ALBERT

Description: Garnet granulite facies xenoliths hosted in Devonian lamprophyres from the Kola Peninsula are interpreted to represent the high-grade metamorphic equivalents of continental flood tholeiites, emplaced into the Baltic Shield Archaean lower crust in early Proterozoic time. Geochronological data and similarities in major and trace element geochemistry suggest that the xenoliths formed during the same plume-related magmatic event that created a widespread Palaeoproterozoic large igneous province (LIP) at 2·4–2·5 Ga. They are, thus, the first samples of the lower crust of a Palaeoproterozoic LIP to be studied in petrological detail. The suite includes mafic granulites (gar + cpx + rutile ± plag ± opx ± phlog ± amph), felsic granulites (plag + gar + cpx + rutile ± qtz ± Kspar ± phlog ± amph) and pyroxenites (± phlog ± amph), but mafic garnet granulites predominate. Although some samples are restites, there is no evidence for a predominance of magmatic cumulates, as is common for Phanerozoic lower-crustal xenolith suites. Metasediments are also absent. Phlogopite and/or amphibole occur in xenoliths of all types and are interpreted to be metasomatic in origin. The K-rich metasomatic event occurred at ∼2·0 Ga, and led to substantial enrichment in Rb, K, LREE/HREE, Th/U, Th/Pb and, to a lesser extent, Nb and Ti. The fluids responsible for this metasomatism were probably derived from a second plume that arrived beneath the region at this time. Evidence for partial melting of mafic crust exists in the presence of migmatitic granulites. The timing of migmatization overlaps that of metasomatism, and it is suggested that migmatization was facilitated by the metasomatism. The metamorphism, metasomatism and migmatization recorded in the Kola granulite xenoliths may be representative of the processes responsible for converting Archaean LIP-generated proto-continents into continental crust.

Description: The Neoproterozoic Aries kimberlite was emplaced in the central Kimberley Basin, Western Australia, as a N–NNE-trending series of three diatremes infilled by lithic-rich kimberlite breccias. The breccias are intruded by hypabyssal macrocrystic phlogopite kimberlite dykes that exhibit differentiation to a minor, high-Na–Si, olivine–phlogopite–richterite kimberlite, and late-stage macrocrystic serpentine–diopside ultramafic dykes. Mineralogical and geochemical evidence suggests that the high-Na–Si, olivine–phlogopite–richterite kimberlite was derived from the macrocrystic phlogopite kimberlite as a residual liquid following extended phlogopite crystallization and the assimilation of country rock sandstone, and that the macrocrystic serpentine–diopside ultramafic dykes formed as mafic cumulates from a macrocrystic phlogopite kimberlite. Chemical zonation of phlogopite–biotite phenocrysts indicates a complex magmatic history for the Aries kimberlite, with the early inheritance of a range of high-Ti phlogopite–biotite xenocrysts from metasomatized mantle lithologies, followed by the crystallization of a population of high-Cr phlogopite phenocrysts within the spinel facies lithospheric mantle. A further one to two phlogopite–biotite overgrowth rims of distinct composition formed on the phlogopite phenocrysts at higher levels during ascent to the surface. Ultra-violet laser 40Ar/39Ar dating of mica grain rims yielded a kimberlite eruption age of 815·4 ± 4·3 Ma (95% confidence). 40Ar/39Ar laser profiling of one high-Ti phlogopite-biotite macrocryst revealed a radiogenic 40Ar diffusive loss profile, from which a kimberlite magma ascent duration from the spinel facies lithospheric mantle was estimated (assuming an average kimberlite magma temperature of 1000°C), yielding a value of ∼0·23–2·32 days for the north extension lobe of the Aries kimberlite.

Description: A multi-method geochronological approach is applied to unravel the dynamics of a paired metamorphic belt in the Coastal Cordillera of central Chile. This is represented by high-pressure–low-temperature rocks of an accretionary prism (Western Series), and a low-pressure–high-temperature overprint in the retro-wedge with less deformed metagreywackes (Eastern Series) intruded by magmas of the coeval arc. A pervasive transposition foliation formed in metagreywackes and interlayered oceanic crust of the Western Series during basal accretion near metamorphic peak conditions (∼350–400°C, 7–11 kbar) at 292–319 Ma (40Ar/39Ar phengite plateau ages). 40Ar/39Ar UV laser ablation ages of phengite record strain-free grain growth and recrystallization with a duration of 31–41 Myr during a pressure release of 3–4 kbar. During early accretion the main intrusion in the arc occurred at 305 Ma (Pb–Pb evaporation; zircon) and the Eastern Series was overprinted by a short high-temperature metamorphism at 3 kbar, 296–301 Ma (40Ar/39Ar muscovite plateau ages). Fission-track ages of zircon (206–232 Ma) and of apatite (80–113 Ma) are similar in both series, indicating synchronous cooling during distinct periods of exhumation. Early exhumation (period I) during continuing basal accretion proceeded with mean rates of ≥0·19–0·56 mm/yr, suggesting that erosion in a tectonically active area was an important unroofing mechanism. At the same time mean rates were 0·03–0·05 mm/yr in the Eastern Series, where crustal thickening was minor. A shallow granite intruded into the Western Series at 224 Ma, at the end of basal accretion activity, when exhumation rates decreased to 0·04–0·06 mm/yr in both series during period II (∼100–225 Ma). Major extension, basin formation and local bimodal dyke intrusion at 138 Ma were accompanied by mean cooling rates of ∼1–2°C/Myr. Accelerated cooling of 3–5°C/Myr at ∼80–113 Ma suggests a mid-Cretaceous convergence event (period III). After 80 Ma cooling rates decreased to 1–2°C/Myr (period IV). The pressure–temperature–deformation–time information for subduction, basal accretion and exhumation in the accretionary wedge of central Chile illustrates that these processes reflect a continuous cyclic mass flow that lasted nearly 100 Myr, while the retro-wedge remained stable. After the cessation of accretion activity a similarly long period of retreat of the subducting slab occurred; this ended with renewed convergence and shortening of the continental margin.