Monazite petrochronology constrains the metamorphic evolution of high-grade metamorphic rocks in the Dai Loc shear zone, Central Vietnam

Compositional zoning of trace elements in garnet serves as a valuable tool for reconstructing petrogenetic evolution, supplementing major element analyses. This is particularly applicable to trace elements exhibiting a strong affinity for garnet and characterized by slow diffusion rates, such as Y and heavy rare earth elements (HREE). The present study examines various zoning patterns of trace elements observed in large garnet porphyroblasts within micaschist samples from the Variscan high-pressure (HP) metamorphic terrain of the Krušné hory Mts. (Saxothuringian zone, Bohemian Massif).Using electron probe micro-analyser and laser ablation-inductively coupled plasma mass spectrometry, three distinct types of compositional zoning in garnet were identified by compositional mapping. These zoning types were classified as a continuous core-to-rim change, concentric annular changes, and overprinting (or mimicking) of a pre-existing distribution. The study focuses on the formation mechanisms of each type of zoning, their dependence on pressure-temperature change, and fluid availability.The significantly elevated concentrations of Sc, Y, and HREE in the garnet's central core suggest a rapid diffusion of these elements from the matrix into the garnet after nucleation, challenging a description solely through Rayleigh fractionation. The observed prograde growth of pressure-temperature (PT) conditions of the rock samples to HP–medium temperature (MT) aligns well with the compositional zoning patterns exhibited by the garnet, encompassing major and trace elements, as well as other minerals. Specific compositional patterns include: (1) gradual increase in Co and Zn contents towards the rim, mirroring Mg and inversely related to Mn, indicative of a continuous rise in temperature; (2) overprint zoning of Ti and partly Ca, Sm, Eu, Gd, and Tb in the central part, transitioning to purely concentric annular zoning in the rim, suggesting an increase in temperature; (3) well-developed overprint zoning of Cr throughout the garnet grain, indicating temperatures only up to MT; and (4) depletion of Y, and most of rare earth elements (HREE, Ho, Dy, Tb, Gd, Eu, and Sm – REE) in the rim, accompanied by enrichment of coupled VIII(Na, Li)+ + IVP5+ substitution elements, experimentally documented from HP to ultra-HP conditions.The observed inverse annular oscillatory distribution of Sc and V is discussed to be attributed to fluctuating oxygen fugacity during garnet growth, influenced by changes in the availability of the fluid matrix medium carrying trace elements. Higher fluid availability corresponds to increased Sc, Y, and REE incorporation into garnet, evident in well-correlated annular elevations, while V exhibits the inverse trend. Elevated trace element contents in garnet are linked to the breakdown of main and accessory phases carrying these elements during garnet growth, incorporating them into the garnet. The presence of a fluid medium in the system appears to predominantly influence the extent and frequency of annular variations in trace element concentrations. Thus, annular zoning in garnet is associated with both the decomposition of trace element-bearing phases and fluid medium availability.Acknowledgement: This work was supported by the Czech Science Foundation (Grant No. 24-12845S), Grant Agency of Charles University (Grant No. 1194019), and by Charles University through the Cooperatio Program (Research Area GEOL).

ABSTRACTA garnet population from the lower Kalak Nappe Complex in Finnmark (Arctic Norway) was characterized using high‐resolution X‐ray micro‐computed tomography, electron probe micro‐analysis and laser ablation inductively coupled plasma mass spectrometry mapping to assess the extent of compositional equilibration and the controlling crystallization mechanisms during garnet growth. The obtained petrological dataset includes the spatial relationships of garnet crystals and the rock matrix fabrics, as well as the two‐dimensional distributions of major and trace elements in differently‐sized garnet crystals. Our results indicate that the observed elongated shape and clustered distribution of garnet resulted from crystallization in a texturally and chemically differentiated matrix, evidenced by the preferred distribution of biotite porphyroblasts. The major component (Fe, Mn, Mg, Ca) zoning presents systematic variations across differently sized garnet crystals, indicative of progressive nucleation and growth of garnet in equilibrium with an evolving matrix composition at increasing ‐ conditions. Annular features with the same Ca concentration in the analysed garnet crystals are used as markers of the contemporaneous growth of specific segments in crystals of different sizes. The slopes of compositional gradients correlate with crystal size, with smaller crystals showing steeper gradients for equivalent segments in the largest crystals of the rock. The chemical signature and microstructural properties of garnet suggest that growth rates were anisotropic, interface‐controlled and size‐dependent. Since similar concentrations and distribution patterns are observed for Sc, Ti, V, Co, Y and rare earth elements (Gd to Lu) across the differently sized crystals at the positions of the markers defined by the low‐Ca annuli in the crystals, the quasi‐equilibration of these elements at the centimetre scale across the intergranular medium can be inferred. A possible explanation for the observed trace element distribution across the garnet population is a sufficiently slow heating rate during prograde metamorphism, which provided the time required for the efficient transport of trace elements in the intergranular medium during garnet growth. Crystallization simulations using equilibrium thermodynamics indicate garnet growth over an interval of approximately 60°C and 1 kbar until peak conditions of approximately 570°C and 4.5 kbar. Previously published Lu‐Hf garnet‐whole rock ages coupled with our ‐ constraints indicate that heating rates could have been as slow as 1.3°C/Myr, suggesting that interface‐controlled, size‐dependent growth is not restricted to metamorphic garnet that crystallized rapidly and at fast heating rates ( C/Myr), as previously observed in a mica schist from the Barrovian garnet zone of Sikkim. The approach developed in this study provides a quantitative means to estimate the minimum length scales of major and trace element equilibration in the intergranular medium. This information is required to critically assess thermodynamic models of metamorphism and the ages used to constrain metamorphic histories. This approach may be helpful to identify crystal growth mechanisms and trace elements' equilibration length scales in natural samples and across the range of metamorphic conditions and durations. The case study presented here emphasizes the importance of the microstructural and chemical characterization of crystal populations for the study of crystallization kinetics and the extent of equilibrium during prograde metamorphism.

Tectonic evolution of high-grade metamorphic terranes in central Vietnam: Constraints from large-scale monazite geochronology

ABSTRACTIntegrated field mapping, phase equilibria modelling and in situ U–Pb monazite geochronology from the northern margin of the Rae craton on Baffin Island document three metamorphic events during the Neoarchean to the middle Paleoproterozoic. The Qimivvik area comprises Neoarchean tonalitic gneiss structurally juxtaposed over Neoarchean metasedimentary rocks along the Paleoproterozoic Qimivvik thrust and associated shear zone. High‐grade metamorphism at ca. 2.56–2.50 Ga supports a footprint for cryptic late Neoarchean metamorphism over a distance of ∼600 km along the northwestern Rae margin from southern Boothia Peninsula to northern Baffin Island. Thermal peak mineral assemblages in the Qimivvik area equilibrated at ca. 1.9 Ga at conditions of ~710°C–790°C and 4.3–5.5 kbar. The dominant Paleoproterozoic foliation is defined by peak metamorphic phases and is reoriented by folds related to the Qimivvik thrust. Peak metamorphism and associated deformation, including the Qimivvik thrust, are interpreted as a manifestation of the Ellesmere‐Inglefield belt of Ellesmere Island and West Greenland, which links with the ca. 1.9 Ga Thelon orogen of western Canada. Partial melting also occurred at ca. 1.8 Ga, possibly resulting from decompression of the Churchill domain following the collisional‐accretionary events related to the late stages of amalgamation of Laurentia and supercontinent Nuna. Quantitative trace element maps (acquired using LA‐ICP‐MS) of monazite reveal distinct trace element signatures associated with each of three growth stages. Ca. 2.5 Ga monazite exhibits complex intragrain compositional zoning, has elevated Y and heavy rare earth elements (HREEs) relative to ca. 1.9 Ga monazite and has higher Th/U overall than both ca. 1.9 Ga and ca. 1.8 Ga monazite. These signatures suggest that ca. 2.5 Ga monazite growth was concomitant with partial melting and preceded the majority of garnet growth. The ca. 1.9 Ga monazite grains are comparatively less zoned and have lower Y + HREE contents than both ca. 2.5 Ga and 1.8 Ga monazite, consistent with the ca. 1.9 Ga monazite forming after most garnet growth. Elevated Y + HREE in the ca. 1.8 Ga monazite imply that it formed after retrograde resorption of garnet rims. In our samples, Y + HREE generally exhibit stronger correlations with monazite age and/or petrographic context than Eu/Eu* and Th/U. As some compositional overlap exists between monazite of different ages and petrographic contexts, quantitative limits (‘cut‐offs’) based on trace element concentrations or ratios (e.g., Th/U, Eu/Eu*, LaCN/YbCN) are unreliable for distinguishing between monazite populations. In addition to providing important constraints on the early tectonic evolution of northeastern Laurentia, our study offers new insights into trace element behaviour in a key accessory mineral during three metamorphic events occurring over a ~700 Ma time period.

The trace elements characteristics of the migmatitic gneisses (biotite-garnet- and hornblende-biotite), granulite facies rocks (charnockitic gneisses) and meta-peridotite in the area of Southwest Obudu Plateau indicate that the area exhibits a high degree of geochemical variability. Compatible trace elements (Ni and Cr) are comparatively high in the granulite facies rocks and meta-peridotite. Ni ranges from 28×10−6 to 266×10−6 whilst Cr ranges from 62×10−6 to 481×10−6 for the granulite facies rocks (charnockitic gneisses), and for the meta-peridotite Ni varies between 2045×10−6 and 2060×10−6. Incompatible trace elements show a higher variability in these rocks. The rocks in the area of Southwest Obudu Plateau generally are characterized by the high concentrations of Ba, Ce, Sr, Rb, Ga, Ni, Cr, Co, Zr, Pr and moderate concentrations of Cu, Sm and Th. The available data show that the charnockitic gneisses are of lower crustal origin. The enrichment of the meta-peridotite in compatible trace elements suggests the primitive mantle would be the source region, with slight contamination during ascent. These diverse origins collaborate the tectonic setting of the area as shown by discrimination diagrams. The diverse tectonic settings range from arc to collisional. Alkaline to sub-alkaline magmatism in the area was probably contemporaneous with the tectonic events that occurred in the area during the Proterozoic.

The pressure-temperature location of garnet and spinel lherzolite facies boundary is often critical for understanding mantle dynamics. We have investigated the boundary by experiments using natural starting materials. For compositions close to the model primitive mantle at constant temperature of 1360°C, garnet-in boundary is approximately 2.3 GPa while spinel-out is at approximately 2.6 GPa. The redistribution of trace elements during the garnet break down reaction (i.e. olivine[ol] + garnet[gt] = orthopyroxene[opx] + clinopyroxene[cpx] + spinel[sp]) lags behind phase equilibrium and product opx and cpx “inherit” the REE abundances from reactant garnet. Diffusion equilibration for trace elements takes several hundred thousand years for pyroxenes over distance of 500µm at 1200°C. The pressure-temperature data for garnet-spinel transformation are inadequate particularly under near solidus conditions in natural peridotite compositions. Previous experimental studies have demonstrated that compositional effects on the location of garnet-spinel facies boundary in simple systems were considerable (e.g. Nickel, 1986; O’Neill, 1981). Addition of chrome shifts the boundary to higher pressures, but the data are not sufficient for extrapolating the results to natural compositions. For natural systems, experimental studies of the transformation boundary are limited in number (Jenkins and Newton, 1979; O’Hara et al., 1971) and their results span over a range approximately 10 kbar. Furthermore, experimental results for the transformation boundary near solidus (above 1300°C) do not exist. Our experiments were aimed at determining where garnet and spinel become stable at near solidus condition (e.g. 1360°C). In addition, the phase rule predicts a range of pressure- temperature conditions for the existence of transitional garnet-spinel lherzolite zone, because the degree of freedom is more than two for the garnet break down reaction in the natural system. This suggests that the garnet-in boundary is located toward the lower pressures than the spinel-out boundary. We circumvent the problem, that is caused by sluggishness of reaction, by monitoring rates of the garnet breakdown and growth reactions. Model parameters corresponding to chemical affinity are determined from time dependent progresses of the reaction. These parameters are used to extrapolate the condition where reaction rate is zero, that is equilibrium. The results show that at 1360°C garnet-in boundary is located at 0.3 GPa lower pressure than spinelout boundary. Garnet and spinel can coexist in the range of pressure. It should be noted that the 0.3 GPa pressure range for the coexistence is difficult to resolve by the conventional reversal reaction method (Koga et al., 1998). Trace elements that reside in garnet have to be redistributed during the garnet break-down reaction. Peridotite samples that have experienced the decompression often display incomplete trace element redistribution in newly formed minerals. Trace element measurements on experimental charges demonstrate that product opx and cpx “inherit” the trace element abundance from the reactant garnet, and there are no recognizable differences in abundance of trace element between opx and cpx. Thus, while major element abundances in pyroxenes are in equilibrium as demonstrated by experimentally calibrated geothermobarometers, trace element redistribution is incomplete. This may be caused by the differences in diffusivities of major and trace elements. The disequilibrium trace element distribution similar to experimental results could be expected in rocks under went rapid decompression. Indeed, The trace element distribution among fine grained rims around garnet consisting opx, cpx, and spinel in peridotite xenoliths from Lashaine, Tanzania closely resembles experimental results. Time scales for equilibration of pyroxenes can be modeled by diffusion. A model involving a spherical grain with a constant composition boundary, and a model with adjacent finite length slabs of opx and cpx, are compared under various diffusivities and boundary conditions in cooling histories. The results show persistence of disequilibrium for 7-800k years at high temperature (1200°C), and fractionated zoning of rare earth elements due to D(Yb) > D(La). Starting materials are mineral separates from garnet lherzolite from Pali Aike, Chile for garnet break down and spinel lherzolite from Kilbourne Hole, USA for garnet production reaction. Experiments are conducted at the conditions at 1360°C and from 1.8 to 3.0 GPa, with the run duration spanning form 2 to 200 hours. The run conditions were achieved by 2.54 cm diameter piston-cylinder type apparatus, with Ba- CO3 as pressure transmitting medium. Pressure calibrations were done by the CaTs break-down reaction (Hays, 1967) and An-Sp lherzolite transformation for CMAS system at subsolidus and above solidus conditions (Kushiro and Yoder, 1966). Volume fractions of product phases are measured from the digitally captured back-scattered electron images with the combination of x-ray images. Major element compositions are measured by electron probe. Trace elements abundances are measured by ion probe.

Trace element (REE, Cr, Ti, Y, Y, and Zr) analysis of garnet from the garnet, staurolite, and lower sillimanite zones of an aluminous schist of the Black Hills, South Dakota, indicates that REE zoning varies as a function of grade. Garnet-zone garnet has high concentrations of REEs, Cr, Ti, Y, Y, and Zr in the cores and low concentrations in the rims. Profiles of heavy REEs contain inflections between the cores and rims, which are approximately symmetric about the cores. Staurolite-zone garnet contains cores enriched with Y and heavy REEs, which decrease toward the rim and increase again at the rim edges but to lower concentrations than in the cores. Cr, Y, Ti, Zr, and light REE zoning is less pronounced than heavy REE zoning and is less symmetric about the garnet cores. Almandine-rich garnet of the lower sillimanite zone displays no major element zonation. Trace element (Ti, Cr, Y, and Zr) concentrations are minimal, and the zoning is irregular and not symmetric about the garnet cores. Garnet from all three zones has core-to-rim Fe/(Fe + Mg) profiles that suggestgarnet growth was uninterrupted with respect to major element components and that Mn zoning formed by a fractionation process. Analysis of trace element zoning in this garnet reveals that the major element zoning was relatively unaffected by volume-diffusion reequilibration. Trace element zonation of all samples of garnet is best explained by a fractionation mechanism in conjunction with limited intergranular diffusion and changing partition coefficients during garnet growth. Heavy REE partitioning is especially dependent on the major element composition of garnet. This research complements previous research by others on the use of trace elements as metamorphic petrogenetic indicators, which demonstrated the importance of bulk-rock composition and phase assemblage on trace element partitioning.

This study focuses on the trace and rare earth elements (REE) geochemistry of the Nkporo and Ekenkpon Shales of the Calabar Flank. The main aim is to infer their depositional environment and the degree of their metal enrichment. The shale samples were analyzed using inductively coupled plasma mass spectrometry. The results indicated that the mean concentrations of K, Na, and Fe in Nkporo and Ekenkpon Shales are 1.45, 0.4, and 4.17 wt%, and 1.11, 0.44, and 5.42 wt%; respectively. The Nkporo Shale is enriched with the following trace elements; P > Mn > Sr > Ba > Zn > Ce > Rb > Zr > V>Cr > Ni and depleted in the following trace elements; Ta > Ge > Sb > Bi > Cd > Ag > Te > In > Hg. While the Ekenkpon Shale is enriched with the following trace elements; P > Mn > Ba > Sr > V>Ce > Zr > Rb > Cr > Zn > Ni and depleted in; Sb > Ge > Bi > Ag > Ce > Te > In > Hg. The concentration of redox-sensitive elements such as V, Ni, Mo, U, Cu, Cr, Re, Cd, Sb, Ti, Mn, and their ratio V/Mo and U/Mo in the black and grey shale samples show different patterns. The REE obtained from the Nkporo and Ekenkpon Shales were PAAS normalized. The Nkporo Shale showed a slightly flat light rare-earth element (LREE), middle rare-earth element (MREE), and heavy rare earth element (HREE) pattern enrichment. Ce/Ce* ranges from 0.95 to 1.09 in Nkporo Shale and 0.67 to 1.40 in Ekenkpon Shale. The Ekenkpon Shale showed a slight LREE, MREE enrichment, and depleted HREE patterns. The Mn contents and U/Mo ratio in Nkporo and Ekenkpon Shales suggests a poor oxygen transitional environment. The V/Mo and V/(V + Ni) ratios indicated that the Nkporo shales were deposited in an anoxic to suboxic conditions and Ekenkpon shales were also deposited under an anoxic to suboxic conditions. The V/Ni ratio indicated that the organic matter in the Nkporo shale is terrigenous while that of the Ekenkpon shales are both terrigenous and marine in origin.

Fresh samples of hypabyssal kimberlite from the five major kimberlite pipes in the Kimberley area of South Africa have been analysed for their bulk-rock major and trace element geochemistry. The geochemical data allow identification of the influence of crustal contamination in certain samples, best illustrated in terms of elevated SiO2, Al2O3, Pb and heavy rare earth element (HREE) contents. Samples devoid of such crustal contamination show coherent major and fluid-immobile trace element variations, whereas fluid-mobile trace elements are scattered. Kimberlites rich in macrocrysts are shown to reflect substantial (up to 35%) entrainment of mantle peridotite, with Ni–SiO2 and Sc–SiO2 variations defining mixing trajectories towards garnet lherzolite. The likely primary magma(s) parental to the Kimberley kimberlites is suggested to have a composition of 26–27 wt % MgO, 26–27 wt % SiO2, ∼2·2 wt % Al2O3 and Mg number 0·86. Subtle differences in chondrite-normalized REE abundance patterns can be explained by small variations in the degree of partial melting within the range 0·4–1·5%, leaving residual garnet. The data are satisfied by melting a source enriched relative to chondrites by a factor of ∼10 in light REE (LREE), with chondritic or lower HREE abundances. Extended normalized trace element diagrams exhibit significant negative K, Rb, Sr and Ti anomalies that are interpreted to be primary magma characteristics, despite evidence for secondary mobility of K, Sr and Rb. A model is proposed in which fluid or melt from a sub-lithospheric source region precipitates phlogopite en route to metasomatizing the overlying subcontinental mantle lithosphere, imprinting its geochemical signature on a source region previously depleted in HREE relative to primitive mantle. Subsequent ∼1% melting of the metasomatized source produces a kimberlite with compatible element characteristics strongly influenced by depleted lithospheric peridotite (high Mg number, high Ni, low HREE), but with incompatible elements (and their isotope ratios) characteristic of the deeper source. The similarity of incompatible element ratios (Nb/U, Nb/Th, Ce/Pb) in the kimberlite magmas to those of ocean island basalts from the South Atlantic suggests an ultimate origin in an upwelling mantle plume.

Geochemistry of trace and rare earth elements during weathering of black shale profiles in Northeast Chongqing, Southwestern China: Their mobilization, redistribution, and fractionation

Garnet commonly accommodates high contents of Mn + Y + heavy rare earth elements (HREE) that follow Rayleigh fractionation during garnet early growth, with the exception of overstepping nucleation (late crystallization owing to reaction overstepping). Because of this, as the garnet porphyroblasts form mostly in equilibrium with the surrounding matrix, the concentration of these elements continuously decreases towards the porphyroblast rims. Yet rapid changes in the reaction progress of a rock during garnet growth, namely the resorption–dissolution of minerals with high concentrations of Y + REE, may create an anomaly or peak in the mantle or rim of garnet grains. In this study we present an example of the resorption of garnet cores and formation of atoll garnet textures in eclogite from the Krušné hory (in the Saxothuringian tectonic zone of the Bohemian Massif). Based on textural relations, we show that the atoll garnet grains in the studied rocks were formed during the prograde stage from blueschist- to eclogite-facies metamorphism. Preliminary observations showed that the full (non-atoll) garnet grains had compositionally different cores (interior, or garnet I) and rims (ring, or garnet II) that were separated by a Y + HREE + medium REE (MREE) concentration peak. The ring garnet II indicated an elevated concentration of Mn in comparison with the marginal parts of the interior garnet I. Therefore, minor elements that were less vulnerable to diffusion than major elements and strongly sensitive to the broad spectrum of geochemical processes, such as Y + REE, were used to track possible mineral reactions during the whole garnet growth path. Thermodynamic modelling indicated the formation of garnet by the breakdown of chlorite and lawsonite/zoisite, and peak-pressure phases were represented by garnet, omphacite, quartz, amphibole, rutile, and talc. To quantify the sources of high Mn concentrations in garnet II and of the Y + HREE + MREE sharp peaks, the sequences of mineral reactions and dissolution of garnet I leading to the formation of the atoll structure were investigated. In addition to thermodynamic modelling and pressure–temperature path constraints, mass-balance calculations of trace elements were also performed. The results combined with the observed compositional and textural relations indicate that the concentrations of Mn + Y + HREE + MREE in garnet II and the concentration peaks at the interface of the two garnet types were controlled by a complex mechanism that included the dissolution of garnet I during the formation of the atoll texture, stepwise growth of garnet during increasing pressure and temperature, and decomposition of phases with high concentrations of trace elements, such as zoisite/epidote or lawsonite.

Rare earth and trace element geochemistry of termite mounds in central and northeastern Namibia: Mechanisms for micro-nutrient accumulation

This study considers the potential of using the U-Pb dating of garnet for determining quantitative P-T-t paths for the late Archean metamorphism in the Pikwitonei granulite domain. Garnets for U-Pb dating were selected mainly from samples that also provide information on pressure and temperature. The garnets used for dating were clear and free of any visible inclusions. Pb concentrations range from 63 ppb to 966 ppb and U from 136 ppb to 1143 ppb. The measured 206Pb/204Pb ratios range from 52.8 to 529.4. The ages are generally discordant with U/Pb ages that may lie above or below concordia. The discordance is caused by a recent disturbance of the U/Pb ratio in the garnets as indicated by replicate analyses on the same garnet separates that reproduce 207Pb/206Pb ages well within analytical uncertainty and in most cases within ±1.5 Ma at 2600–2750 Ma. High grade metamorphism continued over a period of at least one hundred million years, but the garnet-K-feldspar Pb-Pb ages suggest that, during this time, garnet growth has been favored during three distinct periods in the Cauchon Lake area: 2700–2687 Ma 2660–2637 Ma 2605–2591 Ma The ca. 2695 Ma garnet ages from Cauchon Lake date the time of melting and staurolite breakdown during prograde metamorphism, the ca. 2640 Ma ages date the time of extensive migmatization and the last period of metamorphic garnet growth, the ca. 2600 Ma ages date the time of crystallization of igneous garnet in late granitic intrusions. Peak metamorphism occurred around 2640 Ma followed by the intrusions of pegmatites starting at 2629 Ma. The Pb-Pb ages for garnet are similar to the U-Pb ages for zircon that date a leucocratic mobilizate (2695 Ma), a plagioclaseamphibole mobilizate (2637 Ma) and pegmatite (2598 Ma) (Heaman et al. 1986 a; Krogh et al. 1986; this study). Xenocrysts of garnet from 2600 Ma old graphic granites give minimum ages of 2984 Ma and 2741 Ma which are minima for the times of garnet growth in the source of the granites. The agreement of the zircon and garnet ages suggests that the metamorphism may have been punctuated by events that led to the development of melts or encouraged mineral growth at specific times. If so, the prograde and retrograde paths of metamorphism in the area may have contained minor excursions in pressure, temperature or fluid fugacities. In the Natawahunan Lake area some 50 km northwest of Cauchon Lake, garnet growth associated with the prograde breakdown of staurolite occurred at ca. 2744–2734 Ma. This suggests that a similar style of metamorphism may have occurred earlier in the Natawahunan Lake area than at Cauchon Lake area, or higher grades of metamorphism were reached earlier and were of longer duration associated with the somewhat greater depths in the Natawahunan Lake area. These results indicate the these garnets, which are 0.1–1 cm in diameter, have maintained closed system behavior for U and Pb at peak metamorphic conditions, i.e. temperatures up to 800° C and pressures of 7.5 kb.

The lower crust of the Serre massif (Calabria, southern Italy) provides a window into the mid- to lower crust of the south European Variscan orogenic belt. Previously, zircon U-Pb ages were employed to date high-temperature processes affecting this portion of the Variscan crust. The present paper reports new LA-ICP-MS U-Pb data on the zircon of a deformed quartz-monzodiorite dike and of three mafic granulites sampled at the base of the lower crust section. Determination of trace elements on zircon, including rare earth elements (REE), has been also performed. The end of the Variscan exhumation, dated by anatectic zircon from migmatitic metapelites, and the growth-modification of zircon with respect to the growth of Variscan metamorphic garnet have been assumed as “time markers”. The concordant zircon ages of the metamorphic basic rocks cover a range from 744 ± 20 Ma to 231 ± 10 Ma with high age density from 357 ± 11 Ma to 279 ± 10 Ma, a few ages comprised between 418 ± 14 Ma and 483 ± 12 Ma and between 505 ± 11 Ma and 593 ± 14 Ma. Zircon from the deformed quartz-monzodiorite dike evidences a minimum age of emplacement of 323 ± 5 Ma. Most of the analysed zircon domains dated between 357 ± 11 Ma to 279 ± 10 Ma from garnet-bearing metabasic rocks show flat patterns of heavy rare earth elements (HREE), as expected in the case of their simultaneous growth with garnet. This allows to consider (1) zircon domains giving Variscan ages as “metamorphic” with specific geological significance, and (2) zircon domains with ages ranging from 564 ± 17 Ma to 593 ± 14 Ma as dating the emplacement of the magmatic protoliths as shown by internal microtextures, fractionated patterns of HREE and Th/U ratios (0.16–0.19). The Variscan zircon ages (357–279 Ma) reflect effects of crustal thickening, peak metamorphism and subsequent multistage Variscan decompression documented by the statistically significant clusters of ages around 347–340 Ma, 323–318 Ma, 300–294 Ma and 279 Ma. The U-Pb zircon ages of the metabasic rocks suggest a period of about 60–70 Ma for granulite facies metamorphism and anatectic conditions. Literature data indicate that the migmatitic metapelites of the upper part of the Serre crust section also underwent a long period, about 40 Ma, of granulite facies metamorphism and anatectic conditions. A diachronism emerges through the time comparison of the Variscan evolution between the upper and the lower portions of the Serre deep crust. The duration of the Variscan processes defined in Calabria is comparable to that of other south European Variscan blocks.

Geochemistry of Trace and Rare Earth Elements in Red Soils from the Dongting Lake Area and Its Environmental Significance