Granulite-facies Metasediments Research Articles

High-temperature S-type granitoids (charnockites) in the Jining complex, North China Craton: Restite entrainment and hybridization with mafic magma

The Liangcheng garnet granitoids are parautochthonous S-type granites that occur within granulite-facies metasediments in the ultrahigh-temperature (UHT) Jining Complex, eastern part of the Khondalite Belt, North China Craton. These garnet granitoids contain mafic microgranular enclaves and are intimately associated with numerous small intrusions of the Xuwujia gabbronorites, showing magma mingling and mixing textures. Typical mineralogy in the garnet granitoids is plagioclase + K-feldspar + quartz + garnet + biotite ± orthopyroxene. UPb ion microprobe dating of zircon suggests that garnet granitoids and associated gabbronorites were coeval with UHT metamorphism at ~1.93–1.92 Ga, and also recorded ~1.92 Ga metamorphism during slow cooling. Garnet granitoids are characterized by the presence of orthopyroxene, high modal proportion of garnet (up to 25%) and strongly peraluminous (ASI = 1.11–1.55) along with lower SiO2 (57.8–70.1 wt%) and higher FeOt + MgO (5.6–14.4 wt%) contents than typical S-type granites. Mineral chemistry (high Ti biotite and high Al orthopyroxene), high Al-in-orthopyroxene temperature (910–989 °C) and Zr saturation temperature (825–901 °C), and low whole-rock Al2O3/TiO2 ratios (11–28), indicate that garnet granitoids were derived from biotite dehydration melting of metasedimentary rocks under UHT granulite-facies conditions. Textural and compositional evidence suggests that a significant proportion of garnets in the Liangcheng garnet granitoids are potentially a peritectic phase entrained from the source. Garnet aggregates may also represent entrained garnet-rich restites. The large HREE variations of garnet granitoids also support the entrainment of garnet-rich restites. Garnet granitoids display nearly linear trends with the mafic enclaves and gabbronorite for most major elements that demonstrates the involvement of mafic magma in the granitoid petrogenesis. The εHf(t) values (−1.0 − +5.6) and δ18O values (9.2–11.6‰) of zircons in garnet granitoids partly overlap with those in gabbronorites (εHf(t) = −2.9 − +3.5, δ18O = 7.7–10.9‰), suggesting the strong interaction and mixing process between them. Major element modelling suggests that the addition of 20–40% restites (garnetites) and gabbronorites to the anatectic melts was responsible for the large chemical variations. Therefore, the Liangcheng garnet granitoids represent high-temperature (> 900 °C), H2O-undersaturated, strongly peraluminous granitoids, and are produced by large-scale melting of metasedimentary rocks and subsequent mixing between restite-rich anatectic melt and mafic magma. The voluminous high-temperature granitoids, coupled with UHT granulites in the Jining Complex, manifest the thermal response of a granulitic lower crust to the mafic underplating.

A pulse of cryptic granulite-facies metamorphism in the Archean Wyoming Craton revealed by Sm–Nd garnet and U–Pb monazite geochronology

Granulite-facies metamorphism of large tracts of continental crust is a characteristic feature of Archean cratons, where metamorphism ultimately led to crustal stabilization and chemical differentiation. Constraining the timing and duration of this metamorphism can provide insight into the processes behind Archean cratonic evolution, and here we present new Sm–Nd garnet ages from granulite-facies metasediments from the eastern Beartooth Mountains of the Wyoming Craton. The eastern Beartooth Mountains are dominantly comprised of a ∼2.8Ga calc-alkaline batholith known as the Long Lake Magmatic Complex (LLMC). Within this batholith, numerous cm- to km-scale granulite-facies metasedimentary xenoliths and roof pendants have previously been interpreted to represent metamorphism due to contact heating with the LLMC or prior to LLMC emplacement. Garnet ages from five granulite-facies metasediments range from 2669±48Ma to 2681±58Ma: significantly younger than the LLMC and consistent with newly described field relations. Preserved major and trace element garnet zoning suggest that these dates are biased towards late stage garnet growth during granulite-facies metamorphism, and that this metamorphism post-dates LLMC emplacement. U–Pb monazite petrochronology of a metasediment and an tonalite reveals distinct periods of monazite (re)crystallization at 2.79–2.78Ga (consistent with LLMC emplacement) and at 2.69–2.67Ga (coincident with the garnet ages). The Sm–Nd bulk garnet ages exhibit larger errors and high MSWDs, which may result from sampling a polymetamorphic population, as suggested by REE zoning in garnet and supported by the U–Pb monazite data. Diffusion modeling of preserved major element garnet zoning suggests that the timescale at near-peak temperatures during the 2.69–2.67Ga event was <1Ma. Phase equilibria modeling suggests that the LLMC would have experienced minimal partial melting during this cryptic granulite-facies metamorphism, implying that large volumes of crust may have experienced a significant thermal event, but that only some parts of the crust record strong evidence for it. The ages presented here are similar to known granulite-facies metamorphic and mafic magmatic events in other parts of the Wyoming Craton, though no clear evidence of mafic magmas is present locally.

Petrology, phase equilibria and monazite geochronology of granulite-facies metapelites from deep drill cores in the Ordos Block of the North China Craton

Among the various Precambrian crustal blocks in the North China Craton (NCC), the geology and evolution of the Ordos Block remain largely enigmatic due to paucity of outcrop. Here we investigate granulite-facies metapelites obtained from deep-penetrating drill holes in the Ordos Block and report petrology, calculated phase equilibria and in-situ monazite LA-ICP-MS geochronology. The rocks we studied are two samples of cordierite-bearing garnet–sillimanite–biotite metapelitic gneisses and one graphite-bearing, two-mica granitic gneiss. The peak metamorphic age from LA-ICP-MS dating of monazite in all three samples is in the range of 1930–1940Ma. The (U+Pb)–Th chemical ages through EPMA dating reveals that monazite occurring as inclusions in garnet are older than those in the matrix. Calculated metamorphic phase diagrams for the cordierite-bearing metapelite suggest peak P–T conditions ca. 7–9kbar and 775–825°C, followed by decompression and evolution along a clockwise P–T path. Our petrologic and age data are consistent with those reported from the Khondalite Belt in the Inner Mongolia Suture Zone in the northern part of the Ordos Block, suggesting that these granulite-facies metasediments represent the largest Paleoproterozoic accretionary belt in the NCC.

New insights into the provenance of Saudi Arabian Palaeozoic sandstones from heavy mineral analysis and single-grain geochemistry

Saudi Arabian Palaeozoic siliciclastics cover a stratigraphic range from the Cambrian to the Permian. They crop out along the eastern margin of the Arabian Shield and are comprised of highly mature sandstones. Their heavy mineral assemblage reflects their mineralogical maturity and is dominated by the ultra-stable phases zircon, tourmaline and rutile. Less stable accessories are apatite, staurolite and garnet. Standard heavy mineral analysis of samples from two study areas in central/northern (Tabuk area) and southern (Wajid area) Saudi Arabia reveals distinct changes in provenance. Cambrian–Ordovician sandstones are first-cycle sediments, probably sourced from the ‘Pan-African’ basement. The overlying Hirnantian glaciogenic deposits consist of recycled Cambrian–Ordovician material. Devonian–Permian sandstones show a significant influx of fresh basement material, as attested by an increase of meta-stable heavy minerals. Single-grain geochemical analysis of rutile and garnet has proven to be a powerful supplementary technique. Rutile varietal studies reveal distinct differences in host rock lithologies between the two study areas: the Tabuk area contains predominantly felsic rutiles, whereas the Wajid area has more mafic input. Zr-in-rutile thermometry identified granulite-facies detritus in the lower Palaeozoic of the Tabuk area and has the potential to further define source areas. The distribution patterns of garnet host rock lithologies are remarkably similar in both study areas. They are dominated by amphibolite-facies metasediments and intermediate to felsic igneous rocks. Garnets derived from granulite-facies metasediments, which are scarce in the Arabian–Nubian Shield, also occur. Possible source rocks for high-grade garnets can be found in Yemen or farther south in the Mozambique Belt.

Fluid and deformation induced partial melting and melt volumes in low-temperature granulite-facies metasediments, Damara Belt, Namibia

Fluid-present partial melting has generally been regarded a poor candidate for effecting crustal differentiation. In this study we report on granulite-grade metasediments from the Pan-African Damara Belt in Namibia that have undergone fluid-present biotite melting at a relatively low-temperature, yet appear to have lost a significant volume of melt. In situ anatectic features have been identified on the basis of the existence of new generations of cordierite and/or garnet produced as the solid products of incongruent anatexis within or adjacent to leucosomes. Leucosomes occur in lens-shaped pods that are orientated at high angles to the regional stretching lineation that formed during orogen-parallel extrusion of the rocks during the main collisional event in the Damara Belt. Within these sites biotite underwent incongruent melting via the reaction Bt + Qtz + Pl + H 2O = Melt + Grt + Crd. Cordierite nucleated on pre-existing crystals within the bounding gneiss; garnet nucleated within the fracture sites (leucosomes) and typically occurs as individual, large (50 to 120 mm) poikiloblastic crystals. Thermobarometry applied to the anatectic assemblage yields low-temperature, granulite-facies peak conditions of 750 °C, 0.5 GPa. This temperature is approximately 100 °C lower than the accepted conditions for the onset of fluid-absent biotite melting. This, coupled to the focussing of anatexis on dilational sites, suggests that anatexis occurred through water-present biotite incongruent melting. In order to better understand this process, both fluid-absent and water-present partial melting experiments were conducted within the temperature interval 700 to 900 °C at 0.7 GPa. In the fluid-absent experiments, biotite incongruent melting started between 800 and 850 °C to produce melt coexisting with peritectic garnet and cordierite. In contrast, in water-saturated experiments, biotite melted via the reaction Bt + Qtz + Pl + H 2O = Melt + Grt + Crd, between 700 and 750 °C, to produce melt, cordierite and garnet in the proportions 73:24:3. The garnet compositions produced in these low-temperature experiments match very well with the relatively high spessartine content (10%), low Mg# garnets formed with the leucosomes, confirming the water-saturated, low-temperature nature of anatexis in the study area. Mass balance calculations indicate that melting in typical dilational sites has produced 6 dm 3 of melt. This melt has been sourced from some 0.08 m 3 of rock, a volume that coincides well with the typical frequency of extensional fracture spacing. This was probably important in enabling efficient melt extraction which has caused a slight shift towards more refractory compositions in the granulite-facies metapelites over amphibolite-facies equivalents. However, the attendant expansion in chemical equilibration volume has resulted in a marked compositional homogenization of the high-grade gneisses over the rhythmically bedded amphibolite-facies metapelites.

Garnet compositions in Scottish and Norwegian basement terrains: a framework for interpretation of North Sea sandstone provenance

Detrital garnets have proved to be useful discriminators of North Sea sandstone provenance because they show a wide range of potential compositions and are relatively stable during burial diagenesis. It has been less easy to use garnet data to provide a direct link between sediment and source because of the lack of a comprehensive database of garnet compositions in basement terrains forming the North Sea hinterland. This paper presents garnet compositional data from river sediments sourced from northern Scotland and Norway, and demonstrates the presence of marked variations in garnet compositions from different regions related to fundamental differences in basement lithology. The value of the river sediment database is demonstrated by comparing it with garnet compositions in Paleocene sandstones from the North Sea and Møre basins, enabling the establishment of direct links between source and sediment. Paleocene sandstones in the Gannet Field area on the southwestern margin of the Central North Sea were derived from the Dalradian of the Grampian Highlands, those in the northern North Sea were derived from the Moine/Dalradian rocks of Shetland, and those along the Møre Basin margin were derived from western Norway. By contrast, Paleocene sandstones in the Nelson Field (central North Sea) and in the Beryl Embayment (northern North Sea) have a garnet component that cannot be readily traced back to any basement terrain in either Scotland or Norway. This component, which was ultimately derived from granulite-facies metasediments or charnockites, is interpreted as being recycled from Triassic sandstones, which were themselves derived from an exotic source, probably to the west of the British Isles.

Spinel-bearing assemblages and P-T-t evolution of the Central Zone of the Damara Orogen, Namibia

The Central Zone of the Damara Orogen in Namibia is dominated by intrusive granitoids and upper amphibolite to granulite-facies metasediments. High-temperature low-pressure mineral assemblages near the Atlantic Coast have not been previously described with spinel symplectites. In semi-pelitic gneisses, the symplectites are composed of hercynitic spinel and Fe-Ti oxides occurring within the cores of zoned porphyroblasts of antiperthite+sillimanite. In pelitic schists, hercynite-sillimanite symplectites occur together with minor quartz in the core of Mg-cordierite porphyroblasts (X Mg = 0.66–0.72). Thermobarometry determines the P-T conditions of spinel symplectites to be 527 ± 112°C and 4 ± 1 kbar. Textural relationships indicate that the symplectite post-dates the metamorphic peak conditions dated by syn-metamorphic granite at 534 ± 7 Ma (published U-Pb zircon age) but are still relatively early within the metamorphic history. Subsequent isobaric regional heating and partial melting occurred in association with biotite breakdown and garnet growth. Thermobarometry gives 700 ± 50°C and 4 ± 1 kbar for this episode. This regional isobaric heating is equated with the emplacement of voluminous granitoid magmas, dated at post-534 ± 7Ma and pre-508 ± 2 Ma (published U-Pb zircon and monazite ages) and generated during decompression caused by the exhumation of the Central Zone. The system then cooled through 550 ± 25°C and 2 ± 1 kbar at 465 ± 5 Ma (published Rb-Sr and Ar 40/Ar 39 biotite-muscovite ages) on its way to the surface. The occurrence of a later separate thermal overprint as evidenced by spinel symplectites and garnet growth argues in favour of a two-stage metamorphic model. This study confirms the clockwise P-T path for the Central Zone of the Damara Orogen.

Trace-element characteristics and Pb isotopic evolution of metasediments and associated Proterozoic rocks from the amphibolite- to granulite-facies Bamble sector, southern Norway

A study of trace-element distribution and Rb/Sr, Sm/Nd, and Pb isotopic compositions has been performed on amphibolite- and granulite-facies metasediments, mafic granulites, dioritic gneiss, intrusive enderbitic veins and calc-alkaline tonalitic gneisses from the Proterozoic Bamble sector, southern Norway. The LILE content of the granulite-facies metapelites is similar to the average of the upper crust and post-Archaean average shales. The K/Rb ratios of the amphibolite- and granulite-facies semipelites and impure quartzites are similar, and the K/Rb ratios of the metapelites show overall similarities to the ratios of undepleted metapelites (< 300). There is evidence of incomplete Sveconorwegian correlations in the Rb Sr isotopic system, and a poor correlation in the Sm Nd system. The calculated t DM model ages for the metasediments can represent crustal residence times of 1.67 to 2.05 Ga. The present-day lead isotopic data can be reproduced by a three-stage model of Pb evolution in which ( t 0 = 4.57 Ga, t 1 = 1.9 or 1.5 Ga, t2 = 1.1 Ga, μ 1 ( 238U/ 204Pb) = 7.9, κ 1 ( 232Th/ 238U) = 2.5, κ 2 ≤ 4.15), where t 1 and t 2 are related to separation from the mantle and the Sveconorwegian orogeny, respectively. The protolith of the metasediments, the dioritic gneiss and the mafic granulites evolved in high-U/Pb environments prior to Sveconorwegian times (μ 2 = 16−32, 19,and18−45, respectively). Most of these rock units experienced conditions ofμ 2 > μ 3 andκ 2 < κ 3, suggesting a large-scale fractionation in the U Th Pb system connected to U-mobility during the medium- to high-grade metamorphism. There are no unambiguous differences in the Pb isotopic composition between the amphibolite- and granulite-facies rocks, but the mafic granulites from the classical ‘depleted granulite’ area of Tromøy island evolved in overall lowerμ 2 ( 238U/ 204Pb) environments than similar rocks from the neighbouring Hisøy island. This suggests that the similar U content of these rocks largely can be due to local pre-Sveconorwegian 238U/ 204Pb variations. Sveconorwegian intrusive enderbitic dykes and veins crystallized with low LILE and Nb concentrations at granulite-facies P T aCO 2 conditions. Pre-Sveconorwegian granulite-facies tonalitic gneisses show a less depleted trace-element pattern and low Nb concentrations and the hornblende-free tonalitic gneisses have significantly lower U content than their hornblende-bearing counterparts. The investigated metasediments have very low contents of the less mobile elements Nb, Sr, Cu and P 2O 5, which probably represent characteristics of the pre-sedimentary protolith.

Primary positive Eu anomaly in clinopyroxenes of low-crust gabbroic rocks

Positive Eu and Sr anomalies have been found in whole rocks and clinopyroxenes, as well as feldspars, in gabbros of the deep crustal mafic intrusion of the Ivrea Zone (northwestern Italy). These anomalies are particularly pronounced at one stratigraphie level of the intrusion, the upper part of the Upper Zone of the Layered Series (HUZ). The Eu Eu ∗ ratios range from 19 to 40 in plagioclases and from 3 to 14 in the coexisting clinopyroxenes. Mass balance calculations show that metamorphic redistribution of Eu from plagioclase cannot account for the positive Eu anomalies of the clinopyroxenes and that a corresponding anomaly in the parent melt is required. Speculatively, this feature is attributed to mixing of a mantle component with crustal, highly polymerized, melts deriving from advanced anatexis of granulite-facies metasediments. The existence in the deep crust of hybrid melts having a positive Eu anomaly may be important for the Eu balance between the lower and upper crust.

Stable isotope profile across the orthoamphibole isograd in the Southern Marginal Zone of the Limpopo Belt, South Africa

In the Southern Marginal Zone of the Limpopo Belt the transition from granulite- to amphibolite-facies metamorphism is defined by a change from orthopyroxene- to orthoamphibole-bearing assemblages within metasedimentary rocks of the supracrustal Bandelierkop Formation. Oxygen-isotope fractionations among constituent minerals within both amphibolite- and granulite-facies metasedimentary rocks are of similar magnitude implying a similar retrograde thermal history. Temperatures calculated from these fractionations can be arranged in the following order: quartz-garnet ≈ quartz-orthoamphibole ⩾ quartz-pyroxene > quartz-plagioclase = quartz-biotite. The observed temperature sequence is consistent with retrograde metamorphism under “closed” system conditions and oxygen closure of garnet, orthoamphibole and orthopyroxene at or close to peak metamorphism while quartz, plagioclase and biotite crystallized from and/or re-equilibrated via a melt/fluid phase down to temperatures of 560°C. Small fluid/rock ratios are implied by the simultaneous closure of quartz, plagioclase and biotite. Given that quartz is the dominant oxygen-bearing phase during retrogressive exchange and assuming garnet closure to oxygen diffusion at the time of formation then the quartz-garnet fractionations may provide minimum peak temperature estimates of 736 ± 52°C. Coarse-grained concordant felsic veins within the metasediments, texturally interpreted as anatectites, are in isotopic equilibrium with the host rock. The “closed” system, defined here only on the basis of oxygen-isotope systematics, does allow for fluid/melt extraction at high temperatures. In contrast to the metasediments, oxygen-isotope systematics for mafic and ultramafic rocks of the Bandelierkop Formation suggest open system behaviour for at least some of the rocks. Such rocks are commonly cross-cut by coarse-grained felsic veins which are not in isotopic equilibrium with the host rock and contain quartz whose δ 18O values are indistinguishable from those observed in the metasedimentary rocks. The stable isotope data are consistent with a model involving anatexis of metasedimentary rocks at peak metamorphic temperatures. Partial extraction of the anatectic melts and their crystallization elsewhere in the sequence is suggested by the mineralogy and isotopic composition of large discordant pegmatites together with the isotopic disequilibrium and open system behaviour observed in some mafic and ultramafic rocks. In the pelitic rocks, the release of small amounts of fluid from residual melts and/or somewhat larger amounts from partially collected melts during cooling, may account for many of the retrograde mineralogic features observed for these rocks without perturbing the oxygen-isotope systematics. Such retrograde features include the cordierite + orthopyroxene symplectite after garnet and the incipient hydration of orthopyroxene and cordierite within the granulites and possibly the regional retrogression of granulites to form orthoamphibole gneisses. Retrograde kyanite within these rocks thus suggests an isobaric cooling path. Similar δ 13C values of graphite separated from amphibolite- and granulite-facies metasediments together with significant oxygen and carbon isotope heterogeneity among charnockites, metasediments, banded iron formations and most mafic and ultramafic rocks are not consistent with a retrogressive influx of extraneous CO 2-rich fluids as the cause of regional retrogression and orthoamphibole formation in the gneisses nor with charnockitization. The data also argue against a pervasive CO 2 infiltration as the cause of granulite-facies metamorphism.

Chemical composition and fractionation of the continental crust

A new estimate of the bulk continental crust is reported consisting of 57 percent lower crust (60% felsic and 40% mafic granulites) and 43 percent upper crust. The proportions of crustal units are based on petrological observations (Bohlen &Mezger, 1989). The estimate of a bulk composition is intermediate between andesite and tonalite and is higher in Si, K, Rb, Sr, Zr, Nb, Ba, LREE, Pb, Th concentrations and lower in Mg, Ca, Sc, Mn, Fe than the crustal abundances reported byTaylor &McLennan (1985). Equal chemical composition between the upper crust and the felsic part of the lower crust is attained in balance computations if one restores a fraction of 12.5 percent S-type granite from the upper into the lower crust. An example of water-undersaturated partial melting and separation of a fraction of about 35 percent granitic magma at the conversion from amphibolite-into granulite-facies metasediments has been balanced bySchnetger (1988) in the Ivrea area (N. Italy). The worldwide observed discrepancy between a larger negative Eu anomaly in the upper crust compared with the half as large positive anomaly of the lower crust increasing from the early Precambrian to present has been explained by recycling of Ca-rich restite into the upper mantle. The composition of the Archean crust (example: Greenland) does not differ systematically from the post-Archean crust.

The paragenesis of spinel- and garnet-amphibole lherzolite in the Rodel area, South Harris

Synopsis The petrography and mineralogy of spinel- and garnet-amphibole lherzolites occurring in the granulite facies metasediments is described. Chemical analyses are presented for 7 lherzolites, 5 amphiboles, 5 orthopyroxenes, 3 clinopyroxenes, and a garnet. The spinel-amphibole lherzolites and chemically identical garnet-amphibole lherzolite conceivably represent spatially removed accumulates from the metagabbro-anorthosite complex. Garnet generation within the garnet-amphibole lherzolite and not the spinel-amphibole lherzolites is discussed, although both have probably been metamorphosed under identical conditions existing on the pressure-temperature boundary which separates the spinel and garnet lherzolite stability fields.