Béatrice Luais

Dating the Indian continental subduction and collisional thickening in the northwest Himalaya: Multichronology of the Tso Morari eclogites

Multichronometric studies of the low-temperature eclogitic Tso Morari unit (Ladakh, India) place timing constraints on the early evolution of the northwest Himalayan belt. Several isotopic systems have been used to date the eclogitization and the exhumation of the Tso Morari unit: Lu-Hf, Sm-Nd, Rb-Sr, and Ar-Ar. A ca. 55 Ma age for the eclogitization has been obtained by Lu-Hf on garnet, omphacite, and whole rock from mafic eclogite and by Sm-Nd on garnet, glaucophane, and whole rock from high-pressure metapelites. These results agree with a previously reported U-Pb age on allanite, and together these ages constrain the subduction of the Indian continental margin at the Paleocene-Eocene boundary. During exhumation, the Tso Morari rocks underwent thermal relaxation at about 9 ± 3 kbar, characterized by partial recrystallization under amphibolite facies conditions ca. 47 Ma, as dated by Sm-Nd on garnet, calcic amphibole, and whole rock from metabasalt, Rb-Sr on phengite, apatite, and whole rock, and Ar-Ar on medium-Si phengite from metapelites. Ar-Ar analyses of biotite and low-Si muscovite from metapelites, which recrystallized at <5 kbar toward the end of the exhumation, show that the Tso Morari unit was at upper crustal levels ca. 30 Ma. These results indicate variable exhumation rates for the Tso Morari unit, beginning with rapid exhumation while the Indian margin subduction was still active, and later proceeding at a slower pace during the crustal thickening associated with the Himalayan collision.

Sm–Nd disequilibrium in high-pressure, low-temperature Himalayan and Alpine rocks

In order to decipher the causes of Sm–Nd isotopic disequilibrium in high-pressure, low-temperature rocks, Sm–Nd isotopic analyses were carried out on minerals from four Himalayan (Tso Morari unit) and four Alpine (Dora-Maira, Monte Viso, Sesia Lanzo) eclogitic rocks of different lithologies and different intensities of post-eclogitic metamorphism. In most of these

Mantle mixing and crustal contamination as the origin of the high-Sr radiogenic magmatism of Stromboli (Aeolian arc)

Abstract The temporal evolution of the volcanic activity on Stromboli (Aeolian arc) is characterized by high-K calc-alkaline magmatism followed by shoshonitic magmatism. Rocks from the two series can be distinguished by their K and P concentrations, hygromagmaphile element concentrations, and Sr isotopic ratios, which are higher in the shoshonitic series than in high-K calc-alkaline series. The mantle oxygen isotopic ratios of the shoshonitic series (Javoy, 1976 [39]) suggest that these characteristics are generated in the upper mantle. The geochemical and isotopic characteristics of the magmatism on Stromboli are midway between those of calc-alkaline rocks from the Aeolian arc and potassic rocks from Central Italy (Campanian region). The similarity between the most radiogenic and LILE-enriched samples of the shoshonitic series and the low-K magmatism in Central Italy suggests that the volcanism on Stromboli is the result of mantle mixing between a calc-alkaline magma and a radiogenic Sr/hygromagmaphile element-enriched magma, such as the high-K magmatism in Central Italy (Alban Hills). The chemical and isotopic characteristics of the differentiated samples can be explained by crustal contamination during the differentiation of the basic liquid by fractional crystallization. An anorthose-bearing xenolith with high Sr isotopic composition of crustal origin (0.71300) is a likely candidate for this crustal component.