Hornblende Gabbro Research Articles

Petrogenesis and tectonic implications of Seih Syn-tectonic gabbroic intrusion, South Sinia, Egypt: Insights from whole-rock geochemistry, mineral chemistry and P-T estimate

The present work deals with the gabbroic intrusion in the Wadi Seih area as a part of the Neoproterozoic Pan-African basement in the southwestern part of Sinai, Egypt. It comprises as small plutons intruded into Seih metamorphic belt. The intrusions comprise hornblende gabbros, Anorthositic gabbros and sometimes occur as coarse-grained gabbros (appetites). These plutons belong to what is known throughout Egypt as Younger Gabbros. The present study includes petrography, whole-rock geochemistry, mineral chemistry, and geothermobarometry. Geochemically, the gabbroic intrusions are derived from tholeiitic magma with minor calc-alkaline affinity. They have a chemical signature of subduction-related arc magmatism formed at an active convergent plate margin by 15– 30% of partial melting of a garnet lherzolite and to a minor extent of spinel-garnet lherzolite sources, modified by fluids related to a subducting slab. Mineral chemistry indicates that the gabbroic rocks crystallize at a pressure between 2.9 and 4.4 Kb and temperature between 590 °C to 700°C.

Triassic magmatism and tectonic setting of the eastern Tianshan, NW China: Constraints from the Weiya intrusive complex

The Weiya intrusive complex, located in the eastern part of the middle Tianshan terrane can provide crucial constraints on the tectonic evolution of the eastern Tianshan during the Triassic period. Here, we perform detailed zircon U–Pb chronological and Hf–O isotopic as well as whole-rock geochemical and Sr–Nd isotopic analyses on the hornblende gabbro, syenogranite, and granodiorite of the Weiya intrusive complex. The hornblende gabbro with a zircon U–Pb age of 241.6 ± 1.0 Ma is an alkaline gabbro and characterized by enrichments in large iron lithophile elements with depletions of Nb and Ta. It has uniform initial 87Sr/86Sr ratios (0.707173–0.707211) and positive εNd(t) values (+0.58 to +0.61) with zircon δ18O and εHf(t) values ranging from 6.0‰ to 7.6‰ and from +6.1 to +10.5, respectively. Geochemical modelling indicates that the mantle source of the hornblende gabbro was metasomatized by ~10% oceanic sediments associated with previous subduction events. The syenogranite (238.1 ± 2.0 Ma), emplaced nearly simultaneously with the hornblende gabbro, is an A-type granite with slightly metaluminous to peraluminous characteristics (A/CNK = 0.99–1.02), and it has high initial 87Sr/86Sr ratios (0.711740–0.713603) and negative εNd(t) values (−0.94 to −0.76). By contrast, the granodiorite (230.8 ± 0.9 Ma), which contains minor late Carboniferous dioritic enclaves (314.4 ± 3.3 Ma) belongs to the high-K calc-alkaline I-type granite with metaluminous characteristics (A/CNK = 0.95–0.98), and it has relatively low initial 87Sr/86Sr ratios (0.706731–0.707081) and negative εNd(t) values (−1.11 to −1.33). The δ18O and εHf(t) values of zircons from the syenogranite vary from 6.1‰ to 7.5‰ and from +2.8 to +5.3, respectively, while those of zircons from the granodiorite vary from 7.6‰ to 9.0‰ and from −8.1 to −3.4, respectively. These geochemical and isotopic data imply that the syenogranite formed by partial melting of Neoproterozoic calc-alkaline granitoids at high temperatures, whereas the granodiorite was derived from partial melts of (meta)-igneous rocks with additions of supracrustal sedimentary materials. Taking account into all available geological data, we suggest that the eastern Tianshan was in an intra-continental setting during the Triassic period, and the Weiya intrusive complex was generated related to lithospheric extension and thinning.

The Triassic Duobaoshan appinite-granite suite, NE China: Implications for a water-fluxed lithospheric mantle and an extensional setting related to the subduction of the Mongol-Okhotsk Ocean

The appinite suite of rocks offers a unique opportunity to reveal the properties of subarc lithospheric mantle, indicate essential crust-mantle interactions, and ascertain critical geodynamic settings. In this study, we present zircon U–Pb ages, Hf–O isotopes, and whole-rock geochemical data for the newly identified Duobaoshan appinite-granite suite in the eastern Central Asian Orogenic Belt (CAOB). Field and petrographic investigations show that this suite of rocks is composed of hornblende gabbro, quartz monzonite, and granite porphyry. The appinites are abnormally hornblende-rich, and contain spectacular hornblende phenocrysts, pegmatitic textures, and miarolitic cavity structures that indicate an anomalous water-rich origin. The quartz monzonite and granite porphyry show intrusive contact relations with the appinites. Secondary ion mass spectrometry (SIMS) zircon dating reveals that the Duobaoshan appinite-granite suite was emplaced in a very short time interval of 224–223 Ma. These samples show typical subduction related arc magmatic geochemical characteristics that are enriched in large-ion lithophile elements (LILEs) and light rare earth elements (REEs), depleted in high field strength elements (HFSEs) and middle and heavy REEs. All samples exhibit comparative total REEs and no significant evolutionary relationships. The appinites show high Sr/Y ratios (reaching ~100) and positive or zero Eu anomalies, which reveal amphibole fractionation promotion and plagioclase fractionation suppression in a water-rich magma system. The quartz monzonite possesses high Sr/Y and La/Yb ratios that exhibit adakite-like properties and were derived from thickened lower continental crust. The granite porphyry is silica-rich and non-adakitic. All samples have juvenile whole-rock initial 87Sr/86Sr ratios (~0.7038) and ɛNd(t) values. Zircons have depleted mantle-like ɛHf (t) (to +12.0) and δ18O (~ + 5.65‰) values. Combining these geochemical features with observations of the regional geology and tectonic setting, we propose that the Duobaoshan appinite-granite suite was generated under a Triassic back-arc extensional setting related to the southward subduction of the Mongol-Okhotsk Ocean. The appinites were derived from an unusual water-rich mantle source, which was previously metasomatized by slab-derived fluids. The quartz monzonite and granite porphyry were generated from partial melting of the juvenile lower continental crust triggered by coeval appinitic magma underplating.

PETROLOGY OF THE CRUSTAL PLUTONIC ROCKS OF NAWEOBA BLOCK, ZHOB OPHIOLITE, BALOCHISTAN, PAKISTAN

Zhob Ophiolite complex is composed of three detached blocks named Omzha, Ali Khanzai and Naweoba blocks. The crustal plutonic section of the Naweoba block is mapped and divided into gabbro and granite. Based on petrographical studies, the gabbros fell in the domain of gabbro, gabbronorite, and hornblende gabbro while granitic rocks fell in the vicinity of quartz-rich granitic rocks, granodiorite, plagiogranite and tonalite. Gabbroic rocks cover the maximum area of the crustal plutonic section and are usually medium-grained while at many places the grain size is quite large to be seen with naked eyes. Minerlogically gabbroic rocks consist of orthopyroxene, clinopyroxene, amphibole and plagioclase. These rocks maybe the fragments of main crustal plutonic section of the Zhob ophiolite. The granitic rocks having mafic minerals dominted in the eastern portion, while the felsic minerals dominted ones are in the west. The eastern side of the granitic body is compact and massive compared to western portion which is quite altered and shattered. Granitic rocks are composed of plagioclase, alkali feldspar and quartz where rutile and Cr-spinel exist in trace amounts. The gabbros of Naweoba block may have formed in a magma chamber as a result of fractional crystallization. While the granites maybe a late magmatic differentiate from the same magma chamber. The close correlation of gabbroic and granitic rocks of Naweoba block with Muslim Bagh, Khanozai and Bela ophiolites suggests their formation in supra subduction zone setting.

Heavy magnesium isotopes in the Gangdese Magmatic Belt: Implications for magmatism in the Mesozoic subduction system of southern Tibet

The Gangdese magmatic belt of southern Tibet preserves important records of the Mesozoic Neo-Tethyan orogeny. Here we investigate a newly identified Late Cretaceous hornblende gabbro pluton using zircon U–Pb geochronology, whole-rock major and trace elements and Sr-Nd-Hf-O-Mg isotopes with a view to evaluate the magma source characteristics and fluid evolution. The geochronological data and Sr-Nd-Hf isotopes suggest that the Late Cretaceous gabbro crystallized in the Cenomanian stage of the Late Cretaceous (ca. 98 Ma), from magma generated by partial melting of the depleted mantle. We report the first set of Mg isotopic data for the Late Cretaceous and Late Triassic arc-type gabbroid rocks in the Gangdese magmatic belt, where both rocks show homogeneous heavy Mg isotopic compositions with δ26Mg values ranging from 0.01 to 0.12‰ and − 0.09 to 0.09‰, respectively. Their δ26Mg values are higher than the average mantle values (−0.25 ± 0.07‰), suggesting that 26Mg-rich fluids were involved in their mantle sources. Based on Sr-Nd-Hf-O isotopic data and published information on Mg isotopic reservoir, we argue that talc-rich serpentinite fluids played an important role in producing the heavy Mg isotopes in these arc-related intrusions. Integrating data from previous studies and this study, we propose that the northward subduction of the Neo-Tethys oceanic lithosphere and formation of arc-type magmas provide a viable model to account for the heavy Mg isotopic compositions through fluid-rock interaction within the subduction channel. This further indicates that the northward subduction of the Neo-Tethys oceanic lithosphere commenced in the Late Triassic or earlier rather than in the Early Cretaceous (ca. 145 Ma).

Petrogenesis and metallogenic potential of the early devonian Yugusayi mafic-ultramafic complex in Qimantagh, East Kunlun orogeic belt

ABSTRACT The Yugusayi mafic-ultramafic complex, located in the Qimantagh area, East Kunlun orogenic belt, consists of harzburgite, olivine orthopyroxenite, hornblende pyroxenite, and hornblende gabbro, which were once considered to be a part of the Early Palaeozoic ophiolite suite. However, the Yugusayi mafic-ultramafic rocks are characterized by thermal emplacement, accompanied by Cu-Ni sulphide mineralization, which is different from typical ophiolite suite. The LA-ICP-MS zircon U-Pb dating yields a 405 ± 2.8 Ma age for the hornblende gabbro, which belongs to the Early Devonian. All rocks are enriched in LREE but depleted in HREE, with a characteristic of ferrous mafic-ultramafic rocks. The εHf(t) values of the zircons from hornblende gabbro are 5.6–14.8, the εNd(t) values of whole rocks are from −4.27 to 0.08, and the δ34S values of sulphide are 1.36–8.72‰, indicating that the magma originated from the depleted mantle and contaminated by continental crust material during the ascending process. Based on the regional geological background, it is reasonable that the rocks formed in the post-collision extension stage after the closure of the Proto-Tethys Ocean and the continental collision, which represents an important metallogenic period for Cu-Ni sulphide deposits in the East Kunlun oregenic belt. Early Devonian magmatic mafic-ultramafic intrusions are developed in the early Palaeozoic ophiolitic tectonic melange belt of Qimantagh area. Combined with good metallogenic conditions and remarkable mineralization characteristics, it is believed that the Qimantagh area is a potential area for the prospecting of magmatic Cu-Ni sulphide deposits.

Mineralogical and geochemical constraints on the origin of the Sohland-Rožany Ni-Cu-(PGE) sulfide mineralization (Lausitz Block, Bohemian Massif, Germany/Czech Republic)

The Sohland-Rožany Ni-Cu-(PGE) sulfide mineralization is hosted by a small gabbroic dike that is one of several Ni-Cu sulfide-bearing gabbroic intrusions in the northern Bohemian Massif. The intrusions are associated with a Middle Devonian to Early Carboniferous mafic–ultramafic plumbing system that emplaced within Cadomian granodiorites of the Lausitz Block. The Sohland-Rožany dike comprises varitextured olivine- and hornblende-bearing gabbro and hornblende gabbro which originated from differently fractionated magma batches. Fractionation likely took place in deeper parts of the magmatic systems, e.g. within staging magma chambers. Mineralogical features like Ti-rich primary brown hornblende and diopsidic clinopyroxene as well as overall enriched incompatible element contents suggest moderate alkaline basaltic parental magmas with relatively high volatile contents.The magmatic sulfide mineralization is exclusively hosted by the olivine- and hornblende-bearing gabbro and include disseminated, net-textured and massive sulfides. The ore types are predominated by pyrrhotite-pentlandite-chalcopyrite assemblages accompanied by secondary violarite and pyrite as well as by accessory Pt, Pd, Au and Ag phases. The disseminated and net-textured sulfides have highly variable modal proportions of base-metal sulfides with pyrrhotite-dominated beside chalcopyrite-dominated aggregates on the hand specimen scale. The platinum-group mineral (PGM) assemblage includes Pd-Pt-Ni-bearing bismuthotellurides of the melonite–merenskyite and michenerite–testibiopalladite solid solution series as well as sperrylite. The sulfide ores feature predominantly elevated Ni and highly variable Cu and Pt, Pd and Au tenors (3.5–6.4 wt.% Ni, 0.1–16.6 wt.% Cu, 80–3080 ppb Pt, 50–5320 ppb Pd and 20–3770 ppb Au). The magmatic sulfides are locally characterized by a late-stage hydrothermal overprint where sulfides were replaced by hydrous secondary phases (e.g. actinolite–tremolite and chlorite). A significant redistribution of the PGE due to hydrothermal alteration is unlikely as the majority of PGM show a close spatial association to the base-metal sulfides.Textures and chemical compositions of the sulfides suggest that repeated magma batches eroded already fractionated sulfide pools and entrained early crystalized cumulates of monosulfide solid solution (MSS) together with variable fractionated sulfide melts. Sulfide saturation in the Sohland-Rožany magmas was locally controlled by assimilation of the granodioritic wall-rock in the course of thermomechanical erosion. This process was amplified by high mica contents of the granodiorites. Sulfur isotope signatures (–2.0 to –1.2‰ VCDT) and the S-poor nature of the hosting granodiorites exclude a significant contribution of S from the country rocks.

Early Cretaceous magmatism of the Langxian complex in the eastern Gangdese batholith, southern Tibet: Neo-Tethys ocean subduction re-initiation

新特提斯洋长期俯冲消减作用在早白垩世可能经历二次俯冲启动或板片俯冲几何形态的重大转换。确定西藏南部冈底斯岩基早白垩世岩浆作用的岩石地球化学特征和作用方式是甄别上述过程的关键，对理解新特提斯洋的俯冲演化过程至关重要。本文就冈底斯岩基东段朗县杂岩中保存的各类早白垩世岩浆岩，开展了锆石U-Pb地质年代学和Hf同位素、全岩元素和同位素（Sr-Nd）组成分析。数据结果表明：1）基性岩侵位时代为早白垩世晚期（103.6~100.8Ma），为高钾钙碱性偏铝质岩石，锆石ε_Hf（t）=+0.3~+5.7，全岩ε_Nd（t）=-0.8和-0.3，暗示其岩浆源区具有大量俯冲沉积物或流体的混入，为沉积物熔体和流体交代的地幔楔物质部分熔融的产物，经历了一定程度的角闪石分离结晶作用；2）中性岩形成于99.8~97.6Ma，略晚于基性岩，其主量元素与基性岩具有较好的线性关系，全岩ε_Nd（t）=+1.1，具有较多的地幔物质参与，为基性岩浆进一步演化形成；3）酸性岩（脉体）记录了多阶段岩浆作用（124.1~95.3Ma），根据同位素组成不同进一步划分为两类，第一类具有较低的全岩ε_Nd（t）值（-8.3~-6.0），其岩浆源区显示富集特征，t_DM2=1385~1586Ma，由古老地壳物质的再熔融形成；第二类的锆石ε_Hf（t）值（-2.8~+3.2）变化较大，岩脉的锆石ε_Hf（t）=+0.4~+8.1，t_DM=428~906Ma，全岩ε_Nd（t）=+0.1和+0.8，表明岩浆源区具有不均一性，为古老地壳物质被富流体地幔岩浆改造形成；和4）镁铁质包体的主量元素与寄主花岗岩具有较好的线性关系，锆石的Hf同位素组成变化较大（ε_Hf（t）=-9.3~+4.1），变化范围可达13个ε单位，为岩浆混合成因。寄主花岗岩和角闪辉长岩分别作为酸性和基性端元，是基性岩浆与其诱发古老地壳熔融形成的花岗质岩浆经混合形成。结合冈底斯岩基早白垩世岩浆岩的研究结果，朗县杂岩在早白垩世（124~97Ma）的岩浆作用具有明显的岩浆混合现象，锆石Hf和全岩Sr-Nd同位素组成变化较大，可达13个ε单位，其岩浆源区复杂且富含流体，代表了新特提斯洋在早期（240~144Ma）经历漫长的俯冲之后，在早白垩世时期（~120Ma）俯冲带发生跃迁或俯冲角度达到临界点，导致大量俯冲沉积物和流体沿俯冲带俯冲下去，与发生部分熔融的地幔楔物质混合，底侵导致上覆古老地壳物质的再熔融，形成早白垩世复杂的岩浆岩组合，很可能是新特提斯洋二次俯冲开始的标志。;The subduction of the Neo-Tethyan oceanic lithosphere might experienced a major shift in the subdcution geometry in the Early Cretaceous time during its long-lasting subducion. Knowing the geochemical nature and the mode of the Early Cretaceous magmatism in the Gangdese batholith is critical to deccriminate the subdcution processes in this time. Data from zircon U-Pb geochronology and geochemical (whole-rock element and isotope, zircon Hf) analyses on various rock types preserved in the Langxian complex show that: 1) the mafic rocks formed at 103.6~100.8Ma are of high-K calc-alkaline aluminous in composition. They are characterized by relatively low zircon ε_Hf(t) (+0.3~+5.7) and bulk ε_Nd(t) (-0.8 and -0.3), indicating that they were drived from partial melting of depeled mantle wedge metasomatized by fluids from subducted sediment; 2) the intermediate rocks formed at 99.8~97.6Ma and have slightly higher Nd isotope compositions with ε_Nd(t)=+1.1. They display a good linear relationship in major oxides with those in the mafic ones, suggesting that they are derivative products from the mafic suite; 3) the felsic rock (pluton and dike) crystallized at 124.1~95.3Ma. They could be subdivided into two types based on their isotopic compositions. The first type has a lower ε_Nd(t) (-8.3~6.0) but higher t_DM2 (1385~1586Ma). They formed by remelting of ancient crustal materials. In contrast, ε_Hf(t) and ε_Nd(t) in the second type granitic rocks vary widely with ε_Hf(t) ranging from -2.8 to +3.2 for the plutonic granite and from +0.4 to +8.1 for the dike, respectively. Together with relatively low Nd isotope (ε_Nd(t)=+0.1 and +0.8) and t_DM(428~906Ma), these characteristics suggest a relative young source region due to modification of ancient crustal materials by melts derived from fluid-enriched mantle; 4)enclaves hosted by the granitic rocks show good linear relationship in major oxides with the host rocks. They display a rather large shift of ~13 epsilon units in zircon Hf isotope compositions (ε_Hf(t)=-9.3~+4.1) and might represent the melts formed by various degrees of mixing an acidic end member of the hosted granite wih a mafic end member of hornblende gabbro. Combined with literature data of the Gangdese batholith, magmatic rocks from the Langxian complex preserved a key record of a major magmatic process during the Early Cretaceous (122~97Ma). Melting and mixing of various source components (ancient crustal material, fluid-enriched mantle, and depleted mantle) is one of the important features of the Early Cretaceous magmatism due to subduction of the Neo-Tethyan ocean and resulted in large shifts (up to ~13 epsilon units) in zircon Hf as well as in bulk Sr-Nd isotope compositions. To introduce various components into and achieve a large isotopic heterogeneity in the source region, it requires a major change in the subduction geometry in the long subduction of the Neo-Tethyan subdction system. Since the initation of the subduction at ~240Ma, subdcition of the Neo-Tethyan Ocean might experinecd a critical reorganization enabling the introduction of an increased amount of sediments and fluids into the subduting system and the modification of the mantle wedge in the Early Cretaceous (~120Ma). The mafic magmas caused remelting of the overlying ancient crustal material and finally formed the complex magmatic activity in the Early Cretaceous, which is likely to be a sign of the beginning of the second subduction of the Neo-Tethyan Ocean.

Mineralogy of an Appinitic Hornblende Gabbro and Its Significance for the Evolution of Rising Calc-Alkaline Magmas

The magmatic and sub-solidus evolution of calcic amphiboles and Fe–Ti oxides was investigated in the Neoproterozoic Frog Lake pluton, Nova Scotia, Canada, in order to understand the relationship between the history of hydrous magma and the resulting mineralogy. The pluton occurs as sheet-like bodies of hornblende gabbro and hornblendite, with lesser tonalite dykes and granite bodies, interlayed with screens of medium-grade metamorphic country rock. Small, diffuse clots of felsic minerals are present in the gabbro. The subsolidus growth of actinolite occurs in early clinopyroxenes and amphiboles. Ilmenite is the dominant Fe–Ti oxide, as interstitial magmatic crystals. The increase of Mn towards the margin of the ilmenite crystals indicates a gradual increase in oxygen fugacity with time, leading to the precipitation of titanite and ferrohypersthene. The replacement of titanite by ilmenite and ilmenite lamellae in the amphiboles suggests subsequent reducing conditions during the sub-solidus crystallisation. The gabbros in the coeval, but apparently shallower, Jeffers Brook granodiorite laccolith have dominant magnetite and Mg-rich subsolidus amphiboles, which are indicative of high oxygen fugacity. The differences between the two plutons suggest that there was a greater flux of hydrothermal water through the sheet-like architecture of the Frog Lake pluton.

Neoproterozoic metasomatized mantle beneath the western Yangtze Block, South China: Evidence from whole-rock geochemistry and zircon U-Pb-Hf isotopes of mafic rocks

Mafic rocks contain important information about the nature of the mantle source and tectonic settings. Here we present results of whole-rock geochemistry and zircon U-Pb-Hf isotopes for the Early Neoproterozoic mafic rocks (Ganyuhe hornblende gabbros and Guandaoshan gabbro-diorites) in the western Yangtze Block, South China, in order to constrain the petrogenesis and the mantle metasomatism processes under subduction setting. LA-ICP-MS zircon U-Pb dating results display concordant crystallization ages of 857.4 ± 2.0 Ma for the hornblende gabbros and 856.4 ± 2.8 Ma for the gabbro-diorites, respectively. These mafic rocks possess low SiO2 contents (45.5–53.3 wt%) as well as high MgO contents (3.27–9.20 wt%) and Mg# (48.6–62.6) values, and show characteristics of arc-like trace element patterns with enriched Rb, Ba, Sr, K, and Pb but depleted Nb, Ta, Zr, Hf, and Ti. The prominently positive whole-rock εNd(t) (+4.9 to + 5.8) and zircon εHf(t) (+8.48 to + 20.6) values suggest a depleted lithospheric mantle source. We therefore infer that the mafic rocks investigated here were derived from an amphibole-bearing spinel peridotite source that was affected by metasomatism between the overlying mantle wedge and subduction-related fluids. In conjunction with the compiled data of Neoproterozoic mafic rocks from the western Yangtze Block, we propose that there exist Neoproterozoic metasomatized mantle sources beneath the western Yangtze Block during the subduction process in response to the evolution of the supercontinent Rodinia. The Neoproterozoic metasomatic agents in mantle sources include the slab fluids, sediment melts, and oceanic slab melts.

Final-Stage Magmatic Record of Paleo-Asian Oceanic Subduction? Insights from Late Permian to Early Triassic Intrusive Rocks in the Yanbian Area, Easternmost Central Asian Orogenic Belt

This paper reports new zircon LA–ICP–MS U–Pb and Hf isotope data, and whole-rock major and trace element data for Late Permian to Early Triassic intrusive rocks in the Yanbian area, NE China. These data provide new insights into the timing of the final subduction of the Paleo-Asian Ocean beneath the North China Craton. The zircon U–Pb age data indicate that a suite of Late Permian to Early Triassic intrusive rocks related to subduction is present within the Yanbian area. The Late Permian intrusive rocks consist of diorites while the Early Triassic granites and hornblende gabbros constitute a geochemically bimodal igneous rock association. Furthermore, the Early Triassic granites show the geochemical characteristics of shoshonitic rocks. All the rocks are characterized by enrichment in LILEs and LREEs, and depletion in HREEs and HFSEs, suggesting they formed in a subduction setting. Zircons from the Early Triassic gabbros have εHf(t) values and TDM2 ages of +7.6 to +10.7 and 735–1022 Ma, respectively, suggesting that they formed from a primary magma generated by the partial melting of lithospheric mantle material that had been previously modified by subduction-related fluids. The Late Permian diorites have εHf(t) values and TDM2 ages of +0.5 to +9.5 and 853 to 1669 Ma, respectively, while they have high contents of Al2O3, Fe2O3, and low contents of SiO2, Cr, and Ni, indicating Late Permian diorites should derive from the mantle and are influenced by some crustal material. Early Triassic granitic rocks have a wide range of εHf(t) values and TDM2 ages of −4.8 to +9.4 and 852 to 2136 Ma, respectively. Their zircons imply that the Early Triassic granites could be mainly derived from partial melting of the crust, with minor contribution of the crustal material of an ancient crust. The Early Triassic bimodal intrusive rocks in Yanbian area, combined with the regional geologic information; therefore, record a final post-subduction extensional environment due to the break-off of the previously subducted slab.

Emplacement of Young Island Arc Crust Over Older Mantle Along a Cratonic Foreland: Constraints From Multiple Isotopes and Elemental Geochemistry

AbstractMany studies have revealed that mantle‐derived mafic rocks can be more isotopically enriched than coexisting crustal felsic rocks. However, its origin remains to be elucidated. Here, we report on some Early Cretaceous (ca. 130 Ma) mafic rocks in Gan‐Hang Belt (South China) that are more isotopically enriched than the temporally and spatially associated granitoids. Relatively depleted Nd‐Hf isotopic signatures [εNd(t) = −0.4 to +0.6, εHf(t) = 3.4 to 4.8, εHf(t)zircon = −2.3 to +9.3], coupled with low zircon δ18OSMOW values (5.33–6.84 ‰) and geographical proximity to the exposed island arc rocks, indicate that the isotopically depleted Daixi alkali feldspar granites were dominantly derived from the relatively young Neoproterozoic accreted Shuangxiwu island arc sequences. Yet new and published whole‐rock Sr‐Nd‐Hf‐Pb‐O isotopic data [87Sr/86Sr)i = 0.70788–0.70833; εNd(t) = −7.6 to −3.5; εHf(t) = −6.8 to −1.9; (206Pb/204Pb)i = 18.028–18.108, (207Pb/204Pb)i = 15.565–15.577, (208Pb/204Pb)i = 38.198–38.341; δ18OSMOW = 4.5–5.5 ‰] highlight that the isotopically enriched Siling hornblende gabbros were predominantly sourced from an older, metasomatically enriched SCLM of adjacent Yangtze cratonic affinity. The arc crustal allochthon is assumed to have been tectonically emplaced onto the southeast Yangtze cratonic foreland, resulting in isotope inversion with younger crust on top of the older lithospheric mantle. Our results reveal a juxtaposition geometry of continental material after collision and accretion of island arcs by using isotope methods, which is independently supported by tectonic geological and geophysical studies. It suggests that ancient lithospheres should exist at depth within certain orogens associated with island arc accretion.

Petrogeochemical features of the Neogene collision volcanism of the Lesser Caucasus (Azerbaijan)

The article is devoted to the petrogeochemical features of Neogene collision volcanism in the central part of the Lesser Caucasus within Azerbaijan. The main goal of the study is to determine the thermodynamic conditions for the formation of Neogene volcanism in the central part of the Lesser Caucasus using the available petrogeochemical material. Using factor analysis, as well as the “IGPET”, “MINPET”, “Petrolog-3” programs, material balance calculations were performed that simulate the phenocryst fractionation process, the crystallization temperature, pressure, and figurative nature of the rock-forming minerals of the formation rocks were calculated. It was determined that at the early and middle stages of crystallization of the rocks of the andesite-dacite-rhyolite formation, the fractionation of amphibole played an important role in the formation of subsequent differentiates. Based on computer simulation, it was revealed that rocks of the andesite-dacite-rhyolite formation were formed by fractional crystallization of the initial high-alumina basaltic magma of high alkalinity in the intermediate magma foci. The calculations of the balance of the substance, simulating the process of fractionation of phenocrysts, as well as magnetite, confirmed the possibility of obtaining rock compositions from andesites to rhyolites as a result of this process. In this case, the process of crystallization differentiation was accompanied by processes of contamination, hybridism and mixing. Based on the geochemical features of rare and rare-earth elements, changes in their ratios, the nature of the mantle source and the type of fractionation process are determined. It was revealed that the enrichment of formation rocks by light rare earths, as well as by many incoherent elements, is associated with the evolution of enriched mantle material. Under high water pressure, as a result of the fractionation of olivine and pyroxene, high-alumina basalts are formed from primary high-magnesian magma, which can be considered parental magma. It was established that, in contrast to the elevated Transcaucasian zone in the more lowered East Caucasus, under conditions of increased fluid pressure and reduced temperature, the melt underwent fractional crystallization in the intermediate centers, being enriched with alkaline, large-ion lithophilic elements, light REEs, etc. This is evidenced by the presence of large crystals of feldspars, the contamination of these minerals by numerous crystals of biotite, magnetite, several generations of these minerals, zonality, as well as the presence of related “water” inclusions, such as hornblendites, hornblende gabbro, etc. The physicochemical conditions for the formation of Neogene volcanic rocks of the Lesser Caucasus are determined.

Transition from the lithospheric to asthenospheric mantle-derived magmatism in the Early Jurassic along eastern Bangong–Nujiang Suture, Tibet: Evidence for continental arc extension induced by slab rollback

Abstract The transition of the geochemical signature in mafic rocks along the eastern Bangong–Nujiang suture in Tibet contains important information about geodynamic processes in the upper mantle. This study recognized two episodes of Early Jurassic gabbros from the Kaqiong terrane, a microblock within the Bangong–Nujiang suture zone. Early gabbros (ca. 197–191 Ma) appear as lenses in the basement complex and were overprinted by amphibolite/granulite-facies metamorphism at ca. 180 Ma. Later undeformed hornblende gabbros (ca. 177–175 Ma) occur as dikes intruding into the basement complex. The early metagabbros are characterized by arc-like geochemical features and enriched Nd-Hf isotopic compositions (whole rock ∑Nd(t) = –0.7 to +0.3; zircon ∑Hf(t) = –5.7 to –2.2), which suggests formation by partial melting of an enriched lithospheric mantle source. In contrast, the later hornblende gabbros have depleted Nd-Hf isotopic compositions (whole rock ∑Nd(t) = +6.1 to +7.1; zircon ∑Hf(t) = +10.7 to +16.8) and normal mid–oceanic–ridge basalt (N–MORB)-type rare earth element (REE) features. They also show variable enrichments of fluid mobile elements (e.g., Rb, U, Pb), indicative of the input of slab-derived fluids in their mantle source. Thus, the hornblende gabbros were most likely originated from the asthenospheric mantle metasomatized by subducted oceanic slab-derived fluids. The transition in geochemical and isotopic compositions of these mantle-derived magmas reveals a long-lasting lithosphere extension and thinning along the southern margin of the Qiangtang terrane in the Early Jurassic. Combined with geological observations, we propose that this transition has resulted from the southward rollback of the subducting Bangong–Nujiang Tethyan oceanic slab. The slab rollback could have initiated the overriding plate extension and the asthenosphere upwelling. Wider implications of this study are that an onset of slab rollback could be an important trigger for the transition of magmatic geochemistry in subduction zones.

Re-Mapping the Neoproterozoic Basement Rocks of Um Gheig area, Central Eastern Desert, Egypt based on Remote Sensing, Petrography and Field Investigation

Landsat 8 (OLI) ETM+ satellite imagery data are evaluated forimproving and re-mapping the geologic map of Um Gheig area, CentralEastern Desert of Egypt. Different remote sensing techniques includingFCC, band ratio, PCA, supervised classification and spectralcharacteristic analysis were applied on satellite imagery to facilitate thediscrimination of the widely exposed Neoproterozoic basement rocks inthe study area. Best color composite 146 FCC, 762 FCC, band ratio 6/2,4/3, 7/3 and 7/6, 6/5, 4/2 in RGB, as well as PCA 213 and 314 in RGBwere selected and used for detailed mapping of the different lithologicalunits. Spectral characteristic analysis was used to discriminate the rockforming minerals through the different rock units in Umm Gheig area(e.g. amphibole b7/b5, serpentine b5/b7, Quartz b7/b6, muscovite b6/b4and PCA4). The rock units of Um Gheig area are divided four mainlithological units according to field and petrographical studies; ophioliticassociations, foliated metavolcanics, post-granitoids and hammamatsediments, as well as small parts of unmapple metavolcanics andmetgabbro. Ophiolitic associations are subdivided serpentinites,metagabbro, volcanoclastic metasediments and hormblende schist.Granitoids are subdivided El-Deilihimmi granite and Abu Shaddadgranite. Petrographically, serpentinites prevailing by antiguriteserpentinites, metagabrro includes quartz and hornblende gabbro, VCMcomprised different varieties range from biotite schist, biotitehornblende schist, garnet biotite hornblende schist, intercalations ofactionolite schist and highly mylonitic schist, foliated metavolcanicsrepresented by schistose metatuffs and mylonitic schist, metavolcanicsrepresented by meta-andesite and fine metatuff, El-Deilihimmi graniteincludes granodiorite and monzogranite while Abu shaddad granitecomprised alkali feldspar granite and syenogranite, hammamatsediments represented by pebbles of dacite. A new modified geologicalmap is produced for Um Gheig area based on the remote sensing data,field investigation and petrographical description.

Opening and closure history of the Mudanjiang Ocean in the eastern Central Asian Orogenic Belt: Geochronological and geochemical constraints from early Mesozoic intrusive rocks

The tectonic evolution of the ancient Mudanjiang Ocean within the Central Asian Orogenic Belt (CAOB), is strongly debated. The ocean played an important role in the amalgamation of the Songnen and Jiamusi massifs; however, the timings of its opening and closure have remained ambiguous until now. In this study, we analyzed early Mesozoic intrusive rocks from the eastern Songnen and western Jiamusi massifs in the eastern CAOB. The new zircon UPb ages, Hf isotope data, and whole-rock major and trace element data are used to reconstruct the tectonic evolution of the Mudanjiang Ocean. Zircon UPb dating indicates that early Mesozoic magmatism in the eastern Songnen Massif occurred in three stages: Early to Middle Triassic (ca. 250 Ma), Late Triassic (ca. 211 Ma), and Early Jurassic (ca. 190 Ma). The Triassic intrusive rocks typically consist of bimodal rock suites, which include gabbros, hornblende gabbros, and granitoids. The compositional information indicates an extensional environment that was probably related to the final closure of the Paleo-Asian Ocean. We integrated the results with observations from Triassic A-type granitoids and coeval sedimentary formations in the eastern Songnen Massif, as well as depositional ages of metasedimentary rocks from Heilongjiang Complex. We conclude that the opening of the Mudanjiang Ocean took place in the Early to Middle Triassic. The Early Jurassic intrusive rocks are bimodal and include olivine gabbros, hornblendites, hornblende gabbros, gabbro diorites, and granitoids. The bimodal rock suite indicates a back-arc style extensional environment. This setting formed in relation to westward subduction of the Paleo-Pacific plate beneath the Eurasia during the Early Jurassic. Following subduction, the closure of the Mudanjiang Ocean and subsequent amalgamation of the Songnen and Jiamusi massifs happened during the late Early Jurassic to Middle Jurassic. This sequence of events is further supported by ages of metamorphism and deformation acquired from the Heilongjiang Complex. Based on these observations, we conclude that the Mudanjiang Ocean existed between the Middle Triassic and Early Jurassic, making it rather short-lived.

Geological and petrographical studies around Um Taghir area, Сentral Eastern Desert, Egypt

Um Taghir area is located in the northern extreme boundary of Central Eastern Desert of Egypt at the west of Safaga City. Um Taghir is represented by island arc related rocks and late to post tectonic magmatism. The island arc related rocks are represented by metavolcaniclastic sequences and metagabrroic rocks. Metavolcanoclastic rocks are considered as the older rock units of the study are and intruded by the metagabbro. The late to post tectonic magmatism is represented by (dokhan volcanic, gabbro, tonalite-granodiorite, monzogranite, alkali feldspar granites and different types of dikes). Usually, the gabbroic rock is bearing ilmenite lenses or bands in the bottom of the layered; this is related to magma rich of iron oxides. Petrographically, island arc assemblage is classified in to actinolite hornblende schist and metagabbro that show quite different of their content in plagioclase, hornblende, augite, quartz and biotite. Occasionally, the late to post tectonic magmatism represented by andesite, gabbro, tonalite, granodiorite monzogranite, alkali feldspar granites and different types of dikes. Andesite consists of plagioclase, quartz, alkali feldspar and hornblende. Gabbroic rocks are represented by pyroxene hornblende gabbro and leucogabbro. They show quite different of their content in plagioclase, pyroxene and clear difference in the content of both olivine and hornblende in both of them. While tonalite and granodiorite show quite different of their content in plagioclase, quartz, hornblende, alkali felspar and biotite. On the other hand, monzogranite and alkali feldspar granite, they show plagioclase is varying from oligoclase to albite; K-feldspars, quartz and muscovite are relatively more abundant in the alkali feldspar granite. Finally, the different types of dikes classified into granite, andesite, rhyolite and basalt dikes consist of the different mineral compositions.

Tectonic evolution of the northeastern North China Craton: Constraints from geochronology and Sr–Nd–Hf–O isotopic data from Late Triassic intrusive rocks on Liaodong Peninsula, NE China

This paper reports new whole-rock major and trace element, SrNd isotopic, and zircon UPb and HfO isotopic data for Late Triassic intrusive rocks on Liaodong Peninsula, NE China, aiming to constrain the petrogenesis of these rocks in terms of lithospheric thinning in the northeastern North China Craton (NCC). New zircon UPb data indicate that the rocks were emplaced during the Late Triassic (220–211 Ma). Based on petrology, petrography and geochemistry, we divided these rocks into three groups. The Group 1 rocks, including hornblende gabbro, hornblende diorite and mafic vein, show low SiO2, Na2O + K2O and high MgO contents, with (87Sr/86Sr)i (0.7066–0.7071), εNd(t) (−10.6 to −5.7), εHf(t) (−16.6 to +1.5), and δ18O (7.06‰–7.82‰) values, indicating that they were formed by partial melting of lithospheric mantle with addition of lower continent crust materials. The Group 2 rocks, including syenites, display high SiO2, Na2O + K2O and low MgO contents, with (87Sr/86Sr)i (0.7064–0.7073), εNd(t) (−14.8 to −14.5), εHf(t) (−11.7 to −5.7), and δ18O (5.49‰–6.37‰) values, indicating they were originated through partial melting of lithospheric mantle with continent crust materials added, coupled by fractional crystallization of mafic minerals. The Group 3 rocks, including monzogranite and syenogranite, show high SiO2, Na2O + K2O and low MgO contents, with different isotopic signatures: the former shows (87Sr/86Sr)i (0.7085–0.7087), εNd(t) (−17.0 to −16.8), εHf(t) (−17.9 to −4.4) and δ18O (6.10‰–6.95‰) values. The latter shows higher εNd(t) (−9.0 to −8.1), εHf(t) (−3.4 to +1.2) and lower δ18O (5.52‰–7.29‰) values. Combined with the adakitic features of monzogranite and different two model ages, we believe they were derived from partial melting of ancient thickened lower crust and juvenile crust, respectively. Together with the spatial and temporal distribution of the Late Triassic rocks, we believe that these rocks were related to the collision between NCC and Yangtze Craton, and the initiation of lithospheric thinning of NCC in eastern segment began in Late Triassic.

Granitic magmas with enriched isotopic compositions, from enriched mantle source: Implications for continental generation

Granitoids with depleted isotopic compositions are generally considered beneficial to continental generation. However, most granitoids exhibit enriched isotopic characteristics (high initial Sr isotopic ratios, negative ƐNd and ƐHf values) and their role in continental generation is scantly explored. In this contribution, we report petrological and geochemical data of the Permian (ca. 270 Ma) Yabulai granites and hornblende gabbros in the Alxa Block of western North China Craton. Compared to initial mantle-derived basalts, the hornblende gabbros show low MgO (3.74 to 5.68 wt%), Cr (34.9 to 66.2 ppm), Ni (16.6 to 45.6 ppm) contents and Mg# (0.46 to 0.54), with zircon ƐHf values of +3.5 to +6.6 and −8.4 to −4.8. Relative to heavy rare earth elements (HREEs), the hornblende gabbros are enriched in light rare earth elements (LREEs) with the absence of Eu anomalies. In primitive mantle-normalized spidergrams, the hornblende gabbros exhibit notably negative NbTa and positive Pb anomalies. Elemental and isotopic data indicate that they might have originated from a shallow and heterogeneous lithospheric mantle source modified by subducted materials. The granites are metaluminous to weakly peraluminous and can be classified as high-K calc-alkaline I-type, with negative zircon ƐHf values of −8.3 to −5.3. These granitic rocks were likely to be derived from a newly underplated mafic crustal source. Using our data and published data from the granitic rocks in the Alxa Block, we suggest that a Permian crustal generation event has occurred in the Nuru-Langshan belt of the Alxa Block. Our current results reveal that the crustal generation rates at the convergent plate margins might have been underestimated due to the unintended omission of the mantle contribution to the granitoids with enriched isotopic compositions.

Late Devonian to early Carboniferous magmatism in the western Songliao–Xilinhot block, Northeast China: Implications for eastward subduction of the Nenjiang oceanic lithosphere

The timing and direction of narrowing and closure of the Nenjiang Ocean (NJO) by plate subduction remain poorly constrained. This paper presents geochemical, isotopic, and geochronological results for Late Devonian to early Carboniferous intrusive rocks in the western Songliao–Xilinhot block to constrain the evolution of subduction in the NJO region. The Late Devonian syenogranites and monzogranites formed at 366–360 Ma and are classified as A‐type granites. The wide range of εHf(t) values (−17.62 to +8.13) and TDM2 ages (789–2,208 Ma) of the syenogranites indicate that the primary magma was derived from partial melting of multiple sources, including both young accreted continental crust and old lower crust rocks. The early Carboniferous granitic mylonites formed at 342 Ma and are I‐type granites. Their high SiO2 and K2O contents, εHf(t) values of +6.61 to +9.86, and young TDM2 ages of 675 to 857 Ma suggest a preexisting juvenile crustal material source region. The early Carboniferous hornblende gabbros formed at 334 Ma and are part of an appinite suite. The gabbros have low silica and high MgO contents, E‐MORB‐like trace element features, εHf(t) values of +1.34 to +8.97, and young TDM1 ages of 595 to 902 Ma, which together suggest a primitive magmatic source including juvenile MORB and/or enriched lithospheric mantle. Magmatism in the western Songliao–Xilinhot block formed a late Palaeozoic magmatic arc stretching along the eastern Hegenshan–Heihe suture. A new model of the evolution of subduction in the NJO region and associated coeval magmatic events is proposed that includes westward subduction of NJO oceanic crust during the Late Devonian at 366–360 Ma, bidirectional subduction during the early Carboniferous at ~342 Ma, and break‐off/delamination of the eastward subducting slab at ~334 Ma.