High-pressure Granulite Research Articles

Characterization of the inverted metamorphic gradient of the Passos Nappe (SE-Brazil) based on multiple geothermobarometers

Inverted metamorphic gradients are recognized in several Proterozoic to Phanerozoic orogens. However, the mechanisms that are responsible for the formation of these gradients are still a matter of discussion. In the southern Brasília Orogen, an inverted metamorphic gradient is recorded in the Passos Nappe, that comprises a sequence of metasedimentary rocks with minor intercalations of tholeiitic metamafic rocks, metamorphosed under greenschist facies (biotite zone) at the bottom and high-pressure granulite facies at the top. This paper presents a detailed investigation of pressure and temperature conditions along the Passos Nappe by integrating conventional geothermobarometry in metamafic rock and single-trace elements geothermometry (Zr-in-rutile and Ti-in-quartz) in quartzite. Nine lithostratigraphic units are identified in the Passos Nappe, which are informally named A to I, from the bottom to top, adding up to a minimum thickness of 4.56 km. In mafic rocks, the increase of metamorphic conditions is indicated by: (1) chemical variation of amphiboles, from actinolite/edenitic hornblende in Unit C, to pargasitic hornblende in Units E and G, and Fe-pargasite in Units H and I; (2) plagioclase composition varying from An1-13 in Unit C, An15-27 in Unit D to andesine at upper units; and (3) occurrence of clinopyroxene from the top of Unit E upwards. Additional petrographic features in quartzite also corroborate these insights, such as: the increase of grain size of both quartz and rutile, from bottom to top; the morphology of rutile crystals; the mechanism of quartz recrystallization (subgrain rotation dominates in lower units while grain boundary migration dominates in upper units) and the occurrence of rutile needles in quartz in the upper units, which suggest a continuous gradient. Combined application of Zr-in-rutile and Ti-in-quartz geothermometers in quartzite provided the following average temperature and pressure values for each unit: 673 °C and 11.7 kbar for Unit E, 690 °C and 9.5 kbar for Unit G, 747 °C and 11.6 kbar for Unit H and 758 °C and 11.3 kbar for Unit I. Our data indicate that the Zr-in-rutile and Ti-in-quartz systems do not re-equilibrate in metamorphic conditions lower than 600 °C. The extrapolation of these results to the low-grade units indicate: 496 °C and 4.8 kbar at Unit A, 504 °C and 4.8 kbar at Unit B, 502 °C and 4.4 kbar at Unit C. The results obtained with the geothermometer hornblende-plagioclase and the geobarometer garnet-rutile-ilmenite-plagioclase-quartz in metamafic rocks yielded the following average values for each unit: 486 °C and 5.0 kbar at Unit C, 568 °C and 6.0 kbar at Unit D, 622 °C and 8.1 kbar at Unit E, 650 °C and 8.4 kbar at Unit G, 743 °C and 9.4 kbar at Unit H and 723 °C and 11.2 kbar at Unit I. Since the two geothermobarometric approaches are based on independent chemical systems, the results obtained represent robust estimates for the temperature and pressure conditions for the inverted metamorphic gradient of the Passos Nappe. Comparing the inverted metamorphic gradient at the Passos Nappe with classical occurrences at the Himalaya Orogen, it is possible to assume that both origins were similar, which probably involved isotherm inversion due to subduction, channel flow process associated with shear heating and transport of higher pressure rocks over low-pressure rocks by reverse ductile shearing.

Linking granulites, intraplate magmatism, and bi-mineralic eclogites with a thermodynamic-petrological model of melt-solid interaction at the base of anorogenic lower continental crust

This study reports results from thermodynamic phase equilibrium and trace element modelling of mafic magmatic underplating and solid-liquid interaction in the lower continental crust (LCC) in intraplate settings. The arrival of underplating basalt sills into thin (∼30 km at 8 kbar) and thicker (∼45 km at 12 kbar) andesitic and basaltic LCC precursors was simulated with heating and batch-melting to yield refractory residues. Continued magmatic invasion of the LCC was then modelled at the same pressures with hybridisation between the residual solids and basaltic and picritic liquids (at 80:20 and 50:50 proportions). The first finding is that hybridisation with basalt increases the stability fields of 2-pyroxene-plagioclase and garnet-clinopyroxene-plagioclase granulites, by far the dominant LCC xenoliths found in anorogenic settings. The second finding pertains to situations when the liquid proportion of the hybrid system is lower than the fraction of the incoming mafic liquid. The resulting liquids share many characteristics with silicic volcanic rocks in continental flood basalt (CFB) provinces, including low Al and high Ti-concentrations. Stoichiometric melting reactions from 80:20 residual andesite:basalt hybridisation reproduce trace element patterns of silicic eruptives with potential for decoupling of Sr-Nd-Pb isotope systematics, as found in CFB rhyolites and LCC xenoliths. Incongruent melting of hornblende and garnet (the A in AFC) from the residual solid and dominant peritectic fractional crystallisation of plagioclase (the FC in AFC) from the liquid occur. A notable feature of LCC-picrite hybridisation is that solid residues after felsic liquid extraction, when subjected to higher pressure, are buffered to olivine and quartz-free compositions. Consequently, if these high-pressure granulite residues delaminated, they would convert to the bi-mineralic high-Mg eclogites found as xenoliths in kimberlites. The modelled residues also have the Sr-enrichment and positive Eu-anomalies of eclogite xenoliths. Solid-liquid interaction in intraplate settings has been and remains a significant process in crustal growth and evolution.

Neoproterozoic HP granulite and its tectonic implication for the East Kunlun Orogen, northern Tibetan Plateau

The P–T path of high-pressure (HP) granulite is a sensitive indicator providing metamorphic evidence on the convergence process and on the evolutionary history of orogenic belts during the supercontinent circles. The Wulonggou mafic granulites in the East Kunlun Orogen are intercalated as lenses/boudins within the dominant amphibolite facies basement rocks. Based on microstructures and mineralogical observations, three mineral assemblage generations can be recognized in the granulite samples: (1) inclusions within garnet and amphibole comprising cpx + pl + amp + qtz + rt; (2) the matrix assemblage grt + cpx + pl + amp + qtz + rt ± bt ± ilm; and (3) a retrogressive assemblage consisting of kfs + pl + qtz + amp coronas around garnet. Phase equilibrium modeling and conventional thermobarometric calculations reveal that the Wulonggou granulite has experienced (1) first HP granulite facies metamorphism at > 12 kbar and 810–840 °C, (2) dominant HP granulite facies stage of 800–860 °C at 10.7–12.3 kbar, as well as (3) a retrograde stage of 620–650 °C and 6.7–8.1 kbar, implying a clockwise P–T path. Zircon U–Pb geochronological results indicate that the mafic granulites and the gneissic country rocks yield similar Neoproterozoic metamorphic ages of 939–941 Ma. The granulite also yields a zircon U–Pb age of 444 ± 7 Ma and amphibole 40Ar/39Ar plateau age of 433 ± 1 Ma, suggesting an Early Paleozoic metamorphic overprint at ca. 440 Ma. Together with our previous studies on the contemporaneous Neoproterozoic magmatism and metamorphism, all the above lines of evidence constrain that the Wulonggou mafic granulite has experienced high pressure granulite facies metamorphism at ca. 940 Ma, suggesting a Neoproterozoic collisional orogeny in the East Kunlun Orogen, which has been overprinted by the Early Palaeozoic orogenic processes.

Thermodynamic constraints on the composition of orogenically thickened lower crust

Orogenically thickened lower crust is the key site of crustal differentiation, crustal deformation, and Moho modification. However, the composition of thickened lower crust is still highly debated. Here, we calculate a set of pseudosections with mafic lower crust compositions in the Na2O–CaO–K2O–FeO–MgO–Al2O3–SiO2–H2O–TiO2–O2 (NCKFMASHTO) system. Our modeling results show that the maximum thickness of the mafic lower crust increases with the Moho temperature (TMoho). In addition, the lithologies of stable mafic crust are characterized by medium-pressure (MP) to high-pressure (HP) granulites at 40–50 km, HP granulites and garnet-omphacite granulites at 50–60 km, and garnet-omphacite granulites at 60–70 km. Under the Pamir geothermal conditions, mafic rocks with high SiO2 (>50.2 wt%), XMg (>0.70), XCa (>0.49), or low XAl (<0.11) could be stable at 70 km; however, only ~10% of global mafic granulite xenoliths lie within this compositional range. Further modeling indicates that if TMoho reaches 900–1000 °C, neither the lower crust nor the upper mantle has significant strength relative to the upper crust, and that only ~5–37% of mafic materials are gravitationally stable at 70 km. This implies that the base of doubly thickened (70 km) crust is dominated by intermediate-felsic rocks, consistent with the low Vp and Vp/Vs values seismically observed in young orogenic crustal roots. Thus, most mafic materials at >70 km could delaminate into the deep mantle. Our results provide insights on the formation of extremely thick crust with a predominantly intermediate-felsic base and the crustal thickness variation in continental collision zones.

Diverse P‐T‐t Paths Reveal High‐Grade Metamorphosed Forearc Complexes in NW China

AbstractSubduction zones provide important constraints on reconstruction of tectonic configurations and convergent geodynamics. The timing and evolution of the subduction zones at the interaction between the Paleo‐Asian and Tethyan Oceans remain ambiguous, casting significant uncertainty on the reconstructions of proto‐Asia and subduction dynamics. Here, we report new petrologic and geochronologic data of a high‐grade metamorphic complex in the Dunhuang area, NW China, together with thermodynamic modeling, geothermobarometry, and second ion mass spectrometry U‐Pb chronology to reveal the complex metamorphic structure and history of the Paleozoic subduction‐exhumation channel at the intersection of the Paleo‐Asian and Tethyan domains. The subduction zone contains a diverse collection of eclogite, high‐pressure (HP) granulites, amphibolites, and metasedimentary rocks with a broad spectrum of P‐T‐t paths, which were buried and exhumed at different depths at ∼463–411 Ma. Our first dating of the oldest (∼463 Ma) HP granulite extends the orogenic period earlier to the Middle Ordovician. The uneven change in subduction gradients from ∼18°C/km to ∼10°C/km illustrates the thermal evolution of the subduction zone from infancy to maturity. A dramatic drop of the gradient at ∼420 Ma suggests a potential short‐term switch of the subduction dynamics from “hot” to “cool,” possibly due to plate geometry reorganization in response to slab roll‐back. Our data demonstrate that the Dunhuang Complex was chaotically mixed and juxtaposed at different levels in the subduction‐exhumation channel. The southward younging and increase in depth of the HP metamorphism indicate that the Dunhuang Complex was formed by north‐dipping subduction of the Proto‐Tethyan Ocean from the Early Paleozoic.

The Multiple Metamorphism of Mafic Granulites From the East Hebei Terrane, North China Craton: Insights Into the Transition of Tectonic Regimes

The East Hebei terrane from the North China Craton preserves the dome–and–keel structures, which was transected by a later linear belt in the north margin. Mafic granulites from the linear belt and domes record two groups of metamorphic ages at Neoarchean and Paleoproterozoic, but their accurate metamorphic peak conditions and paths have not been well addressed. Three samples of mafic granulites, including two-pyroxene granulite (JD15120), garnet-bearing two-pyroxene granulite (YC8-43), and garnet clinopyroxene granulite (JD1546), were documented for detailed metamorphic studies. Two-episode metamorphism can be recognized. The first-episode recovered from JD15120 and YC8-43 is represented by peak assemblage of medium-grained clinopyroxene, orthopyroxene, amphibole, plagioclase, and ilmenite, which yields ultrahigh temperature (UHT) conditions of 940–960°C at 7.5–8.5 kbar and 950–990°C at 8 kbar, respectively, constrained by contours of the maximum anorthite (XAn) in plagioclase cores. The post-peak evolution is dominated by cooling with decompression, constrained mostly from the measured core-to-rim decreasing XAn in plagioclase. By contrast, the second-episode overprinting is recognized in all samples, but exhibits varying textures. In garnet-bearing samples (YC8-43 and JD1546), the overprinting assemblages are characterized by poikilitic garnet that occurs either as coronae around the first-episode pyroxenes, forming “red-eye socket” textures, or as grains in equilibrium with tiny-grained clinopyroxene, plagioclase, amphibole, rutile, and quartz, forming high-pressure (HP) granulite assemblages. These HP granulite assemblages show peak conditions of ∼12 kbar/860°C and ∼12.6 kbar/835°C, constrained by contours of the maximum grossular (XGrs) in garnet cores and the minimum XAn in plagioclase cores. The post-peak evolution is dominated by isothermal decompression, constrained from the outward decreasing XGrs in garnet and increasing XAn in plagioclase. LA-ICP-MS U-Pb zircon dating on JD15120 and JD1546 suggests two metamorphic ages of ∼2.49 Ga and ∼1.78 Ga, being considered to be correlated with the UHT and HP granulite metamorphism, respectively. Tectonically, the late Neoarchean UHT granulite metamorphism may correlate a vertical sagduction regime, whereas the late Paleoproterozoic HP granulite metamorphism is favored to register the continental collision in the northern margin of the North China Craton. This study may have indications for the Neoarchean–Paleoproterozoic tectonic transition of the craton.

Contrasting mechanisms and timescales of subduction and exhumation as recorded by Paleoproterozoic and late Paleozoic high-pressure granulites in the North China Craton

Abstract The North China Craton records multiple metamorphic events related to supercontinent assembly during the Paleoproterozoic, forming Columbia, and again during the late Paleozoic, forming Pangea. Here we show that the Paleoproterozoic high-pressure granulites (HPGs) formed from enriched mid-ocean ridge basalt protoliths and record a clockwise pressure-temperature-time (P–T–t) path with prograde metamorphism at 7.8–10.0 kbar and 780–820 °C, peak granulite-facies metamorphism at 12–12.3 kbar and ∼860–880 °C, and retrograde metamorphism at 8.7–9.1 kbar and 850–855 °C. Subduction initiated prior to 1.90 Ga, with final collision and orogeny at 1.88 Ga, followed by post-collision/exhumation at 1.80 Ga, defining a prolonged exhumation period (∼90 m.y.) that occurred at a slow velocity of ∼0.16 ± 0.08 mm/y. Late Paleozoic HPGs are normal mid-ocean ridge basalt type and record a near clockwise P–T–t path, with peak/post-peak amphibolitefacies metamorphism at 11.0–12.5 kbar and 860–890 °C, isothermal decompression to 7.2–7.5 kbar and 810–820 °C, and retrogression to 5.5–7.2 kbar and 805–850 °C. Subduction initiated earlier than ca. 340 Ma, exhumation and uplift initiated at 335–309 Ma and continued to 297–287 Ma. The exhumation was short-lived (∼50 m.y.) and relatively fast (0.38 ± 0.14 mm/y). When compared to granulite-facies metamorphism documented in many Paleoproterozoic HPGs, late Paleozoic HPGs appear to commonly form with an initial period of steep subduction leading to eclogite-facies metamorphism, with subsequent exhumation to middle/lower levels of the crust. Our results further reveal that the exhumation velocity for supercontinent collision was facilitated and duration shortened through time, and that the exhumation mechanism might have been controlled by subduction angle, compression pressure, and temperature.

HT overprint of HP granulites in the Oisans–Pelvoux massif: Implications for the dynamics of the Variscan collision in the external western Alps

The Oisans–Pelvoux massif belongs to the Paleozoic basement of the External western Alps and records high temperature (HT) metamorphism associated with intense migmatization during the syn to post-collisional stages of the Variscan orogeny. Metamorphic assemblages related to the early collision stages have been obliterated making it difficult to constrain the earlier evolution of the unit. In this study, we report the finding of new bodies of high pressure (HP) mafic granulites, which preserve relics of the prograde evolution. Relics of Mn-rich garnet cores containing inclusions of epidote, titanite, chlorite and rare white micas constrain an early prograde stage at 460–550 °C and 0.4–1.0 GPa. The HP assemblage consists in garnet + clinopyroxene+ quartz + rutile ± plagioclase ± amphibole ± biotite and yield peak-P conditions at 650–730 °C and 1.5–1.7 GPa. Decompression was associated with heating to HT granulite-facies conditions of 800–870 °C and 0.6–0.9 GPa, which led to the development of granoblastic polycrystalline mosaics over the resorbing HP assemblage. The U-Pb dating of magmatic zircon cores constrains the emplacement age of the mafic protoliths at 479 ± 5 Ma. Timing of the prograde to HP evolution is estimated around 345–330 Ma based on rutile U-Pb dating, which is contemporaneous with HP recorded in the other crystalline massifs of the western Alps. Zircon metamorphic rims yields U-Pb dates scattering between 337 and 294 Ma, which are interpreted to record crystallization during decompression and heating to granulite facies. Metamorphic conditions during the early prograde stage precludes scenarios involving subduction of a cold passive margin and are better reconciled with thickening of an orogenic wedge during collision. In contrast with other Variscan HP relics of the external Alps, the HP rocks of the Oisans–Pelvoux massif record heating to HT granulitic conditions during decompression, which may result from longer residence time in the lower crust before exhumation.

Deformation mechanisms and characteristics of the meta-BIFs from an early Proterozoic shear system of the Southern Granulite Terrane (SGT), India

We studied three samples of Banded Iron Formation (BIF) rocks from Southern Granulite Terrane (SGT) of India, which was metamorphosed at high-pressure granulite facies and exhumed from 40 to 50 km depth. We examined the microstructure, texture, and deformation mechanisms of the primary minerals (quartz, magnetite, and hematite). Microstructures, variations in size and shape of quartz grains suggest that the three samples deformed to different intensities and temperatures. The EBSD derived CPOs for quartz and magnetite grains are strong, implying dislocation creep as the dominant mechanism. The presence of distinct subgrains in magnetite, rare in natural samples, attests to the same. Furthermore, twinning on {111}, as evident from the misorientation angle and axis distributions, indicates ductile deformation of magnetite below 400 °C. Subgrain boundary trace analysis in selected magnetite grains reveals that multiple-slip planes {111}, {110}, and {100} were operational. Topotaxial replacement, via redox reaction, of magnetite grains along their octahedral planes by hematite is evident. Well-developed CPOs but low (<1°) GOS values of both magnetite and hematite grains suggest that recovery processes in these were rapid. Post-replacement deformation was weak and did not affect the hematite CPOs that were inherited from the relatively more deformed host magnetite grains.

High‐pressure granulite facies re‐equilibration and zoisite–biotite dehydration melting during decompression of an ultrahigh‐pressure garnet clinopyroxenite from the island of Fjørtoft, Norway

AbstractA garnet clinopyroxenite from the island of Fjørtoft, Western Gneiss Region (WGR) in Norway, has experienced a complex metamorphic evolution demonstrated by studies involving petrographic observations, mineral chemistry, phase equilibria modelling and geothermobarometry. This rock, originally an eclogite, documents ultrahigh‐pressure (UHP) conditions higher than 3.5 GPa (Stage 1), witnessed by Ca‐rich garnet inclusions in kyanite. The UHP assemblage was pervasively re‐equilibrated at high‐pressure (HP) granulite facies conditions around 1.75 GPa and 870°C, during which and/or somewhat before Ca‐poor garnet, sodian diopside, zoisite and biotite formed (Stage 2a). Dehydration melting of zoisite and biotite (Stage 2b) took place at similar pressure–temperature conditions. The produced melt was poor in Si and rich in alkalis and crystallized mainly to analcime + plagioclase + K‐feldspar. Oriented composite inclusions of quartz + amphibole + phengite ± garnet, found in clinopyroxene, are interpreted to have resulted from interaction between a fluid (hydrous melt and/or aqueous fluid) and clinopyroxene at the early retrogression Stage 3. Stage 1 resulted from deep subduction of the rock. Stage 2 followed after early exhumation probably due to stagnation at a hot and thickened orogenic root. Consequently, other HP granulite facies metabasites, which are similar to the studied garnet clinopyroxenite and widespread in the northwestern WGR, could have been transformed from former eclogite at this stage, too. In addition, this study provides a direct link between the generation of peralkaline melts and crustal anatexis in collision zones and new insights into the retrograde formation of oriented mineral inclusions in HP‐UHP minerals.

Clockwise P–T–t paths with considerable heating processes during decompression from high-pressure mafic granulites in the Wuhe Complex, southeastern North China Craton

The Wuhe Complex lies in the southern section of the Jiao–Liao–Ji mobile/orogenic belt (JLMB/JLJB), critical for understanding the Proterozoic tectonic evolution of the southeastern North China Craton (NCC). Detailed microstructural observations, mineral chemistry, phase equilibria modeling, and zircon and titanite chronological dating were conducted on the mafic granulites from the Wuhe Complex. Five stages of mineral assemblages were identified: the prograde stage (Pl1 + Cpx1 + Amp1 + Ilm1 + Ap1 ± Qz1 ± Kfs1 ± Ttn1 ± Py1), the pressure–peak stage (the core of matrix plagioclase and Grt2a + Cpx2a + Rt2a + Qz2a + Liq2a ± Amp2a ± Kfs2a ± Ttn2a), the temperature–peak stage (the rim of garnet porphyroblast and Pl2b + Cpx2b + Amp2b + Liq2b ± Ilm2b ± Qz2b ± Rt2b), the retrograde stage (Pl3 + Cpx3 + Opx3 + Amp3 + Ilm3 + Ap3 ± Grt3 ± Qz3 ± Scp3), and the cooling stage (Mag4 ± Chl4 ± Py4 ± Ccp4). Traditional thermobarometry calculations and phase equilibria modeling illustrated that clockwise metamorphic pressure–temperature (P–T) paths were retrieved, passing from 9.4 to 13.4 kbar/701 ℃–781 ℃ (prograde stage) through 16.3–17.2 kbar/807 ℃–846 ℃ (pressure–peak stage), and through 10.2–10.8 kbar/895 ℃–912 ℃ (temperature–peak stage) to 5.6–9.1 kbar/803 ℃–883 ℃ (retrograde stage). Hence, the high-pressure (HP) granulites show clockwise P–T paths with heating processes (∼80 ℃) from the lower crust (pressure–peak stage) to the middle-lower crust (temperature–peak stage). Therefore, HP mafic granulites might be buried to the depth of ∼ 57 km, with a slow exhumation rate. The metamorphic zircon and titanite grains from HP mafic granulites yield U–Pb ages of 1991–1758 Ma and 1832–1810 Ma, respectively. The 1832–1810 Ma might represent the retrograde time when HP mafic granulites cooled to 820 ℃–800 ℃. Therefore, the HP mafic granulites of the Wuhe Complex record the Late Paleoproterozoic subduction/collision orogenic event of the southeastern NCC; therefore, the JLMB/JLJB is a prolonged orogenic belt.

Contrasting P-T-t paths of basement and cover within the Búzios Orogen, SE Brazil – Tracking Ediacaran-Cambrian subduction zones

Deeply eroded collisional orogens show complex structural and inverted stratigraphic relations with juxtaposition of rock stacks from distinct crustal levels, origins and ages, hence with contrasting P-T-t paths during convergent tectonics. This paper presents petrochronological data on Paleoproterozoic (basement) and Ediacaran (cover) gneisses tectonically interleaved during the Ediacaran-Cambrian Búzios Orogeny, in southeastern Brazil. U-Pb in zircon and EPMA U-Th-Pb in monazite data, coupled with geothermobarometric data, plus Zr-in-rutile, reveal that at a first orogenic stage (ca. 530–520 Ma), these units were at distinct crustal levels. Samples within the Paleoproterozoic basement show metamorphic near-peak conditions of ∼800 °C and 10 kbar, at a depth of c.37 km. Contrastingly, Ediacaran kyanite-orthoclase-garnet-biotite granulite with retrometamorphic sillimanite (cover) reached near-peak conditions of 15 kbar and 818 °C–785 °C at depths of c. 55 km, in high-pressure granulite facies. This deep burial of Ediacaran sediments in less than 20 m.y. would be compatible with a low angle subduction zone active from ca. 550 to 530 Ma. Intrusion of ca. 550 Ma tholeiitic dykes in the Paleoproterozoic gneiss indicates a high geothermal gradient for this subduction setting, which is consistent with a low subduction rate. In a second orogenic stage (ca. 520–500 Ma), these distinct stratigraphic units were placed tectonically side by side during a fast exhumation, preserving an inverted metamorphic stack. The cover underwent retrometamorphic conditions of 800 °C and 10 kbar on a clockwise return path due to decompression. It is proposed here that the contact between reworked units within a Paleoproterozoic continental crust and Ediacaran magmatic and sedimentary units represent the suture of an Ediacaran NW-subduction of the Angola continental paleomargin below the Oriental Terrane of the Ribeira belt. This paper reports the highest pressure recorded in Ediacaran-Cambrian metamorphic rocks from the Brasiliano belts along the actual South Atlantic continental margins. The suture we propose here is aligned along strike, with a medium to high-pressure Ediacaran metamorphic occurrence 700 km to the SW, in the Curitiba Terrane.

Polyphase post-Variscan thinning of the North Pyrenean crust: Constraints from the P-T-t-deformation history of the exhumed Variscan lower crust (Saleix Massif, France)

As part of the Variscan orogen, the Pyrenean realm has undergone several rifting episodes, starting from the Permian-Carboniferous post-orogenic collapse and culminating with mid-Cretaceous lithospheric mantle exhumation. We present the Pressure-Temperature-time-deformation (P-T-t-d) evolution of the tonalite bodies (here described for the first time) intruded in the late Carboniferous high pressure (HP) granulites of the Saleix Complex, forming the inner envelope of the Lherz peridotites (Aulus Basin, Ariège, France), that is used as a proxy to reconstruct the post-Hercynian evolution of the Pyrenean continental crust. By integrating textural investigations with metamorphic thermobarometry, and zircon and titanite UPb geochronology, we (i) constrain timing and thermo-baric conditions related to the transition from magma emplacement to solid-state syn-tectonic deformation in tonalite(s); and (ii) discuss implications on the polyphase exhumation history of the granulite host rocks. Emplacement of the Saleix tonalite(s) took place in the early Permian (ca. 281 Ma), in a thermo-baric environment (P = 0.5 ± 0.1 GPa and T = 750–880 ± 20 °C) attesting for an important thinning of the Pyrenean crust at the end of the Variscan cycle. Solid-state deformation occurred during the mid-Cretaceous (ca. 96 Ma), at shallow crustal depths (≤10 km) and under amphibolite facies metamorphic conditions. These results document that rifting events had already thinned the Pyrenean crust prior to the Cretaceous mantle exhumation and that the lower crust was affected by ductile deformation during the Cretaceous rifting. Our study demonstrates that in the Pyrenean realm the Permian crustal thinning played a very important role and that mantle exhumation resulted from polyphase post-Variscan, Permian-to-Cretaceous crustal thinning episodes.

High pressure granulite facies metamorphism at the interface of the Archean Bastar craton and the Proterozoic Eastern Ghats Belt, India

Nd model ages show that the suture between the Archean Bastar craton and Proterozoic Eastern Ghats Province (a part of the Eastern Ghats Belt), India is marked by a broad shear zone that contains a mixture of rocks from both the blocks. In this suture zone, amphibolite facies rocks of the Bastar craton were subjected to high pressure granulite facies metamorphism possibly at ~500 Ma (published isotopic data). New petrological, geothermobarometric and P-T pseudosection studies show that dehydration-melting of amphibole (in mafic protolith) and of biotite (in quartzofeldspathic protolith) along a clockwise P-T path led to peak metamorphic conditions of 9.75 ± 0.5 kbar; 875 ± 30 °C. Mafic granulites from the Eastern Ghats Province had an earlier metamorphic history of isobaric cooling from unknown peak metamorphic conditions, but currently record P-T maximum values of 9.25 ± 0.25 kbar; 825 ± 25 °C. Followed by cooling through ~100 °C from the peak metamorphic conditions, mafic granulites from the Bastar craton were exhumed to mid-crustal levels by decompression-cooling, probably as a tectonic wedge. This decompression-cooling is also shared by charnockitic rocks of the Bastar craton and mafic granulites of the Eastern Ghats Province. We interpret further burial of the Bastar craton rocks as a result of underthrusting beneath the Eastern Ghats Province, consistent with available seismic data. Total absence of any evidence of ocean opening or closure between the Bastar craton and the Eastern Ghats Province at around 500 Ma, coupled with interpretation of deduced P-T paths, suggest the shear zone resulted from far-field stress related to the Kuunga orogeny in an intraplate setting. This would support a model of Tonian age amalgamation of the Bastar craton and the Eastern Ghats Province as a part of the Greater Indian Landmass.

Inhomogeneities of the earth’s crust of the Aldan-Stanovoy shield along the 3-DV profile (Eastern Siberia, Russia

Profile 3-DV (Skovorodino-Tommot) crosses in the sublatitudinal direction the Stanovoy and Aldan megablocks of the Aldan-Stanovoy shield. As the basic elements of the Earth’s crust section along the profile 3-DV, a technique was adopted for identifying regional inhomogeneities of the lithosphere based on the results of the analysis of seismic and gravimetric data with subsequent typification of their nature. According to the SRM-CMP data, in the upper part of the section (up to 35 km) of the Aldan megablock, the Yakokut and Chulman heterogeneities are distinguished, and the Stanovoy megablock — the Kalara-Dzhugdzhur heterogeneity. The Yakokut and Chulman seismic inhomogeneities in the gravitational field correspond to minima with an the amplitude of up to 25 mGal. The gravitational field of the Kalara-Dzhugdzhur heterogeneity is mosaic and reflects its block structure.&#x0D; It is shown that the deep structure of the Aldan megablock in the area of the 3-DV profile is determined by the Yakokut granite-gneiss dome and Chulman sublateral decompaction zone, and the upper part (0—25 km) of the Stanovoy megablock is represented by the Kalar-Dzhugdzhur structure, composed of the Stanovoy complex of rocks and blocks of highpressure granulites. A significant (up to 10 km) increase in the thickness of the earth’s crust of the Aldan megablock is explained by the presence of the upper layer juvenile crust formed in the Paleoproterozoic as a result of regional metamorphism of igneous rocks. The Earth’s crust of the Stanovoy megablock is tectonically rebuilt for almost the entire thickness of up to 40 km during the Mesozoic collision of the Precambrian North Asian and Sino-Korean cratons.&#x0D; The Yakokut granite-gneiss dome, in accordance with the proposed model of the structure of the Earth’s crust of the Aldan megablock, is the ore-controlling structure of the Central Aldan gold-bearing region, and highpressure granulites of the Zverevsky block of the Kalara-Dzhugdzhur heterogeneity of the Stanovoy megablock served as a source of gold in the Chako-Berkakit ore cluster.

Metamorphism and geochronology of high-pressure mafic granulites (retrograded eclogites?) in East Cathaysia terrane of South China: Implications for Mesozoic tectonic evolution

AbstractHigh-pressure mafic granulites (retrograded eclogites?) were discovered as minor lenses enclosed in garnet-kyanite-cordierite gneiss from the Badu Complex of the East Cathaysia terrane in South China. These rocks consist mainly of garnet, clinopyroxene, hornblende, quartz, and rutile/ilmenite with or without omphacite pseudomorphs that are indicated by clinopyroxene + sodic plagioclase symplectic intergrowths. Mineral textures and reaction relationships suggest three metamorphic stages: (1) an eclogite-facies stage (M1) characterized by the mineral assemblage of garnet + clinopyroxene (omphacite) + hornblende + rutile + quartz; (2) a high-pressure granulite-facies (M2) stage mainly represented by garnet + clinopyroxene + plagioclase + hornblende + rutile + quartz in the matrix; and (3) an amphibolite retrograde stage (M3) defined by hornblende + plagioclase + ilmenite + quartz symplectites surrounding garnet porphyroblasts. Conventional geothermometers and geobarometers in combination with phase equilibria modeling constrain metamorphic P–T conditions of 15.8–18.2 kbar/625–690 °C (M1), 11.8–14.5 kbar/788–806 °C (M2), and 5.4–6.4 kbar/613–668 °C (M3), respectively. Two-staged decompression processes are defined after the peak pressure, which suggests a two-staged exhumation of these deeply buried rocks. Secondary ion mass spectrometry (SIMS) zircon U-Pb dating and trace element analysis show that the high-pressure metamorphism occurred at 240–244 Ma. Complete early Mesozoic orogenic processes characterized by initial subduction and/or crustal thickening and subsequent exhumation followed by rapid uplift are reconstructed for this part of the East Cathaysia terrane, South China.

Multi-phase metamorphism in the northern margin of the North China Craton: Records from metapelite in the Hongqiyingzi Complex

It is controversial about the tempo–spatial distribution and tectonic attributes of the Paleoproterozoic orogenic belts in the North China Craton (NCC). The Hongqiyingzi Complex in the northern margin of the NCC, comprising orthogneiss, supracrustal sequence and an ophiolitic mélange, was investigated to have experienced multiple tectonic evolution from Paleoproterozoic to Mesozoic. However, the depositional age and metamorphic P–T path of the supracrustal sequence have not been well addressed. Garnet mica schists are investigated on the basis of petrography, mineral chemistry, phase equilibria modelling and zircon and monazite dating, which record an extreme case with four phases of metamorphism. The first-phase is characterized by high pressure (HP) granulite facies with the peak conditions of 10–11 kbar/>780 °C, which occurred at ~1.95 Ga. It is considered to correspond to a collisional orogeny with crust thickening widespread in the Paleoproterozoic orogenic belts of the NCC, followed by an exhumation with heating at ~1.92 Ga. The second-phase is marked by the overprinting of staurolite-bearing assemblages with the peak conditions of 6–7 kbar/610–630 °C, which occurred at ~1.85 Ga. It may correlate a separate orogeny occurred along the northern margin of the NCC, following a ~1.90 Ga oceanic subduction. Phase modelling suggests this overprinting over the first-phase HP granulites is mainly controlled by water infiltration. The third-phase andalusite-type and the fourth-phase greenschist facies metamorphism are confined to local equilibrium domains and their ages have not been well constrained. With a comparison of regional geology, the third-phase metamorphism is inferred to be related with the 320–290 Ma extension in the northern part of the NCC during Late Paleozoic, and the fourth-phase metamorphism is correlated with the 250–230 Ma collision occurred along the Solonker Suture Zone in the Central Asia Orogenic Belt during Early Mesozoic. Moreover, detrital magmatic zircons yield ages of 2.0–2.1 Ga, providing a definite upper limit on the deposition of the Hongqiyingzi supracrustal sequence.

On the origin of high-pressure mafic granulite in the Eastern Himalayan Syntaxis: Implications for the tectonic evolution of the Himalayan orogen

The Himalayan orogen, resulting from the Early Cenozoic collision of the Indian and Asian plates, is an ideal vehicle to study active orogenic processes and test geodynamic models of how the crust responds to collisional orogeny. This paper focused on migmatitic high-pressure (HP) mafic granulite and associated leucosome from the Greater Himalayan Crystallines (GHC) in the Eastern Himalayan Syntaxis (EHS) in order to understand the conditions and timescales over which high-grade rocks and partial melts were produced during the Himalayan orogeny. Combining with previous study results from the Western and Central Himalayas and Trans-Himalayan magmatic arc, we obtained the following conclusions: (1) The mafic granulites from the EHS underwent HP and high-temperature (HT) granulite facies metamorphism and partial melting, with peak metamorphic conditions of 15–17 kbar and 820–880 °C. The GHC, at least its western part of the EHS, underwent coherent HP granulite-facies metamorphism. (2) The HP mafic granulites experienced long-lived dehydration melting of amphibole from ~40 Ma to ~20 Ma during prograde metamorphism and generated up to ~16 vol% partial melt. The variable degrees of dehydration melting of the HP mafic, pelitic and felsic granulites in the EHS generated voluminous granitic melts with distinct compositions, and provided the source for the Himalayan granites. (3) Peak metamorphic pressure of the GHC gradually decreases, whereas the metamorphic temperature progressively increases from the Western to Eastern Himalayas. This indicates that the Indian continental crust deeply subducted into the mantle in the Western Himalaya after the Indo-Asia collision, whereas the Indian crust underthrusted or relaminated beneath the Asian continental crust, and formed the thickened lower crust in the Central and Eastern Himalayas and Gangdese arc. (4) The melts derived from the underthrusted Indian crust probably resulted in isotopic compositional enrichment of the Early Cenozoic mantle- and juvenile crust-derived magmatic rocks of the Gangdese arc.

Chicheng high-pressure granulites record the paleoproterozoic tectonic evolution in the northern North China Craton

High-pressure (HP) mafic granulites are widely distributed along the Trans-North China Orogen (TNCO), and they provide important insights into the timing and tectonic processes involved in the formation of the TNCO. We discovered three types of HP granulite, including mafic, intermediate, and felsic granulites, exposed in Chicheng area, in the northern segment of the TNCO. Their mineral assemblages of prograde, peak, and post-peak metamorphic stages were identified based on petrographic investigations. SIMS U-Pb zircon analyses yielded an upper intercept age of 2547 ± 38 Ma and a lower intercept age of 1916 ± 100 Ma for one HP mafic granulite and a discordant 207Pb/206Pb age of 2479 Ma with a weighted mean 207Pb/206Pb age of 1905 ± 9 Ma for the other HP mafic granulite. For the HP intermediate granulite, an upper intercept age of 2544 ± 20 Ma was obtained. The HP felsic granulite yielded an upper intercept age of 2524 ± 23 Ma and a concordant 207Pb/206Pb age of 1906–1907 Ma. These results reveal that the protoliths of HP granulites were formed at ~ 2.5–2.55 Ga and underwent high pressure granulite facies metamorphism at ~ 1.9 Ga. The elevated Hf isotopic compositions of the metamorphic zircons from two granulite samples indicate that metamorphic fluids that have high 176Hf/ 177Hf ratios but low 176Lu/177Hf ratios were involved in their metamorphism. The low 18O isotopic features of the zircons from the intermediate granulite suggest alteration by δ18O-depleted seawater or meteoric fluids at high temperatures. By integrating geochronological data from previous studies, we infer that the formation of the TNCO may have begun as early as ~ 1.9 Ga and lasted until ~ 1.8 Ga, supporting the hypothesis that it was formed by a long-lived tectono-metamorphic event

Improved resolution of Paleoproterozoic orogenesis: Multi-directional collision tectonics in the Sodankylä belt of northern Finland

The geodynamic system of Earth is controlled by the horizontal motion of fragmented tectonic plates due to complex interaction between the mantle and the crust. Land and marine paleomagnetic signatures, along with extensive petrochronological and structural data, suggest non-continuous, multi-directional convergence histories for modern orogenic belts, as seen in the Himalayas and Betic Cordillera, among others. The consequences of radical changes in the direction of plate motions are obscured by complex structural settings and repeated cycles of metamorphism in orogenic belts. Although, subduction was operational on Earth in the Paleoproterozic, one of the major questions is: how far back in geological time can we trace the scale of modern-style subduction, collisional tectonics and poly-metamorphism? The metasedimentary and volcanic rocks (2.5 to 2.05 Ga) of the Sodankylä belt of northern Finland were deformed and metamorphosed during a prolonged period, now represented by the Svecofennian orogen (ca. 1.92–1.77 Ga). The orogenic belt represents a window into the deep levels of a Paleoproterozoic crust, where the interplay between deformation and metamorphism can be compared with the modern orogenic settings. In this study, five discrete heterogeneously developed superimposed, near-orthogonal, deformation events (D1 to D5) have been identified and mapped based on the high-resolution aeromagnetic data and field studies. Regional correlation of tectonic fabrics, along with metamorphic textures across the Sodankylä belt and northern Finland, suggest that each deformation event was a regional-scale phenomenon rather than a local one. Previous geochronological data obtained from the Sodankylä belt and adjacent Central Lapland Granitoid Complex on different Paleoproterozoic deformed rocks allow us to propose a time-constrained tectono-metamorphic model for the Sodankylä belt. The sequence of near-orthogonal deformation history and the polymetamorphic evolution offer comparisons with modern-style orogenic settings. The most notable aspect is the occurrence of Mesoarchean and Paleoproterozoic eclogitic high-pressure granulites reported from the Belomorian belt of Fennoscandian Shield, which lies to the east of the Sodankylä belt. From a tectono-metallogenic viewpoint, the Sodankylä belt is located in the centre of Ni-Cu-PGE, orogenic gold-cobalt deposits of the northern part of Fennoscandian Shield, therefore, the tectonic processes and overprinting role of multiple deformation phases need to be well understood as they directly link mineral exploration to the timing and controls on mineralization.