Arc-like Trace Element Patterns Research Articles

Carbonate metasomatism in sub-continental lithospheric mantle: Insights from decoupled Nd[sbnd]Hf isotopes of Mesozoic gabbros in Jinan, eastern North China Craton

The ancient sub-continental lithospheric mantle (SCLM) of the eastern North China Craton (NCC) underwent metasomatism in the Triassic due to fluids derived from subducted Yangtze continental crust, followed by further metasomatism during the Jurassic–Cretaceous caused by fluids originating from subducted Paleo-Pacific oceanic slabs. However, the occurrence of carbonate metasomatism during these two geological events is a subject of debate, and the origin of carbonate-bearing metasomatic agents remains ambiguous. Here, we present an integrated study of petrography, whole-rock trace element data and Sr–Nd–Hf–Pb isotope data for the Jinan gabbros in the Luxi block of the eastern NCC. The Jinan gabbros show arc-like trace element patterns and enriched Sr–Nd–Hf–Pb isotopic compositions, suggesting that they originated from partial melting of the enriched ancient SCLM. In addition, there are remarkable similarities in Sr–Nd–Hf–Pb isotopes between the Jinan gabbros and HP–UHP metamorphic rocks in the Dabie–Sulu orogenic belt, indicating the involvement of continental crustal components from the subducted Yangtze block in the mantle source of the Jinan gabbros. The gabbro samples show decoupled NdHf isotopes with positive ∆εHf values (∆εHf = εHf – 1.55 × εNd – 1.21; Vervoort et al., 2011), which were mainly caused by a metasomatic agent with high Nd/Hf ratios. The increase in εNd values with increasing Hf/Hf* (Hf/Hf* = HfPM/[(NdPM × SmPM)0.5]) and Nb/La further indicates that the metasomatic agent in the mantle source was depleted in high field strength elements (HFSEs) but enriched in rare earth elements (REEs), resembling the mantle-derived carbonatites in the Luxi area. The Jinan gabbros show significant variations in εNd and εHf, which cannot be explained by the involvement of C–O–H fluids and pure carbonatitic melts in their mantle source. Instead, carbonate-bearing felsic melts from the deeply subducted Yangtze continental crust are proposed to have acted as metasomatic agents. Carbonate metasomatism can significantly weaken the strength and decrease the melting temperature of the overlying ancient SCLM. Therefore, carbonate metasomatism that occurred in the lithospheric mantle of the eastern NCC during the Triassic, likely associated with deep subduction of the Yangtze block, might have played a significant role in promoting lithospheric thinning and mafic magmatism in the late Mesozoic.

Two episodes of mid-Neoproterozoic mafic magmatism in the Hong'an Terrane, Central China: Records of continental rifting during the breakup of the Rodinia supercontinent

The mid-Neoproterozoic (800–635 Ma) tectonic regime along the northern margin of the Yangtze Block is still debated, particularly the timing and mode of transition from a subduction-related setting to a rifting-related setting. The new identification of two episodes of mafic magmatism, represented by (I) 736–710 Ma Daleishan and (II) 635 Ma Hong'an intrusions in the Hong'an Terrane, central China, provides new constraints on the nature of their mantle source and tectonic evolution of the northern Yangtze Block. (I) Daleishan mafic rocks are tholeiitic in composition and exhibit N-MORB- to arc-like trace element patterns. They have variable whole-rock initial 87Sr/86Sr (0.7041 to 0.7092), εNd(t) (−2.98 to +2.17), and εHf(t) (+3.37 to +10.25) values. Their magmatic zircons preserve relatively homogeneous δ18O (+4.50 to +6.01 ‰) and highly variable εHf(t) (+1.3 to +13.1) values. Chemical compositions and modeling results suggest that Daleishan rocks were derived from a depleted asthenospheric mantle source with involvement of recycled sediment component, and the magma subsequently underwent 5–15% contamination by the wall-rocks during its emplacement. (II) Hong'an mafic rocks mainly show E-MORB- to arc-like geochemical features with more enriched HfNd isotopes [zircon εHf(t) = −3.0 to +3.6; whole-rock εNd(t) = −6.87 to −0.75, and εHf(t) = −0.50 to +2.00], suggesting they were derived from the interactions between the asthenosphere and the previously-metasomatized lithospheric mantle. Integrating the new data with previous results for the Neoproterozoic rocks in the northern margin of the Yangtze Block (including the Qinling-Dabie orogen), we emphasize that episodic extension-related asthenosphere upwelling in a continental rifting setting, coupled with interactions between the asthenosphere and the lithosphere, played a prominent role in the formation of the 780–635 Ma bimodal-type magmatic rocks and diverse geochemical features in the mafic counterparts.

Oligocene High-MgO Alkali Basalts in Central Tibet: Implications for Magma–Mush Mixing and Mantle Processes

Abstract High-MgO (>9 wt %) basaltic rocks can be primary magmas and are used to constrain the geochemistry and temperature of the mantle. However, high MgO contents can also result from mixing between evolved melts and antecrysts or xenocrysts, and thus, the whole-rock composition might not represent the solidified equivalents of primary magma. Whether such mixing with crystals can result in erroneous interpretations of mantle processes remains unclear. This study presents a petrological and geochemical investigation of the post-collision high-MgO (>9 wt %) Lugu volcanic rocks in the southern Qiangtang terrane, central Tibet. The Lugu volcanic rocks comprise porphyritic and intersertal alkali basalts. Zircon U–Pb ages and 40Ar/39Ar dating suggest that the two types of alkali basalts were erupted at c. 29 Ma. Based on detailed petrographic observations and geochemical analysis, the porphyritic alkali basalts may represent near-primary melts, which are characterised by low SiO2 contents (40.9–45.1 wt %), high CaO/Al2O3 ratios (1.1–1.5), and arc-like trace element patterns. We suggest these basalts were derived by partial melting of enriched garnet peridotite (>3 GPa) in the presence of H2O and CO2. These geochemical features are different from those of the c. 30-Ma (ultra)-potassic rocks in the Qiangtang terrane, indicating that a heterogeneous lithospheric mantle existed beneath the Qiangtang terrane during the Oligocene. In contrast, although the intersertal alkali basalts have high MgO contents (>9 wt %), evidence from mineral chemistry indicates that the whole-rock compositions of the intersertal alkali basalts represent mixtures of evolved residual melts and cumulate crystals. They were the product of polybaric fractional crystallisation and the subsequent mixing of crystals and residual melts in a magmatic plumbing system. Furthermore, when intersertal alkali basalts are assumed to be primary melts, they would have been derived by partial melting of shallow (~2.5 GPa) CO2-poor pyroxenite or peridotite. These conditions are different from interpretations of the nature of the mantle source and melting conditions for porphyritic alkali basalts. Our results highlight that the interpretation of petrogenetic processes should be preceded by detailed mineralogical investigations.

Origin of composite Ni-Cu sulfide- and Fe-Ti oxide-bearing mafic-ultramafic intrusions in the Central Asian Orogenic Belt, China: Role of externally derived crustal organic carbon in controlling mineralization of post-collisional mafic magmatism

Composite Ni-Cu sulfide- and Fe-Ti oxide-bearing mafic-ultramafic intrusions have been discovered in convergent margin settings worldwide, but their origins remain enigmatic. Permian Kebu mafic-ultramafic intrusion in the Central Asian Orogenic Belt hosts both Ni-Cu sulfide and Fe-Ti oxide mineralization and formed in a post-collisional, Andean-style arc setting. The Ni-Cu sulfide- and Fe-Ti oxide-bearing rocks in the intrusion have similarly arc-like trace element patterns on the primitive mantle−normalized trace element diagram and negative εNd(t) values (−16 to −5), which indicate that the parental magma of the intrusion may have been derived from the enriched, metasomatized lithospheric mantle. The magma oxygen fugacity (fO2) was calculated to be FMQ+1.3∼FMQ+2.0, which can be attributed to the evolution of hydrated parental magma derived from the metasomatized lithospheric mantle. Also, the rocks all have negative δ13C values (−28.5‰ to −20‰), which indicates incorporation of external-derived crustal organic carbon into magmas. However, the Ni-Cu sulfide-bearing rocks contain more high-temperature carbon than the Fe-Ti oxide-bearing rocks. Modeling results based on alphaMELTS show that the magmas forming the Ni-Cu sulfide-bearing rocks may have assimilated more organic carbon than those that formed the Fe-Ti oxide-bearing rocks. Based on the estimated amounts of organic carbon in the two types of rocks, we deduced that initially oxidized magma pulses may have been reduced in variable degrees, leading to the formation of composite Ni-Cu sulfide and Fe-Ti oxide mineralization in one intrusive body. Mafic magmas in subduction-related, convergent margin settings are commonly highly oxidized due to metasomatism of the mantle wedge by slab-derived fluids/melts, and this may explain why the mafic-ultramafic intrusions associated with composite Ni-Cu sulfide and Fe-Ti oxide mineralization often occur in convergent margin settings.

Cadomian tectonic evolution of Iran: records of an unusually hot and broad extensional convergent margin on the northern margin of Gondwana

ABSTRACT The continental crust of Iran is dominated by abundant calc-alkaline and alkaline plutonic and volcanic rocks and by rifted basins filled with mostly terrigenous sedimentary rocks that formed at a Late Ediacaran to Cambrian extensional convergent plate margin along the northern margin of Gondwana. Here we present new zircon U–Pb age, geochemical, and isotopic data from plutonic (granite-granodiorite) and metamorphic (gneiss) rocks in the Kariznou region of NE Iran to provide insights into the nature of the Cadomian convergent margin of Iran. Geochemical data indicate calc-alkaline signatures, characterized by strong depletions in Nb, Ta, P, and Ti and arc-like trace element patterns. New zircon U–Pb ages show that calc-alkaline granitoids and granitic gneiss formed at ~564 to 537 Ma and 538 Ma, respectively. Bulk rock Sr-Nd isotopic data of calc-alkaline rocks have εNd(t) = –5.42 to −5.53 and −6.93 to −7.43 for granite and gneiss, respectively. The gneisses show stronger interaction with and/or re-melting of older continental crust than do granitic rocks. We interpret Kariznou magmatic rocks as forming in association with strong extension accompanied by crustal assimilation. Extension initiated ~570 Ma with the deposition of Late Ediacaran sediments, and magmatism began at 545–535 Ma, generating calc-alkaline magmas. The tectonomagmatic evolution of the Kariznou region encapsulates the prolonged transition of Cadomian Iran from a strongly extensional convergent margin, possibly as a result of oblique oceanic subduction and slab roll-back of the subducting Proto-Tethys oceanic lithosphere, culminating in the formation of an Early Palaeozoic passive margin on the northern side of Gondwana.

Identification of metasomatic components in lithospheric mantle under multiple tectonic regimes: Insights from Early Cretaceous mafic rocks in the northeastern North China Craton

Identification of mantle metasomatic components under multiple tectonic regimes is crucial for reconstructing the subduction history of multiple plates. Representative Early Cretaceous (125–122 Ma) gabbroic diabase from the northeastern North China Craton (NCC), provides a geochemical record to explore the genetic relationship between mantle metasomatic agents and tectonic evolution. Samples from gabbroic diabase exhibit low SiO2 and high MgO contents as well as arc-like trace element patterns, suggesting a sub-continental lithospheric mantle (SCLM) origin. Their inherited zircons and isotope variations indicate the existence of two crustal components with different tectonic affinities that were recycled into the SCLM: Paleo-Asian oceanic crust and Yangtze Craton continental crust. The metasomatic signatures of the two crustal components are previously imprinted on the geochemical compositions of the studied mafic rocks. Subsequently, the melting of the metasomatic SCLM is triggered by the subduction of the Paleo-Pacific Plate. Based on the identification of mantle metasomatic components in this study and previous geological evidence, the spatial extents of the subduction influence of the Paleo-Asian oceanic plate and Yangtze Craton on the SCLM beneath the northeastern NCC are broader than previously considered. These new findings signify a critical role of the mantle metasomatic components that can effectively identify the geological relics of the previous plate subduction. Thus we believe that the former mantle metasomatism and latter mantle melting for mafic rocks suggest two geological implications.

Zircon U–Pb Dating and Geochemical Characteristics of Chahannuo Gabbros in the Northern Margin of Qaidam Basin, Northern Tibetan Plateau

Late Paleozoic–early Mesozoic intrusive rocks are distributed widely along the northern margin of the Qaidam Basin in the northern Tibetan Plateau. To constrain the tectonic evolution, we carried out petrological, chronological, and geochemical studies of the Chahannuo gabbros. LA-ICP-MS Zircon U–Pb dating yields an age of 255.0 ± 0.9 Ma for the gabbros, which confirms the existence of Indosinian tectono-magmatic activity on the northern margin of the Qaidam Basin. The Chahannuo gabbros have low whole-rock SiO2, Fe2O3 contents, and high Al2O3 contents, which suggests a calc-alkaline affinity. In addition, the gabbros have high MgO, Cr, and Ni contents and Mg#, similar to those predicted of the regional basaltic melts, and indicating that they were affected mainly by fluid from the subducted slab. The Chahannuo gabbros are characterized by arc-like trace element patterns, with enrichment in LREE and LILE, and depletion in HREE and HFSE. No obvious negative Eu anomalies also indicate that no significant magmatic differentiation has occurred. The low Nb/La ratio and Ti content in gabbros samples suggests that the Chahannuo gabbros were partially contaminated by the crust during their formation. The Chahannuo gabbros have high incompatible element ratios (Rb/Sr, Th/Nd, and Th/La), which are closer to the category of enriched mantle. Combing our data with previous data from contemporaneous magmatism in the region, we suggest that the Chahannuo gabbros formed in a continental arc environment related to the northward subduction of the Paleo-Tethyan oceanic plate.

Geochemical signature of a deeply subducted slab in the source of post-collisional magmatism: A case study from the Western Tianshan NW China

The contributions of deeply subducted slabs to post-collisional magmatism are poorly constrained. Here, we present a study of the post-collisional, Early Permian (ca. 279 Ma), Hamisite basalts from the Western Tianshan NW China to trace the geochemical fingerprints of deeply subducted slab. The Hamisite basalts have arc-like trace element patterns but high Nb/Ta ratios (18.4−23.0). They have slightly enriched Sr−Nd isotopic compositions (87Sr/86Sr = 0.7048−0.7052; εNd[t] = 2.02−3.85), which indicates that the basalts were derived from an enriched mantle source. Extremely low δ7Li values (−5.76‰ to 0.20‰) imply the involvement of a deeply subducted slab (i.e., eclogites) in the source. Correlations among Nb/Ta and (Dy/Yb)N ratios, and rare earth element contents, indicate that the deeply subducted slab contained residual rutile and garnet but no epidote. Given that melting of eclogites requires an anomalously hot mantle source, we propose that mantle upwelling during post-collisional extension triggered melting of the deeply subducted slab. Our results indicate that melting of a deeply subducted slab can generate a metasomatized mantle source with high Nb/Ta ratios, which could be a hidden, high-Nb/Ta reservoir on Earth. Low δ7Li values and high Nb/Ta ratios could be a fingerprint of recycling of a deeply subducted slab.

Genesis of Mesozoic high-Mg dioritic rocks from the eastern North China Craton: Implications for the evolution of continental lithosphere

Abstract Pre-Cenozoic high-Mg andesites (HMAs) are mostly present in continental interiors, but their genetic relationship with continental lithosphere evolution remains unclear because of uncertainties of their mantle source, magmatic processes, and physicochemical conditions of formation. Early Cretaceous high-Mg dioritic rocks (HMDs, analogs of HMAs) of the Jinling complex in the Luxi area are typical intra-plate intrusions of the eastern North China Craton (NCC) and can be subdivided into two groups (Group-I and -II) on the basis of their petrographic and geochemical features. Group-I HMDs show low SiO2 contents (52.47–56.10 wt%) and Sr/Y (34.5–39.6) and (La/Yb)N (10.3–13.6) ratios but high contents of MgO (7.86–9.13 wt%), Y (18.3–20.3 ppm), Yb (1.43–1.47 ppm), and compatible elements (Cr = 407–585 ppm; Ni = 117–216 ppm), classifying as sanukitic rocks. Group-II HMDs are characterized by high SiO2 contents (63.81–64.87 wt%) and Sr/Y (47.1–63.4) and (La/Yb)N (16.1–17.5) ratios with low MgO (2.90–3.08 wt%), Y (0.88–1.04 ppm), Yb (0.88–1.04 ppm), and compatible elements (Cr = 201–213 ppm; Ni = 55–57 ppm) contents, belonging to adakitic rocks. Group-I and Group-II HMDs of the Jinling complex are closely related in spatial and temporal distribution, and all have enriched Sr-Nd isotopic compositions and arc-like trace element patterns with abundant hydrous minerals. Therefore, the Jinling HMDs should share a common source of ancient sub-continental lithospheric mantle that was metasomatized by aqueous fluids derived from the subducted Paleo-Pacific slab. The Jinling HMDs were not formed from interaction between slab-derived melts and mantle-wedge peridotites but were instead derived from partial melting of hydrous mantle peridotites in the continental interior of the eastern NCC. The distinctly different petrography, geochemistry, and mineralogy of the two groups of rocks resulted mainly from differing magmatic processes at crustal depths. Thus, Pre-Cenozoic intra-plate HMAs/HMDs are genetically distinct from Cenozoic HMAs that were mostly present in arc settings and generally represent juvenile crust growth. In a way, Archean tonalitic-trondhjemiticgranodioritic rocks (TTG) and sanukitoids, geochemically similar to HMAs/HMDs, could also be derived from interaction between slab-derived melts and mantle-wedge peridotites in arc settings or partial melting of hydrous mantle peridotites in continental interiors, and thus might not always be related with continental crustal growth and the onset of plate subduction.

Geochemical evidence for the Eocene surface uplift of the southern Lhasa subterrane, southern Tibet and the implications

Whether the crust of southern Tibet has been thickened during the Eocene and if so, the mechanisms of crustal thickening in this region need to be further investigated. This study presents geochronological and geochemical data for Eocene (45–41 Ma) granitic plutons, including the Aima diorites in the eastern part and Huo'er pluton (granites and mafic enclaves) in the western part of the southern Lhasa subterrane. The Aima diorites show adakitic affinities, such as moderate SiO 2 contents and relatively high Sr/Y values, with depleted 87 Sr/ 86 Sr (i) (0.704039–0.704054), εNd (t) (+2.9 to + 3.3) isotopic compositions, indicating that they are derived from a juvenile continental crust. The Huo'er host granites have high SiO 2 and low MgO contents. Meanwhile, they are characterized by the enriched initial 87 Sr/ 86 Sr (i) (0.707899–0.708618), εNd (t) (from − 6.4 to − 5.3) and variable zircon εHf (t) (from − 6.8 to + 5.0) values. These geochemical characteristics and the presence of mafic enclaves demonstrate that the Huo'er host granites were the result of melting of crust (both ancient and juvenile) of southern Lhasa subterrane with the involvement of mantle-derived materials. The Huo'er mafic enclaves display an igneous texture and contain acicular apatite, suggesting that they are quenching of mafic magmas that have co-mingled with the host granites. The mafic enclaves have arc-like trace-element patterns, together with their enriched 87 Sr/ 86 Sr (i) (0.707608–0.707826), εNd (t) (from − 4.8 to − 4.6) and zircon εHf (t) (from − 4.0 to − 1.7) isotopes, revealing that they were derived from an enriched lithospheric mantle source. Taking the regional tectonic evolution into consideration, we propose that the Eocene Aima diorites and Huo'er pluton are the products of the last stage of magmatism related to break-off of the subducted Neo-Tethyan oceanic slab. Our study of these two plutons, along with results from previous studies, suggests that the crust of the southern Lhasa subterrane was thickened during the Eocene. The thickened crust was mainly caused by underplating of mafic magmas and, to a lesser extent, by thrust-related accretion of continental crust of the Indian terrane to the Lhasa terrane. We further propose that the Eocene thickened crust of southern Lhasa subterrane has triggered the surface uplift of this region during the Eocene, which may have non-negligible impact on the Cenozoic evolution of the Indian Monsoon system. • The Eocene Aima diorites and Huo'er pluton were in response to the slab break-off. • The crust of the southern Lhasa subterrane became thickened during the Eocene. • Magma underplating and accretion of Indian crust contributed to crustal thickening. • The Eocene surface uplift is synchronous with intensification of the Indian Monsoon.

New geochronological and geochemical data for the Eocene shoshonitic trachyandesites in NW Iran: Constraints on their petrogenesis and tectonic setting

The Eocene magmatic flare-up in NW Iran provides valuable information on the tectonic evolution of the region. This study presents detailed geochronological and geochemical data from Eocene trachyandesitic rocks from the Ahar region of NW Iran. 40 Ar/ 39 Ar ages indicate that the Ahar trachyandesites were produced during the Eocene (44 Ma). The Ahar trachyandesites have a limited range of SiO 2 contents (53.4–54.4 wt%) and low MgO contents (1.57–2.78 wt%), and they belong to the shoshonitic series with high Na 2 O + K 2 O contents (9.1–9.8 wt%) and K 2 O/Na 2 O ratios (0.4–1). The Ahar trachyandesites are characterized by arc-like trace element patterns, including enrichment in large-ion lithophile and light rare earth elements and depletion in high field strength elements. They have depleted 87 Sr/ 86 Sr (i) ratios (0.704616–0.704736) and εNd (t) (+2.0 to +3.5) and εHf (t) (+6.6 to +6.8) values and high Th/La and Ba/La ratios, similar to those of previously studied Eocene K-rich mafic–intermediate rocks in NW Iran. The geochemical characteristics of the Ahar trachyandesites suggest that they originated from an enriched, fertile mantle source that was produced by the reaction between mantle wedge peridotite and oceanic slab-derived components (sediment-derived melts and slab-derived fluids) during the subduction of the Neo-Tethys Oceanic slab along the Zagros segment. Taking the regional tectonic evolution into consideration, we propose that NW Iran was in an extensional setting caused by the rollback of the Neo-Tethys Oceanic slab during the Eocene. Partial melting of the enriched, fertile mantle source would have produced the Ahar trachyandesites and other contemporaneous K-rich mafic–intermediate rocks in NW Iran. Accordingly, the Ahar trachyandesites and contemporaneous K-rich mafic–intermediate rocks provide a geochemical record of interaction between the crust and mantle in an oceanic subduction channel. • The Ahar shoshonitic trachyandesites were emplaced during the Eocene (44 Ma). • The mantle source of Ahar trachyandesites was enriched by slab-derived components. • The extensional setting for Eocene magmatism was triggered by the slab rollback.

A newly defined, long-lived Paleozoic intra-oceanic arc in the South Tianshan (NW China): Implications for multiple accretionary tectonics in the southern Altaids

Abstract The South Tianshan Orogenic Belt marks the final assembly of the southern Altaids and the Karakum-Tarim Cratons. Integrated petrological, geochemical, and geochronological studies were carried out on the Akeyazi high-alumina basaltic lavas (HABs) and their host sandstones from the southern margin of the Central Tianshan Block. Given their relative high alumina contents (Al2O3 > 16 wt%) and geochemical similarity to basalts from modern arcs (e.g., Aleutian, Kamchatka, Izu, and Honshu arc), the Akeyazi basaltic lavas are classified as HABs. The Akeyazi HABs are distinguished by strong enrichment in large ion lithophile elements (LILE) and negative Nb-Ta anomalies. Furthermore, their mid-oceanic ridge basalt (MORB)-like Sr-Nd-Hf (87Sr/86Sr(i) ~0.705766, εNd(t) ~+2.5, εHf(t) ~+9.5), along with arc-like trace element patterns, indicate a petrogenesis derived from a mantle wedge metasomatized by pelagic, sediment-derived melts (2%–5% melts mixed, as confirmed by melt modeling). Zircon U-Pb ages of Akeyazi HABs and associated arc-related gabbros suggest that they were mainly erupted during ca. 415 Ma. Moreover, host sandstones of Akeyazi HABs exhibit a unimodal distribution with a peak at 480–410 Ma in the detrital zircon spectrum, which is notably distinct from those of the Central Tianshan Block, South Tianshan Accretionary Complex, and North Tarim Craton, each of which is multimodal. The maximum depositional age (MDA) of the meta-sandstones is 351.0 ± 4.3 Ma, with the weighted average of three oldest Paleozoic grains at 502 ± 26 Ma. Collectively, our data, for the first time, discloses an intra-oceanic arc setting from ca. 502–351 Ma during the northward subduction of the South Tianshan Ocean, which further reveals the history of multiple episodes of accretion in the southern Altaids.

Mercury isotopes as a useful tracer of magma sources: An example from the Daocheng-Cuojiaoma batholith, eastern Tibetan Plateau

Magma mixing between mafic and felsic melts is a common magmatic process. Tracing the sources of different magma endmembers is often challenging using traditional isotopes, given the source isotopic signatures are often altered due to isotope fractionation. Mass-independent fractionation of mercury (Hg), which is predominantly generated during Hg photochemical reactions on Earth's surface with little influence from other processes, has the potential to be used as a petrogenetic tracer. This study investigates the Hg isotopic composition of mafic microgranular enclaves (MMEs) and their host granitoids from the Daocheng-Cuojiaoma batholith (DCGB), eastern Tibetan Plateau. Zircon U-Pb dating indicates both the host granitoids and two types of MMEs formed coevally at ca. 216–217 Ma, coherent to the subduction of Garzê–Litang paleo-Tethyan ocean. The host granitoids exhibit negative to slightly positive ∆199Hg values (−0.20 to 0.02‰), indicating that their source magma was a mixture of terrestrial sediments- and mantle-derived melts. Type-I MMEs display arc-like trace element patterns, low SiO2 (53.8 to 55.0 wt%) and positive ∆199Hg values (0.00 to 0.10 ‰), indicating their derivation from a subduction-related fluid/melt metasomatized mantle source. Type-II MMEs show intervening concentrations of major/trace elements, and intermediate ∆199Hg values (−0.18 to 0.02‰), suggesting they were generated via mixing of magmas for Type-I MMEs and granitoid. This study demonstrates the powerful use of Hg isotope as a tracer of magma sources during crustal-mantle interactions.

Two episodes of Late Paleozoic mafic magmatism in the western Tianshan Orogen: From Carboniferous subduction to Permian post-collisional extension

The evolution of the western Tianshan Orogen in the southern Central Asian Orogenic Belt involved prolonged accretion and collision processes in response to the closure of the Paleo-Asian Ocean during the Late Paleozoic. However, the timing for the tectonic transition from subduction to post-collision is still poorly constrained. Herein, we address this issue based on geochronological, geochemical and Sr-Nd-Hf isotopic investigations on representative mafic dikes from the Awulale Mountains in the heart of the western Tianshan. Zircon U-Pb dating suggest two episodes of mafic magmatism during the Late Carboniferous (ca. 324–310 Ma) and Early Permian (ca. 299–290 Ma). The Late Carboniferous gabbro dikes exhibit large variations in MgO (2.79–10.97 wt%) and Ni (7.58–244 ppm), and arc-like trace element patterns with high Th/Yb (0.55–2.26) ratios, relatively low εNd(t) (+0.2 to +2.5) and variable εHf(t) (+1.3 to +10.4) values, indicating that they were derived from fractional crystallization of a peridotite-bearing mantle source metasomatized by sediment-derived hydrous melts. In contrast, the Early Permian diabase dikes have slightly variable MgO (5.88–7.78 wt%) and Ni (59.90–104 ppm) with apparently low Th/Yb ratios (0.20–0.34, except for one sample: 0.86) and depleted εNd(t) (+5.0 to +7.8) and εHf(t) (+8.0 to +14.6) values, which indicates derivation from partial melting of a juvenile lithospheric mantle source with contributions from early subduction metasomatic components. In context with relevant published data, our study indicates that the Late Carboniferous magmatism of the Awulale Mountains was likely induced by breakoff of the North Tianshan oceanic slab during ongoing subduction; conversely, the Early Permian magmatism may have originated from delamination of the lower crust and lithospheric mantle in the subsequent post-collisional setting. Furthermore, the tectonic transition from Carboniferous subduction to Permian post-collisional extension of the western Tianshan suggests that terminal closure of the North Tianshan Ocean took place at Carboniferous-Permian boundary at ∼ 300 Ma.

Tectonic switch from a lithospheric rift to an active continental margin in the Paleoproterozoic: Evidence from low δ18O granites from the Trans-North China Orogen in the North China Craton

Tectonic switch from rift zones to subduction zones is common along convergent plate boundaries. While this process is susceptible to retrieving from Phanerozoic rock records, the difficulty has been encountered for Precambrian rock records because of the relative lack of characteristic geological signatures. Nevertheless, such a difficulty can be overcome by finding of specific geochemical signatures in ancient rock records. This paper reports our finding of low δ18O zircons from the Trans-North China Orogen (TNCO) in the North China Craton (NCC), where the tectonic switch would occur during the early to middle Paleoproterozoic in association with the amalgamation of supercontinent Columbia. A combined study of zircon U-Pb ages and Hf-O isotopes as well as whole-rock major-trace elements and Nd isotopes were carried out for magmatic rocks from the Taiyue complex in the southern part of the TNCO. Zircon U-Pb dating yields two episodes of magmatism at ca. 2.34–2.31 Ga and ca. 2.2–2.1 Ga. These magmatic rocks are dominated by the ca. 2.18–2.16 Ga granites, 2.18–2.17 Ga diorites and 2.17–2.11 Ga mafic–ultramafic cumulates that intruded the 2.34–2.30 Ga granites and diorites. The 2.2–2.1 Ga diorites, mafic-ultramafic cumulates, and the regional mafic dykes/intrusions exhibit continuously varying major element compositions, arc-like trace element patterns, and consistent zircon Hf and whole-rock Nd isotope compositions, indicating their derivation from the same suite of continental arc magmas. Such primitive arc magmas would evolve through fractionation and accumulation of pyroxenes and plagioclase into dioritic magmas. The two episodes of granites are similar in major and trace element compositions, generally belonging to alkali-calcic or calc-alkalic A-type granitoids. Although both groups show a small difference in their zircon Hf isotope compositions, they exhibit a big difference in their zircon O isotope compositions. The 2.31 Ga granites show variably low zircon δ18O values of 3.4–5.5‰, mostly lower than normal mantle zircon values. The 2.18 Ga granites also exhibit variable zircon δ18O values from 3.6 to 6.0‰, but mostly mantle-like values. It is inferred that the 2.31 Ga granites would acquire their low δ18O signatures from partial melting of the high-T seawater-hydrothermally altered Archean crust in an early Paleoproterozoic continental rift. This rift would extend for at least 300 km along the TNCO. The 2.18 Ga granites are closely associated with the 2.2–2.1 Ga diorites and mafic–ultramafic cumulates. They would be most likely to form through differentiation of the 2.2–2.1 Ga continental arc magma, with their low 18O signatures being contaminated by the low δ18O 2.3 Ga granites. The low 18O signatures in the 2.3 Ga and 2.18 Ga granites indicate that the southern part of the TNCO would have probably evolved from a lithospheric rift to an active continental margin during ca. 2.3–2.1 Ga.

Nature of the Late Cretaceous mantle source beneath the western Lhasa terrane, southern Tibet: Insights from the newly discovered mafic intrusion

Recent studies indicate that the mantle source of eastern Lhasa terrane was still depleted during the late stage of oceanic subduction (100–50 Ma). However, the mantle source beneath the western Lhasa terrane during this period was poorly constrained mainly due to the rare exposure of mafic rocks in this region. Here, we report the detailed petrological, geochronological, geochemical, and Sr–Nd–Hf–Pb isotopic data for recently identified gabbroic rocks in the Taruocuo area, western Lhasa terrane, southern Tibet. The zircon U-Pb dating results indicate that the Taruocuo gabbroic rocks were emplaced during the Late Cretaceous (~90 Ma). These gabbroic rocks have low SiO2 (50.9–55.0 wt%), and moderately high MgO (4.10–5.60 wt%) contents with arc-like trace-element patterns, such as enrichment in large ion lithophile elements, light rare earth elements, and depletion in high field strength elements. Meanwhile, they have high Th/La, low Ba/La values, and exhibit relatively enriched and uniform Sr–Nd–Hf–Pb isotopic compositions (87Sr/86Sr(i) = 0.7048–0.7058, εNd(t) = −1.8 to −1.2, εHf(t) = +3.1 to +3.8, and 206Pb/204Pb = 18.60 to 18.66). Such geochemical features of the Taruocuo gabbroic rocks suggest that they would originate from an enriched and fertile mantle source that was generated by the metasomatic reaction between the mantle wedge peridotite and the subducted Neo-Tethyan oceanic sediment-derived melts. Taking the regional tectonic evolution into consideration, we favor the model of Neo-Tethyan oceanic-ridge subduction having taken place to account for the Late Cretaceous (~90 Ma) magmatic event in the Lhasa terrane. Under such geological background, partial melting of this enriched mantle would have produced the Taruocuo gabbroic rocks in the western Lhasa terrane. Given that the Late Cretaceous Taruocuo gabbroic rocks have the relatively enriched Sr–Nd–Hf isotopic compositions, distinctive from the contemporaneous mafic rocks in the eastern Lhasa terrane that have the depleted Sr–Nd–Hf isotopic compositions, we propose that the mantle source beneath the Lhasa terrane was probably isotopically segmented during the Late Cretaceous (~90 Ma).

Triassic calc-alkaline lamprophyre dykes from the North Qiangtang, central Tibetan Plateau: evidence for a subduction-modified lithospheric mantle

AbstractPrimitive lamprophyres in orogenic belts can provide crucial insights into the nature of the subcontinental lithosphere and the relevant deep crust–mantle interactions. This paper reports a suite of relatively primitive lamprophyre dykes from the North Qiangtang, central Tibetan Plateau. Zircon U–Pb ages of the lamprophyre dykes range from 214 Ma to 218 Ma, with a weighted mean age of 216 ± 1 Ma. Most of the lamprophyre samples are similar in geochemical compositions to typical primitive magmas (e.g. high MgO contents, Mg no. values and Cr, with low FeOt/MgO ratios), although they might have experienced a slightly low degree of olivine crystallization, and they show arc-like trace-element patterns and enriched Sr–Nd isotopic composition ((87Sr/86Sr)i = 0.70538–0.70540, ϵNd(t) = −2.96 to −1.65). Those geochemical and isotopic variations indicate that the lamprophyre dykes originated from partial melting of a phlogopite- and spinel-bearing peridotite mantle modified by subduction-related aqueous fluids. Combining with the other regional studies, we propose that slab subduction might have occurred during Late Triassic time, and the rollback of the oceanic lithosphere induced the lamprophyre magmatism in the central Tibetan Plateau.

Mantle wedge enrichment beneath southern Tibet during the late stage (100–45 Ma) of oceanic subduction: Geochemical constraints from mantle-derived intrusions

The Miocene K-rich mafic rocks in the Lhasa Terrane of the southern Tibetan Plateau are considered to be related to either the subduction of the Neo-Tethyan oceanic slab or Indian continental slab. In order to investigate how subduction of the Neo-Tethyan oceanic slab has geochemically affected the mantle beneath southern Tibet, we obtained geochronological and geochemical data for the late Early Cretaceous Cangguo gabbros (97 Ma) and early Eocene Langka diorites (51 Ma) in southern Tibet. Although both the Cangguo gabbros and Langka diorites are characterized by arc-like trace element patterns, with enrichment in light rare earth and large-ion lithophile elements (e.g., Ba, Pb, and U), depletion in high-field-strength elements (e.g., Nb and Ta), and depleted Sr–Nd–Hf isotopic compositions, there are some other major geochemical differences (e.g., Ba/La and Th/La ratios) between these two suites of rocks. These geochemical characteristics of the Cangguo gabbros and Langka diorites indicate they were produced by melting of the depleted mid-ocean ridge mantle (DMM) that was enriched by oceanic sediment-derived fluids and melts, respectively. Our data and previous studies suggest that dehydration and melting of subducted Neo-Tethyan oceanic sediments beneath the Lhasa Terrane occurred in the late stage (100–45 Ma) of Neo-Tethyan oceanic slab subduction, which may have enriched the mantle beneath the Lhasa Terrane. However, the mantle beneath the Lhasa Terrane that was enriched by the subduction of oceanic slab during 100–45 Ma is still isotopically more depleted than the source of Miocene post-collisional K-rich mafic rocks in the Lhasa Terrane. This indicates that the enriched component in the mantle source of the K-rich mafic rocks cannot be simply attributed to materials derived from the Neo-Tethyan oceanic slab, and other additional materials are required.

Permian to Cretaceous granites and felsic volcanics from SW Vietnam and S Cambodia: Implications for tectonic development of Indochina

Zircon ages and geochemistry are presented for igneous rocks from SW Vietnam and S Cambodia. Four main age groupings occur: Cretaceous (107–75 Ma), Early Jurassic (195 Ma), Late Triassic (230–222 Ma), and Permian (294–265 Ma). Cretaceous and Jurassic samples are amphibole-bearing biotite granodiorites to biotite granites and occur mainly east of the Kampot Fold Belt. Pre-Cretaceous samples occur within the Kampot Fold Belt and are dominantly felsic volcanics. The rocks are primarily high-K calc-alkaline, weakly peraluminous rhyolites or granites, with similar arc-like trace element patterns. Cretaceous granites are similar to Dalat Zone Granites and formed during Pacific subduction. Two Cretaceous granites have adakite-like signatures. The youngest Cretaceous granite has an A-type signature and may represent post-collision activity following the cessation of subduction. Jurassic granites are also linked to Pacific subduction. Two Triassic samples could be affiliated to similarly aged rocks in the Chanthaburi Terrane, or linked to the Loei Fold Belt and related to closure to the Sa Kaeo back arc. We cannot exclude that they are related to Pacific Plate subduction. Correlation of Permian volcanics to paleo-Tethys subduction and the Sukhothai-Chanthaburi arc is not clear as the Cambodian volcanics are older than Chanthaburi rocks, and there are stratigraphic contrasts between S Cambodia and Chanthaburi. Furthermore, such a correlation requires extension of the Sa Kaeo suture into S Cambodia and SW Vietnam at a high angle to regional structures. We propose that the Permian volcanics are also related to Paleo-Pacific subduction as shown in regional reconstructions of the region.

Recycling of subducted continental crust: Geochemical evidence from syn-exhumation Triassic alkaline mafic rocks of the southern Liaodong Peninsula

Syn-exhumation mafic magmatism during continental collision provides insights into the crust-mantle reaction during deep subduction and the nature of orogenic lithospheric mantle in collisional orogens. In this study, we present a comprehensive data set of zircon U-Pb ages and whole-rock major-trace elements as well as Sr-Nd-Pb isotopes of alkaline mafic rocks from the southern Liaodong Peninsula, eastern China. Zircon U-Pb analyses yield Late Triassic age of 213 ± 3 to 217 ± 3 Ma, younger than the Middle Triassic ultrahigh-pressure metamorphic rocks of the Dabie-Sulu orogen. Thus, the alkaline mafic rocks are products of syn-exhumation magmatism during continental collision of the South and North China blocks. The rocks show shoshonitic affinities with high K2O (3.78–5.23 wt%) and K2O/Na2O (0.71–1.22). They are characterized by arc-like trace-element patterns with enriched LILE, Pb, and LREE, and depleted HFSE. They exhibit enriched Sr-Nd isotopic compositions with high initial 87Sr/86Sr isotopic ratios of 0.7058–0.7061 and negative εNd(t) values of −13.0 to −15.1. These results suggest involvement of recycled continental crust in their mantle source. The mantle source likely formed by the metasomatic reaction of subducted continental crust-derived melts with the overlying subcontinental lithospheric mantle during the Triassic continental collision. Decompressional melting of this metasomatized mantle formed syn-exhumation mafic magmas during the transition from convergent to extensional tectonics in the Late Triassic. Accordingly, mafic rocks from the southern Liaodong Peninsula provide a geochemical record of the subduction and recycling of continental crust into the mantle and melt-mantle reaction induced metasomatism within the orogen.