Fractionated REE Patterns Research Articles

Mesozoic lithospheric mantle evolution in South China: Evidence from Ar-Ar dating, geochemistry and Sr-Nd-Pb isotopes of lamprophyres

Understanding the Mesozoic lithospheric mantle beneath South China is challenging, primarily because mantle-derived magmatic rocks are rare, whereas granitic magmatism is far more abundant. In this study, we present new 40Ar/39Ar geochronology, major and trace elemental composition, and Sr-Nd-Pb isotopic data for lamprophyres from northern and central Guangxi Province, South China. Our results reveal two intrusion phases: an earlier phase at 207.7 ± 1.2 Ma, and a later phase between 95.6 ± 0.6 Ma and 102.3 ± 0.7 Ma. The earlier phase, represented by the Danyang lamprophyres, is marked by low Cr (330–355 ppm) and Ni (241–258 ppm) contents and strongly fractionated REE patterns with (La/Yb)n ratios of 57.5 to 62.8. The later phase, represented by the Leidong, Nongchang, and Longchang lamprophyres, shows higher Cr (490–781 ppm) and Ni (237–377 ppm) contents, with less fractionated REE patterns and (La/Yb)n ratios of 20.2 to 27.8. Despite their temporal differences, the lamprophyres share similar isotopic characteristics, including high initial 87Sr/86Sr (0.707940–0.721310), negative εNd(t) (−9.19 to −5.31), and radiogenic Pb isotopic compositions (206Pb/204Pb: 18.200–18.331, 207Pb/204Pb: 15.62–15.72, 208Pb/204Pb: 38.53–38.84). These features suggest derivation from low-degree partial melting of a phlogopite-bearing, refractory peridotite lithospheric mantle, modified by olivine and clinopyroxene fractionation. This refractory mantle was metasomatically enriched by Proterozoic subduction processes beneath the Yangtze Craton. By synthesizing published Sr-Nd-Pb isotopic data, we propose that the lithospheric mantle beneath South China shifted from enriched to depleted mantle between ∼220 Ma and ∼ 100 Ma, possibly due to extension associated with the subduction and rollback of the Paleo-Pacific Plate.

Prolonged arc accretion during growth of juvenile crust in the Arabian Nubian Shield: Insights from the granitoids of northern Ethiopia

The Arabian Nubian Shield (ANS) is one of the largest juvenile continental crust formed after the Archean. To determine the timing and understand the mechanism of its early crustal growth, we conducted geochronological and geochemical studies on granitoids from northern Ethiopia. A plagiogranite, dated at 929.7 ± 2.2 Ma, represents one of the early granitoids from the ANS. It shows low K2O, TiO2, and REE contents, flat REE pattern and absence of Eu anomaly. Moreover, it exhibits (87Sr/86Sr)i, ƐNd(t) and ƐHf(t) values of 0.70341, +5.0 and +8.4, respectively. Its zircon δ18O value is 4.8‰, lower than the mantle value. These characteristics suggest that it was derived from partial melting of altered oceanic crust. The oxygen fugacity obtained from zircon trace elements is ΔFMQ+1.3, consistent with a subduction setting.Other plutons have much younger ages ranging from 860 to 840 Ma and are mainly medium K, I-type granitoids. These granitoids show low MgO, Ni and Cr contents, low Sr/Y, (La/Yb)N, Nb/Ta and Zr/Sm ratios. They exhibit whole-rock (87Sr/86Sr)i values of 0.70236–0.70304, ƐNd(t) values of 4.2–5.2, ƐHf(t) values of 5.8–7.6 and zircon ƐHf(t) values of 5.5–10.4, slightly lower than coeval depleted mantle. Furthermore, they exhibit fractionated REE patterns, enrichments in LILE and depletion in HFSE. These geochemical features suggest that these granitoids were generated by remelting of arc crust. Compiled data show that the associated basaltic rocks are of arc affinity, which confirms the coeval development of arc magmatism in the southern ANS. Our results suggest that the growth of juvenile crust in the southern ANS started from the early Neoproterozoic and peaked at ca. 880 to 800 Ma, much earlier than those in the northern ANS. The prolonged history accounts for the growth of the vast juvenile crust in the ANS.

Geochronology and Petrogenesis of Late Carboniferous to Early Permian Basalts in the Central Lhasa Subterrane, Southern Tibet: Implications for the Evolution of the Sumdo Paleo‐Tethys Ocean

AbstractBasalts from the Late Carboniferous to Early Permian are extensively developed in the central Lhasa subterrane, southern Tibet. Studying the petrogenesis of these rocks may have implications for the late Paleozoic arc magmatism along the central Lhasa subterrane uncovering more of the evolution of the Sumdo Paleo‐Tethys Ocean and its dynamic mechanism. Basalt samples from the Luobadui Formation in the Leqingla area, NW of Linzhou City in the central Lhasa subterrane, southern Tibet exhibit arc‐like geochemical signatures in a subduction‐zone tectonic setting characterized by high Al2O3 and low TiO2 contents, fractionated REE patterns with low Nb/La ratios and high LREE concentrations, and negative HFSE anomalies. Based on their higher Th/Ce, Nb/Zr, and lower Ba/Th, Pb/Nd ratios, slightly negative to positive εNd(t) values, and the relatively high Sr‐Pb isotopic compositions, these samples were probably derived from partial melting of a depleted mantle source of garnet + spinel lherzolite, metasomatized by subducted sediments around 297 Ma. Modeling of the trace elements indicates that these basalts experienced fractional crystallization of olivine, clinopyroxene and minor plagioclase during magma ascent and eruption. It is proposed that these Late Carboniferous–Early Permian basalts are associated with the northward subduction of the Sumdo Paleo‐Tethys Ocean seafloor along the southern margin of the central Lhasa subterrane.

Back-arc magmatism of the Proto-Tethys Ocean in the West Kunlun Orogenic Belt: evidence from the Ordovician Nanpingxueshan Nb-enriched gabbros

ABSTRACT The West Kunlun Orogenic Belt (WKOB) in the northwestern Tibetan Plateau records key information about the tectonic evolution of the Proto-Tethys Ocean. To further constrain the Early Palaeozoic tectonic evolution of the WKOB, we report petrography, geochronology, whole-rock geochemistry, and Sr-Nd-Hf-O isotope data of the newly identified Ordovician Nanpingxueshan gabbros in the southern WKOB. Laser ablation-inductively coupled plasma-mass spectroscopy (LA-MC-ICP-MS) U-Pb zircon dating reveals that the gabbroic intrusion was emplaced during 476–471 Ma. Geochemically, the gabbroic rocks exhibit a tholeiitic signature and can be subdivided into two groups, i.e. normal arc-like gabbros (AGs) and Nb-enriched gabbros (NEGs). The AGs have TiO2 (1.20–1.64%), P2O5 (0.18–0.25%), and Nb (4.60–7.64 ppm) contents, fractionated REE patterns ((La/Yb)N = 2.64–4.11), and Nb-Ta depletion (Nb/La = 0.37–0.50). Compared with AGs, the NEGs have higher TiO2 (1.74–2.68%), P2O5 (0.34–0.55%), and Nb (10.79–14.08 ppm) contents and Nb/La ratios of 0.45–0.53, which are similar to typical Nb-enriched arc basalts. Isotopically, the AGs have depleted whole-rock εNd(t) values of 3.22 to 4.04 and negative to positive zircon εHf(t) values of −1.8 to 7.8, while the NEGs exhibit slightly lower whole-rock εNd(t) values of 0.69 to 3.18, zircon εHf(t) values of −7.8 to 4.6, and mostly δ18O values greater than those of normal mantle zircon. Integrated analyses reveal that the AGs and NEGs can be attributed to a similar mantle source metasomatized by different proportions of subducted sediment-derived melt during the subduction process. Our new data, combined with the results of previous studies, suggest that the southward subduction of the Proto-Tethys Ocean under the WKOB initiated at ca. 530 Ma. The long-term southward subduction led to the formation of the massive Early Palaeozoic accretionary complex in the South Kunlun Terrane, while trench retreat led to migration of the arc magmatism northward from the Mazar – Tianshuihai Terrane to the South Kunlun Terrane, as well as back-arc extension in the Mazar – Tianshuihai Terrane.

Post-collisional magmatism and local lithospheric thinning in the Neoproterozoic Alto Pajeú Domain, northeastern Brazil: Geochemical and isotopic evidence

The 595–560 Ma post-collisional magmatism in the Alto Pajeú Domain, Borborema Province, northeastern Brazil, is characterized by the intrusion of voluminous high-K calc-alkaline, shoshonitic and ultrapotassic magmas. The nature of the source rocks and the geodynamic context associated with the generation and emplacement of this post-collisional magmatism is still enigmatic. In this study, we report new UPb ages, elemental geochemistry, mineral chemistry and SrNd isotope data for rocks from the Pajeú batholith. LA-ICP-MS zircon UPb dating revealed that this batholith was formed by at least two distinct magma pulses. The oldest pulse (ca. 592 Ma) consists mainly of porphyritic quartz monzonites to monzogranites with abundant mafic microgranular enclaves (MMEs), while the younger pulse (ca. 568 Ma) is composed of equigranular biotite granites. The porphyritic monzogranites are magnesian, have intermediate to acidic compositions (SiO2 = 62.2–67.6 wt%) and high Mg# (44–55) values. In contrast, biotite granites are ferroan, have high SiO2 (69.3–73.1 wt%) and low Mg# (16–36) values. Both rock types are characterized by enrichment in LREE and LILE and depletion in HREE and HFSE, but with more fractionated REE patterns for biotite granites ([La/Yb]N = 67.4–101.5) than for porphyritic rocks ([La/Yb]N = 35.8–54.8). The SrNd isotopic values for the different rock types are roughly similar. However, biotite granites show slightly higher initial 87Sr/86Sr (0.71265–0.71412) ratios and more negative εNd (t) (−18.45 to −18.67) values than porphyritic monzogranites (initial 87Sr/86Sr ratios from 0.71077 to 0.71155 and εNd (t) from −16.04 to −16.96). These geochemical and isotopic signatures, together with mineral chemistry data, suggest different sources for the two rock types. The biotite granites are of purely crustal origin, derived by partial melting of a Paleoproterozoic amphibolitic lower continental crust, while a mixture of melts derived from the Paleoproterozoic continental crust and magmas derived from an enriched lithospheric mantle could explain the origin of porphyritic monzogranites. Based on the spatial and temporal distribution of magmatic rocks from the Alto Pajeú Domain, we suggest that a local lithospheric thinning model could explain the evolution of post-collisional magmatism in this region of the Borborema Province.

Alkaline Silicate Metasomatism Recorded through Fe-Ti-Rich Mantle Xenoliths from the Calatrava Volcanic Field (Spain)

Much of the lithospheric subcontinental mantle (SCLM) sampled in the Calatrava Volcanic Field (CVF) shows refertilization by alkaline metasomatic agents. The Cerro Pelado and El Palo ultramafic xenolith suites record the best evidence of this type of metasomatism in this volcanic field. Several groups of peridotite (lherzolite, wehrlite, and dunite) and pyroxenite (clinopyroxenite and websterite) xenoliths have been distinguished. Despite having scarce phlogopites and amphiboles as modal metasomatic phases, all studied xenoliths present a variable cryptic metasomatism, highlighted by the strong Fe-Ti enrichment and fractionated REE patterns in the most evolved wehrlite and pyroxenite varieties. They show a common trend of an Fe-Ti-Ca increase, whereas the pyroxenites are more depleted in Fe compared to the lherzolites and wehrlites. Trace-element (REE and multi-trace) patterns are roughly similar among them, suggesting different interactions and refertilization degrees by alkaline silicate melts. The same Sr–Nd isotopic EAR composition, combined with trace-element chemistry of metasomatic xenolith phases and phenocrysts from the Calatrava volcanics, highlights the main role of this magmatism in percolation processes beneath Central Iberia. These mantle xenoliths also show variable amounts of interstitial glass that originated by in situ partial melting, favored by the enriched chemical nature of cryptically metasomatized clinopyroxene during their volcanic transport. This alkaline-refertilized mantle type represents the main domain within the SCLM beneath Central Iberia, as was also recorded in other Western European Cenozoic volcanic fields.

The December 2018 eruption at Etna volcano: a geochemical study on melt and fluid inclusions

This study focus on the Mt Etna December 2018 eruption with the aim of investigating the geochemical characteristics of the feeding magma. New data on major and trace element geochemistry of olivine-hosted melt inclusions (MI) in volcanic products are presented together with the noble gas geochemistry of fluid inclusions (FI) in olivines. The noble gas geochemistry of fluid inclusions (FIs) in olivines was also investigated. The major element composition of MIs is variable from tephrite/trachybasalt to phonotephrite/basaltic trachyandesite, with SiO2 = 45.51–52.72 wt%, MgO = 4.01–6.02 wt%, and CaO/Al2O3 = 0.34–0.72. Trace element patterns of MIs present a typical enrichment in LILE and LREE, depletion in HFSE, and relatively fractionated REE patterns: (La/Lu) N= 18.8–41.08, with Eu/Eu* = (0.5–1.8). Positive anomalies in Sr (Sr/Sr* = 0.8–2.3) and Ba can be ascribed to the assimilation of plagioclase-rich cumulates in the magmatic reservoir. The variable Ba/La (9.8–15.8), K/Nb (260–1037), Ce/Nb (1.9–3.4), Rb/La (0.4–1.6), and Ba/Nb (10.8–25.8) ratios reveal mixing between two types of end-member magmas comparable to those emitted from 1) the 2001 Upper Vents and 2002–03 Northern Fissures (Type-1) and 2) the 2001 Lower Vents and 2002–03 Southern Fissures (Type-2), respectively. Type-2 represents a magma that was under the influence of a crustal component, whereas Type-1 is compatible with a HIMU–MORB-type heterogeneous mantle source. It appears that the 2018 MIs have captured the two different types of magmas, and the lack of homogenization may imply a very fast ascent (a few months). Compatible with the contemporary presence of primordial HIMU–MORB and crust-contaminated end-members are the data on noble gases from FI that highlighted an 3He/4He value of 6.5–6.6Ra. The hypothesis of two different types of magmas, identified by the trace element geochemistry in MIs, is, thus, reinforced by helium isotopic data on FI of the 2018 eruption together with data from other Etnean eruptions and allows the inference of a bicomponent magma mixing.

Mesoarchean synchronous emplacement of TTG gneisses and potassic granitoids in the Nyabessane granite-greenstone terranes, NW Congo Craton (southern Cameroon): Zircon U[sbnd]Pb geochronology, petrogenesis and tectonic implications

Archean granitoids played key roles in the generation and differentiation of the Archean continental crust and provide clues to understand crustal processes in the early Earth. Abundant Mesoarchean granitoids were emplaced in the Nyabessane granite-greenstone terrane (NGB), part of the Ntem Complex of the Northwest Congo Craton. They include charnockites, tonalite-trondhjemite-granodiorite (TTG), granitic and monzogranitic gneisses. Here, we present a geochemical and geochronological (zircon LA-ICP-SF-MS UPb) study of these granitoids to determine their petrogenesis and to better constrain the crustal evolution of the Ntem Complex. Field and petrographic observations indicate that most of these granitoids underwent extensive metamorphism and deformations, associated with anatexis. Zircon UPb dating results suggest that the charnockite, TTGs and granitic gneisses, and monzogranites have emplacement ages of 2910 ± 11 Ma, 2870–2865 Ma and 2852 ± 31 Ma, respectively. The charnockites have low SiO2 (55–58 wt%) and high Al2O3 (16–18 wt%), CaO (7–8 wt%) and MgO (∼ 4.5 wt%) content with Mg# ∼ 54, and exhibit magnesian, metaluminous characteristics of the Cordilleran granitoid-type formed in magmatic arc. The 2.87–2.86 Ga TTG gneisses are silica-rich (55–58 wt% SiO2), sodic (3–5 wt% Na2O, Na2O/K2O = 1–3), with HREE-depleted, and display the typical Archean medium- to low pressure TTG geochemical features groups. Their chemical compositions are characteristics of TTG-like melts derived from the partial melting of hydrated low- to high-K metabasic/thickened lower crust at various depths followed by magmatic differentiation during ascent. When compared to the TTG gneisses in the NGB, the ∼2.87 Ga granitic gneisses are K-rich, and high Gd/Yb, Th/Yb, Th/Nb, Sr/Y and La/Yb ratios but lower MgO, Yb, V, Y, Cr, Ni and Sr contents, matching typical Archean hybrid and potassic granitic rocks. We propose that the granitic gneisses were derived from the intra-crustal melting of a pre-existing felsic crust. The granitic and the TTG gneisses were generated contemporaneously through the same magmatic event at ∼2.87–2.86 Ga. The ∼2852 Ma monzogranitic gneisses are ferroan and metaluminous rocks, and show LREE enrichment with strongly fractionated REE patterns and positive Eu anomalies. Geochemical features, together with the presence of ∼2.9 Ga-old inherited zircon grains are consistent with the remelting of Mesoarchean granitoids. Considering the petrogenetic, regional geological and geochronological data, the Mesoarchean granitoid magmatism of the Ntem Complex was likely generated via complex transitional geodynamic regimes involving subduction and accretion processes.

A Long-Lived Accretionary Process during the Amalgamation of the North China Craton: Insights from Neoarchean–Paleoproterozoic Polyphase Magmatism in the Lüliang Complex

Abstract There has been a long debate regarding the timing of the final amalgamation of the North China Craton, which is considered to have occurred either during the Neoarchean or Paleoproterozoic era. One major point of contention is whether there existed a long-lived subduction lasting through the Neoarchean to Paleoproterozoic. The Lüliang Complex contains multiphases of magmatism and thus represents the most viable region to address this controversy. In this study, we carried geochronological and geochemical analysis on the representative granitoids. Secondary ion mass spectrometry U–Pb dating revealed four distinct granitoid groups emplaced at 2531 ± 4, 2189–2173, 2027 ± 25, and 1852 ± 41 Ma, respectively. Notably, the 2531 Ma granitic gneiss was identified for the first time in this region. Based on the geochemical characteristics, the granitoids can be divided into two types. The 2531 and 2027 Ma groups display I-type features, while the 2189–2173 and 1852 Ma groups exhibit A-type geochemical affinities. Both I-type groups exhibit enrichment in Rb, depletion in Nb, Ta, and Ti, moderate fractionated REE patterns, substantial negative Eu anomalies, low Sr/Y ratios, and positive εHf(t) (+3.51 to +5.53 and +5.59 to +7.32, respectively), indicating that they were generated from partial melting of the juvenile mafic crust. In contrast, the 2189–2173 Ma granitoids belong to A2-type and were most likely generated by the partial melting of felsic rocks in the back-arc region, while the 1852 Ma granitoids belong to A1-type and were most possibly the result of partial melting of mafic-intermediate rocks during the post-collisional stage. Based on the records of A-type granitic magmatism and the ~1950 Ma peak metamorphism throughout the Trans-North China Orogen, we propose that a long-lived subduction process (2531–1950 Ma) can mostly explain the existing geological phenomena. It is likely that the subduction between the Eastern and Western Blocks should have commenced at ~2531 Ma, followed by a long-lived subduction. The two blocks ultimately collided with each other to form the North China Craton at ~1950 Ma, which triggered post-collisional exhumation and partial melting at ~1852 Ma.

Petrogenesis of Eocene Lamprophyre Dykes in Northern Qiangtang Terrane, Tibetan Plateau: Implications for the Tethyan Mantle Metasomatism and Tectonic Evolution

Post-collisional (ultra)potassic lamprophyre dykes are the key probes for understanding mantle metasomatism and reconstructing tectonic evolution. In this study, we present new petrological, geochronological, geochemical and zircon Lu-Hf isotopic data for lamprophyre dykes in the northern Qiangtang terrane (central Tibet), aiming to constrain their petrogenesis and geodynamic setting. The studied lamprophyres are minettes with phenocrysts of siderophyllite and phlogopite, which intrude into Triassic granite of 236.9 Ma. These lamprophyres yield zircon U-Pb ages of 39.7–40.9 Ma. They exhibit high contents of K2O (7.61–8.59 wt.%) and ultrapotassic features with high K2O/Na2O (11.43–14.38) ratios. They are characterized by increased values of Mg# (69.1 to 72.1) and high concentrations of compatible elements (e.g., Cr = 277–529 ppm, Ni = 232–322 ppm), which are diagnostic of mantle-derived primitive magma. The studied lamprophyres have a high abundance of rare earth elements (∑REE = 902–1061 ppm) with significantly fractionated REE patterns ((La/Yb)N = 66.3–100.6), and they are enriched in large ion lithophile elements (LILE) and light rare earth elements (LREE), but depleted in high field strength elements (HFSE) (e.g., Nb, Ta and Ti) and heavy rare earth elements (HREE) with enriched zircon Hf isotopes (εHf(t) from −6.40 to 3.80). This indicates their derivation from an enriched mantle source which was metasomatized by subduction-related fluids and sediment-derived melts. A petrogenetic study suggests that the lamprophyres were generated by the partial melting of a phlogopite-bearing lherzolite within the garnet stability field. We propose that the Cenozoic ultrapotassic mafic rocks in the central Tibetan Plateau originated in the lithospheric mantle metasomatized by the subduction-related components, and are the magmatic response to the detachment of the subducted Tethyan slab.

Petrology and geochemistry of Late Triassic rapakivi-type granites in the Dupangling area, South China: Role of fluid-rock interaction in the origin of rapakivi texture

Rapakivi-type granites not only document magmatic processes but also archive subsolidus interactions between granitic rock and fluid, and thereby provide crucial information about fluid infiltration in the Earth's crust. However, the key process to generate rapakivi texture is still debated. Here, we report comprehensive mineralogical, geochemical and Hf isotopic compositions of the Dupangling rapakivi granites (210.4–214.9 Ma) in the South China Block to investigate the effect of fluid-rock interaction on the origin of rapakivi feldspar. Dupangling rapakivi granites have high SiO2 (72.63–78.21 wt%) and alkali contents (Na2O + K2O = 6.14–9.06 wt%) with high K2O/Na2O ratios (1.37–2.64), characteristic of weakly peraluminous (A/CNK = 1.04–1.10) high-K calc-alkaline granites. Samples have weakly fractionated REE patterns with negative Eu anomalies (δEu = 0.05–0.38). They have negative whole-rock εNd(t) (−8.45 to −7.91), variable zircon εHf(t) (−9.7 to −1.0) and two-stage Hf isotope model ages (1.32–1.84 Ga), due to their derivation from a major early Mesoproterozoic crustal source with a subordinate late Paleoproterozoic component. Rapakivi feldspars in Dupangling granites are composed of an alkali-feldspar core and a plagioclase mantle. Fractures, pores and secondary inclusions (biotite, sericite, epidote and hematite) are closely associated with the mantles and oligoclase patches within alkali-feldspar cores. These pervasive microtextural features and variations of Be, Rb, Ba, Sr and LREE within rapakivi feldspars are interpreted to have formed through subsolidus fluid-induced dissolution-reprecipitation replacement processes. This study highlights that fluid-rock interaction plays an important role in generating rapakivi texture.

Archean Crustal Evolution of the Alxa Block, Western North China Craton: Constraints from Zircon U-Pb Ages and the Hf Isotopic Composition

The Alxa Block is an important component of the North China Craton, but its metamorphic basement has been poorly studied, which hampers the understanding of the Alxa Block and the North China Craton. In this study, we conducted geochronological and geochemical studies on three TTG (tonalite–trondhjemite–granodiorite) gneisses and one granitic gneiss exposed in the Langshan area of the eastern Alxa Block to investigate their crustal evolution. The zircon U-Pb dating results revealed that the protoliths of the TTG and granitic gneisses were formed at 2836 ± 20 Ma, 2491 ± 18 Ma, 2540 ± 38 Ma, and 2763 ± 42 Ma, respectively, and were overprinted by middle–late Paleoproterozoic metamorphism (1962–1721 Ma). All gneiss samples had high Sr/Y ratios (41–274) and intermediate Mg# values (44.97–55.78), with negative Nb, Ta, and Ti anomalies and moderately to strongly fractionated REE patterns ((La/Yb)N = 10.6–107.1), slight Sr enrichment, and positive Eu anomalies, displaying features of typical high-SiO2 adakites and Archean TTGs. The magmatic zircons from the 2.84 Ga and 2.49 Ga TTG rocks had low εHf(t) values of −1.9–1.7, and −3.83–2.12 with two-stage model ages (TDMC) of 3.24–3.11 Ga and 3.10–3.01 Ga, respectively, whereas those from the 2.54 Ga TTG rock exhibited εHf(t) values ranging from −1.1 to 3.46 and TDMC from 3.0 Ga to 2.83 Ga, suggesting that the crustal materials of the basement rocks in the eastern Alxa Block were initially extracted from the depleted mantle during the late Paleoarchean to Mesoarchean era and were reworked in the late Mesoarchean and late Neoarchean era. By contrast, the Alxa Block probably had a relative younger crustal evolutionary history (<3.24 Ga) than the main North China (<3.88 Ga), Tarim (<3.9 Ga), and Yangtze (<3.8 Ga) Cratons and likely had a unique crustal evolutionary history before the early Paleoproterozoic era.

Petrogenesis and Physicochemical Conditions of Fertile Porphyry in Non‐arc Porphyry Mineralization: A Case from Habo Porphyry Cu‐Mo Deposits, SW China

AbstractThe Habo deposit is a typical porphyry Cu‐Mo deposit in the Ailaoshan–Red River metallogenic belt. Ore minerals in the Habo deposit typically occur as veins in the monzonite porphyry. Zircon U‐Pb dating suggests that the monzonite porphyry formed at 35.07 ± 0.38 Ma. The monzonite porphyry is characterized by high SiO2, Al2O3, K2O and Na2O contents, with A/CNK ratios ranging from 0.97 to 1.02. All samples exhibit fractionated REE patterns, characterized by high (La/Yb)N ratios (9.4–13.6, average of 11.2). They show adakite‐like geochemical features, high Sr concentrations (627–751 ppm, average of 700 ppm), low Y concentrations (15.13–16.86 ppm, average of 15.81 ppm) and high Sr/Y values (39.5–47.4, average of 44.3). These samples have high initial 87Sr/86Sr ratios (0.7074–0.7076) and negative εNd(t) values (–5.1 to −3.7), whereas the zircon εHf(t) values range from −2.2 to +0.4, suggesting that the monzonite porphyry was derived from the partial melting of a thickened juvenile lower crust. The oxygen fugacity, calculated on the basis of the chemical composition of the amphiboles, shows ΔNNO values ranging from +1.65 to +2.16 (average of 1.94) and lg(fO2) ranging from −12.72 to −11.99 (average of −12.25), indicating that the monzonite porphyry has high oxygen fugacity. Zircons have high Ce4+/Ce3+ ratios (29.29–164.24, average of 84.92), with high ΔFMQ values ranging from +0.50 to +1.51 (average of 0.87) and high lg(fO2) values ranging from −14.72 to −12.85 (average of −14.07), which also indicates that the oxygen fugacity of the magma was high. The dissolved water content of the Habo monzonite porphyry is 9.5–11.5 wt%, according to the geochemical characteristics, zircon‐saturation thermometry (692–794°C) and the mineral phases (amphibole, no plagioclase) in the deep magma chamber. Combined with previous studies, we propose that the high oxygen fugacity and high water content of magma played key roles in controlling the formation of the Habo and other Cu‐Mo‐Au deposits in the Ailaoshan–Red River metallogenic belt.

A peridotite source for strongly alkalic ultrabasic HIMU lavas of the Oslo Rift, Norway

High alkali (Na2O + K2O) low SiO2 lavas generally have highly fractionated trace element compositions and their petrogenesis is debated. Both pyroxene-rich (pyroxenite) and olivine-rich (peridotite) mantle sources have been proposed for such lavas. HIMU (high 238U/204Pb) lavas usually have such compositions and are often interpreted as being derived from garnet-pyroxenite (eclogite) residue of subducted recycled oceanic crust, possibly including a carbonate component. In some cases, an origin from a garnet peridotite source has also been considered.We present new Mg, Pb, Nd, and Sr isotopic data and elemental data for alkalic ultrabasic lavas (melilitites, nephelinites) from the Oslo Rift, Norway. The magmatic province of the Permo-Carboniferous Oslo Rift is interpreted as part of the much larger Skagerrak-Centered Large Igneous Province (SCLIP) which may be related to a mantle plume source originating from the African Large Low Shear Velocity Province (LLSVP) at the core-mantle boundary. The range of δ26Mg is narrow (−0.32 to −0.28) and is similar to typical mantle values, and therefore is inconsistent with a carbonated source as has been suggested for other similar lavas. The 206Pb/204Pb values of all the lavas are high, which identifies them as HIMU lavas. The εNd of +1 to +2 are substantially lower than for typical HIMU lavas, and in the εNd-εSr diagram they plot on the LoNd array between HIMU and EMI. The lavas are more like archetypal kimberlites (formerly, Group I kimberlites), as these also have εNd slightly higher than the chondritic value and Pb isotopes consistent with a HIMU source. The REE patterns of the lavas are also almost as fractionated as in Group I kimberlites.We used major and trace elements to infer the primary magma composition of these lavas. This primary magma composition is compared with experimental melts of both carbonated peridotite and carbonated eclogite source compositions and shows that their major element compositions are only consistent with either a very high degree of melting of eclogite or a very low degree melting of garnet peridotite. The strongly fractionated REE patterns of the lavas can only be made by low-degree melting of either eclogite or garnet peridotite, which therefore effectively rules out eclogite as a major component of the mantle source of these lavas. Further trace element modeling points to a mantle source composition with >97% garnet peridotite that was lightly carbonated (<0.5 wt% CO2) and melted to a very low degree (∼0.4%) to produce these lavas. The modeled source trace element pattern was constrained by the Nd, Sr, and Pb isotopic compositions of the lavas. The measured high Ti contents of these lavas (3.8–6 wt%) are quantitatively shown to be the result of enrichment by both partial melting and fractional crystallization, using experimentally determined DTi for melting of this type of source. We also show that a low degree of melting of a slightly carbonated source is sufficient to result in the likely very high CO2 content (10–15 wt%) of the primary magma.The rather uniform Pb isotope composition of HIMU magmas suggests that the HIMU source is rather uniform in composition, but the Pb isotope composition requires this source to form late in Earth's history. The similarity of these lavas and kimberlites suggests a very deep source, possibly in the LLSVP that gave origin to the SCLIP. This means that LLSVPs do not represent some primordial layer around the core that escaped homogenization of the mantle by the Moon-forming giant impact. Instead, this layer must have formed late in Earth's history. The constraint that this source contains <3% of garnet-pyroxenite (eclogite) residue of recycled basaltic oceanic crust shows that this layer is primarily not a subducted slab graveyard. The process by which the LLSVPs formed, however, remains obscure.

Carbon Enrichment in the Lithospheric Mantle: Evidence from the Melt Inclusions in Mantle Xenoliths from the Hainan Basalts

AbstractIt is generally believed that the lithospheric mantle and the mantle transition zone are important carbon reservoirs. However, the location of carbon storage in Earth's interior and the reasons for carbon enrichment remain unclear. In this study, we report CO2‐rich olivine‐hosted melt inclusions in the mantle xenoliths of late Cenozoic basalts from the Penglai area, Hainan Province, which may shed some light on the carbon enrichment process in the lithospheric mantle. We also present a detailed petrological and geochemical investigation of the late Cenozoic basalts and mantle xenoliths from northern Hainan Island. The collected samples of late Cenozoic Hainan Island basalts belong to both alkaline and subalkaline series, showing fractionated REE patterns with high (La/Yb)Nvalues of 3.52–11.77, which are typical for OIB. Based on Al‐in‐olivine thermometry, the temperatures estimated for the mantle xenoliths can be divided into two groups. One group has temperatures of less than 1050°C, and the other group has temperature ranging from 1050°C to 1282°C. Clinopyroxene (La/Yb)N–Ti/Eu and clinopyroxene Ca/Al–Mg#diagrams indicate that the mantle peridotite experienced metasomatism from both silicate and carbonate melts. Melt inclusions in the olivine of mantle xenoliths include (1) CO2bubble–rich melt inclusions; (2) multiphase melt inclusions (glass + CO2bubble + daughter minerals); (3) pure glass melt inclusions. Magnesite is a daughter mineral in the olivine‐hosted melt inclusions, which could be interpreted as a secondary mineral formed by the interactions of CO2‐rich fluids with an olivine host, due to post‐entrapment effects. The glasses in olivine‐hosted melt inclusions have high SiO2contents (60.21–77.72 wt%). Our results suggest that a considerable amount of CO2‐rich melt inclusions are captured in the lithospheric mantle during metasomatism. The lithospheric mantle can therefore act as is a ‘carbon trap’, with much CO2being absorbed by the lithospheric mantle in this way.

Mineral associations and in-situ major and trace element compositions of dalyite from charoitites, Murun complex, Siberia

The rare potassium Zr-silicate dalyite has been found in the charoitites of Murun alkaline complex as an accessory mineral and the main Zr phase. Electron probe microanalysis (EPMA) and laser ablation inductively coupled plasma mass spectrometry (LA-ICP-MS) were used to measure major and trace element compositions of dalyite, respectively. Dalyite from charoitites is defined by significant Ti ↔ Zr substitution and low extent of Na–K solid solution and mineral formula can be presented as K2(Zr1-xTix)Si6O15. The Ti ↔ Zr substitution is a geothermometer in many minerals, however, the presence of zoning with increasing Ti from core towards the rim in dalyite from charoitites might indicate that element activity was more important factor than temperature. The first trace element analysis of dalyite shows that this mineral is enriched in lithophile elements, has strongly fractionated REE patterns but no Eu anomaly. Dalyite shares some common geochemical features with magmatic zircon: it shows HREE-enrichment and positive Ce anomaly relative to chondrite values, which could indicate that rare earth elements occur in similar structural position. Dalyite is an important mineral of peralkaline agpaitic rocks, a typical mineral of magmatic rocks, and likely indicates low-pressure conditions. Our new mineralogical and geochemical results on the dalyite clarify the role of Zr-silicates in agpaitic rocks and provide better understanding of the formation of charoitites.

Petrogenesis of Neoarchean granitoids beneath the Koyna-Warna region, Deccan Volcanic Province, India

• A new find of ca. 2.5 Ga granitoids in the western Dharwar craton. • These I-type clac-alkaline granitoids are bimodal in composition. • Trace element patterns are identical to Archean TTGs. • Precursor magmatic sources evolved in a Neoarchean subduction zone. The continental crust that is concealed beneath the voluminous flood basalt provinces of the Deccan Traps (India) is inaccessible except by few deep bore-holes. The well-core extracted from the deep borehole KBH-8 (> 1000 m) penetrating the massive flood basalt cover provides an exclusive opportunity to examine the Precambrian continental crust in this part of the Indian shield. Here, we present detailed petrology, geochemistry and zircon U-Pb geochronology of the KBH-8 granitoids, which are granodiorite and monzogranites by their normative composition showing a magnesian calc-alkaline metaluminous I-type geochemical affinity. The granitoids are categorically identical to low-and high-HREE TTGs. Their fractionated REE patterns exhibit positive Eu anomaly. Besides the negative Nb and Ti anomalies, they exhibit positive Sr and Zr anomalies. Such attributes are atypical of the granitoids formed from a reworked older (> 3.0 Ga) TTG crust. The geochemistry of these granitoids cannot be reconciled with assimilation and fractional crystallization of the felsic crust in the Dharwar craton. Alternatively, the Ba+Sr enrichments in these granitoids are ascribed to melting of a slab fluid/melt metasomatized mantle wedge. Further, the Sr-Y and La-Yb systematics are consistent with melting of a low-Mg amphibolite source with garnet in the restite. In addition to the above, the variable Gd N /Yb N =1-2.3 ratios and Nb-Zr systematics suggest that the melts were produced from variable depths involving both depleted and undepleted segments of the sub-arc mantle. Consequently, we propose that the KBH-8 granitoids were sourced from a heterogeneous thickened arc-like crust in a Neoarchean subduction zone.

Petrogenesis of the ca. 2.32 Ga low-δ18O gabbroic diorites and granites in the Xiaoshan area, southern North China Craton: Implications for the early Paleoproterozoic tectonic evolution

The database of global zircon ages shows a marked age gap at 2.45–2.2 Ga, which was postulated associated with the global magmatic and plate tectonic “shutdown” or “slowdown”. But in recent years, a great deal of ∼ 2.3 Ga magmatism has been identified in the North China Craton (NCC). Nevertheless, tectonic evolutionary scenario of the NCC during early Paleoproterozoic is still in hot debate. This study focuses on the gabbroic diorites and biotite granites in the Xiaoshan area, southern margin of the NCC. They were emplaced at ∼ 2.32 Ga. The gabbroic diorites have low SiO2, high Mg#, Cr, Ni values, and characterized by LREE-enriched, HREE-depleted trends, slightly positive Eu anomalies with pronounced negative Nb-Ta anomalies. Meanwhile, the samples have low-δ18O value of 2.05–5.35 ‰, they show negative zircon εHf (t) (−5.8 to − 4.1) and whole-rock εNd (t) (−2.36 to − 1.77) values, with TDM1 ages of 2.81–2.88 Ga and 2.80–2.86 Ga respectively, and crustal assimilation was limited during magma ascent, indicating a lithospheric mantle (CLM) source, which has been refertilized by metasomatic processes. The biotite granites have low 10000 Ga/Al ratios, Zr, Zr + Nb + Ce + Y concentrations, and calculated zircon saturation temperature (697–776 °C), belonging to metaluminous I-type granite. Their highly fractionated REE patterns, high Sr/Y and La/Yb ratios, low Dy/Yb and Gd/Yb ratios indicating that they were derived from partial melting of a lower crust source with both garnet and amphibole in the residue. Besides, they have δ18O values ranging from 2.41 ‰ to 5.11 ‰, variable zircon εHf (t) (−1.1 to + 1.4) and whole-rock εNd (t) (−0.76 to + 0.29) values, implying they were formed mainly by reworking of the pre-existing ancient crust, which was probably triggered by underplated mantle-derived mafic magmas. In combination with previous studies on the great diversity 2.45–2.2 Ga mafic-felsic rock assemblages and their geochemical features, we prefer that the extensional tectonic regime, which can result in thermal anomaly to account for the lithospheric mantle and crustal melting, is reasonable in the early Paleoproterozoic Era, at least for the southern NCC.

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

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

Late Cretaceous adakitic intrusive rocks in the Laimailang area, Gangdese batholith: Implications for the Neo-Tethyan Ocean subduction

Abstract The Gangdese batholith, emplaced from the Cretaceous to the Eocene in the southern Lhasa terrane of Tibet, provides critical constraints on the tectonic-magmatic evolution of the Neo-Tethyan Ocean and the India-Asia continental collision. In this article, we report new data for the Laimailang monzogranite in the southern Gangdese, including major and trace element analyses, zircon U–Pb dating, and in situ Hf isotope analyses. In situ zircon U–Pb dating of sample yielded Late Cretaceous ages (ca. 81 Ma). The monzogranite is characterized by relatively high-silica (≥56 wt% SiO2), Na-rich, and high-Al granitoids that are characterized by high Sr, low Y and HREE contents and strongly fractionated REE patterns, with no significant Eu anomaly, indicating that they are consistent with the definition of adakite. These monzogranite have high K2O (2.92–6.5%) and negative ε Hf(t) (−2.1 to −5.4), suggesting that these rocks were likely derived from the partial melting of the lower continental crust. We conclude that the Laimailang rocks of adakitic affinity were derived due to the melting of the thickened lower continental crust in the late Cretaceous.