Raymond Montigny

Deccan flood basalts at the Cretaceous/Tertiary boundary?

Abstract Joint consideration of new paleomagnetic, paleontological and geochronological data from the Deccan continental flood basalts in India and critical discussion of earlier results lead us to suggest that volcanic activity may have lasted less than 1 Ma, thus possibly ranking as one of the largest volcanic catastrophes in the last 200 Ma. Available data are best satisfied if volcanism spanned the Cretaceous/Tertiary boundary, followed shortly afterwards by rifting of the Arabian Sea. These results point out the need for further work which may help in choosing between “external” and “internal” models of the Cretaceous/Tertiary boundary events.

Oligo-Miocene rotation of Sardinia: KAr ages and paleomagnetic data of Tertiary volcanics

Abstract Conventional K-Ar dating was carried out on mineral separates, mostly plagioclases, from Middle Tertiary volcanics of Sardinia along with paleomagnetic studies. the close agreement of dates from the two methods establishes the rotation age of Sardinia relative to Europe within a rather short time span of 1.5 Ma between 20.5 and 19 Ma. From correlation of our results with the geomagnetic time scale, it is suggested that the rotation took place shortly after the magnetic anomaly 6.

How does the Alpine belt end between Spain and Morocco

The Betic-Rif arcuate mountain belt (southern Spain, northern Morocco) has been interpreted as a symmetrical collisional orogen, partly collapsed through convective removal of its lithospheric mantle root, or else as resulting of the African plate subduction beneath Iberia, with further extension due either to slab break-off or to slab retreat. In both cases, the Betic-Rif orogen would show little continuity with the western Alps. However, it can be recognized in this belt a composite orocline which includes a deformed, exotic terrane, i.e. the Alboran Terrane, thrust through oceanic/transitional crust-floored units onto two distinct plates, i.e. the Iberian and African plates. During the Jurassic-Early Cretaceous, the yet undeformed Alboran Terrane was part of a larger, Alkapeca microcontinent bounded by two arms of the Tethyan-African oceanic domain, alike the Sesia-Margna Austroalpine block further to the northeast. Blueschist- and eclogite-facies metamorphism affected the Alkapeka northern margin and adjacent oceanic crust during the Late Cretaceous-Eocene interval. This testifies the occurrence of a SE-dipping subduction zone which is regarded as the SW projection of the western Alps subduction zone. During the late Eocene-Oligocene, the Alkapeca-Iberia collision triggered back-thrust tectonics, then NW-dipping subduction of the African margin beneath the Alboran Terrane. This Maghrebian-Apenninic subduction resulted in the Mediterranean basin opening, and drifting of the deformed Alkapeca fragments through slab roll back process and back-arc extension, as reported in several publications. In the Gibraltar area, the western tip of the Apenninic-Maghrebian subduction merges with that of the Alpine-Betic subduction zone, and their Neogene roll back resulted in the Alboran Terrane collage astride the Azores-Gibraltar transpressive plate boundary. Therefore, the Betic-Rif belt appears as an asymmetrical, subduction/collision orogen formed through a protracted evolution straightfully related to the Alpine-Apenninic mountain building.

Paleomagnetism and age determinations of the Deccan Traps (India): Results of a Nagpur-Bombay Traverse and review of earlier work

Review of available radiometric age determinations of the Deccan traps (India) shows a spectrum of K-Ar ages that is highly polluted by argon loss. Stepwise 40Ar-39Ar age determinations include estimates of data quality and thus avoid contaminated results. The absolute age of the Deccan traps determined using 22 40Ar-39Ar plateau age spectra is 65.5 ± 2.5 Ma. Paleontological data on infratrappean and intertrappean sediments constrain Deccan age to between the A. mayaroensis zone, in the Upper Maestrichtian (about 67 Ma), and the P2 foraminifer zone, in the Lower Paleocene (about 60.5 Ma). Paleomagnetic study of a Nagpur-Bombay traverse (preliminary results of which were used by Courtillot et al. (1986a, b) for a general discussion about Deccan volcanism and the Cretaceous-Tertiary boundary) is presented in detail. All available paleomagnetic results from the Deccan traps (563 flows) are then compiled. Results considered to be transitional or to come from suspicious sites are removed leaving 485 flow results. This extensive data set from a single geological unit allowed us to look in some detail at its statistical distribution. The virtual geomagnetic poles (VGP) are approximately Fisher distributed but present a complex asymmetry. No regional variation can be seen (to within paleomagnetic uncertainties). Although the 3.5° angular difference between the separate normal (pole) and reversed (antipole) data is not statistically significant, it can be explained by either a 2.1 m.y. drift along the apparent polar wander path (APWP) of the Indian plate assuming a normal-reverse-normal (N-R-N) magnetostratigraphy, or a 3.5% contamination by a present field overprint, or a slight nondipole field component. A quality coefficient has been assigned to each result on the basis of existence and value of published 95% confidence angle. Because the normal and reversed mean poles become more precisely antipodal with higher-quality data and with more recent publication date (as a consequence of the evolution of paleomagnetic techniques), we favor the overprint hypothesis. The angular standard deviation of our VGP set is 20% larger than the value predicted by the paleosecular variation model of McFadden and McElhinny (1984). Finally, our best estimate of the overall mean pole is located at 281.3°E, 36.9°N, A95 = 2.4° (calculated from the 163 highest-quality flow VGPs). It is in remarkable agreement with the reference APWP of Besse and Courtillot (1991), leading to an independent estimate of the age of Deccan volcanism at 67.2 ± 6.6 Ma. The same comparison following Acton and Gordon (1989) provides an estimated age at 67 ± 5 Ma. These estimates which are based on paleomagnetic data only are in perfect agreement with both radiometric and paleontological ages.

Geochemistry of the mantle beneath the Rodriguez Triple Junction and the South-East Indian Ridge

Rare earth element abundances and Sr, Nd. Pb isotope compositions have been measured on zero-age dredge samples from the Rodriguez Triple Junction (RTJ) and the South-East Indian Ridge (SEIR), Along the SEIR. the geochemical “halo” of the St. Paul hot spot has a half-width of about 400 km and the data may be fairly well accounted for by a binary mixing between an Indian MORB-type component (87Sr/86Sr = 0.7028. 143Nd/ 144Nd = 0.51304. 206Pb/204Pb = 17.8) and the plume-type St. Paul component (0.7036, 0.5129, and 18.7 respectively). The alignment of the lead isotope data is particularly good with an apparent age of 1.95 ± 0.13 Ga and Th/U source value of 3.94. One sample dredged on the ridge 60 km southeast of St. Paul bears a definite Kerguelen isotopic signature. The RTJ has distinctive geochemical properties which contrast with those of the adjacent ridge segments. Low 206Pb/204Pb ratios which plots to the left of the geochron, rather high 208Pb/204Pb and 87Sr/87Sr ratios (17.4. 37.4, and 0.7031 respectively), a striking isotopic homogeneity, and variable LREE/HREE fractionation with (La/Sm)N, = 0.3–0.8 make this triple junction an anomalous site. The geochemical properties of the Indian Ocean basats have been examined using a three-component mantle model involving (a) a normal MORB-type source though to represent the depleted upper mantle matrix, (b) an OIB-type source of uncertain parentage (recycled oceanic crust?), and (c) a component with low μ. low Sm/Nd. high Rb/Sr (time-averaged value) which is tentatively assigned to ancient hydrothermal and abyssal sediments recycled in the mantle. The high 208Pb/204Pb and 87Sr/86Sr ratios typical of the Dupal anomaly are likely due to the widespread distribution of this latter component in the basalt source from this area. including that for MORBs.

K-Ar dating of some infra-ophiolitic metamorphic soles from the Eastern Mediterranean: New evidence for oceanic thrustings before obduction

Abstract K-Ar dating on separated minerals from the infra-peridotitic metamorphic soles outcropping beneath various eastern Mediterranean ophiolite complexes (Pindos nappe in Greece; Lycian, Antalya, Beysehir-Hoyran, Mersin and Pozanti-Karsanti nappes in the Taurides belt. Turkey; Baer-Bassit nappe in Syria) shows that the main metamorphic recrystallization of those rocks occurred 25–30 m.y. prior to the tectonic emplacement of the ophiolite complexes onto a continental platform. These data support the hypothesis of an early slicing of the oceanic crust before its obduction, advocated recently by several authors.

The Rif mountain building (Morocco); a new tectonic scenario

The building of the Alpine Rif belt (southern limb of the Betic-Rif orocline) is restored, mostly based on the Tertiary stratigraphic and metamorphic data set. The Betic-Rif Internal zones derive from an exotic Alboran Terrane partly involved in a S-dipping Betic subduction during the Late Cretaceous ?-Eocene. Incipient collision of the terrane against Iberia triggered back-thrust tectonics south of the Internal mountain belt during the latest Eocene-Oligocene. A N-dipping Maghrebian subduction developed from that time up to Middle Miocene, responsible for the rifting of the internal Alboran Terrane. Docking of the extending Alboran Terrane onto the North African margin occurred during the Neogene through the closure of the Maghrebian Flysch oceanic trough, with southwestward growth of the external accretionary prism, and foredeep subsidence. Subduction zone westward roll back associated with delamination of the dense lithosphere seem to account for the Betic-Rif late orogenic evolution.

Is the Vourinos Complex an island arc ophiolite

Abstract A geochemical study has been undertaken on the Vourinos ophiolites, northern Greece, a complex long known for its unusual characteristics such as an environment of acidic rocks and a calc-alkaline chemical affinity. The Nd-Sr isotopic ratios and the Hf/Th and Ta/Th ratios are indicative of an island arc origin for Vourinos as opposed to the mid-oceanic ridge origin inferred for other ophiolites such as Inzecca, Corsica. Other data on trace elements confirm that the cumulative suite and the lavas originated from the same magma through a simple fractional crystallization process and show that this magma would have formed through partial melting of an already highly-depleted material. It is thus possible to distinguish ophiolites with MORB characteristics from island arc ophiolites such as the Vourinos Complex, the existence of the latter type imposing new constraints on the possible tectonic processes for emplacement.

Pétrologie du complexe alcalin sous-saturé de Kokoumi (Cameroun)

The Cameroon Line was created by the rejuvenation, at the beginning of the opening of the Atlantic Ocean, of a Pan-African N070 degrees E fracture zone [Moreau et al., 1987], which acted as a huge lithospheric crack taping a hot asthenospheric zone [Deruelle et al., 1998; Marzoli et al., 2000]. The Kokoumi anorogenic pluton belongs to the E-W Garoua rift structure, which represents the easternmost extension of the Benue trough. The Garoua rift opened during the Neocomian-Lower Aptian ages [Benkhelil, 1988] through the rejuvenation of Pan-African normal faults. The rift subsided, was partially filled by conglomerates and sandstones, and the ensemble was folded in the Cretaceous period [Guiraud, 1993]. Post-Cretaceous faulting affected these sediments. Intrusion of the Kokoumi anorogenic complex through the Cretaceous sandstones was favoured by N-S, N070 degrees E, E-W and N135 degrees E faults and N030 degrees E extension [Moreau et al., 1987]. The Kokoumi complex was first described by Koch [1959]. It is composed of a plutonic gabbro-nepheline monzosyenite-nepheline syenite series and of lamprophyric dykes (monchiquites and camptonites). One trachyte dyke is also observed. The gabbros are olivine (Fo 70 )-, nepheline-, or kaersutite-bearing gabbros. They also contain Ti-Al-rich diopside, Ti-rich biotite, titanite, ilmenite, Ti-magnetite and apatite. The nepheline monzosyenites contain diopside, Fe-diopside, kaersutite, Fe-kaersutite, titanite and apatite. The nepheline syenites contain aegirine-augite, F-rich arfvedsonite and aenigmatite. Kaersutite and clinopyroxene predominate in the lamprophyres. Monchiquites and gabbros, camptonites and monzosyenites, display respective similar mineralogy. Monchiquites contain carbonate ocelli. The trachyte does not contain ferromagnesian minerals. For gabbros and monchiquites, equilibrium Fe-Ti oxide temperatures are between 650 and 750 degrees C (+ or -40 degrees C) and oxygen fugacities between 10 (super -15) and 10 (super -14) (+ or -0.5 X 10 (super -15) ) atmospheres, according to Spencer and Lindsley [1981]. Nepheline crystallized below 700 degrees C, according to Hamilton [1961]. All the rocks (except the trachyte) are nepheline normative (Ne 6 to Ne 40 ). Major and trace element distributions in MgO-element diagrams for the two series merge together into a single trend, from monchiquites to nepheline syenites. Nevertheless, the monchiquites trends have different slopes. We deduce the evolution from gabbros to nepheline syenites on the one hand and from monchiquites to camptonites on the other from primitive mantle normalized multi-element diagrams. Multi-element diagrams for the trachyte and the nepheline syenite are strictly similar. Patterns for Kokoumi gabbros are similar to those for basalts of the Kapsiki plateau [Ngounouno et al., 2000] and the Garoua rift [Ngounouno et al., 1997] with typical negative K and positive Zr and Ti anomalies. Patterns for nepheline monzosyenites display negative anomalies in Sr, P, Eu and Ti and those for nepheline syenites and trachyte display greater anomalies in these elements and Ba. Compared to gabbros, nepheline monzosyenites are enriched in all REE with a concave upward pattern and no Eu-anomaly. Nepheline syenites have a range of broadly similar REE patterns to nepheline monzosyenites with steep slope from La to Sm, strong Eu negative anomaly (Eu/Eu (super *) nearly equal 0.15) and heavy-REE spoon-shape. REE patterns for monchiquites, camptonites, and trachyte are respectively similar to those for gabbros, monzosyenites, and nepheline syenite. Initial Sr-isotope ratios of 0.7033 (recalculated from the measured ratios for an age of 39 Ma for plutonic rocks and 20 Ma for the lamprophyres and the trachyte) are similar to those obtained for basalts from the continental segment of the Cameroon Line [Halliday et al., 1988; Ngounouno et al., 2000; Demaiffe et al., unpubl.], whereas nepheline syenites and trachyte are distinctly more radiogenic with values between 0.7128 and 0.7251. Amphibole and whole-rock K-Ar analyses (table III) yield 39.0+ or -0.9 Ma and 36.6+ or -0.9 Ma respectively. Since amphibole is a reliable chronometer in K-Ar dating, we propose the first age as the probable time of emplacement of the gabbros. Whole-rock analysis of nepheline syenite 99 displays an age of 33.1+ or -0.5 Ma. Field and geochemical observations suggest that gabbros and nepheline syenite are cogenetic and hence contemporaneous.

Supra-ophiolitic formations from the Thessaloniki nappe (Greece), and associated magmatism : An intra-oceanic subduction predates the Vardar obduction

Abstract The Vardar ophiolites from Chalkidiki (Thessaloniki nappe) show autochthonous cover formations. These formations include reefal limestones of Kimmeridgian-Tithonian age, and clastic deposits with spilite, andesite and granodiorite elements, suggesting an intra-oceanic island arc setting. The earliest continental clasts are found in post-Tithonian layers. A granodioritic intrusion associated with granitoid dykes crosscutting the ophiolite corresponds to deep parts of the arc. The Chaikidiki arc, 150–140 My old, can be followed northward for at least 150 km (Guevgueli). The Vardarian obduction would have occurred during the Early Cretaceous owing to the arc-continent collision.