Granulite-facies Overprint Research Articles

Changes in the [formula omitted] ratios of fluids as evidence for different metamorphic episodes in high grade gneisses from the Konovalov Mountains area (Rayner Complex, East Antarctica)

Abstract Rocks of the Rayner complex in East Antarctica have suffered a complex multiphase metamorphic evolution. Radiogenic age constraints taken from the literature suggest at least two metamorphic events, the `main Rayner event' accompanied by charnockite emplacement 963 Myr ago and an amphibolite facies overprint 500–550 Myr ago. Some authors have suggested that a second high temperature metamorphic event was connected with the widespread intrusion of granitic magmas at about 770 Myr. Petrological and stable isotope thermometry is consistent with three metamorphic events, (i) a first granulite facies metamorphism, as preserved in the cationic equilibria of minerals in enderbitic gneisses yielding 810°C and 6.9 kbar; (ii) a regionally pervasive overprint, also at high temperatures (730°C and 5.9 kbar), previously regarded as the `main Rayner' event and; (iii) an amphibolite facies overprint followed by a greenschist facies retrogression, which is especially pronounced in late shear zones. We show that the two older events can be distinguished by the isotopic composition of the metamorphic fluids. During the first granulite facies event, the rocks preserved their inherited isotopic signatures, whereas the second granulite facies overprint was accompanied by pervasive flux of fluids of magmatic (possibly mantle) origin. During the third, amphibolite grade event, the fluids trapped during the second event were remobilised and led to partial reequilibration.

Age and Petrogenesis of the Sondalo Gabbroic Complex (Central Alps, Northern Italy): Implications for the Origin Of Post-Variscan Magmatism

The Late Carboniferous-Early Permian gabbroic complexes of the Alpine belt are commonly ascribed to the extensional tectonics which postdate the Variscan orogeny (Dal Piaz, 1993). The best examples are provided by the Main Gabbro from the Ivrea-Verbano Zone (Quick et al., 1994) and by the Malenco Gabbro (Hermann et al., 1997). However, both Main and Malenco Gabbro have a widespread granulite-facies metamorphic overprint, which does not allow a complete petrological characterization of their igneous features. Very little is known about the Sondalo gabbroic complex of the Languard-Campo nappe (Austroalpine nappe system of Central Alps), although it is volumetrically important (it occupies an area of about 40 km 2) and almost devoid of metamorphic recrystallization. There are no recent studies on its petrogenesis and a determination of the intrusion age is lacking, despite geochronological determinations on adjacent granitoid masses suggesting that the Sondalo gabbroic complex was emplaced in Permian times (Del Moro and Notarpietro, 1987). The Sondalo gabbroic complex is a stock intrusive into high-grade metapelites of the Languard-Campo nappe (Austroalpine nappe system of the Central Alps). The pluton broadly consists of norites and minor troctolites at the core, and diorites and subordinate granodiorites at the rim. The pluton frequently contains xenoliths of granulite-facies restitic rocks and is crosscut by muscovite-bearing pegmatitic to aplitic dykes. This work aims to verify whether the Sondalo gabbroic complex is related to the Alpine gabbroic complexes which postdate the Variscan orogeny. For this purpose, we carried out a petrological study (petrography, major and trace element mineral chemistry, and Nd-Sr-O isotopic compositions) of the troctolite/norite association, thus unraveling the intrusion age, the affinity of the parental liquids and the role of crustal contamination. Combining the data presented with the literature information on the Late Carboniferous-Early Permian gabbroic complexes from the Alpine belt, we give some insights into the origin of the postVariscan magmatism. Troctolitic rocks commonly consist of euhedral to subhedral plagioclase and olivine (Fo = 73-78 mol %), and minor poikilitic clinopyroxene. The variation trend of anorthite content of plagioclase versus the forsterite content of olivine differs from that of arcrelated troctolitic rocks (Beard, 1986) and is similar to that of troctolitic rocks from slow spreadingridges. Noritic rocks are usually composed of euhedral to subhedral plagioclase and orthopyroxene, and minor poikilitic clinopyroxene. Moreover, noritic rocks may contain relatively high percentages of interstial ilmenite and biotite. Both troctolitic and noritic rocks have up to 15 % of titanian pargasite, which occurs as discontinuous rims around mafic minerals or as poikilitic grains. Pyroxenes and Tipargasite from troctolitic rocks have higher Mg, A1 and Cr than those from noritic rocks, whereas plagioclase does not show systematic compositional variations (An about 65 mol %). Plagioclase, pyroxene and Ti-pargasite were analysed for REE and selected trace elements by ion microprobe. Plagioclase shows a regular decrease from LREE to HREE and a marked positive Eu anomaly. Orthopyroxene contains low REE, with a steady increase from LREE to HREE. Clinopyroxene has depleted LREE, negative Eu anomaly and slightly depleted to almost fiat HREE. The REE pattern of titanian pargasite mimics that of the associated clinopyroxene, but at higher abundance levels. Chondrite normalization of incompatible trace elements in Ti-pargasite reveals that K, Ba and Sr (LILE) are slightly depleted relative to LREE, and Nb is commonly enriched relative to K and Ba. The trace element compositions of parental liquids,

Trace Element and Isotope Geochemistry of Gabbro-Derived and Felsic Granulites from the Northern Apennines (Italy): Evidence for Assimilation/Fractional Crystallization in the Lower Crust

The Late Cretaceous sedimentary melanges from the External Liguride Units of the Northern Apennines include large slide blocks of mantle peridotites, MOR-basalts and lower and upper continental crust rocks representative of a continent-ocean transition between the Internal Liguride oceanic domain (Late Jurassic Western Tethys) and the thinned continental margin of the Adria plate (Marroni et al., 1998). The slide-blocks of lower continental crust consist of gabbro-derived and felsic granulites, which locally preserve primary contacts. The gabbro-derived granulites show variable degree of recrystallisation related to the granulite-facies re-equilibration. However, rocks which preserve a gabbroic fabric are found locally. The most frequent rock-types are olivine-bearing gabbronorites and Fe-Ti-oxidebearing gabbronorites; troctolites and anorthosites are also present. The felsic granulites include quartzo-feldspathic granulites and rare quartz-poor to quartz-free rocks. The gabbroic protoliths of the granulites were emplaced at about 290 Ma at deep crustal levels, where they underwent slow cooling and recrystallization under granulite-facies conditions (P = 0.7-0.8 GPa, T = 800-900~ They were exhumed to upper levels, in association with the felsic granulites, in late Triassic-middle Jurassic times (Marroni et al., 1998). Permian gabbroic rocks with granulite-facies metamorphic overprint are widespread in the Western Alps, in the remnants of the Adria plate margin. These gabbroic complexes are ascribed to the extensional tectonics which postdate the Variscan orogeny. In particular, striking similarities between the gabbro-derived granulites from Northern Apennines and the Ivrea Mafic Body (Pin, 1990) can be shown, i.e. similar ages of emplacement, close paragenetic and compositional resemblances and comparable retrograde metamorphic evolution. This work shows a geochemical investigation of gabbro-derived and felsic granulites from the Northern Apennines, in order to unravel (i) the affinity of the parental liquids of the gabbroic protoliths, (ii) the role of fractional crystallization and crustal assimilation in the igneous differentiation process, (iii) the nature of the mantle source. Trace element and Sr-Nd isotopic analyses have been performed on selected granulite samples, and clinopyroxene and plagioclase cores from undeformed gabbro-derived granulites were analysed for trace elements by ion microprobe. Relics of igneous textures, mineral and bulk-rock compositional variations indicate a comagmatic origin for the gabbro-derived granulites. They can be generally recognized as cumulitic rocks with negligible amounts of residual trapped liquid, on the basis of low SiO2/A1203 ratios (Kempton and Harmon, 1992) and overall low contents of incompatible trace elements. The mg# value ranges from 80 to 52, and points to negative correlations with TiO2 (up to 5 %) and MnO, thus indicating a tholeiitic differentiation trend. Major element variations are consistent with a process of fractional crystallization of plagioclase + olivine as liquidus phases, followed by plagioclase + clinopyroxene + orthopyroxene. In addition, petrographic relations in undeformed gabbroic rocks suggest the growth of AIspinel and Fe-Ti-oxides as intercumulus igneous minerals. The range of incompatible trace element ratios (Zr/ Y = 0.7-2.9, Y/Nb <~ 9, Zr/Nb ~< 10) encompasses that of subalkaline basaltic liquids. The R E E distribution (Fig. 1) is characterized by: (i) slightly LREE-enr iched patterns showing decreasing Eu positive anomaly with increasing absolute R E E

Eclogites with a short-lived granulite facies overprint in the Moldanubian Zone, Czech Republic: petrology, geochemistry and diffusion modelling of garnet zoning

The petrology and geochemistry of a newly discovered suite of high-pressure garnet + clinopyroxene-bearing rocks from the Monotonous Series of the Moldanubian Zone of the Bohemian Massif, southwest Czech Republic have been investigated. Three types [common eclogites, quartz ± kyanite ± (clino)zoisite eclogites and garnet-hornblende-clinopyroxenites] are distinguished by petrography and geochemistry. All underwent a significant degree of partial breakdown under granulite and amphibolite facies conditions during exhumation. Important features include the growth of orthopyroxene in breakdown domains after garnet and omphacite and anorthite + spinel ± corundum ± exceedingly peraluminous sapphirine replacing kyanite. Garnet zoning and inclusion patterns support a prograde evolution from low pressures for at least some of the samples. The post-eclogite stage granulite facies overprint indicates that high temperatures prevailed during exhumation, but preservation of zoning in some garnets and the results of diffusion modelling suggest that this overprint took place over a very short time-scale. The geochemical and petrological results allow characteristic differences to be recognized between these eclogites and metabasites found in other tectonic units of South Bohemia and consequently the assigning of all high-pressure rocks to a single, now disrupted, tectonic unit is a gross simplification that seriously misrepresents the tectono-metamorphic history of the region.