Showing papers by "Yong-Fei Zheng published in 2019"

Subduction zone geochemistry

Abstract: Crustal recycling at convergent plate boundaries is essential to mantle heterogeneity. However, crustal signatures in the mantle source of basaltic rocks above subduction zones were primarily incorporated in the form of liquid rather than solid phases. The physicochemical property of liquid phases is determined by the dehydration behavior of crustal rocks at the slab-mantle interface in subduction channels. Because of the significant fractionation in incompatible trace elements but the full inheritance in radiogenic isotopes relative to their crustal sources, the production of liquid phases is crucial to the geochemical transfer from the subducting crust into the mantle. In this process, the stability of specific minerals in subducting crustal rocks exerts a primary control on the enrichment of given trace elements in the liquid phases. For this reason, geochemically enriched oceanic basalts can be categorized into two types in terms of their trace element distribution patterns in the primitive mantle-normalized diagram. One is island arc basalts (IAB), showing enrichment in LILE, Pb and LREE but depletion in HFSE such as Nb and Ta relative to HREE. The other is ocean island basalts (OIB), exhibiting enrichment in LILE and LREE, enrichment or non-depletion in HFSE but depletion in Pb relative to HREE. In either types, these basalts show the enhanced enrichment of LILE and LREE with increasing their incompatibility relative to normal mid-ocean ridge basalts (MORB). The thermal regime of subduction zones can be categorized into two stages in both time and space. The first stage is characterized by compressional tectonism at low thermal gradients. As a consequence, metamorphic dehydration of the subducting crust prevails at forearc to subarc depths due to the breakdown of hydrous minerals such as mica and amphibole in the stability field of garnet and rutile, resulting in the liberation of aqueous solutions with the trace element composition that is considerably enriched in LILE, Pb and LREE but depleted in HFSE and HREE relative to normal MORB. This provides the crustal signature for the mantle sources of IAB. The second stage is indicated by extensional tectonism at high thermal gradients, leading to the partial melting of metamorphically dehydrated crustal rocks at subarc to postarc depths. This involves not only the breakdown of hydrous minerals such as amphibole, phengite and allanite in the stability field of garnet but also the dissolution of rutile into hydrous melts. As such, the hydrous melts can acquire the trace element composition that is significantly enriched in LILE, HFSE and LREE but depleted in Pb and HREE relative to normal MORB, providing the crustal signature for the mantle sources of OIB. In either case, these liquid phases would metasomatize the overlying mantle wedge peridotite at different depths, generating ultramafic metasomatites such as serpentinized and chloritized peridotites, and olivine-poor pyroxenites and hornblendites. As a consequence, the crustal signatures are transferred by the liquid phases from the subducting slab into the mantle.

Ultrahigh-pressure metamorphic rocks in the Dabie-Sulu orogenic belt: compositional inheritance and metamorphic modification

Abstract: Abstract The Dabie–Sulu orogenic belt in east-central China contains one of the largest ultrahigh-pressure (UHP) metamorphic terranes in the world. The UHP eclogites are associated with gneiss, peridotite and marble. But all these rocks underwent in situ UHP metamorphism during the continental collision in the Triassic. Although fluid action is not significant during cold subduction, it becomes prominent during hot exhumation of UHP slices. Whereas the composition of UHP metamorphic rocks is primarily inherited from their protoliths, they were locally modified by partial melting to varying extents. The partial melting of UHP rocks is highly heterogeneous along the collisional orogen, and achieves a maximum during rifting orogeny at the post-collisional stage. This paper outlines the petrographical features and geochemical compositions of UHP metamorphic rocks, and presents an overview of collisional modification and post-collisional reworking of the subducted continental crust. Further discussions are devoted to continental subduction tectonics, as well as to subduction style, subduction polarity and exhumation mechanism. The results also provide insights into the initiation of continental rifting in a Wilson cycle. Consequently, studies of the Dabie–Sulu orogenic belt have contributed greatly to our understanding of tectonic processes, fluid regime and chemical geodynamics in continental subduction zones.

Hydrothermal ore deposits in collisional orogens

Abstract: Hydrothermal ore deposits at convergent plate boundaries represent extraordinary metal enrichment in the continental crust. They are generally associated with felsic magmatism in extensional settings at high thermal gradients. Although their formation is common during accretionary orogeny, more and more ore deposits have been discovered recently in the collisional orogens of China. Because collisional orogeny was operated in a compressional regime at low thermal gradients, it is not favorable for mobilization of ore-forming elements and thus for the production of hydrothermal ore deposits. Nevertheless, continental collision is generally preceded by oceanic subduction, which enables the preliminary enrichment of ore-forming elements in the mantle wedge due to chemical metasomatism by subducting slab-derived fluids. This gave rise to metal pre-enriched domains in the overriding lithosphere, which may be reactivated by extensional tectonism for hydrothermal mineralization either immediately during accretionary orogeny or at a later time during and after collisional orogeny. It is these tectonic processes that have resulted in the progressive enrichment of ore-forming elements through the geochemical differentiation of the subducting oceanic crust, the metasomatic mantle domains and the mafic juvenile crust, respectively, at different depths. Finally, the reactivation of metal pre-enriched domains by continental rifting in the orogenic lithosphere is the key to the metallogenesis of collisional orogens.

The Origin of Garnets in Anatectic Rocks from the Eastern Himalayan Syntaxis, Southeastern Tibet: Constraints from Major and Trace Element Zoning and Phase Equilibrium Relationships

Abstract: Amphibolite- and granulite-facies metamorphic rocks are common in the eastern Himalayan syntaxis of southeastern Tibet. These rocks are composed mainly of gneiss, amphibolite and schist that underwent various degrees of migmatization to produce leucogranites, pegmatites and felsic veins. Zircon U–Pb dating of biotite gneiss, leucocratic vein and vein granite from the syntaxis yields consistent ages of ∼49 Ma, indicating crustal anatexis during continental collision between India and Asia. Garnets in these rocks are categorized into peritecitc and anatectic varieties based on their mode of occurrence, mineral inclusions and major- and trace-element zoning. The peritectic garnets mainly occur in the biotite gneiss (mesosome layer) and leucocratic veins. They are anhedral and contain abundant mineral inclusions such as high-Ti biotites and quartz, and show almost homogeneous major-element compositions (except Ca) and decreasing HREE contents from core to rim, indicating growth during the P- and T-increasing anatexis. Peak anatectic conditions at 760–800°C and 9–10·5 kbar are well constrained by phase equilibrium calculations, mineral assemblages, and garnet isopleths. In contrast, anatectic garnets only occur in the vein granite. They are round or subhedral, contain quartz inclusions, and exhibit increasing spessartine and trace-element contents from core to rim. The garnet–biotite geothermometry and the garnet–biotite–plagioclase–quartz geobarometry suggest that the anatectic garnets crystallized at ∼620–650°C and 4–5 kbar. Some garnet grains show two-stage zoning in major and trace elements, with the core similar to the peritectic garnet but the rim similar to the anatectic garnet. Mineralogy, whole-rock major- and trace-element compositions and zircon O isotopes indicate that the two types of leucosomes were produced by hydration (water-present) melting and dehydration (water-absent) melting, respectively. The leucocratic veins contain peritectic garnet but no K-feldspar, have lower whole-rock K2O contents and Rb/Sr ratios, higher whole-rock CaO contents and Sr/Ba ratios, and show homogeneous δ18O values that are lower than those of relict zircons, indicating that such veins were produced by the hydration melting. In contrast, the vein granite contains peritectic garnet and K-feldspar, has higher whole-rock K2O contents and Rb/Sr ratios, lower whole-rock CaO contents and Sr/Ba ratios, and shows comparable δ18O values with those of relict zircons, suggesting that this granite were generated by the dehydration melting. Accordingly, both hydration and dehydration melting mechanisms have occurred in the eastern Himalayan syntaxis.

Enhanced weathering as a trigger for the rise of atmospheric O 2 level from the late Ediacaran to the early Cambrian

Abstract: A shift toward a higher oxygen level in both ocean and atmosphere systems during the late Ediacaran to the early Cambrian has been suggested from multiple indirect proxies. However, the mechanism and magnitude of this oxidation remain unclear. To solve this issue, we measured carbon isotopes in both carbonate and organic matter as well as their trace element compositions for an Ediacaran-Cambrian sequence in the Lower Yangtze basin, South China. The δ13Corg and δ13Ccarb excursions of this sequence are coupled and can be compared with contemporaneous global carbon isotope curves. A 2‰ rise in Δ13Ccarb-org occurred from the late Ediacaran to the early Cambrian, suggesting a substantial increase in atmospheric oxygen level from 16% to 30% of the present atmospheric level (PAL). Furthermore, the distribution pattern of rare earth elements and the concentrations of water-insoluble elements in the carbonates indicate a sudden enhancement in chemical weathering of the continental crust during the early Cambrian, which may be a trigger for the rise of atmospheric O2 level. Both the supply of a large amount of nutrients due to the enhanced continental weathering and the contemporary increase of atmospheric oxygen concentrations may have promoted the appearance of large metazoans in the early Cambrian.

Geochemical Evidence for Hydration and Dehydration of Crustal Rocks During Continental Rifting

Abstract: To understand the temporal sequence of geological processes such as magmatism, water‐rock interaction, and metamorphism in fossil continental rifts, a combined study of petrography, mineral geochemistry, in situ garnet O isotopes, in situ zircon U‐Pb ages and O isotopes, and pseudosection calculations was conducted for metagranites from a Neoproterozoic continental rift generated during the Rodinia breakup. The results provide insights into the operation of hydration and dehydration during continental rifting. In the metagranites from the northern margin of South China, three types of garnet (Garnet‐I to ‐III) are distinguished. They were sequentially produced by hydrothermal alteration, metamorphic dehydration, and fluid metasomatism. All of these garnets show negative δ18O values of −19.3‰ to −14.5‰, in contrast to mantle‐like δ18O values for magmatic zircon. The extremely negative δ18O values of hydrothermal Garnet‐I require infiltration of the continental deglacial meltwater during the continental rifting, and before that zircon crystallized from normal δ18O magmas. Once the rocks were hydrothermally altered, the extreme 18O depletion was retained in all later products such as metamorphic Garnet‐II and metasomatic Garnet‐III. Pseudosection calculations indicate that the metamorphic dehydration occurred at 1.0–3.0 kbar and 630–690 °C during a reheating stage, corresponding to high thermal gradients of >60 °C/km. The high‐temperature/low‐pressure metamorphic rocks produced by such high thermal gradients are indicative of the continental rift setting. The mineral geochemistry records not only the temporal sequence of rift magmatism, water‐rock interaction, and rift metamorphism but also the evolution of temperature and water action in the crust during the continental rifting.