Dolerite Dykes Research Articles

Petrogenesis of the Cretaceous Intraplate Mafic Intrusions in the Eastern Tianshan Orogen, NW China

In this study, we conducted zircon U-Pb dating, and whole-rock geochemical and Sr-Nd isotope analyses on the Late Mesozoic dolerite dykes in the Bailingshan Fe deposit (Eastern Tianshan Orogen, NW China) to unravel their petrogenesis and regional tectonic significance. Zircon U-Pb dating on the dolerite yielded an Early Cretaceous age of 129.7 ± 1.4 Ma. The dolerite is calc-alkaline sodic (Na2O/K2O = 4.71 to 6.80), and enriched in LILEs (Rb, K, Sr, and Pb) but depleted in HFSEs (Nb, Ta, and Ti). The intermediate Nb/U (16.7 to 18.5) and Ce/Pb (6.33 to 6.90) values, and the presence of xenocrystic zircons in these dolerite dykes suggest crustal assimilation during the magma evolution. Petrological modeling suggests fractionation of olivine, pyroxene, garnet, and spinel. All the dolerite samples have low initial 87Sr/86Sr (0.7041 to 0.7043) and positive εNd(t) (+ 4.6 to + 5.1) values, indicative of a depleted asthenospheric mantle source. Partial melting modeling suggests that the melting has occurred in the spinel-garnet stability field. Integrating the data from ore deposit geology, geochronology, geochemistry and Sr-Nd isotopes, we proposed that the Late Cretaceous Eastern Tianshan mafic magmatism was developed in an intraplate extension setting.

Compositional signatures of dolerite dykes from the Purang ultramafic massif, Tibet: Implications for garnet-bearing components in the Neo-Tethyan mantle

We present geological, whole-rock geochemical, Sr–Nd isotopic and U–Pb geochronological data for the mafic dykes from the Purang ultramafic massif, South Tibet (China). The mafic dykes are composed of dolerite within a mylonitized harzburgite body. These mafic dykes are dated at 138.0–138.5 Ma using zircon and apatite U–Pb methods, and are characterized by enrichments in large-ion lithophile elements (LILEs) and high-field-strength elements (HFSEs), resembling those of typical ocean island basalts (OIB). They have initial 87Sr/86Sr ratios of 0.70612 to 0.70670, initial 143Nd/144Nd ratios of 0.51231 to 0.51233 and high εNd(t) values of −3.1 to −2.6. Geochemical modeling suggests that the parental melts of dolerites were derived by ~50% melting of garnet pyroxenite or eclogite veins in the lithospheric mantle. The formation of the dyke-filling melts can thus be linked to melting of a thin mantle layer containing old low buoyancy plume (LPB)-components in the form of garnet-bearing veins. Our results provide a scenario for the tectono-magmatic evolution of the oceanic lithosphere beneath the Neo-Tethys in the South Tibet. We conclude that the existence of a thermal plume might not be essential to the petrogenesis of the Yarlung-Zangbo Suture Zone (YZSZ) ophiolites.

Geochronological and Geochemical Constraints on the Petrogenesis of Permian Dolerite Dyke Swarms in the Beishan Orogenic Belt, NW China

Extensive Early Permian mafic-ultramafic intrusions, doleritic dykes, and basalts crop out within the Beishan area, southern Central Asian Orogenic Belt (CAOB). We present new geochronological and geochemical data for Gubaoquan dolerite dyke swarms in the Beishan orogenic belt. Zircon U-Pb Dating of the Gubaoquan dykes indicates that they were emplaced during the Early Permian (280.7 ± 4 Ma), that was coeval with Yinaoxia and Podong mafic dykes in Beishan area. The dykes are characterized by low Mg# (47–84) in the clinopyroxene crystals, and the content of whole-rock Fe2O3 (t), MgO, and alkali (Na2O + K2O) range from 12.5–17.4, 4.06–5.51, and 2.8–4.4 wt.%, respectively. The samples from the Gubaoquan dykes have high and variable Ba/La (5.93–14.2) and Ba/Nb (15.0–37.3) ratios but low Th/Yb (0.17–0.24) ratios. The rocks show slightly enrichments in LREE, HFSE, Th, and Hf, and depletion in Nb and Ta. The εNd (t = 280 Ma) values and initial 87Sr/86Sr ratios of the Gubaoquan dykes show variations ranging from 6.4 to 6.8 and 0.706240 to 0.707546, respectively. These data suggest that the parental magmas for the Gubaoquan dykes were probably derived from partially decompressed melting of upwelling depleted asthenosphere mantle that was metasomatized previously by subducted fluids.

First palaeointensity data from the cryogenian and their potential implications for inner core nucleation age

SUMMARY The timing of inner core nucleation is a hugely significant event in Earth's evolution and has been the subject of intense debate. Some of the most recent theoretical estimates for the age of nucleation fall throughout the Neoproterozoic era; much younger than previously thought. A young inner core requires faster recent core cooling rates and a likely hotter early core; knowledge of its age would be invaluable in understanding Earth's thermal history and total energy budget. Predictions generated by numerical dynamo models need to be tested against such data, but records are currently much too sparse to constrain the event to a precise period of time. Here, we present results from 720 Ma dolerite dykes (and one sill) from the Franklin Large Igneous Province, which fall within a crucial 300 Myr gap in palaeointensity records. This study uses three independent techniques on whole rocks from 11 sites spread across High Arctic Canada and Greenland to produce virtual dipole moments ranging from 5 to 20 ZAm2 (mean 11 ZAm2); almost one order of magnitude lower than the present-day field. These weak-field results agree with recent ultralow palaeointensity data obtained from Ediacaran rocks formed ∼150 Myr later and may support that the dynamo was on the brink of collapse in the Neoproterozoic prior to a young inner core formation date.

GEOLOGICAL AND GEOMORPHOLOGICAL ANALYSIS AND HYDROGEOCHEMICAL ENVIRONMENT OF AQUIFERS OF THE SOUTHERN WEST COAST OF KARNATAKA, INDIA

Dakshina kannada district is situated in peninsular region. The peninsula is composed of geologically ancient rocks of diverse original and most of them have undergone metamorphism. The early Precambrian tonalitic gneisses invaded by granites, granulites and dolerite dykes. Granulites are mostly restricted to areas south of Mangalore. High grade alumina rich (corundum bearing) metamorphic schists have been encountered and younger alkaline intrusive rocks like Aegerine syenites have been reported. There are ve rivers and estuaries. Number of lineaments cut across each other and some lineaments are parallel to each other. The Arabian sea class is the largest among other land cover features in the study area. The river/tidal creek land cover appear as long irregular and sinous in outline. Mulki river, Netravati river, Gurupur river at southern terrain. The qualities of groundwater at sandy aquifer are good, lateritic/weathered gneissic rocks it is sweet.

Baddeleyite Pb[sbnd]Pb geochronology and paleomagnetic poles for ~1.89–1.86 Ga mafic intrusions from the Dharwar craton, India, and their paleogeographic implications

We present new key paleomagnetic pole at 13°S, 152°E (k = 21, A95 = 7.8°) for recently identified 1864.4 ± 2.7 Ma (weighted mean age of four PbPb ages) mafic magmatic event, based on a detailed paleomagnetic study of dolerite dykes and sills intruding Archean basement rocks and Tadipatri formation of the Cuddapah basin, Dharwar craton respectively. The PbPb baddeleyite geochronology yields a crystallisation age of 1867.1 ± 1.0 Ma (MSWD = 1.02) for N77°E trending dyke in the southern region to Cuddapah basin. This new age obtained, confirms the presence of ~1864 Ma magmatic episode with a spatial extent of ~400 km in the Eastern Dharwar craton, within the brief period of ~5 Ma. The paleomagnetic results in these dykes revealed reverse polarity magnetisation direction with mean D = 107°, I = 24° (N = 13 sites, α95 = 10°). Here, we also update the normal polarity magnetic directions on ~1.89 Ga swarm, and the corresponding paleopole situated at 21°N, 336°E (N = 79 sites, A95 = 3.6°). The paleoposition of India is constrained around the equator during ~1.89–1.86 Ga time. The paleogeographic reconstructions were also been attempted at ~1.89 Ga and ~ 1.86 Ga with available key poles from other cratons, indicates the possibility of single plume acting as a source for two distinguishable radial emplacement of mafic dyke swarms across India (Dharwar and Bastar craton) and Western Australia (Yilgarn craton) within a time span of ~35 Ma. The individual movement of India, Baltica and Siberia with a drift rate of ~5.55 cm/yr towards the south, whereas Amazonia craton has moved rapidly to the north (~24.9 cm/yr), do not suggest the amalgamation of a supercontinent (Columbia/ Nuna) during ~1.88–1.86 Ga time.

Sr-Nd isotopic study of dolerite dykes in the Western Dharwar craton, southern India: Implications for the evolution of the subcontinental lithospheric mantle in late Archean

Mafic dyke swarms are one of the major geologic features that represent crustal extension and rifting episodes which result in the transfer of large volumes of mantle materials to the crust. The rapid emplacement of dykes within a short span of time helps to preserve the composition of subcontinental lithospheric mantle (SCLM) and understand its evolution over time. Here we report major and trace element geochemistry and Sr-Nd isotopic characteristics of dykes from the Tiptur area in the late Archean Western Dharwar craton of southern India. The studied mafic dykes are un-metamorphosed and preserve pristine igneous textures and mineralogy. Three different groups of dykes were identified and categorized as olivine dolerites, dolerites and REE enriched dolerites, based on the distinct petrographical and geochemical characteristics. Olivine dolerites are having a high Mg# suggesting a lower degree of fractionation and is indicative of a less evolved magma. Dolerites have comparatively lower Mg# than the olivine dolerites indicating that the former have undergone significant fractional crystallization as confirmed by the higher Ni contents. The low initial Sr isotopic ratio for olivine dolerites (87Sr/86Sr = 0.70088) with negative epsilon Nd values are suggesting a late Archean enriched subcontinental lithospheric mantle (SCLM) in the source region. The dolerites (87Sr/86Sr = 0.70132 to 0.70327) and REE enriched dolerites (87Sr/86Sr = 0.70405 to 0.70653) show signatures of a more enriched source. The evolution of SCLM from a DMM source enriched by the subduction of the mafic oceanic crust in the late Archean to a mantle enriched by continent-derived sediments towards the early Proterozoic are reflected in the dolerite and REE enriched dykes.

Mantle-derived gold scavenged by bismuth-(tellurium)-rich melts: Evidence from the mesozoic wulong gold deposit in the north china craton

Mesozoic gold deposits associated with large-scale cratonic destruction of the North China Craton (NCC) made up a giant gold province that is unique among ancient cratons worldwide. The Au sources of these deposits are highly debated among Archean to Proterozoic strata, Mesozoic granitoid plutons, subducting slab-derived fluids, and mantle-derived fluids. Here we chose the Wulong gold deposit, a typical lode gold deposit in the NCC, to constrain the sources of gold and sulfur. Sulfides from auriferous quartz veins in the Wulong gold deposit have restricted Δ33S (−0.1 to +0.2‰), without diagnostic signature of Archean sedimentary sulfur. Their mantle-like δ34S (−0.9 to +3.3‰) and restricted Pb isotopic ratios are remarkably different from those of sulfides from Paleoproterozoic strata and Mesozoic granitoid plutons. Sulfides from the Mesozoic doleritic dykes in the Wulong district have nearly identical δ34S (+1.7 to +2.0‰) to that of auriferous quartz veins, indicating that they were derived from similar mantle sources. We interpret that the sulfur and gold of the Wulong gold deposit were likely sourced from mantle-derived fluids released from devolatilization of the hydrated, subcontinental lithospheric mantle (SCLM). However, the enrichment of Au is not directly linked to an anomalously Au-rich source in the SCLM beneath the NCC which contains 0.39 ppb Au on average. Neither Mesozoic igneous rocks (0.22 ppb Au on average) nor Paleoproterozoic strata (0.19 ppb Au on average) in the Wulong district are enriched in Au. In addition, sulfides from auriferous quartz veins are not enriched in Au (commonly < 1 ppm Au), indicating that the mantle-derived fluids are likely Au-undersaturated during the deposition of sulfides. On the other hand, positive correlation of Au and Bi of auriferous quartz veins and close association of native gold with native bismuth and Bi-Te-S minerals consistently indicate that Au enrichment is likely related to Bi-(Te)-rich melts. The Bi-(Te)-rich melts may have precipitated from the Au-undersaturated auriferous fluids and scavenged Au from the fluids, leading to the Au enrichment. The insignificant fractionation of δ34S among sulfides and typical reduced assemblage of native bismuth, joséite and pyrrhotite in auriferous quartz veins consistently indicate that the occurrence of Bi-(Te)-rich melts is related to the reduction of fluids. This study demonstrates that Au in the mantle-derived fluids can be scavenged and significantly enriched by Bi-(Te)-rich melts, even if the fluids were Au-undersaturated, leading to the formation of giant lode gold deposits.

U-Pb baddeleyite age for the Ottfjället Dyke Swarm, central Scandinavian Caledonides: new constraints on Ediacaran opening of the Iapetus Ocean and glaciations on Baltica

ABSTRACT The Ottfjället Dyke Swarm (ODS) is a prominent component of the Ediacaran mafic magmatism associated with opening of the Iapetus ocean, and hosted in the Särv Nappe, Middle Allochthon of the Scandinavian Caledonides. A U-Pb baddeleyite age of 596.3 ± 1.5 Ma for a thick, well preserved, plagioclase-phyric dolerite dyke in Härjedalen, Sweden, dates emplacement of the swarm. The age represents a robust, inheritance-free reference age for variably deformed and metamorphosed tholeiitic dykes in sandstone-dominated sequences of the lower part of the Middle Allochthon, representing the proximal, rifted Baltoscandian margin preceding the opening of Iapetus. The new age is within the narrow time span between 610 and 595 Ma defined by the most reliable age estimates for mafic dykes in structurally higher nappes (upper part of Middle Allochthon), representing the distal margin during the opening of Iapetus. The Ottfjället Dyke Swarm cuts the Tossåsfjället Group succession, which includes sabkha-related carbonate platform and diamictite couples, one of several correlated Neoproterozoic glaciogenic successions in Scandinavia. The intrusion age of ca. 596 Ma therefore sets a minimum age for the glaciogenic successions. It implies that Neoproterozoic glaciations in Scandinavia predate the ca. 580 Ma Gaskiers glaciation event and are probably part of the ca. 635 Ma Marinoan “Snowball-Earth”-type glaciation.

Trace and rare earth element geochemistry of clinopyroxene in mafic dykes from Western Dharwar craton, southern India

The major and trace element characteristics of the clinopyroxene from two generations of dykes, dolerite and olivine dolerite dykes of Western Dharwar Craton were investigated by using electron microprobe and LA� ICPMS. The clinopyroxene in dolerite dykes show a compositional zoning with Mg# decreasing from the core to the rim (85 to 51) and Cr2O3contents decrease towards the rim. The trace and rare earth element (REE) pattern of the core shows lower concentrations of REEs compared to the rim. The clinopyroxene in the olivine dolerites are devoid of zoning and compositionally more primitive than the dolerites as visible in the trace element and REE concentrations. The dolerite and olivine dolerite are formed from different source magmas and the fractional crystallization of clinopyroxene is dominant in dolerites. The present estimated melt composition in equilibrium with clinopyroxene is consistent with the bulk rock composition for dolerite and olivine dolerite. © 2021. All Rights Reserved.

The setting and style of manganese mineralization in the Constantiaberg Massif, Cape Peninsula, South Africa

Abstract Manganese oxyhydroxide mineralization is widespread in the Constantiaberg Massif. It is largely hosted by west-northwest – east-southeast trending brittle structures in the competent Ordovician arenites of the Peninsula Formation of the Cape Supergroup. Manganese is also found impregnating more porous Peninsula Formation arenites and Quaternary scree. This study proposes that the more significant deposits at Hout Bay and Constantiaberg differ from most of the mineralization in that they are spatially associated with saprolithic dolerite dykes belonging to the 132 Ma False Bay dolerite dyke swarm. It is suggested that this deep weathering may be related to a Miocene palaeo-landsurface, yielding a maximum age for these more important Mn deposits. However, there is evidence for mineralization of different ages through the Quaternary Period. Apart from Mn-mineralization hosted by rare breccias containing hydrothermal quartz, most of the mineralization is supergene, having been leached and transported in reduced acidic groundwater and precipitated at or near a redox front in a near-surface environment.

Prospective policy safeguards to mitigate hydrogeological risk pathways in advance of shale gas development in the Karoo basin, South Africa

Policies surrounding energy development are frequently implemented in response to known or perceived problems. South Africa is in a unique position to develop prospective policies that build on knowledge gained from elsewhere. This paper provides a prospective analysis of hydrogeological risk pathways and vulnerability attributes in advance of anticipated shale gas operations in the Karoo Basin of South Africa. The ‘hazard-pathways-receptors’ approach is applied to define the hydrogeologic system in the context of potential sources of water resource contamination. This case study focuses on two critical hydrogeological risk pathways: regional groundwater flow and discrete structural features. Depending on the targeted area (hydro-litho-structural domain, depth of target, presence of aquitard, intrusion ratio), the capacity of deep natural pathways to enable hydraulic fracturing fluids, chemicals, or produced water to reach shallow groundwater, will be reduced or difficult. Deep artesian water could however be intercepted at different depths and, based on past groundwater exploration in the Karoo, water could flow into horizontal fractures or openings characterising the shallow aquifers: sills-ring complexes, lithological contacts, and transgressive fractures across dolerite dykes. However, above-ground water and wastewater management safeguards are needed to protect shallow groundwater from potential water quality degradation due to a) flowback or produced water b) stray gas and/or c) spills or illicit discharges that could introduce contaminants into groundwater resources. This paper describes a systematic approach to evaluate hydrogeologic risk pathways and informs adaptive management policies to protect South African groundwater resources.

Distribution of the Pan‐African Domains in East Antarctica and Adjacent Areas

AbstractThe Pan‐African event is widely distributed in East Antarctica craton. Many terranes or outcrops of the craton bear more or less signs of the event. From characteristics of the shear zones, granites, pegmatites, feature and time of high grade metamorphism and detrital zircon ages peaks of the downflowing sediments from the plateau, the Pan‐African event in the East Antarctica and adjacent areas in the Gondwana reconstruction, like SE Africa, southern India and SW Australia, wasdistributed as special zones or areas in many localities, including both the coastal regions and interior of the East Antarctica. In geochemistry, the granites are generally anorogenic, ocassionally with some gabbros or dolerite dykes, showing sign of bi‐modal feature. The water or fluid available along the shear zones were responsible for retrogression of the earlier, e.g., Grenville age, high grade outcrops to later Pan‐African amphibolite facies metamorphism. Meanwhile, the Pan‐African event has influenced most isotopic systems, including the U‐Pb, Sm‐Nd, Rb‐Sr and Ar‐Ar systems, giving younger apparent ages. Manifestation of the Pan‐African event is distributed from possibly locally granitic magmatism, to wider medium‐high grade metamorphism, and mostly widespread in resetting for some isotope systems, suggesting the prevailing thermal effect.Before Gondwana formation, local depressions in the East Antarctica could be filled with sediments, implying the initial breakup period of the Rodinia. The later Pan‐Gondwana counterrotating cogs shaped the interstitial fold belts between continent blocks and formed a set of shear zones. The mafic underplating in the Gondwana may be responsible for the widespread granites, pegmatites and more or less isotopic resetting due to strong thermal effect from the deep. That is, the Pan‐African event is a possible response of the plate movement surrounding the continent swarms in the non‐stable interior of theyet consolidated Gondwana. The Pan‐African event may be an overwhelmingly extensional and transcurrent tectonics in mechanism.

Magnetic Signature of Serpentinization at Zedang in the South Tibetan Ophiolite Belt

AbstractMagnetic signature of serpentinized mantle peridotite has crucial importance in understanding the serpentinization process and interpreting the origin of strong magnetization anomalies at ultramafic‐hosted hydrothermal settings. However, different groups of serpentinized peridotites from both ocean drillings and ophiolite complexes have shown considerable variations in the abundance of magnetite (Oufi et al., 2002; Bonnemains et al., 2016; Li et al., 2017). We examined the magnetic properties, petrography and mineral chemistry of variably serpentinized peridotites from Zedang ophiolite in the eastern Yarlung‐Zangbo suture in south Tibet to evaluate the conditions of serpentinization and magnetite formation as well as magnetic sources in suture zones.The studied samples were 0–90% serpentinized with densities from 3.316 to 2.593 g cm–3 and show typical mesh textures of olivine replaced by serpentine on thin sections of core specimen. Serpentines were divided into type‐1 Fe‐poor serpentine mesh (1.84–2.88 wt% FeO) associated with magnetite in the early stage and type‐2 Fe‐rich serpentine cores (3.92–5.12 wt% FeO) with no formation of magnetite in the late serpentinization. Brucite vein appeared in central serpentine veins and show Mg/(Mg+Fe) values of 0.74–0.87 at ∼50–70% of serpentinization. Pure magnetite was identified as the main magnetic carrier by thermomagnetic analyses, but minor Cr‐magnetite (∼0.8 mole fractions of Fe3O4) was also detected due to oxidation of early spinel.All the peridotite samples show a rapid increase of magnetic susceptibility from ∼0.001 to ∼0.03 SI before 40–50% of serpentinization and a following flat trend in values 0.02–0.03 SI at &gt; 50% of serpentinization. This density–susceptibility relationship differs from the rapid production of magnetite above 60–70% of serpentinization for many abyssal peridotites (Oufi et al., 2002; Bach et al., 2006) and suggests that magnetite formation was coupled with hydration of olivine in the early serpentinization but the two decoupled at ∼ 40–50% of serpentinization. This transition is consistent with the petrographic observation that magnetite‐free serpentinization was developed in higher degrees (&gt; 50%) of serpentinization.Prior studies suggested that serpentinization of &lt; 200°C would generate Fe‐rich brucite, serpentine and little magnetite, whereas magnetite‐rich serpentinization was associated with Fe‐poor brucite and occurred at higher temperatures of 200–300°C (Klein et al., 2014). The petromagnetic features of serpentinized peridotites from the Zedang ophiolite indicate that the serpentinization process took place initially above 250°C (estimate from brucite composition) and continued to lower temperatures of &lt; 200°C, probably during the mantle lithosphere cooling down in forearc settings (Xiong et al., 2017). These serpentinized peridotites have higher magnetization intensities (average 2.26 Am–1) than mafic dolerite dykes and basaltic volcanic rocks (mostly &lt; 1 Am–1) and should be significant sources of aeromagnetic highs in the Yarlung‐Zangbo suture.

Petrology and geochemical framework of dolerites dykes of Temté, North Cameroon, Central Africa

The Temte basement in North Cameroon is crosscut by dyke swarms with N 20°–40° trending, including dykes 15–30 m wide, up to 3 km-long. Representative rocks exhibit intersertal to sub-ophitic textures. Electron microprobe analyses identified diopside, augite, pargasite, biotite, Ti-magnetite, plagioclase, and sanidine. Whole-rock ICP-MS and ICP-AES chemical analyses showed compositions of basaltic andesite, basaltic trachyandesite and trachyandesite in composition. Igneous differentiation was likely governed by fractional crystallization associated with limited fluid metasomatism. Some lavas could have been moderately contaminated by crustal materials during feeding of local cracks through turbulent magma flows. Discrimination geochemical diagrams and immobile trace and REE element ratios show that the mantle source of Temte dolerites was a deep phlogopite-bearing EMII mantle component and has undergone moderate to high partial melting rate. Correlations of fieldwork and analytical data with previous results evidence the Temte dolerite dyke swarms as fingerprints of crustal extension accompanying regional uplift in an active continental margin when early rifting led to the formation of the Poli marginal basin.

Carbonates at the supergiant Olypmic Dam Cu-U-Au-Ag deposit, South Australia part 2: Sm-Nd, Lu-Hf and Sr-Pb isotope constraints on the chronology of carbonate deposition

Mineralization at the supergiant Cu-U-Au-Ag Olympic Dam deposit (South Australia), the ‘uranium endmember’ of the iron-oxide copper–gold (IOCG) spectrum of ore deposits, is hosted in a breccia complex developed entirely within granite of the 1.59 Ga Hiltaba Suite (Gawler Craton). Earlier studies suggested brecciation and mineralization occurred within a magmatically-driven hydrothermal system at 1.59 Ga, with a critical role for mafic–ultramafic intrusions. In contrast, recent radiometric dating of the breccia complex indicates a prolonged, multi-stage history of brecciation and mineralization from 1.59 to 0.5–0.4 Ga. Ca-Fe-Mg-Mn-carbonate gangue minerals are associated with ore minerals at virtually every stage of mineralization. In a companion study (Apukhtina et al., 2020), this mineralogically, texturally and compositionally diverse carbonate mineral suite was assigned to seven associations defined on the basis of host lithology and texture. Here we report Sm-Nd, Pb-Pb and Lu-Hf isotope ages for these carbonates, which are used to examine the chronology of carbonate deposition. Initial Sr-Nd isotopic compositions are used to place constraints on fluid sources. Sm-Nd and Pb-Pb isotope systematics of calcite veins in ~1.59 Ga IOCG ore indicate 1.59–1.55 Ga deposition ages. Likewise, locally abundant laminated siderites have Sm-Nd ages in this age interval. A world-first attempt to apply Lu-Hf dating to carbonate gangue in an ore deposit yields ages that are 70–100 Ma younger than corresponding Sm-Nd ages, presumably reflecting isotopic exchange of carbonate Lu-Hf isotope systems with host rocks. Sm-Nd ages for carbonates assigned to other carbonate associations (hosted in highly altered inferred 1.59 Ga basalt and picrite; diverse settings within granite-dominated breccia; locally abundant megaclasts of green and red bedded sandstone/mudstone sequences; ~0.82 Ga doleritic dykes) are more diverse and range from ~1.59 to 0.5 Ga. The structurally youngest carbonates (unbrecciated fluorite-barite veins; carbonate matrix in polymict conglomerate above the breccia complex) yield ~0.50 Ga Sm-Nd ages. Inferred carbonate ages are broadly consistent with radiometric dates for other hydrothermal minerals (e.g., hematite, uraninite, apatite, fluorite). They suggest that mineralization initiated at 1.59 Ga was reworked and possibly increased in size in response to large-scale tectonic, magmatic, sedimentary and hydrothermal events. Initial 87Sr/86Sr in the carbonates is higher and more variable (0.710–0.752, average ~0.721) than could be explained by ore and gangue mineral formation from magmatic-hydrothermal fluids during a single event at 1.59 Ga, a model favored in several earlier studies. By contrast, carbonate formation over a long period, as inferred from the Sm-Nd chronology presented here, would allow ingrowth of 87Sr in the granitic host rocks to develop the heterogeneous initial 87Sr/86Sr recorded in the carbonates. Carbonate-bearing fluids appear to have sourced Nd (and most likely also Sr) locally, within the host granite and breccia, with contributions from mafic rocks. The emerging evidence for protracted, multi-stage mineralization implies that single-stage models for Olympic Dam need to be revisited and that all studies of sulfide and gangue minerals in this deposit require careful radiometric dating. We speculate that the polymetallic nature and unusually large metal reserves of the Olympic Dam mineralization are related to its multi-stage formation history.

Mineralogy and geochemistry of the In Allarene layered mafic-ultramafic igneous complex (In Ouzzal terrane, western Hoggar, South Algeria)—petrogenesis and geodynamic implications

The In Allarene mafic-ultramafic complex is located at the southern part of the In Ouzzal terrane and was known as the unique Pan-African mafic-ultramafic intrusion in this terrane. The complex shows a ring-shaped intrusion of about 20 km2 and concentric structures. Petrological observations and mineralogical analyses show that the core of the intrusion is composed of harzburgites and dunites surrounded by lherzolites, whereas the outer part consists of fine- to coarse-grained gabbros that are crosscut by dolerite dykes. The mineralogy and geochemistry of these rocks show that they are mainly cumulates, but some gabbros and dolerites have characteristics indicating that they correspond to magmatic liquids or they have trapped a variable amount of magmatic liquid. They correspond to the least magnesian and the most alkaline-rich gabbros and dolerites. The geochemistry of the gabbros and dolerites is very similar to the mafic-ultramafic complexes linked to arc zones and they exhibit all the features of the subduction zones magmatism as showing in the N-MORB normalization, with LILE enrichment relatively to HFSE, and Nb-Ta and Ti negative anomalies. The chemical compositions of clinopyroxene and spinel support this hypothesis. This is also the case for all geodynamic setting discrimination diagrams, both those using major elements and trace elements. Several of these diagrams also show that the studied rocks can be distinguished from intracontinental plate tholeiites, which can be very close geochemically to continental arc tholeiites. These results support the hypothesis of the existence of an arc-type metasomatized mantle under the In Ouzzal terrane during Pan-African orogeny.

Application of Landsat 8 OLI for Mapping the Deccan Traps of Kachchh, Gujarat

In the present study, Landsat 8 Operational Land Imager multispectral data covering Gujarat's Kachchh district have been analyzed to map the Deccan Traps. The study area is covered by different lithological units from Middle Jurassic to Holocene rock formation. Deccan Volcanics of Upper Cretaceous to Eocene age consist of Alkali Gabbro, Andesitic Trachyte, Basalt Flows, Intertrappean sediments, Dolerite dykes and Olivine Gabbro. The study aims to distinguish the basalts of Deccan Volcanics using remote sensing data. The basalts were segregated using image processing techniques such as false color composite based on the optimal index factor, band rationing, minimum noise fraction and principal component analysis. Conclusively, entire techniques used differentiated the basalts from other lithological units. The data were validated using laboratory reflectance spectra of rock samples. The map of basalt in Kachchh district was created from the processed Landsat 8 OLI image manifesting different flows, and it covers an area of total 1913.95 sq km.

Petromagnetic Characteristics of Serpentinization and Magnetite Formation at the Zedang Ophiolite in Southern Tibet

AbstractVariably serpentinized peridotites from the Zedang ophiolite in southern Tibet were magnetically and petrologically examined to understand the serpentinization process and evaluate the origin of magnetic anomalies in ultramafic‐hosted tectonic settings. Magnetite occurs in the serpentine and brucite veins and is identified as the dominant magnetic carrier by thermomagnetic and petrological analyses. The magnetic susceptibility increases rapidly from &lt;0.001 to ~0.02 SI for the &lt;50% serpentinized samples followed by nearly constant values of 0.02–0.03 SI above 50% serpentinization. This transition corresponds with the formation of Fe‐poor serpentine mesh (2–3 wt% FeO) and magnetite in the early stages and the replacement of mesh center olivine by Fe‐rich serpentine (4–5 wt% FeO) without magnetite in the late stages. Brucite veins occur in the 50–70% serpentinized samples and indicate serpentinization temperatures from ~250 to &lt;100°C. The serpentinization may initiate at an oceanic spreading ridge center under high temperatures (&gt;250–300°C) to produce magnetite and subsequently continue at lower temperatures (&lt;200–250°C) in near‐seafloor settings and limit the magnetite formation, possibly associated with ophiolite emplacement. These serpentinized peridotites have higher magnetization intensities (average 2.26 Am−1) than dolerite dykes and basaltic volcanics (mostly &lt;1 A m−1) in the area and should be the major source of aeromagnetic highs in the south Tibetan ophiolite belt.

LA–ICP–MS zircon U[sbnd]Pb dating, Lu[sbnd]Hf, Sm[sbnd]Nd geochronology and tectonic setting of the Mesoarchean mafic and felsic magmatic rocks in the Sangmelima granite-greenstone terrane, Ntem Complex (South Cameroon)

The Ntem Complex represents the northwestern margin of Congo Craton in southern Cameroon. It consists mainly of charnockites, TTG suites, potassic grantitoids, minor supracrustal and mafic rocks. The latter include gabbronorite enclave or sill, gabbro and dolerite dykes. The metagabbro are basaltic to andesitic in composition and show geochemical features of low-La Archean tholeiitic basalts, and LREE-depleted (LaN/SmN < 1) akin to modern N-MORB. The doleritic dykes show high TiO2/P2O5 (7.50–11.47) and (La/Yb)N ratios, and low NbTa values similar to modern intraplate continental basalts and intermediate-La Archean tholeiitic basalt, least affected by contamination from granitic continental crust. The partial melting of low-to high-K mafic rocks of oceanic and continental arc affinity generated the charnockites and low−/medium-pressure TTGs. The potassic granites were formed by partial melting of charnockite or TTG, and inherited many geochemical features of their sources. New LA–ICP–MS UPb zircon dates suggest that the charnockites and TTG suites formed at ~3155–2850 Ma. The gabbro intrusion was dated at 2866 ± 6 Ma. Migmatisation of TTG-gneiss was coeval whit the regional high–grade metamorphism event at 2843 ± 7 Ma. The potassic granites were probably crystallized at ~2758 Ma. The garnet-whole rock SmNd age of 2744 ± 31 Ma may represent the age of the second anatexis event coeval with D2 deformation. Literature data indicate the disruption or open isotopic systems in the Ntem Complex around 2500–2600 Ma.