Steep HREE Patterns Research Articles

Zircon petrochronological evidences for diachronous tectonic switch and orogenic keel collapse of the Tongbai-Dabie-Sulu orogens

Migmatites can be produced in different stages of orogeny and thus have a great significance to constrain the evolution of an orogenic belt. Migmatites widely occur in the Tongbai orogen, but their formation ages and geological significance have not been well constrained. In this study, a combination of cathodoluminescence (CL) imaging, U-Pb-Hf-O isotopes and trace elements of zircons were carried for migmatites in the Tongbai orogen. Most inherited cores in a melanosome show variable UPb ages and Hf isotope compositions with high δ18O values, suggesting they might be detrital zircons from its protolith. Whereas other relict cores in this sample have nebulously zoning or no zoning, flat HREE patterns with insignificant Eu anomalies and relatively low formation temperatures, indicating their formation under eclogite-facies conditions. They yield UPb ages of 228 ± 3 Ma, providing direct evidence for the involvement of Triassic subducted continental materials. The overgrowth rims and some new grown grains in the migmatites show euhedral shapes, low CL luminescence, high Th/U ratios, clear negative Eu anomalies and steep HREE patterns, suggesting they are anatectic zircons formed during partial melting. They yield UPb ages from 139 ± 2 to 132 ± 1 Ma. Combined with previous studies, we suggest that migmatization mainly occurred at 139–130 Ma in the Tongbai orogen. The anatectic zircons in the leucosomes show much lower δ18O values (5.79 ± 0.23‰ vs. 8.94 ± 0.16‰) and higher 176Hf/177Hf ratios (0.282302–0.282648 vs. 0.281728–0.282504) than the zircons in adjacent melanosomes, suggesting that the leucosomes might be externally sourced. According to a compilation of the geochronological information of migmatite and magmatism, the Tongbai-Dabie-Sulu orogens have a spatial and temporal variation of partial melting and magma activities in the post-collisional stage. These might result from the diachronical tectonic switch and collapse of the orogenic lithospheric keels caused by the subduction and rollback of the Paleo-Pacific slab.

The effect of crystal fractionation on the geochemical composition of syn-exhumation magmas: Implication for the formation of high δ56Fe granites in collisional orogens

Syn-exhumation magmatism in collisional orogens is an important process for crustal differentiation and crust-mantle interaction at convergent plate boundaries. It is intriguing to elucidate which factors control the geochemical composition of such magmatic products. To answer this question, a combined study of whole-rock major-trace elements and Nd-Fe isotopes, mineral O isotopes, and zircon U–Pb ages and trace elements was carried out for syn-exhumation granites from the Sulu ultrahigh-pressure (UHP) metamorphic belt in east-central China. Several granitic plutons of Triassic age were newly found in this typical collisional orogen, with a total exposure area of ∼10 km2, suggesting large-scale partial melting of the deeply subducted continental crust in the terminal stage of continental collision. Field observations indicate that the massive granites show little deformation. The granites are high-K calc-alkaline and exhibit enrichment in LILE and LREE but depletion in HFSE relative to HREE in trace element distribution patterns. Zircons in these granites contain relict magmatic cores characterized by steep HREE patterns with strong negative Eu anomalies. These cores show middle Neoproterozoic U-Pb ages of 724–779 Ma, consistent with the protolith age of UHP metaigneous rocks in the Dabie-Sulu orogenic belt. Zircon rims exhibit oscillatory zoning, and are characterized by Triassic U-Pb ages of 210–216 Ma, low LREE contents and steep HREE patterns with significant negative Eu anomalies, suggesting their growth from granitic magmas during exhumation of the deeply subducted continental crust. The granites have low δ18O values of 2.0–6.1‰ for quartz, 1.1–5.2‰ for whole-rock, and 0.9–3.2‰ for zircon cores. The low δ18O values and Neoproterozoic U-Pb ages of zircon cores in the granites are characteristic feature of the deeply subducted continental crust in the northern margin of the South China Block. One coesite inclusion was found in the anatectic zircon rim, pointing to the deep origin of anatectic melts at a subarc depth of >80 km. Whole-rock SiO2 contents are correlated with major and trace elements, which are ascribed to crystal fractionation of mainly biotite and plagioclase during magma evolution. In particular, these granites exhibit highly variable δ56Fe values from 0.05 to 0.30‰, which are correlated with Fe3+/ΣFe, SiO2, Al2O3, Nb/Ta and Eu/Eu*. This suggests that the crystal fractionation of biotite would have controlled the Fe isotope variation in the granites. Therefore, the syn-exhumation granites experienced significant geochemical differentiation mainly through the fractional crystallization during the magma evolution. A compilation of syn-exhumation granites in the typical collisional orogens of the world shows that such granites were mainly formed through dehydration melting of hydrous minerals during decompressional exhumation. They fall into the magnesian group with relatively low REE and HFSE (like Ti, Nb and Zr) contents, distinct from the geochemical compositions of A-type granites. Therefore, syn-exhumation granites record crustal differentiation in the terminal stage of continental collision.

Geochemistry of high-pressure to ultrahigh-pressure granitic melts produced by decompressional melting of deeply subducted continental crust in the Sulu orogen, east-central China

The production of granitic melts by decompressional melting of deeply subducted crustal rocks under high-pressure (HP) to ultrahigh-pressure (UHP) conditions is important for the crust-mantle interaction at convergent plate boundaries. However, little is known on the geochemical composition of natural HP to UHP granitic melts from collisional orogens. This problem is partially resolved here through an integrated study of the Yangkou granitic veins in the Sulu orogen, which provide the excellent records of HP to UHP granitic melts from the deeply subducted continental crust. In trace element composition, these granitic veins show enrichment in LREE and LILE but depletion in HFSE with remarkable negative anomalies of Nb, Ta, P and Ti. They have low Sr/Y (5.92–26.3) and (La/Yb)N ratios (12.7–45.6), which are primarily controlled by the solubility of accessory minerals such as apatite and epidote/allanite in anatectic melts. Phengite in the granitic veins have high Si (3.29–3.42 p.f.u.) and Mg (0.16–0.35 p.f.u.) with Mg# of 37.7–68.1 and calculated pressures of 1.7–3.1 GPa, indicating their crystallization under HP to UHP conditions. Geochemical variables such as K2O/Na2O, A/CNK and LREE for whole-rock exhibit a positive correlation with pressure, indicating the significant pressure effect on the mineral stability that in turn controls the composition of anatectic melts. Furthermore, the granitic veins can be divided into two groups based on their formation pressures and geochemical compositions. Group I granitic veins formed at low pressures of 1.7–2.4 GPa and exhibit higher SiO2 and Na2O contents, but lower contents of K2O, MgO, TiO2, P2O5 and most trace elements (e.g., Rb, Ba, LREE) with lower K2O/Na2O ratios and A/CNK values compared to Group II granitic veins that formed at high pressure of 2.6–3.1 GPa. Zircon from the granitic veins give concordant U–Pb ages of 755 ± 10 to 769 ± 9 Ma with steep HREE patterns for relict magmatic cores and 216 ± 4 to 222 ± 4 Ma with steep to flattened HREE patterns for anatectic rims. In comparison to concordant U–Pb ages of 220 ± 3 to 227 ± 3 Ma for newly grown zircon in the host eclogites, it appears that these granitic veins were crystallized from anatectic melts produced by crustal anataxis during the exhumation of the deeply subducted continental crust. The anatectic rims are characterized by the absence of not only oscillatory zoning in CL images but also Eu anomalies and low U contents, indicating their growth from anatectic melts under HP to UHP conditions. The granitic veins show different Sr–Nd–O isotope compositions from the host UHP eclogites, indicating that the former did not originate from partial melting of the latter. This is also supported by the distinct δ18O and εHf(t) values for the Triassic zircon from the granitic veins and eclogites. Instead, there are comparable isotopic compositions between the granitic veins and regional UHP granitic gneisses, indicating that the granitic veins would be derived from partial melting of the underlying UHP granitic gneisses. Group II granitic veins have higher εNd(t) values than Group I ones and the granitic gneisses by 3–5 εNd unit, indicating that the anatectic melts forming Group II granitic veins are in Nd isotope disequilibrium with their source rocks. This can be caused by the behavior of accessory minerals like apatite and epidote/allanite during crustal anataxis at different pressures. The contributions of apatite together with garnet can also account for the increased Hf isotopes in the anatectic zircon. Therefore, the accessory minerals play a key role in constraining the geochemical features of incompatible-element enriched granitic melts during the crustal anatexis under HP to UHP conditions.

Zircon evidence for the Eoarchean (~3.7 Ga) crustal remnant in the Sulu Orogen, eastern China

Zircon provides one of the best records of the formation and reworking of continental crust in the early Earth. However, Hadean to Eoarchean zircons are relatively scarce worldwide. Here we present the first report of relict Eoarchean magmatic zircons in granitic gneisses from the Sulu Orogen, eastern China. Based on internal structures, trace element contents, and U–Pb ages, we identified four groups of zircon domains with U–Pb ages of ~ 3.7 Ga (Group I), ~2.1 Ga (Group II), ~790 Ma (Group III), and ~720 Ma (Group IV). Group I domains exhibit variable Th/U ratios, steep HREE patterns, and negative Eu anomalies. They yield lower intercept U–Pb ages of 1.82–1.95 Ga and discordia upper intercept ages of 3.65–3.69 Ga that are similar to the oldest concordant spot age of 3680 ± 29 Ma. This indicates their growth from an Eoarchean magma and reworking during the Paleoproterozoic. The oldest Eoarchean domains with U–Pb ages of 3606 ± 28 to 3680 ± 29 Ma have low P contents of 216–563 ppm and high (Y + REE)/P molar ratios of 1.13–3.34, consistent with an igneous source. They show εHf(t) values of –2.8 to –0.9 at 3.67 Ga and TCHUR2 ages of 3.7–4.0 Ga, suggesting the growth of juvenile crust during the early Eoarchean. Group II to IV domains have consistent TDM2 ages of 2.6–3.0 Ga, suggesting that they grew during multiple reworkings of the Archean crust. Group II domains have variable Th/U ratios and steep to flattened HREE patterns that suggest growth during Paleoproterozoic crustal anatexis. Groups III and IV zircon domains have Th/U ratios and trace element contents that indicate growth from magmas that formed during Neoproterozoic continental rifting. In view of the unique feature of Neoproterozoic rifting magmatism in South China, the relict Eoarchean magmatic zircons would have originated in the Yangtze Craton and then undergone multiple phases of reworking during the Paleoproterozoic and Neoproterozoic. The results indicate the presence of ~3.7 Ga relict magmatic zircons in the Sulu Orogen, and they represent the oldest remnants of crustal material in the Yangtze Craton.

Paleoproterozoic tectonic evolution of the Yangtze Block: New evidence from ca. 2.36 to 2.22 Ga magmatism and 1.96 Ga metamorphism in the Cuoke complex, SW China

The infrequent early Paleoproterozoic rocks are essential for investigating the early evolution of the Yangtze Block and its reconstruction in the global supercontinent. Here, we carried out an integrated study of petrology, whole-rock geochemistry, as well as zircon U-Pb-Hf isotope compositions for granitoids from the recently discovered Cuoke complex in the southwestern Yangtze Block. SHRIMP zircon U-Pb dating yields magmatic crystallization ages of ca. 2.22 and 2.35 Ga for two monzogranites, ca. 2.36 Ga for a granodiorite, and ca. 2.34 Ga for a granitic gneiss. The granitic gneiss also contains metamorphic zircons that give an age of ca. 1.96 Ga. These results, compiled with published geochronological data, suggest that the western part of the Yangtze Block (WYB) features extensive magmatism with ages of ca. 2.5–2.2 Ga and multiple regional metamorphism with peaks ca. 2.50, 2.36, 1.96 and 1.85 Ga. This characteristic is significantly different from the eastern part of the Yangtze Block (EYB), which is dominated by the ca. 2.15–1.95 Ga subduction- and collision-related metamorphism and magmatism with a peak ~2.0 Ga. Our studied granitoids show negative zircon εHf(t) values from −14.0 to −1.9, with corresponding two-stage model ages of ca. 3.50–2.93 Ga, indicative of their derivation by reworking of ancient crustal materials. Geochemically, the ca. 2.22 Ga monzogranites have high HFSEs (Zr + Nb + Ce + Y = 777–1136 ppm) and 10,000*Ga/Al ratios (3.73–4.56) with dramatically negative Eu, Ba, Sr and Ti anomalies. Together with high Y/Nb (2.75–3.15) and Yb/Ta (2.11–2.94) ratios, they are classified as A2-type granites. The ca. 2.36–2.35 Ga granitoids have high CIPW-normative corundum contents of 4.92–6.06 vol% and are strongly peraluminous with A/CNK values of 1.46–1.64, similar to S-type granites. They are enriched in LREEs ((La/Yb)N = 14.55–25.33) with moderately negative Eu anomalies (Eu/Eu* = 0.56–0.66), but depleted in Nb, Sr and Ti. The ca. 2.34 Ga granitic gneisses show intermediate geochemical signatures between S-type and I-type granites. They are featured by no discernible negative Eu and Sr anomalies and relatively steep HREE patterns with (Gd/Yb)N ratios between 1.94 and 2.50. In combination with available observations, our new geochronological and geochemical dataset indicates that these ca. 2.36–2.34 Ga granitoids represent the post-collisional magmatism immediately following a possible collision event in the WYB, while the 2.22 Ga A2-type monzogranites were likely formed in a back-arc extensional setting. In addition, we further suggest that the Yangtze Block and NW Laurentia share a common succession of major Paleoproterozoic tectonothermal events, favoring their tight spatial linkage in the global Columbia supercontinent.

Petrological and zircon evidence for the Early Cretaceous granulite‐facies metamorphism in the Dabie orogen, China

AbstractAn integrated study of petrology, mineralogy, geochemistry, and geochronology was carried out for contemporaneous mafic granulite and diorite from the Dabie orogen. The results provide evidence for granulite‐facies reworking of the ultrahigh‐pressure (UHP) metamorphic rock in the collisional orogen. Most zircons from the granulite are new growth, and their U‐Pb ages are clearly categorized into two groups at 122–127 Ma and 188.2 Ma. Although these two groups of zircons show similarly steep HREE patterns and variably negative Eu anomalies, the younger group has much higher U, Th and REE contents and Th/U ratios, much lower εHf(t) values than the older group. This suggests their growth is associated with different types of dehydration reactions. The older zircon domains contain mineral inclusions of Grt, Cpx and Qz, indicating their growth through metamorphic reactions at high pressures. In contrast, the young zircon domains would have grown through peritectic reaction at low to medium pressures. The younger granulite‐facies metamorphic age is in agreement not only with the adjacent diorite at 125.1 Ma in this study but also the voluminous emplacement of coeval mafic and felsic magmas in the Dabie orogen. Mineral separates from both mafic granulite and its adjacent diorite show uniformly lower δ18O values than normal mantle, similar to those for UHP eclogite‐facies metaigneous rocks in the Dabie orogen. In combination with major‐trace elements and zircon Lu‐Hf isotope compositions, it is inferred that the protolith of mafic granulites shares with the source rock of diorites, both being a kind of mafic metasomatites at the slab‐mantle interface in the continental subduction channel. This provides a direct link in petrogenesis between the granulitic, migmatic and magmatic rocks in the collisional orogen to active continental rifting, whereby high heat flow was transferred from the asthenospheric mantle into the thinned orogenic lithosphere for partia melting.

Reply to “Comment on “Isotopic insight on paleodiet of …” by Bocherens et al. (Gondwana Research, 48(1), 7–14)”

We report here for the first time the occurrence of sapphirine-bearing rocks within the Paleoproterozoic granulite complex of the Angara-Kan block (SW Siberian craton) and provide strong evidence for crustal metamorphism at ultrahigh-temperature (UHT) conditions in this region. The UHT rocks contain sapphirine, high alumina orthopyroxene, orthopyroxene + sillimanite assemblage, mesoperthitic feldspar and pseudomorphoses after osumilite crystals. The peak temperature of metamorphism was estimated using Al2O3 contents in Opx (up to 10 wt%) and the ternary feldspar composition, and falls in a range of 1050–1100 °C. Observed mineral microstructures indicate post-peak cooling of rocks without a considerable decrease in pressure down to a temperature of about 900 °C. Another type of rock related with UHT metamorphism is represented by orthopyroxene-sillimanite gneisses, which recorded a peak temperature of about 950 °C, decreasing down to 900 °C during post-peak stage. The UHT gneisses record microstructural evidence of melt injection and matrix-melt interaction near the peak metamorphic conditions. Sapphirine grains occur in the gneiss with relatively low Mg bulk composition. It is suggested that sapphirine occurs in the Mg-Al-rich domains which have been formed as a result of partial melting and precipitation of Fe-oxides from the melt. High concentrations of Al2O3 in orthopyroxene are the only sign of UHT metamorphism in Fe-rich rock matrix which is not affected by melt. New SHRIMP U-Pb zircon data from two UHT gneisses indicate the presence of two well-defined age peaks at ca. 1.8 and 1.9 Ga from rims and metamorphic grains, and older dates from ca. 1.91 to 3.15 Ga from detrital cores. The prominent feature of the metamorphic zircons is depletion of U with high Th/U ratio up to 6.0. Steep HREE patterns of the metamorphic zircons suggest their growth in the presence of a melt. The metamorphic zircons from the UHT gneiss record a broad range of temperatures between 1000 °C and 810 °C indicating protracted zircon growth from near peak conditions through retrograde stage of PT-path. The growth of the youngest zircons occurred at higher temperature indicating that the UHT metamorphic conditions were reached at 1.78–1.8 Ga. The timespan of the granulite-UHT metamorphic event is near 50 Ma and is limited by intrusions of late leucogranites and charnockites at 1.75–1.73 Ga. Protracted interval of granulite-UHT metamorphism conforms with isobaric cooling P-T path constructed by mineral assemblages.

Sapphirine-bearing Fe-rich granulites in the SW Siberian craton (Angara-Kan block): Implications for Paleoproterozoic ultrahigh-temperature metamorphism

We report here for the first time the occurrence of sapphirine-bearing rocks within the Paleoproterozoic granulite complex of the Angara-Kan block (SW Siberian craton) and provide strong evidence for crustal metamorphism at ultrahigh-temperature (UHT) conditions in this region. The UHT rocks contain sapphirine, high alumina orthopyroxene, orthopyroxene+sillimanite assemblage, mesoperthitic feldspar and pseudomorphoses after osumilite crystals. The peak temperature of metamorphism was estimated using Al2O3 contents in Opx (up to 10wt%) and the ternary feldspar composition, and falls in a range of 1050–1100°C. Observed mineral microstructures indicate post-peak cooling of rocks without a considerable decrease in pressure down to a temperature of about 900°C. Another type of rock related with UHT metamorphism is represented by orthopyroxene-sillimanite gneisses, which recorded a peak temperature of about 950°C, decreasing down to 900°C during post-peak stage. The UHT gneisses record microstructural evidence of melt injection and matrix-melt interaction near the peak metamorphic conditions. Sapphirine grains occur in the gneiss with relatively low Mg bulk composition. It is suggested that sapphirine occurs in the Mg-Al-rich domains which have been formed as a result of partial melting and precipitation of Fe-oxides from the melt. High concentrations of Al2O3 in orthopyroxene are the only sign of UHT metamorphism in Fe-rich rock matrix which is not affected by melt. New SHRIMP U-Pb zircon data from two UHT gneisses indicate the presence of two well-defined age peaks at ca. 1.8 and 1.9Ga from rims and metamorphic grains, and older dates from ca. 1.91 to 3.15Ga from detrital cores. The prominent feature of the metamorphic zircons is depletion of U with high Th/U ratio up to 6.0. Steep HREE patterns of the metamorphic zircons suggest their growth in the presence of a melt. The metamorphic zircons from the UHT gneiss record a broad range of temperatures between 1000°C and 810°C indicating protracted zircon growth from near peak conditions through retrograde stage of PT-path. The growth of the youngest zircons occurred at higher temperature indicating that the UHT metamorphic conditions were reached at 1.78–1.8Ga. The timespan of the granulite-UHT metamorphic event is near 50Ma and is limited by intrusions of late leucogranites and charnockites at 1.75–1.73Ga. Protracted interval of granulite-UHT metamorphism conforms with isobaric cooling P-T path constructed by mineral assemblages.

Zircon geochemical constraints on the protolith nature and metasomatic process of the Mg-rich whiteschist from the Western Alps

The Mg-rich silicic crustal rocks characterized by high SiO2 and MgO contents are widespread in the Alps, providing rare targets to investigate the origin of Mg-rich fluids in the continental subduction zone. However, it is a prerequisite to elucidate the protolith nature of such Mg-rich rocks, including the possible alteration process experienced before the Alpine metamorphism. For this purpose, we carried out a detailed study of trace elements, O-Hf isotopes and U-Pb ages for zircons from the coesite-bearing whiteschist and its country rock metagranite at Dora-Maria in Western Alps. The whiteschist is characterized by much higher MgO contents than the adjacent metagranite. Relict magmatic zircons from the two types of rocks give essentially the same U-Pb ages of ~262Ma, similar REE contents and patterns, the same range of εHf(t1) values from −6.4 to 2.8 and δ18O values from mostly 9 to 11‰. These features provide compelling evidence that the whiteschist and metagranite share the same protolith, i.e., the Permian granite. All the above geochemical compositions are consistent with the argument that the high MgO content of the whiteschist was acquired from high Mg fluids from dehydration of the serpentinized peridotite at the slab-mantle interface in the continental subduction channel. The hypothesis of a pre-Alpine hydrothermal alteration of the granitic protolith to cause the MgO enrichment is not favorable. Zircon in the metagranite does not show any metamorphic overgrowth during the Alpine orogeny, in contrast to the extensive overgrowth of metamorphic rims at ~35Ma in the whiteschist. The metamorphic rims show relatively low REE contents with variably flattened HREE patterns, different from the relict magmatic zircons that exhibit relative high REE contents with steep HREE patterns. This difference suggests that the breakdown of hydrous minerals plays a crucial role in dictating mineral reactions for zircon growth during the continental subduction-zone metamorphism. This interpretation is also consistent with the local occurrence of garnet coronae along the grain margins of biotite in the metagranite. Therefore, this study provides not only the constraints on the protolith nature and metasomatic process of Mg-rich whiteschist from the Western Alps, but also insights into the role of metamorphic fluids in controlling mineral reactions for the metamorphic growth of both zircon and garnet in the continental subduction zone.

Petrological and zircon evidence for the Early Cretaceous granulite-facies metamorphism in the Dabie orogen, China

An integrated study of petrology, mineralogy, geochemistry, and geochronology was carried out for contemporaneous mafic granulite and diorite from the Dabie orogen. The results provide evidence for granulite-facies reworking of the ultrahigh-pressure (UHP) metamorphic rock in the collisional orogen. Most zircons from the granulite are new growth, and their U-Pb ages are clearly categorized into two groups at 122–127 Ma and 188±2 Ma. Although these two groups of zircons show similarly steep HREE patterns and variably negative Eu anomalies, the younger group has much higher U, Th and REE contents and Th/U ratios, much lower εHf(t) values than the older group. This suggests their growth is associated with different types of dehydration reactions. The older zircon domains contain mineral inclusions of garnet + clinopyroxene ± quartz, indicating their growth through metamorphic reactions at high pressures. In contrast, the young zircon domains only contain a few quartz inclusions and the garnet-clinopyroxene-plagioclase-quartz barometry yields pressures of 4.9 to 12.5 kb. In addition, the clinopyroxene-garnet Fe-Mg exchange thermometry gives temperatures of 738–951 °C. Therefore, the young zircon domains would have grown through peritectic reaction at low to medium pressures. The younger granulite-facies metamorphic age is in agreement not only with the adjacent diorite at 125±1 Ma in this study but also the voluminous emplacement of coeval mafic and felsic magmas in the Dabie orogen. Mineral separates from both mafic granulite and its adjacent diorite show uniformly lower δ18O values than normal mantle, similar to those for UHP eclogite-facies metaigneous rocks in the Dabie orogen. In combination with major-trace elements and zircon Lu-Hf isotope compositions, it is inferred that the protolith of mafic granulites shares with the source rock of diorites, both being a kind of mafic metasomatites at the slab-mantle interface in the continental subduction channel. The spatial and temporal distribution of Early Cretaceous granulite-facies metamorphic rocks in this region is associated with the bimodal magmatism within a short period of 120–130 Ma in the postcollisional stage. This provides a direct link in petrogenesis between the granulitic, migmatic and magmatic rocks in the collisional orogen to active continental rifting, whereby high heat flow was transferred from the asthenospheric mantle into the thinned orogenic lithosphere for partial melting.

Late Paleozoic-Early Mesozoic tectonic evolution of the Paleo-Asian Ocean: geochronological and geochemical evidence from granitoids in the northern margin of Alxa, Western China

The Paleo-Asian Ocean (Southern Mongolian Ocean) ophiolitic belts and massive granitoids are exposed in the Alxa block, in response to oceanic subduction processes. In this work, we report petrographic, geochemical, and zircon U-Pb age data of some granitoid intrusions from the northern Alxa. Zircon U-Pb dating for the quartz diorite, tonalite, monzogranite, and biotite granite yielded weighted mean 206Pb/238U ages of 302±9.2 Ma, 246.5±4.6 Ma, 235±4.4 Ma, and 229.5±5.6 Ma, respectively. The quartz diorites (~302 Ma) exhibit geochemical similarities to adakites, likely derived from partial melting of the initially subducted Chaganchulu back-arc oceanic slab. The tonalites (~246.5 Ma) display geochemical affinities of I-type granites. They were probably derived by fractional crystallization of the modified lithospheric mantle-derived basaltic magmas in a volcanic arc setting. The monzogranites (~235 Ma) are characterized by low Al2O3, but high Y and Yb with notably negative Eu anomalies. In contrast, the biotite granites (~229.5 Ma) show high Al2O3 but low Y and Yb with steep HREE patterns and the absence of negative Eu anomalies. Elemental data suggested that the biotite granites were likely derived from a thickened lower crust, but the monzogranites originated from a thin crust. Our data suggested that the initial subduction of the Chaganchulu oceanic slab towards the Alxa block occurred at ~ 302 Ma. This subduction process continued to the Early Triassic (~246 Ma) and the basin was finally closed before the Middle Triassic (~235 Ma). Subsequently, the break-off of the subducted slab triggered asthenosphere upwelling (240–230 Ma).

Timing of anatexis and melt crystallization in the Socorro–Guaxupé Nappe, SE Brazil: Insights from trace element composition of zircon, monazite and garnet coupled to U[sbnd]Pb geochronology

The timing of partial melting and melt crystallization in granulite facies rocks of the Socorro–Guaxupé Nappe (SGN), Brazil is constrained using a combination of imaging techniques, LA-ICP-MS and EPMA dating, trace element geochemistry and thermobarometry. (Orthopyroxene)-garnet-bearing migmatite that records extensive biotite dehydration melting shows evidence for a clockwise P–T–t path. UHT peak conditions were attained at 1030±110°C, 11.7±1.4kbar, with post-peak cooling to 865±38°C, 8.9±0.8kbar. Cryogenian igneous inheritance of ca. 720–640Ma is identified in oscillatory zoned zircon cores (n=167) with steep HREE patterns. Resorbed, Y-rich monazite cores preserve a prograde growth stage at 631±4Ma prior to the partial melting event, providing an upper age limit for the granulite facies metamorphism in the SGN. REE-rich, Th-depleted monazite related to apatite records the initial stages of decompression at 628±4Ma. Multiple monazite growth episodes record melt crystallization events at 624±3Ma, 612±5Ma and 608±6Ma. Stubby, equant “soccer ball” zircon provide evidence for melt crystallization at 613±2Ma and 607±4Ma. The excess scatter in zircon and monazite age populations between 629±4 and 601±3Ma is interpreted as discontinuous and episodic growth within this age range, characterizing a prolonged metamorphic event in the SGN lasting ca. 30m.y. The development of Y+HREE-rich monazite rims at ca. 600Ma documents retrograde garnet breakdown, extensive biotite growth and the final stages of melt crystallization. Th-rich, Y+HREE-poor monazite rims at ca. 590Ma record monazite recrystallization.

Uncoupled U/Pb and REE response in zircon during the transformation of eclogite to mafic and intermediate granulite (Blanský les, Bohemian Massif)

ABSTRACTAn eclogite–mafic granulite occurs as a rare boudin within a felsic kyanite–K‐feldspar granulite in a low‐strain zone. Its boundary is marked by significant metasomatism–diffusional gain of potassium at the centimetre‐scale, and probable infiltration of felsic melt on a larger scale. This converted the eclogite–mafic granulite into an intermediate‐composition, ternary‐feldspar‐bearing granulite. Based on inclusions in garnet, the peak P–T conditions of the original eclogite are 18 kbar at 850–950 °C, with later matrix re‐equilibration at 12 kbar and 950 °C. Four samples from the transition of the eclogite–mafic granulite through to the intermediate granulite were studied. In the eclogite, REE patterns in the garnet core show no Eu anomaly, compatible with crystallization in the absence of plagioclase and consistent with eclogite facies conditions. Towards the rim of garnet, LREE decrease, and a weak negative Eu anomaly appears, reflecting passage into HP granulite facies conditions with plagioclase present. The rims of garnet next to ternary feldspar in the intermediate granulite show the lowest LREE and deepest Eu anomalies. Zircon from the four samples was analysed by LASS (laser ablation–split‐stream inductively coupled plasma–mass spectrometry). It shows U–Pb ages from 404 ± 4.0 to 331 ± 3.3 Ma, with a peak at 340 ± 4.0 Ma corresponding to the likely exhumation of the rocks to 12 kbar. Older ages from zircon with steep HREE patterns indicate the minimum age of the protolith, and ages <360 ± 4.0 Ma are interpreted to correspond to the eclogite facies metamorphism. Only some zircon grains ≤350 ± 4.0 Ma have flat HREE patterns, suggesting that these are primarily modified protolith grains, rather than new zircon crystallized in the eclogite‐ or granulite facies. The metasomatic processes that converted the eclogite–mafic granulite to an intermediate granulite may have facilitated zircon modification as zircon in the intermediate granulite has flat HREE and ages of 340 ± 4.0 Ma. The difference between the oldest and youngest ages with flat REE patterns indicates a 16 ± 5.6 Ma period of zircon modification in the presence of garnet.

The tectonic transition from oceanic subduction to continental subduction: Zirconological constraints from two types of eclogites in the North Qaidam orogen, northern Tibet

In the plate tectonics theory, continental subduction is pulled by subduction of dense oceanic crust. In practice, however, it is not easy to demonstrate that preceding oceanic crust exposes as oceanic-type eclogite together with continental-type eclogite in collisional orogens. The North Qaidam orogen in northern Tibet is an ultrahigh-pressure (UHP) metamorphic belt that contains the two types of eclogites, providing us with an excellent opportunity to study the tectonic transition from oceanic subduction to continental subduction. In order to constrain the protolith nature and metamorphic evolution of eclogites, we performed a combined study of zircon U–Pb ages, trace elements, mineral inclusions and O–Hf isotopes for various eclogites from the orogen. We discriminate the two types of eclogites by their differences in zircon U–Pb ages and O–Hf isotopes. CL-dark zircon domains exhibit high Th/U ratios, steep HREE patterns and significantly negative Eu anomalies, indicating that they are protolith zircons of magmatic origin with different extents of metamorphic recrystallization. Relict magmatic zircon domains in Type I eclogites yield Neoproterozoic protolith ages of >830Ma and Hf model ages of 850–1100Ma, whereas those in Type II eclogites yield Cambrian protolith U–Pb ages of >489Ma and Hf model ages of 500–650Ma. Most of the CL-bright zircon domains show low Th/U ratios, flat HREE patterns and no negative Eu anomalies, and contain mineral inclusions of garnet, omphacite and rutile, indicating their growth under eclogite-facies metamorphic conditions. These metamorphic domains have consistent eclogite-facies metamorphic ages of 433–440Ma throughout the North Qaidam orogen, regardless of the eclogite types and locations. The metamorphic zircon domains in Type I eclogites mostly exhibit δ18O values higher than normal mantle values, whereas Type II eclogites mostly have δ18O values lower than the normal mantle values. The difference in the δ18O values indicates that their protoliths underwent different temperatures of hydrothermal alteration at different tectonic settings. Combining zircon U–Pb ages and O–Hf isotope compositions with local tectonics, it is inferred that Type I eclogites were metamorphosed from Neoproterozoic continental mafic rocks, whereas Type II eclogites were metamorphosed from oceanic mafic rocks that were subducted prior to the continental subduction. The consistent eclogite-facies metamorphic ages for the two types of eclogites indicate that the exhumed oceanic-type eclogite was detached from the subducted oceanic crust and then entrained by the exhuming continental crust. Therefore, the coexistence of oceanic- and continental-type eclogites in the North Qaidam orogen demonstrates the tectonic transition from oceanic subduction to continental collision in the early Paleozoic.

Geochemical constraints on the protoliths of eclogites and blueschists from North Qilian, northern Tibet

An integrated study of whole-rock geochemistry, mineral O isotope geochemistry and zirconology was carried out for low-temperature eclogites and blueschists from the North Qilian orogen in northern Tibet. The results demonstrate that these oceanic-type eclogites and blueschists were produced by metamorphism of backarc basin basalts rather than mid-ocean ridge basalts as commonly thought. These high pressure metabasites show significant heterogeneity in their major and trace element compositions, varying from island arc basalts-like, mid-ocean ridge basalts-like to oceanic island basalts-like, respectively, in trace element distribution diagrams. Such variable compositions are comparable with those of backarc basin basalts at different stages. Whole-rock Nd isotope analyses indicate that the compositional difference between the metabasite protoliths can be ascribed to involvement of different amounts of subducted sediment-derived melts in the backarc mantle. Oxygen isotope fractionations between coexisting minerals are not at equilibrium; garnet O isotope analyses indicate that whole-rock δ18O values are either higher or lower than normal mantle value, which is attributable to seawater-hydrothermal alteration during seafloor spreading to open the backarc basin. Relict magmatic zircon domains show oscillatory zoning, high Th and U contents, high Th/U ratios, and steep HREE patterns, with protolith U–Pb ages of 486–496Ma. Metamorphic zircon domains exhibit lowered Th, U and HREE contents and Th/U ratios than the relict magmatic domains and contain mineral inclusions of omphacite, rutile and phengite, giving concordant U–Pb ages of 463±10Ma for eclogite-facies metamorphism. Zircons have markedly depleted Hf isotope compositions with Hf model ages slightly or significantly older than the protolith ages, suggesting incorporation of crustal components into the magma sources of these metabasite protoliths. Most zircons have δ18O values different from normal mantle values, and the distinct zircon O isotopes in different samples are in accordance with garnet δ18O values for the same sample, pointing to a localized fluid source for the metamorphism. Therefore, the different metabasites with different compositions were metamorphosed from different basaltic rocks generated in a backarc basin in the early Paleozoic. Their occurrence in the same region indicates that a backarc basin was transformed to an oceanic subduction zone where eclogite-facies metamorphism occurred.

Anatexis of ultrahigh-pressure eclogite during exhumation in the North Qaidam ultrahigh-pressure terrane: Constraints from petrology, zircon U-Pb dating, and geochemistry

Plagioclase-rich leucosomes and K-feldspar–rich veins are widely distributed within retrogressed eclogite in the Xitieshan area of the North Qaidam ultrahigh-pressure metamorphic terrane, western China. In this contribution, a combined study of petrology, zircon internal structure, zircon U-Pb age, zircon trace-element composition, whole-rock geochemistry, and Sr-Nd isotopes in plagioclase-rich leucosomes and K-feldspar–rich veins provides insight into the nature and timing of partial melting in these rocks. Petrological evidence for partial melting is provided by elongated, highly cuspate feldspars, quartz along grain boundaries (e.g., muscovite, garnet, and clinopyroxene), and felsic veinlets composed of quartz and feldspar along the boundaries of garnet, clinopyroxene, and clinopyroxene-plagioclase symplectite in the metabasite. Whole-rock geochemistry suggests that the plagioclase-rich leucosomes have lower contents of both rare earth elements (REEs) and high field strength elements (HFSEs) but higher large ion lithophile elements (LILEs; e.g., Rb, Ba, K, Sr, and Pb) than the metabasite hosts. The plagioclase-rich leucosomes may be divided into two subgroups according to their distinct REE patterns: (1) higher total REE content with or without weak negative Eu anomalies, and (2) lower total REE content with conspicuous positive Eu anomalies. The Eu-rich group also shows higher Sr content than those in the Eu-poor group. However, the K-feldspar–rich veins show higher REE, HFSE, and LILE concentrations than the plagioclase-rich leucosomes and metabasites. The inherited cores of K-feldspar–rich veins are of typical magmatic origin (oscillatory zoning with high Th/U ratios, enriched heavy [H] REEs, and negative Eu anomalies), yielding 206 Pb/ 238 U ages ranging from 904 ± 8 Ma to 915 ± 28 Ma. The zircon cores of plagioclase-rich leucosomes and mantles of K-feldspar–rich veins exhibit no zoning or weak zoning, with flat HREE patterns and no negative Eu anomalies, and they contain mineral inclusions of garnet and clinopyroxene, implying that these zircon domains were formed during an episode of eclogite-facies metamorphism with 206 Pb/ 238 U ages ranging from 444 ± 10 Ma to 452 ± 9 Ma. Internal textures and mineral inclusions (quartz, plagioclase, and K-feldspar) and trace-element systematics (steep HREE patterns with negative Eu anomalies and low Th/U ratios) of the zircon rims from plagioclase-rich leucosomes and K-feldspar–rich veins are similar to anatectic zircon. These anatectic zircon domains yielded 206 Pb/ 238 U ages ranging from 430 ± 10 Ma to 440 ± 12 Ma, and 406 ± 12 Ma to 430 ± 13 Ma, respectively. An integrated study of petrology, geochronology, and geochemistry demonstrated that the metabasites of the North Qaidam ultrahigh-pressure terrane likely experienced initial partial melting with plagioclase-rich leucosome formation under eclogite-facies conditions and partial melt crystallization during the granulite-facies stage, triggered by dehydration melting involving zoisite and rare muscovite. The K-feldspar–rich veins within the metabasites may be the result of injected melts that were derived from partial melting of nearby gneisses.

Geochronology and geochemistry of felsic xenoliths in lamprophyre dikes from the southeastern margin of the North China Craton: implications for the interleaving of the Dabie–Sulu orogenic crust

Lamprophyre dikes emplaced in a Jurassic granite at the southeastern margin of the North China Craton (NCC) carry different types of xenoliths. Here, we report a combined study of zircon U–Pb ages and whole-rock geochemistry of the xenoliths, as well as an Ar–Ar age of the lamprophyre, providing constraints on the sources of the magmatism and tectonic evolution in the southeastern margin of NCC. Phlogopite from the lamprophyre dike gave a 40Ar/39Ar plateau age of 116.15 ± 0.33 Ma. The felsic xenoliths can be classified into three groups: monzogranite, banded biotite granitic gneiss, and garnet-bearing gneiss. The internal structures of zircons from the banded biotite granitic gneiss xenolith show complex growth patterns. The mantles of these zircons display low luminance with a concordant SHRIMP U–Pb age of 227 ± 10 Ma. The rims with lighter luminance provide a SHRIMP U–Pb age of 213 ± 10 Ma, both ages are identical within error and significantly different from the ages of the basement rocks in the surrounding Bengbu uplift. However, the age is identical to those of the ultrahigh-pressure metamorphic rocks in the adjacent Dabie–Sulu orogen. In addition, zircon mantle domains of the banded biotite granitic gneiss xenolith have low Th/U ratios (>0.01) and flat HREE patterns ((Yb/Dy)n < 10), which suggest growth in an assemblage with garnet during HP metamorphism. The rim domains show very low Th/U ratios (<0.01) and steep HREE patterns ((Yb/Dy)n > 10), implying growth during exhumation in the absence of a garnet. Our studies show that the banded biotite gneiss represents vestiges of the subducted South China crust injected into or thrust below the North China Craton and provides constraints on the process of underthrusting in a continental collision zone, as well as the Mesozoic tectonic history of the southeastern margin of the NCC.

Geochronology and geochemistry of leucogranites from the southeast margin of the North China Block: Origin and migration

Systematic geochronological, geochemical and geological data for the Late Jurassic granitic intrusions from the Bengbu area in the southeastern margin of North China Block (NCB) are presented in this paper. The very low FeO+MgO+TiO2 contents and ferromagnesian mineral abundance as well as low Al-saturation indexes (ASI=0.92–1.09) of the rocks document that these granitic intrusions are metaluminous leucogranites. The Bengbu leucogranites contain abundant both inherited Neoproterozoic igneous and Triassic metamorphic zircons, showing a high agreement in ages with the ultrahigh-pressure (UHP) metamorphic rocks from the adjacent Dabie–Sulu orogen, but significant different from the country rocks. The magmatic overgrown zircon rims give the ages of 167–148Ma, consistent with the time of Jurassic migmatization in the Sulu orogenic belt. Mineral inclusions such as quartz, feldspar, apatite, titanite, biotite, muscovite and phengite (Si=3.58) without coesite occurring within the Triassic metamorphic zircons suggest that the source rocks of the leucogranites might have experienced only high-pressure (HP) rather than UHP metamorphism. Furthermore, steep HREE patterns of the inherited metamorphic zircons, plus very low bulk REE contents and Sr–Nd–Pb isotopic characteristics, suggest that the leucogranites were most likely derived from partial melting of subducted felsic gneiss rather than basaltic eclogite in Dabie–Sulu orogen, with residual allanite in the source. Their low Rb, high Sr contents and low Rb/Sr ratios suggest that the metaluminous leucogranites are derived from mica-poor orthogneiss. Melting temperature for the leucogranites is about 700–710°C as estimated using combined Ti-in-zircon and zirconium saturation thermometries. Inherited metamorphic zircons yielded metamorphic Ti-in-zircon temperature of 681±60°C, representing that for protolith-forming, which shows no significant difference from the melting temperature. H2O-present melting is thus required to generate the low-temperature melt. Sedimentary facies analysis of the Jurassic strata in the Hefei basin suggests that the Sulu orogen had been northward moved to the east of the Bengbu uplift by the Tan–Lu Fault in the Late Jurassic, resulting into a contrasting topography in this area and lateral pressure gradient in the local middle-lower crust. This lateral pressure gradient may drive the partially molten crust (migmatite) in Sulu orogen westward flow into the middle-lower crust of the Bengbu uplift in NCB. Gravity then drove melt to ascent into the middle-upper crust of the Bengbu uplift to form the leucogranitic intrusions. The NWW–SEE distribution of the later Jurassic leucogranites in Bengbu uplift may indicate the direction of ancient channel flow.

Zircon captures exhumation of an ultrahigh-pressure terrane, North-East Greenland Caledonides

Zircon from the North-East Greenland ultrahigh-pressure (UHP) terrane formed over a 45million year period from peak UHP conditions through the amphibolite facies. Our study utilizes sensitive high resolution ion microprobe-reverse geometry (SHRIMP-RG) mass spectrometry to assess the multiple ages and trace element patterns preserved in zircon from samples chosen to capture the exhumation history of these rocks. Peak UHP conditions from 365 to 350Ma are derived from coesite-bearing samples, while a suite of progressively retrogressed quartzofeldspathic host gneisses and late-stage, leucocratic melts emplaced into the gneisses track exhumation. Melting occurred during all stages of exhumation, beginning with H2O-absent dehydration melting of phengite on the decompression path. A garnet-bearing leucosome in the neck of a kyanite-eclogite boudin that gives an age of 347Ma is taken as the beginning of phengite melting. Leucosomes formed in HP granulite to amphibolite facies gneisses between 350 and 340Ma, and fluid assisted melting continued until 320Ma in the form of late, cross cutting pegmatites. Changes in the zircon trace element patterns are linked to decreasing temperature, and show that significant new zircon grew during melting on the exhumation path. Zircon cores recording protolith ages generally preserve magmatic temperatures (700°C) and typical igneous REE patterns (Yb/Gd=10). UHP/HP eclogite-facies zircon records higher T (900°C) and flat HREE patterns (Yb/Gd=1). Granulite to amphibolite facies zircon in quartzofeldspathic gneisses records both flat (Yb/Gd=1) and steep (Yb/Gd=100) HREE patterns at ca 700°C suggesting the variable effects of garnet during decompression. Amphibolite facies pegmatites and leucosomes document a transition from moderate HREE (Yb/Gd=10) at 700°C to steep HREE (Yb/Gd=100–1000) patterns at 600°C. The pronounced steepening of the HREE patterns is attributed to garnet breakdown during amphibolite-facies metamorphism. The 30–50million year spread of ages observed in individual samples records multiple periods of zircon growth and is interpreted as a characteristic signature of slowly exhumed UHP terranes. The data show that zircon ages combined with trace element and textural characterization of zircon from a broad suite of samples can successfully define the exhumation history of UHP terranes.

Felsic veins within UHP eclogite at xitieshan in North Qaidam, NW China: Partial melting during exhumation

Felsic veins within the Xitieshan UHP eclogite in North Qaidam, NW China vary from 1cm or less to several decimetres in width, exhibiting coarse granular in texture and heterogeneous distribution in mineral assemblages. They encircle or cut the deformed sub-lentoid eclogite body. Some fine veins extended into and ended in the eclogite, without visible link to their country rocks. They consist mainly of feldspar and quartz with minor amounts of muscovite, biotite and amphibole. To constrain their formation age, the conditions and source of vein-forming fluid/melt, and the relationship between the veins and host UHP eclogite, a combined study of zircon U–Pb dating and mineral inclusions was carried out on the felsic veins and their host eclogite. Zircon grains from the felsic veins are perfectly euhedral with zonations, characterized by very low Th/U ratios and steep HREE patterns without Eu anomalies. These zircons contain micro-inclusions of plagioclase, muscovite, garnet and quartz, which are consistent with mineral assemblage of the veins except for garnet captured from the host eclogite, indicating that these zircons crystallized during the veining. LA-ICPMS U–Pb dating yielded a weighted mean 206Pb/238U age of 428±2Ma, representing the formation age of veins. Zircon grains from the host eclogite are oval to round with homogeneous and/or cloudy internal-structure; residual core can be observed locally. They contain eclogite–facies micro-inclusions of garnet, omphacite, rutile and kyanite in metamorphic domains. LA-ICPMS U–Pb dating gave a metamorphic age of 458±2Ma. Geochemical analyses show that the felsic veins are tonalitic with high Na/K ratios, high Sr contents but low Y and Yb contents, and LREE-enriched patterns, consistent with partial melting of omphacite in eclogite at elevated pressures. Therefore, the felsic veins were most likely derived from partial melting of the host UHP eclogite. There is a considerable difference of about 30 Myr in zircon U–Pb age between the UHP eclogite–facies metamorphism and the felsic veining, indicating that the partial melting took place in the exhumation stage of the UHP terrain.