High Closure Temperature Research Articles

Multi-stage ultrahigh temperature metamorphism in the lower crust of the Kaapvaal craton recorded by U–Pb ages of garnet

U–Pb ages were determined by split-stream LA-SF/MC-ICPMS in garnets from UHT granulite xenoliths (Star mine, South Africa; 124 Ma). They give a considerable age range of 400 million years with well-defined maximas at 3.09, 3.01 and 2.75 Ga. The oldest peak overlaps with the changeover from tonalites to K-granites at 3.14–3.04 Ga and with zircon ages of the mid-crustal granulites of the Vredefort dome (3.1 Ga) in the wake of the 3.2 Ga collision of three terrains that compose the Witwatersrand block. Subduction (or sagduction) of the uppermost crust in an ultrahot orogen setting brought shales and greenstones to the lower crust. Ultrahigh temperature (UHT) conditions are the result of high mantle potential temperatures and self- heating by the radioactive inventory of the subducted lithologies. Metamorphism, anatexis to very high degrees and melt extraction left UHT granulites as residue. Rejuvenation of UHT conditions was brought about by Dominion Group magmatism between 3.0 and 2.95 Ga. Magmatic uprise caused intense shearing in the lower crust followed by recrystallisation of the shear zones to generate the younger garnet age group. Ventersdorp flood basalt volcanism caused similar processes at around 2.72 Ga and generated the third garnet age group. Zircon gives U–Pb ages mainly around 2.72 Ga (both literature and our own data) i.e. zircon adjusted or newly crystallized at the youngest UHT event. Only few zircon grains retained older ages up to 2.94 Ga. Still unconstrained, but very high closure temperatures (≥ 1100 °C) for the U–Pb system in garnet keep the memory of the oldest ages in UHT granulites. Such ages can only be reset by recrystallization. This way, garnet records a prolonged high-temperature history of the lower crust of the Kaapvaal craton.

Melting processes, cooling rates, and tectonic settings of the Neo-Tethyan mantle: the in-situ mineral chemical record

Peridotites in the Yarlung Zangbo ophiolite (YZO) have the potential to elaborate the mantle dynamics that operated below the Neo-Tethyan oceanic crust. Here, we measured major and trace element concentrations of minerals (e.g., clinopyroxene, orthopyroxene, olivine, and spinel) in YZO harzburgites and lherzolites to trace the origin and evolution of the YZO. The rare earth element (REE) patterns of clinopyroxenes in lherzolites and harzburgites plot within the field of abyssal and forearc peridotites, respectively, indicating that the latter experienced a higher degree of partial melting. Based on the relative extents of light (LREE) and heavy (HREE) REE depletions in clinopyroxene, we classified the lherzolites into two groups (I and II). The group I clinopyroxenes are depleted in HREEs and enriched in LREEs and MREEs compared to those of group II. Meanwhile, the REE patterns of harzburgite clinopyroxenes are identical to that of group II, although they display lower REE concentrations. Models based on REE abundances in clinopyroxene suggest that group I lherzolites experienced ~8–11% partial melting in the spinel domain, whereas group II lherzolites formed after 5% melting in the garnet domain followed by 6–9% and 9–16% partial melting in the spinel domain, respectively. These high degrees of melting may suggest that the harzburgites formed in a supra-subduction zone (SSZ) setting. Surprisingly, the REE-in-two-pyroxenes thermometer (TREE) shows that the YZO peridotites record high-temperature cooling at 991–1218 °C which is lower than the conditions below mid-oceanic ridges (MORs), but compatible with recent reports of high closure temperatures in forearc environments. Furthermore, our TREE results constrain the cooling rate of the YZO peridotites to ~0.01–0.5 °C/yr, overlapping with rates reported for SSZ, MOR, and abyssal peridotites. By integrating the petrological, geochemical, and thermal features, we tentatively interpret the group I lherzolites accreted in an Early Cretaceous forearc during subduction initiation, whereas the group II lherzolites and the harzburgites formed by further melting of the earlier lherzolites in a SSZ setting.

Al and H incorporation and Al-diffusion in natural rutile and its high-pressure polymorph TiO 2 (II)

Abstract Rutile is an important accessory mineral in metamorphic rocks and is used as a geothermobarometer or geochronometer. This study aims to bridge the gap between diffusion studies in simplified and complex natural systems by investigating the incorporation and mobility of Al in natural rutile and its high-pressure polymorph TiO 2 (II). Experiments were performed at 0.1 MPa to 7 GPa, 1223–1373 K, at buffered μ(Al 2 O 3 ) and with f O 2 constrained to ≤CCO, which is the equilibrium between graphite and a CO-CO 2 gas phase. Based on electron probe microanalysis, secondary ion mass spectrometry and Fourier transform infrared analyses, we suggest a complex combination of mechanisms to explain the incorporation of Al and H in natural rutile and TiO 2 (II). This includes: (1) the incorporation of Al 3+ on octahedral Ti-sites charge balanced by the formation of oxygen vacancies; and (2) the incorporation of oxygen in interstitial positions charge balanced by hydrogen interstitials. Determined Al-diffusivities in natural TiO 2 are approximately eight to nine orders of magnitude faster compared to previously published data. A possible explanation includes a significantly enhanced rate of ionic diffusion through the combined effect of hydrolytic weakening, enhanced Al-diffusion through extended defects and to a minor extent oxygen fugacity variations. Consequently, results of this study question that the inferred high closure temperatures for the Al-in-rutile geothermobarometer can be applied to all natural systems.

Effect of heterogeneity on the diffusion of Pb in apatite for petrochronological applications: A multiscale approach to characterizing the influence of apatite chemistry and anisotropy on Pb diffusion

The investigation of various factors effecting the Lead (Pb) diffusion in phosphate minerals such as apatite is still challenging in the interpretation of (UTh)/Pb geochronology. For (UTh)/Pb system, apatite minerals have closure temperatures in the range of 375 to 600 °C and therefore can be used for the investigation of mid-temperature thermochronological and/or petrochronological questions i.e., the reconstruction of thermal events in Earth's crust. There is still uncertainty whether Pb diffusion in apatite is characterized by thermally activated volume and/or anisotropic diffusion profiles or is instead impacted by novel growth processes and recrystallization (chemical substitutions). As the apatite structure support extensive compositional variability, including partial or total substitution of both the cationic and anionic sites and forms solid solutions therefore, it necessitates a thorough examination of these effects and anisotropy on Pb diffusivity and (UTh)/Pb geochronometric system. For this, a multi-scale study is carried out to examine the effects of chemical composition, anisotropy, and growth structure on the diffusion of Pb in order to better understand the behaviour of Pb diffusion in apatite. This study employed computational techniques like Density Functional Theory (DFT) and Transition State Theory (TST) at the atomic level and integrates it with the Kinetic Monte Carlo (KMC) simulations at the macroscopic level. Models of this study shows that Pb diffusion is completely anisotropic along the preferred z-axis or [001] direction and Pb readily escapes faster from Na-substituted apatite when compared to pure F-apatite and Cl-substituted apatite. Because of this anisotropy and chemical substitutions, Pb diffusivity in apatite either increases by opening of diffusion channels or decreases by blocking the diffusion channels depending on the site and type of chemical substitution. Further, in case of blocking effect the Pb diffusion occurs through workaround pathways and approaches towards the isotropic diffusion. For Na-substituted apatite, the impact of Na occupation on anisotropic Pb diffusion is significantly greater while in case of Cl-substituted apatite the Cl occupation mostly leads towards isotropic diffusion by opening the diffusion paths along other directions (mostly along the in-plane direction). Furthermore, the high closure temperatures (Tc) (e.g., ∼1370 °C) of the modelled apatites (except the perfect Na-substituted apatite e.g., ∼500 °C) of this study when compared to the Tc of Durango apatite obtained experimentally for the effective grain size of 100 μm and cooling rates of 10 °C/Ma indicate that the effective closure temperature dominantly depends on the degree and types of chemical substitutions and play a crucial role for the closure or opening of Pb diffusion/loss in apatites.

Microstructures, geochemistry, and geochronology of mica fish: Review and advances

Shear zones are one of the most conspicuous structures in orogenic and rifting settings, accommodating high strain, rock displacement and influencing magma emplacement and fluid flow. Since shear zones may be simultaneously or sequentially affected by multiple processes involving mineral reactions, variations in pressure-temperature conditions, fluid-rock interaction, and diffusion, determining the timing of such structures has been one of the major challenges for modern geochronology. Although low- (up to lower greenschist facies) and high-temperature shear zones (above amphibolite facies) are well-dated through low- and high-closure temperature minerals, medium-temperature shear zones developed within the critical temperature window of ∼450–550 °C, in which conventional chronometers such as 40Ar/39Ar and Rb–Sr applied to mica fish may or may not record the timing of deformation for multiple reasons (e.g., grain size, cooling rate, mineral composition, fluid activity, deformation, neo- and recrystallization). Here, we review the current knowledge on the evolution of mica fish and the effect of deformation on its chemical and isotopic systems. We evaluate the effect on the widely deployed in situ 40Ar/39Ar technique. Furthermore, we demonstrate the potential to assess mica fish evolution applying high-spatial resolution microstructural and chemical mapping techniques such as electron backscatter diffraction (EBSD), time-of-flight secondary ion mass spectrometry (ToF–SIMS) and in situ Rb–Sr via triple quadrupole inductively coupled plasma mass spectrometry (TQ-ICP-MS) to a case study of medium-temperature mylonites from the well-characterized Taxaquara shear zone, SE Brazil. We show that mica fish display complex microstructures with variable strain intensity, commonly with low strain inner cores and high strain edges and along kink planes. Strain shadows in mica fish are commonly characterized by low-strain fine-grained muscovite, suggesting recrystallization coeval with ductile deformation. Despite being intensely deformed, muscovite fish Rb–Sr retain the protolith age (c. 600 Ma), whereas recrystallized fine-grained muscovite yields the timing of deformation (c. 550–540 Ma). Synthetic shear bands cross-cutting coarse-grained muscovite fish induce muscovite recrystallization consistent with their distinct chemistry, with recrystallized muscovite characterized by higher Fe–Mg and lower Na suggesting fluid-assisted recrystallization under lower temperature compared to the muscovite fish host. We propose that these shear bands across mica fish play an important role by accommodating grain size reduction and subsequent deformation, leading to the formation of smaller individual mica fish. Grain size reduction, likely enhanced by dynamic precipitation (i.e., coeval crystal-plastic deformation and dissolution-precipitation creep), appears as the key recrystallization mechanism that allows low strain muscovite in strain shadows and shear bands to record the timing of deformation in medium-temperature shear zones, consistent with qualitative Sr diffusion modelling.

The youngest lunar zircon reveals an extremely fractionated nature of Chang’e-5 basalt

The U(Pb)-Pb age of zircon is commonly used to represent the crystallization age for igneous rocks due to its high closure temperature and robust resistance to impact disturbance. However, no zircon crystallization age has yet been reported for Chang'e-5 (CE-5) basalt due to their limited occurrence in mare basalt. In this study, rare zircon grains from CE-5 lunar samples were investigated by the in-situ Pb isotopic analysis, and a precise zircon crystallization age of 2036 ± 19 Ma was determined from Pb-Pb isochron. This is hitherto the youngest reported crystallization age of lunar zircon, similar to the ages of CE-5 lunar basalts obtained by zirconium (Zr)-bearing minerals such as baddeleyite, tranquillityite, and zirconolite. Petrographic evidence and rare-earth element geochemistry indicate that zircon in the CE-5 lunar basalts were formed by the reaction of early-formed baddeleyite with SiO2 melt within the latest residue of extreme fractionation of a non-KREEP (an acronym for potassium, REE, and phosphorus) basaltic magma. In contrast to the prevailing view that lunar zircon have formed in late-stage enriched melts resulting from extensive fractional crystallization of the Lunar Magma Ocean, this study shows that zircon could be derived from extreme fractionation of non-KREEP basaltic magma unrelated to Lunar Magma Ocean.

Apatite laser ablation Lu[sbnd]Hf geochronology: A new tool to date mafic rocks

Mafic rocks are the most common type of igneous rocks on Earth, however, constraining the crystallization age of mafic rocks can be challenging. Apatite is a common accessory phase in mafic rocks and is amenable to dating using the U–Pb system. However, the U–Pb system in apatite has a relatively low closure temperature (∼350°-550 °C) and is therefore prone to resetting by later thermal and metasomatic events. Here, a recently developed LuHf dating method using laser ablation reaction-cell mass spectrometry is applied to apatite from mafic rocks. The LuHf system in apatite has a higher closure temperature (∼650°-750 °C) compared to U–Pb, increasing the chances of obtaining primary crystallization ages. Furthermore, the laser-ablation method allows rapid data collection compared to traditional solution-based LuHf dating techniques. Four study areas were selected to compare the LuHf vs U–Pb systematics of apatite in mafic igneous rocks: the Paleoproterozoic Sudbury Igneous Complex (Canada), the Neoproterozoic Borborema Province (NE Brazil), the Paleoproterozoic Fennoscandian Shield (Finland), the Archean Yilgarn Craton and adjacent Mesoproterozoic Albany Fraser Orogen (Western Australia). For all analyzed samples that have apatite trace element compositions typical of an undisturbed primary mafic igneous lithology, the LuHf system retains primary igneous apatite crystallization ages, whereas the U–Pb system in the same grains often records isotopic disturbance or a cooling age. In few cases, the LuHf system has also been disturbed in response to recrystallization, however, such disturbance is readily detected with trace element data. Hence, this study demonstrates the potential of laser ablation apatite LuHf dating to obtain primary crystallization ages for otherwise difficult to date mafic rocks.

In-situ U[sbnd]Pb geochronology of vesuvianite in skarn deposits

The timing of hydrothermal activity is a key factor in any predictive genetic model of skarn mineralization, but direct dating is often hindered by the absence of suitable minerals to date. Vesuvianite, a common mineral in skarn deposits, can incorporate U and has a high Pb closure temperature (>650 °C), and consequently may be suitable for dating for skarn deposits. Vesuvianite collected from nine Chinese skarn deposits, including the Jinchuantang SnBi, Shizhuyuan W–Sn–Mo–Bi polymetallic, Jiepai CuW, Dashunlong Sn, Xianghualing Sn, Furong Sn, and Huangshaping W–Sn–Mo–Cu polymetallic deposits in Nanling Range (Southern China), and the Kaerqueka and Saishitang Cu deposits in East Kunlun Orogen (Northwestern China), have high U concentrations (median 66.8 ppm for 460 LA-ICP-MS analysis spots). The flat and stable time-resolved signal spectra for U during LA-ICP-MS analysis, combined with the absence of nanoscale mineral inclusions, porosities, or lattice defects in the JCT vesuvianite, with high and consistent U concentrations, revealed by the HAADF-STEM images, and the marked positive relationships between U and REE collectively suggest that the U incorporation into vesuvianite was primarily controlled by replacement of Ca2+ in eight-fold coordination, confirming that vesuvianite may be a suitable UPb geochronometer. The JCT vesuvianite is characterized by high and consistent U concentrations (260–526 ppm, median 307 ppm) and homogeneous internal texture and elemental distribution, and yields an ID-TIMS UPb concordia age (2σ, 153.9 ± 0.2 Ma, MSWD = 1.6, n = 11), demonstrating that it can be taken as a potential LA-ICP-MS UPb reference material, which can be freely distributed to other laboratories. Using the JCT vesuvianite as a UPb reference material, the Kaerqueka, Saishitang, Jiepai, Dashunlong, Xianghualing, Furong, and Shizhuyuan vesuvianite yielded well-defined LA-ICP-MS UPb ages (239.0–152.1 Ma, 2σ), consistent with previously published age constraints of mineralization and cogenetic igneous rocks. Vesuvianite from the Huangshaping W–Sn–Mo–Cu polymetallic deposit, yielded UPb ages of 157.3 ± 1.4 Ma (2σ, MSWD = 1.8) and 161.0 ± 1.3 Ma (2σ, MSWD = 1.9), providing reliable constraints on the overprinting sequence of the skarn W–Sn–Mo and Cu mineralization events at Huangshaping, which was not previously detectable using other mineralization chronometers. Combined, these ages suggest that vesuvianite is a robust and reliable geochronometer, and offers a new tool for dating skarn deposits.

Decoupling of metamorphic zircon U-Pb ages and P-T paths in the Dunhuang metamorphic complex, northwestern China

U-Pb dating of metamorphic zircon is commonly used to determine the timing of high-grade metamorphism, but linking zircon U-Pb ages to metamorphic P-T paths can be challenging, especially in metamorphic terranes where multiple metamorphic events result in various degrees of re-equilibration and complex metamorphic zircon growth/recrystallization. Here we carried out a detailed P-T path study, as well as zircon/titanite/apatite U-Pb dating and mineral Sm-Nd isochron dating, for garnet amphibolites from the Dunhuang Block, northwestern China, where late Paleoproterozoic metamorphism was extensively overprinted by a Paleozoic tectonothermal event and the P-T conditions of the earlier event remain elusive. Petrographic observation, phase equilibria modeling and geothermobarometry show that the garnet amphibolites share a clockwise P-T path, from 675 – 689 °C/10.0 – 11.6 kbar to 780 – 807 °C/9.9 – 11.8 kbar for the prograde to peak metamorphism, followed by significant cooling and decompression to 612 – 689 °C/4.1 – 6.0 kbar, indicating an orogenic setting. U-Pb dating of metamorphic zircons yields two groups of upper intercept ages of ca. 1.82 – 1.85 and 1.95 Ga. The ca. 1.82 – 1.85 Ga metamorphic zircons have flat HREE patterns and weak Eu* anomalies, suggesting zircon growth when abundant garnet but little plagioclase was present, consistent with high-pressure metamorphism. In contrast, the ca. 1.95 Ga zircons have steep HREE and negative Eu anomalies, suggesting formation at relatively low-pressures. These zircon results provide evidence for 1.8 – 1.9 Ga metamorphism, possibly due to variable crustal thickening during a late Paleoproterozoic orogenic event. However, garnet porphyroblasts and the matrix minerals yield a Sm-Nd isochron age of 333.5 ± 4.0 Ma, whereas metamorphic titanites record a U-Pb age of 345.4 ± 2.7 Ma. Considering the high closure temperatures of the garnet Sm-Nd (650 – 720℃) and titanite U-Pb (710 – 780℃) systems, our data suggest that the late Paleoproterozoic metamorphic mineral assemblages were likely completely re-equilibrated by a Paleozoic tectonothermal event and that the P-T path documented above is a result of Paleozoic orogeny. In addition, apatite U-Pb dating yields 236.6 ± 4.3 and 300 ± 10 Ma for samples from the southern and central Dunhuang area, respectively, indicating different cooling histories. Combining with previous results, we propose that the Paleozoic metamorphism occurred in a long-lived hot continental arc with slow cooling rates (0.3 – 5℃/Myr), rather than an subduction–accretionary complex. Our study highlights the decoupling of metamorphic zircon U-Pb ages and P-T paths, which has significant implications for P-T-t path studies in polyphase metamorphic terranes.

Detrital apatite Lu–Hf and U–Pb geochronology applied to the southwestern Siberian margin

AbstractApatite is increasingly used in sedimentary provenance studies. However, detrital apatite U–Pb geochronology can be challenging due to the presence of non‐radiogenic Pb, its intermediate closure temperature (~350–550°C) and/or age‐resetting by metamorphic/metasomatic processes. The Lu–Hf system in apatite has a higher closure temperature (~675–750°C) and is, therefore, more robust to thermal resetting. Here we present the first detrital apatite Lu–Hf age spectra. We have developed a laser‐ablation Lu–Hf dating technique, using reaction‐cell mass spectrometry, that allows rapid cost‐effective analysis, required for detrital apatite studies. The method is best suited to Precambrian detritus, permitting greater radiogenic Hf ingrowth. Using samples from Siberia, we demonstrate: (1) excellent correlations between U–Pb and Lu–Hf dates for apatites from igneous protoliths; and (2) that Lu–Hf dating can detect primary age information in metamorphic grains. Hence, when used in tandem with U–Pb zircon and apatite geochronology, Lu–Hf apatite dating provides a powerful new tool for provenance studies.

Temperatures and Cooling Rates Recorded by the New Caledonia Ophiolite: Implications for Cooling Mechanisms in Young Forearc Sequences

AbstractTo unveil how forearc lithosphere cools and re‐equilibrates, we carried out a comprehensive geothermometric investigation of the New Caledonia ophiolite, which represents a rare example of proto‐arc section generated during subduction infancy. A large data set, including more than 80 samples (peridotites and mafic‐ultramafic intrusives), was considered. Closure temperatures calculated for the lherzolites using slow (TREE‐Y) and fast diffusing (TCa‐in‐Opx, TBKN, TCa‐in‐Ol, TOl‐Sp) geothermometers provide some of the highest values ever documented for ophiolitic peridotites, akin to modern sub‐oceanic mantle. Cooling rates deduced from TREE‐Y and TBKN yield values of ≈10−3°C/y, similar to those obtained with TCa‐in‐Ol. These features are consistent with a post‐melting history of emplacement, possibly along a transform fault, and thermal re‐equilibration via conduction. Cpx‐free harzburgites register a high‐T evolution, followed by quenching and obduction. The relatively high TCa‐in‐Ol, TOl‐Sp and cooling rates computed from TCa‐in‐Ol (≈10−3°C/y) are atypical for this geodynamic setting, mirroring the development of an ephemeral subduction system, uplift and emplacement of the Peridotite Nappe. Temperature profiles across the crust‐mantle transect point to high closure temperatures, with limited variations with depth. These results are indicative of injection and crystallization of non‐cogenetic magma batches in the forearc lithosphere, followed by thermal re‐equilibration at rates of ≈10−4–10−3°C/y. Our study shows that the thermal conditions recorded by forearc sequences are intimately related to specific areal processes and previous lithospheric evolution. Thus, detailed sampling and exhaustive knowledge of the geological background are critical to unravel the cooling mechanisms in this geodynamic setting.

Chronological constraints on the thermal evolution of ordinary chondrite parent bodies from the 53Mn-53Cr system

The 53Mn-53Cr isotope systematics in ordinary chondrites constrains the accretion and thermal history of their parent bodies. Mineralogical observations and olivine-spinel geothermometry suggest that chromite in ordinary chondrites formed during prograde thermal metamorphism with the amount of chromite increasing with petrologic grades in type 3 to type 6 ordinary chondrites. Assuming a chondritic evolution of the respective parent bodies, 53Cr/52Cr model ages for chromite range from 3.99-0.79+0.93 to 11.1-2.8+6.0 Ma after the formation of calcium-aluminium-rich inclusions (CAIs). Chromite and silicate-metal-sulphide isochrons define an age range from 2.78-0.50+0.55 to 15.4-1.6+2.4 Ma. Both chromite model ages and isochron ages correlate with the petrological grade of the samples, which is consistent with an onion-shell structure of the chondrite parent bodies. The study shows that unlike the isochron ages, which are prone to impact-related disturbances or partial re-equilibration during cooling from high temperatures, the chromite model ages are not easily affected by thermal metamorphism or later events and yield robust mineral growth ages. The results are consistent with a homogenous distribution of 53Mn and an initial canonical 53Mn/55Mn = 6.28 × 10−6. The estimated closure temperatures for the Mn-Cr system in chromites range from ~760 °C for type 6 to ~540–620 °C for type 3 ordinary chondrites. The high closure temperatures estimated for type 3 and type 6 ordinary chondrites imply that the chromite ages correspond to the peak metamorphic temperature reached during the thermal history of the chondrite parent bodies. The oldest chromite model age obtained for type 3 samples along with the established Al-Mg chondrule formation ages constrain the accretion of the parent bodies to >2.1 Ma after CAI formation, implying that planetesimal accretion immediately followed chondrule formation.

Building an Orogen: Review of U-Pb Zircon Ages from the Calabria–Peloritani Terrane to Constrain the Timing of the Southern Variscan Belt

The application of zircon dating to the reconstruction of orogenic systems is invaluable since time constraints of the geological evolution of orogens are crucial for the proposal of geodynamic and paleogeographic models. Zircon is one of the most promising accessory minerals in geochronology of crystalline basements because of its high-closure temperature. Moreover, U-Pb data of relict and recrystallized grains indicate the maximum sedimentation age as well as the timing of metamorphism in metasediments. In addition, the U-Pb ages of magmatic zircons constrain the timescale of magmatism. The Calabria–Peloritani terrane (CPT) represents a key area in the Southern Variscan Belt, whose reconstruction is still unresolved. Therefore, a review of literature zircon age data accompanied with new data from six samples of orthogneisses, paragneisses, amphibolites, and actinolite schists, helps to constrain the evolution of this Cadomian fragment, affected by metamorphic and magmatic Variscan events. A revisiting of the timing of the geological events from Paleo-proterozoic to Permian is revealed by comparing the internal textures of zircons and their U-Pb age clusters. The detected age peaks at 2500 Ma, 1600 Ma, and 1000 Ma in the CPT were related to a provenance from West and East Gondwana realms. A sedimentation age around 630 Ma emerges for the middle-deep crust terranes of the CPT, affected by Ediacaran (579–540 Ma) intrusions, accompanied by metamorphism dated at 556–509 Ma in the host metasediments. In the following, during Ordovician–Silurian extensional tectonics, the former Cadomian terranes were at least locally affected by fluid-assisted metamorphism (around 450 Ma) whereas the upper extensional basins that formed, were infilled by sediments along with interspersed volcanic to subvolcanic products. All these pre-Silurian terranes were involved in the subduction process of the Palaeotethys–Gondwana margin beneath Laurussia. The compressive phase began around 347 Ma, with under-thrusting of the formerly Gondwana substrate that was subjected to middle-high grade metamorphism, while the Ordovician–Silurian sediments were scraped off along the front of the Southern Variscan Belt and affected by low-grade metamorphism. Decompression of the whole Variscan orogenic system started around 320 Ma, together with uplifting of the chain and emplacement of widespread granitic intrusions which ended around 280 Ma and completed the Variscan orogenic cycle in the CPT.

Division of crustal units in China using grid-based clustering and a zircon U–Pb geochronology database

Zircon U–Pb dating is one of the most effective and widely used methods due to the widespread existence of zircon as an accessory mineral in rocks and its high closure temperature. Throughout history, each region of the earth has experienced its own crustal evolution. Using zircon U–Pb geochronology data, researchers can reveal the crustal growth history and regional discrimination. In this study, a large spatial analysis was performed across China, involving more than 120,000 data records taken from the zircon U–Pb geochronology database. By applying the Gaussian multipeak fitting method, nine age peaks corresponding to crustal growth events on the continent of China were identified, which centered at 45, 132, 249, 441, 815, 1236, 1908, 2505, and 2933 Ma. Using a grid-based clustering algorithm, grids with similar crustal growth event combinations were gathered and their features extracted. Based on the time series of the crustal growth events, the gathered grid clusters were further integrated into larger crustal units. We used various grid sizes in our algorithm, including 0.325 ° × 0.325 °, 0.4 ° × 0.4 °, 0.6 ° × 0.6 °, 0.8 ° × 0.8 °, 1.0 ° × 1.0 °, 1.2 ° × 1.2 °, and 1.4 ° × 1.4 °. Although the shape and number of the crustal units varied depending on grid size, six large entities were clearly identified and were best visualized using the 0.8 ° × 0.8 °, 1.0 ° × 1.0 °, 1.2 ° × 1.2 °, and 1.4 ° × 1.4 ° grids. These six entities include the Tibet-Sichuan-Yunnan, North Xinjiang, Northeast, Gansu-Qinghai, North China, and South China units. Furthermore, while there were some variations, the crustal unit results found herein were in agreement with tectonic unit research, displaying a new potential avenue of research.

Closed system behaviour of argon in osumilite records protracted high‐Tmetamorphism within the Rogaland–Vest Agder Sector, Norway

AbstractHere, we present results of the first40Ar/39Ar dating of osumilite, a high‐Tmineral that occurs in some volcanic and high‐grade metamorphic rocks. The metamorphic osumilite studied here is from a metapelitic rock within the Rogaland–Vest Agder Sector, Norway, an area that experienced regional granulite facies metamorphism and subsequent contact metamorphism between 1,100 Ma and 850 Ma. The large grain size (~1 cm) of osumilite in the studied rock, which preserves a nominally anhydrous assemblage, increases the potential for large portions of individual grains to have remained essentially unaffected by the effects of diffusive argon loss, potentially preserving prograde ages. Step‐heating diffusion experiments yielded a maximum activation energy of ~461 kJ/mol and a pre‐exponential factor of ~8.34 × 108 cm2/s for Ar diffusion in osumilite. These parameters correspond to a relatively high closure temperature of ~620°C for a cooling rate of 10°C/Ma in an osumilite crystal with a 175 µm radius. Fragments of osumilite separated from the sample preserve a range of ages betweenc. 1,070 and 860 Ma. The oldest ages are inferred to date the growth of coarse‐grained osumilite during prograde granulite facies regional metamorphism, which pre‐date contact metamorphism that has historically been ascribed to the growth of osumilite in the region. The majority of fragments record ages betweenc. 920 and 860 Ma, inferred to reflect the growth of osumilite and/or diffusive argon loss during contact metamorphism. The retention of old40Ar/39Ar dates was facilitated by the low diffusivity of Ar in osumilite (i.e. a closed system), large grain sizes, and anhydrous metamorphic conditions. The ability to date osumilite with the40Ar/39Ar method provides a valuable new thermochronometer that may constrain the timing and duration of high‐Tmagmatic and metamorphic events.

A Potential (U‐Th)/He Zircon Reference Material from Penglai Zircon Megacrysts

In this study (U‐Th)/He dating of the Penglai zircons, which occur as abundant megacrysts in Neogene alkaline basalts in northern Hainan Province, south‐eastern China, was undertaken. A weighted mean age of 4.06 ± 0.35 Ma (2s) with a mean square weighted deviation (MSWD) of 1.79 was obtained from eighteen fragments of four zircon megacrysts using single‐crystal laser fusion He determinations and the U‐Th isotope dilution (ID) method. The (U‐Th)/He ages are consistent, homogeneous and systematically slightly younger than the preferred206Pb/238U age of 4.4 ± 0.1 Ma (95% confidence interval) determined byID‐TIMSand subsequently published U‐Pb results. The U‐Pb isotopic system in zircon has a high closure temperature of ~ 900 °C, and the preferred U‐Pb age may record both the time since eruption and the zircon residence time in the magma chamber. In contrast, the closure temperature of the zircon (U‐Th)/He system is ~ 190 °C and the zircon megacrysts were brought quickly to the surface by the host basaltic magma. Thus, the (U‐Th)/He age represents the timing of the eruption. Based on the unlimited quantity, large grain size, mostly weak broad zoning, rapid cooling and homogenous (U‐Th)/He ages, we consider the Penglai zircons suitable for use as a reference material in (U‐Th)/He isotope geochronology.

Trace element thermometry of garnet-clinopyroxene pairs

We present major and trace element data on coexisting garnet and clinopyroxene from experiments carried out between 1.3 and 10 GPa and 970 and 1400 °C. We demonstrate that the lattice strain model, which was developed for applications to mineral-melt partitioning, can be adapted to garnet-clinopyroxene partitioning. Using new and published experimental data we develop a geothermometer for coexisting garnet and clinopyroxene using the concentration of rare earth elements (REE). The thermometer, which is based on an extension of the lattice strain model, exploits the tendency of minerals at elevated temperatures to be less discriminating against cations that are too large or too small for lattice sites. The extent of discrimination against misfit cations is also related to the apparent elasticity of the lattice site on which substitution occurs, in this case the greater stiffness of the dodecahedral X-site in garnet compared with the eightfold M2-site in clinopyroxene. We demonstrate that the ratio of REE in clinopyroxene to that in coexisting garnet is particularly sensitive to temperature. We present a method whereby knowledge of the major and REE chemistry of garnet and clinopyroxene can be used to solve for the equilibrium temperature. The method is applicable to any scenario in which the two minerals are in equilibrium, both above and below the solidus, and where the mole fraction of grossular in garnet is less than 0.4. Our method, which can be widely applied to both peridotitic and eclogitic paragenesis with particular potential for diamond exploration studies, has the advantage over commonly used Fe-Mg exchange thermometers in having a higher closure temperature because of slow interdiffusion of REE. The uncertainty in the calculated temperatures, based on the experimental data set, is less than ±80 °C.

Variable helium diffusion characteristics in fluorite

Precise analysis of the diffusion characteristics of helium in fluorite is crucial for establishing the new fluorite (U–Th–Sm)/He thermochronometer (FHe), which potentially provides a powerful tool for dating ore deposits unsuitable for the application of conventional geochronometers. Incremental helium outgassing experiments performed on fluorites derived from a spectrum of geological environments suggest a thermally activated volume diffusion mechanism. The diffusion behaviour is highly variable and the parameters range between logD0/a2=0.30±0.27–7.27±0.46s−1 and Ea=96±3.5–182±3.8kJ/mol. Despite the fact that the CaF2 content of natural fluorites in most cases exceeds 99 weight percent, the closure temperature (Tc) of the fluorite (U–Th–Sm)/He thermochronometer as calculated from these diffusion parameters varies between 46±14°C and 169±9°C, considering a 125μm fragment size. Here we establish that minor substitutions of calcium by rare earth elements and yttrium (REE+Y) and related charge compensation by sodium, fluorine, oxygen and/or vacancies in the fluorite crystal lattice have a significant impact on the diffusivity of helium in the mineral. With increasing REE+Y concentrations F vacancies are reduced and key diffusion pathways are narrowed. Consequently, a higher closure temperature is to be expected. An empirical case study confirms this variability: two fluorite samples from the same deposit (Horni Krupka, Czech Republic) with ca. 170°C and ca. 43°C Tc yield highly different (U–Th–Sm)/He ages of 290±10Ma and 79±10Ma, respectively. Accordingly, the fluorite sample with the high Tc could have quantitatively retained helium since the formation of the fluorite-bearing ores in the Permian, despite subsequent Mesozoic burial and associated regional hydrothermal heating. In contrast, the fluorite with the low Tc yields a Late Cretaceous age close to the apatite fission track (AFT) and apatite (U–Th)/He ages (AHe) from the same locality. Remarkably, thermal modelling of FHe yields comparable results to the well-established modelling based on AFT and AHe.

Highly retentive core domains in K-feldspar and their implications for 40Ar/39Ar thermochronology illustrated by determining the cooling curve for the Capoas Granite, Palawan, The Philippines

K-feldspar from the late Miocene Capoas Granite on Palawan in The Philippines appears to contain highly retentive diffusion domains that are closed to argon diffusion at near-solidus temperatures during cooling of this ∼7 km-diameter pluton. This is an important result, for K-feldspar is commonly considered not retentive in terms of its ability to retain argon. Closure temperatures for argon diffusion in K-feldspars are routinely claimed to be in the range ∼150–400°C but the release of 39Ar from irradiated K-feldspar during furnace step-heating experiments in vacuo yields Arrhenius data that imply the existence of highly retentive core domains, with inferred closure temperatures that can exceed ∼500–700°C. These high closure temperatures from the Capoas Granite K-feldspar are consistent with the coincidence of 40Ar/39Ar ages with U–Pb zircon ages at ca 13.5 ± 0.2 Ma. The cooling rate then accelerated, but the rate of change had considerably slowed by ca 12 Ma. Low-temperature (U–Th)/He thermochronology shows that the cooling rate once again accelerated at ca 11 Ma, perhaps owing to renewed tectonic activity.

Magma mixing and crust–mantle interaction in Southeast China during the Early Cretaceous: Evidence from the Furongshan granite porphyry and mafic microgranular enclaves

The petrogenesis and tectonic setting of Early Cretaceous granitoids and their enclaves emplaced in the Gan-Hang Tectonic Belt are still controversial. Here, we investigate mafic microgranular enclaves (MMEs) and their host granite porphyry from the Furongshan caldera to elucidate magma mixing and crust–mantle interaction in the Gan-Hang Tectonic Belt. The Furongshan granite porphyry is characterized by enrichments of alkalis, REE, Zr+Nb+Ce+Y contents (averaging 377ppm), and high zircon saturation temperatures (793–843°C), suggesting A-type granitic affinities. The granite porphyry can be further classified as an A2 subtype granite based on high Y/Nb ratios (averaging 1.37). Zircon cores from the Furongshan MMEs exhibit the same εHf(t) values (−10.0 to −3.0) and U–Pb ages (127–129Ma) as zircons form the granite porphyry, implying that they were captured from the felsic magma as xenocrysts. Petrological and mineralogical characteristics (such as needle-like apatite and disequilibrium feldspar xenocryst) suggest that the Furongshan MMEs and host granite porphyry were formed by magma mixing rather than restite, xenolith or fractional crystallization of mafic magma. The Furongshan granite porphyry samples have initial 87Sr/86Sr ratios of 0.7073–0.7099 and εNd(t) values of −3.7 to −3.3, which are similar to those of the MMEs (0.7068–0.7077 and −3.2 to −2.9, respectively). Similar trace element and Sr–Nd isotopic compositions imply a high degree of geochemical equilibration between the granite porphyry and its MMEs, and hence intense magma mixing, although some element contents and zircons εHf(t) values differ due to high zircon closure temperature and rapid cooling of commingled magmas. A binary mixing model based on Sr–Nd isotopes indicates a contribution of ∼50% basaltic melt to the hybrid magma of the Furongshan granite porphyry. A compilation of Sr–Nd–Hf isotopic data of the granitoids and MMEs from the Xiangshan, Furongshan and Muchen areas suggest that the basaltic components in the hybrid felsic magma increased from 0–20% at ∼135Ma to 50–60% during 127–112Ma in the Gan-Hang Tectonic Belt. This is consistent with the observation that zircon εHf(t) values of these granitoids simultaneously increased from −10.0 to 0. This increased input of mantle-derived magma through time suggested progressively intensified crust–mantle interaction during the Early Cretaceous in Southeast China, which may have been a response to ongoing slab roll-back during paleo-Pacific plate subduction.