0000000000594292
AUTHOR
Patrick J. O'brien
Zircon ages for high pressure granulites from South Bohemia, Czech Republic, and their connection to Carboniferous high temperature processes
Petrological and isotopic investigations were undertaken on high pressure granulites of granitic to mafic composition from the Prachatice and Blanský les granulite complexes of southern Bohemia, Czech Republic. The predominant felsic granulites are quartz + ternary feldspar (now mesoperthite)-rich rocks containing minor garnet, kyanite and rutile, and most show a characteristic mylonitic fabric formed during retrogression along the exhumation path. Three high temperature reaction stages at distinctly different pressures are recognized. Rare layers of intermediate to mafic composition, containing clinopyroxene, best record a primary high pressure–high temperature stage (>15 kbar, >900 °C), a…
Zircon geochronology and metamorphic evolution of mafic dykes in the Hengshan Complex of northern China: Evidence for late Palaeoproterozoic extension and subsequent high-pressure metamorphism in the North China Craton
Abstract Magmatic and metamorphic zircons have been dated from ductilely deformed gabbroic dykes defining a dyke swarm and signifying crustal extension in the northern part of the Hengshan Complex of the North China Craton. These dykes now occur as boudins and deformed sheets within migmatitic tonalitic, trondhjemitic, granodioritic and granitic gneisses and are conspicuous due to relics of high-pressure granulite or even former eclogite facies garnet + pyroxene-bearing assemblages. SHRIMP ages for magmatic zircons from two dykes reflect the time of dyke emplacement at ∼1915 Ma, whereas metamorphic zircons dated by both SHRIMP and evaporation techniques are consistently in the range 1848–18…
Combined thermodynamic and rare earth element modelling of garnet growth during subduction: Examples from ultrahigh-pressure eclogite of the Western Gneiss Region, Norway
Abstract Major and trace element zonation patterns were determined in ultrahigh-pressure eclogite garnets from the Western Gneiss Region (Norway). All investigated garnets show multiple growth zones and preserve complex growth zonation patterns with respect to both major and rare earth elements (REE). Due to chemical differences of the host rocks two types of major element compositional zonation patterns occur: (1) abrupt, step-like compositional changes corresponding with the growth zones and (2) compositionally homogeneous interiors, independent of growth zones, followed by abrupt chemical changes towards the rims. Despite differences in major element zonation, the REE patterns are almost…
Linking growth episodes of zircon and metamorphic textures to zircon chemistry: an example from the ultrahigh-temperature granulites of Rogaland (SW Norway)
In-situ U-Th-Pb analyses by ion-microprobe on zircon in intact textural relationships are combined with backscatter and cathodoluminescence imaging and trace element analyses to provide evidence for growth episodes of zircon. This approach helps: (a) to unravel the polymetamorphic history of aluminous migmatitic and granitoid gneisses of the regional contact aureole around the Rogaland anorthosite-norite intrusive complex; and (b) to constrain the age of M 2 ultrahigh-temperature (UHT) metamorphism and the subsequent retrograde M 3 event. All samples yield magmatic inherited zircon of c. 1035 Ma, some an additional group at c. 1050 Ma. This suggests that loss of Pb by volume diffusion in no…
Fluid migration above a subducted slab - Thermodynamic and trace element modelling of fluid-rock interaction in partially overprinted eclogite-facies rocks (Sesia Zone, Western Alps)
Abstract The amount and composition of subduction zone fluids and the effect of fluid–rock interaction at a slab–mantle interface have been constrained by thermodynamic and trace element modelling of partially overprinted blueschist-facies rocks from the Sesia Zone (Western Alps). Deformation-induced differences in fluid flux led to a partial preservation of pristine mineral cores in weakly deformed samples that were used to quantify Li, B, Sr and Pb distribution during mineral growth, -breakdown and modification induced by fluid–rock interaction. Our results show that Li and B budgets are fluid-controlled, thus acting as tracers for fluid–rock interaction processes, whereas Sr and Pb budge…
Petrological and Isotopic Studies on Palaeozoic High-pressure Granulites, Gory Sowie Mts, Polish Sudetes
2154·7-1·4 Ma, whereas 19 analyses of grain fractions and single grains of multi-faceted zircons interpreted, from their char- acteristic spherical 'football' shape, as newly crystallized meta-
Fluid Migration above a Subducted Slab-Constraints on Amount, Pathways and Major Element Mobility from Partially Overprinted Eclogite-facies Rocks (Sesia Zone, Western Alps)
The Western Alpine Sesia-Lanzo Zone (SLZ) is a sliver of eclogite-facies continental crust exhumed from mantle depths in the hanging wall of a subducted oceanic slab. Eclogite-facies felsic and basic rocks sampled across the internal SLZ show different degrees of retrograde metamorphic overprint associated with fluid influx. The weakly deformed samples preserve relict eclogite-facies mineral assemblages that show partial fluid-induced compositional re-equilibration along grain boundaries, brittle fractures and other fluid pathways. Multiple fluid influx stages are indicated by replacement of primary omphacite by phengite, albitic plagioclase and epidote as well as partial re-equilibration a…
The petrology of two distinct granulite types in the Hengshan Mts, China, and tectonic implications
Abstract The Archean to Proterozoic Hengshan Complex (North China Craton), comprises tonalitic and granodioritic gneisses with subordinate mafic lenses, pegmatites and granites. Amphibolite facies assemblages predominate, although granulite-facies relics are widespread, and greenschist-facies retrogression occurs in km-wide shear zones. Mafic lenses, locally abundant, occur as strongly deformed amphibolite (hornblende+plagioclase) boudins or sheets. In contrast to previously published models we find two series of mafic rocks with distinctly different granulite-facies evolutions. In the north of the Complex, relict high-pressure mafic granulites are garnet+clinopyroxene-bearing rocks with a …