OIB-like Trace Element Patterns Research Articles

Ca. 830 Ma alkaline mafic dykes in the southeastern Guizhou Province, South China: New constraints on the Neoproterozoic evolution of the Yangtze Block

The Neoproterozoic alkaline mafic rocks in the Yangtze Block are critical to investigate the magmatic and tectonic evolution of South China. However, these rocks are rarely exposed. We here present a detailed study of ca. 830 Ma alkaline mafic dykes newly identified from a drilled borehole at the Nage area in the southern Yangtze Block. The dykes have SIMS zircon U-Pb ages of 827 ± 8 Ma. They show coherent variations in major and trace elements, characterized by enrichment in LREE and incompatible elements and no significant Nb-Ta and Zr-Hf anomalies, suggesting that their parental magmas have experienced varying degrees of fractionation crystallization with negligible crustal contamination. Modeling calculations based on their εNd(T) values (-0.55–3.23), SiO2 contents (48.38–53.61 wt%) and elemental ratios (e.g., La/Ta and Nb/La) suggest that the Nage mafic dykes were probably generated from an asthenospheric mantle with previously input of a plume-derived enrich component occurred within the source region rather than during ascent. Rocks from the Nage dykes exhibit OIB-like trace element and REE patterns as well as trace elemental compositions, which are different from arc-related igneous rocks but similar to alkaline basalts and mafic dykes formed in mantle plume related rifting systems around the world. In combination with tectonostratigraphic evidences, the Nage dykes in this study are suggested to have been generated in an intraplate rift which was probably induced by a mantle plume beneath the Yangtze Block.

The newly recognized ca. 1.23–1.21 Ga dolerite sills and flood basalts from Fanhe Basin in the northeastern North China Craton: Petrogenesis and tectonic implications

The Ectasion Period (1400–1200 Ma) represents a transition stage from Columbia (Nuna) to Rodinia supercontinents. LIPs and mafic magmatic events during this period can provide important constraints on deep processes and paleogeographic reconstructions during this transition. In this paper, we firstly reported some ca. 1.23–1.21 Ga flood basalts coeval with mafic sills from the Fanhe Basin in the northeastern North China Craton (NCC). New zircon/baddeleyite U-Pb/Pb-Pb dating on dolerite sills emplaced into the Fanhe Group yields emplacement ages from 1231 ± 9 Ma to 1206 ± 20 Ma. Whole-rock 40Ar/39Ar dating of two basalts from the Erdaogou Formation yields approximate total gas ages of ca. 1250 Ma and ca. 1210 Ma. These new geochronological results revealed that the Erdaogou basalts are coeval with some dolerite sills emplaced into the Kuangzhuangzi, Guanmenshan and Tongjiajie Formations and the dolerite sills emplaced into the Hutouling and Erdaogou Formations. These newly recognized ca. 1.23–1.21 Ga mafic rocks can be linked to the broader Licheng LIP of the previous studies. The ca. 1.23–1.21 Ga mafic rocks in the Fanhe basin can be divided into alkaline and sub-alkaline series and these two series exhibit distinct trace element and Nd isotopic compositions. The alkaline rocks show strongly fractionated chondrite-normalized REE patterns with high LaN/YbN values of 12.69–15.05. The sub-alkaline samples display less fractionated REE patterns with lower LaN/YbN values of 2.74–10.13. The alkaline rocks have OIB-like trace element patterns and slightly negative to positive εNd(t) values from −0.7 to 0.7. An OIB-like, asthenospheric mantle source is suggested as the source of the alkaline rocks. In contrast, the sub-alkaline rocks exhibit low negative εNd(t) values from −10.9 to −4.6 and are interpreted to be derived from a subduction-modified subcontinental lithospheric mantle. By comparisons with the coeval mafic magmatism in other parts of the NCC and other cratons, we suggest that the ca. 1.23–1.21 Ga mafic magmatism or LIP in the NCC was either related to the mantle plume that resulted in the Marnda Moorn LIP in Western Australia or related to a back-arc extension prior to the formation of a Grenvillian orogen located in another continent connected to the southeastern margin of the NCC during assembly of the Rodinia supercontinent.

Petrology and geochemistry of early Permian mafic–ultramafic rocks in the Wajilitag area of the southwestern Tarim Large Igneous Province: Insights into Fe-rich magma of mantle plume activity

The majority of global Fe–Ti-oxide deposits hosted in mafic–ultramafic layered intrusions are closely related to mantle plume activity. The metallogenesis of these Fe–Ti-oxide deposits is still debated, especially with regard to their mantle source and relationship to magmatism. Here we report detailed petrology and geochemistry for early Permian (ca. 280 Ma) mafic–ultramafic igneous rocks in the Wajilitag area of the western Tarim Large Igneous Province (TLIP) to establish the relationships among mantle source, magmatism, and metallogenesis in mantle plume activity. The mafic rocks (i.e., gabbro and diabase) show alkaline affinities and OIB-like trace-element patterns. Clinopyroxenites closely related to Fe–Ti-oxide deposits were derived from the same magmatic system as that of the mafic rocks. The primary magma of the Wajilitag mafic rocks was enriched in FeOT and TiO2 and depleted in SiO2, and has high FC3MS (FeOT/CaO − 3MgO/SiO2) and Fe/Mn, indicating the involvement of Si-poor pyroxenite/eclogite in the mantle source. Primary olivines from the Wajilitag gabbro have consistently high NiO and Fe/Mn. Wajilitag mafic rocks have variable Sr–Nd–Hf isotopes owing to their derivation from a hybrid mantle source with the involvement of subducted oceanic crust and sediments. Subducted slabs within the upper mantle are inferred to have been entrained by the upwelling mantle plume and preferentially melted to produce mainly Fe-rich magma. The occurrence of an adjacent oceanic subduction event just prior to the operation of a mantle plume played a crucial role in the formation of large-scale Fe–Ti-oxide deposits in LIPs.

Magnesium and zinc isotopic anomaly of Cenozoic lavas in central Myanmar: Origins and implications for deep carbon recycling

Carbon in the form of carbonate in sediments and altered oceanic crust can be transported into the mantle by subducted slabs, but how to identify recycled carbonate and decipher its storage depths in the mantle still requires to be investigated for typical subduction zones. Here we investigate possible carbonate recycling related to subduction of the Neo-Tethyan oceanic slab, by a combined study of Mg and Zn isotopes on three types of Cenozoic mantle-derived lavas from central Myanmar. The calc-alkaline lavas have arc-like geochemical characteristics and originated from the mantle wedge. These rocks have high δ26Mg (−0.20‰ to +0.03‰), interpreted to be caused by the input of Mg-rich fluids from dehydration of altered subducted slab. The alkali basalts of suite I are depleted in Nb and Ta and were derived from partial melting of the lithospheric mantle, which have normal δ26Mg and δ66Zn. By contrast, the alkali basalts of suite II display OIB-like trace element patterns and originated from the asthenospheric mantle. They possess extremely low δ26Mg (−0.66‰ to −0.48‰) and high δ66Zn (0.57‰ to 0.77‰) that is typically characterized by marine carbonates. No known processes (e.g., magmatic processes and diffusion effects) can account for the coupling of low δ26Mg and high δ66Zn except source heterogeneity caused by recycled magnesian carbonates (magnesite ± dolomite). This is supported by Mg-Zn-Sr isotopic mixing modelling that suggests a “magnesian carbonate + pyroxenite” source. Since calcic carbonate transforms to magnesite at depths of >300 km during subduction, carbonates carried by the Neo-Tethyan oceanic slab may have been recycled into the mantle transition zone as revealed by seismic tomography observations on central Myanmar.

Contrasting compositions between phenocrystic and xenocrystic olivines in the Cenozoic basalts from central Mongolia: Constraints on source lithology and regional uplift

AbstractTwo Cenozoic prominent features are spatio-temporally associated in central Mongolia, i.e., the continental basalts and regional uplift, but their genesis and relationship remain unclear. This study presents major- and trace-element compositions for olivine phenocrysts and xenocrysts, as well as data of bulk-rock geochemistry and Sr-Nd-Hf isotopes for the host basalts. The studied basalts mostly have trachybasalt compositions with high total alkali (Na2O + K2O = 5.1–8.2 wt%) contents and all display OIB-like trace element patterns (e.g., spikes of Ba, Nb, and Ta and troughs of Th and U) and EM1-like Sr-Nd-Hf isotopic compositions. Compared to the partial melting products of mantle peridotite, these basaltic samples have higher FeO/MnO, Zn/Mn, and Zn/Fe ratios. Meanwhile, phenocrystic olivines are characterized by lower Ca, Mn, Mn/Zn, and Mn/Fe but higher Ni than their counterparts in the peridotitic melts, indicating a pyroxenite-rich mantle source. The above geochemical data suggest that the source of the studied basalts was mainly made up of secondary pyroxenite produced by the reaction of recycled oceanic crust with its ambient mantle peridotite. The calculated magma oxygen fugacities (ΔFMQ-0.26 to +0.42) and mantle melting temperatures (1343–1430 °C) do not support a genetic link with the stagnant Pacific slab or with a deep mantle plume. Instead, the far-field effect of India-Eurasia convergence possibly tapped the upper asthenospheric mantle, subsequent melting of which gave rise to the dispersive Cenozoic basalts. On the other hand, the xenocrystic olivines exhibit zoned textures with high-Fo (up to 92) cores and low-Fo (down to 76) rims, reflecting the melt-rock interaction. Preservation of zoned olivine xenocrysts indicates rapid magma ascent and widespread melt-rock reaction in the mantle lithosphere, which may modify the rheology and accelerate the mechanical erosion of mantle lithosphere. Consequently, mass deficit in the lithosphere could have caused isostatic uplift of central Mongolia in the Cenozoic.

Cadomian Magmatic Rocks from Zarand (SE Iran) Formed in a Retro-Arc Basin

A major arc-related magmatic episode is recorded in the Ediacaran-Early Cambrian (Cadomian) crust of Iran and Anatolia due to the southward subduction of Prototethyan oceanic lithosphere beneath N Gondwana. This magmatism was generated at a convergent margin, fragments of which can be traced for thousands of kilometres, from west Avalonia to the Lhasa terrane. Cadomian crustal tracts in Iran and Tauride-Anatolia are represented by abundant arc-like plutonic and volcanic rocks and a series of retro-arc, rifted basins dominated by thick sequences of terrigenous rocks as well as both calc-alkaline and alkaline magmatic rocks. This paper presents new zircon U-Pb as well as geochemical-isotopic data from plutonic (granite to gabbro) and volcanic (basalt to rhyolite) magmatic rocks of a Cadomian retro-arc rifted basin from Zarand in SE Iran. Geochemical data indicate two different geochemical signatures; high K calc-alkaline-shoshonitic, characterized by strong depletions in Nb, Ta, P, Ti, and alkaline rocks (and A1-type intrusions), with strong OIB-like trace element patterns. Cadomian OIB-like rocks- as well as A1-type intrusions- are rare in the basements of Iran and Anatolia and are only documented as minor gabbroic intrusions and/or (meta-) volcanic rocks from Saghand (central Iran), NE Iran and from exotic blocks from Ediacaran salt domes. Moreover, A2-type granites are also present in the Cadomian crust of Iran. Zarand volcanic rocks are interlayered with Rizu-Dezu terrigenous sedimentary rocks, whereas plutonic rocks intruded these metasediments. New zircon U-Pb ages show that high K calc-alkaline-shoshonitic rocks formed ~ 537-536 Ma, whereas A1-type granites crystallized at ~535 Ma. Most zircons from A1-type granites show positive εHf(t) values from +1.1 to +5.1, mostly higher than high K calc-alkaline-shoshonitic rocks with εHf (t) between −6.6 and +8.1. Bulk rock Nd-Sr isotopic data (e.g., ɛNd(t)= +0.3 to +4.0) for OIB-like rocks confirm that these rocks originated from an enriched OIB-like mantle source, whereas high K calc-alkaline-shoshonitic rocks (with εNd(t) = −7.7 to −6.2) show strong interaction with, and/or re-melting of a continental crust. Our results are consistent with an interpretation that the Zarand volcanic-plutonic rocks as well as associated thick sequences of sedimentary strata rocks formed in a retro-arc rifted basin behind the Cadomian magmatic arc. This basin seems to have developed during the Ediacaran at ~570-560 Ma with deposition of Ediacaran Morad to Lower Cambrian Rizu-Dezo sedimentary rocks. The retro-arc extensional basin seems to have become magmatically active at 540-535 Ma, as shown by the occurrence of both high K calc-alkaline- shoshonitic and OIB-like magmatic rocks. This basin was flanked by a cratonic hinterland towards Gondwana and a magmatic arc to the N. The cratonic hinterland fed detritus with detrital zircons older than 0.6 Ga into the basin, whereas the magmatic arc shed detritus with 500-600 Ma old zircons. Our compiled and new data from Ediacaran-Early Paleozoic magmatic rocks also show that Cadomian high magmatic fluxes occurred differently in parts of Iran-Anatolia, with overlaps in some areas.

Recycled oceanic crust in the form of pyroxenite contributing to the Cenozoic continental basalts in central Asia: new perspectives from olivine chemistry and whole-rock B–Mo isotopes

Cenozoic continental basalts are widespread in central Asia. To explore their source nature and petrogenesis, this study presents an integrated study of olivine chemistry, bulk-rock 40Ar/39Ar age and geochemistry as well as Sr–Nd–Pb–B–Mo isotopes for the Miocene (ca. 15.5 Ma) Halaqiaola basalts in the Chinese Altai, central Asia. The Halaqiaola basalts mostly have basanite compositions with high total alkali (Na2O + K2O = 6.89–8.01 wt%) contents and high K2O/Na2O (0.87–1.39) ratios. Compared with partial melting products of mantle peridotite, the basaltic samples possess lower CaO and CaO/Al2O3 but higher TiO2, Zn/Mn and Zn/Fe values. Meanwhile, olivine phenocrysts from these basalts are characterized by lower Ca, Ni and Mn contents but higher Fe/Mn ratios than their counterparts in the peridotitic melts, suggesting a pyroxenite-rich source. Moreover, these rocks show OIB-like trace element patterns (e.g., spikes of Ba, Sr, Nb and Ta and troughs of Th and U), and constant Nd but variable Sr and EM1-like Pb isotopic compositions, and yield light δ11B (– 11.0 to – 8.1‰) and δ98Mo (– 0.40 to – 0.06‰) values. The above geochemical data suggest that secondary pyroxenite was likely produced by reaction of recycled oceanic crust with its ambient peridotite and subsequently became the main source for the basanite. Furthermore, their light and variable δ98Mo values probably reflect that recycled oceanic crust involved in such pyroxenite was altered with different degrees. In combination with available data from adjacent regions, we propose that the far-field effect of India–Eurasia collision was the first-order factor for the upwelling of dispersive asthenospheric mantle beneath central Asia, subsequent melting of which gave rise to the widespread Cenozoic volcanism.

Two Types of mafic rocks in southern Tibet: A mark of tectonic setting change from Neo-Tethyan oceanic crust subduction to Indian continental crust subduction

We collated existing data for the Eocene Langshan mafic rocks (Eocene mafic rocks) and the Miocene potassic–ultrapotassic mafic rocks (Miocene mafic rocks) in southern Tibet to investigate the tectonic transition from Neo-Tethyan oceanic crust subduction to Indian continental crust subduction. The Eocene mafic rocks have high Na2O contents (K2O/Na2O = 0.03–0.2) and show OIB-like trace element patterns (e.g., positive Nb and Ta anomalies) and depleted radiogenic Sr–Nd isotope compositions (87Sr/86Sr of apatite = 0.7031, εNd(t) = +5.1 to +6.1). In contrast, the Miocene mafic rocks have high K2O contents (K2O/Na2O = 1.9–8.5) and exhibit arc-like trace element patterns (enrichment in LILEs and depletion in HFSEs) and enriched radiogenic Sr–Nd isotope compositions (87Sr/86Sr = 0.7115–0.7362, εNd(t) = −16 to −12.4). The mantle source for the Eocene mafic rocks was generated by reactions between asthenospheric mantle wedge and felsic melts from subducted Neo-Tethyan oceanic crust (outside the field of rutile stability). In contrast, the mantle source of the Miocene mafic rocks was generated by reactions between asthenospheric (or lithospheric) mantle wedge and felsic melts from subducted Indian continental crust. Taking into account the regional tectonic evolution, we propose that break-off of the Neo-Tethyan oceanic slab and roll-back and/or break-off of the Indian continental slab were the most likely geodynamic mechanisms that led to the production of the Eocene and Miocene mafic rocks, respectively. Therefore, the transition from the Eocene to Miocene mafic rocks in southern Tibet provides an opportunity to understand the tectonic transition from Neo-Tethyan oceanic to Indian continental crust subduction.

The geodynamic setting of Dulan eclogite-type rutile deposits in the North Qaidam orogen, western China

Abundant eclogite-type rutile deposits have recently been discovered along the North Qaidam ultrahigh-pressure (UHP) metamorphic belt in China. The identified titanium reserves exceed 3 million tons and range in grade from 1.0% to 4.5%. The rutile deposits of economic interest are found within two types of eclogite. However, the origin and original geodynamic setting of the two types of rutile-bearing eclogites remain controversial. This study therefore investigates ore and mineralization bodies belonging to the two types by way of detailed field work and geochronological, geochemical and Sr–Nd–Hf isotopic compositional analysis, taking the Dulan eclogite-type rutile deposit as its case study. Type-1 rutile-bearing eclogites enclosed in granitic gneisses are found to have Neoproterozoic protolith ages of 845 ± 4 Ma and an eclogite-facies metamorphic age of ca. 433 Ma. Their high TiO2, TFe2O3, and P2O5 contents cannot have been produced by rutile-formation during UHP metamorphism but could have arisen through the accumulation of ilmenite and apatite in a high-Ti protolith. They have ocean-island basalt (OIB)-like trace element patterns, with enrichment in light rare-earth elements (LREEs) and high field strength elements (HFSEs) (Nb, Ta, Zr, and Hf). The Hf isotopic compositions of the residual magmatic zircon cores in Type-1 rutile-bearing eclogites (εHf (845) = +1.4 to +6.0, TDM = 1.2–1.5 Ga) indicate that their parental magmas were derived from the Proterozoic mantle. However, they show initial 87Sr/86Sr values of 0.71059–0.71326 and εNd (845) values of −1.5 to −2.5, suggesting significant crustal contamination. Thus, the Type-1 rutile-bearing eclogites may have originated through partial melting of an OIB-type mantle plume source during the Neoproterozoic rifting of the Rodinia supercontinent. In contrast, the Type-2 rutile-bearing eclogites, which are hosted in a mafic–ultramafic ophiolitic complex, have protolith ages of 449 ± 3 Ma and eclogite-facies metamorphic ages of ca. 435–433 Ma. Correlations between the whole-rock MgO and the major and trace element contents of Type-2 rutile-bearing eclogites suggest that they are derived from cumulates with alternating olivine and pyroxene-rich and ilmenite-rich layers. They have mid-ocean-ridge basalt (MORB) and OIB-like trace element patterns, indicating extensive fractional crystallization and the possibility that there were multiple injections of magma during the formation of the protolith. Meanwhile, the +0.6 to +4.3 εNd (449) values and −1.8 to +11.8 εHf (449) values of the Type-2 rutile-bearing eclogites indicate that the protoliths may have been generated by partial melting of a mantle wedge source metasomatized by slab-derived components. The short interval (ca. 14 my) between magma generation and eclogite-facies metamorphism, in combination with the type of rock assemblage within which they are found, indicates that the Type-2 rutile-bearing eclogites originated in a supra-subduction zone (SSZ) ophiolite. Type-1 and Type-2 rutile-bearing eclogites have similar eclogite-facies metamorphic ages of ca. 435–433 Ma, indicating that the Type-1 and SSZ-related Type-2 protoliths were incorporated into a subduction mélange that was dragged down the subduction channel and underwent eclogite-facies metamorphism via subduction of the Qaidam Block, eventually forming eclogite-type rutile deposits.

Late Cenozoic basaltic lavas from the Changbaishan-Baoqing Volcanic Belt, NE China: Products of lithosphere-asthenosphere interaction induced by subduction of the Pacific plate

Cenozoic intraplate basalts are low in volume but widespread in eastern China. They are predominantly alkaline and have oceanic island basalt (OIB)-like trace element compositions. Despite numerous studies, the origin of Cenozoic basalts in eastern China remains elusive. Possible roles of lithosphere thickness, subduction of Pacific plate and lithosphere-asthenosphere interaction in triggering spatial geochemical variations have not yet been clarified. Here, we have carried out mineral chemistry, major and trace element and SrNdHf isotope analyses of the late Cenozoic (<20 Ma) basaltic rocks from the Changbaishan-Baoqing Volcanic Belt (CVB), NE China, which revealed clear spatial compositional variations. The North CVB is dominated by basanites and alkali basalts with OIB-like trace element patterns and depleted SrNdHf isotopic compositions (87Sr/86Sr = 0.7039–0.7047, εNd = 3–5.6; εHf = 6.7–12), which may have been derived from low degree partial melting of a depleted source from asthenospheric mantle beneath a thick lithosphere. On the other hand, the South CVB consists of both alkali and sub-alkali lavas (alkali basalts, tholeiites and basaltic andesites) that display generally higher SiO2, Sm/Nd, Ba/Nb, Th/U, and lower Nb/Th, La/Sm and more enriched SrNdHf isotopic compositions (87Sr/86Sr = 0.7038–0.7056, εNd = −2.4 – +3.2; εHf = 3–8.2). These rocks may have been produced by larger degrees of partial melting of asthenospheric mantle beneath a relatively thin lithosphere. Ancient metasomatized lithospheric mantle might also have contributed to their genesis. In addition, the similar ranges of Mn, Ni and Fe/Mn for olivine phenocrysts from both the North and the South CVB suggest that they may have been derived from hybrid mantle sources containing similar proportions of peridotite and pyroxenite/eclogite components. We propose that decompression melting of upwelling asthenosphere and mechanical-chemical erosion of basal lithosphere related to lithosphere-asthenosphere interaction responsible for the genesis of the CVB magmas were likely associated with upper mantle convection and back-arc extension induced by deep subduction of the Pacific plate and its stagnancy in the transition zone.

Constraining lithospheric removal and asthenospheric input to melts in Central Asia: A geochemical study of Triassic to Cretaceous magmatic rocks in the Gobi Altai (Mongolia)

Throughout northeast China, eastern and southern Mongolia, and eastern Russia there is widespread Mesozoic intracontinental magmatism. Extensive studies on the Chinese magmatic rocks have suggested lithospheric mantle removal was a driver of the magmatism. The timing, distribution and potential diachroneity of such lithospheric mantle removal remains poorly constrained. Here, we examine successions of Mesozoic lavas and shallow intrusive volcanic plugs from the Gobi Altai in southern Mongolia that appear to be unrelated to regional, relatively small-scale deformation; at the time of magmatism, the area was ~200km from any active margin, or, after its Late Jurassic-Early Cretaceous closure, from the suture of the Mongol-Okhotsk Ocean. 40Ar/39Ar radiometric age data place magmatic events in the Gobi Altai between ~220 to 99.2Ma. This succession overlaps Chinese successions and therefore provides an opportunity to constrain whether Mesozoic lithosphere removal may provide an explanation for the magmatism here too, and if so, when.We show that Triassic to Lower Cretaceous lavas in the Gobi Altai (from Dulaan Bogd, Noyon Uul, Bulgantiin Uul, Jaran Bogd and Tsagaan Tsav) are all light rare-earth element (LREE) and large-ion lithophile element (LILE)-enriched, with negative Nb and Ta anomalies (NbLa and TaLa≤1). Geochemical data suggest that these lavas formed by low degrees of partial melting of a metasomatised lithospheric mantle that may have been modified by melts derived from recycled rutile-bearing eclogite. A gradual reduction in the involvement of garnet in the source of these lavas points towards a shallowing of the depth of melting after ~125Ma.By contrast, geochemical and isotope data from the youngest magmatic rocks in the area — 107-99Ma old volcanic plugs from Tsost Magmatic Field — have OIB-like trace element patterns and are interpreted to have formed by low degrees of partial melting of a garnet-bearing lherzolite mantle source. These rocks did not undergo significant crustal contamination, and were derived from asthenospheric mantle. The evidence of a gradual shallowing of melting in the Gobi lava provinces, culminating in an asthenospheric source signature in the youngest magmatic rocks is similar to examples from neighboring China, emphasising the wide-scale effect of a regional Mesozoic magmatic event during similar time periods. We suggest that Mongolia underwent lithospheric thinning/delamination during the Mesozoic (between ~125 and ~107Ma) with patchy areas thinning sufficiently to enable the generation of relatively small-scale asthenospheric-derived magmatism to predominate in the late Cretaceous.

Late Cretaceous transition from subduction to collision along the Bangong-Nujiang Tethys: New volcanic constraints from central Tibet

This study deals with arc-type and subsequent bimodal volcanic rocks interbedded with (late) Cretaceous sedimentary formations near Gaize, central Tibet that shed new light on the Tethyan evolution along the Bangong-Nujiang suture. Unit I consists of trachyandesites interbedded with fine-grained sandstone, slate, and limestone. Zircon dating on a trachyandesite sample yields a 206Pb/238U age of 99±1Ma. The trachyandesites are characterized by strong enrichment in LILE but depletion in HFSE, low zircon saturation temperatures (TZr=642–727°C), and high oxygen fugacity (∆FMQ=−0.67–0.73), indicating their arc affinities. Unit II comprises a bimodal basalt-rhyolite suite interbedded with coarse-grained sandstone and conglomerate. Zircon dating on two rhyolitic samples yield 206Pb/238U ages of 97.1–87.0Ma. In contrast with Unit I trachyandesites, Unit II basalts and rhyolites exhibit OIB-like trace element patterns, high temperatures (T=1298–1379°C for basalts, TZr=855–930°C for rhyolites), and low oxygen fugacity (∆FMQ=−3.37~−0.43), suggesting that Unit II bimodal volcanic rocks probably formed in an extensional setting. The Sr-Nd isotopes of both Unit I (87Sr/86Sri=0.7052–0.7074, εNd(t)=−2.21–1.02) and Unit II (87Sr/86Sri=0.7057–0.7098, εNd(t)=−3.22–0.88) rocks are similar to mantle-wedge-derived volcanic rocks within the southern Qiangtang block. The parental magma of Unit I trachyandesites was formed by fluid induced melting of the mantle wedge above the subducted Bangong-Nujiang Tethyan slab, and contaminated by crustal materials during MASH process within a deep crustal hot zone; and Unit II bimodal volcanic rocks were derived by melting of upwelling asthenosphere and a mildly metasomatized mantle wedge during the Lhasa-Qiangtang collision. We propose that the transition from the Bangong-Nujiang Tethyan subduction to the Lhasa-Qiangtang collision occurred during the Late Cretaceous in central Tibet.

1.01–0.98 Ga mafic intra-plate magmatism and related Cu-Au mineralization in the eastern Jiangnan orogen: Evidence from Liujia and Tieshajie basalts

The Grenvillian-aged mafic magmatism is critical for understanding the regional tectonic settings and testing different tectonic models of South China. However, the geochronology, petrogenesis and geodynamic setting of Grenvillian-aged mafic rocks in the Jiangnan orogen remain unclear. An integrated study on LA−ICP−MS zircon U−Pb geochronology, elemental and zircon Hf isotopic geochemistry has been conducted on the Liujia and Tieshajie basalts in the eastern Jiangnan orogen. Two samples yielded zircon U-Pb ages of ca. 1012–980Ma. The Liujia and Tieshajie basalts show typical OIB-like trace element patterns, and have high Ti concentrations and positive ɛHf(t) values, implying a fertile asthenospheric mantle as their magma source. The Liujia and Tieshajie basalts have relative low Mg# values (36–58) indicating that their parental magma experienced moderately to strong fractional crystallization of olivine and clinopyroxene. Most samples of the basalts have high Nb/La (1.05–1.39) and Nb/Th (19.04–19.53) values, indicating an insignificant crustal contamination. The 1.01–0.98Ga intra-plate mafic magmatism argued against the existence of Grenvillian orogeny in the Jiangnan orogen. The geochemical signatures of mafic rocks along the Jiangnan orogen display systematic changes from 1.01–0.98Ga OIB-like mafic rocks to 0.97–0.83Ga island arc and arc-like basalts to 0.80–0.76Ga OIB- and arc-like mafic rocks. The systematic geochemical variations of mafic rocks demonstrate that tectonic switch occurred from an intra-plate regime to an oceanic subduction-related setting to a post-orogenic environment. With respect to Cu-Au mineralization, this study suggests that the Tieshajie deposit is one of the oldest regional mineralization events and the formation of coeval metal-rich juvenile crust is a critical step for the generation of later regional large-scale Cu-Au mineralization.

Cenozoic basalts in SE China: Chalcophile element geochemistry, sulfide saturation history, and source heterogeneity

Cenozoic basalts in SE China may be derived from a mixture of depleted MORB mantle (DMM) and enriched mantle 2 (EM2) sources, but whether these basalts share a common mantle source or magmatic history remains unknown. To investigate these unresolved issues, this study sampled basalts from Niutoushan and Mingxi (Fujian province), Xilong (Zhejiang province), and Penghu (Taiwan) for geochemical analysis. The basalt samples show OIB-like trace element patterns and have low PGE contents, with 0.02–0.7ppb Ir and Pd, 0.05–1.4ppb Ru, 0.01–0.2ppb Rh, and 0.06–1.1ppb Pt. All samples have high Cu/Pd ratios ranging from ~69,000 to 3,500,000, and low Cu/Zr ratios ranging from 0.1 to 0.8, suggesting sulfur-saturated fractionation. Model calculations indicate that the basalts are depleted in PGE due to the retention of 0.001% to 0.1% sulfide in the mantle and the removal of up to 0.0022% sulfide during magma ascent. The crystallization of olivine and spinel, and partial melting are insufficient to account for the observed PGE variation in these basalts. Thus, the distinct PGE patterns in basalts with different ages may reflect the heterogeneity of the mantle source beneath SE China. The source heterogeneity may be due to compositional heterogeneity, particularly variations in oxygen fugacity and PGE mineral phases, or due to variable fluid/melt metasomatic agents in the sub-continental lithospheric mantle. This heterogeneity is possibly related to the westward subduction of the Paleo-Pacific Plate.

The origin of Cenozoic continental basalts in east-central China: Constrained by linking Pb isotopes to other geochemical variables

Cenozoic continental basalts in east-central China are characterized by OIB-like trace element patterns with more depleted to less enriched Sr–Nd isotope compositions. Such geochemical signatures are attributable to variable contributions to their mantle sources from crustal components in the oceanic subduction zone. A combined study of basalt Pb isotope variations with other geochemical variables indicates that four mantle and crustal components were involved in the basalt petrogenesis. Model calculations verify the geochemical transfer from the subducted crustal components to the mantle sources. The depleted MORB mantle component is indicated by the depleted Sr–Nd isotope compositions of basalts. Relatively high 206Pb/204Pb and low Δ8/4 ratios are ascribed to contributions from the igneous oceanic crust with high U/Pb and low Th/U ratios, low 206Pb/204Pb and high Δ8/4 ratios are ascribed to the lower continental crust, and high 206Pb/204Pb and high Δ8/4 ratios are linked to the seafloor sediment. This generates different compositions of mantle sources for these OIB-like continental basalts. The basalts with the most depleted Sr–Nd isotope compositions show Pb isotope compositions distinct from Pacific MORB but similar to Indian MORB, suggesting the occurrence of Indian type asthenospheric mantle beneath the continental lithosphere of eastern China. The depleted MORB mantle would be metasomatized by the three crustal components at the slab-mantle interface in oceanic subduction channel, generating the mantle sources that are enriched in melt-mobile incompatible trace elements and their pertinent radiogenic isotopes. Nevertheless, the crustal components were not directly incorporated in the forms of crustal rocks into the mantle sources, but underwent partial melting to produce the felsic melts that predominate the composition of those trace elements and their pertinent radiogenic isotopes in the basalts. As such, the depleted MORB mantle component was contributed to the mantle sources in the form of peridotite, dominating the composition of major elements in the basalts. In is inferred that the westward subduction of Pacific slab beneath the North China Craton would lead not only to replacement of the ancient refractory lithospheric mantle by the depleted MORB mantle but also to incorporation of the crustal components into the mantle sources of continental basalts. Therefore, a series of processes from subduction, anatexis and reaction to storage and heating (SARSH) would be operated for the origin of continental basalts.

Petrology and geochemistry of Cenozoic intra-plate basalts in east-central China: Constraints on recycling of an oceanic slab in the source region

Cenozoic mafic rocks in Jiangsu and Anhui Provinces, east-central China are chiefly basanites and alkali olivine basalts with subordinate tholeiites, which were erupted in three stages; Paleogene, Neogene and Quaternary. The rocks become increasingly alkaline as they become younger. On a primitive mantle-normalized multi-element plot, these lavas exhibit typical OIB-like trace element patterns, including enrichment in most incompatible elements (LILE and HFSE) and negative K and Pb anomalies. The compositions of the mafic rocks indicate that they were derived from a mantle source mainly containing clinopyroxene and garnet, most probably a mixture of pyroxenite/eclogite and peridotite. A mineral equilibrium projection shows that all the mafic magmas were produced at pressures of 3–4GPa, implying an asthenospheric origin. Their positive Ba and Sr anomalies and relatively high 87Sr/86Sr ratios suggest derivation from an EM1-type mantle source. However, poor correlations between 87Sr/86Sr and 143Nd/144Nd indicate an isotopically heterogeneous source for the magmas, including DMM, EM1 and EM2, representing mantle peridotite, recycled ancient oceanic crust and seafloor sedimentary rocks, respectively. Variable correlations between 87Sr/86Sr and 143Nd/144Nd ratios, CaO–MgO contents and Eu/Eu* and Ce/Ce* anomalies with rock type imply that marine sediments (plus variable amounts of oceanic crust) and peridotites were the dominant source lithologies of the basanites, whereas recycled oceanic crust (pyroxenite/eclogite) was the main source of the weakly alkaline basalts. This hypothesis is supported by seismic tomographic images of the mantle beneath the region, which show the presence of a stagnant subducted slab in the mantle transition zone. Thus, we propose a petrological model in which a hybrid magma column originated from the mantle transition zone and assimilated some of the overlying peridotite during upwelling, to become the parental magmas of these mafic rocks. This process may relate to the documented lithospheric thinning in East China in the Mesozoic and Cenozoic.

Tectonic transition from Late Carboniferous subduction to Early Permian post-collisional extension in the Eastern Tianshan, NW China: Insights from geochronology and geochemistry of mafic–intermediate intrusions

The closure of the Junggar Ocean between the Central Tianshan and Junggar terranes is essential in understanding the final assembly of the southern Central Asian Orogenic Belt. This study presents new whole-rock geochemical, Sr–Nd and zircon U–Pb–Hf isotopic data for mafic–intermediate intrusions from the Central Tianshan block to provide robust constraints on the final closure of the ocean. LA-ICP-MS U–Pb dating on magmatic-type zircons yields weighted mean 206Pb/238U ages of ca. 310Ma and ca. 290Ma, which are interpreted as the crystallization ages of the intrusions. Petrographic and geochemical analyses of the Late Carboniferous mafic–intermediate rocks, characterized by typical subduction-related signatures, low Sm/Yb (<2.0) but high Lu/Hf (≥0.2) ratios and positive εNd(t) (+1.2 to +3.1) and zircon εHf(t) (+4.1 to +7.8) values, suggest that their parental magmas were most likely generated by the partial melting of a metasomatized lithospheric mantle wedge in the spinel stability field and emplaced in a continental arc setting. This consideration is consistent with the occurrence of Carboniferous ophiolitic and arc-related granitoids in the region, probably as a result of the southward subduction of the Junggar oceanic plate. In contrast, the Early Permian mafic magmatism exhibits typical within-plate basalt affinities, such as high TiO2 (2.7–3.2wt.%) contents, elevated Ti/V (86.0–115.1) and Zr/Y (4.9–9.3) ratios, OIB-like trace element patterns and high εNd(t) (+1.1 to +4.5) and zircon εHf(t) (+3.0 to +9.8) values. In association with previous investigations, we suggest that their protoliths were most probably derived from the partial melting of an asthenospheric mantle source in the garnet stability field, plausibly induced by asthenosphere upwelling during the slab break-off of the Junggar oceanic plate, which agrees well with the linear distributions of Permian mafic–ultramafic rocks in the Eastern Tianshan. Collectively, our data pinpoint a tectonic transition from oceanic subduction to post-collisional extension during Late Carboniferous to Early Permian time, probably triggered by the closure of the Junggar Ocean and subsequent arc–continent collision between the Central Tianshan and Junggar terranes that gave rise to the final assembly of the Eastern Tianshan.

Lithospheric mantle xenoliths sampled by melts from upwelling asthenosphere: The Quaternary Tasse alkaline basalts of southeastern British Columbia, Canada

The Quaternary Tasse basalts are exposed near the north shore of Quesnel Lake in southeastern British Columbia. They host a variety of mantle xenoliths consisting predominantly of spinel lherzolite with minor dunite and pyroxenite. Mineralogically, the xenoliths are composed of olivine, orthopyroxene, clinopyroxene and spinel characterized by forsterite (Fo87–93), enstatite (En90–92), diopside (En45–50–Wo40–45–Fs5), and Cr-spinel (6−11wt.% Cr), respectively. All of the mantle xenoliths are coarse-grained and show granoblastic textures. Clinopyroxene and spinel display textural evidence for chemical reactions with percolating melts.The mantle xenoliths are characterized by restricted Mg-numbers (89−92) and low abundances of incompatible elements (Ba=2−11ppm; Sr=3−31ppm) and Yttrium (1−3ppm). On the basis of REE patterns, the xenoliths are divided into three groups reflecting the various degrees of mantle metasomatism: (1) Group 1 consists of concave-up LREE patterns (La/Smcn=0.48−1.16; Gd/Ybcn=0.71−0.92); (2) Group 2 possesses flat to moderately LREE-enriched patterns (La/Smcn=1.14−1.92; Gd/Ybcn=0.87−1.09); and (3) Group 3 is characterized by strongly LREE-enriched patterns (La/Smcn=1.53−2.45; Gd/Ybcn=1.00−1.32). On MORB-normalized trace element diagrams, the majority of the xenolith samples share the enrichment of LILE (Rb, Ba, K), U, Th, Pb, Sr and the depletion of HFSE (Nb, Ta, Ti, Y) relative to REE. These geochemical characteristics are consistent with a compositionally heterogeneous subcontinental lithospheric mantle source that originated as subarc mantle wedge peridotite at a convergent plate margin.The Tasse basalts have alkaline compositions characterized by low SiO2 (44−46wt.%) and high alkali (Na2O+K2O=5.1−6.6wt.%) contents. They are strongly enriched in incompatible elements (TiO2=2.4−3.1wt.%; Ba=580−797ppm; Sr=872−993ppm) and, display OIB-like trace element patterns (La/Smn=3.15−3.85; Gd/Ybn=3.42−4.61). They have positive εNd (+3.8 to +5.5) values, with 338−426Ma depleted mantle model ages, and display uniform OIB-like Sr (87Sr/86Sr=0.703346−0.703591) and Pb (206Pb/204Pb=19.40−19.58; 207Pb/204Pb=15.57−15.60; 208Pb/204Pb=38.99−39.14) isotopic compositions. The basalts erupted discontinuously along a >1000km long SE-NW-trending linear belt with minimal compositional variation indicative of a homogenous mantle source. The Sr−Nd−Pb isotope and trace element systematics of the alkaline basalts suggests that they originated from partial melting of an upwelling asthenospheric mantle source. Melting of the asthenospheric mantle might have stemmed from extension of the overlying lithosphere in response to the early stages of back-arc basin opening in the Omineca and Intermontane belts. Ridge subduction beneath the Canadian Cordillera might have played an important role in the weakening of the lithospheric mantle prior to its extension. Alternatively, melting of the upwelling asthenosphere in response to the delamination of the lithospheric mantle beneath the Rocky Mountain Trench might have generated the alkaline lavas.

Early Permian Tarim Large Igneous Province in northwest China

Tarim Large Igneous Province (TLIP) is the second Late Paleozoic LIPs in China after the recognition of Emeishan LIP, and is a hot research topic in geosciences. On the basis of the analysis of research history about TLIP, this paper summarizes the research result during last twenty years and suggests the key research area in the future. The residual distribution range of TLIP is up to 250000 km2, and the largest residual thickness is 780 m. The eruption of basalt happened during 290–288 Ma and belongs to LIPs magmatic event with fast eruption of magma. The lithological units of the TLIP include basalt, diabase, layered intrusive rock, breccia pipe mica-olivine pyroxenite, olivine pyroxenite, gabbro, ultramafic dyke, quartz syenite, quartz syenite porphyry and bimodal dyke. The basalt and diabase of TLIP exhibit OIB-like trace element patterns and enrichment of LILE and HFSE, and mainly belong to high TiO2 series. There is an obvious difference in isotope among the basalt from Keping and the basalt and dibase from the northern Tarim Basin. The basalt from Keping with negative ɛ Nd and high REE value derives from enriched mantle, and the diabase and basalt from the northern Tarim Basin with positive ɛ Nd and low REE value are related to depleted mantle. The crust uplifting in the Early Permian and the development of picrite and large scale dyke and formation of large scale V-Ti-Magnetite deposit in Wajilitag area support the view that the TLIP is related to mantle plume. The TLIP has a temporal-spatial relationship with Permian basic to ultra-basic igneous rock, which is distributed widely in Central Asia, and they represent a tectono-magmatic event with very important geodynamic setting. This paper also suggests that the deep geological process, the relation with mantle plume, mineralization, the relation with environmental change and biological evolution, and the geodynamics of the TLIP will be the key research topics in the future.

Post-orogenic extension in the eastern part of the Jiangnan orogen: Evidence from ca 800–760 Ma volcanic rocks

We present a systematic geochronological and geochemical study on ca 800–760Ma volcanic rocks in the eastern part of the Jiangnan orogen. The Xucun composite dykes are dated at ca 805Ma; the mafic components have OIB-like trace-element patterns and positive anomalies in Zr and Hf. The least-contaminated sample has relatively depleted Nd isotopic features, suggesting the Xucun mafic dykes may have been generated from the partial melting of OIB-like asthenosphere with later crustal contamination. The Xucun felsic dykes have decoupled Nd–Hf isotopes, and the Hf-isotope compositions of zircons indicate that the dykes may be derived from the partial melting of the early Neoproterozoic juvenile crustal materials, with minor incorporation of Paleoproterozoic crustal components. The ca 800–790Ma Shangshu volcanics include two compositional series: calc-alkaline and tholeiitic. The Shangshu calc-alkaline volcanics in the Minjiawu area have low abundances of LILE, HFSE and high Na2O contents and Sr/Y ratios, similar to adakitic rocks. The evident arc-like geochemical features and radiogenic Nd isotopes (ɛNd(t) values of +3.7 to +4.8) suggest that these rocks may have been generated from the partial melting of juvenile lithospheric mantle metasomatized by Na-rich melts released from the subducted slab. The tholeiitic mafic rocks from the Shangshu bimodal volcanics represent two different magma sources. The partial melting of metasomatized lithospheric mantle led to the formation of arc-like basalts with low TiO2 contents, negative anomalies in Zr and Hf, and high values of Mg# and ɛNd(t) (+6.2), whereas the partial melting of asthenospheric mantle generated volcanic rocks with high TiO2 contents and low positive ɛNd(t) (+1.4 to +2.7), without negative anomalies of Nb, Ta, Zr and Hf. The Shangshu felsic rocks were formed by the reworking of early Neoproterozoic juvenile arc crustal materials. The ca 760Ma mafic rocks from the Puling bimodal volcanics generally have low TiO2 contents (<0.9wt%), nearly flat REE distributions and arc-like trace-element patterns. They may have been generated from the high-degree partial melting of metasomatized lithospheric mantle. One sample has a high TiO2 content (2.41wt%) and high ɛNd(t) (+6.2), with overall OIB-like trace-element patterns, implying the local partial melting of asthenospheric mantle. The occurrence of significant volumes of bimodal volcanics in the eastern part of the Jiangnan orogen suggests an extensional setting in the period ca 800–760Ma. The evident partial melting of newly-metasomatized lithospheric mantle and subordinate partial melting of asthenosphere suggest that post-orogenic extension shortly after the Neoproterozoic orogenesis may be a better explanation for the genesis of the mid-Neoproterozoic magmatic rocks in the eastern part of the Jiangnan orogen. Post-orogenic extension may be diachronous along the whole orogenic belt, and probably has no direct relationship with the Rodinia rifting event. A more detailed model is presented to illustrate the evolution of the eastern part of the Jiangnan orogen.