Ancient Subcontinental Lithospheric Mantle 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.

Recycling of subducted Indian continental crust constrained by late Cretaceous mafic dykes in Central Lhasa block of the Tibetan plateau

The question of whether subducted continental crust can be recycled into post-collisional magmatism in continental collisional zones remains controversial. Post-collisional mantle-derived ultrapotassic rocks are widespread in western part of central Lhasa block (WCL) of the Tibet-Himalaya orogen and show arc-type trace-element signatures and very enriched SrNd isotopes, which have been explained by the recycling of subducted Indian continental crust. However, due to the lack of late Cretaceous mafic magmatism in the WCL, the nature of the ancient subcontinental lithospheric mantle (SCLM) beneath the WCL has not been well constrained. It is therefore still unclear whether the enriched components of these ultrapotassic rocks were derived from subducted Indian continental crust or inherited from the ancient SCLM. Here we report the mafic dykes from the TangraYumco area in the WCL, which formed at ∼90 Ma (whole-rock 40Ar39Ar, and zircon and titanite UPb ages). The diabase-porphyrites and diorite-porphyrites are similar to intraplate Nb-enriched basalts and magnesium adakitic rocks, respectively. This rock association reveals the lithospheric foundering process. Furthermore, the diabase-porphyrites generated by the interaction between the ancient SCLM and asthenosphere provide important constraints on the nature of the ancient SCLM, which shows distinct Nd isotopes to the post-collisional ultrapotassic rocks. In addition, the NW Lhasa block was in a rear-arc environment since the late Cretaceous and thus this SCLM was not significantly influenced by the ongoing northward subduction of the Indus-YarlungZangbu Neo-Tethys ocean. Finally, we propose that the post-collisional ultrapotassic rocks cannot be sourced from the ancient SCLM, but instead were derived from a relatively depleted mantle (e.g., juvenile lithospheric mantle) metasomatized by subducted Indian continental crust. Thus, this study confirms the recycling of subducted continental materials in continental collisional orogens.

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.

Shallow recycling of lower continental crust: The Mahoney Seamount at the Southwest Indian Ridge

The Mahoney Seamount is a recently discovered volcanic edifice located 4 km north of the ultra-slow spreading Southwest Indian Ridge (SWIR). The SWIR is one of the slowest spreading ridges worldwide with a full spreading rate of ∼14 mm/year and low magmatic productivity. We report that highly vesicular basalts from the Mahoney Seamount have unradiogenic Nd-Hf together with radiogenic Sr isotopic compositions. Their distinct low 206Pb/204Pb isotope signature combined with high 207Pb/204Pb and 208Pb/204Pb is best explained by melting of a mantle that has been strongly influenced by stranded lower continental crust. The geographic distribution of the isotopic variability favors the idea of shallow recycling of lower continental crust isolated for a longer period contributing to melts forming Mahoney Seamount through off-axis fault systems. The isotopic composition of Mahoney Seamount lavas shares many characteristics with EM-1 sources and the DUPAL signature. Previous isotopic studies of the SWIR basalts proposed recycling of ancient subcontinental lithospheric mantle (SCLM) or pelagic sediments with oceanic crust to be responsible for this enriched isotopic signature. Lu/Hf and Sm/Nd ratios of pelagic sediments would result in decoupled 143Nd/144Nd and 176Hf/177Hf ratios. This decoupling is also observed in Ejeda-Bekily dikes from Madagascar, but those are believed to sample the SCLM dispersed in the Indian Ocean. However, Mahoney Seamount shows no decoupling in those isotopic systems and the restricted occurrence of the extreme lower continental crustal signature at Mahoney Seamount implies that the enriched isotopic signature has a different origin.

Petrogenesis of Lava from Christmas Island, Northeast Indian Ocean: Implications for the Nature of Recycled Components in Non-Plume Intraplate Settings

Lava samples from the Christmas Island Seamount Province (CHRISP) record an extreme range in enriched mantle (EM) type Sr-Nd-Pb-Hf isotope signatures. Here we report osmium isotope data obtained on four samples from the youngest, Pliocene petit-spot phase (Upper Volcanic Series, UVS; ~4.4 Ma), and four samples from the earlier, Eocene (Lower Volcanic Series, LVS; ~40 Ma) shield building phase of Christmas Island. Osmium concentrations are low (5–82 ppt) with initial Os isotopic values (187Os/188Osi) ranging from (0.1230–0.1679). Along with additional new geochemical data (major and trace elements, Sr-Nd-Pb isotopes, olivine δ18O values), we demonstrate the following: (1) The UVS is consistent with melting of shallow Indian mid-ocean ridge basalt (MORB) mantle enriched with both lower continental crust (LCC) and subcontinental lithospheric mantle (SCLM) components; and (2) The LVS is consistent with recycling of SCLM components related to Gondwana break-up. The SCLM component has FOZO or HIMU like characteristics. One of the LVS samples has less radiogenic Os (γOs –3.4) and provides evidence for the presence of ancient SCLM in the source. The geochemistry of the Christmas Island lava series supports the idea that continental breakup causes shallow recycling of lithospheric and lower crustal components into the ambient MORB mantle.

Mineralogical and Re-Os isotope constraints on fluid– and melt–rock interactions and the origin of mantle peridotites from the Amdo ophiolite, northern Tibet

Ophiolites from the Bangong–Nujiang Suture Zone in the central part of the Tibetan Plateau represent relics of Neo-Tethyan oceanic lithosphere. Here we present the results of a detailed mineralogical and Os isotopic study of mantle peridotites of the Amdo ophiolite, which provide new insights into the origin of the peridotites and their diverse melt/fluid–peridotite reactions. The Amdo ophiolite comprises mantle peridotites, mafic–ultramafic cumulates, massive gabbros, diabases, and pillow basalts. The mantle peridotites are mainly refractory harzburgites, composed of olivine (Ol, 65%–85%), orthopyroxene (Opx, 12%–35%), clinopyroxene (Cpx, <4%), and Cr-spinel (Cr-Spl, ~2%). Most olivine grains (Ol-I) are protogranular, locally enclosed in orthopyroxene and Cr-spinel, and have high Fo values (91.1–92.9) and NiO contents (0.29–0.44 wt%), typical of residual origin. In contrast, interstitial olivines (Ol-II) have low Fo values (67.2–84.7) and NiO contents (<0.05 wt%), but extremely high MnO contents (0.28–1.65 wt%), suggesting metasomatism by carbonate-bearing fluids. Hypidiomorphic-granular clinopyroxene grains (Cpx-I) have high Mg# values (~94.5) and low Al2O3 (average 1.26 wt%) and Na2O (<0.19 wt%) contents, typical of residual phases after melting. However, clinopyroxene coronas (Cpx-II) around orthopyroxene have higher Na2O contents (0.24–1.06 wt%), indicative of mantle metasomatism by melts. Osmium isotopic compositions in the harzburgites vary from subchondritic to suprachondritic (187Os/188Os ratios: 0.11716–0.13284). The subchondritic Os isotopic compositions (0.11716–0.12367) correspond to TRD model ages of 0.76–1.56 Ga, suggesting Mesoproterozoic mantle depletion events. On the other hand, several harzburgites with low Os (0.29–0.86 ppb) and suprachondritic 187Os/188Os ratios (0.12823–0.13284) reflect the overprinting of original Os budgets and Os isotopic compositions by percolating melts or fluids. The combined mineralogical and Os isotopic compositions suggest that the Amdo peridotites represent a mixture of ancient subcontinental lithospheric mantle (SCLM) and Mesozoic oceanic lithosphere, which were subsequently infiltrated by carbonate-bearing fluids and silicate melts in a supra-subduction zone.

Development of a complex arc–back-arc basin system within the South Tianshan Ocean: Insights from the Wuwamen ophiolitic peridotites

Ophiolites within the South Tianshan orogenic belt directly record the nature and tectonic evolution of the South Tianshan Ocean (STO), which is key for understanding the accretionary history of the Central Asian Orogenic Belt. Here we present petrological and Os isotopic data for the Wuwamen mantle peridotites from the South Tianshan orogenic belt, which provide new insights into the tectonic origin of this ophiolite and evolution of the STO. The lherzolites have low spinel Cr# (0.1–0.2), similar to those of fertile oceanic or continental peridotites that have undergone low degrees of melting. However, the extremely high Na2O (1.5–1.8 wt%) and Al2O3 (5.5–6.2 wt%) contents of the clinopyroxenes are atypical of oceanic peridotites, and resemble those of subcontinental peridotites. The harzburgites have more refractory compositions, suggesting higher degrees of melt extraction. Both the lherzolites and harzburgites have low initial 187Os/188Os ratios (< 0.121), which reflect Proterozoic melt depletion and a long-term lithospheric evolution. These data confirm an ancient subcontinental lithospheric mantle origin for the Wuwamen peridotites. The replacive dunites have moderate spinel Cr# (0.3–0.4), reduced Os and enriched 187Os/188Os (0.142–0.160), indicative of reaction between host lherzolites and percolating mid-ocean ridge basalt (MORB)-type melts with radiogenic Os isotopic signatures, presumably in a subduction-related setting. This and the associated back-arc basin basalt (BABB)–type mafic crust suggest the Wuwamen ophiolite formed in a continental margin back-arc setting. A comparison with other ophiolites in the South Tianshan orogenic belt further indicates that a complex arc–back-arc system existed along the southern margin of the Central Tianshan, which was associated with northward subduction within the STO.

Accretion kinematics and driving mechanism of the eastern Central Asian Orogenic Belt: Insights from seismic tomography and middle Permian–Middle Triassic magmatism in central Jilin Province

The amalgamation of accretionary terranes plays an important role in the evolution of ancient accretionary orogens, but the kinematic process and its driving mechanism are commonly ambiguous. Here we provide new constraints on the accretion kinematics and associated driving mechanism of the eastern Central Asian Orogenic Belt (CAOB), using seismic tomography and magmatism. Permian to Middle Triassic magmatism was linked to two distinct accretionary orogens, that existed along the northern margin of the North China Craton. The geochemistry of middle Permian to Middle Triassic igneous rocks indicates a magmatic arc origin at an active continental margin. Based on the horizontal zoning of magmatism and south-dipping stagnant oceanic slab imaged by seismic tomography, we conclude that southward subduction of the oceanic slab resulted in the accretion and collision of multiple arc systems along the continental margin. Our seismic tomographic models indicate that formation and growth of the eastern CAOB occurred as the result of the continuous accretion and amalgamation of arc assemblages with addition of crustal material. Tomographic models and previous Re-Os geochronological studies indicate the existence of a deep crustal root and ancient sub-continental lithospheric mantle in most regions of the eastern CAOB. Taking into account the magmatic evolution and sedimentary sequences, we conclude that the terrane accretion should be the driving mechanism of accretionary orogenesis in the eastern CAOB.

In situ characterization of forearc serpentinized peridotite from the Sulu ultrahigh-pressure terrane: Behavior of fluid-mobile elements in continental subduction zone

Serpentinized peridotites in the Yangkou (YK), Suoluoshu (SLS) and Hujialin (HJL) areas in the Sulu ultrahigh-pressure terrane represent the relic of ancient subcontinental lithospheric mantle below the North China Craton. Their protoliths, harzburgite and dunite, were variably hydrated by aqueous fluids released from subducting Yangtze continent. The rocks are enriched in fluid-mobile elements (FME) including Sb (42–333 times the depleted mantle value) and Pb (30–476 times). The degrees of the FME enrichment are comparable to that of the Himalayan forearc serpentinites, and greater than forearc mantle serpentinites from Marianas, suggesting that the degrees of FME enrichment in the forearc serpentinites are greater in continental subduction zones than those in the oceanic subduction zones. Lizardite after olivine in the SLS serpentinite shows higher degrees of enrichment in Sb and As than those for antigorite after both olivine and orthopyroxene in the YK area. The antigorite has highly enriched in Pb, U, Cs, and LREE, but not for the lizardite. The abundance of FME in two different species of serpentine reflects the different temperature of hydration. At temperature lower than 300 °C, formed lizardite at shallow depths of the mantle wedge incorporates elements that are fluid mobile at low temperatures, such as Sb and As. When the temperature greater than 300 °C, formed antigorite at a relatively deep mantle wedge incorporate more FME from the subducting continental slab (or fragments), including Pb, U, Cs, LREE as well as Sb and As. The eventual breakdown of antigorite (600–700 °C) in prograde metamorphism would discharge water as well as FME into the subducting channel and/or the overlying mantle.

Petrogenesis of Pleistocene Basalts from the Western Snake River Plain, Idaho

Abstract We present new geochemical, Sr, Nd, and Pb isotope, and 40Ar/39Ar data from Pleistocene basalts of the Western Snake River Plain (WSRP), Idaho, USA to explore their petrogenesis and to investigate the nature of the lithosphere at the western boundary of the North American craton. The basalts are divided into three groups based on their geochemical and isotopic characteristics. Prior to ∼1 Ma, volcanoes in the WSRP erupted iron-rich tholeiites (FeB1), but subsequent volcanism was dominated by concurrent eruptions of mildly alkaline, alumina-rich lavas (AlB) and iron-rich tholeiites (FeB2) with isotopic signatures similar to the AlB lavas. New 40Ar/39Ar dates of AlB and FeB2 basalts range from 0·920 ± 0·049 to 0·287 ± 0·014 Ma. MELTS models of FeB1 differentiation trends indicate that the range of compositions in this suite can be produced by 10–15 % crystallization of olivine and plagioclase at low pressure using the least evolved FeB1 composition as a parental magma; isotopic ratios can be produced via combined assimilation of a Miocene rhyolite and fractional crystallization. Additional modeling suggests that parental magmas at AlB centers were produced by 3–12 % equilibrium melting of a garnet–spinel-enriched mantle source, slightly different from that proposed for the youngest mildly alkaline lavas of the eastern and central Snake River Plain. Our new geochemical, isotopic, and geochronological data for the FeB2 basalts suggests that they are related to AlB-type magmas via a combination of fractional crystallization and assimilation of evolved mafic crust. MELTS models suggest that crystallization of an AlB parental melt at a depth of 6–8 km (2·5 kbar) could produce residual liquids having many of the major oxide characteristics of FeB2 ferrobasalts. Sr–Nd–Pb isotopic signatures of these three suites indicate a dominant contribution from an enriched plume source. FeB1 lavas are probably products of mixing between melts of an enriched plume mantle source (represented by Imnaha and Steens Basalts of the Columbia River Basalt Group) and isotopically heterogeneous sub-continental lithospheric mantle (SCLM) that has been isolated from the convecting mantle since the Archean. Isotopic ratios of FeB2 and AlB lavas capture mixing between enriched plume mantle and a more isotopically homogeneous ancient SCLM domain characteristic of the eastern and central Snake River Plain, with a coupled decrease in lithospheric contribution and degree of partial melting through time to the present. Mixtures of enriched asthenospheric reservoirs with lithospheric mantle have been proposed for neighboring volcanic fields to the east along the strike of the Yellowstone–SRP hotspot track, and to the west owing to differences in the mantle underlying the boundary of the North American craton and accreted terranes. Our petrogenetic model for the Pleistocene WSRP basalts suggests that there is also a lateral, across-strike gradient in the geometry and interaction of enriched plume mantle and ancient lithosphere. We reiterate suggestions that the WSRP is a lithosphere-scale conduit connecting initial plume-head impingement in east–central Oregon with the subsequent Yellowstone–SRP hotspot plume-tail track.

Re-Os Isotope Systematics of Sulfides in Chromitites and Host Lherzolites of the Andaman Ophiolite, India

Laser ablation MC-ICP-MS was used to measure the Os-isotope compositions of single sulfide grains, including laurite (RuS2) and pentlandite [(Fe,Ni)9S8], from two chromitite bodies and host lherzolites from ophiolites of North Andaman (Indo-Burma-Sumatra subduction zone). The results show isotopic heterogeneity in both laurite (n = 24) and pentlandite (n = 37), similar to that observed in other chromitites and peridotites from the mantle sections of ophiolites. Rhenium-depletion model ages (TRD) of laurite and pentlandite reveal episodes of mantle magmatism and/or metasomatism in the Andaman mantle predating the formation of the ophiolite (and the host chromitites), mainly at ≈0.5, 1.2, 1.8, 2.1 and 2.5 Ga. These ages match well with the main tectonothermal events that are documented in the continental crustal rocks of South India, suggesting that the Andaman mantle (or its protolith) had a volume of lithospheric mantle once underlaying this southern Indian continental crust. As observed in other oceanic lithospheres, blocks of ancient subcontinental lithospheric mantle (SCLM) could have contributed to the development of the subduction-related Andaman–Java volcanic arc. Major- and trace-element compositions of chromite indicate crystallization from melts akin to high-Mg IAT and boninites during the initial stages of development of this intra-oceanic subduction system.

Oceanization of the subcontinental lithospheric mantle recorded in the Yunzhug ophiolite, Central Tibetan Plateau

The Yunzhug ophiolite, in the middle sector of the Shiquanhe-Namco ophiolite belt (SNOB), has generally been interpreted as a remnant of the Neo-Tethyan oceanic lithosphere, composed of mantle peridotites and mafic-ultramafic magmatic rocks. However, detailed study reveals that the mantle parts are ultra-depleted peridotites with affinities to subcontinental lithospheric mantle (SCLM), including major harzburgites and minor dunites. The olivine in dunite has the highest Fo values, up to 93.8, and the harzburgite is divided into three groups. Olivine included in Opx in Group I has Fo 92.4–93.3, close to the dunite. In Group II, olivine in the peridotites has Fo between 91.8 and 92.5. Group III olivine occurs in vein-like clusters with small grains of Cpx and Spl within Group II peridotites, and has the lowest Fo, mainly at 91.5 and the Cr# of Spl as low as 50 with MOR-like features. The chemical composition of Opx is consistent with that from on-craton garnet lherzolites, while the Cpx displays lower Na2O and Al2O3, and elevated Cr2O3 contents with an embryonic oceanic-crust affinity. The high Mg# values and low 187Os/188Os (0.11301–0.12374; TRD = 2.30 Ga − 0.54Ga) of whole rocks and the high Fo in Ol of Group I and II suggest that the peridotites originated from ancient SCLM. The fine-grained mineral clusters in Group III reflect modification of the peridotite massif, representing the oceanization process during the initial rifting of the Gondwana continent.

A telltale signature of Archean lithospheric mantle in the Paraná continental flood basalts genesis

The origin of the Paraná Continental Flood Basalts (PCFB), which constitute one of largest magmatic provinces in the world, remains a very controversial subject, particularly whether melts are sourced in the mesosphere (deep mantle plume), asthenosphere and/or lithosphere. In an effort to scrutinize those competing hypotheses, new measurements of Os, Nd, Sr and Pb isotopes, along with major and trace elements are presented for low-TiO2 tholeiites from the province. This suite occurs discreetly in the central-north of the province (termed Ribeira magma-type), whose lithospheric structure is characterized by higher P- and S- wave velocities, and high electrical resistivity. The initial 187Os/188Os isotopic compositions in Ribeira lavas, ranging from 0.10660 to 0.12575, are more unradiogenic than the other magma-types of the PCFB, as well as the estimates of the contemporary Depleted Mantle and are lower than any osmium isotopic ratio yet reported for continental flood basalts. These remarkably unradiogenic 187Os/188Os ratios preclude significant continental crust contamination and require the involvement of an ancient subcontinental lithospheric mantle source that evolved in a very low Re/Os environment. The fact that the Ribeira rocks occur on a peculiar lithospheric geophysical structure and have a unique unradiogenic osmium isotope signature reveals for the first time the existence of Archean lithosphere concealed by the Paraná Basin, inserted in the Paranapanema fragmented lithosphere. On the other hand, the other magma-types of the province, whose osmium isotopic signature is similar to the modern fertile mantle, occur close to more electrically conductive lithospheric structures. Thus, osmium isotopic information integrated with recent crustal and upper mantle geophysical soundings provides crucial information about the PCFB mantle sources. To account for the combined Os, Nd, Sr and Pb isotopic compositions of the Ribeira rocks, we propose that the primary melts formed from the heterogeneous lithospheric mantle (including Archean lithosphere fragments) that was variably hybridized by melts derived from recycling of eclogites related to multiple Neoproterozoic-Cambrian suture zones that surround the Paraná Basin and subduction processes.

Mesozoic lithospheric modification and replacement beneath the Cathaysia Block: Mineral chemistry and water contents of the Daoxian peridotite xenoliths

Water contents (a key parameter controlling the stability of a continent) of the Mesozoic sub-continental lithospheric mantle (SCLM) beneath the Cathaysia Block (South China) are still poorly constrained. Here, we report mineral chemistry and water contents of peridotite xenoliths entrained by Mesozoic basalts from the Daoxian area in the Cathaysia Block. The peridotite xenoliths are spinel-facies lherzolites with porphyroclastic textures and have olivine Mg# (Mg2+/(Mg2++Fe2+)) of 88.0–89.3, suggesting an origin from a fertile lithosphere mantle. Clinopyroxenes in the lherzolites show LREE-depleted patterns and negative anomalies in Ba, Nb, Pb, Sr, Zr and Ti. The contents of HREE and Y in clinopyroxenes suggest low degrees of melt extraction (e.g., 1–5% fractional melting) from a primitive mantle source. The weak enrichments in Th, U show later weak metasomatism by silicate melts. The water contents of clinopyroxenes and orthopyroxenes in the xenoliths are 182–417 ppm and 53–123 ppm, respectively. Calculated whole-rock water contents, based on the mineral modes and partition coefficients, are 35–82 ppm. The water contents are similar to those estimated for the MORB source mantle and the newly accreted fertile SCLM beneath the Cathaysia Block. Because the Daoxian area is located at the Ningyuan-Jianghua fault region, the translithospheric weak zone can provide a channel for the upwelling of asthenosphere and the subsequent accretion of fertile SCLM. The Mesozoic replacement of the ancient SCLM by fertile one therefore re-shaped the lithospheric architecture of the Cathaysia Block.

Widespread Os-isotopically ultradepleted mantle domains in the Paleo-Asian oceanic upper mantle: evidence from the Paleozoic Tianshan ophiolites (NW China)

The scale and extent of isotopic heterogeneities in the Paleo-Asian oceanic mantle remain poorly constrained. Here, we report Os isotopic data for mantle peridotites from the Paleozoic Gangou and Kumishi ophiolites, which are preserved in the western Chinese Tianshan sutures (Xinjiang, China) and considered to sample the Paleo-Asian oceanic upper mantle. These heavily serpentinized peridotites display positive correlations between Al2O3, Ti, and Yb contents, indicating variable degrees of melt extraction from fertile mantle sources. However, enriched light rare-earth element patterns in the peridotites are inconsistent with simple residues of partial melting, thus reflecting the effect of post-melting melt percolation. Peridotites from both ophiolites show wide variation of Os isotopic compositions, several of which are characterized by highly unradiogenic 187Os/188Os values (0.117–0.118), reflecting Proterozoic melt depletion. Such feature is common in modern oceanic peridotites and most Phanerozoic ophiolites, meaning that Os-isotopically ultradepleted domains should be widespread in the Paleo-Asian oceanic mantle. The overall similarity of Os-isotope distribution between the Paleozoic Tianshan ophiolites and modern oceanic peridotites suggests that they derive from the same heterogeneous upper mantle reservoir. This might support the hypothesis that isotopically ultradepleted domains are inherent components of the convecting upper mantle and have not been sufficiently homogenized by convection. Alternatively, Re-depletion ages (TRD) for these ancient domains are broadly coincident with the major peaks in crustal zircon ages that are thought to reflect episodes of continental crust growth in the adjacent Yili–Central Tianshan microcontinents. This suggests that the ultradepleted mantle domains in some of the Tianshan ophiolites may derive from ancient subcontinental lithospheric mantle, which detached and were exhumed to form the substrate of ocean basins.

Genesis of porphyry Mo deposits linked to gradually dehydrating subcontinental lithospheric mantle metasomatised by previous subduction in northeastern China

Many Mesozoic Mo-bearing granites, present along the northern margin of the Precambrian North China Block (NCB) and Phanerozoic Xing'an-Mongolian Belt (XMB) within the Central Asian Orogen (CAO) have hydrated sources. These granites are temporally and spatially related to biotite-rich mafic, alkali syenitic plutons, are highly evolved, have an arc-like trace element geochemical pattern, and were emplaced between ca. 245–129 Ma bracketed by A2- and A1-type granites. This conflicts temporally with the current model involving the westward subduction of the Pacific oceanic plate. We use Sm/Nd and Th/Yb ratios of Mesozoic nepheline-bearing pyroxene-biotite syenites (biotite-syenites), mantle-peridotite xenoliths from Palaeozoic diamondiferous kimberlites, and Mesozoic to Cenozoic alkali basalts to define a hydrous melt-extraction trend. The results indicate a Palaeozoic mantle-peridotite source that is more metasomatised than the lower continental crust and the source of the Mesozoic (<129 Ma) to Cenozoic mantle-peridotite xenoliths (Th/Yb < 1). This indicates that the sub-continental lithospheric mantle (SCLM) beneath northeast China has been gradually dehydrating since the Palaeozoic, and the lower continental crust and Cenozoic SCLM in northern NCB are too depleted to be the magmatic source for the Mo-bearing granites. The results of the study also indicate that the Palaeozoic SCLM beneath the northern margin of the NCB was more metasomatised than beneath its central zone. The development of Mo-bearing granites in both the Precambrian northern margin of NCB and Phanerozoic XMB indicate that the presence of an ancient SCLM is not necessary for the genesis of Mo mineralisation. From these observations, it is proposed that the genesis of Mo mineralisation in the region is attributed to a hydrated SCLM metasomatised by the subduction of the Paleo-Asian oceanic plate rather than the younger Pacific oceanic plate.

Sequence and petrogenesis of the Jurassic volcanic rocks (Yeba Formation) in the Gangdese arc, southern Tibet: Implications for the Neo-Tethyan subduction

The Yeba Formation volcanic rocks in the Gangdese arc recorded important information regarding the early history of the Neo-Tethyan subduction. To explore their magmatic evolution and tectonic significance, we performed a systematic petrological, geochronological and geochemical study on these volcanic rocks. Our data indicated that the Yeba Formation documents a transition from andesite-dominated volcanism (which started before 182 Ma and continued until ~176 Ma) to bimodal volcanism (~174–168 Ma) in the earliest Middle Jurassic. The early-stage andesite-dominated volcanics are characterized by various features of major and trace elements and are interpreted as the products of interactions between mantle-derived arc magmas and lower crustal melts. Their positive εNd(t) and εHf(t) values suggest a significant contribution of asthenosphere-like mantle. The late-stage bimodal volcanism is dominated by felsic rocks with subordinate basalts. Geochemical signatures of the basalts indicate a composite magma source that included a “subduction component”, an asthenosphere-like upper mantle domain and an ancient subcontinental lithospheric mantle component. The felsic rocks of the late stage were produced mainly by the melting of juvenile crust, with some ancient crustal materials also involved. We suggest that the occurrence and preservation of the Yeba Formation volcanic rocks were tied to a tectonic switch from contraction to extension in the Gangdese arc, which probably resulted from slab rollback of the subducting Neo-Tethyan oceanic slab during the Jurassic.

Sr–Nd–Pb isotope systematics of the Permian volcanic rocks in the northern margin of the Alxa Block (the Shalazhashan Belt) and comparisons with the nearby regions: Implications for a Permian rift setting?

The petrogenesis of the Permian magmatic rocks in the Shalazhashan Belt is helpful for us to understand the tectonic evolution of the Central Asian Orogenic Belt (CAOB) in the northern margin of the Alxa Block. The Permian volcanic rocks in the Shalazhashan Belt include basalts, trachyandesites and trachydacites. Our study shows that two basalt samples have negative εNd(t) values (–5.4 to −1.5) and higher radiogenic Pb values, which are relevant to the ancient subcontinental lithospheric mantle. One basalt sample has positive εNd(t) value (+10) representing mafic juvenile crust and is derived from depleted asthenosphere. The trachyandesites are dated at 284 ± 3 Ma with εNd(t) = +2.7 to +8.0; ISr = 0.7052 to 0.7057, and they are generated by different degrees of mixing between mafic magmas and crustal melts. The trachydacites have high εNd(t) values and slightly higher ISr contents, suggesting the derivation from juvenile sources with crustal contamination. The isotopic comparisons of the Permian magmatic rocks of the Shalazhashan Belt, the Nuru-Langshan Belt (representing the northern margin of the Alxa Block), the Solonker Belt (Mandula area) and the northern margin of the North China Craton (Bayan Obo area) indicate that the radiogenic isotopic compositions have an increasingly evolved trend from the south (the northern margins of the Alxa Block and the North China Craton) to the north (the Shalazhashan Belt and the Solonker Belt). Three end-member components are involved to generate the Permian magmatic rocks: the ancient subcontinental lithospheric mantle, the mafic juvenile crust or newly underplated mafic rocks that were originated from depleted asthenosphere, and the ancient crust. The rocks correlative with the mafic juvenile crust or newly underplated mafic rocks are predominantly distributed along the Shalazhashan Belt and the Solonker Belt, and the rocks derived from ancient, enriched subcontinental lithospheric mantle are mainly distributed along the northern margins of the Alxa Block and the North China Craton. The magmatic rock types, isotopic features and their temporal, spatial distributions suggest an extensional regime probably related to rifting.

Mesozoic mafic magmatism in North China: Implications for thinning and destruction of cratonic lithosphere

The North China Craton (NCC) has been thinned from >200 km to in its eastern part. The ancient subcontinental lithospheric mantle (SCLM) has been replaced by the juvenile SCLM in the Meoszoic. During this period, the NCC was destructed as indicated by extensive magmatism in the Early Cretaceous. While there is a consensus on the thinning and destruction of cratonic lithosphere in North China, it has been hotly debated about the mechanism of cartonic destruction. This study attempts to provide a resolution to current debates in the view of Mesozoic mafic magmatism in North China. We made a compilation of geochemical data available for Mesozoic mafic igneous rocks in the NCC. The results indicate that these mafic igneous rocks can be categorized into two series, manifesting a dramatic change in the nature of mantle sources at ~121 Ma. Mafic igneous rocks emplaced at this age start to show both oceanic island basalts (OIB)-like trace element distribution patterns and depleted to weakly enriched Sr-Nd isotope compositions. In contrast, mafic igneous rocks emplaced before and after this age exhibit both island arc basalts (IAB)-like trace element distribution patterns and enriched Sr-Nd isotope compositions. This difference indicates a geochemical mutation in the SCLM of North China at ~121 Ma. Although mafic magmatism also took place in the Late Triassic, it was related to exhumation of the deeply subducted South China continental crust because the subduction of Paleo-Pacific slab was not operated at that time. Paleo-Pacific slab started to subduct beneath the eastern margin of Eruasian continent since the Jurrasic. The subducting slab and its overlying SCLM wedge were coupled in the Jurassic, and slab dehydration resulted in hydration and weakening of the cratonic mantle. The mantle sources of ancient IAB-like mafic igneous rocks are a kind of ultramafic metasomatites that were generated by reaction of the cratonic mantle wedge peridotite not only with aqueous solutions derived from dehydration of the subducting Paleo-Pacific oceanic crust in the Jurassic but also with hydrous melts derived from partial melting of the subducting South China continental crust in the Triassic. On the other hand, the mantle sources of juvenile OIB-like mafic igneous rocks are also a kind of ultramafic metasomatites that were generated by reaction of the asthenospheric mantle underneath the North China lithosphere with hydrous felsic melts derived from partial melting of the subducting Paleo-Pacific oceanic crust. The subducting Paleo-Pacific slab became rollback at ~144 Ma. Afterwards the SCLM base was heated by laterally filled asthenospheric mantle, leading to thinning of the hydrated and weakened cratonic mantle. There was extensive bimodal magmatism at 130 to 120 Ma, marking intensive destruction of the cratonic lithosphere. Not only the ultramafic metasomatites in the lower part of the cratonic mantle wedge underwent partial melting to produce mafic igneous rocks showing negative e Nd( t ) values, depletion in Nb and Ta but enrichment in Pb, but also the lower continent crust overlying the cratonic mantle wedge was heated for extensive felsic magmatism. At the same time, the rollback slab surface was heated by the laterally filled asthenospheric mantle, resulting in partial melting of the previously dehydrated rocks beyond rutile stability on the slab surface. This produce still hydrous felsic melts, which metasomatized the overlying asthenospheric mantle peridotite to generate the ultramafic metasomatites that show positive e Nd( t ) values, no depletion or even enrichment in Nb and Ta but depletion in Pb. Partial melting of such metasomatites started at ~121 Ma, giving rise to the mafic igneous rocks with juvenile OIB-like geochemical signatures. In this context, the age of ~121 Ma may terminate replacement of the ancient SCLM by the juvenile SCLM in North China. Paleo-Pacific slab was not subducted to the mantle transition zone in the Mesozoic as revealed by modern seismic tomography, and it was subducted at a low angle since the Jurassic, like the subduction of Nazca Plate beneath American continent. This flat subduction would not only chemically metasomatize the cratonic mantle but also physically erode the cratonic mantle. Therefore, the interaction between Paleo-Pacific slab and the cratonic mantle is the first-order geodynamic mechanism for the thinning and destruction of cratonic lithosphere in North China.

Cretaceous basalts of the High Arctic large igneous province at Axel Heiberg Island (Canada): Volcanic stratigraphy, geodynamic setting, and origin

The Cretaceous (Cenomanian, ~96 Ma) basaltic rocks of the Strand Fiord Formation of the Sverdrup Basin of the Canadian Arctic Archipelago, Nunavut, Canada, are a part of the High Arctic large igneous province. The basaltic suite reaches a thickness of almost 1,000 m on Axel Heiberg Island in its depocentre and over 700 m at the studied section. The rocks are variably fractionated tholeiitic basalts, which are geochemically similar to many other continental flood basalts, particularly to low Ti basalts of flood basalt provinces. Geochemical as well as Sr (87Sr/86Srinitial ~ 0.7045 to 0.7067) and Nd (ƐNd(t) ~ +1.3 to +4.3) isotopic signatures of the basalts were inherited during partial melting of spinel peridotite of an ancient subcontinental lithospheric mantle, which was modified around 0.7–0.9 Ga by a metasomatic event. The melting generating the Cretaceous basalts was probably triggered by a mantle plume but was also associated with basin rifting and stretching. There is evidence of plume‐induced doming and uplift of the crust prior to and during the eruption of the basalts as documented by the progression from marine sedimentation to subaerial/lacustrine and then back to marine sedimentation in the Sverdrup Basin centre. The basalts represent parts of the Early Cretaceous igneous province that is dispersed around the Arctic Ocean due to the opening of the Canada Basin, which is inferred to be related to a mantle plume.