The Biotite to Phengite Reaction and Mica-dominated Melting in Fluid + Carbonate-saturated Pelites at High Pressures

Subsolidus and melting experiments were performed at 2·0-3·7 GPa and 750-1300°C on a carbonated pelite in the model system K2O-CaO-MgO-Al2O3-SiO2-H2O-CO2 to define stabilities of potassic micas and fluid-present melting reactions. The biotite to phengite reaction occurs at pressures between 2· 4 and 2·6 GPa for temperatures of 750-850°C, and the amphibole to clinopyroxene reaction from 2·0 GPa, 875°C to 2·5 GPa, 740°C. Dolomite is the carbonate phase stable at subsolidus conditions. The biotite to phengite reaction preserves K2O, but is not H2O conservative, as a fluid is produced from the decomposition of zoisite. Phengite + quartz control fluid-saturated melting at a pressure (P) >2·6 GPa, whereas biotite + quartz dominate at P <2· 4 GPa. Incongruent melting occurs through the reactions phengite or biotite + zoisite + quartz/coesite + fluid = silicate melt + clinopyroxene + kyanite. Overstepping of the solidus, located at 850-950°C, results in 7-24 wt % metaluminous K-rich granitic melts. The experiments define the melting surface of the model system, projected from kyanite + quartz/coesite + fluid onto the K2O-CaO-MgO plane. The solidus melts in the studied system occur at a peritectic point consuming mica + zoisite and forming clinopyroxene. With increasing temperature (T), carbonated pelites then evolve along a peritectic curve along which further clinopyroxene is produced until zoisite is exhausted. This is then followed by a peritectic curve consuming clinopyroxene and producing garnet. A comparison of CO2-bearing with CO2-free experiments from the literature suggests that the main effect of adding calcite to a continental sediment is not the minor shift of typically 20-30°C of reactions involving fluid, but the change in bulk Ca/(Mg + Fe) ratio stabilizing calcic phases at the expense of ferromagnesian phases. The experiments suggest that in most subduction zones, CO2, H2O and K2O will be carried to depths in excess of 120-150 km through carbonates and K-micas, as partial melting occurs only at temperatures at the uppermost end of thermal models of subduction zones. Nevertheless, the release of fluid through P-induced decomposition of amphibole and zoisite provides some H2O for arc magma formation. Melting at higher temperatures (e.g. resulting from slower burial rates or from incorporation of subducted crust into the mantle) will produce potassic granitic melts and provide a substantial volatile and K source for the formation of arc magma

The Malolotsha Klippe: Large‐Scale Subhorizontal Tectonics Along the Southern Margin of the Archean Barberton Greenstone Belt, Eswatini

Whether Archean tectonics were horizontally or vertically dominated is controversially discussed because arguments bear on the kinematics and thermal state of the Archean mantle and constrain the mode of formation of the earliest continental crust. Highly deformed strata of Archean greenstone belts figure prominently in this debate because they record long periods of time and multiple deformation phases. Among the best-preserved greenstone belts counts the Barberton Greenstone Belt (BGB) of southern Africa. Geological mapping of part of the southern BGB in Eswatini (Swaziland), combined with U-Pb zircon dating, shows that the region preserves a tightly re-folded imbricate thrust stack in which metavolcanic and -volcaniclastic strata of the Onverwacht Group, deposited at 3.34–3.29 Ga, have been thrust on top of ca. 3.22 Ga siliciclastic strata of the Moodies Group. The structurally highest element, the Malolotsha Syncline, forms a tectonic klippe of substantial size and is >1,450 m thick. Forward modeling of a balanced cross section indicates that this thrust stack was part of a northwestward-verging orogen along the southern margin of the BGB and records a minimum horizontal displacement of 33 km perpendicular to its present-day faulted, ductily strained and multiply metamorphosed margin. Because conglomerate clasts indicate a significantly higher degree of prolate strain which extends further into the BGB than at its northern margin, late-stage tectonic architecture of the BGB may be highly asymmetrical. Our study documents that the BGB, and perhaps other Archean greenstone belts, preserves a complex array of both vertically- and horizontally-dominated deformation styles that have interfered with each other at small regional and short temporal scales

Geological processes defining the formation of plumasite-type corundum in the Paleoproterozoic Isertoq Terrane, South-East Greenland

Plumasite-type corundum occurrences in the Nattivit area in South-East Greenland offer a unique opportunity to study corundum formation in-situ where pegmatites intruded into metamorphosed lherzolite and dunite of the Archean-Paleoproterozoic continental crust. The Nattivit area, located in the Isertoq Terrane of North Atlantic Craton, forms part of the overriding plate during convergence of the Nagssugtoqidian orogen (1910-1840 Ma). New field observations and elemental and isotopic geochemical analysis provide further insights in the history of crustal convergence, its exhumation and how corundum was formed. The continental crust in the area consists of metamorphosed mafic to ultramafic rocks and tonalite-trondhjemite-granodiorite (TTG) gneisses, where the mafic rocks in the Isertoq Terrane yield a εNd T$_{DM}$ model age of 3000–2800 Ma. Dunite and lherzolite sills/dikes intruded the mafic rocks before the intrusion of the TTG sheets. The intrusion ages for the TTG obtained from zircon U-Pb geochronology are 2818 ± 8 Ma, 2760 ± 13 Ma to 2667 ± 7 Ma. U-Pb zircon data, zircon textures and Th/U ratios indicate metamorphism occurred at 2698 ± 7 Ma to 2629 ± 11 Ma, 2500–2400 Ma and 1900–1600 Ma. Whole rock geochemical data of mafic to ultramafic rocks show a continental arc affinity, with negative Ta, Nb and positive Pb anomalies. A metasomatic event at 2390 ± 70 Ma partly reset the isotopic signature in the mafic to ultramafic rocks. A marked absence of ages between 2350 and 2100 Ma in the TTG zircon age populations exists, indicating a period with minimal magmatic and/or metamorphic activity. The metamorphic mineral assemblages of the schist, amphibolite, ultramafic rocks and metasomatic reaction zones in ultramafic rocks indicate upper to medium–high amphibolite facies conditions. Kyanite in the metasomatic reaction zones in ultramafic rocks indicate the higher end of the temperature and pressure range above 4.2–10 kbar and 530–800 °C, similar to estimates from dolerite dikes in the Kitak area. The syn-tectonic pegmatites with an intrusion age of 1843 ± 4 Ma formed corundum. The new data indicate that the pegmatite melt/fluid and the geotectonic setting are defining factors for generating plumasite-type corundum

U-Pb dating identifies titanite precipitation in Paleogene sandstones from a volcanic terrane, East Greenland

Titanite (CaTiSiO5) occurs as a rare mineral in magmatic and metamorphic rocks. It is commonly found in clastic sedimentary rocks as an accessory heavy mineral – a mineral of high density.&nbsp;Recently, U-Pb dating of single-grains of detrital titanite has been shown to be a useful tool in sedimentary provenance studies (e.g. McAteer&nbsp;et al. 2010; Thomsen&nbsp;et al. 2015). Titanite U-Pb geochronologies can add important information to constrain the sediment sources of rocks and basins, and can help date precipitation of titanite. However, there are a number of complicating factors that must be taken into consideration for reliable application of titanite U-Pb dating in provenance studies. First, titanite is less stable than zircon – the most commonly employed dating target. For example, in Palaeocene sediments in the North Sea, titanite rarely occurs as detrital grains at burial depths greater than 1400 m (Morton 1984). It can also show dissolution features due to weathering and burial diagenesis (e.g. Morton 1984; Turner &amp; Morton 2007). Second, titanite may precipitate during burial diagenesis, which would reflect the burial history of sediments and not their provenance. Precipitation of authigenic titanite is documented from deeply buried (i.e. at temperatures greater than 100°C) volcaniclastic sandstones and mudstones (Helmond &amp; Van de Kamp 1984; Milliken 1992) and intrusion-associated mineralisation in volcanic Permian sandstones (van Panhuys-Sigler &amp; Trewin 1990). Moreover, titanite also occurs in shallow-buried Jurassic sandstones with no volcanic affinity (Morad 1988). Thus, the formation of titanite is not necessarily linked to a volcaniclastic source, but nevertheless, the presence of volcanic material seems to promote titanite precipitation. If authigenic titanite precipitation was incorrectly identified as detrital, this would have considerable implications for provenance investigations, as apparently titanite-rich source rocks would be wrongly inferred to be present in the sediment source area. Here, we present examples from the Kangerlussuaq Basin in southern East Greenland of what appeared to be detrital titanite. However, new U-Pb dating reveals that the titanite formed authigenically, and hence contributed to the burial history, and not the provenance, of the sediments

jAgeDisplay: software for evaluation of data distributions in U-Th-Pb geochronology

During the past 10–15 years, analytical innovations in geochronology have greatly enhanced the application of geochronological data to geological problems. The advances are mainly driven by developments in laser ablation inductively coupled plasma mass spectrometry (LA-ICPMS) which allows for rapid determination of U-Th-Pb ages of mineral grains in large sample sets. LA-ICPMS has now become the most common tool in the application of zircon geochronology to a host of different geological problems

Investigations of detrital zircon, rutile and titanite from present-day Labrador drainage basins: fingerprinting the Grenvillean front

A multidisciplinary provenance study was conducted on stream sediment samples from major rivers in the eastern part of Labrador, Canada (Fig. 1). The purpose was to fingerprint the sources that deliver material to the stream sediments and to the reservoir sand units deposited off shore in the sedimentary basins in the Labrador Sea. We used a multimineral U-Pb geochronological approach employing rutile and titanite in addition to zircon to obtain unbiased age data. The purpose of this was to characterise the different igneous and metamorphic episodes that occurred in Labrador, which is an area with highly variable geology characterised by the Palaeoproterozoic south-eastern Churchill province in the north-west, the Archaean Nain plutonic suite in the north-east, the Palaeoproterozoic Makkovik province in the east and the Mesoproterozoic Grenville Province to the south. The field work was carried out in 2012 and 2013 and the study is a collaborative project between the Geological Survey of Denmark and Greenland and the Geological Survey of Newfoundland and Labrador. In this paper we focus on three samples from the southern part of the study area where two parts of the Grenville orogeny are found (Fig. 1)

The Malolotsha Klippe: Large‐Scale Subhorizontal Tectonics Along the Southern Margin of the Archean Barberton Greenstone Belt, Eswatini

Whether Archean tectonics were horizontally or vertically dominated is controversially discussed because arguments bear on the kinematics and thermal state of the Archean mantle and constrain the mode of formation of the earliest continental crust. Highly deformed strata of Archean greenstone belts figure prominently in this debate because they record long periods of time and multiple deformation phases. Among the best‐preserved greenstone belts counts the Barberton Greenstone Belt (BGB) of southern Africa. Geological mapping of part of the southern BGB in Eswatini (Swaziland), combined with U‐Pb zircon dating, shows that the region preserves a tightly re‐folded imbricate thrust stack in which metavolcanic and ‐volcaniclastic strata of the Onverwacht Group, deposited at 3.34–3.29 Ga, have been thrust on top of ca. 3.22 Ga siliciclastic strata of the Moodies Group. The structurally highest element, the Malolotsha Syncline, forms a tectonic klippe of substantial size and is >1,450 m thick. Forward modeling of a balanced cross section indicates that this thrust stack was part of a northwestward‐verging orogen along the southern margin of the BGB and records a minimum horizontal displacement of 33 km perpendicular to its present‐day faulted, ductily strained and multiply metamorphosed margin. Because conglomerate clasts indicate a significantly higher degree of prolate strain which extends further into the BGB than at its northern margin, late‐stage tectonic architecture of the BGB may be highly asymmetrical. Our study documents that the BGB, and perhaps other Archean greenstone belts, preserves a complex array of both vertically‐ and horizontally‐dominated deformation styles that have interfered with each other at small regional and short temporal scales.Plain Language Summary: Worldwide, only a few regions exist where ancient rock strata document how earth cooled, surface strata deformed, and continents grew. It is debated whether vertical movements dominated (akin to a lava lamp) and when major horizontal motions (as they dominate Earth today) began; certainly, there was also overlap between these regimes. Radiometric age dating of zircons extracted from strata along the southern margin of one of the best‐preserved ancient regions in southern Africa, the Barberton Greenstone Belt in Eswatini, show that older strata were thrust there over younger strata for at least 33 km distance subhorizontally. Then they were shingled, and then folded. The results show that even at a time when Earth's oldest continents were just forming, significant horizontal displacements existed already.Key Points: U‐Pb zircon dating and geological mapping confirm a folded thrust‐stack along part of the southern margin of the Archean Barberton Greenstone Belt (BGB). Forward modeling of a balanced cross‐section indicates >33 km of horizontal shortening toward the northwest. Vertically‐ and horizontally‐dominated tectonics interfered with each other in the BGB and may have done so in other Archean greenstone belts as well.Friedrich‐Schiller‐University Jenahttps://doi.org/10.5880/fidgeo.2022.03