Pre-Alpine eclogites in the Pennine Basement Complex of the Eastern Alps

An occurrence of quartz-eclogite is described from the Inner Schieferhülle unit of the Pennine Basement Complex in the SE Tauern Window, Austria.
Field relations strongly suggest a pre-Alpine age for the primary eclogitic mineral assemblage (garnet + omphacite + quartz + rutile). This implies that there was no connection between the formation of these eclogites and the late Cretaceous and Tertiary tectonic evolution of the Eastern Alps. The quartz-eclogite mineral assemblage crystallized under conditions of 620 ± 100°C and at pressures in excess of 12 kbar, and suffered amphibolitic overprinting of Alpine and possibly Hercynian age.
A four-stage polymetamorphic history is proposed for the Inner Schieferhülle:     

Evolution of Alpine eclogites in the Eastern Alps: Implications for Alpine geodynamics

In the Eastern Alps Alpine eclogites are generally associated with rocks of continental lithosphere, while eclogites that are associated with oceanic assemblages are restricted to minor exposures. Such eclogites are exposed both in the Penninic unit of the Tauern Window and in the Austroalpine nappe complex. (1) In the central southern part of the Tauern Window (Eclogite Zone) eclogites and associated high pressure metasediments of a distal continental margin are intercalated between Penninic basement units. A mylonitic eclogitic foliation and stretching lineation are contemporaneous to the high pressure metamorphism and are related to the subduction of distal Penninic continental margin sequences. Continuous subduction of cool lithosphere resulted in blueschist facies overprint of the whole Penninic nappe pile. (2) Within the Middle-AustroAlpine Koralm/Saualm region most eclogites are eclogitic mylonites documenting plastic deformation of omphacite and garnet. The meso- and macroscale structures indicate an overall extensional regime possibly related to a large-scale SE-directed ductile low-angle normal shear zone. The eclogites are associated with migmatite-like structures and are intruded by pegmatites. This indicates decreasing pressure, but isothermal or even increasing temperature conditions during exhumation.These relationships argue for the subduction of Penninic continental lithosphere in the foot-wall of the Austroalpine unit at the time of exhumation of the Koralm/Saualm eclogites. Formation of the Austroalpine eclogites is explained by subduction of continental lithosphere, and subsequent, rapid exhumation in an upper plate tectonic position within an extensional regime.     

含榴花岗片麻岩 Lu-Hf 年代学初探

对西大别四道河含石榴子石花岗片麻岩进行了锆石 U-Pb 和石榴子石 Lu-Hf 年代学测试,锆石 U-Pb谐和年龄为(223±1) Ma,石榴子石-全岩 Lu-Hf 等时线年龄为(212.2±0.7) Ma.石榴子石具有极高的母子体同位素比值(176Lu/177Hf =~300).结合锆石和石榴子石的微量元素特征,该锆石 U-Pb 年龄代表的可能是超高压/高压变质时间,石榴子石 Lu-Hf 年龄代表的是石榴子石重结晶时间,可能指示了后期退变质作用流体活动     

Retrograde P-T-t path for the Voltri Massif eclogites (Ligurian Alps, Italy): some tectonic implications

The retrograde P-T trajectory of the eclogitic Fe-Ti-gabbros from the Ligurian Alps is constrained by the appearance of mineral parageneses post-dating the Na-clinopyroxene + garnet eclogitic assemblage and indicating the following sequence of metamorphic events: (1) amphibolitic stage— edenite/katophorite + plagioclase (An_33–43) + oxides in symplectitic aggregates; (2) glaucophanic stage— a porphyroblastic glaucophanic amphibole has overgrown the symplectite, winchite also occurs as thin rims around glaucophane and both amphiboles are, sometimes, armoured by atoll garnets; (3) albite-amphibolite stage—barroisite/katophorite + albite + epidote + oxides ± chlorite overprint the glaucophanic stage minerals; (4) greenschist stage—represented by actinolite + albite + epidote + oxide paragenesis.
The metamorphic evolution is complex and the decompression path, on a P–T diagram, is significantly different from those defined in the literature for the Voltri eclogites. The main features inferred from the P–T path are the following: (1) the pressure climax does not match the thermal climax, the maximum temperature conditions are in fact achieved during the early stage of uplift; (2) a decrease in temperature, suggested by the appearance of glaucophane after the amphibolitic symplectite; (3) successive uplift, probably accompanied by an increase in temperature. The complexity of the P-T path drawn for the Voltri eclogites can be explained with a mechanism of successive underthrusts propagating from the innermost to the outermost sector of the Ligurian Alps.     

Cr-rich magnesiochloritoid eclogites from the Monviso ophiolites (Western Alps, Italy)

Cr-rich magnesiochloritoid in the eclogitized ophiolites of the Monviso massif occurs in the least differentiated rocks of the gabbroic sequence (troctolites to melatroctolites). Chloritoid ( X _Mg=0.63–0.85; Cr≤0.55, atoms) co-exists with omphacite, talc and garnet. Minor, syn-eclogitic minerals are chromite, rutile and sometimes magnesite and Cr–Ti oxides.
Coronitic textures, indicative of a static recrystallization, characterize the analysed samples. Layers of variable mineral composition develop among igneous plagioclase, olivine, clinopyroxene and spinel. The minerals in the coronitic layers display sharp compositional zonings. The igneous minerals are commonly not preserved; their presence in the original assemblage is inferred from the mineralogical composition of the pseudomorphs.
Syn-eclogitic volatile components are indicated by the development of OH-bearing minerals (e.g. chloritoid & talc) and carbonates (e.g. magnesite), and supported by the presence of coarse-grained and fibrous mineral growths. The complex pseudomorphic replacements of igneous minerals suggest that these rocks changed their mineralogical composition prior to the eclogite facies recrystallization, most likely during ocean-floor metamorphism. It is suggested that syn-eclogitic fluids formed by breakdown reactions of pre-eclogitic volatile-bearing minerals.
Geothermobarometry indicates that the investigated rocks recrystallized at a depth corresponding to 2.4  GPa and temperatures of 620±50  °C. The attainment of high-pressure conditions is supported by the presence of magnesiochloritoid, magnesite and garnet with high pyrope content (up to 58  mol%). P–T  estimates point to a very low thermal gradient (about 9  °C km⁻¹), comparable to that deduced in the adjacent Dora-Maira ultra-high pressure unit.     

Crystallization and very rapid exhumation of the youngest Alpine eclogites (Tauern Window,Eastern Alps) from Rb/Sr mineral assemblage analysis

Multimineral Rb/Sr internal isochrons from eclogite facies rocks of the Eclogite Zone (Tauern Window, Eastern Alps) consistently yield an Early Oligocene age of 31.5±0.7 Ma. This age has been obtained both for late-prograde, dehydration-related eclogitic veins, and for rocks variably deformed and recrystallized under eclogite facies conditions (2.0–2.5 GPa, 600°C). Initial Sr-isotopic equilibria among all phases indicate absence of significant post-eclogitic isotope redistribution processes, therefore the ages date eclogite facies assemblage crystallization. Equilibria also prove that no prolonged pre-eclogite facies history is recorded in the rocks. Instead, subduction, prograde mineral reactions, and eclogitization proceeded rapidly. Fast exhumation immediately after eclogitization, with minimum rates >36 mm/a is inferred from a 31.5±0.5 Ma internal mineral isochron age of a post-eclogitic greenschist facies vein assemblage. Such rates equal typical subduction rates. Late Eocene to Early Oligocene subduction of the European continental margin, with subsequent rapid exhumation of high-pressure nappe complexes has previously been recognized only in the Western Alps. The new data signify synchronous continental collision all along the Alpine belt. Our results demonstrate the unique potential of Rb/Sr assemblage system analysis for precise dating of both eclogite facies and post-eclogitic events, thus for precisely constraining exhumation rates of deep-seated rocks, and for straightforward linkage of petrologic evidence with isotopic ages.     

Geochemistry and geochronology of meta-igneous rocks from the Tokat Massif, north-central Turkey: implications for Tethyan reconstructions

Located in the eastern Pontides of the Sakarya Zone in north-central Turkey, the Tokat Massif records the closure of both the Paleo-Tethyan (Karakaya Complex) and Neo-Tethyan ocean basins. Meta-igneous samples collected from the region were studied to determine their sources and ages. We find significant geochemical differences between metagabbros of the Karakaya and Neo-Tethyan units in terms of their trace elements: Neo-Tethyan rocks are consistent with generation in an island arc setting, whereas Karakaya assemblages were likely generated in an oceanic spreading-center environment. Karakaya metagabbros also contain glaucophane, consistent with subduction subsequent to formation. Small (2–50 μm) zircon and baddeleyite grains from four Karakaya metagabbros were dated in thin section using an ion microprobe. The results demonstrate the reliability of the method to directly constrain the tectonomagmatic history of these types of assemblages. The rocks yield Late Permian/Early Triassic ²³⁸U/²⁰⁶Pb crystallization ages of 258 ± 14 Ma (±1σ, zircon) and 254 ± 8 Ma (±1σ, baddeleyite) and an Early Cretaceous minimum metamorphic age of 137 ± 8 Ma (±1σ, zircon). Some zircon grains and baddeleyite grains with zircon overgrowths yield Early to Middle Jurassic ages. Here we present a model in which metamorphism and deformation in this region occurred during northward subduction and closure of a Paleo-Tethyan ocean basin and accretion of the Karakaya units to the Laurasian continental margin. This was followed by the onset of closure of the Neo-Tethys during the Campanian-Paleocene and accretion of island arc units to the Tokat region.     

Glaucophanites and eclogites from Val Chiusella,Sesia-Lanzo zone (Italian Alps)

In the metabasites of Val Chiusella, metamorphic assemblages are present, corresponding to the glaucophane schist facies, i.e. garnet glaucophanites to omphacite-garnet glaucophanites, as well as to the eclogite facies, i.e., glaucophane eclogites, eclogites, and omphacite felses. Both groups of assemblages are divided by the critical reaction 1 zoisite +1 glaucophane 1.2 omphacite+0.8 garnet+0.7 paragonite +1.4 quartz+0.8 H₂O. From textural evidence it is clear that in the investigated area this reaction proceeded to the right according to a prograde metamorphism. Correspondingly, K ^garn-cpx _D(Fe/Mg) values of coexisting garnet-omphacite pairs in the glaucophane schist facies assemblages are higher than in the eclogite facies assemblages and reflect a temperature increase from about 450 ° C to about 550 ° C at minimum water vapour pressures of 12 to 16 kb.     

Coupled Lu–Hf and Sm–Nd geochronology constrains garnet growth in ultra-high-pressure eclogites from the Dabie orogen

Ultra-high-pressure eclogites from the Dabie orogen that formed over a range in temperatures (∼600 to > 700 °C) have been investigated with combined Lu–Hf and Sm–Nd geochronology. Three eclogites, sampled from Zhujiachong, Huangzhen and Shima, yield Lu–Hf ages of 240.0 ± 5.0, 224.4 ± 1.9 and 230.8 ± 5.0 Ma and corresponding Sm–Nd ages of 222.5 ± 5.0, 217.6 ± 6.1 and 224.2 ± 2.1 Ma respectively. Well-preserved prograde major- and trace-element zoning in garnet in the Zhujiachong eclogite suggests that the Lu–Hf age mostly reflects an early phase of garnet growth that continued over a time interval of c. 17.5 Myr. For the Huangzhen eclogite, despite preserved elemental growth zoning in garnet, textural study reveals that the Lu–Hf age is biased towards a later garnet growth episode rather than representing early growth. The narrow time interval of <6.6 Myr defined by the difference between Lu–Hf and Sm–Nd ages indicates a short final garnet growth episode and suggests a rapid cooling stage. By contrast, the rather flat element zoning in garnet in the Shima eclogite suggests that Lu–Hf and Sm–Nd ages for this sample have been reset by diffusion and are cooling ages. The new Lu–Hf ages point to an initiation of prograde metamorphism prior to c . 240 Ma for the Dabie orogen, while the exact peak metamorphic timing experienced by specific samples ranges between c . 230 to c. 220 Ma.     

Correlation by Rb-Sr geochronology of garnet growth histories from different structural levels within the Tauern Window,Eastern Alps

In order to evaluate rates of tectonometamorphic processes, growth rates of garnets from metamorphic rocks of the Tauern Window, Eastern Alps were measured using Rb-Sr isotopes. The garnet growth rates were determined from Rb-Sr isotopic zonation of single garnet crystals and the Rb-Sr isotopic compositions of their associated rock matrices. Garnets were analyzed from the Upper Schieferhülle (USH) and Lower Schieferhülle, (LSH) within the Tauern Window. Two garnets from the USH grew at rates of 0.67 _–0.13 ^+0.19 mm/million years and 0.88 _–0.19 ^+0.34 mm/million years, respectively, indicating an average growth duration of 5.4±1.7 million years. The duration of growth coupled with the amount of rotation recorded by inclusion trails in the USH garnets yields an average shear-strain rate during garnet growth of 2.7 _–0.7 ^+1.2 ×10^-14 s^-1. Garnet growth in the sample from the USH occurred between 35.4±0.6 and 30±0.8 Ma. The garnet from the LSH grew at a rate of 0.23±0.015 mm/million years between 62±1.5 Ma and 30.2±1.5 Ma. Contemporaneous cessation of garnet growth in both units at 30 Ma is in accord with previous dating of the thermal peak of metamorphism in the Tauern Window. Correlation with previously published pressure-temperature paths for garnets from the USH and LSH yields approximate rates of burial, exhumation and heating during garnet growth. Assuming that theseP — T paths are applicable to the garnets in this study, the contemporaneous exhumation rates recorded by garnet in the USH and LSH were approximately 4 _–2 ⁺³ mm/year and 2±1 mm/year, respectively.     

Geochemistry of high-grade eclogites and metarodingites from the Central Alps

Analyses for major, minor, and trace element contents of metamorphosed, variably rodingitized mafic rocks demonstrate substantial removal of Na and as much as three-fold gains in Ca as a consequence of rodingitization. Modest declines in Si and Fe can be explained in terms of dilution effects. Losses in K and Ba do not correlate with Ca% and may have been caused by an alteration process not related to the rodingitization. The Ca-metasomatism was not accompanied by a gain in Sr. The relative contents of Ti, Zr, Hf, Y, Co, Sc, and heavy REE show no readily detectable changes, despite the rodingitization (±other alteration) and subsequent metamorphisms, namely, eclogite facies (T800° C, P 20kbar) followed by amphibolite facies, sillimanite zone. Protoliths were tholeiitic basalt or diabase, and gabbro, with trace element contents indicative of a spreading center origin. Trace element and REE patterns indicate low-pressure fractionation of this magma, with plagioclase stable. This petrogenesis is consistent with prior conclusions on the shallow crustal origin of the protolith of the eclogite-metarodingite-garnet lherzolite suite in the Cima Lunga-Adula nappe, Central Alps. Based on their bulk chemical composition, the mafic rocks in this suite could be the equivalent of Mesozoic ophiolitic rocks in the more external parts of the Alps.     

Zircon formation during fluid circulation in eclogites (Monviso, Western Alps): implications for Zr and Hf budget in subduction zones

The zircons from an eclogite and an enclosed eclogite-facies vein from the Monviso ophiolite (Western Alps) display contrasting chemical and morphologic features and document different stages of the evolution of the ophiolite. The zircons from the eclogite show a typical magmatic zoning and are enriched in heavy rare earth elements (HREEs) over middle rare earth elements (MREEs) and have an accentuated negative Eu anomaly, which indicates that the grains co-crystallised with plagioclase. These magmatic zircons document the formation of oceanic crust at 163 ± 2 Ma. In contrast, zircons from the vein contain inclusions of garnet, omphacite, and rutile, which indicate that they crystallised under eclogite-facies conditions. The vein zircons have Th/U ratios < 0.09, lack Eu anomalies, and are only weakly enriched in HREE with respect to MREE. These features are consistent with a garnet-bearing, plagioclase-free, i.e., eclogite-facies paragenesis. Vein zircons yield an age of 45 ± 1 Ma, which is evidence for Eocene subduction-zone metamorphism of the Monviso ophiolite.In the vein, the apparent coexistence of zircon, omphacite, and garnet permits the determination of a set of trace element distribution coefficients among these minerals at high pressure. This set of partitioning can demonstrate chemical equilibrium among these phases in rocks that show less clear evidence of textural equilibrium. In addition, zircon age can now be linked to sensors of metamorphic pressure-temperature conditions. The presence of zircon and rutile in the vein is another example of high field strength element (HFSE) mobility over short distances in aqueous fluids at eclogite-facies conditions. However, the concentrations of Zr and Hf in the aqueous fluid are estimated to be at least a factor of 10 less than primitive mantle values.Mass balance calculations demonstrate that zircon hosts > 95% of the bulk Zr, 90% of Hf, and ∼25% of U in the vein. Zircon is a residual phase in subducted basalts and sediments up to temperatures of at least 800 to 900 °C. Therefore, residual zircon in subducted crust, together with rutile, control the HFSE in liberated subduction zone fluids/melts and might be partly responsible for negative Zr and Hf anomalies in subduction zone magmas.     

Carbonation and decarbonation of eclogites: the role of garnet

Carbonates are potentially significant hosts for primordial and subducted carbon in the Earth's mantle. In addition, the coexistence of carbonate with silicates and reduced carbon (diamond or graphite), allows constraints to be placed on the oxidation state of the mantle. Carbonate-silicate-vapor reactions control how carbonate + silicate assemblages may form from carbon-bearing vapor + silicate assemblages with increasing pressure. In olivine-bearing rocks such as peridotite, considered the dominant rock type in the upper mantle, the lowest-pressure carbonate-forming reactions involve olivine (±clinopyroxene) reacting with CO₂ (e.g., Wyllie et al. 1983). In eclogitic rocks, the essential mineral assemblage is omphacitic clinopyroxene + garnet, without olivine. Therefore, alternative carbonate-forming reactions must be sought. The carbonation of clinopyroxene via the reaction dolomite + 2 coesite = diopside + 2 CO₂ was studied experimentally by Luth (1995). The alternative possibility that garnet reacts with CO₂ is explored here by determining the location of the reaction 3 magnesite + kyanite + 2 coesite = pyrope + 3 CO₂ between 5 and 11 GPa in multi-anvil apparatus. At the temperatures ≥1200 °C, carbonation of eclogitic rocks with increasing pressure will proceed initially by reaction with clinopyroxene, because the pyrope-carbonation reaction lies at higher pressures for a given temperature than does the diopside-carbonation reaction. Diluting the pyrope component of garnet and the diopside component of clinopyroxene to levels appropriate for mantle eclogites does not change this conclusion. At lower temperatures, appropriate for “cold” slabs, it is possible that the converse situation will hold, with initial carbonation proceeding via reaction with garnet, but this possibility awaits experimental confirmation. Decarbonation of an eclogite under “normal mantle” geothermal conditions by a decrease in pressure, as in an ascending limb of a mantle convection cell, would be governed by the formation of clinopyroxene + CO₂. At higher pressure than this reaction, any CO₂ produced by the breakdown of magnesite reacting with kyanite and coesite would react with clinopyroxene to produce dolomite + coesite. Release of CO₂ from eclogite into mantle peridotite would form carbonate at sub-solidus conditions and produce a dolomitic carbonate melt if temperatures are above the peridotite-CO₂ solidus. Received: 4 May 1998 / Accepted: 23 December 1998     

Water in garnet of garnetite (metarodingite) and eclogite from the Erzgebirge and the Lepontine Alps

Little is known about water in nominally anhydrous minerals of orogenic garnet peridotite and enclosed metabasic rocks. This study is focused on peridotite-hosted eclogite and garnetite (metarodingite) from the Erzgebirge (EG), Germany, and the Lepontine Alps (LA), Switzerland. Newly discovered, peridotite-hosted eclogite in the Erzgebirge occurs in the same ultra-high pressure (UHP) unit as gneiss-hosted coesite eclogite, from which it is petrologically indistinguishable. Garnet is present in all mafic and ultramafic high pressure (HP) rocks providing for an ideal proxy to compare the H₂O content of the different rock types. Garnet composition is very similar in EG and LA samples and depends on the rock type. Garnet from garnetite, compared to eclogite, contains more CaO (garnetite: 10.5–16.5 wt%; eclogite: 5–11 wt%) and is also characterized by an anomalous REE distribution. In contrast, the infrared (IR) spectra of garnet from both rock types reveal the same OH absorption bands that are also identical to those of previously studied peridotitic garnet from the same locations. Two groups of IR bands, SW I (3,650 ± 10 cm⁻¹) and SW II (3,570–3,630 cm⁻¹) are ascribed to structural hydroxyl (colloquially ‘water’). A third, broad band is present in about half of the analysed garnet domains and related to molecular water (MW) in submicroscopic fluid inclusions. The primary content of structural H₂O, preserved in garnet domains without fluid inclusions (and MW bands), varies systematically—depending on both the location and the rock type. Garnet from EG rocks contains more water compared to LA samples, and garnet from garnetite (EG: 121–241 wt.ppm H₂O; LA: 23–46 wt.ppm) hosts more water than eclogitic garnet (EG: 84 wt.ppm; LA: 4–11 wt.ppm). Higher contents of structural water (SW) are observed in domains with molecular water, in which the SW II band (being not restricted to HP conditions) is simultaneously enhanced. This implies that fluid influx during decompression not only led to fluid inclusions but also favoured the uptake of secondary SW. The results signify that garnet from all EG and LA samples was originally H₂O-undersaturated. Combining the data from eclogite, garnetite and previously studied peridotite, H₂O and CaO are positively correlated, pointing to the same degree of H₂O-undersaturation at peak metamorphism in all rock types. This ubiquitous water-deficiency cannot be reconciled with the derivation of any of these rocks from the lowermost part of the mantle wedge that was in contact with the subducting plate. This agrees with the previously inferred abyssal origin for part of the rocks from the LA (Cima di Gagnone). A similar origin has to be invoked for the Erzgebirge UHP unit. We suggest that all mafic and ultramafic rocks of this unit not only shared the same metamorphic evolution but also a common protolith origin, most probably on the ocean floor. This inference is supported by the presence of peridotite-hosted garnetite, representing metamorphosed rodingite.     

Lu-Hf年代学研究——以大别榴辉岩为例

本文利用多接收电感耦合等离子体质谱仪（MC-ICP-MS）进行了同位素稀释法Lu-Hf年代学研究,建立了全岩Lu、Hf的分离条件.分离后Lu溶液中^176Lu/^176Yb大于30,分离后Hf溶液中Lu和Yb对Hf的干扰（^176Lu/^176Hf和^176Yb/^176Hf分别小于2×10^-6和2×10^-4）采用指数法则进行校正。建立了MC-ICP-MS进行Lu、Hf同位素高精度准确测试的校正方法.天然样品中Hf同位素^176Hf/^177Hf的内部测试精度优于0.0015%,外部精密度优于0.0010%.应用本文建立的方法获得大别双河榴辉岩石榴子石-全岩Lu-Hf等时线年龄为254±16Ma（2σ）.年龄误差稍大与该样品石榴子石中较低的Lu含量（1.1 μg/g）和石榴子石-全岩的未充分分开的^176Lu/^177Hf比值（分别为0.05和0.01）有关。     

Geodynamic models for the Alpine type of orogeny (Test-case II: the Alps in central Europe)

Two modern geodynamic models on the Alps are tested: plate tectonics (mechanical effects of colliding lithospheric mega-units), and mantle diapirism (mechanical and geochemical effects of ultra-lowvelocity bodies, rising from the low-velocity layer or asthenosphere). Verification occurs by means of comparing the expectations of these models (their ‘prognoses’) with the great wealth of available ‘diagnostic’ facts on the geology and geophysics of the Alps. It appears that the picture of the Alpine structural evolution, drafted by plate tectonics, is inadequate to describe the observed reality, whereas the idea of mantle diapirism, combined with crustal corrosion (geochemical oceanization) and gravity tectonics, provides a functionally correct model, that logically and coherently explains the entire Alpine cycle of orogeny.In the concluding remarks comparisons are made with the formation of island arcs in the western Pacific, the origin of the basin-and-range topography and volcanicity along the eastern margin of the Pacific, as well as the Sunda arc of Indonesia.     

On the origin of oriented rutile needles in garnet from UHP eclogites

Although oriented rutile needles in garnet have been reported from several ultrahigh‐pressure (UHP) rocks and considered to be important UHP indicators, their crystallographic features including growth habit and lattice correspondences with garnet host have never been properly characterized. This paper presents a detailed analytical electron microscopic (AEM) study on evenly distributed oriented rutile needles in garnet of two eclogitic rocks from Sulu. Some garnet in one UHP diamondiferous quartzofeldspathic rock from the Saxonian Erzgebirge, and in one high‐pressure (HP) felsic granulite from Bohemia also contain a few unevenly distributed oriented rutile needles. They have also been studied for the purpose of comparison. Despite different distribution patterns, AEM revealed that all rutile needles are oriented along the 〈111〉 directions of garnet with their lateral sides surrounded by the {110} planes of garnet, and that the growth directions of most needles are close to the normal of the {101} planes of rutile. No other specific crystallographic orientation relationships between rutile and garnet host were observed, and there is no pyroxene associated with rutile, as necessitated by the precipitation reaction of rutile in garnet as previously proposed. A simple solid‐state precipitation scenario for the formation of the rutile needles in garnet in these two eclogitic rocks is not justified. Three alternative mechanisms are considered for the formation of oriented rutile needles: (i) the rutile needles may be inherited from precursor minerals; (ii) the rutile needles may be formed by a dissolution–reprecipitation mechanism; and (iii) the rutile needles may be formed by cleaving and healing of garnet with rutile deposition. None of these mechanisms can fully explain the observations, although the first one is less likely and the third one is preferred. This study presents an example where the presence of oriented/aligned inclusions in minerals does not necessarily imply a precipitation origin.