Multiple Thermal Events Research Articles

Comparative studies on the nature and kinetics of phase transitions in plastic crystals: Hydrogen-bonded adamantane derivatives

In this paper, the results of thermal, structural, and spectroscopic studies on 3-amino-1-adamantanol (3-NH2-1-OH-ADM) were discussed with those recently published for 1-hydroxyl and 1-amine derivatives of adamantane (1-OH-ADM, 1-NH2-ADM). Calorimetric measurements showed that the examined compound, like 1-NH2-ADM, is characterized by multiple thermal events in the thermogram measured on heating and cooling. Further X-ray diffraction, Fourier-transform infrared, and Raman spectroscopy investigations suggested that two of them correspond to the transition from the ordinary crystal (OC) to the plastic crystal (PC) phase I and the transition between two PC phases (PC(I) and PC(II)), while the third subtle one is rather not related to a phase transition. Interestingly, ambient pressure dielectric studies performed during heating and cooling cycles revealed changes in the imaginary and real parts of the complex dielectric permittivity at temperatures close to the temperatures of phase transitions detected by the calorimetry. Such behavior resembled that reported for 1-NH2-ADM. Finally, isothermal time-dependent dielectric and infrared studies, supported by the analysis of asynchronous 2D-IR correlation spectra, indicated that the first endothermic peak, visible in the thermogram of 3-NH2-1-OH-ADM over an enormously large temperature interval and related to the OC-PC(I) phase transition, similarly to the one, assigned to the PC(I)-PC(II) transition in 1-NH2-ADM, has a character of the kinetic process. Importantly, during the annealing of 3-NH2-1-OH-ADM at T = 328–343 K, we observed a clear change in the dc-conductivity as well as intramolecular dynamics related to the vibrations of CH, NH2, and OH moieties that occur at different rates.

Identifying and characterizing missing source orogens for syn-orogenic basins based on detrital accessory mineral U-Pb geochronology and trace element geochemistry

Abstract Identifying source orogens for syn-orogenic sediments in dispersed fragments of a supercontinent provides key information on the tectonic evolution of the orogens involved in supercontinent assembly and on paleogeographic reconstruction. An approach using U-Pb geochronology and trace elements of multiple detrital minerals including zircon, rutile, apatite, and monazite has major advantages over detrital zircon studies alone. Application of this multi-pronged approach to Ediacaran and Cambrian clastic sedimentary rocks from the West Cathaysia terrane of the South China block demonstrates that these rocks were deposited in two distinct, but overlapping, syn-orogenic basins related to the assembly of eastern Gondwana. The Ediacaran rocks have detritus likely sourced from the Paterson-Petermann orogen (PPO) in Australia, and the dominant 650–600 Ma populations and trace element compositions of detrital rutile and apatite indicate a significant Ediacaran thermal and high-pressure metamorphic event in the orogen. The appearance of Cambrian and Tonian detrital rutile and apatite populations in the Cambrian rocks indicates derivation from a different source orogen characterized by multiple thermal events, most likely the Kuunga-Pinjarra orogen (KPO). Our data suggest that West Cathaysia was located adjacent to both the PPO and the KPO and support the hypothesis that the PPO was a collisional, rather than a long-lived post-Grenvillian intraplate, orogen. Our data also suggest a poly-metamorphic history for the poorly preserved northern part of the KPO and a larger expanse of Greater India.

Zircon U–Pb ages and geochemistry of deep-seated crustal xenoliths from Yangyuan: Implications for the evolution of lower crust beneath the Trans North China Orogen

Deep-seated crustal xenoliths captured by volcanic rocks play a rare opportunity in understanding the composition and evolution of the lower continental crust. Here, we present mineral chemistry, whole-rock major and trace elements and Sr–Nd isotopes, and in-situ zircon U–Pb ages and Hf isotope data for a suite of granulite and biotite gneiss xenoliths entrained in the Oligocene basalts from the Yangyuan county in the central part of the Trans North China Orogen (TNCO) with aims to decipher the composition and evolutionary history of the lower crust beneath this area. Calculated equilibrium temperatures (841–933 °C) and pressures (9.7–14.3 kbar) suggest an origin of the Yangyuan granulites from the lower crust. The dominantly Archean–Proterozoic zircon ages and low Mg# values (29.0–68.4) of the Yangyuan crustal xenoliths indicate that they represent the ancient crust and the fractional crystallization plays a key role during the protolith formation. Furthermore, the zircon U–Pb–Hf isotopes data reveal multiple thermal events, including two main peaks of crustal growth at ∼2.7 Ga and ∼2.5 Ga, ∼1.85–1.80 Ga and the Phanerozoic accretion and reworking of the lower crust beneath Yangyuan. In addition, a comparison study of crustal xenoliths from Yangyuan, Nangaoya and Hannuoba areas demonstrates that the lower crust beneath the central–northern part of the TNCO experienced as a whole the Neoarchean crustal growth, Paleoproterozoic metamorphism and Phanerozoic magmatic underplating. More importantly, the absence of U–Pb age peak of ∼1.95 Ga and Hf model age peaks of ∼3.9–3.8 Ga, ∼2.9–2.8 Ga, ∼2.2–2.1 Ga and ∼700–600 Ma in Yangyuan shows a heterogeneous evolution of the lower crust beneath the TNCO, which could pave the way for a more comprehensive picture of the crustal evolutional history beneath the North China Craton.

Response of Sm–Nd isotope systematics to complex thermal histories: A case study from 3.58 Ga gneisses of the Pilbara Craton

In felsic igneous rocks, the parent and daughter elements in the widely used Sm–Nd and Lu–Hf isotope tracer systems are mainly hosted in accessory phases. Recrystallisation and/or breakdown of these minerals during metamorphism, deformation and weathering potentially compromises the chemical and isotopic composition of the respective whole rocks, impeding the utility of such information for deducing the timing, rates and processes of crust-mantle differentiation in the early Earth. The different abilities of zircon and REE-rich minerals to withstand metamorphism have been suggested as a reason for the decoupling of the Lu–Hf and Sm–Nd isotope systems observed in a number of ancient gneiss terranes. The controls on element mobility and subsequent isotopic disturbance during recrystallisation and breakdown of LREE-rich accessory minerals are, however incompletely understood. Here, we use petrography, element mapping, and microanalysis of accessory minerals, in tandem with whole rock Sm–Nd data, to assess the reliability of the Sm–Nd system in the 3.59–3.58 Ga Mount Webber Gabbros, the oldest rocks in the Pilbara Craton (Western Australia). We show that despite multiple thermal events, which reset the mineral Sm–Nd systematics, and decomposition of the REE-rich mineral allanite, the Mount Webber rocks retained the Sm–Nd isotope signatures of their magmatic protoliths at the whole-rock scale. We show that the allanite breakdown occurred during modern, near-surface weathering processes at low temperature, such that the REE were sequestered into secondary minerals rather than escaping in higher temperature metamorphic fluids. The whole rock Sm–Nd, and zircon O–Hf signatures, together with new 142Nd isotope data, suggest derivation of the Mount Webber rocks from undifferentiated mantle sources that preserve no evidence for Hadean silicate Earth differentiation. This study highlights the benefits of a combined analytical approach using both in-situ and whole-rock isotope analyses to obtain a more complete record of the source and thermal evolution of ancient, highly metamorphosed igneous rocks.

Spatial-derivative-based compression approach for distributed temperature data.

The new generation of distributed optical sensors with improved interrogation, multiplexing, and acquisition techniques with the possibility of performing measurements with high spatial resolution over tens of kilometers of optical fiber has led to the accumulation of a vast volume of data that can present a big challenge to process and store all this data. Looking for simple solutions to this problem, we present in this paper a data compression method for distributed temperature sensors. This compression approach performs the spatial derivative of the temperature signal, constituting a simple and effective method to remove redundant information. Also, this compression methodology is suitable for temperature data, as it follows thermal variations over time and can be applied to any temperature profile with multiple thermal events along the sensing fiber, whether in heating or cooling circumstances. Tests performed with a large amount of experimental data showed that an average compression ratio of 1.5× can be obtained by removing redundant spatial temperature variations without losing spatial resolution.

Depth‐Profiling Determinations of Rare Earth Element Abundances and U‐Pb Ages from Zircon Crystals Using Sensitivity‐Enhanced Inductively Coupled Plasma‐Time of Flight‐Mass Spectrometry

Depth‐profiling measurements of both the abundances of rare earth elements (REEs) and U‐Pb ages from zircons were conducted using a time‐of‐flight based inductively coupled plasma‐mass spectrometer (ICP‐ToF‐MS) coupled with laser ablation sampling. To improve ion transmission, a dry plasma cone from Nu Instruments was applied to the ICP‐ToF‐MS system employed. The signal‐to‐noise ratios for the analytes were further improved using a multiple spot‐laser ablation (msLA) protocol. Moreover, constructing a ‘moat’ around an analysis area obviated the risk of mixing of particles released from different layers. With the technique developed here, reproducible analyses of REE abundances and U‐Pb ages for three reference material zircons (91500, Plešovice and OD‐3) were performed for sampling depths in the range 0.59–0.66 μm. The present technique was also applied to zircon samples collected from the Himalayan orogen. The combination of REE abundances and U‐Pb isotopic ratios suggested that the discordant trend in U‐Pb isotopic ratios observed in this study was mainly due to mixing among three components in grains (thin outer layer, inner rim and core). The geochemical data obtained demonstrate that the combination of the msLA technique and ICP‐ToF‐MS system is a powerful tool to decode the multiple thermal events contributing to sample formation.

Exhumation of the passive margin of the DR Congo during pre- and post- Gondwana breakup: Evidence from low-temperature thermochronology, geology and geomorphology

The Lower Congo region encompasses the South Atlantic passive margin of the Democratic Republic of Congo (Central Africa). It is host to the lowermost course of the Congo River, cutting through the Central African Atlantic Swell (CAAS). The region is underlain by litho-structural units of the Pan-African West Congo Belt, which consists of different tectono-metamorphic domains. The Precambrian basement is covered to the west by marine deposits of the South Atlantic Ocean and to the east by continental deposits of the Congo Basin. In this study we aim to constrain the timing of uplift and exhumation of the CAAS by using apatite fission track (AFT) thermochronology in combination with an updated overview of the geology and geomorphology of the Lower Congo region. AFT ages vary widely between 108 and 312 Ma. Short track lengths (11–12 μm) and broad, complex track length distributions indicate mixed ages between multiple thermal events. We derive the timing of exhumation from inverse thermal history models, that show that the Lower Congo region experienced a first exhumation event before Gondwana breakup during the Carboniferous to Middle Jurassic. This event is probably related to compressive forces at the boundaries of Gondwana. Both rifting and subsequent opening of the South Atlantic Ocean do not seem to have had a pronounced effect on the CAAS region. During the Late Cretaceous to Palaeogene, a slight reheating is suggested and could be due to subsidence and consequential modest reburial of the Lower Congo rocks. A second phase of exhumation initiated around the Palaeogene–Neogene and eventually emplaced the sampled rocks at surface temperatures. The multi-phased differential denudation results from reactivation of fault-bounded tectono-metamorphic blocks of the Precambrian basement, controlled by the combination of two systems of faults related to the Cretaceous South Atlantic Ocean opening and Pan-African orogeny respectively. Differential denudation of the Lower Congo region is also well-marked by independent qualitative indicators of the present-day geomorphology including distinct knickpoints and steep valleys along the course of the Lower Congo River, reworking of erosion surfaces and associated laterite crust and contrasting karst morphology.

Two-Dimensional Correlation Analysis of iPP Bead Foaming Thermal Features Modeled by Fast Scanning Calorimetry.

The bead foaming of semicrystalline polymers is a complex thermal process involving the formation of multiple crystalline populations, which serve the dual purposes of ensuring the structural integrity of beads while also allowing bead sintering at the interface. The quality of this "double melting peak" structure is determined by the temperature and duration of the isothermal treatment of the beads as well as the quenching rate following the isotherm. Currently, the intricacies of the quenching process are not very well-known due to the challenge of replicating these rapid cooling rates in a laboratory setting. Fast-scanning calorimetry was used to reproduce these conditions for isotactic polypropylene (iPP), revealing optimal quenching rates for the bead foaming of iPP. We further probed these thermal features using two-dimensional correlation analysis as a tool to understand the dynamics, interdependence, and relative contributions of multiple thermal events such as glass transition, mesophase formation, cold crystallization, and melting in response to the perturbation of the quenching rate.

Multiple thermal events recorded in IIE silicate inclusions: Evidence from in situ U–Pb dating of phosphates in Weekeroo Station

Silicate-bearing iron meteorites record metal–silicate separation and mixing processes of planetesimals. Of particular interest is the IIE group as its silicate inclusions show great petrological diversity, probably resulting from the complex petrogenesis involving early differentiation and impact-induced dynamic mixing, re-melting, and reduction. These geologic records may have been documented by individual minerals to various extents, which blurs the chronological records on the formation of IIEs. Here, by combining X-ray elemental mapping with ion microprobe dating technique, we show that two groups of phosphates are present in the Weekeroo Station IIE iron meteorite. They have distinct petrography, mineralogy, Th/U ratios, and Pb isotopic compositions, which reflects their different formation mechanisms. One group (Group 1) has a fine-grained, euhedral or acicular habit, suggesting rapid crystallization from a melt during the metal–silicate mixing. By contrast, the other group (Group 2) is intimately associated with the decomposition and reduction of pyroxene under subsolidus conditions. The Group 1 phosphates yield a precise Pb–Pb age of 4528 ± 11 Ma and they provide the first robust constraint on the timing of impact-induced metal–silicate mixing on the IIE iron meteorite parent body. The Group 2 phosphates, on the other hand, probably record a Pb–Pb age at least 50 Myr younger, and they give insight into the role of subsequent impacts on the alteration of IIE irons. Combining our results with published chronological data suggests that the formation of IIEs irons is consistent with early partial differentiation followed by multiple impact-driven metal–silicate mixing and that most of them were not shielded from subsequent impacts.

SULFIDE RECRYSTALLIZATION AND GOLD REMOBILIZATION DURING THE 2.0 GA STAGE OF THE MINAS OROGENY: IMPLICATIONS FOR GOLD MINERALIZATION IN THE QUADRILÁTERO FERRÍFERO AREA, BRAZIL

Abstract The historic Quadrilátero Ferrífero mining district in Brazil is a polydeformed Archean terrane recording several tectonic events. This history has hampered the accurate dating of its important gold deposits, given that most of the geochronological clocks have been affected by multiple thermal events. To determine an accurate time constraint for the mineralizing events in this region, we provide the first Re-Os dating for a gold deposit in the Quadrilátero Ferrífero, obtained from 13 mineral separates of disseminated sulfide phases (pyrrhotite, coarse-grained arsenopyrite, fine-grained arsenopyrite, and pyrite) at the recently discovered São Sebastião deposit (northwest Quadrilátero Ferrífero). Three distinct successive sulfidation stages are interpreted at the deposit. The final stage, texturally associated with gold, is marked by fine-grained arsenopyrite and pyrite (assemblage III) and is associated with high-temperature (~600°C) features. A Re-Os errorchron is obtained when considering the data of all samples collectively; however, a comprehensive Re-Os isochron age of 1987 ± 72 Ma (2σ; n = 4; IsoplotR model 3; initial 187Os/188Os = 2.1 ± 0.7) is obtained from assemblage III. A 187Re-187Osr isochron at 1988 ± 56 Ma (mean square of weighted deviates = 0.1, initial 187Osr = –0.1 ± 2.6 ppt; n = 4) is obtained if an initial 187Os/188Os composition of ca. 2.1, is used, this being consistent with the fact that the 187Os in the sulfides in assemblage III is largely radiogenic (187Osr). Earlier assemblages (I and II) individually show variable initial 187Os/188Os, indicative of disturbance to the Re-Os systematics at ~2.0 Ga. This age is considered the best estimate for the timing of gold mineralization at São Sebastião, being coincident with the waning stages of the Minas orogeny, the thermal effects of which are restricted to the southern Quadrilátero Ferrífero. Hence, we consider that the hot fluids from which assemblage III crystallized were responsible for resetting the Re-Os systematics in assemblages I and II and were central to Paleoproterozoic gold deposition in the region. This ~2.0 Ga age challenges previous notions of a late Archean, ~2.7 Ga mineralizing event in the Quadrilátero Ferrífero and makes it possible that gold remobilization occurred elsewhere in the region, particularly in areas representing deeper crustal levels.

From sink to source: Using offshore thermochronometric data to extract onshore erosion signals in Namibia

AbstractProducts of onshore passive continental margin erosion are best preserved in offshore sedimentary basins. Therefore, these basins potentially hold a recoverable record of the onshore erosion history. Here, we present apatite fission track (AFT) data for 13 samples from a borehole in the southern Walvis basin, offshore Namibia. All samples show AFT central ages older or similar to their respective stratigraphic ages, while many single grain ages are older, implying none of the samples has been totally annealed post‐deposition. Furthermore, large dispersion in single grain ages in some samples suggests multiple age components related to separate source regions. Using Bayesian mixture modelling we classify single grain ages from a given sample to particular age components to create ‘subsamples’ and then jointly invert the entire dataset to obtain a thermal history. For each sample, the post‐depositional thermal history is required to be the same for all age components, but each component (‘subsample’) has an independent pre‐depositional thermal history. With this approach we can resolve pre‐ and post‐depositional thermal events and identify changes in sediment provenance in response to the syn‐ and post‐rift tectonic evolution of Namibia and southern Africa. Apatite U‐Pb and compositional data obtained during the acquisition of LA‐ICP‐MS FT data are also presented to help track changes in provenance with time. We constrain multiple thermal events linked to the exhumation and burial history of the continental and offshore sectors of the margin over a longer timescale than has been possible using only onshore AFT thermochronological data.

Multiple episodes of tectono-thermal disturbances in the Huayangchuan U-Nb-Pb polymetallic deposit in the Xiaoqinling region, central China and their significances on metallogeny

The Huayangchuan U-Nb-Pb polymetallic deposit is hosted in a suite of Neoarchean to Paleoproterozoic gneisses in the Xiaoqinling region, central China. The deposit is composed mainly of discrete small-scale carbonatite veins emplaced largely along shallow-lying brittle fractures. Various regimes of deformation from ductile flow to brittle fracturing are documented in the deposit and its adjacent areas. These include an earliest phase of contracting deformation (D1) manifested as well-developed boudinages, abundant relict rootless folds and penetrative compositional bands, i.e., gneissosity generally trending E-W, a subsequent phase of disperse dextral ductile shear (D2) along the E-W-trending gneissosity, and a following phase of general NE-SW compression (D3) resulting in local contraction deformation bands and abundant sinistral ductile shear along the E-W-trending gneissosity, all of which are affected by an overwhelming NNE-SSW compression (D4) with hemi-ductile to brittle deformation behaviors. The deformation after D4 is generally brittle and may arise from an earlier regime of dominant compression (NW-SE) (D5) and a later regime of extensive extension (D6). Multiple thermal events are superimposed on gneisses as revealed by 40Ar/39Ar dating on biotite collected from various lithologies in the deposit and its adjacent areas, which yields ages ranging from Late Triassic (238–203 Ma), through earliest Cretaceous (139–134 Ma) and middle Cretaceous (117–92 Ma), to latest Cretaceous (72 Ma) and even early Cenozoic (53 Ma). This sequence of thermal disturbances unlikely predates the D4 deformation given that it is the last ductile deformation experienced by the Precambrian rocks. Ore-bearing carbonatite veins have their shallowly lying attitudes consilient with the D4 and D5 deformation and ages compatible with the first two thermal overprints. Accordingly, a two-stage mineralization model is proposed for the Huayangchuan polymetallic deposit with an early phase of ore veining in the Late Triassic to latest Jurassic during the nearly N-S collision between the South and North China blocks and a late phase of ore veining or more possibly reworking in the Early Cretaceous related to the Yanshanian orogeny due to a generally NW-directed subduction of the Paleo-Pacific oceanic plate beneath the eastern Asia continent.

Changing the Electronic Polarizability of Monolayer MoS2 by Perylene‐Based Seeding Promoters

AbstractLarge area molybdenum disulfide (MoS2) monolayers are typically obtained by using perylene‐3,4,9,10‐tetracarboxylic acid tetrapotassium salt (PTAS) as organic seeding promoter in chemical vapor deposition (CVD). However, the influence of the seeding promoter and the involvement of the functional groups attached to the seed molecules on the physical properties of the MoS2 monolayer are rarely taken into account. Here, it is shown that MoS2 monolayers exhibit remarkable differences in terms of the electronic polarizability by using two representative cases of seeding promoter, namely, the commercial PTAS and a home‐made perylene‐based molecule, N,N‐bis‐(5‐guanidil‐1‐pentanoic acid)‐perylene‐3,4,9,10‐tetracarboxylic acid diimide (PTARG). By thermogravimetric analysis, it is verified that the thermal degradation of the promoters occurs differently at the CVD working condition: with a single detachment of the functional groups for PTAS and with multiple thermal events for PTARG. As a consequence, the promoter‐dependent electronic polarizability, derived by free charges trapped in the monolayer, impacts on the photoluminescence emission, as well as on the electrical performances of the monolayer channel in back‐gated field‐effect transistors. These findings suggest that the modification of the electronic polarizability, by varying the molecular promoter in a pre‐growth stage, is a path to engineer the MoS2 opto‐electronic properties.

Partial resetting of U–Pb ages during experimental fluid-induced re-equilibration of xenotime

Experimentally metasomatized xenotime was investigated in terms of preservation of U–Pb ages during fluid-induced alterations at 250, 450, 550 and 650 °C. Starting materials for experiments included xenotime (38.4 ± 0.6 Ma), albite, K-feldspar, biotite, muscovite, garnet, SiO2, CaF2, Na2Si2O5 and H2O that roughly replicated metapelitic to granitic composition with high Na, Ca, Si and F activities in the fluid. Despite partial replacement of xenotime by secondary phases including (Y,REE)-rich fluorapatite to Y-rich fluorcalciobritholite in all runs, and an unnamed (Y,HREE)-rich silicate in 250 °C experiment, only two experiments at 550 °C, 800 MPa and 650 °C, 600 MPa resulted in development of faint to moderate patchy zoning in the xenotime. Experimental results indicate higher chemical-stability of xenotime at lower P-T conditions compared to those corresponding to amphibolite facies conditions. The xenotime domains metasomatized via coupled dissolution-reprecipitation reactions are characterized by depletion of U and Pb and younger 206Pb/238U dates of ca. 15–34 Ma, demonstrating preferential removal of Pb and thus the partial resetting of the age record. The U–Pb data for the first time experimentally confirm that xenotime may be affected by re-equilibration induced by fluid or melt with high Na, Ca and F activities via coupled dissolution-reprecipitation reactions that result in removal of radiogenic Pb under temperature conditions of 550–650 °C. These findings have important implications for U–Pb geochronology of igneous (particularly in pegmatites) and metamorphic processes in terms of preservation of U–Pb ages of xenotime that was affected by multiple thermal events in the presence of alkali-rich fluid and/or melt.

Constraints on the timing of multiple thermal events and re-equilibration recorded by high-U zircon and xenotime: Case study of pegmatite from Piława Górna (Góry Sowie Block, SW Poland)

The application of zircon and xenotime geochronometers requires knowledge of their potential and limitations related to possible disturbance of the age record. The alteration of the intergrown zircon and xenotime in pegmatite from the Góry Sowie Block (SW Poland) was studied using the electron microprobe analysis, X-ray WDS compositional mapping, micro-Raman analysis, and LA-ICP-MS U-Pb dating of zircon and xenotime, as well as the U-Th-total Pb dating of uraninite. These microanalytical techniques were applied to understand the formation mechanisms of the secondary textures related to post-magmatic processes in the zircon and xenotime intergrowth, and to constrain their timing. Textural and compositional features combined with U-Pb data indicate that the pegmatite-related crystallization of the zircon and xenotime intergrowth occurred ca. 2.09 Ga (2086 ± 35 Ma for zircon and 2093 ± 52 Ma for xenotime), followed by the re-equilibration of zircon and xenotime ca. 370 Ma (373 ± 18 Ma and 368 ± 6 Ma, respectively) during the formation of the younger pegmatite. The zircon and xenotime were most likely derived from Precambrian basement rocks and emplaced in the pegmatite as a restite. The zircon preserved textures related to diffusion-reaction processes that affected its high-U core (up to ca. 9.6 wt% UO2), which underwent further metamictization and amorphization due to self-radiation damage. The zircon rim and xenotime were affected by coupled dissolution-reprecipitation processes that resulted in patchy zoning, age disturbance and sponge-like textures. Xenotime was also partially replaced by fluorapatite or hingganite-(Y) and Y-enriched allanite-(Ce). The termination of the low-temperature alteration was constrained by the U-Th-total Pb age of the uraninite inclusions that crystallized in zircon at 281 ± 2 Ma, which is consistent with the age of 278 ± 15 Ma obtained from the youngest cluster of U-Pb ages in the re-equilibrated high-U zircon domains. This study demonstrates the importance of the careful examination of compositional, microtextural and geochronological data obtained using microanalytical techniques to reconstruct the complex thermal histories recorded by accessory minerals.

Thermal history of the northern Olympic Domain, Gawler Craton; correlations between thermochronometric data and mineralising systems

Multi-method thermochronology applied to the northern Olympic Domain of South Australia reveals a complex thermal evolution with multiple thermal events. Apatite U/Pb (closure temperature ~550–350 °C) and muscovite 40Ar/39Ar ages (~400–350 °C) record post magmatic cooling from 1850 to 750 Ma for the ~1850 Ma Donington Suite, and from ~1560–1500 Ma for the ~1590 Ma Hiltaba Suite and the Gawler Range Volcanics. Potassium feldspar 40Ar/39Ar ages (~350–150 °C) record disturbed spectra that are likely related to hydrothermal alteration at ~1000–650 Ma in the Neoproterozoic Adelaide Rift Complex. Apatite fission track (~60–120 °C), zircon (U-Th-Sm)/He (~180–200 °C), and apatite (U-Th-Sm)/He (~45–75 °C) ages reveal regional low temperature thermal events at ~1000 Ma, ~430–400 Ma, 350–330 Ma, and ~200 Ma, in addition to localised thermal events during the Cretaceous. The ~1000 Ma apatite fission track ages are amongst the oldest recorded for South Australia and are only preserved near the margins of the Olympic Domain. The ~430–400 Ma and ~350–330 Ma cooling events are interpreted to be driven by the Alice Springs Orogeny. The Mesozoic thermochronometric ages are interpreted to record localised thermal perturbations, possibly related with hydrothermal activity within the northern Olympic Domain. The presence of abundant IOCG deposits near these young thermal anomalies may indicate that these zones record more favourable thermal conditions and/or exposure levels for IOCG discoveries in the study area.

Continental outbuilding along the margin of an Archean cratonic nucleus in the North China Craton

The history of crustal growth in the Early Earth, formation of cratonic nuclei and the building of large continents have important implications in understanding the secular evolution of our planet. The North China Craton (NCC) is envisaged in recent models as a collage of several Archean microblocks which were amalgamated along multiple zones of ocean closure during the late Neoarchean, and offers window to gain insights on the origin of cratonic nuclei and their subsequent outbuilding. Here we investigate a suite of magmatic rocks developed at the periphery of one of these microblocks, the Qianhuai Block. We present petrological, geochemical and zircon U–Pb geochronological and Lu-Hf isotopic data on a suite of rocks ranging from hornblendite through gabbro and dioritic gabbro to TTG (tonalite-trondhjemite-granodiorite) and granitoids exposed in the Miyun area, and representing the cross section of the lower to mid segments of a continental magmatic arc. Geochemically, the felsic units of the suite straddle from monzonite through granodiorite to granite with dominantly metaluminous affinity, magnesian composition and arc-related features. The metagranites and TTG gneisses are characterized by negative Nb-Ta anomalies and positive K and Pb anomalies. The diorite, gabbros and hornblendite display negative anomalies of Th-U, Nb-Ta and Zr-Hf and positive anomalies at Ba, Pb and Sm with negative Eu anomalies and minor positive Ce anomalies, attesting to arc-related origin. In the tectonic discrimination diagrams, the rocks plot in the volcanic arc field, indicating formation within a subduction-related setting.The oldest magmatic population with ages of ca. 2560 and 2529 Ma from the hornblendite and the TTG gneiss represent the magma emplacement ages with fractional crystallization of underplated basalts derived from slab melts, leaving hornblendite as the cumulate. Magmatic zircon grains from the metagranite and granodiorite show upper intercept ages of 2457 ± 21 Ma and 2475 ± 12 Ma whereas those from the TTG gneisses yield ages of 2529 ± 19 Ma and 2522 ± 38 Ma. Younger age groups of 2444.4 ± 8.3 Ma, 2418 ± 16 Ma and 2161 ± 31 Ma also occur in these rocks indicating multiple thermal events. The zircon grains from diorite and gabbros show 207Pb/206Pb weighted mean ages of 2411 ± 11 Ma–2486 ± 13 Ma with younger ages of 2.0–2.37 Ga. These rocks also carry older xenocrystic grains of ca. 2.51–2.65 Ga. The zircons in hornblendite show an upper intercept age of 2560 ± 33 Ma and a lower intercept age of 1947 ± 54 Ma. Young groups of ca. 2.2 Ga and 1.8 Ga are also found in this rock. The ca. 1.9 to 1.8 Ga ages correspond to the timing of metamorphism. The Hf isotopic data on zircons from the various rock types of the Miyun magmatic suite show both negative and positive εHf(t) values ranging from −9.64 to +7.04 with Hf model ages in the range of 2606–2844 Ma (TDM1) and 2635–3147 Ma (TDM2), suggesting crustal growth through both juvenile magmas from depleted mantle sources and reworked Meso- to Neoarchean crustal components. Our study reveals that the ca. 2.7–3.0 Ga core of the Qianhuai microcontinent represents one of the ancient cratonic nuclei in the NCC that underwent extensive continental outbuilding during late Neoarchean – early Paleoproterozoic through multiple magmatic events mostly related to convergent margin arc-type processes. It is possible that similar processes operated around the other Archean microcontinents in the NCC in the process of building the major crustal blocks which were finally amalgamated at the end of the Paleoproterozoic.

Microblock amalgamation in the North China Craton: Evidence from Neoarchaean magmatic suite in the western margin of the Jiaoliao Block

The Archaean Earth is considered to have been characterized by microcontinents that formed, dominantly, through the accretion of oceanic arcs and plateaus. The North China Craton (NCC) provides a typical case where at least seven ancient microcontinental nuclei with distinct lithological features and independent tectonic histories were amalgamated into the cratonic framework at the end of the Archaean. Here we investigate a suite of magmatic rocks developed at the periphery of one of these microblocks, the Jiaoliao Block that forms part of the composite Eastern Block of the NCC. We present petrological, geochemical and zircon U–Pb geochronological data from the Taipingzhai charnockite suite, and associated amphibolites, metagabbros and orthogneisses from the Qianxi Complex. Geochemically the rocks show a wide range of SiO2 (charnockite suite: 52.57–75.50wt.%; metagabbro: 43.71wt.%; amphibolite: 50.24wt.%; garnet-bearing biotite: 63.73wt.%), and MgO (charnockite suite: 0.89–5.01wt.%; metagabbro: 3.99wt.%; amphibolite: 6.23wt.%; garnet-bearing biotite: 2.08wt.%). The composition of the felsic units straddle from diorite through syeno-diorite to granite with both alkalic and subalkalic affinity, with dominantly magnesian composition and arc-related features. Their immobile trace element relationships suggest calc-alkaline affinity. They show positive Pb, Ba, La, Nd, and Gd and negative Nb, Ta, Sr, Th and Ti anomalies with slightly negative anomalies of Ce and Y, attesting to arc-related features. In tectonic classification diagrams, the rocks plot in the VAG+syn-COLG field or the VAG area suggesting subduction-related origin.The dominant population of zircons in all these rocks displays magmatic crystallization features including high Th/U values with core-rims textures indicating subsequent thermal events. The zircon U–Pb data yield upper intercept ages of 2587±10Ma to 2543±17Ma and 207Pb/206Pb mean ages of 2578±7.3Ma to 2536±8Ma for the charnockite suite, marking the timing of emplacement of the arc magmas. The overgrowth rims as well as discrete neoformed grains are interpreted as dating subsequent metamorphism and yield 207Pb/206Pb ages between 2533Ma to 2490Ma. Zircons in the metagabbro preserve upper intercept ages of 2556±20Ma representing the crystallization age of this rock. The younger ages of 2449±58Ma (upper intercept age) and 1845±25Ma (207Pb/206Pb spot age) are interpreted to represent subsequent multiple thermal events in this area. Zircons in the amphibolite preserve the 207Pb/206Pb mean age of 2539±9Ma, representing the crystallization age of this rock. The garnet-bearing biotite gneiss shows an upper intercept age of 2562±10Ma (MSWD=0.66; N=36) and the 207Pb/206Pb mean age of 2561±9Ma (MSWD=0.63; N=33) which is taken to represent the crystallization age of this rock. Some inherited zircons are also identified with 207Pb/206Pb ages of 2664±26Ma and 2628±26Ma. Zircon Lu–Hf data show dominantly positive εHf(t) values and combined with crustal residence ages, the results suggest Mesoarchean to Neoarchean juvenile crust formation in the NCC. We interpret the data presented here to represent a phase of major late Neoarchaean arc magmatism along the western margin of the Jiaoliao Block related to the birth of microcontinental nuclei within the NCC. Our data suggest that the Western and Eastern Blocks might not have existed as discrete crustal blocks, and that the construction of the NCC is a result of the assembly of several microblocks or terranes at the end of Archaean. Similar Archean cratonic nuclei in other regions of the world might have formed part of a primitive supercontinent in the early Earth.

Retrograde metamorphism of the eclogite in North Qaidam, western China: Constraints by joint 40Ar/39Ar in vacuo crushing and stepped heating

Two amphiboles and a syn-metamorphic quartz vein from the Yuka terrane, North Qaidam, western China, have been analyzed by joint 40Ar/39Ar crushing in vacuo and stepwise heating techniques. The crushing in vacuo results provide information to directly constrain the timing of fluid activity and the age of amphibolite-facies retrogression. The stepwise heating results could further be used to decipher the thermal history of the UHP rocks. Amphiboles from amphibolites and quartz vein within garnet-amphibolite lens analyzed by in vacuo crushing yield similarly shaped age spectra and exhibit relatively flat age plateaus for the last several steps. The characteristics of gas release patterns and geochronological data testify to the presence of significant excess 40Ar within the fluid inclusions. The age plateaux with weighted mean ages (WMA) ranges from 488 to 476 Ma for amphiboles and 403 Ma for quartz (2σ). These data points constitute amphibole WMA yielding excellent isochrons with isochron ages of 469 and 463 Ma with initial 40Ar/36Ar ratios of 520 and 334, respectively. The isochron ages are interpreted to represent initial amphibolite-facies retrogression. The data points constituting the quartz age plateaux give an isochron age of 405 Ma with initial 40Ar/36Ar ratio of 295, recording a significant aqueous fluid flow episode during the early Devonian. Age spectra obtained by stepwise heating of amphibole residues remaining after crushing experiments are characterized by younger and relatively complex age spectra, which are probably influenced by the combined effects of resetting argon and/or mineral inclusions. Nevertheless, we note that the spectra shapes have features in common: excluding the last two steps, minimum apparent ages are found at temperatures of around 500 °C, corresponding to 319 and 249 Ma, perhaps representing the time of isotopic resetting or resulting from release gas from mineral inclusions of, e.g., biotite or feldspar. Maximum apparent ages are obtained at temperatures of around 800 °C, corresponding to 418 and 413 Ma, which probably reflect mixed ages of radiogenic resetting and original amphibole. These results indicate that the Yuka eclogites and their retrogressed equivalents were overprinted by multiple thermal events in the Silurian and possibly as young as the Triassic.

Sr and Nd isotopic compositions of apatite reference materials used in U–Th–Pb geochronology

Apatite is an important common U- and Th-bearing accessory mineral in igneous, metamorphic and clastic sedimentary rocks. The advent of in situ U–Th–Pb apatite geochronology by the SIMS and LA-(MC)-ICP-MS methods has demonstrated the importance of having uniform and homogeneous reference materials. Recently, it has been shown that Sr and Nd isotopic data combined with U–Pb age and trace element concentration data can provide important constraints on apatite paragenesis because this phase usually exhibits high Sr and REE concentrations but has low Rb/Sr ratios which result in negligible corrections for the ingrowth of radiogenic Sr. However, as apatite can potentially have complex internal structures resulting from multiple thermal events, such as inherited cores and metamorphic overgrowths, requires that the Sr and Nd isotopic data should be measured with high spatial resolution. However isobaric interferences hamper the precise determination of Sr or Nd isotopic compositions in LA-MC-ICP-MS analysis. In this work we undertook in situ measurements of Sr and Nd isotopic compositions of eleven apatite reference materials (AP1, AP2, Durango, MAD, Otter Lake, NW-1, Slyudyanka, UWA-1, Mud Tank, McClure Mountain and SDG) commonly used in U–Th–Pb geochronology. Our obtained Sr and Sm–Nd isotopic compositions for these apatite samples are consistent with those values obtained by solution-based methods (isotope dilution and ion chromatography) using MC-ICP-MS or TIMS, which demonstrates the reliability and robustness of our analytical protocol.