Tholeiitic Mafic Rocks Research Articles

Exotic vs local Caribbean origin for the arc-related Mabujina Amphibolite Complex (Cuba): implications for segmentation of the Caribbean arc during the Cretaceous

ABSTRACT The Mabujina Complex exposed in south-central Cuba consists of an elongated body of metamorphosed volcanic and subvolcanic mafic rocks and tonalitic gneisses. The complex is largely composed of greenschist facies rocks (Porvenir Formation) directly below the Cretaceous volcanic arc section and of a deeper sequence of deformed amphibolite facies rocks (Mabujina Amphibolite Complex). The origin of the protolith of the Mabujina Complex has been the subject of intense debate in the past. While some authors consider the Mabujina Complex to represent the deepest and oldest (Early Cretaceous) exposed section (root) of the Cretaceous Caribbean volcanic arc others suggested an exotic (non-Caribbean related) Early Cretaceous volcanic arc section. The later proposal implies the tectonic emplacement underneath the Caribbean arc of a segment of the Pacific-related Early Cretaceous Mexican Guerrero volcanic arc terrane. A new set of whole-rock geochemical analyses presented in this study shows that the protoliths of the Mabujina Amphibolite Complex constitute an island-arc sequence with a) less abundant island-arc tholeiitic mafic rocks similar to the Early Cretaceous tholeiitic island arc of the Caribbean and b) dominant transitional to calc-alkaline mafic and felsic igneous rocks that resemble the Early to Late Cretaceous magmatism of the Caribbean arc. The high initial Nd ratios of the rocks from the Mabujina Amphibolite Complex are indistinguishable from the tholeiitic to calc-alkaline sequences from the Caribbean arc, reflecting a depleted MORB mantle source with limited subduction-related component input. The present data, together with published geochemical and geochronological data of the protoliths, allow identifying a Caribbean origin for the Mabujina Complex instead of an exotic provenance in the Guerrero Arc of western Mexico. However, even if the interpretation of the geochemical data favours a Caribbean origin, the Mabujina Amphibolite Complex cannot represent the root of the volcanic arc. Here, we propose that the Mabujina Amphibolite Complex is a fragment of the Early to Mid-Cretaceous Caribbean arc that was tectonically emplaced below this same arc as a likely consequence of arc segmentation and strike-slip trans-compression triggered by oblique subduction during the mid-Cretaceous.

A greenstone belt in southeast Tibet: An accreted middle–late Permian oceanic plateau

Studies of accreted oceanic plateau sections provide crucial information on their structures, compositions, and origins. We investigate the petrogenesis of ultramafic–mafic rocks in the Tangjia–Sumdo greenstone belt of southeast Tibet using petrography, whole-rock geochemistry, and U-Pb zircon geochronology. These rocks are divided into four groups based on geochemical characteristics that include depleted and tholeiitic mafic rocks, transitional mafic rocks, enriched and alkaline mafic rocks, and picritic ultramafic rocks. Depleted and tholeiitic mafic rocks have the oldest crystallization ages (∼272 Ma), followed by picritic ultramafic rocks (∼270 Ma), transitional mafic rocks (267–254 Ma), and enriched and alkaline mafic rocks (252–250 Ma). Hafnium and neodymium isotope ratios of depleted and tholeiitic mafic rocks (εHf(t) = +13.1–+16.9; εNd(t) = +6.9–+7.1), transitional mafic rocks (εHf(t) = +1.8–+16.9; εNd(t) = +0.8–+5.5), enriched and alkaline mafic rocks (εHf(t) = +0.5–+5.4; εNd(t) = −1.5 to +1.9) and picritic ultramafic rocks (εHf(t) = +14.9–+17.2; εNd(t) = +7.8–+9.0) are similar to those of N-MORB, E-MORB, OIB and depleted-type picritic mafic rocks in other oceanic plateaus, respectively. The geochemical characteristics of the depleted and tholeiitic mafic rocks suggest that they formed by partial melting of depleted spinel lherzolite in a mid-ocean ridge setting, whereas the picritic ultramafic rocks suggest a high degree of partial melting of depleted lherzolite in a hot mantle plume head. The transitional mafic rocks formed by partial melting of moderately enriched garnet lherzolite. The youngest rocks (enriched and alkaline mafic rocks) formed by partial melting of a more enriched garnet lherzolite (compared to transitional mafic rocks) at relatively low temperatures. We propose that the depleted and tholeiitic mafic rocks represent normal oceanic crust of the Sumdo Paleo-Tethys Ocean and the transitional mafic rocks, enriched and alkaline mafic rocks and picritic ultramafic rocks are the fragments of the oceanic plateau, which were related to middle–late Permian mantle plume activity in the Sumdo Paleo-Tethys Ocean. We further suggest that the majority of the Tangjia–Sumdo greenstone belt represents a middle–late Permian oceanic plateau that reflects a previously unrecognized middle–late Permian mantle plume.

Neoproterozoic metavolcanic suites in the Micangshan terrane and their implications for the tectonic evolution of the NW Yangtze block, South China

The Micangshan terrane is located along the NW margin of the Yangtze block and contains the second oldest Precambrian basement in South China. It comprises the Beiba and the Wangcang-Nanjiang segments, while the oldest suites (∼2.1 Ga Houhe complex) occur in the Beiba. The Hekou and the Wangjiaping metamorphic volcano-sedimentary sequences in the Wangcang are characterized by intensive metamorphism and deformation, long being regarded as the equivalent of the Houhe suite. This contribution presents an integrated study of zircon U-Pb geochronology and Hf isotope, whole rock elemental and Sr-Nd isotope geochemistry on volcanic rocks of the metastrata. It reveals that they formed at ∼ 865–860 Ma and underwent upper amphibolite facies metamorphism at ∼ 815 Ma with lower limits in P-T conditions of ∼ 5.0 kbar and ∼ 650–660 °C. The Hekou metavolcanics comprise dacitic-rhyolitic and alkaline basaltic rocks with minor tholeiitic mafics, whereas those of the Wangjiaping are composed of dacitic and tholeiitic mafic rocks. The Hekou alkaline metamafics show affinity to high-Nb to Nb-enriched basalts (HNB-NEB) with εNd(t) values of + 5.80 to + 6.25, whereas tholeiitic metamafics of the both sequences display characteristics typifying high-Al basalt (HAB) with εNd(t) value ∼+2.66. The Hekou and the Wangjiaping metafelsics have εNd(t) ranges of −2.33 to + 5.60 and −0.82 to + 0.99 (except one + 2.29), respectively, and the dacitic rocks show features akin to the high Mg-number andesites. The ∼ 865–860 Ma magmatism is suggested to have occurred in an extensional basin with a post-subduction affinity. It was induced by upwelling of convective asthenosphere succeed the ceased slab subduction: the Wangjiaping metafelsics were derived from partial melting of juvenile lower crust, the Hekou dacitic rocks were produced by partial melts of the relict of subducted oceanic crust interacted with mantle wedge; the Hekou HNB-BEB were sourced by metasomatized mantle wedge, whereas the HAB lavas were generated by anhydrous partial melts of the upwelling asthenosphere. The ∼ 815 Ma metamorphism was caused by an amalgamation between the South Qinling belt and the proto margin of the northwestern Yangtze block. This event was followed by an expanding rift from the Micangshan successively to the South Qinling.

Petrogenesis of Late Carboniferous-Early Permian mafic-ultramafic-felsic complexes in the eastern Central Tianshan, NW China: The result of subduction-related transtension?

Knowledge of the Late Paleozoic accretionary processes responsible for the Altaids (or southern Central Asian Orogenic Belt) is critical for a better understanding of continental growth in Central Asia. Here, we systematic study the Late Carboniferous mafic-ultramafic-felsic complexes in eastern Central Tianshan to understand the tectonic evolution of the southern Altaids. Two gabbros yield zircon crystallization ages of 301.5 ± 2.3 Ma and 300.7 ± 2.6 Ma and one granite is dated as 301.6 ± 1.5 Ma. The gabbros are tholeiitic and characterized by typical subduction-related geochemical signatures with low TiO2 contents, enrichments in Rb, Ba, U, K, Pb and Sr, negative Nb and Ta anomalies, and negative to positive Eu and Ti anomalies. The granites are the A2-type granites with high contents of SiO2, K2O + Na2O, Ga, Zr, Nb, Y, HFSE, REE, and high ratios of Fe/Mg and Y/Nb (>1.2). The gabbros and granites have high εNd(t) values of +1.64 to +5.03, zircon εHf(t) values of −2.8 to +9.1, and low (87Sr/86Sr)i (0.704007–0.705437) values. These geochemical characters demonstrate that the rocks in the complexes are fractionation genesis and that the A-type granitic rocks are the highest fractionation facies of tholeiitic mafic rocks. These results suggest that they were originated from the asthenospheric garnet-free mantle in a supra-subduction zone. Combined with regional data, we suggest that the oblique subduction with slab break-off of the Southern Tianshan oceanic plate plausibly induced asthenospheric upwelling to form the tholeiitic magma, and which intrude along the strike-faults the Central Tianshan-Yamansu arc. Accordingly, these results reveal the Paleo-Asian Ocean continued to subduct at the southern Altaids in the Late Carboniferous.

Geology, physical-chemical and geodynamic conditions for the formation of Sokolovsk and Krasnokamensk granitoid massifs (South Ural)

The article describes the geological structure of the Sokolovsk and Krasnokamensk massifs located in the central part of the Western subzone of the Chelyabinsk-Adamovka zone of the Southern Urals. They are of Lower Carboniferous age and break through the volcanogenic-sedimentary deposits of the Krasnokamensk (D3kr) and Bulatovo (S1-D1bl) strata. It was found that these intrusions belong to the gabbro-syenite complex and are composed of gabbroids (phase I) and syenites, quartz monzonites, less often monzodiorites (phase II). The rocks of the second phase predominate (90–95%). Gabbros belong to the normal alkaline series of the sodium series and are close to tholeiitic mafic rocks, the formation of which is associated with riftogenic structures; syenites correspond to moderately alkaline series with K-Na type of alkalinity. It has been proved that in terms of their petrographic, petrochemical, geochemical, and metallogenic features (content of TiO2, K2O, Na2O, Rb, Sr, distribution of REE, the presence of skarn-magnetic mineralization), the rocks of the massifs under consideration undoubtedly belong to the gabbro-granite formation. Crystallization of the Sokolovsk and Krasnokamensk intrusions occurred at a temperature of 880–930 °С in the mesoabyssal zone at a depth of about 7–8 km (P = 2.2–2.4 kbar). At the postmagmatic stage, the transformation parameters of the initially igneous rocks were, respectively, T = 730–770 °C, P = 4.0–4.2 kbar. The fact that these massifs belong to the gabbro-granite formation makes it possible to include them, together with Bolshakovsk, Klyuchevsky, Kurtmaksky and Kambulatovo, into the Chelyabinsk-Adamovka segment of the South Ural Early Carboniferous rift system.

Early Cambrian multiple-sourced plutonism in the Eastern Sierras Pampeanas, Córdoba, Argentina: Implications for the evolution of the early Paleozoic Gondwana margin

Four major igneous suites with different geochemical characteristics are identified in the accretionary complex of the Pampean orogeny from the southern Sierras de Córdoba, Argentina. Crystallization ages of gabbroic and trondhjemitic plutonic rocks are Early to Middle Cambrian. The plutonic suites include mafic to intermediate alkaline OIB-like rocks, mafic tholeiitic N-MORB rocks, transitional mafic rocks, and minor trondhjemitic rocks with adakitic geochemical signature. The multiple igneous suites are spatially and temporally related to what reveals simultaneous tapping of distinct source reservoirs with their respective melts remaining distinctive from extraction to emplacement at middle crustal depths. The alkaline mafic-intermediate rocks with OIB-like geochemistry were generated by the partial melting of an enriched or metasomatized mantle source. By contrast, tholeiitic mafic rocks with N-MORB geochemical signature came out from a depleted mantle source. Trace element abundances of the transitional mafic magmas reflect variable mixtures between OIB-like and MORB-type magmas. Trondhjemitic rocks exhibit geochemical features highlighting their parent melts formed by low degree melting of a metasomatized subducting oceanic crust. The most reliable setting to generate all these diverse magmatic suites at the same time is a convergent margin where the opening of a slab window followed ridge-trench interaction. A corollary is that widespread mafic magmatism enhanced heat advection and gave rise to extensive intra-crustal anatexis in an accretionary complex of crustal extension.

Stratigraphic setting and timing of the Montagne d’Or deposit, a unique Rhyacian Au-rich VMS deposit of the Guiana Shield, French Guiana

In French Guiana, the Montagne d’Or gold deposit (5 Moz at 1.5 g/t Au) is located in the northern branch of the Rhyacian Paramaca Greenstone Belt. The sulphide deposit is hosted by a south-facing bimodal volcanic and volcaniclastic sequence that is highly strained and affected by a penetrative E-W striking and steeply south-dipping foliation.The volcanic sequence is composed of three members, (1) the Lower unit in the stratigraphic footwall, (2) a bimodal mafic-felsic formation hosting the orebody, and (3) the Upper sedimentary and volcanic rocks unit in the stratigraphic hanging-wall. The bimodal formation is dominated by calc-alkaline felsic volcanic rocks in the west, interbedded/interdigitated with tholeiitic mafic rocks in the east. Pillowed mafic flows and graded-bedded felsic volcaniclastic rocks collectively indicate effusive to explosive volcanic activity in a submarine environment. The mineralization consists of thick and laterally extensive sulphide zones forming (1) stratiform sulphide disseminations, (2) structurally-transposed stringer stockworks, and (3) thin layers of deformed semi-massive sulphides. The gold-rich sulphide mineralization consists mainly of pyrite, pyrrhotite and chalcopyrite with minor sphalerite, magnetite, galena and arsenopyrite, hosted by chlorite-sericite-rich alteration zones mainly developed in the felsic tuff facies.U-Pb zircon geochronology shows that most of the Montagne d’Or volcanic and intrusive sequence has crystallized during a multi-cycle magmatic event, from ca. 2152 Ma to ca. 2130 Ma. A porphyry intrusion crosscutting the ore yields a U-Pb zircon age of 2117.6 ± 5.1 Ma, hence constraining a minimum age for the auriferous sulphide mineralization. This also indicates that the sulphide mineralization was coeval with arc magmatism, demonstrating the volcanogenic nature of the Montagne d’Or gold deposit.

Sm–Nd and Sr isotope fingerprinting of iron mining tailing deposits spilled from the failed SAMARCO Fundão dam 2015 accident at Mariana, SE-Brazil

This work presents the first Sm–Nd and Sr isotope TIMS data on the fine-grained fraction (<74 μm) of iron-rich deposits related to the November 5th, 2015 Mariana accident, caused by the rupture of the Fundão dam, an iron tailings pond operated by SAMARCO Mining. The failure of the Fundão dam provoked the gravitational flow of approximately 62 Mi m3 of a quartz and iron oxide-rich slurry composed of water, fine sand and silt, that flowed into the Gualaxo do Norte and Carmo rivers, and eventually along the whole Doce River. Samples were collected between 5 and 80 km downstream from the Fundão Dam, along the Santarém creek and the Gualaxo do Norte and Carmo rivers. XRD analyses show that samples are >90% composed of quartz and iron oxides (hematite, goethite and magnetite), with minor phyllosilicates (kaolinite and mica). Owing to sediment transport processes, heavy (hematite, goethite, magnetite) to light (quartz, phyllosilicates) mineral fractionation is observed. As with XRD data, major and trace element compositions also closely match those of the original Paleoproterozoic Cauê Formation BIF iron ore. The studied samples yielded δNd between −17.0 and −19.7 pptt (part per ten thousand), with the exception of one sample, with δNd of −23.2 pptt. Strontium 87Sr/86Sr ratios show variability between 0.7351 and 0.7729. The TDM ages scatter between 1.94 Ga and 2.75 Ga, with a negative correlation with phyllosilicate contents, indicating that the latter has preferential provenance from younger, more juvenile sources than the Cauê Fm itabirites. This interpretation is corroborated by two component mixing models of Nd and Sr isotope ratios, where the Nd–Sr isotopic signatures of the studied samples are compatible with >98% of the Cauê Fm itabirite component, mixed with a Neoproterozoic juvenile mafic rocks component, such as those from the ophiolitic assemblages of the distal Macaúbas Group, or tholeiitic mafic rocks of the Pedro Lessa suite. The reported Sm–Nd and Sr isotope compositions closely match those of the original iron ore and contrast with either those of the upriver or downriver Precambrian to Ordovician bedrock units. The firstly defined isotope signature can thus be potentially used as a proxy for ongoing suspended sediment dispersion studies farther downstream along the Doce River and in adjacent marine environment.

Geochemistry of metamorphosed volcanic rocks in the Neoarchean Qingyuan greenstone belt, North China Craton: Implications for geodynamic evolution and VMS mineralization

The Qingyuan greenstone belt (QGB) situated in the northeastern section of the North China Craton (NCC) is considered to be the unique metallogenic province in China that hosts both banded iron formations (BIFs) and volcanogenic massive sulfide (VMS) deposits. Traditionally, the QGB is divided into the Hunbei and Hunnan terranes separated by the Hunhe fault and grouped into the lower Shipengzi, middle Hongtoushan, and upper Nantianmen formations, which have undergone amphibolite- to granulite-facies metamorphism. Exposed supracrustal rocks are composed of plagioclase amphibolite and biotite plagioclase gneiss with minor amounts of BIF, marble, and schist. In this study, detailed field and petrographic observations, lithogeochemical, U-Pb geochronological, as well as whole-rock Sm-Nd isotope analyses were conducted on the metamorphosed mafic and felsic volcanic rocks (i.e. plagioclase amphibolite and biotite plagioclase gneiss, respectively) in the QGB in order to discuss implications for tectonic evolution and comitant VMS mineralization.The supracrustal rocks of both Hunbei and Hunnan regions were formed synchronously from 2.57 to 2.53 Ga and then had undergone 2.52–2.48 Ga regional metamorphism. Specifically, the lower section of the Shipengzi Formation is composed dominantly of transitional-tholeiitic basalts with negative Nb anomalies, suggesting that they were likely generated by partial melting of a mantle source that was previously altered by variable amounts of slab-derived fluids or melts. In contrast, the lithologic assemblages grade upward into interlayered bimodal volcanic rocks (i.e. N-MORB-type tholeiitic basalts and FI- to FII-type dacites of transitional to calc-alkalic affinities), representing a change in tectonic regime related to subduction from compression to extension. Among them all the basalts are characterized by consistently positive initial Nd compositions (εNd(t) = +2.7 to +3.4) further indicate a depleted mantle source; whereas those dacites possess negative to positive initial Nd values (εNd(t) = −4.6 to +3.0), suggesting they were formed by partial melting of depleted mafic source (likely lower mafic crust) and accompanied with variable addition of old crustal material. Significantly, VMS deposits are genetically linked to this extensional event and closely associated with the FII-type dacites that were generated by higher melting degree at high-temperature conditions and shallow crustal level (likely 10–15 km). In all, geochemical features (e.g., negative Nb anomalies and εNd value), together with contamination from continental crusts for tholeiitic mafic rocks and inherited zircons from these volcanic rocks, imply a continental arc setting where the QGB was evolved.

Late Neoarchean supracrustal rocks from the Anshan-Benxi terrane, North China Craton: New geodynamic implications from the geochemical record

The late Neoarchean supracrustal rocks of the Anshan-Benxi (AnBen) terrane in the northeastern part of the North China Craton (NCC) have important regional tectonostratigraphic implications for the development of large-scale banded iron formations (BIFs). These supracrustal rocks underwent polyphase deformation and upper greenschist to upper amphibolite facies metamorphism and are subdivided into two main lithotectonic units in terms of differing ages, metamorphic grades, and lithological assemblages: the Benxi unit (∼2.56–2.53 Ga) composed of tholeiitic to transitional mafic rocks with minor siliciclastic sedimentary rocks, and the Anshan unit (∼2.53–2.51 Ga) consisting of siliciclastic sedimentary rocks with minor tholeiitic mafic rocks. In this study, we report the results of field investigations and petrographic, whole-rock major and trace element geochemical and Sm-Nd isotopic analyses, as well as zircon U-Pb-Hf-O isotopic analyses for these volcano-sedimentary rocks, and integrate the results with previously published data to provide a new geodynamic synthesis for the setting of the BIFs. Three groups of amphibolites (meta-basalts) in the vicinity of BIFs were recognized in the AnBen area on the basis of their REE and HFSE characteristics: (a) Group 1 is characterized by nearly-flat to slightly depleted LREE patterns and an absence of significant Nb anomalies, similar to those of modern N-MORB, suggesting that their magmatic precursors were derived from partial melting of a strongly depleted mantle; (b) Group 2 displays relatively flat to slightly enriched LREE patterns and minor negative Nb anomalies, which are comparable to those of back-arc basin basalts and indicate that they were probably generated by partial melting of depleted mantle modified by slab-derived fluids; and (c) Group 3 exhibits LREE-enriched patterns and pronounced negative Nb anomalies, which are interpreted to be the result of a depleted mantle-derived melt interacting with older upper continental crust. The overall field and geochemical signatures suggest that these basaltic rocks were erupted in a back-arc basin within a continental volcanic arc setting with a protracted history, consistent with Nd isotope, inherited zircon age, and zircon Hf-O isotope data that require variable inputs of older crustal sources for their formation. Diagnostic geochemical features and detrital zircon age spectra further indicate that the source rocks for felsic to mafic detritus in the Anshan unit may have been derived dominantly from coeval volcanic rocks of the Benxi unit and/or distal volcanic arc edifices. Collectively, the evidence indicates that the tectonic evolution of the AnBen terrane took place in two main stages: the first stage involved the generation of ∼2560 to 2530 Ma tholeiitic to transitional mafic rocks and associated siliciclastic sedimentary rocks of the Benxi unit, which is interpreted to have formed during opening of the back-arc basin, whereas the second stage corresponded to closure of the back-arc basin (∼2530–2510 Ma) and involved extensive chemical and siliciclastic sedimentary rocks with minor emplacement of tholeiitic mafic lavas.

Petrogenesis of Eocene granitoids and microgranular enclaves in the western Tengchong Block: Constraints on eastward subduction of the Neo-Tethys

Eocene granitic and related igneous rocks in the western Tengchong Block are considered to be the result of eastward subduction of Neo-Tethyan oceanic lithosphere beneath the Tengchong Block. In this paper we show that the granitic and mafic rocks in the western Tengchong Block exhibit a systematic compositional variation from west to east, with Na-rich granodiorites in the Nabang area (west) that differ from coeval high-K calc-alkaline granodiorites in the Bangwan area (east), and with tholeiitic mafic rocks in the Nabang area that differ from shoshonitic mafic microgranular enclaves (MMEs) in granodiorites of the Bangwan area. In addition, high-silica biotite granites were intruded into the granodiorites in the Bangwan area. The host granodiorites, MMEs, and biotite granites in the Bangwan area yield zircon U–Pb ages of ca. 50Ma. The MMEs have relatively low SiO2 contents (53.1–64.95wt%) and Mg# values (37–45), and high K2O (4.14–5.02wt%) and ∑REE contents (331–509ppm); the MMEs contain acicular apatites that indicate quenching. The host granodiorites also have high K2O (4.48–5.95wt%) and ∑REE compositions (320–459ppm), and together with the MMEs they are enriched in Th but depleted in Nb and Ti. The Sr–Nd–Pb isotopic compositions of the host granodiorites and the MMEs are similar, with εHf(t) values of −1.0 to −10.8 and 3.3 to −11.1, respectively. The geochemical data and igneous textures suggest that the MMEs represent a mafic magma that was derived from the partial melting of mantle pyroxenite, with the melting induced by the influx of fluids/melts from the recycling of sediments in the subducted slab. The mafic melts then caused the partial melting of lower crustal tonalitic rocks to produce granodioritic magma that was subsequently mixed with mafic magma. The biotite granites have relatively high SiO2 contents and low Mg# values that indicate a purely crustal origin and derivation from the partial melting of upper crustal metagraywacke. The similar isotopic compositions of the biotite granites and the granodiorites can be interpreted to reflect the influence of fluids released during crystallization of the granodiorites. A comparison of contemporaneous Eocene granitic and mafic rocks in Nabang, Tongbiguan, Longchuan, and Bangwan in the Tengchong Block reveals that the mafic rocks vary from tholeiitic through high-K calc-alkaline to shoshonitic in composition from west to east, while the granitic rocks vary from Na- to K-rich. These systematic compositional variations indicate that the arc magmatism in the western Tengchong Block was the result of the eastward subduction of the Neo-Tethyan oceanic lithosphere. Fluids or/and melts from the subducted slab (including sediments) are likely to have played an important role in producing these compositional variations.

Old and juvenile source of Paleozoic and Mesozoic basaltic magmas in the Acatlán and Ayú complexes, Southern Mexico: Nd isotopic constraints

The Neoproterozoic–Paleozoic Acatlán Complex and the Mesozoic Ayú Complex of southern Mexico consist of clastic rocks and rift-related igneous rocks inferred to have originated along a rifted passive margin in southwestern Mexico, either in an inactive (Neoproterozoic–Ordovician) or an active (Devonian–Carboniferous and Triassic–Early Jurassic) tectonic setting. The latter formed on the inner margin of a backarc basin. These passive margin rocks were partly underthrust beneath the Acatlán Complex, into which they were subsequently extruded: extrusion was synchronous with the backarc basin development. Thus, (i) the Neoproterozoic–Ordovician rocks underwent underthrusting, high-pressure metamorphism, and extrusion during the Late Devonian–Carboniferous (365–330Ma); (ii) the Carboniferous rocks underwent underthrusting, amphibolite facies metamorphism, and extrusion during the Permian and Triassic; and (iii) the Triassic–Lower Jurassic rocks underwent underthrusting, amphibolite facies metamorphism, and extrusion during the Jurassic. Nd isotopic data from tholeiitic mafic rocks on either side of the HP extrusion zone reveal that both were underlain by similar peri-Rodinian subcontinental lithospheric mantle in the Neoproterozoic–Ordovician, which was supplemented in the Devonian–Carboniferous and Triassic–Early Jurassic by a juvenile depleted mantle source. The alternation of underthrusting and backarc rifting accompanied by extrusion may be related to flattening and steepening of the Beniof zone, respectively.

Volcanic Stratigraphy, Geochronology, and Gold Deposits of the Archean Hope Bay Greenstone Belt, Nunavut, Canada

The Archean Hope Bay greenstone belt is located in the Bathurst Block of the northeastern part of the Slave Structural Province, a predominantly Archean, granite-greenstone-metasedimentary terrane. The greenstone belt is dominated by mafic volcanic rocks with less common felsic volcanic and volcaniclastic products and subordinate ultramafic bodies and metasedimentary rocks. Three main Au deposits have been defined in the greenstone belt, with a cumulative resource of over 10 million ounces (Moz) of Au, as of April 2007. The Hope Bay greenstone belt can be divided into a series of coherent panels of strata that tend to be fault bounded and collectively are used to construct a composite stratigraphic column of the belt constrained by U-Pb geochronology on felsic suites. The oldest felsic suite, the Flake Lake suite (ca. 2700 Ma), is a succession of felsic volcanic rocks that has a tholeiitic geochemical affinity, interpreted as a product of rift-related volcanism. Overlying this suite is a series of well-constrained calc-alkaline, mainly felsic volcanic rocks, which have ages of ca. 2690 Ma (Square Lake suite), ca. 2686 Ma (Windy felsic suite), ca. 2677 Ma (Koignuk suite), and ca. 2662 Ma (Clover Lake suite), all of which are interpreted as products of arc volcanism. The transition from rift- to arc-related volcanism divides the greenstone belt into upper and lower volcanic cycles with the transition occurring at about ca. 2690 Ma. The older cycle of volcanic rocks has a distinctive group of mafic rocks consisting of mafic pillowed flows which have an Fe-rich tholeiitic geochemical affinity which is not recognized in the younger volcanic cycle. These Fe-rich tholeiitic mafic volcanic rocks commonly show a spatial relationship to Au mineralization acting as host strata to the main Au deposits in the greenstone belt. The younger cycle (<ca. 2690 Ma) of mafic volcanic rocks tends to have a tholeiitic affinity with some minor andesitic volcanic rocks near the top of the composite stratigraphic column. Detrital zircon work was carried out for all the main sedimentary successions recognized in the Hope Bay greenstone belt. The detrital zircon results show that in each case the sediments were derived mainly from local sources, as very few anomalously old zircons were recognized. A conglomerate outcropping in the north part of the belt is dominated by locally derived material, with detrital zircon work supporting this interpretation. The three main Au deposits at Hope Bay are all associated with the older cycle of volcanic rocks, interpreted as products of rift-related volcanism. The structural setting of each deposit is different but each is related to D2 strain in structural and/or stratigraphic settings which provided dilational environments to localize auriferous vein-related mineralization or localized alteration and mineralization of favorable host lithologies. The older cycle of tholeiitic felsic and Fe-rich mafic volcanic rocks are rift-related volcanic products, which are time-equivalent to the Kam Group, in the Yellowknife Supergroup. Overlying these rocks are calc-alkaline felsic volcanic rocks and tholeiitic mafic volcanic rocks, which represent the transition to calc-alkaline arc-related volcanism, which correlates to the Banting Group of the Yellowknife Supergroup. Overlying the younger (ca. 2677 Ma) felsic volcanic rocks are a succession of conglomerates consisting mainly of locally derived clasts, representing collapse of the arc into a fluvial-dominated environment. This sedimentary succession is possibly equivalent to the regionally extensive Burwash Basin of the Yellowknife Supergroup.

Post-orogenic extension in the eastern part of the Jiangnan orogen: Evidence from ca 800–760 Ma volcanic rocks

We present a systematic geochronological and geochemical study on ca 800–760Ma volcanic rocks in the eastern part of the Jiangnan orogen. The Xucun composite dykes are dated at ca 805Ma; the mafic components have OIB-like trace-element patterns and positive anomalies in Zr and Hf. The least-contaminated sample has relatively depleted Nd isotopic features, suggesting the Xucun mafic dykes may have been generated from the partial melting of OIB-like asthenosphere with later crustal contamination. The Xucun felsic dykes have decoupled Nd–Hf isotopes, and the Hf-isotope compositions of zircons indicate that the dykes may be derived from the partial melting of the early Neoproterozoic juvenile crustal materials, with minor incorporation of Paleoproterozoic crustal components. The ca 800–790Ma Shangshu volcanics include two compositional series: calc-alkaline and tholeiitic. The Shangshu calc-alkaline volcanics in the Minjiawu area have low abundances of LILE, HFSE and high Na2O contents and Sr/Y ratios, similar to adakitic rocks. The evident arc-like geochemical features and radiogenic Nd isotopes (ɛNd(t) values of +3.7 to +4.8) suggest that these rocks may have been generated from the partial melting of juvenile lithospheric mantle metasomatized by Na-rich melts released from the subducted slab. The tholeiitic mafic rocks from the Shangshu bimodal volcanics represent two different magma sources. The partial melting of metasomatized lithospheric mantle led to the formation of arc-like basalts with low TiO2 contents, negative anomalies in Zr and Hf, and high values of Mg# and ɛNd(t) (+6.2), whereas the partial melting of asthenospheric mantle generated volcanic rocks with high TiO2 contents and low positive ɛNd(t) (+1.4 to +2.7), without negative anomalies of Nb, Ta, Zr and Hf. The Shangshu felsic rocks were formed by the reworking of early Neoproterozoic juvenile arc crustal materials. The ca 760Ma mafic rocks from the Puling bimodal volcanics generally have low TiO2 contents (<0.9wt%), nearly flat REE distributions and arc-like trace-element patterns. They may have been generated from the high-degree partial melting of metasomatized lithospheric mantle. One sample has a high TiO2 content (2.41wt%) and high ɛNd(t) (+6.2), with overall OIB-like trace-element patterns, implying the local partial melting of asthenospheric mantle. The occurrence of significant volumes of bimodal volcanics in the eastern part of the Jiangnan orogen suggests an extensional setting in the period ca 800–760Ma. The evident partial melting of newly-metasomatized lithospheric mantle and subordinate partial melting of asthenosphere suggest that post-orogenic extension shortly after the Neoproterozoic orogenesis may be a better explanation for the genesis of the mid-Neoproterozoic magmatic rocks in the eastern part of the Jiangnan orogen. Post-orogenic extension may be diachronous along the whole orogenic belt, and probably has no direct relationship with the Rodinia rifting event. A more detailed model is presented to illustrate the evolution of the eastern part of the Jiangnan orogen.

The mafic–silicic layered intrusions of Saint-Jean-du-Doigt (France) and North-Guernsey (Channel Islands), Armorican Massif: Gabbro–diorite layering and mafic cumulate–pegmatoid association

The Saint-Jean-du-Doigt (France) and North-Guernsey (Channel Islands) Intrusive Complexes (hereafter referred to as SJIC and NGIC, respectively) are examples of mafic–silicic layered intrusions in the Armorican Massif. Both are characterized by the occurrence of (1) a basal/peripheral gabbroic unit interlayered with sheets (generally dioritic in composition, occasionally gabbroic) and crossed by leucocratic diapirs and pipes (from monzodioritic to Q-monzonitic in composition), (2) peripheral pegmatoids associated with mafic cumulates and (3) coeval granitoids. Beside these main similarities, some contrasted features lead us to propose two distinct models of formation. The Variscan SJIC includes tholeiitic mafic rocks (monzogabbro) that locally mingle and mix with leucocratic components (monzonite or Q-monzonite). The Cadomian NGIC is calc-alkaline. The SJIC sheet-bearing gabbro is homogeneous from a petrologic point of view, whereas the NGIC exhibits gabbroic macrorhythmic sequences with mineral layering. The Sr–Nd isotopic compositions of the SJIC gabbros are significantly different from those of the associated dioritic layers. This is not the case in the NGIC where the magmas could be cogenetic. We argue that the SJIC gabbro was a liquid that crystallized in situ without significant crystal settling. By contrast, the rhythmic sequences of the NGIC are consistent with crystal accumulation. Subsequently, both can be seen as mafic reservoirs which were repeatedly invaded by magmas of intermediate composition. We interpret the sheets in the SJIC as the result of horizontal spreading of dioritic metastable magmas into a gabbroic reservoir crystallizing from below, at levels of neutral buoyancy. Injections and convection in the central part of the reservoir possibly resulted in spectacular mixing/mingling structures. In the NGIC, the emplacement of the dioritic sheets was rather controlled by pre-existing rhythmic cumulative structures. In both intrusions, late differentiated diapirs were extracted from the dioritic sheets. Associated peripheral pegmatoids are thought to result from the crystallization of liquids issued from a mafic intercumulus melt in the presence of a fluid phase. This extraction might have been enhanced by the disruption of the peripheral cumulate stack, perhaps following pressure drops.

Lower Cryogenian calc-alkaline mafic rocks of the Western Anti-Atlas (Morocco): An example of orogenic-like magmatism in an extensional setting

The lower Cryogenian mafic magmatism from the Igherm, Ifni and Kerdous inliers (Moroccan Western Anti-Atlas) have calc-alkaline, tholeiitic and alkaline affinities. The calc-alkaline dolerite dykes and gabbros bodies emplaced before the conglomeratic formations of the Upper Cryogenian and after the tholeiitic mafic rocks that characterize the pre-Pan-African rifting. They are similar to rocks from orogenic setting and characterized by high LILE, Th, Ce, P, Sm contents and La/Nb ratio and a low HFSE content with negative anomalies in Nb, Zr and Ti. The geodynamic environment of the sedimentary country rocks corresponds to that of a passive margin in a distensive tectonic context. The calc-alkaline affinity of these magmas can be attributed to the influence of a Palaeoproterozoic subduction zone that contributed to the enrichment of the sub-continental mantle. During the extensional event of the Pan-African orogenesis, the mantle would have produced tholeiitic, alkaline and/or transitional magmas before melting (caused by adiabatic decompression) reached the enriched sub-continental mantle. This mantle was previously enriched during the Eburnean subduction, where it would have generated calc-alkaline magmas.

SHRIMP Zircon Age and Geochemical Constraints on the Origin of Lower Jurassic Volcanic Rocks from the Yeba Formation, Southern Gangdese, South Tibet

We present SHRIMP zircon dating, bulk-rock geochemical, and Sr-Nd-Pb isotopic results for Yeba volcanic rocks and a mafic dike from Southern Gangdese (SG), southern Tibet, in order to constrain their tectonic setting and origin. Yeba volcanic rocks span a continuous compositional range from basalt to dacite, although andesites are minor, and mafic and felsic rocks are volumetrically predominant. New SHRIMP zircon dating for a dacite coupled with previous SHRIMP zircon dating for a mafic dike and fossil constraints for the sedimentary sequence indicate that Yeba volcanic rocks were emplaced in the Early Jurassic (174-190 Ma). Yeba tholeiitic mafic rocks possess compositional diversity and are divided into three groups based on concentrations of MgO, Al2O3, and La. Mafic samples are all characterized by marked negative Nb, Ta, and Ti anomalies and positive εNd(T) values (+ 2.4 to + 4.5). Yeba calc-alkaline felsic rocks are characterized by coherent, concave-upward MREE patterns and negative anomalies in Nb, Ta, P, and Ti, with positive εNd(T) values (+ 0.3 to + 2.6). Sr-Nd-Pb isotopes overlap among the different groups of Yeba mafic rocks; Pb isotopic compositions in both mafic and felsic rocks are nearly identical. These features are consistent with a subduction-related origin, most likely in an arc built on thin, immature continental crust. Yeba volcanic rocks are interpreted as having been created by northward subduction of Neo-Tethyan oceanic crust in Early Jurassic time. Geochemical signatures and quantitative modeling indicate that fractional crystallization and crustal assimilation played insignificant roles in the generation of Yeba mafic magmas, and that their geochemical diversity was probably produced by variable degrees of partial melting from a common but heterogeneous mantle source, which had been metasomatized by variable contributions of sediments/fluids released from the subducted Neo-Tethyan oceanic crust. Yeba felsic rocks were probably generated by moderate degrees of partial melting of juvenile basaltic lower crust, which consists of dominant underplated magmas (similar to Yeba mafic rocks in composition) and variable contributions from ancient lower crust beneath the Gangdese Back-Arc fault uplift belt (GBAFUB).

Hercynian post-collisional A-type granites of the Kokshaal Range, Southern Tien Shan, Kyrgyzstan

The Tien Shan (Tianshan) orogen formed during Late Palaeozoic collision between the Karakum–Tarim continent and the Paleo-Kazakhstan continent, a Caledonian component of the Altaid Collage. The Southern Tien Shan terrane represents an intensely deformed fold and thrust belt formed after the final closure of the Paleo-Turkestan ocean. In the Late Carboniferous–Early Permian the Tien Shan was affected by regionally developed diverse post-collisional magmatism. Post-collisional granites of the Kokshaal Segment (350 × 100 km), the easternmost segment of Southern Tien Shan in Kyrgyzstan, include some 20 post-tectonic intrusions varying in composition from biotite–hornblende rapakivi granite to biotite and topaz-bearing leucogranite. The granites are associated with coeval tholeiitic mafic rocks and with alkaline undersaturated syenites. The granites have a pronounced A-type affinity: they are metaluminous with high concentrations of Na 2O + K 2O, Rb, HFSE and high K 2O/Na 2O and Fe/(Fe + Mg) ratios, and comprise a single geochemical trend explained by fractionation of K-feldspar and hornblende. The U–Pb zircon SIMS ages of the Djangart, Uch-Koshkon, Mudryum and Kok-Kiya intrusions of the Kokshaal are 296 ± 4 Ma, 279 ± 8 Ma, 281 ± 2 Ma and 281 ± 3 Ma, respectively. The oldest age was obtained for rapakivi granites in the eastern part of Kokshaal while the leucogranites in its western part were dated at 280 Ma. This time span closely corresponds to known ages of post-collisional granitoids and related gold deposits of the Tien Shan. The granites have negative initial ε Nd values (− 1.6 to − 6.9 at 280 Ma) and depleted mantle model ages of 1.05–1.43 Ga indicating a mixing of asthenospheric mantle and Precambrian crustal components. Precambrian Mesoproterozoic crust comprises the basement of the Tarim microcontinent underthrusted to the north beneath the sedimentary piles of Kokshaal. Mafic rocks associated with the granites may represent juvenile asthenospheric melts underplating the Tarim margin at the post-collisional stage and providing heat for generation of A-type granites from its Precambrian crust. Lithospheric plate-scale strike–slip shear zones that formed in the region subsequent to collision provided suitable conduits for ascending asthenospheric material. On a regional scale, available Nd data for the Tien Shan show that most Paleozoic felsic rocks originated from Precambrian crustal sources. The Northern Tarim rocks, exposed in the Kokshaal Range, and rocks of the southern Paleo-Kazakhstan margin have similar Proterozoic depleted mantle model ages.

1.8 Ga magmatism in the Fennoscandian Shield; lateral variations in subcontinental mantle enrichment

The ca. 1.8 Ga post-collisional magmatism in the southern part of the 2.1–1.86 Ga Svecofennian domain of the Fennoscandian Shield have been studied with particular reference to the character and sources of the mafic rocks in a traverse from the Archaean craton margin in east, across the juvenile Svecofennian domain to its western margin. For this purpose three key areas were selected in the eastern, central and western parts of the domain: (i) in the western part of the domain, the Tjällmo-Vättern zone (southern Sweden) of the Transscandinavian Igneous Belt (TIB) consists of extensive areas of dominantly alkali-calcic granitoids associated with calc-alkaline to tholeiitic mafic rocks. Initial ɛ Nd for the mafic rocks vary from around 0 to above +3. (ii) In the central part of the domain (SW Finland), the post-collisional rocks are represented by small intrusions consisting of calc-alkaline high-Ba–Sr granites associated with shoshonitic lamprophyres and their plutonic equivalents. Initial ɛ Nd for the rocks in this series vary between 0 and +1. (iii) In the easternmost part of the Svecofennian domain (Russian Karelia and SE Finland) shoshonitic associations occur, comprising lamprophyres and their plutonic equivalents (apatite and magnetite-rich monzodiorites), which are related to syenites and high-Ba–Sr granites by fractional crystallization. All the rock types in this shoshonitic association have strongly elevated contents of P 2O 5, LREE and LILE. Initial ɛ Nd for all rocks in Karelia fall between 0 and −1. Geochemical and isotopic results indicate that the post-collisional rocks in the central and eastern part of the domain stem from lithospheric mantle sources that were enriched during the preceding Svecofennian orogeny. The HFSE depletion, combined with the strong Sr, LILE and LREE enrichment, recall signatures of increasingly carbonate-dominated metasomatism of the mantle eastwards towards the craton margin. In contrast, the mainly LILE enriched mafic rocks from TIB in the west signal sources subjected to H 2O-dominated metasomatism, that could in part be coeval with the magmatism. In all areas the rocks carry a subduction type chemistry with continental arc affinity, however, with strongly increasing enrichment levels eastwards. Rocks in the west are derived by relatively larger degrees of melting at shallower levels, from previously depleted spinel–phlogopite–amphibole lherzolites/harzburgites, while going eastwards successively smaller melt fractions were tapping deeper, more enriched mantle sections in the garnet stability field. The enrichment agents are interpreted to be LILE-bearing H 2O-dominated fluids from dewatering slabs in the west, changing to an increasing role for CO 2-dominated fluids/melts derived mainly from subducted Svecofennian metasediments eastwards. A convergent continental margin setting with transpressional shearing was active during TIB formation in the west. Whether this shearing was instrumental in the formation of the 1.8 Ga magmatism further continentwards, or if the magmatism in the central and eastern areas was the result of extensional collapse or plume activity is presently not known.

Archean Molasse Basin Evolution and Magmatism, Wabigoon Subprovince, Canada

The late‐orogenic 2703–2696 Ma Stormy basin of western Wabigoon Subprovince, 20 m.yr. younger than the surrounding greenstone belt, is a 1.5–2‐km‐thick volcano‐sedimentary sequence. The basin, bounded by a major fault to the north and an unconformity to the south, contains an association of subaerial felsic and mafic massive to brecciated flows, shoshonitic dykes, and coarse clastic fluvial deposits. The volcanic basement, represented by the ca. 2732–2722 Ma Wapageisi Group and Boyer Lake Group, is dominated by tholeiitic mafic flows. In addition, it contains minor intermediate to felsic components that have calc‐alkaline characteristics. The Wapageisi Group is similar to a back‐arc sequence, whereas Boyer Lake Group is typical of an oceanic island arc setting. The Stormy basin displays bimodal volcanism with calc‐alkaline porphyry stocks and felsic flows along the basin margins and tholeiitic mafic rocks in the center. The subaerial mafic flows compositionally resemble rift‐related continental tholeiites. The amphibole‐phyric shoshonites are similar to modern shoshonites as well as to other Archean shoshonitic rocks. Parental magmas of both the intrabasinal tholeiites and shoshonites were generated at a significantly greater depth (>60–80 km) than those of the older volcanic basement. The crustal thickening prior to basin formation reflects the transition from incipient arc formation to mature arc evolution. Although convergent‐style magmatism was dominant at the arc/back‐arc stage, rift‐related volcanism is characteristic of the arc fragmentation stage. Collectively, the setting and evolution is analogous to evolving modern oceanic island and continental arcs, in which terminal arc stages were affected by rifting.