Jacques Leterrier

Clinopyroxene composition as a method of identification of the magmatic affinities of paleo-volcanic series

A set of discrimination diagrams have been derived from a statistical study of the Ti, Cr, Ca, Al and Na contents of over 706 analyses of calcic clinopyroxene phenocrysts from recent volcanic rocks of various magmatic types from various tectonic settings. Using these diagrams three major basaltic groups can be distinguished with greater than 80% confidence: alkali basalts and related rocks, tholeiites from spreading centres and orogenic basalts. Thesediagrams can be used to identify the magmatic affinities of paleo-volcanic series where, in spite of metamorphic or metasomatic transformations, primary clinopyroxene phenocrysts are commonly preserved. This method is applied successfully to six examples of Mesozoic to Paleozoic metabasites and spilitized volcanic rocks.

Magmatic source enrichment by slab-derived melts in a young post-collision setting, central Mindanao (Philippines) ☆

Abstract Central Mindanao was the locus of a Pliocene (4–5xa0Ma old) arc–arc collision event followed by basaltic to dacitic magmatism starting at 2.3xa0Ma, representing the most voluminous volcanic field in the Philippines. Lava compositions range from calc-alkaline to shoshonitic. Adakites and Nb-enriched basalts are among the magmatic products. All the lavas are Na-rich (up to 4.88%), with Na 2 O/K 2 O ratios from 2.5 to 6.5. Sr, Nd and Pb isotopic compositions are similar to MORB, except for some shoshonitic lavas that have slightly less radiogenic Nd ratios. K-enrichment in basalts can be related to both fractional crystallization (FC) at moderate pressures and to partial melting of an enriched source. Trace element systematics indicate that the sub-central Mindanao mantle is characterized by the presence of garnet, phlogopite, amphibole, and perhaps some titanate phase. The enrichment of this source is attributed to the interaction of slab-derived melts, i.e., adakites, with the arc mantle. This would explain the presence of Nb-enriched basalts, transitional adakites and high-magnesium andesites, as well as the bulk Na-enrichment and relatively unradiogenic character of the central Mindanao lavas. We envision an ion-exchange type of enrichment, in which the HFSE, LILE and LREE, mobilized during slab melting, are preferentially enriched in the metasomatized mantle, resulting in a diversity of post-collision magma compositions. The MORB-like isotopic signatures of the central Mindanao lavas preclude important contributions of slab-derived hydrous fluids, sediments, continental crust or an OIB-type contaminant. Slab melting after cessation of subduction is deemed possible by thermal rebound of previously depressed geotherms. Initial contributions to mantle enrichment in post-collision sites may thus come from slab melts. In most other cases of post-collision magmatism, however, this signature can be easily masked by enrichments coming from other sources, e.g., the continental lithosphere.

U–Pb zircon and monazite geochronology of post-collisional hercynian granitoids from the Central Iberian Zone (Northern Portugal)

Abstract In the Central Iberian Zone (CIZ) of the Iberian Massif large volumes of granitoids were emplaced during the post-collisional stage of the Hercynian orogeny (syn- to post-D3, the last ductile deformation phase). Twelve granitic units and a quartz monzodiorite were selected for a U–Pb zircon and monazite geochronological study. They represent successive stages of the D3 event. The Ucanha-Vilar, Lamego, Sameiro and Refoios do Lima plutons are coeval (313±2 Ma, 319±4 Ma, 316±2 Ma and 314±2 Ma, respectively) and belong to the earliest stage. Later on the Braga massif was emplaced, its different units yielding the same age: 309±3 Ma for the Braga granite, 309±1 Ma for the Gonca granite and 311±5 Ma for a related quartz monzodiorite. The Braga massif is subcontemporaneous with the Agrela and Celeiros plutons (307±3.5 Ma and 306±2 Ma, respectively), in agreement with field data. The Briteiros granite is younger (300±1 Ma), followed by the emplacement of the Peneda–Geres massif (Geres, Paufito, Illa and Carris granites). The Geres granite, emplaced at 296±2 Ma, seems to represent a first magmatic pulse immediately followed by the intrusion of the Paufito granite at 290±2.5 Ma. For the Carris granite a minimum emplacement age of 280±5 Ma was obtained. Based on these results the following chronology is proposed: (1) syn-D3 biotite granitoids, 313–319 Ma; (2) late-D3 biotite-dominant granitoids, 306–311 Ma; (3) late- to post-D3 granitoids, ca. 300 Ma; (4) post-D3 granitoids, 290–296 Ma. These chronological data indicate that successive granitic intrusions were emplaced in the CIZ during a short time span of about 30 Ma that corresponds to the latest stages of the Hercynian orogeny. A rapid and drastic change occurred at about 300 Ma, between a compressive ductile tectonic regime (D3, ca. 300–320 Ma) associated to calc-alkaline, monzonitic and aluminopotassic plutonism and a fragile phase of deformation (D4) which controlled the emplacement of the subalkaline ferro-potassic plutonism at 290–296 Ma.

The geochemistry and tectonic setting of the northern section of the Luzon arc (The Philippines and Taiwan)

Abstract The Luzon arc consists of a 1200 km chain of stratovolcanoes and volcanic necks stretching from Mindoro (13° N) to the Coastal Range of Taiwan (24° N). This study is concerned with three of the five major segments along the arc: the Northern Luzon, Babuyan, and Taiwan segments. The late Tertiary to Quaternary volcanics of these segments are primarily andesitic but range in composition from basalt to rhyolite and are typical arc volcanics: porphyritic plagioclase textures, primarily calc-alkaline with a few tholeiitic volcanic centers, low TiO 2 concentrations, and low high-field strength element (HFSE) to large-ion lithophile element (LILE) ratios. There is a large range in K 2 O and other LILEs from low-K tholeiites to high-K calc-alkaline suites. Calayan island and Mt. Tabungon (volcanic substratum of Mt. Cagua) volcanics in the Babuyan segment and most rocks from the Taiwan Coastal Range are low-K tholeiites (e.g., nearly flat REE patterns, low LREE and LILE concentrations, and low Th/U ratios). The dominant calc-alkaline series ranges from medium-K to high-K rocks. The high-K calc-alkaline rocks are mainly young and are found on Batan and Lutao islands. When the low-K tholeiitic and medium- and high-K calc-alkaline rocks are associated in the same region, the low-K rocks are usually older. There is a general relationship between K 2 O (and other LILE concentrations) and K-Ar radiometric dates. To some extent, 87 Sr 86 Sr ratios also increase with time throughout the arc. There is a correlation between latitude and 87 Sr 86 Sr , and a positive correlation between 87 Sr 86 Sr and [La Sm] CN . An increase in both these geochemical parameters has been associated with an increase in the input of continental crustal material in other arc regions. The lowest 87 Sr 86 Sr ratios found along the entire Luzon arc (the samples from Baguio) are equivalent to those found in samples from mid-Tertiary plutons from Northern Luzon and the Bicol arc. Both groups of rocks are associated with westward subduction along the Philippine Trench where presumably deep oceanic sediments with little or no continental crustal component have been subducted. The absence of continental crust below the arc in the Northern Luzon and Babuyan segments has been suggested by several groups of researchers. This, together with the fact that metasomatized ultramafic nodules (probably of mantle origin) in disequilibrium with their Batan host lavas (but with similar 87 Sr 86 Sr and [La Sm] CN ratios to the host rocks), suggests that upper level assimilation (AFC processes) does not appear to be the major influence on the geochemical signatures. A collision zone between the upper crustal block of Eastern China and Taiwan and the Manila Trench has been recorded in the northern section of the arc (Taiwan). Sediments have also been shown to decrease in thickness along the South China Sea basin nearly parallel to the Manila Trench from north to south. The sediment source region is most probably Taiwan, and perhaps Eastern China. This latitudinal in variation sediment thickness may explain the crustal signature related to both geochemical and age parameters. We suggest that this crustal input has taken place via subduction of sediment rich in a crustal component.

Trace element behavior in the alkali basalt-comenditic trachyte series from Mururoa Atoll, French Polynesia

Abstract Numerous drill holes have penetrated close to 1000 m of the volcanic pile of Mururoa atoll. The rocks are a typical example of mildly alkaline intraplate basaltic volcanics ranging from Mg-rich compositions to comenditic trachytes. Evolved basalts and hawaiites are the dominant rock types. Benmoreites and trachytes are relatively uncommon. The available Kue5f8Ar ages indicate a long period of activity between 11.8 and 10.7 Ma. Nevertheless, all the volcanic rocks studied are cogenetic in a broad sense (identical 143 Nd 144 Nd and initial 87 Sr 86 Sr ratios, constancy of ratios of highly incompatible elements). No indications of the occurrence of assimilation or magma mixing have been found. The Mururoa series is thus suitable for the study of fractionation-related processes. Three crystallization/fractionation models have been tested using trace elements, the mineral proportions taken from mass-balance calculations on major elements and individual distribution coefficients determined from trace element data on phenocrysts/host rock pairs. Closed-system fractional crystallization (CSF) is consistent with all of the trace element data. It satisfactorily reproduces the trends observed for compatible and incompatible elements, including the complex behavior of Y and rare-earth elements which are fractionated by kaersutite and apatite in mugearitic and benmoreitic magmas. In situ crystallization and equilibrium crystallization produce trends very close to Rayleighs law model for most incompatible elements. In contrast, in situ and equilibrium crystallization models produce significant underdepletions (compared to CSF) in compatible elements (Sc, Cr, Co, Ni). The corresponding patterns are not consistent with our analytical data in intermediate and evolved rocks. Cooling calculations indicate, however, that the length of volcanic activity in Mururoa ( ≥ 1 Ma) is hardly compatible with closed-system fractional crystallization of a single magma batch. Various models of open-system fractionation have thus been tested, but generally they do not fit the observed trace element patterns. However, one peculiar case of open-system fractionation in a periodically replenished magma chamber is consistent with the data. In this model, batches of mantle-derived parent liquids filling up the chamber at the beginning of each cycle evolve by fractional crystallization. All the corresponding residual liquids either crystallize or erupt at the end of each cycle, before the next replenishment. Thus, the apparently cogenetic Mururoa series is likely to result from fractional crystallization occurring under similar conditions through time, either in several magma chambers or in a single periodically refilled reservoir.

The subalkaline potassic magmatism of the Ballons massif (Southern Vosges, France): shoshonitic affinity

Abstract A systematic geochemical study of the Ballons plutonic complex (Southern Vosges, France) indicates that the border facies of the massif are composed of two continuous differentiation series ranging from gabbros to quartz monzonites and very similar to the shoshonitic associations (high K2O/Na2O ratios as well as high Ba, Sr, U and Th contents). The existence of a geochemical discontinuity between the two series and the monzogranite, which is the principal component of the massif, argues against the monzogranite being directly related to the two differentiation series by fractional crystallization. The zoned structure of the monzogranite can be attributed to in situ crystal fractionation.

Géochronologie U-Pb sur zircon de granitoïdes éburnéens et panafricains dans les boutonnières protérozoïques d'Igherm, du Kerdous et du Bas Drâa (Anti-Atlas occidental, Maroc)

Abstract U-Pb ages obtained from zircons of granitoids from the Proterozoic western Anti-Atlas confirm the existence of Palaeoproterozoic magmatism in the inliers of Igherm (Ait Makhlouf granite: 2 050±6 Ma) and Bas Drâa (Sidi Said granite: 1 987±20 Ma) and neoproterozoic magmatism in the inliers of Bas Drâa (Taourgha granite: 575±4 Ma) and Kerdous (Tarcouate granodiorite: 583±11 Ma, Tarcouate gabbro-diorite: 560±2 Ma). The emplacement of these granitoids is therefore polycyclic: Eburnian and Panafrican.

Granitoïdes de la zone houillère briançonnaise en Savoie et en Val d'Aoste (Alpes occidentales): géologie et géochronologie U-Pb sur zircon

Resume La Zone Houillere Brianconnaise (ZHB) est la premiere unite litho-tectonique continue surmontant le Front Pennique a lEst. Elle est constituee de terrains dâge namurien a Stephanien et de lambeaux de couverture permo-triasique. Comme le reste du domaine brianconnais et du Grand Saint Bernard, elle comporte des unites ≪ gneissiques ≫ dont lâge, lorigine et la position tectonique sont controverses. Les ≪ gneiss du Sapcy ≫ et les metagranites de la Costa Citrin (Val dAoste) sont etudies dans cette note. Levolution tectonique alpine de la ZHB consiste en un ecaillage precoce accompagne dune foliation non penetrative dans les formations houilleres, suivi par une phase majeure (schistosite regionale) et par des phases tardives ou lextension joue un role important. Les parageneses de facies amphibolite observees dans les gneiss du Sapey. malgre les deformations et recristallisalions alpines indiquent quil sagit dun socle polymetamorphique antealpin. La datation U-Pb de zircons de trois echantillons de gneiss du Sapcy confirme cette interpretation: âges minimum (intercepts interieurs) plus vieux que. 360 Ma et heritage dâge prolerozoique. Par contre, deux metagranites de la Costa Citrin ont fourni des âges viseens de 324±17 Ma et 323±8 Ma. Lexistence de lambeaux de socle hercynien precoce ou plus ancien, dun episode magmatique dâge viseen a namurien dans la ZHB ainsi que lâge namurien et westphalien recemment revise des sediments appuient lhypothese de lexotisme du domaine brianconnais par rapport aux domaines externes. En effet la ZHB avec son socle dâge ante-carbonifere et son magmatisme anterieur a synchrone par rapport a une sedimentation dâge carbonifere moyen doit etre opposee aux bassins stephaniens tardi-orogeniques et a levolution magmatique dâge Stephanien observes dans le Massif Central oriental et les Massifs Cristallins Externes des Alpes. La signification du plutonisme viseen-namurien de la Costa Citrin est discutee dans le cadre de la chaine hercynienne europeenne: il est la consequence soit dune subduction tardi-hercynienne de lu Paleo-Tethys. soit plutot dun stade dextension precoce anterieur au collapse post-orogenique.

Potassic volcanism in Central Java and South Sulawesi, Indonesia

Abstract This study deals with fifteen Neogene and Quaternary K-rich volcanic series from the back-are area of Central Java and from South Sulawesi. The rock associations have been subdivided into three types: (1) silica-saturated or -oversaturated potassic series (SK) either potassic calc-alkaline (Lasem, Java) or shoshonitic (Patiayam, Java; Parepare, Sopeng 2 and part of Lompobatang, South Sulawesi); (2) weakly silica-saturated (ne 10%) ultrapotassic series, usually leucite-bearing (UK: Muria 2, Bawean in Java; Camba 2b Formation, Sopeng 1 Formation in Sulawesi). Comparative geochemistry of the SK, AK and UK associations show that the above distinction is consistent with the behaviour of incompatible elements (increase of K, Rb, Sr, Ba and of La/Yb ratios from SK to UK series). All these rock series show, moreover, “orogenic” chemical characteristics compatible with a subduction-related environment. However, in South Sulawesi, their emplacement clearly post-dates the latest known subduction event; in Central Java they do not fit with any model of increasing K 2 O with depth of the Benioff plane, and the location of the UK series is independent from the latter (the Quaternary UK Series on Bawean is located away from the 600 km isobath). Thus we strongly prefer a genetic model for the studied K-rich volcanic series by melting of mantle sources enriched in incompatible elements during previous subduction events, and possibly involving a contribution of subcontinental mantle (Central Java).

Âge Paléozoïque inférieur (U—Pb sur zircon) de métagranophyres de la nappe du Grand-Saint-Bernard (zona interna, vallée d'Aoste, Italie)

Abstract Early Paleozoic U—Pb age of zircons from metagranophyres of the Grand-Saint-Bernard Nappe (zona interna, Aosta Valley, Italy). The U—Pb age of a metagranophyre from the Val de Rhemes (Brianconnais basement of the Aosta valley) of 511 ± 9 Ma suggests that the micaschists which dominate the pre-alpine basement of the Grand-Saint-Bernard Nappe are, in part, of Early Paleozoic age. The Brianconnais domain is characterized by a Cambrian to Ordovician alkaline to subalkaline (monzonitic) magmatism, similar in age to orthogneisses known in other parts of the Variscan belt and generally interpreted as related to an extensional event.