Coupling thermodynamic modeling and high-resolution in situ LA-ICP-MS monazite geochronology: evidence for Barrovian metamorphism late in the Grenvillian history of southeastern Ontario

The Flinton Group is a greenschist to upper amphibolite facies package of metasediments in southeastern Ontario that was metamorphosed during the Ottawan Orogeny. Thermodynamic modeling of metapelitic mineral assemblages suggests an increase in peak conditions of metamorphism across the 40 km wide study area from 3.5 to 7.9 kbar and 540 to 715 °C. Garnet isopleth thermobarometry applied to the cores of compositionally zoned porphyroblasts reveals remarkably similar P-T conditions of initial crystallization at approximately 3.7–4.0 kbar and 512–520 °C, corresponding to a relatively high geothermal gradient of ca. 34–45 °C km^?1. It is inferred from modeling and reaction textures that metamorphism was along Barrovian P-T paths. Major and trace element zoning in garnet from one sample records a complex growth history as evidenced by major and trace element zoning and the distribution of xenotime, allanite and monazite inclusions. High-resolution (6 μm) LA-ICP-MS U-Pb geochronology performed on monazite in the rock matrix and included in the outer 150 μm of garnet rim-ward of a Y annulus revealed an age of 976?±?4 Ma. The age is interpreted to reflect monazite growth at the expense of allanite and apatite late in garnet’s growth history over the P-T interval 4.5–6.8 kbar and 540–640 °C. This new age estimate for near peak metamorphism fits well into the regional framework but is significantly younger than previously reported ages for Ottawan metamorphism. Based on microstructures this new age suggests that compressional tectonics were operating much later in the history of the Grenville of southeastern Ontario than previously thought.     

Detrital zircon provenance of upper Cambrian-Permian strata and tectonic evolution of the Ellsworth Mountains,West Antarctica

Detrital zircons from the upper Cambrian-Devonian sandstones (Crashsite Group; n = 485) and Carboniferous tillite (Whiteout Conglomerate; n = 81) of the Ellsworth Mountains, Antarctica record a steady supply of Neoproterozoic (“Pan-African”) orogeny (~ 550–600 Ma), Grenville (~ 1000 Ma) and Neoarchean (~ 3000–3500 Ma) zircons into the northern marginal basin of Gondwana. The overlying Permian Glossopteris-bearing Polarstar Formation shales (n = 85) have the same zircon provenance as underlying units but also include a dominance of depositional-age (263 Ma) euhedral zircons which are interpreted to be of local, volcanic arc origin. Modeling of detrital zircon provenance suggests that source areas were present in Pan-African and Laurentian crust throughout the Paleozoic. We also report calcite twinning strain results (12 strain analyses; n = 398 twins) for the Cambrian Minaret Fm. in the Heritage range which is predominantly a layer-parallel shortening strain in the direction (WSW-ENE) of Permian Gondwanide orogen thrust transport. There is a secondary, sub-vertical twinning strain overprint. The initiation of localized lower-middle Cambrian rifting (Heritage Group deposition) in Grenville-aged crust as Gondwana amalgamated and the subsequent Jurassic counterclockwise rotation of the Ellsworth-Whitmore terrane out of the Permian Gondwanide belt into central Antarctica each remain tectonic curiosities.     

Conditions for veining and origin of mineralizing fluids in the Alpi Apuane (NW Tuscany,Italy): Evidence from structural and geochemical analyses on calcite veins hosted in Carrara marbles

This work deals with structural and geochemical (chemical and isotopic) analyses of calcite veins hosted in Carrara marbles in the Alpi Apuane, NW Tuscany, Italy. Geometric features and spatial distribution of veins provided estimations of stress ratio (Φ = (σ2 − σ3)/(σ1 − σ3)), driving stress ratio (R′ = (Pf − σ3)/(σ1 − σ3)) and fluid overpressure (Δs_i = Pf − σ3) at the time of vein formation. The obtained values of Φ = 32 and R′ = 0.43 reveal that fluid pressure was higher than the intermediate principal stress at the time of veins formation, whereas the estimated Δs_i ranging from 129 to 207 MPa indicates that veins formed under supra-hydrostatic to lithostatic pressure conditions. Carbon (δ¹³C_V-PDB = 1.81–2.10‰ for veins and 1.95–2.51‰ for host marbles), oxygen (δ¹⁸O_V-SMOW = 28.71–29.57‰ for veins and 28.90–29.36‰ for host marbles) and strontium (⁸⁷Sr/⁸⁶Sr = 0.707716–0.707985 for veins and 0.0707708–0.707900 for host marbles) isotope compositions in vein/host marble pairs were internally quite consistent. Combining our structural and geochemical data, a modeling approach was performed to investigate the compositional features and temperatures of calcite depositing fluids. The results of our studies give evidence that (1) pore-fluids in Carrara marble, consisting of metamorphic formation waters, were re-mobilized during veining event and migrated within the veins in closed system conditions, (2) veins formed after ductile folding phases and before high-angle brittle faulting events, at temperature and pressure around 250 °C and 210 MPa, and finally (3) about 12 g H₂O/m³ marble are calculated to have been available as vein parental fluid at the time of vein formation.     

13C12C exchange between calcite and graphite: A possible thermometer in Grenville marbles

The fractionation of ¹³C between calcite and graphite, Δ(Cc-Gr). is consistently small (2.6–4.8 permil) in 34 assemblages from upper amphibolite- and granulite-facies marbles of the Grenville Province. In 25 samples from the Adirondack Mountains, New York, it decreases regularly with increasing metamorphic temperature. The fractionations are independent of absolute δ¹³C values of calcite (?2.9 to +5.0). For T = 600–800°C, the Adirondack data are described by Δ(Cc-Gr) = ?0.00748T (°C) + 8.68. This good correlation between Δ and T suggests that carbon isotope equilibrium was attained in these high-grade marbles and that the theoretical calculations of this fractionation by Bottinga are approximately 2 permil too large in this temperature range. Because of the relatively high temperature sensitivity suggested by these results and by Bottinga's calculations, and the pressure independence of isotope fractionation, Δ(Cc-Gr) may provide a very good thermometer for high-grade marbles.Comparison of this field calibration for Δ(Cc-Gr) vs temperature with results from other terranes supports the utility of Δ(Cc-Gr) for geothermometry and suggests that graphite is much more sluggish to exchange than calcite, that exchange between calcite and graphite occurs at temperatures as low as 300°C, and that equilibrium may normally be attained only when peak metamorphic temperatures are greater than 500–600°C.Because ¹³C exchange is an unavoidable metamorphic process at temperatures above 300°C, high values of δ¹³C(Gr) in moderate- to high-grade carbonate-bearing rocks do not provide a sufficient criterion to infer an abiogenic origin for the graphite.     

The Grenville orogenic cycle of southern Laurentia: Unraveling sutures,rifts, and shear zones as potential piercing points for Amazonia

The magnetic anomaly map of North America serves as a useful base from which to attempt palinspastic reconstruction of terranes accreted during the Elzevirian orogeny (1250–1200 Ma); the Shawinigan (1200–1150 Ma), Ottawan (1080–1020 Ma), and Rigolet (1020–1000 Ma) phases of the Grenvillian orogeny; and post-Grenvillian magmatism (760–600 Ma) and deformation prior to Iapetan rifting at 565 Ma. Accreted terranes had unique histories prior to amalgamation and share common tectonic events afterwards. Comparisons with magnetic signatures of the Paleozoic craton–craton suture, sutures of accreted terranes, and the Jurassic rifted-margin for the southern-central Appalachians provide a basis for discriminating among alternative Grenvillian sutures beneath the Appalachian orogen.The Elzevirian suture is partially preserved beneath the Appalachians where it separates the Reading Prong terrane from Laurentia (i.e., Adirondacks and composite-arc terrane and Canadian Grenville Province). The Shawinigan suture is partially preserved in the Llano area (Texas), but separated the now-fragmented and allochthonous Amazonian (as indicated from Pb-isotope data) blocks of the outboard Blue Ridge terrane from the Reading Prong terrane in the Appalachians. Isolated blocks of the Sauratown Mountains terrane are interpreted as outboard of the Blue Ridge terrane, but were also accreted during the Shawinigan phase. Within present-day Laurentia, the only fragment of a terrane believed to have been accreted during the main Ottawan phase is the Mars Hill terrane (North Carolina–Tennessee). This suggests that the outboard Ottawan suture may have served as the locus of Iapetan rifting along much of Laurentia. The Rigolet phase (1020–1000 Ma) is characterized by widespread “Basin and Range” type extension (NW–SE) associated with sinistral or dextral movement on the NY-AL lineament, mobilization of core-complexes (Adirondack Highlands), and AMCG magmatism along the outboard flank of the extensional region. Following the Rigolet phase, the Appalachian region continued to be characterized by NW–SE extension during the passage of a possible hotspot along a NE-track (760–600 Ma) across the Blue Ridge and other terranes, and during initial Iapetan rifting (565 Ma). The palinspastic rifted-margin of Laurentia crosses many of these terranes and sutures as well as the possible region of Rigolet extension and the possible hotspot track, thus providing many potential piercing points within the Grenville orogen for comparison with Paleozoic terranes like the Precordillera in South America.     

Zircon U–Pb ages and O–Nd isotopic composition of basement rocks in the North Qinling Terrain, central China: evidence for provenance and evolution

The Qinling Group was previously interpreted as the oldest Precambrian basement unit of the North Qinling Terrain, recording its formation and early crustal evolution. The Qinling Group consists predominantly of gneisses, amphibolites, and marbles, which underwent multi-phase deformation and metamorphism. In order to better constrain the provenance and tectonic setting of this group and the evolution of the North Qinling orogenic belt, in situ U–Pb dating and oxygen isotopic analysis of zircons in combination with whole-rock geochemistry and Sr–Nd isotope analysis was performed on the two dominant rock types, amphibolite and felsic gneiss. Felsic gneisses exhibit enrichment of LREEs and LILEs (Rb, Ba, Th, K, Pb), negative Eu anomalies and depletion of HFSEs (Nb, Ta, P, Ti). The rocks have slightly elevated δ¹⁸O values (6.5–9.3 ‰) and initial ε _Nd values of ?4.6 corresponding to two-stage Nd model age of 1.99 Ga. Amphibolites are also enriched in LILEs and LREEs and depleted in Nb and Ta and have homogeneous δ¹⁸O values (5.0–6.0 ‰), but higher initial ε _Nd values (2.8–3.3) and younger two-stage Nd model ages (1.29–1.24 Ga) compared to the gneisses. The zircon age record indicates that the gneisses and amphibolites were formed in a ~960 Ma volcanic arc environment rather than in a rift setting as previously suggested. A major metamorphic event took place during the Early Paleozoic. Based on the age spectrum of detrital zircons, the Qinling Group is interpreted as an autonomous geological unit, which was mainly derived mostly from 1,000 to 900 Ma old granitoid rocks. The North Qinling Terrain can be regarded as a remnant of the Grenville orogenic belt with an early Neoproterozoic evolution different from that of the North and South China blocks.     

Tectonic evolution of Cretaceous extensional basins in Zhejiang Province,eastern South China: structural and geochronological constraints

Widespread Cretaceous volcanic basins are common in eastern South China and are crucial to understanding how the Circum-Pacific and Tethyan plate boundaries evolved and interacted with one another in controlling the tectonic evolution of South China. Lithostratigraphic units in these basins are grouped, in ascending order, into the Early Cretaceous volcanic suite (K_1V), the Yongkang Group (K_1-2), and the Jinqu Group (K₂). SHRIMP U-Pb zircon geochronological results indicate that (1) the Early Cretaceous volcanic suite (K_1V) erupted at 136–129 Ma, (2) the Yongkang Group (K_1-2) was deposited from 129 Ma to 91 Ma, and (3) the deposition of the Jinqu Group (K₂) post-dated 91 Ma. Structural analyses of fault-slip data from these rock units delineate a four-stage tectonic evolution of the basins during Cretaceous to Palaeogene time. The first stage (Early to middle Cretaceous time, 136–91 Ma) was dominated by NW–SE extension, as manifested by voluminous volcanism, initial opening of NE-trending basins, and deposition of the Yongkang Group. This extension was followed during Late Cretaceous time by NW–SE compression that inverted previous rift basins. During the third stage in Late Cretaceous time, possibly since 78.5 Ma, the tectonic stress changed to N–S extension, which led to basin opening and deposition of the Jinqu Group along E-trending faults. This extension probably lasted until early Palaeogene time and was terminated by the latest NE–SW compressional deformation that caused basin inversion again. Geodynamically, the NW–SE-oriented stress fields were associated with plate kinematics along the Circum-Pacific plate boundary, and the extension–compression alternation is interpreted as resulting from variations of the subducted slab dynamics. A drastic change in the tectonic stress field from NW–SE to N–S implies that the Pacific subduction-dominated back-arc extension and shortening were completed in the Late Cretaceous, and simultaneously, that Neo-Tethyan subduction became dominant and exerted a new force on South China. The ongoing Neo-Tethyan subduction might provide plausible geodynamic interpretations for the Late Cretaceous N–S extension-dominated basin rifting, and the subsequent Cenozoic India–Asia collision might explain the early Palaeogene NE–SW compression-dominated basin inversion.     

Remarkably uniform oxygen isotope systematics for co-existing pairs of gem-spinel and calcite in marble,with special reference to Vietnamese deposits

Oxygen isotope systematics for co-existing pairs of gem-spinel and calcite in marble from Vietnam and other worldwide deposits have been determined in order to characterize the O-isotope fractionation between calcite and spinel. In Vietnam, the Δ¹⁸O_cc–sp (= 3.7 ± 0.1‰ for six samples from the An Phu and Cong Troi deposits) is remarkably constant. The combination of these data with those obtained on calcite–spinel pairs of Paigutan (Nepal, n = 2), Ipanko (Tanzania, n = 1), and Mogok (Myanmar, = 2) are also consistent with an overall Δ¹⁸O_cc–sp of 3.6 ± 0.3‰ for all the spinel samples (n = 11). The straight line correlation δ¹⁸O_cc = 0.96 δ¹⁸O_sp + 4.4 is excellent despite their worldwide geographic spread. The increment method of calculating oxygen isotope fractionation gave a geologically unreasonable temperature of formation for both minerals at 1374 °C when compared to temperatures obtained by mineral assemblage equilibrium of these marble type deposits, between 610 and 750 °C. The constant Δ¹⁸O_cc–sp reflects a constant temperature for this amphibolite facies assemblage, whose current best estimate is calculated at 620 ± 40 °C, but unquantified uncertainties remain.     

Intragrain oxygen isotope zoning in titanite by SIMS: Cooling rates and fluid infiltration along the Carthage‐Colton Mylonite Zone,Adirondack Mountains,NY, USA

Oxygen isotopes are an attractive target for zoning studies because of the ubiquity of oxygen‐bearing minerals and the dependence of mineral ¹⁸O/¹⁶O ratios on temperature and fluid composition. In this study, subtle intragrain oxygen isotope zoning in titanite is resolved at the 10‐μm scale by secondary ion mass spectrometry. The patterns of δ¹⁸O zoning differ depending on microstructural context of individual grains and reflect multiple processes, including diffusive oxygen exchange, partial recrystallization, grain‐size reduction, and grain growth. Using the chronological framework provided by structural relations, these processes can be related to specific events during the Grenville orogeny. Titanite was sampled from two outcrops within the Carthage‐Colton Mylonite Zone (CCMZ), a long‐lived shear zone that ultimately accommodated exhumation of the Adirondack Highlands from beneath the Adirondack Lowlands during the Ottawan phase (1090–1020 Ma) of the Grenville orogeny. Titanite is hosted in the Diana metasyenite complex, which preserves three sequentially developed fabrics: an early NW‐dipping protomylonitic fabric (S₁) is crosscut by near‐vertical ultramylonitic shear zones (S₂), which are locally reoriented by a NNW‐dipping mylonitic fabric (S₃). Texturally early titanite (pre‐S₂) shows diffusion‐dominated δ¹⁸O zoning that records cooling from peak Ottawan, granulite‐facies conditions. Numerical diffusion models in the program Fast Grain Boundary yield good fits to observed δ¹⁸O profiles for cooling rates of 50 ± 20 °C Ma^?1, which are considerably faster than the 1–5 °C Ma^?1 cooling rates previously inferred for the Adirondack Highlands from regional thermochronology. High cooling rates are consistent with an episode of rapid shearing and exhumation along the CCMZ during gravitational collapse of the Ottawan orogen at c. 1050 Ma. Texturally later titanite (syn‐S₂) has higher overall δ¹⁸O and shows a transition from diffusion‐dominated to recrystallization‐dominated δ¹⁸O zoning, indicating infiltration of elevated‐δ¹⁸O fluids along S₂ shear zones and continued shearing below the blocking temperature for oxygen (~≤500 °C for grain sizes at the study site). The texturally latest titanite (post‐S₃) has growth‐dominated δ¹⁸O zoning, consistent with porphyroblastic grain growth following cessation of shearing along the Harrisville segment of the CCMZ.     

Graphite content and isotopic fractionation between calcite-graphite pairs in metasediments from the Mgama Hills,Southern Kenya

Amphibolite-grade metasediments from the Mgama Hills region, Kenya, contain conspicuous quantities of graphite, most probably derived from organic progenitor materials. The highest graphite contents (5.1–20.4%) are found in schists whereas calcite marbles intercalated in the sequence contain relatively low amounts (0.1–2.0%). The graphitic constituents are consistently enriched in ¹³C relative to common sedimentary organic material, with the highest isotopic ratios in graphite from the marbles (δ¹³C = ?7.3 ± 5.0%.; n = 10). Carbon isotope fractionations between calcite and graphite mostly vary between 3.3 and 7.1‰, which comes close to both empirically recorded and thermodynamically calculated fractionations in the temperature range of the upper amphibolite faciès (550–650°C). However, larger values occasionally encountered in the marbles suggest that complete isotopic equilibrium is not always attained in amphibolite-facies metamorphism.     

Humite- and scapolite-bearing assemblages in marbles and calcsilicates of Dronning Maud Land, Antarctica: new data for Gondwana reconstructions

This study investigates marbles and calcsilicates in Central Dronning Maud Land (CDML), East Antarctica. The paleogeographic positioning of CDML as part of Gondwana is still unclear; however, rock types, mineral assemblages, textures and P–T conditions observed in this study are remarkably similar to the Kerala Khondalite Belt in India. The CDML marbles and calcsilicates experienced a Pan-African granulite facies metamorphism at c. 570 Ma and an amphibolite facies retrogression at c. 520 Ma. The highest grade assemblage in marbles is forsterite+spinel+calcite+dolomite, in calcsilicates the assemblages are diopside+spinel, diopside+garnet, scapolite+wollastonite+clinopyroxene±quartz, scapolite±anorthite±calcite+clinopyroxene+wollastonite. These assemblages constrain the peak metamorphic conditions to 830±20 °C, 6.8±0.5 kbar and X _CO2>0.46. During retrogression, highly fluoric humite-group minerals (humite, clinohumite, chondrodite) replaced forsterite, and garnet rims formed at the expense of scapolite during reactions with wollastonite, calcite or clinopyroxene but without involvement of anorthite. Metamorphic conditions were about 650 °C, 4.5±0.7 kbar, 0.2< X _CO2^fluid<0.36, and the co-existence of garnet, clinopyroxene, wollastonite and quartz constrains f_O2 to FMQ-1.5 log units. Mineral textures indicate a very limited influx of H₂O-rich fluid during amphibolite facies retrogression and point to significant variations of fluid composition in mm-sized areas of the rock. Gypsum was observed in two samples; it probably replaced metamorphic anhydrite which appears to have formed under amphibolite facies conditions. The observed extensive anorogenic magmatism (anorthosites, A-type granitoids) and the character of metamorphism between 610 and 510 Ma suggest that the crustal thermal structure was characterized by a long-lived (50–100 Ma) rise of the crustal geotherm probably caused by magmatic underplating.     

1.1 Ga K-rich alkaline plutonism in the SW Grenville Province

U–Pb zircon and baddeleyite dating of six syenitic stocks establishes that the ultrapotassic, potassic alkaline and shoshonitic magmatism with island-arc affinities in the Central Metasedimentary Belt (CMB) of the southwestern Grenville Province, Canada took place between 1089 and 1076 Ma, along a 400-km-long, northeast-trending plutonic belt. These ages indicate that ultrapotassic rocks with arc affinities are not unique to the Phanerozoic. West to east emplacement ages along a northern and southern cross-section of this belt range from 1083±2 Ma (Kensington), through 1081±2 Ma (Lac Rouge) to 1076 _–1 ⁺³ Ma (Loranger) in the north, and from 1089 _–3 ⁺⁴ Ma (loon Lake) and 1088±2 Ma (Calabogie), to 1076±2 Ma (Westport) in the south. Although closely spaced in time, in detail these ages suggest a slight younging of this magmatic activity to the southeast. Integration of the geochronological data with the spatial extent and potassic character of the plutons shows that the K-rich alkaline suite is distinct from the nepheline-syenite belt of the Bancroft terrane and from the syenite-monzonite suite of the Frontenac terrane of the CMB, and it is considered to be a magmatic episode unique to the Elzevir terrane and its Gatineau segment. The timing and the postmetamorphic emplacement of these plutons indicate that the regional greenschist to granulite-facies metamorphism of the country rock (precise age unknown) is older than 1089 Ma throughout the entire Elzevir terrane. The potassic magmatism is interpreted as the initiation of the 1090–1050 Ma Ottawan Orogeny in the Elzevir terrane; thus, the regional metamorphism in this terrane, previously assigned to the Ottawan Orogeny, is an earlier event. The contemporaneous emplacement of this postmetamorphic plutonic belt with Keweenawan volcanism is at variance with current tectonic models which consider the Keweenawan rift to be formed at the same time as regional metamorphism in the CMB.     

Petrogenesis of early Silurian intrusions in the Sanchakou area of Eastern Tianshan,Northwest China,and tectonic implications: geochronological,geochemical, and Hf isotopic evidence

ABSTRACT

Palaeozoic intrusions in Eastern Tianshan are important for understanding the evolution of the Central Asian Orogenic Belt (CAOB). The Sanchakou intrusions situated in Eastern Tianshan (southern CAOB), are mainly quartz diorite and granodiorite. A comprehensive study of zircon U–Pb ages, zircon trace elements, whole-rock geochemistry, and Lu–Hf isotopes were carried out for the Sanchakou intrusive rocks. LA-ICP-MS zircon U–Pb dating yielded crystallization ages of 439.7 ± 2.5 Ma (MSWD = 0.63, n = 21) for the quartz diorite, and 430.9 ± 2.5 Ma (MSWD = 0.21, n = 21) and 425.5 ± 2.7 Ma (MSWD = 0.04; n = 20) for the granodiorites. These data, in combination with other Silurian ages reported for the intrusive suites from Eastern Tianshan, indicate an early Palaeozoic magmatic event in the orogen. In situ zircon Hf isotope data for the Sanchakou quartz diorite shows ε_Hf(t) values of +11.2 to +19.6, and the two granodioritic samples exhibit similar ε_Hf(t) values from +13.0 to +19.5. The Sanchakou plutons show metaluminous to weakly peraluminous, arc-type geochemical and low-K tholeiite affinities, and display trace element patterns characterized by enrichment in K, Ba, Sr, and Sm, and depletion in Nb, Ta, Pb, and Ti. The geochemical and isotopic signatures indicate that the Sanchakou dioritic and granodioritic magmas were sourced from a subducted oceanic slab, and subsequently underwent some interaction with peridotite in the mantle wedge. Combined with the regional geological history, we suggest the Sanchakou intrusions formed due to the northward subduction of the Palaeo-Tianshan Ocean beneath the Dananhu–Tousuquan arc during early Silurian time.     

The Ediacaran–Early Cambrian detrital zircon record of NW Iberia: possible sources and paleogeographic constraints

Ediacaran and Early Cambrian sedimentary rocks from NW Iberia have been investigated for detrital zircon U–Pb ages. A total of 1,161 concordant U–Pb ages were obtained in zircons separated from four Ediacaran samples (3 from the Cantabrian Zone and one from the Central Iberian zone) and two Lower Cambrian samples (one from the Cantabrian Zone and one from the Central Iberian Zone). Major and trace elements including REE and Sm–Nd isotopes were also analyzed on the same set of samples. The stratigraphically older Ediacaran sequence in the Cantabrian Zone has a maximum sedimentation age of ca. 600 Ma based on detrital zircon content and is intruded by ca. 590–580 Ma granitoids constraining the deposition of this part of the sequence between ca. 600 and 580 Ma. The stratigraphically younger Ediacaran sequence in the Cantabrian Zone has a maximum sedimentation age of ca. 553 Ma. The Ediacaran sample from the Central Iberian Zone has an identical within error maximum sedimentation age of ca. 555 Ma. The detrital zircon U–Pb age patterns are very similar in all the Ediacaran samples from both zones including the main age groups ca. 0.55–0.75 Ga, ca. 0.85–1.15 Ga and minor Paleoproterozoic (ca. 1.9–2.1 Ga) and Archean (ca. 2.4–2.6 Ga) populations. Kolmogorov–Smirnov statistical tests performed on this set of samples indicate that they all were derived from the same parent population (i.e., same source area). The same can be said on the basis of Nd isotopes, REE patterns and trace element concentrations. The two Cambrian samples, however, show contrasting signatures: The sample from the Cantabrian Zone lacks the ca. 0.85–1.15 Ga population and has a high proportion of Paleoproterozoic and Archean zircons (>60 %) and a more negative ε _Nd and higher T _DM values than the Ediacaran samples. The Early Cambrian sample from the Central Iberian Zone has the same U–Pb detrital zircon age distribution (based on KS tests) as all the Ediacaran samples but has a significantly more negative ε _Nd value. These data suggest apparently continuous sedimentation in the NW Iberian realm of northern Gondwana between ca. 600 and 550 Ma and changes in the detrital influx around the Ediacaran–Cambrian boundary. The nature and origin of these changes cannot be determined with available data, but they must involve tectonic activity on the margin as evidenced by the angular unconformity separating the Ediacaran and Lower Cambrian strata in the Cantabrian Zone. The absence of this unconformity and the apparent continuity of detrital zircon age distribution between Ediacaran and Cambrian rocks in the Central Iberian Zone suggest that the margin became segmented with significant transport and sedimentation flux changes in relatively short distances. As to the paleoposition of NW Iberia in Ediacaran–Early Cambrian times, comparison of the data presented herein with a wealth of relevant data from the literature both on the European peri-Gondwanan terranes and on the terranes of northern Africa suggests that NW Iberia may have lain closer to the present-day Egypt–Israel–Jordan area and that the potential source of the hitherto enigmatic Tonian–Stenian zircons could be traced to exposed segments of arc terranes such as that described in the Sinai Peninsula (Be’eri-Shlevin et al. in Geology 40:403–406, 2012).     

Deformation correlations,stress field switches and evolution of an orogenic intersection: The Pan-African Kaoko-Damara orogenic junction,Namibia

Age calibrated deformation histories established by detailed mapping and dating of key magmatic time markers are correlated across all tectono-metamorphic provinces in the Damara Orogenic System.Correlations across structural belts result in an internally consistent deformation framework with evidence of stress field rotations with similar timing,and switches between different deformation events.Horizontal principle compressive stress rotated clockwise ~180°in total during Kaoko Belt evolution,and~135° during Damara Belt evolution.At most stages,stress field variation is progressive and can be attributed to events within the Damara Orogenic System,caused by changes in relative trajectories of the interacting Rio De La Plata,Congo,and Kalahari Cratons.Kaokoan orogenesis occurred earliest and evolved from collision and obduction at ~590 Ma,involving E-W directed shortening,progressing through different transpressional states with ~45° rotation of the stress field to strike-slip shear under NW-SE shortening at ~550-530 Ma.Damaran orogenesis evolved from collision at ~555-550 Ma with NW-SE directed shortening in common with the Kaoko Belt,and subsequently evolved through ~90°rotation of the stress field to NE-SW shortening at ~512-508 Ma.Both Kaoko and Damara orogenic fronts were operating at the same time,with all three cratons being coaxially convergent during the 550-530 Ma period;Rio De La Plata directed SE against the Congo Craton margin,and both together over-riding the Kalahari Craton margin also towards the SE.Progressive stress field rotation was punctuated by rapid and significant switches at ~530-525 Ma,~508 Ma and ~505 Ma.These three events included:(1)Culmination of main phase orogenesis in the Damara Belt,coinciding with maximum burial and peak metamorphism at 530-525 Ma.This occurred at the same time as termination of transpression and initiation of transtensional reactivation of shear zones in the Kaoko Belt.Principle compressive stress switched from NW-SE to NNW-SSE shortening in both Kaoko and Damara Belts at this time.This marks the start of Congo-Kalahari stress field overwhelming the waning Rio De La Plata-Congo stress field,and from this time forward contraction across the Damara Belt generated the stress field governing subsequent low-strain events in the Kaoko Belt.(2)A sudden switch to E-W directed shortening at ~508 Ma is interpreted as a far-field effect imposed on the Damara Orogenic System,most plausibly from arc obduction along the orogenic margin of Gondwana(Ross-Delamerian Orogen).(3)This imposed stress field established a N-S extension direction exploited by decompression melts,switch to vertical shortening,and triggered gravitational collapse and extension of the thermally weakened hot orogen core at ~505 Ma,producing an extensional metamorphic core complex across the Central Zone.     

Late Paleozoic to Mesozoic extension in southwestern Fujian Province,South China: Geochemical,geochronological and Hf isotopic constraints from basic-intermediate dykes

The tectonic evolution of SE China block since late Paleozoic remains debated.Here we present a new set of zircon U-Pb geochronological,Lu-Hf isotopic data and whole-rock geochemistry for two stages of basicintermediate dykes from the southwestern Fujian.The samples were collected from the NE-trending(mainly diabases) and NW-trending(mainly diabasic diorites) dykes and yielded zircon U-Pb ages of 315 and 141 Ma,with ε_(Hf)(t) values of-8.90 to 7.49 and-23.39 to-7.15(corresponding to T_(DM2) values of 850 to 1890 Ma and 737 to 2670 Ma),respectively.Geochemically these rocks are characterized by low TiO_2(0.91-1.73 wt.%)and MgO(3.04-7.96 wt.%),and high Al_2O_3(12.5-16.60 wt.%) and K_2O(0.60-3.63 wt.%).Further they are enriched in LREEs and ULEs(Rb,Ba,Th and K).but depleted in HFSEs(Nb,Ta and Zr).The tectonic discrimination analysis revealed that the dykes were formed in an intraplate extensional environment.However,the NW trending dykes show crust-mantle mixed composition,which indicate an extensional tectonic setting with evidence for crustal contamination.The SE China block experienced two main stages of extensional tectonics from late Carboniferous to early Cretaceous.The tectonic evolution of the SE China block from late Devonian to Cretaceous is also evaluated.     

Re–Os and Pb–Pb geochronology of the Archean Salobo iron oxide copper–gold deposit, Carajás mineral province, northern Brazil

Rhenium–osmium ages were determined for two molybdenite samples and a Pb–Pb age was derived from bornite–chalcopyrite–magnetite at the Salobo iron oxide copper–gold deposit to determine the timing of mineralization and its relation to the nearby Old Salobo Granite. Rhenium–osmium dating of molybdenite spatially associated with copper sulfide minerals yields ages with weighted means of 2576±8 and 2562±8 Ma. Removing the error multiplier introduced by the decay constant uncertainty, appropriate for comparing ages from the same isotopic system, these data convincingly argue for two temporally separated pulses of molybdenite deposition at 2576.1±1.4 Ma (n=2) and 2561.7±3.1 Ma (n=3). The 2576±8 Ma age coincides with a previously published U–Pb age of 2573±2 Ma for the Old Salobo Granite, suggesting that main stage ore formation may have been contemporaneous with granite magmatism. The slightly younger 2562 Ma age most likely represents new molybdenite precipitation associated with the development or reactivation of local shear zones. Lead–lead stepwise leaching of copper sulfide minerals yields a less precise isochron age of 2579±71 Ma, and supports an Archean age for the Salobo ores. This is the first documentation of an Archean iron oxide copper–gold deposit, and the Re–Os and Pb–Pb geochronology herein support 2580–2550 Ma estimates for basement reactivation and regional granite magmatism associated with the development of brittle–ductile shear zones.     

Analysis of relationships between geochemical parameters of petroleum potential in related source and carbonaceous materials

A worldwide data set of 1,085 samples containing organic matter of the type II/III kerogen from Carboniferous to Cenozoic was used to analyse the evolution of the hydrogen index (HI), quality index (QI), and bitumen index (BI) with increasing thermal maturity. The HI_max, QI_max and BI_max lines were defined, based on statistical analysis and cross-plots of HI, QI and BI versus the vitrinite reflectance (%Ro) and T _max (°C). The constructed HI, QI and BI bands were broad at low maturities and gradually narrowed with increasing thermal maturity. The petroleum generation potential is completely exhausted at a vitrinite reflectance of 2.0–2.2 % and T _max of 510–520 °C. An increase in HI and QI suggests extra petroleum potential related to changes in the structure of the organic material. A decline in BI signifies the start of the oil window and occurs within the vitrinite reflectance range 0.75–1.05 % and T _max of 440–455 °C. Furthermore, petroleum potential can be divided into four different parts based on the cross-plot of HI versus %Ro. The area with the highest petroleum potential is located in “Samples and methods” with %Ro = 0.6–1.0 %, and HI > 100. Oil generation potential is rapidly exhausted at “Results and discussion” with %Ro > 1.0 %. This result is in accordance with the regression curve of HI and QI with %Ro based on 80 samples with %Ro = 1.02–3.43 %. The exponential equation of regression can thus be achieved: HI = 994.81e^?1.69Ro and QI = 1,646.2e^?2.003Ro (R ² = 0.72). The worldwide organic material data set defines two range of oil window represented by the upper and lower limits of the BI band: %Ro 0.75–1.95 %, T _max 440–525 °C, and %Ro 1.05–1.25 %, T _max 455–465 °C, respectively.     

First results of GPS measurements on the Ulaanbaatar geodynamic testing area

First results of the analysis of GPS measurement data obtained from 18 sites of two local networks in the vicinity of Ulaanbaatar (Mongolia) for the period 2010–2015 have been presented. Horizontal velocity vectors are consistent with each other in the ITRF2014 system and with the velocities of the IGS permanent station ULAB. The sites move in the E–SE direction at a rate of 25–30 mm/yr, with the displacement azimuth averaging 105°. With respect to Eurasia, the vectors for most of the sites are slighly turned to the south, but they are still of SE orientation with the azimuth range of 130°–150° and magnitude values of 2–4 mm/yr. Relative horizontal velocities tend to decrease towards southeast that determines a zonal distribution of different type of relative strain patterns. The western part of the Ulaanbaatar network is characterized by the W–E oriented extension with the elongation rate ε₁ = 12–16 × 10^–8 yr^–1. The shortening NW–SE trending strain with calculated value ε₂ = 22.4 × 10^–8 yr^–1 characterizes the network’s eastern part. The highest values of the maximum shear strains (ε_max = 10–14 × 10^–8 yr^–1) form an extended area in the center of the testing ground, which is elongated in the northeastern direction, conformably with the strike of the major geologic structures. The strain distribution pattern of the Emeelt network located within the eponymous seismogenic structures is characterized by the crustal elongation (5 × 10^–6 yr^–1) trending SE–NW and less pronounced shortening in the SW–SE directions.The axial part of the fault crossing the network in the NW direction exhibits maximum deformations.     

Is there Link between the Type of the Volumetric Strain Curve and Elastic Constants, Porosity, Stress and Strain Characteristics ?

The stress [crack damage stress (σ _cd) and uniaxial compressive strength (σ _c)] and strain characteristics [maximum total volumetric strain (ε _cd), axial failure strain (ε _af)], porosity (n) and elastic constants [elastic modulus (E) and Poisson’s ratio (ν)] and their ratios were coordinated with the existence of two different types (type 1 and type 2) of volumetric strain curve. Type 1 volumetric strain curve has a reversal point and, therefore, σ _cd is less than the uniaxial compressive strength (σ _c). Type 2 has no reversal point, and the bulk volume of rock decreases until its failure occurs (i.e., σ _cd = σ _c). It is confirmed that the ratio between the elastic modulus (E) and the parameter λ = n/ε _cd strongly affects the crack damage stress (σ _cd) for both type 1 and type 2 volumetric strain curves. It is revealed that heterogeneous carbonate rock samples exhibit different types of the volumetric strain curve even within the same rock formation, and the range of σ _cd/σ _c = 0.54–1 for carbonate rocks is wider than the range (0.71 < σ _cd/σ _c < 0.84) obtained by other researchers for granites, sandstones and quartzite. It is established that there is no connection between the type of the volumetric strain curve and values of n, E, σ _cd, ν, E/(1 ? 2ν), M _R = E/σ _c and E/λ. On the other hand, the type of volumetric strain curve is connected with the values of λ and the ratio between the axial failure strain (ε _af) and the maximum total volumetric strain (ε _cd). It is argued that in case of small ε _af/ε _cd–small λ, volumetric strain curve follows the type 2.