High-pressure Mafic Granulites Research Articles

Neoarchean clockwise P-T trajectories with evolution sequence from HP to UHT metamorphism during decompression: Evidence from petrology, mineralogy, and phase equilibria modeling of mafic granulites in the Sittampundi Complex, southern India

The dispute of tectonic environments such as subduction-collision setting versus extension tectonic setting, is a common phenomenon in the Precambrian. Despite the ongoing controversy, the pressure–temperature (P–T) paths recorded by metamorphic rocks are often used as an indicator for distinguishing between subduction-collision and extension tectonic settings in the early Precambrian. It is widely acknowledged that high-pressure (HP) granulites may have formed in crustal-thickening-dominated collision/subduction environment. Conversely, ultra-high temperature (UHT) granulites may have formed in the elevated mantle heat flux environment. The Sittampundi Complex (SC), where HP granulites and UHT granulites coexist, plays a crucial role in unraveling the late-Neoarchean to early-Paleoproterozoic tectonic evolution of northern South Granulite-facies Terrane (NSGT) of India. In this study, we present new P–T paths from three HP mafic granulites overprinted by UHT granulite facies metamorphism in the SC and provide more data to unravel metamorphic evolution of NSGT. Phase equilibria modeling and traditional thermobarometry calculations illustrated that clockwise metamorphic pressure–temperature (P–T) paths were yielded, passing from 3.3 to 9.3 kbar/631–669 °C (prograde stage) through 11.9–18.5 kbar/828–986 °C (pressure-peak stage), and through 6.6–11.1 kbar/1019–1085 °C (temperature-peak stage) to 4.4–8.1 kbar/845–930 °C (retrograde stage). The HP mafic granulites exhibit clockwise P–T paths with a considerable heating process (ca. 150–180 °C) from the pressure-peak stage (HP-granulite sub-facies) to the temperature-peak stage (UHT-granulite sub-facies). Zircon U-Pb dating yields an age of 2513 ± 29 indicates that the metamorphism took place in the Neoarchean. The uncommon P–T paths from Neoarchean HP granulite in the NSGT suggest that during late Neoarchean, the NSGT had experienced the crustal-thickening dominated collision/subduction (assembly of micro-blocks of Dharwar Craton) and subsequently mantle diapirism process.

Considerable heating during clockwise decompression of a long-lived Paleoproterozoic hot orogen: Evidence from the Xuanhua Complex, North China Craton

Granulites comprise the root of collisional orogens, and decoding their evolution may provide key information on orogenic processes from collisional thickening to post-collision extensional thinning. However, the records of collisional orogeny in most cases have been obscured or even erased by later high-/ultrahigh-temperature (HT/UHT) overprinting. Here, we report new petrological, geochemical and geochronological data for granulite-facies rocks in the Xuanhua Complex of the northern Trans-North China Orogen (TNCO). We reveal the complex metamorphic features and tectonic evolution of a long-lived Paleoproterozoic hot orogen as well as their link with Phanerozoic orogens. The rocks are composed mainly of garnet + clinopyroxene + orthopyroxene ± amphibole + plagioclase + quartz + rutile + ilmenite with symplectitic intergrowths of clinopyroxene + orthopyroxene ± amphibole + plagioclase ± quartz. Pseudosection modeling and thermobarometric calculations suggest that the high-pressure (HP) mafic granulites record clockwise pressure–temperature (P-T) paths, from the prograde stage (M1) to the pressure-peak stage (M2; 13.5–15.0 kbar and 850–875 °C), followed by considerable heating during decompression to the temperature-peak stage (M3a; 8.0–9.6 kbar and 910–950 °C) and final cooling to the M3b stage. Magmatic zircon U-Pb dating reveals c. 2.1–2.0 Ga protolith ages for the granulites. Metamorphic zircon and titanite U-Pb results, and trace elements in zircons indicate that the age of c. 1.92 Ga is possibly close to the lower limit of HP granulite-facies pressure-peak, and c. 1.84–1.81 Ga ages are related to the timing of the final cooling stage. Hence, by comparison with existing geological data, the synthesized metamorphic P-T-t data of the Xuanhua granulites indicate two stages of tectonic evolution, including: (1) crustal thickening by continental collision between the Western and Eastern Blocks at c. 1.95–1.92 Ga, resulting in an early HP granulite-facies metamorphism at relatively low thermal gradients of 16–19 °C/km; and (2) the collision-thickened orogenic lithosphere was thinned controlled by continental extension and gravitational instability, which might provide additional heat in the interval c. 1.90–1.84 Ga, resulting in a later UHT overprinting at reduced pressures and higher gradients of 30–36 °C/km. In addition, the post-collisional extension accompanying exhumation decompression shows a long-lived timescale of > 80 Myr, corresponding roughly to a slow exhumation rate of < 0.3 mm/yr. To sum up, these features of long-lived tectono-thermal evolution (tectonic transition from crustal thickening to lithospheric thinning, and prolonged orogeny and exhumation), slow exhumation rates, and the occurrence of UHT overprinting, are becoming more commonly identified in Precambrian hot orogens like the TNCO.

Metamorphic evolution and tectonic significance of Neoarchean high-pressure mafic granulites in the Central Limpopo Belt, South Africa

ABSTRACT The Sand River area is in the northeastern part of the Central Zone (CZ) of the Limpopo Belt, and represents a key terrane to reveal early Precambrian tectonic processes in southern Africa. Based on petrology, U–Pb zircon ages, and major and trace element data of high-pressure (HP) mafic granulites from the Sand River area, the tectonic evolution of the Central Zone of the Limpopo Belt is re-evaluated here that allows revision of previous interpretations. The protoliths of the HP mafic granulites and amphibolites are sub-alkaline tholeiitic basalts similar to those formed at a mid-ocean ridge and intruded at 2.8 Ga. Based on the distinct mineral assemblages, three metamorphic stages (M1–M3) are recognized in the studied samples: (1) a peak HP granulite facies stage M1 (~1.17–1.32 GPa/~825–870°C), (2) a post-peak near-isothermal decompression stage M2 (~0.70–0.73 GPa/~835–880°C), and (3) a later, independent metamorphic stage M3 (~0.53–0.69 GPa/~793–801°C). M1 and M2 define together a clockwise P–T path. It reveals important information related to the Neoarchean collisional orogeny, and its later overprinted by an independent orogeny with subsequent exhumation and cooling. U–Pb zircon dating of two mafic granulites samples yielded protolith formation of the mafic granulites is prior to ~2.72 Ga, and the two metamorphic age peaks of ~2.72 (M1) and ~2.0 Ga (M3), which present the peak and post-peak cooling stages, respectively. The metamorphic evolution of HP mafic granulites in the Sand River area as reconstructed in this study suggests the Neoarchean collision of the Kaapvaal and Zimbabwe cratons and a discrete Paleoproterozoic orogenic overprint followed by post-orogenic exhumation and cooling.

Origin of clinopyroxene-ilmenite symplectites in mafic granulites from eastern parts of the Chotanagpur granite gneissic complex, East Indian shield

Abstract In this manuscript, we report a rare occurrence of rod-like intergrowths of clinopyroxene-ilmenite that variably replace titanite in a suite of high-pressure mafic granulites from the Chotanagpur Granite Gneissic Complex, Eastern Indian Shield. Garnet proximal to the clinopyroxene-ilmenite intergrowth is invariably replaced with symplectic clinopyroxene-plagioclase or a rind of plagioclase. Textural modeling of the reaction textures and mineral compositions suggests that the garnet-titanite became unstable and the ilmenite-clinopyroxene preferentially develop after titanite. The presence and texture of halogen rich apatites within the ilmenite-clinopyroxene symplectite are consistent with a fluid mediated process. Thermodynamic modeling in the NCFMAST (+H2O) system, demonstrates that the clinopyroxene-ilmenite symplectite was formed along a steeply decompressive retrograde P-T path (from ~13 to ~7 kbar, at ~800 °C), in the presence of partial melts. The study demonstrates that relative to Fe, Mg, and Ca, Ti was less mobile and that the mobility of Ti was restricted within the confines of titanite being replaced by the clinopyroxene-ilmenite symplectite. This study implies that besides the ligand activity, the fluid/rock ratio exerts a strong control on the length scale of Ti transport in natural rocks.

Geochemistry, geochronology and metamorphism of high-pressure mafic granulites in the Huai'an Complex, North China Craton: Implications for the tectonic evolution of the Paleoproterozoic orogeny

The Paleoproterozoic was a critical tectonic transition stage for the North China Craton (NCC), with several collisional orogenic belts identified. However, the tectonic boundaries, temporal-spatial evolution, and geodynamic mechanisms of these orogens remain controversial. In this contribution, we document a newly identified largest high-pressure (HP) mafic granulite outcrop at Xizhaojiayao in the Huai'an Complex, which is composed of metamorphosed supracrustal (volcano-sedimentary) sequences, including HP mafic granulites (metabasalts), marbles (metalimestones), quartzites (metasilicalites), and pelitic granulites (metapelites). They occur as tectonic slices within Neoarchean migmatized tonalitic gneisses. The Xizhaojiayao HP mafic granulites show different field occurrences and rock assemblages from the dike-type HP mafic granulites that are widely documented in the Huai'an and Hengshan Complexes. Microstructural studies, geothermobarometer calculations, and phase equilibrium modeling of the HP mafic granulites defined a clockwise pressure–temperature (P–T) path with prograde evolution from amphibolite-facies (M1, 5.2–5.5 kbar/538–668 °C) to HP granulite-facies (M2, 11.2–12.2 kbar/850–880 °C), followed by a significant near-isothermal decompression to medium-low-pressure granulite-facies (M3-1, 8.4–9.1 kbar/800–900 °C; M3-2, 4.9–5.6 kbar/779–805 °C). Zircon/titanite U–Pb dating constrains peak HP metamorphism, granulite-facies decompression, and amphibolite-facies cooling retrograde at 1902–1909 Ma, ∼1830 Ma, and < 1818 Ma, respectively. Whole-rock major and trace-element compositions, as well as positive εNd(t) (+2.0 to + 4.9) and zircon εHf(t) values (+2.9 to + 8.8) of these HP mafic granulites, indicate a mid-ocean ridge basalt geochemical affinity, whereas an island-arc signature is common for the dike-type HP mafic granulites. The Xizhaojiayao supracrustal sequences, which represent dismembered and metamorphosed upper oceanic crustal fragments, were subducted from the upper crust to depths of 35–40 km, followed by rapid exhumation to the middle-shallow crust and were tectonically involved in the melt-weakened tonalitic crust. These tectono-metamorphic scenarios illustrate the Paleoproterozoic orogenic evolution of a consecutive subduction-collision-exhumation process from 1.95 to 1.80 Ga, which led to the final assembly of the NCC.

Diverse metavolcanic sequences in the Cambrian accretionary complex at the Pamir Syntax: Implications for tectonic evolution from Proto-Tethys to Paleo-Tethys

The Pamir Plateau contains critical information on the tectonic evolution from the Proto- to Paleo-Tethys. In this contribution, we report geochronology, geochemistry and Sr-Nd isotopic compositions of diverse metavolcanic rocks in the Bulunkuole Group (BG) in the NE Pamir, to construct the architecture of the thick Cambrian volcanic-sedimentary sequence in the NE Pamir and to provide new insights into the tectonic evolution from Proto-Tethys to Paleo-Tethys. Our newly obtained zircon U-Pb ages combined with ages from literature reveal that these volcanic-sedimentary sequences were deposited during the Cambrian and metamorphosed during the Early Jurassic. The bimodal volcanic rocks (ca. 530 Ma) at the lowest BG show typical arc-like geochemical features, representing the initial arc produced by the southward subduction of the Proto-Tethys Ocean. Amphibolites (ca. 526 Ma) from the Maeryang area show an OIB-like affinity with high Nb/La ratios (0.75–1.01) and positive εNd(t) values (4.19–4.76), representing a seamount component in the Proto-Tethys Ocean. High-pressure mafic granulites to the north of Tashkorgan County exhibit E-MORB geochemical features. Combined with recent advances in the NE Pamir geology, we suggest that the southward subduction of the Proto-Tethys slab under the Mazar-Tianshuihai terrane initiated earlier than 530 Ma. Subsequent trench retreat led to the formation of the massive early Paleozoic accretionary wedge (BG) followed by the northward migration of arc magmatism to the BG. The Proto-Tethys Ocean at Pamir did not close in the early Paleozoic. Instead, it was finally closed during the early Mesozoic as revealed by the Early Jurassic amphibolite-facies metamorphism in the BG.

Juxtaposition of different-grade metamorphic rocks in an ancient orogen: Evidence from the Chengde Complex of the Trans-North China Orogen, North China Craton

Juxtaposition of different-grade metamorphic slices is a typical feature of Phanerozoic orogens but is relatively scarce in Precambrian orogens. Here, we focus on the Chengde Complex at the northern segment of the late Paleoproterozoic Trans-North China Orogen, the North China Craton as a case, to explore how different-grade orogeny-related metamorphism was juxtaposed in Precambrian orogens. The Trans-North China Orogen is a typical Precambrian collisional orogen and records abundant information about the Paleoproterozoic orogeny. High-pressure (HP) mafic granulite and two types of amphibolite samples were collected from the Chengde Complex, and we conducted a combined study involving litho-structural assemblage investigation, metamorphic petrology, and geochronology to decipher their metamorphic P-T−t history. HP mafic granulite and amphibolite occur as enclaves or dikes within felsic gneisses. HP mafic granulite records clockwise P-T paths with isothermal decompression (ITD) segments, and the peak metamorphic P-T conditions are 13.0−14.9 kbar/790−830 °C. Peak metamorphic P-T conditions retrieved from garnet-bearing amphibolite are 8.3 kbar/675 °C, whereas peak metamorphic P-T conditions retrieved from garnet-free amphibolite are 4.0−5.5 kbar/500−510 °C. Zircon U-Pb dating of HP granulite and amphibolite yield different prograde (1914−1871 Ma), peak (1869−1816 Ma), and retrograde (ca. 1830 Ma) metamorphic ages, and all of these data indicate that these metamorphic rocks were diachronously transferred to different depths in the subduction channel and subsequently exhumed at shallower crustal levels. Therefore, we conclude that the Chengde Complex is composed of imbricate rocks with different metamorphic grades and ages, and such an unordered juxtaposition of diverse metamorphism could also be a typical feature of Precambrian orogens.

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.

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.

Phase equilibria modelling and zircon U-Pb ages of the Paleoproterozoic high-pressure mafic granulites in the Jianping Complex and tectonic implications

The late Paleoproterozoic metamorphic evolution of the North China Craton (NCC) has been a controversial issue, and bears significant implications for the final cratonization of the NCC. The Jianping Complex (JPC) is located at the boundary between the Paleoproterozoic Trans-North China Orogen and the Eastern Block, and thus is an ideal site for studying the tectonic evolution of the NCC. Petrographic study reveals that mafic granulites exposed along the western margin of the JPC are characterized by the peak stage mineral assemblage of garnet + plagioclase + clinopyroxene + amphibole + ilmenite + quartz and post-peak growth of orthopyroxene/amphibole + plagioclase + magnetite + quartz around peak-stage garnet. Two representative mafic granulite samples were selected for phase equilibria modelling using THERMOCALC 3.45. The modelling results, combined with petrographic observations, indicate that these mafic granulites recorded clockwise P-T paths involving near isothermal decompression (ITD) process or decompression with thermal relaxation. Isopleths of Ti content in amphibole and XAn in plagioclase define the peak metamorphic P-T condition as 13.0–13.5 kbar/850–900 °C. The post-peak final solidus assemblages of samples 19JP7 and 19JP32 were stable at the conditions of 7.5–8.0 kbar/750–800 °C and 10.5–11.5 kbar/880–920 °C, respectively. Metamorphic zircons from sample 19JP7 yield a metamorphic age of ∼1.82 Ga, while zircons from sample 19JP32 yield magmatic age of ∼2.1 Ga and metamorphic age of ∼1.85 Ga, representing the timing of emplacement and high-pressure metamorphism. Based on the metamorphic study and zircon U-Pb ages, we conclude that the Paleoproterozoic high-pressure metamorphism of the JPC resulted from the collision of the Eastern and Western Blocks along the Trans-North China Orogen at ∼1.85 Ga.

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.

Validation of clinopyroxene-garnet magnesium isotope geothermometer to constrain the peak metamorphic temperature in ultrahigh-temperature ultramafic-mafic granulites

Abstract Conventional Fe-Mg exchange geothermometers generally are not effective means to measure the peak metamorphic temperatures of granulites because of Fe-Mg diffusion during the re-equilibration during the exhumation and cooling. Therefore, it is essential to find alternative geothermometers. In this study, we investigated the magnitude of Mg isotope fractionation between the coexisting clinopyroxene and garnet in garnet pyroxenites and high-pressure mafic granulites from southern India. The clinopyroxene and garnet from the garnet pyroxenites have δ26Mg values of −0.04 to −0.07‰, and −0.65 to −0.64‰, respectively, with Δ26MgCpx-Grt = δ26MgCpx − δ26MgGrt = 0.62 and 0.57‰. The δ26Mg values of the coexisting clinopyroxene and garnet in the high-pressure mafic granulites are 0.03 to 0.07‰ and −0.54 to −0.55‰, respectively, with Δ26MgCpx-Grt = 0.57 and 0.62‰. The inter-mineral Mg isotope fractionations between the clinopyroxene and garnet of the garnet pyroxenites are similar to those of the high-pressure mafic granulites and more or less fall within the equilibrium fractionation lines at peak metamorphic temperatures. The measured peak temperatures for all four samples indicate equilibrium Mg isotope fractionations between the clinopyroxene and garnet. The clinopyroxene-garnet Mg isotope geothermometer of Li et al. (2016) yields temperatures of 994 ± 60 °C and 1048 ± 89 °C for the garnet pyroxenites, and 1048 ± 89 °C and 994 ± 65 °C for the high-pressure mafic granulites, which are slightly lower than those at both 10 and 12 kbar from the clinopyroxene-garnet Mg isotope thermometer of Huang et al. (2013). Compared with the peak metamorphic conditions from phase equilibrium modeling, the clinopyroxene-garnet Mg isotope thermometry yielded temperatures corresponding to the peak metamorphic conditions of the garnet pyroxenites and the high-pressure mafic granulites, whereas the conventional clinopyroxene-garnet Fe-Mg exchange thermometry yielded lower temperatures corresponding to the retrograde metamorphism. These results underscore the inter-mineral Fe-Mg exchange between clinopyroxene and garnet during the retrograde cooling of the ultrahigh temperature (&amp;gt;900 °C), which, however, did not disturb their Mg isotope fractionation equilibrium attained during the ultrahigh-temperature metamorphic condition. Therefore, we conclude that clinopyroxene-garnet Mg isotope thermometry, together with phase equilibrium modeling, is a valid tool to constrain the peak metamorphic temperature conditions even for ultramafic-mafic granulites that have undergone ultrahigh-temperature metamorphism.

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.

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

Garnet perspectives on the metamorphic history and tectonic significance of Paleoproterozoic high-pressure mafic granulites from the northern Hengshan, North China Craton

Reconstructing the pressure-temperature (P-T) evolution of early Precambrian high-pressure granulites and eclogites is critical for decoding the tectono-metamorphic evolution of the early Earth. However, debates continue regarding the P-T history of these rocks because of various modifications by peak and retrograde rehydration metamorphism. As a significant rock-forming mineral in high-grade metamorphic rocks, garnet not only grows with increasing pressure or temperature and develops variably zoned patterns along given P-T loops, but its properties are stable subsquent to its formation when compared to other minerals in the matrix. Consequently, garnet is excellent for monitoring the thermo-metamorphic history of its host rock during orogeny. Selected garnet grains from the Paleoproterozoic high-pressure mafic granulites in the northern Hengshan area of the North China Craton, which exhibit variable microstructural and chemical zoning, present an excellent example for extracting metamorphic evolution of the host rocks. Microstructural zoning displays a zonal distribution of frozen inclusion-type minerals in garnet. Chemically, the garnet shows pronounced compositional zoning, with bell-shaped XSps, initially flat and then increasing/decreasing XPyr and Fe#, initially increasing and then decreasing XGrs from core to mantle, and resorbed rims. Both zoning patterns document multiple generations of radial growth history. The progressive evolution of the host rocks can be further divided into early prograde (M1–1), late prograde (M1–2), near peak (M2–1), and peak (M2–2) stages. The resorption textures indicate the succeeding decompression (M3) and cooling (M4) stages.The pseudosection modeling indicates P-T conditions from M1–1 to M2–1 of 6–9 kbar/600–700 °C, 9–11.2 kbar/680–740 °C and 12–15 kbar/735–785 °C, respectively. Although the peak metamorphic information of M2–2 is lost because of the breakdown of the garnet rim, plausible P–T results are estimated at ~15–17 kbar/~790–810 °C. These conditions prevailed to the transitional eclogite-granulite facies through qualitative reconstruction of the original composition of clinopyroxene, modal proportion, and rim composition of the garnet. The subsequent conditions of decompression (M3) and cooling (M4) stages were roughly estimated at ~7–10 kbar/~800–850 °C and 3–5 kbar/ 650–680 °C. Together, they define a clockwise P-T path in response to the Paleoproterozoic orogeny involved in the assembly of the North China Craton. The zoned garnet suggests that the host mafic granulite documents a relatively complete progressive metamorphic history, which probably correlates with subduction processes. The resorbed rim sequentially records exhumation and cooling processes. Furthermore, the preservation of growth zoning in granulitic garnet and near-isothermal decompression P-T path suggests that the host rocks must have experienced a very short peak metamorphism and a rapid exhumation process to the middle crust. These findings suggest that the tectonic processes involved in the formation of the Paleoproterozoic northern Hengshan granulite terrane might be similar to those of the Phanerozoic continental collisional orogens under modern plate tectonic regimes; however, they record relatively higher apparent thermal gradients (~14–16 °C /km).

Metamorphic P-T-t evolution of amphibolite in the north Hengshan terrane, North China Craton: Insights into the late Paleoproterozoic tectonic processes from initial collision to final exhumation

AbstractAmphibolite retrograded from high-pressure (HP) mafic granulite can provide valuable insight into exhumation of deeply buried crust in orogenic belts. In the north Hengshan terrane of the North China Craton, amphibolite occurs as rims of HP mafic granulite block or as smaller homogeneous boudins representing retrograde products of the granulite. Three amphibolite samples were selected. The rocks are mainly composed of hornblende + plagioclase + quartz + biotite + ilmenite with or without garnet pseudomorph consisting of plagioclase + hornblende + ilmenite and symplectite of hornblende + plagioclase ± clinopyroxene. The pseudomorph-, symplectite-bearing sample experienced a post-peak isothermal decompression at &amp;gt;800 °C that was accompanied by breakdown of garnet and clinopyroxene. Isopleths of the maximum An in plagioclase and Ti in hornblende were used to constrain the Tmax stage of ca. 6 kbar/825–850 °C, which was followed by cooling and post-cooling decompression. For the sample showing an “equilibrated” mineral assemblage, a medium-temperature decompression from &amp;gt;6.8 kbar/685 °C to 3.6–4.8 kbar/640–660 °C was inferred. P-T evolution of the north Hengshan terrane is characterized by two discrete (post-peak and post-cooling) decompression processes. Zircon U-Pb dating of amphibolite yields a metamorphic age of 1868 ± 15 Ma, which is interpreted to record the timing of late amphibolization. Synthesized metamorphic P-T-t data in the Hengshan-Wutai area indicate a complicated tectonic evolution that includes an older collisional orogeny at ca. 1.95 Ga and a younger metamorphism at ca. 1.85 Ga. The post-cooling decompression path of the amphibolite may reflect final exhumation of the north Hengshan terrane through the late deformation/metamorphism of the Zhujiafang shear zone.

Geochronology, geochemistry, origin, and tectonic implications of high‐pressure mafic granulites of the Amdo region, Central Tibet

The Amdo Terrane, Central Tibet, was previously located in the middle of the Bangong–Nujiang Ocean (BNO), and is key to understanding the subduction history of the BNO. However, the high‐pressure (HP) mafic granulites of the Amdo Terrane remain poorly studied. Here, we report the petrology, geochemistry, and zircon U–Pb geochronology of the Amdo HP mafic granulites. The granulites occur mainly as enclaves or deformed dikes in granitic gneisses of the Amdo Terrane. Zircons from the granulites yield two clusters of ages, at 193–192 and 180 Ma. The former represents the best estimate of the age of peak metamorphism, and the latter represents the age of retrograde amphibolite‐facies metamorphism. Although the contents of mobile elements in the granulites, including the large‐ion lithophile elements (Rb, Sr, Cs, Pb, K), were significantly altered during metamorphism, the contents of the high‐field‐strength elements (HFSEs; Th, Nb, Zr, Ti) and rare earth elements (REEs) remained largely unchanged. The granulites belong to the tholeiitic basalt series, with SiO2 contents of 47.05–51.03% and Mg# of 35–50. The REE contents of the granulites are characterised by flat chondrite‐normalised patterns (LaN/YbN = 0.96–1.92; LaN/SmN = 1.0–1.59) and weak Eu anomalies (Eu/Eu* = 0.84–0.99). Most of the granulite samples have high TiO2 contents, clear negative Nb and Ta anomalies, and high Pb enrichment, with positive zircon εHf(t) values (+8.14 to +16.02). The granulites exhibit both normal mid‐ocean ridge basalt (N‐MORB) and island‐arc basalt (IAB) characteristics. Using the data presented here and previous data, we suggest that the protolith of the granulites were back‐arc basin basalts derived from high‐degree partial melting of depleted mantle in the spinel stability field, which records the early evolution of the BNO. Northward subduction of the BNO oceanic plate led to back‐arc spreading and the formation of back‐arc basin basalts on the northern margin of the Amdo Terrane. The back‐arc basin extended further to form an ocean basin, which separated the Amdo Terrane and the Southern Qiangtang–Baoshan (SQB) Terrane, resulting in the Amdo Terrane becoming an isolated microcontinent in the BNO.

Discovery of kyanite in typically cordierite/sillimanite-bearing low- to medium-pressure pelitic granulites from the Jiaobei terrain, North China Craton

The pelitic granulites along the Paleoproterozoic Jiao-Liao-Ji belt (JLJB) are dominated by cordierite/sillimanite-bearing low- to medium-pressure (LMP) conditions. Recent studies have recognized a few typical high-pressure (HP) pelitic granulites without retrograde cordierite from the Jiaobei terrane, the southern part of the JLJB. The relationship between the dominant LMP and rare HP pelitic granulites is not well constrained, and whether the LMP types are retrograde products of the HP ones remains uncertain. This uncertainty precludes a full understanding of the original spatial distribution of HP pelitic granulites and the tectonic affinity of the JLJB. We identify in this study for the first time, relict kyanite and the mineral assemblage of kyanite + K-feldspar + rutile + garnet from typically cordierite/sillimanite-bearing LMP pelitic granulites within the Jiaobei terrane. Integrated results from petrology, pseudosection modeling, geothermobarometry, and zircon and monazite U-Pb geochronology suggest that these LMP pelitic granulites experienced an earlier HP granulite-facies metamorphism possibly at ~1.95 Ga, with minimum P-T conditions of 13–14 kbar and 860–870 °C. Subsequently, these rocks underwent near isothermal decompression to 4–7 kbar and >840 °C, followed by isobaric cooling to ~600 °C and 4–6 kbar. The decompression and cooling triggered LMP granulite-facies retrogression at ~1.85 Ga that eclipsed the earlier HP granulite-facies records. The clockwise P-T-t path defined is consistent with those reported from typical HP pelitic and mafic granulites elsewhere in the Jiaobei terrane. The results confirm that at least some portions of the typically cordierite/sillimanite-bearing LMP pelitic granulites are retrograded from the HP types. This greatly expands the spatial distribution of HP pelitic granulites and suggests a linear HP granulite-facies metamorphic belt within the Jiaobei terrane. The linear distribution of HP pelitic granulites whose protoliths are sedimentary rocks from Earth’s surface and the consistent clockwise P-T-t paths require Paleoproterozoic subduction, collision, and exhumation processes similar to those operating in modern plate tectonics. Our results from the JLJB thus provide new metamorphic constraints on the nature of Paleoproterozoic plate tectonics, and have implications for other Precambrian high-grade terranes worldwide where pelitic granulites are dominated by LMP types.

Diverse subduction and exhumation of tectono-metamorphic slices in the Kalatashitage area, western Paleozoic Dunhuang Orogenic Belt, northwestern China

High-pressure mafic granulite and garnet amphibolite are identified as small-scale tectonic slices within pelitic or semi-pelitic gneiss in the Kalatashitage area, which is located in the western Paleozoic Dunhuang Orogenic Belt, northwestern China. These rocks retain three generations of metamorphic mineral assemblages: prograde assemblage (M1) preserved as inclusions within garnet porphyroblasts, metamorphic peak assemblage (M2) consisting of matrix minerals and garnet porphyroblasts, and retrograde assemblage (M3) mainly represented by the symplectic minerals surrounding the embayed garnet and the retrograded hornblende rimming matrix-type clinopyroxene. Metamorphic pressure and temperature (P-T) paths of high-pressure mafic granulite, amphibolite, and metapelite retrieved by thermobarometry are all clockwise, passing from 640 to 720 °C/6.2–12.6 kbar (M1) through 840–920 °C/14.6–16.2 kbar (M2) to 750–815 °C/5.5–7.9 kbar (M3) for high-pressure mafic granulite, from ~650 °C/5.7 kbar (M1) through ~750 °C/9.2 kbar (M2) to ~780 °C/8.1 kbar (M3) for amphibolite, and from ~615 °C/7.9 kbar (M1) through 730–820 °C/8.6–11.7 kbar (M2) to 675–740 °C/5.4–8.7 kbar (M3) for pelitic and semi-pelitic gneiss. Furthermore, pseudosection modeling of high-pressure mafic granulite indicates that the growth zonation of garnet porphyroblast exhibits prograde metamorphism in a P-T range of 510–800 °C/8.5–13 kbar and demonstrates peak metamorphic P-T conditions of ~850 °C/16 kbar, which are consistent with the thermobarometric estimates. The significant pressure differences in peak metamorphism observed in different rocks indicate that the rocks initially subducted to remarkably different depths and were subsequently juxtaposed at shallower crustal levels during exhumation. Sensitive high-resolution ion microprobe (SHRIMP) analysis and laser ablation inductively coupled plasma mass spectrometry (LA-ICP-MS) UPb dating of metamorphic zircon indicates that the metamorphic events occurred at ca. 430–420 Ma (M2) and ca. 400–390 Ma (M3), respectively. Metamorphism was followed by the intrusion of granitic dykes at ca. 244 Ma. Moreover, the metamorphic evolution indicates that the Kalatashitage area was involved in the subduction, collision and subsequent tectonic exhumation in the Paleozoic. Combined with previous literature, it is inferred that the discrepant subduction and exhumation of high-grade metamorphic rocks is a universal phenomenon in the Paleozoic Dunhuang Orogenic Belt, supporting the ubiquitous existence of subduction-collision complexes in this orogenic belt.