Peak Granulite Facies Metamorphism Research Articles

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.

Building a continental arc section: Constraints from Paleozoic granulite-facies metamorphism, anatexis, and magmatism in the northern margin of the Qilian Block, northern Tibet Plateau

AbstractContinental arcs in active continental margins (especially deep-seated arc magmatism, anatexis, and metamorphism) can be extremely significant in evaluating continent building processes. In this contribution, a Paleozoic continental arc section is constructed based on coeval granulite-facies metamorphism, anatexis, and magmatism on the northern margin of the Qilian Block, which record two significant episodes of continental crust growth. The deeper layer of the lower crust mainly consists of medium-high pressure mafic and felsic granulites, with apparent peak pressure-temperature conditions of 11–13 kbar and 800–950 °C, corresponding to crustal depths of ~35–45 km. The high-pressure mafic granulite and local garnet-cumulate represent mafic residues via dehydration melting involving breakdown of amphibole with anatectic garnet growth. Zircon U-Pb geochronology indicates that these high-grade metamorphic rocks experienced peak granulite-facies metamorphism at ca. 450 Ma. In the upper layer of the lower crust, the most abundant rocks are preexisting garnet-bearing metasedimentary rocks, orthogneiss, and local garnet amphibolite, which experienced medium-pressure amphibolite-facies to granulite-facies metamorphism at depths of 20–30 km at ca. 450 Ma. These metasedimentary rocks and orthogneiss have also experienced partial melting involving mica and rare amphibole at 457–453 Ma. The shallow to mid-crust is primarily composed of diorite-granodiorite batholiths and volcanic cover with multiple origin, which were intruded during 500–450 Ma, recording long-term crustal growth and differentiation episode. As a whole, two episodes of continental crust growth were depicted in the continental arc section on the northern margin of the Qilian Block, including: (a) the first episode is documented in a lithological assemblage composing of coeval mafic-intermediate intrusive and volcanic rocks derived from partial melting of modified lithospheric mantle and subducted oceanic crust during southward subduction of the North Qilian Ocean at 500–480 Ma; (b) the second episode is recorded in mafic rocks derived from partial melting of modified lithospheric mantle during transition from oceanic subduction to initial collision at 460–450 Ma.

Late Eocene-Oligocene granulite facies garnet-sillimanite migmatites from the Mogok Metamorphic belt, Myanmar, and implications for timing of slip along the Sagaing Fault

The Mogok Metamorphic Belt (MMB) in Myanmar is a polymetamorphic, mainly Paleogene granulite-uppermost amphibolite facies terrane consisting mainly of marbles and calc-silicates hosting spinel, ruby and sapphire. Jurassic charnockite-syenite intrusions, as well as Eocene-Miocene leucogranite intrusions are also present. Pelitic rocks are uncommon, and where present, have sillimanite, both as primary inclusions in garnet and as secondary Bt + Sil coronas around garnet. Core samples from the Kyi-Tauk-Pauk gold mine at Thabeikkiyin, north of Mandalay, are mostly Grt + Bt + Sill gneisses and migmatites with uncommon interbanded Opx + Grt + Bt gneisses. Pseudosection modelling suggests prograde garnet growth occurred by biotite-dehydration melting that reached peak P–T conditions of 870–970 °C and ~ 0.9 GPa, and was followed by garnet breakdown forming coarse retrograde Bt + Sil coronas. UPb monazite data show an early high-grade granulite event at 43–32 Ma, and a later upper amphibolite sillimanite-grade event peaking at 23–20 Ma, with a change in monazite chemistry after c. 22 Ma that is consistent with fluid/(melt) interaction and garnet breakdown. Elevated Th/U ratios from ~35 to 22 Ma, and at ~18 Ma are compatible with melt influx at that time, timing that is similar to the age of the regional Kabaing leucogranite in the Mogok valley area. Our data show that peak granulite facies metamorphism along the Mogok Metamorphic belt was mainly Middle Eocene-Early Oligocene, with upper amphibolite facies metamorphism extending to earliest Miocene. The MMB is cut abruptly by the Sagaing fault, a large-scale dextral fault that extends from the Andaman Sea north to the East Himalayan syntaxis. Our new UPb monazite data constrain the oldest age of initiation of the eastern branch of the cross-cutting Sagaing dextral strike-slip fault at <20–18 Ma.

Metamorphic evolution of high-grade granulite-facies rocks of the Mashan Complex, Liumao area, eastern Heilongjiang Province, China: Evidence from zircon U–Pb geochronology, geochemistry and phase equilibria modelling

The nature and age of the Mashan Complex (or Mashan Group) in northeast China is key for determining the Precambrian geological evolution and origins of the Jiamusi Massif. Here we present a detailed investigation of high-grade granulite-facies rocks of the Mashan Complex from the Liumao area, Heilongjiang Province, China. This includes zircon U–Pb dating, petrological and geochemical analysis, geothermobarometry, phase equilibria modelling, and a P–T path. The metamorphic rocks consist mainly of two groups: pelitic and felsic granulites; amphibolite-facies Sil–Grt–Bi, Sil–Grt, and Grt–Bi gneisses. The protoliths of the metamorphic rocks were clay rocks and sandstones, which were derived dominantly from silicic rocks in a continental island arc setting. In the pelitic granulites, four metamorphic assemblages (M1–M4) were recognised. Three metamorphic assemblages (M2–M4) were identified in the felsic granulites. Geothermobarometric data and phase equilibria modelling indicate that the M2, M3, and M4 assemblages in the pelitic granulite were metamorphosed at 9.2 kbar/845–865 °C, 4.7–5.1 kbar/820–865 °C, and 4.0–4.8 kbar/660–720 °C, respectively. These features imply a collisional orogenic process with a clockwise P–T path with nearly isothermal decompression segments. Similar results were obtained for the felsic granulite, with the metamorphic P–T conditions of the peak assemblage (M2) evaluated as 9.0–9.2 kbar/845–870 °C. The similar P–T paths indicate that the rocks underwent an evolutionary process that started with crustal thickening (M2), followed by rapid crustal uplift (M3), and then final cooling (M4). Based on our new zircon U–Pb age data, the peak of granulite-facies metamorphism in the Liumao area of the Jiamusi Massif occurred at ca. 530 Ma and final cooling occurred at ca. 490 Ma.

New 40Ar/39Ar geochronology data of the Fuping and Wutai Complexes: Further constraints on the thermal evolution of the Trans-North China Orogen

The Trans-North China Orogen has witnessed the assembly of the Western and Eastern blocks of the North China Craton during Late Paleoproterozoic. Numerous studies have focused on the subduction/collision timing and metamorphic evolution of the Trans-North China Orogen, whereas the thermal evolution and exhumation history are still poorly understood. The Fuping and Wutai Complexes are widely exposed of middle- to high-grade metamorphic rocks, which are excellent regions to investigate the thermal evolution and exhumation history of the Trans-North China Orogen. Previous U-Pb geochronological data of metamorphic zircon and monazite suggest that the peak granulite facies metamorphism in the Fuping Complex occurred during 1.92–1.80 Ga, and peak amphibolite facies metamorphism in the northeastern part of the Wutai Complex happened at 1.85–1.82 Ga. Here we report newly obtained hornblende 40Ar/39Ar age of 1813–1806 Ma in the Fuping Complex, and biotite 40Ar/39Ar age of 1802–1786 Ma in the northeastern part of the Wutai Complex, which representing the cooling ages. Combined with previous thermochronological data, the cooling rate of 2.0–8.5 °C/Ma was obtained for the middle to southern segments of the Trans-North China Orogen, possibly suggesting a moderate to fast exhumation relative to other Proterozoic orogens.

Petro-tectonic evolution of metamorphic sole of the Semail ophiolite, UAE

The Semail ophiolite located in the eastern part of the Arabian platform preserves remnants of ocean plate stratigraphy and related metamorphic sole. To understand the petro-tectonic evolution of a metamorphic sole during subduction to obduction processes, here we investigate the garnet metagabbros from the metamorphic sole and the tonalites which intruded the mantle section of the Khor Fakkan Block. We present results from petrology, geochemistry, zircon U-Pb, Hf and O isotope analyses and phase equilibria modeling. The garnet metagabbro samples have E-MORB-type enriched-mantle compositions with zircon dates of ca. 89–96 Ma, and positive εHf(t) values ranging from 5.6 to 10.0. The tonalite is peraluminous with those range of ca. 87–92 Ma, and a range of positive εHf(t) values of 5.1–10.0. The similarity in εHf values from both the garnet metagabbro and tonalite samples suggests a strong relevance to their mantle source, indicating the role of subducted material during their formation. In contrast, the δ18O(zircon) values show distinctly different values of high δ18O(zircon) of ~13–16‰ for the tonalite and ~ 5–8‰ for the metagabbro samples, reflecting variations in the role of surface-derived source materials. The phase equilibria modeling of the garnet metagabbro shows high-pressure amphibolite facies metamorphism that preceded the peak granulite facies metamorphism, followed by lower pressure hydration and decompression. This clockwise P-T path might reflect partial melting and differentiation of mantle wedge section above subducted slab. Our results provide insights into the complex processes within a supra-subduction zone, implying differences in degree of partial melting of the ocean plate stratigraphic sequences including recycled oceanic slab and surface-derived marine sediments that were subsequently interacted with hydrothermally altered mantle at a mantle wedge during subduction to obduction processes that formed the Semail ophiolite during the Upper Cretaceous.

U–Th-total Pb ages of monazite from the Eckergneiss (Harz Mountains, Germany): evidence for Namurian to Westfalian granulite facies metamorphism at the margin of Laurussia

Four samples of garnet-bearing metapsammitic gneisses and one sample of a kinzigitic sillimanite-garnet-cordierite gneiss from the Eckergneiss Complex (Harz Mountains, Germany) were studied in detail for U–Th-total Pb dating with the electron microprobe. Monazite in all samples occurs in the matrix and show similar chemical compositional variation. A single stage monazite growth during the highest stages of metamorphism was deduced and chemical ages of all samples are between 313 ± 4 and 328 ± 6 Ma. These ages are interpreted as the time of granulite facies metamorphism in the Eckergneiss Complex. This metamorphism post-dates the formation of the main foliation, presumably developed under amphibolite facies conditions, as indicated by overprinting static grain growth and partial melt microstructures. It pre-dates the significantly younger intrusion of the Harzburg Gabbronorite and the Brocken Granite that are of post-Variscan age. Our results, in combination with P–T conditions for the peak of granulite facies metamorphism, show that the Eckergneiss Complex must have resided at a depth of at least 15 km during the Namurian to Westfalian. Later exhumation of the Eckergneiss Complex and its tectonic emplacement within the low-grade metamorphic Rhenohercynian rock units of the NW-Harz most likely occurred along the Acker-Bruchberg Thrust Zone, which marks the transition between the allochthonous and autochthonous domains of the Harz Mountains. At Early Permian time, this imbricated sequence was intruded and sealed by the post-Variscan intrusions.

Late Paleoproterozoic granulite-facies metamorphism in the North Altyn Tagh area, southeastern Tarim craton: Pressure-temperature paths, zircon U-Pb ages, and tectonic implications

Abstract Granulite occupies the root of orogenic belts, and understanding its formation and evolution may provide critical information on orogenic processes. Previous studies have mainly focused on garnet-bearing high-pressure and medium-pressure granulites, whereas the metamorphic evolution and pressure-temperature (P-T) paths of garnet-absent, low-pressure granulites are more difficult to constrain. Here, we present zircon U-Pb ages and mineral chemistry for a suite of newly discovered two-pyroxene granulites in the North Altyn Tagh area, southeastern Tarim craton, northwestern China. Conventional geothermobarometry and phase equilibrium modeling revealed that these rocks experienced a peak granulite-facies metamorphism at T = 790–890 °C and P = 8–11 kbar. The mineral compositions and retrograde symplectites record a clockwise cooling and exhumation path, possibly involving near-isothermal decompression followed by near-isobaric cooling. Zircon U-Pb dating yielded a ca. 1.97 Ga metamorphic age, which likely represents the initial cooling age, based on Ti-in-zircon thermometry. Combined with regional geological records, we interpret that these granulites originated from the basement rocks of a late Paleoproterozoic magmatic arc that was subsequently involved in a collisional orogen in the southern Tarim craton, presumably related to the assembly of the Columbia/Nuna supercontinent. The clockwise P-T paths of the granulites record crustal thickening and burial followed by crustal thinning and exhumation in the upper plate of the collisional orogen. Our data indicate that the initial exhumation of this orogen probably occurred no later than ca. 1.97 Ga, which is supported by widespread 1.93–1.85 Ga postorogenic magmatism in this area.

Early Paleozoic Granulite-Facies Metamorphism and Magmatism in the Northern Wulan Terrane of the Quanji Massif: Implications for the Evolution of the Proto-Tethys Ocean in Northwestern China

The nature and evolution of the Proto-Tethys Ocean originated from the breakup of the supercontinent Rodinia remain controversial. Early Paleozoic magmatism and metamorphism can provide important constraints on the closure of the Proto-Tethys Ocean. This paper reports on a set of geological, petrographical, geochronological, mineralogical and geochemical data for Early Paleozoic granite, gabbro, granulite and granitic leucosome in the northern Wulan terrane of the Quanji Massif. Zircon LA-ICP-MS U-Pb dating reveals two episodes of magmatism, with the emplacement of a granitic pluton at 476.7±2.8 Ma and a gabbroic dike at 423±2 Ma. Whole-rock geochemistry suggests an arc affinity for the magma of the granitic pluton but a post-collisional extension setting for the gabbroic dike. Zircon LA-ICP-MS U-Pb dating also shows that the peak granulite-facies metamorphism and anatexis occurred at ~475 Ma, coeval with the formation of the granitic pluton in the Quanji Massif as well as the early lawsonite-bearing eclogites in the North Qaidam high-pressure and ultrahigh-pressure (HP-UHP) metamorphic belt to the south. The granulite-facies metamorphism with peak P-T conditions at 718–729 oC and 0.46–0.53 GPa is characterized by an anticlockwise P-T path. Our data provide compelling evidence for Early Paleozoic paired metamorphic belts with HP-UHP metamorphism in the North Qaidam to the south and low P/T metamorphism in the Quanji Massif as a continental arc to the north, hence suggesting a northward subduction polarity for the Proto-Tethys oceanic plate. The intrusion of the post-collisional gabbroic dike supports for the closure of the Proto-Tethys Ocean in northwestern China before 423 Ma.

Discovery of granulite-facies metamorphic rocks in the Ji’an area, northeastern Jiao–Liao–Ji Belt, North China Craton: Metamorphic P–T evolution and geological implications

The Ji’an area in the Jiao–Liao–Ji Belt of the North China Craton is characterized by granulite-facies metamorphic rocks, including orthopyroxene–cordierite–garnet pelitic granulites and clinopyroxene–orthopyroxene mafic granulites. The pelitic granulites contain orthopyroxene, cordierite, garnet, biotite, and feldspar, whereas the mafic granulites contain coarse-grained orthopyroxene, clinopyroxene, amphibole, biotite, plagioclase, and quartz. Petrographic observations, traditional thermobarometry, and quantitative phase equilibria modeling of the pelitic granulites reveal a clockwise P–T path involving near-isothermal decompression and decompressional cooling that followed the peak granulite-facies metamorphism. The peak metamorphic assemblage is interpreted to be garnet+biotite + plagioclase+quartz+ilmenite+magnetite with P–T conditions of 860–870°C and ∼10kbar. Peak metamorphism was followed by a post-peak near-isothermal decompression with the development of biotite+cordierite+garnet+orthopyroxene+plagioclase+quartz+magnetite at 850–890°C and ∼7.5kbar, resulting in the formation of matrix orthopyroxene and cordierite associated with biotite, due to decompressional dehydration melting reactions involving biotite. Thermobarometric data for the mafic granulites show high temperatures of 821–872°C that are similar to the peak temperatures of the orthopyroxene–cordierite–garnet pelitic granulites. The results indicate that the granulite facies metamorphism of the rocks in the Ji’an area of the Jiao–Liao–Ji Belt was a response to continent–continent subduction and collision of the Longgang and Nangrim blocks of the North China Craton, followed by exhumation.

Petrology, phase equilibria modelling and zircon U–Pb geochronology of Paleoproterozoic mafic granulites from the Fuping Complex, North China Craton

AbstractThe Fuping Complex is one of the important basement terranes within the central segment of the Trans‐North China Orogen (TNCO) where mafic granulites are exposed as boudins within tonalite–trondhjemite–granodiorite (TTG) gneisses. Garnet in these granulites shows compositional zoning with homogeneous cores formed in the peak metamorphic stage, surrounded by thin rims with an increase in almandine and decrease in grossular contents suggesting retrograde decompression and cooling. Petrological and phase equilibria studies including pseudosection calculation using thermocalc define a clockwise P–T path. The peak mineral assemblages comprise garnet+clinopyroxene+amphibole+quartz+plagioclase+K‐feldspar+ilmenite±orthopyroxene±magnetite, with metamorphic P–T conditions estimated at 8.2–9.2 kbar, 870–882 °C (15FP‐02), 9.6–11.3 kbar, 855–870 °C (15FP‐03) and 9.7–10.5 kbar, 880–900 °C (15FP‐06) respectively. The pseudosections for the subsequent retrograde stages based on relatively higher H2O contents from P/T–M(H2O) diagrams define the retrograde P–T conditions of &lt;6.1 kbar, &lt;795 °C (15FP‐02), 5.6–5.8 kbar, &lt;795 °C (15FP‐03), and &lt;9 kbar, &lt;865 °C (15FP‐06) respectively. Data from LA‐ICP‐MS zircon U–Pb dating show that the mafic dyke protoliths of the granulite were emplaced at c. 2327 Ma. The metamorphic zircon shows two groups of ages at 1.96–1.90 Ga (peak at 1.93–1.92 Ga) and 1.89–1.80 Ga (peak at 1.86–1.83 Ga), consistent with the two metamorphic events widely reported from different segments of the TNCO. The 1.93–1.92 Ga ages are considered to date the peak granulite facies metamorphism, whereas the 1.86–1.83 Ga ages are correlated with the retrograde event. Thus, the collisional assembly of the major crustal blocks in the North China Craton (NCC) might have occurred during 1.93–1.90 Ga, marking the final cratonization of the NCC.

Mid-crustal shear zone development under retrograde conditions: pressure–temperature–fluid constraints from the Kuckaus Mylonite Zone, Namibia

Abstract. The Kuckaus Mylonite Zone (KMZ) forms part of the larger Marshall Rocks–Pofadder shear zone system, a 550 km-long, crustal-scale strike-slip shear zone system that is localized in high-grade granitoid gneisses and migmatites of the Namaqua Metamorphic Complex. Shearing along the KMZ occurred ca. 40 Ma after peak granulite-facies metamorphism during a discrete tectonic event and affected the granulites that had remained at depth since peak metamorphism. Isolated lenses of metamafic rocks within the shear zone allow the P–T–fluid conditions under which shearing occurred to be quantified. These lenses consist of an unsheared core that preserves relict granulite-facies textures and is mantled by a schistose collar and mylonitic envelope that formed during shearing. All three metamafic textural varieties contain the same amphibolite-facies mineral assemblage, from which calculated pseudosections constrain the P–T conditions of deformation at 2.7–4.2 kbar and 450–480 °C, indicating that deformation occurred at mid-crustal depths through predominantly viscous flow. Calculated T–MH2O diagrams show that the mineral assemblages were fluid saturated and that lithologies within the KMZ must have been rehydrated from an external source and retrogressed during shearing. Given that the KMZ is localized in strongly dehydrated granulites, the fluid must have been derived from an external source, with fluid flow allowed by local dilation and increased permeability within the shear zone. The absence of pervasive hydrothermal fractures or precipitates indicates that, even though the KMZ was fluid bearing, the fluid/rock ratio and fluid pressure remained low. In addition, the fluid could not have contributed to shear zone initiation, as an existing zone of enhanced permeability is required for fluid infiltration. We propose that, following initiation, fluid infiltration caused a positive feedback that allowed weakening and continued strain localization. Therefore, the main contribution of the fluid was to produce retrograde mineral phases and facilitate grain-size reduction. Features such as tectonic tremor, which are observed on active faults under similar conditions as described here, may not require high fluid pressure, but could be explained by reaction weakening under hydrostatic fluid pressure conditions.

New constraints on metamorphism in the Highjump Archipelago, East Antarctica

AbstractThe age and conditions of metamorphism in the Highjump Archipelago, East Antarctica, are investigated using samples collected during the 1986 Australian Antarctic expedition to the Bunger Hills–Denman Glacier region. In situ U-Pb dating of monazite from three metasedimentary rocks yields ages between c. 1240–1150 Ma and a weighted mean 207Pb/206Pb age of 1183±8 Ma, consistent with previous constraints on the timing of metamorphism in this region and Stage 2 of the Albany–Fraser Orogeny in south-western Australia. This age is interpreted to date the development of garnet ± sillimanite ± rutile-bearing assemblages that formed at c. 850–950°C and 6–9 kbar. Peak granulite facies metamorphism was followed by decompression, evidenced largely by the partial replacement of garnet by cordierite. These new pressure–temperature determinations suggest that the Highjump Archipelago attained slightly higher temperature and pressure conditions than previously proposed and that the rocks probably experienced a clockwise pressure–temperature evolution.

Petrotectonic framework of granitoids and associated granulites at Nagavalli Shear Zone (NSZ), Eastern Ghats Belt: Evidence of a late transpression orogeny

Megacrystic granitoids associated with migmatitic and metasedimentary gneisses occurring around Nagavalli Shear Zone (NSZ) preserve complex metamorphic and deformation history. Thinly laminated discontinuous banding of quartzofeldspathic layer (S1) in the migmatites is the product of first incipient melting during prograde M1–D1 tectonothermal event. Peak M2–D2 event is manifested by the development of S2 gneissic foliation in all rocks, which is axial planar to rootless folds on S1. Porphyroblastic garnet mantled by leucosomal melt fraction in granitoids, suggest that the rock suffered peak granulite facies metamorphism along with host migmatitic gneisses. The subsequent D3 event deforms differently the massive granitoids and the migmatitic granulite gneisses. The D4 deformation acted as transpression with broad northwest–southeast compression that develops strong discontinuous regional-scale anastomosing shear zones transecting the earlier gneissosity (S2) in the granitoids with prominent sinistral shear sense. It deforms the axial plane of regional folds in migmatites and develops superposed non-plane non-cylindrical folds in outcrop to regional scale. Thus we infer megacrystic granitoids were possibly emplaced in pre- to syn-peak metamorphic event within the host granulites. Granitoids and associated migmatitic gneisses of Late Meso- to Neoproterozoic age suffered subsequent petrotectonic events followed by a sinistral transpression acted along NSZ.

Metamorphic P–T–t evolution of mafic HP granulites in the northeastern segment of the Tarim Craton (Dunhuang block): Evidence for early Paleozoic continental subduction

The Dunhuang block, which constitutes the northeastern segment of the Tarim Craton, is located between the Central Asian Orogenic Belt (CAOB) to the north and the Altyn–Qilian Orogenic Belt to the south. The present study reveals that the early Paleozoic HP mafic granulites from the Dunhuang block underwent four stages of metamorphism: prograde amphibolite-facies (M1), peak high-pressure (HP) granulite-facies (M2) and two late stages of amphibolite-facies retrograde metamorphism. Based on phase equilibrium modeling, P–T conditions of the four stages of metamorphism are estimated at 720–750°C and 11–13kbar, 760–800°C and 14–16kbar, 690–720°C and 8–8.5kbar, and at <6kbar and <640°C respectively. Peak granulite-facies metamorphism is characterized by a low geothermal gradient of ca. 16°C/km. Zircon U–Pb dating shows that the HP granulite-facies metamorphism occurred at ca. 431Ma and the early retrograde amphibolite-facies overprint at ca. 403Ma. Thus the investigated mafic rocks here reveal a clockwise P–T–t path involving burial heating before peak granulite-facies metamorphism and subsequent decompression–cooling with an uplift rate of ca. 0.8km/Ma. This, together with a continental affinity of the HP metamorphic rocks, indicates that the Dunhuang block experienced a collisional orogenesis during the early Paleozoic.

Multiple migmatite events and cooling from granulite facies metamorphism within the Famatina arc margin of northwest Argentina

The Famatina margin records an orogenic cycle of convergence, metamorphism, magmatism, and extension related to the accretion of the allochthonous Precordillera terrane. New structural, petrologic, and geochronologic data from the Loma de Las Chacras region demonstrate two distinct episodes of lower crustal migmatization. The first event preserves a counterclockwise pressure‐temperature path in kyanite‐K‐feldspar pelitic migmatites that resulted in lower crustal migmatization via muscovite dehydration melting at ∼12 kbar and 868°C at 461 ±1.7 Ma. The shape of the pressure temperature path and timing of metamorphism are similar to those of regional midcrustal granulites and suggest pervasive Ordovician migmatization throughout the Famatina margin. One‐dimensional thermal modeling coupled with regional isotopic data suggests Ordovician melts remained at temperatures above their solidus for 20–30 Ma following peak granulite facies metamorphism, throughout a time period marked by regional oblique convergence. The onset of synconvergent extension occurred only after regional migmatites cooled beneath their solidus and was synchronous with the cessation of Precordillera terrane accretion at ∼436 Ma. The second migmatite event was regionally localized and occurred at ∼700°C and 12 kbar between 411 and 407 Ma via vapor saturated melting of muscovite. Migmatization was synchronous with extension, exhumation, and strike‐slip deformation that likely resulted from a change in the plate boundary configuration related to the convergence and collision of the Chilenia terrane.

LA–ICP–MS monazite U–Pb age and trace element constraints on the granulite-facies metamorphism in the Tongbai orogen, central China

Granulite-facies metamorphic rocks in the Tongbai orogen have great implications for the evolution of the whole Qinling–Tongbai–Dabie orogenic belt. However, the age and tectonic setting of the granulite-facies metamorphism have not been well constrained yet. In this contribution, BSE imaging, trace element analysis and U–Pb dating of monazite were undertaken for three felsic granulites, a gneiss and a leucosome from the Tongbai orogen. Most monazite grains from the granulite and gneiss samples exhibit no zoning, relatively low Th/U ratios, significant depletion in HREEs and Y, and negative Eu anomalies, indicating their formation under granulite-facies metamorphic conditions. Their formation ages range from 423±2 to 430±3Ma with a weighted mean of 426±5, which is taken as the timing of the peak granulite-facies metamorphism. In the leucosome sample, the monazite shows a euhedral to subhedral shape, coupled with a core and rim structure. Besides being depleted in HREEs and Y, both of them have significantly negative Eu anomalies and variable U contents (1220–13323ppm) and Th/U ratios (7.2–136.7). This suggests that their formation was coeval with the crystallization of feldspar, metamorphic zircon, and garnet. The monazites yield a weighted mean age of 426±3Ma, suggesting nearly coeval partial melting during granulite-facies metamorphism. The relationship between metamorphic monazite and zircon within the same metamorphic sample is complex as revealed by metamorphic monazite and zircon U–Pb ages for the gneiss, granulite and leucosome samples. In the other two granulite samples, the monazite yields U–Pb ages of 423±2 and 425±3Ma, whereas only little overgrowth or recrystallization of metamorphic zircon. This indicates that monazite is more sensitive than zircon in recording of metamorphic age even at the granulite-facies conditions. Combined with previous metamorphic zircon U–Pb age results, the prograde, peak, and retrograde stages of the granulite-facies metamorphism in the Tongbai orogen can be constrained at ca. 440, 426, and 415Ma, respectively. Therefore, the granulite-facies metamorphism would have lasted for more than 20Myr. The prolonged granulite-facies metamorphism might result from the continuous northward subduction of the Paleotethyan oceanic crust beneath the North Qinling terrane.

Metamorphic P–T path and tectonic implications of pelitic granulites from the Daqingshan Complex of the Khondalite Belt, North China Craton

Pelitic granulites are cropped out in the Daqingshan Complex of the Khondalite Belt, a Paleoproterozoic tectonic belt in the Western Block of the North China Craton. Petrological studies show that these granulites contain four distinct metamorphic assemblages. The core of a garnet porphyroblast, along with fine-grained inclusions of biotite+plagioclase+K-feldspar+quartz±sillimanite±ilmenite, defines the prograde metamorphic (M1) stage. The peak (M2) assemblage consists of garnet (mantle)+biotite+K-feldspar+plagioclase+quartz±sillimanite±orthopyroxene±ilmenite±magnetite±rutile. Peak metamorphism was followed by a near-isothermal decompression (M3) and the development of coronae of garnet+biotite+cordierite+plagioclase+quartz±K-feldspar±sillimanite±spinel±ilmenite±magnetite (M3) in the Crd–Grt–Bt gneisses during the following garnet breakdown reactions: garnet+sillimanite+melt→cordierite+biotite+Fe-oxide and garnet+melt→biotite+quartz+plagioclase. Retrograde cooling (M4) assemblages are represented by biotite+muscovite+sillimanite+quartz+plagioclase+K-feldspar±ilmenite. Quantitative phase equilibria modeling in the system Na2O–CaO–K2O–FeO–MgO–Al2O3–SiO2–H2O–TiO2–Fe2O3 was applied to obtain P–T conditions of <780°C and <9kbar for M1, 840–880°C and 9–11kbar for M2, 800–870°C and 5.0–7.5kbar for M3, and <660°C and 4.1–6.9kbar for M4. The combination of the mineral assemblages, mineral compositions, and metamorphic reaction histories in the Daqingshan pelitic granulites defines a clockwise P–T path that involves periods of near-isothermal decompression and late cooling that followed the peak granulite-facies metamorphism. This result is consistent with the tectonic history of the Daqingshan Complex in the Khondalite Belt, which involved continent–continent subduction and collision followed by exhumation and cooling. This suggests a continent–continent collisional event between the Yinshan and Ordos blocks, which became amalgamated to form the Paleoproterozoic Khondalite Belt in the Western Block of the North China Craton.

SHRIMP ZIRCON AGES AND PETROLOGY OF LOWER CRUSTAL GRANULITE XENOLITHS FROM THE LETSENG-LA-TERAE KIMBERLITE, LESOTHO: FURTHER EVIDENCE FOR A NAMAQUA-NATAL CONNECTION

Recrystallized zircons extracted from intermediate granulites of the Letseng-la-Terae Diamond Mine in northeastern Lesotho provide evidence that the Tugela Terrane of the Mesoproterozoic high-grade Namaqua-Natal mobile belt may extend ~200 km farther west than previously thought. The rock-forming minerals in the investigated lower crustal granulites contain garnet, clinopyroxene, plagioclase and quartz or garnet, wollastonite, plagioclase and quartz. Calculated values for peak granulite-facies metamorphism are ~13 kbar and ~850°C. Thermal overprint and therein granulite-facies metamorphism occurred during the late Mesoproterozoic and is documented by SHRIMP metamorphic zircon ages of 1018 ± 18, 1126 ± 10 and 1127 ± 6 Ma. These ages are in good agreement with the timing of other events in the Namaqua-Natal belt. High-grade metamorphism was probably caused by crustal thickening resulting from continent-arc collision, followed by underplating of hot mantle melts.

Contrasting metamorphic records and their implications for tectonic process in the central Sør Rondane Mountains, eastern Dronning Maud Land, East Antarctica

AbstractMetamorphic rocks in the central part of Sør Rondane Mountains, eastern Dronning Maud Land, East Antarctica, are classified into three types based on petrological characteristics. (i) The Austkampane area preserves c. 800 °C and 0.5–0.6 GPa peak metamorphic conditions followed by decompression and subsequent isobaric cooling and later hydration (A-type). (ii) The Brattnipene and eastern Menipa area preserve peak P–T conditions of c. 800 °C and 0.7–0.8 GPa with subsequent isobaric cooling and later hydration (B-type). (iii) The area including Lunckeryggen, southern Walnumfjella and western Menipa preserves an amphibolite–facies peak metamorphic condition with signatures of prograde metamorphism (L-type), which are typically unaffected by the retrograde hydration event. Peak granulite–facies metamorphism of A- and B-type rocks are contemporaneous at c. 640–600 Ma, but a difference in the P–T paths between these rocks can be explained by thrusting of the A-type rock unit onto the B-type rock unit. By contrast, the timing of the metamorphism of the L-type rocks is significantly younger at c. 550 Ma, possibly related to the intrusion of pegmatites and granitoids. These metamorphic records in the central part of the Sør Rondane Mountains can be a test ground for the regional tectonic processes proposed for the orogeny related to Gondwana formation.Supplementary material:Representative mineral compositions are listed at www.geolsoc.org.uk/SUP18623