Ultramafic Intrusive Rocks Research Articles

Geochemistry, geochronology, and radiogenic isotopes of the Balmer and Confederation assemblages of the Laird Lake Area, Red Lake greenstone belt, Canada: implications for Archean tectonic evolution

The Laird Lake property, southwest Red Lake greenstone belt, straddles the contact between the Balmer (2.99–2.96 Ga) and the Confederation (2.74–2.73 Ga) assemblages. The property is 10 km along strike from the Madsen and Starrat–Olsen Au mines that are hosted near the contact. The Balmer assemblage consists of fine-grained, aphyric, locally pillowed mafic volcanic rocks, ultramafic intrusive and volcanic rocks with flow breccia textures hosting local spinifex-bearing clasts, and banded-iron formations. In contrast, the Confederation assemblage comprises porphyritic mafic volcanic rocks intercalated with intermediate to felsic volcanic rocks that include crystal lapilli tuffs, crystal tuffs, and tuffs. The Balmer assemblage is composed of tholeiitic mafic volcanic rocks with minor Al-undepleted komatiites, whereas the Confederation assemblage is calc–alkalic. Neodymium isotopes, in conjunction with trace element geochemistry, suggests that parts of the Balmer assemblage were weakly contaminated by an older intermediate basement. Both arc and back-arc volcanism occurs in the Confederation assemblage, with the arc rocks showing a stronger crustal component than the back-arc rocks. A maximum U–Pb age of 2741 ± 19 Ma for a crystal tuff and an age of 2737.68 ± 0.79 Ma for a diorite are consistent with a Confederation assemblage affinity for the intermediate calc–alkaline rocks south of the Au-bearing horizon. The Balmer assemblage represents an oceanic plateau formed by plume magmatism on the margins of the North Caribou Terrane, whereas the Confederation assemblage at Laird Lake formed in an oceanic arc setting where both arc and back-arc volcanism occurred simultaneously.

Identification of the Sedimentary Sources and Origin of Uranium for Zhiluo Formation of the Tarangaole U Deposit, Northeastern Ordos Basin

The large-sized Tarangaole uranium deposit and its neighboring Daying and Nalinggou deposits, located in the northeastern margin of the Ordos Basin, constitutes a major uranium resource base in northern China. In order to further clarify the sedimentary material source, uranium source and regional sediment–tectonic setting of the uranium-fed clastic rocks (i.e., Zhiluo Formation(J2z)) in the district, this paper carried out whole-rock geochemistry, heavy minerals composition and in situ U-Pb dating of detrital zircons for sandstones from the lower section of the Zhiluo Formation. The results have shown that the average chemical differentiation index (CIA) for the host rocks is 73.16 and the chemical weathering degree is moderate. Heavy minerals are mainly composed of ilmenite, garnet, chlorpyrite, zircon, pyrite, apatite, hematite, etc. The U-Pb dating of detrital zircon generally indicates three age peaks, i.e., 260~Ma, 1850~Ma and 2450~Ma, respectively. In conclusion, the source rocks may have been formed at active continental margins, e.g., in a continental margin arc environment. The sedimentary materials mainly come from khondalite series, TTGs, granulite, and mafic–ultramafic intrusive rocks distributed among the Daqing–Ula Mountains and adjacent areas, etc. The Late Paleozoic U-rich intermediate and acidic magmatic rocks spreading over the eastern part of the Ula–Daqing and Wolf mountains have provided the main uranium sources for the formation of major U deposits in the northern Ordos Basin.

A Seismic Tomography, Gravity, and Flexure Study of the Crust and Upper Mantle Structure of the Hawaiian Ridge: 1

AbstractThe Hawaiian Ridge has long been a focus site for studying lithospheric flexure due to intraplate volcano loading, but crucial load and flexure details remain unclear. We address this problem using wide‐angle seismic refraction and reflection data acquired along a ∼535‐km‐long profile that intersects the ridge between the islands of Maui and Hawai'i and crosses 80–95 Myr‐old lithosphere. A tomographic image constructed using travel time data of several seismic phases reveals broad flexure of Pacific oceanic crust extending up to ∼200–250 km either side of the Hawaiian Ridge, and vertically up to ∼6–7 km. The P‐wave velocity structure, verified by gravity modeling, reveals that the west flank of Hawaii is comprised of extrusive lavas overlain by volcanoclastic sediments and a carbonate platform. In contrast, the Hāna Ridge, southeast of Maui, contains a high‐velocity core consistent with mafic or ultramafic intrusive rocks. Magmatic underplating along the seismic line is not evident. Reflectors at the top and bottom of the pre‐existing oceanic crust suggest a ∼4.5–6 km crustal thickness. Simple three‐dimensional flexure modeling with an elastic plate thickness, Te, of 26.7 km shows that the depths to the reflectors beneath the western flank of Hawai'i can be explained by volcano loading in which Maui and the older islands in the ridge contribute ∼43% to the flexure and the island of Hawai'i ∼51%. Previous studies, however, revealed a higher Te beneath the eastern flank of Hawai'i suggesting that isostatic compensation may not yet be complete at the youngest end of the ridge.

Petrology of Palaeoarchaean mafic–ultramafic rock suites of the western Iron Ore Group, Singhbhum Craton, eastern India, using the chemistry of minerals

AbstractThe petrogenesis of Palaeoarchaean mafic–ultramafic rock suites from the western Iron Ore Group, Singhbhum Craton, eastern India, has been evaluated based on the chemistry of constituent mineral phases. The rock suites include basaltic rocks and mafic (gabbro) to ultramafic (serpentinized peridotite) intrusive rocks, which occur in host rocks covering phyllite, ferruginous shale, banded haematite quartzite and jasper. The constituent clinopyroxene shows dominant uralitization, whereas plagioclase grains are generally saussuritized, being marked by relatively tiny granular aggregates of albite, chlorite, epidote and K-feldspar. The ultramafic intrusive rocks are overwhelmingly serpentinized. Clinopyroxene compositions are augitic, whereas relict plagioclase is typically bytownite. Amphiboles of the investigated rock suites are divisible into the ‘uralite’ type (occurring peripherally to clinopyroxene) and the ‘completely changed-over amphibole’ type (with no traces of initial clinopyroxene). Both the amphibole types belong to the ‘calcic group’, showing a compositional spectrum from actinolite–magnesiohornblende–ferrohornblende–ferroactinolite. Opaque minerals include magnetite (both Cr-magnetite and Al-magnetite), chromite (Al-chromite) and ilmenite, whereas serpentine (belonging to the ultramafic intrusive rocks) corresponds to lizardite. Looking at the mineral compositions of the pyroxene, glass and amphibole, the studied rock suites show a wide equilibration temperature–pressure domain range (∼750 °C to ∼1400 °C at ∼0.26 kbar to ∼21 kbar), which corresponds to an ascending magma that underwent a ‘hydration event’ at a shallow level. The assessment of the clinopyroxene and spinel chemistry characteristically suggests an arc setting for the parent magma that has undergone both equilibrium and fractional crystallization in the course of magmatic evolution. During differentiation, the magmatic density remains almost constant, with variable oxygen fugacity.

Late Tonian explosive volcanism and hyaloclastites in northern Paraguay Belt, Central Brazil: A record of Rodinia break-up in western Gondwana

Explosive volcanism and oceanic crust generation are key aspects to understanding the break-up history of the Rodinia. Its impact on the extreme glaciations of the Cryogenian was marked by ocean chemistry fluctuations in these times, which are still poorly known and preserved in western Gondwana. Here we describe in detail and provide a series of U-Pb ages for the Nova Xavantina Metavolcanosedimentary Sequence, where extrusive rocks, such as hyaloclastite and metabasalts, as well as mafic and ultramafic intrusive rocks and ignimbrites, are associated with diamictites, limestones, and iron formations in the Paraguay belt, Central Brazil. Two styles of pyroclastic eruptions were identified in the studied area. Basic-ultrabasic Surtseyan eruption was enhanced by magma-water interaction and is represented by hyaloclastites and incorporated bombs of scoria. Felsic Plinian eruption, in its turn, was formed by violent explosions in a subaerial environment and is represented by widely exposed acidic tuffs and ignimbrites. Shallow subaqueous environments, such as lakes, salt lakes, and possibly proto-oceans, developed in this precursor basin concomitant with bimodal volcanism. High resolution U-Pb geochronology results indicate they were formed in at least three magmatic events, starting with the formation of ca. 745 Ma seafloor metabasalts and felsic volcanics dated at ca. 735 Ma, and intrusive gabbros at ca. 714 Ma. Alkaline ultramafic rocks of ca. 577 Ma intrude the region and may be associated with similar rocks that crop out regionally as the Planalto da Serra Alkaline Complex. Our results from Nova Xavantina volcanism explain the main Neoproterozoic source until now unrecognized and previous recorded in detrital zircons from the Paraguay belt sediment provenance.

Origin of highly variable and unusually low δ7Li in mineral separates from ultramafic intrusive rocks in a convergent tectonic setting in the Tibetan plateau

Lithium isotopes are increasingly used as a tracer for the sources of mafic-ultramafic intrusive rocks, but the validity of such approach has never been evaluated directly. We have addressed this issue by analyzing mineral separates from an arc-type mafic-ultramafic complex (namely Xiarihamu) in the Tibetan plateau for Li isotopes. The Xiarihamu complex consists of a gently-dipping, ultramafic chonolith in the center and older gabbro in the margins. The chonolith was interpreted to be a feeder for olivine-charged magma by previous studies. Our new data show minor to strong isotopic disequilibrium between coexisting minerals in the chonolith, with lower δ7Li in clinopyroxene than in coexisting orthopyroxene by up to 7‰. The total range of δ7Li in the pyroxene and olivine separates together is from 5.0‰ to −16.2‰, which is significantly different from that of known arc lavas in the world (3.6 ± 1.2 ‰). Samples from a subvertical drill core of ∼190 m in length penetrating the chonolith show a concentric decrease of δ7Li in orthopyroxene from −16.2‰ in the middle to +1.5‰ and + 3.8‰ in both ends. The δ7Li values of mineral separates show no correlation with whole-rock SrNd isotopes and incompatible trace element ratios, as well as olivine forsterite contents and orthopyroxene Mg/(Mg + Fe) molar ratios, indicating that the observed great variability of δ7Li in the minerals was not caused by variable subduction inputs or magma differentiation alone. A positive correlation exists between orthopyroxene δ7Li and whole-rock LOI, illustrating that serpentinization-talc alteration caused δ7Li in the residual mineral to increase, contributing to the great variability of δ7Li on the mineral scales. Subducting sediment input during magma generation and crustal contamination together cannot adequately account for the observed extremely low δ7Li (−2 to −16.2‰) in the minerals from many of the rock samples as well as the unusually low average δ7Li (−3.5‰) of all of the samples, because the δ7Li of the crust and subducting sediments are much higher. Kinetic fractionation of Li isotopes in response to a temperature gradient across the chonolith as well as to subsequent diffusion of Li from interstitial melt to the mineral phases is the best explanation. Since the kinetic fractionation is independent of tectonic settings, such effect is inevitably present in all mafic-ultramafic intrusions on Earth, illustrating that the effect of kinetic Li isotope fractionation must be considered before using Li isotopic data to constrain mantle sources and magmatic evolution, especially if mineral rather than whole-rock data are employed.

New Insights into Interpretation of Aeromagnetic Data for Distribution of Igneous Rocks in Central Iran

Abstract —New insights in the aeromagnetic data over the Central Iranian Microcontinent (CIM) have revealed interesting results for future studies and exploration. This work presents the interpretation of different magnetic analyses and the calculated 3D inversion model to provide important insights into the distribution of igneous rocks in the area that may be traced under significant cover. By analyzing several hundred magnetic susceptibility data points and aeromagnetic anomalies of known igneous rocks over the area, it was determined that mafic–ultramafic intrusive rocks generally have a high magnetic susceptibility and produce a strong magnetic response. Intermediate–felsic intrusive rocks have a low magnetic susceptibility and show a smooth gradient variation and commonly regular shape. Volcanic rocks show a wide range of magnetic susceptibility; therefore, the aeromagnetic anomalies are often random or show strong amplitude with high frequency signals and are rapidly eliminated when an upward continuation is applied. Based on the results of analysis of different magnetic maps and 3D inversion of data, and combining this information with known outcropped of igneous rocks, we revealed 1215 concealed intrusive rocks and 528 volcanic rocks in the area. We also renewed the boundaries of tens outcropped igneous rocks. The known and new mapped igneous rocks can be identified as 12 regions (or zones) for intrusive rocks and 4 regions for volcanic rocks. The results indicate that the mafic–ultramafic rocks are mainly located in the Sistan suture zone of eastern Iran along the Nehbandan fault zone. They also show that the many parts of the Lut block as the main structure of CIM have been under magmatic events, so that most of concealed igneous rocks are distributed in the middle and southern part of the Lut block. Volcanic rocks are widespread in the southeastern and northern parts of the area such as the Urumieh-Dokhtar Magmatic Arc, North Lut, and Bam region.

Petrogenesis of Late Carboniferous–Early Permian mafic igneous series in the Baoshan block: Implications to birth of Neo-Tethys and generation of magmatic sulfide deposit

The petrogenesis of the rift-related Late Carboniferous–Early Permian mafic igneous rocks (basaltic lavas, dolerite dykes and mafic–ultramafic intrusive rocks) from the Sibumasu block in the eastern end of Cimmeria was still ambiguous. This petrogenesis is related to understand the drive for the formation of Neo-Tethys which was developed along the margin of Cimmeria, and to the generation of magmatic sulfide deposit via partial melting of previously subduction-modified mantle induced by rifting of continental margin. Most of the mafic rock samples belong to the subalkaline series, with lithologies varying from basalt to basaltic andesite. Magmatic zircons from the doleritic dykes yield weighted mean 206Pb/238U ages of 308.7 ± 2.5 Ma. The εHf(t) values of zircon are ranging from 3.2 to 11.8, which are much lower than those of the coeval depleted mantle (εHf(t) ≈ 16 at 305 Ma). The igneous series share similar features, such as elevated light rare earth elements, negative Nb-Ta anomalies and enriched Sr-Nd-Hf isotopes. Based on geochemical analyses of the mafic igneous series in the northern part of Sibumasu, we suggest that the primary melts of the mafic igneous rocks were derived from a fertile subcontinental lithospheric mantle (SCLM) which was previously modified by oceanic subduction, and these rocks were intruded and erupted in a passive plate margin during plate extension. The subsequent melting-induced softening of SCLM triggered the evolution of continental margin from the rift to the open of Neo-Tethys Ocean. The proposed model emphasizes that the melt generated from previously-modified SCLM in a continental rift setting is beneficial for the generation of magmatic sulfide deposit.

Geologic-tectonic setting of the gold-endowed Kilo Terrane in the eastern Central Kibalian Superterrane, northeastern Democratic Republic of Congo

Solid geological mapping, granite geochemistry and robust U-Pb SHRIMP geochronology confirm our subdivision of the eastern Central Kibalian Superterrane (ECKS) into two terranes: the ca 2570–2552 Ma Western Ituri Terrane and the ca 2680–2570 Ma Kilo Terrane. The ECKS is part of the Central, Northern and Southern Kibalian Superterranes, first-order interpretative crustal blocks, which constitutes the Kibalian Greenstone Region in the Bomu-Kibalian craton. The Kilo Terrane comprises: 1) the lower Kibalian lithostratigraphic units of deep-sea facies mafic–ultramafic volcanic rocks, interlayers of pelitic-sedimentary rocks and mafic granite and diorite-tonalite complexes in the Galaya-Kilo Sub-Terrane; and 2) the upper Kibalian lithostratigraphic units of intermediate-felsic volcanic-sedimentary, mafic–ultramafic intrusive rocks and widespread quartz-dioritic, tonalitic to granodioritic granites in the Balazana-Bunia Sub-Terrane.Mafic granite and diorite-to-tonalite magmatism in the Kilo Terrane is demonstrated by: 1) the ca 2680–2640 Ma syn-volcanic mafic-dioritic granite magmatism, including the ca 2650–2640 Ma albitic granites extensive in the Nizi-Tsi Belt; 2) the ca 2640–2600 Ma syn-volcanic hornblende tonalite-to-granodiorite magmatism in the Kilo and Moto-Zani Terranes; and 3) a ca 2600–2570 Ma syenite, quartz-monzonite and felsic granite intrusions along extensional zones and reactivated major terrane boundaries. The coincidence between the ca 2603 Ma tectonic uplifting and ca 2605–2590 Ma hydrothermal activity in the Kilo Terrane is associated with the gold mineralization.The 2711–2790 Ma xenocryst ages point to potential existence of the > 2790 Ma basement rocks extending from underneath the Western Ituri Terrane into underneath the Bunia West Domain. It’s absence in the gold-hosting domains of the Kilo Terrane does not exclude the existence of > 2.8 Ga basement, but instead it invokes on the existence of an unconformity separating the basement rocks from rootless, ca 2680–2570 Ma complexly folded and faulted juvenile granite-greenstone domains, sutured together by first- to second-order fault/shear zones. Protracted periods of Archean to post-Archean tectonic uplifting and inversion were responsible for preservations and erosion of upper crustal-level granite greenstone belts, including orogenic gold depositsThe ca 2680–2570 Ma juvenile crustal addition in the Kilo Terrane occurred some 30 m.y. after the ca 2850–2600 Ma juvenile crustal addition events in well-researched Archean cratons, worldwide.

Sub-arc mantle heterogeneity in oxygen isotopes: evidence from Permian mafic–ultramafic intrusions in the Central Asian Orogenic Belt

Integrated zircon–olivine O–Hf isotope data have been successfully used to unravel the nature of the source mantle for the early Permian post-collisional mafic–ultramafic intrusive rocks in the southern margin of the Central Asian Orogenic Belt in NW China. Olivine crystals with forsterite (Fo) contents varying from 91 to 87 mol% from the Permian Pobei mafic–ultramafic complex in the region yield highly elevated δ18O from 6.0 to 7.2‰. These values are much higher than typical mantle values (~ 5.3‰) and are apparently at odds with the mantle-like eNd(t) values of whole rocks (4.9–5.4). Magmatic zircon crystals from troctolite and gabbroic rocks show divergent oxygen and hafnium isotopic compositions: mantle-like eHf(t) values from 5.1 to 11.9 and crust-like δ18O values from 7.6 to 10.1‰. The observed increase of δ18O values from olivine (an early crystallizing phase) to zircon (a late crystallizing phase) in the mafic–ultramafic rocks is generally consistent with an AFC process. However, this process cannot fully explain the highly elevated δ18O values (6–7‰) for the most primitive olivine containing Fo as high as mantle olivine (> 90 mol%) and the mantle-like Hf isotope composition of zircon. Mixing calculation indicates that such highly unusual isotope compositions can be explained by the previous source mantle contamination with subducted sediment-derived melts and slab-derived fluids. Our results show that the combination of zircon O–Hf isotopes and olivine oxygen isotopes is more effective than the data of zircon or olivine alone to distinguish the effect of AFC process from source contamination. The results from this study provide a new line of evidence that the sub-arc mantle is not homogeneous in oxygen isotopes.

Evidence for Neoarchean Ni-Cu-bearing mafic intrusions along a major lithospheric structure: A case study from the south Rae craton (Canada)

Deformed and metamorphosed mafic to ultramafic intrusive rocks are common along and adjacent to the Snowbird Tectonic Zone, a major Paleoproterozoic structure separating the Rae and Hearne cratons. On the Rae side of this major lithospheric structure, we investigated a series of mafic bodies that intrude high-grade paragneiss and are associated with Ni–Cu ± PGE mineralization. By linking field observations, geochronology and metamorphic petrology, our results indicate Archean (∼2.63 Ga) crystallization ages for these mafic intrusions over a minimum along-strike extent of ∼500 km and a later high-temperature (∼820 °C and ∼7.5 kbar), Paleoproterozoic metamorphic overprint at t < 1.94 Ga. The youngest reproducible ages of detrital zircon from the host paragneiss indicate a maximum depositional age of ∼2.73 Ga for the paragneiss. These results indicate that the sedimentary rocks hosting the mineralized mafic intrusions are older than the mixed siliciclastic and carbonate sedimentary units previously described along the Snowbird Tectonic Zone (<2.07 Ga). Furthermore, in two localities (Thye and Yotin-Shagory lakes, in Northwest Territories and northern Saskatchewan), zircon from both the mafic and pelitic samples exhibit an unusual within-run fluctuation in the lead isotope signal during SHRIMP analysis, suggesting inhomogeneous redistribution of radiogenic Pb within the crystal lattice. Redistribution is interpreted to have occurred during the ∼1.94–1.90 Ga tectonometamorphic event. This behaviour, documented only in ∼10 (Ultra)High-Temperature localities worldwide, has not been observed in the other two studied localities (Currie and Axis lakes in northern Saskatchewan), suggesting either i) a different metamorphic history or ii) different re-crystallization processes occurring during the Paleoproterozoic high-temperature evolution (or a combination of the two). Our data indicate that (U)HT metamorphism and the absence of a fluid/melt phase are not a requirement for lead redistribution in zircon.

四国東部および紀伊半島西部の三波川帯の苦鉄質・超苦鉄質貫入岩について : (その1)四国東部の三成川帯の苦鉄質・超苦鉄質貫入岩について

四国東部および紀伊半島西部の三波川帯の苦鉄質・超苦鉄質貫入岩について : (その1)四国東部の三成川帯の苦鉄質・超苦鉄質貫入岩について

U–Pb ages of zircons from Mesozoic intrusive rocks in the Yanbian area, Jilin Province, NE China: Transition of the Paleo-Asian oceanic regime to the circum-Pacific tectonic regime

Geology in the Yanbian area, NE China, is a key to understanding the relationship between the Paleo-Pacific tectonic regime and the older Paleo-Asian oceanic regime, especially the transition between the regimes during the Mesozoic. A compilation of 131 U–Pb zircon ages, including 18 obtained in the present study, shows that the late Paleozoic to Mesozoic magmatism in the Yanbian area took place from 285 to 105Ma (mainly 205–160Ma) and that there was a peak of activity at ∼175Ma and a hiatus from the Late Jurassic to Early Cretaceous (150–130Ma). Provisionally, we subdivide the magmatic activity into four dominant stages: Middle Permian to Middle Triassic (280–240Ma), Late Triassic (230–205Ma), Early to Middle Jurassic (205–160Ma) and Early Cretaceous (130–105Ma). Middle Permian intrusive rocks, including pyroxenite, gabbro, diorite, granodiorite, tonalite and monzogranite, are interpreted to have been emplaced along an Andean-style continental margin in association with the subduction of the Paleo-Asian oceanic plate. The Late Permian–Middle Triassic intrusive rocks are mainly quartz monzonite, monzogranite and syenogranite, and are considered to have been the result of syn-orogenic/post-collisional activities. The Late Triassic magmatism involved not only the emplacement of mafic–ultramafic intrusive rocks, but also coeval I- and A-type granitoids, indicating an extensional environment after the final closure of the Paleo-Asian Ocean. Since the Jurassic, the tectonic setting has been dominated by subduction of the Paleo-Pacific oceanic plate. Previously published data for the Yanbian area and age data from surrounding regions show that the entire region from the Yanbian area to the Korean Peninsula and the Japanese Islands experienced peaks in igneous activities that ranged from 200 to 165Ma during the Jurassic, and 120 to 110Ma and 90 to 65Ma during the Cretaceous. The Jurassic intrusive rocks were emplaced mainly in the west and Cretaceous intrusive rocks in the east. In other words, there is a southeastwards younging of Mesozoic intrusive rocks that appears to have been closely associated with the evolution of the Paleo-Pacific Ocean. Taking all this into account, we conclude that the transition from the Paleo-Asian oceanic regime to the circum-Pacific tectonic regime took place during the Late Triassic to Early Jurassic.

Isotope and trace element studies of the Xingdi II mafic–ultramafic complex in the northern rim of the Tarim Craton: Evidence for emplacement in a Neoproterozoic subduction zone

The Xingdi II mafic–ultramafic complex is located in the northern rim of the Tarim Craton, Northern Xinjiang, NW China. This complex is mainly composed of gabbro, gabbronorite, websterite and lherzolite, plus minor granodiorites and granites. The geodynamic setting of this complex and other Neoproterozoic mafic–ultramafic complexes in the region is debated, with opinions varying from mantle plume to a continental arc. A new zircon U–Pb age from this study reveals >7-myr difference for two gabbroic intrusive phases in the Xingdi II mafic–ultramafic complex and up to a 21-myr difference between this complex and the nearby mafic–ultramafic complexes that occur <12km from this complex. The age changes for the Neoproterozoic (730–820Ma) mafic–ultramafic complexes in the area occur in different directions and hence a mantle plume origin for the complexes is unlikely. The Xingdi II mafic–ultramafic intrusive rocks are characterized by moderate light-REE enrichments and pronounced negative Nb anomaly, which are common for mafic–ultramafic rocks in convergent tectonic settings, and negative εNd(t) values (−9.2 to −5.1) and elevated initial 87Sr/86Sr ratios (0.7061 to 0.7086), which indicate enriched source mantle plus crustal contamination. The εHf(t) values of zircon crystals from the Xingdi II mafic–ultramafic complex are between −2.8 and −7.0. The trace element and isotope data together indicate that this complex formed from multiple pulses of contaminated subduction-related basaltic magma that were emplaced separately with a time gap of at least 7myr. The results from this study support the viewpoint that in the Neoproterozoic the northern rim of the Tarim Craton was an active continental margin that belongs to the Circum-Rodinia Subduction System.

Discovery of Early Permian radiolarian fauna in the Solon Obo ophiolite belt, Inner Mongolia and its geological significance

The Solon Obo ophiolite belt in Inner Mongolia is located close to the border between China and Mongolia. It is an integral part of the Solon Obo tectonic suture formed by the closure of the Paleo-Asian Ocean between Siberia Craton and North China Craton in the Paleozoic. This ophiolite belt is a rare remnant of the Paleo-Asian Ocean and thus provides important samples for the study of tectonic evolution in the eastern part of the Central Asian Orogenic Belt in North China. The age of the tectonic event of the Solon Obo tectonic suture has been debated for a long time. Some researchers believe that it was from Silurian to Devonian whereas other researchers believe it was from Early Carboniferous to Early Permian. Here we report the first discovery of Early Permian radiolarians in the cherts of the ophiolite belt. The Solon Obo ophiolite belt is E-W trending, with a total length of ~100 km. It is mainly composed of ultramafic intrusive rocks such as harzburgite, lherzolite, dunite and pyroxenite, and gabbro, diabase, basalts and cherts. On the base of the field geological investigation and 1:10000 structure-lithology mapping in the Solon Obo area, we took cherts samples from the Solon Obo ophiolite belt in the Subuergen Hudag area of northern Urad Middle Banner, Inner Mongolia, and carried out studies including petrologic characteristics of cherts, separation and identification of the radiolarian fossils in cherts. Early Permian radiolarian fauna is first found in the chert of Solon obo ophiolite belt. This fauna is composed of 5 genera and 6 species (including 4 unidentified species and 1 new species), Pseudoalbaillella bulbosa , Ps. solonensis Wang sp. nov., Stigmosphaerostylus sp., Ruzhencevispongus sp., Cenosphaera sp., Latentifistula sp. etc. The new species, Pseudoalbaillella solonensis can be distinguished from Ps. u-forma by its holes in the end of the pseudoabdominal segment. Pseudoalbaillella bulbosa , found in Late Carboniferous-Early Permian cherty strata of Japan, Thailand, Chile, Western Coast of North America and South China is regarded as a zonal species of early Early Permian time in China and has become one mark of the boundary between Carboniferous and Permian cherty strata in open marine facies. This radiolarian fauna is characterized by rich Pseudoalbaillellids and Entactinids which have possessed about 45% of this fauna respectively. The Pseudoalbaillellids are mostly composed of the apical cones and many pseudothoraxes with smaller entire individuals. The Entactinids are numerous in number and their spines are longer, but incomplete due to break. Other radiolarians, such as Latentifistalids , Cenosphaerids and Ruzhencevispongids are rare. These radiolarian fossils possessing over 50% in whole rocks are considered as the radiolite which is a component of the ophiolite. Therefore, the time determined by radiolarian fossils may represent forming age of the ophiolites and this set of radiolite is known as a part of the Solon Obo ophiolite belt. The discovery of this radiolarian fauna has provided a key evidence about forming age of the Solon Obo ophiolite lasted until Early Permian. This new discovery, together with the previously reported zircon U-Pb ages of 279–296 Ma for the associated mafic-ultramafic rocks in the Solon Obo ophiolite belt, confirms that the final closure of the Paleo-Asian Ocean took place no early than Early Permian.

Protolith of ultramafic rocks in the Kluane Schist, Yukon, and implications for arc collisions in the northern Cordillera

Mafic and ultramafic rocks crop out as decimetre- to centimetre-sized bodies of talc–antigorite–olivine (±orthopyroxene) and chlorite–amphibole schists interleaved in the pelitic Kluane Schist of southwestern Yukon. The metamorphic assemblages in ultramafic rocks exposed at Doghead Point overprint two generations of cleavage and are consistent with metamorphism reaching &gt;550 °C (talc + olivine) and &gt;750 °C (olivine + enstatite) in the contact aureole of the Eocene Ruby Range batholith. The bulk rock major and trace element patterns in the ultramafic schists (&gt;40 wt.% MgO, Mg/(Mg + Fe) &gt; 0.90) are unlike residual mantle from partial melting (i.e., ophiolite, orogenic massif, abyssal ocean floor) but are similar to peridotite or pyroxenite cumulates from arc magmas. Identical trace element concentrations and patterns are observed in several late Triassic basalts, pyroxenites, and websterites occurring to the southeast in Stikinia (present coordinates). A highly discordant U–Pb zircon date for one antigorite–talc–olivine schist sample (200–210 Ma) is within the range of U–Pb zircon ages for the late Triassic Lewes River – Stuhini arc in northwestern Stikinia (200–208 Ma, 216–220 Ma). When combined with other published age information, the ultramafic rocks in the Kluane Schist are interpreted as fragments of deeper arc-related mafic and ultramafic intrusive rocks introduced to the Kluane forearc basin between 95 and 82 Ma by exhumation along shear zones in northwestern Stikinia, most likely the Wann River or Llewellyn Faults. The Kluane Schist represents a west-facing forearc basin bordered to the east by arc-parallel strike-slip fault(s) that served to exhume and imbricate large knockers into the accretionary prism.

U–Pb zircon geochronology and geochemistry from NE Vietnam: A ‘tectonically disputed’ territory between the Indochina and South China blocks

Volcanoplutonic complexes in NE Vietnam have recently been interpreted as intraplate products of the Emeishan plume. Alternatively, mafic–ultramafic rocks have been considered as dismembered Palaeotethyan ophiolites juxtaposed along a tectonic mélange zone. New U–Pb zircon geochronological and geochemical datasets presented here suggest a complex geological history that records collision between the Indochina–South China blocks. Mafic–ultramafic rocks exposed within a tectonic mélange (Song Hien Tectonic Zone) include sub-alkaline pillow basalts that define two geochemically distinct ophiolitic suites (SH-1: N-MORB-like, SH-2: transitional E-MORB-like). Both suites have geochemical signatures suggestive of crustal contamination, compatible with a volcanic passive margin/rift setting. We suggest that SH-1 basalts may correlate with the Devonian–Carboniferous Jinshajiang–Ailaoshan–Song Ma branch of the Palaeotethys and form part of the associated Dian–Qiong belt, whereas SH-2 basalts are co-magmatic with Middle–Late Permian mafic–ultramafic intrusive rocks (dolerites, gabbros, peridotites) that developed in a rift basin, most likely on the margin of the down-going South China plate during west-vergent subduction beneath Indochina. During continental orogenesis and thrust stacking, these ophiolitic rocks were juxtaposed with other lithotectonic blocks within the Song Hien Tectonic Zone. Post-collisional relaxation led to the development of a rift basin (Song Hien rift) comprising Late Permian–Triassic volcano-sedimentary strata including <270–265Ma terrigenous sandstones, <252Ma mudstones, and c. 254–248Ma felsic effusives. Granites and granodiorites were emplaced across NE Vietnam between c. 252 and 245Ma in a syn- to post-collisional setting. The Late Permian–Early Triassic felsic magmatic rocks best correlate with coeval rocks in SW Guangxi and the Central and Western Ailaoshan fold belts (China) and the Truong Son fold belt (Vietnam); together they signal the final to post-collisional stages of Indochina–South China collision. We demonstrate that the analysed magmatic rocks in the Lo-Gam–Song Hien domains of NE Vietnam are not genetically linked to the Emeishan Large Igneous Province in the Yangtze block of South China, as has been previously widely proposed.

Role of asthenosphere and lithosphere in the genesis of the Early Permian Huangshan mafic–ultramafic intrusion in the Northern Tianshan, NW China

The Huangshan mafic–ultramafic intrusion hosts a large Ni–Cu sulfide deposit and is situated in the Northern Tianshan at the southern margin of the Central Asian Orogenic Belt (CAOB). The Early Permian intrusion consists of lherzolite, websterite, gabbronorite, gabbro and diorite. The Huangshan deposit contains ~80.4Mt of ore grading 0.54wt.% Ni and 0.3wt.% Cu and is the second largest magmatic sulfide deposit in Northern Xinjiang, China. The Huangshan intrusive rocks are enriched in large ion lithophile elements and depleted in high field strength elements relative to N-MORB, with low Nb/U (1.53–5.27) and high Ba/Nb (1.68–121) ratios, indicating that the primary magma was derived from partial melting of a metasomatized mantle source. The mafic–ultramafic rocks in the Northern Tianshan are characterized by lower Ca contents (<1000ppm) in olivine, more depleted Nb and Ta, lower Nb/U ratios, and higher εΝd(t) than those of the Tarim mafic–ultramafic intrusive rocks and ocean island basalts (OIB). The range of εΝd(t) values of the mafic–ultramafic rocks in the Northern Tianshan over time suggests a greater role for upwelling asthenospheric mantle in the younger rocks. This implies that they were produced by interactions between metasomatized lithospheric mantle and depleted asthenospheric melts rather than a mantle plume. The linear distribution of Permian mafic–ultramafic intrusions along the Kangguer fault in the Northern Tianshan suggests that slab breakoff played a key role in the genesis of the mantle-derived magma in a syn-collisional setting.

A modified genetic model for the Huangshandong magmatic sulfide deposit in the Central Asian Orogenic Belt, Xinjiang, western China

The Huangshandong Ni–Cu deposit is the largest magmatic sulfide deposit discovered to date in the Central Asian Orogenic Belt in northern Xinjiang, western China. The host intrusion is a 274-Ma composite mafic–ultramafic intrusion consisting of four separate intrusive units: a large layered gabbroic sequence (phase I), a sheet-like ultramafic body (phase II), a dyke-like gabbronorite body (phase III), and an irregular ultramafic unit (phase IV). Important sulfide mineralization is present in the last three intrusive units, predominantly as disseminated and net-textured sulfides (pyrrhotite, pentlandite, and chalcopyrite). The Huangshandong mafic–ultramafic intrusive rocks are characterized by arc-like geochemical signatures such as low Ca content in olivine and negative Nb–Ta anomalies in whole rocks. This, together with a post-subduction setting for the East Tianshan in the Permian, suggests that the source mantle was modified previously by slab-derived fluids in the Carboniferous. The mantle-derived magma was ponded in a staging chamber in the lower part of the newly formed arc crust. The first batch of magma to arrive at Huangshandong was most fractionated and depleted in Ni, crystallizing Fe-rich and Ni-depleted olivine (Fo67, <300 ppm Ni). The second batch of magma was more primitive, crystallizing more primitive olivine (Fo81–84). The third batch of magma was also highly fractionated and depleted in Ni, crystallizing Fe-rich and Ni-depleted olivine (Fo72, ~600 ppm Ni). The final batch of magma became more primitive again, crystallizing the most primitive olivine (Fo81–86). The occurrence of rounded sulfide inclusions in olivine primocrysts in the Huangshandong ultramafic rocks indicates that immiscible sulfide liquid droplets were present during olivine crystallization. The Ni tenors of disseminated sulfide ores in the gabbronorite dyke vary mainly between 5 and 8 wt%, which are too high to have been produced by the parental magma of the dyke. The Ni, Cu, and platinum-group elements (PGE) tenors of disseminated sulfide ores in the dyke (phase III) and the ultramafic sheet (phase II) are remarkably similar. These observations, together with the sequence of magma emplacement, suggest that the sulfide liquids entrapped in the magma of the dyke formed at depth by a previous pulse of more primitive magma. The estimated parental magma for the most primitive lherzolites in the Huangshandong intrusion contains 10 wt% MgO. Modeling shows that sulfide saturation in the parental magma of the Huangshandong lherzolites could have resulted from fractional crystallization. Significant PGE depletions relative to Ni and Cu in the disseminated sulfide ores of the Huangshandong deposit may be due to sulfide retention in the source mantle.

ASTER spectral sensitivity of carbonate rocks – Study in Sultanate of Oman

Remote sensing satellite data plays a vital role and capable in detecting minerals and discriminating rock types for explorations of mineral resources and geological studies. Study of spectral absorption characters of remotely sensed data are under consideration by the exploration and mining companies, and demonstrating the spectral absorption characters of carbonates on the cost-effective multispectral image (rather than the hyperspectral, Lidar image) for easy understanding of all geologists and exploration communities of carbonates is very much important. The present work is an integrated study and an outcome of recently published works on the economic important carbonate rocks, includes limestone, marl, listwaenites and carbonatites occurred in parts of the Sultanate of Oman. It demonstrates the spectral sensitivity of such rocks for simple interpretation over satellite data and describes and distinguishes them based on the absorptions of carbonate minerals in the spectral bands of advanced spaceborne thermal emission and reflection radiometer (ASTER) for mapping and exploration studies. The study results that the ASTER spectral band 8 discriminates the carbonate rocks due to the presence of predominantly occurred carbonate minerals; the ASTER band 5 distinguishes the limestones and marls (more hydroxyl clay minerals) from listwaenite (hydrothermally altered rock) due to the presence of altered minerals and the ASTER band 4 detects carbonatites (ultramafic intrusive alkaline rocks) which contain relatively more silicates. The study on the intensity of the total absorptions against the reflections of these rocks shows that the limestones and marls have low intensity in absorptions (and high reflection values) due to the presence of carbonate minerals (calcite and dolomite) occurred in different proportions. The listwaenites and carbonatites have high intensity of absorptions (low reflection values) due to the occurrence of Mn-oxide in listwaenites and carbonates in carbonatites apart the influence of major carbonate minerals that occurred predominantly in these rocks. The study of ASTER thermal infrared (TIR) spectral bands distinguished the marls have low emissivity of energy due to the presence of hydroxyl bearing alumina-silicate minerals from the other rocks such as limestones, listwaenites and carbonatites which have high emissivity due to the absence of hydroxyl bearing alumina-silicate minerals and the presence of carbonate minerals and carbonates.Further, the study demonstrates and confirms the spectral sensitivity of marls and carbonatites. Marls have high reflectivity in ASTER visible near infrared (VNIR) and shortwave infrared (SWIR) spectral bands and low emissivity of energy in ASTER TIR spectral bands due to the presence of hydroxyl bearing alumina-silicate minerals. Carbonatites have low reflectivity in ASTER VNIR–SWIR spectral bands and high emissivity in ASTER TIR spectral bands due to the absence of hydroxyl bearing alumina-silicate minerals and the presence of the carbonate minerals and carbonates. These have been discussed by providing the grey scale color image of 14 ASTER spectral bands of the study sites. The study is based on the interpretation of image spectra of multispectral image conducted to map such economic valuable carbonate rocks. It provides a simple methods and basic knowledge, which are of great help to the geology and exploration communities. It is recommended to the geologists, industrialists, exploration communities of carbonates and mine owners to take up the knowledge for economic exploration of such deposits. Further, the study has proved that the technique is time and cost effective in mapping of such deposits and can be used to the areas which have extremely rugged topography occurred in similar arid region, where difficult to do exhaustive sampling and not reachable for conventional geological mapping.