Jean-Yves Cottin

Isotopic (O, Sr, Nd) and trace element geochemistry of the Laouni layered intrusions (Pan-African belt, Hoggar, Algeria): evidence for post-collisional continental tholeiitic magmas variably contaminated by continental crust

The three layered intrusions studied in the Laouni area have been emplaced within syn-kinematic Pan-African granites and older metamorphic rocks. They have crystallized at the end of the regional high-temperature metamorphism, but are free from metamorphic recrystallization, revealing a post-collisional character. The cumulate piles can be interpreted in terms of two magmatic liquid lines of descent: one is tholeiitic and marked by plagioclase-olivine-clinopyroxene cumulates .troctolites or olivine bearing gabbros , while the other is calc-alkaline and produced orthopyroxene-plagioclase rich . . . cumulates norites . One intrusion WL West Laouni -troctolitic massif , shows a Lower Banded Zone where olivine-chro- mite orthocumulates are interlayered with orthopyroxene-rich and olivine-plagioclase-clinopyroxene cumulates, whereas the Upper Massive Zone consists mainly of troctolitic and gabbroic cumulates. The other two massifs are more . homogeneous: the WL-noritic massif has a calc-alkaline differentiation trend whereas the EL East Laouni -troctolitic massif has a tholeiitic one. Separated pyroxene and plagioclase display similar incompatible trace element patterns, regardless of the cumulate type. Calculated liquids in equilibrium with the two pyroxenes for both noritic and troctolitic cumulates are characterized by negative Nb, Ta, Zr and Hf anomalies and light REE enrichment inherited from the parental

Hydrothermal fluid interaction in basaltic lava units, Kerguelen Archipelago (SW Indian Ocean)

Hydrothermally altered basaltic lava-units in the northern Kerguelen Archipelago contain a wide variety of secondary silicate and carbonate minerals, including zeolites, hydrothermal calcite, dolomite and magnesite, as well as celadonite, orthoclase (adularia) and quartz. Petrography, fluid-inclusion microthermometry, trace-elements geochemistry, Sr isotopes and stable-isotope compositions indicate hydrothermal fluid cells derived from meteoric water interacting with basalts and Rb-rich subvolcanic peralkaline rocks at temperatures ranging from 50 to 200 °C associated with the cooling of the lava pile. The calculated δ 18 O values of meteoric-hydrothermal waters in fossil hydrothermal systems are identical to those in present-day hot springs, suggesting that meteoric recharge was continuous throughout the igneous cooling cycles of the 28–23 Ma older host basalts and the younger 15–5 Ma old peralkaline intrusions. The Kerguelen northern coastline hydrothermal system in the basaltic pile demonstrates that much of the silicate mineralogy and almost all carbonate secondary minerals in altered basalts were derived from meteoric-hydrothermal fluids, rather than products of seawater interaction, or even magmatic-hydrothermal fluids associated with peralkaline intrusions.

A mineralogical and geochemical study of polymict eucrite discovered in Sahara of southwest Algeria

NWA2268 is a polymict eucrite discovered in the Sahara, at southwest Algeria, close to the region of Tindouf. This meteorite weighs 65xa0g and presents a thin black fusion crust. The rock is fine- to medium-grained breccia and contains mineral fragments of plagioclases, pyroxenes, spinel, olivine and silica. The rock contains some basaltic fragments with sub-ophitic or cumulative textures, constituted by plagioclases and exsolved pigeonite. Pyroxferroite grains are present and locally destabilised in an association of hedenbergite, fayalite and silica. It also presents unequilibrated eucritic clast with heterogeneous pyroxenes and plagioclases compositions. Pyroxenes in the all of the other clasts have equilibrated composition, with exolved pigeonites with augite lamellaes. This polymict eucrite contains also partially devitrified glass that represents impact melts linked to impact event. None recrystallization of this glass confirms a lack of post-brecciation metamorphism. Diogenitic fragments are less abundant than 10xa0%. The oxygen isotopic composition of NWA2268 is Δ17O (−0.43). This meteorite is interpreted as belonging to the HED group attributed to the 4-Vesta asteroid.