Metavolcanic Research Articles

Petrogenesis and tectonic significance of Paleoproterozoic metavolcanic rocks in the Khondalite Belt, North China Craton

As a typical continent–continent collision belt, the Khondalite Belt (KB) of the North China Craton (NCC) represents a Paleoproterozoic orogenic belt that underwent subduction and collision. Although many high-grade metamorphic rocks have been well studied, the low to medium grade metamorphic rocks well preserved in KB are less studied, which hinders a better understanding of the initiation of Paleoproterozoic subduction and collision processes. The Erdaowa Group is a representative of such a metamorphic rock series. Zircon U-Pb dating of the metavolcanic rocks in the Erdaowa Group yields the eruption ages of 2465 ± 24 Ma and 2350 ± 34 Ma, and the metamorphic ages of 1933 ± 32 Ma and 1844 ± 30 Ma. The geochemical results show that the metavolcanic rocks of the Erdaowa Group belong to tholeiitic basalts series. The enrichment of large ion lithophile elements (LILEs) such as Rb and Ba, and the depletion of high field strength elements (HFSEs) such as Ta, Nb and Ti indicate that the metavolcanic rocks have arc geochemical affinities and may have been formed at an active continental margin associated with subduction. Zircon Lu-Hf isotopic analysis shows that εHf(t) values are −0.15 – + 8.72, and the single-stage depleted mantle model age is 2448 – 2707 Ma, indicating that the primary magma originated from partial melting of the depleted lithospheric mantle and experienced slight crustal contamination. The results further prove that the low and medium grade rocks were directly involved in the subduction and collision between the Yinshan Block and the Ordos Block.

Absence of hydrothermal oxygen isotope variations in host rocks supports magmatic origin of the giant Gr\xe4ngesberg iron oxide\u2013apatite (IOA) deposit, Central Sweden

The origin of Kiruna-type iron oxide–apatite ores is controversial, and debate presently centres on a ‘magmatic’ versus a ‘hydrothermal’ mode of formation. To complement recent investigations on the Grängesberg iron oxide–apatite ore deposit in the northwestern part of the Palaeoproterozoic Bergslagen ore province in central Sweden, we investigated the oxygen isotope composition of the host rocks of this large iron oxide–apatite ore body. As the metavolcanic and metagranitoid country rocks around the Grängesberg ore body either pre-date or are coeval with ore formation, they would be expected to record an extensive isotopic imprint if the ore body had formed by large-scale hydrothermal processes involving an externally sourced fluid. A direct magmatic formation process, in turn, would have produced localized alteration only, concentrated on the immediate vicinity of the ore body. Here, we test these two hypotheses by assessing the oxygen isotope variations in the host rocks around the main Grängesberg iron oxide–apatite ore body. We analysed oxygen isotopes in quartz from metavolcanic (n = 17) and metagranitoid host rocks (n = 14) from the vicinity of the ore body, and up to 2 km distance along and across the strike of the ore body. Remarkably, we find no significant variation in δ18O values with distance from the ore body, or any deviations in country rock δ18O from common magmatic and/or regional values. Only two samples show shifts to values more negative than the common magmatic range, indicating highly localized hydrothermal overprint only. As a large-scale, low-temperature hydrothermal origin of the ore body through voluminous fluid percolation would be expected to have left a distinct imprint on the oxygen isotope values of the country rocks, our results are more consistent with an ortho-magmatic origin for the Grängesberg iron oxide–apatite ore.

The Statherian Natividade Basin evolution constrained by U–Pb geochronology, sedimentology, and paleogeography, central Brazil

The tectonostratigraphic evolution of the intracontinental Natividade Basin is evaluated by zircon U/Pb analysis of one sample of acidic metavolcanic rock observed at the bottom of the depositional succession and three samples of metasedimentary rocks collected at different stratigraphic levels. The metavolcanic sample yielded a 1824 Ma zircon U–Pb age, and the youngest detrital zircon grains from the metasedimentary sequence resulted in a maximum depositional age of 1776 Ma for the paleobasin. The isotopic data associated with the analysis of stratigraphy, paleogeography, and sedimentary environments converge to the interpretation that the Natividade Group evolved from the late Paleoproterozoic (Statherian), possibly entering the early Mesoproterozoic. The sedimentation is interpreted to have occurred between the deposition of the Araí and the Traíras/Serra da Mesa groups. This understanding is due to the absence of continental sediments in the Natividade Basin (which could corroborate the correlation with the Araí Basin) and the absence of detrital zircon grains younger than 1.5 Ga (which would support the correlation with the Traíras/Serra da Mesa groups). The metavolcanic rocks are observed only locally and are interpreted as the result of melting by crustal rifting that was greatest in the south, related to the Araí Group, and decreased northwards of the study region. The mechanical subsidence responsible for the deposition of the Araí Group continental sediments was not observed in the Natividade Basin. The thermo-flexural subsidence responsible for the Natividade Basin evolution allowed the deposition of complex sedimentary environments related to mixed siliciclastic platforms and shallow water turbidite facies. Based on bibliographic studies and fieldwork data, our geological model suggests that the Almas Block was possibly a high paleorelief area, separating the Araí Basin to the south and the Natividade Basin to the north. This assertion is based on the following facts for the study area: i) there is no occurrence of the Veadeiros Supergroup sediments; ii) Nowadays in the region, there are many high relief areas related to quartzite layers and several granitic units that are resistant to denudation processes; iii) deposition of the shallow water turbidite present at the Natividade Group needed elevated source areas to control the sedimentation; iv) The predominance of carbonates and mass flow deposits in the southern portion of Natividade Group, near to Almas Block, suggests a lower depth of the sea to the south of the basin. Further north, the predominance of fine-grained terrigenous deposits suggest a deeper basin; v) it is common in rift-sag environments the preservation of elevated blocks as an isostatic compensation mechanism. The integrated analysis conducted allowed the following conclusions: the Natividade Basin evolution is not coeval with the Araí or the Traíras/Serra da Mesa basins and is classified as a sag-type basin in which the sedimentary environments resulted in massive carbonate accumulation in a complex platform controlled by the basement paleorelief.

Coexisting arc and MORB signatures in the Sonakhan greenstone belt, India: late Neoarchean – early Proterozoic subduction rollback and back-arc formation

Differentiation of rock suites related to mid-ocean ridge and subduction zone in Archean greenstone belts is important in tracing back tectonic processes related to evolution of these belts. The late Neoarchean – early Paleoproterozoic Sonakhan greenstone belt (SGB) lying between Mesoarchean gneisses of the Bastar craton and the Mesoproterozoic Chattisgarh Supergroup in central India was earlier interpreted to have arc-like affinity. New data from the SGB is presented to reinterpret the Archean tectonic setting. NNW-SSE trending SGB is constituted of three domains. The Baghmara domain in the west is dominantly a mafic metavolcanic rock succession (BGMV group), with repeated cycles of massive to pillowed basalts, pillow breccia and thin chert-BIF-shale and greywacke interlayers, representing an oceanic back-arc system. The Bilari domain in the east, with mixed mafic and felsic metavolcanic rocks (BLMV group) and minor clastic metasediments, presents an ancient magmatic arc. Overlapping these, a polymictic conglomerate-sandstone (greywacke) intercalation of the Arjuni Formation occurs in the central part of steep fold-fault belt of the SGB. Basic to intermediate intrusives (SMI group) and syn- to late-tectonic granitoids occur in all three domains. The BGMV group samples are low-K tholeiites and characterized by modern MORB like major element composition and near-flat REE patterns, reminiscent of some basalts of back-arc spreading centres, such as Parece Vela off Mariana arc. These features together with plots in Sm/Yb versus La/Sm diagram suggest derivation of their parental magmas from primitive spinel lherzolite mantle source with an N-MORB affinity that subsequently fractionated under low-pressure conditions. The BLMV and SMI samples with calc-alkaline major element composition are characterized by E-MORB type REE profiles, with enriched LREE and fractionated HREE patterns, and enrichment in trace elements more incompatible than Ti, relative to N-MORB. In addition, plots in Sm/Yb versus La/Sm diagram indicate derivation of parental magmas from partial melting of enriched garnet lherzolite mantle source at different depths, less and more deep for the BLMV and SMI groups, respectively. The BLMV magmas evolved via crystal fractionation under high water pressure conditions. The intermediate to acidic intrusives of the SGB are calc-alkaline and metaluminous, similar to I-type granites. Although in Th/Yb versus Nb/Yb diagram all the SGB mafic rocks plot above MORB array, restriction of the BGMV samples near N-MORB – PM field and distribution of the BLMV and SMI samples along AFC curve above the MORB array confirm juxtaposition of two contrasting suites, with oceanic back-arc and arc affinities, in the SGB. The Arjuni Formation apparently represents an accretionary wedge lodged in between the Baghmara and Bilari domains. Based on geological and geochemical characteristics, we suggest influence of subduction rollback and oceanic back-arc spreading in the tectonic evolution of the Sonakhan greenstone belt, which may have been common in other late Neoarchean – early Proterozoic greenstone belts.

New geochronologic and geochemical constraints for the Pedro Pereira metavolcanosedimentary sequence: Evidence for a 2.77 Ga oxygen oasis record in the São Francisco-Congo paleocontinent

Exposed within the reworked basement of the Araçuaí orogen, eastern Brazil, the Pedro Pereira metavolcanosedimentary sequence (PPMS) comprises a lower unit dominated by metaultramafic/mafic rocks, an intermediate unit dominated by metarhyolites and a predominant upper unit of acid metavolcanites associated with chemical and clastic metasedimentary rocks. This sequence has been defined and investigated in the ’80s when it was considered as a Mesoarchean greenstone belt. Since then, few studies have addressed these rocks, and due to the lack of reliable geochronological data, a Paleoproterozoic age has also been suggested for the sequence. To reevaluate the age and evolution of the unit, new whole-rock geochemical and zircon U–Pb (LA-ICPMS) geochronologic data were obtained for the iron formation and acid metavolcanic rocks, respectively. When normalized to post-Archean Australian Shale standard, BIF samples can be divided into three groups. Group 1 BIF samples are marked by significant enrichment of the LREE, very high ∑REE + Y, positive La anomaly, extreme negative Ce anomaly, negative Eu anomaly, and subchondritic Y/Ho ratio. In contrast, Group 2 BIFs are marked by the enrichment of HREE, low ∑REE + Y, positive La anomaly, negative Eu anomaly, slight negative Ce anomaly, and predominantly super- and subchondritic Y/Ho ratios. The massive iron formation (MIF) group shows very high middle-to-heavy REE + Y enrichment, strong negative Ce anomaly, negative Eu anomaly, very high ∑REE + Y, and variable Y/Ho ratios. The modification of the iron formation by post-depositional hydrothermal alteration might have disturbed the original basin water signature. However, Group 2 is more compatible with pure chemical precipitates and its slight negative anomalies suggest an oxic environment. A PPMS metarhyolite from the upper portion of the sequence interlayered with the BIFs yielded the Concordia Age of 2771 ± 8.4 Ma. This age rules out a Paleoproterozoic origin for the PPMS, as suggested/considered by recent papers. The results imply that PPMS possibly represents one of the oldest records of oxygenation previous to the GOE in the geologic archives and could provide information to understand the early evolution of localized pockets of oxygen before the Great Oxidation Event.

Archean to Paleoproterozoic evolution of the Crixás greenstone belt, Central Brazil: Insights from two contrasting assemblies of metaigneous rocks

The Archean-Paleoproterozoic Crixás greenstone belt is one of the most important components of the Crixás-Goiás Domain, the oldest crustal segment of the Central Brazil Goiás Massif. This gold-bearing greenstone comprises amphibolite facies ultramafic-mafic metavolcanic rocks overlaid by a low-grade metasedimentary succession associated with rocks of intermediate composition. We present new integrated geochemical and isotopic data of the metaigneous rocks to characterize the stages of magmatism and delineate the petrogenetic processes and tectonic environment of the different units. The data give important insights into the geological evolution of the Crixás greenstone belt during the Archean and Paleoproterozoic, including that the Crixás greenstone belt consists of genetically and temporal unrelated rocks that are spatially associated in a structural framework marked by stacking and imbrication. Two lithological assemblies were described: i) a Mesoarchean komatiite-tholeiite association formed by heterogeneous mantle plume melting in an oceanic plateau setting, and ii) a Rhyacian intraoceanic arc association. These units are allochthonous and override an Archean crustal basement dominated by granites and gneisses of mostly TTG composition. Chemical composition of the komatiites is similar to the Barberton-type Al-depleted komatiites, whilst the basalts consist of high-Mg tholeiites with flat chondrite-normalized REE patterns and immobile trace element systematics that resemble Phanerozoic oceanic plateau lavas. Eruption age of the ultramafic and mafic lavas is roughly estimated at ~3.0 Ga, based on previously published SmNd and PbPb isotopic studies, whereas whole-rock ReOs isotopic data of samples of komatiite and basalt presented here yielded a 2609 ± 65 Ma isochron age. This younger age may reflect a Neoarchean overprinting event associated with metasomatic Re-addition recorded in the lower ultramafic-mafic metavolcanic rocks, possibly related to the late stages of the cratonization of the Crixás-Goiás Domain during the emplacement of the youngest Archean granites of the region (~2.70 Ga). Newly described andesites and diorites possess a subduction zone fingerprint. LA-ICP-MS UPb zircon dating of an andesite sample provided an upper intercept Concordia age of 2172.2 ± 12.7 Ma, constraining the age of the intermediate magmatism. Therefore, these rocks are correlated with the Rhyacian syn-orogenic poorly-sorted graywackes and carbonaceous phyllites of the greenstone's upper stratigraphy. Andesites and diorites have homogeneous Paleoproterozoic two-stage Nd model ages of 2.28 to 2.26 Ga and positive initial εNd values of +2.34 to +2.69, indicating a juvenile nature for the arc magmatism. ReOs geochronology of gold-related arsenopyrite from a metasedimentary rock-hosted massive sulfide provided an age of 2137 ± 11 Ma, suggesting that the mineralization is also coeval to the Rhyacian collisional stage.

Aeromagnetic and remote sensing characterization of structural elements influencing iron ore deposits and other mineralization in Kabba, southwestern Nigeria

Remote sensing and aeromagnetic data analyses were conducted for the characterization of basement structural elements that influence iron ore deposits and other mineralization over kabba and its surroundings. Spectral analysis suggests depth to magnetic sources range from deep, intermediate, and shallow with depths (i.e., 2.8 km, 1.5 km, and 0.3 km respectively). Three lithological zones were revealed on the Analytic Signal (AS) image which includes Meta-Sedimentary Complex, migmatite-gneiss-quartzite complexes, and undifferentiated schist and metavolcanic rocks. Structures interpreted based on first vertical derivatives (FVD), total horizontal derivatives (THD), and AS of the aeromagnetic images enabled delineation of long, deep, regional, inter-terrain linear structural features that are different from shallow, relatively short local intra-terrain structures. Typical potential mineralized zones are regions where the delineated structures on the AS coincide with FVD anomalies vis-a-vis zones exhibiting THD peak anomalies and high magnetic susceptibility. These characterized zones also coincide with the location of known geologically deposits of iron - ore and marble. Mineral deposits tend to be dominated regionally by the dominant structural patterns (NE-SW to ENE-WSW, NW-SE to WNW-ESE, and E-W) delineated from SRTM and aeromagnetic analysis.

Crustal architecture and structural evolution of a Neoarchean sedimentary basin: geological and geophysical evidence from Metal Earth Chicobi transect in the Abitibi Subprovince, Superior Province, Quebec, Canada

Our understanding of crustal architecture and tectonic evolution has advanced due to integrated geological and geophysical research on Precambrian cratons. However, it is still a heated debate whether Archean crust structure developed by an allochthonous model similar to modern plate tectonics or autochthonous model emphasizing in-situ evolution of supracrustal belts. The < ca. 2696 Ma Chicobi belt is a sedimentary basin in the central Abitibi Subprovince of the Superior Province, and as such provides a unique opportunity to contribute to this debate. It comprises both Porcupine-type turbidite and Timiskaming-type polymictic conglomerate and cross-bedded sandstone, and unconformably overlies ca. 2728–2711 Ma metavolcanic rocks, including the Amos Group to the south. The metasedimentary rocks were deposited after an early (D1 > ca. 2696 Ma) deformation event, which is expressed by internally foliated clasts in conglomerate. The Chicobi belt was regionally folded and cut by shear zones during a second (D2) deformation event, and was re-folded by Z-shaped folds during a third (D3) dextral shearing event. The D2 event is constrained between ca. 2696 Ma, the pre-existing age of detrital zircon in the Scapa Group along strike of the Chicobi belt, and ca. 2681 Ma, the new age of the cross-cutting syenitic Gémini-Saint-Éloi Pluton.New high-resolution seismic reflection and magnetotelluric surveys were conducted along the same transect across the Chicobi belt. The seismic reflection survey imaged subhorizontal to shallowly north-dipping reflective bands at depths of 3–7 km. Regional-scale inversion of new and pre-existing gravity data indicates that these reflective bands are linked to the Beauchamp Fault south of the Amos Group. Magnetotelluric inversion model shows high resistivity (>1000 Ω·m) zones under metasedimentary rocks, as well as a synform defined by low resistivity (10–100 Ω·m) zones, which aligns with folded metavolcanic rocks south of the Chicobi belt.Integration of the geophysical data with surface mapping suggests that the upper-middle crustal architecture of the Chicobi belt is dominated by D2 upright folds overlying a fault zone. We suggest that deformation of the Chicobi belt is dominated by thrusting and folding, a deformation style compatible with allochthonous growth of the Abitibi Subprovince.

Petrological, whole–rock geochemical, geochronological, and Lu–Hf isotope constraints on the regional tectonic evolution of the Laoling Group in northeastern Jiao–Liao–Ji Belt, North China Craton

This study examines the original nature, tectonic setting, and evolution of Paleoproterozoic metasedimentary rocks in the Laoling Group of northern Jiao–Liao–Ji Belt (JLJB), in the northeastern margin of the Eastern Block of the North China Craton (NCC), using new petrological, whole–rock geochemical, geochronological, and Lu–Hf isotope data. The youngest detrital zircons constrain the maximum deposition age of the metasedimentary rocks to 1.97 Ga, and two metamorphic zircon age peaks correspond to 1.91 and 1.84 Ga for their metamorphism. The protoliths of the metasedimentary rocks in the Laoling Group were medium- to fine-grained clastic sediments, dominated by shale, graywacke, and claystone, with a smaller amount of sandstone, derived from Paleoproterozoic granites and coeval acidic meta-volcanic rocks, with minor contributions from Mesoarchean–Neoarchean basement rocks and Paleoproterozoic meta-mafic rocks. The protoliths of the metasedimentary rocks in the Dalizi Formation of the Laoling Group were deposited in a convergent margin setting, and subduction along the northern JLJB likely ended before ca. 1.91 Ga when the Longgang and Nangrim blocks collided. The εHf(t) values of ca. 2.14 Ga zircons mostly lie between the depletion mantle and the chondrite evolution line, coupled with extensive coeval magmatism, are consistent with the ca. 2.14 Ga significant continental crust growth in this region. An integrated study of petrology, geochemistry, metamorphic P–T paths, and tectonic evolution suggests that the JLJB experienced two metamorphic events at ca. 1.91 and 1.84 Ga.

Geochemistry and Petrography of the Sediments From the Marginal Areas of Qinghai Lake, Northern Tibet Plateau, China: Implications for Weathering and Provenance

The provenance study of the sediments from Qinghai Lake is of great significance for the understanding of geological and climatic evolution processes of the Tibet Plateau on the one hand and for evaluating the controlling factors of the sediment components on the other hand. The samples were collected from five rivers, foreshore, beach, beach bar, and aeolian sand dune in the Qinghai Lake. The bulk geochemical composition, petrography, and mineralogy features of the samples are analyzed. The results show that: 1) Qinghai Lake sediments experienced low-intensity chemical weathering from the source areas to the deposition sites and were affected by some recycled detrital materials and 2) the source rocks for the sediments include felsic rocks (granite, granodiorite, and felsic volcanic rocks), carbonate, metamorphic rocks (marble and meta-volcanic rocks), and clastic rocks with the felsic source rocks to have the most important impact on the chemical compositions of the sediments. The geochemical indicator of Al2O3/TiO2 reflects that the provenance of fine-grained sediments from the center of Qinghai Lake is more mafic than the coarse-grained sediments from the margin of the Qinghai Lake, suggesting that the hydraulic sorting of grain size probably plays an important role in the geochemical compositions of the sediments. The mafic elements were probably preferentially enriched in muds.

Precambrian and Early Palaeozoic metamorphic complexes in the SW part of the Central Asian Orogenic Belt: Ages, compositions, regional correlations and tectonic affinities

Precambrian metamagmatic complexes of the Zhingeldy and Kendyktas blocks of the Zheltau and Chu-Kendyktas terranes of the SW part of the Central Asian Orogenic Belt (CAOB) consist of predominant Neoproterozoic orthogneisses with the estimated protolith ages of ca. 790–800 Ma, and subordinate epidote and garnet amphibolites, metabasalts and metadolerites of the lower metamorphic grades. The orthogneisses are characterized by negative εNd(t) values of −12 to −4 with Nd model ages of 2.3–1.7 Ga, indicating their protoliths could have formed by a reworking of the Palaeoproterozoic crustal materials. Precambrian metavolcanic rocks in the studied region are rhyolites with the estimated crystallization ages of ca. 800–830 Ma, and are associated with metabasalts. Metasedimentary complexes in the studied region consist of intensively retrogressed garnet-mica schists interlayered with marmorized limestones, metasilicic rocks, epidote and garnet amphibolites and strongly altered eclogites. The inferred protoliths of the garnet-mica schists are terrigenous rocks accumulated during the latest Ediacaran – middle Cambrian and formed from Mesoproterozoic (ca. 1.0 Ga) and to a lesser extent Palaeoproterozoic felsic source rocks (ca. 1.6–1.7 Ga and ca. 2.5 Ga). The garnet-mica schists of the Kendyktas block have negative εNd(t) of −13.4 and a model age tNd(DM) of 2.2 Ga, implying formation of their protolith from Palaeoproterozoic crustal source. The studied Zheltau and Chu-Kendyktas terranes from the SW part of the CAOB have much in common in terms of Precambrian and Early Palaeozoic evolution and chemical compositions of the key rock types. The presence of the Early Precambrian rocks (Zheltau terrane), an abundance of the Late Mesoproterozoic (1000–1100 Ma) detrital zircons in the Ediacaran-Cambrian metasedimentary complexes and a wide distribution of the Late Neoproterozoic magmatism, are distinctive features of the studied terranes. We refer such Precambrian terranes of the western CAOB to the Ulutau-Moyunkum group where tectono-magmatic evolution is consistent with that observed from the northern part of the Tarim craton. Our results also aid comprehensive correlation of metamorphic complexes of the Precambrian terranes from the SW CAOB.

Reconstructing the Lancang Terrane (SW Yunnan) and implications for early Paleozoic Proto-Tethys evolution at the northern margin of Gondwana

The Lancang Group in SW Yunnan constitutes a pivotal component of the continental margin bounding the Proto- and Paleo-Tethys oceans, yet its formation timing, composition and tectonic affinity remain ill-defined. In this contribution, we present new zircon U-Pb age and whole-rock geochemistry data from volcanic and sedimentary rock units, which are compiled with published data, to refine the depositional age, provenance and nature of the group and its host terrane. The Lancang Group consists of five formations, two of which (Manlai and Huimin) contain abundant metavolcanic rocks. The metavolcanic rocks show a secular change in geochemistry, from EMORB-like calc-alkaline series of the ca. 495 Ma Manlai Formation, to arc-like high-K series of the Huimin Formation. Five metavolcanic samples from the Huimin Formation yielded zircon U-Pb ages between 478 ± 5 Ma and 442 ± 5 Ma. The late Cambrian-Ordovician arc magmatism is related to an active continental margin setting associated with subduction of the Proto-Tethys Ocean. The time-integrated geochemical variation shown by the volcanic rocks likely reflects evolving maturity of the arc system. The youngest detrital zircon ages of the five formations, in conjunction with age constraints from the interbedded volcanic rocks, suggest that the Lancang Group was largely accumulated during the early Paleozoic. The Lancang Terrane is reconstructed as a continental block, which formed part of the magmatically-active margin of the Proto-Tethys during the early Paleozoic. Provenance analysis based on detrital zircon age spectra and Hf isotope data indicates Gondwana affinities of the terrane. Furthermore, xenocrystic zircon ages and zircon Hf and whole-rock Nd model ages from igneous rocks support presence of Neoarchean-Mesoproterozoic basement underlying the terrane. The reconstructed Lancang Terrane bears important implications for understanding the configuration of the northern margin of Eastern Gondwana and the relationship between the Proto- and Paleo-Tethys oceans.

Petrology and geochemistry of the Archean Huangling greenstone belt in the Yangtze Craton, South China

The Archean Huangling greenstone belt (HGB) is preserved as amphibolites in the Huangling granite–greenstone terrane, Yangtze Craton. Geologically and geochemically, the amphibolites are metavolcanic rocks and can be subdivided into amphibole schist, massive amphibolite and banded amphibolite, with komatiite, komatiitic basalt and tholeiitic basalt protoliths, respectively. The komatiites have high MgO (20–30%), Cr, Co and Ni contents; low Rb, Sr, Ba and ΣREE contents; and slight LREE depletion and HREE enrichment, characterizing them as the Al-undepleted type according to the criteria (CaO/Al2O3)adj ˃ 20, (Al 2O3/TiO2)adj and (Gd/Yb)PM < 1. The komatiitic basalts and tholeiitic basalts have lower MgO (5–16%), Cr, Co and Ni contents; higher Rb, Sr, Ba and ΣREE contents; and slight LREE enrichment and HREE depletion. The geochemical plots (Zr/Y vs. Nb/Y, Nb/Yb vs. Th/Yb, La/Ta vs. La/Sm and Y vs. Zr/Y), negative Nb anomalies, wide ƐNd values (−8.0 to +6.2), negative zircon ƐHf values (−11.59 to −3.98), and old TNd (DM) (3.9–2.9 Ga) and zircon THf (DM) ages (3.9–3.3 Ga) of the amphibolites reflect a mantle source with slight crustal contamination and show that the komatiitic basalts and tholeiitic basalts are more closely related to each other than to the komatiites. The tholeiitic basalts originated from moderate-degree melting of a mantle plume head, the komatiitic basalts originated from the fractional crystallization of tholeiitic basalt melts as cumulates, and the komatiites originated from high-degree melting of the mantle plume tail with garnet entering the melt phase. These high-Mg lavas erupted in a continental rift when the mantle plume impinged on the continental lithosphere, generating rifting and subsequent volcanic eruptions. These ultramafic–mafic rocks (3.2–3.0 Ga) formed the HGB, which was subsequently intruded by TTG (3.0–2.9 Ga), together forming the Archean Huangling granite–greenstone terrane in the Yangtze region.

Adakites and associated granitoids from the Serra da Prata Arc: Evidence for a Tonian subduction setting within the Araçuaí-Ribeira orogenic system (AROS), SE Brazil

Within deeply eroded orogenic belts, the geochemical signatures of metamorphosed igneous rocks can be used to constrain modern-like tectonic models operating in Precambrian times. One important aspect of this discussion is the presence or not of pre-collisional arc-related rocks that would represent the magmatic product of subduction processes imprinted on the upper plate of the convergent setting. New detailed geological and geochemical data of the northern segment of the Tonian to Cryogenian juvenile Outer Arc of the Araçuaí-Ribeira Orogenic System (AROS) are presented. Results were combined with recently published U–Pb and Hf data to contribute to a comprehensive tectonic model. The arc-related rocks (Serra da Prata Complex), transformed in foliated orthogneisses, are interlayered with dolomitic marbles, amphibolites, and psammo-pelitic paragneisses (Italva Group). This association is similar to the one described in the southern part of the orogenic system, pointing to the extension of the Outer Arc system until the Rio Doce River, in the transition zone between the Araçuaí and Ribeira segments of the AROS. New geochemical analyses together with reinterpreted published data indicate the presence of typical adakites that, together with normal calc-alkaline expanded series, island arc tholeiitic series, and A-type granitoids, corroborate with a modern style plate tectonic model. The envisaged model started in the Tonian, with E-verging subduction that resulted in the production of very juvenile arc-related rocks and associated fore-to back-arc basins filled with carbonatic rocks, meta-volcanic rocks, and MORB-like basaltic rocks. The typical adakites are considered the result of partial melting of young and hot oceanic lithosphere under moderate to high pressures in which garnet was an important stable residue. Potassic C-type adakitic rocks formed by the partial melting of thickened lower crust occur associated with the typical adakites. The occurrence of both adakites, together with A-type granitoids, characteristic of the investigated terminal sector of the Serra da Prata arc, is interpreted to result from a slab window gap, either associated with slab tearing/ridge subduction or slab roll-back, hampering an intracontinental model for the Ribeira orogen.

Carboniferous mafic metavolcanic rocks in the Northern Gemeric Unit: Petrogenesis, geochemistry, isotope composition and tectonic implication

Carboniferous mafic metavolcanic rocks in the Northern Gemeric Unit: Petrogenesis, geochemistry, isotope composition and tectonic implication

Late Palaeozoic extensional volcanism along the northern margin of Gondwana in southern Turkey: implications for Palaeotethyan development

The Late Palaeozoic–Early Mesozoic Tethyan development of the Eastern Mediterranean region remains debatable, especially in Turkey, where alternative northward and southward subduction hypotheses are proposed. Relevant to this debate, new whole-rock geochemical data are provided here for early Carboniferous (Late Tournaisian-Late Visean; c. 340–350 Ma) tuffaceous sedimentary rocks within the Çataloturan thrust sheet (Aladağ nappe), eastern Taurides. The tuffs accumulated from evolved alkaline volcanism, variably mixed with terrigenous and radiolarian-rich sediments. In addition, Late Palaeozoic meta-volcanic rocks, c. 150 km farther NE, within the Binboğa (= Malatya) metamorphics (a low-grade high-pressure unit), are indicative of a within-plate setting. An impersistent geochemical subduction signature in these volcanics may represent an inherited, rather than contemporaneous, subduction influence, mainly because of the absence of a continental margin arc or of arc-derived tuff. Both the Binboğa metamorphics and the Çataloturan thrust sheet (Aladağ nappe) restore generally to the north of the relatively autochthonous Tauride carbonate platform (Geyik Dağ), within the carbonate platform bordering north-Gondwana. The Çataloturan thrust sheet is interpreted, specifically, as a c. E–W, deep-water, volcanically active rift that progressively infilled. Regional geological evidence suggests that melange units (Konya Complex, Afyon zone), Teke Dere unit, Lycian nappes), and Chios–Karaburun melange, E Aegean) accreted to the north-Gondwana continental margin during the late Carboniferous; this was coupled with localised calc-alkaline granitic magmatism (Afyon zone of Anatolide crustal block). We propose an interpretation in which Late Devonian–Carboniferous alkaline intra-plate volcanism relates to extension/rifting along the north-Gondwana margin. In contrast, the melange accretion and granitic magmatism could relate to short-lived late Carboniferous southward subduction that accompanied the diachronous closure of Palaeotethys.

Provenance Characterization and Palaeoenvironmental Analysis of the Meta-Sedimentary Rocks of Sonaghati Formation, Betul District, Madhya Pradesh Using Geochemical Approach

Betul belt, ENE-WSW trending, 135 km long, prominent litho-tectonic unit exposed in the central part of Central Indian Tectonic Zone (CITZ) is composed of meta-sedimentary &amp; meta-volcanic rocks intruded by mafic-ultramafic and granitic suite of rocks, belonging to Palaeoproterozoic to Neoproterozoic age. This belt is traversed by several ENE-WSW trending, sub-vertical ductite shear zones. The meta-sedimentary rocks of Sonaghati Formation were geochemically characterized and their geochemical composition was interpreted for provenance characterization and paleo-environmental assessment. The weathering indices including Chemical index of Alteration, Chemical index of Weathering, Plagioclase Index of Alteration and Weathering Index of Parker indicate that theses meta-sedimentary rocks have witnessed the substantial amount of weathering at the source without any evidence of potash metasomatism. The Bivariate plots using the major and trace element composition show co-linear trends, which reflect that all these samples belong to co-genetic population and the visible compositional variation could be attributed to chemical, mineralogical and textural maturity. The Sonaghati metasedimentary rocks are enriched in REE with negative Eu anomaly. The LREE enrichment varies from 122 to 174 times and that of the HREE enrichment ranges from 12 to 31 times of Chondrite indicating highly varied protoliths. The provenance characterization was attempted using the large ion lithophile elements and high field strength elements. The results show that the precursor for these meta-sedimentary litho-units are mixed source with the major contributor being felsic to intermediate and minor contribution has come from the mafic end members. These meta-sedimentary rocks were deposited in the overall semi arid climate with a sequential transition, suggesting the variable climatic conditions ranging from semi-arid to arid. The Cu/Zn, V/Cr ratios, and presence of pyrites dissemination and stringers eventually indicate the prevalence of reducing environmental conditions during the deposition of these meta-sediments.

The Paleoarchean Northern Mundo Novo Greenstone Belt, São Francisco Craton: Geochemistry, U–Pb–Hf–O in zircon and pyrite δ34S-Δ33S-Δ36S signatures

Greenstone belts contain several clues about the evolutionary history of primitive Earth. Here, we describe the volcano-sedimentary rock association exposed along the eastern margin of the Gavião Block, named the Northern Mundo Novo Greenstone Belt (N-MNGB), and present data collected with different techniques, including U–Pb–Hf–O isotopes of zircon and multiple sulfur isotopes (32S, 33S, 34S, and 36S) of pyrite from this supracrustal sequence. A pillowed metabasalt situated in the upper section of the N-MNGB is 3337 ± 25 Ma old and has zircon with εHf(t) = −2.47 to −1.40, Hf model ages between 3.75 Ga and 3.82 Ga, and δ18O = +3.6‰ to +7.3‰. These isotopic data, together with compiled whole-rock trace element data, suggest that the mafic metavolcanic rocks formed in a subduction-related setting, likely a back-arc basin juxtaposed to a continental arc. In this context, the magma interacted with older Eoarchean crustal components from the Gavião Block. Detrital zircons from the overlying quartzites of the Jacobina Group are sourced from Paleoarchean rocks, in accordance with previous studies, yielding a maximum depositional age of 3353 ± 22 Ma. These detrital zircons have εHf(t) = −5.40 to −0.84, Hf model ages between 3.66 Ga and 4.30 Ga, and δ18O = +4.8‰ to +6.4‰. The pyrite multiple sulfur isotope investigation of the 3.3 Ga supracrustal rocks from the N-MNGB enabled a further understanding of Paleoarchean sulfur cycling. The samples have diverse isotopic compositions that indicate sulfur sourced from distinct reservoirs. Significantly, they preserve the signal of the anoxic Archean atmosphere, expressed by MIF-S signatures (Δ33S between −1.3‰ to +1.4‰) and a Δ36S/Δ33S slope of −0.81 that is indistinguishable from the so-called Archean array. A BIF sample has a magmatic origin of sulfur, as indicated by the limited δ34S range (0 to +2‰), Δ33S ~ 0‰, and Δ36S ~ 0‰. A carbonaceous schist shows positive δ34S (2.1‰–3.5‰) and elevated Δ33S (1.2‰–1.4‰) values, with corresponding negative Δ36S between −1.2‰ to −0.2‰, which resemble the isotopic composition of Archean black shales and suggest a source from the photolytic reduction of elemental sulfur. The pillowed metabasalt displays heterogeneous δ34S, Δ33S, and Δ36S signatures that reflect assimilation of both magmatic sulfur and photolytic sulfate during hydrothermal seafloor alteration. Lastly, pyrite in a massive sulfide lens is isotopically similar to barite of several Paleoarchean deposits worldwide, which might indicate mass dependent sulfur processing from a global and well-mixed sulfate reservoir at this time.

The Serrinha granite-greenstone terrane revisited: A reevaluation of its evolution based on the current U–Pb age database

This paper aims to summarize the current knowledge of the Serrinha Nucleus and add new U–Pb geochronological data for this area. New ages were obtained for granitoids and a metadacite belonging to the Felsic Volcanic Unit of the Rio Itapicuru Greenstone Belt (RIGB). The age of 3185 ± 9 Ma obtained at the Pedra Alta granodiorite-tonalite pluton corresponds to the interval of emplacement (2.9–3.2 Ga) of the TTG granites in the Serrinha Nucleus during the Archean. As expected for granites belonging to the Paleoproterozoic TTG/calc-alkaline group, Barrocas and Lagoa dos Bois plutons yielded U–Pb zircon ages of 2168 ± 12 Ma and 2158 ± 13 Ma, respectively. These 2.15–2.17 Ga ages are attributed to a syn-orogenic evolution of the calc-alkaline felsic volcanism of the RIGB during the Rhyacian period, which is related to the closure of the Itapicuru basin. A sample of a biotite-granite dyke collected at the Araci TTG Pluton, which also is part of the Archean TTGs group of granites, yields an age related to the post-orogenic Morro do Lopes magmatism. Its age of 2076 ± 6 Ma also coincides with those attributed to the hydrothermal event that resulted in the gold mineralization at the RIGB and reinforces the widespread efects of this thermal post-orogenic alkaline magmatic event. The metadacite age of 2147 ± 38 Ma is considered the minimum crystallization age for the felsic volcanic arc of the RIGB, being compatible with regional ages obtained for other metavolcanic rocks and granitoids from this area. The ages obtained in the present work, added to the geochronological database of Serrinha Nucleus, show that the closure of the Itapicuru basin occurred between approximately 2.15 and 2.17 Ga, period corresponding to the first deformation phase that affected the orogen. The age obtained for the intrusive Barrocas Pluton (2168 ± 12 Ma), which outcrops at the southern tip of the greenstone, suggests that the volcanic-sedimentary rocks of Itapicuru should be older and highlights a new model for the evolution of Serrinha Nucleus and the concept of the “Rio Itapicuru Greenstone Belt”.

Geochemistry, zircon U–Pb geochronology, and Hf isotopes of the metavolcanic rocks in the Tongbai orogen of central China: Implication for Neoproterozoic oceanic subduction to slab break-off

Abundant metavolcanic rocks (leptynite, leptite, and amphibolite) occur in the Tongbai orogen, central China, but their age, origin, and tectonic significance are poorly constrained. Here we report geochemistry, zircon U–Pb ages, and Hf isotopes of these metavolcanic rocks to determine their petrogenesis and geodynamic implications. Our data indicate that all the leptynites, leptites, and amphibolites were initially formed at 760–720 Ma and then underwent later metamorphism. These rocks exhibit arc-related geochemical compositions characterized by enriched large ion lithophile elements and light rare earth elements and depleted heavy rare earth elements. Leptynites and leptites present similar characteristics, including high SiO2 (72.14–77.24 wt%), MgO (0.19–0.44 wt%), and Sr (239–827 ppm) compositions and low Y (6.9–16 ppm) and Yb (0.70–1.17 ppm), along with high Sr/Y and (La/Yb)N values. The leptynites exhibit no or slightly negative Eu anomalies in both the whole-rock and zircon. Based on the high K2O (3.75–4.31 wt%) and slightly lower Al2O3 (<15 wt%) contents, as well as high ε(Hf)t values (−0.53 to 5.80), the leptynites conform to C-type adakites and were possibly derived from partial melting of the thickened juvenile mafic lower crust. The leptites demonstrate a distinct negative Eu anomaly and low εHf(t) values of −13.19 to −6.60, implying an ancient middle–lower crustal source. In contrast, the amphibolites exhibit lower SiO2 (52.62–53.33 wt%) and higher MgO (3.88–4.32 wt%) contents, as well as higher Y (19.5–30.0 ppm) and Yb (1.89–3.26 ppm) concentrations, indicating of island arc volcanic rocks. Furthermore, the higher Mg# (31–35), Cr (14–18 ppm), V (200–272 ppm), and Ni (37–59 ppm) contents of the amphibolites indicate more substantial amounts of mantle components in their magma source. The variable zircon εHf(t) values from −10.1 to 1.95 in the amphibolites suggest an enriched mantle source. Based on these new data, we propose that the Tongbai region underwent a continuous oceanic subduction before 750 Ma, followed by a subducted slab break-off during 750–720 Ma, which is inconsistent with the previously proposed single slab break-off model for the South Qinling region