A TEM study of the oriented orthopyroxene and forsterite inclusions in garnet from Otrøy garnet peridotite,WGR, Norway: new insights on crystallographic characteristics and growth energetics of exsolved pyroxene in relict majoritic garnet

Regularly oriented orthopyroxene (opx) and forsterite (fo) inclusions occur as opx + rutile (rt) or fo + rt inclusion domains in garnet (grt) from Otrøy peridotite. Electron diffraction characterization shows that forsterite inclusions do not have any specific crystallographic orientation relationships (COR) with the garnet host. In contrast, orthopyroxene inclusions have two sets of COR, that is, COR‐I: <111>_grt//<001>_opx and {110}_grt～//～{100}_opx (～13° off) and COR‐II: <111>_grt//<011>_opx and {110}_grt～//～{100}_opx (～14° off), in four garnet grains analysed. Both variants of orthopyroxene have a blade‐like habit with one pair of broad crystal faces parallel/sub‐parallel to {110}_grt plane and the long axis of the crystal, <001>_opx for COR‐I and <011>_opx for COR‐II, along <111>_grt direction. Whereas the lack of specific COR between forsterite and garnet, along with the presence of abundant infiltrating trails/veinlets decorated by fo + rt at garnet edges, provide compelling evidence for the formation of forsterite inclusions in garnet through the sequential cleaving–infiltrating–precipitating–healing process at low temperatures, the origin of the epitaxial orthopyroxene inclusions in garnet is not so obvious. In this connection, the reported COR, the crystal habit and the crystal growth energetics of the exsolved orthopyroxene in relict majoritic garnet were reviewed/clarified. The exsolved orthopyroxene in a relict majoritic garnet follows COR‐III: {112}_grt//{100}_opx and <111>_grt//<001>_opx. Based on the detailed trace analysis on published SEM images, these exsolved orthopyroxene inclusions are shown to have the crystal habit with one pair of broad crystal faces parallel to {112}_grt//{100}_opx and the long crystal axis along <111>_grt//<001>_opx. Such a crystal habit can be rationalized by the differences in oxygen sub‐lattices of both structures and represents the energetically favoured crystal shape of orthopyroxene inclusions in garnet formed by solid‐state exsolution mechanism. Considering the very different COR, crystal habit, as well as crystal growth direction, the orthopyroxene inclusions in garnet of the present sample most likely had been formed by mechanism(s) other than solid‐state exsolution, regardless of their regularly oriented appearance in garnet and the COR specification between orthopyroxene and garnet. In fact, the crystallographic characteristics of orthopyroxene and the similar chemical compositions of garnet at opx + rt inclusion domains, fo + rt inclusion domains/trails and garnet rim suggest that the orthopyroxene inclusions in the garnet are most likely formed by similar cleaving‐infiltration process as forsterite inclusions, though probably at an earlier stage of metamorphism. This work demonstrates that the oriented inclusions in host minerals, with or without specific COR, can arise from mechanism(s) other than solid‐state exsolution. Caution is thus needed in the interpretation of such COR, so that an erroneous identification of exhumation from UHP depths would not be made.     

Oriented inclusions of pyroxene,amphibole and rutile in garnet from the Lüliangshan garnet peridotite massif,North Qaidam UHPM belt,NW China: an electron backscatter diffraction study

Oriented inclusions of clinopyroxene, orthopyroxene, sodic amphibole and rutile have been identified in garnet from the Lüliangshan garnet peridotite massif in the North Qaidam ultrahigh‐pressure metamorphic (UHPM) belt, northern Tibetan Plateau, NW China. Electron backscatter diffraction (EBSD) analyses demonstrate that nearly half of the measured intracrystalline clinopyroxene (8 out of 17) have topotactic crystallographic relationships with host garnet, that is, (100)_Cpx//{112}_Grt, (010)_Cpx//{110}_Grt and [001]_Cpx//<111>_Grt. One‐fifth of the oriented sodic amphibole (23 out of 110) inclusions of have topotactic crystallographic relationships with host garnet, that is, (010)_Amp//{112}_Grt, (100)_Amp//{110}_Grt and [001]_Amp//<111>_Grt. Over a third of rutile (36 out of 99) inclusions also show a close crystallographic orientation relationship with host garnet in that one <103>_Rt and one <110>_Rt parallel to two <111>_Grt while the axes of [001]_Rt exhibit small girdles centred the axes of <111>_Grt. But, no ‘well‐fit’ crystallographic relationship was observed between orthopyroxene inclusions and host garnet. Considering a very long and complex history for the Lüliangshan garnet peridotite, we suggest that the low fit rates for these oriented minerals may result from several possible assumptions including different generations or multi‐stage formation mechanisms, heterogeneous nucleation and growth under non‐equilibrium conditions, and partial changes of initial crystallographic orientations of some inclusions. However, the residual quantitative ‘well‐fit’ crystallographic information is sufficient to indicate that the nucleation and growth of many pyroxene, amphibole and rutile are controlled by the lattice of the host garnet. The revealed close topotactic relationships accompanied by clear shape orientations provide quantitative microstructural evidence demonstrating a most likely exsolution/precipitate origin for at least some of the oriented phases of pyroxene, sodic amphibole and rutile from former majoritic garnet and support an ultra‐deep (>180 km depth) origin of the Lüliangshan garnet massif.     

Precipitation of pyroxenes and Mg2SiO4 from majoritic garnet: simulation of peridotite exhumation from great depth

Our experimental simulations of the exhumation path of mantle peridotites show that high‐temperature (1400 °C) decompression of lherzolite from 14 to 13 and 12 GPa results in exsolution of interstitial blebs of diopside and Mg₂SiO₄ (wadsleyite) lamellae from majoritic garnet. At lower pressures (from 8 to 5 GPa, at T = 1400 °C) only enstatite exsolves as blebs at garnet boundaries. Continuous high‐temperature decompression from 14 to 7 GPa produces zoned majoritic garnet containing blebs of exsolved pyroxenes inside garnet rims. No intracrystalline precipitation of pyroxene was observed in garnet, although such lamellae are found in some natural garnet peridotites. The explanation appears to be the three orders of magnitude difference in grain size between experimental and natural specimens. Our data suggest that Mg₂SiO₄ and diopside exsolutions reflect the deepest point of the exhumation path of garnet peridotites, whereas enstatite precipitation may be restricted to garnets with less majoritic component at shallower depths.     

Opx–Cpx exsolution textures in lherzolites of the Cretaceous Purang Ophiolite (S. Tibet,China), and the deep mantle origin of Neotethyan abyssal peridotites

ABSTRACT

We investigated lherzolitic peridotites in the Cretaceous Purang ophiolite along the Yarlung Zhangbo suture zone (YZSZ) in SW Tibet to constrain their mantle–melt evolution history. Coarse-grained Purang lherzolites contain orthopyroxene (Opx) and olivine (Ol) porphyroclasts with embayments filled by small olivine (Ol) neoblasts. Both clinopyroxene (Cpx) and Opx display exsolution textures represented by lamellae structures. Opx exsolution (Opx1) in clinopyroxene (Cpx1) is made of enstatite, whose compositions (Al₂O₃ = 3.85–4.90 wt%, CaO = <3.77 wt%, Cr₂O₃ = 0.85–3.82 wt%) are characteristic of abyssal peridotites. Host clinopyroxenes (Cpx1) have higher Mg#s and Na₂O, with lower TiO₂ and Al₂O₃ contents than Cpx2 exsolution lamellae in Opx, and show variable LREE patterns. Pyroxene compositions of the lherzolites indicate 10–15% partial melting of a fertile mantle protolith. P–T estimates (1.3–2.3 GPa, 745–1067°C) and the trace element chemistry of pyroxenes with exsolution textures suggest crystallization depths of ~75 km in the upper mantle, where the original pyroxenes became decomposed, forming exsolved structures. Further upwelling of lherzolites into shallow depths in the mantle resulted in crystal–plastic deformation of the exsolved pyroxenes. Combined with the occurrence of microdiamond and ultrahigh-pressure (UHP) mineral inclusions in chromites of the Purang peridotites, the pyroxene exsolution textures reported here confirm a multi-stage partial melting history of the Purang lherzolites and at least three discrete stages of P-T conditions in the course of their upwelling through the mantle during their intra-oceanic evolution.     

Exsolution textures of orthopyroxene and clinopyroxene in high-grade BIF of the Voronezh Crystalline Massif: evidence of ultrahigh-temperature metamorphism

Archaean banded iron formation (BIF) of the Voronezh Crystalline Massif (VCM) contains coexisting clino‐ and orthopyroxenes with exsolution textures. The pyroxene in the VCM BIF is found in two generations, with only the first generation containing such textures. Clinopyroxene contains large (up to 5–10 μm) (0 0 1) orthopyroxene (Opx1L) lamellae in a host clinopyroxene (Cpx1H). This host Cpx, in turn, exsolves into thin (～1 μm) (1 0 0) lamellae of orthopyroxene (Opx2L) and clinopyroxene (Cpx2H). Orthopyroxene exhibits similar exsolution textures with large (up to 50 μm) (0 0 1) clinopyroxene (Cpx1L) lamellae developed in a host orthopyroxene (Opx1H). This clinopyroxene Cpx1L shows further exsolution of thin (1 0 0) Opx2LL lamellae and clinopyroxene (Cpx2LH). Point microprobe analysis, raster‐mode microprobe analysis, and microprobe element mapping of mineral grains with a large number of point analysis were used to determine the composition of the exsolution products and the primary chemistry of the coexisting clinopyroxene (CaO = 14.86–17.26 wt%) and pigeonite (CaO = 4.45–6.23 wt%). These pyroxenes crystallized during the peak of metamorphism, and application of the Lindsley geothermometer suggested that they formed at extremely high temperatures of about 1000 °C. Primary very dense CO₂‐rich fluid inclusions (ρ = 1.152 g cm^?3, T_h = ?49.2 °C) were discovered for the first time in these BIF. With these data, the metamorphic pressure was estimated as 10–11 kbar (depth 36–40 km). Such ultrahigh temperature–high pressure (UHT–HP) conditions for the regional metamorphism of the Precambrian BIF have previously been reported only for Archaean meta‐ironstone from the Napier Complex (Enderby Land, Antarctica). They give an insight into the peak metamorphic conditions of the BIF of the VCM, their burial under thickened continental crust during this period of Earth evolution and suggest a more complicated multistage metamorphic and tectono‐thermal history for the region than has previously been postulated.     

High-grade regional metamorphism of ultramafic and mafic rocks from the Archaean Sargur terrane,Karnataka, South India

Deformed and metamorphic ultramafic to mafic rocks emplaced into the Archaean Sargur supracrustal series (>3.0 Ga) in Karnataka, southern India, represent layered igneous bodies. The terrane has been affected by several episodes of deformation and metamorphism in the time span from 3.4 to 2.5 Ga ago.During the regional metamorphism about 2.5 Ga ago the igneous bodies re-equilibrated partly or completely at conditions of the upper amphibolite to granulite facies. The development of sagvandites with enstatite + magnesite and anthophyllite + magnesite-bearing assemblages, and of mafic garnet—pyroxene charnockites indicates the presence of CO₂-rich intergranular fluids (X_CO2 ? 0.5) in these rocks during metamorphism.The physical conditions of metamorphism have been estimated by applying methods of geothermobarometry to the recrystallized ultramafic assemblages with olivine, pyroxenes and spinel and to the charnockitic assemblages with garnet, pyroxenes, plagioclase and quartz. A best temperature estimate of 700 ± 30°C was derived with the geothermometers of Evans and Frost (Ol—Spi), Fabriès (Ol—Spi), Wells (Opx—Cpx), Powell (Opx—Cpx), Ellis and Green (Gra—Cpx), Lal and Raith (Gra—Opx), and Danckwerth and Newton (Al₂O₃-content in opx). A mean pressure estimate of 8.6 ± 0.8 kbar has been obtained with the models of Perkins and Newton (Gar—Opx/Cpx—Plag—Qtz). The P—T data indicate a minimum crustal thickness of about 35 km at c. 2.5 Ga in this part of the South Indian Archaean craton.     

Super-silicic garnet microstructures from an orogenic garnet peridotite, evidence for an ultra-deep (>6 GPa) origin

We report the field, petrographic and mineral chemical characteristics of relict super‐silicic (=majoritic) garnet microstructures from the Otrøy peridotites in the Western Gneiss Region, Norway. The evidence for the former existence of super‐silicic garnet consists of two‐pyroxene exsolution microstructures from garnet. Estimates of the initial composition of the super‐silicic garnet imply pressures of 6–6.5 GPa, indicating that the Otrøy garnet peridotites were derived from depths >185 km. The garnet peridotites consist of inter‐banded variable compositions with c. 50% garnet peridotite and 50% garnet‐free peridotite. Two distinct garnet types were identified: (a) normal matrix garnet, grain‐size ≤4 mm, and (b) large isolated single garnet crystals and/or (polycrystalline) garnet nodules up to 10 cm in size. Large garnet nodules occur only within limited bands within the garnet peridotites. The relicts of super‐silicic garnet were exclusively found in some (not all) of the larger garnet nodules. Petrographic observations revealed that the microstructure of nodular garnet consists of the following four characteristic elements. (1) Individual garnet nodules are polycrystalline, with grain sizes of 2–8 mm. Garnet grain boundaries are straight with well‐defined triple junctions. (2) Some garnet triple junctions and garnet grain boundaries are decorated by interstitial orthopyroxene. (3) Cores of larger polycrystalline garnet contain two‐pyroxene exsolution microstructures. (4) Precipitation‐free rims (2 mm thick) surround garnet cores containing the exsolved pyroxene microstructure. Pyroxene exsolution from super‐silicic garnet was subsequently followed by brittle–ductile deformation of garnet. Both exsolved pyroxene needles and laths become undulous or truncated by fractures. Simultaneous garnet plasticity is indicated by the occurrence of high densities of naturally decorated dislocations. Transmission electron microscopy observations indicate that decoration is due to Ti‐oxide precipitation. Estimates of the P–T conditions for mineral chemical equilibration were obtained from geothermobarometry. The mineral compositions equilibrated at mantle conditions around 805±40 °C and 3.2±0.2 GPa. These P–T estimates correspond to cold continental lithosphere conditions at depths of around 105 km. From a combination of both depth estimates it can be concluded that the microstructural memory of the rock extends backwards to twice as great a depth range as obtained by thermobarometric methods. Available geochronological and geochemical data of Norwegian garnet peridotites suggest a multi‐stage, multi‐orogenic exhumation history.     

Relict Majoritic Garnet Microstructures from Ultra-Deep Orogenic Peridotites in Western Norway

Protogranular, porphyroclastic and equigranular (or equant-polygonal)garnet microstructures from Mg–Cr type orogenic garnetperidotites, Otrøy, Western Gneiss Region, Norway, havebeen studied using naked eye, light-optical, electron-opticaland confocal laser (fluorescence) microscopy techniques. Protogranularand porphyroclastic garnets contain microstructural evidencefor the former existence of majoritic (or super-silicic) garnet.The microstructural evidence consists of exsolution texturesinvolving pyroxene. Two types of exsolution microstructuresoccur—needles parallel to <111>_grt and interstitialgrains. The maximum volume percentage for intra-crystallinepyroxene exsolution is 2·7, and 3·6 for inter-crystallinepyroxene exsolution. The maximum pyroxene total volume percentagemeasured in one single protogranular or porphyroclastic garnetis 4·0. This value, at 1200°C, corresponds to minimumpressures of 6·4 GPa (     

Origin of rutile needles in star garnet and implications for interpretation of inclusion textures in ultrahigh‐pressure metamorphic rocks

The asterism effect of star garnet has been attributed to the oriented distribution of needle‐like rutile inclusions. Rutile needles occur in garnet from a wide range of metamorphic settings and rock bulk compositions, and their origin has been ascribed to different mechanisms, such as exsolution, and used to interpret petrological and tectonic processes. Results from an optical and transmission electron microscopy of Idaho star garnet indicate a co‐precipitation origin. It was found that rutile needles are predominantly oriented along the <103>_rt//<111>_grt and <001>_rt//<001>_grt directions following multiple crystallographic orientation relationships (CORs); i.e. COR‐1, 2, 2′, 3, 4 and 5 in 6‐ray star garnet, and are oriented solely along the <103>_rt//<111>_grt directions following exclusively COR‐2 in 4‐ray star garnet. The sole presence of COR‐2 <111>_grt needles in the common 4‐ray star garnet, in contrast to the presence of both <111>_grt and <001>_grt needles with multiple CORs in the rare 6‐ray star garnet, suggests that the COR‐2 <111>_grt needle probably is the energetically most favoured variant, as is also supported by the coincidence site lattice considerations. The unique crystallography‐controlled microstructures of 4‐ray star garnet, including the cloudy domains behind the {111}_grt or {100}_grt fronts with abundant inclusions of rutile needle, rutile compound needle and multiple‐phase‐inclusion, as well as the clear domains behind the {110}_grt fronts with only a few above inclusions concentrated exclusively within the linear, <110>_grt‐oriented, continuous tube‐like domains, further suggest that the COR‐2 <111>_grt needles in 4‐ray star garnet most likely have a growth‐in origin, co‐precipitating with garnet at its growth fronts close to thermodynamic equilibrium conditions. The 6‐ray star garnet, on the other hand, most likely formed under far‐from equilibrium conditions, thereby yielding a maximum of 99 crystallographic variants of rutile needles with multiple CORs in a single crystal. In the light of these findings, along with the common occurrences of the sole COR in many inclusion‐host systems owing to the requirement to minimize the energy barrier in an exsolution process, the presence of both <103>_rt//<111>_grt and <001>_rt//<001>_grt needles with multiple CORs in garnet of Sulu eclogite and Erzegebirge quartzofeldspathic rock would therefore cast doubt on the assertion of an exsolution origin of rutile needles in garnet from these ultrahigh‐pressure rocks.     

Rutile inclusions in garnet from a dissolution‐reprecipitation mechanism

Metamorphic garnet commonly contains needle‐like rutile inclusions as well as equant rutile inclusions that surround quartz inclusions and range in size from submicrometer to nanometer. Although the origin of these equant rutile inclusions, that is, exsolution or non‐exsolution, has important implications for petrological and tectonic processes, the crystallographic characteristics of these inclusions have rarely been studied because of the small sizes and analytical difficulties involved. Here, we report the crystallographic characteristics pertinent to the genetic origin of minute equant rutile inclusions in cloudy, nearly spherically shaped garnet domains with Ti‐depleted compositions surrounding quartz inclusions in ultrahigh‐pressure garnet from several diamondiferous Erzgebirge quartzofeldspathic gneissic rock samples. TEM analyses show that the equant rutile crystals in cloudy garnet domains are partially bounded by the low‐energy {100}_rt ± {110}_rt ± {101}_rt facets and have rather random crystallographic orientation relationships (CORs) with the garnet host, with preferential alignment of low‐energy lattice planes, for example, {100}_rt//{112}_grt, for some rutile crystals. Although the rather random CORs are unlikely to be attributed to solid‐state exsolution subjected to the stringent topotactic garnet lattice constraints, the characteristic subhedral {100}_rt ± {110}_rt ± {101}_rt crystal forms of rutile can be rationalized by a metasomatic dissolution‐reprecipitation mechanism via a fluid phase. In this scenario, the quartz+fluid inclusions in garnet were first subjected to decompression microcracking during rock exhumation, followed by dissolution of Ti‐bearing garnet matrix at the crack tips or along the crack surfaces and subsequent reprecipitation of rutile, apatite, gahnite, akdalaite, and Ti‐depleted garnet. The rapid coalescence between rutile and garnet crystals in fluid or direct attachment of rutile crystals onto the dissolving crack surfaces would then yield the rather random CORs as reported here. These results, along with previous work on rutile needles, indicate rather diverse genesis of rutile inclusions in various crystal forms, thus shedding light on the controversial exsolution origin for other inclusion suite/microstructure in minerals.     

Petrological constraints on the cooling history of high-temperature garnet peridotite massifs in lower Austria

High-temperature peridotite massifs occur as lensoid bodies with high-pressure granulites in the southern Bohemian massif. In lower Austria the peridotites comprise garnet lherzolites lacking primary spinel, rare garnet and garnet-spinel harzburgites, and harzburgites containing Cr-rich primary spinel instead of garnet. These phase assemblages suggest initial high-pressure equilibration and are consistent with results from garnet-orthopyroxene geobarometry indicating equilibration at around 3–3.5 GPa. Maximum temperature estimates obtained on core compositions of coexisting minerals from the peridotites are not higher than ca. 1100 °C. In contrast, pyroxene megacryst compositions, garnet exsolution textures in the garnet pyroxenites, and results from geothermometry indicate much higher original equilibration temperatures in most of the pyroxenites (up to 1400 °C). High temperatures, modal zoning, the occasional presence of Mg-rich garnetites and chemical evidence suggest that the pyroxenites are cumulates which crystallized from low-degree melts derived from the sub-lithospheric mantle. Isothermal interpolation of the high temperatures to an upper mantle adiabat suggests that the melts were derived from a minimum depth of 180–200 km. The formation of small garnet II grains and garnet exsolution lamellae in the pyroxenites and pyroxene megacrysts may reflect isobaric cooling of the cumulates from temperatures above 1400 °C to ca. 1100–1200 °C (at 3–3.5 GPa) to approach the ambient lithospheric isotherm. This model differs from other models in which the formation of garnet II was explained by an increase in pressure during cooling in a subduction zone. Isobaric cooling was followed by near-isothermal decompression from 3–3.5 GPa to 1.5–2 GPa at 1000–1200 °C, as indicated by the increase of Al in pyroxenes near garnet. Further cooling in the spinel lherzolite stability field is indicated by spinel exsolution lamellae in pyroxenes from lherzolites. The formation of symplectites and kelyphites indicate sub-millimetre scale re-equilibration during exhumation in the course of the Carboniferous collision in the Bohemian massif. The peridotite massifs represent fragments of normal (non-cratonic) lithospheric mantle from a Paleozoic convergent plate margin. Received: 22 July 1996 / Accepted 28 February 1997     

Carbonation and melting reactions in the system CaO–MgO–SiO2–CO2 at mantle pressures with geophysical and petrological applications

Bowen's petrogenetic grid was based initially on a series of decarbonation reactions in the system CaO-MgO-SiO₂-CO₂ with starting assemblages including calcite, dolomite, magnesite and quartz, and products including enstatite, forsterite, diopside and wollastonite. We review the positions of 14 decarbonation reactions, experimentally determined or estimated, extending the grid to mantle pressures to evaluate the effect of CO₂ on model mantle peridotite composed of forsterite(Fo)+orthopyroxene(Opx)+clinopyroxene(Cpx). Each reaction terminates at an invariant point involving a liquid, CO₂, carbonates, and silicates. The fusion curves for the mantle mineral assemblages in the presence of excess CO₂ also terminate at these invariant points. The points are connected by a series of reactions involving liquidus relationships among the carbonates and mantle silicates, at temperatures lower (1,100–1,300° C) than the silicate-CO₂ melting reactions (1,400–1,600° C). Review of experimental data in the bounding ternary systems together with preliminary data for the system CaO-MgO-SiO₂-CO₂ permits construction of a partly schematic framework for decarbonation and melting reactions at upper mantle pressures. The key to several problems in the peridotite-CO₂ subsystem is the intersection of a subsolidus carbonation reaction with a melting reaction at an invariant point near 24 kb and 1,200°C. There is an intricate series of reactions between 25 kb and 35 kb involving changes in silicate and carbonate phase fields on the CO₂-saturated liquidus surfaces. Conclusions include the following: (1) Peridotite Fo+Opx+Cpx can be carbonated with increasing pressure, or decreasing temperature, to yield Fo+Opx+Cpx+Cd (Cd=calcic dolomite), Fo+Opx+Cd, Fo+Opx+Cm (Cm=calcic magnesite), and finally Qz+Cm. (2) Free CO₂ cannot exist in subsolidus mantle peridotite with normal temperature distributions; it is stored as carbonate, Cd. (3) The CO₂ bubbles in peridotite nodules do not represent free CO₂ in mantle peridotite along normal geotherms. (4) CO₂ is as effective as H₂O in causing incipient melting, our preferred explanation for the low-velocity zone. (5) Fusion of peridotite with CO₂ at depths shallower than 80 km produces basic magmas, becoming more SiO₂-undersaturated with depth. (6) The solubility of CO₂ in mantle magmas is less than about 5 wt% at depths to 80 km, increasing abruptly to about 40 wt% at 80 km and deeper. (7) Deeper than 80 km, the first liquids produced are carbonatitic, changing towards kimberlitic and eventually, at considerably higher temperatures, to basic magmas. (8) Kimberlite and carbonatite magmas rising from the asthenosphere must evolve CO₂ at depths 100-80 km, which contributes to their explosive emplacement. (9) Fractional crystallization of CO₂-bearing SiO₂-undersaturated basic magmas at most pressures can yield residual kimberlite and carbonatite magmas.     

中国东部及西秦岭地区新生代岩石圈地幔中的相转变带及其温压条件

五相(橄榄石 斜方辉石 单斜辉石 石榴石 尖晶石)共存的地幔橄榄岩捕虏体是来自岩石圈地幔相转变带的直接样品。中国东部及西秦岭地区晚第三至第四纪碱性火山岩携带的少量五相共存的地幔橄榄岩捕虏体为探讨这些地区新生代岩石圈地幔中相转变带提供了宝贵的样品。本文根据地幔橄榄岩捕虏体中石榴石和尖晶石的产出状况,将这些橄榄岩捕虏体分为三类:第一类橄榄岩中尖晶石为粒状残核,尖晶石外缘被石榴石的反应边包围。这种橄榄岩捕虏体代表尖晶石一石榴石相转变带的上限,故称为尖晶石带橄榄岩;第二类橄榄岩中尖晶石和石榴石以单颗粒零散分布为特征,二者共存但未见明显的相转变关系。这类橄榄岩多位于相转变带中部,拟称为尖晶石-石榴石过渡带橄榄岩;第三类橄榄岩中以石榴石为主,尖晶石和辉石等微晶构成石榴石反应边。这类橄榄岩代表尖晶石-石榴石相转变带的下限,故称为石榴石带橄榄岩。因此,根据不同类型橄榄岩捕虏体中矿物的组成,结合温度压力估算即可确定岩石圈地幔中相转变带的深度和厚度。本文通过对中国东部及西秦岭地区晚第三至第四纪碱性火山岩携带的尖晶石-石榴石二辉橄榄岩捕虏体的温度压力估算来进一步厘定中国东部新生代岩石圈地幔中的相转变带深度和厚度。     

Petrogenesis of garnet-bearing ultramafic rocks and associated eclogites in the Su-Lu ultrahigh-P metamorphic terrane, eastern China

Ultramafic blocks that themselves contain eclogite lenses in the Triassic Su-Lu ultrahigh-P terrane of eastern China range in size from hundreds of metres to kilometres. The ultramafic blocks are enclosed in quartzofeldspathic gneiss of early Proterozoic age. Ultramafic rocks include garnetiferous lherzolite, wehrlite, pyroxenite, and hornblende peridotite. Garnet lherzolites are relatively depleted in Al₂O₃ (<3.8wt%), CaO (<3.2%) and TiO₂ (<0.11 wt%), and are low in total REE contents (several p.p.m.), suggesting that the rocks are residual mantle material that was subjected to low degrees of partial melting. The eclogite lenses or layers within the ultramafic rocks are characterized by higher MgO and CaO, lower Al₂O₃ and TiO₂ contents, and a higher CaO/Al₂O₃ ratio compared to eclogites enclosed in the quartzofeldspathic gneiss. Scatter in the plots of major and trace elements vs. MgO, REE patterns and La, Sm and Lu contents suggest that some eclogites were derived from melts formed by various degrees (0.05–0.20) of partial melting of peridotite, and that other eclogites formed by accumulation of garnet and clinopyroxene ± trapped melt in the upper mantle. Both ultramafic and eclogitic rocks have experienced a complex metamorphic history. At least six stages of recrystallization occurred in the ultramafic rocks based on an analysis of reaction textures and mineral compositions. Stage I is a high temperature protolith assemblage of Ol + Opx + Cpx + Spl. Stage II consists of the ultrahigh-pressure assemblage Ol + Cpx + Opx + Grt. Stage III is manifested by the appearance of fine-grained garnet after coarse-grained garnet. Stage IV is characterized by formation of kelyphitic rims of fibrous Opx and Cpx around garnet, and replacement of garnet by spinel and pargasitic-hornblende. Stage V is represented by the assemblage Ol + Opx + Prg-Hbl + Spl. The mineral assemblages of stages VI_A and VI_B are Ol + Tr-Amp + Chl and Serp + Chl ± talc, respectively. Garnet and orthopyroxene all show a decrease in MgO with retrogressive recrystallization and Na₂O in clinopyroxene also decreases throughout this history. Eclogites enclosed within ultramafic blocks consist of Grt + Omp + Rt ± Qtz ± Phn. A few quartz-bearing eclogites contain rounded and oval inclusion of polycrystalline quartz aggregates after coesite in garnet and omphacite. Minor retrograde features include thin symplectic rims or secondary amphiboles after Cpx, and ilmenite after rutile. P-T estimates indicate that the ultrahigh-metamorphism (stage II) of ultramafic rocks occurred at 820-900d? C and 36-41 kbar and that peak metamorphism of eclogites occurred at 730-900d? C and >28 kbar. Consonant with earlier plate tectonic models, we suggest that these rocks were underplated at the base of the continental crust. The rocks then underwent ultrahigh-pressure metamorphism and were tectonically emplaced into thickened continental crust during the Triassic collision between the Sino-Korean and Yangtze cratons.     

Oriented growth of garnet by topotactic reactions and epitaxy in high‐pressure,mafic garnet granulite formed by dehydration melting of metastable hornblende‐gabbronorite (Jijal Complex,Kohistan Complex,north Pakistan)

Garnet growth in high‐pressure, mafic garnet granulites formed by dehydration melting of hornblende‐gabbronorite protoliths in the Jijal complex (Kohistan palaeo‐island arc complex, north Pakistan) was investigated through a microstructural EBSD‐SEM and HRTEM study. Composite samples preserve a sharp transition in which the low‐pressure precursor is replaced by garnet through a millimetre‐sized reaction front. A magmatic foliation in the gabbronorite is defined by mafic‐rich layering, with an associated magmatic lineation defined by the shape‐preferred orientation (SPO) of mafic clusters composed of orthopyroxene (Opx), clinopyroxene (Cpx), amphibole (Amp) and oxides. The shape of the reaction front is convoluted and oblique to the magmatic layering. Opx, Amp and, to a lesser extent, Cpx show a strong lattice‐preferred orientation (LPO) characterized by an alignment of [001] axes parallel to the magmatic lineation in the precursor hornblende‐gabbronorite. Product garnet (Grt) also displays a strong LPO. Two of the four 〈111〉 axes are within the magmatic foliation plane and the density maximum is subparallel to the precursor magmatic lineation. The crystallographic relationship 〈111〉_Grt // [001]_Opx,Cpx,Amp deduced from the LPO was confirmed by TEM observations. The sharp and discontinuous modal and compositional variations observed at the reaction front attest to the kinetic inhibition of prograde solid‐state reactions predicted by equilibrium‐phase diagrams. The P–T field for the equilibration of Jijal garnet granulites shows that the reaction affinities are 5–10 kJ mol.^?1 for the Grt‐in reaction and 0–5 kJ mol.^?1 for the Opx‐out reaction. Petrographic and textural observations indicate that garnet first nucleated on amphibole at the rims of mafic clusters; this topotactic replacement resulted in a strong LPO of garnet. Once the amphibole was consumed in the reaction, the parallelism of [001] axes of the mafic‐phase reactants favoured the growth of garnet crystals with similar orientations over a pyroxene substrate. These aggregates eventually sintered into single‐crystal garnet. In the absence of deformation, the orientation of mafic precursor phases conditioned the nucleation site and the crystallographic orientation of garnet because of topotaxial transformation reactions and homoepitaxial growth of garnet during the formation of high‐pressure, mafic garnet‐granulite after low‐pressure mafic protoliths.     

Petrology and Ni-Cu-Cr-PGE Mineralization of the Largest Mafic Pluton in Europe: The Early Proterozoic Burakovsky Layered Intrusion,Karelia, Russia

The largest mafic pluton in Europe (area = 630 km², thickness = 5 to 7 km) is the early Proterozoic (2445 ± 4 Ma; Pb-Pb) Burakovsky Layered Intrusion (BLI). It is located in the southern part of Russian Karelia, in the SE part of the Baltic Shield, within an Archean granite-greenstone terrain. The BLI is overlain by Quaternary deposits, and our present understanding of its character, composition, and internal structure is based on geophysical surveys and the nature of the rocks at depth, as sampled by diamond drill core. In order to better understand the petrogenesis of the BLI, we present geologic, petrographic, mineral-chemical, and whole-rock chemical analyses from throughout the stratigraphic sequence.

The BLI is a lopolith-like body and is divided into two major units: the Layered Series (LS), which exhibits layering that is discordant to the contact, and the Border Group (BG), with layering that conforms to the contact surface. The LS constitutes most of the BLI and consists of, from bottom to top: the ultramafic zone (UZ), 3 to 3.5 km thick, with a lower dunite (01 ± Chr) and an upper peridotite subzone (01 ± Chr, 01 + Opx ± Chr, and rare Chr cumulates); the pyroxenite zone (PZ) (Opx ± Chr ± 01, Opx + Cpx ± Chr ± 01), 0.2 km thick; the gabbronorite zone (GZ), 1.1 km thick and composed of a lower banded subzone (Opx, Opx + Cpx ± Chr, Opx + Pig ± Cpx cumulates) and an upper massive subzone (Opx + Cpx + Pig and Pig cumulates); the pigeonite-gabbronorite zone (PGZ) (Pig + inverted Pig, Pig-Aug + Pig cumulates), 1.2 km in thickness; and the magnetite-gabbrodiorite zone (MGZ) (Ti-Mt + Pig + Pig + Cpx), 0.8 km thick.

A quite unusual feature of the BLI is an irregular conformable body of clinopyroxenite, in the lower part of the PZ, which consists of inverted Pig-Aug with a non-cumulate texture. The pyroxenites contain occasional relict cumulate structures. The pyroxenes contain small oval-shaped quartz-carbonate inclusions. We speculate that the clinopyroxenites may be the result of subsolidus metasomatism by fluids likely generated within the intrusion.

Economic mineralization is represented by chromite, Ti-V-Mt, and platinum-group minerals (PGMs). Chromite mineralization is well developed in the upper peridotite subzone of the UZ. The largest chromite seam—the main chromite horizon (MCH)—is 3 to 4 m thick and is situated at the top of the UZ, in contact with the PZ. Potential reserves of Ni, in both sulfides and olivine, are present in the UZ. Syngenetic platinum-group-element (PGE) mineralization is observed in chromitite layers in the layered subzone of the GZ and in the rocks of the Border Group. In the MCH, Os, Ir, and Rh occur as small inclusions of sulfides in chromite; these inclusions have compositions that would place them within the aurite-erlichmanite and isoferroplatinum-awaruite series. Low-sulfur PGE mineralization (mainly Pd and Pt) occurs in the banded subzone of the GZ and in sulfide-bearing rocks of the BG. PGMs consist of Pd- and Pt-bearing tellurides and bismuthides: moncheite, merenskyite, froodite, sobolevskite, and kotulskite. Epigenetic PGE mineralization also occurs in tectonic breccias.

The BLI is similar in many respects to other early Proterozoic layered mafic intrusions (i.e., the Bush veld, Monchetundra, and Koilismaa intrusions), especially in terms of the general rock sequence and economic mineralization. The main differences are the very thick dunite cumulates (comprising half of the intrusion) and the relatively minor proportion of pyroxenerich and plagioclaserich cumulates. Another contrasting feature is the predominance of refractory PGE (Os, Ir, and Rh), over Pt and Pd, in the MCH. This observation leads to the interpretation that the MCH has a non-cumulate (deuteric hydrothermal?) origin.     

The system tonalite-peridotite-H2O at 30 kbar,with applications to hybridization in subduction zone magmatism

We present data on the phase relationships of mixtures between natural tonalite and peridotite compositions with excess H₂O at 30 kbar, and on the composition of the piercing point where the peridotite-tonalite mixing line intersects the L(Ga,Opx) reaction boundary. These data, in conjunction with earlier analogous data along peridotite-granite and basalt-granite mixing lines, permit construction of a pseudoternary liquidus projection that is relevant to interaction of peridotite with slab-derived magmas. Knowledge of the liquidus phase and temperature for a range of compositions within this projection enables us to map primary crystallization fields for quartz, garnet, orthopyroxene, clinopyroxene, and olivine, and to estimate the distribution of isotherms across the projection. Using this projection, we explore the consequences of peridotite assimilation by mafic to intermediate (basalt to dacite) hydrous slab-derived melts. Progressive assimilation under isothermal conditions results in garnet precipitation as the melt composition traverses the garnet liquidus surface and then garnet+orthopyroxene crystallization once the melt reaches the L(Ga,Opx) field boundary. The melt is constrained to remain on this field boundary and further assimilation of peridotite simply results in continued precipitation of garnet+orthopyroxene until the melt is consumed. The product is a hybrid solid assemblage consisting of Ga+ Opx. It is noteworthy that this process drives the melt composition in a direction nearly perpendicular to the mixing line between peridotite and the initial melt. If assimilation occurs with increasing temperature (as might occur if a slab-derived magma rises into the hotter mantle wedge), intermediate magmas (e.g. andesites) will again precipitate garnet until they reach the L(Ga,Opx) reaction boundary at which point Ga re-dissolves and orthopyroxene precipitates as the melt composition moves up-temperature along this boundary. The product of this process is a hybrid solid assemblage with garnet subordinate to orthopyroxene. For more mafic initial compositions (e.g. basalts) originally plotting in the Cpx field, it appears possible to avoid field boundaries involving garnet and shift in composition more directly toward peridotite, if assimilation is accompanied by a sharp increase in temperature. Considering published REE evidence (arguing against garnet playing a significant role in the genesis of many subduction-related magmas) in light of our results, it appears unlikely that peridotite assimilation by intermediate magmas under conditions of constant or increasing temperature is an important process in subduction zones. However, if assimilation is accompanied by an increase in temperature, our data do permit the derivation of high-Mg basalts from less refractory precursors (e.g. high-Al basalts) by peridotite assimilation.     

Quantitative Untersuchungen mit der Elektronen-Mikrosonde an Pyroxenen aus Basalten und Peridotit-Einschlüssen

The chemical composition of the pyroxenes and olivines of 12 basaltic rocks and 5 lherzolite nodules was determined quantitatively by electron micro-probe analysis. The composition of the pyroxenes depends on the type of basalt in which they occur. Tholeiitic basalts with normative quartz contain three pyroxenes: orthorombic pyroxenes, pigeonites and augites. All pyroxene phases are zoned and do not show any exsolution. Their Ti and Al contents (Ca-Tschermaks and Ti-augite molecules) are small. All pyroxene phases were formed under disequilibrium with each other and with the melt because of rapid quenching. The sequence of crystallization: orthopyroxene—pigeonite—augite could be established by their Cr content.The alkali olivine basalts undersatured in SiO₂ and the olivine nephelinites are characterized by Ti and Al-rich clinopyroxenes. The distribution of Ti and Al in the pyroxenes of the alkali olivine basalts shows a differentiation trend from the cores of the phenocrysts to their outer zones and to the crystals of the ground mass. Thereby the Ca-Tschermaks molecule is being replaced more and more by the Ti-augite molecule. The Ti content of the pyroxenes of the olivine nephelinites decreases in the last stage of differentiation because simultaneously increasing amounts of titaniferous magnetite crystallize.The pyroxenes of lherzolite peridotite nodules are characterized by high Al and low Ti contents which differ according to the type of basalt (alkali olivine basalt or olivine nephelinite) in which the nodules occur. The homogeneous distribution of the elements within the single grains indicates crystallization under equlibrium conditions. The conditions of their formation are comparable to those of Al-pyroxene peridotites in the upper mantle. The composition of pyroxenes of early accumulates of alkali basaltic melts differ from those of peridotite nodules. Therefore lherzolite nodules can be taken as residues of deeper peridotite masses.     

Chrome-diopside Megacryst-bearing Lamprophyre from the Late Cretaceous Mundwara Alkaline Complex,NW India: Petrological and Geodynamic Implications

The occurrence of a rare mantle-derived chrome-diopside megacryst (～8 mm), containing inclusions of olivine, in a lamprophyre dyke from the late Cretaceous polychronous (～100 – 68 Ma) Mundwara alkaline complex of NW India is reported. The olivine inclusions are forsteritic (Fo: 85.23) in composition, and their NiO (0.09 wt%) and CaO (0.13 wt%) contents imply derivation from a peridotitic mantle source. The composition of the chrome diopside (Cr₂O₃: 0.93 wt ) (Wo_45.27 En_48.47 Fs_5.07 and Ac_1.18) megacryst is comparable to that occurring in the garnet peridotite xenoliths found in diamondiferous kimberlites from Archaean cratons. Single pyroxene thermobarometry revealed that this chrome diopside megacryst was derived from a depth range of ～100 km, which is relatively much deeper than that of the chrome-diopside megacrysts (～40–50 km) reported in spinellherzolite xenoliths from the alkali basalts of Deccan age (ca. 66–67 Ma) from the Kutch, NW India. This study highlights that pre- Deccan lithosphere, below the Mundwara alkaline complex, was at least ～100 km thick and, likely, similar in composition to that of the cratonic lithosphere.     

Experimental tests of garnet peridotite oxygen barometry

We have performed experiments aimed at testing the calibration of oxygen barometers for the garnet peridotite [garnet (Gt)-olivine (Ol)-orthopyroxene (Opx)] phase assemblage. These involved equilibrating a thin layer of garnet sandwiched between layers of olivine and orthopyroxene at 1300°C and 23–35 kbar for 1–7 days. Oxygen fugacity was controlled (but not buffered) by using inner capsules of Fe?Pt alloy or graphitc or molybdenum sealed in welded Pt outer capsules. Post-experiment measurement of fO₂ was made by determining the compositions of Pt-Fe alloy sensors at the interface between garnet and olivine + orthopyroxene layers. The composition of alloy in equilibrium with olivine + orthopyroxene was approached from Fe-oversaturated and Fe-undersaturated conditions in the same experiment with, in general, excellent convergence. Product phase compositions were determined by electron microprobe and a piece of the garnet layer saved for ⁵⁷Fe Mössbauer spectroscopy. The latter gave the Fe³⁺ content of the garnet at the measured P-T-fO₂ conditions. Approach to equilibrium was checked by observed shifts in Fe³⁺ content and by the approach of garnet-olivine Fe?Mg partitioning to the expected value. The compositions of the phases were combined with mixing properties and thermodynamic data to calculate an apparent fO₂ from two possible garnet oxybarometers:- (1) $\begin{gathered} 2Ca_3 Fe_2 Si_3 O_{12} + 2Mg_3 Al_2 Si_3 O_{12} + 4FeSiO_3 = 2Ca_3 Al_2 Si_3 O_{12} \hfill \\ Gt Gt Opx Gt \hfill \\ + 8FeSi_{0.5} O_2 + 6MgSiO_3 + O \hfill \\ Ol Opx \hfill \\ \end{gathered} $ and (2) $\begin{gathered} 2Fe_3 Fe_2 Si_3 O_{12} = 8FeSi_{0.5} O_2 + 2FeSi_3 O_2 \hfill \\ Gt Ol Opx \hfill \\ \end{gathered} $ Comparison of calculated fO₂s with those measured by the Pt-Fe sensors demonstrated that either barometer gives the correct answer within the expected uncertainty. Data from the first (Luth et al. 1990) has an uncertainty of about 1.6 logfO₂ units, however, while that from equilibrium (2) (Woodland and O'Neill 1993) has an error of +/- 0.6 log units, comparable to that of the spinel peridotite oxybarometer. We therefore conclude that equilibrium (2) may be used to calculate the fO₂ recorded by garnet peridotites with an uncertainty of about +/- 0.6 log units, providing the potential to probe the oxidation environment of the deep continental lithosphere. Preliminary application based on data from Luth et al. (1990) indicates that garnet peridotite xenoliths from Southern Africa record oxygen fugacities about 3.0 log units below the FMQ (fayalite-magnetite-quartz) buffer. These are substantially more reducing conditions than those recorded by continental spinel lherzolites which typically give oxygen fugacities close to FMQ (Wood et al. 1990).