Mafic Units Research Articles

Tonalite-Dominated Magmatism in the Abitibi Subprovince, Canada, and Significance for Cu-Au Magmatic-Hydrothermal Systems

In Archean greenstone belts, magmatism is dominated by intrusive and volcanic rocks with tholeiitic affinities, as well as tonalite- and granodiorite-dominated large-volume batholiths, i.e., tonalite–trondhjemite–granodiorite (TTG) suites. These intrusions are associated with poorly documented mineralization (Cu-Au porphyries) that, in the Neoarchean Abitibi Subprovince (>2.79 to ~2.65 Ga), Superior Province, Canada, are associated with diorite bearing plutons, i.e., tonalite–trondhjemite–diorite (TTD) suites. The importance of TTG versus TTD suites in the evolution of greenstone belts and of their magmatic-hydrothermal systems and related mineralization is unconstrained. The aim of this study was to portray the chemistry and distribution of these suites in the Abitibi Subprovince. The study used data compiled by the geological surveys of Québec and Ontario to evaluate the chemistry of TTG and TTD suites and uncovered two coeval magmas that significantly differentiated (fractional crystallization mostly): 1) a heavy rare earth elements (HREE)-depleted tonalitic magma from high pressure melting of an hydrated basalt source; and 2) a hybrid HREE-undepleted magma that may be a mixture of mantle-derived (tholeiite) and tonalitic melts. The HREE-depleted rocks (mostly tonalite and granodiorite) display chemical characteristics of TTG suites (HREE, Ti, Nb, Ta, Y, and Sr depletion, lack of mafic unit, Na-rich), while the other rocks (tonalite and diorite) formed TTD suites. Tonalite-dominated magmatism, in the Abitibi Subprovince, comprises crustal melts as well as a significant proportion of mantle-derived magmas and this may be essential for Cu-Au magmatic-hydrothermal mineralizing systems.

Lithostratigraphy of the Palaeoproterozoic Verena Granite

Abstract The Verena Granite forms part of the Palaeoproterozoic Lebowa Granite Suite of the Bushveld Complex and was named after the village of Verena in the Mpumalanga Province of South Africa. It occurs over an area of ~600 km2 and is intrusive into the Rooiberg Group, the Rashoop Granophyre Suite and the Klipkloof Granite. It is in turn intruded by the Makhutso Granite, the youngest known granite of the Lebowa Granite Suite. The Verena Granite is characterised by its coarse to very coarse-grained nature, its pinkish to reddish colours and its porphyritic texture defined by the presence of large perthitic K-feldspar phenocrysts within a finer grained groundmass of plagioclase (An8-15) and quartz. Geochemically it can be classified as an A-type granite that straddles the boundary between metaluminous and peraluminous compositions. The granite is enriched in REEs relative to chondrite and shows strong fractionation of the LREEs, a distinct negative Eu anomaly and little fractionation of the HREEs. U-Pb dating presented here places the age of the Verena Granite at 2052 ± 9 Ma, which is the same as that of the published 2054 ± 2 Ma age of the Nebo Granite. Currently no consensus exists regarding the petrogenesis of the Verena Granite. Doubts have been cast on a genetic link between the Verena Granite and the remainder of the Nebo Granite. A genetic link between the Klipkloof Granite and the Verena Granite appears likely, with the former possibly representing the rapidly chilled roof of the magmas that crystallised to form the latter. Lu-Hf isotope data on zircons are consistent with that from other units of the Lebowa Granite Suite. It also supports the unconventional model involving a common enriched mantle origin for all mafic and felsic units of the Bushveld Complex, with minimal input from older crust.

Caught between two continents: First identification of the Ediacaran Central Iapetus Magmatic Province in Western Svalbard with palaeogeographic implications during final Rodinia breakup

The final fragmentation of Rodinia occurred during the Ediacaran period as the continental blocks of Baltica and Laurentia, as well as Amazonia and West Africa, rifted and drifted apart. It was along this progressively rifted margin that the Iapetus Ocean opened and subsequently closed, creating the Caledonian orogeny (sensu lato), some 100 and 200 million years later. The 0.62–0.54 Ga Central Iapetus Magmatic Province (CIMP), accompanying this break-up, is variously manifested in northern Europe and north-eastern North America as mafic dykes and sills, volcanic rocks, as well as carbonatites. This magmatism is interpreted to be broadly coincident with the so-called Marinoan and Gaskiers glaciations, and the subsequent evolution of metazoans. While the various Precambrian basement blocks of Svalbard are known to carry a Caledonian overprint, the proximity of various parts of Svalbard to either Baltica or Laurentia during final Rodinia breakup in the Ediacaran has yet to be established. Petrographic, geochronological and geochemical data from the variably deformed and metamorphosed coarse-grained mafic units of Oscar II Land (OIIL) from Spitsbergen, Svalbard, are presented here to determine the source of the magma and to elucidate on their palaeogeographic affinity with adjacent crustal blocks/units. An alkali and OIB-like affinity of the mafic units is confirmed by their whole-rock geochemical composition. An ID-TIMS U-Pb baddeleyite crystallisation age determination of 560 ± 12 Ma, obtained from one such mafic unit, contrasts with the published Caledonian (sensu lato) age determinations. These data for OIIL compare well with the broadly coeval and geochemically similar 570–560 Ma Norwegian Seiland Igneous Province, which may have formed a part of Baltica. Temporally and compositionally similar mafic units also occur within the North American Appalachian Belt, as well as the Sept-Îles intrusion, which is part of Laurentia. However, any links between these intrusions with those of Svalbard are yet to be unequivocally demonstrated.

U–Pb geochronology and Hf isotope data from the Late Cretaceous Mawat ophiolite, NE Iraq

The Mawat ophiolite, NE Iraq, is one of the Neo-Tethyan ophiolites within the Iraqi Zagros orogen. It consists of many metre-to kilometre-sized tectonic slices of serpentinized dunite, peridotite, gabbro, basaltic rocks and associated oceanic metasediments. Felsic intrusions crosscut the ophiolite. We present U–Pb zircon and monazite ages and Hf zircon isotopes from two crosscutting felsic dykes and a gabbro from the mantle section of the ophiolite. Zircons from the felsic dykes contain spongy domains and xenotime and monazite inclusions. They give ages from 222 to 46 Ma. The age range is interpreted to be caused by secondary processes such as radiogenic Pb mobility and Pb loss. The monazite age of 94.6 ± 1.2 Ma is considered to give a crystallisation age of the felsic dykes. The gabbro zircons give ages between 81 to 38 Ma of which the two oldest grains give the weighted average age of 81.2 ± 2.5 Ma which we interpret to be the crystallisation age of the gabbro. The zircon initial εHf values in the felsic dykes are negative (averages -2.7 and -3.1) while they in the gabbro are positive (average + 3.5), indicating that the felsic magma comes from an older source while the mafic magma comes from a juvenile one. Two mafic units of different ages were identified: the older unit is cut by the 95 Ma felsic dykes and the younger one is represented by the 81 Ma gabbro located within a thrust zone. The youngest ages of 40 Ma are considered to be related to crustal extension.

The Ribeirão da Folha ophiolite-bearing accretionary wedge (Araçuaí orogen, SE Brazil): New data for Cryogenian plagiogranite and metasedimentary rocks

The Araçuaí orogen and West Congo belt make up a singular confined orogenic system, embraced by the San Francisco – Congo craton: the AWCO. It includes a southern sector with ophiolite bodies and magmatic arc, and a northern sector free of them, suggesting the precursor basin was an embayment partially floored by oceanic crust. The northernmost ophiolitic rock-assemblage found in the Araçuaí orogen comprises metamafic and meta-ultramafic rocks with signatures of ocean-floor magmas, and associated pelagic to oceanic metasedimentary rocks of the Ribeirão da Folha Formation. Although tectonically dismembered and metamorphosed, those rocks resemble the classical ophiolite pseudostratigraphy. The Ribeirão da Folha Formation comprises rocks expected to be found in the upper units of an ophiolite edifice, like Al-rich micaschist (pelagic pelite), graphite-rich schist (black shale), sulfide-bearing metachert, diopsidite with massive sulfide, and banded iron formations (chemical-exhalative sediments), and sulfide-bearing fine-grained ortho-amphibolite with thin metachert intercalations (mafic volcanic unit). That formation hosts tectonic slices of banded ortho-amphibolite (dolerite) with plagiogranite veins, and coarse-grained massive ortho-amphibolite (gabbro), representing dismembered slivers from deeper mafic units, and slices of meta-ultramafic rocks from the deepest ophiolite units. Zircon crystals from a plagiogranite vein yielded the U-Pb SHRIMP age of 645 ± 10 Ma, providing a new time constraint for ocean-floor emplacement. Micaschist samples show chemical attributes typical of distal passive margin pelites. Among three progressive deformation phases, the main ductile phase (Dn) shows kinematic indicators related to top to SW mass transport, associated with intermediate P-T (St, Ky, Sil) metamorphic zoning. Although the few youngest grains of detrital zircon from three siliciclastic samples have distinct ages (around 599 Ma, 741 Ma, and 816 Ma), their wide-range age spectra and Hf signatures suggest similar sediment sources. The wide lithological variety and stratigraphic complexity along with the intricate tectonic framework of the Ribeirão da Folha region, comprising thrust slices of ophiolitic rocks tectonically interleaved with older rift-related rocks, characterize an accretionary wedge that was scrapped off the subducted slab and involved in collisional tectonics, marking the AWCO suture zone for some 250 km between the Guanhães basement block (lower plate) and Rio Doce magmatic arc (upper plate).

U-Pb geochronology, petrogenesis and tectonomagmatic evolution of uppermost Neoproterozoic- lower Cambrian intrusive rocks in Kaboodan area, NE of Iran

ABSTRACT This study presents new U-Pb zircons ages, trace element geochemistry, and Nd and Sr isotope compositions of intrusive rocks from the Kaboodan area, NE of Iran. The Kaboodan area embraces two geologically distinct zones of Taknar and Sabzevar. All intrusive rocks can be divided into two age groups, uppermost Neoproterozoic and Cretaceous, respectively. Uppermost Neoproterozoic and lower Cambrian plutonic rocks are found in the Taknar Zone section and include different mafic and felsic types. Mafic bodies include gabbroic- dioritic lithologies with ages around 552–545 Ma and low 87Sr/86Sri (0.7032–0.7041) and high ɛNdt (+4.82 to +8.49) values. These lithologies may have been generated by shallow depth partial melting of a spinel peridotite mantle source in an arc setting. Felsic intrusive rocks show two distinct U-Pb ages. The older granitoids (ONG) have uppermost Neoproterozoic age as the mafic rocks (549–547 Ma) while younger samples (YCG) have been formed ca. 20 m.y. later in the lower Cambrian (531–528 Ma). The ONG and YCG rocks show completely different geochemical characteristics. The ONG samples exhibit I-type granitoid properties with lower 87Sr/86Sri (0.704–0.707) and positive ɛNdt (+2.49 to +3.68) values, while YCG rocks display S-type granitoid signatures with higher 87Sr/86Sri (0.710–0.714) and negative ɛNdt (−4.54 and −6.95). The field relations, Sr-Nd isotope systematics, and geochemical variations indicate the interaction of the uppermost Neoproterozoic mafic and ONG units. The ONG melts were originated from the partial melting of mantle-derived material in a subduction zone. The trace element composition of the YCG samples is similar to within plate granitoids and was generated by the melting of meta-sedimentary crustal rocks during syn- to post-collisional events indicative of the end of Cadomian subduction at ∼ 530 Ma.

The Secret Life of Granite: A Story of Emplacement, Alteration, Subduction and Metamorphism

Ultra-high pressure (UHP) metamorphism has long been associated with deep subduction. UHP minerals, such as coesite and microdiamond, are often found in discrete mafic units hosted in larger scale ...

Geochemical and tectonic significance of Arbat alkali gabbro-monzonite-syenite intrusions, Urumieh–Dokhtar Magmatic Arc, Iran

The Oligocene Arbat alkali intrusions of the Eastern Miandoab are located in the northwestern part of Iran and belong to the Urumieh–Dokhtar Magmatic Arc (UDMA). The intrusions show a ring structure with gabbro-monzogabbro-monzodiorite (mafic units) on the edges, with monzonite-monzosyenite-syenite (felsic units) gradually going towards the central parts. The textures in different rock types are cumulate, granular and laminated. The high values of (La/Sm)n and (La/Yb)n, contents of K, Rb and Cs (positive anomalies normalized on the basis of the primitive mantle), low concentrations of Hf, Nb, Zr and Ta (negative anomalies), and the changes in Th/Nb, Th/Ta, La/Nb and Ce/Pb ratios along with the geochemical and tectonic setting evidence exhibit a subduction-modified mantle origin for the formation of these rocks. Accordingly, the intrusions were formed between the Central Iran and the Arabian plates as a result of the partial melting of a mantle wedge at a syn-collision or post-collision arc-related environment. Our data suggested that, after the end of the oblique Neotethys subduction and during/after the continental collision, the break-off or rollback of the Neotethys slab beneath western Iran, in the Oligocene, might have occurred. Such a process led to the change in the geothermal gradient of the mantle wedge because of the subduction fluids, transtension, pressure reduction along the SE-trending lateral depth strike-slip fault zones in the upper part of the mantle wedge, decompression partial melting at the mantle, and the resulting formation of a mafic potassium-rich melt. The mafic magma was injected into crustal magma chambers; probably, the fractional crystallization and partial contamination occurred with crustal components, forming the intermediate and felsic rocks in the intrusions. Geochemical evidence related to the variations in the ratios of Th/Yb, Ta/Yb, Rb/Y, and Nb/Y and Harker variation diagrams along with the spider diagrams confirmed fractional crystallization and partial FC (fractional crystallization) and AFC (assimilation and fractional crystallization) in the intrusions.

Variations of Major and Minor Elements in Pt–Fe Alloy Minerals: A Review and New Observations

Compositional variations of major and minor elements were examined in Pt–Fe alloys from various geological settings and types of deposits, both lode and placer occurrences. They included representatives of layered intrusions, Alaskan-Uralian-(Aldan)-type and alkaline gabbroic complexes, ophiolitic chromitites, and numerous placers from Canada, USA, Russia, and other localities worldwide. Pt–Fe alloy grains in detrital occurrences are notably larger in size, and these are considered to be the result of a special conditions during crystallization such as temperature, pressure, geochemistry or time. In addition, the number of available statistical observations is much greater for the placer occurrences, since they represent the end-product of, in some cases, the weathering of many millions of tonnes of sparsely mineralized bedrock. Typically, platinum-group elements (PGE) present in admixtures (Ir, Rh, and Pd) and minor Cu, Ni are incorporated into a compositional series (Pt, PGE)2–3(Fe, Cu, Ni) in the lode occurrences. Relative Cu enrichment in alloys poor in Pt implies crystallization from relatively fractionated melts at a lower temperature. In contrast to the lode deposits, the distribution of Ir, Rh, and Pd is fairly chaotic in placer Pt–Fe grains. There is no relationship between levels of Ir, Rh, and Pd with the ratio Σ(Pt + PGE):(Fe + Cu + Ni). The compositional series (Pt, PGE)2–3(Fe, Cu, Ni) is not as common in the placer occurrences; nevertheless, minor Cu and Ni show their maximums in members of this series in the placer grains. Global-scale datasets yield a bimodal pattern of distribution in the Pt–Fe diagram, which is likely a reflection of the miscibility gap between the ordered Pt3Fe structure (isoferroplatinum) and the disordered structure of native or ferroan platinum. In the plot Pt versus Fe, there is a linear boundary due to ideal Pt ↔ Fe substitution. Two solid solution series are based on the Ir-for-Pt and Pd-for-Pt substitutions. The incorporation of Ir is not restricted to Pt3Fe–Ir3Fe substitution (isoferroplatinum and chengdeite, plus their disordered modifications). Besides, Ir0 appears to replace Pt0 in the disordered variants of (Pt–Ir)–Fe alloys. There is a good potential for the discovery of a new species with a Pd-dominant composition, (Pd, Pt)3Fe, most likely in association with the alkaline mafic-ultramafic or gabbroic complexes, or the mafic units of layered intrusions. The “field of complicated substitutions” is recognized as a likely reflection of the crystallochemical differences of Pd and Ir, extending along the Ir-Pd axis of the Ir–Pd–Rh diagram. The inferred solid solution extends approximately along the line Ir–(Pd:Rh = 2:3). Minor Pd presumably enters the solid solution via a coupled substitution in combination with the Rh. An Ir-enrichment trend in Pt–Fe alloys typically occurs in the Alaskan-type complexes. The large size of the Pt–Fe nuggets associated with some of these complexes is considered to be related to an ultramafic-mafic pegmatite facies, whereas significant Pd-enrichment is characteristic of gabbroic source-rocks (e.g., Coldwell Complex), resulting in a markedly different trend for the Pt versus Fe (wt.%). However, based on our examination of a large dataset of Pt–Fe alloys from numerous origins, we conclude that they exhibit compositional overlaps that are too large to be useful as reliable index-minerals.

Stratigraphy and Geochemistry of the Goedgenoeg and Makwassie Formations, Ventersdorp Supergroup, in the Bothaville area of South Africa

Abstract The Neoarchaean Ventersdorp Supergroup represents a fully evolved bimodal volcano-sedimentary succession on the Kaapvaal Craton. Stratigraphic information was mainly derived from exploration drilling for gold in the underlying Witwatersrand Supergroup. Following the basal flood basalts of the Klipriviersberg Group, the Platberg Group mostly comprises extensive and voluminous intermediate to felsic volcanics of the Goedgenoeg and Makwassie Formations. Detailed stratigraphic and geochemical reports of these two formations were previously mostly restricted to the so-called Bothaville gap, between the Free State and Klerksdorp Gold Fields. The lower part of the Goedgenoeg Formation succession consists of non-porphyritic andesitic lava flows, followed upwards by dacitic phenocryst-rich feldspar porphyries. The basal lava flows are frequently interbedded with sedimentary rocks of the underlying Kameeldoorns Formation. Andesitic lava-flow units persist upwards, occurring interbedded with the Goedgenoeg Formation feldspar porphyries. The feldspar porphyries originated as ash flows; most are highly welded. The upper feldspar porphyries in the succession are absent in large parts of the Bothaville gap. The overlying Makwassie Formation consists of quartz porphyries, also with intermittent interbedded mafic lava units throughout the succession. The upper part of the succession is composed of phenocryst-rich crystal tuffs, only preserved over limited areas of the Bothaville gap. The quartz porphyries are highly welded, high-temperature ash flows with individual flows of tens of metres in thickness, typically separated by thin tuffs and sediments. The Goedgenoeg and Makwassie interbedded andesitic lavas have similar geochemical composition (the geochemical composition of the overlying Rietgat Formation lavas is also similar to that). The Goedgenoeg dacitic feldspar porphyries are separated into two geochemical groups: a lower high-Zr group and an upper low-Zr group. The Makwassie quartz porphyries are of dacitic compositions, while the upper part is rhyolitic. Two geochemical groups can be distinguished in the rhyolites, based on the Zr/Cr ratio. Although each of the two groups occur as distinct stratigraphic units, in some boreholes the low-Zr/Cr group form the lower part of the rhyolite succession overlain by the high-Zr/Cr group, while in other areas the sequence is reversed. A significant hiatus may have existed between eruption of the Goedgenoeg and Makwassie volcanics. The andesitic mafic lavas persistently co-erupted with the Goedgenoeg dacites and with the Makwassie dacites and rhyolites, indicating a stable mafic magma source that was re-mobilised from time to time to initiate eruption of the more viscous dacite and rhyolite crustal melts. The Goedgenoeg and Makwassie dacites are mixed magmas of the mafic andesitic source with crustal melts, while the Makwassie Formation rhyolites are a distinct crustal melt.

A Neoproterozoic hyper-extended margin associated with Rodinia's demise and Gondwana's build-up: The Araguaia Belt, central Brazil

The Araguaia Belt encloses a poorly constrained Pan-African (Brasiliano Cycle) continental suture marked by a series of (~750 Ma) ophiolitic units which, when properly characterized, could provide important informations on its geological history, closely linked with the Rodinia demise and further western Gondwana amalgamation. We present new bulk-rock and mineral major and trace element compositions for these ultramafic and mafic units. They mainly consist in fully serpentinized harzburgite, scarce dunite lenses and chromite pods, tectonically overlain by basaltic pillow lavas. Low Al2O3/SiO2 ratios (0.01 to 0.06), rather high MgO concentrations (42.28 to 45.29 wt%) and spinels' Cr# and Mg# ratios comprised between 0.36 and 0.51 and 0.59 and 0.72, respectively, indicate a depleted oceanic-like protolith. MORB-peridotite interactions are evidenced both by pyroxenite, olivine gabbro and diabase occurrences in the serpentinites and by high TiO2 (up to 0.42 wt%) contents in spinels from some Serra do Quatipuru serpentinites. These observations support that the Araguaia Belt ophiolitic bodies are the remnants of the upper mantle section of a MOR or subcontinental lithosphere. The serpentinites whole-rock REE content can be modeled as resulting from a dry partial melting involving 14 to 24% of melt extraction, coupled with refertilization by fertile melts, generated deeper in the mantle. Such an oceanic-like setting is also supported by the N-MORB signature of Serra do Tapa and Morro do Agostinho pillow lavas basalts. All together, these results tend to infirm the supra-subduction zone (SSZ) setting previously proposed for these ophiolitic units. Important LILE, B and Li enrichments in the serpentinites likely result from a metasomatic event involving sediments-derived fluids that occurred during the obduction of the units on the Amazonian Craton. Our results combined with (1) the apparent scarcity of igneous crustal rocks, (2) the proximal nature of the metasedimentary rocks hosting the ophiolitic units, and (3) the occurrences of Amazonian Craton fragments eastward of the ophiolitic bodies, allow us to propose that the Araguaia Belt comprises a fossil ocean-continent transition (OCT) accreted on the eastern border of the Amazonian Craton.

Petrology, physical volcanology and geochemistry of a Paleoproterozoic large igneous province: The Hekpoort Formation in the southern Transvaal sub-basin (Kaapvaal craton)

The ∼2.23 Ga Hekpoort Formation (Transvaal sub-basin) and the ∼2.43 Ga Ongeluk Formation (Griqualand West sub-basin) represent voluminous Paleoproterozoic igneous events on the Kaapvaal craton of South Africa that predate the emplacement of the ∼2.055 Ga Bushveld Complex, and probably covered most of the craton at the time of their extrusion. In this contribution, we present field, petrological and geochemical studies of the Hekpoort Formation and compare it with the Ongeluk Formation. The Hekpoort Formation consists of a thick subaerial volcanic sequence in which volcanoclastic rocks occur mainly at the base. Rare, localized hyaloclastites and variolitic rocks record the presence of ponded water, while interbedded sedimentary rocks and paleo-weathered flow tops suggest prolonged time-breaks in volcanic activity. The Hekpoort rocks underwent metamorphism up to greenschist facies but also episodes of metasomatism and silicification. Preserved primary magmatic minerals are clinopyroxene (pigeonite, augite and diopside), and rarely plagioclase (labradorite). Both the variable whole rock Mg# (evolving from 69 to 50) and the changes in clinopyroxene composition attest to magmatic fractionation. Lava units of both the Hekpoort and Ongeluk formations are mostly basalts, with silicification responsible for increased SiO2 contents. Lava units of both formations also display remarkably similar trace elements patterns, which is noteworthy for units separated by 200 million years, and unique among the Precambrian mafic magmatic units of the Kaapvaal craton that we evaluated.Similar to other Precambrian mafic magmatic units of the Kaapvaal craton, the Hekpoort Formation shows an arc-like trace element signature, mainly represented by negative Nb-Ta anomalies (in normalized trace element patterns). The Hekpoort (and Ongeluk), together with three other Paleoproterozoic mafic units of the craton older than 2.2 Ga, exhibit relatively high contents of Th and U, which sharply contrasts with Archean units. The data suggest that a subduction process marked the Archean-Proterozoic boundary on the Kaapvaal craton.

Petrogenesis of plagiogranites in the Muslim Bagh Ophiolite, Pakistan: implications for the generation of Archaean continental crust

AbstractHigh-SiO2 rocks referred to as oceanic plagiogranites are common within the crustal sequences of ophiolites; however, their mode of petrogenesis is controversial with both late-stage fractional crystallization and partial melting models being proposed. Here, we present new whole-rock data from plagiogranitic dyke-like bodies and lenses from the lower and middle sections of the sheeted dyke complex of the Cretaceous Muslim Bagh Ophiolite, northwestern Pakistan. The plagiogranites have similar geochemical signatures that are inconsistent with them being the fractionation products of the mafic units of the Muslim Bagh Ophiolite. However, the plagiogranites all display very low TiO2 contents (<0.4 wt%), implying that they formed by partial melting of mafic rocks. Melt modelling of a crustal gabbro from the Muslim Bagh Ophiolite shows that the trace-element signature of the plagiogranites can be replicated by 5–10% melting of a crustal hornblende gabbro with amphibole as a residual phase, resulting in a concave-up middle rare Earth element pattern. Compositional similarities between the Muslim Bagh Ophiolite plagiogranites and Archaean TTG (trondhjemite–tonalite–granodiorite) has implications for the generation of juvenile Archaean continental crust. As the Muslim Bagh Ophiolite was derived in a supra-subduction zone, it is suggested that some Archaean TTG may have been derived from melting of mafic upper crust in early subduction-like settings. However, due to the small volume of Muslim Bagh Ophiolite plagiogranites, it is inferred that they can be instructive on the petrogenesis of some, but not all, Archaean TTG.

Silicic, high- to extremely high-grade ignimbrites and associated deposits from the Paraná Magmatic Province, southern Brazil

The Cretaceous trachydacites and dacites of Chapecó type (ATC) and dacites and rhyolites of Palmas type (ATP) make up 2.5% of the ~800.000km3 of volcanic pile in the Paraná Magmatic Province (PMP), emplaced at the onset of Gondwana breakup. Together they cover extensive areas in southern Brazil, overlapping volcanic sequences of tholeiitic basalts and andesites; occasional mafic units are also found within the silicic sequence. In the central region of the PMP silicic volcanism comprises porphyritic ATC-type, trachydacite high-grade ignimbrites (strongly welded) overlying aphyric ATP-type, rhyolite high- to extremely high-grade ignimbrites (strongly welded to lava-like). In the southwestern region strongly welded to lava-like high-grade ignimbrites overlie ATP lava domes, while in the southeast lava domes are found intercalated within the ignimbrite sequence. Characteristics of these ignimbrites are: widespread sheet-like deposits (tens to hundreds of km across); absence of basal breccias and basal fallout layers; ubiquitous horizontal to sub-horizontal sheet jointing; massive, structureless to horizontally banded-laminated rock bodies locally presenting flow folding; thoroughly homogeneous vitrophyres or with flow banding-lamination; phenocryst abundance presenting upward and lateral decrease; welded glass blobs in an ‘eutaxitic’-like texture; negligible phenocryst breakage; vitroclastic texture locally preserved; scarcity of lithic fragments. These features, combined with high eruption temperatures (≥1000°C), low water content (≤2%) and low viscosities (104–7Pas) suggest that the eruptions were characterized by low fountaining, little heat loss during collapse, and high mass fluxes producing extensive deposits.

Naturaliste Plateau: constraints on the timing and evolution of the Kerguelen Large Igneous Province and its role in Gondwana breakup

ABSTRACTVolcanism associated with the Kerguelen Large Igneous Province is found scattered in southwestern Australia (the ca 136 to ca 130 Ma Bunbury Basalts, and ca 124 Ma Wallaby Plateau), India (ca 118 Ma Rajmahal Traps and Cona Basalts), and Tibet (the ca 132 Ma Comei Basalts), but apart from the ∼70 000 km2 Wallaby Plateau, these examples are spatially and volumetrically minor. Here, we report dredge, geochronological and geochemical results from the ∼90 000 km2 Naturaliste Plateau, located ∼170 to ∼500 km southwest of Australia. Dredged lavas and intrusive rocks range from mafic to felsic compositions, and prior geophysical analyses indicate these units comprise much of the plateau substrate. 40Ar/39Ar plagioclase ages from mafic units and U–Pb zircon ages from silicic rocks indicate magmatic emplacement from 130.6 ± 1.2 to 129.4 ± 1.3 Ma for mafic rocks and 131.8 ± 3.9 to 128.2 ± 2.3 Ma for silicic rocks (2σ). These Cretaceous Naturaliste magmas incorporated a significant component of continental crust, with relatively high 87Sr/86Sr (up to 0.78), high 207Pb/204 Pb ratios (15.5–15.6), low 143Nd/144Nd (0.511–0.512) and primitive-mantle normalised Th/Nb of 11.3 and La/Nb of 3.97. These geochemical results are consistent with the plateau being underlain by continental basement, as indicated by prior interpretations of seismic and gravity data, corroborated by dredging of Mesoproterozoic granites and gneisses on the southern plateau flank. The Cretaceous Naturaliste Plateau igneous rocks have signatures indicative of extraction from a depleted mantle, with trace-element and isotopic values that overlap with Kerguelen Plateau lavas reflect crustal contamination. Our chemical and geochronological results therefore show the Naturaliste Plateau contains evidence of an extensive igneous event representing some of the earliest voluminous Kerguelen hotspot magmas. Prior work reports that contemporaneous correlative volcanic sequences underlie the nearby Mentelle Basin, and the Enderby Basin and Princess Elizabeth Trough in the Antarctic. When combined, the igneous rocks in the Naturaliste, Mentelle, Wallaby, Enderby, Princess Elizabeth, Bunbury and Comei-Cona areas form a 136–124 Ma Large Igneous Province covering >244 000 km2.

The late Neoproterozoic Dahanib mafic-ultramafic intrusion, South eastern Desert, Egypt: Is it an Alaskan-type or a layered intrusion?

In Egypt, mafic-ultramafic complexes have been classified into three major types: incomplete ophiolite sequences; Alaskan-type intrusions, concentrically-zoned bodies formed in a subduction arc environment; and layered intrusions, vertically-zoned bodies intruded in post-collisional tectonic environments and rift-related bodies associated with the opening of the Red Sea. We present new field work, geochemical data, mineral chemistry and interpretations for the late Neoproterozoic Dahanib mafic-ultramafic intrusion in the South Eastern Desert of Egypt (northernmost Arabian–Nubian Shield, ANS). The Dahanib intrusion shows no evidence of metamorphism or deformation, with excellent preservation of intrusive contacts, well-preserved textures and primary mineralogy. Field relations indicate that it is younger than the surrounding metamorphic rocks and syn-tectonic granitoids. The intrusion is composed of a basal suite of ultramafic rocks (dunite, lherzolite, wehrlite and pyroxenite) and an overlying suite of mafic rocks (olivine gabbronorite, gabbronorite and anorthosite). It displays evident layering of modal abundance, visible directly in outcrop, as well as cryptic layering discernible through changes in mineral compositions. The western and eastern lobes of the Dahanib intrusion occur in the form of a lopolith with readily correlated layers, especially in the upper mafic unit. The present-day dip of the layering decreases from the ultramafic units into the mafic sequence. Structural and compositional relations show that the ultramafic units are cumulates from a high-Mg tholeiitic parent magma emplaced at deep crustal levels and evolved via fractional crystallization rather than any kind of residual mantle sequence. Fo content of olivine and Mg# of pyroxenes display a systematic decrease from ultramafic to mafic rocks, well-correlated with whole-rock Mg#. Spinels in ultramafic samples vary from Cr-rich to Al-rich and have Mg# and Fe^(3+)# similar to spinels from typical stratiform complexes and clearly different from those found in ophiolitic and Alaskan-type complexes. Although the mafic and ultramafic units are clearly related and can be derived from common parent magma, they were not emplaced coevally; rather, they represent different pulses of magma. The Dahanib mafic–ultramafic intrusion does not display any features that convincingly identify it as a typical Alaskan-type body, particularly the lack of clinopyroxenite and hornblendite, rarity of primary hornblende, and the notable abundance of orthopyroxene and plagioclase in its rocks. Our results confirm that it is more akin to a layered mafic-ultramafic intrusion with a multistage evolution. It was emplaced into a stable post-orogenic cratonic setting, with a trace element signature indicating contamination of the mantle source by previous subduction events.

Metal accumulation in soils derived from volcano-sedimentary rocks, Rio Itapicuru Greenstone Belt, northeastern Brazil

Many countries and some Brazilian regions have defined the guideline values for metals in soils. However, the local geological features may be so heterogeneous that global or even regional guideline values cannot be applied. The Greenstone Belts are worldwide geological formations of vast extension, containing mineralization of various metals (e.g., Au, Cr, Ni, and Ag). Natural concentrations of soils must be known to correctly assess the impact of mining. We studied the soils of the Rio Itapicuru Greenstone Belt (RIGB), of Paleoproterozoic age, sampling at 24 sites (0–0.20m) in the areas not or minimally human impacted, equally distributed in the three units of the RIGB: Volcanic Mafic Unit (VMU), Volcanic Felsic Unit (VFU), and Volcano-clastic Sedimentary Unit (SU). The natural pseudo-total concentrations of Cr, Ni, Cu, Zn, Pb, Fe, and Mn were obtained by acid digestion (EPA3050b) both in the soil and the particle-size fractions (sand and clay+silt). The concentrations of metals in RIGB soils, especially Cr and Ni, are generally higher than those reported for other regions of Brazil or other countries. Even the sedimentary soils have relatively high metal values, naturally contaminated by the VMU of the RIGB; a potential impact on Mesozoic and Cenozoic sedimentary rocks located near the study region is highly expected. Metals are concentrated (80%) in the fine particle-size fraction, implying an easy availability through surface transport (wind and runoff). We introduced a new index, called the Fe-independent accumulation factor – AF-Fe, which reveals that 90–98% of the dynamics of the trace metals is associated with the iron geochemical cycle. We primarily conclude that determining the guideline values for different soil classes in variable geological/geochemical environment and under semiarid climate is meaningless: the concentration of metals in soils is clearly more related to the source material than to the pedogenesis processes.

Compositional variations of several Early Permian magmatic sulfide deposits in the Kalatongke district, southern Altai, western China: With genetic and exploration implications

A Permian magmatic Ni-Cu sulfide deposit cluster occurs in the Kalatongke district in the Southern Chinese Altai Orogenic Belt, western China. These deposits are associated with the mafic units of the Y1, Y2, Y3, Y9 and G21 mafic-intermediate complexes. In this paper we report the first zircon U-Pb ages for the Y3 and G21 intrusions, which are 283.3±1.3Ma and 281.1±1.5Ma, respectively. Our new age data confirm that the sulfide-bearing mafic units of the Y1, Y2 (connected with Y1 at depth), Y3, Y9 and G21 intrusions all formed in Early Permian between ∼281 and ∼287Ma. New and existing petrological-geochemical data show some important regular variations between these deposits. The host lithologies change from olivine-bearing rocks for the Y1-Y2-Y9 deposits to olivine-free rocks such as norite for the Y3 deposit and leucogabbro for the G21 deposit. The olivine Fo contents of the Y1 deposit are up to 82mol%, which are slightly higher than those of the Y2 deposit (up to 81mol%) and the Y9 deposit (up to 79mol%). The average plagioclase An contents of the olivine-bearing Y1-Y2-Y9 deposits are higher than those of the olivine-free Y3-G21 deposits. Among the three deposits (Y1, Y2 and Y3) that occur closely along the same structural lineament, the Ni/Cu ratios of bulk sulfides decrease from the olivine-bearing deposits (Y1 and Y2) to the olivine-free deposit (Y3). The PGE tenors of these deposits (Y1, Y2 and Y3) and the nearby coeval deposits (Y9 and G21) are extremely low, indicating that their parental magmas are severely depleted in PGEs. The variations of PGE tenors within a single deposit as well as among the different deposits are mainly due to variable R factors. The host rocks of these deposits are all characterized by elevated initial 87Sr/86Sr ratios from 0.7045 to 0.7047, positive εNd values from 4.95 to 6.86, positive εHf values of zircon from 9 to 16, and elevated δ18O values of zircon from 6.15 to 6.7‰. The isotope data indicate that the parental magmas for these deposits experienced up to ∼15wt% crustal contamination. The δ34S values of the sulfide minerals from these deposits are from −3.1‰ to 0.4‰, with a peak at −2.2‰, indicating the involvement of crustal sulfur. The isotope data and mineral chemistry together indicate that both olivine fractional crystallization and addition of crustal sulfur played a role in triggering sulfide saturation in the parental magmas for these deposits. Based on higher Ni/Cu ratios of sulfide mineralization in the olivine-bearing intrusions (Y1, Y2, Y9) than in the coeval olivine-free intrusions (Y3, G21), we recommend that Ni exploration in the region focus on the olivine-bearing intrusions that were emplaced in the Early Permian.

Petrogenesis and Metallogenesis of Malek Siah Kuh Adakite-Like Rocks and Associated Hydrothermal Mineralization (Sistan Area, Iran)

The granitoids-diroretic complex and associated volcanic suits of Malek Siah Kuh (eastern Iran) were examined by RS, XRD, XRF, AAS and ICP-MS methods. The Multispectral data were used for differentiating of lithological units, main alterations, structural trends, and hydrothermally altered minerals. Based on the results of remote sensing, field studies and petrographic data, the complex classified into three main groups: 1) mafic unit (gabbro-basalt), 2) intermediate rocks (diorite-andesite suite) and 3) felsic unit (granitoide rocks). The complex is characterized by high Al2O3 and Sr contents as well as high Sr/Y and La/Yb ratios, while low in Y and Yb contents. The geochemical characteristics of these rocks indicate that they are similar to adakite like rocks. The complex is high calc-alkaline to shoshonitic nature. The results indicated derivation from similar silicate melt source followed by differentiation and assimilation of crustal rocks. Based on set of petrological and geochemical relations, 3 distinct stages of mineralization process can be recognized. 1) It is differentiation during magma evolution as well as metal enrichment and subsolidus leaching by hot fluids. 2) It occurred by ascending of the gas-rich magma and release of S and As. 3) It was accompanied by hydrothermal activities leading to the generation of sulfidation and associated polymetallic (Cu, Au, Fe) mineralization.

Crustal exhumation during ongoing compression in the Variscan Maures-Tanneron Massif, France-Geological and thermo-mechanical aspects

The Maures-Tanneron Massif (MTM), together with Corsica and Sardinia, represent the Southeastern branch of the huge European Variscan belt. A continuous evolution from continental collision to exhumation is described from ca. 350Ma to ca. 320Ma, based on an extended compilation of available geological and geochronological data. This geological compilation is complemented with thermo-mechanical modelling that tests the conditions under which deep-seated HP units can melt and exhume massively during continued convergence. We simulate in two-dimensions the development of gravitational instabilities in a partially melting crust, depending on rheological layering and heat sources, and accounting for temperature dependent elasto-visco-plastic rheologies. In order to reproduce PT exhumation patterns in the MTM, over the appropriate time-scale (>20My) and spatial extent (>100km), a best fit was obtained with a convergence rate of 0.5cm/yr and moderate diffusive surface processes. The contribution of radiogenic heating in the pre-stacked felsic crustal units is crucial. A lateral alternation with mafic units is also required in order to prevent lateral spreading of the orogen. A low viscosity partially molten crust (1020Pa·s, thus rather felsic) accompanies crustal exhumation in a decoupled mode from the deeper mantle. A shallow asthenosphere below the orogen (LAB at ~70km depth, e.g. shallow slab break-off) produces too warm and sudden exhumation as opposed to a competent mantle lithosphere (120km depth LAB) that restrains it. The MTM witnesses the typical competition between far-field plate convergence and internal body forces, and our study pleads for a progressive evolution of transpression towards perpendicular extension from ca. 320Ma.