Subduction-collision Zone Research Articles

U–Pb Age and Mineral Inclusions in Zircon from Diamondiferous Garnet–Pyroxene Rocks of the Kumdy-Kol Microdiamond Deposit in Northern Kazakhstan

Abstract ––This paper describes the results of studying inclusions and determining the U–Pb age in zircon from the diamondiferous garnet–pyroxene rock of the Kumdy-Kol metamorphogenic diamond deposit, located in the Kokchetav subduction-collision zone. The distribution of rare earth elements in garnet and clinopyroxene is used as a basis for estimating the equilibrium pressure (5.5 ± 0.3 GPa) and temperature (993 ± 24 °C), which correspond to the diamond stability field. The composition of mineral inclusions in zircon indicates its formation at both the progressive and the regressive stage of metamorphism. The concordia diagram shows that the figurative points of zircon lie on a discordia with an upper intersection at 1953 ± 139 Ma and a lower intersection at 512 ± 4 Ma. The main peak on the graph of the probability density distribution of zircon ages corresponds to an age of 519 Ma. The presence of grossular–almandine garnet inclusions in zircon confirms the previous assumption that the basement rocks of the Kokchetav massif act as protoliths of garnet–pyroxene rocks.

Rejuvenation of the lithospheric mantle beneath the orogens: Constraints from elemental geochemistry and Os isotopes in mantle xenoliths

Continental orogens that involve subduction and collision are important target areas for studying the growth, stabilization and transformation of Earth’s continents. In particular, the nature of the lithospheric mantle and its relationship to the overlying crust in these active subduction-collision zones are key factors to be constrained. Mantle xenoliths entrained by Cenozoic basalts from the Indochina Block, the Sukhothai Arc and the Inthanon Zone (IB-SA-IZ) provide direct deep-seated samples of the lithospheric mantle derived from part of the Tethyan orogenic belt, offering a unique opportunity to study mantle processes during orogenesis in response to the closure of the Paleo-Tethys Ocean. The studied mantle xenoliths comprise 32 spinel lherzolites, six harzburgites, and one olivine websterite. We present a comprehensive study of the elemental geochemistry and Os isotopes of this sample suite. Their lithophile element systematics indicate that they represent mantle residues after varying degrees of melt extraction (5–24%), followed by melt refertilization. Although the highly siderophile element abundances have been affected to some extent by melt refertilization through removal of sulfides, the Os isotopic compositions of the studied xenoliths were minimally disturbed. Lherzolites and the sole olivine websterite from the IB-SA-IZ share similar Re-Os isotope systematics (187Os/188Os: 0.121–0.137) to global abyssal peridotites, representing juvenile lithospheric mantle accreted from the upwelling asthenosphere, most likely associated with arc-continent collision in response to the closure of the Paleo-Tethys Ocean and back-arc basin during the Middle-Late Triassic. In contrast, the refractory harzburgites from the Indochina Block and Sukhothai Arc are characterized by substantially lower 187Os/188Os ratios (187Os/188Os: 0.117–0.122) than the lherzolites, yielding Mesoproterozoic Re-depletion Os model ages, which can be interpreted as either ancient lithosphere relics beneath these regions or accreted ancient mantle debris from the upwelling asthenosphere. Although the interpretation for the origin of the older harzburgites is ambiguous, we can conclude that rejuvenation of the lithospheric mantle commonly occurred beneath different tectonic units of the Tethyan orogenic belt: i.e., the pre-existing lithospheric mantle beneath different tectonic units of the Tethyan orogenic belt has been largely replaced by upwelling asthenospheric mantle during the Middle-Late Triassic orogenesis. This mantle was then further refertilized to varying degrees by asthenosphere-derived melts. Both processes play key roles in defining the nature of the lithospheric mantle in orogenic settings. This study also highlights that the geodynamic processes in the lithospheric mantle beneath orogenic belts can be well constrained by integrated evidence from lithophile and highly siderophile elements and their isotopes (e.g., Os isotopes) of mantle xenoliths.

Thermobaric Conditions for Exhumation of Ti-clinohumite Garnetites of the Kokchetav Subduction-Collision Zone (Northern Kazakhstan)

Abstract —A new variety of Ti-clinohumite garnetites (‘garnet peridotites’) of the diamond-bearing Kumdy Kol terrane in the Kokchetav massif has been studied. An ultrahigh-pressure phase, Ti-chondrodite, orthopyroxene and contrast-zoned garnets of retrograde zoning have been found in these rocks for the first time. Ti-chondrodite and orthopyroxene were not described in these rocks earlier, and garnet composition was supposed to be homogeneous. Based on the character of chemical zoning of garnet and orthopyroxene, the peak and three retrograde parageneses for these minerals have been distinguished. Using the garnet–orthopyroxene geothermobarometry, for the Kumdy Kol terrane, based on the quantitative P–T estimates of Ti-clinohumite garnetites, a detailed retrograde P–T trend has been constructed for the first time; this trend includes the UHP peak stage (55–57 kbar, 1075–1120 °С), and three stages of retrograde metamorphism under UHP (32 kbar, 830 °С), HP (21 kbar, 750 °С), and MP (12 kbar, 740 °С) conditions. For the Kumdy Kol terrane this is the most detailed Р–Т trend indicating subisothermal decompression during crustal rock exhumation from the mantle depths (~170 km) to the lower-crust conditions (~36 km). The above-mentioned mineralogical findings allowed reconstructing all the described metamorphism stages, using the same rock and various generations of the same minerals (garnet and orthopyroxene) formed at different stages of tectono–metamorphic evolution of the Kumdy Kol terrane. Preservation of the relics for mineral zoning of the early metamorphic stages seemed to be due to high uplift velocities resulted in rapid decrease in P–T parameters and short duration of regressive metamorphism. The virtually complete identity of the obtained P–T trend for the Kumdy Kol terrane and the P–T trend for the Barchi terrane supports the idea of the common tectono–metamorphic evolution of the whole western (diamond-bearing) part of the Kokchetav massif.

Low-grade metamorphic rocks of the Tethys subduction–collision zone in the Medvednica Mt. (NW Croatia)

Low-grade metamorphic rocks of the Tethys subduction–collision zone in the Medvednica Mt. (NW Croatia)

Compositional and metamorphic controls on tectonic erosion along a continental subduction-collision zone: Implications from mafic granulites in the northern Sulu orogen

Abstract Subduction erosion has been reported recently in continental subduction-collision zones, and the response of the upper plate remains enigmatic. The Dabie-Sulu orogen is considered to have formed by deep northward subduction of the Yangtze block (lower plate) beneath the North China block (upper plate). However, within the northern Sulu ultrahigh-pressure (UHP) belt, the Haiyangsuo complex has intriguingly been accepted as Neoarchean–Paleoproterozoic metamorphic basement from the North China block, implying subduction-erosion could have occurred. Here, we examined new petrographic, mineral chemical, and published geochronological data from the Haiyangsuo mafic granulites to decipher their multiphase metamorphic evolution. Pressure-temperature (P-T) estimates from pseudosection modeling and geothermobarometry show two major metamorphic events: a medium-pressure granulite-facies event (7.3–8.3 kbar and 830–895 °C) and a later high-pressure granulite-facies event (12.2–16.6 kbar and 800–875 °C). Geochronology and mineral geochemistry link these events to Paleoproterozoic and Triassic metamorphism, respectively. Peak Triassic high-pressure granulite-facies metamorphism was followed by near-isothermal decompression and then near-isobaric cooling. These rocks derive from the upper plate and were metamorphosed at the same time as UHP eclogites, but maximum pressures were substantially lower, and P-T paths did not converge until late-stage exhumation. Evidently, metamorphic basement from the North China block was dragged to lowercrustal to upper-mantle depths (50–60 km) while that from the Yangtze block was subducted deeply, and then they were tectonically juxtaposed during exhumation. These data corroborate thermal-mechanical models that predict tectonic erosion as a major process during continental subduction and collision.

Magmatic and metamorphic evolution of a layered gabbro-anorthosite complex from the Coorg Block, southern India: Implications for Mesoarchean suprasubduction zone process

As one of the oldest crustal blocks in Southern Peninsular India, the Coorg Block has been the focus of investigations related to crustal evolution in the early history of the Earth with implications on Mesoarchean plate tectonic processes. The Coorg Block is dominantly composed of arc magmatic rocks and bordered on the north by the Mercara Suture Zone, a Mesoproterozoic subduction-collision zone hosting extruded high-pressure and ultra-high temperature metapelitic and meta-mafic rocks. Here, we report the occurrence of a dismembered gabbro-anorthosite complex corresponding to a layered intrusion from the northern margin of the Coorg Block. We present results from an integrated study on the petrology, mineral chemistry, P-T phase equilibria, whole-rock geochemistry, and zircon and monazite U–Pb geochronology to understand the magmatic and metamorphic evolution of the layered intrusion and associated rocks. The geochemical data suggest that the parental magma was generated within a suprasubduction zone setting. Phase equilibrium modelling of garnet-bearing metagabbro from the Coorg Block suggests metamorphic P–T range of 10.5–11 kbar at 1000–1100 °C corresponding to ultra-high temperature conditions. The zircon U-Pb data from the various rock types of the mafic–ultramafic suite yield weighted mean ages at 3176 Ma, 3174 Ma, 3143 Ma, and 3124 Ma whereas the associated charnockite shows a slightly older age of 3319 ± 12 Ma. Metamorphic zircon from charnockite yield an age of 3101 Ma, close to the monazite age of 3110 ± 24 Ma from the same rock, constraining the timing of metamorphism as Mesoarchean, and marking the collisional event between the Coorg Block and the Dharwar Craton along the Mercara Suture Zone. We propose a tectonic model involving southward subduction of the Western Dharwar Block beneath the northern margin of the Coorg Block that can explain the extensive arc magmatism and suprasubduction zone rock suites, followed by ocean closure and collision along the Mercara Suture Zone, accompanied by high P-T metamorphism.

Isotope-Geochemical Evidence of the Nature of the Protoliths of Diamondiferous Rocks of the Kokchetav Subduction–Collision Zone (Northern Kazakhstan)

Abstract —The isotope-geochemical features of diamondiferous metamorphic rocks of the Kokchetav subduction–collision zone (KSCZ) show that both the basement rocks and the sediments of the Kokchetav massif were their protoliths. A whole-rock Sm–Nd isochron from the diamondiferous calc-silicate, garnet–pyroxene rocks and migmatized granite-gneisses of the western block of the KSCZ yielded an age of 1116 ± 14 Ma, while an age of 1.2–1.1 Ga was obtained by U–Pb dating of zircons from the granite-gneiss basement of the Kokchetav microcontinent. Based on these data, we assume that the protoliths of the calc-silicate, garnet–pyroxene rocks and the granite-gneisses of the KSCZ were the basement rocks sharing an initially single Nd source, which was not influenced by high- to ultrahigh-pressure metamorphism (~530 Ma). Therefore, their geochemical features are probably not directly related to ultrahigh-pressure metamorphism. The corresponding rock associations lack isotope-geochemical evidence of partial melting that would occur during ultrahigh-pressure metamorphism, which suggesting that they were metamorphosed under granulite-facies conditions. At the same time, the high-alumina diamondiferous rocks of the Barchi area (garnet–kyanite–mica schists and granofelses), which were depleted to different degrees in light rare-earth elements (REE) and K, have yielded a Sm–Nd whole-rock isochron age of 507 ± 10 Ma indicating partial melting of these rocks during their exhumation stage. The close ɛNd (1100) values of the basement rocks and garnet–kyanite–mica schist with geochemical characteristics arguing against its depletion during high-pressure metamorphism indicate that the basement rocks were a crustal source for high-alumina sediments.

Tracking a continental deep subduction and exhumation from granulitized kyanite eclogites in the South Altyn Tagh, northern Qinghai-Tibet Plateau, China

Kyanite eclogites can provide insights into global orogenic processes within subduction-collision zones. Here we present an integrated study of the petrography, mineral chemistry, zircon U-Pb geochronology, and whole-rock geochemistry of granulitized kyanite eclogites from South Altyn Tagh, located at the northern margin of the Qinghai-Tibet plateau, China. Our results suggest that the rocks experienced eclogite-facies metamorphism (27–37 kbar, 1030–1068 °C), followed by a high pressure-ultra-high temperature (HP-UHT) stage (17–20 kbar, 945–1033 °C), isothermal decompression, and a medium pressure-(ultra)-high temperature (MP-(U)HT) (10–12 kbar, 810–950 °C) granulite-facies overprint.Zircon U-Pb dating yielded a weighted mean U-Pb age of ca. 500 Ma for the metamorphic domains, which is interpreted as the timing of HP-UHT metamorphism. In contrast, the age of ca. 800 Ma recorded by the inherited cores of zircon grains is considered to represent the protolith formation age. High Al2O3 and CaO and relatively low MgO and FeO contents, LREE enrichment and nearly flat HREE patterns, and enriched Sr-Nd isotopic compositions suggest that the protoliths of the granulitized kyanite eclogites were likely derived from an enriched mantle source affected by crustal contamination. These results, together with previous research on these associations suggested that the protoliths of granulitized kyanite eclogites and associated garnet peridotite in the Bashiwake area are derived from mafic-ultramafic intrusions composed of plagioclase-rich gabbroic rocks and olivine-rich cumulates. These mafic-ultramafic intrusions were possibly generated by plume-induced melting related to the rifting of the Rodinia supercontinent during the Neoproterozoic, and were later subjected to early Paleozoic metamorphism. The geodynamic model proposed envisages continental subduction followed by relamination of felsic and mafic-ultramafic associations at the base of the lower crust, which were subsequently exhumed, forming diapir-like bodies. Our results, combined with those from previous studies, not only provide insights into the processes of continental collision to exhumation, but also provide significant constraints on the geotectonic processes leading to the formation of HP rocks within subduction-collision orogens.

Petrogenesis and tectonic significance of the Early Devonian lamprophyres and diorites in the Alxa Block, NW China

The North Qilian orogenic belt (NQOB) has been defined as a subduction-collision zone between the Alxa Block and the Qilian Block during the Early Paleozoic. To constrain the post-collisional tectonic evolution of the NQOB, analyses of zircon U-Pb-Hf isotopes, whole-rock major, trace element and Sr-Nd-Pb isotope compositions of the newly discovered Early Devonian lamprophyres and diorites dikes from the Longshoushan area in southwestern margin of the Alxa Block were conducted. Zircon U-Pb dating yields emplacement ages of 400 ± 4 Ma and 403 ± 6 Ma for two lamprophyre dikes and 391 ± 3 Ma for two diorite dikes. The lamprophyres dikes are shoshonitic-high-K (calc-alkaline) in nature, and are characterized by SiO2 contents of 53.6–56.3 wt %, (87Sr/86Sr)i ratios of 0.7064 to 0.7072, εNd(t) values of 0.1–1.0, and zircon εHf(t) values of −8.0 to −2.9. The diorite dikes are high-K (calc-alkaline), and are characterized by MgO contents of 6.32–6.98 wt %, (87Sr/86Sr)i ratios of 0.7089–0.7137, εNd(t) values of −3.8 to −3.5, and zircon εHf(t) values of −9.9–0.4. Both the lamprophyre and diorite dikes show parallel enrichments in LREEs and LILEs and depletions in HREEs and HFSEs and have similar ratios of (206Pb/204Pb)i (17.587–18.133), (207Pb/204Pb)i (15.518–15.584) and (208Pb/204Pb)i (37.676–38.058). Geochemical and isotopic data suggest that the lamprophyre and diorite dikes were derived from low-degrees melts of amphibole- and phlogopite-bearing lherzolite and phlogopite-bearing lherzolite, respectively, in the spinel-garnet transition facies. Their parental magmas both experienced extensive fractional crystallization in a deep magma chamber and negligible crustal contamination during their ascent. Regarding the Palaeozoic tectonic development of the North Qilian orogenic belt, we propose that the Early Devonian lamprophyres and diorites possibly are related to North Qilian orogen unrooting and collapse and marking the end of the North Qilian orogenic events.

Geochemistry and zircon ages of the Yushigou diabase in the Longshoushan area, Alxa Block: implications for crust–mantle interaction and tectonic evolution

AbstractThe North Qilian orogenic belt in North China has been defined as a subduction–collision zone between the Alxa Block and the Qilian Block. We present petrography, zircon U–Pb geochronology, major- and trace-element geochemistry, and Sr–Nd–Pb–Hf isotope analysis for the Yushigou diabase from the Longshoushan area, which is located SW of the Alxa Block, aiming to understand its petrogenetic link to subduction processes. The Yushigou diabase belongs to the tholeiite series, and shows enrichment in light rare earth and large-ion lithophile elements, and a depletion in heavy rare earth and high-field-strength elements. Laser ablation – inductively coupled plasma – mass spectrometry U–Pb zircon dating yielded an emplacement age of 414 ± 9 Ma, with an ϵHf(t) value in the range of −10.3 to 1.8. The whole-rock initial 206Pb/204Pb, 207Pb/204Pb and 208Pb/204Pb ratios of the diabase range over 16.811–17.157, 15.331–15.422 and 37.768–37.895, respectively. The (87Sr/86Sr)i ratios vary between 0.7086 and 0.7106, and ϵNd(t) values vary between −14.4 and −13.4, which are significantly higher than the ϵHf(t) value (Nd–Hf decoupling). An interpretation of the elemental and isotopic data suggests that the Yushigou diabase was derived from partial melting of an enriched mantle I (EM-I) -type lithospheric mantle in the spinel–garnet transitional zone. Based on the geochemical features and previous regional geological data, we propose that the Silurian magmatism was most likely triggered by slab break-off after the closure of the North Qilian Ocean, and ancient continental materials from the subduction slab metasomatized the overlying lithospheric mantle during exhumation.

Isotopic-geochemical evidence for crustal contamination of eclogites in the Kokchetav subduction-collision zone

This paper reports isotopic and geochemical studies of eclogites from the western ultrahigh pressure (UHP) and eastern high-pressure (HP) blocks of the Kokchetav subduction-collision zone. These HP and UHP eclogites exhumed in two stages: (1) The rocks of the western block metamorphosed within the field of diamond stability (e.g., Kumdy-Kol and Barchy); (2) In contrast, the metamorphic evolution of the eastern block reached the pressure peak within the stability field of coesite (e.g., Kulet, Chaglinka, Sulu-Tyube, Daulet, and Borovoe). The eclogites vary widely in the ratios of incompatible elements and in the isotope ratios of Nd (143Nd/144Nd = 0.51137-0.513180) and Sr (87Sr/86Sr = 0.703930.78447). The Sulu-Tyube eclogites display isotope-geochemical features close to N-MORB, while those from the other sites are compositionally similar to E-type MORB or island arc basalts (IAB). The model ages TNd(DM) of eclogites vary between 1.95 and 0.67 Ga. The Sulu-Tyube eclogite yields the youngest age; it has the values of εNd(T) (7.2) and 87Sr/86Sr (0.70393) close to the depleted mantle values. The crustal input to the protolith of the Kokchetav eclogites is evident on the εNd(T)-86Sr/87Sr and εNd(T)-T plots. The eclogites make up a trend from DM to country rocks. Some eclogites from the Kulet, Kumdy-Kol, and Barchy localities display signs of partial melting, such as high Sm/Nd (0.65-0.51) and low (La/Sm)N (0.34-0.58) values. The equilibrium temperatures of these eclogites are higher than 850 °C. The geochemical features of eclogites testify to the possibility of the eclogite protolith formation in the tectonic setting of passive continental rift margin subducted to depths over 120 km.

MG-CR-TYPE SPINEL PERIDOTITES IN THE WESTERN PART OF THE CENTRAL ASIAN OROGENIC BELT (ZHELTAU MASSIF, SOUTHERN KAZAKHSTAN): THE FIRST DATA ON P-T PATHS AND PROTOLITHS

Ultramafic and mafic lithologies, attributed to the orogenic terranes and formed under ultrahigh-pressure (UHP) and high-pressure (HP) conditions, have been intensively studied for the last decades. It is mainly related to a particular significance of these rocks for geodynamics, since they contain an important information on the fluid-rock interactions and element redistribution in the subduction-collision zones and could shed the light on the tectonic evolution of the studied region.

Multistage exhumation and partial melting of high-T ultrahigh-pressure metamorphic rocks in continental subduction-collision zones

High-temperature (HT; >850°C) metamorphism in continental collision orogens, particularly for those ultrahigh-pressure (UHP) metamorphic rocks, has become one of the remarkable topics in Earth science. It has bearing on the element and isotope behaviors of UHP rocks, their partial melting and related geodynamic effects during exhumation. In this paper, five representative continental collision orogens with typical HT/UHP rocks, including the Dabie orogen in China, the Kokchetav in Kazakhstan, the Caledonides in Greenland, the Rhodope in Greece, and the Erzgebirge in Germany are introduced, and their HT/UHP metamorphism and evolution processes are summarized. In addition, metamorphic P-T-t paths, multistage exhumation processes, and partial melting and preservation and retrogression of UHP index minerals during exhumation and their possible mechanisms are discussed. On this basis, the forthcoming key fields and scientific subjects of HT/UHP rocks within continental subduction channel are proposed.

Structural location, composition, and geodynamic nature of diamond-bearing metamorphic rocks of the Kokchetav subduction-collision zone of the Central Asian Fold Belt (northern Kazakhstan)

We present data on different aspects of geology, mineralogy, petrology, geochemistry, and geochronology of diamond-bearing metamorphic rocks of the Kumdy-Kol terrane, which show the similarity of their protolith to the sedimentary rocks of the Kokchetav microcontinent. The structural location of the studied objects in the accretion-collision zone evidences that the subduction of the Kokchetav microcontinent beneath the Vendian-Cambrian Ishim-Selety island arc is the main mechanism of transport of graphite-bearing terrigenous-carbonate rocks to zones of their transformation into diamond-bearing metamorphic rocks. The sedimentary rocks of the Kokchetav microcontinent, which are enriched in graphite and iron sulfides and carbonates, contain all components necessary for diamond crystallization in deep-seated subduction zone. This is in agreement with the experimental data and the compositions of fluid-melt inclusions in the minerals of diamond-bearing rocks.

Formation of the Kokchetav subduction–collision zone (northern Kazakhstan): Insights from zircon U–Pb and Lu–Hf isotope systematics

The Kokchetav subduction–collision zone is located in the western part of the ancestral Central Asian Orogenic Belt. This zone is built up by the Precambrian Kokchetav microcontinent which includes a HP–UHP metamorphic belt, and the North Kokchetav tectonic zone (NKTZ) which represents an accretionary complex between the Kokchetav microcontinent and the adjacent Stepnyak island-arc. The entire region is widely intruded by Palaeozoic granitoids which were emplaced after the collision of the Stepnyak island-arc with the Kokchetav microcontinent. We present zircon U–Pb ages and Lu–Hf systematics in zircon to better characterize the tectonic evolution of the Kokchetav subduction–collision zone.The Lu–Hf results indicate that the Kokchetav basement rocks are derived from late Neoarchaean — early Palaeoproterozoic (~2.5Ga) crust. For the granite-gneiss basement of the Kokchetav microcontinent, early Mesoproterozoic (Grenville-age, ~1.17–1.14Ga) zircon U–Pb crystallization ages were obtained. For the NKTZ, two main age-components were recognized: (1) an oldest Mesoproterozoic age-component (~1.20–1.05Ga) similar as for the Kokchetav microcontinental zircons, and (2) a younger Early Cambrian (540–520Ma) or Late Cambrian – Early Ordovician (~490–480Ma) age-component. Th/U ratios (<0.1) are indicative of a metamorphic origin for both Early Cambrian and Late Cambrian – Early Ordovician zircon populations. The oscillatory zoned Mesoproterozoic zircons have higher Th/U ratios (>0.1) and are typical of a magmatic protolith. The distinction between both zircon types is supported by characteristic cathodoluminescence images. These results confirm previous observations, indicating early Palaeozoic high-grade metamorphism (~540–520Ma) and collisional deformation (~490–480Ma) of the Mesoproterozoic Kokchetav zone as a result of subduction–accretion and collision of the Stepnyak island-arc to the Kokchetav microcontinent. For two additional samples of the Balkashin granitic complex Early Devonian (~415–395Ma) zircon crystallization ages (magmatic Th/U ratios) were obtained. The occurrence of a Mesozoic xenocryst within these leucogranites may indicate that they were emplaced in a continental-arc setting.

The Murchison Greenstone Belt, South Africa: Accreted slivers with contrasting metamorphic conditions

This paper presents new petrological and geochronological data for the ∼3.09–2.92Ga Murchison Greenstone Belt (MGB), located in South Africa's Kaapvaal Craton, and discusses their geotectonic implications. The MGB is made of three tectono-metamorphic units: the Silwana Amphibolites, the Murchison Unit and the La France Formation. They underwent contrasting clockwise pressure–temperature–deformation (P–T–D) histories, and are separated from each other by relatively narrow, high-strain shear zones, with a sinistral, transpressive top-to-the-south movement, consistent with the deformation patterns observed throughout the belt. These patterns are explained by a N–S compressional stress field, affecting the Murchison Belt between 2.97 and 2.92Ga. Results of new petrological investigations indicate that ultramafic to felsic volcano-sedimentary rocks of the Murchison Unit underwent a greenschist- to lower-amphibolite-facies metamorphism at maximum P–T conditions of 5.6±0.6kbar at 570°C, along a relatively hot, minimum apparent geotherm of ∼30°C/km. In contrast, the Silwana Amphibolites and the La France Formation were metamorphosed at much higher peak metamorphic conditions of 8.7–10kbar, 630–670°C, and 8–9kbar, 600–650°C, respectively, and require a colder apparent geotherm of ∼20°C/km. A retrograde, nearly isothermal–decompression P–T path followed by isobaric cooling is also inferred for the La France Formation. The timing of the structural–metamorphic overprint is bracketed between 2.97 and 2.90Ga, which is constrained by U–Pb zircon ages of a syn-deformation granite within the Murchison Unit and the post-deformation Maranda granite, respectively. Monazite and xenotime from La France metapelites yield much younger ages of ca. 2.75Ga, with few inherited components at 2.92 Ga. They point to a later activation of the MGB, perhaps related with tectono-thermal events in the Rooiwater Complex and the Pietersburg Greenstone Belt. The relatively cold apparent geotherms recorded in the Silwana and La France rocks, the contrasted peak P–T conditions between the different units, and the near isothermal decompression of the La France Formation indicate that the Kaapvaal craton crust must have been cold enough to enable significant crustal thickening and strain localisation along narrow shear zones and, as a consequence, fast tectonic juxtaposition of rocks metamorphosed at different crustal depths. These features are similar to those observed along Palaeozoic or modern day, oblique subduction–collision zones, but different to those of hot Archaean provinces. We therefore interpret the MGB as representing part of an oblique collision-zone between two terrains of the Kaapvaal craton: the Witwatersrand and Pietersburg terrains.

Analysis of natural tectonic systems coupled with numerical modelling of the polycyclic continental lithosphere of the Alps

Subduction–collision zones are characterized by the amalgamation and disaggregation of lithospheric slices; such processes work in competition in constructing the tectonic architecture of metamorphic belts. Determination of contours for thermally and structurally characterized units is crucial to define the variations in sizes of such slices involved in the dynamic evolution of an active margin. The dimensions of these entities change over time and must be reconstructed using the structural and metamorphic evolution of the basement rocks as tracers, rather than by simply relying on lithologic associations. They constitute tectono-metamorphic units (TMUs) and represent discrete portions of the orogenic crust influenced by a sequence of metamorphic and textural changes. Their translational trajectories and shape changes during deformation cannot simply be derived from the analysis of the geometries and kinematics of tectonic units but from a joint reconstruction of quantitative P-T-d-t paths. The TMU investigation tool bears a marked thermo-tectonic connotation and, through modelling, offers the opportunity to test the physical compatibilities of interconnected variables, such as density, viscosity, and heat transfer, with the interpretative geologic history. Comparison between modelling predictions and natural data obtained by this analytical approach has helped solve longstanding ambiguities on the pre-Alpine and Alpine geodynamic evolution of the different continental units of the Central and Western Alps and explore the crustal levels of protolith derivation. Three-dimensional estimation of structurally and chemically re-equilibrated volumes aids in the evaluation of physical parameters chosen for the numerical modelling.

Stratigraphy in the continental crust: lithologic and tectonic records

Preservation of stratigraphic settings in continents is mostly confined to uppermost crustal levels, in which prevailing deformation is translational and internal strain of tectonic units is weak. This does not specially depend on association of metamorphism to tectonic processes, but rather on mechanical properties of deforming multilayers. Scattered findings of sedimentary features, not supported by structural investigation on the actual nature of lithologic layering, may lead to failure in determining size of preserved sedimentary sequences. In the intermediate and lower crust, interaction of metamorphism and deformation down to granular scale facilitates construction of new types of lithologic layers and segregation of thick differentiated mineral layerings that mimic stratigraphic sequences. Critical examples of difficulties encountered in different tectonic contexts when assessing the sedimentary origin and related stratigraphic meaning of variously deformed layered sequences are summarised. In deep subduction-collision zones similar or contrasted lithostratigraphies are of little help in definition of tectonic units; the structural and metamorphic reworking of rocks of contrasted origin constructs tectonic units that repeatedly couple and decouple from similar adjacent sequences and their actual relative mechanical paths may be disclosed by a combined structural and petrologic analytical tool delimiting volumes that experienced equivalent structural histories and metamorphic signatures (contouring of tectono-metamorphic units=TMUs); these units constitute valuable elements of correlation in metamorphic belts and for the investigation of mechanisms of lithosphere dynamics.

Quantitative P–T–X constraints on orthopyroxene‐bearing high‐pressure granulites in felsic–metapelitic rocks: evidence from the Huangtuling granulite, Dabieshan Orogen

AbstractHigh‐pressure granulites are generally characterized by the absence of orthopyroxene. However, orthopyroxene is reported in a few high‐pressure, felsic–metapelitic granulites, such as the Huangtuling felsic high‐pressure granulite in the North Dabie metamorphic core complex in east‐central China, which rarely preserves the high‐pressure granulite facies assemblage of garnet + orthopyroxene + biotite + plagioclase + K‐feldspar + quartz. To investigate the effects of bulk‐rock composition on the stability of orthopyroxene‐bearing, high‐pressure granulite facies assemblages in the NCKFMASHTO (Na2O–CaO–K2O–FeO–MgO–Al2O3–SiO2–H2O–TiO2–Fe2O3) system, a series of P–T–X pseudosections based on the melt‐reintegrated composition of the Huangtuling felsic high‐pressure granulite were constructed. Calculations demonstrate that the orthopyroxene‐bearing, high‐pressure granulite facies assemblages are restricted to low XAl [Al2O3/(Na2O + CaO + K2O + FeO + MgO + Al2O3) &lt; 0.35, mole proportion] or high XMg [MgO/(MgO + FeO) &gt; 0.85] felsic–metapelitic rock types. This study also reveals that the XAl values in the residual felsic–metapelitic, high‐pressure granulites could be significantly reduced by a high proportion of melt loss. We suggest that orthopyroxene‐bearing, high‐pressure granulites occur in residual overthickened crustal basement under continental subduction–collision zones and arc–continent collision belts.

Evolution of Precambrian continental lithosphere in Western Canada: results from Lithoprobe studies in Alberta and beyond

The Precambrian lithosphere of western Canada was assembled into the present crustal configuration between ca. 2.0–1.78 Ga by plate collisions, sometimes accompanied by arc magmatism, with subsequent cooling of the lithosphere since ca. 1.7 Ga. Collisional processes inferred along preserved plate sutures include (1) subduction of oceanic lithosphere and accretion of Proterozoic arc crust to the western Rae Province; (2) marginal basin consumption and tectonic entrapment of the Hearne Province between coeval subduction–collision zones; and (3) amagmatic marginal basin closure, perhaps analogous to the roots of small collisional orogens, such as the Pyrenees. Seismic reflection profiles acquired during the Lithoprobe Alberta Basement Transect have captured images of syn- to post-collisional structures along these sutures and evidence for crustal-scale thrust imbrication and rigid body accretion of Archean crust with preservation of precollisional tectonic fabric. The degree to which lithospheric mantle beneath Archean crustal blocks was preserved during these collisions is unknown, although tectonic geometries imply significant thermal and (or) mechanical interaction. Post-collisional, intrusive mafic magmatism is imaged widely in both seismic reflection and refraction surveys. These magmatic events are demonstrably Proterozoic, based on crosscutting relationships seen on seismic reflection profiles and geochronology of lower crustal xenoliths, and are comparable in scale to Phanerozoic igneous provinces (e.g., large igneous provinces) but have little preserved surface manifestation. Reactivation of Precambrian basement structures is limited or very subtle, reflecting strength control by the mantle on stress transmission and crustal failure. Long-wavelength elastic deformation of the crust during the Phanerozoic occurred in regions associated with, or adjacent to, Proterozoic mafic magmatism, suggesting local rheologic control of anomalous Phanerozoic paleotopography.