Olivier Bruguier

Inherited zircon and titanite UPb systems in an Archaean syenite from southwestern Australia: implications for UPb stability of titanite

Abstract Inherited zircon and titanite have been identified in a syenite from the Archaean of southwestern Australia. Conventional and SHRIMP analyses on euhedral zoned zircon and zoned rims on complex grains define a crystallisation age of 2654 ± 5 Ma for the syenite. In addition, SHRIMP analyses on zircon cores and unzoned subhedral zircons show that zircon has a ca. 3250 Ma inherited component. Conventional UPb ages on titanite also fall between ca. 3250 Ma and ca. 2650 Ma, demonstrating that inherited titanite as well as zircon is present in the syenite. The syenite has been affected by regional upper amphibolite facies metamorphism at an estimated temperature of 625–650°C. Retention of the inherited radiogenic Pb in the titanite is evidence that the closure temperature for titanite is greater than 650°C. The presence of inherited titanite and zircon also demonstrates a crustal source component for the syenite and indicates it originated by partial assimilation of crustal rocks by a potassic magma at

Origin of the island arc Moho transition zone via melt-rock reaction and its implications for intracrustal differentiation of island arcs: Evidence from the Jijal complex (Kohistan complex, northern Pakistan)

If the net fl ux to the island arc crust is primitive arc basalt, the evolved composition of most arc magmas entails the formation of complementary thick ultramafi c keels at the root of the island arc crust. Dunite, wehrlite, and Cr-rich pyroxenite from the Jijal complex, constituting the Moho transition zone of the Kohistan paleo‐island arc (northern Pakistan), are often mentioned as an example of high-pressure cumulates formed by intracrustal fractionation of mantle-derived melts, which were later extracted to form the overlying mafi c crust. Here we show that calculated liquids for Jijal pyroxenites-wehrlites are strongly rare earth element (REE) depleted and display fl at or convex-upward REE patterns. These patterns are typical of boninites and are therefore unlike those of the overlying mafi c crust that have higher REE concentrations and are derived from light rare earth element (LREE)‐enriched melts similar to island arc basalt. This observation, along with the lower 208 Pb/ 204 Pb and 206 Pb/ 204 Pb ratios of Jijal pyroxenites-wehrlites relative to gabbros, rejects the hypothesis that gabbros and ultramafi c rocks derive from a common melt via crystal fractionation. In the 208 Pb/ 204 Pb versus 206 Pb/ 204 Pb diagram, ultramafi c rocks and gabbros lie on the same positive correlation, suggesting that their sources share a common enriched mantle 2 (EM2) signature but with a major depleted component contribution for the ultramafi c rocks. These data are consistent with a scenario whereby the Jijal ultramafi c section represents a Moho transition zone formed via melt-rock reaction between subarc mantle and incoming melt isotopically akin to Jijal gabbroic rocks. The lack in the Kohistan arc of cogenetic ultramafi c cumulates complementary to the evolved mafi c plutonic rocks implies either (1) that a substantial volume of such ultramafi c cumulates was delaminated or torn out by subcrustal mantle fl ow from the base of the arc crust in extraordinarily short time scales (0.10‐0.35 cm/yr), or (2) that the net fl ux to the Kohistan arc crust was more evolved than primitive arc basalt.

Evolution of an Archean Metamorphic Belt: A Conventional and SHRIMP U-Pb Study of Accessory Minerals from the Jimperding Metamorphic Belt, Yilgarn Craton, West Australia

U-Pb TIMS analyses of zircon, monazite, titanite, and apatite combined with SHRIMP U-Th-Pb analyses of zircons from magmatic and metasedimentary rocks of the Jimperding Metamorphic Belt in the southwestern part of the Yilgarn craton (Western Australia), provide reliable information on the evolution of this belt. SHRIMP analyses on single detrital zircon grains suggest the sediments originated from Archean sources similar in age to the Narryer Gneiss Complex. Deposition of the clastic metasediments is bracketed by the youngest concordant grain (

Record of a Palaeogene syn-collisional extension in the north Aegean region: evidence from the Kemer micaschists (NW Turkey)

3055 \pm 12 Ma

Late Visean hidden basins in the internal zones of the Variscan belt: U-Pb zircon evidence from the French Massif Central

) and the minimum age of a possible amphibolite dike crosscutting the sediments (

Crustal-scale structure of South Tien Shan: implications for subduction polarity and Cenozoic reactivation

2711 \pm 7 Ma