozoic times.