Contemporaneous Volcanic Rocks Research Articles

Silurian post-orogenic volcanic rocks of the South Kunlun Belt, NW China: links to the Proto- to Paleo-Tethyan tectonic transition

ABSTRACT Newly discovered early Palaeozoic intraplate volcanic rocks within the southern Kunlun Orogen provide an opportunity to examine the middle to late Silurian tectonic setting of this area. This study presents new whole-rock geochemical and zircon U–Pb geochronological and Lu–Hf isotope data for the Silurian volcanic rocks of the Wanbaogou area, South Kunlun Belt. These volcanic rocks are split into trachytic and basaltic groups based on field observations and petrographic and geochemical data. Laser ablation–inductively coupled plasma–mass spectrometry (LA–ICP–MS) U–Pb dating of zircons from four samples from this area yields similar ages of 428.2 ± 3.4, 426.4 ± 4.2, 426.1 ± 3.2, and 424.5 ± 3.2 Ma, indicating this volcanic complex formed during the middle to late Silurian. The basalts in this area have high Nb/La ratios, generally positive εHf(t) values and hump-shaped ocean island basalt (OIB)-like intraplate compositions that are free of negative Nb, Ta, and Ti anomalies, indicating the magmas that formed these units underwent at most only slight crustal contamination. In comparison, the trachytes have incompatible trace element concentrations that are higher than the concentrations found in typical arc-type magmas. The melting of the underplated young basaltic crust is a more likely explanation for the formation of the trachytes. These igneous rocks are all of within-plate affinity, were derived either from the asthenospheric mantle or from a mantle plume, and underwent some crustal contamination prior to their emplacement. Combining these data with the results of previous research on the Kunlun high to ultrahigh-pressure (UHP) belt and contemporaneous volcanic and intrusive rocks elsewhere in this region suggests that the Silurian volcanic rocks of the Wanbaogou area formed in a post-orogenic setting as a response to the final closure of the Proto-Tethys Ocean and the transition from Proto- to Paleo-Tethyan tectonic regimes within the East Kunlun Orogen. The geology of this region indicates that middle–late Silurian post-orogenic extension was not just limited to the Wanbaogou area but probably extended across the entire EKO.

Volcanic–plutonic connection and associated Au‐Cu mineralization of the Tulasu ore district, Western Tianshan, NW China: Implications for mineralization potential in Palaeozoic arc terranes

Constructing the plutonic–volcanic connections in volcanic arc setting is essential for understanding a variety of magmatic processes from continental crust differentiation to formation of magmatic‐hydrothermal ore deposits. Based on the combination of zircon U–Pb geochronology and Hf isotope and whole‐rock geochemistry and Sr‐Nd isotopes, we evaluated the potential connections between volcanic and plutonic rocks from the Tulasu Basin, the largest epithermal gold ore district in the Chinese Western Tianshan. The peak of volcanism coincides well with the major plutonism, representing an important magma event at ~347–367 Ma. These contemporaneous volcanic and plutonic rocks show broadly similar arc‐related geochemical signatures and zircon in‐situ Hf (εHf[t] = −1.0–14.3) and whole‐rock Nd (εNd[t] = −4.2–4.6) isotopic compositions, indicative of formation from parental magmas derived from a common depleted mantle source metasomatized by slab‐derived melts, despite diverse consequent processes (fractional crystallization, crustal contamination, and magma mixing/mingling) during their differentiation in a subduction‐related setting. Volcanic rocks from the Axi caldera in the centre of the Tulasu district are geochemically less evolved than the coeval porphyritic granite, which indicates that the intrusive rocks might originate from residual melts left behind by extraction of erupted magmas. In contrast, the porphyry Cu mineralized Kexiaxi pluton, which occurs in the margin of the Tulasu district, is compositionally indistinguishable from the coeval volcanic suites. This implies that the Kexiaxi pluton represents the intrusive equivalent of a common magma reservoir feeding the volcanic eruptions, and magma fluxes may have controlled their differentiation. We thus suggest that the depleted mantle derived calc‐alkaline magma, modified during slab‐related melt metasomatism, crustal contamination, and magma mixing may have played an important role in copper enrichment at Kexiaxi. Finally, the relatively water‐rich and fertile nature of arc magma of the Tulasu district indicates high exploration potential for significant porphyry Cu‐Au deposits. The existing alteration‐immobile fertility proxy (V/Sc) for intrusive rocks may also be useful to assess the fertility of cogenetic volcanic rocks, in turn with implications for unexposed porphyry Cu deposits at depths.

Geochemistry and U-Pb geochronology of 1590 and 1550 Ma mafic dyke swarms of western Laurentia: Mantle plume magmatism shared with Australia

Three new UPb ID-TIMS isotopic ages confirm previous evidence for two geochemically distinct, early Mesoproterozoic pulses of mafic magmatism along the western margin of Laurentia. The first is a UPb baddeleyite age of 1590 ± 5 Ma from a dyke swarm in the Tobacco Root Mountains of western Laurentia. It is the first evidence for magmatism of this age in west-central Laurentia. The second is a UPb baddeleyite age of 1592.4 ± 2.5 from the Western Channel Diabase, 2000 km to the north in NW Laurentia, which supports two previous UPb baddeleyite ages of ca. 1590 Ma for these intrusions. The third is a UPb baddeleyite age of 1551 ± 5 Ma, also from the Tobacco Root Mountains, and provides evidence for a distinct younger pulse of mafic magmatism.We propose that the ca. 1590 Ma mafic intrusions in northwestern and west-central Laurentia represent components of a large igneous province (LIP), which we name the “ca. 1590 Ma Mammoth-Western Channel LIP.”This 1590 Ma LIP is geochemically similar to the contemporaneous volcanic rocks in the Gawler Craton and Curnamona Province of Australia. Furthermore, the 1599 Ma Wernecke Breccias near the Western Channel Diabase are geologically similar to the 1590 Ma Olympic Dam Breccias on the South Australian craton. We propose that a mantle plume at ca. 1590 Ma centered between the Laurentian and Southern Australian cratons, located by converging dyke swarms, fed the intrusions on Laurentia plus the Gawler Range Volcanics, the Hiltaba Suite granites and the Ninnerie Supersuite on the South Australian craton. Additionally, magmatic underplating from the plume set up the hydrothermal system responsible for the formation of the Wernecke and Olympic Dam Breccias. The younger 1551 Ma magmatism in the Tobacco Root Mountains, which shows less contamination of a metasomatized subcontinental lithospheric mantle than the older 1590 Ma pulse, may represent an early stage of rifting that pre-dates the ca. 1470 Ma Belt-Purcell Basin extension of western Laurentia.Felsic magmatism, hypothesized to have accompanied the 1590–1550 Ma LIP magmatism, could be a potential source for detrital zircon, thereby reducing the requirement for a non-Laurentian source for detrital zircon ages within the 1610–1490 Ma “North American Magmatic Gap.” Additionally, the 1590 Ma and 1550 Ma ages on western Laurentia dykes provide tighter constraints for previous paleomagnetic studies.

Petrogenesis and tectonic implication of the Late Triassic post-collisional volcanic rocks in Chiang Khong, NW Thailand

The volcanic rocks exposed within the Chiang Khong–Lampang–Tak igneous zone in NW Thailand provide important constraints on the tectonic evolution of the eastern Paleotethys ocean. An andesite sample from the Chiang Khong area yields a zircon U–Pb age of 229±4Ma, significantly younger than the continental-arc and syn-collisional volcanic rocks (ca. 238–241Ma). The Chiang Khong volcanic rocks are characterized by low MgO (1.71–6.72wt.%) and high Al2O3 (15.03–17.76wt.%). They are enriched in LILEs and LREEs and depleted in HFSEs, and have 87Sr/86Sr (i) ratios of 0.7050–0.7065, εNd (t) of −0.32 to −1.92, zircon εHf (t) and δ18O values of 3.5 to −11.7 and 4.30–9.80 ‰, respectively. The geochemical data for the volcanic rocks are consistent with an origin from the enriched lithospheric mantle that had been modified by slab-derived fluid and recycled sediments. Based on available geochronological and geochemical evidences, we propose that the Late Triassic Chiang Khong volcanic rocks are equivalent to the contemporaneous volcanic rocks in the Lancangjiang igneous zone in SW China. The formation of these volcanic rocks was possibly related to the upwelling of the asthenospheric mantle during the Late Triassic, shortly after slab detachment, which induced the melting of the metasomatized mantle wedge.

Petrogenesis of the flood basalts from the Early Permian Panjal Traps, Kashmir, India: Geochemical evidence for shallow melting of the mantle

The Early Permian Panjal Traps of northern India represent a significant eruption of volcanic rocks which occurred during the opening of the Neotethys Ocean. Basaltic, basaltic-andesites, dacitic and rhyolitic rocks collected from Guryal Ravine and Pahalgam show evidence for subaerial and subaqueous eruptions indicating that they are contemporaneous with the formation of a shallow marine basin. The major and trace element geochemistry of the basalts is consistent with a within-plate setting and there are basalts which have high-Ti (TiO2 > 2.0 wt.%) and low-Ti (TiO2 < 1.8 wt.%) compositions. The ‘high-Ti’ basalts are similar to OIB whereas the ‘low-Ti’ basalts are similar to continental tholeiites. The identification of ‘high- and low-Ti’ basalts within the Panjal Traps is analogous to other large igneous provinces (e.g. Karoo, Deccan, Parana, Emeishan). The Sr–Nd isotopic values (εNd(T) = − 5.3 to + 1.3; ISr = 0.70432 to 0.71168) of both types of basalts overlap indicating that the rocks may have originated from the same ancient subcontinental lithospheric (i.e. EMII-like) mantle source (TDM = ~ 2000 Ma). The two groups of basalts can be modeled by using a primitive mantle source and different degrees of partial melting where the high-Ti rocks are produced by ~ 1% partial melting of a spinel peridotite source whereas the low-Ti rocks are produced by ~ 8% partial melting. Trace elemental and isotope modeling indicates that some of the basalts assimilated ≤ 10% crustal material. In contrast, the basaltic-andesites are likely formed by mixing between basaltic magmas and crustal melts which produced rocks with higher SiO2 (~ 55 wt.%) content and enriched isotopic signatures (εNd(T) = − 6.1; ISr = 0.70992). The Panjal Trap volcanism was likely due to partial melting of the SCLM within a passive extensional setting related to the rifting of Cimmeria from Gondwana. Contemporaneous volcanic and plutonic granitic rocks throughout the Himalaya are probably not petrogenetically related but are likely part of the same regional tectonic regime.

Petrogenesis of late Mesozoic granitoids in southern Jilin province, northeastern China: Geochronological, geochemical, and Sr–Nd–Pb isotopic evidence

We present geochronological, geochemical, and Sr–Nd–Pb isotope data for late Mesozoic granitoids in southern Jilin province, northeastern China, with the aim of ascertaining the genesis of the granitoids, and further constraining the nature of the lower crust and Mesozoic tectonic evolution of the eastern North China Craton (NCC). The results of LA-ICP-MS zircon U–Pb dating for seven granitoid plutons (from northeast to southwest, the Liudaogou, Toudaogou, Xingfu, Ji'an, Yushulinzi, Lujiangchun, and Zhenjiang plutons) indicate that they formed in the Early Cretaceous (130–122Ma). The granitoids consist of granodiorite, monzogranite, syenogranite, and alkali feldspar granite. Geochemically, they have SiO2=61.8–76.2%, MgO=0.04–3.27%, Mg# [100(Mg/(Mg+Fe2+)]=7–55, and Al2O3=12.7–16.2%, and they are characterized by enrichment in light rare earth elements (LREEs) and large ion lithophile elements (LILEs, such as Rb, Cs, and K), and depletion in heavy rare earth elements (HREEs) and high field strength elements (HFSEs, such as Nb and Ta). Their initial 87Sr/86Sr ratios, εNd(t) values, and initial 206Pb/204Pb ratios range from 0.70627 to 0.71067 (except for one sample), −1.9 to −13.9, and 17.84 to 18.47, respectively. Compared with the Zhenjiang, Yushulinzi, Lujiangchun, and Ji'an plutons from the southwestern part of the study area [εNd(t)=−9.8 to −13.9 and TDM2=1726–2058Ma], those from the northeastern part of the study area (the Xingfu, Toudaogou, and Liudaogou plutons) have relatively high εNd(t) values (−1.9 to −8.8), low TDM2 (1081–1641Ma). The Early Cretaceous granitoids with high SiO2 contents and low Mg# from the southwestern part of the study area, together with those with high SiO2 contents and low Mg# from the northeastern part of the study area, were possibly mainly derived from the partial melting of Paleoproterozoic lower crust, with a minor contribution from younger material. The latter granitoids, together with the Zhenjiang granodiorites from the southwestern part of the study area, may have formed via mixing between crust- and mantle-derived melts. The rock associations and geochemistry of the southern Jilin granitoids, together with the contemporaneous volcanic rocks in this area, indicate that they formed in an active continental margin setting associated with the oblique subduction of the Paleo-Pacific Plate beneath the Eurasian continent in the Early Cretaceous.

Distribution, Mineralogy, and Geochemistry of Selenium in Felsic Volcanic-Hosted Massive Sulfide Deposits of the Finlayson Lake District, Yukon Territory, Canada

Three significant base metal deposits were recently discovered in the Finlayson Lake district, Yukon Territory, Canada; however, widely varying selenium contents (e.g., 1,100 ppm avg Se at Wolverine, 200 ppm avg Se at Kudz Ze Kayah, and 7 ppm avg Se at GP4F) have hindered their rapid exploitation. Observations and data from the deposits in the Finlayson Lake district show a strong positive correlation between high-temperature (>300°C), copper-rich sulfide assemblages at the base of volcanic-hosted massive sulfide (VHMS) and the selenium content of all sulfide minerals. Thermodynamic calculations for selenide and sulfide minerals indicate significantly higher temperatures of formation and lower solubilities of selenide versus sulfide minerals, which is consistent with petrographic observations and can explain the distribution of selenium in zone-refined VHMS and volcanic sediment-hosted massive sulfide deposits. Volumetric mass-balance calculations indicate that some selenium could be sourced from contemporaneous volatile degassing of SeO 4 , its rapid reduction to H 2 Se, and subsequent entrainment into hydrothermal ore fluid. However, estimates for the Kudz Ze Kayah and Wolverine deposits suggest an additional large Se reservoir. Volumetric mass-balance estimates indicate that contemporaneous volcanic rocks and black shales are likely selenium reservoirs for the Kudz Ze Kayah and Wolverine deposits, respectively.

Nd Isotopes and Geochemistry of Phanerozoic Clastic Sedimentary Rocks from the Yangtze Block and Their Tectonic Implications

This article presents Sm-Nd and geochemical data on fine-grained sediments of the northern margin from the Yangtze block, China, to understand the variations of Nd isotopic compositions and crustal evolution history in this area. The results are as follows: (1) Nd isotopic compositions for clastic sedimentary rocks of the Middle-Late Proterozoic have relatively positive Nd( t) values (+ 2.72 to + 0.69), with Nd model ages from 1.38 Ga to 1.55 Ga, corresponding to the contemporaneous volcanic rocks from the Xixiang ( ▪) Group. This indicates that the arc-related materials from Middle-Late Proterozoic dominate the provenances of the Middle-Late Proterozoic periods. (2) The gradual decrease in ɛNd( t) during the Cambrian-Carboniferous periods is likely to reflect the progressively increasing proportion of erosion materials from the Foping ( ▪) and Qinling ( ▪) complexes, corresponding to a gradually decreasing trend in the La/Th ratios. (3) A prominent increase in the ɛNd( t) value of the Late Permian strata probably reflects the significant incorporation of the mantle-derived materials. The trace element data are compared with data of the Emeishan ( ▪) flood basalts. These data indicate that the volcanic dust has been added to the Late Permian strata during the Late Permian, represented by periods of extremely high Emeishan flood basalt activity in the south-eastern margin of the Yangtze block.

Neoproterozoic and Cambrian arc magmatism along the eastern margin of the Victoria Lake Supergroup: A remnant of Ganderian basement in central Newfoundland?

The eastern margin of the composite and structurally complex Victoria Lake Supergroup has previously been mapped where the Silurian Rogerson Lake conglomeratic rocks unconformably overlie Cambrian Tally Pond volcanic rocks. However, the recognition that the form of this contact changes between an unconformity and a brittle-ductile fault, in conjunction with the Neoproterozoic age of the Crippleback Intrusive Suite, dictate that the above interpretation of the nature of this margin requires revision. These discrepancies are resolved by the recognition of a Neoproterozoic, largely bimodal volcanic sequence (Sandy Brook Group), which is unconformably overlain by Rogerson Lake rocks to the east, with these combined units in fault contact with the Tally Pond Group to the west. The Sandy Brook Group and Crippleback Intrusive Suite form a terrane of ca. 563 Ma rocks with continental arc-like geochemical signatures. The distinctly negative ɛ Nd,563 values for the felsic rocks (mean −3) of this terrane imply derivation from a crystalline basement older than the largely Pan-African crust that underlies contemporaneous volcanic rocks in West Avalonia, but similar to that inferred for the Gander Zone. Thus this terrane is likely to be the leading edge of the peri-Gondwanan microcontinent Ganderia. The ca. 511 Ma Tally Pond Group arc magmatism represent some of the oldest products of Iapetean subduction in Newfoundland. Although the Tally Pond Group are only observed to be in fault contact with the ca. 50 million year older Sandy Brook Group, the high proportion of felsic volcanic rocks, Sandy Brook/Crippleback age inherited zircons and Pb isotopic data all support the inference that the Tally Pond Group was formed on Sandy Brook/Crippleback continental crust.

Single zircon ages for two Archean banded migmatitic gneisses from central Zimbabwe

We report new single zircon ages for two Archean banded gneisses from the Shangani Batholith in central Zimbabwe. The ages of ~2.77 and ~2.84 Ga do not support previous views that the area west of the current exposure of the Tokwe Segment was part of an early- to mid-Archean (>2.95 Ga) domain (Sebakwe Proto-craton). The data correspond to ~2.9 to 2.8 Ga dates for TTG granitoids of the Chingezi Suite. Members of this suite occur as intrusions within the south-central part of the Zimbabwe Craton and are associated with contemporaneous felsic volcanic rocks. On the basis of their age distribution, isotopic characteristics and outcrop pattern with respect to the Tokwe Segment, the formation of these igneous rocks is consistent with the development of a volcanic arc along the western margin of the Sebakwe proto-craton between ~2.9 to 2.8 Ga.

Geology and Neoarchean tectonic setting of the Central Hearne supracrustal belt, Western Churchill Province, Nunavut, Canada

The Central Hearne supracrustal belt, one of the largest Neoarchean “greenstone” terranes in the Canadian Shield, occurs in the Hearne domain of the Western Churchill Province, one of the largest, poorly known fragments of Archean crust on Earth. The belt contains two isotopically juvenile volcano–plutonic assemblages (I: ∼2710–2690 Ma, and II: ∼2685–2680 Ma), separated in time by localised, ∼2690 Ma, greenschist-facies deformation (D1), and overlain by ∼2680 Ma Archean siliciclastic and chemical sedimentary rocks. Extensive, penetrative, greenschist-facies regional deformation (D2) occurred at ∼2680 Ma, with amphibolite-facies metamorphism localised in the aureoles of isotopically juvenile synkinematic plutons. In many respects, the Central Hearne supracrustal belt is similar to other Neoarchean “greenstone” belts that have been interpreted in terms of arc-subduction systems, e.g. the Abitibi greenstone belt of the Superior Province. However, the principal tectonic characteristics of the Central Hearne supracrustal belt include: (i) location in an anomalously wide (>225 km) swath of penecontemporaneous juvenile crust that extends across much of the Hearne domain; (ii) close, primary intercalation of contemporaneous volcanic rocks of MORB-like and arc-like geochemical signatures, coupled with highly discontinuous volcanic map units; (iii) abundant intermediate to felsic volcanic rocks that do not represent a localised, laterally extensive volcanic arc edifice; and (iv) the development of isolated, independent, felsic volcanic centres throughout the magmatic history of the belt. These features are incompatible with oceanic arc or plateau models. We propose that the early history (assemblage I) of the Central Hearne supracrustal belt may be analogous with a modified extensional, suprasubduction “infant arc” model, such as that described for the earliest (Eocene) phase of construction of the Izu–Marianas–Bonin and Tonga arc-trench systems of the Southwest Pacific Ocean. The later history (assemblage II) may reflect attempted initiation of classical subduction and arc construction.

Crustal growth in southern Arizona: U-Pb geochronologic and Sm-Nd isotopic evidence for addition of the Paleoproterozoic Cochise block to the Mazatzal Province

Terranes composed of arc-related igneous and sedimentary rocks were accreted onto the continental margin of southwestern North America from 1.9 to 1.6 Ga. They formed the Yavapai-Mazatzal province in Arizona, of which the so called ‘Pinal Schist’ and associated rocks in southeastern Arizona form an integral part. 148 U-Pb analyses on magmatic and detrital zircons were performed on 21 samples to constrain timing of geologic events in Paleoproterozoic time in the area of southeastern Arizona. 25 Sm-Nd whole rock analyses were made to infer sources of sedimentary and igneous rocks. Our results allow us to delineate a suture zone between the Pinal tectonostratigraphic unit (Pinal block), which consists primarily of basinal metaturbiditic rocks, and a southeastern allochthonous juvenile volcano-sedimentary domain. The location of the suture is constrained by lithologic contrast across the boundary, an increase in age of detrital zircons, a marked decrease of ε_Nd(t), and an intervening subduction complex (Swift and Force, this issue). We propose the name Cochise block for the southeastern part, because it represents a faultbound, allochthonous tectonic unit. In the Cochise block, we document volcanic activity from about 1630 to 1647 Ma. Quartzitic sedimentary rocks were largely derived from almost contemporaneous volcanic rocks and show detrital zircon ages from 1630 to 1729 Ma, with the majority (85 percent) between 1630 to 1674 Ma. Quartzites and intermediate to felsic metavolcanic rocks show a tight cluster of ε_Nd(t)-values (2.9–3.8) close to the depleted mantle model and therefore represent juvenile additions to the crust. Post-tectonic metabasalts with ε_Nd(1500) = 4.5 are interpreted as being derived from a depleted upper mantle source around 1.5 Ga. In the Pinal block, detrital zircons yield values from 1678 to 1731 Ma. ε_Nd(t)-values of metapelites and wackes range from −0.2 to 2.3. Values decrease from southeast to northwest, indicating progressive input of more evolved material toward the craton. Our data suggest that sedimentation within the Pinal block was proximal to the continental margin and occurred later than 1.68 Ga and ended before the intrusion of granitoids at 1.65 Ga. We show that the North American crust grew rapidly and progressively by addition of juvenile but evolved metavolcanic arc rocks and derived quartz-rich metasedimentary rocks as documented in the Cochise block. Timespans between depleted mantle model age and zircon crystallization on the order of 50 my attest to rapid crustal processing during this significant time of crustal growth.

Barite-hyalophane sulfidic ores at Rozna, Bohemian Massif, Czech Republic; metamorphosed black shale-hosted submarine exhalative mineralization

Strata-bound barite-hyalophane sulfidic orebodies recently discovered at Rozna (in the Moldanubian zone of the Bohemian Massif, Czech Republic) are hosted within the high-grade metamorphosed complex composed of migmatitic biotite or sillimanite-biotite gneiss and amphibolite, with rare intercalations of marble and calcsilicate gneiss. The orebodies and the host rocks have a complex metamorphic history. The last Variscan amphibolite facies metamorphic event is recorded in the quartz-biotite oxygen isotope fractionation in the host gneiss, which indicates temperatures from 600 degrees to 700 degrees C. Metamorphic textures in the orebodies include granoblastic annealing-recrystallization of barite and sulfides, pressure solution seams, and local development of shear zones near the margins of the orebodies. Despite intense metamorphism, the ore is generally strata bound, with sharp host-rock contacts. At a few places, tectonically modified offsets (usually enriched in sulfides) extend up to 2 m across the foliation of the surrounding rocks.Based on ore mineralogy and texture, the orebodies can be divided into three types: (1) massive baritehyalophane sulfidic rock, (2) calc-silicate gneiss with minor barite and sulfides, and (3) disseminated sulfides in marble. Massive barite ore, which is the only type of mineralization of economic interest (with estimated reserves of over 2 million tons [Mt]), contains an average of 62.2 percent barite, 3.2 percent Zn, and 1.8 percent Pb. The relatively simple mineral assemblage of massive ore includes barite, hyalophane, calcite, quartz, pyrite, pyrrhotite, sphalerite, and galena, as well as minor amounts of pyrargyrite, Ag tetrahedrite, fluorite, pyrophanite, and ilmenite. Calc-silicate gneiss with sulfides and barite usually forms the outer parts of the barite orebodies hosted in gneiss. It consists mainly of pyroxene, hyalophane, and quartz, with minor garnet, calcite, forsterite, minerals of the zoisite group, and sulfides. Disseminated Pb-Zn mineralization in marbles occurs only near the contact of marbles with massive barite orebodies.The delta 34 S values of sulfide minerals are heavy in all mineralization types (7.2-14.8ppm), probably indicating their formation in an anoxic, restricted marine environment, where the isotopic composition of sulfate and sulfide gradually shifted to more positive values as a result of the fractionation of S isotopes during bacterial reduction of sulfate to sulfide and the removal of isotopically light S by sulfide sedimentation. The restricted character of the sedimentary environment during barite formation is supported by the elevated content of graphite in the host rocks (0.01-2.0%) and by very heavy delta 34 S values of pyrite and anhydrite in a small (up to 1 m thick) anhydrite lens that occurs in a host-rock complex. The 87 Sr/ 86 Sr ratios in barite ore (0.7077-0.7079) are close to those for anhydrite (0.7080). Because the anhydrite probably represents a meta-evaporite, the isotopic composition of both barite and anhydrite strontium is close to that of contemporaneous seawater. The isotopic composition of lead in galenas from massive barite and calc-silicate ore is very homogeneous ( 206 Pb/ 204 Pb from 17.65 to 17.66, 207 Pb/ 204 Pb from 5.47 to 15.51, and 206 Pb/ 204 Pb from 37.36 to 37.45), and lead isotope ratios prove the pre-Variscan age of the orebodies.It is proposed that the Ron strata-bound mineralization formed by mixing of the hydrothermal solutions carrying Ba, Zn, and Pb with seawater in a restricted oceanic or intra-arc trough filled with clastic sediments derived mostly from contemporaneous volcanic rocks. The restricted nature of the trough resulted in low oxygen contents in the bottom waters and low recycling rates for the organic matter. During the subsequent thermal events, the mineralization and its host rocks were deformed and metamorphosed to the upper amphibolite grade and the carbonate-enriched orebodies were transformed to calc-silicate ore. Following deformation and retrograde metamorphic events at rather superficial tectonic levels caused retrograde shearing at the margins of the orebodies. At a waning stage of the metamorphism, the ore and host rocks were metamorphosed under greenschist facies conditions.

Palaeomagnetic study of block rotations in the Niksar overlap region of the North Anatolian Fault Zone, central Turkey

This palaeomagnetic study investigates crustal deformation within, and adjacent to, the Niksar overlap area of the North Anatolian Fault Zone (NAFZ) in central-east Turkey. The studied rock formations comprise: (1) red limestones of Late Cretaceous age (3 sites); (2) mafic lavas of Eocene age on the north side (13 sites) and south side (9 sites) of the NAFZ; and (3) volcanic rocks of Pliocene-Quaternary age from the Niksar pull-apart basin within the NAFZ (8 sites). Comparisons with reference palaeofield directions computed from apparent polar wander paths of the Eurasian and Afro-Arabian plates identify two scales of regional and local tectonic rotation: 1. (1) A pre-tilting remanence in the Eocene volcanic rocks south of the NAFZ ( D I = 144.1 −47.5° , β 95 = 7.6°) is interpreted to reflect counterclockwise rotation by 30–40° from the reference palaeofields. Contemporaneous volcanic rocks from the north side of the NAFZ have the same reverse polarity recorded in pre-tilting magnetisations. The remanence is also rotated counterclockwise ( D I = 152.4 −42.5° , α 95 = 11.3°), but by about 8° less than the volcanics on the south side of the NAFZ. Hence similar amounts of rotation are observed on both sides of the NAFZ and are interpreted to reflect motions during the pre-Middle Miocene collisional history in this sector of the Pontides. No distributed clockwise rotations anticipated from subsequent dextral motion along the NAFZ intracontinental transform are observed. The slightly larger anticlockwise rotation found on the south side of the NAFZ probably records relative rotation of en-echelon wedges by continental escape during post-Middle Miocene strike slip along the transform. 2. (2) Within the narrow zone of intense deformation along the NAFZ, Cretaceous limestones appear to be rotated clockwise by dextral strike-slip motion whilst Plio-Quaternary lavas within a fault-bounded block in the overlap region associated with the Niksar pull-apart basin, have magnetisations consistently directed 240–270°E. Magnetic inclinations are not diagnostic of polarity but both polarity solutions identify rapid clockwise rotation at rates in excess of 50°/m.y. A normal polarity solution is favoured and implies that a block (ca. 5 km across) has undergone a strike-slip displacement of around 12 km within the NAFZ during the last polarity chron. Cretaceous-Eocene palaeolatitudes are closer to those predicted from Eurasia than Afro-Arabia, but a study of older rocks is required to resolve affinities of this sector of the Anatolian block. Theoretical models of crustal deformation across intracontinental transforms obeying power-law behaviour and treating the lithosphere as a viscous medium predict that distributed clockwise rotations should be observed about as broad across the NAFZ. These rotations are not observed. Instead intense clockwise rotation is confined to a narrow zone close to the major fault break. The concentration of historic seismic activity here also implies that the bulk of the dextral motion between the Eurasian Plate and the Anatolian block is accommodated by slip along faults close and parallel to the main trace of the NAFZ.

Palaeoproterozoic intraplating exposed by resultant gravity overturn near Kiruna, northern Sweden

The tectonic history of the Kiruna area is reexamined on the basis of four newly available digitised geophysical maps, a literature survey and some first-hand field observations.A basement complex of Archaean gneisses was unconformably overlain by a Palaeoproterozoic sequence of metabasalt, andesite and keratophyric porphyries with interstratified tuffs and sediments. The cover was then detached from its basement by an orogeny with a northerly tectonic grain. A pile of ductile nappes separated by mylonites in the east pushed westward an imbricate fold-thrust belt that may have decoupled over an evaporite unit of regional extent. Conglomerates and quartzites accumulated in a foredeep migrating in front of a wedge of emergent thrusts.The allochthonous cover then developed a vertical north-trending foliation in high-T/low-P conditions as it foundered among buoyant diapirs of remobilised granitoid basement. The near-constant lateral spacing of the centres of these diapirs, their mushroom shapes, and the absence of contemporaneous volcanic rocks, suggest that these diapirs rose in the solid state.The region continued to distort episodically even after diapir emplacement. Ductile shear zones evolved through vein systems to faults as the region cooled and was uplifted before the Cambrian. The youngest fault movements clearly displace 9000-years-old glacial deposits.Post- or late-orogenic plutons in the Kiruna area appear to be contemporaneous with syn-Svecofennian plutons further south. We therefore suggest reinstating the label Svecokarelian for the orogeny in the Kiruna area. Heat advected by melts from the mantle is invoked to account for the Archaean basement risking in two convective stages, the first as porphyritic lavas, the second as solid-state diapirs. Melts rose rapidly and erupted before the Svecokarelian orogeny, while the slower diapirs ended it. We attribute the current southwestern dip of the diapir zone to extension of Svecokarelian crust before the Svecofennian magmatic arc was built above it.

Palaeogeographic implications of the heavy mineral distribution in Miocene sandstones of the North Sumatra Basin

Heavy minerals record major changes in the provenance of Miocene sandstones in the North Sumatra Basin, related to important tectonic events in the basin hinterland. Early Miocene sandstones fo the Belumai (Peutu Formation) were derived from the east or southeast, from a predominantly granitic terrain in the Asahan Arch or Malaysian Peninsula area. The uplift of the Barisan Mountain range in the early Middle Miocene led to the introduction of sand from the west or southwest, forming th sandstones of the Keutapan Formation. The source terrain can be reconstructed as consisting of regionally metamophosed pelitic rocks heavily intruded by granites. Contemporaneous volcanic rocks of intermediate to acidic character were also involved, and the presence of diaspore shows that tropical weathering conditions, generating lateritic soils, were prevalent. Chrome spinel is abundant in the Lower Keutapang Member but is rare in the Upper Keutapang Member. This indicates that ultramafic rocks were an important component of the Barisan Mountain source in the Middle Miocene, but were relatively insignificant by Late Miocene times. These ultramafic rocks were either removed through erosion or by northwestward-directed strike-slip motion. In the latter case, the outcrops of the Woyla Group in northern Aceh may represent the remnants of the ultramafic source that supplied sediment to the North Sumatra Basin during the late Middle Miocene, This study demonstrates the importance of obtaining provenance information from basin-fill sequences in constraining tectonic events.

Geology of the Bayah area: implications for the Cenozoic evolution of the West Java, Indonesia

The geological setting of the Bayah area in the Paleogene is indicated by the presence of alternating sedimentary and volcanic rocks. The sedimentary rocks consist of fine to coarse clastic sediments, which suggest regressive deposition in a shallow basin. In the Neogene time, sediments with open marine characteristics and contemporaneous volcanic rocks were deposited in the transition area at the southern border of the basin. The clasts of Neogene sedimentary rocks were derived from a southern source area. The boundary between the Paleogene and the Neogene sequences is characterized by an unconformity, indicated in the field by an erosion surface. Three further tectonic phases can be recognized in this area. They occurred in the Late Paleogene, late Middle Miocene and latest Miocene age. The Tertiary volcanic rocks can be divided into a lower sequence of Late Eocene to Early Oligocene age (LOA) and upper sequence of Oligocene to Miocene age (UOA). All the rocks, except for one sample from the LOA, are rocks typical of a calc-alkaline island arc. The LOA consists of basalts and basaltic andesites with characteristics that suggest a more primitive magma than those the UOA. The LOA rocks probably correspond to fractional crystallization and contamination with a component of low-Th concentration of the intermediate lower crust. The LOA was generated in the volcanic front and appears to be related to a period of slower subduction. The UOA, which ranges from basalt to rhyolite composition, correspond to an increase of differentiation. They experienced contamination of the parental magma terrigenous sediments and occur in high crustals levels.

Geochemical, oxygen, and neodymium isotope compositions of metasediments from the Abitibi greenstone belt and Pontiac Subprovince, Canada: Evidence for ancient crust and Archean terrane juxtaposition

The Abitibi greenstone belt (AGB) and Pontiac Subprovince (PS) in the southwestern Superior Province are adjacent greenstone-plutonic and metasedimentary-dominated terranes, respectively, separated by a major fault zone. Metasediments from these two contrasting terranes are compared in terms of major- and trace-element and O- and Nd-isotope compositions, and detrital zircon ages. The following two compositional populations of metasediments are present in the low-grade Abitibi southern volcanic zone: 1. (1) a mafic-element-enriched population (MEP) characterized by flat, depleted REE patterns; enhanced Mg, Cr, Co, Ni, and Sc; low-incompatible-element contents; and minor or absent normalized negative troughs at Nb, Ta, and Ti; and 2. (2) a low-mafic-element population (LMEP) featuring LREE-enriched patterns; enhanced Rb, Cs, Ba, Th, and U contents; and pronounced normalized negative troughs at Nb, Ta, and Ti. These geochemical features are interpreted to indicate that the MEP sediments were derived from an ultramafic- and mafic-dominated oceanic provenance, whereas the LMEP sediments represent mixtures of mafic and felsic arc source rocks. The PS metasediments are essentially indistinguishable from Abitibi LMEP on the basis of majorelement and transition metal abundances, suggesting comparable types of source rocks and degrees of maturity, but are distinct in terms of some trace elements and O-isotope compositions. The Pontiac metasediments are depleted in 18O and enriched in Cs, Ba, Pb, Th, U, Nb, Ta, Hf, Zr, and total REE and also have higher ratios of Rb K , Cs Rb , Ba Rb , Ta Nb , Th La , and Ba La relative to the Abitibi LMEP. Two subtypes of REE patterns have been identified in PS metasediments. Both have moderately fractionated distributions, with La Yb n = 3.9–17 and 7.8–13.6, respectively; but the first has minor Eu anomalies ( Eu Eu ∗ = 0.89–1.16 ), whereas the second features pronounced negative anomalies ( Eu Eu ∗ = 0.52–0.81 ) coupled with high concentrations of incompatible elements such as K, Rb, Th, U, Nb, and Ta. The first subtype is interpreted to be derived from provenances of mixed mafic and felsic volcanic rocks, whereas the Eu-depleted type has geochemical features that are typical of post-Archean sediments or Archean K-rich granites and volcanic equivalents. Abitibi metasediments have ϵ Nd (2.7 Ga) values of −0.71 to +1.83, corresponding to model T CHUR = 2.82−2.55 Ga, with the majority around 2.65 Ga, close to the depositional age of ~2.7 Ga. In contrast, PS metasediments are characterized by ϵ Nd (2.7 Ga) = +0.95 to −3.51, corresponding to T dhur = 2.7 to ~3.1 Ga, consistent with a detrital zircon age spectrum of ~2.69 Ga to ~3.1 Ga. Pontiac Subprovince and Abitibi metasediments have contrasting provenances, suggesting their separate, initial geodynamic settings and later tectonic juxtaposition. Major and trace elements, O- and Nd-isotopic compositions, and detrital zircon ages collectively suggest that some of the PS metasediments were derived mainly from juvenile rocks; whereas others require various degrees of contribution from older (3.0–3.5 Ga) evolved crust mixed with juvenile (~2.7 Ga) rocks. Older components were derived either directly from erosion of an ancient continental arc or from contemporaneous volcanic rocks which may contain recycled older components. If the PS represents in part an accretionary prism to the Abitibi southern volcanic zone, then oblique subduction may have been involved, which allowed the Pontiac Subprovince to juxtapose or sample exotic source rocks.

Spatial variations of Sr and Nd isotope ratios of Cretaceous-Paleogene granitoid rocks, Southwest Japan Arc

Cretaceous-Paleogene granitoid rocks and contemporaneous volcanic rocks are widely distributed in the Inner Zone of Southwest Japan. This intense intermediate to felsic magmatism is considered to have taken place on the eastern margin of the Eurasian Continent, before the Southwest Japan Arc drifted away from the continent in the middle Miocene, resulting in the opening of the Japan Sea. The granitoid rocks show regional variations in terms of their radiometric age, petrography, Sr, Nd and O isotope ratios. Based on Sr and Nd isotope ratios, granitoid rocks can be divided into three zones (South, Transitional and North) between the Median Tectonic Line and the Japan Sea. Granitoid rocks and associated gabbros of the North Zone have low initial Sr isotope ratios (0.7048 to 0.7068) and high initial eNd values (+3 to-2.2), whereas granitoid rocks and gabbros from the South Zone have high initial Sr isotope ratios (0.7070 to 0.7088) and low initial eNd values (-3.0to-8.0). Most granitoid rocks from the Transitional Zone have Sr and Nd isotope ratios that lie between those of the North and South Zones, although there is some overlap. Contamination of magmas by upper crust cannot explain this geographical variation in Sr and Nd isotopes. Instead, the regional variation is attributed to compositionally different, magma sources (probably upper mantle and lower crust), beneath the North and South Zones. This is supported by the Sr and Nd isotopic ratios of upper mantle and lower crustal xenoliths included in Cenozoic volcanic rocks in the North and South Zones. These ratios are similar to those of the granitoid rocks in the respective zones. It is suggested that a micro-continent or island arc consisting of continental crust was underthrust beneath the South Zone before or during the Cretaceous, resulting in compositionally distinct sources for granitoid rocks of the North and South Zones. The large variation observed in Sr and Nd isotope ratios for Transitional Zone granitoid rocks is explained by variable proportions of the two different crustal and upper mantle components.

Crustal outgassing and LILE enrichment in major lithosphere structures, Archean Abitibi greenstone belt: evidence on the source reservoir from strontium and carbon isotope tracers

Major structural discontinuities in the Abitibi greenstone belt acted as conduits for outgassing of the Archean crust, as reflected in fixation of a select group of lithophile elements including Si, C, K, Rb, Ba, Li, Cs, B and Pb, in metasomatized faults. For two of the largest structures, the Destor-Porcupine (DP) and Kirkland Lake — Cadillac (KC) fault zones ∼6×1015 g Si, 3×1015 g CO2 and 1015 g K were introduced into the faults during expulsion of an estimated 6×1018 g aqueous fluids. Strontium isotope ratios of tourmaline, piemontite, actinolite and scheelite mineral separates, characterized by Rb/Sr≤0.02, are concordant with respect to 87Sr/86Sr initial ratios over local sectors of the faults. The Sr isotope data record geographic variations which, from east to west on the KC fault is 0.7031–0.7041 (Val d'Or), 0.7008–0.7022 (Bourlemaque), 0.7017–0.7019 (Bousquet), 0.7029–0.7031 (Noranda), and 0.7013 to 0.7015 (Kirkland Lake). At Timmins, on the PD fault, 87Sr/86Sr initial ratios cluster at 0.7010 to 0.7020. Metasomatised fault zones are systematically more radiogenic than contiguous host lithologies, and imply a source reservoir (0.7010 to 0.7041) generally more radiogenic than the upper mantle at 2690 Ma (0.700±0.001), or contemporaneous volcanic rocks of mafic to ultramafic composition (0.700 to 0.7012). Whereas certain minerals are concordant and retentive, Rb-Sr isochrons based on suites of rocks at progressive intensities of metasomatism, have been systematically reset over an elpased time of ∼200 Ma after termination of outgassing, due to disturbance accompanying incremental displacements on structures.