Ti Anomalies Research Articles

Post-Collisional Silica-Undersaturated Bamaoqiongzong Volcanic Rocks from Northern Qiangtang: Indicators of the Mantle Heterogeneity and Geodynamic Evolution of Central Tibet

AbstractThe formation of post-collisional mantle-derived rocks in the Tibetan Plateau has been linked to the deep geodynamic processes that cause surface uplift. Co-existing silica-oversaturated to silica-undersaturated mantle-derived rocks have been identified in the northern Qiangtang Terrane (NQT). However, the origins of silica-undersaturated magmas are controversial, and the mechanisms responsible for variable silica activity in the mantle-derived rocks are unclear. Here, we present 40Ar/39Ar chronology, mineral chemistry, and whole-rock geochemical data for the Bamaoqiongzong (BMQZ) volcanic rocks of the NQT. The BMQZ volcanic rocks consist of olivine leucitites, trachybasalts, and phonolites and were erupted at ca. 29 Ma. All samples are unsaturated in silica and characterized by enrichment in light rare earth elements and large-ion lithophile elements, depletion in high-field-strength elements, and the presence of negative Nb–Ta–Ti anomalies and positive Pb anomalies. All samples show limited variation in (87Sr/86Sr)i (0.7079–0.7085) and εNd(t) values (−6.9 to −5.3). The geochemical compositions of the BMQZ volcanic rocks indicate that they were produced by partial melting of carbonated phlogopite–lherzolite within the lithospheric mantle. The formation of the olivine leucitites-trachybasalts-phonolites suite was controlled by fractional crystallization and magma mixing in a magmatic plumbing system. This plumbing system included several independent reservoirs and conduits within the crust. The enriched mantle sources of the BMQZ volcanic rocks were formed by the addition of carbonate-rich melts released from the southward-subducted Songpan–Ganzi Terrane after the Late Cretaceous. Our new results, together with published data, reveal systematic variations in geochemical compositions between silica-undersaturated and silica-oversaturated rocks in the NQT, which are ascribed to variations in the nature of the subducted continental materials added during intracontinental subduction. Carbonate-rich melts that were formed by the breakdown of carbonate minerals helped to generate the mantle sources of silica-undersaturated rocks, whereas silicate melts produced by the partial melting of sediment diapirs contributed to the generation of the silica-oversaturated rocks. On the basis of published numerical modelling of continental subduction and crustal deformation records in the NQT, we suggest that intracontinental subduction and lithospheric thinning together contributed to the generation of post-collisional mantle-derived rocks in the Tibetan Plateau.

Tracing decarbonated eclogite in the mantle sources of Tarim continental flood basalts using Zn isotopes

Recycled oceanic crust is believed to have played an important role in the formation of continental flood basalts, whereas the involvement of large amounts of CO2 derived from recycled marine carbonate in their mantle sources is highly debated. Zn isotopes have great potential to trace recycled carbonate due to the distinctly different δ66Zn values between marine carbonates and the mantle. Representative continental flood basalt samples from Keping (Group1) and Yingmai and Shengli (Group 2) in Tarim, NW China, were collected to investigate their mantle sources, and their Zn isotopes were studied systematically for the first time. The Zn isotope values of Keping basalts (between 0.29‰ ± 0.03‰ and 0.32‰ ± 0.05‰) are higher than values in the primitive mantle (δ66Zn = 0.16‰ ± 0.06‰) but similar to those of mid-ocean-ridge basalt (MORB; δ66Zn = 0.24‰−0.31‰). Considering their high fractionation (e.g., Mg# = 0.28−0.37) and potential involvement of crustal contamination (87Sr/86Sri between 0.70720 and 0.70779; εNd between −3.2 and −1.9), their Zn isotopes may not conclusively point to a carbonated mantle source. In contrast, Yingmai and Shengli basalts show heavy Zn isotope values (between 0.32‰ ± 0.03‰ and 0.39‰ ± 0.03‰), nonradiogenic 87Sr/86Sri (0.70459‒0.70518) and εNd(t) (between −1.3 and 0.1) signatures, and less fractionation (Mg# = 0.46−0.71), implying the involvement of recycled carbonate components in their mantle source. Nonetheless, the lack of negative Zr, Hf, and Ti anomalies, low CaO/Al2O3 ratios, and high SiO2 contents preclude direct melting of carbonate-bearing mantle. Alternatively, these features may suggest the melting of decarbonated subducted eclogite and variable interaction with subsolidus peridotite. This assumption is consistent with the positive correlations of δ66Zn with Gd/Yb and Sm/Yb ratios and Zn content as a result of mixing between eclogite-derived high-δ66Zn melt and peridotite-derived mantle-like δ66Zn melt. Our study provides a new model to reconcile the geochemical features of the Tarim continental flood basalts and highlights the potential role of decarbonated eclogite in the formation of continental flood basalts.

Opening of the Algerian Basin: Petrological, geochemical and geochronological constraints from the Yaddene Complex (Lesser Kabylia, Northeastern Algeria)

The Yaddene complex, located in Lesser Kabylia (northeastern Algeria), is composed of Tertiary mafic and ultramafic rocks that outcrop within syn-rift sedimentary rocks of the Algerian marginal basin, in the Maghrebide belt of North Africa. Petrographic observations show that the Yaddene complex is composed mainly of two distinct lithologies: (1) a layered quartz-bearing gabbro at the bottom, which consists mainly of plagioclase (An93), clinopyroxene (altered to actinolite) and rare interstitial quartz; and (2) layered plagioclase-bearing lherzolites composed of olivine (Fo86-88) orthopyroxene (En86–87), clinopyroxene (diopside and augite), rare plagioclase (anorthite) and amphibole of pargasitic composition.The trace element concentrations in pyroxenes and olivine of the Yadenne complex show a depletion in light rare earth elements (LREE) similar to basalts of the Mid-Ocean Ridge (N-MORB), which reflects the depletion of the mantle source. In addition, high #Cr in spinel reflects a high degree of partial melting of the mantle source. On the other hand, slight enrichment in Large Ion Lithophile Elements (Cs, Rb, Ba), weak but significant negative Nb, Ta, Zr, Hf and Ti anomalies, and positive Pb spikes are observed in both whole-rocks and minerals. These patterns are best explained by interaction between parental mantle and fluids and/or siliceous melts derived from a subducting oceanic slab. The tectonic discrimination diagrams of minerals and whole rocks indicate that the most likely geodynamic setting for emplacement of the Yaddene Complex is a back arc basin environment.In situ U-Pb dating of euhedral zircons and titanite from a quartz-bearing gabbro yields an age of 18.97 ± 0.12 Ma (2σ), which is interpreted as the crystallization age of magmatic zircons and the gabbroic magma. Zircons from a plagioclase-bearing lherzolite gave a consistent age of 19.05 ± 0.39 Ma (2σ), which is attributed to zircon growth during metamorphic/metasomatic processes coeval to gabbroic magma emplacement. The mafic/ultramafic rocks of the Yaddene complex are intercalated within fine-grained sediments of the syn-rift “Oligo-Miocene Kabyle” and the obtained early Burdigalian ages are interpreted as the early stages of an extensional regime that ultimately led to the opening of the oceanic Algerian Basin.

Petrogenesis of late Paleoproterozoic post-collisional magmatism in southern north China Craton: Insights from geochemistry and Nd–Hf isotopic compositions of A-type granites

A-type granites are produced commonly in extensional settings and they are important proxies to track breakup of continent. Here we report systematic studies on geochronology and geochemical features of A-type granites in Songxian–Ruyang region, southern margin of the North China Craton (NCC), in order to constrain their petrogenesis and tectonic implications. Zircons from these granites show oscillatory zoning and high Th/U ratios (0.12 to 1.26), indicating a magmatic origin. LA–ICP–MS zircon U–Pb dating shows that they were emplaced at 1813 to 1789 Ma, indicating that they were formed in late Paleoproterozoic. Geochemically, these rocks are high in SiO2 (67.51–74.42 wt%) and total alkalis (Na2O + K2O = 7.76–10.19 wt%), and low in MgO (0.09–1.31 wt%), CaO (0.09–1.11 wt%) and P2O5 (0.12–0.26 wt%) contents, with negative Nb, Ta, Sr, P and Ti anomalies. They are characterized by high FeOT/(MgO + FeOT), 10000 Ga/Al ratios (1.57–3.15) and Zr + Nb + Ce + Y contents (566–835 ppm). They are also characterized by high calculated Zr saturation temperatures (892–918 °C), high Ti-in-zircon temperatures (693–912 °C) and metaluminous to weakly peraluminous natures, which are typical features of aluminous A-type granites, further belonging to the A2 granites. The low zircon εHf(t) values (–13.8 to –8.6), along with the low whole rock εNd(t) values (–8.62 to –6.10) and low apatite εNd(t) values (–8.31 to –6.56), indicate that the Songxian–Ruyang A-type granites were derived from partial melting of the calc-alkaline tonalitic–trondhjemitic–granodioritic gneisses in Taihua Complex, and they were formed in post-collisional extensional setting. Combined with coeval regional granitoids, the south margin of the NCC is in a continuous extensional setting during 1.8–1.6 Ga, and the Songxian–Ruyang A-type granites can represent a response of the breakup of Columbia supercontinent.

Ordovician continental arc magmatism in the Tam Ky‐Phuoc Son Suture Zone, Central Indochina Block, Southeast Asia

The Indochina Block is considered as a part of the Indian and Australian margins of Gondwana, and the Tam Ky‐Phuoc Son Suture Zone (TPSZ) lies in its central region as the boundary between the Truong Son Belt and Kon Tum Massif (KTM). However, the early Palaeozoic tectonic nature and timing of magmatism within the TPSZ are still controversial. To tackle the issue, we present herein the zircon U–Pb ages with their trace‐element compositions of five dioritic–granitic granitoids in the TPSZ. U–Pb age dating of magmatic zircons from them suggests emplacement ages of ca. 447–450 Ma. Whole‐rock geochemistry of the samples shows low TiO2 contents (0.36–1.18 wt%) and Mg# (42.04–53.07). They display enrichment in light rare earth elements ([La/Yb]N = 4.10–15.19), pronounced negative Nb, Ta and Ti anomalies relative to the chondrite‐normalized rare earth element patterns and primitive mantle‐normalized trace‐element distribution diagrams. The whole‐rock geochemical characteristics together with zircon trace‐element compositions from the same samples revealed their petrogenesis of a continental arc tectonic setting. Combining the results from this study together with available data from the TPSZ and its neighbouring units, we suggest accretionary orogenesis was responsible for generation of the Ordovician magmatic arc in the KTM, which will provide better understanding of the Early Palaeozoic evolution of the eastern Indochina Block.

Permian-Triassic flood basalts in the pre-Jurassic basement of the Arctic zone of the West Siberian platform

Research subject. Permian-Triassic flood basalts in the basement of the Arctic zone of the West Siberian Platform locate mainly in graben-rift structures. Flood basalts in this region remain to be understudied in comparison with other areas of its distribution, mainly due to the significant depth of their occurrence (4–6 km).Materials and methods. 36 core samples from 11 superdeep and deep boreholes were studied. Isotopic ratios were measured on mass spectrometers NEPTUNE PLUS (Nd, Sm) and TRITON PLUS (Rb, Sr). Bitumen were studied using a Raman spectrometer LabRAM HR800 Evolution. The Raman spectra were deconvoluted (“Peak fitting” procedure), and the bitumen conversion temperature was estimated.Results. About half of the samples of volcanic rocks underwent metamorphism of the prehnite-pumpellite and locally greenschist facies or intense low-temperature hydrothermal alteration. The studied basalts are close to typical flood basalts and are somewhat similar to island-arc volcanic rocks in terms of their geochemical characteristics. For the first time, thin inclusions of bitumen were found in the amygdalae of Permian-Triassic basalts in the superdeep borehole Tyumenskaya SG-6 at a depth of 7310.6 m. A high similarity of the studied volcanics by geochemical characteristics and the isotopic composition of Sr and Nd with the flood basalts of the Siberian platform is shown.Conclusions. The presence of a negative Ta, Nb, Ti anomaly, as well as a negative Ce anomaly, in some of the analyzed samples indicates a possible contamination of the basalts by island arc volcanics and volcanogenic-sedimentary rocks. The temperature of transformation of bitumen in inclusions in basalts from the well Tyumenskaya SG-6 according to Raman spectroscopy is estimated at 150–300°C and generally corresponds to the temperature of metamorphism of the host basalts. The presence of bitumen in the amygdalae may indicate the migration of hydrocarbons through the basalts.

Modern-style plate tectonics manifested by the late Neoarchean TTG-sanukitoid suite from the Datong-Huai'an Complex, Trans-North China Orogen

When modern-style plate tectonics started and operated on a specific old craton has been a key point involving the early Earth's evolution. In order to address this issue on the evolution of the Neoarchean North China Craton (NCC), we investigated a representative successive magmatic rock suite of tonalite-trondhjemite-granodiorite (TTG) and sanukitoid in the Datong-Huai'an Complex. Geochemically, TTG gneisses in the Datong-Huai'an Complex can be divided into high-pressure (HP) and low-pressure (LP) TTG rocks. The HP TTGs emplaced at ∼2538–2497 Ma and 2441 Ma, respectively. Geochemical data shows that they are characterized by steep rare earth element (REE) patterns, obvious negative Nb, Ta, Ti anomalies and positive δEu anomalies, high Sr/Y, Nb/Ta and (La/Yb) N ratios, and positive ɛ Hf (t) (+2.1 to +9.1) and δ 18 O values (avg. 5.5‰ to 6.0‰). Their protolith is interpreted as the consequence of partial melting of a subducted oceanic slab with garnet and minor rutile as residual phases. In contrast, the LP TTGs emplaced at ∼2518 Ma. They display flat REE patterns, slightly negative Nb, Ta, Ti anomalies and negative δEu anomalies, low Sr/Y, Nb/Ta and (La/Yb) N ratios, positive ɛ Hf (t) (+3.4 to +5.9) and high δ 18 O values (avg. 5.9‰ to 6.1‰). They were most likely generated by partial melting of the mafic lower crust with residual plagioclase and amphibolite in the source region. The sanukitoid rocks emplaced at 2517–2485 Ma and show high MgO, Cr and Ni concentrations but relatively low (La/Yb) N values, positive ɛ Hf ( t ) values (+1.5 to +5.4), and higher δ 18 O values (avg. 6.8‰ to 7.5‰), suggesting that they were originated from partial melting of the mantle peridotite that was previously modified by slab-derived or sediment-derived melts. Combined with previous petrological, geochronological and geochemical data, we propose that the Datong-Huai'an Complex recorded the prolonged magmatism through ca. 2.56 Ga to 2.44 Ga, which was characterized by the high-angle subduction of oceanic lithosphere, leading to the opening of a mantle wedge, slab rollback and back-arc extension. All these lines of evidence manifested that a modern-style plate tectonics regime started along the western margin of the NCC Eastern Block during the late Neoarchean, though the interior Eastern Block was still dominated by the mantle plume-induced crustal growth and vertical tectonism. • Coeval TTGs and sanukitoids were recognized in the Datong-Huai’an Complex. • They recorded the prolonged high-angle subduction with rollback and back-arc extension. • Modern-style plate tectonics started along the margin of NCC Eastern Block in late Neoarchean.

Geochemistry of basalts in unravelling the mantle processes and crustal evolution: Insights from the greenstone belts of western Dharwar Craton

Basalts are an integral part of all the greenstone belts of Dharwar Craton which played a vital role in the thermo-tectonic evolution and mantle processes during the Archean time. Here, we evaluate the basalt geochemistry from the Holenarsipur, Shimoga and Chitradurga greenstone belts of western Dharwar Craton wherein both Sargur and Dharwar Supergroups are present reflecting a gradual change in the geodynamic processes. They have overlapping petrographic and distinct geochemical characteristics with slightly fractionated chondrite normalized rare earth element patterns for the Shimoga basalts, low order negative Eu anomalies and negative Nb and Ti anomalies with a hump at Zr-Hf. The Holenarsipur basalts display nearly flat REE patterns with low LREE/HREE [(La/Yb)N = 0.26-0.97] and negative to positive Eu (Eu/Eu* = 0.93-1.05) anomalies whereas the Chitradurga basalts show flat REE patterns [(La/Yb)N = 0.92-1.28] analogous to MORB. The U-Pb zircon age of the Holenarsipur basalts indicate 3235 ± 54 Ma with multiple thermal overprints after their deposition due to Paleoproterozoic to Mesozoic dyke emplacement. The Holenarsipur and Chitradurga basalts show partial melting trend whereas the Shimoga basalts follow fractional crystallization trajectory. Nb/La (0.13-1.33) and La/Yb (0.38-6.76) ratios of all these basalts represent the derivation of their primary magma from the lithospheric mantle with a significant asthenospheric contribution reflecting on mantle recycling. Furthermore, Nb/Yb and La/Yb relationship indicates that the Holenarsipur and Chitradurga basalts are derived from N-MORB type mantle whereas Shimoga basalts are derived from slightly enriched mantle. Nb/Yb and Th/Yb relationship show back arc basin affinity for the Chitradurga basalts whereas Holenarsipur basalts endorse oceanic arc characteristics and the Shimoga basalts display continental arc affinity. These basalts follow the spinel to spinel-garnet lherzolite melting regime and the degree of melting ranges from 10-30% resembling the global counterparts. Considering the regional geology, geochemistry and geochronology of these three greenstone belts, it is proposed that the WDC experienced three different tectono-magmatic events viz. the oceanic arc, back arc basin and continental arc during the Meso-Neoarchean which played a key role in the crustal evolution of Dharwar Craton.

Ancient crust-derived syenitic and A-type granitic intrusions in the Emeishan large igneous province, SW China

Within the inner zone of the Emeishan large igneous province (ELIP), SW China, there are several small syenitic and A-type granitic intrusions not spatially related to giant Fe–Ti–V-deposit-bearing mafic–ultramafic layered intrusions. The representative Heime syenitic and Huili granitic intrusions as well as their spatially associated mafic dykes were developed at ca. 260 Ma, contemporaneous with the ELIP. The Heime metaluminous syenites are K-rich and have moderate SiO2 (60.1–61.4 wt%) and high total-alkali (Na2O + K2O = 9.68–10.7 wt%) contents with high K2O/Na2O ratios (1.18 to 1.74). The Huili metaluminous granites are K-rich A-type granites with moderate–high SiO2 (62.7–68.9 wt%), high total-alkali (7.96–8.62 wt%) and Zr (324–853 ppm) contents, and high 10,000 × Ga/Al ratios (2.68–3.22). Both of them are enriched in high-field-strength elements and light rare-earth elements, with significantly negative Nb–Ta–Sr–P–Ti anomalies. They also have highly evolved whole-rock SrNd isotopic compositions (Heime syenites, (87Sr/86Sr)i = 0.7092–0.7105, εNd(t) = −7.77 to −6.09; Huili granites, (87Sr/86Sr)i = 0.7116–0.7146, εNd(t) = −6.34 to −5.32). Zircon δ18O values of the Heime syenites (5.73‰–6.40‰) are slightly higher than those of the mantle zircons, and lower than those of the Huili granites (5.98‰–7.52‰). Zircon Hf 8isotopic compositions of the Huili granites (εHf(t) = −3.30 to +0.73) are higher than those of the Heime syenites (εHf(t) = −7.08 to −2.97), possibly indicating a more contribution of mantle component. In addition, the spatially associated mafic dykes display considerably higher εNd(t) values (Heime mafic dykes, −3.95 to −3.01; Heime mafic dykes, −0.58 to −0.51) than those of the Heime syenites and Huili granites. The parental magmas of both the Heime syenites and the Huili A-type granites show a close affinity to the K-rich, metaluminous syenitic magmas. Based on the Sr–Nd–O–Hf isotopic compositions of the Heime syenites, Huili granites, and spatially associated mafic dykes, we suggest that the K-rich syenitic magmas parental to the Heime syenites were produced mainly by remelting of Paleo–Mesoproterozoic mafic meta-igneous rocks, whereas those parental to the Huili A-type granites were generated by the remelting of Paleo–Mesoproterozoic basement rocks and mixing with small amounts of coeval mantle-derived basaltic melts.

Forearc and back-arc mantle characteristics of the mafic-ultramafic rocks of Simlipal complex, Singhbhum Craton, India

The Simlipal complex of Singhbhum Craton is enigmatic in terms of its shape, stratigraphy, age and geodynamic setting. The circular-shaped Simlipal complex has predominant mafic volcanic rocks and quartzites associated with komatiites, komatiitic basalts, picrites, boninites, andesites and other ultramafic rocks. The mafic rocks are subalkaline tholeiitic basalts to basaltic andesites with moderate SiO2 (45.94–54.26 wt%), MgO (6.09–12.21 wt%), Fe2O3 (10.02–16.44 wt%), CaO (8.33–12.12 wt%), Mg # (32–58), low TiO2 (0.4–1.02 wt%), P2O5 (0.08–0.34 wt%), Ni (112–241 ppm) and Cr (104–506 ppm). The chondrite normalized rare earth element (REE) patterns are slightly fractionated [(La/Yb)N = 2.97–5.44] with slight negative to positive Eu anomalies (Eu/Eu* = 0.84–1.46). Their primitive mantle normalized multielement patterns exhibit negative Th, Nb, Ta and positive to negative Ti anomalies. The ultramafic rocks characterized as harzburgites, have high MgO (34.77–38.11 wt%), Ni (1974–2453 ppm), Cr (3875–4610 ppm) and Mg# (80–85), low SiO2 (40.61–44.18 wt%), TiO2 (0.08–0.14 wt%), Al2O3 (2.24–3.79 wt%), Fe2O3 (7.24–9.77 wt%) and P2O5 (∼0.05 wt%). Their chondrite normalized REE patterns exhibit fractionated LREE [(La/Sm)N = 2.07–2.37], depleted MREE, almost flat HREE [(Gd/Yb)N = 0.99–1.14] and a pronounced trough at Nb-Ta; ZrHf and Ti on the primitive mantle normalized multielement patterns. The basalt gave a zircon UPb age of 2843 ± 16 Ma (MSWD = 0.72) which is the crystallization age. HFSE relationship of the basalts indicates back-arc nature and their melting in the spinel-lherzolite field. The peridotites are arc cumulates, exhibiting boninitic characteristics and forearc affinity. Their PGE systematics indicate retention of sulphides in the residue during the partial melting and fractionation of the mantle. These peridotites and basalts were generated in forearc and back-arc environments during the Neoarchean time, subsequently juxtaposed with plume-derived volcanic sequences and the passive margin sediments as the arc progressed towards the continent. The plume-arc accretion model (PAA) is a viable tectonic setting with the presence of komatiites, komatitic basalts, picrites, tholeiitic basalts and peridotites. Further, the metasediments from the passive continental margin accreted with PAA products in the Simlipal complex. The above findings also support the existence of Archean smaller plate configuration and the proximity of the arc to the continental margin. After the formation of the volcano-sedimentary sequences, the meteoritic impact appears to have obliterated the accretionary imprints in the Simlipal complex.

Petrogenesis of the Alubaogeshan intrusion in the Maodeng–Xiaogushan area, southern great xing'an range, NE China: Implications for magma evolution and tin–polymetallic mineralization

• The Alubaogeshan intrusion formed at ca. 134 Ma and belongs to transitional I–A 2 -type granite . • The Alubaogeshan intrusion was formed by partial melting of a Neoproterozoic juvenile crust. • The syenogranite porphyry evolved from the porphyritic monzogranite. • Fluid–melt interaction is crucial for W mineralization. • The magma evolutionary degree influences the size of the Sn–polymetallic deposits in the SGXR. The Alubaogeshan intrusion, consisting of porphyritic monzogranite in the central part and syenogranite porphyry in the edge, is genetically related to the Maodeng–Xiaogushan Sn–polymetallic deposit. This paper aims to investigate the genetic link between these two granite phases and the relationship between magma evolution and Sn mineralization. LA-ICP-MS zircon U–Pb dating of the porphyritic monzogranite and syenogranite porphyry yielded ages of 134 ± 1.0 Ma and 133 ± 0.5 Ma, respectively. The Alubaogeshan intrusion has high SiO 2 and total alkali (Na 2 O + K 2 O) contents, with low Al 2 O 3 , Fe 2 O 3 , FeO, and MgO contents, belonging to high-K calc-alkaline metaluminous–weakly peraluminous granites. Chondrite-normalized rare earth element (REE) patterns are characterized by light REE enrichment and heavy REE depletion, with extremely negative Eu anomalies. In addition, the intrusion was also enriched in Rb, Th, U, K, Zr, Hf, and light REE and strongly depleted in Ba, Sr, P, and Ti. Mineralogical and geochemical characteristics, together with high zircon saturation temperature, indicate that the Alubaogeshan intrusion belongs to transitional I–A 2 -type granites. Compared with the porphyritic monzogranite, the syenogranite porphyry reveals higher SiO 2 content and lower TiO 2 , Al 2 O 3 , Fe 2 O 3 , FeO, CaO, MgO, and P 2 O 5 contents, with more obvious negative anomalies of Eu, Ba, Sr, P, and Ti. The temporal and spatial relationship and geochemical characteristics of the two rocks indicate that the syenogranite porphyry evolved from the porphyritic monzogranite. The modeling results show that the fractional crystallization of amphibole, plagioclase, K-feldspar, biotite, apatite, monazite, and allanite occurred during the magma evolution. The granitic magma differentiation plays a key role in the formation of the Maodeng–Xiaogushan Sn–polymetallic deposit. The ε Hf ( t ) values of zircon range from 2.58 to 7.08, and the two-stage Hf model ages range from 619 Ma to 850 Ma, implying that the Alubaogeshan intrusion was predominantly derived from partial melting of a Neoproterozoic juvenile crust. The TE 1,3 values of the ore-related granites from W–(Sn) deposits are generally higher than 1.1, suggesting that fluid–melt interaction is crucial for W mineralization. The degree of magmatic evolution is one of the controlling factors for the size of Sn–polymetallic deposits in the southern Great Xing′an Range.

Geochemistry and Sr-Nd isotopic studies of Precambrian gneisses from central Aravalli Craton, NW India: Implications for crustal evolution and reworking

The Aravalli Craton of the Indian shield constitutes heterogeneous basement lithologies (Banded Gneissic Complex; BGC), and among them, the granitoids are the most voluminous lithology. The BGC comprises two lithotectonic units, viz., BGC-I and BGC-II. The BGC-II has been further classified as amphibolite facies Mangalwar and granulite facies Sandmata Complexes. In the present study, the gneisses of the Mangalwar Complex are geochemically categorized into (i) low-and high-pressure sodic gneisses and (ii) potassic gneisses. The sodic gneisses are metaluminous and characterized by high Sr/Y and LaN/YbN ratios; and exhibit subduction-related negative anomalies of Nb and Ti. The εNd (t = 2992 Ma) ranges from +2.3 to +3.1, with an average Nd-depleted mantle model age (TDM) of 3.06 Ga. The whole-rock Sm-Nd isochron age is ∼3.0 Ga (2992 ± 340 Ma). Genetically, the sodic gneisses originated from the melting of an enriched precursor (oceanic plateau) in an arc environment. These gneisses show strong correlations with the gneisses from BGC-I depicting similar geochemical signatures. In contrast, the potassic gneisses are characterized by slightly higher SiO2 along with high K2O and high large-ion lithophile elements and negative Eu anomalies along with negative εNd (t = 1.7 Ga) (−13.2 to −3.9), higher initial 87Sr/86Sr isotopic ratios and average TDM = 2.87 Ga. These geochemical features of the potassic gneisses indicate that they were derived from the reworking of the pre-existing TTG-like (sodic gneisses) crust during the Paleoproterozoic Era.

Geochemistry of Khandadharpahar-Thakuranipahar metabasites from the western Singhbhum Craton, eastern India: implications for subduction-zone tectonics and mantle-wedge metasomatism

The identification of new rock types in the volcano-sedimentary sequences of the Singhbhum Craton has attractedmuch attention in recent years. The present study deals on newly identified Nb-Enriched Basalts (NEB) from theKhandadharpahar-Kadakala-Thakuranipahar (KKT) section, western Singhbhum Craton, which is comparablein composition to basalts-basaltic andesites and calk-alkaline in character. These metabasites have a porphyritictexture with phenocrysts of pyroxene and plagioclase, as well as a groundmass that has metamorphosed to thegreenschist facies. High Nb contents (7.5-22.8ppm) combined with high (Nb/Th)PM (0.28-0.59), (Nb/La)PM (0.40-0.69) and Nb/U (11.7-34.4) ratios, compared to arc basalts ((Nb/Th)PM= 0.10-1.19; (Nb/La)PMn 0.17-0.99, Nb/U<10), characterized them as NEB. Negative Nb, Zr, Hf and Ti anomalies, and Nb/Th vs La/Nb and Th/Nb vs. La/Sm relationships, collectively indicate typical arc volcanics. The available geochemical parameters suggest a genesisof KKT metabasites through i) slab melt migration from the downgoing oceanic crust, ii) low-degree melting ofthe garnet-bearing peridotite in the mantle wedge metasomatized by the slab melts, iii) slab melt - peridotiteinteraction triggering increasing Nb concentrations and iv) NEB generation in an arc-related environment. Thediscovery of KKT NEB sheds new information on Paleoproterozoic subduction-zone processes and crustal growthin the Singhbhum craton.

Magnesium and Calcium Isotopic Geochemistry of Silica‐Undersaturated Alkaline Basalts: Applications for Tracing Recycled Carbon

AbstractCarbon plays important roles in the evolution of the atmosphere and biosphere, and geochemical differentiation in Earth's interior. Most subducted C is recycled to the deep mantle and returns to the surface by degassing from erupted basalts and associated fluids. The Mg–Ca isotopic systems have been widely used in tracing recycled C. However, the idea that these geochemical proxies truly reflect the deep C cycle has been challenged. Here we present whole‐rock geochemical and Mg–Ca isotopic compositions of Miocene silica‐undersaturated alkaline basalts of the Western Qinling orogen, China. These alkaline basalts are associated with carbonatites and are characterized by low SiO2 (39.0–43.2 wt.%) and Al2O3 (6.98–9.15 wt.%) contents, high CaO/Al2O3 ratios (1.4–1.8), and positive Nb–Ta and negative Pb–Zr–Hf–Ti anomalies, suggesting they were derived by partial melting of carbonatite‐metasomatized asthenospheric mantle. The studied samples have δ26Mg values of −0.24‰ to −0.44‰, ranging from mantle‐like values (δ26Mg = −0.25‰ ± 0.07‰) to lower values. This implies that carbonatite metasomatism does not always produce low‐δ26Mg anomalies. The samples have δ44/40Ca values (0.59‰–0.77‰; relative to the standard SRM915a) that are lower than Bulk Silicate Earth (0.94‰ ± 0.05‰), which are attributed to the involvement of low δ44/40Ca recycled carbonate in the mantle source. We suggest that the shift in mantle δ26Mg values during carbonatite metasomatism is controlled by the type and amount of carbonatite involved, while Ca isotope variations depend largely on the δ44/40Ca values of subducted carbonates. Mg or Ca isotopes alone, however, may not be sufficient to track the deep carbon cycle.

Petrogenesis of continental intraplate alkaline basalts in the Tuoyun Basin, western Central Asian Orogenic Belt: implications for deep carbon recycling

Abstract The origin of intraplate volcanic provinces in continental interiors remains equivocal. One of the major gaps is the absence of a clear understanding of thermal state and the role of recycled carbon. Here we present results from detailed temperature estimation, trace element content of olivine and integrated Sr-Nd-O-Mg-Zn isotopic studies on Meso-Cenozoic basalt and basanite from the Tuoyun Basin in the western Central Asian Orogenic Belt. The calculated crystallization temperatures are in the range of 1101-1273 °C for basalt and 1037-1295 °C for basanite, consistent with ambient upper mantle-derived melts. Olivine Zn/Fe, Mn/Zn and Fe/Mn ratios fall in the hybrid source region composed of peridotite and pyroxenite for both basalt and basanite, which is also in agreement with their bulk-rock CaO-MgO trends, FeO/MnO ratios (62.8-77.4 for basalt and 57.8-65.1 for basanite) and FCKANTMS values (overall major element compositions; 0.52-0.68 for basalt and 0.43-0.64 for basanite). Trace elemental modeling shows that the basalt was produced by higher degrees of partial melting with more pyroxenite contribution than basanite. Mg and Zn isotopes show lighter Mg (-0.45 to -0.55 ‰ for basalt and -0.45 to -0.47 ‰ for basanite) and heavier Zn (0.37-0.41 ‰ for basalt and 0.38 to 0.48 ‰ for basanite) isotopic compositions than primitive mantle, indicating the possible involvement of recycled sedimentary carbonate in mantle source. Conversely, the basalt and basanite lack typical “carbonatitic fingerprints” such as high CaO/Al2O3 ratios and negative Zr, Hf and Ti anomalies. Furthermore, the Sr-O isotopes display typical mantle-affinity without explicit crustal signatures. Such a decoupling is attributed to carbonate species transition and decarbonation reactions between carbonate and surrounding silicate during plate subduction at a depth of > 150 km where the following reactions take place: CaMg(CO3)2 (dolomite) = MgSiO3 (magnesite) + CaCO3 (aragonite), and MgCO3 (magnesite) + SiO2 (coesite) → MgSiO3 (enstatite) + CO2. When the recycled carbonates are exhausted, crustal Sr-O is released but the Mg-Zn isotopic anomalies are inherited by the silicate products (“ghost carbonate”). These silicates are expected to be involved in the subsequent mantle upwelling that induces decompression melting to generate intraplate magmas. We therefore propose recycled carbonates in the mantle sources of the Tuoyun lavas underwent recycling at depths > 150 km and were incorporated as “ghost carbonate” with recycled Mg and Zn elements in the partial melting process rather than directly in the carbonate form.

Palaeoproterozoic arc related supracrustal units from the Tasiilaq Region, SE Greenland: Insights into the convergence of the Rae and North Atlantic Cratons

The Tasiilaq region in SE Greenland contains Palaeoproterozoic supracrustal rocks, which record the collision and suturing of the Rae Craton and North Atlantic Craton during the 1.9 Ga Nagssugtoqidian orogeny. The original mineralogy and textures of the supracrustal rocks have been largely overprinted by amphibolite facies metamorphism, obscuring the original protoliths and complicating the interpretation of the early Earth surface environment in which these rocks formed. Here, we present new major and trace element data for the Schweizerland, Kuummiut and Isertoq Terranes, with the former two representing the Rae craton, and the latter the northern margin of the North Atlantic Craton. Supracrustal lithologies include metapelites, marbles, calc-silicate rocks and mafic to felsic volcanic rocks and/or shallow intrusive rocks. Pelites normalised to Post Archaean Australian shales have (sub)horizontal rare earth element patterns, with La/SmCN: 1.61 – 23.91 and Eu/Eu* 0.34 – 1.83 and Th/Sc 1. The volcanic rocks are enriched relative to the primitive mantle (PM) with parallel to sub parallel PM-normalised heavy rare earth elements. They have a suprasubduction zone signature with distinct Nb-Ta troughs. The amphibolites can be further divided into those that interacted with the crust; back-arc basalts with subtle Nb-Ta troughs; and fore-arc basalts with distinct Nb-Ta and Zr-Hf troughs and negative Ti anomalies. The marbles consist of a Mg-rich dolomite group with δ13C 0.48 to 0.91 ‰ and δ18O −6.13 to −7.57 ‰; and Mg-poor calcitic carbonates with δ13C −0.74 to 0.17 ‰ and δ18O −10.81 to −14.87 ‰. This isotopic composition suggests formation at ∼ 2050 Ma during the low δ13C excursion following the Great Oxidation Event. We propose that the supracrustals of the Tasiilaq region were deposited at 2050 Ma in a series of interconnected basins during dual subduction convergence of the North Atlantic and Rae cratons, with development of an island arc with back arc basin in the north, and a continental arc in the south. These basins were inverted during continental collision, thrust, and folded into the Archaean Kuummiut and Isertoq TTG-dominated terrane basement, before being subjected to the region-wide amphibolite facies metamorphism at ∼ 1840 Ma during the Nagssugtoqidian orogeny.

Petrogenesis of late Permian to Middle Triassic magmatic rocks on northern Hainan Island, South China: Implications for crust–mantle interaction and the tectonic evolution of the Paleo-Tethys

Geochronological, geochemical, and Sr–Nd–Hf isotope data are reported for the late Permian to Middle Triassic Juding gabbros, the Xinxing gabbroic–dioritic rocks, and granites and mafic enclaves from the northern part of Hainan Island, South China. Our data were used to constrain the petrogenesis of these rocks, the crust–mantle interactions involved, and the implications for the tectonic evolution of the area. Zircon U–Pb dating gave crystallization ages of ca. 235 Ma for the Juding gabbros, ca. 252 Ma for the Xinxing gabbroic–dioritic rocks, and 253–238 Ma for the granites and mafic enclaves. The Juding gabbros are characterized by weak negative Nb, Ta, and Ti anomalies, low initial 87Sr/86Sr ratios (0.704603–0.704630), and high εNd(t) values (+3.7 to +3.8). The Juding gabbros are inferred to have been derived by partial melting of depleted asthenospheric mantle, with these melts mixing with melts produced from enriched lithospheric mantle. The Xinxing gabbroic–dioritic rocks have relatively intermediate compositions, with initial 87Sr/86Sr ratios of 0.706600–0.708664, εNd(t) values of −6.3 to −5.1, and εHf(t) values of −3.3 to −0.6. The Xinxing gabbroic-dioritic rocks probably derived by partial melting of enriched lithospheric mantle that was modified by subduction-related melts and/or fluids. The granites from the northern part of Hainan Island have extremely high SiO2, low MgO and Ni contents, with initial 87Sr/86Sr ratios of 0.704794–0.711158, εNd(t) values of −8.5 to −6.6, and εHf(t) values of −15.0 to −1.2. They were probably derived by partial melting of a variety of ancient crustal sources with some addition of mantle material. The mafic enclaves in these granites have relatively low SiO2, high MgO and Ni contents, with initial 87Sr/86Sr ratio of 0.707882, εNd(t) value of −6.4, and εHf(t) values of −9.8 to −3.9. They seem to represent the remnants of a mafic component derived from enriched mantle. The gabbroic–dioritic rocks and gabbros formed in response to the subduction and break off of the Paleo-Tethyan oceanic slab. The late Permian to Middle Triassic magmatic rocks from the northern part of Hainan Island record complex multistage crust–mantle interactions and lithosphere–asthenosphere interaction, probably related to subduction during closure of the Paleo-Tethys Ocean and the consequential collision of the South China and Indochina blocks.

Petrogenesis of Early Cretaceous granitoids in Liyang volcanic basin, eastern China: Geodynamic implications for Paleo-Pacific slab rollback

Late Mesozoic magmatism in eastern China was extensive and closely related to the subduction of the Paleo-Pacific plate, but genetic relationship between magmatism and subduction remains unclear. Cretaceous magmatic rocks distributed along the eastern coast of China can provide an opportunity to evaluate the influence of subduction on the evolution of the magmatism. Here, a combined study of whole-rock major-trace elements and Sr-Nd-Pb-Hf isotopes as well as zircon U-Pb ages was carried out for the Cretaceous granitoids in Liyang volcanic basin, eastern China. Zircon U-Pb dating results indicate that the Liyang granitoids were emplaced in the Early Cretaceous (ca. 130–125 Ma). These samples (SiO2 = 70.8–75.3 wt%) are mainly composed of high-K calc-alkaline granite and granite porphyry. Their geochemical characteristics are similar to I-type granites, with enrichment in LREEs, Rb, Th and U, and negative Eu (δEu = 0.04–0.51), Ba, Sr, P and Ti anomalies. All samples show negative whole-rock εNd(t) (−9.8 to −5.7) and zircon εHf(t) (−15.7 to −3.60) values (corresponding crustal Hf model ages of 1.40–2.17 Ga), and high radiogenic Pb isotopic compositions. The integrated interpretation of geochemical data reveal that rollback of the Paleo-Pacific subducted slab resulted in lithospheric extension and thinning, and the upwelling of hot asthenosphere in eastern China, which induced more intensive underplating of the mantle-derived mafic magma. The ascent of heat and mafic magma, intruded in Meso-Neoproterozoic crust, and triggered partial melting of the Meso-Neoproterozoic crust. The Liyang I-type granitoids were the result of the Meso-Neoproterozoic crustal components mixing with a small number of mantle-derived magmas in the extensional tectonic setting, which was response to slab rollback of the Paleo-Pacific plate.

Late Permian rift-related volcanic rocks from the Zhongba Terrane, Southern Tibet: implications for the opening of the Neotethys

ABSTRACT The questions of how and when the Neotethys Ocean opened are crucial for understanding the evolution of the Tethys Realm and the break-up of Gondwana. However, the evolution of the Neotethys remains controversial. Herein, we report the zircon U–Pb ages, whole-rock major as well as trace elements, and Sr–Nd isotopic data for the basaltic volcanic rocks from the Zhongba Terrane in the western Yalung–Zangbo Suture Zone. The results demonstrate that the Saga basalts were extruded during the Late Permian (257–258 Ma). All samples are characterized based on the low SiO2 content (44.03–50.33 wt%) and belong to the high-K calc-alkaline series. Furthermore, they exhibit light rare earth element enrichment ((La/Yb)N = 12.7–26.7); slightly positive Eu anomaly(Eu/Eu* = 1.01–1.14); prominent Nb and Ta positive anomalies; slightly negative Zr, Hf, and Ti anomalies and exhibit high ratios of Ta/Hf (0.93–1.21), Zr/Y (8.68–10.63), and Nb/Zr (0.39–0.45). The (87Sr/86Sr)i ratio varied from 0.7050 to 0.7063, with relatively constant and depleted εNd(t) values ranging from 0.52 to 1.08. The rocks were probably sourced from a low-degree partial melting of the asthenospheric mantle in a rift environment and had undergone the process of fractional crystallization. These characteristics indicate that the Saga basalts differ from the rift-related magmatic rocks produced by the Panjal Traps. Consequently, combined with the recently reported detailed fieldwork, the Zhongba Terrane disintegrated from the northern margin of Gondwana during the Late Permian, promoting the opening of the Neotethys Ocean, which is most likely caused by the northward subduction of the Paleotethys Ocean.

Geochronology, petrogenesis and metallogenic implications of granitoids in the Xiaotuergen Cu deposit, Northern Chinese Altai Orogen, NW China: Constraints from zircon, apatite and whole-rock geochemistry

The Xiaotuergen Cu deposit is the first Cu deposit described in the Northern Chinese Altai Orogen, Xinjiang, NW China. The multiple phases of granitic intrusions occur in the Xiaotuergen region, including the intrusions of granodiorite porphyry, granite porphyry, and biotite monzogranite. Among these types of intrusions, Cu mineralization is considered to be closely related to the early Hercynian granodiorite porphyry. However, the precise emplacement ages, petrogenesis, redox states, and tectonic setting of the three Xiaotuergen granitoids and the relationship between these granitoids and mineralization was unclear. To resolve these issues, zircon U–Pb dates, zircon in situ trace element concentrations, Hf isotopic compositions, apatite in situ major element concentrations, and whole-rock geochemistry were determined in this study. Three different intrusive phases are recognized within the district; in order of deceasing age, these phases are biotite monzogranite, granodiorite porphyry, and granite porphyry, which have the ages of 400.2 ± 1.6 Ma to 399.6 ± 2.4 Ma, 390.3 ± 1.6 Ma to 377.1 ± 1.6 Ma, 381.0 ± 2.0 Ma to 378.7 ± 1.2 Ma, respectively, suggesting that they were emplaced during the Early-Middle Devonian. Geochemical data show that these intrusions are metaluminous to weakly peraluminous (A/CNK = 0.91–1.28), calc-alkalic to shoshonitic I-type granitoids. The two Harker plot trends reveal derivation from two magma sources (biotite monzogranite vs granodiorite porphyry and granite porphyry) that experienced different evolutionary processes. Multiple indicators, including the zircon Ce4+/Ce3+ ratio and oxygen fugacity (logfO2) values, apatite Mn oxybarometer, and whole-rock Fe2O3/FeO ratio, consistently indicate that the parental magma of the fertile granodiorite porphyry was more oxidized than that of the two barren intrusions (biotite monzogranite and granite porphyry). Furthermore, apatite grains from the fertile granodiorite porphyry have higher Cl contents and Cl/F ratios than those of the ore-barren biotite monzogranite and granite porphyry, which may have resulted from the higher water content of the parental magma and increased lithospheric mantle-derived fluid involvement in their generation. Thus, higher Cl/F ratios of apatite can be used to discriminate fertile granitoid intrusions that host Cu mineralization in the Northern Altai orogen. Zircon Hf isotopic compositions show widely ranging and mostly positive εHf (t) values (−4.2 to +11.4, mainly concentrated between +2 and +8). These isotope characteristics in combination with young zircon TDMC (Hf) ages (655–1,659 Ma; mainly concentrated between 850 Ma and 1250 Ma) indicate that the Xiaotuergen granitoids were most likely derived from the partial melting of juvenile lower crust, with some interaction of mantle materials from the underlying mantle wedge. The three Xiaotuergen granitoids were enriched in light rare earth elements and large ion lithophile elements, depleted in heavy rare earth elements, and showed negative Nb, Ta, Sr, P, and Ti anomalies, displaying typical arc geochemical affinities. Combined with the regional tectonic evolution, we propose that the Xiaotuergen granitoids were generated in a continental arc setting associated with subduction processes, which were the products of syn-accretion orogenic processes.