Pb Hf Isotope Research Articles

Petrogenesis and Geodynamic Implications of Late Triassic Mogetong Adakitic Pluton in East Kunlun Orogen, Northern Tibet: Constraints from Zircon U–Pb–Hf Isotopes and Whole-Rock Geochemistry

Adakites or adakitic rocks usually show special geochemical signatures and are petrological probes to reveal the tectono–magmatic evolutionary history of paleo–orogenic belts. Here, we present a comprehensive study on the zircon U–Pb geochronology, whole-rock geochemistry, and zircon Lu-Hf isotopes of Mogetong adakitic pluton in East Kunlun orogen, Northern Tibetan Plateau, to constrain its petrogenesis and tectonic setting, and thus to reveal its implications for the Paleo–Tethyan orogeny. The studied pluton comprises of quartz monzonite porphyry with zircon U–Pb crystallization age of ca. 215 Ma, which is coeval to their diorite enclaves (ca. 212 Ma). The quartz monzonite porphyries have intermediate SiO2 (63.31–65.74 wt%), relatively high Al2O3 (15.52–16.02 wt%), K2O (2.83–3.34 wt%), and Sr (462–729 ppm), but low Y (9.14–15.7 ppm) and Yb (0.73–1.39 ppm) with high Mg# (47–55), Sr/Y (30–57) and La/Yb ratios resembling typical high–K calc-alkaline and high Mg# adakitic rocks. Zircon Lu–Hf isotopes show that the studied samples have weakly juvenile zircon Lu-Hf isotopes (εHf(t) = 1.80–4.03) with older model age (1.00–1.14 Ga). The relative low content of Cr (14–59 ppm) and Ni (8–30 ppm), as well as the petrological, geochemical, and Lu-Hf isotopic data, indicates that the Mogetong adakitic rocks were generated by partial melting of thickened lower crust with a certain contribution of the underplated mantle-derived magma in slab break-off setting. This study shows that the Late Triassic adakitic magmatism in East Kunlun orogen may be the response of tectonic transition from oceanic subduction to post–subduction extension, and the reworking of ancient continental crust with subsequent variable crust-mantle magma mixing is the major mechanism of continental crust evolution in the Paleo–Tethyan orogenic belt.

Geochemical constraints on source nature and recycled oceanic crust in the mantle of the Celebes Sea

Indian-type mantle is extensively distributed in the marginal basins of the western Pacific; however, the origin of this mantle remains unclear. The Celebes Sea is located between the Pacific and Indian oceans, and its igneous crustal rocks provide crucial constraints on if and on how this anomalous mantle developed in the region. However, limited knowledge has been obtained on the origin of mantle source of the Celebes Sea because only normal-type mid-ocean ridge basalts (N-MORBs) at ODP Site U770 in this basin have been examined. In this study, both N-MORBs and enriched (E)-MORBs were identified at Site U770, which were analyzed for whole-rock major and trace elements, Sr–Nd–Pb–Hf isotopes, and in situ olivine geochemistry. Site U770 basalt samples are Indian-type (i.e., plot above the Northern Hemisphere reference line [NHRL] on plots of 208Pb/204Pb and 207Pb/204Pb versus 206Pb/204Pb) MORBs, based on their Sr–Nd–Pb–Hf isotopic compositions. An enriched mantle end-member exists in the MORB-type depleted mantle source, as evidenced by correlations between ɛNd and SmN/YbN, Sr/Sr*, and Eu/Eu*. Recycled igneous oceanic crust that experienced plagioclase fractionation can account for the geochemically enriched end-member in the mantle source, based on the positive correlation of ɛNd vs. Sr/Sr* and Eu/Eu*. The recycled igneous oceanic crust plus minor amounts of pelagic sediments can also explain the origins of Indian-type mantle in the Celebes Sea. The E-MORBs have higher Zn/Mn and Zn/Fe ratios, and similar olivine Ni, Ca, and Mn contents as compared to global MORBs, which is also consistent with a recycled oceanic crust-derived, eclogite-rich, mantle source. This suggests recycled components contribute to the lithological and geochemical heterogeneity in the mantle beneath the Celebes Sea.

Late Neoarchean high-grade regional metamorphism in the eastern North China Craton: New constraints from monazite dating in northern Liaoning

Late Neoarchean tectono-thermal events in the eastern North China Craton have been comprehensively characterized in terms of mineralogy, petrology, geochemistry, metamorphic evolution, geophysics and geochronology. The Late Neoarchean metamorphic event is debated to correlate with vertical and/or horizontal tectonism, potentially an indicator as initiation of plate tectonics. With a growing number of zircon geochronological investigations, the precise timing of the Late Neoarchean metamorphic event should be questioned due to the complexity of post-Archean multiple tectono-thermal overprints. Here we demonstrate the capabilities of LA–ICP–MS U–Pb and LA–MC–ICP–MS Sm–Nd isotope measurements applied to monazite endowed with LREE-enriched geochemical peculiarities as well as zircon U–Pb–Hf isotope, to reevaluate the Late Neoarchean metamorphic event in northern Liaoning, eastern North China Craton. Monazites within the biotite two-feldspar gneiss from the Qingyuan greenstone belt are Ce-monazite in composition, with negligible amounts of the huttonite and brabantite components. The internally homogeneous and anhedral rounded granular monazites show textural equilibration in contact with plagioclase, quartz, and K-feldspar, indicating that monazite is a product of metamorphism. The apparent ages and Eu/Eu* values of monazite analyses show a positive correlation, which confirms the continuous formation of monazite and coeval feldspar during the metamorphic evolution. These metamorphic monazites yield a weighted mean 207Pb/206Pb age of 2493 ± 8 Ma and an upper intercept age of 2496 ± 8 Ma, consistent with the metamorphic zircon dates of 2492–2468 Ma. These results, along with monazite Sm–Nd isochron of 2459 Ma, thus provide a critical clue to understand the Late Neoarchean high-grade regional metamorphism in the eastern North China Craton. As a consequence, the Qingyuan greenstone belt was only partially influenced by the Paleoproterozoic tectono-thermal events, not largely as previously thought by some workers.

Age and genesis of the Jinshan gold deposit in the Chinese North Tianshan: A link to large-scale strike–slip shearing events

Large-scale transcrustal strike–slip shearing, orogenic gold deposits, and magmatic Ni–Cu sulfide deposits are prominent features in the Chinese North Tianshan in the northeastern segment of the Tianshan Orogenic Belt. Therelationshipbetween the shearing events and the different styles of mineralization, however, remains ambiguous. Here, structural data, zircon U–Pb–Hf isotopes, bulk-rock geochemistry, muscovite 40Ar/39Ar age dating, and S–Pb isotopes of pyrite from the Jinshan gold deposit are utilized in conjunction with the Pb isotope composition of mineralized mafic–ultramafic rocks of the Huangshandong Ni–Cu sulfide deposit to characterize thelinkbetweenstrike–slipshearing events, andorogenicAuandmagmaticNi–Cu sulfidemineralization. The new zircon U–Pb age results indicate that the host diorite of the gold orebodies formed at ca. 335 Ma. These diorites are enriched in light rare earth elements and large ion lithophile elements (e.g., Ba, U, K, and Pb), but depleted in high field strength elements (e.g., Nb, Ta, and Ti), and have low MgO (4.45–5.17 wt%) contents, Mg# (0.32–0.36), and (La/Yb)CN ratios (4.81–6.90). Based on the lack of ancient crustal basement rocks in the Qoltag Arc, the depleted Nd–Hf isotope composition (εHf(t) = 10.82–15.38; εNd(t) = 6.17–6.67), the juvenile Hf depleted-mantle model ages (350–530 Ma), and the enrichment in Na2O (4.45–5.17 wt%) relative to K2O (1.05–2.20 wt%) of the diorite, it is inferred that the primary magma from which the diorite crystallized was derived from partial melting of metasomatized mantle wedge in an oceanic arc environment. Two muscovite samples separated from an auriferous quartz lode yield 40Ar/39Ar plateau ages of 248.6 ± 0.8 Ma and 249.6 ± 0.9 Ma, representing the age of the gold mineralization event. The similarity in S–Pb isotope composition of pyrite in the Jinshan gold deposit and pyroxenite in the Huangshandong Ni–Cu sulfide deposit suggests that the auriferous fluids required for the formation of the Jinshan gold deposit could represent a mixture of deuteric fluids with limited upper crustal metamorphic fluids from the Kanggur–Huangshan Shear Zone.

Zircon U–Pb chronology and Hf isotopes of the Lebowa Granite Suite and petrogenesis of the Bushveld Complex, South Africa

Zircon U–Pb chronology and Hf isotopes of the Lebowa Granite Suite and petrogenesis of the Bushveld Complex, South Africa

Geochemical Compositions and Detrital Minerals of Stream Sediments around the Zijinshan Copper-Gold Orefield and Their Implications

Regional geochemical anomalies in stream sediments often have close spatial relationships with metallogenic provinces or ore districts, but the relationships between them have not been examined in depth. In this study, stream sediments were collected around the Zijinshan Copper-Gold Orefield, Fujian Province, China. Element geochemistry, U–Pb geochronology and Hf isotope compositions of detrital zircons, and electron microprobe and LA-ICP-MS analyses of iron oxides were conducted. The aims of this study were to investigate the relationship between the provenance of the stream sediments and ore-bearing magmatic rocks in the Zijinshan Copper-Gold Orefield, and to explore the enrichment mechanism of the ore-forming elements in stream sediments. The results show that the ore-forming elements and their associated elements are most significantly enriched in stream sediments near the orefield. U–Pb ages and Hf isotopic compositions of detrital zircons in the sediments closest to the orefield carry information on the ore- bearing magmatic rocks in the orefield. However, as the stream sediments are relatively far from the orefield, the degree of enrichment of ore-forming elements and the detrital zircon U–Pb age signals of the ore-bearing magmatic rocks in the orefield rapidly weaken. This weakening of the geochemical signals may have been affected by many factors, such as lithological resistance to weathering, vegetation coverage, micro-topographic conditions, etc. In-situ elements analysis of iron oxides and elemental correlation analysis of stream sediments indicate iron oxides and clay minerals are the main carrier minerals for the migration of ore-forming elements.

Early Devonian Arc-Related Volcanic Rocks in the Haerdaban, North Margin of the Yili Block: Constraint on the Southward Subduction of the Junggar Ocean

The origin and tectonic implication of Early–Middle Devonian magmatism in the northern margin of YB (Yili Block) remain enigmatic and are important for understanding Late Paleozoic evolution of the Junggar Ocean and southern Kazakhstan Orocline. Here, we present the systematic study of whole-rock geochemical and Sr–Nd isotope features as well as U–Pb–Hf isotope characteristics of zircon crystals for newly identified Early Devonian volcanic rocks from the northern margin of YB. The volcanic rocks are composed of rhyolite, rhyolite porphyry, and rhyolitic tuff. Zircon U-Pb age dating indicates they were formed at ca. 407~418 Ma. They have high SiO2 (70.16–77.52 wt.%) and alkali (5.10–9.56 wt.%) contents, and high Zr + Nb + Ce + Y content (~456 ppm), indicative of A-type magma. Their relative depletion of Nb, Ta, and Ti, and enrichment of LILEs show arc affinity. Their low initial 87Sr/86Sr ratios (0.699708–0.709822) and negative εNd(t) values (−1.8 to −4.0) indicate a mainly continental magma source and their positive εHf(t)values (+6.13 to +14.81) are possibly due to the garnet effect. All these above reveal that volcanic rocks were generated by re-melting of lower crust under a high temperature condition, which was induced by long-lived heat accumulation with no or minimal basalt flux. Combined with active continental margin inference evidenced by contemporaneous sedimentary rocks, we attribute the generation of the volcanic rocks to a continental arc setting related to the southward subduction of Junggar oceanic crust. Thus, we infer the Early–Middle Devonian arc-related magmatic rocks in the northern margin of YB are eastward counterparts of the southern limb of the Devonian Volcanic Belt, which resulted from a relatively steady-state southward subduction.

Crustal reworking and growth during India–Asia continental collision: Insights from early Cenozoic granitoids in the central Lhasa Terrane, Tibet

Studying granitoids can provide important knowledge on the composition of continental crust and the geodynamic processes of continental growth. Nevertheless, the mechanisms of granitoids formation during continental collision remain uncertain. In this paper, we present new zircon U–Pb–Hf isotope and whole‐rock Sr–Nd isotope compositions, and major‐ and trace‐element data for granitoids from the Chacangka area of the central Lhasa Terrane, southern Tibet. The Chacangka quartz monzonites crystallized at ca. 61 Ma, whereas the monzonites and granites were emplaced at ca. 51 Ma. The quartz monzonites are peraluminous and have high 87Sr/86Sri ratios (0.7129–0.7131), and their low εNd(t) (−7.65 to −7.10) and zircon εHf(t) (−9.3 to −7.5) values are more depleted than those of ancient crustal basement beneath the central Lhasa Terrane, which suggests that they originated from a mixed source of juvenile and ancient lower crust. Compared with the quartz monzonites, the monzonites have much higher contents of MgO (2.34–3.62 wt%) and values of Mg# (42.3–46.3) and zircon εHf(t) (−6.9 to 1.9) and show more depleted Sr–Nd isotopes, indicating that more juvenile materials were involved in their source. The granites have Sr–Nd isotope compositions that are similar to those of the coeval monzonites but have higher SiO2 contents, lower CaO and MgO contents, and lower values of Mg#, suggesting that they were differentiates of the monzonites. The elemental and isotopic compositions of the Chacangka granitoids exhibit a marked transformation from 61 to 51 Ma, probably indicating breakoff of the Neo‐Tethyan oceanic slab. The Neo‐Tethyan slab breakoff not only induced the upwelling of deep material and eruption of magma but also caused re‐melting and destruction of ancient crustal basement of the central Lhasa Terrane.

Origin of the dioritic porphyrite and its associated Matouniu skarn gold polymetallic deposits in the Eastern Hebei Province, North China: Evidence from geochronology, geochemistry, and C–O–S–Pb–Hf isotopes

The Matouniu gold polymetallic deposit in the Eastern Hebei Province (Jidong) in the northern margin of the North China Craton (NCC) is a skarn‐type ore deposit. Geochemical data, zircon U–Pb, and Hf isotope analysis of the mineralization‐related dioritic porphyrite and C–O–S–Pb isotopes from the deposit were used to investigate the origin, magmatic evolution, mineralization, and tectonic setting of the Matouniu gold polymetallic deposit. Zircon U–Pb dating reveals an emplacement age of 146.7 ± 1.1 Ma, indicating that the magmatism and mineralization occurred during the Late Jurassic. Geochemical, Hf, and Pb isotopic compositions indicate that the dioritic porphyrite was formed by the partial melting of the ancient mafic lower crust, leaving behind residual plagioclase, amphibole, and garnet, with traces of mantle material involvement. The S and Pb isotopic compositions revealed that both the ore‐forming materials and the dioritic porphyrite were derived from a deep‐seated magmatic source, which mainly originated from the lower crust, with minor input from the mantle. The C–O isotopic compositions suggest that the early ore‐forming fluids had a mainly deep‐seated magmatic source with minor meteoric water involvement, and were accompanied by continuous meteoric water involvement and water‐rock interaction during the fluid migration. The Matouniu gold polymetallic deposit and Late Jurassic dioritic porphyrite associated with skarn mineralization formed under geodynamics involved a compressional regime that was associated with the continuous subduction of the Palaeo‐Pacific Plate beneath the East Asia Plate.

Combining detrital zircon shape and U–Pb–Hf isotope analyses for provenance studies – An example from the Aquiri region, Amazon Craton, Brazil

Provenance studies are commonly based on in situ U–Pb dating and Hf isotope analyses of detrital zircon grains, while leaving shape parameters unconsidered, even though these can provide valuable information about the hinterland and detritus transport. This is demonstrated herein by presenting a comprehensive data set derived from approximately 950 detrital zircon grains from three quartzite samples of the Cuxiú Formation of the Aquiri region, which is part of the eastern realm of the Amazon Craton. The data set combines results of zircon U–Th–Pb–Hf isotope analyses with zircon shape, and some physical parameters, including length, width, aspect ratio, roundness, typology, alpha-decay dose and density. Zircon U–Pb ages and field relationships indicate that pebbly sandstones of the Cuxiú Formation were deposited during the Neoarchaean between 2705 and 2665 Ma, likely in a fluvial or near-shoreline environment of a rift basin, with zircon grains supplied from igneous rocks of the surrounding Amazon Craton. The detrital zircon grains record crystallisation ages of 3100–2800 Ma (74%), and subordinately of 2800–2700 Ma (13%) and 3600–3100 Ma (3%). Hafnium isotope analyses further suggest crust formation in the hinterland at 4.01–3.65 Ga (14%), 3.60–3.10 Ga (85%) and 2.95 Ga (1%). Well preserved euhedral shapes of more than 50% of the detrital zircon grains, independent of U–Pb ages, suggest fast, direct supply from heterogeneous Archaean basement sources to the sedimentary basin, with a short-distance transport, plausibly in a fluvial setting. Correlation of zircon roundness with width in the size range from 50 to 150 µm indicates more intense abrasion of larger zircon grains compared to smaller ones, reflecting difference in transport mechanisms – i.e., abrasive-rolling transport vs. non-abrasive suspension transport. Similar average values for length (~120 µm), width (~66 µm), aspect ratio (1.9) and roundness (~2.5), obtained from zircon grains of very different ages (3600–2700 Ma), as well as log-normal distribution of the width parameter, suggest thorough hydrodynamic sorting during transport. Zircon typology analysis reveals a wide range of combinations of {100}:{110} prism faces with{101}:{301}:{211} pyramids, pointing to zircon crystallisation from 900 to 600 °C (average 755 °C). Uranium–Th contents indicate low α-decay doses of ≪1 × 10−15 decay events/mg for all zircon grains by the time of deposition, providing an additional explanation for the excellent zircon preservation.

An iron isotope perspective on back-arc basin development: Messages from Mariana Trough basalts

The geology and magmatism of the northern Mariana Trough (NMT) represent a type example for the development of a back-arc basin (BAB). It is a mature spreading center in the south and propagating northward via rifting of arc and rear arc lithosphere in response to trench retreat and overriding plate extension in space and time. It is thus essential to fully characterize its tectonomagmatic processes that are potentially of global significance. For this reason, we carried out a comprehensive geochemical study on basaltic samples (glasses) from the NMT axis from 17 to 23°N by analyzing a full suite of major and trace elements, Sr–Nd–Pb–Hf radiogenic isotopes and stable Fe isotopes. The Fe isotopes, in particular, have the potential to offer novel perspectives beyond the aforementioned geochemical variables. From south to north, the δ56Fe of the NMT basalts (NMTB) decrease systematically, varying from a MOR-like value of +0.12‰ to an arc-like value of 0.00‰. Importantly, incompatible element abundances and Sr–Nd–Pb–Hf isotope compositions of the NMTB also show systematic along-latitude variations. Positively correlated with δ56Fe, the abundances of fluid-insoluble elements (e.g., Nb, Ta, Zr, Hf, Ti and heavy rare earth elements) show the south-to-north decrease, suggesting that the NMTB δ56Fe systematics are inherited from the magma source variation. The source variation is most consistent with an asthenosphere source in the south and progressively more depleted harzburgitic lithosphere source of prior melt depletion towards north. On the other hand, there are scattered but significant enrichment trends towards north in terms of Sr–Nd–Pb–Hf radiogenic isotopes and fluid-soluble incompatible elements (e.g., Ba, Rb, Cs, Th, U, K, Pb, Sr and light rare earth elements), which is consistent with south-to-north increase in slab material (e.g., slab fluids, sediments etc.) contribution. These north-to-south magma source systematics defined by the NMTB can be readily understood as a tectonic response to the back-arc basin development with time from initial arc-magmatism dominated by sub-arc lithospheric mantle melting with significant slab material contributions to back-arc rifting and to back-arc spreading center magmatism characterized by increasing asthenospheric mantle melting, decreasing slab material contribution and diminishing sub-arc lithospheric mantle participation. This tectonic-magma source evolution in space and time is manifested by the snapshot of the NMTB magmatism in terms of the geology, petrology and geochemistry.The NMTB δ56Fe values and fluid-insoluble incompatible element abundances are unaffected by slab materials, but controlled by the harzburgitic residues of previous melt extraction depleted in these elements with low δ56Fe at the early arc-magmatism stage in the north and by the asthenosphere of MORB mantle characteristics in the south with well-developed back-arc spreading system. Importantly, these observations affirm the understanding that compositionally depleted and physically buoyant lithosphere is prerequisite as the overriding plate for subduction initiation at its edges.

Late Ordovician fore-arc ophiolitic mélange in the southern margin of the Bainaimiao arc: constraints from zircon U–Pb–Hf isotopes and geochemical analyses

AbstractThe Harihada–Chegendalai ophiolitic mélange, which is located between the Bainaimiao arc and the North China Craton, holds significant clues regarding the tectonic setting of the southern margin of the Central Asian Orogenic Belt. The ophiolitic mélange is mainly composed of gabbroic and serpentinized ultramafic rocks. Here, zircon U–Pb dating, in situ zircon Hf isotopic, whole-rock geochemical and in situ mineral chemical data from the ophiolitic mélange are reported. The zircons in the gabbroic rocks yielded concordia U–Pb ages of 450–448 Ma and exhibited slightly positive ϵHf(t) values (0.87–4.34). The geochemical characteristics of the gabbroic rocks indicate that they were generated from a mantle wedge metasomatized by subduction-derived melts from sediments with continental crust contamination, in a fore-arc tectonic setting. These rocks also experienced the accumulation of plagioclase. The geochemical characteristics of the ultramafic rocks and their Cr-spinels indicate that they may constitute part of residual mantle that has experienced a high degree of partial melting and has interacted with fluids/melts released from the subducted slab in the same fore-arc tectonic setting. The ophiolitic mélange may therefore have formed in this fore-arc tectonic setting, resulting from the northward subduction of the South Bainaimiao Ocean beneath the Bainaimiao arc during Late Ordovician time, prior to the collision between the Bainaimiao arc and the North China Craton during the Silurian to Carboniferous periods.

Constraints of magmatism on the Ergu Fe–Zn polymetallic metallogenic system in the central Lesser Xing’an Range, NE China: evidence from geochronology, geochemistry and Sr–Nd–Pb–Hf isotopes

AbstractThe medium-sized Ergu Fe–Zn polymetallic skarn deposit is located in the central Lesser Xing’an Range, NE China. The ore bodies are mainly hosted in the contact zone between granodiorite intrusions and lower Cambrian dolomitic crystalline limestones or skarns. To reveal the magmatic influence on the mineralization, resource potential and metallogenic geodynamic process of this deposit, a systematic study of the geology, petrology, zircon U–Pb dating, element geochemistry, amphibole geochemistry and Sr–Nd–Pb–Hf isotopes of the Ergu deposit intrusives was conducted. The results show the following: (1) The major rock types in the mine area are medium-grained granodiorite and porphyritic granite, and the rock related to mineralization is medium-grained granodiorite. Zircon U–Pb dating suggests that the granodiorite and porphyritic granite formed at 181.9–183.8 Ma and 182.7 Ma, respectively. Thus, an Early Jurassic magmatic event led to the formation of the Ergu deposit. (2) The granodiorite and porphyritic granite are high-K calc-alkaline I-type granites that formed by comagmatic evolution with varying degrees of fractional crystallization and were likely derived from partial melting of the lower crust. The Ergu deposit occurred in an active continental-margin tectonic setting. (3) The high water content (5.69 wt % H2O), high oxygen fugacity (ΔFMQ = +1.75 to +1.82) and intermediate-plutonic emplacement (3.13 km) of the granodioritic magma are key factors in the formation of the Ergu deposit. The porphyry granite is characterized by high water content (>4 wt % H2O), reduced oxygen fugacity (ΔFMQ = −0.47) and shallow emplacement (<3 km).

Geochemistry and Sr–Nd–Hf–Pb isotope systematics of late Carboniferous sanukitoids in northern West Junggar, NW China: Implications for initiation of ridge-subduction

The study of Carboniferous magmatism in northern West Junggar plays an important role in understanding the tectonic evolution of that part of the Central Asian Orogenic Belt. In this study, we present petrology, zircon U–Pb geochronology, mineral and whole-rock geochemistry, and the Sr–Nd–Hf–Pb isotope compositions of volcanic rocks from the Hamutusi area of northern West Junggar. LA–ICP–MS zircon U–Pb analysis of a representative andesite yielded an early to late Carboniferous age of 324.4 ± 6.9 Ma. The volcanic rocks are calc-alkaline, with high SiO2 (54.2–57.8 wt%), MgO (5.97–8.33 wt%), Mg# (60–69), Cr (147–448 ppm), and Ni (29.1–120 ppm) contents, and are enriched in large ion lithophile elements (LILE) and light rare earth elements (LREE), but depleted in high field strength elements (HFSE), these characteristics are similar to those of typical sanukitoids within the Setouchi volcanic belt in Japan. All samples have radiogenic initial Sr and Pb isotopic compositions, and low εNd(t) and εHf(t) values, indicating the sanukitoids were generated by partial melting of subducting sediments in which the melts interacted with the mantle. Geochemical modeling calculations indicate a proportion of 3–10% sediment melt and slab-derived fluids were mixed with the depleted mantle to produce the bulk of the Hamutusi rocks. We conclude that the studied rocks from north West Junggar record the initiation of ridge subduction related to a transition from normal subduction to subduction of young and hot oceanic lithosphere between the early and late Carboniferous.

Paleo - position of the South China Craton in the Rodinia Supercontinent: Evidence from the U - Pb age and Hf isotope of detrital zircon from the Nam Co Complex

To constrain the paleo - positions of the South China Cratons in the Rodinia Supercontinent during the Neoproterozoic, the in - situ U - Pb dating, and Hf isotope analysis of the detrital zircon from the Nam Co Complex, Song Ma Suture zone, northwestern Vietnam was performed. The U - Pb isotopic dating on detrital zircons shows that the Nam Co Complex demonstrates the major population (>50%) of around ~850 Ma while the minor population is scattered between ~1.2÷3.0 Ga. The Neoproterozoic age spectrum exhibits a large range of the εHf(t) from strongly negative to positive values ( - 17.418022÷ 14.600527), indicating that the source of the magma for this age range has been not only derived from reworking of the Archean basement rocks, but also generated from the juvenile material. The U - Pb age distribution patterns and Hf isotopic data of the detrital zircon in the Nam Co Complex are compatible with those of the South China Craton rather than those of the Indochina Craton. The data also indicate that sedimentary protoliths of the Nam Co Complex were deposited in a convergent - related basin along the southwestern margin of the South China Craton during the Neoproterozoic. Combined with the similarities of the detrital zircon age between western Cathaysia, Indochina, East Antarctica and East India, it is proved that the South China Craton was situated at the margin of the Rodinia Supercontinent and in close proximity to the Indochina, East Antarctica and East India.

Early Paleozoic tectono–magmatic evolution in the South Altun orogenic belt, northwest China: Insights from zircon U–Pb geochronology, Hf isotope and geochemistry of the granitoids

ABSTRACT Precise timing of granitoids and constraints of their magma sources are critical to understanding the subduction–collision tectonic evolution of the Altun orogenic belt (AOB), a vital part of the Qinghai-Tibet plateau in northwestern China. We provide in-situ zircon U–Pb age, Hf isotopic composition and whole-rock geochemistry of the Yusupualeke granitic pluton in the South Altun orogenic belt (SAOB), an integral unit of the AOB, to determine the tectono-magmatic evolution of the SAOB at early Palaeozoic. The Yusupualeke granitic pluton comprises the medium-coarse grained porphyritic monzogranite and medium-fine grained granodiorite. The monzogranite sample yielded a weighted mean 206Pb/238U age of 476.8 ± 3.6 Ma (MSWD = 0.59), while the granodiorite yielded a weighted mean 206Pb/238U age of 453.2 ± 4.7 Ma (MSWD = 0.013). Zircon U–Pb ages suggest that the subduction–collision process of the SAOB remained from the middle stage of the early Palaeozoic to the end. Both of the studied granitoids belong to metaluminous to weakly peraluminous series and show typical I-type granite characteristics with depletions in Ba, Nb, Sr, P, and Ti, and enrichments in light rare earth elements (LREEs), Rb, Th, K, and slightly negative Eu anomalies. Based on the geochronological data and regional geological background, we believe that the porphyritic monzogranites were formed during the northward subduction process of the South Altun ocean (SAO), while the granodiorites were formed during the tectonic regime transition stage. Besides, combing the previous research achievements with our newly obtained data, we put forward a new division and geodynamic model of the early Palaeozoic tectono-magmatic evolution for the South Altun orogenic belt.

Peritectic assemblage entrainment (PAE) model for the petrogenesis of Neoproterozoic high-maficity I-type granitoids in the western Yangtze Block, South China

Under the condition of paucity of mantle-derived ferromagnesian components, peritectic assemblage entrainment (PAE) model can properly explain a large variation of Fe, Mg, Ti, and Ca contents as well as positive linear trends for Ti and Ca plotted against maficity (molar Fe + Mg) exhibited by more mafic I-type granitic melts. Herein we present a comprehensive study of zircon U–Pb–Hf isotopes, whole-rock major and trace elements, and Sr–Nd isotopes for newly identified Neoproterozoic high-maficity I-type granitoids collected from the Yonglang area in the western Yangtze Block, South China, in order to evaluate PAE process acting on the petrogenesis of these more mafic granitoids. Zircon U-Pb geochronological results show that the Yonglang granitoids, including the granodiorites, monzogranites, and biotite granites, yield the weighted mean 206Pb/238U ages of ca. 850–825 Ma. They are geochemically characterized by high SiO2 (64.63–72.62 wt.%) and K2O (3.49–6.59 wt.%) contents as well as variable A/CNK (0.94–1.09) and molar Fe + Mg values (0.0452–0.1210), and display a negative correlation of A/CNK and Fe + Mg, indicating an I-type affinity. Isotopically, they display negative whole-rock εNd(t) (−4.06 to−6.38) and highly evolved zircon εHf(t) values (−9.36 to +1.92) as well as old two-stage Nd model ages (1.69–1.85 Ga) and crustal Hf model ages (1608–2139 Ma), implying a crustal source. In comparison with the geochemical composition of fluid-absent experimental melts, the Yonglang I-type granitoids have Fe, Mg, Ti, and Ca contents that are too high to equilibrium with those of pure crustal melts, indicating the additional incorporation of a ferromagnesian, Ti-, and Ca-rich component into primary melts. Like the Cotoncello–Monte Capanne granitic complex in Elba Island and Verdenburg I-type granite in South Africa, the Yonglang I-type granitoids exhibit highly positive linear correlations for Ti vs. maficity and Ca vs. maficity as well as negative correlation for A/CNK vs. maficity, suggesting that these high-maficity I-type granitoids predominantly underwent the peritectic assemblage entrainment of Ca-Fe-Mg-Ti bearing minerals (e.g., clinopyroxene, plagioclase, ilmenite, etc.) during coupled melting of biotite and hornblende phases in crustal source. The process of peritectic assemblage entrainment thus play a vital role in moulding geochemical compositions of more mafic I-type granites.

Origin of arc-like intraplate volcanism by melting of lithospheric mantle pyroxenite of the South China continental margin

Volcanic arcs usually produce silica-rich magmas that may modify the peridotitic sub-arc mantle, but direct observations on the lithology and geochemistry of the sub-arc lithospheric mantle are rare. An Andean-type arc system is inferred on the continental margin of southeastern China during the Mesozoic time; however, direct petrologic evidence for such an ancient arc is lacking. In this study, we have analyzed whole-rock major and trace elements, Sr–Nd–Pb–Hf isotope, and high-precision olivine geochemistry of the basaltic rocks of the Leizhou Peninsula and Hainan Island, on the South China continental margin. Unlike Hainan Island with only typical ocean island basalt (OIB)-type basalts, the Leizhou Peninsula has volcanic rocks ranging from typical OIB-type basalts to island arc basalt (IAB)-type with enrichment in large-ion lithophile elements and depletion in high-field-strength elements. The IAB-like rocks of the Leizhou Peninsula have less radiogenic Nd and Hf isotopes than the Hainan OIB-type basalts and the sub-continental lithospheric mantle. The IAB-like rocks can be explained by interaction of the Hainan plume with a subduction-derived component based on the plots of Nb/Nb* vs. Ce/Pb, Ba/Nb, Pb/Pb⁎, and SiO2. Olivine phenocrysts with anomalously high Ni and low Ca and Mn contents are consistent with a pyroxenite-rich mantle source for the IAB-like rocks. We suggest that the pyroxenite-rich mantle component of the IAB-like rocks formed by metasomatic reaction between silica-rich arc melts and the sub-continental lithospheric mantle during an ancient plate subduction event. We suggest that these arc-like volcanic rocks record melting of lithospheric mantle pyroxenite triggered by the Hainan plume. Our results are consistent with an Andean-type active continental margin related to subduction of the paleo-Pacific Plate beneath the southeastern China during the Mesozoic.

Nature of the Mantle Plume Under the Emeishan Large Igneous Province: Constraints From Olivine‐Hosted Melt Inclusions of the Lijiang Picrites

AbstractOlivine‐hosted melt inclusions have the potential to provide direct evidence about the nature of the mantle sources of magmas. In this study, we report the major and trace element and Sr–Nd–Pb–Hf isotope compositions of the Lijiang picrites from the Emeishan large igneous province, as well as major and trace element and Pb isotopic compositions of olivine‐hosted melt inclusions in the picrites. Using these data, we infer the nature of the mantle sources for the Lijiang high‐Ti picrites (Ti/Y = 645–654), and compare them with the Dali low‐Ti picrites (Ti/Y = 351–408). Lijiang melt inclusions have highly heterogeneous Pb isotope compositions (208Pb/206Pb = 2.015–2.201, 207Pb/206Pb = 0.812–0.882, 260 Ma age‐corrected) that overlap the majority of the fields from FOZO to enriched mantle 1 (EM1). The primitive mantle‐normalized trace element patterns of these melt inclusions reveal an enriched composition with depletions in Ba, Nb, Ta, Zr, and Hf. We suggest that an old recycled GLOSS II‐like sediment was probably the main component that imparted the EM1 signature into the sources of the Lijiang picrites. Furthermore, we reverse calculated the mantle potential temperature (Tp) beneath Lijiang and Dali using the Al‐in‐ol temperature. The estimated mantle Tp beneath Lijiang is lower than Dali. We infer that the Lijiang picrites might have originated from the mantle plume with lower thermal energy, corresponding to the waning stage of the axis of the Emeishan mantle plume. Our study suggest that high‐Ti magmas of the Emeishan large igneous province were derived from those parts or stages of the mantle plume with lower thermal energy.

Petrogenesis of Early Cretaceous granitoids and mafic microgranular enclaves from the giant Tonglushan Cu–Au–Fe skarn orefield, Eastern China

Late Mesozoic high-K calc-alkaline and high Sr/Y granitoids related to the skarn–porphyry–stratabound Cu–Au polymetallic deposits are widespread in the Middle–Lower Yangtze River Belt (MLYRB), Eastern China. However, the role of mantle-derived magmas in the petrogenesis of these granitoids remains controversial. In this study, we present new zircon U–Pb–Hf isotopes, and whole-rock geochemical and Sr–Nd isotopes data for quartz monzodiorites (porphyry), their hosting mafic microgranular enclaves (MMEs) and associated dykes from Tonglushan orefield in the MLYRB. Field relationships and geochronological data show that these intrusive rocks cluster around 141–139 Ma, indicating a short duration of magmatism, rather than a prolonged magmatism (~30 My) as previously thought. Crystallization age of the MMEs is identical to that of their quartz monzodiorite host. Petrographical and geochemical observations suggest that the MMEs represent mafic melts that injected into the quartz monzodioritic magmas, and magma mixing and (previously) fractional crystallization were involved in producing their compositional diversity. All the Tonglushan granitoids and MMEs show trace element geochemical and Sr–Nd–Hf isotope compositions similar to the coeval K-rich mafic rocks in the MLYRB. This indicates that these granitoids and MMEs were mainly sourced from the enriched subcontinental lithospheric mantle (SCLM) that metasomatized by slab derived fluids/melts, and had experienced significant fractional crystallization. The high oxygen fugacity and water content of the early Tonglushan quartz monzodiorite (porphyry) indicate melting of metasomatized SCLM and subsequently fractional crystallization and recharging/mixing in magmatic system have the potential to Cu–Au polymetallic mineralization. This study also highlights that the SCLM beneath the eastern Yangtze plate is one of the likely sources for Late Mesozoic magmatic rocks in the MLYRB.