REE partitioning between monazite and garnet: Implications for petrochronology

Rare‐earth element and Y partitioning between garnet and monazite was measured in metamorphic rocks from western Norway to provide more confidence in tying monazite U/Th–Pb dates to P–T conditions recorded in garnet. A subset of samples has low‐Y garnet mantles and low‐Y monazite cores that gave Y‐partitioning temperatures similar to independently determined metamorphic temperatures. In combination with previously published data, these monazite–garnet pairs have temperature‐dependent partitioning of the HREE from Dy to Lu, and nonsystematic partitioning of the LREE from La–Gd. The temperature‐dependent partitioning must be considered when using HREE to assess which portions of garnet and monazite might have coexisted, but experiments are needed to place the dependence on a firm footing.     

Protracted garnet growth in high‐P eclogite: constraints from multiple geochronology and P–T pseudosection

Understanding convergent margin processes requires determination of the onset and the termination of subduction, the duration of subduction‐zone metamorphism, and the subduction zone polarity. Garnet growth and intracrystalline zonation can be used to constrain the timing, duration and kinetics of tectonometamorphic processes. An eclogite from the Huwan shear zone in the Hong'an orogen was investigated with combined pseudosection analysis and multiple geochronologies. The pseudosection analysis illustrates that garnet growth is continuous and along an early near‐isothermal trajectory followed by a near‐isobaric heating path from 1.9 GPa/500 °C to 2.4 GPa/575 °C and subsequent near‐isothermal decompression. ⁴⁰Ar/³⁹Ar dating of an amphibole inclusion in garnet from the eclogite yielded an age of 310 ± 5 Ma, which is consistent with a U–Pb age of 305 ± 3 Ma for the metamorphic zircon within uncertainty. Garnet core and rim material produced Lu–Hf ages of 296.9 ± 3.8 and 256.9 ± 3.9 Ma respectively; the latter is consistent with its Sm–Nd age of 254.3 ± 4.6 Ma for the same aliquots. Similarly, limited zircon U–Pb ages of c. 257 Ma were obtained in zircon rims with garnet inclusions. These ages were interpreted to bracket the period of garnet growth and the difference of up to c. 40 Ma is best explained by protracted garnet growth. We propose that the rocks represent detachment of part of the downgoing slab and remained free of significant compression/decompression or heating/cooling close to the subduction channel, most likely underplating the mantle wedge, for a long time. These rocks were incorporated into the following subduction channel due to the successive entry of the buoyant materials, and exhumed at some time later than c. 254 Ma. The increasing observations of protracted garnet growth and long‐lived subduction in various orogens worldwide demand more sophisticated geodynamic models.     

Does ternary feldspar constrain the metamorphic conditions of high‐grade meta‐igneous rocks? Evidence from orthopyroxene granulites,Bohemian Massif

The presence of ternary feldspar in high‐grade meta‐igneous rocks, and the recognition of the thermometric significance of this mineral, has led recent researchers to postulate peak metamorphic temperatures in excess of 1000 °C. However, it needs to be established that such ternary feldspar is not in fact a survivor of the original high‐temperature crystallization of the igneous protolith. After exsolution, the host and lamellae in the ternary feldspar grains may be stable throughout subsequent history as long as recrystallization does not occur. Such a history may involve rehydration and metamorphism, including H₂O‐saturated conditions, with the compositions and proportions of the host and lamellae being modified to reflect the P–T conditions experienced. In the case of the high‐grade meta‐igneous rocks from the Moldanubian of the Bohemian Massif, some samples that contain ternary feldspar preserve a substantial measure of their igneous heritage. Orthopyroxene‐bearing granulites not only include types that are barely affected by the metamorphism, but also others that have undergone hydration of the igneous protolith prior to the development of a metamorphic overprint. A key to establishing the igneous origin of the ternary feldspar grains is their preservation in garnet that is either itself igneous, or of a relatively low‐temperature metamorphic origin. Applying the logic to the other ternary feldspar‐bearing meta‐igneous rocks deprives the Moldanubian of its ultrahigh temperature (UHT) metamorphic status.     

Experimental determination of partition coefficients for rare earth and high-field-strength elements between clinopyroxene,garnet, and basaltic melt at high pressures

Clinopyroxene/melt and garnet/melt partition coefficients have been determined for Ti, Sr, Y, Zr, Nb, Hf, and rare earth elements from 19 doped experiments on 1921 Kilauea basalt. The experiments were carried out from 2.0 to 3.0 GPa and 1310° to 1470 °C. The purpose was to derive a set of partition coefficients for high-field-strength elements (HFSE) and rare earth elements (REE) in a systematic, linked set of experiments at P and T conditions relevant to basalt petrogenesis. These data are used in melting models to understand the development of negative HFSE anomalies observed in many abyssal peridotite clinopyroxenes. It is shown that melting can account for the observed trace element patterns in some residual peridotites, but that other processes may also be needed to account for most residual mantle compositions in mid-ocean ridge systems. It is also shown that REE are more strongly fractionated by garnet at these P-T conditions than previously thought. Received: 1 July 1997 / Accepted: 11 May 1998     

胶南新元古代花岗片麻岩中淡色脉体的锆石年代学和稀土元素研究

苏鲁造山带中出露了大面积的新元古代花岗片麻岩,这些花岗片麻岩中赋存规模不等、形态各异的变质岩包体,已有资料表明它们均受到印支期高压-超高压变质作用的强烈影响.在花岗片麻岩及包体中经常可见以白色为主的长英质淡色脉体,脉体通常较窄,宽度多在1厘米以内,少数可达几个厘米,它们一般随片麻理方向延展,亦可见切穿片麻理的淡色脉,通常认为这类淡色脉体是母岩部分熔融的产物.作者等对出露于山东诸城市石河头乡新元古代花岗片麻岩中淡色脉体的锆石年代学及地球化学进行了探索性研究,取得了宝贵的地质信息.研究结果表明,淡色脉体中锆石SHRIMP法U-Pb年龄为(147.6±2.5)Ma,同时锆石具有高铀、极低钍和重稀土富集等特征.文中在简略介绍地质背景的基础上,着重对淡色脉体中锆石的特征、锆石微区年龄和REE测定结果的地质意义进行了讨论.     

Relating zircon and monazite domains to garnet growth zones: age and duration of granulite facies metamorphism in the Val Malenco lower crust

Zircon from a lower crustal metapelitic granulite (Val Malenco, N‐Italy) display inherited cores, and three metamorphic overgrowths with ages of 281 ± 2, 269 ± 3 and 258 ± 4 Ma. Using mineral inclusions in zircon and garnet and their rare earth element characteristics it is possible to relate the ages to distinct stages of granulite facies metamorphism. The first zircon overgrowth formed during prograde fluid‐absent partial melting of muscovite and biotite apparently caused by the intrusion of a Permian gabbro complex. The second metamorphic zircon grew after formation of peak garnet, during cooling from 850 °C to c. 700 °C. It crystallized from partial melts that were depleted in heavy rare earth elements because of previous, extensive garnet crystallization. A second stage of partial melting is documented in new growth of garnet and produced the third metamorphic zircon. The ages obtained indicate that the granulite facies metamorphism lasted for about 20 Myr and was related to two phases of partial melting producing strongly restitic metapelites. Monazite records three metamorphic stages at 279 ± 5, 270 ± 5 and 257 ± 4 Ma, indicating that formation ages can be obtained in monazite that underwent even granulite facies conditions. However, monazite displays less clear relationships between growth zones and mineral inclusions than zircon, hampering the correlation of age to metamorphism. To overcome this problem garnet–monazite trace element partitioning was determined for the first time, which can be used in future studies to relate monazite formation to garnet growth.     

Experimental determination of REE partition coefficients between amphibole and basaltic to andesitic liquids at high pressure

Amphibole/liquid partition coefficients for the REE(D^amph/liq_REE) obtained from natural rocks increase systematically with bulk rock compositional change from basalt to rhyolite and are higher for the middle to heavy REE. Five new experimentally determined sets of D_REE (La, Sm, “Eu²⁺”, Ho, Lu)and 4 published sets are similar to these data and provide values for use in geochemical modelling of magmatic processes involving amphibole, over a range of temperature, pressure and oxygen fugacity. The partition coefficients increase significantly with decreasing temperature, and increase slightly with increasing oxygen fugacity. Pressure does not appear to have a major effect, although one data set suggests increased pressure lowers D^amph/liq_REE in a basaltic composition.     

Temporal links between pluton emplacement,garnet granulite metamorphism,partial melting and extensional collapse in the lower crust of a Cretaceous magmatic arc,Fiordland, New Zealand

Garnet granulite facies mid‐to lower crust in Fiordland, New Zealand, provides evidence for pulsed intrusion and deformation occurring in the mid‐to lower crust of magmatic arcs. ²³⁸U‐²⁰⁶Pb zircon ages constrain emplacement of the ～595 km² Malaspina Pluton to 116–114 Ma. Nine Sm‐Nd garnet ages (multi‐point garnet‐rock isochrons) ranging from 115.6 ± 2.6 to 110.6 ± 2.0 Ma indicate that garnet granulite facies metamorphism was synchronous or near synchronous throughout the pluton. Hence, partial melting and garnet granulite facies metamorphism lasted <5 Ma and began within 5 Ma of pluton emplacement. Garnet granulite facies L‐S tectonites in the eastern part of the Malaspina Pluton record the onset of extensional strain and arc collapse. An Sm‐Nd garnet age and thermobarometric results for these rocks directly below the amphibolite facies Doubtful Sound shear zone provide the oldest known age for extension in Fiordland at ≥112.8 ± 2.2 Ma at ～920 °C and 14–15 kbar. Narrow high Ca rims in garnet from some of these suprasolidus rocks could reflect a ≤ 1.5 kbar pressure increase, but may be largely a result of temperature decrease based on the Ca content of garnet predicted from pseudosections. At peak metamorphic conditions >900 °C, garnet contained ～4000 ppm Ti; subsequently, rutile inclusions grew during declining temperature with limited pressure change. Garnet granulite metamorphism of the Malaspina Pluton is c. 10 Ma younger than similar metamorphism of the Pembroke Granulite in northern Fiordland; therefore, high‐P metamorphism and partial melting must have been diachronous for this >3000 km² area of mid‐to‐lower crust. Thus, two or more pulses of intrusion shortly followed by garnet granulite metamorphism and extensional strain occurred from north to south along the axis of the lower crustal root of the Cretaceous Gondwana arc.     

Metamorphic petrology of a high‐T/low‐P granulite terrane (Damara belt,Namibia) – Constraints from pseudosection modelling and high‐precision Lu–Hf garnet‐whole rock dating

Migmatites comprise a minor volume of the high‐grade part of the Damara orogen of Namibia that is dominated by granite complexes and intercalated metasedimentary units. Migmatites of the Southern Central Zone of the Damara orogen consist of melanosomes with garnet+cordierite+biotite+K‐feldspar, and leucosomes, which are sometimes garnet‐ and cordierite‐bearing. Field evidence, petrographic observations, and pseudosection modelling suggest that, in contrast to other areas where intrusion of granitic magmas is more important, in situ partial melting of metasedimentary units was the main migmatite generation processes. Pseudosection modelling and thermobarometric calculations consistently indicate that the peak‐metamorphic grade throughout the area is in the granulite facies (~5 kbar at ~800°C). Cordierite coronas around garnet suggest some decompression from peak‐metamorphic conditions and rare andalusite records late, near‐isobaric cooling to <650°C at low pressures of ~3 kbar. The inferred clockwise P–T path is consistent with minor crustal thickening through continent–continent collision followed by limited post‐collisional exhumation and suggests that the granulite facies terrane of the Southern Central Zone of the Damara orogen formed initially in a metamorphic field gradient of ~35–40°C/km at medium pressures. New high‐precision Lu–Hf garnet‐whole rock dates are 530 ± 13 Ma, 522.0 ± 0.8 Ma, 520.8 ± 3.6 Ma, and 500.3 ± 4.3 Ma for the migmatites that record temperatures of ~800°C. This indicates that high‐grade metamorphism lasted for c. 20–30 Ma, which is compatible with previous estimates using Sm–Nd garnet‐whole rock systematics. In previous studies on Damara orogen migmatites where both Sm–Nd and Lu–Hf chronometers have been applied, the dates (c. 520–510 Ma) agree within their small uncertainties (0.6–0.8% for Sm–Nd and 0.1–0.2% for Lu–Hf). This implies rapid cooling after high‐grade conditions and, by implication, rapid exhumation at that time. The cause of the high geothermal gradient inferred from the metamorphic conditions is unknown but likely requires some extra heat that was probably added by intrusion of magmas from the lithospheric mantle, i.e., syenites that have been recently re‐dated at c. 545 Ma. Some granites derived from the lower crust at c. 545 Ma are the outcome rather than the cause of high‐T metamorphism. In addition, high contents of heat‐producing elements K, Th, and U may have raised peak temperatures by 150–200°C at the base of the crust, resulting in the widespread melting of fertile crustal rocks. The continuous gradation from centimetre‐scale leucosomes to decametre‐scale leucogranite sheets within the high‐grade metamorphic zone suggests that leucosome lenses coalesced to form larger bodies of anatectic leucogranites, thereby documenting a link between high‐grade regional metamorphism and Pan‐African magmatism. In view of the close association of the studied high‐T migmatites with hundreds of synmetamorphic high‐T granites that invaded the terrane as metre‐ to decametre‐wide sills and dykes, we postulate that crystallization of felsic lower crustal magma is, at least partly, responsible for heat supply. Late‐stage isobaric cooling of these granites may explain the occurrence of andalusite in some samples.     

Uncoupled U/Pb and REE response in zircon during the transformation of eclogite to mafic and intermediate granulite (Blanský les,Bohemian Massif)

An eclogite–mafic granulite occurs as a rare boudin within a felsic kyanite–K‐feldspar granulite in a low‐strain zone. Its boundary is marked by significant metasomatism–diffusional gain of potassium at the centimetre‐scale, and probable infiltration of felsic melt on a larger scale. This converted the eclogite–mafic granulite into an intermediate‐composition, ternary‐feldspar‐bearing granulite. Based on inclusions in garnet, the peak P–T conditions of the original eclogite are 18 kbar at 850–950 °C, with later matrix re‐equilibration at 12 kbar and 950 °C. Four samples from the transition of the eclogite–mafic granulite through to the intermediate granulite were studied. In the eclogite, REE patterns in the garnet core show no Eu anomaly, compatible with crystallization in the absence of plagioclase and consistent with eclogite facies conditions. Towards the rim of garnet, LREE decrease, and a weak negative Eu anomaly appears, reflecting passage into HP granulite facies conditions with plagioclase present. The rims of garnet next to ternary feldspar in the intermediate granulite show the lowest LREE and deepest Eu anomalies. Zircon from the four samples was analysed by LASS (laser ablation–split‐stream inductively coupled plasma–mass spectrometry). It shows U–Pb ages from 404 ± 4.0 to 331 ± 3.3 Ma, with a peak at 340 ± 4.0 Ma corresponding to the likely exhumation of the rocks to 12 kbar. Older ages from zircon with steep HREE patterns indicate the minimum age of the protolith, and ages <360 ± 4.0 Ma are interpreted to correspond to the eclogite facies metamorphism. Only some zircon grains ≤350 ± 4.0 Ma have flat HREE patterns, suggesting that these are primarily modified protolith grains, rather than new zircon crystallized in the eclogite‐ or granulite facies. The metasomatic processes that converted the eclogite–mafic granulite to an intermediate granulite may have facilitated zircon modification as zircon in the intermediate granulite has flat HREE and ages of 340 ± 4.0 Ma. The difference between the oldest and youngest ages with flat REE patterns indicates a 16 ± 5.6 Ma period of zircon modification in the presence of garnet.     

大别山变质岩锆石微区稀土元素和Th,U特征

对进行过微区U-Pb定年和阴极发光成像研究的大别山辛店榴辉岩、双河榴辉岩、黄镇榴辉岩和双河硬玉石英岩中锆石,进行了微区核部与边部稀土元素测定.结合U-Pb年龄和CL图象,探讨了超高压变质过程中稀土元素从原岩锆石到变质锆石的变化.结果表明,原岩锆石和变质锆石有很不相同的稀土元素含量,它取决于变质锆石是由变质重结晶还是变质增生作用形成及形成时间.一般说来,边部变质锆石比原岩锆石亏损稀土元素,特别是重稀土,并且有更低的Th/U比.变质锆石的稀土元素和Th/U比可以为变质锆石形成时的物理化学环境和变质锆石成因提供重要信息.     

内蒙古阿尔哈达地区花岗岩锆石LA-ICP-MSU-Pb年龄、稀土元素、Hf同位素组成及其地质意义

内蒙古阿尔哈达地区位于西伯利亚板块东南缘查干敖包-奥尤特-朝不楞岩浆岩带东段,该区发育大量晚古生代花岗岩,主要岩石类型为似斑状花岗岩和中细粒花岗岩。对该区1件似斑状花岗岩、2件中细粒花岗岩进行了锆石LA-ICP-MSU-Pb同位素定年,获得成岩年龄分别为319.0±5.0 Ma(MSWD=4.9)、324.0±2.0 Ma(MSWD=1.3)和332.0±8.5 Ma(MSWD=5.7)。定年结果显示所有花岗岩的形成时代均为早石炭世,属于海西中期。锆石的原位微区稀土元素分析表明,所有花岗岩中的锆石具有轻稀土元素亏损、重稀土元素富集、强烈的正Ce异常和负Eu异常特征,与岩浆锆石的特点一致。Lu-Hf同位素测试结果显示,锆石的εHf(t)值为-0.71~+10.21,二阶段模式年龄为709~1 436 Ma,表明其主要起源于中新元古代增生的地壳。根据已获得的同位素年龄和锆石稀土元素数据,结合野外地质调查和区域构造演化,可以推测阿尔哈达地区可能存在时限为310~330 Ma的构造岩浆活动。     

Age and duration of ultrahigh-temperature metamorphism in the Anápolis–Itauçu Complex, Southern Brasília Belt, central Brazil – constraints from U–Pb geochronology, mineral rare earth element chemistry and trace-element thermometry

Integrated petrographic and chemical analysis of zircon, garnet and rutile from ultrahigh‐temperature (UHT) granulites in the Anápolis–Itauçu Complex, Brazil, is used to constrain the significance of zircon ID‐TIMS U–Pb geochronological data. Chondrite‐normalized rare earth element (REE) profiles of zircon cores have positive‐sloping heavy‐REE patterns, commonly inferred to be magmatic, whereas unambiguous metamorphic grains and overgrowths have flat to slightly negatively sloping heavy‐REE patterns. However, in one sample, a core of zircon interpreted as having formed prior to garnet crystallization and a metamorphic zircon formed within microstructures involving garnet breakdown both display elevated heavy‐REE (and Y) with positive‐sloping patterns. D_REE(zrc/grt) partition coefficients suggest an approximation to equilibrium between zircon and garnet cores, although progressive enrichment in heavy REE towards garnet rims occurs in two of the samples investigated. Titanium‐in‐zircon thermometry indicates zircon growth during both the prograde and post‐peak evolution, but not at peak temperatures of the UHT metamorphism. By contrast, zirconium‐in‐rutile thermometry of inclusions armoured by garnet records crystallization temperatures, based on the upper end of the interquartile range of the data, of ～890 to 870 °C and maximum temperature around 980 °C, indicating prograde and retrograde growth close to and after peak conditions. Rutile located in retrograde microstructures records crystallization temperatures of 850 to 820 °C. Rutile intergrown with ilmenite and included within orthopyroxene, which is associated with exsolved zircon, records temperatures ～760 °C, consistent with Ti‐in‐zircon crystallization temperatures. ID‐TIMS U–Pb geochronological data from two of the four samples investigated define upper intercept ages of 641.3 ± 8.4 Ma (MSWD 0.91) and 638.8 ± 2.5 Ma (MSWD 1.03) that correlate with periods of zircon growth along the prograde segment of the P–T path. Individual zircon U–Pb dates retrieved from all samples range from 649 to 634 Ma, indicating a maximum duration of c. 15 Myr for the UHT event. This period is interpreted as recording modest thickening of hot backarc lithosphere located behind the Arenópolis Arc at the edge of the São Francisco Craton consequent upon terminal collision of the Parána Block with the arc during the amalgamation of West Gondwana.     

Integrated garnet and zircon–titanite geochronology constrains the evolution of ultra‐high–pressure terranes: An example from the Sulu orogen

Dating ultra‐high–pressure (UHP) metamorphic rocks provides important timing constraints on deep subduction zone processes. Eclogites, deeply subducted rocks now exposed at the surface, undergo a wide range of metamorphic conditions (i.e. deep subduction and exhumation) and their mineralogy can preserve a detailed record of chronologic information of these dynamic processes. Here, we present an approach that integrates multiple radiogenic isotope systems in the same sample to provide a more complete timeline for the subduction–collision–exhumation processes, based on eclogites from the Dabie–Sulu orogenic belt in eastern China, one of the largest UHP terranes on Earth. In this study, we integrate garnet Lu–Hf and Sm–Nd ages with zircon and titanite U–Pb ages for three eclogite samples from the Sulu UHP terrane. We combine this age information with Zr‐in‐rutile temperature estimates, and relate these multiple chronometers to different P–T conditions. Two types of rutile, one present as inclusions in garnet and the other in the matrix, record the temperatures of UHP conditions and a hotter stage, subsequent to the peak pressure (‘hot exhumation') respectively. Garnet Lu–Hf ages (c. 238–235 Ma) record the initial prograde growth of garnet, while coupled Sm–Nd ages (c. 219–213 Ma) reflect cooling following hot exhumation. The maximum duration of UHP conditions is constrained by the age difference of these two systems in garnet (c. 235–220 Ma). Complementary zircon and titanite U–Pb ages of c. 235–230 Ma and c. 216–206 Ma provide further constraints on the timing of prograde metamorphism and the ‘cold exhumation' respectively. We demonstrate that timing of various metamorphic stages can thus be determined by employing complementary chronometers from the same samples. These age results, combined with published data from adjacent areas, show lateral diachroneity in the Dabie–Sulu orogeny. Three sub‐blocks are thus defined by progressively younger garnet ages: western Dabie (243–238 Ma), eastern Dabie–northern Sulu (238–235 Ma) and southern Sulu terranes (225–220 Ma), which possibly correlate to different crustal slices in the recently proposed subduction channel model. These observed lateral chronologic variations in a large UHP terrane can possibly be extended to other suture zones.     

Evolution of high-Mg–Al granulites from Sunkarametta, Eastern Ghats, India: evidence for a lower crustal heating–cooling trajectory

A suite of high-Mg–Al granulites from Sunkarametta, Eastern Ghats Belt, India, shows contrasting prograde assemblages of extremely aluminous orthopyroxene+cordierite+sapphirine and similarly aluminous orthopyroxene+Ti-rich spinel in closely associated domains. Textural and compositional characteristics indicate that both were derived from prograde dehydration–melting of biotite–plagioclase–quartz-bearing protoliths. The former assemblage was stabilized at relatively more magnesian bulk composition. Geothermobarometric data and petrogenetic grid considerations place 'peak' metamorphic conditions at c. 950 °C and 9 kbar. Subsequent to peak metamorphism, the rocks cooled to c . 700–750 °C, with slight lowering of pressure, and the retrograde reactions also involved melt–solid interaction. The inferred P – T  trajectory is one of heating–cooling at lower crustal (25–30 km) depths.     

The strain‐dependent spatial evolution of garnet in a high‐P ductile shear zone from the Western Gneiss Region (Norway): a synchrotron X‐ray microtomography study

Reaction and deformation microfabrics provide key information to understand the thermodynamic and kinetic controls of tectono‐metamorphic processes, however, they are usually analysed in two dimensions, omitting important information regarding the third spatial dimension. We applied synchrotron‐based X‐ray microtomography to document the evolution of a pristine olivine gabbro into a deformed omphacite–garnet eclogite in four dimensions, where the 4^th dimension is represented by the degree of strain. In the investigated samples, which cover a strain gradient into a shear zone from the Western Gneiss Region (Norway), we focused on the spatial transformation of garnet coronas into elongated garnet clusters with increasing strain. The microtomographic data allowed quantification of garnet volume, shape and spatial arrangement evolution with increasing strain. The microtomographic observations were combined with light microscope and backscatter electron images as well as electron microprobe (EMPA) and electron backscatter diffraction (EBSD) analysis to correlate mineral composition and orientation data with the X‐ray absorption signal of the same mineral grains. With increasing deformation, the garnet volume almost triples. In the low‐strain domain, garnet grains form a well interconnected large garnet aggregate that develops throughout the entire sample. We also observed that garnet coronas in the gabbros never completely encapsulate olivine grains. In the most highly deformed eclogites, the oblate shapes of garnet clusters reflect a deformational origin of the microfabrics. We interpret the aligned garnet aggregates to direct synkinematic fluid flow, and consequently influence the transport of dissolved chemical components. EBSD analyses reveal that garnet shows a near‐random crystal preferred orientation that testifies no evidence for crystal plasticity. There is, however evidence for minor fracturing, neo‐nucleation and overgrowth. Microprobe chemical analysis revealed that garnet compositions progressively equilibrate to eclogite facies, becoming more almandine‐rich. We interpret these observations as pointing to a mechanical disintegration of the garnet coronas during strain localization, and their rearrangement into individual garnet clusters through a combination of garnet coalescence and overgrowth while the rock was deforming.     

Low‐P and high‐T metamorphism of basalts: Insights from the Sudbury impact melt sheet aureole and thermodynamic modelling

Low‐pressure and high‐temperature (LP–HT) metamorphism of basaltic rocks, which occurs globally and throughout geological time, is rarely constrained by forward phase equilibrium modelling, yet such calculations provide valuable supplementary thermometric information and constraints on anatexis that are not possible to obtain from conventional thermometry. Metabasalts along the southern margin of the Sudbury Igneous Complex (SIC) record evidence of high‐grade contact metamorphism involving partial melting and melt segregation. Peak metamorphic temperatures reached at least ~925°C at ~1–3 kbar near the SIC contact. Preservation of the peak mineral assemblage indicates that most of the generated melt escaped from these rocks leaving a residuum characterized by a plagioclase–orthopyroxene–clinopyroxene–ilmenite‐magnetite±melt assemblage. Peak temperatures reached ~875°C up to 500 m from the SIC lower contact, which marks the transition to metabasalts that only experienced incipient partial melting without melt loss. Metabasalts ~500 to 750 m from the SIC contact are characterized by a similar two‐pyroxene mineral assemblage, but typically contain abundant hornblende that overgrew clino‐ and orthopyroxene along an isobaric cooling path. Metabasalts ~750 to 1,000 m from the SIC contact are characterized by a hornblende–plagioclase–quartz–ilmenite assemblage indicating temperatures up to ~680°C. Mass balance and phase equilibria calculations indicate that anatexis resulted in 10–20% melt generation in the inner ~500 m of the aureole, with even higher degrees of melting towards the contact. Comparison of multiple models, experiments, and natural samples indicates that modelling in the Na₂O–CaO–FeO–MgO–Al₂O₃–SiO₂–H₂O–TiO₂–O₂ (NCFMASHTO) system results in the most reliable predictions for the temperature of the solidus. Incorporation of K₂O in the most recent amphibole solution model now successfully predicts dehydration melting by the coexistence of high‐Ca amphibole and silicate melt at relatively low pressures (~1.5 kbar). However, inclusion of K₂O as a system component results in prediction of the solidus at too low a temperature. Although there are discrepancies between modelling predictions and experimental results, this study demonstrates that the pseudosection approach to mafic rocks is an invaluable tool to constrain metamorphic processes at LP–HT conditions.     

SIMS determination of trace element partition coefficients between garnet, clinopyroxene and hydrous basaltic liquids at 2–7.5 GPa and 1080–1200°C

Trace element partition coefficients (D's) for up to 13 REE, Nb, Ta, Zr, Hf, Sr and Y have been determined by SIMS analysis of seven garnets, four clinopyroxenes, one orthopyroxene and one phlogopite crystallized from an undoped basanite and a lightly doped (200 ppm Nb, Ta and Hf) quartz tholeiite. Experiments were conducted at 2–7.5 GPa, achieving near-liquidus crystallization at relatively low temperatures of 1080–1200°C under strongly hydrous conditions (5–27 wt.% added water). Garnet and pyroxene D_REE show a parabolic pattern when plotted against ionic radius, and conform closely to the lattice strain model of Blundy and Wood (Blundy, J.D., Wood, B.J., 1994. Prediction of crystal–melt partition coefficients from elastic moduli. Nature 372, 452–454). Comparison, at constant pressure, between hydrous and anhydrous values of the strain-free partition coefficient (D₀) for the large cation sites of garnet and clinopyroxene reveals the relative importance of temperature and melt water content on partitioning. In the case of garnet, the effect of lower temperature, which serves to increase D₀, and higher water content, which serves to decrease D₀, counteract each other to the extent that water has little effect on garnet–melt D₀ values. In contrast, the effect of water on clinopyroxene–melt D₀ overwhelms the effect of temperature, such that D₀ is significantly lower under hydrous conditions. For both minerals, however, the lower temperature of the hydrous experiments tends to tighten the partitioning parabolas, increasing fractionation of light from heavy REE compared to anhydrous experiments.

Three sets of near-liquidus clinopyroxene–garnet two-mineral D values increase the range of published experimental determinations, but show significant differences from natural two-mineral D's determined for subsolidus mineral pairs. Similar behaviour is observed for the first experimental data for orthopyroxene–clinopyroxene two-mineral D's when compared with natural data. These differences are in large part of a consequence of the subsolidus equilibration temperatures and compositions of natural mineral pairs. Great care should therefore be taken when using natural mineral–mineral partition coefficients to interpret magmatic processes.

The new data for strongly hydrous compositions suggest that fractionation of Zr–Hf–Sm by garnet decreases with increasing depth. Thus, melts leaving a garnet-dominated residuum at depths of about 200 km or greater may preserve source Zr/Hf and Hf/Sm. This contrasts with melting at shallower depths where both garnet and clinopyroxene will cause Zr–Hf–Sm fractionation. Also, at shallower depths, clinopyroxene-dominated fractionation may produce a positive Sr spike in melts from spinel lherzolite, but for garnet lherzolite melting, no Sr spike will result. Conversely, clinopyroxene megacrysts with negative Sr spikes may crystallize from magmas without anomalous Sr contents when plotted on mantle compatibility diagrams. Because the characteristics of strongly hydrous silicate melt and solute-rich aqueous fluid converge at high pressure, the hydrous data presented here are particularly pertinent to modelling processes in subduction zones, where aqueous fluids may have an important metasomatic role.