Amphibolite Facies Metamorphic Event Research Articles

Neoarchean magmatism and Rhyacian reworking in the northeast of the Amapá Block, southeastern Guiana Shield, Brazil: Geochemistry, U–Pb geochronology, and Hf–Nd isotopes

This study investigates the age, geochemical nature, sources, and Rhyacian reworking of the Neoarchean magmatism at the northeastern portion of the Amapá Block, southeastern Guiana Shield (northeast Amazonian Craton). This block consists of a large Archean continental landmass strongly reworked during the Transamazonian orogeny (2.26–1.95 Ga). We provide the very first geochemical results for Archean basement units of the Amapá Block, which are the Mesoarchean Tumucumaque and Neoarchean Guianense complexes, and Neoarchean Mungubas Granite. The first two exhibited geochemical affinity with enriched TTG-type and hybrid granitoids, respectively, while the Mungubas Granite showed affinity with biotite granites. Such affinity with C-type granitoids indicates that the genesis of these units involves partial melting of crustal sources. LA-MC-ICP-MS zircon U–Pb and Lu–Hf, and whole-rock Sm–Nd data were obtained for the Neoarchean Mungubas Granite and Guianense Complex. In both units, zircon U–Pb dating provided a distribution of dates that span ∼50 Ma forming two groups with distinct mean 207Pb/206Pb ages of ∼2.70 Ga and ∼2.65 Ga, respectively. The older age is considered as the crystallization/emplacement age of the Mungubas Granite and the Guianense orthogneiss protolith, while the younger one is ascribed to concealed Pb loss. The recognition of Mesoarchean inherited zircon core and xenocryst with ages of 2.84 Ga and 2.99 Ga suggest that these units reworked older basement rocks from the Amapá Block. Subchondritic ԐHf(2.70 Ga) of zircon and ԐNd(2.70 Ga) of whole-rock, along with their respective Hf-TDMC (3.6–3.2 Ga) and Nd–TDM (3.6 Ga) model ages, for both units, reflect the substantial crustal reworking of predominant Paleoarchean sources during the Neoarchean in the northeastern Amapá Block. In addition, U–Pb dating on titanite crystals from the Guianense orthogneiss yielded an unprecedented ∼2.05 Ga age, arguing that a Late Rhyacian metamorphic event affected the Archean orthogneisses. This 2.05 Ga event coincides with the emplacement of Rhyacian granitoids in the area, as well as with the 2.06–2.04 Ga late orogenic HT granulite facies and amphibolite facies metamorphic events identified, respectively, at the northern border and in the central-southern portion of the Amapá Block.

Garnet–Quartz Inclusion Thermobarometry and Lu–Hf Chronology Detail the Pre-Ultra-High Pressure Metamorphic History of the Grapesvare Nappe, Scandinavian Caledonides

Abstract The subduction–exhumation history of the Grapesvare nappe in the northern Seve Nappe Complex (Scandinavian Caledonides) is recorded by late Cambrian/Early Ordovician ultra-high pressure (UHP) and subsequent amphibolite facies metamorphic events. Records of these events obscured earlier metamorphic episodes that are important for understanding the tectonics of the orogen. To extract the pre–UHP metamorphic records, garnet Lu–Hf geochronology, Titanium-in-Quartz thermobarometry, and Quartz-in-Garnet elastic thermobarometry were applied to garnet porphyroblasts in metasedimentary rocks and eclogite. Metasedimentary rocks contain chemically homogeneous garnet (Grt-M1) with shape-matured quartz inclusions. In some rocks, these garnets are overgrown by garnet with bell-shaped Mn-zoning (Grt-M2) containing irregularly-shaped quartz inclusions. This evolution is interpreted as partial dissolution of Grt-M1 and subsequent growth of Grt-M2. Garnet in the eclogite is volumetrically dominated by eclogite-facies garnet (Grt-E1) that envelope remnants of an older, chemically distinct generation (Grt-E0) with highly irregular and diffuse boundaries. Shape-matured quartz inclusions are present within both garnet generations and define a zoning pattern that is not reflective of the chemical zoning. Collectively, these characteristics are interpreted as replacement of Grt-E0 by Grt-E1 via interface-coupled dissolution–reprecipitation, with the latter inheriting the shape-matured quartz inclusions of the former. Pressure–temperature (P–T) conditions extracted from the quartz inclusions in Grt-M1 and Grt-E0/E1 are 1.08 to 1.21 GPa at 645°C to 695°C and 0.94 to 1.03 GPa at 605°C to 640°C, respectively. These conditions are interpreted as cooling of the rocks from a high temperature metamorphic history, altogether preceding subduction of the Grapesvare nappe. The quartz inclusions in Grt-M2 record 1.04 to 1.21 GPa at 620°C to 675°C, interpreted as prograde metamorphic growth of Grt-M2 during subduction at 495.7 ± 3.2 Ma. Subsequent eclogite-facies metamorphism was responsible for the formation of Grt-E1 at the expense of Grt-E0. The collective results indicate a prolonged polymetamorphic history of the Grapesvare nappe prior to UHP metamorphism that has not been recognized previously.

Different Tectonic Evolution of Fast Cooling Ophiolite Mantles Recorded by Olivine-Spinel Geothermometry: Case Studies from Iballe (Albania) and Nea Roda (Greece)

Mg-Fe2+ diffusion patterns in olivine and chromite are useful tools for the study of the thermal history of ultramafic massifs. In the present contribution, we applied the exponential modeling of diffusion patterns to geothermometry and geospeedometry of chromitite ores from two different ophiolite contexts. The Iballe ophiolite (Northern Albania) hosts several chromitite pods within dunites. Primary and re-equilibrated Mg#, estimated by using an exponential function, provided re-equilibration and primary temperatures ranging between 677 and 996 °C for chromitites and between 527 and 806 °C for dunites. Cooling rates for chromitites are higher than for dunites, suggesting a different genesis for the two lithologies, confirmed also by spinel mineral chemistry. Chromitites with MORB affinity formed in a SSZ setting at a proto-forearc early stage, explaining the higher cooling rates, while dunites, with boninitic affinity, were formed deeper in the mantle in a more mature subduction setting. At the Nea Roda ophiolite (Northern Greece) olivine in chromitites do not show Mg-Fe variations, and transformation into ferrian chromite produced “fake” diffusion patterns within chromite. The absence of diffusion patterns and the low estimated temperatures (550–656 °C) suggest that Nea Roda chromitites were completely re-equilibrated during an amphibolite-facies metamorphic event that obliterated all primary features.

Amphibolite facies metamorphic event within the Upper Sebtides tectonic units (Internal Rif, Morocco): a record of a hyperextended margin at the border of the western Tethys

Located at the westernmost tip of the Mediterranean, the Gibraltar Arc is a segment of the Alpine belt that exhibits peridotite massifs associated with high-grade crustal metamorphic units. The timing and mechanisms of exhumation of these crust–mantle associations are still debated in the Rif in Morocco as well as in the Betic Cordilleras in Spain. A structural, petrographic, and geochronological study ( 40 Ar– 39 Ar method) is performed in the Beni Bousera region (Internal Rif) on the Upper Sebtides units that consist of Paleozoic basement and Permo-Triassic metasedimentary rocks. We point out that, in the metagreywackes, an early assemblage of K-feldspar, plagioclase, quartz, muscovite, biotite, and garnet, which is diagnostic of amphibolite facies metamorphism. By dating white mica porphyroclasts from this metamorphic assemblage, we provide the first evidence of internal Rif metamorphism of Triassic age. We assume that these crustal units belong to the ancient continental margin of the Western Tethys, and thus, we interpret this metamorphic event in the framework of a hyperextended margin that is coeval with the exhumation of the subcontinental mantle during the Upper Triassic. Afterward, these units underwent greenschist facies metamorphism during the Lower Miocene due to a thermal event related to the back-arc opening of the Alboran Basin.

Tectonics of the Isua Supracrustal Belt 1: P‐T‐X‐d Constraints of a Poly‐Metamorphic Terrane

AbstractThe Eoarchean Isua supracrustal belt (ISB) has been interpreted as one of the earliest records of subduction processes, leading to the conclusion that a plate tectonic geodynamic system was likely operating since the early Archean. However, proposed tectonic models remain difficult to evaluate as our understanding of the metamorphic and structural evolution remains fragmentary. Here, we present a metamorphic study of the supracrustal rocks of the ISB. We used petrographic and microstructural observations, phase equilibria, isopleth geothermobarometry, and conventional thermometry to explore the prograde, peak, and retrograde metamorphic evolution of the northeastern ISB. Our results show that the ISB records a syn‐tectonic, amphibolite facies metamorphic event (M1) with peak conditions of 550°C–600°C and 0.5–0.7 GPa. M1 was followed by a static, lower amphibolite facies metamorphic event (M2; <540°C and <0.5 GPa). Published constraints suggest that M1 and M2 occurred in the late Eoarchean (>3.5 Ga) and the Neoarchean (<2.9 Ga), respectively. These events are partially overprinted by late low temperature (<500°C) retrogression (M3) that is most intensely developed in the northeastern part of the belt; it typically overprints some peak mineral phases while preserving the peak fabric. Our findings are consistent with spatially homogeneous syn‐tectonic amphibolite facies metamorphism and macroscale folding. Such features are predicted by a heat‐pipe tectonic model. Therefore, our findings permit the interpretation of the ISB as a record of early nonuniformitarian tectonic processes.

New geochronological evidences of late Neoarchean and late Paleoproterozoic tectono-metamorphic events in the Miyun area, North China Craton

The Miyun area is located in the western margin of the Eastern Block of the North China Craton (NCC), and granulite- to amphibolite facies tectono-metamorphic events occurred in the Precambrian. In this contribution, high-resolution secondary ion mass spectrometry (SIMS) U-Pb dating of metamorphic zircon from garnet-pyroxene amphibolite yielded the metamorphic age of 2507 ± 4 Ma. SIMS U-Pb dating of monazite from garnet-biotite gneiss yielded the metamorphic ages of 2496 ± 16 Ma and 1834 ± 21 Ma, respectively. Furthermore, 40Ar/39Ar dating of hornblende from amphibolite yielded plateau ages of 1817 ± 41–1795 ± 19 Ma, and the biotite yielded a plateau age of 1744 ± 12 Ma. Combined with previous studies and the closure temperatures of the dating minerals, it is inferred that the Miyun area experienced a late Neoarchean granulite-facies metamorphic event occurred at ~2.50 Ga, followed by a late Paleoproterozoic amphibolite-facies metamorphic event occurred at ~1.83–1.74 Ga. These two tectono-metamorphic events are ascribed to the Precambrian orogenic-related environments.

Paleoproterozoic and Triassic metamorphic events in the Jiaobei Terrane, Jiao-Liao-Ji Belt, China: Hidden clues on multiple metamorphism and new insights into complex tectonic evolution

The Laixi Complex located on the margin between the Jiao-Liao-Ji orogenic belt (JLJB) and the Sulu ultra-high pressure (UHP) belt preserves the records of complex tectonic evolution of both belts from the Paleoproterozoic to the Early-Middle Triassic. With an outcrop area of ca. 240 km2, the complex is dominantly composed of metasedimentary rocks including Grt-Ky-St-schists, (Grt)-Mus-schists, Mus-Qtz-schists, quartzites and marbles intruded by amphibolite dykes and Mus-bearing granitic gneisses. Here we present an integrated study of petrography, metamorphic paragenesis, mineral inclusions and cathodoluminescence (CL) features of zircon grains together with U-Pb ages, and muscovite Ar-Ar geochronology. Our results constrain the depositional age of the metasedimentary rocks as ca. 2.1–1.96 Ga. Although Paleoproterozoic magmatic protoliths including ca. 1.87–1.85 Ga amphibolites and 1.87–1.86 Ga granitic gneisses are identified, Neoproterozoic (ca. 800–650 Ma) protolith ages that are common in the Sulu UHP rocks is absent in this complex. Two discrete metamorphic events are distinguished for the first time from the metasedimentary rocks. The first episode of granulite facies metamorphism was characterized by a mineral assemblage of Grt + Sil + Bt + Kfs ± Pl + Qtz as inclusions preserved in the metamorphic zircons. In contrast, the second phase of lower amphibolite facies metamorphism is recorded by Grt ± Ky ± St + Mus ± Bt + Kfs ± Pl + Qtz in the matrix of the host rocks. Metamorphic zircon U-Pb and muscovite Ar-Ar dating constrains the two metamorphic events in the Laixi Complex as Paleoproterozoic (ca. 1.94–1.88 Ga) and Early-Middle Triassic (ca. 252–244 Ma). The metamorphic units in the Laixi Complex show similar features of lithology, geochronologic and metamorphic evolution to that of the Paleoproterozoic Jiaobei Terrane (JBT) in the southwestern JLJB, and are clearly distinguished from the Sulu UHP rocks. We therefore consider the Laixi Complex to be an integral component of the JBT during the collisional orogeny between Longgang and Langrim blocks in Paleoproterozoic. The Early-Middle Triassic lower amphibolite facies metamorphic event identified for the first time in the Laixi Complex coincides with that (249–230 Ma) of the adjacent Jiaodong Terrane in the northeastern Sulu UHP belt. These findings suggest that the Laixi Complex was also involved in Triassic collision-subduction between the North China and South China blocks, and experienced low-grade metamorphism at the middle crustal level. Combined with the observations in previous studies, we propose a multiphase metamorphic belt extending at least 300 km from Yantai through Laixi to Wulian, parallel to the Yantai-Qingdao-Wulian Fault.

Metamorphic Evolution and Zircon U-Pb Ages of the Nanshankou Mafic High Pressure Granulites from the Jiaobei Terrane, North China Craton

Petrological analysis and LA-ICP-MS zircon U-Pb dating were conducted on high-pressure mafic granulites, which occured as xenolith within TTG gneisses, from the Nanshankou Village of the Jiaobei terrane, Shandong Peninsula in the north-eastern part of the North China Craton (NCC). The mafic HP granulite is composed of garnet, clinopyroxene, orthopyroxene, amphibole and symplectitic clinopyroxene, orthopyroxene, plagioclase, ilmente and magnetite which were formed after the decomposition of porphyroblastic garnet and clinopyroxene. Four stages of metamorphic mineral assemblages for the mafic HP granulites were constrained by detail petrological and mineralogical investigations. The early prograde assemblage is represented by the mineral inclusions within garnet and clinopyroxene porphyroblasts (Opx1+Pl1+Qtz1), recording the metamorphic conditions at ~754–757 °C, 0.63–0.71 GPa; peak metamorphic conditions were determined at ~874–891 °C, 1.32–1.35 GPa with the mineral assemblage of Grt2+Cpx2+Amp2+Pl2+Qtz2. Retrograde minerals derived from symplectitic assemblage Opx3+Cpx3+Amp3+Pl3+Qtz3+Ilm3±Mag3 were formed at 693–796 °C, 0.60–0.84 GPa. A final greenschist to sub-greenschist facies event was recorded by the exsolution of actinolite and albite within a retrograded clinopyroxene, as well as the occurrence of prehnite, chlorite and calcite minerals. Accordingly, a clockwise P-T path was concluded on the basis of the different stages of mineral asseblage. Cathodoluminescence imaging, trace element and U-Pb dating of zircons from the mafic HP granulites recorded similar charactistics for three episodes of Paleo–Meso Proterozoic metamorphic events. These are the metamorphic events preserved in mafic and pelitic granulites in the Jiao-Liao-Ji belt (JLJB) with 207Pb/206Pb ages of 2.0‒1.9 Ga for peak metamorphism and of 1.86‒1.84 Ga for decomposing process, followed by a retrograde amphibolite facies metamorphic event related to the post-orogenic extension at the age of 1.76‒1.74 Ga, resulting the exhumation of the granulite to the upper crust level.

Synkinematic emplacement of the magmatic epidote bearing Major Isidoro tonalite-granite batholith: Relicts of an Ediacaran continental arc in the Pernambuco–Alagoas domain, Borborema Province, NE Brazil

The Neoproterozoic Major Isidoro batholith (∼100 km2), composed of metaluminous to slightly peraluminous magmatic epidote-bearing tonalite to granite, is part of the Águas Belas–Canindé composite batholith, which intruded the Pernambuco–Alagoas Domain of the Borborema Province, northeastern Brazil. These rocks contain biotite, amphibole, titanite and epidote that often shows an allanite core as key mafic mineral phases. K-diorite mafic enclaves are abundant in this pluton as well as are amphibole-rich clots. The plutonic rocks are medium-to high-K calc-alkaline, with SiO2 varying from 59.1 to 71.6%, Fe# from 0.6 to 0.9 and total alkalis from 6.1 to 8.5%. Chondrite-normalized REE patterns are moderately fractionated, show (La/Lu)N ratios from 13.6 to 31.8 and discrete negative Eu anomalies (0.48–0.85). Incompatible-element spidergrams exhibit negative Nb–Ta and Ti anomalies.This batholith was emplaced around 627 Ma (U–Pb SHRIMP zircon age) coevally with an amphibolite-facies metamorphic event in the region. It shows Nd-model age varying from 1.1 to 1.4 Ga, average εNd(627Ma) of −1.60 and back-calculated (627 Ma) initial 87Sr/86Sr ratios from 0.7069 to 0.7086. Inherited zircon cores that yielded 206Pb/238U ages from 800 to 1000 Ma are likely derived from rocks formed during the Cariris Velhos (1.1–0.9 Ga) orogenic event. These isotopic data coupled with calculated δ18O(w.r.) value of +8.75‰ VSMOW indicate an I-type source and suggest a reworked lower continental crust as source rock. A granodioritic orthogneiss next to the Major Isidoro pluton, emplaced along the Jacaré dos Homens transpressional shear zone, yielded a U–Pb SHRIMP zircon age of 642 Ma, recording early tectonic movements along this shear zone that separates the Pernambuco–Alagoas Domain to the north, from the Sergipano Domain to the south. The emplacement of the Major Isidoro pluton was synkinematic, coeval with the development of a regional flat-lying foliation, probably during the peak of metamorphism related to the convergence/contractional deformation of the São Francisco craton and Pernambuco–Alagoas block during the Brasiliano Orogeny.

Pressure–temperature evolution of Neoproterozoic metamorphism in the Welayati Formation (Kabul Block), Afghanistan

The Welayati Formation, consisting of alternating layers of mica-schist and quartzite with lenses of amphibolite, unconformably overlies the Neoarchean Sherdarwaza Formation of the Kabul Block that underwent Paleoproterozoic granulite-facies and Neoproterozoic amphibolite-facies metamorphic events. To analyze metamorphic history of the Welayati Formation and its relations to the underlying Sherdarwaza Formation, petrographic study and pressure–temperature (P–T) pseudosection modeling were applied to staurolite- and kyanite-bearing mica-schists, which crop out to the south of Kabul City. Prograde metamorphism, identified by inclusion trails and chemical zonation in garnet from the micaschists indicates that the rocks underwent burial from around 6.2kbar at 525°C to maximum pressure conditions of around 9.5kbar at temperatures of around 650°C. Decompression from peak pressures under isothermal or moderate heating conditions are indicated by formation of biotite and plagioclase porphyroblasts which cross-cut and overgrow the dominant foliation. The lack of sillimanite and/or andalusite suggests that cooling and further decompression occurred in the kyanite stability field. The results of this study indicate a single amphibolite-facies metamorphism that based on P–T conditions and age dating correlates well with the Neoproterozoic metamorphism in the underlying Sherdarwaza Formation. The rocks lack any paragenetic evidence for a preceding granulite-facies overprint or subsequent Paleozoic metamorphism. Owing to the position of the Kabul Block, within the India–Eurasia collision zone, partial replacement of the amphibolite-facies minerals in the micaschist could, in addition to retrogression of the Neoproterozoic metamorphism, relate to deformation associated with the Alpine orogeny.

Late Paleoproterozoic multiple metamorphic events in the Quanji Massif: Links with Tarim and North China Cratons and implications for assembly of the Columbia supercontinent

The Quanji Massif preserves important records of late Paleoproterozoic metamorphic events that provide insights into the tectonic history of the Tarim Craton and its relation with the North China Craton. Here we investigate amphibolites from two regions in the Quanji Massif. U–Pb ages from metamorphic zircons in the amphibolites, combined with previously published ages from this region, define distinct metamorphic events during Paleoproterozoic. The earlier medium-P/T type upper amphibolite-facies metamorphic event occurred at ca. 1.90–1.96Ga in a collisional setting. The second event took place with low-P/T type amphibolite-facies metamorphism in an arc setting at ca. 1.82–1.85Ga, with a subsequent medium-P/T type metamorphism during arc–continental collision at ca. 1.80–1.82Ga. Our data reveal for the first time that the late Paleoproterozoic metamorphic events in the Quanji Massif are closely comparable with those documented from the Khondalite Belt within the Inner Mongolia Suture Zone and the Trans-North China Orogen in the North China Craton. We therefore propose that the Quanji Massif and Tarim Craton probably shared similar tectonic histories with those of the North China Craton in the late Paleoproterozoic, coinciding with the global assembly of the Columbia supercontinent.

Late Neoproterozoic thermal events in the northern Lhasa terrane, south Tibet: Zircon chronology and tectonic implications

The high-grade metamorphic rocks from the Lhasa terrane, south Tibet, have been traditionally grouped as the Precambrian metamorphic basement. However, a number of recent studies show that these metamorphic rocks from the southern part of Lhasa terrane, including granulite- and eclogite-facies rocks, were metamorphosed during the Mesozoic and Cenozoic. Based on the LA–ICP–MS in situ zircon U–Pb chronology, we report for the first time late Neoproterozoic metamorphic and magmatic events from the Lhasa terrane. The rocks analyzed in this study occur in the northern part of Lhasa terrane, and include amphibolite, garnet-bearing muscovite schist and marble, with the mineral assemblages of hornblende + andesine + quartz, muscovite + garnet + plagioclase + quartz and calcite + diopside + quartz, indicating an amphibolite-facies metamorphic event. Zircons from a marble and an amphibolite sample show typical features of metamorphic origin, and yield peak-metamorphic ages of ca. 676 Ma and ca. 661 Ma, respectively. Zircons from another amphibolite sample possess inherited magmatic cores and metamorphic overgrown rims, yielding a protolith age of ca. 856 Ma and a metamorphic age of ca. 683 Ma. Zircons from the schist include detrital magmatic cores showing a wide spectrum of Proterozoic ages from ca. 1747 to 789 Ma, representing derivation from multiple sources. However, the metamorphic rims of these zircons yield an age population of ca. 690 Ma. Based on the geochemical and geochronological data presented in this work and from a synthesis of data from previous studies, we propose the location of the Lhasa terrain in a reconstruction of the Neoproterozoic Rodinia supercontinent. We correlate the late Neoproterozoic metamorphic and magmatic events with an Andean-type orogeny that resulted from oceanic subduction beneath the northwestern margin of the Rodinia supercontinent. We correlate the orogeny with a global-scale late Proterozoic arc magmatism widely represented in the Seychelles, Madagascar, northwestern India and South China.

Major-element and Li, Be compositional evolution of tourmaline in an S-type granite-pegmatite system and its country rocks: an example from Ikaria, Aegean Sea, Greece

The chemical evolution of tourmaline in a granite – pegmatite – hydrothermal system and its metamorphic country-rock on the island of Ikaria, in Greece, has been investigated to elucidate the range of compositions generated from a single intrusive system, and the relation between genetic types of tourmaline and their chemical composition. On Ikaria, an S-type granite intruded metapelites, and several aplitic and pegmatitic dykes originating from the granite cross-cut the granite as well as the country rock. Quartz–tourmaline rocks occur in both the S-type granite and in the metapelite. Tourmaline in rocks genetically related to the granite have Li concentrations between 9.3 and 473 μg/g and Be concentrations between 1.5 and 41.8 μg/g. Tourmaline in rocks genetically related to the metapelite have more than an order of magnitude lower concentrations (Li: 2.0–5.8 μg/g, Be: 0.3–0.9 μg/g). Tourmaline in these rocks is typically zoned, with abrupt changes in composition between core and rim. Generally, the cores have a high Al and low minor-element concentrations. Oscillatory-zoned tourmaline rims formed by diffusion-limited growth. Dykes in which the magma travelled far through the metapelitic country-rocks are more differentiated than others, and contain tourmaline with very low Mg/Fe in the core. The dykes interacted with the surrounding rocks in places, leading to Mg enrichment in the dykes. This interaction also introduced Mn into the dykes; spessartine-rich garnet formed with Li concentrations as high as 166 μg/g. Boron-rich fluids segregating from the metapelite during an amphibolite-facies metamorphic event produced quartz–tourmaline rocks in the metapelite, which are probably unrelated to the S-type granite. The metamorphic fluids in the metapelites interacted with marble units where they were in close vicinity, leading to the formation of Ca-rich tourmaline in those quartz–tourmaline rocks. Concentrations of Li, Be and B in associated minerals decrease as follows: Li: St > Bt > Ms > Grt > Tur, Kfs > Pl; Be: St > Pl > Ms > Tur > Bt > Kfs > Grt; B: Tur >> Ms > Bt > Pl > Grt > Kfs, St.

Paleoarchaean tonalites in the Orekhovo-Pavlogradskaya palaeoproterozoic collsional zone (Ukrainian Shield)

The novel detailed geological and geochronological data for the tonalitie rocks and products of its structural and metamorphic overprint in the Orekhovo-Pavlogradskaya collision zone (Ukrainian Shield) are discussed. Magmatic crystallization of tonalites (3500 ± 13 Ma) followed by amphibolite (2863 ± 22 Ma) and granulite (2105 ± 40 Ma) facies metamorphic events, resulting in biotite-garnet gneisses formation. The last date also indicating the time of collision event. The earliest age of 3.64 Ga was found in the zonal zircon fragment within 3.5-Ga old magmatic zircon from tonalite.

Temporal evolution of the Angavo and related shear zones in Gondwana: Constraints from LA-MC-ICP-MS U–Pb zircon ages of granitoids and gneiss from central Madagascar

Granitoids of Neoproterozoic age are well exposed in the Precambrian basement rocks of Madagascar, in the vicinity of and within the Neoproterozoic dextral Angavo shear zone (ASZ). Pre-kinematic granitoids, a syn-kinematic granite, and a granitic gneiss have been dated using Laser Ablation Multicollector Inductively Coupled Plasma Mass Spectrometry (LA-MC-ICP-MS). One pre-kinematic granitoid yields a weighted mean 206Pb/ 238U igneous zircon crystallization age of 823.6 ± 6 Ma, and a second a 756.6 ± 9 Ma igneous crystallization age and a 563 ± 11 Ma metamorphic age. A syn-kinematic granite gives a weighted mean 206Pb/ 238U zircon age of 549.8 ± 4 Ma, which points to the time of movement in the ASZ. A granitic gneiss yields a weighted mean 206Pb/ 238U protolith age of 732 ± 13 Ma and records a 558.8 ± 4 Ma metamorphic age. Accordingly, the ASZ was active and associated with the high-temperature metamorphism of the Angavo-Nondiana belt during the Ediacaran. The late Proterozoic ages of igneous intrusions recorded by the granites show that pre-tectonic granites were sheared in the ASZ, and recorded granulite–amphibolite facies metamorphic events. These U–Pb data suggest a link between the ASZ and the Achankovil shear zone in India, which were adjacent in several proposed Gondwana reconstructions.

Ancient graphite in the Eoarchean quartz–pyroxene rocks from Akilia in southern West Greenland I: Petrographic and spectroscopic characterization

Because all known Eoarchean (>3.65 Ga) volcano-sedimentary terranes are locked in granitoid gneiss complexes that have experienced high degrees of metamorphism and deformation, the origin and mode of preservation of carbonaceous material in the oldest metasedimentary rocks remain a subject of vigorous debate. To determine the biogenicity of carbon in graphite in such rocks, carbonaceous material must be demonstrably indigenous and its composition should be consistent with thermally altered biogenic carbon as well as inconsistent with abiogenic carbon. Here we report the petrological and spectroscopic characteristics of carbonaceous material, typically associated with individual apatite grains, but also with various other minerals including calcite, in a >3.83 Ga granulite-facies ferruginous quartz-pyroxene unit (Qp rock) from the island of Akilia in southern West Greenland. In thin sections of the fine-grained parts of Akilia Qp rock sample G91-26, mapped apatites were found to be associated with graphite in about 20% of the occurrences. Raman spectra of this carbonaceous material had strong G-band and small D-band absorptions indicative of crystalline graphite. Three apatite-associated graphites were found to contain curled graphite structures, identified by an anomalously intense second-order D-band (or 2D-band) Raman mode. These structures are similar to graphite whiskers or cones documented to form at high temperatures. Raman spectra of apatite-associated graphite were consistent with formation at temperatures calculated to be between 635 and 830 °C, which are consistent with granulite-facies metamorphic conditions. Three graphite targets extracted by focused ion beam (FIB) methods contained thin graphite coatings on apatite grains rather than inclusions sensu stricto as inferred from transmitted light microscopy and Raman spectroscopy. TEM analyses of graphite in these FIB sections showed a (0 0 0 2) interplanar spacing between 3.41 and 3.64 Å for apatite-associated graphite, which is larger than the spacing of pure graphite (3.35 Å) and may be caused by the presence of non-carbon heteroatoms in interlayer sites. Samples analyzed by synchrotron-based scanning transmission X-ray microscopy (STXM) also confirmed the presence of crystalline graphite, but abundances of N and O heteroatoms were below detection limit for this method. Graphite in the Akilia Qp rock was also found to occur in complex polyphase mineral assemblages of hornblende ± calcite ± sulfides ± magnetite that point to high-temperature precipitation from carbon-bearing fluids. These complex mineral assemblages may represent another generation of graphitization that could have occurred during the amphibolite-facies metamorphic event at 2.7 Ga. Several observations point to graphitization from high-temperature fluid-deposition for some of the Akilia graphite and our results do not exclude a biogenic source of carbon in graphite associated with apatite, but ambiguities remain for the origin of this carbon.

Garnet reaction rims from the breakdown of Staurolite in polymetamorphic micashists from the Rappold complex, Austroalpine basement, Eastern Alps

In polymetamorphic pelites of the Rappold complex in the Wolz crystalline basement (Eastern Alps) reaction rim garnets at staurolite-quartz interfaces (type I) and single grain garnets along previous staurolite-white mica interfaces (type II) were formed. The garnet reaction rims were formed during the Cretaceous amphibolite facies metamorphic overprint of the pre-existing mineral assemblages comprising garnet, staurolite, and kyanite from an amphibolite facies metamorphic event probably of Variscian age. The newly formed garnet may take the form of reaction rims along the margins of large pre-existing staurolite blasts. The initial growth increments of garnet have low grossular content, and reaction rim growth was controlled by the transfer of Fe, Mg and Mn components from the staurolite-garnet interface to the quartz-garnet interface. Later garnet growth increments have relatively high grossular content due to consumption of matrix plagioclase, which was destabilized by successive pressure increase. The grossular content of newly formed garnet shows systematic increase towards sites where plagioclase breaks down indicating that transport of calcium through the matrix was sluggish. On the basis of reaction microstructures it is demonstrated that the mineral assemblage garnet + kyanite + biotite + paragonite was formed at the conditions of eo-alpine amphibolite facies overprint while staurolite and plagioclase broke down successively with increasing pressure.

Metamorphic reworking of a high pressure–low temperature mélange along the Motagua fault, Guatemala: A record of Neocomian and Maastrichtian transpressional tectonics

The Guatemala suture zone is a major east–west left-lateral strike slip boundary that separates the North American and Caribbean plates in Guatemala. The Motagua fault, the central active strand of the suture zone, underwent two major collisional events within a system otherwise dominated by strike–slip motion. The first event is recorded by high-pressure/low temperature (HP/LT) eclogites and related rocks that occur within serpentinites both north and south of the Motagua fault. Lawsonite eclogites south of the fault are not significantly retrograded and give 40Ar/ 39Ar ages of 125–116 Ma and Sm–Nd mineral isochrons of 144–132 Ma. Eclogites north of the fault give similar Sm–Nd isochron ages (131–126 Ma) but otherwise differ in that they are strongly overprinted by a lower pressure assemblage and, along with associated HP/LT rocks, give much younger 40Ar/ 39Ar ages of 88–55 Ma indicating a later amphibolite facies metamorphic event. We propose therefore that all serpentinite hosted eclogites along the Motagua fault formed at essentially the same time in different parts of a laterally extensive Lower Cretaceous forearc subduction system, but subsequently underwent different histories. The southern assemblages were thrust southwards (present coordinates) immediately after HP metamorphism whereas the northern association was retrograded during a later collision that thrust it northward at ca. 70 Ma. They were subsequently juxtaposed opposite each other by major strike slip motion. This model implies that the HP rocks on opposing sides of the Motagua fault evolved along a plate boundary that underwent both dip slip and strike slip motion throughout the Late Cretaceous as a result of oblique convergence. The juxtaposition of a convergent and strike slip system means that HP/LT rocks within serpentinites can be found at depth along much of the modern Guatemala suture zone and its eastward extension into the northern Caribbean. Both sets of assemblages were exhumed relatively recently by the uplift of mountain ranges on both sides of the fault caused by movement along a restraining bend. Recent exhumation explains the apparently lack of offset of surface outcrops along a major strike slip fault.

A zircon study from the Rhodope metamorphic complex, N-Greece: Time record of a multistage evolution

Zircons from an eclogite and a diamond-bearing metapelite near the Kimi village (north-eastern Rhodope Metamorphic Complex, Greece) have been investigated by Micro Raman Spectroscopy, SEM, SHRIMP and LA-ICPMS to define their inclusion mineralogy, ages and trace element contents. In addition, the host rocks metamorphic evolution was reconstructed and linked to the zircon growth domains. The eclogite contains relicts of a high pressure stage (ca. 700 °C and > 17.5 kbar) characterised by matrix omphacite with Jd 40–35. This assemblage was overprinted by a lower pressure, higher temperature metamorphic event (ca. 820 °C and 15.5–17.5 kbar), as indicated by the presence of clinopyroxene (Jd 35–20) and plagioclase. Biotite and pargasitic amphibole represent a later stage, probably related to an influx of fluids. Zircons separated from the eclogite contain magmatic relicts indicating Permian crystallization of a quartz-bearing gabbroic protolith. Inclusions diagnostic of the high temperature, post-eclogitic overprint are found in metamorphic zircon domain Z2 which ages spread over a long period (160 – 95 Ma). Based on zircon textures, zoning and chemistry, we suggest that the high-temperature peak occurred at or before ca. 160 Ma and the zircons were disturbed by a later event possibly at around 115 Ma. Small metamorphic zircon overgrowths with a different composition yield an age of 79 ± 3 Ma, which is related to a distinct amphibolite-facies metamorphic event. The metapelitic host rock consists of a mesosome with garnet, mica and kyanite, and a quartz- and plagioclase-bearing leucosome, which formed at granulite-facies conditions. Based on previously reported micro-diamond inclusions in garnet, the mesosome is assumed to have experienced UHP conditions. Nevertheless, (U)HP mineral inclusions were not found in the zircons separated from the diamond-bearing metapelite. Inclusions of melt, kyanite and high-Ti biotite in a first metamorphic zircon domain suggest that zircon formation occurred during pervasive granulite-facies metamorphism. An age of 171 ± 1 Ma measured on this zircon domain constrains the high-temperature metamorphic event. A second, inclusion-free metamorphic domain yielded an age of 160 ± 1 Ma that is related to decompression and melt crystallization. The similar age data obtained from the samples indicate that both rock types recorded a high-T metamorphic overprint at granulite-facies conditions at ca. 170 – 160 Ma. This age implies that any high pressure or even ultra-high pressure metamorphism in the Kimi Complex occurred before that time. Our findings define new constraints for the geodynamic evolution for the Alpine orogenic cycle within the northernmost Greek part of the Rhodope Metamorphic Complex. It is proposed that the rocks of the Kimi Complex belong to a suture zone squeezed between two continental blocks and result from a Paleo-ocean basin, which should be located further north of the Jurassic Vardar Ocean.

Proterozoic metamorphism and cooling in the southern Lake Superior region, North America and its bearing on crustal evolution

Metamorphism along the southern margin of the Archean Superior Province has been historically attributed to the Penokean orogeny. A narrow corridor of amphibolite facies rocks north of the main suture does record 1.83–1.80 Ga metamorphic monazite U–Th–Pb ages that mark the culmination of arc accretion. However, subsequent widespread amphibolite facies metamorphism and associated magmatism is recorded along the regions of greatest Penokean crustal thickening: the tectonically buried Archean–Proterozoic continental margin. In Minnesota, new monazite geochronology reveals a profound midcrustal metamorphic imprint caused by emplacement of the ∼1.775 Ga East-central Minnesota batholith at moderate depths. In northern Wisconsin and upper peninsula Michigan metamorphic monazite growth at 1.78–1.745 Ga (and far from geon 17 intrusions) reflect a previously little recognized regional amphibolite facies metamorphic event associated with ca. 1.76 Ga Yavapai-interval accretion, not solely Penokean induced crustal collapse. South of the Penokean suture, Penokean terrane rocks were twice metamorphosed to upper greenschist facies; first during Yavapai accretion and again during geon 16 Mazatzal accretion. Geon 16 overprinting also affected a small part of the continental margin in the northeast orogen, the Peavy metamorphic node. South-directed basement thrusts there likely accommodated substantial Mazatzal foreland shortening, suggesting thick-skinned deformation. Mazatzal amphibolite facies metamorphism occurred throughout Iowa and southernmost Wisconsin (south of the Baraboo quartzite). 40Ar/ 39Ar mineral cooling ages from eastern Wisconsin reveal a limited metamorphic aureole associated with the intrusion of the 1.47 Ga Wolf River batholith, in part reflecting its rapid emplacement at shallow crustal levels. A local area of anomalously young <1.20 Ga 40Ar/ 39Ar mica ages away from known Midcontinent Rift exposures probably reflect shallow crustal level reheating during rifting, not exhumation. The pattern and degree of Proterozoic metamorphism preserved across the northern interior of the North American midcontinent is highly variable and is a reflection of multiple accretionary and intrusive events between 1.87 and 1.47 Ga. Collectively, the geo- and thermochronologic data reveal a progressive tectonic younging toward the south and overall waning of Proterozoic metamorphism in this region. These results demonstrate a changing tectonic environment from active margin to continental interior as Laurentia grew southward and stabilized over a few hundred million years.