Zircon ages in granulite facies rocks: decoupling from geochemistry above 850 °C?

Granulite facies rocks frequently show a large spread in their zircon ages, the interpretation of which raises questions: Has the isotopic system been disturbed? By what process(es) and conditions did the alteration occur? Can the dates be regarded as real ages, reflecting several growth episodes? Furthermore, under some circumstances of (ultra-)high-temperature metamorphism, decoupling of zircon U–Pb dates from their trace element geochemistry has been reported. Understanding these processes is crucial to help interpret such dates in the context of the P–T history. Our study presents evidence for decoupling in zircon from the highest grade metapelites (> 850 °C) taken along a continuous high-temperature metamorphic field gradient in the Ivrea Zone (NW Italy). These rocks represent a well-characterised segment of Permian lower continental crust with a protracted high-temperature history. Cathodoluminescence images reveal that zircons in the mid-amphibolite facies preserve mainly detrital cores with narrow overgrowths. In the upper amphibolite and granulite facies, preserved detrital cores decrease and metamorphic zircon increases in quantity. Across all samples we document a sequence of four rim generations based on textures. U–Pb dates, Th/U ratios and Ti-in-zircon concentrations show an essentially continuous evolution with increasing metamorphic grade, except in the samples from the granulite facies, which display significant scatter in age and chemistry. We associate the observed decoupling of zircon systematics in high-grade non-metamict zircon with disturbance processes related to differences in behaviour of non-formula elements (i.e. Pb, Th, U, Ti) at high-temperature conditions, notably differences in compatibility within the crystal structure

The Pikwitonei granulite domain: A lower crustal level along the Churchill-Superior boundary in central Manitoba

The greenschist to amphibolite facies tonalite-greenstone terrain of the Gods Lake subprovince grades - in a northwesterly direction - into the granulite facies Pikwitonei domain at the western margins of the Superior Province. The transition is the result of prograde metamorphism and takes place over 50 - 100 km without any structural or lithological breaks. Locally the orthopyroxene isograd is oblique to the structural grain and transects greenstone belts, e.g., the Cross Lake belt. The greenstone belts in the granulite facies and adjacent lower grade domain consist mainly of mafic and (minor) ultramafic metavolcanics, and clastic and chemical metasedimentary rocks. Typical for the greenstone belts crossed by the orthopyroxene isograd are anorthositic gabbros and anorthosites, and plagiophyric mafic flows. The Pikwitonei granulite domain has been interpreted as to represent a lower crustal level which was uplifted to the present level of erosion. On the basis of gravimetric data this uplift has been modelled as an obduction onto the Churchill Province during the Hudsonian orogeny, similar to the Ivrea Zone. The fault between the Churchill and Superior Province is described

Trace element and isotope constraints on crustal anatexis by upwelling mantle melts in the North Atlantic Igneous Province: an example form the Isle of Rum, NW Scotland

Sr and Nd isotope ratios, together with lithophile trace elements, have been measured in a representative set of igneous rocks and Lewisian gneisses from the Isle of Rum in order to unravel the petrogenesis of the felsic rocks that erupted in the early stages of Palaeogene magmatism in the North Atlantic Igneous Province (NAIP). The Rum rhyodacites appear to be the products of large amounts of melting of Lewisian amphibolite gneiss. The Sr and Nd isotopic composition of the magmas can be explained without invoking an additional granulitic crustal component. Concentrations of the trace element Cs in the rhyodacites strongly suggests that the gneiss parent rock had experienced Cs and Rb loss prior to Palaeogene times, possibly during a Caledonian event. This depletion caused heterogeneity with respect to 87Sr/86Sr in the crustal source of silicic melts. Other igneous rock types on Rum (dacites, early gabbros) are mixtures of crustalmelts and and primarymantle melts. Forward Rare Earth Element modelling shows that late stage picritic melts on Rum are close analogues for the parent melts of the Rum Layered Suite, and for the mantle melts that caused crustal anatexis of the Lewisian gneiss. These primary mantle melts have close affinities to Mid-Oceanic Ridge Basalts (MORB), whose trace element content varies from slightly depleted to slightly enriched. Crustal anatexis is a common process in the rift-to-drift evolution during continental break-up and the formation of Volcanic Rifted Margins systems. The ‘early felsic–later mafic’ volcanic rock associations from Rum are compared to similar associations recovered from the now-drowned seaward-dipping wedges on the shelf of SE Greenland and on the Vøring Plateau (Norwegian Sea). These three regions show geochemical differences that result from variations in the regional crustal composition and the depth at which crustal anatexis took place

Nature and origin of fluids in granulite facies metamorphism

The various models for the nature and origin of fluids in granulite facies metamorphism were summarized. Field and petrologic evidence exists for both fluid-absent and fluid-present deep crustal metamorphism. The South Indian granulite province is often cited as a fluid-rich example. The fluids must have been low in H2O and thus high in CO2. Deep crustal and subcrustal sources of CO2 are as yet unproven possibilities. There is much recent discussion of the possible ways in which deep crustal melts and fluids could have interacted in granulite metamorphism. Possible explanations for the characteristically low activity of H2O associated with granulite terranes were discussed. Granulites of the Adirondacks, New York, show evidence for vapor-absent conditions, and thus appear different from those of South India, for which CO2 streaming was proposed. Several features, such as the presence of high-density CO2 fluid inclusions, that may be misleading as evidence for CO2-saturated conditions during metamorphism, were discussed

Pressure, temperature and time constraints on tectonic models for southwestern Sweden

In this work, a number of key localities have been investigated in detail in order to provide precise constraints on models for the tectonic evolution of southern Sweden. The new data presented in this thesis show that there are large differences in terms of pre-Sveconorwegian tectonic evolution between the Eastern and Median-Western Segments situated on either side of the Mylonite Zone, a major shear zone as well as a structural and lithological boundary. This has a direct influence on the possible tectonic scenarios that can be suggested when reconstructing the formation of the south-western part of the Fennoscandian Shield. At Viared in the central Eastern Segment, Sveconorwegian eclogite facies metamorphism is dated at 0.97 Ga using mainly U-Pb on zircon. This is similar to other localities showing high-pressure granulite or eclogite facies metamorphism in the Eastern Segment and suggests that this high grade event was a regional feature east of the Mylonite Zone. On the well exposed Nordön Island in the Western Segment, both pre-Sveconorwegian and Sveconorwegian metamorphism and deformation was dated using several isotope methods, including U-Pb SIMS zircon and Sm-Nd garnet dating. In the Median Segment, veining was dated using U-Pb SIMS zircon methods. In addition to age determinations, thermobarometry was done on several samples and the results compared with published data. The results show that Sveconorwegian peak metamorphism reached amphibolite to granulite facies west of the Mylonite Zone at 1.02-1.04 Ga. This is 50-70 Ma before the orogenic activity started in the Eastern Segment. In the central Eastern Segment, Pre-Sveconorwegian veining is dated at 1.42 Ga, thus belonging to the 1.42-1.46 Ga Hallandian veining found elsewhere in the Eastern Segment. Pre-Sveconorwegian veining and isoclinal folding in the Western Segment is dated at 1.55 Ga. Neither the 1.55 Ga nor the 1.02-1.04 Ga events have been found in the Eastern Segment. Further, the 1.42-1.46 Ga veining event documented in the Eastern Segment has not been found in the Western or Median Segments. Only a few 0.97 Ga zircon U-Pb ages have been found in the Western and Median Segments west of the Mylonite Zone. In the Western Segment, Ar-Ar dating of muscovite related to late Sveconorwegian uplift gives 981±4 Ma, interpreted to reflect the crystallization of muscovite below 400 °C. This indicates that the Western Segment already was exhumed at the time when the Eastern Segment experienced its Sveconorwegian high-pressure peak metamorphism. Additionally this work shows that there is an age difference between the 1.34 Ga Askim, the 1.30 Ga Göta and 1.31 Ga Kärra granites west of the Mylonite Zone and the 1.38-1.40 Ga granites and monzonites east of the Mylonite Zone, thus disproving the previous concept of “stitching granites” that was used as an argument for a pre-Sveconorwegian correlation between the Eastern and Western Segments. The differences in pre-Sveconorwegian history between the segments east and west of the Mylonite zone suggest that the zone originally formed as a Sveconorwegian first order crustal suture. However, no ophiolites have been found along the zone and no calk-alkaline magmatism related to subduction of oceanic crust slightly before 0.97 Ma has yet been documented. If such features were found it would strengthen the idea that the Mylonite Zone is a crustal suture, however their lack does not preclude it. Geothermobarometry on retrograde eclogite facies rocks exposed at Viared indicates that those rocks experienced metamorphic conditions of 15.0–16.7 kbar at temperatures of 719 to 811°C. The equivalent burial depth of ~50 km is compatible with metamorphism in a subduction setting. The partial preservation of a high pressure paragenesis indicates rapid exhumation after burial. A two-dimensional model involving continental subduction of the Eastern Segment is proposed to explain the cycle of eclogite and high-pressure granulite facies that took place during the ~0.97 Ga Falkenberg phase of the Sveconorwegian Orogeny

U-Pb SHRIMP zircon dating of Grenvillian metamorphism in Western Sierras Pampeanas (Argentina) : correlation with the Arequipa-Antofalla craton and constraints on the extent of the Precordillera Terrane

The Sierras Pampeanas of Argentina, the largest outcrop of pre-Andean crystalline basement in southern South America, resulted from plate interactions along the proto-Andean margin of Gondwana, from as early as Mesoproterozoic to Late Paleozoic times (e.g., Ramos, 2004, and references therein). Two discrete Paleozoic orogenic belts have been recognized: the Early Cambrian Pampean belt in the eastern sierras, and the Ordovician Famatinian belt, which partially overprints it to the west (e.g., Rapela et al., 1998). In the Western Sierras Pampeanas, Mesoproterozoic igneous rocks (ca. 1.0–1.2 Ga) have been recognized in the Sierra de Pie de Palo (Fig. 1) (McDonough et al., 1993 M.R. McDonough, V.A. Ramos, C.E. Isachsen, S.A. Bowring and G.I. Vujovich, Edades preliminares de circones del basamento de la Sierra de Pie de Palo, Sierras Pampeanas occidentales de San Juán: sus implicancias para el supercontinente proterozoico de Rodinia, 12° Cong. Geol. Argentino, Actas vol. 3 (1993), pp. 340–342.McDonough et al., 1993, Pankhurst and Rapela, 1998 and Vujovich et al., 2004) that are time-coincident with the Grenvillian orogeny of eastern and northeastern North America (e.g., Rivers, 1997 and Corrievau and van Breemen, 2000). These Grenvillian-age rocks have been considered to be the easternmost exposure of basement to the Precordillera Terrane, a supposed Laurentian continental block accreted to Gondwana during the Famatinian orogeny (Thomas and Astini, 2003, and references therein). However, the boundaries of this Grenvillian belt are still poorly defined, and its alleged allochthoneity has been challenged (Galindo et al., 2004). Moreover, most of the Grenvillian ages so far determined relate to igneous protoliths, and there is no conclusive evidence for a Grenvillian orogenic belt, other than inferred from petrographic evidence alone (Casquet et al., 2001). We provide here the first evidence, based on U–Pb SHRIMP zircon dating at Sierra de Maz, for a Grenville-age granulite facies metamorphism, leading to the conclusion that a continuous mobile belt existed throughout the proto-Andean margin of Gondwana in Grenvillian times

Heat transfer by fluids in granulite metamorphism

The thermal role of fluids in granulite metamorphism was presented. It was shown that for granulites to be formed in the middle crust, heat must be advected by either magma or by volatile fluids, such as water or CO2. Models of channelized fluid flow indicate that there is little thermal difference between channelized and pervasive fluid flow, for the same total fluid flux, unless the channel spacing is of the same order or greater than the thickness of the layer through which the fluids flow. The volumes of volatile fluids required are very large and are only likely to be found associated with dehydration of a subducting slab, if volatile fluids are the sole heat source for granulite metamorphism

Cl-rich minerals in Archean granulite facies ironstones from the Beartooth Mountains, Montana, USA: Implications for fluids involved in granulite metamorphism

The implications of Cl-rich minerals in granulite facies rocks are discussed. Results from ironstones of the Beartooth Mountains, Montana are discussed. It is suggested that CO2-brine immiscibility might be applicable to granulite facies conditions, and if so, then aqueous brines might be preferentially adsorbed onto mineral surfaces relative to CO2

U–Pb dating and Sm–Nd isotopic analysis of granitic rocks from the Tiris Complex : new constaints on key events in the evolution of the Reguibat Shield, Mauritania

The Reguibat Shield of N Mauritania and W Algeria represents the northern exposure of the West African Craton. As with its counterpart in equatorial West Africa, the Leo Shield, it comprises a western Archaean Domain and an eastern Palaeoproterozoic Domain. Much of the southern part of the Archaean Domain is underlain by the Tasiast-Tijirit Terrane and Amsaga Complex which, along with the Ghallaman Complex in the northeast, preserve a history of Mesoarchaean crustal growth, reworking and terrane assembly. This study presents new U–Pb and Sm–Nd data from the Tiris Complex, a granite–migmatite–supracrustal belt, that intervenes between these units and the Palaeoproterozoic Domain to the northeast. New U–Pb geochronology indicates that the main intrusive events, broadly associated with formation of dome-shaped structures, occurred at around 2.95–2.87 Ga and 2.69–2.65 Ga. This study also recognises younger regional metamorphism and intrusion of syn-tectonic granites located within major shear zones at around 2.56–2.48 Ga. Sm–Nd depleted mantle model ages indicate that magmatism involved recycling of crustal source components older than at least 3.25 Ga in age. Comparison with other Archaean units in the Reguibat Shield and in the Leo Shield illustrate the importance of deformation and tectonism of a regional greenstone-sedimentary province prior to around 3.00 Ga as well as subsequent magmatic episodes broadly equivalent in age to those in the Tiris Complex

Continental crustal composition and lower crustal models

The composition of the upper crust is well established as being close to that of granodiorite. The upper crustal composition is reflected in the uniform REE abundances in shales which represent an homogenization of the various REE patterns. This composition can only persist to depths of 10-15 km, for heat flow and geochemical balance reasons. The composition of the total crust is model dependent. One constraint is that it should be capable of generating the upper granodioritic (S.L.) crust by partial melting within the crust. This composition is based on the andesite model, which assumes that the total crust has grown by accretion of island arc material. A representation of the growth rate of the continental crust is shown. The composition of the lower crust, which comprises 60-80% of the continental crust, remains a major unknown factor for models of terrestrial crustal evolution. Two approaches are used to model the lower crust