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SHRIMP U-Pb ages have been obtained for zircon in granitic gneisses from the aureole of the Rogaland anorthosite-norite intrusive complex, both from the ultrahigh temperature (UHT; >900 °C pigeonite-in) zone and from outside the hypersthene-in isograd. Magmatic and metamorphic segments of composite zircon were characterised on the basis of electron backscattered electron and cathodoluminescence images plus trace element analysis. A sample from outside the UHT zone has magmatic cores with an age of 1034 ± 7 Ma (2{sigma}, n = 8) and 1052 ± 5 Ma (1{sigma}, n = 1) overgrown by M1 metamorphic rims giving ages between 1020 ± 7 and 1007 ± 5 Ma.In contrast, samples from the UHT zone exhibit four major age groups:(1) magmatic cores yielding ages over 1500 Ma(2) magmatic cores giving ages of 1034 ± 13 Ma (2{sigma}, n = 4) and 1056 ± 10 Ma (1{sigma}, n = 1)(3) metamorphic overgrowths ranging in age between 1017 ± 6 Ma and 992 ± 7 Ma (1{sigma}) corresponding to the regional M1 Sveconorwegian granulite facies metamorphism, and(4) overgrowths corresponding to M2 UHT contact metamorphism giving values of 922 ± 14 Ma (2{sigma}, n = 6). Recrystallized areas in zircon from both areas define a further age group at 974 ± 13 Ma (2{sigma}, n = 4).This study presents the first evidence from Rogaland for new growth of zircon resulting from UHT contact metamorphism. More importantly, it shows the survival of magmatic and regional metamorphic zircon relics in rocks that experienced a thermal overprint of c. 950 °C for at least 1 Myr. Magmatic and different metamorphic zones in the same zircon are sharply bounded and preserve original crystallization age information, a result inconsistent with some experimental data on Pb diffusion in zircon which predict measurable Pb diffusion under such conditions. The implication is that resetting of zircon ages by diffusion during M2 was negligible in these dry granulite facies rocks. Imaging and Th/U-Y systematics indicate that the main processes affecting zircon were dissolution-reprecipitation in a closed system and solid-state recrystallization during and soon after M1.
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 similar to 1915 Ma, whereas metamorphic zircons dated by both SHRIMP and evaporation techniques are consistently in the range 1848-1888 Ma. The Youngest granitoid gneiss yet dated in the Hengshan has an emplacement age of 18 2 17 Ma. These results complement recent geochronological studies from the neighbouring Wutai and Fuping Complexes, to the SE of the Hengshan, showing that a crustal extension event Occurred in the late Palaeoproterozoic. This preceded a major high-pressure collision- type metamorphic event in the central part of the North China Craton that occurred in the Palaeoproterozoic and not in the late Archaean as previously thought. Our data support recent suggestions that the North China Craton experienced a major, craton-wide orogenic event in the late Palaeoproterozoic after which it became cratonized and acted as a stable block.
The type-locality granulites from the Granulitgebirge of Saxony, Germany, are rocks of broadly granitic composition containing minor garnet and kyanite within a commonly mylonitised matrix of feldspars and quartz. Petrographic evidence indicates a primary assemblage of ternary feldspar + quartz + garnet + kyanite + rutile, most likely resulting from partial melting of a granitic protolith, for which equilibrium temperature and pressure conditions of > 1000 degrees C and > 1.5 GPa have been deduced. These extreme (for crustal rocks) conditions, and the inferred peak assemblage, are supported by the newly-developed Zr-in-rutile geothermometer and experimental studies on the same bulk composition, respectively. As these conditions lie above those required for plagioclase stability in quartz tholeiites, they are thus in the eclogite facies. Widespread modification of the peak assemblage, for example mesoperthite formation after ternary feldspar, deformation-induced recrystallisation of perthites to two-feldspar + quartz aggregates, biotite replacing garnet, Ca-loss at garnet rims, sillimanite replacing kyanite or secondary garnet growth, makes reliable interpretation of equilibrium assemblages and compositions very difficult and explains the spread of published pressure- temperature values and consequent confusion about formation depths and the validity of tectonometamorphic models. Such extreme metamorphic conditions in rock compositions typical for the upper continental crust, reflecting a hot subduction environment, has important consequences for understanding some collisional orogens
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 and/or overgrowths in phengite and sodic amphibole, producing characteristic step-like compositional zoning patterns. The observed textures, together with the map-scale distribution of the samples, suggest open-system, pervasive and reactive fluid flux across large rock volumes above the subducted slab. Thermodynamic modelling indicates a minimum amount of fluid of 0 center dot 1-0 center dot 5 wt % interacting with the wall-rocks. Phase relations and reaction textures indicate mobility of K, Ca, Fe and Mg, whereas Al is relatively immobile in these medium-temperature-high-pressure fluids. Furthermore, the thermodynamic models show that recycling of previously fractionated material, such as in the cores of garnet porphyroblasts, largely controls the compositional re-equilibration of the exhumed rock body.
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, Stand Pb distribution during mineral growth, -breakdown and modification induced by fluid-rock interaction. Our results show that Li and 13 budgets are fluid-controlled, thus acting as tracers for fluid-rock interaction processes, whereas Stand Pb budgets are mainly controlled by the fluid-induced formation of epidote. Our calculations show that fluid-rock interaction caused significant Li and B depletion in the affected rocks due to leaching effects, which in turn can lead to a drastic enrichment of these elements in the percolating fluid. Depending on available fluid-mineral trace element distribution coefficients modelled fluid rock ratios were up to 0.06 in weakly deformed samples and at least 0.5 to 4 in shear zone mylonites. These amounts lead to time integrated fluid fluxes of up to 1.4-10(2) m(3) m(-2) in the weakly deformed rocks and 1-8-10(3) m(3) m(-2) in the mylonites. Combined thermodynamic and trace element models can be used to quantify metamorphic fluid fluxes and the associated element transfer in complex, reacting rock systems and help to better understand commonly observed fluid-induced trace element trends in rocks and minerals from different geodynamic environments.
Eclogites from the main borehole of the Chinese Continental Scientific Drilling project yield highly precise Lu-Hf garnet-clinopyroxene ages of 216.9 +/- 1.2 Ma (four samples) and 220.5 +/- 2.7 Ma (one sample). The spatial distribution of the rare earth elements in garnet is consistent with the preservation of primary growth zoning, unmodified by diffusion, which supports the interpretation that the Lu-Hf ages date the time of formation of garnet, the major rock forming mineral in the eclogites. The preservation of primary REE-zoning, despite peak metamorphic temperatures around 800-850 degrees C. indicates that the Lu-Hf chronometer is perfectly suitable to date garnet-forming reactions in high grade rocks. The range of Lu-Hf ages for eclogites in the Dabie-Sulu UHP terrane point to episodic rather than continuous growth of garnets and thus punctuated metamorphism during the collision of the North China Block and the Yangtze Block. The U-Pb ages and Hf-isotope systematics of zircon grains from one eclogite sample imply a protracted geologic history of the eclogite precursors that started around 2 Ga and culminated in the UHP metamorphism around 220 Ma.
The pressures required for diamond and coesite formation far exceed conditions reached by even the deepest present-day orogenic crustal roots. Therefore the occurrence of metamorphosed continental crust containing these minerals requires processes other than crustal thickening to have operated in the past. Here we report the first in situ finding of diamond and coesite, characterized by micro-Raman spectroscopy, in high-pressure granulites otherwise indistinguishable from granulites found associated with garnet peridotite throughout the European Variscides. Our discovery confirms the provenance of Europe's first reliable diamond, the "Bohemian diamond," found in A.D. 1870, and also represents the first robust evidence for ultrahigh-pressure conditions in a major Variscan crustal rock type. A process of deep continental subduction is required to explain the metamorphic pressures and the granulite-garnet peridotite association, and thus tectonometamorphic models for these rocks involving a deep orogenic crustal root need to be significantly modified.
Garnets in continentally derived high-pressure (HP) rocks of the Sesia Zone (Western Alps) exhibit three different chemical zonation patterns, depending on sample locality. Comparison of observed garnet zonation patterns with thermodynamically modelled patterns shows that the different patterns are caused by differences in the water content of the subducted protoliths during prograde metamorphism. Zonation patterns of garnets in water-saturated host rocks show typical prograde chemical zonations with steadily increasing pyrope content and increasing XMg, together with bell-shaped spessartine patterns. In contrast, garnets in water-undersaturated rocks have more complex zonation patterns with a characteristic decrease in pyrope and XMg between core and inner rim. In some cases, garnets show an abrupt compositional change in core-to-rim profiles, possibly due to water-undersaturation prior to HP metamorphism. Garnets from both water-saturated and water-undersaturated rocks show signs of intervening growth interruptions and core resorption. This growth interruption results from bulk-rock depletion caused by fractional garnet crystallization. The water content during burial influences significantly the physical properties of the subducted rocks. Due to enhanced garnet crystallization, water-undersaturated rocks, i.e. those lacking a free fluid phase, become denser than their water-saturated equivalents, facilitating the subduction of continental material. Although water-bearing phases such as phengite and epidote are stable up to eclogite-facies conditions in these rocks, dehydration reactions during subduction are lacking in water-undersaturated rocks up to the transition to the eclogite facies, due to the thermodynamic stability of such hydrous phases at high P-T conditions. Our calculations show that garnet zonation patterns strongly depend on the mineral parageneses stable during garnet growth and that certain co-genetic mineral assemblages cause distinct garnet zonation patterns. This observation enables interpretation of complex garnet growth zonation patterns in terms of garnet-forming reactions and water content during HP metamorphism, as well determination of detailed P-T paths.
Metabasites were sampled from rock series of the subducted margin of the Indian Plate, the so-called Higher Himalayan Crystalline, in the Upper Kaghan Valley, Pakistan. These vary from corona dolerites, cropping out around Saif- ul-Muluk in the south, to coesite-eclogite close to the suture zone against rocks of the Kohistan arc in the north. Bulk rock major- and trace-element chemistry reveals essentially a single protolith as the source for five different eclogite types, which differ in fabric, modal mineralogy as well as in mineral chemistry. The study of newly-collected samples reveals coesite (confirmed by in situ Raman spectroscopy) in both garnet and omphacite. All eclogites show growth of amphiboles during exhumation. Within some coesite-bearing eclogites the presence of glaucophane cores to barroisite is noted whereas in most samples porphyroblastic sodic-calcic amphiboles are rimmed by more aluminous calcic amphibole (pargasite, tschermakite, and edenite). Eclogite facies rutile is replaced by ilmenite which itself is commonly surrounded by titanite. In addition, some eclogite bodies show leucocratic segregations containing phengite, quartz, zoisite and/or kyanite. The important implication is that the complex exhumation path shows stages of initial cooling during decompression (formation of glaucophane) followed by reheating: a very similar situation to that reported for the coesite-bearing eclogite series of the Tso Morari massif, India, 450 km to the south-east.