Deep Subduction Of Continental Crust Research Articles

Nitrogen behavior during fluid-rock interaction in a continental subduction zone: Results from the UHP Dora-Maira Massif whiteschists and Eastern Alps leucophyllites

Deep-Earth cycling of nitrogen (N) along the subduction pathway, remains relatively poorly understood, particularly that related to deep subduction of continental crust. The ultrahigh-pressure (UHP) Dora-Maira Massif whiteschists in the Western Alps and their lower-P-T equivalent leucophyllites in the Eastern Alps show a decrease in N concentration and an increase in δ15N relative to their presumed country rock protoliths. On average, the whiteschists contain 20.7 ± 10.5 ppm N (mean ± 1σ) with δ15Nair = +2.7 ± 2.5‰ (mean ± 1σ) and the leucophyllites contain 59.8 ± 12.3 ppm N with δ15Nair = +4.3 ± 2.0‰, whereas the metagranitic country rocks from Dora-Maira contain 41.3 ± 12.5 ppm N with δ15Nair = −1.9 ± 3.2‰ and the country rocks from the Eastern Alps contain 91.1 ± 42.5 ppm N with δ15Nair = +1.7 ± 2.1‰. The loss of N and isotope shift in the whiteschists and leucophyllites from these two localities was accompanied by loss of LILE (Rb, Ba), Sr, CaO, Na2O, and FeOtotal. The other stable isotope systems applied to study both suites (Mg, Fe, O, Li and Ba) show no obvious correlation with variations in δ15N. Infiltration by and interaction with a serpentinite-derived fluid depleted in N and LILE could have led to leaching of N from the country rock protoliths and production of the +4.6‰ (Dora-Maira Massif) and +2.6‰ (Eastern Alps) isotope shifts adequately explained using a Rayleigh distillation model. The N exchange and leaching process is best explained as involving N2-NH4 or NH3-NH4 at about 550–600 °C, the latter the approximate temperatures at which the fluid infiltration is thought to have occurred. These results provide new insights regarding N behavior and isotopic fractionation during fluid-rock interactions occurring in UHP rocks and point to the importance of H2O-rich fluids from serpentinite dehydration as agents of metasomatism along deep subduction interfaces. Furthermore, the data for the metagranites indicate the potential of this lithology to retain significant amounts of N to at least 100 km and beyond sub-arc depths.

The early Paleozoic Wuyi–Yunkai orogeny in South China: a collisional orogeny with a major lag in time between onset of collision and peak metamorphism in subducted continental crust

Abstract Collisional orogeny is characterized by deep subduction of continental crust and major crustal thickening, leading to high-pressure/high-temperature metamorphism and anataxis of the subducted continental crust. Since conductive heating of large slabs of cold, subducted continental crust is a slow process, heating to a temperature that is high enough to generate significant partial melting can take tens of millions of years. Where the spatial and temporal relationships are obscured due to later modification (e.g. post-collisional rifting), the peak metamorphism and magmatism may be interpreted as an orogeny that is separate from the collision, or may be interpreted as an intraplate orogeny as no contemporaneous arcs or ophiolite may be present. We propose here that this is the case for the early Paleozoic orogeny in South China. In our model, the West Cathaysia terrane of South China was part of a continent (possibly Australia) on the lower plate and collided with another continent (possibly India) in the Cambrian–Ordovician, at a late stage of Gondwana assembly, and the late Ordovician–Silurian Wuyi–Yunkai orogeny, characterized by amphibolite–granulite facies metamorphism and extensive anataxis, was a continuation of the Cambrian–Ordovician collisional orogeny. In this interpretation, the Wuyi–Yunkai orogen was part of the Kuunga orogen before Gondwana break-up.

First finding of continental deep subduction in the Sesia Zone of the Western Alps and implications for subduction dynamics.

Continental deep subduction after the closure of large oceanic basins is commonly ascribed to the gravitational pull of the subducting oceanic slab. However, it is not clear how continental lithosphere adjacent to small oceanic basins was subducted to mantle depths. The Sesia Zone in the Western Alps provides an excellent target for exploration of subduction dynamics in such a tectonic setting. Here we report the first finding of coesite in a jadeite-bearing orthogneiss from the Sesia Zone, providing the first evidence for deep subduction of the continental crust to mantle depths for ultrahigh-pressure (UHP) metamorphism in this zone. Three coesite inclusions were identified by laser Raman spectroscopy in two garnet grains. Based on zircon U-Pb dating and trace element analysis, the UHP metamorphic age was constrained to be 76.0±1.0 Ma. The phase equilibrium modeling yields peak metamorphic pressures of 2.8-3.3GPa, demonstrating the continental deep subduction to mantle depths of >80km. The subducted continental crust was a rifted hyperextended continental margin, which was converted to the passive continental margin during seafloor spreading and then deeply subducted during the oblique convergence between the Adria microplate and Eurasian plate in the Late Cretaceous. Because the slab pull could only play a limited role in closing small oceanic basins for continental collision, the distal push of either continental breakup or seafloor spreading is suggested as the major driving force for the deep subduction of continental crust in the Western Alps. Therefore, deep subduction of the continental crust bordering small oceanic basins would have been induced by the far-field stress of compression, whereas that bordering large oceanic basins was spontaneous due to the oceanic slab pull. This provides a new insight into the geodynamic mechanism of continental deep subduction.

Did the Neoproterozoic Revolution Extend to the Deep Mantle?

Did the Neoproterozoic Revolution Extend to the Deep Mantle?

Microdiamond on Åreskutan confirms regional UHP metamorphism in the Seve Nappe Complex of the Scandinavian Caledonides

AbstractMetamorphic diamond in crustal rocks provides important information on the deep subduction of continental crust. Here, we present a new occurrence of diamond within the Seve Nappe Complex (SNC) of the Scandinavian Caledonides, on Åreskutan in Jämtland County, Sweden. Microdiamond is found in situ as single and composite (diamond+carbonate) inclusions within garnet, in kyanite‐bearing paragneisses. The rocks preserve the primary peak pressure assemblage of Ca,Mg‐rich garnet+phengite+kyanite+rutile, with polycrystalline quartz surrounded by radial cracks indicating breakdown of coesite. Calculated P–T conditions for this stage are 830–840 °C and 4.1–4.2 GPa, in the diamond stability field. The ultrahigh‐pressure (UHP) assemblage has been variably overprinted under granulite facies conditions of 850–860 °C and 1.0–1.1 GPa, leading to formation of Ca,Mg‐poor garnet+biotite+plagioclase+K‐feldspar+sillimanite+ilmenite+quartz. This overprint was the result of nearly isothermal decompression, which is corroborated by Ti‐in‐quartz thermometry. Chemical Th–U–Pb dating of monazite yields ages between 445 and 435 Ma, which are interpreted to record post‐UHP exhumation of the diamond‐bearing rocks. The new discovery of microdiamond on Åreskutan, together with other evidence of ultrahigh‐pressure metamorphism (UHPM) within gneisses, eclogites and peridotites elsewhere in the SNC, provide compelling arguments for regional (at least 200 km along strike of the unit) UHPM of substantial parts of this far‐travelled allochthon. The occurrence of UHPM in both rheologically weak (gneisses) and strong lithologies (eclogites, peridotites) speaks against the presence of large tectonic overpressure during metamorphism.

Devonian subduction and syncollisional exhumation of continental crust in Lofoten, Norway

When continents collide, continental crust of the lower plate may be subducted to mantle depth and return to the surface to form eclogite facies metamorphic terranes, as typified by the Western Gneiss Complex of the Scandinavian Caledonides. Proterozoic basement of the Lofoten Islands, located northeast and along strike of the Western Gneiss Complex, contains Caledonian eclogite, although Caledonian deformation is only minor. Previous dating suggested that Lofoten eclogites formed at ca. 480 Ma, i.e., ∼50 Ma before the collision between the major continents Baltica and Laurentia, and that the Lofoten basement may not originate from Baltica but rather represents a stranded microcontinent. Newly discovered kyanite eclogites from the Lofoten Islands record deep subduction of continental crust during the main (Scandian) stage of Baltica-Laurentia collision ca. 400 Ma. Thermobarometry and thermodynamic modeling yield metamorphic conditions of 2.5–2.8 GPa and ∼650 °C. Lu-Hf geochronology yields 399 ± 10 Ma, corresponding to the time of garnet growth during subduction. Our results demonstrate that the Lofoten basement belonged to Baltica, was subducted to ∼90 km depth during the collision with Laurentia, and was exhumed at an intermediate to high rate (>6 mm/yr) while thrusting of a Caledonian allochthon (Leknes Group) was still ongoing. This supports the challenging conclusions that (1) subducted continental crust may stay rigid down to a depth of ∼90 km, and (2) it may be exhumed during ongoing collision and crustal shortening.

Eclogite and garnet pyroxenite from Stor Jougdan, Seve Nappe Complex, Sweden: implications for UHP metamorphism of allochthons in the Scandinavian Caledonides

AbstractUltrahigh‐pressure metamorphism (UHPM) has recently been discovered in far‐travelled allochthons of the Scandinavian Caledonides, including finding of diamond in the Seve Nappe Complex. This UHPM of Late Ordovician age is older and less recognized than that in the Western Gneiss Region of southwestern Norway, which was related to terminal collision between Baltica and Laurentia. Here we report new evidence of UHPM in the Lower Seve Nappe, recorded by eclogite and garnet pyroxenite from the area of Stor Jougdan in northern Jämtland, central Sweden. Peak‐metamorphic assemblage of eclogite, garnet + omphacite + phengite + rutile + coesite? yields P–T conditions of 2.8–4.0 GPa and 750–900 °C, constrained by conventional geothermobarometry and thermodynamic modelling in the NCKFMTASH system. The prograde metamorphic evolution of the eclogite is inferred from inclusions of zoisite and amphibole in garnet, which are stable at lower pressure, whereas the retrograde evolution is recorded by formation of diopsidic clinopyroxene + plagioclase symplectites after omphacite, growth of amphibole replacing these symplectites, and of titanite around rutile. In garnet pyroxenite the peak‐metamorphic assemblage consists of garnet + orthopyroxene + clinopyroxene + olivine. P–T conditions of 2.3–3.8 GPa and 810–960 °C have been derived based on the conventional geothermobarometry and thermodynamic modelling in the CFMASH and CFMAS systems. Retrograde evolution has been recognized from replacement of pyroxene and garnet by amphibole. The results show that eclogite was metamorphosed during deep subduction of continental crust, most probably derived from the continental margin of Baltica, whereas the origin and tectonic setting of the garnet pyroxenite is ambiguous. The studied pyroxenite/peridotite of Baltican subcontinental affinity could have been metamorphosed as a part of the subducting plate and exhumed due to the downward extraction of a forearc lithospheric block.

Early Palaeozoic Deep Subduction of Continental Crust in the Kyrgyz North Tianshan: Evidence from Field Relationships and Geochronology

Early Palaeozoic Deep Subduction of Continental Crust in the Kyrgyz North Tianshan: Evidence from Field Relationships and Geochronology

Paleozoic HP granulite-facies metamorphism and anatexis in the Dulan area of the North Qaidam UHP terrane, western China: Constraints from petrology, zircon U–Pb and amphibole Ar–Ar geochronology

HP mafic and felsic adakitic granulite bodies in the Dulan area of the North Qaidam ultrahigh-pressure (UHP) terrane record high-pressure (HP) granulite-facies metamorphism and anatexis and provide temporal and tectonic constraints on deep subduction of continental crust and its subsequent exhumation. Mafic HP granulite components dominate the main outcrop and preserve features diagnostic of anatexis and locally may be described as migmatite. The HP mafic granulites comprise garnet, clinopyroxene, plagioclase and quartz. The felsic granulite (leucosome) is mainly composed of K-feldspar+plagioclase+quartz+kyanite+garnet. Detailed zircon U–Pb and amphibole Ar–Ar geochronology, combined with trace element geochemistry, indicate peak metamorphism for the mafic HP granulite at 434±3 to 435±3Ma and peak metamorphism and partial melting for the felsic HP granulite at 433±5 to 438±4Ma, which overlaps the ages of UHP metamorphism for adjacent eclogite (430–446Ma). 40Ar/39Ar amphibole ages of 423 to 432Ma represent amphibolite-facies retrograde metamorphism and indicate rapid cooling during exhumation of the HP granulite bodies. Our geochronological data, combined with field relationships, petrology and geochemistry suggest that HP granulite-facies metamorphism and the partial melting that produced adakitic melts represent the same tectonic event. In this case, the felsic HP granulites (leucosome) formed from an adakitic melt derived from partial melting of mafic HP granulite in the overriding plate in a relatively higher geothermal gradient (15–18°C/km), leaving garnet-cumulate and/or meta-ultramafic (mainly garnet pyroxenite) as the residual component. In contrast, the nearby UHP eclogite is thought to have formed in the subducted plate in a relatively lower geothermal gradient (6–10°C/km). Penecontemporaneous metamorphic ages but different geothermal gradients between HP granulites and related UHP eclogite define a possible paired metamorphic belt generated in a subduction–collision setting associated with the North Qaidam continental collisional orogeny during the Late Ordovician–Early Silurian.

Deep subduction of continental crust in accretionary orogen: Evidence from U–Pb dating on diamond-bearing zircons from the Qinling orogen, central China

Accretionary orogenic belts are considered the most important sites for the generation of continental crust. However, it is still not very clear whether in such settings the continental crust has been significantly transported back to the mantle accompanied with its lateral growth. Continental UHP rocks, the best indicators of the recycling of the continental crust, have rarely been discovered in accretionary orogenic belts, which hinder the exploration of this essential process. The possible occurrence of early Paleozoic UHP metamorphism in the Qinling orogen provides an excellent opportunity to address this question. This article reports an integrated study of U–Pb age, trace element and mineral inclusion of zircon from an amphibolite sample in the Qinling orogen. The zircon crystals show typical metamorphic growth zoning, low Th/U ratios, flat HREE patterns, and insignificant Eu anomalies. They give a weighted mean U–Pb age of 490.4±5.8Ma. Most importantly, an in situ diamond inclusion was identified from one of the zircon crystals. Therefore, the obtained 490.4±5.8Ma age was taken as registering the peak UHP metamorphism. The discovery of in situ diamond inclusion provides important evidence for the UHP metamorphism of the Qinling orogen and indicates that the North Qinling microcontinent was subducted to mantle depths of >120km when it collided with the Erlangping arc attached to the southern margin of the North China Block. Combining with previous results, we suggested that the North Qinling UHP terrane underwent fast exhumation from >120km to ca. 30km within 10Myr intervals. As a few examples of UHP metamorphism have been discovered in some accretionary orogens, more attention needs to be paid to the occurrence of continental UHP metamorphism during accretionary orogenic process. Furthermore, evident crustal growth has been documented in the North Qinling orogen during the early Paleozoic. It is inferred that the accretionary orogenic belts may play important roles in recycling of continental crust into the mantle besides generation of continental crust.

Early Palaeozoic deep subduction of continental crust in the Kyrgyz North Tianshan: evidence from Lu–Hf garnet geochronology and petrology of mafic dikes

High-pressure and ultrahigh-pressure (UHP) eclogite-bearing metamorphic assemblages in the North Tianshan of Kyrgyzstan are known from the Aktyuz and Makbal areas, where eclogites and garnet amphibolites are associated with continental rocks such as granitoid gneisses in Aktyuz and shallow-water clastic (passive margin?) metasediments in Makbal. We present the first Lu–Hf isotope data for an eclogite and two garnet amphibolite samples from the two metamorphic terranes which, combined with petrological analysis, tightly constrain the age of high-pressure metamorphism in the Kyrgyz North Tianshan. A five-point isochron for an Aktyuz eclogite sample provides a Lu–Hf age of 474.3 ± 2.2 Ma, and a four-point isochron on a Makbal sample corresponds to 470.1 ± 2.5 Ma. A prograde, subduction-related path is inferred for both samples with peak P–T conditions ranging from 1.4 to 1.6 GPa and 610–620 °C. A further Makbal sample provided a significantly older Lu–Hf age of 486 ± 5.4 Ma, most likely due to late alteration in the sample (late addition of unradiogenic Hf). We conclude that garnet growth in all three samples occurred around ca. 474 Ma and that these rocks likely experienced UHP metamorphism contemporaneously. Our results support previous geochronological evidence for an Early Ordovician collision belt in the North Tianshan and allow refinement of a tectonic model involving subduction of thinned continental crust to considerable depth along the margin of a small microcontinent.

Metamorphic Conditions in Orogenic Belts: A Record of Secular Change

In general, Archean rocks exhibit rather ordinary moderate-P-high-T facies series metamorphism; neither blueschists nor any record of deep continental subduction and return are documented. However, the abundance and scale of ultrahigh-temperature (UHT) metamorphic belts from the Neoarchean to the Cambrian imply a significant change in geodynamics during the Neoarchean Era, after which transient sites of high heat flow were available at intervals throughout this period of Earth evolution. Many Neoproterozoic-Cambrian UHT metamorphic belts appear to have developed in settings analogous to modern backarcs that were closed and inverted during crustal aggregation and formation of the Gondwana supercontinent. If backarcs were the general setting for UHT metamorphism, then on a hotter Earth the cyclic formation of supercratons (in the Neoarchean Era) and supercontinents (in the Proterozoic Eon) required the destruction of oceans floored by thinner lithosphere that may have generated hotter backarcs than those associated with the current destruction of the Pacific Ocean on the modern Earth. The inherent weakness of the lithosphere in a hotter thermal regime inevitably localized magmatism and deformation at these sites contemporaneously with UHT metamorphism. Medium-temperature eclogites of crustal derivation and associated highpressure granulites are also first recognized in the Neoarchean, for example within the Belomorian Mobile Belt, and they occur at intervals throughout the Proterozoic, for example in the Orosirian Usagaran Orogen and in the Grenvillian belts of the Proto-Atlantic region, and Paleozoic, for example in the circum-North Atlantic Caledonides and European Variscides. Eclogite-high-pressure granulite (E-HPG) metamorphism is predominantly a Proterozoic-Paleozoic phenomenon—complementary to but sparser than UHT metamorphism to begin with, but extending further into the Paleozoic than does UHT metamorphism—that is inferred to record subduction-to-collision orogenesis. Blueschists appear in the Neoproterozoic Era, becoming common through the Phanerozoic Eon; they record the low thermal gradients associated with modern subduction. Lawsonite-bearing blueschists and eclogites, and ultrahigh-pressure (UHP) metamorphism characterized by coesite or diamond are predominantly Phanerozoic phenomena related to deep subduction within subductionto-collision orogens. In general, UHP metamorphic belts in subduction-to-collision orogens are not associated with a contemporary magmatic arc in the hanging wall; this suggests that deep subduction of continental crust may inhibit the generation of calc-alkaline magmas, a feature that may have enabled preservation during exhumation of the mineralogical evidence for extreme pressures. However, an enigma concerning UHP metamorphism is the first evidence of deep subduction of continental crust in the rock record. At issue is the recycling implied by the geochemistry of Archean and Proterozoic diamonds (since entrained from mantle to upper crust in younger magmatic events), which requires a supracrustal component. Are these diamonds evidence of deep subduction of continental crust and an early record of UHP metamorphism, or was some other mechanism (e.g., delamination of underthrust lithosphere, some form of slab break-off) responsible for taking a supracrustal component deep into the mantle source? The Archean and Proterozoic eons were characterized by higher but decreasing mean mantle temperatures and radioactive heat production (RHP), and a thinner thermal boundary layer (TBL) with a shorter residence time than modern Earth. Modeling the effect of increased RHP on the thermal evolution of crust instantaneously doubled in thickness predicts that metamorphic rocks in Archean collisional orogens should have experienced maximum temperatures several hundreds of degrees Celsius higher than those recorded by metamorphic rocks in modern collisional orogens. However, there is no evidence of this—the Archean record is dominated by rather ordinary P-T conditions and crustal melting at relatively low temperatures probably fluxed by water. I argue that a duality of metamorphic belts—reflecting a duality of thermal environments—is the characteristic metamorphic imprint of plate tectonics in the rock record, and it appears only since the Neoarchean Era. Based on the occurrence of both UHT and E-HPG metamorphic belts since the Archean-to-Proterozoic transition, I suggest this transition records the onset of a "Proterozoic plate tectonics regime," although the total ridge length and the number of plates undoubtedly were larger in the beginning than on modern Earth and the style of tectonics likely involved some differences. The Neoproterozoic transition to the "modern plate tectonics regime" registers a change to subduction of continental crust deeper into the mantle and its (partial) return from depths of up to 300 km, a change perhaps related to whole mantle convection as oceanic lithosphere became thicker with decreased thermal gradients. Although there was overlap in time and space between E-HPG and UHP metamorphism during the Paleozoic, since the Carboniferous UHP metamorphism has dominated, and extreme thermal metamorphism is rare.

Metamorphism, Plate Tectonics, and the Supercontinent Cycle

Granulite facies ultrahigh temperature metamorphism (G-UHTM) is documented in the rock record predominantly from Neoarchean to Cambrian; G-UHTM facies series rocks may be inferred at depth in younger, particularly Cenozoic orogenic systems. The first occurrence of G-UHTM in the rock record signifies a change in geodynamics that generated transient sites of very high heat flow. Many G-UHTM belts may have developed in settings analogous to modern continental backarcs. On a warmer Earth, the cyclic formation of supercontinents and their breakup, particularly by extroversion, which involved destruction of ocean basins floored by thinner lithosphere, may have generated hotter continental backarcs than those associated with the modern Pacific rim. Medium-temperature eclogite, high-pressure granulite metamorphism (E-HPGM), is also first recognized in the Neoarchean rock record and occurs at intervals throughout the Proterozoic and Paleozoic rock record. E-HPGM belts are complementary to G-UHTM belts and are generally inferred to record subduction-to-collision orogenesis. Blueschists become evident in the Neoproterozoic rock record; they record the low thermal gradients associated with modern subduction. Lawsonite blueschists and eclogites (high-pressure metamorphism, HPM) and ultrahigh pressure metamorphism (UHPM) characterized by coesite (±lawsonite) or diamond are predominantly Phanerozoic phenomena. HPM-UHPM registers the low thermal gradients and deep subduction of continental crust during the early stage of the collision process in Phanerozoic subduction-to-collision orogens. Although perhaps counterintuitive, many HPM-UHPM belts appear to have developed by closure of small ocean basins in the process of accretion of a continental terrane during a period of supercontinent introversion (Wilson cycle ocean basin opening and closing). A duality of metamorphic belts—reflecting a duality of thermal regimes—appears in the record only since the Neoarchean Era. A duality of thermal regimes is the hallmark of modern plate tectonics and the duality of metamorphic belts is the characteristic imprint of plate tectonics in the rock record. The occurrence of both G-UHTM and E-HPGM belts since the Neoarchean manifests the onset of a ‘Proterozoic plate tectonics regime’, although the style of tectonics likely involved differences. The ‘Proterozoic plate tectonics regime’ evolved during a Neoproterozoic transition to the ‘modern plate tectonics regime’ characterized by colder subduction and subduction of continental crust deep into the mantle and its (partial) return from depths of up to 300 km, as chronicled by the appearance of HPM-UHPM in the rock record. The age distribution of metamorphic belts that record extreme conditions of metamorphism is not uniform, and metamorphism occurs in periods that correspond to amalgamation of continental lithosphere into supercratons (e.g. Superia/Sclavia) or supercontinents (e.g. Nuna (Columbia), Rodinia, Gondwana, and Pangea).

Protolith of eclogites in the north Qaidam and Altun UHP terrane, NW China: Earlier oceanic crust?

An early Paleozoic ultrahigh pressure metamorphic belt occurs in the north Qaidam–Altun mountains and was offset about 400 km southwestward by the Altyn Tagh strike-slip fault. Eclogites in the belt consist of major basaltic and minor picritic rock types and can be subdivided into three groups: high TiO 2 (2–5 wt%), medium TiO 2 (1–2 wt%) and low TiO 2 (<1 wt%). Geochemical evidence shows that the protoliths of the eclogite were basaltic rocks from different tectonic environments, including mid-ocean ridge basalt, island arc basalt and ocean island basalt. The Nd isotopes of these rocks are similar to those of MORB, characterized by mainly positive and minor negative εNd(0) values, providing further evidence that the eclogite protoliths were ocean floor basalts to which minor crustal components were added during subduction. Geochronological data indicate that the UHPM occurred about 500–440 Ma ago, and that there were two eclogite protoliths with ages of 800–750 and ∼1000 Ma, respectively. Geochemically and isotopically, the eclogites are similar to basaltic rocks in Luliangshan, north Qaidam mountains, which have Sr and Nd ages of 768±39 and 780±22 Ma. The age and composition of these volcanic rocks suggest that they were one type of the protoliths for the eclogites. A later series of calc-alkaline, island arc volcanic rocks was formed at about 500 Ma ago, essentially contemporaneous with the UHP metamorphism. We propose the following tectonic model for the evolution of the north Qaidam UHP belt. At about 1000 Ma ago, a number of continents were amalgamated to form the Rodinian continent in this general area, which contained some oceanic volcanic rocks, possibly as ophiolitic fragments. This part of Rodinia was then rifted at about 800–750 Ma ago to form an oceanic basin with a variety of MORB and ocean island basalts. Closure of this ocean basin produced the Neoproterozoic ophiolites and granitic gneisses in the north Qaidam mountains. In the early Paleozoic another Qilian ocean basin formed and subduction of this oceanic lithosphere formed the island arc volcanic rocks at about 500 Ma ago and the subduction-related granites (470–450 Ma). Once the oceanic lithosphere was consumed, the subsequent continent–continent collision led to deep subduction of continental crust, which was tectonically mixed with the eclogites, to cause the ultrahigh pressure metamorphism.

Duality of thermal regimes is the distinctive characteristic of plate tectonics since the Neoarchean

Ultrahigh-temperature (UHT) granulite metamorphism is documented predominantly in the Neoarchean to Cambrian rock record, but UHT granulite metamorphism also may be inferred at depth in Cenozoic orogenic systems. The first occurrence of UHT granulite metamorphism in the record signifies a change in geodynamics that generated transient sites of very high heat flow. Many UHT granulite metamorphic belts may have developed in settings analogous to modern continental backarcs; on a warmer Earth, destruction of oceans floored by thinner lithosphere may have generated hotter backarcs than those associated with the modern Pacific ring of fire. Medium-temperature eclogite–high- pressure (EHP) granulite metamorphism is documented in the Neoarchean rock record and at intervals throughout the Proterozoic and Paleozoic record. EHP granulite metamorphic belts are complementary to UHT granulite metamorphic belts in that they are generally inferred to record subduction-to-collision orogenesis. Blueschists become evident in the Neoproterozoic rock record, but lawsonite blueschist–eclogite metamorphism (high pressure [HP]) and ultrahigh-pressure metamorphism (UHP) characterized by coesite or diamond are predominantly Phanerozoic phenomena. HP-UHP metamorphism registers the low thermal gradients and deep subduction of continental crust during the early stage of subduction-to-collision orogenesis. A duality of metamorphic belts—reflecting a duality of thermal regimes—appears in the record only since the Neoarchean Era. A duality of thermal regimes is the hallmark of modern plate tectonics, and the duality of metamorphic belts is the characteristic imprint of plate tectonics in the rock record. The occurrence of both UHT and EHP granulite metamorphism since the Neoarchean marks the onset of a “Proterozoic plate tectonics” regime, which evolved during a Neoproterozoic transition to the modern plate tectonics regime, characterized by colder subduction as chronicled by HP-UHP metamorphism.

Ultrahigh‐pressure metamorphism and exhumation of garnet peridotite in Pohorje, Eastern Alps

AbstractNew evidence for ultrahigh‐pressure metamorphism (UHPM) in the Eastern Alps is reported from garnet‐bearing ultramafic rocks from the Pohorje Mountains in Slovenia. The garnet peridotites are closely associated with UHP kyanite eclogites. These rocks belong to the Lower Central Austroalpine basement unit of the Eastern Alps, exposed in the proximity of the Periadriatic fault. Ultramafic rocks have experienced a complex metamorphic history. On the basis of petrochemical data, garnet peridotites could have been derived from depleted mantle rocks that were subsequently metasomatized by melts and/or fluids either in the plagioclase‐peridotite or the spinel‐peridotite field. At least four stages of recrystallization have been identified in the garnet peridotites based on an analysis of reaction textures and mineral compositions. Stage I was most probably a spinel peridotite stage, as inferred from the presence of chromian spinel and aluminous pyroxenes. Stage II is a UHPM stage defined by the assemblage garnet + olivine + low‐Al orthopyroxene + clinopyroxene + Cr‐spinel. Garnet formed as exsolutions from clinopyroxene, coronas around Cr‐spinel, and porphyroblasts. Stage III is a decompression stage, manifested by the formation of kelyphitic rims of high‐Al orthopyroxene, aluminous spinel, diopside and pargasitic hornblende replacing garnet. Stage IV is represented by the formation of tremolitic amphibole, chlorite, serpentine and talc. Geothermobarometric calculations using (i) garnet‐olivine and garnet‐orthopyroxene Fe‐Mg exchange thermometers and (ii) the Al‐in‐orthopyroxene barometer indicate that the peak of metamorphism (stage II) occurred at conditions of around 900 °C and 4 GPa. These results suggest that garnet peridotites in the Pohorje Mountains experienced UHPM during the Cretaceous orogeny. We propose that UHPM resulted from deep subduction of continental crust, which incorporated mantle peridotites from the upper plate, in an intracontinental subduction zone. Sinking of the overlying mantle and lower crustal wedge into the asthenosphere (slab extraction) caused the main stage of unroofing of the UHP rocks during the Upper Cretaceous. Final exhumation was achieved by Miocene extensional core complex formation.

Metallogenesis of superlarge gold deposits in Jiaodong region and deep processes of subcontinental lithosphere beneath North China Craton in Mesozoic

The study of ore-forming chronology indicates that the superlarge gold deposits in the Jiaodong region were formed in 120±10 Ma. Sr-Nd-Pb isotopic compositions from typical gold deposits suggest that ore-forming materials were derived from the multisources, mantle component was partly involved in mineralization, the deep dynamic processes are the major geological background of large-scale metallogenesis in the Jiaodong region in Mesozoic. The deep processes mainly include the effect of post deep-subduction of continental crust of the central orogen belt and the distant effect of subduction of the paleo-Pacific plate underneath the Eurasian continent. However, lithosphere thinning, crust-mantle interaction, crustal extension and formation of large-type ore-controlling structures would be the comprehensive consequences of the above-mentioned geodynamic processes in the region.

On Continent‐Continent Point‐Collision and Ultrahigh‐Pressure Metamorphism

Abstract Up to now it is known that almost all ultrahigh‐pressure (UHP) metamorphism of non‐impact origin occurred in continent‐continent collisional orogenic belt, as has been evidenced by many outcrops in the eastern hemisphere. UHP metamorphic rocks are represented by coesite‐ and diamond‐bearing eclogites and eclogite facies metamorphic rocks formed at 650–800°C and 2.6–3.5 GPa, and most of the protoliths of UHP rocks are volcanic‐sedimentary sequences of continental crust. From these it may be deduced that deep subduction of continental crust may have occurred. However, UHP rocks are exposed on the surface or occur near the surface now, which implies that they have been exhumed from great depths. The mechanism of deep subduction of continental crust and subsequent exhumation has been a hot topic of the research on continental dynamics, but there are divergent views. The focus of the dispute is how deep continental crust is subducted so that UHP rocks can be formed and what mechanism causes it to be subducted to great depths and again exhumed to the shallow surface. Through an analysis of the continental process and mechanical boundary conditions of the Dabie collisional belt—an UHP metamorphic belt where the largest area of UHP rocks in the world is exposed, this paper discusses the variations of viscous stresses and average pressure in the viscous fluid caused by tectonism with rock physical properties and the contribution of the tectonic stresses to production of UHP. Calculation indicates that the anomalous stress state on the irregular boundary of a continental block may give rise to stress concentration and accumulation at local places (where the compressional stress may be 5–9 times higher than those in their surroundings). The tectonic stresses may account for 20–35% of the total UHP. So we may infer that the HP (high‐pressure)‐UHP rocks in the Dabie Mountains were formed at depths of 60–80 km. Thus the authors propose a new genetic model of UHP rocks—the point‐collision model. This model conforms to the basic principles of the mechanics and also to the geologic records and process in the Dabie orogenic belt. It can explain why UHP rocks do not exist along the entire length of the collisional orogen but occur in some particular positions. The authors also propose that the eastern and western corners of the Himalaya collision zone are typical point‐collision areas and that almost all UHP metamorphism of continental crustal rocks occurred in the two particular positions.

Geochronological Evidence for the In situ Tectonic Relationship in the Dabie UHP Metamorphic Terrane, Central China

The Dabie Mountains and the Su-Lu region in central China are known to contain the largest distribution of ultrahigh pressure metamorphic (UHPM) rocks in the world. They are parts of the Qinling-Dabie orogen formed by collision between the Sino-Korean and Yangtse cratons. The preservation of UHP minerals has inspired different models to explain the deep subduction of continental crust and the mechanisms for subsequent unroofing of the UHPM rocks. In the absence of good constraints from geochemical, isotopic and age data for different lithotectonic units, numerous tectonic models have been proposed. It appears that the greatest controversy to date concerns the primary age and the nature of eclogite protoliths, their tectonic relation with the associated ultramafic rocks and enclosing granitic gneisses ('in situ' vs 'foreign' hypothesis), the areal extent of the UHPM rocks, and their mode of exhumation. Here we report new geochemical and Sr-Nd for the host gneisses of the largest coesite-bearing eclogite body in this terranne the Bixiling Complex. These data will be used to tackle the problem of eclogitegneiss tectonic relationship. The Bixiling Complex has well preserved its UHP paragenesis of which coesite and quartz pseudomorph occur as inclusions within garnet or omphacite. In contrast, the host gneisses exhibit a typical phase assemblage of the albite-epidote amphibolite facies (plagioclase + biotite + zoisite + K-feldspar + quartz + muscovite + apatite) corresponding to the retrograde metamorphism of the Bixiling Complex. However, the presence of garnet, though very minor in amount, may suggest a previous higher metamorphic condition. The country rocks are paragneisses or orthogneisses of TTG composition (tonalite-trondhjemite-granodiorite). They are all characterized by negative Ti, P, Ta and Nb anomalies in their spidergrams, hence suggesting their protolith formation in ancient arc environments. Much of the protoliths of the gneisses from the Dabie UHP terrane was probably formed in late Proterozoic (750-800 Ma; Ames et al., 1996; Rowley et al., 1997; Hacker et al., 1998). However, there is indication of older protoliths from Sm-Nd model age (TDM) data for gneisses from Shuanhe (Chen and Jahn, 1998). The Bixiling gneisses have TDM of 1.3 to 1.5 Ga, and and(800 Ma) = +4.2 _+ 0.4, indicating their late Proterozoic formation and juvenile character. Geochronological constraints are obviously essential for understanding the gneiss-eclogite tectonic relationship. Rb-Sr isotope analyses of whole-rock and constituent phases for five gneisses yielded a tight range of Ms-WR-Zo isochron ages from 194 to 198 Ma, and Bt-Zo isochron ages from 170 to 184 Ma (Fig. l and 2). On face values, the Ms Rb-Sr ages are only slightly younger than the Phg-WR Rb-Sr ages obtained on the coesite-bearing eclogite from the Bixling Complex (Fig. 3; 198 to 223 Ma; Chavagnac and Jahn, 1996). Moreover, the Bt RbSr ages are also in agreement with that of an amphibolite near the Bixiling Complex. Thus, when combined the new age information with the published zircon age data (Ames et al., 1996;