Low TiO2 Contents Research Articles

Alaskan-type nature and PGE mineralization of the Wuxing mafic–ultramafic complex in eastern part of the Central Asian Orogenic belt

The Wuxing mafic–ultramafic complex is located in the eastern part of the Central Asian Orogenic Belt (CAOB). It consists of hornblende-olivine clinopyroxenite, hornblende clinopyroxenite, clinopyroxenite, gabbro and diorite. Hornblende occurs in all the rock types of the complex and is pargasite in composition. Clinopyroxene is diopsidic in composition with high CaO and low TiO2 contents. The flat REE and sub-parallel trace element patterns of both clinopyroxene and hornblende suggest that these rocks were formed by fractional crystallization from common magma with no significant crustal contamination. Enrichment in large ion lithophile elements relative to high field strength elements of the silicate minerals, together with the variable sulfur isotope compositions (δ34S = -2.55 ~ 11.01‰) of the sulfides, indicate the involvement of subducted oceanic materials into the mantle source. These petrological, mineralogical and geochemical features strongly suggest the Alaskan-type nature of the Wuxing complex. The hydrous parental magma of the Wuxing complex was derived from partial melting of the mantle wedge, which had been metasomatized by subducted melts/fluids.The Wuxing complex is a unique Pt-Pd-rich Ni-Cu sulfide deposit in the CAOB. The ore minerals, which are dominated by pyrite, pyrrhotite, pentlandite, chalcopyrite and platinum-group mineral (PGM), mainly occur in clinopyroxenite and hornblende clinopyroxenite. The sulfide ores are predominantly present as net-textured and disseminated textures. Pentlandite and chalcopyrite associated with pyrrhotite commonly occur in the marginal zone of the pyrrhotite grains. The PGMs occur as interstitial grains at the contact between base-metal sulfides and silicate minerals or along the crack of pyrrhotite or clinopyroxene, or as inclusions in pyrrhotite. The occurrences of sulfides and PGMs in the Wuxing complex, together with sulfur isotope compositions and PGE patterns, reveal that the PGE enrichment of the Alaskan-type complexes can be attributed to the high-degree partial melting of the mantle source, sulfur involvement in the source, and PGE transportation of hydrous melts.

Petrogenesis of the Middle Jurassic andesitic dikes in the giant Dexing porphyry copper ore field, South China: Implications for mineralization

The giant Dexing porphyry copper ore field, containing 9.7 Mt copper, is the largest porphyry copper system in South China. The petrogenesis of andesitic dikes in the Dexing ore field and their genetic link with porphyry mineralization remain unclear. This paper presents LA-ICP-MS zircon U-Pb dating, elemental and Sr-Nd-Hf isotopic data of the andesitic dikes in the Dexing porphyry copper ore field. The Dexing andesitic dikes were emplaced during the Middle Jurassic at ca. 166 Ma. The Dexing andesitic dikes have relative high MgO contents and Mg# ratios but low TiO2 contents. They also have obvious Nb, Ta and Ti depletion in the primitive mantle normalized spider diagrams. The geochemical affinity of arc magma and low Ti contents indicate that the magma source is probably lithosphere mantle modified by subduction process. A non-arc setting during the Middle Jurassic is inferred due to the absence of typical arc rocks and the presence of rifting-related magmatic rocks in the interior of South China. The non-arc setting implies that the Neoproterozoic Jiangnan Orogen was likely responsible for the arc geochemical features. The concentration of copper in the andesitic dikes is not high, supporting that remelting of the metal-rich Neoproterozoic juvenile lower continental crust instead of the mantle is the critical factor for the giant porphyry style mineralization. The mantle derived mafic magma may play a key role for providing significant heat for remelting of the metal-rich lower continental crust via underplating into the crust-mantle boundary.

Syn-kinematic ferroan high-K I-type granites from Dschang in southwestern Cameroon: U–Pb age, geochemistry and implications for crustal growth in the late Pan-African orogeny

Abstract The Dschang granites, SW Cameroon, are post-Panafrican granitoids, hosted in Paleoproterozoic orthogneiss in the Western domain of the Central African fold belt. They include coarse-grained K-feldspar biotite granite and fine-grained magnetite granite. Magnetite forms nodules surrounded by a quartzo-feldsparthic halo. Major and trace element data of both granites show high SiO 2 , Al 2 O 3 , total alkalis and Ba and low TiO 2 , MgO, P 2 O 5 and Nb contents, conferring a fractionated I-type high-K alkali-calcic and weakly peraluminous character. Both granites are enriched in light rare earth elements and large ion lithophile elements, depleted in high field strength elements, and show a rare earth element pattern with Eu/Eu* = 0.3 to 0.9. Primitive mantle-normalized spidergrams display negative Nb, Ta and Ti anomalies, typical of their derivation from subduction-related post-collisional melt. Laser inductively coupled plasma mass spectrometry U–Pb data on zircons yielded crystallization ages of 578 +7/−11 Ma and 563+8/−3 Ma for the biotite and magnetite granites respectively. Inherited zircon (1991–1442 Ma), whole-rock Nd model ages in the range 2.1–1.9 Ga, highly negative ε Ndt (−16 to −18) and initial 87 Sr/ 86 Sr ratios in the range 0.7075–0.7114, support the remobilization of a Paleoproterozoic crust. The felsic magmas were probably produced by partial melting of Paleoproterozoic orthogneiss host rocks, under oxidizing conditions. Magnetite granite is also known from the Nkambé region. Such granites from both the Dschang and Nkambé regions suggest the presence of a Paleoproterozoic domain rich in iron mineralization. This domain potentially represents a Cu, Pb, Zn and Mo metallogenic province, heavily reworked during the Pan-African orogeny.

Petrogenesis of Late Triassic mafic enclaves and host granodiorite in the Eastern Kunlun Orogenic Belt, China: Implications for the reworking of juvenile crust by delamination-induced asthenosphere upwelling

Late Triassic granitoids containing abundant mafic microgranular enclaves (MMEs) occur widely in the Eastern Kunlun Orogenic Belt (EKOB). In this work, we present mineral chemistry, zircon U-Pb ages and L-Hf isotopes, whole-rock chemistry and S-Nd isotope compositions of the MMEs and host granodiorite from the Huda pluton in the Elashan area within the easternmost domain of the EKOB. These rocks contain inherited (Meso- to Neoproterozoic) and xenocrystic (ca. 240 Ma) zircon grains that yield apparent older ages, whereas the magmatic zircons from MMEs and granodiorite yield similar weighted mean ages around 224 Ma, which are interpreted as their crystallization ages. The MMEs have low SiO2 but high TiO2, TFe2O3, CaO, MgO and MnO concentrations with relatively high Mg# values (48–54) and 100MnO/(MnO + MgO + TFe2O3) ratios (1.2–1.6). They display identical Sr-Nd-Hf isotope compositions to the host granite. Combined with petrological evidence, we suggest that the MMEs are cognate cumulates that formed by pressure quenching during the late stage of magma evolution from the same parental magma of the host granodiorite, rather than a magma mixing origin. The granodiorite is calc-alkaline to high-K calc-alkaline, metaluminous I-type granite. They show relatively low SiO2 and MnO, but high MgO, Al2O3, CaO and TFe2O3 contents with Mg# values of 45–50. They are enriched in light rare earth elements (LREEs) and large ion lithophile elements (LILEs), such as Rb, Th, K and Pb, and are depleted in P and high field strength elements (HFSE) including Nb, Ta and Ti. These rocks display slightly negative Eu anomalies and low Sr/Y and La/Yb ratios. Together with the rim-ward chemically evolved nature of some phenocrysts, the comparatively high initial Sr isotope (0.70888–0.70912), low whole-rock εNd(t) (−5.6 to −6.0) and zircon εHf(t) (−3.3 to −0.1) values, and low Nb/Th (0.11–0.26) and Ta/U (0.53–0.68) ratios, we suggest that the granodiorite magma was sourced from the lower crust. Considering their comparatively young two-stage Nd and Hf model ages (1.42–1.49 Ga and 1.13–1.42 Ga, respectively) and same trace element character with the juvenile crust beneath the EKOB, we interpret the juvenile lower crust as the dominant source rocks for the granodiorite. Based on our data and regional geological evidence, we suggest that the partial melting of juvenile crust resulted from delamination-related asthenosphere mantle upwelling. The latter process resulted in extensive melting of the lower crust, producing a major Late Triassic magmatic flare-up event in the EKOB.

Petrology of a Neoproterozoic mantle peridotite–chromitite association from Abu Dahr area, Eastern Desert, Egypt: Infiltration of a boninitic melt in highly depleted harzburgite

Peridotites of Abu Dahr represent the main litho–unit of a Neoproterozoic dismembered ophiolite sequence and are among the best–preserved and well–exposed mantle rocks in South Eastern Desert of Egypt. Here, we present new geochemical and mineral chemical data for peridotites and associated pyroxenites and for chromitites and their platinum–group minerals to constrain their petrogenesis and geotectonic setting. The Abu Dahr ophiolite mantle section consists mainly of harzburgites, cut by pyroxenite dykes and containing dunite–chromitite lenses. The harzburgites are composed of olivine, orthopyroxene, spinel and minor clinopyroxene (˂1.0 vol %) and amphibole. Olivine from harzburgites is highly magnesian (Fo 91–93) and Cr–spinel shows a wide–range of Cr2O3 and Al2O3 contents. The enstatite component of orthopyroxene decreases from harzburgite (En = 90–91) to orthopyroxenite (En = 84–87). Amphiboles are represented by magnesiohornblende and tschermakite. The chromitites are massive to disseminated and composed of magnesiochromite with high Cr# (83–93) and Mg# (66–79), and low TiO2 (<0.1 wt%) content. Solid inclusions in chromite include olivine, orthopyroxene and hornblende. Laurite (RuS2) is the most common PGM detected in the investigated chromitite samples and forms micrometer–size inclusions in fresh chromite. Various Ni–sulfides are found both in fresh chromite and along serpentine veinlets. Harzburgites have a refractory composition with a very low Al2O3 (0.4–0.8 wt%) and CaO (0.2–1.6 wt%) contents and high bulk–rock Mg# (89–92). Geochemical data suggest that the Abu Dahr peridotites are highly depleted SSZ peridotites formed in a forearc mantle wedge setting by high degrees of hydrous partial melting and emplaced as a result of the collision of the intra–oceanic arc with the Beitan gneisses. The podiform chromitites and orthopyroxenites were formed due to impregnation of mantle wedge harzburgites by boninitic melt. The highly depleted nature of the harzburgite is responsible for the small reserves of chromite ore at Abu Dahr and in the South Eastern Desert in general.

Generation of Jurassic high-Mg diorite and plagiogranite intrusions of the Asa area, Tibet: Products of intra-oceanic subduction of the Meso-Tethys Ocean

Intra-oceanic subduction is a fundamental process on Earth, the study of which can improve understanding of plate tectonic processes and the history of continental growth. Several studies of the intra-oceanic subduction of the Meso-Tethys Ocean have been published in recent years, particularly relating to the Bangong–Nujiang suture zone (BNSZ) and the Shiquanhe–Nam Tso mélange zone (SNMZ) in central Tibet. Here, we report newly discovered occurrences of Jurassic diorite (161.2–160.6 Ma) and plagiogranite (161.0 Ma) from the western part of the central SNMZ. The diorite samples have moderate SiO2 and high MgO contents and Mg# values; high Ba/Th and Ba/La ratios; low K2O, TiO2, and REE contents; low (La/Yb)N, (La/Sm)N, and Th/Yb ratios; and positive εHf(t) values, similar to high-Mg diorite formed by partial melting of a subduction-modified mantle wedge. The plagiogranite intrudes gabbro of the ophiolite suite and has high SiO2 and Na2O contents; low Al2O3, K2O, P2O5, TiO2, and total REE contents; and clear negative Eu anomalies, similar to shear-related oceanic plagiogranite, which can be formed by partial melting process of gabbro in the high temperature shear zone near the expansion center. The formation processes of Asa high-Mg diorite and shear-related plagiogranite are consistent with initial subduction within the Meso-Tethys ocean. Based on these data and those for sedimentary and Jurassic magmatic rocks of the region, we propose that, during subduction, the Northern Lhasa terrane was formed from a collage of oceanic island arcs, oceanic plateaus, and accretionary complexes.

Physico-chemical conditions of crystallization and composition of source magma of the Grenvillian post-collisional mafic–ultramafic rocks in the Chhotanagpur Gneissic Complex, Eastern India

In the Chhotanagpur Gneissic Complex (CGC) of Eastern India a suite of mafic and ultramafic rocks occurs as sills, dykes and enclaves within porphyritic granitoid pluton. These mafic and ultramafic rocks and host porphyritic granitoids were emplaced in a post-collisional setting around 998 ± 10 Ma ago. Field occurrence, petrology and mineral chemistry of the mafic–ultramafic rocks have been studied. Both the mafic (Pl + Hyp + Di + Hbl + Bt + Mag + Spn ± Ol ± Spl) and ultramafic rocks (Di + Hyp + Bt ± Hbl ± Ol ± Pl ± Spl ± Ep ± Spn) are composed of same minerals but in different modal proportions. Plagioclase, clinopyroxene, orthopyroxene, amphibole, biotite and rarely olivine and spinel are important primary minerals of mafic–ultramafic suite. Primary amphiboles, biotites and pyroxenes show their affinity with shoshonitic lamprophyres. Chemically these rocks are similar to the kentallenite (of appinite suite) and are enriched in both compatible (Fe, Mg, Ni, and Cr) and incompatible (K, Ba, Rb, and LREE) elements and show crust-like trace element patterns. Crystallization of clinopyroxene before labradorite and presence of primary hornblende and biotite suggest high water content while biotite–magnetite–sphene assemblage suggests high fO2 of the magma. Liquidus temperature (975–1088°C) of the parental magma of the mafic–ultramafic rocks was obtained by two-pyroxene thermometer. The pressure (2.9–5.7 kbar) and near-solidus temperature (782–819°C) of crystallization were determined using the amphibole–plagioclase geothermobarometry. Similar range of values of pressure, temperature and fO2 values were obtained using other thermobarometers. High H2O and fO2 (>NNO buffer) of the magma are characteristics of convergent setting. The mafic–ultramafic rocks of the suite probably crystallized from a magma which had high SiO2 (48.16–67.64 wt%), high CaO (3.01–11.73 wt%), high K2O (1.34–4.49 wt%) and low TiO2 (0.04–2.71 wt%) contents and intermediate Mg# (46.73 and 59.78).

Early Cretaceous Wulong intermediate-mafic dike swarms in the Liaodong Peninsula: Implications for rapid lithospheric delamination of the North China Craton

The North China Craton (NCC) underwent a dramatic change in the lithospheric architecture, and more than 100 km of the ancient and refractory lithospheric mantle was removed. However, the timing and mechanism of the lithospheric thinning remain controversial. New zircon UPb ages and whole-rock/in-situ geochemical and isotopic data of the Early Cretaceous intermediate-mafic dike swarms in the Liaodong Peninsula have been carried out to decipher the timing and mechanism of lithospheric thinning of the NCC. The dike swarms are chemically subdivided into the ca. 126 Ma diorite porphyry and coeval (ca. 119 Ma) low-Ti (TiO2 < 1.20 wt%, Ti/Y < 375) and high-Ti (TiO2 > 1.90 wt%, Ti/Y > 580) diabase dikes. The diorite porphyries exhibit a moderate silica (56.71–61.13 wt%) and high-K calc-alkalic signature, with low TiO2 and total Fe2O3 and high Cr and Ni compositions. They are enriched in large ion lithophile elements (LILEs) (e.g., Rb, K, Pb, and Sr) and depleted in high field strength elements (HFSEs) (e.g., Nb, Ta, Ti, and P). They have high radiogenic Sr isotopes (whole-rock (87Sr/86Sr)i = 0.711553–0.714284, in-situ (87Sr/86Sr)i of plagioclase = 0.71392), low εNd(t) values (−19.9 to −14.5), and old Nd model ages (1.81–2.25 Ga). Meanwhile, some plagioclase phenocrysts have reverse zonings with Na-rich cores (An: 35–48) and Ca-rich rims (An: 48–57). These features imply that the diorite porphyries were generated by mixing between magmas from the Archean–Paleoproterozoic lower crust and the lithospheric mantle. The low-Ti diabase dikes are characterized by medium to high-K calc-alkaline series with high Al2O3 but low TiO2 contents. They show arc-like trace element signatures, such as enrichment in LILEs (Rb, K, Pb, and Sr) and depletion in HFSEs (Nb, Ta, Zr, Hf, Ti, and P). They also have relatively lower initial 87Sr/86Sr ratios (0.706331–0.708315 and 0.70837 for the whole-rock and in-situ Sr isotopes, respectively) compared to the porphyries and negative εNd(t) values (−13.6 to −5.0), indicating that they were derived from partial melting of an enriched lithospheric mantle. Their variable Th/Yb and Th/Nb ratios with constant Sr/Nd and U/Th values further suggest that the mantle source might have been previously modified by the subduction-related melt. In contrast, the high-Ti diabase dikes belong to the high-K calc-alkaline to shoshonitic series, with higher TiO2 and total Fe2O3 but lower Al2O3 contents. They have OIB-like trace elements (e.g., slight enrichments of Nb and Ta with no HFSE depletion, high Nb/U values (42 ± 5)) and SrNd isotopic compositions (e.g., positive whole-rock εNd(t) values (+3.1 to +4.3)), suggesting that they were generated from an asthenospheric mantle source. The coexisting Wulong low-Ti and high-Ti mafic dikes with similar formation ages (ca. 119 Ma) may indicate a transition from lithospheric to asthenospheric mantle source. In combination with previous studies on the Mesozoic–Cenozoic mafic rocks in the Eastern Block of the NCC, we propose that the lithospheric thinning beneath the Liaodong Peninsula initiated at ca. 119 Ma and this process beneath the NCC lasted for a narrow duration (ca. 13 Ma). The unsteady mantle flows due to the rapid subduction and rollback of the Paleo-Pacific plate might trigger the thickened, modified lithosphere to lose its gravitational stability, eventually leading to lithospheric delamination and thinning of the NCC.

WEAR BEHAVIOR OF HYBRID COMPOSITE REINFORCED WITH TITANIUM DIOXIDE NANOPARTICLES

The aim of the present study is to investigate the influence of polymerization times, normal loads and titanium dioxide nanoparticles content on the wear of hybrid composite resin reinforced with titanium dioxide (TiO2) nanoparticle. The materials used are hybrid composite resin and titanium dioxide nanoparticles (TiO2nanoparticles). Four groups of composite resin samples are prepared; one as received and three groups reinforced with TiO2nanoparticles in contents of 0.1, 0.2 and 0.3 wt. %. Hybrid composites resin is manually mixed with TiO2 nanoparticles. The resulting blend is packed into plastic cylindrical bars. The samples are then cured from both sides for 10, 20, 40, 60 and 80 seconds using a visible light curing unit. To determine the wear, all samples are tested with two-body abrasion, reciprocating sliding apparatus at room temperature. The test conditions are; sliding velocity= 60 strokes/min, load= 6, 8, 10, 12 and 14 N, time= 60 seconds. Results show that the wear decreases with the increase of polymerization time at all normal loads. Results showed also that wear depends on the normal load. The wear initially decreased and reached a minimum at a concentration of 0.1 wt. % TiO2 nanoparticles, above 0.1 wt. % TiO2 nanoparticles, the aggregation of TiO2 nanoparticles led to increase of the wear. The present study suggests that in order to minimize wear, it is important to use relatively higher polymerization time, lower TiO2content and lower force of chewing.

Origin of the Triassic Lincang granites in the southeastern Tibetan Plateau: Crystallization from crystal mush

The crustal generation of the Paleo-Tethys orogenic belt in the southeastern Tibetan Plateau is a major unresolved geological problem. The Changning-Menglian suture in the southeastern Tibetan Plateau represents a remnant of the Paleo-Tethys Ocean. The Lincang batholith and associated volcanic rocks (the Manghuai rhyolites and Xiaodingxi basalts) are located to the east of the Changning-Menglian suture, and represent important constraints for the abovementioned geological problem. New zircon UPb ages, together with previously published results, indicate that the Manghuai rhyolites were emplaced at 229–239 Ma and the Lincang batholith yielded longer age span (211–239 Ma). Broad age span of zircons (to 28 million years) from these rocks record a long-lived crustal magmatic system in the Changning-Menglian Paleo-Tethys orogenic belt. These intrusive and associated volcanic rocks have significant differences between them in terms of geochemistry. In this study, these rocks are geochemically classified into three groups: (1) Group 1 comprises biotite granite and granodiorite rocks; (2) Group 2 includes alkali feldspar granite, two-mica granite, and rhyolite, which are high-silica (generally >70 wt%) rocks that have low TiO2, Al2O3, FeOtot, MgO, CaO, P2O5, Eu, and Sr contents, compared to those of Group 1. Based on their geochemical relationships and mineral textures, Groups 1 and 2 include cumulative- and fractionated-type granitoids, respectively; (3) while Group 3 is comprised of dioritic enclaves and the Xiaodingxi basalts. Based on more robust evidence, dioritic enclaves are derived from a mantle source magma. The Lincang granites have high and variable initial 87Sr/86Sr ratios (0.703122–0.728476), significantly negative εNd (t = 220 Ma) values (−13.4 to −7.5), negative εHf (t) values (−17.7 to −6.8), and old TDM2 ages. The Manghuai rhyolites and Xiaodingxi basalts have lower initial 87Sr/86Sr ratios (0.695970–0.709398), and higher εNd (t) values (−3.4 to −0.6) and εHf (t) values (−1.2 to +3.7) than those of the Lincang granites. Geochemical data suggest that the Manghuai rhyolites could be seen as fractionated and erupted melts from crystal mush reservoirs, which later crystallized to form the Lincang granites. The Manghuai rhyolites provide an instantaneous snapshot of the state of the crystal mush at the time of eruption. The Lincang granites are interpreted to record prolonged histories of open-system magmatic evolution, involving magma mixing, wall-rock assimilation, and multiple cycles of crystallization from the crystal mush reservoirs. Magmatic evolution was likely related to mantle upwelling in a post-collisional setting, forming large-volume granites and volcanics in the Changning-Menglian Paleo-Tethys orogenic belt. Thus, this crystal mush model provides an integrated picture of the Triassic silicic magmatism, linking the evolution of the Lincang batholith and Manghuai rhyolites before storage in the Changning-Menglian Paleo-Tethys orogenic belt.

Late Neoproterozoic mafic sills in the Suizhou area of the South Qinling block: constraints for the tectonic evolution of the northern margin of the Yangtze craton

ABSTRACT Late Neoproterozoic gabbroic-diabasic sills are widely distributed in the eastern part of the South Qinling block. Here, a comprehensive geochronological and geochemical study of these mafic sills has been conducted. LA-ICP-MS zircon U-Pb results indicate that the gabbroic-diabasic sills were formed at 632 ± 2 Ma. The sills consist mainly of tholeiitic basaltic rocks with relatively low TiO2 contents (0.81–1.07 wt.%) and high Fe2O3 T/MgO ratios. These rocks display relatively flat to moderate light rare earth elements (LREE) enrichment patterns and positive Eu anomalies (Eu/Eu* = 1.15–1.41). On the primitive mantle normalized spider diagram, the samples show E-MORB-like geochemical features. All of the studied samples display similar Sm-Nd isotopic compositions with negative εNd (t) values ranging from −6.2 to −5.5. Most clinopyroxenes from the mafic sills have relatively low Al/Ti ratios similar to those of typical rift-related cumulates. Therefore, Whole-rock and mineral geochemical features of the studied samples suggest that the mafic sills might have been derived from partial melting of a metasomatised subcontinental lithospheric mantle source in a continental rift setting. It can be advocated that the break-up of the Rodinia supercontinent in the northern part of the Yangtze craton might have been a long-lasting process. By comparison of the coherent Neoproterozoic magmatic events in the northern Yangtze craton and the Tarim block, it can be suggested that the Yangtze craton might have been adjacent to the Tarim block in the reconstruction model of the Rodinia supercontinent.

Suprasubduction-zone origin of the podiform chromitites of the Bir Tuluhah ophiolite, Saudi Arabia, during Neoproterozoic assembly of the Arabian Shield

The ultramafic section of a dismembered ophiolite is exposed at Bir Tuluhah, in the north-central part of the Arabian Shield. It is penetratively serpentinized and locally carbonate-altered to talc‑carbonate and quartz‑carbonate rocks (listvenite) along shear zones and fault planes. Despite the high degree of mineral replacement, preserved mesh and bastite textures and fresh relics of primary Cr-spinel and olivine show that the protoliths were mainly harzburgite with minor dunite, with sparse massive chromitite bodies of various forms and sizes. Olivine inclusions in the chromitite lenses have higher forsterite content and NiO concentrations than fresh olivine relics in the host harzburgites and dunites, due to subsolidus re-equilibration. Cr-spinels in the chromitites have higher Cr# (0.74–0.82) than those hosted in dunite (0.72–0.76) or harzburgite (0.55–0.66). The scarce Cr-spinel crystals in harzburgite that have Cr# < 0.6 are interpreted to represent the population least affected by melt-rock interaction. The chromitite bodies are interpreted to have formed just below the contact between the oceanic crust and mantle sections (i.e., the petrologic Moho). The primary olivine (high Fo and Ni content) and Cr-spinel core compositions (high Cr# and low TiO2 content) of the Bir Tuluhah serpentinized peridotite are typical of modern supra-subduction zone (SSZ) fore-arc peridotites and consistent with crystallization from boninitic magma. The multistage petrogenesis leading to the chromitite bodies begins with moderate to high degrees of melt extraction from the protoliths of the serpentinized harzburgites, followed by reaction with melt compositions that evolved from tholeiite to boninite and left dunite residues. The massive Cr-rich chromitites in the Bir Tuluhah ophiolite are most probably the residues of such interaction between depleted harzburgite and ascending melts; mixtures of the reacted melts formed boninites, which became saturated with chrome-rich spinel and crystallized chromite pods before ascending past the Moho. We offer a novel thermodynamic model of this mixing and reaction process that quantifies the yield of Cr-spinel.

Late Triassic back-arc spreading and initial opening of the Neo-Tethyan Ocean in the northern margin of Gondwana: Evidences from Late Triassic BABB-type basalts in the Tethyan Himalaya, Southern Tibet

The tectonic evolution of the Neo-Tethyan oceanic plate between the Indian Plate and the Lhasa Terrane is a key issue for the evolution of Gondwana and formation of the Tibetan Plateau. In this paper we report on the geochemistry, SrNd isotope compositions, and zircon UPb dating of Late Triassic basalts from the Nieru Formation (T3n) in the Kampa region, southern Tibet. The basalts have relatively low contents of MgO, TiO2, and total alkalis (K2O + Na2O), and they have affinities to tholeiitic basalt. They exhibit weakly fractionated rare earth element (REE) patterns with slight depletions in LREEs and slightly negative Eu anomalies on a chondrite-normalized diagram. On a primitive-mantle-normalized spider diagram, they are characterized by slight enrichments in large ion lithophile elements (LILEs) and relatively flat patterns of high field strength elements (HFSEs), except for depletions in Ba, Nb, Ta, and Ti. Their initial 87Sr/86Sr ratios vary from 0.718133 to 0.738977 and εNd(t) values are relatively depleted (4.09–5.22), similar to mid-ocean ridge basalts in the Yarlung Zangbo ophiolite. We propose the T3n basalts were derived from a shallow and depleted mantle source by relatively high-degree partial melting in the spinel stability field. They underwent slight crustal contamination as well as the fractional crystallization of clinopyroxene. The T3n basalts are similar to typical back-arc basin basalts (BABB) such as the Okinawa BABB. This indicates an extensional back-arc-basin setting along the northern margin of Gondwana during the Late Triassic. The initial opening of the Neo-Tethyan Ocean was related to the southwards subduction of the Paleo-Tethyan oceanic plate and back-arc-basin spreading during the Late Triassic. Magmatic activity along the passive continental margin records key information on continental break-up and incipient ocean development.

EPMA and LA-ICP-MS Data of Clinopyroxene and Chromite from the Wadi Ghadir Ophiolites, Eastern Desert, Egypt

New EPMA and LA-ICP-MS data of pyroxene and chromite are used to assess the so far published tectonic setting of Wadi Ghadir Ophiolite (WGO) in the Central Eastern Desert of Egypt. The low contents of TiO2 (< 0.46 wt%) and high MgO (13.39-15.17 wt%) concentrations in the analyzed clinopyroxene suggest a depleted residual mantle source material. The trace element composition and REE patterns reflect transitional IAT-boninitic geochemical affinities in a supra-subduction zone setting. Chromite, on the other hand, shows high Cr#(Cr/(Cr+Al)) (0.83-0.88 mol) and relatively constant Mg#(Mg/(Mg+Fe+2)) (0.66-0.70 mol), indicating a high degree of partial melting of the uPP.er mantle source. These features are typical for supra-subduction zone in a back-arc or inter‐arc basin setting. The micro-analytical data of relict magmatic mineral phases in the WGO and other ophiolitic rocks in the Arabian-Nubian Shield can help in interpreting their precise tectonic setting as far as data of the immobile, incompatible elements are used.

Mineral chemistry, geochemistry, Raman spectroscopy and geotectonic significance of Neoproterozoic ophiolitic peridotites and pyroxenites from Kab Amiri district, central Eastern Desert, Egypt

The small gold mining Kab Amiri district ultramafic ophiolitic section (central Eastern Desert) comprises highly serpentinized peridotites (serpentinites) and few pyroxenites. It lies within a region of Neoproterozoic ophiolitic rocks and associated derivatives. The chromian spinel textural and chemical compositional features suggest transitional greenschist-amphibolite up to lower amphibolite facies metamorphism and indicates that they represent highly-depleted mantle residues. The presence of the Si-Ob-Si antisymmetric ν1(A1), symmetric ν2(A1) bands and the hydroxyl group (OH) modes are conclusive for the discrimination of antigorite. Furthermore, the speculation of serpentines as antigorite can also be indicated from the hydroxyl group modes. The Kab Amiri ophiolite suite (central Eastern Desert) represents fragments of oceanic lithosphere that are developed in forearc setting in a supra-subduction zone (SSZ) environment. Thus the low TiO2 contents, the enrichment in LILE and LREE and the Nb depletion reflect the remelting of a highly depleted mantle source and suggest their formation in a SSZ environment (i.e forearc).

Origin of Devonian mafic magmatism in the East Kunlun orogenic belt, northern Tibetan Plateau: implications for continental exhumation

AbstractThis paper presents a comprehensive study of the zircon geochronology, geochemistry and Sr–Nd isotope geology of Devonian mafic rocks developed in the East Kunlun orogenic belt, northern Tibetan Plateau, and reveals their mantle sources, petrogenesis and geodynamic implications for continental exhumation. The zircon geochronology of typical samples indicates that these mafic rocks crystallized at 406∼408 Ma. They can be classified into two different groups based on petrographic observations and geochemical compositions. Group 1 rocks exhibit low TiO2 and FeOt contents and Nb/Y ratios and have enriched mid-ocean ridge basalt (E-MORB)-like compositions with slight negative Nb and Ta anomalies. However, Group 2 rocks have distinctly high TiO2 and FeOt contents and Nb/Y ratios, comparable to typical Fe–Ti-rich mafic rocks worldwide. All the samples exhibit weak enrichments in light rare earth elements, Nb and Ta relative to the primitive mantle. Based on geochemical and isotopic studies, Group 1 rocks are suggested to be derived from depleted asthenospheric mantle that was metasomatized by c. 3–5 % continental crustal components, while Group 2 rocks originated from partial melting of enriched lithospheric mantle. The high contents of Fe, Ti and Nb for Group 2 rocks could be attributed to a high degree of olivine crystallization under low fO2 conditions with delayed nucleation of Fe–Ti oxides. Combining those results with other geological data, we conclude that slab break-off was the key factor causing exhumation of eclogites and triggering flare-up of the Devonian magmatism, and that continental collision or continental subduction may have initiated at 431∼436 Ma.

Kaersutite lamprophyres of the Pokrovo-Kyriivo massif (the Azov area, Ukraine): petrographic features, mineral composition, geochemistry

The paper represents results of the petrographical, geochemical and mineralogical studies of a dyke of porphyry-like mafic rock from the Pokrovo-Kyriivo massif of the Azov area, the Ukrainian Shield. This dyke of fine-grained rock has unique mineral composition as the main mineral in the groundmass is kaersutite. According to mineralogical and petrographic features, it can be identified as kaersutite lamprophyre. The Devonian Pokrovo-Kyriivo massif is located at a junction of the Azov Domain of the Ukrainian Shield and the folded structure of Donbas. This massif is composed of subalkaline (pyroxenite, gabbro) and alkaline (malignites, nepheline syenite) rocks which are cut by numerous dykes varying in composition from ultramafic to intermediate. The dyke of finegrained kaersutite lamprophyre was discovered in the bottom hole of the drillhole 36, where it is intruded into mediumgrained gabbroids and pyroxenites. The groundmass of the rock is composed of kaersutite, titanomagnetite, albite, chlorite, titanite, pyrite, chalcopyrite, apatite, monazite. The phenocrysts are represented by chlorite and talc pseudomorphoses after a diamond-shaped mineral with shortened obtuse angles. According to electron microprobe analysis, kaersutite has low TiO2 content and intermediate mg# values of 0.54—0.67. According to the chemical composition, studied kaersutites differ from the same mineral in the microgabro dyke in the Pokrovo-Kyriivo massif, and from kaersutites in camptonites of the Khlibodarivka open pit in the Azov area. At the same time, it is similar to kaersutites from camptonites of the Novoukrainka dyke field of the Inhul terrane. Kaersutite lamprophyre also differ from dyke rocks of the Pokrovo-Kyriivo massif by the absence of clinopyroxene, mica, ilmenite and low content of TiO2 in titanomagnetite, which does not exceed 6 %. This is due to a low TiO2 content in the rocks (up to 3.5 %), relative to other dyke gabbro of the area. According to mineralogical-petrographic and chemical characteristics, kaersutite lamprophyres are unique rocks that have no direct counterparts in the Azov area or on the Ukrainian Shield in the whole.

Geochemical characteristics and Sr‐Nd‐Hf isotope compositions of Late Triassic post‐collisional A‐type granites in Sarudik, SW Sumatra, Indonesia

AbstractLate Triassic A‐type granites are identified in this study in Sarudik, SW Sumatra. We present new data on zircon U–Pb geochronology, whole‐rock major and trace elements and Sr‐Nd‐Hf isotope geochemistry, aiming to study their petrogenesis and tectonic implications. LA‐ICP‐MS U–Pb dating of zircon separated from one biotite monzogranite sample yields a concordia age of 222.6 ±1.0 Ma, indicating a Late Triassic magmatic event. The studied granites are classified as weakly peralumious, high‐K calc‐alkaline granites. They exhibit high SiO2, K2O + Na2O, FeO/(FeO + MgO) and Ga/Al ratios and low Al2O3, CaO, MgO, P2O5 and TiO2 contents, with enrichment of Rb, Th and U and depletion of Ba, Sr, P and Eu, showing the features of A‐type granites. The granites have zircon εHf(t) values from −4.6 to −0.4 and whole‐rock εNd(t) values from −5.51 to −4.98, with Mesoproterozoic TDM2 ages (1278–1544 Ma) for both Hf and Nd isotopes. Geochemical and isotopic data suggest that the source of these A‐type granites is the Mesoproterozoic continental crust, without significant incorporation of mantle‐derived component, and their formation is controlled by subsequent fractional crystallization. The Sarudik A‐type granites are further assigned to A2‐type formed in post‐collisional environment. Combined with previous knowledge on the western SE Asia tectonic evolution, we conclude that the formation of the Late Triassic A‐type granites is related to the post‐collisional extension induced by the crustal thickening, gravitational collapse, and asthenosphere upwelling following the collision between the Sibumasu and the East Malaya Block.

Geochronology, geochemistry, and Sr–Nd–Hf–O isotopes of the Zhongqiuyang rhyolitic tuff in eastern Guangdong, SE China: Constraints on petrogenesis and tectonic setting

A‐type magmas generally occur in extension‐related environments and shed new light on the tectonic setting by metallogenic and petrological studies of A‐type felsic volcanic rocks. The Zhongqiuyang porphyry Cu deposit is located in eastern Guangdong, SE China. Here, we present zircon U–Pb geochronology of rhyolitic tuffs, which yields a weighted mean206Pb/238U age of 164.7 ± 1.3 Ma, consistent with the age of the Jurassic porphyry Cu mineralization in the southeastern coastal metallogenic belt. The Zhongqiuyang rhyolitic tuffs are characterized by high FeOT/MgO ratios and low MgO, CaO, and TiO2contents and enrichments in Rb, Th, and U and depletions in Ba, Nb, Ta, Sr, P, and Ti, showing an A2‐subtype affinity. The Sr–Nd–Hf–O isotopic compositions imply that the rhyolitic tuffs were generated by partial melting of the crustal melts (~90% of the Palaeoproterozoic metaigneous rocks and 10% of the Palaeoproterozoic metasedimentary rocks) in the Cathaysia Block, with input of 10~20% mantle‐derived materials. Element variations indicate that crystal fractionation was also required for the generation of the studied samples after partial melting. In combination with previous studies, we propose that a window or a break‐off of the subducted slab triggered by upwelling asthenosphere during the Middle to Late Jurassic underneath the Cathaysia Block, and the ascending mafic magmas not only supplied heat for crustal remelting but also added mantle‐derived melts, resulting in the formation of I‐ or A‐type felsic intrusions related to Cu–Au/Cu–Mo deposits along the active continental margin of SE China.

Early Jurassic Mafic Intrusions in the Southern Youjiang Basin, SW China: Petrogenesis, Tectonic and Metallogenic Implications

A suite of mafic intrusions, composed of diabase and micro-gabbro outcrops in the Jingxi area of southern Youjiang Basin, SW China. This study conducts geochronological, geochemical, and Sr–Nd isotopic analyses on the mafic intrusions in Jingxi with the aim of determining their petrogenesis, tectonic setting, and metallogenic implications. Zircon U–Pb dating for the mafic intrusions yielded an age of 183 ± 3 Ma (MSWD = 2.3), which is coeval with the Carlin-like gold mineralization in the Youjiang Basin. The mafic intrusions are alkaline in composition and characterized by low TiO2 (1.25–1.87 wt %) contents and low Ti/Y ratios (410–550). They exhibit OIB-like patterns of trace element distribution and they have low (87Sr/86Sr)i ratios of 0.704341 to 0.705677, slightly negative εNd(t) values of −0.30 to −2.16, low La/Ta (11.57–15.66) and La/Nb (0.77–1.06) ratios, with [La/Yb]N = 6.52–10.63. The geochemical characteristics, combined with regional considerations, suggest that the mafic intrusions originated from partial melting of upwelling asthenosphere within the garnet-spinel transition zone, as a result of intracontinental back-arc extension triggered by the steep subduction of the Paleo-Pacific plate beneath the South China Block. Moreover, the new data not only suggest Early Jurassic magma was a possible heat source, but also support a magmatism-related model for the Carlin-like gold mineralization in the Youjiang Basin.