Mafic Bodies Research Articles

Emplacement origins of coarsely-crystalline mafic rocks hosted in greenstone belts: Examples from the 2.7 Ga Yilgarn Craton, Western Australia

Coarsely crystalline mafic rocks, commonly referred to as dolerite and diabase, make up a substantial proportion of Archean greenstone belts and are hosts to major orogenic gold deposits, yet their internal architecture and contact relationships remain poorly documented and their emplacement origins are assumed rather than supported with evidence. We present results from drill core logging of several late Archean coarsely crystalline packages from the Eastern Goldfields Superterrane of the Yilgarn Craton, Western Australia, including the Golden Mile Dolerite. These large mafic bodies comprise mostly medium- to coarsely crystalline mafic rocks, but also include a variety of other products of differentiation. The bodies range from ∼75 to 460 m thick and extend for tens of kilometers, are primarily hosted in mudstones and basalt and appear to be concordant to stratigraphy. As such, this study excludes mafic dykes and extremely thick and internally complex bodies such as the Bushveld Complex. From base to top, the internal stratigraphy generally consists of a lower unit of aphyric basalt, dolerite/gabbro and peridotite ± pyroxenite, then a voluminous middle dolerite/gabbro, and an upper quartz dolerite/gabbro, and a topmost aphyric basalt. Granophyric veins and domains are common in the upper third of the mafic body. Contacts are gradational between all lithofacies, with the exception of granophric veins, which have sharp contacts. Within this overall stratigraphic framework there is variability in the relative thickness of lithofacies, number of lithofacies, as well as repetition of some lithofacies (several examples also include basalt in the interior). In well exposed examples, the key evidence for distinguishing intrusions from extrusions is provided by the top contact. However, the top contact from these examples is ambiguous. To overcome this gap, we review the character of well-studied thick ponded lavas from the literature. Most aspects of the internal character are not unique to intrusions, although the maximum groundmass grain size (excluding granophyric veins) of the Archean examples (>3 mm) is significantly coarser than that for any known ponded lavas (≪1 mm). The ability to distinguish intrusions from extrusions has important implications for reconstructing Archean greenstone stratigraphy and mineral exploration.

Genesis of Manganese Ore Occurrences of the Olkhon Terrane the Western Baikal Region

Geological and mineralogical data are reported on the manganese occurrences of the Olkhon terrane (Western Baikal region), which are localized in metadolerites of the Ustkrestovsky Complex, high-temperature mafic hornfels, granites, calcitic marbles and calciphyres, and occasionally are developed as separate veins in gneiss granites or small lenses in quartzites. Most of them are made up of high-temperature mineral assemblages (Opx + Cpx + Pl + Ilm ± Grt ± Bt ± Amp), the main manganese carriers in which are ferrorhodonite (33–36 wt % MnO), orthopyroxene (6–12 wt % MnO), and ilmenite (3–16 wt % MnO). Obtained data are in conflict with traditional concepts that these rocks are gondites (manganese-rich metamorphosed sediments) or that manganese flux in carbonate sediments was related to the volcanic activity that occurred simultaneously with sedimentation at about 500 Ma. The diversity of manganese occurrences was produced by metasomatic processes that occurred almost simultaneously with regional metamorphism and emplacement of subalkaline mafic bodies during collisional tectonogenesis (about 470 Ma).

Ore component mobility, transport and mineralization at mid-oceanic ridges: A stable isotopes (Zn, Cu and Fe) study of the Rainbow massif (Mid-Atlantic Ridge 36°14′N)

Ultramafic hosted hydrothermal deposits are ubiquitous along slow-spreading ridges such as the Mid-Atlantic Ridge (MAR; e.g., Ashadzé, Rainbow, Lost City) where they exert a major control on the cycling of economically important elements (e.g., Zn, Cu, Ni). However, the origin of metal mobility in these environments remains unclear. Here we use Zn (δ66Zn), Cu (δ65Cu) and Fe (δ56Fe) stable isotopes to explore the mobility of metals during (1) the serpentinization of the Rainbow massif basement in a seawater dominated system at low temperature (<250 °C) and (2) the subsequent high temperature (>350 °C) mineralization of serpentinites through seawater-derived fluids that interacted with gabbro prior to interacting with serpentinite near hydrothermal sites (stockworks).The Rainbow samples display among the largest range of isotopic variations ever reported for ultramafic rocks (−0.10‰ ≤ δ66Zn ≤ +0.47‰; −0.93‰ ≤ δ65Cu ≤ +0.24‰; −0.15‰ ≤ δ56Fe ≤ +0.25‰). These variations reflect a two-stage process. (1) Serpentinization of the ultramafic basement is accompanied by a decrease in Zn (26–41 ppm) and Cu (1–13 ppm) concentrations and an increase of δ66Zn (+0.30–+0.47‰) in peridotites relative to primitive mantle (Zn ∼ 55 ppm, Cu ∼ 20 ppm, δ66Zn ∼ +0.16‰). Striking correlations between δ66Zn and indices of serpentinization (LOI and Fe/3+ΣFe) show preferential leaching of isotopically light Zn by fluids during the serpentinization of the massif. This isotopic fractionation is controlled by the dissolution of both mantle sulfides and/or spinels and Zn complexation with chlorine in fluids. At this stage, Fe seems to be immobile as attested by correlations between δ56Fe and indices of peridotite fertility (e.g., Al2O3/SiO2). (2) The mineralization of serpentinites near the Rainbow stockwork is accompanied by an increase in Fe/3+ΣFe (>0.7), FeO (up to 19.8 wt%), Zn (≫50 ppm) and Cu (≫20 ppm) concentrations. The δ66Zn and δ65Cu values progressively decrease with indices of serpentinite mineralization (e.g., Zn, Cu, Fe/3+ΣFe), while several samples display abnormally high δ56Fe (up to 0.25‰) relative to primitive mantle (δ56Fe ∼ 0.025‰), suggesting a high mobility of Zn, Cu and Fe in high temperature hydrothermal fluids. These isotopic fractionations can be explained by the local oxidation of sulfur bearing fluids in contact with seawater. This process enhances metal precipitation as well as the formation of Fe3+-bearing phases, such as magnetite, beneath the stockwork, explaining the presence of magnetic anomalies below the Rainbow hydrothermal field. Our study shows that the mobility of metals in hydrothermal fluids can be enhanced by both peridotite interaction with seawater or with fluid that interacted with deeper mafic bodies and then flowed to the surface. These processes may generate hydrothermal deposits with distinct metal signatures.

Mantle composition, age and geotherm beneath the Darby kimberlite field, west central Rae Craton

The Rae Craton, northern Canada, contains several diamondiferous kimberlite fields that have been a focus of episodic diamond exploration. Relatively little is known about the deep mantle lithosphere underpinning the architecturally complex crust. We present bulk and mineral element and isotopic compositional data for peridotite and pyroxenite/eclogite xenoliths from the Darby kimberlites representing fragments of the west central Rae lithosphere, as well as the first kimberlite eruption age of 542.2±2.6 Ma (2 σ; phlogopite Rb-Sr isochron). Darby peridotites have low bulk Al2O3 contents with highly-depleted olivine (median Mg# = 92.5) characteristic of cratonic lithosphere globally, but more depleted than peridotites from other Rae Craton localities. One peridotite xenolith contains a harzburgitic G10D garnet. Re-Os TRD model ages appear to be the oldest measured to date from peridotites of the Rae lithosphere, having a mode in the early Neoarchean and ranging to the Paleoproterozoic (~2.3 Ga). Concentrate clinopyroxene defines a well constrained mantle geotherm indicating the existence of a ~200 km thick lithosphere at the time of kimberlite eruption, greater than the lithospheric thickness beneath Somerset Island and in good agreement with modern seismic constraints. Nickel-in-garnet thermometry in grains that record temperatures below the mantle adiabat, indicates mantle sampling dominantly in the graphite stability field whereas Al-in-olivine thermometry shows a distinct mantle sampling mode in the diamond stability field. Abundant pyroxenite and eclogite xenoliths are recovered across the Darby property and low-Cr garnet (Cr2O3 < 1 wt%) is the most abundant garnet type recovered in kimberlite concentrate. Based on thermometry, these rocks range in likely depths of equilibration, from the lower crust into the shallow lithospheric mantle. They give variable Os model ages, with the oldest ages in the Mesoarchean. The abundance of this mafic material reflects derivation from a large mafic body possibly evident in the layered structure of the Rae Craton mantle root defined by new seismic studies.

Basement geology of the southern Thomson Orogen

The diverse geological and geophysical data sets compiled, interrogated and interpreted for the largely undercover southern Thomson Orogen region reveal a Paleozoic terrane dominated by deformed metasedimentary rocks intruded by S- and I-type granites. An interpretive basement geology map and synthesis of geochronological constraints allow definition of several stratigraphic packages. The oldest and most widespread comprises upper Cambrian to Lower Ordovician metasedimentary rocks deposited during the vast extensional Larapinta Event with maximum depositional ages of ca 520 to ca 496 Ma. These units correlate with elements of the northern Thomson Orogen, Warburton Basin and Amadeus Basin. The degree of deformation and metamorphism of these rocks varies across the region. A second major package includes Lower to Middle Devonian volcanic and sedimentary units, some of which correlate with components of the Lachlan Orogen. The region also includes a Middle to Upper Ordovician package of metasedimentary rocks and a Devonian or younger package of intermediate volcaniclastic rocks of restricted extent. Intrusive units range from diatremes and relatively small layered mafic bodies to batholithic-scale suites of granite and granodiorite. S-type and I-type intrusions are both present, and ages range from Ordovician to Triassic, but late Silurian intrusions are the most abundant. Two broad belts of intrusions are recognised. In the east, the Scalby Belt comprises relatively young (Upper Devonian) intrusions, while in the west, the Ella Belt is dominated by intrusions of late Silurian age within a curvilinear, broadly east–west trend. The stratigraphic distributions, characteristics and constraints defined by this interpretive basement mapping provide a basic framework for ongoing research and mineral exploration.

Tectonostratigraphic and petrogenetic setting of late Mississippian volcanism in eastern Canada

In the aftermath of the Middle Devonian Acadian Orogeny, a thick mafic body is inferred to have been emplaced at the base of the crust during development of the Maritimes Basin complex in eastern C...

Evidence for the Upwelling of Mafic Bodies into the Upper Crust beneath the N40-50&amp;amp;#176;E Branch of the Pan-African Central Cameroon Shear Zone from Gravity Studies

The existence of mafic bodies at mid crustal level beneath the Pan-African Central Cameroon Shear Zone is still a matter of debate. To provide additional constrains on this issue, the crust of the west region of Cameroon has been investigated using gravity data. Analyses of these data show N40-50°E oriented iso-anomal contours in the Bafoussam area, interpreted as the N40-50°E branch of the Central Cameroon Shear Zone. In addition, spectral analysis and 2.5D gravity modeling reveal intrusions of mafic bodies at depth between 3.2 and 14.2 km under N40-50°E aligned volcanic centers, namely Mt Bambouto and Mt Mbapit. The above observations suggest a structurally controlled emplacement of the mafic bodies. In the light of the recent geophysical data, the interaction between the NE-ward channel flow operating at the bottom of the lithosphere or the asthenosphere upwelling and the Cameroon Shear Zone could better explain the magma upwelling in the upper crust. This result is the novelty of the present work.

Geochemical and Geodynamic Constrain of Tholeiitic Volcanism and Related Intrusions in the Kampti Gold District, Southwest Burkina Faso: Implication for Mineral Exploration

The Kampti serie is a volcano-plutonic complex in the south-eastern corner of the Houndé belt, closed to the border with Ivory Coast. The stratigraphy comprises initially: a thick sequence of tholeiitic and pillowed basalt followed by pyroclastic projections derivated from bimodal volcanism; and flowing mostly to the south. The volcanic sequence is cross-cut by mafic cumulate body, stocks of gabbro, diorite, subvolcanic dykes and lately intrude by granitoid. It is bordered to the west by the pelitic schist of the Bambela basin with minor intercalation of Tarkwa type sediment. volcanoclastic facies is dominantly plagioclase-phyric (albite +/- oligoclase), zoned plagioclase has a core of anorthoclase. Secondary mineral infilled (quartz, kutnahorite, ripidolite, clinozoisite) of spherulites and oolite highlight a general low grade metamorphism of greenschist facies affecting the complex. Based on trace element chemistry, the tholeiitic rocks present flat REE pattern contrasting with the felsic rocks more enriched in LREE and depleted in HREE. The style of the magmatism in the Kampti serie is compatible with an island arc model, describe elsewhere in the birimian. Gold mineralisation and base metal occurrences associated to the nature of rocks and tectonics highlight a polymetallic district.

Crustal-scale subsidence and uplift caused by metamorphic phase changes in the lower crust: a model for the evolution of the Loppa High area, SW Barents Sea from late Paleozoic to Present

The Loppa High area has been subject to several events of uplift and subsidence from the Late Paleozoic to Present. The driving mechanisms behind the vertical movements, however, are not fully understood. We propose that uplift and subsidence were influenced by the combination of density changes caused by metamorphic phase changes in a 90 × 140 km wide mafic lower crustal body below the Loppa High and local (rift-related) and far-field stress. Through a numerical modelling approach we analyse the tectonically induced variations in pressure and temperature in the lower crust, their influence on phase changes in the mafic body and the affiliated vertical movements. Results show that (1) densification of the mafic body caused by far-field compression associated with the late Triassic westward translation of Novaya Zemlya could cause surface subsidence, (2) heat and fluid influx provided by early Cretaceous rifting could trigger density reduction and surface uplift and (3) the present-day geometry of the Loppa High as observed in seismic data can be reproduced by combining the modelled effect of rift flank uplift and phase changes in the mafic body. Phase change-driven vertical movements may also have affected other structural highs in the western Barents Sea, including the Stappen High.

Mineralogical-geochemical features of contact zone rocks of the Shivey alkaline granite-quartz-syenite and Chadal gabbroic massifs (Eastern Tuva)

The Kaakhem (> 30,000 km2) is one of the major magmatic areal within the Altai-Sayan folded area (Eastern Tuva, Russia). Formations of the Kaakhemsky area were more than 250 million years with change of geotectonic conditions. The last stage is bound to intraplatform magmatism (300 Ma ago). Massifs of bimodal association – gabbroic and alkaline granitoids particularly the Chadal gabbroic and the Shivey alkaline granite-quartz-syenite massifs were formed as a result of its activities within the Kaakhem magmatic areal. The object of the study are rocks breccia-like and drip-like structurs in the contact zone of the alkali-granitoid Shivey and gabbroid Chadal massifs in the Kaakhem magmatic. Predecessors repeatedly noted that such structurs relationships have typical features of mingling: flexuous, scalloped contours of mafic bodies with orientation of protrusions towards granitoids; oval, rounded form of basal fragments; signs of plastic deformation in the basices and others. Geochemical analysis of rare and scattered elements contents in contact zone rocks showed strongly marked character of hybridization. Results of scanning electron microscope analysis and X-ray microanalysis let consider the probable sequence of mineralization in contact zone of massifs of bimodal association of gabbroic and alkaline gran itoids of the Kaakhem magmatic areal. Relationship established between microtexture and geochemical inherity indicate the interaction of basic and granitic magmatic meits and it was formation of middle group rocks.

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.

Geochronological and geochemical constraints on sulfide mineralization in the Qingmingshan mafic intrusion in the western part of the Proterozoic Jiangnan orogenic belt along the southern margin of the Yangtze Craton

The Qingmingshan magmatic Fe-Ni-Cu sulfide deposit is located in the southern margin of the Yangtze Craton, in the western part of the Proterozoic Jiangnan orogenic belt. Sulfide mineralization occurs as disseminated zones in a dike-like mafic body intruding Early Neoproterozoic meta-sedimentary rocks. The host intrusion can be further divided into an upper gabbronorite unit, a middle dolerite unit and a lower gabbronorite unit. Each unit contains at least one disseminated sulfide zone. Sulfide mineral assemblages are composed of pyrrhotite, pentlandite and chalcopyrite. Zircon U–Pb dating yields a crystallization age of 847.8±3.8Ma (2σ) for the intrusion. The host rocks are characterized by enrichment in light rare earth elements (REE) relative to heavy REE and show pronounced negative Nb-Ta anomalies, similar to the characteristics of arc mafic igneous rocks worldwide. Mixing calculations indicate that the variations of (Th/Yb)N and (Th/Nb)N in the intrusion are consistent with between 10 and 20wt% crustal contamination of the parental magma. The sulfide separates are characterized by elevated δ34S values from 1.5 to 3.5‰, with a peak at 2.6‰, and by γOs values from 2 to 89, indicating addition of crustal sulfur and Os to the parental magma. The trace element data and S-Os isotopes support the premise that sulfide saturation in the Qingmingshan magma was triggered by crustal contamination. Variations of PGE tenors in different sulfide zones can be explained by variable R-factors from 300 to 1000 during sulfide segregation from the magma. Mass balance calculations indicate that the parental magma was severely depleted in PGE, which could be a primary signature of arc basaltic magma formed by low degrees of partial melting or due to previous sulfide segregation at depth. The lithological and mineralization zonation across the dike-like Qingmingshan mafic intrusion indicates that at least three separate pulses of sulfide-charged magma were involved in the formation of the Qingmingshan magmatic sulfide deposit. The Qingmingshan mafic intrusion is interpreted to be a part of a sub-arc magma plumbing system. Coeval ultramafic intrusions formed from more primitive magma in the region are good exploration targets for magmatic sulfide deposits with higher Ni/Cu ratios than the Qingmingshan deposit.

Ductile–brittle deformation effects on crystal-chemistry and U–Pb ages of magmatic and metasomatic zircons from a dyke of the Finero Mafic Complex (Ivrea–Verbano Zone, Italian Alps)

A detailed microstructural, geochemical and geochronological study was performed on zircon grains from plagioclase-rich dioritic dykes discordantly intruded within meta-diorites/gabbros forming the External Gabbro unit of the Finero Mafic Complex (Italian Alps). This unit is exposed as part of a near complete crustal section spanning from mantle rocks to upper crustal metasediments (Val Cannobina, Ivrea–Verbano Zone, Italy). The leucocratic dykes consist mainly of plagioclase (An18–24Ab79–82Or0.3–0.7) with subordinate amounts of biotite and spinel defining melanocratic layers. Zircon and corundum are common accessory phases. Both the dykes and the surrounding meta-diorites/gabbros show evidence of ductile deformation under amphibolite-facies conditions.Zircon grains/fragments (up to 2mm in length) occur as porphyroclasts surrounded by fine-grained plagioclase within the leucocratic domains and may occur within the melanocratic layers completely or partially surrounded by biotite and spinels. Fractures are common within zircon, define networks and have associated displacements occasionally and/or they can be filled by secondary minerals.Cathodoluminescence (CL) images reveal that zircon grains from the leucocratic layers show relicts of primary magmatic (i.e. oscillatory and or sector) zoning generally related with the crystal shapes or crystallographic orientation, whereas those from the melanocratic domains do not. In both cases, zircon shows secondary CL features, i.e. mosaic-like textures, due to deformation. EBSD maps confirmed a profuse mosaic texture, resulting in an internal misorientation of ca. 10°, generally associated with fractures. Locally, zircon shows clear evidence of crystal-plastic deformation at the edges, with a gradual misorientations of up to 12°, suggesting an origin prior fragmentation.Trace elements and U–Pb analyses were carried out by LA-ICP-MS directly on petrographic thin sections. Such in situ investigations revealed a strong correlation between internal zircon structures, chemistry, U–Pb isotope ratios and mylonitic fabric.U–Pb data return highly discordant and variable ages. The 206Pb/238U ages may range from ca. 297 to 198Ma within the same zircon grain from the leucocratic layers, whereas 206Pb/238U younger than 250Ma were systematically obtained from zircon within the melanocratic layers. The 206Pb/238U data younger than ca. 240Ma from zircon grains within the leucocratic layers were obtained from narrow axial stripes observed in CL images and oriented parallel or at low-angle with respect to the foliation planes. These stripes are characterized by an overall HREE, Y, U and Th enrichment possibly reflecting deformation of the grain in presence of interstitial fluid phases. Combining U–Pb data, microstructure and zircon CL features, we suggest a multistage evolution of the dykes whereby the melanocratic layers are the result of a Late Permian metasomatic event promoting modification of the pre-existing mineral assemblage of the (Late Carboniferous–Early Permian?) dykes and deformation and partial resetting of zircon porphyroclasts. This has important implications in the geology of the Ivrea–Verbano Zone because, having the dyke intruded the External Gabbro unit before Triassic (probably during Carboniferous–Permian), the latter cannot be considered a Triassic intrusion. At least part of the External Gabbro unit is Carboniferous–Permian as the other mafic bodies of the Ivrea–Verbano Zone.

Evidence for brittle deformation events at eclogite-facies P-T conditions (example of the Mt. Emilius klippe, Western Alps)

Eclogitic rocks are crucial for the understanding of tectonic processes as they provide key constraints on both the P-T-t evolutions and the deformation modes sustained by rocks in subduction zones. Here we focus on eclogitised and deformed mafic bodies that are exposed within granulites from the continental basement slice of the Mt. Emilius klippe (Western Alps, Italy). These eclogites exhibit highly deformed garnetite and clinopyroxenite layers. In some places, these deformed rocks (up to mylonitic grade) can be found as clasts within meter-thick brecciated fault rocks that formed close to the lawsonite-eclogite facies peak P-T conditions. Garnet-rich layers are dominated by brittle features, whereas deformation within clinopyroxene-rich layers is accommodated by both creep and fracturing. We present a petro-structural study of these eclogites, that allows to track the brittle deformation history associated with chemical evolution. Based on these data, we propose a new tectono-metamorphic model for these rocks, related to the alpine eclogitic stage. This model is consistent with the coexistence of both ductile and brittle features that developed at similar P-T conditions (i.e., at P~2.15–2.40GPa and T~500–550°C), and closely associated with fluid circulations. Our study demonstrates that crustal material, buried along the subduction interface at HP-LT conditions, can record several successive brittle events in places where deformation is classically envisioned as ductile. We suggest, based on our observations, that strain-rate increase along plate interface shear zones may trigger fracturing and fluid infiltration which in turn enables brittle-ductile instabilities along these deformation networks.

Fluid pathways and high‐P metasomatism in a subducted continental slice (Mt. Emilius klippe, W. Alps)

AbstractThe Mt. Emilius klippe (Western Alps, Italy) corresponds to a segment of the stretched Adriatic continental margin metamorphosed at granulite facies during Permian. This slice was subducted during the early Cenozoic Alpine subduction with the underlying eclogite facies remnants of the Tethyan seafloor (Zermatt‐Saas zone). Near the base of the Mt. Emilius massif, there is a shear zone with eclogite facies hydrofracture systems associated with deformation‐induced re‐equilibration of granulites during high‐P metamorphism. In the basal part of the massif, a pluri‐hectometre domain of sheared mafic boudins is hosted in the granulitic paragneiss. In these mafic boudins, there are garnetites, garnet veins and clinopyroxenites, as well as clinozoisite and calcite veins. These features record multiple events of fracture opening, brecciation, boudinage and parallelization of structures coevally with fluid–rock interaction, metasomatism and volume change. This integrated petrological, micro‐textural and geochemical investigation illustrates the multiplicity and the chemical variability of fluid sources during prograde to peak metamorphic evolution in the lawsonite–eclogite‐facies field (at ~2.15–2.4 GPa, 500–550 °C) during subduction of the Mt. Emilius slice. The calcite veins crosscutting the garnetites have relatively low δ18OVSMOW values (∼+6.5‰) near those for marble layers (and nearby calcsilicates) embedded within the metasomatized granulites (+8 to +10‰). It is proposed that infiltration of externally‐derived H2O‐rich fluids derived from the plate interface flushed the marbles, promoting decarbonation followed by short‐distance transport and re‐precipitation along garnetite fractures. This study highlights the importance of inherited structural heterogeneities (such as mafic bodies or sills) in localizing deformation, draining fluids from the downgoing plate and creating long‐lasting mechanical instabilities during subduction zone deformation.

Zircon in amphibolites from Naxos, Aegean Sea, Greece: origin, significance and tectonic setting

AbstractWe report U–Pb zircon ages of c. 700–550 Ma, 262–220 Ma, 47–38 Ma and 15–14 Ma from amphibolites on Naxos Island in the Aegean extensional province of Greece. The zircon has complex internal structures. Based on cathodoluminescence response, zoning and crosscutting relationships a minimum of four zircon growth stages are identified: inherited core, magmatic core, inner metamorphic (?) rim and an outer metamorphic rim. Trace element compositions of the amphibolites suggest igneous differentiation and crustal assimilation. Zircon solubility as a function of saturation temperatures, Zr content and melt composition indicates that the zircon did not originally crystallize in the mafic bodies but was inherited from felsic precursor rocks, and subsequently assimilated into the mafic intrusives during emplacement. Zircon inheritance is corroborated by the complex, xenocrystic nature of the zircon in one sample. Ages of c. 700–550 Ma and 262–220 Ma are assigned to inherited zircon. Available geochemical data suggest that the 15–14 Ma metamorphic rims grew in situ in the amphibolites, corresponding to a high‐grade metamorphic event at this time. However, the geochemical data cannot conclusively establish if the c. 40 Ma zircon rims also grew in situ, or whether they were inherited along with the xenocrystic cores. Two scenarios for emplacement of the mafic intrusives are discussed: (i) Intrusion during late‐Triassic to Jurassic ocean basin development of the Aegean realm, in which case the 40 Ma zircon rims would have grown in situ, and (ii) emplacement in the Miocene as a result mafic underplating during large‐scale extension. In this case, only the 15–14 Ma metamorphic outer rims would have formed in situ in the amphibolitic host rocks.

New insights from geophysical data on the regional structure and geometry of the southwest Thelon Basin and its basement, Northwest Territories, Canada

Detailed analysis of gravity and aeromagnetic data covering the southwest Thelon Basin, Northwest Territories, Canada, has provided insight into basement geology that has significance to exploration for uranium and possibly other economic metals in a remote frontier region. Interpretation of basement geology has been constrained by the calibration of gravity and magnetic signatures with Precambrian geology adjacent to the basin and sparse seismic data within the basin, creating the first basement geologic map of the southwest Thelon Basin. The basement to the overlying sedimentary units is dominated by magnetic felsic and mafic bodies variably overlying and intruding the gneissic crystalline basement. Supracrustal belts located outside the basin margins are interpreted to continue below the basin fill. Major structures have been delineated geophysically including the Howard Lake Shear Zone and the Bathurst and McDonald fault systems. Northwest-trending structures forming part of the Bathurst fault system appear to control the unconformity surface morphology and the location of basin depocenters. The geologic interpretations are corroborated by joint gravity and magnetic modeling of profiles that reveal the deepest part of the Thelon Basin reaches depths of [Formula: see text] in an area of subdued magnetic and gravimetric response to the north. The basin is a focus of active exploration for uranium, and we have found that areas along the south and eastern margins underlain by U-rich granitoid rocks may have significant potential where intersected by reactivated faults.

Subduction or sagduction? Ambiguity in constraining the origin of ultramafic–mafic bodies in the Archean crust of NW Scotland

The Lewisian Complex of NW Scotland is a fragment of the North Atlantic Craton. It comprises mostly Archean tonalite–trondhjemite–granodiorite (TTG) orthogneisses that were variably metamorphosed and reworked in the late Neoarchean to Paleoproterozoic. Within the granulite facies central region of the mainland Lewisian Complex, discontinuous belts composed of ultramafic–mafic rocks and structurally overlying garnet–biotite gneiss (brown gneiss) are spatially associated with steeply-inclined amphibolite facies shear zones that have been interpreted as terrane boundaries. Interpretation of the primary chemical composition of these rocks is complicated by partial melting and melt loss during granulite facies metamorphism, and contamination with melts derived from the adjacent migmatitic TTG host rocks. Notwithstanding, the composition of the layered ultramafic–mafic rocks is suggestive of a protolith formed by differentiation of tholeiitic magma, where the ultramafic portions of these bodies represent the metamorphosed cumulates and the mafic portions the metamorphosed fractionated liquids. Although the composition of the brown gneiss does not clearly discriminate the protolith, it most likely represents a metamorphosed sedimentary or volcano-sedimentary sequence. For Archean rocks, particularly those metamorphosed to granulite facies, the geochemical characteristics typically used for discrimination of paleotectonic environments are neither strictly appropriate nor clearly diagnostic. Many of the rocks in the Lewisian Complex have ‘arc-like’ trace element signatures. These signatures are interpreted to reflect derivation from hydrated enriched mantle and, in the case of the TTG gneisses, partial melting of amphibolite source rocks containing garnet and a Ti-rich phase, probably rutile. However, it is becoming increasingly recognised that in Archean rocks such signatures may not be unique to a subduction environment but may relate to processes such as delamination and dripping. Consequently, it is unclear whether the Lewisian ultramafic–mafic rocks and brown gneisses represent products of plate margin or intraplate magmatism. Although a subduction-related origin is possible, we propose that an intraplate origin is equally plausible. If the second alternative is correct, the ultramafic–mafic rocks and brown gneisses may represent the remnants of intracratonic greenstone belts that sank into the deep crust due to their density contrast with the underlying partially molten low viscosity TTG orthogneisses.

Assessing the potential of two typical layered complexes from the Arabian Shield for hosting Ni-Cu mineralization through stream sediment geochemistry

Two late Proterozoic (< 630 Ma) mafic layered complexes (Al-Jilani and Shai) from the central and southern Arabian Shield were chosen for a preliminary rock and stream sediment survey to test their potential for hosting Ni–Cu sulfide mineralization. The choice was made on the basis of the highly differentiated and exceptionally well-layered nature of these intrusives, with the ultimate aim of drawing inferences on the mineralization potential of other mafic bodies of the same age and setting. The results showed a marked depletion of the target elements and their pathfinders (Ni, Cu, Co, Cr, and PGE), except for a single anomalous location at Shai which appears to lithologically induced rather than representing a true sulfide signature. The two major problems in the stream sediment results were the masking effect from the host lithologies and the dilution by felsic detritus. Applying factor analysis to the data set failed to yield an “ore factor” that excludes mafic lithophile elements, but revealed the important role of the Fe–Mn oxyhydroxides in adsorbing certain metal cations even in the arid climate of the Arabian Shield, thus opening up the possibility of future work utilizing partial leach techniques in this part of the world. Overall, the two complexes do not seem to satisfy the necessary requirements for Ni–Cu sulfide enrichment in that there is a general lack of an external source of sulfur, in addition to the absence of a concentration mechanism for any produced sulfides.

Mineral chemistry of a Cenozoic igneous complex, the Urumieh-Dokhtar magmatic belt, Iran: Petrological implications for the plutonic rocks

The Niyasar plutonic complex, one of the Cenozoic magmatic assemblages in the Urumieh-Dokhtar magmatic belt, was the subject of detailed petrographic and mineralogical investigations. The Niyasar magmatic complex is composed of Eocene to Oligocene mafic rocks and Miocene granitoids. Eleven samples, representing the major rock units in the Niyasar magmatic complex and contact aureole were chosen for mineral chemical studies and for estimation of the pressure, temperature, and oxygen fugacity conditions of mineral crystallization during emplacement of various magmatic bodies. The analyzed samples are composed of varying proportions of quartz, plagioclase, K-feldspar, hornblende, biotite, titanite, magnetite, apatite, zircon, garnet, and clinopyroxene. Application of the Al-in-hornblende barometer indicates pressures of around 0.2 to 0.4 kbar for the Eocene–Oligocene mafic bodies and around 0.5 to 1.7 kbar for the Miocene granitoids. Hornblende-plagioclase thermometry yields relatively low temperatures (661–780 °C), which probably reflect late stage re-equilibration of these minerals. The assemblage titanite–magnetite–quartz as well as hornblende composition were used to constrain the oxygen fugacity and H2O content during the crystallization of the parent magmas in the Miocene plutons. The results show that the Miocene granitoids crystallized from magmas with relatively high oxygen fugacity and high H2O content (~5 wt% H2O). The Miocene granitoids show similar range of oxygen fugacity, H2O contents and mineral chemical compositions, which indicate a common source for their magmas. Although the crystallization pressures of the Miocene plutons discriminate various categories of plutonic bodies emplaced at depths of about 5.7–6.5 km (Marfioun pluton), about 4.2 km (Ghalhar pluton) and 1.9–2.3 km (Poudalg pluton), they were later uplifted to the same level by vertical displacement of faults. The emplacement depths of the Niyasar plutons suggest that the central part of the Urumieh-Dokhtar magmatic belt has experienced an uplift rate of ca. 0.25–0.4 mm/yr from the Miocene onwards. Urumieh-Dokhtar火成岩帯の新生代火成岩類の一つであるNiyasar深成複合岩体について詳細な記載岩石学・鉱物学的研究を行った.Niyasar火成複合岩体は始新世から漸新世のマフィック岩と中新世の花崗岩類からなる.Niyasar火成複合岩体とその接触変成帯から採取した11試料について鉱物組成を分析し,各岩体の定置時の晶出温度・圧力,酸素分圧を推定した.分析試料は石英,斜長石,カリ長石,普通角閃石,黒雲母,チタナイト,磁鉄鉱,燐灰石,ジルコン,ザクロ石,単斜輝石からなる.普通角閃石Al圧力計は始新世―漸新世マフィック岩体について0.2―0.4 kbar,中新世花崗岩類について0.5―1.7 kbarを示す.普通角閃石―斜長石温度計は比較的低温(661–780 ˚C)を示すが,これは後の再平衡によるものと思われる.中新世深成岩類のチタナイト―磁鉄鉱―石英組合せと普通角閃石組成からマグマが固化した時の酸素分圧と含水量を制約した.その結果,中新世花崗岩類は比較的高い酸素分圧と高含水量(およそ5 wt%)のマグマから晶出したことがわかった.中新世花崗岩類は同様の酸素分圧,含水量,鉱物組成を示すことから,同じ起源物質に由来すると考えられる.中新世深成岩類の晶出圧力は様々な定置深度5.7―6.5 km(Marfioun深成岩体), ~4.2 km(Ghalhar深成岩体),1.9–2.3 km(Poudalg深成岩体)を示すが,後の断層運動によって同じ深さにまで上昇した.Nayasar深成岩体の定置深度はUrumieh-Dokhtar火成岩帯中央部が中新世以降年間0.25―0.4 mmの速度で上昇したことを示唆する. Please note: The publisher is not responsible for the content or functionality of any supporting information supplied by the authors. Any queries (other than missing content) should be directed to the corresponding author for the article.