Paleoproterozoic Granulites Research Articles

Trace Element Composition of Discordant Zircon as a Reflection of the Fluid Regime of Paleoproterozoic Granulite Metamorphism (Khapchan Terrane, Anabar Shield)

New data on the U–Pb age (SHRIMP-II) and trace element composition (SIMS) of zircon from gneisses of the Khapchan Group of the Khapchan terrane of the Anabar Shield are presented. Zircon grains contain relicts of magmatic zircon, the protolith and source areas of which are difficult to specify. The only zircon in this group with the least altered core preserved the 207Pb/206Pb protolith age of 1971 ± 19 Ma. During granulite metamorphism, the zircon was subjected to the impact of fluid enriched in incompatible elements. The disurbances affected both the U–Pb isotope system (zircon age was “reset” at metamorphic age of 1920–1930 Ma) and composition of zircon, magmatic cores of which were significantly recrystallized in a solid state or dissolved by fluid up to the practically complete disappearance of primary zircon. In both the cases, zircon was sharply enriched in incompatible elements (Ca, Ti, Pb, Sr, Ba, and some others) owing to fluid effect, while preserved cores seemed to be rimmed by a new zircon population (CL-black). The REE distribution patterns in the recrystallized cores acquired a “bird’s wing” profile atypical for zircon. When the fluid lost its reactivity, the main part of the zircon grain crystallized, typical of granulite zircon. This zircon population is less enriched in incompatible element than the cores are. The horizontal pattern of HREE distribution is consistently repeated, which indicates the co-crystallization of zircon and garnet. The estimated crystallization temperature of the main part of zircon varies in a narrow range of 800–830°C. All zircon domains in the Wetherill concordia diagram form a single trend with a zero lower intercept and an upper intercept confirmed by concordant cluster with an age around 1920–1930 Ma. This value corresponds to the age of regional Paleoproterozoic granulite metamorphism. A unique feature of zircon from the Khapchan gneisses is that its cores did not retain the age marks of the protolith, but were completely reset during metamorphism both in terms of the U-Pb system and the trace element composition, which can be explained by the extremely high intensity of fluid impact during the granulite facies metamorphism superimposed on the rocks of the Khapchan terrane of the Anabar Shield.

Graphitization in Paleoproterozoic granulites from the Taihua Complex, North China Craton: Constraints from laser Raman spectroscopy, carbon isotopes and geochronology

The presence of graphite as a refractory form of solid carbon in high grade meta-sedimentary rocks serves as a valuable tool for identifying carbon sources and assessing the movement of carbon through the Earth's interior. In this study, we investigate the graphite formation processes from different lithologies (marbles, metapelites) from the Paleoproterozoic granulites in the southern margin of North China Craton. By utilizing a range of analytical techniques, including petrographic, mineral-chemical, laser Raman spectroscopic, carbon stable isotopic, and geochronological studies, we characterize the textural and isotopic variety of graphite and the timing of metamorphism associated with graphitization. The results suggest a progressive growth and graphitization of graphite during medium to high temperature metamorphic facies. The δ13C values of crystalline graphite in metapelites and marble range in −24.00‰ to −20.10‰, and − 22.50‰ to −21.60‰, respectively, both close to organic carbon averages. The calcite from the graphite-bearing marble shows δ13C values in the range of −2.90‰ to −2.80‰, which corresponds to marine carbonates. In contrast, graphite-bearing quartzo-feldspathic gneiss and diopside/augite-bearing marble show heavier isotopic values of −14.4‰ to −16.5‰, suggesting graphite precipitation from fluids of both organic and magmatic origin. The spectral and isotopic signatures of graphite in these rocks indicate a dominant biogenic signature with mixture of abiogenic carbon sources. Zircon UPb geochronology marks the timing of high-grade metamorphism at ca. 1.93 to 1.92 Ga, associated with graphite formation during subduction and accretion process associated with the final assembly of the North China Craton. We propose that crystalline graphite was formed during the continental collision-triggered medium to high temperature metamorphic event, consistent with a global graphite metallogenic event in the Paleoproterozoic.

ТИПЫ ЗОЛОТО-ПОЛИМЕТАЛЛЬНОЙ МИНЕРАЛИЗАЦИИ В ПОРОДАХ ДОКЕМБРИЯ НИМНЫРСКОГО ТЕРРЕЙНА АЛДАНО-СТАНОВОГО ЩИТА (ЮГ СЕВЕРО-АЗИАТСКОГО КРАТОНА)

The study of material composition of the Early Precambrian continental earth crust contributes to the identification of its metallogeny. Deposits of graphite, phlogopite, and iron have been discovered in the Paleoproterozoic granulite complexes of the Aldan-Stanovoy Shield on the southern margin of the North Asian Craton, and the first gold deposit has recently been discovered. Deposits are confined to the extended boundary of the infra- and supracrustal complexes within which multistage deformations, intense metasomatosis and metamorphism of various facies are observed. Modern methods of microanalysis allow scientists to significantly supplement data on the mineral composition of deposits, characterize conditions of their formation and make a comparison. The paper considers the geological structure of granulite complexes of the central part of the Aldan-Stanovoy Shield and ore mineralization in graphite-bearing quartzites, phlogopite-containing skarns, gold-bearing crystalline shales and diaphthorites. Ore and wallrock mineral associations have been identified. The comparison was made with well-studied ore deposits near the border of the Karelian Craton. The types of mineralization and directions of comparative metallogenic analysis are discussed.

Structurally controlled mineralization in parts of Aravalli craton, India: Constraints from gravity and magnetic data

Aravalli craton of central Rajasthan comprises Mangalwar Complex and Sandmata Complex with Archean to Proterozoic basement, well known for the mineralization. Recent geological studies have also revealed that the Aravalli mountains of the Banded Gneiss Complex are composed of Paleoproterozoic granulite and amphibolite-facies. Extensive geophysical surveys comprising gravity and magnetic were conducted to assess the occurrence of potential mineral sources in the uplifted crustal blocks of the Aravalli Fold belt. The calculated Bouguer gravity anomalies trending in a NE-SW direction and three broad gravity highs (∼7–10 mGal) were observed. These gravity highs may be due to the uplifting of the basement or the presence of high-density contrast material near the subsurface. A gradient in the gravity contours at the northwestern part of the study area is due to the fault/structural contact trending along the NE-SW direction. The 2D inversion technique is used to model the gravity data perpendicular to fault/contact. The horizontal gradient reflects the distribution of structures and intrusive bodies, which will give new insight for further future mineral exploration. The spectral analysis depicts the three depth interfaces, ∼7, ∼3.1, and ∼1.1 km, representing the basement and shallow depth interfaces. Further, the subsurface ore bodies geometry was obtained through 2D modelling of gravity data incorporating the constraints from the rock sample's physical property (density). We established that the structure and lithology of the host rocks were responsible for controlling mineralization by integrating the geochemical findings with geophysical measurements. In addition, the Archean to the Proterozoic basement of the study area is undulating at a depth of ∼3–5 km.

ORE MINERALIZATION IN PALEOPROTEROZOIC GRANULITE COMPLEXES OF THE NORTH OF THE ALDAN-STANOVOY SHIELD

The problems of metallogenic specialization of the Early Precambrian continental crust in metamorphic complexes of various composition and conditions of formation are of urgent importance for regional metallogeny development. The paper considers the geology, features of the mineral and material composition of deposits and points of mineralization in the Paleoproterozoic granulite complexes of the Aldan-Stanovoy shield. A comparison of the features of composition with those of the ore deposits in the Svecofennides of the Baltic Shield as an well studied folded region has been performed. The conclusion was made about the similarities of metallogenic specialization of the metamorphic complexes.

Phase equilibria modelling and zircon U-Pb geochronology of Paleoproterozoic mafic granulites from the Chengde Complex, North China Craton

The Chengde Complex is located in the northern margin of the Trans-North China Orogen (TNCO) of the North China Craton (NCC), and consists mainly of tonalitic-trondhjemitic-granodioritic (TTG) gneisses, garnet-bearing amphibolites and granulites. Mafic granulites are widely distributed as enclaves or boudins within the Chengde Complex. Three representative mafic granulite samples (CD95, CD84, and CD07-2) with different reaction textures were selected for decoding the metamorphic evolution of the Chengde Complex, through an integrated investigation of petrography, mineral chemistry, phase equilibria modelling and zircon U-Pb dating. Petrographic observations and phase equilibria modeling suggest that these mafic granulites have experienced granulite facies metamorphism with characteristic of clockwise P-T paths. Their metamorphic peak P-T conditions were constrained to be 8.4–12.9 kbar/ 850–920 °C. Zircon U-Pb dating for sample CD95 gives an inherited age of ∼ 2.55 Ga as well as a metamorphic age of ∼ 1.84 Ga, while the age of ∼ 2.51 Ga may represent a metamorphic age or mixed age. Zircons in sample CD84 yield three age groups of ∼ 2.52 Ga, ∼ 2.46 Ga and ∼ 1.83 Ga, interpreted to represent the captured zircon grains from country TTG rocks, metamorphic age of the late Neoarchean, and metamorphic age of the Paleoproterozoic, respectively. In sample CD07-2, zircon cores and rims recorded metamorphic ages of ∼ 1.84 Ga and ∼ 1.81 Ga, respectively, which may represent metamorphic peak timing and the later cooling event. Combined with the zircon HREE patterns and previous investigations, we conclude that these mafic rocks in the Chengde Complex experienced the Paleoproterozoic granulites facies metamorphism. The clockwise P-T paths of mafic granulites in the Chengde Complex imply an orogenic process of thickening followed by cooling and uplift-due to the assembly of the Eastern Block and the Western Block.

Pressure–temperature–time constraints on gneiss dome formation in an intracontinental orogen

AbstractThe Entia Gneiss Complex represents the mid‐crustal core of the intracontinental Alice Springs Orogen. Located in the Harts Range, central Australia, it is characterized by the development of a domal structure including two sub‐domes separated by a steeply dipping median high‐strain zone. Dominantly, orthogneiss basement crops out through a structurally overlying cover sequence represented by the Harts Range Group and records evidence of hydration, recrystallization, and partial melting of precursor Paleoproterozoic granulite facies assemblages at amphibolite facies conditions. The structurally lowest parts of the Harts Range Group were metamorphosed to peak conditions of 10.5 kbar and ~880°C during rift‐related magmatism at 480–460 Ma, immediately prior to the onset of the Alice Springs Orogeny at 450–300 Ma. By contrast, the underlying Entia Gneiss Complex records widespread metamorphism and wet melting at upper amphibolite facies conditions. Phase equilibrium modelling and in situ LA–ICP–MS geochronology of rare, low‐variance kyanite–garnet‐bearing metapelites indicate that maximum P–T conditions of ~9 kbar and ~680°C were reached between c. 360–330 Ma, demonstrating a distinctive metamorphic and temporal evolution relative to the overlying Harts Range Group that can be linked to rheological stratification. Based on the integration of our results with existing monazite, zircon, and titanite geochronology, we interpret doming of the Entia Gneiss Complex to have involved compressive ascent of rheologically weakened crust below a region of extending upper crust near the termination of the Alice Springs Orogeny. In this way, the Harts Range Group represents a cooled, locally extending thrust sheet over ductile basement quasi‐concurrent with doming. Texturally‐late sillimanite combined with increasing Y content in monazite indicates high‐temperature, kyanite‐grade metamorphism was closely followed by decompression (3–4 kbar drop from peak conditions) and rapid cooling below 600°C during emplacement of the Entia Gneiss Complex at shallower crustal levels. The findings of this study highlight the feedback between hydration, retrogression, rheological weakening, and strain accommodation, thus allowing better evaluation of the thermomechanical history of gneiss dome formation within a narrow (<80 km wide) preconditioned intracontinental corridor.Highlights First modern P–T–t framework for gneiss dome formation in the Entia Gneiss Complex, central Australia. The onset of metamorphism, partial melting and ductile flow at c. 360 Ma was catalysed by hydration of granulite facies basement during rift inversion. Rapid exhumation of partially molten crust from depths of >20 km is marked by fluid‐mediated resetting of monazite down to c. 310 Ma. Rheological stratification enabled and accelerated exhumation of the highest‐grade corridor of a narrow intracontinental orogen.

Structural, petrological, and tectonic constraints on the Loch Borralan and Loch Ailsh alkaline intrusions, Moine thrust zone, northwestern Scotland

During the Caledonian orogeny, the Moine thrust zone in northwestern Scotland (UK) emplaced Neoproterozoic Moine Supergroup rocks, metamorphosed during the Ordovician (Grampian) and Silurian (Scandian) orogenic periods, westward over the Laurentian passive margin in the northern highlands of Scotland. The Laurentian margin comprises Archean–Paleoproterozoic granulite and amphibolite facies basement (Scourian and Laxfordian complexes, Lewisian gneiss), Proterozoic sedimentary rocks (Stoer and Torridon Groups), and Cambrian–Ordovician passive-margin sediments. Four major thrusts, the Moine, Ben More, Glencoul, and Sole thrusts, are well exposed in the Assynt window. Two highly alkaline syenite intrusions crop out within the Moine thrust zone in the southern Assynt window. The Loch Ailsh and Loch Borralan intrusions range from ultramafic melanite-biotite pyroxenite and pseudoleucite-bearing biotite nepheline syenite (borolanite) to alkali-feldspar–bearing and quartz-bearing syenites. Within the thrust zone, syenites intrude up to the Ordovician Durness Group limestones and dolomites, forming a high-temperature contact metamorphic aureole with diopside-forsterite-phlogopite-brucite marbles exposed at Ledbeg quarry. Controversy remains as to whether the Loch Ailsh and Loch Borralan syenites were intruded prior to thrusting or intruded syn- or post-thrusting. Borolanites contain large white leucite crystals pseudomorphed by alkali feldspar, muscovite, and nepheline (pseudoleucite) that have been flattened and elongated during ductile shearing. The minerals pseudomorphing leucites show signs of ductile deformation indicating that high-temperature (~500 °C) deformation acted upon pseudomorphed leucite crystals that had previously undergone subsolidus breakdown. New detailed field mapping and structural and petrological observations are used to constrain the geological evolution of both the Loch Ailsh and the Loch Borralan intrusions and the chronology of the Moine thrust zone. The data supports the interpretation that both syenite bodies were intruded immediately prior to thrusting along the Moine, Ben More, and Borralan thrusts.

Orthopyroxene-sillimanite granulites of the Angara-Kan block (SW Siberian craton): Constraints on timing of UHT metamorphism

Abstract We report here the evidence of protracted metamorphic evolution of orthopyroxene-sillimanite granulite culminating in UHT metamorphism within the Paleoproterozoic granulite complex of the Angara-Kan block (SW Siberian craton). The Opx-Sil granulites were formed during UHT metamorphism (T = 1070–1100 °C and P = 9 kbar) at ca. 1.8 Ga which was followed by retrograde isobaric cooling down to the temperature around 900–950 °C. At ca. 1.74 Ga Opx-Sil granulites were trapped and partly reworked by a granitic melt with crystallization of perthitic feldspar at about 750 °C. The integration of all data on metamorphism and granitoid magmatism of the Angara-Kan block indicates two-staged metamorphic history. The first stage of the granulite facies metamorphism occurred in the time range of 1.89–1.85 Ga and was terminated by the emplacement of the 1.84 Ga post-collisional granites, the second stage of metamorphism occurred at UHT conditions and culminated in 1.78–1.8 Ga. The UHT metamorphism including the isobaric cooling retrograde stage and granite magmatism had a duration of 30 to 60 million years from 1.78–1.8 to 1.74–1.75 Ga.

Geology, geochemistry, and geochronology of the paleoproterozoic Donggouzi mafic-ultramafic complex: Implications for the evolution of the North China craton

The Paleoproterozoic tectonic setting and spatial-temporal evolution of the northern North China and Central Orogenic belt constrain the assembly of the North China craton. Here we describe a new Paleoproterozoic mafic-ultramafic unit and intermediate-mafic intrusions located between the Khondalite belt and Central Orogenic belt, which we refer to as Donggouzi mafic-ultramafic complex. This complex is structurally complicated, and comprises a mixture of peridotite, amphibole pyroxenite, and websterite blocks, with associated alkali granitoid and gabbroic dikes, which were intruded into Paleoproterozoic granulites. Zircon UPb geochronology of the granulite, intruding syenite and alkali granites together with the field observations constrain the formation of the Donggouzi mafic-ultramafic complex to between 1849 and 1845 Ma, i.e. 1.85 Ga. Geochemical and SrNd isotopic data indicate that the Paleoproterozoic mafic-ultramafic rocks were sourced from enriched metasomatized mantle by the fluid from a subduction slab. Emplacement of the complex occurred in a post-orogenic extensional setting, possibly associated with the collision along the northern margin of the North China craton at ca. 1.9–1.88 Ga. The ca. 1767 Ma gabbroic dike was emplaced in an intraplate extensional setting and experienced crustal contamination, which suggests that an upwelling mantle plume produced magmas that interacted with the lithosphere during the late Paleoproterozoic.

Ultra-high temperature overprinting of high pressure pelitic granulites in the Huai'an complex, North China Craton: Evidence from thermodynamic modeling and isotope geochronology

Paleoproterozoic granulite facies rocks are widely distributed in the North China Craton (NCC). The Huai'an terrane, located within the northern segment of the Trans-North China Orogen (TNCO), a major Paleoproterozoic collisional belt in the central NCC expose mafic and pelitic granulites as well as TTG (tonalite-trondhjemite-granodiorite) gneisses. Here we investigate the pelitic granulites from this complex and identify four distinct mineral assemblages corresponding to different metamorphic stages. The prograde metamorphism (M1) is recorded by relict biotite and the compositional profile of Xca (grt) isopleths. The Pmax (M2) is distinguished by the Xca (grt) isopleths, which corresponds to the kyanite stable area with an inclusion mineral assemblage of Grt-c–(Ky)-Qz-Rt-Kfs-liq suggesting that the pressures were higher than 12 kbar with a temperature below 900 °C. However, kyanite is absent in thin sections suggesting its consumption during later stages. The Tmax metamorphism (M3) is characterized by the assemblage: Grt-m-Qz-Pl-Rt-Kfs-Sil-liq in the garnet mantle and also reflected in the compositional profile. Two-feldspar geothermometry yields a P-T range of 940 °C–950 °C and 9.5–10.5 kbar, indicating ultra-high temperature (UHT) metamorphic overprinting. The subsequent retrograde metamorphic stage (M4) is characterized by Grt-r-Bt-Sil-Kfs-Pl-Qz ± Rt ± Ilm with symplectites of Bt-Sil-Qz in the garnet rim suggesting garnet breakdown with P-T conditions estimated as 770 °C–840 °C and 6.5–8 kbar. The pelitic granulites show a clockwise path, with P-T estimates higher than those in estimated in previous studies using conventional techniques.LA-ICP-MS U–Pb analysis of metamorphic zircon grains yield two groups of ages at 1972.9 ± 8.1 Ma and 1873.3 ± 9.9 Ma. We suggest that the protoliths of the Manjinggou HP-UHT granulites were deep subducted where they experienced HP metamorphism associated with the collision of the Ordos and Yinshan blocks at ca. 1.97 Ga. Subsequently, the UHT metamorphic overprint occurred during the assembly of the unified Western and Eastern Blocks of the NCC along the TNCO at ca. 1.87 Ga.

Reply to “Comment on “Isotopic insight on paleodiet of …” by Bocherens et al. (Gondwana Research, 48(1), 7–14)”

We report here for the first time the occurrence of sapphirine-bearing rocks within the Paleoproterozoic granulite complex of the Angara-Kan block (SW Siberian craton) and provide strong evidence for crustal metamorphism at ultrahigh-temperature (UHT) conditions in this region. The UHT rocks contain sapphirine, high alumina orthopyroxene, orthopyroxene + sillimanite assemblage, mesoperthitic feldspar and pseudomorphoses after osumilite crystals. The peak temperature of metamorphism was estimated using Al2O3 contents in Opx (up to 10 wt%) and the ternary feldspar composition, and falls in a range of 1050–1100 °C. Observed mineral microstructures indicate post-peak cooling of rocks without a considerable decrease in pressure down to a temperature of about 900 °C. Another type of rock related with UHT metamorphism is represented by orthopyroxene-sillimanite gneisses, which recorded a peak temperature of about 950 °C, decreasing down to 900 °C during post-peak stage. The UHT gneisses record microstructural evidence of melt injection and matrix-melt interaction near the peak metamorphic conditions. Sapphirine grains occur in the gneiss with relatively low Mg bulk composition. It is suggested that sapphirine occurs in the Mg-Al-rich domains which have been formed as a result of partial melting and precipitation of Fe-oxides from the melt. High concentrations of Al2O3 in orthopyroxene are the only sign of UHT metamorphism in Fe-rich rock matrix which is not affected by melt. New SHRIMP U-Pb zircon data from two UHT gneisses indicate the presence of two well-defined age peaks at ca. 1.8 and 1.9 Ga from rims and metamorphic grains, and older dates from ca. 1.91 to 3.15 Ga from detrital cores. The prominent feature of the metamorphic zircons is depletion of U with high Th/U ratio up to 6.0. Steep HREE patterns of the metamorphic zircons suggest their growth in the presence of a melt. The metamorphic zircons from the UHT gneiss record a broad range of temperatures between 1000 °C and 810 °C indicating protracted zircon growth from near peak conditions through retrograde stage of PT-path. The growth of the youngest zircons occurred at higher temperature indicating that the UHT metamorphic conditions were reached at 1.78–1.8 Ga. The timespan of the granulite-UHT metamorphic event is near 50 Ma and is limited by intrusions of late leucogranites and charnockites at 1.75–1.73 Ga. Protracted interval of granulite-UHT metamorphism conforms with isobaric cooling P-T path constructed by mineral assemblages.

Sapphirine-bearing Fe-rich granulites in the SW Siberian craton (Angara-Kan block): Implications for Paleoproterozoic ultrahigh-temperature metamorphism

We report here for the first time the occurrence of sapphirine-bearing rocks within the Paleoproterozoic granulite complex of the Angara-Kan block (SW Siberian craton) and provide strong evidence for crustal metamorphism at ultrahigh-temperature (UHT) conditions in this region. The UHT rocks contain sapphirine, high alumina orthopyroxene, orthopyroxene+sillimanite assemblage, mesoperthitic feldspar and pseudomorphoses after osumilite crystals. The peak temperature of metamorphism was estimated using Al2O3 contents in Opx (up to 10wt%) and the ternary feldspar composition, and falls in a range of 1050–1100°C. Observed mineral microstructures indicate post-peak cooling of rocks without a considerable decrease in pressure down to a temperature of about 900°C. Another type of rock related with UHT metamorphism is represented by orthopyroxene-sillimanite gneisses, which recorded a peak temperature of about 950°C, decreasing down to 900°C during post-peak stage. The UHT gneisses record microstructural evidence of melt injection and matrix-melt interaction near the peak metamorphic conditions. Sapphirine grains occur in the gneiss with relatively low Mg bulk composition. It is suggested that sapphirine occurs in the Mg-Al-rich domains which have been formed as a result of partial melting and precipitation of Fe-oxides from the melt. High concentrations of Al2O3 in orthopyroxene are the only sign of UHT metamorphism in Fe-rich rock matrix which is not affected by melt. New SHRIMP U-Pb zircon data from two UHT gneisses indicate the presence of two well-defined age peaks at ca. 1.8 and 1.9Ga from rims and metamorphic grains, and older dates from ca. 1.91 to 3.15Ga from detrital cores. The prominent feature of the metamorphic zircons is depletion of U with high Th/U ratio up to 6.0. Steep HREE patterns of the metamorphic zircons suggest their growth in the presence of a melt. The metamorphic zircons from the UHT gneiss record a broad range of temperatures between 1000°C and 810°C indicating protracted zircon growth from near peak conditions through retrograde stage of PT-path. The growth of the youngest zircons occurred at higher temperature indicating that the UHT metamorphic conditions were reached at 1.78–1.8Ga. The timespan of the granulite-UHT metamorphic event is near 50Ma and is limited by intrusions of late leucogranites and charnockites at 1.75–1.73Ga. Protracted interval of granulite-UHT metamorphism conforms with isobaric cooling P-T path constructed by mineral assemblages.

Two phases of granulite facies metamorphism during the Neoarchean and Paleoproterozoic in the East Hebei, North China Craton: Records from mafic granulites

Archean cratons are commonly reworked during the late Paleoproterozoic orogeny, which may result in multi-phase granulite facies metamorphism in many domains. The East Hebei terrane of the North China Craton is developed with two phases of granulite facies metamorphism in the Neoarchean and Paleoproterozoic. The Neoarchean granulite facies metamorphism is recognized mainly in the rafts or slivers of supracrustal rocks from the Taipingzhai ovate-structural domain, characteristic of anticlockwise P-T paths. The mafic granulites at this domain contain two-pyroxene granulite assemblages which are commonly overprinted by a late phase of granulite facies marked by tiny-grained vermicular orthopyroxene-bearing assemblages. They only record the Neoarchean metamorphic ages at 2.53–2.52Ga and 2.51–2.48Ga as indicated by zircon U-Pb data, para-simultaneously with the emplacement of TTG rocks. In contrast, the Paleoproterozoic granulite facies metamorphism can be well recognized in the Saheqiao linear-structural belt, which is characterized by high-pressure granulite assemblages in mafic rocks with clockwise P-T paths. In this belt, the Archean two-pyroxene granulite assemblages are locally preserved, showing “red-eye socket” overprinting textures consisting of garnet coronas or fine-grained garnet and clinopyroxene bearing assemblages surrounding the early-stage pyroxenes. Zircon U-Pb data indicate that mafic granulites from the Saheqiao belt record not only the Neoarchean metamorphic ages of ∼2.50Ga but also some Paleoproterozoic ages of 1.97–1.83Ga. The first application of Lu-Hf garnet-whole rock isochron method to garnet-bearing mafic granulites from the Saheqiao belt yields ages of 1766.6±7.0 and 1787±22Ma, confirming that the overprinting of garnet-bearing assemblages occurred in the late Paleoproterozoic. Thus, the Saheqiao linear-structural belt may represent a strongly reworked domain during the Paleoproterozoic orogeny, rather than an Archean high-grade greenstone belt as previously considered.

ARCHEAN AND PALEOPROTEROZOIC HISTORY OF ROCK METAMORPHISM IN THE URALS CRUSTAL SEGMENT

In the Urals, Archean and Paleoproterozoic formations were found in several polymetamorphic complexes which crop out as relatively small tectonic blocks (within the first thousands sq. km in area). It is most likely that the rocks forming the polymetamorphic complexes located within the paleocontinental Ural area extending to the west of the Main Uralian Fault belong to the Archean-Paleoproterozoic section. These complexes are interpreted as fragments of the crystalline basement of the Ural part of the East European Craton, which are included in the structure of the Uralides. Two stages of the granulite facies metamorphism - earlier Neoarchean and later Paleoproterozoic - were established in the rocks of the polymetamorphic complexes in the paleocontinental (and partially paleooceanic) Ural region. High-pressure mineral parageneses are the products of the earliest endogenous rock transformational processes in eclogite-bearing complexes, where high-pressure metamorphism is apparently complementary to the Paleoproterozoic granulite metamorphism.

Paleoproterozoic granulites from the Xinghe graphite mine, North China Craton: Geology, zircon U–Pb geochronology and implications for the timing of deformation, mineralization and metamorphism

The Xinghe graphite deposit is located in the north-central part of the North China Craton (NCC) where Archean gray gneiss, metasedimentary rocks (khondalites) and mafic (high-pressure) granulites constitute the dominant rock types. In this study, we report results of detailed geological and structural mapping around the Xinghe graphite mine. We identify three stages of deformation (D1–D3) and the graphite mineralization is correlated with D1 or between D1 and D2. The first stage of deformation represented by rootless intrafolial folds whose axial surface cuts across the regional penetrative gneissosity (S2) of D2, was constrained at ca. 1.95–1.90Ga by U–Pb age of zircon grains from a mafic high-pressure granulite. The second stage of deformation is represented by tight folds with formation of regional penetrative foliation and strong ductile shearing, when garnet broke down to plagioclase+pyroxene. This stage is dated as ca 1.85Ga, from U–Pb analysis of zircons in leucosome from a migmatitic gneiss. The third stage of deformation is represented by large scale folding, which probably formed between 1.85 and 1.80Ga, as constrained from the ca. 1.80Ga emplacement age of an undeformed pegmatite that cuts across the host metasediments. Prior to the high-pressure granulite metamorphism, voluminous ca. 2.15–2.0Ga granites were emplaced within the khondalites. The graphitization postdates the emplacement of these granites as deduced from the presence of flaky graphites in the metagranites. The protolith of the mafic high-pressure granulite dike that intruded the khondalite at ca. 2.0Ga was also metamorphosed at 1.95–1.90Ga, and is part of the exploited graphite ore body. We therefore constrain the timing of graphite mineralization to be between ca. 1.95 and 1.85Ga. Our geological and geochronological data indicate that all the rock units in the Xinghe graphite mine were subjected to high-pressure granulite facies metamorphism at ca. 1.95–1.90Ga.

Paleoproterozoic granulites from Heling'er: Implications for regional ultrahigh-temperature metamorphism in the North China Craton

The Inner Mongolia Suture Zone (IMSZ, incorporating the Khondalite Belt) forms part of an accretionary orogen developed during the Paleoproterozoic collision between the Yinshan Block and Ordos Blocks within the North China Craton (NCC). The granulites within the IMSZ are characterized by high-temperature (HT) to ultrahigh-temperature (UHT) metamorphism with diagnostic mineral assemblages for UHT conditions preserved in some localities such as Tuguiwula and Wuchuan. We report here a new locality in Heling'er, located within the southern domain of the IMSZ where spinel–garnet–sillimanite-bearing granulites record UHT metamorphic conditions. The prograde assemblages occurring as inclusions in porphyroblastic minerals in these rocks include sillimanite+quartz+spinel, whereas coarse grained garnet+sillimanite+quartz+spinel+cordierite+plagioclase+K-feldspar+magnetite+ilmenite represent the peak assemblages. The retrograde stage is marked by orthopyroxene and cordierite rims developed on garnet and spinel. Biotite marks the final hydration stage of the high-grade metamorphism. Spinel and quartz occur in direct contact, with the spinel showing low Zn, Cr, and Fe3+. Application of Ti-in-zircon and ternary feldspar geothermometers yields temperatures up to ~940°C for the peak stage. Mineral phase equilibria modeling based on the bulk chemistry and mineral assemblages shows that the peak assemblage of garnet+plagioclase+spinel+cordierite+sillimanite+quartz+ilmenite+magnetite was stabilized at 1030°C at 6.5–7.5kbar, thus we infer a 940–1030°C temperature range for peak metamorphism in these rocks. The newly discovered UHT rocks in Heling'er, dated as 1.92Ga, together with the two previously reported UHT localities at Tuguiwula and Wuchuan, suggest that the Paleoproterozoic UHT metamorphism occurred on a regional scale within the IMSZ, probably in association with the subduction–accretion–collision tectonics and the incorporation of the NCC within the Columbia supercontinent.

CHIME dating and age mapping of monazite in granulites and paragneisses from the Hwacheon area, Korea: implications for correlations with Chinese cratons

The basement of the Hwacheon area in the northern part of the Gyeonggi massif in the Korean Peninsula comprises granulite and gneiss complexes that are separated by an extensive mylonite zone. Granulites distant from the mylonite zone show little evidence of a retrograde overprint beyond minor growth of biotite along garnet grain boundaries. Monazite grains enclosed in garnet show a core to rim decrease in Y, and those in the matrix show a peripheral enrichment of Y on low-Y interiors. Similarly, garnet grains decrease in Y from core to rim, with peripheral Yenrichment, which can be attributed to early to peak garnet growth and post-growth modification, respectively. The three chemical domains representing stages of monazite growth show no detectable age difference, together yielding a CHIME age of 1868 ± 24 Ma. Granulites near the mylonite zone show growth of cordierite and biotite aggregates around garnet grains, and yield Permo-Triassic monazite domains on and within predominantly Paleoproterozoic monazite grains. Mylonitized granulites have a metamorphic overprint characterized by the growth of the sillimanite and K-feldsper assemblage at ca. 241 Ma, subsequent to ductile shearing. Pelitic gneisses in the gneiss complex contain zircon grains with Neoproterozoic detrital cores and monazite grains with unequivocal metamorphic ages of ca. 245 Ma. Geological, petrological and chronological evidence suggests differing evolutional histories between two crustal domains prior to amalgamation. The widespread occurrence of Permo-Triassic paragneisses and mylonites in the Gyeonggi massif can be explained by detachment of the metamorphosed upper crust of the South China block, which was thrust over the Paleoproterozoic granulite complex at ca. 241 Ma during the Sulu orogeny. Permo-Triassic metamorphic overprints on the latter are attributed to contact metamorphism from the hot allochthon.

Proposta litoestratigráfica e considerações paleoambientais sobre o Grupo Maricá (Neoproterozóico Iii), Bacia Do Camaquã, Rio Grande Do Sul

The Marica Group (Rio Grande do Sul State, Southern Brazil) contains the first sedimentary successions deposited after the denudation and tectonic subsidence of the basement units of the Gaucho Shield that was formed during the Brasiliano Orogeny. This subsidence event lead to the generation of the Camaqua Basin (Neoproterozoic III / Early Paleozoic), which posteriorly evolved to a rift system characterized by volcanosedimentary successions (Bom Jardim group) and coarse-grained siliciclastics (Santa Barbara and Guaritas groups). The Marica group consists of 2000-2500 m of arkosean sandstones and, in smaller proportion, conglomerates and pelites. Three lithostratigraphic units are here defined: Passo da Promessa formation (fluvial sandstones and conglomerates), Sao Rafael formation (marine sandstones and rhythmites) and Arroio America formation (fluvial sandstones and conglomerates). Stratigraphic and provenance analyses of the alluvial units, as well as paleocurrent data, show that: (i) sedimentary transport was towards the North during the evolution of these units; (ii) source areas were composed of granitic rocks, both foliated and unfoliated; (iii) there was an important contribution of a volcanic source, comprehending mainly rhyolitic and pyroclastic rocks, which frequency of occurrence increase towards the top of the succession, also including rare andesites in the Arroio America formation. The stratigraphic relationship among the Marica formation and the basement units shows that the basin was not related to the Brasiliano Orogeny, as revealed by the fact that the Marica group overlies both the cratonic foreland regions and the collisional suture, represented respectively by paleoproterozoic granulites (Rio the La Plata Craton) and a neoproterozoic subduction complex (Rio Vacacai Terrane).

Post-Collisional Alkaline Magmatism on the Taquarembó Plateau: A Well-Preserved Neoproterozoic–Cambrian Plutono-volcanic Association in Southern Brazil

The Taquarembó Plateau plutono-volcanic association (TPPVA), a magmatic association related to a silica-saturated alkaline series, represents a portion of the last episode of post-Brasiliano/Pan-African collisional magmatism in southern Brazil. It was preceded by a postcollisional high-K calc-alkaline and shoshonitic magmatism, which is more pronounced near the collisional belt. The TPPVA is a sequence of volcanic, volcaniclastic, and volcanogenic sedimentary deposits with hypabyssal associated rocks, lying on Paleoproterozoic granulites. Isotope data suggest that this alkaline postcollisional magmatism occurred over the period from 580 to 537 Ma. Two sequences of magmatic liquids, which evolved by mineral fractionation from low-Ti-P and high-Ti-P basaltic magmas, were identified. The former includes mildly alkaline silica-saturated basalts, metaluminous monzodioritic intrusions, and rhyolitic lavas, whereas the second includes hawaiites, mugearites, syenitic intrusions, and peralkaline to intermediate lava flows of comenditic affinity. Trace-element and isotope data suggest that both parental magmas were produced from the same EMI-type mantle sources, representing different melt fractions, which preserve the trace-element signature inherited from metasomatism caused by a previous (∼780 to 700 Ma) crustal subduction.