Record of Permian–Early Triassic continental arc magmatism in the western margin of the Jiamusi Block,NE China: petrogenesis and implications for Paleo-Pacific subduction

International Journal of Earth Sciences - In this paper, we report zircon U–Pb ages, Hf isotopes and whole-rock geochemical data for the Permian to Early Triassic granitoids from the western...     

Geochronology and geochemistry of Late Triassic bimodal igneous rocks at the eastern margin of the Songnen–Zhangguangcai Range Massif,Northeast China: petrogenesis and tectonic implications

Abstract

New zircon laser ablation inductively coupled plasma mass spectrometry and secondary ion mass spectroscopy U–Pb ages, and Hf isotope and whole-rock geochemical data are reported for Mesozoic igneous rocks from the eastern margin of the Songnen–Zhangguangcai Range Massif, Northeast China, in order to document the petrogenesis of the igneous rocks and reconstruct the early Mesozoic tectonic setting of the region. Zircons from five representative igneous rocks are euhedral–subhedral and display oscillatory growth zoning or striped absorption in cathodoluminescence images, suggesting a magmatic origin. The dating results indicate that granite, gabbro, and rhyolite from the eastern Songnen–Zhangguangcai Range Massif formed during Late Triassic (204–211 Ma). The Late Triassic granitoids and rhyolites have an affinity to A-type granites or rhyolites. Their zircon ε_Hf(t) values and Hf two-stage model ages range from –3.8 to +3.8 and from 999 to 1485 Ma, respectively, indicating that their primary melts were derived from the partial melting of the Meso-Proterozoic crust. The geochemistry of coeval gabbros, which reflects primary magma composition, shows a significant large ion lithophile element (e.g. Ba and Sr) enrichment and high field strength element (i.e. Zr, Hf, Nb, Ta, and Ti) depletion. Based on zircon ε_Hf(t) values (–4.2 to +2.8) and Hf single-stage model ages (746–1031 Ma), we conclude that the mafic magma is the product of partial melting of lithospheric mantle that was metasomatically enriched by fluids derived from the subducted oceanic crust. The Late Triassic magmatism along the eastern margin of the Eurasian continent has bimodal magma compositions, indicating an extensional setting after the final closure of the Palaeo-Asian Ocean rather than being related to subduction of the Palaeo-Pacific Plate beneath the Eurasian continent. The occurrence of Late Triassic igneous rocks on the eastern side of the Mudanjiang Fault suggests that this fault does not represent the suture zone between the Songnen–Zhangguangcai Range and Jiamusi massifs.     

Geochemistry and geochronology of Upper Permian–Upper Triassic volcanic rocks in eastern Jilin Province,NE China: implications for the tectonic evolution of the Palaeo-Asian Ocean

We present zircon U–Pb dating, whole-rock geochemistry, and Sr–Nd isotope results for the Upper Permian–Upper Triassic volcanic rocks to constrain the timing of the final closure of the eastern segment of the Palaeo-Asian Ocean. The volcanic rocks were mainly collected from the Yanbian area in eastern Jilin Province, northeastern China. The zircon U–Pb dating results indicate that the samples can be classified as Upper Permian–Lower Triassic basalts (ca. 262–244 Ma) and Upper Triassic dacites (ca. 216 Ma). The whole-rock geochemical results indicate that the rocks predominately belong to the medium-K and high-K calc-alkaline series. The basalts are enriched in large ion lithophile elements (LILEs, e.g. Ba and K) and depleted in high field strength elements (HFSEs, e.g. Nb and Ta), with weak positive Eu anomalies. The dacites are enriched in LILEs (e.g. Rb, Ba, Th, and K) and light rare earth elements (LREEs) and marked depletion in some HFSEs (e.g. Nb, Ta, and Ti), with significant negative Sr, P, and Eu anomalies. Moreover, the Upper Permian–Lower Triassic basalts have low initial ⁸⁷Sr/⁸⁶Sr ratios (0.7037–0.7048) and high ε_Nd values (4.4–5.4). In contrast, the Upper Triassic dacites possess relatively high initial ⁸⁷Sr/⁸⁶Sr ratios (0.7052) compared with their low ε_Nd values (1.4). The basaltic magma likely originated from the partial melting of a depleted mantle wedge metasomatized by subduction-related fluids, and the felsic magmas likely originated from the partial melting of a dominantly juvenile source with a minor component of ancient crust. Taken together, the Upper Permian–Lower Triassic basalts (ca. 262–244 Ma) are arc basalts that formed in an active continental margin setting, and the Upper Triassic dacites (ca. 216 Ma) are A-type granitic rocks that formed in an extensional setting. Therefore, the final closure of the Palaeo-Asian Ocean occurred during the Middle–Late Triassic.     

Late Paleocene–early Eocene granitoids in the Jiamusi Massif,NE China: Zircon U–Pb ages,geochemistry, and tectonic implications

Northeast (NE) China is characterized by large areas of Mesozoic and Paleozoic granitoids, whereas Cenozoic granitoids are scarce. This paper reports LA-ICP-MS zircon U–Pb ages and whole-rock geochemical data for late Paleocene–early Eocene granitoids from the Jiamusi Massif, NE China, in order to determine their petrogenesis and tectonic implications. Geochronological data indicate that the granodiorite and dioritic porphyry from the Wudingshan pluton formed at 51.5 ± 0.3 Ma and 56.3 ± 0.8 Ma, respectively. The biotite–quartz diorite, biotite granodiorite, and dioritic porphyry have high SiO₂ (68.38–70.06 wt.%), Al₂O₃ (15.34–15.79 wt.%), and Na₂O (3.96–4.49 wt.%) contents, low MgO contents (1.10–1.26 wt.%), A/CNK ratios of 0.99–1.11, and are classified as medium- to high-K calc-alkaline and weakly peraluminous I-type granitoids. They are enriched in LREEs and LILEs, and depleted in HFSEs, with Nb/Ta ratios of 10.4–15.0. Moreover, they have negative Nb–Ta–Ti anomalies, indicating that they were derived from continental crust. Combining with the previously published isotopic data and regional geological results, we suggest that the late Paleocene–early Eocene granitoids (56–52 Ma) were probably derived from partial melting of juvenile lower crust, and formed in an active continental margin setting, possibly related to subduction slab rollback of the Paleo-Pacific Plate.     

Permian basaltic rocks in the Tarim basin,NW China: Implications for plume–lithosphere interaction

There are large areas of Permian basaltic rocks in the Tarim basin (PBRT) in northwestern China. Precise Ar–Ar dating of these rocks revealed an eruption age span of 262 to 285 Ma. Most of the PBRT is composed of alkaline basaltic rocks with high TiO₂ (2.43%–4.59%, weight percent), high Fe₂O₃ + FeO (12.63%–17.83%) and P₂O₅ (0.32%–1.38%) contents. Trace elements of these rocks have affinities with oceanic island basalts (OIB), as shown in chondrite normalized rare earth elements (REE) diagrams and primitive mantle normalized incompatible elements diagrams. The rocks show complex Sr–Nd isotopic character based on which they can be subdivided into two distinct groups: group 1 has relatively small initial (t = 280 Ma)⁸⁷Sr/⁸⁶Sr ratio (∼ 0.7048) and positive εNd(t) (3.42–4.66) values. Group 2 has relatively large initial ⁸⁷Sr/⁸⁶Sr ratio (0.7060–0.7083) and negative εNd(t) (from − 2.79 to − 2.16) values. Lead isotopes are even more complex with variations of (²⁰⁶Pb/²⁰⁴Pb)t, (²⁰⁷Pb/²⁰⁴Pb)t and (²⁰⁸Pb/²⁰⁴Pb)t ranging from 17.9265 to 18.5778, 15.4789 to 15.6067 and 37.2922 to 38.1437, respectively. Moreover, these two groups have different trace elements ratios such as Nb/La, Ba/Nb, Zr/Nb, Nb/Ta and Zr/Hf, implying different magmatic processes. Based on the geochemistry of basaltic rocks and an evaluation of the tectonics, deformation, and the compositions of crust and lithospheric mantle in Tarim, we conclude that these basaltic rocks resulted from plume–lithosphere interaction. Permian mantle plume caused an upwelling of the Tarim lithosphere leading to melting of the asthenospheric mantle by decompression. The magma ascended rapidly to the base of lower crust, where different degrees of assimilation of OIB-like materials and fractionation occurred. Group 1 rocks formed where the upwelling is most pronounced and the assimilation was negligible. In other places, different degrees of assimilation and fractionation account for the geochemical traits of group 2.     

Granitoids of the Tyrma–Bureya complex in the northern Bureya–Jiamusi superterrane of the Central Asian fold belt: age and geodynamic setting

According to U–Pb dating, the granitoids of the Tyrma–Bureya complex in the northern Bureya–Jiamusi superterrane of the Central Asian fold belt are not of Paleozoic, as previously thought, but of Mesozoic age (Nizhnyaya Stoiba massif, 218 ± 2 Ma; Talakan and Ust’-Dikan massifs, 185 ± 1 Ma). They formed at the early stages of collision between the North Asian and Sino-Korean cratons and the intervening Amur superterrane.     

Permian age of the Wudaogou Group in eastern Yanbian: detrital zircon U–Pb constraints on the closure of the Palaeo-Asian Ocean in Northeast China

The Wudaogou Group in eastern Yanbian, Northeast China, plays a key role in constraining the timing and eastward termination of the Solonker–Xra Moron River–Changchun Suture, where the Palaeo-Asian Ocean closed. The Wudaogou Group consists of schist, gneiss, amphibolite, metasedimentary, and metavolcanic rocks, all of which underwent greenschist- to epidote–amphibolite-facies regional metamorphism, with some hornfels resulting from contact metamorphism. To determine the age of deposition, the timing and grade of metamorphism, and the tectonic setting of the Wudaogou Group, we investigated the petrography and geochronology of the metamorphic rocks in this group. Zircons from the metasedimentary rocks of this group can be divided into metamorphic zircons and detrital zircons of magmatic origin. U–Pb ages of metamorphic zircons dated by LA-ICP-MS vary from 249 ± 4 to 266 ± 4 Ma, approximating the age of regional metamorphism in the eastern Yanbian area. Detrital zircons yield U–Pb ages ranging from 253 ± 5 to 818 ± 5 Ma, and indicate that the provenance of the Wudaogou Group experienced four tectonic–thermal events between 818 and 253 Ma: Neoproterozoic (ca. 818–580 Ma), Cambro–Ordovician (ca. 500–489 Ma), Devonian–Carboniferous (ca. 422–300 Ma), and middle–late Permian (ca. 269–253 Ma). The youngest detrital zircon, with a U–Pb age of 253 ± 5 Ma, defines the maximum depositional age of the Wudaogou Group. The presence of the Cambro-Ordovician and Neoproterozoic detrital zircons implies that the source of the Wudaogou Group had an affinity with Northeast China, which leads us to conclude that the Solonker–Xra Moron River–Changchun Suture extends from Wangqing to Hunchun in eastern Yanbian, and that the Palaeo-Asian Ocean may have closed at the end of the Permian or Early Triassic period.     

Geochemistry and Sr–Nd–Hf isotopes of Middle Devonian igneous rocks of the Sarsuk polymetallic Au deposit: implications for understanding the tectonic evolution of the south Altay Orogenic Belt,Northwest China

The medium-tonnage Sarsuk polymetallic Au deposit is located in the Devonian volcanic–sedimentary Ashele Basin of the south Altay Orogenic Belt (AOB), Northwest China. Within the deposit, the rhyolite porphyries and diabases are widespread, emplaced into strata. The orebodies are hosted by the rhyolite porphyries. We studied the petrography, geochemistry, and Sr–Nd–Hf isotopes of the rhyolite porphyries and diabases, in order to understand the petrogenesis of these rocks and their tectonic significance. They display typical bimodality in geochemistry compositions. The diabases are characterized by SiO₂ contents of 44.84–59.77 wt.%, high Mg# values (43–69), enrichment in large ion lithophile elements (LILE) and light rare earth elements (LREE), depletion in Nb and Ta, low (⁸⁷Sr/⁸⁶Sr)_i (0.706687–0.707613) values, positive ε_Nd(t) (4.8–6.8) values, and positive and high ε_Hf(t) (7.15–15.19) values, suggesting a depleted lithosphere mantle source that might have been metasomatized by subduction-related components. The rhyolite porphyries show affinity to sanukitoid magmas contents [high SiO₂ (78.6–81.82 wt.%) and MgO (3.38–5.94 wt.%, one sample at 0.61 wt.%), and enrichments in LILE and LREE], they were derived from the equilibrium reactions between a mantle source and subducted oceanic crust materials. Those characteristics together with the positive ε_Nd(t) (4.1–8.4) and ε_Hf(t) (2.88–15.17) values indicate that the diabases and rhyolite porphyries were generated from the same mantle peridotite source. But the rhyolite porphyries underwent fractional crystallization of Fe–Ti oxides, plagioclase, and apatite due to their negative Eu (δEu = 0.21–0.28) and P anomalies. According to the geochemical and isotopic data, the Sarsuk Middle Devonian igneous rocks are considered to be the products of the juvenile crustal growth in an island arc setting. The Sarsuk polymetallic Au deposit formed slightly later than the Ashele Cu–Zn deposit in the Ashele Basin, but they have the same tectonic setting, belonging to the trench–arc–basin system during extensional process in the south AOB.     

Implications of subduction and subduction zone migration of the Paleo-Pacific Plate beneath eastern North China,based on distribution,geochronology, and geochemistry of Late Mesozoic volcanic rocks

Several major volcanic zones are distributed across the eastern North China Craton, from northwest to southeast: the Greater Xing’an Range, Jibei-Liaoxi, Xishan, and Songliao Basins, and the Yanji, Huanghua, and Ludong volcanic zones. The Huanghua depression within the Bohai Bay Basin was filled by middle Late Mesozoic volcanic rocks and abundant Cenozoic alkaline basalts. Zircon LA-ICP-MS and SHRIMP U–Pb dating show that basic–intermediate volcanic rocks were extruded in the Early Cretaceous of 118.8 ± 1.0 Ma (weighted mean ²⁰⁶Pb/²³⁸U age), before Late Cretaceous acid lavas at 71.5 ± 2.6 Ma. An inherited zircon from andesite has a Paleoprotoerozoic core crystallization age of 2,424 ± 22 Ma (²⁰⁶Pb/²⁰⁷Pb age) indicating that the basement of the Bohai Bay Basin is part of the North China Craton. Early Cretaceous basic and intermediate lavas are characterized by strong enrichments in LREE and LILE and depletions in HREE and HFSE, indicating a volcanic arc origin related to oceanic subduction. Depletion in Zr only occurs in basic and intermediate volcanic rocks, while depletions in Sr and Ti exist only in acid samples, indicating that the acid series is not genetically related to the basic–intermediate series. Formation ages and geochemical features indicate that the Late Cretaceous acid lavas are products of crustal remelting in an extensional regime. Combined information from all these volcanic zones shows that subduction-related volcanic rocks were generated in the Jibei-Liaoxi and Xishan volcanic zones during the Early Jurassic, about 60 Ma earlier than their analogues extruded in the Huanghua and Ludong volcanic zones during the Early Cretaceous. This younging trend also exists in the youngest extension-related volcanism in each of these zones: Early Cretaceous asthenosphere-derived alkaline basalts in the northwest and Late Cretaceous in the southeast. A tectonic model of northwestward subduction and continuous oceanward retreat of the Paleo-Pacific Plate is proposed to explain the migration pattern of both arc-related and post-subduction extension-related volcanic rocks. As the subduction zone continuously migrated, active continental margin and backarc regimes successively played their roles in different parts of North China during the Late Mesozoic (J₁–K₂).     

Ni–Cu–(PGE) magmatic sulfide deposits in the Yangliuping area,Permian Emeishan igneous province,SW China

The Ni–Cu–PGE sulfide deposits in the Yangliuping area, SW China, are hosted in mafic–ultramafic sills. The four mineralized sills are located in the Yangliuping tectonic dome and intrude Devonian carbonaceous marble, graphitic schist. The sills are 200–300 m thick and 1,000–2,000 m in strike length and now consist chiefly of serpentinite, talc schist, tremolite schist, and meta-gabbro. Disseminated Ni–Cu sulfide mineralisation occurs in the serpentinite in the lower parts of the sills. Massive sulfide mineralisation is located in the base of the sills and in the footwall along fractures beneath the mineralized serpentinite. Although the sulfide ores have been modified by hydrothermal activity, there are relict cumulate textures in the disseminated sulfides indicating a magmatic origin for the ores. The Yangliuping Intrusions and the Dashibao Formation have similar primitive-mantle normalized trace element and platinum group element (PGE) patterns, indicating that they are derived from a common parental magma type. The positive correlation between Cu concentrations and Cu/Zr ratios of the Dashibao Formation basalts indicates that the chalcophile elements were removed before eruption. We propose that fractional crystallization of the Yangliuping magma accompanied by the introduction of S and CO₂ from the wall rocks caused the magma to become S-saturated leading to the segregation of magmatic sulfides that became enriched in Ni–Cu–(PGE). The sills acted as conduits for the overlying Dashibao Formation basalts with the sulfide liquid, along with early crystallizing olivine and pyroxene, segregating from the magma as it passed through the conduits prior to eruption.Editorial handling: H.E. Frimmel     

Paleoproterozoic (2.0–1.97 Ga) subduction-related magmatism on the north–central margin of the Yeongnam Massif,Korean Peninsula,and its tectonic implications for reconstruction of the Columbia supercontinent

The Muju area, located on the north–central margin of the Yeongnam Massif, mainly consists of Precambrian orthogneisses (granitic, leucogranitic, augen and dioritic gneisses) with minor migmatite. Zircon U–Pb dating indicates that the protoliths of the orthogneisses intruded at ca. 2.00–1.97 Ga and were metamorphosed at ca. 1.87–1.86 Ga. Magmatic zircon grains within the orthogneisses have positive to negative εHf(t) values (−7.63 to +3.3) and a Neoarchean two-stage model age (T_DM2 = 2.78 Ga), indicating that the protoliths of most of the orthogneisses may have been derived from Archean crustal material. The results of geochemical analysis indicate that the protoliths of the orthogneisses formed by partial melting of metagraywacke and mafic igneous rocks in an arc-related tectonic setting. The intrusion ages and geochemical data of the Paleoproterozoic orthogneisses in the study area match well with those of Paleoproterozoic (ca. 2.00–1.97 Ga) orthogneisses in the northeastern Yeongnam Massif, indicating the presence of regional Paleoproterozoic subduction zones along the northern margin of the Yeongnam Massif at ca. 2.00–1.97 Ga. Meanwhile, ca. 2.00–1.97 Ga subduction-related magmatism has not been reported from the northern Gyeonggi and Nangrim Massifs in the Korean Peninsula or the Jiao–Liao–Ji belt in the eastern North China Craton, indicating that the Yeongnam Massif may not be correlatable with the northern Gyeonggi and Nangrim Massifs or the Jiao–Liao–Ji belt. The Yeongnam Massif may be correlated with the Cathaysia Block in the South China Craton and may have been located near Laurentia and the Siberian Craton within the Columbia supercontinent.     

Geochemistry,geochronology, and Sr–Nd isotopic compositions of Permian volcanic rocks in the northern margin of the North China Block: implications for the tectonic setting of the southeastern Central Asian Orogenic Belt

International Journal of Earth Sciences - The southeastern part of the Central Asian Orogenic Belt (CAOB), which records the collision of the North China Block (NCB) with the South Mongolian...     

Early Permian Age of Nepheline Syenites of the Korgere–Daba Massif (Sangilen Highlands,Tuva)

Doklady Earth Sciences - This paper reports on geochronological U–Pb studies of baddeleyite from nepheline syenite of the Korgere-Daba alkaline massif, which is the largest massif within the...     

Early Cretaceous continental arc-related volcanic rocks in the Duobuzha area,northern Tibet: implications for evolution history of the Bangong–Nujiang Ocean

New whole-rock major and trace elements data, zircon laser ablation inductively coupled plasma mass spectrometry (LA-ICP-MS) U–Pb ages, and zircon Hf isotope compositions were analysed for Early Cretaceous volcanic rocks, also called Meiriqieco Formation (MF) in the Duobuzha area of the Southern Qiangtang–Baoshan Block (SQBB), northern Tibet. Our aim is to clarify their petrogenesis and tectonic setting, and constrain the evolution process on the northern margin of Bangong–Nujiang suture zone (BNSZ) during Early Cretaceous time. The MF volcanic rocks are mainly composed of andesites with subordinate basalts and rhyolites with high-K calc-alkaline affinity. Zircon LA-ICP-MS U–Pb dating for two andesite and one rhyolite samples give uniform ages within error of ca.113, 114, and 118 Ma, respectively, indicating they were erupted on the Early Cretaceous. The MF andesites have variable zircon ε_Hf(t) values (+0.5 to +10.5), which is different from those of MF rhyolites (+7.9 to +10.7). All the MF rocks are enriched in large ion lithophile elements, and depleted in high field strength elements, yielding the affinity of arc rocks. The MF basalts were most likely derived from the mantle wedge that was metasomatized by fluids released from subducting slab with the involvement of subducted sediments. The MF rhyolites were generated by partial melting of the juvenile mafic lower crust. The MF andesites are interpreted to have formed by mixing of the magmas that parental of the MF basalts and the MF rhyolites. In addition, a couple of distinctly magmatic sources are identified in the SQBB, and this may be related to mantle components injected into the continental crust. Combined with published geological data in the BNSZ and SQBB, we consider that the MF volcanic rocks are formed in a continental arc setting, suggesting that BNO were subducting during the Early Cretaceous time in the Duobuzha area.     

Geochronological data of igneous and metamorphic rocks from the Xing’an-Mongolia Orogenic Belt of the eastern Central Asian Orogenic Belt: implications for the final closure of the Paleo-Asian Ocean

International Journal of Earth Sciences - In order to better constrain the evolution of the Xing’an-Mongolia Orogenic Belt and the resulting closure of the Paleo-Asian Ocean, we conducted an...     

Early Mesozoic southward subduction history of the Mongol–Okhotsk oceanic plate: Evidence from geochronology and geochemistry of Early Mesozoic intrusive rocks in the Erguna Massif,NE China

In this paper we present new zircon U–Pb ages, Hf isotope data, and whole-rock major and trace element data for Early Mesozoic intrusive rocks in the Erguna Massif of NE China, and we use these data to constrain the history of southward subduction of the Mongol–Okhotsk oceanic plate, and its influence on NE China as a whole. The zircon U–Pb dating indicates that Early Mesozoic magmatic activity in the Erguna Massif can be subdivided into four stages at ~ 246 Ma, ~ 225 Ma, ~ 205 Ma, and ~ 185 Ma. The ~ 246 Ma intrusive rocks comprise a suite of high-K calc-alkaline diorites, quartz diorites, granodiorites, monzogranites, and syenogranites, with I-type affinities. The ~ 225 Ma intrusive rocks consist of gabbro–diorites and granitoids, and they constitute a bimodal igneous association. The ~ 205 Ma intrusive rocks are dominated by calc-alkaline I-type granitoids that are accompanied by subordinate intermediate–mafic rocks. The ~ 185 Ma intrusive rocks are dominated by I-type granitoids, accompanied by minor amounts of A-types. These Early Mesozoic granitoids mainly originated by partial melting of a depleted and heterogeneous lower crust, whereas the coeval mafic rocks were probably derived from partial melting of a depleted mantle modified by subduction-related fluids. The rock associations and their geochemical features indicate that the ~ 246 Ma, ~ 205 Ma, and ~ 185 Ma intrusive rocks formed in an active continental margin setting related to the southward subduction of the Mongol–Okhotsk oceanic plate. The ~ 225 Ma bimodal igneous rock association formed within an extensional environment in a pause during the subduction process of the Mongol–Okhotsk oceanic plate. Every magmatic stage has its own corresponding set of porphyry deposits in the southeast of the Mongol–Okhotsk suture belt. Taking all this into account, we conclude the following: (1) during the Early Mesozoic, the Mongol–Okhotsk oceanic plate was subducted towards the south beneath the Erguna Massif, but with a pause in subduction at ~ 225 Ma; and (2) the southward subduction of the Mongol–Okhotsk oceanic plate not only caused the intense magmatic activity, but was also favorable to the formation of porphyry deposits.     

Geochemistry of sedimentary rocks from Permian–Triassic boundary sections of Tethys Himalaya: implications for paleo-weathering,provenance, and tectonic setting

The geochemical characteristics of two sections—the Permian–Triassic boundary (PTB) Guryul Ravine section, Kashmir Valley, Jammu and Kashmir, India; and the Attargoo section, Spiti Valley, Himachal Pradesh, India—have been studied in the context of provenance, paleo-weathering, and plate tectonic setting. These sections represent the siliciclastic sedimentary sequence from the Tethys Himalaya. The PTB siliciclastic sedimentary sequence in these regions primarily consists of sandstones and shales with variable thickness. Present studied sandstones and shales of both sections had chemical index of alteration values between 65 and 74; such values reveal low-to-moderate degree of chemical weathering. The chemical index of weathering in studied samples ranged from 71 to 94, suggesting a minor K-metasomatism effect on these samples. Plagioclase index of alteration in studied sections ranged from 68 to 92, indicating a moderate degree of weathering of plagioclase feldspars. The provenance discriminant function diagram suggests that the detritus involved in the formation of present studied siliciclastic sedimentary rocks fall in quartzose sedimentary and felsic igneous provenances. These sediments were deposited in a passive continental margin plate tectonic setting according to their location on a Si₂O versus K₂O/Na₂O tectonic setting diagram.     

Origin of the early Permian Wajilitag igneous complex and associated Fe–Ti oxide mineralization in the Tarim large igneous province,NW China

The Wajilitag igneous complex is part of the early Permian Tarim large igneous province in NW China, and is composed of a layered mafic–ultramafic intrusion and associated syenitic plutons. In order to better constrain its origin, and the conditions of associated Fe–Ti oxide mineralization, we carried out an integrated study of mineralogical, geochemical and Sr–Nd–Hf isotopic analyses on selected samples. The Wajilitag igneous rocks have an OIB-like compositional affinity, similar to the coeval mafic dykes in the Bachu region. The layered intrusion consists of olivine clinopyroxenite, coarse-grained clinopyroxenite, fine-grained clinopyroxenite and gabbro from the base upwards. Fe–Ti oxide ores are mainly hosted in fine-grained clinopyroxenite. Forsterite contents in olivines from the olivine clinopyroxenite range from 71 to 76 mol%, indicating crystallization from an evolved magma. Reconstructed composition of the parental magma of the layered intrusion is Fe–Ti-rich, similar to that of the Bachu mafic dykes. Syenite and quartz syenite plutons have ε_Nd(t) values ranging from +1.4 to +2.9, identical to that for the layered intrusion. They may have formed by differentiation of underplated magmas at depth and subsequent fractional crystallization. Magnetites enclosed in olivines and clinopyroxenes have Cr₂O₃ contents higher than those interstitial to silicates in the layered intrusion. This suggests that the Cr-rich magnetite is an early crystallized phase, whereas interstitial magnetite may have accumulated from evolved Fe–Ti-rich melts that percolated through a crystal mush. Low V content in Cr-poor magnetite (<6600 ppm) is consistent with an estimate of oxygen fugacity of FMQ + 1.1 to FMQ + 3.5. We propose that accumulation of Fe–Ti oxides during the late stage of magmatic differentiation may have followed crystallization of Fe–Ti-melt under high fO₂ and a volatile-rich condition.