Permian Tarim Large Igneous Province Research Articles

Triassic Thermal Pulse of TARIM Mantle Plume: Evidence from Geochronology, Geochemistry, and Nd Isotopes of the Mafic Dikes from the Halaqi Area, Xinjiang, China

Owing to the paucity of research on synchronous mafic rocks in the Tarim Basin, the late Paleozoic–early Mesozoic tectonic development of this region is not well defined. The Halaqi region is situated on Tarim’s northwest edge, and numerous mafic dikes can be found cross-cutting the Permian strata. The whole-rock geochemistry, zircon U–Pb age, and Sr–Nd isotopic signature of these mafic rocks have never been reported before, and this contribution can offer geochronological and petrogenetic investigations that provide fresh insight into the geodynamic development of the area. Major oxide contents in the Halaqi mafic rocks vary, including SiO2 (45.74–50.31 wt.%), Al2O3 (13.28–14.8 wt.%), FeOT (16.48–19.19 wt.%), MgO (7.58–10.32 wt.%), CaO (7.19–12.39 wt.%), Na2O (2.97–4.50 wt.%), K2O (0.24–0.63 wt.%), TiO2 (1.11–1.29 wt.%), MnO (0.14–0.16 wt.%), and P2O5 (0.13–0.17 wt.%). The mafic rocks are enriched in high-field-strength elements (e.g., Zr and Hf) and large-ion lithophile elements (e.g., Sr, Th, and U) but depleted in Nb, Ta, and P. The total REEs in the rocks are lower (ΣREE = 72.80–86.85 ppm), and HREEs are somewhat depleted in comparison to LREEs, with positive Eu anomalies (Eu/Eu* = 1.05–1.17) but weak negative Ce anomalies (Ce/Ce* = 0.91–0.93). Zircon U–Pb ages of 201–247 Ma were obtained from a total of 18 magmatic zircon grains found in the mafic rocks that were studied. These results point to a middle-to-late Triassic emplacement. The mafic dikes exhibit somewhat enriched Nd isotopic compositions (εNd(t) = –1.6~–0.2) and an older Nd model age (TDM = 1.24–1.37 Ga). The Halaqi middle–late Triassic mafic dikes are thought to have originated from the same tectonic background as the Permian Tarim Large Igneous Province, along with similar geochemical and isotopic compositions. This suggests that they are all products of the interaction between asthenospheric and lithospheric mantles in an intraplate extensional environment. Research indicates that the Triassic mafic magmatism in northwest Tarim could be the product of the continuous thermal pulse of the Tarim mantle plume and be a part of the Tarim LIP.

Late Magmatic Activity in the Tarim Large Igneous Province: Evidence from the Puchang Granites.

The magmatic evolution sequence and tectonic activity background of the Permian Tarim large igneous province were discussed based on the research of petrogeochemistry, chronology, and zircon Lu-Hf isotope of two types of granite distributed in the Puchang V-Ti-magnetite deposit in the northwestern margin of Tarim. The results show that the two types are characterized by high silica, alkali, and potassium content. Both types of granites are enriched with light rare earth elements, including porphyritic granites with a negative anomaly of Eu and biotite monzogranites with a weak positive anomaly of Eu. The trace elements generally show that the large-ion lithophile elements are enriched relative to the high-field-strength elements, P and Ti are obviously depleted, and Rb, U, and K are relatively enriched. The zircon U-Pb age of porphyritic granites is 275.7 ± 1.0 Ma, the εHf(t) value ranges from -6.6 to -3.3, and the tDM2 ranges from 1469 to 1684 Ma. The zircon U-Pb age of biotite monzogranites is 268.9 ± 1.4 Ma. The εHf(t) value ranges from -1.2 to 0.5, and the tDM2 ranges from 1232 to 1340 Ma. The two types of granite were formed in a postcollisional extensional environment at the end of the early Permian, which is related to the Permian Tarim mantle activity and asthenosphere upwelling. The porphyritic granite was derived from an enriched lithospheric mantle; the biotite monzogranite was formed by the melting of mantle with crust contamination; and basalt and intrusive complexes in the research area were formed in an extensional intraplate rift environment.

Petrogenesis of the alkali basalt and trachy-andesite suite in the northern Tarim Basin, NW China: Implications for crust-mantle interactions controlled by the Permian mantle plume

The Permian Tarim large igneous province (LIP), which contains both large volumes of basalt and silicic volcanic sequences, has been intensively studied in the past decades, but some issues still remain controversial, including the petrogenesis of each component in the LIP, their interrelationships, and the geodynamic processes that produced them. In the Halahatang area of the northern Tarim Basin, a well-developed eruptive suite composed of basalt and trachy-andesite has been penetrated by numerous boreholes, which provide us an opportunity to better understand these issues. In this paper, we carried out an integrated study on the eruptive suite, including petrography, geochronology and geochemistry. The results indicate that the eruptive suite formed at around ∼290 Ma, generally synchronous with the main episode of magmatism within the Permian Tarim LIP. The basalts have similar compositions to typical oceanic island basalts (OIBs), which have alkaline, high-Ti and sodic compositions and show enrichments in light rare earth elements (LREEs) and large-ion lithophile elements (LILEs). Whole-rock Sr–Nd isotopes are slightly enriched, with εNd(t) and (87Sr/86Sr)i values from −4.5 to −3.4 and 0.7065 to 0.7086, respectively. The trachy-andesites are alkaline, metaluminous and ferroan, and display highly enriched LREEs and LILEs. High Ga/Al ratios and Zr + Nb + Ce + Y values constrain their A-type affinity and A1-type nature. They have εNd(t) values from −8.2 to −4.5 and (87Sr/86Sr)i values from 0.7083 to 0.7189, more enriched than those of the basalts. Based on these results, we proposed that the different components in the igneous suite have close petrogenetic relationships. The alkali basalts may have been formed by mantle plume-derived melts coupled with somewhat assimilation of continental crust, while the trachy-andesites were products of variable fractionation of the basaltic magmas involving more continental crustal components in the magma evolution. In combination with the existing results in Tarim, we further suggested that the alkali basalt and trachy-andesite suite may have been generated via a crust–mantle interaction process related to the Permian mantle plume activity.

A Mantle Plume Connection for Alkaline Lamprophyres (Sannaites) from the Permian Tarim Large Igneous Province: Petrological, Geochemical and Isotopic Constraints

AbstractThe origin of lamprophyres associated with large igneous provinces (LIPs) remains controversial, particularly whether they are derived by direct melting of mantle plumes, or from previously metasomatized domains in thermally perturbed subcontinental lithosphere. Here, we report the petrological and geochemical characteristics of a recently identified suite of alkaline lamprophyres (sannaites) that represent the final pulse of magmatism in the Permian Tarim LIP in NW China. The sannaites display porphyritic texture with phenocrysts of olivine, clinopyroxene, hornblende, phlogopite, and titanomagnetite in a groundmass of plagioclase, clinopyroxene, nepheline, hornblende, biotite, and titanomagnetite with minor pyrite and apatite. Carbonate ocelli and almost pure albite in the groundmass are interpreted to have crystallized from immiscible carbonate and hydrous fluids, respectively, produced by late-stage magmatic segregation. The rocks show low to moderate SiO2 (37.7–49.3 wt.%) and MgO (2.74–9.91 wt.%), together with high Fe2O3T (up to 22.7 wt.%) and alkali contents (up to 9.02 wt.% Na2O + K2O). They are characterized by high incompatible element abundances, especially a marked enrichment in large-ion lithophile elements (Rb and Ba) and light rare-earth elements (e.g. La and Ce) relative to P and high-field-strength elements (e.g. Ti). They show a relatively restricted range of δ66Zn values between 0.22‰ and 0.46‰ with an average of 0.37 ± 0.04‰ (2SE, n = 10), which is marginally heavier than that of MORBs (0.27 ± 0.05‰). Their (87Sr/86Sr)t values range from 0.7035 to 0.7061, εNd(t) from −0.97 to +5.62, and δ26Mg from −0.36‰ to −0.17‰ (n = 8), the latter being consistent with those of global MORBs. Based on the correlation between Zn isotopes and TiO2–FeO concentrations, we infer that the heavy Zn isotopes in some of the sannaites resulted from fractional crystallization of Fe–Ti oxide minerals. The whole rock geochemical features of these rocks (negative K anomalies and enrichment in large-ion lithophile elements) and rhyolite–MELTS simulations suggest that the primary magmas of the sannaites were derived from an amphibole-bearing enriched lithospheric mantle source. Metasomatism and related formation of amphibole-bearing metasomatized mantle may be linked to sublithospheric melts/fluids derived from the Tarim plume in the earlier stages of plume activity, rather than slab-derived fluids or carbonate melts as suggested in previous studies for other alkaline mantle-derived magmas. Partial melting may have been triggered by the thermal input from the Tarim plume during a later stage. This study suggests that exotic, alkali-rich magmas can be produced during the multi-stage evolution of large mantle plumes, involving complex cycles of lithospheric mantle metasomatism and later melting of previously enriched domains.

Hydrogen Concentrations and He Isotopes in Olivine From Ultramafic Lamprophyres Provide New Constraints on a Wet Tarim Plume and Earth's Deep Water Cycle

AbstractWater enters Earth's mantle via subduction of hydrated oceanic slab and largely returns to the ocean‐atmosphere system through arc volcanism. However, the extent to which H2O is transferred into the deep mantle is poorly constrained. Here, we address this question by combining mineral chemistry and bulk‐rock geochemistry data for aillikites related to the deep mantle plume which generated the Permian Tarim large igneous province (NW China). The water contents of olivine phenocrysts are 75–168 ppm H2O and positively correlated with Ti contents. These results, combined with infrared hydroxyl peaks at 3,572 and 3,525 cm−1, suggest that H is mainly present in the form of Ti‐clinohumite‐like point defects. Hydrogen concentration profiles across olivine reveal that H loss during decompression was limited to the outermost rims, and yield dehydration durations of 15–417 min. Based on the water contents of the highest‐Fo olivine cores, the water contents of the primitive aillikite melts and their mantle source are estimated as 1.6–4.7 wt% and 150–1,200 ppm H2O, respectively. 3He/4He ratios (5.31–5.84 Ra) of olivine phenocrysts are slightly lower than MORBs and suggest involvement of recycled slab containing U (and hence radiogenic 4He) in the plume source. This interpretation is consistent with Pb isotope compositions of the aillikites which are intermediate between PREMA (Prevalent Mantle) and EM (Enriched Mantle) compositions. These lines of evidence combined with the depleted Sr‐Nd isotopes and moderately radiogenic Os isotopes of the aillikites suggest that water in these rocks derived from a plume source marginally contaminated by deeply subducted hydrated material.

The Kalba batholith dyke swarms (Eastern Kazakhstan): Mafic magmas effect on granite formation

The Kalba granitoid batholith formed in Early Permian is one of the largest in the western part of the Central Asian Orogenic Belt. Batholith granites are intruded by mafic and intemediate dykes forming several swarms. Previously it was believed that the dykes were Mesozoic and had no relationship with the formation of the batholith. Our geochronological studies allow to determine the age of the dykes at 279 Ma (also Early Permian). Dyke rocks are represented by dolerites, variolites, monzodiorites, and plagioclase porphyrites. The petrographic study of plagioclase porphyrites allow to find the sign of magma mixing. The another indicator of magma mixing are the contacts between mafic and felsic rocks in largest dykes of complex structure. A model for the interaction of mafic magmas with crustal substrates is proposed. Mafic magmas intruded from sublithospheric mantle, provoked a partial melting and then interacted with granitic melts at the basement level of granite batholith. Dyke swarms were formed after the cooling and consolidation of main volume of Kalba batholith by inrtusion of mafic and hybride magmas from the basement level of granite batholith. Chemical diversity of the mafic dykes was caused mainly by the differentiation of parental mafic magmas and also a lesser extent by their interaction with the granitoid magmas. Comparison with data on coeval magmatism in neighboring regions suggests that the Kalba batholith may be included in areal of Early Permian Tarim Large Igneous Province. • The dyke swarms cut the granites of the Kalba batholith in Eastern Kazakhstan. • The dykes and the Kalba batholith granites are Early Permian. • Dyke rocks composition was caused mainly by the differentiation of mafic magmas. • Parent mafic magmas formed in lithospheric mantle and provoked melting in crust. • Geodynamic setting was a post-orogenic extension with effect of Tarim mantle plume.

Phonotephrite and phonolite in the Tarim Large Igneous Province, northwestern China: Petrological, geochemical and isotopic evidence for contrasting mantle sources and deep carbon recycling

The igneous units of Permian Tarim Large Igneous Province (TLIP) is typically characterized by alkaline affinity. Here we report for the first time the petrological, geochemical and isotopic characteristics of phonotephrite and phonolite dykes from the Wajilitag complex in the northwestern margin of the TLIP. Our data, especially Sr-Nd-Mg isotopes, suggest that the dykes have different mantle sources. Phonotephrite with mantle-like Mg isotope values (−0.295 to −0.382 ‰), relatively enriched Sr isotopes (0.70487 to 0.70574) and slightly lower εNd(t) values (−0.23 to +2.06), were possibly derived from a lithospheric mantle source that was metasomatized by calcitic carbonate melt. The phonotephrites are highly evolved rocks formed through fractional crystallization from a parental magma which is similar to that of the mafic–ultramafic intrusions. Phonolites show relatively depleted Sr isotopes (0.70362 to 0.70409), higher εNd(t) values (+1.27 to +3.45), markedly lighter Mg isotopes (−0.851 to −0.906‰) than the normal mantle indicating a mantle plume source with the involvement of carbonated eclogite. The parental magma of phonolites may be related to the liquid immiscibility with calciocarbonatites present in the Wajilitag complex due to their similar extremely light Mg isotopic compositions. The distinct geochemical features between two series of dykes in the Wajilitag complex reflect the diversity of mantle source of the TLIP, and their Sr-Nd-Mg isotopic characteristics indicate the role of subducted recycled sedimentary carbonates at different depths in the mantle.

Constraining the duration of the Tarim flood basalts (northwestern China): CA-TIMS zircon U-Pb dating of tuffs

Abstract The Early Permian Tarim large igneous province (LIP) in northwestern China comprises voluminous basaltic lava flows, as well as ultramafic and silicic intrusions. The age and duration of the Tarim LIP remains unclear, and thus the rate of magma production and models of potential environmental effects are uncertain. Here we present high-precision chemical abrasion–isotope dilution–thermal ionization mass spectrometry zircon U-Pb ages for three newly discovered tuff layers interlayered with lava flows in the Kupukuziman and Kaipaizileike formations in the Keping area (Xinjiang, northwest China). The volcanism of the Kupukuziman Formation is constrained to a short duration from 289.77 ± 0.95 to 289.41 ± 0.52 Ma. An age for the overlying Kaipaizileike Formation is 284.27 ± 0.39 Ma, bracketing the duration of the entire eruptive phase of the Tarim flood basalts at ~5.5 m.y. The low eruption rate and relatively long duration of magmatism is consistent with a plume incubation model for the Tarim LIP.

Discovery of Permian crustal carbonatite in the northwestern margin of Tarim Basin and its geological significance

碳酸岩是自然界较为特殊的一类火成岩，由于其在上侵过程中不易受到地壳物质的混染，可以很好地保留源区原始特征，因而在揭示岩浆源区属性和地球深部碳循环等方面具有重要的研究价值。塔里木盆地西北缘阿克苏-乌什南部地区发育多条侵位到新元古界南华系中的火成碳酸岩岩墙或岩脉。这些岩脉的锆石U-Pb测年结果显示出多个年龄峰值，其中最年轻一组（3颗）岩浆锆石的谐和加权年龄为272±4Ma，另外还有400Ma、450Ma、790Ma、850Ma等年龄峰，而这些锆石可能均为碳酸岩岩浆捕获成因。结合区域上发育大量早二叠世辉绿岩岩床及岩墙等侵入事件，推测这些碳酸岩很可能形成于早二叠世（~270Ma）。碳酸岩主要由方解石（90%以上）构成，并含少量的白云石、重晶石、天青石、赤铁矿等其它矿物，可见方解石与白云石之间明显的出溶结构以及野外宏观上碳酸岩包裹围岩等现象。元素地球化学分析显示，碳酸岩CaO含量高（44.40%~50.40%），SiO₂含量低（1.83%~7.97%）；稀土元素总量很低（ΣREE=21.67×10^-6~91.32×10^-6），轻重稀土分异明显（LREE/HREE=3.90~8.40），具有中等负铕异常（δEu=0.61~0.68）和中等至弱负铈异常（δCe=0.59~0.98）；大离子亲石元素Sr、Rb、Ba等相对富集，Nb、Ta、Ti等高场强元素相对亏损；稳定同位素δ¹³C_V-PDB值高（-2.69‰~-2.93‰）、δ¹⁸O_V-SMOW值高（17.33‰~17.86‰）。这些特征与壳源碳酸岩特征相似，而与幔源碳酸岩差别较大。Sr-Nd同位素结果显示高的（⁸⁷Sr/⁸⁶Sr）_i值（0.710106~0.710558），低的ε_Nd（t）值（-8.46~-12.80），也进一步证实碳酸岩为地壳来源。综合分析认为这些碳酸岩很可能是塔里木二叠纪大火成岩省晚期岩浆事件的组成部分，系由塔里木西北缘地幔柱减压熔融后形成高温的基性岩浆向上侵位运移时，导致下地壳的碳酸盐储库部分熔融形成碳酸岩岩浆，并沿着断裂快速向上侵位形成。本文研究不仅丰富了塔里木大火成岩省的岩石组成认识，而且有助于增进对二叠纪深部复杂壳幔作用过程的理解。;Igneous carbonatites is a kind of special igneous rock in nature. Because they are not easily contaminated by crustal materials in the process of upwelling, they can well retain the original characteristics of the magma source. Therefore, they have important research value in revealing the properties of the magma source and the carbon cycle of the deep earth. Several carbonatite dikes or dykes are found intruded into the Neoproterozoic Nanhua System in the southern Aksu-Wushi area, the northwestern margin of Tarim Basin. Zircon U-Pb geochronology dating of these carbonatites yield multiple peak ages, including a youngest one at 272±4Ma obtained by three zircons, and other ones at 400Ma, 450Ma, 790Ma, 850Ma, respectively. However, all these zircons were probably captured by carbonate magma. Considering the intrusive events of massive Early Permian diabase sills and dikes in the region, we speculate that these carbonatite dikes were probably formed in the Early Permian (~270Ma). The carbonatites are mainly composed of calcite (over 90%) and a small amount of dolomite, barite, celestite, hematite and other minerals. In addition, the carbonatites have obvious exsolution structures between calcite and dolomite, and the country rock breccias are found wrapped by the carbonatites in the field. The element geochemistry analysis results show that carbonatites have high CaO content (44.40%~50.40%), low SiO₂ content (1.83%~7.97%), extremely low total rare earth elements content (21.67×10^-6~91.32×10^-6) with obvious differentiation of LREE and HREE (LREE/HREE=3.90~8.40) and weak negative Eu and Ce anomalies (δEu=0.61~0.68; δCe=0.59~0.98). And the large ion lithophile elements like Sr, Rb, Ba are relatively enriched, and high field-strength elements such as Nb, Ta, Ti are relatively depleted. The stable isotope of δ¹³C_V-PDB and δ¹⁸O_V-SMOW are high (-2.69~-2.93‰ and 17.33~17.86‰, respectively). All these characteristics are similar to the crustal carbonatite, but quite different from the mantle carbonatite. The Sr-Nd isotopic results of high (⁸⁷Sr/⁸⁶Sr)_i values (0.710106~0.710558) and low ε_Nd(t) values (-8.46~-12.80) also confirm that the Aksu carbonatites are derived from the crust. After comprehensive analysis, we propose that these carbonatites are probably the components of the late magmatic event in the Permian Tarim Large Igneous Province. When the mantle plume in the northwestern margin of Tarim was decompressed, it would form high-temperature basic magma and emplaced upward. These basic magma would heated the carbonate reservoir in the lower crust and partially melted it to form carbonatites magma, which was rapidly emplaced upward along the extension fault. This study not only enriched the petrological compositions of the Permian Tarim Large Igneous Province, but also promoted the understanding of the complex crust-mantle interaction processes in the deep area of Permian Tarim Basin.

The Character and Petrogenetic Model of the Permian Tarim Large Igneous Province

AbstractTarim Large Igneous Province (TLIP) is the second Late Paleozoic LIPs in China after the recognition of the Emeishan LIP. The residual distribution range of TLIP is up to 250000 km2, and the largest residual thickness is 780 m. The eruption of basalt happened during 290–288 Ma and belongs to LIPs magmatic event with fast eruption of magma. The lithological units of the TLIP include basalt, diabase, layered intrusive rock, breccia pipe mica‐olivine pyroxenite, olivine pyroxenite, gabbro, ultramafic dyke, quartz syenite, quartz syenite porphyry and bimodal dyke. The basalt and diabase of TLIP exhibit OIB‐like trace element patterns and enrichment of LILE and HFSE, and mainly belong to high TiO2 series. There is an obvious difference in isotope among the basalt from Keping and the basalt and dibase from the northern Tarim Basin. The basalt from Keping with negative Nd and high REE value derives from enriched mantle, and the diabase and basalt from the northern Tarim Basin with positive Nd and low REE value are related to depleted mantle. The crust uplifting in the Early Permian and the development of picrite and large scale dyke and formation of large scale V‐Ti‐magnetite deposit in Wajilitag area support the view that the TLIP is related to mantle plume. The TLIP has a temporal‐spatial relationship with Permian basic‐ultrabasic igneous rock, which is distributed widely in Central Asia, and they represent a tectono‐magmatic event with very important geodynamic setting. This study also systematically demonstrates the two‐stage melting model for the TLIP based on our previous research work and predecessor achievements, and highlights the two types of magmatic rocks within the TLIP. The two‐stage melting model suggests that the formation of the TLIP is mantle plume related. The early hot mantle plume caused the low‐degree partial melting of the lithosphere mantle, while in the later stage, the plume partially melted due to adiabatic uplift and decompression. Therefore, this model carries signatures of both the “Parana” and “Deccan” models in terms of mantle plume activity. During the early stage, the mantle plume provided the heat required for partial melting of sub‐continental lithosphere mantle (SCLM), similar to the “Parana Model”, while later the plume acted as the main avenue for melting, as in the “Deccan Model”. Basalts that erupted in the first stage have higher 87Sr/86Sr, lower 143Nd/144Nd ratios, and are enriched in large ion lithophile elements and high field strength elements, indicating a possible origin from the enriched continental lithosphere mantle, similar to the Parana type geochemical features. The basic‐ultrabasic intrusive rocks in the second stage exhibit lower 87Sr/86Sr, higher 143Nd/144Nd ratios relative to the basalts, consistent with the involvement of a more depleted asthenospheric material, such as a mantle plume, similar to the Deccan type geochemical features. The first stage basalts can be further subdivided into two categories. Developing this petrogenetic model for the TLIP aids in comprehensively understanding its magmatism and deep geological and geodynamic processes. Furthermore, this work enriches the theories describing the origin of large igneous province and mantle plume activity.

Petrogenesis of the Piqiang mafic-ultramafic layered intrusion and associated Fe-Ti-V oxide deposit in Tarim Large Igneous Province, NW China

ABSTRACTThe Piqiang intrusion is one of the two important mafic-ultramafic layered intrusions that host giant Fe-Ti-V oxide deposits in the Permian Tarim Large Igneous Province, NW China. The intrusion mainly consists of gabbro, anorthosite and minor plagioclase-bearing clinopyroxenite in the marginal zone. Disseminated to massive Fe-Ti oxide ores occur as layers and lenses within the gabbro. SHRIMP zircon U-Pb results from both a gabbro from the Piqiang intrusion and a granite from the surrounding granitic dyke yield ages of ~270 Ma. Geochemically, the Piqiang silicate rocks are enriched in light rare earth elements (LREE) and large ion lithophile elements (LILE), moderately depleted in high field strength elements (HFSE), and have a limited range of Sr-Nd-Hf isotopic compositions. The similar mineralogy, mineral compositions, and trace element characteristics of the layered units suggest that all the rocks are co-magmatic. The parental magma is Fe-Ti-rich and is akin to the most primitive diabasic dyke which is associated with the Piqiang intrusion. Partial melting of the Tarim mantle plume with involvement of a subduction-metasomatized lithospheric mantle source best explains the geochemistry and petrogenesis of the parental magmas of the Piqiang intrusion. We propose that the lithospheric mantle source may have been metasomatized by subduction-related materials and the metasomatic enrichment of this source region which may be correlated with oceanic sediment recycling during southward subduction of the South Tianshan oceanic slab during the Early-Middle Paleozoic. Crystal settling and mechanical sorting is the predominant process responsible for the formation of the massive Fe-Ti oxide ores in the Piqiang intrusion. Central to ore formation is a combination of the protracted differentiation history of a Fe-Ti-enriched parental magma and the later addition of external H2O from the country rocks to the slowly cooling magma chamber.

Crustal melting above a mantle plume: Insights from the Permian Tarim Large Igneous Province, NW China

Low NbTa rhyolites from the Permian Tarim Large Igneous Province (LIP) represent the melting products of continental crust above a mantle plume. Zircon UPb dating reveals that the Kuchehe and Dawanqi rhyolites were erupted at 292.6 ± 0.5 Ma and 274 ± 2 Ma respectively. Both rhyolite groups are characterized by pronounced negative NbTa anomalies, crust-like trace element ratios and negative εNd-Hf(t) values, consistent with a crustal origin. Newly acquired data, combined with available data in the literature, clearly show that there is an increasing trend of zircon saturation temperature (TZr) from 794 to 827 °C to 886–936 °C for the early to middle stage (293–287 Ma), followed by a decreasing TZr of 803–908 °C for the late stage (287–274 Ma) of the Tarim magmatism. The temporal changes in TZr for the Tarim rhyolites are interpreted to reflect the crustal melting behavior above a mantle plume, such that fusible materials in the crust melt at an earlier stage and refractory materials melt at a later stage. The gradual decrease in TZr, towards the final stage of the Tarim rhyolitic magmatism, reflects the gradual ‘decay’ of thermal input from mantle plume to the lithosphere in the region. This thermal model is also applicable to describe the crustal melting behavior in the Chon Aike silicic LIP of Patagonia and the Snake River Plateau-Yellowstone Plateau volcanic provinces.

Compositions of olivine from the Wajilitag mafic-ultramafic intrusion of the Permian Tarim Large Igneous Province, NW China: Insights into recycled pyroxenite in a peridotite mantle source

The Tarim large igneous province (TLIP) comprises a complex suite of lithologies including voluminous ferrobasalts, although the petrogenetic relation of these rocks is still poorly understood. In this study, we report the chemical compositions of olivine in two types of olivine pyroxenites (porphyritic and coarse-grained) from the Wajilitag mafic-ultramafic intrusion in the northwestern TLIP, aiming to characterize the mantle sources of magmas. Olivine in the porphyritic olivine pyroxenites shows low 100 * Mn/Fe (∼1.3) and Mn/Zn (<13) ratios, whereas that in the coarse-grained olivine pyroxenites is characterized by markedly higher ratios of 100 * Mn/Fe (>1.9) and Mn/Zn (>14), suggesting a heterogeneous mantle source beneath the Wajilitag area. The low olivine V/Sc values (<1.5) in the two types of olivine pyroxenites reflect high oxygen fugacity conditions during magma crystallization. The maximum temperatures of crystallization of porphyritic olivine pyroxenites and olivine pyroxenites estimated based on the Al-in-olivine thermometer are 1350 °C and 1190 °C, respectively. A comparison of the results from our study in terms of mantle source and oxygen fugacity suggests less potential for metallogeny in this region, as against the Panxi area in the Emeishan Large Igneous Province.

Petrogenetic model of the Permian Tarim Large Igneous Province

Over the last two decades great strides have been made in characterizing the spatial distribution, time sequence, geochemical characteristics, mantle sources, and magma evolution processes for various igneous rocks in the Early Permian Tarim Large Igneous Province (TLIP). This work has laid a solid foundation for revealing the evolutionary processes and genetic models of large igneous provinces (LIPs). This study systematically demonstrates the two-stage melting model for the TLIP based on our previous research work and predecessor achievements, and highlights the two types of magmatic rocks within the TLIP. The two-stage melting model suggests that the formation of the TLIP is mantle plume related. The early hot mantle plume caused the low-degree partial melting of the lithosphere mantle, while in the later stage, the plume partially melted due to adiabatic uplift and decompression. Therefore, this model carries signatures of both the “Parana” and “Deccan” models in terms of mantle plume activity. During the early stage, the mantle plume provided the heat required for partial melting of sub-continental lithosphere mantle (SCLM), similar to the “Parana Model”, while later the plume acted as the main avenue for melting, as in the “Deccan Model”. Basalts that erupted in the first stage have higher 87Sr/86Sr, lower 143Nd/144Nd ratios, and are enriched in large ion lithophile elements and high field strength elements, indicating a possible origin from the enriched continental lithosphere mantle, similar to the Parana type geochemical features. The basic-ultrabasic intrusive rocks in the second stage exhibit lower 87Sr/86Sr, higher 143Nd/144Nd ratios relative to the basalts, consistent with the involvement of a more depleted asthenospheric material, such as a mantle plume, similar to the Deccan type geochemical features. The first stage basalts can be further subdivided into two categories, i.e., Group 1 and Group 2 basalts. Group 2 basalts have lower 87Sr/86Sr and higher 143Nd/144Nd ratios than Group 1 basalts, and lie between compositions of the Group 1 basalts and second stage magmatism. Group 2 basalts may be the intermediate component of the TLIP, and the whole TLIP is the result of plume and lithosphere interaction. Developing this petrogenetic model for the TLIP aids in comprehensively understanding its magmatism and deep geological and geodynamic processes. Furthermore, this work enriches the theories describing the origin of large igneous province and mantle plume activity.

Paleomagnetic directional groups and paleointensity from the flood basalt in the Tarim large igneous province: implications for eruption frequency

We present paleomagnetic secular variation and paleointensity from the Early Permian Tarim large igneous province, NW China. The studied sections comprise a total of ~400 m of basaltic flows. Paleomagnetic directions were determined for 11 flows. Four successive flows with a cumulative thickness of ~150 m showed a statistically identical paleomagnetic direction. Assuming a paleosecular variation speed similar to that of the present day, the ~150-m-thick basalt was estimated to have erupted within the past few centuries. Paleointensity experiments were performed on both whole-rock and single plagioclase samples. Although alterations during the experiment and/or weak remanence degraded the data quality, the flows with the same paleomagnetic direction revealed similar paleointensity estimates, supporting the hypothesis that the eruption of these flows was rapid. More generally, flows from the Lower Kupukuziman Formation seem to record lower paleointensity compared to flows from the overlying Kaipaizileike Formation.Graphical abstractLeft the extent of the Tarim large igneous province. Center in-situ paleomagnetic directions obtained from the Tarim large igneous province. Subashi and Yingan represents directions reported in a previous research. Right tilt-corrected paleomagnetic directions. Tight clustering of the direction indicate fast eruption relative to the paleomagnetic secular variation (PSV) speed.

High water content in primitive continental flood basalts.

As the main constituent of large igneous provinces, the generation of continental flood basalts (CFB) that are characterized by huge eruption volume (>105 km3) within short time span (<1–3 Ma) is in principle caused by an abnormally high temperature, extended decompression, a certain amount of mafic source rocks (e.g., pyroxenite), or an elevated H2O content in the mantle source. These four factors are not mutually exclusive. There are growing evidences for high temperature, decompression and mafic source rocks, albeit with hot debate. However, there is currently no convincing evidence of high water content in the source of CFB. We retrieved the initial H2O content of the primitive CFB in the early Permian Tarim large igneous province (NW China), using the H2O content of ten early-formed clinopyroxene (cpx) crystals that recorded the composition of the primitive Tarim basaltic melts and the partition coefficient of H2O between cpx and basaltic melt. The arc-like H2O content (4.82 ± 1.00 wt.%) provides the first clear evidence that H2O plays an important role in the generation of CFB.

Origin of high-An plagioclase in the early Permian (~280 Ma) Xiaohaizi wehrlite, Northwest China: insights from melt inclusions in clinopyroxene macrocrysts and zircon oxygen isotopes

ABSTRACTThe Xiaohaizi wehrlite intrusion in the early Permian Tarim Large Igneous Province, Northwest China, is characterized by unusual high-An (up to 86) plagioclases. It has been suggested that H2O may have exerted a major control on their formation, but this interpretation requires further direct evidence. Moreover, it remains unclear where the water came from. In order to unravel these questions, we present electron microprobe analyses of minerals and melt inclusions in clinopyroxene macrocrysts in the dikes crosscutting the Xiaohaizi wehrlite intrusion and in situ oxygen isotope data of zircons from the Xiaohaizi wehrlite. The homogenized melt inclusions have restricted SiO2 (45.5–48.7 wt.%) and Na2O + K2O (2.4–3.8 wt.%) contents, displaying sub-alkaline affinity. This is inconsistent with the alkaline characteristic of the parental magma of the clinopyroxenes, suggesting significant modification of melt inclusions by contamination of the host clinopyroxene due to overheating. Nevertheless, the Ca/Na ratios (2.9–4.7) of melt inclusions are the upper limit of the parental magma of the clinopyroxenes due to high CaO (21.5–23.0 wt.%) and very low Na2O (0.22–0.34 wt.%) contents in the host clinopyroxenes. Thermodynamic calculation suggests that under fixed P (2.7 kbar) and T (1000°C), and assumed H2O (~1.5 wt.%) conditions, the Ca/Na ratio of the parental magma cannot generate high-An plagioclase in the wehrlite. The results confirm that H2O exerts a major control. Zircon δ18O (VSMOW) values (2.99–3.71‰) are significantly lower than that of mantle-derived zircon (5.3 ± 0.6‰). Such low zircon δ18O values may be due to incorporation of large amounts of low-δ18O, hydrothermally altered oceanic crust. However, geochemical and Sr-Nd-Pb isotopic data do not support recycled oceanic crust in the mantle source of the Xiaohaizi intrusion. Alternatively this can be explained by incorporation of meteoritic water in the magma chamber. This will increase the H2O content of the liquid that finally crystallize high-An plagioclases.

Petrology and Sr–Nd Isotopic Disequilibrium of the Xiaohaizi Intrusion, NW China: Genesis of Layered Intrusions in the Tarim Large Igneous Province

Layered mafic intrusions (LMI) are sporadically distributed in the Early Permian Tarim large igneous province (LIP), NW China, and are crosscut by numerous contemporaneous dykes. The Xiaohaizi wehrlite intrusion is composed mainly of olivine (Fo69–75), clinopyroxene (Mg# = 75–84), intercumulus plagioclase (An53–86) and Fe–Ti oxides. Both petrography and mineral compositions suggest that olivine and clinopyroxene crystallized earlier than plagioclase and Fe–Ti oxides. The dykes are of alkali basalt to trachyandesite with low Mg# (35–39). The least-contaminated dykes display strong rare earth element (REE) fractionation, enrichment of Nb and Ta, and depletion of Pb relative to other similarly incompatible elements, bearing strong similarity to ocean island basalts (OIB). This, together with their positive εNdi values (4·3–4·8), is consistent with derivation from an enriched asthenospheric mantle source. Clinopyroxenes in the wehrlites display convex-upward chondrite-normalized REE patterns. The melts in equilibrium with these clinopyroxenes have very similar trace element compositions to those of the crosscutting dykes, suggesting a similar mantle source shared by the Xiaohaizi wehrlite intrusion and dykes. The Xiaohaizi wehrlite intrusion is characterized by Sr–Nd isotopic disequilibrium between clinopyroxene and plagioclase separates: 87Sr/86Sri (0·7038–0·7041) and εNdi (1·0–1·9) of clinopyroxene are lower and higher than the respective ratios of intercumulus plagioclase (87Sr/86Sri = 0·7042–0·7043, εNdi = 0·4–1·0). The 87Sr/86Sri and εNdi of clinopyroxene separates correlate positively and negatively with Zr/Nb, respectively, implying variable degrees of crustal contamination during the formation of the Xiaohaizi wehrlite intrusion. 87Sr/86Sri increases and εNdi decreases with increasing Ca content of plagioclase, indicating that higher An plagioclases experienced higher degrees of contamination. This can be explained by assimilation of continental crust through a turbulent magma ascent (ATA) process. However, this ATA model fails to account for the positive correlation between the Mg# and εNdi of clinopyroxene separates. The isotopic disequilibrium in the Xiaohaizi LMI is more probably generated during an assimilation and fractional crystallization process involving Archean–Neoproterozoic basement and carbonates as contaminants.

Peperites in the Permian Tarim large igneous province in Northwest China and their constraints on the local eruption environments

Peperites are special kinds of volcaniclastic materials generated by mingling of magma and unconsolidated sediments. They directly demonstrate the contemporaneity of volcanism and sedimentation, and hence they can be used to constrain the local paleoenvironments during volcanic eruptions. We identified peperites in the lower sequence of the northwest outcrops (Inggan-Kalpin area) of Permian Tarim large igneous province (TLIP), Northwest China. In Inggan, blocky peperites were observed at the base of lava flows generated in the second eruption phase. This kind of peperites is generated by quenching of magma in a brittle fragmentation mechanism. While in Kalpin, both the second and the fourth eruption phases preserved peperites in the base of lava flows. Not only blocky but also fluidal peperites can be observed in Kalpin. The fluidal peperites were generated in vapor films, which insulated the magmas from cold sediments and avoided direct thermal shock, and therefore kept the fluidal forms of magma. All of these peperites are hosted by submarine carbonates. In lava sequences generated in the same eruption phases but located in Kaipaizileike, ∼15 km east to Inggan, terrestrial flood basalts developed while peperites are absent, implying a paleoenvironmental transition between Kaipaizileike and Inggan-Kalpin area. Gathering information from observed peperites, TLIP lava flows, and the Lower Permian sedimentary strata, we precisely constrained the spatial distribution and temporal evolution of sedimentary facies of the early stage of TLIP. As a result, two marine transgressions were identified. The first transgression occurred contemporaneous with the second eruption phase. The transition from submarine to subaerial is located between Kaipaizileike and Inggan. The second transgression occurred contemporaneous with the forth eruption phase, and the transition from submarine to subaerial occurred between Inggan and Kalpin.

Zircon U–Pb dating, geochemistry and Sr–Nd–Pb–Hf isotopes of the Wajilitag alkali mafic dikes, and associated diorite and syenitic rocks: Implications for magmatic evolution of the Tarim large igneous province

Abstract The Early Permian Tarim large igneous province (Tarim LIP) consists mainly of basaltic lavas, mafic–ultramafic intrusions including dikes and, syenite bodies in the Tarim Basin, NW China. A major unit of the Tarim LIP, the Wajilitag intrusive complex, consists of olivine pyroxenite, clinopyroxenite and gabbro units (from bottom to top), diorite and syenite rocks occurred in the upper part of the complex and alkali mafic dikes intrude the clinopyroxenite phase. Here we report the zircon U–Pb age and Hf isotopes, geochemical characteristics and Sr–Nd–Pb isotopic data of the alkali mafic dikes, and diorite, aegirine–nepheline syenite and syenite porphyry units in the Wajilitag intrusive complex. Zircons from the diorite and alkali mafic rocks yield concordant crystallization ages of 275.2 ± 1.2 Ma and 281.4 ± 1.7 Ma, respectively. The diorite and syenitic rocks in Wajilitag area have a narrow range of SiO 2 contents (51.9–57.3 wt.%), and are enriched in total alkalis (Na 2 O + K 2 O = 8.3–14.3 wt.%), among which the aegirine–nepheline syenite and syenite porphyry have the geochemical affinity of A-type granites. The alkali mafic rocks and syenitic rocks have high Al 2 O 3 (19.4–21.1 wt.%), Zr, Hf, Ba contents, total rare earth element abundances and LREE/HREE ratios and low Mg# value, K, P and Ti contents. Diorites have lower Al 2 O 3 contents, total REE abundances and LREE/HREE ratios and higher Mg# values than the alkali mafic rocks and syenitic rocks. The diorites and syenitic rocks have low initial 87 Sr/ 86 Sr ratios (0.7034–0.7046), and high eNd( t ) values (0.1–4.1) and zircon eHf( t ) values (− 0.9–4.4). All the diorites and syenitic rocks show the 206 Pb/ 204 Pb ratios ranging of 18.0–19.5, 207 Pb/ 204 Pb of 15.4–15.6 and 208 Pb/ 204 Pb of 38.0–39.9. Sr–Nd isotopic ratios indicate a FOZO-like mantle source for the diorite and syenitic rocks, similar to that of the mafic–ultramafic rocks in the Wajilitag complex. In contrast, zircon Hf isotopes of basalt and syenite elsewhere in the Tarim LIP indicate a FOZO-like component may also contribute to Tarim LIP magmatism. Geochemical and Sr–Nd–Pb–Hf isotopic features reflect that diorites and syenitic rocks are probably derived from a FOZO-like mantle source, consistent with a plume mantle origin and then underwent crystal fractionation process.