Inclusion-localised crystal-plasticity,dynamic porosity,and fast-diffusion pathway generation in zircon

A population of oscillatory zoned, igneous zircon grains in a Javanese andesite contains fluid and mineral inclusions (up to 10 μm across) trapped during zircon growth. Orientation contrast imaging and orientation mapping by electron backscatter diffraction reveal that crystal-plastic deformation overprints growth zoning and has localized around 1–10 μm pores and inclusions. Cumulative crystallographic misorientation of up to 25° around pores and inclusions in zircon is predominantly accommodated by low-angle (<5°) orientation boundaries, with few free dislocations in subgrain interiors. Low-angle boundaries are curved, with multiple orientation segments at the sub-micrometer scale. Misorientation axes associated with the most common boundaries align with the zircon c-axis and are consistent with dislocation creep dominated by <100>(010) slip. A distinctly different population of sub-micron pores is present along subgrain boundaries and their triple junctions. These are interpreted to have formed as a geometric consequence of dislocation interaction during crystal-plasticity. Dislocation creep microstructures are spatially related to differences in cathodoluminescence spectra that indicate variations in the abundance of CL-active rare earth elements. The extent of the modification suggests deformation-related fast-pathway diffusion distances that are over five orders of magnitude greater than expected for volume diffusion. This enhanced diffusion is interpreted to represent a combination of fast-diffusion pathways associated with creep cavitation, dislocations and along low-angle boundaries. These new data indicate that ductile deformation localised around inclusions can provide fast pathways for geochemical exchange. These pathways may provide links to the zircon grain boundary, thus negating the widely held assumption that inclusions in fracture-free zircon are geochemically armoured once they are physically enclosed.     

Recrystallisation of oscillatory zoned zircon: some geochronological and petrological implications

Oscillatory zoning is a common feature in zircons from acid igneous rocks and is believed to form during crystallisation of zircons from a magma by a mechanism which is not yet understood. Many zircons with oscillatory zoning also show a patchwork replacement of zoned by unzoned zircon. The unzoned zircon occurs as rounded, transgressive patches distributed throughout the zoned zircon and as areas of transitional replacement where zoned zircon is progressively replaced by unzoned zircon such that only faint traces of original zones remain. This structure is interpreted as a progressive recrystallisation of the oscillatory zoned zircon made unstable by the incorporation of high concentrations of contaminant elements during magmatic crystallisation. Recrystallisation overprints oscillatory zones and appears to have occurred after completion of primary crystallisation. It is accompanied by loss of U, Th and Pb and the removal of oscillatory zones. The recrystallised unzoned zircon is extremely stable with respect to later Pb loss and tends to retain a concordant or slightly discordant U–Pb age. Recrystallisation provides a mechanism for resetting zircon U–Pb ages which is independent of the degree of radiation damage of the zircon lattice. This differs from other models of discordance which involve a leaching of radiogenic Pb as a consequence of a progressive breakdown of the zircon structure through time-integraded radiation damage further enhanced by high concentrations of trace-element contaminants. The U–Pb age of the unzoned zircon may date the recrystallisation event, which may be close to the age of primary crystallisation or reflect a later metamorphism.Dedicated to Borwin Grauert on the occasion of his sixtieth birthday     

The difficulties of dating mafic dykes: an Antarctic example

Archaen gneisses of the Vestfold Hills of East Antarctica are transected by several compositionally discrete suites of tholeiitic dykes. A representative of one of those suites, which has been dated in the present study, shows that not only Rb–Sr whole-rock isochrons, but also U–Pb zircon techniques (if not properly applied) can produce erroneous crystallisation ages. Two zircon populations were recovered from the mafic dyke itself, one of which is 2,483±9 Ma in age and clearly of xenocrystic origin. The other yields an age of 1,025±56 Ma, which is not ascribed to the magmatic crystallisation of the dyke, but rather to the time that it underwent metamorphic/metasomatic alteration as a response to high-grade tectonism in the adjacent mobile belt. It is presumed that the zircon in question formed by the breakdown of another mineral or minerals (possibly magmatic baddeleyite), due either to ingress of a siliceous fluid, or more probably by the release of silica from the breakdown of pyroxene to amphibole. A cogenetic 1–2 cm wide felsic vein, of late magmatic/early hydrothermal origin, also contains two zircon populations. Again, most of the grains therein, which are interpreted as of xenocrystic origin, grew at 2,483±9 Ma. However, a few euhedral zircons with very high U and Th contents grew at 1,248±4 Ma, which is taken to be the formation age of both the felsic vein and the enclosing mafic dyke.     

A SIMS U-Pb (zircon) and Re-Os (molybdenite) isotope study of the early Paleozoic Macquarie Arc,southeastern Australia: Implications for the tectono-magmatic evolution of the paleo-Pacific Gondwana margin

New SIMS U-Pb (zircon) data for intrusive rocks of the Macquarie Arc and adjacent granitic batholiths of the Lachlan Orogen (southeastern Australia) provide insight into the magmatic and tectonic evolution of the paleo-Pacific Gondwana margin in the early Paleozoic. These data are augmented by Re-Os dates on molybdenite from four Cu-Au mineralised porphyry systems to place minimum age constraints on igneous crystallization. The simplicity of the zircon age distributions, and absence of older inheritance, stands in contrast to previous geochronological studies. The earliest magmatism within the Macquarie Arc is registered by a ca. 503 Ma gabbro from the Monza igneous complex, whereas a monzodiorite from the same drillhole records the youngest (ca. 432 Ma). Igneous activity in the Macquarie Arc thus overlapped deformation and magmatism in the craton-proximal Delamerian Orogen to the west, and the emplacement of the Lachlan granitic batholiths at 435–430 Ma; the thermal pulse associated with the latter may have triggered the formation of richly mineralised Silurian porphyries in the Macquarie Arc. The juvenile Hf isotope signature of the Monza Gabbro, together with the lack of zircon inheritance and the radiogenic Hf-Nd isotope systematics of Ordovician Macquarie Arc rocks, is consistent with early development of the arc, or a precursor magmatic belt, in an oceanic setting remote from continental influences, and with the arc being built on primitive Cambrian mafic crust. Outboard arc magmatism in the Cambrian may have initiated in response to convergent Delamerian orogenesis adjacent the Gondwana margin. Overlapping radiogenic isotope-time trends are consistent with the evolution of the Macquarie Arc and the Gondwana continental margin being linked from the Cambrian to the Silurian. These data provide further evidence for the growth of continental crust along the southeastern Australian segment of this margin being related to the dynamics of an extensional accretionary orogenic system.     

Polyphase magmatic pulses along the Northern Gondwana margin: U-Pb zircon geochronology from gneiss domes of the Pyrenees

Alkaline granitic dikes intruding the metasedimentary mantle and orthogneiss cores of the Aston and Hospitalet domes of the Axial Zone of the Pyrenees are subjects of a laser ablation ICP-MS U-Pb zircon geochronology study. The age spectra recorded by detrital, magmatic xenocrystic and inherited zircons reveal a more complex, nearly continuous Paleozoic magmatic history of the Variscan basement of the Pyrenees than previously known. Inherited and detrital zircons of Mesoarchean, Paleoproterozoic to Ediacaran ages attest to the Peri-Gondwana location of the Cambrian sediments that later form the metamorphic core of the Variscan Pyrenees. The youngest magmatic zircon ages fall into the late Carboniferous and earliest Permian, ranging from ca. 306–297 Ma, and represent the emplacement ages of the dikes and small granite intrusions. The age spectra of magmatic xenocrystic zircons contain several maxima, middle (475–465 Ma) and late Ordovician (455–445 Ma), early (415–402 Ma) and late Devonian (385–383 Ma), early (356–351 Ma) and middle Carboniferous (ca. 328 Ma). Middle Ordovician and middle Carboniferous ages are obtained from xenocrystic zircons that were assimilated from the rocks the dikes intruded, the Aston and Hospitalet orthogneisses and the Soulcem granite. The presence of early-mid Carboniferous magmatic zircons in several samples lends further support to a wide-spread early Variscan magmatic activity in the central Pyrenees. The other age peaks do not have equivalent igneous or metaigneous rocks in the central Axial Zone, but are thought to be present in the Pyrenean crust, not exposed and yet to be identified. The diversity of Ordovician, Devonian and Carboniferous up to Permian magmatic ages indicates polyphase emplacement of intrusive bodies during pre-Variscan and Variscan orogenies. The source of the heat for the Devonian to early-mid Carboniferous magmatic activity remains elusive and may involve intracontinental subduction zone, lithospheric-scale shearing or a mantle plume (TUZO).     

Dating mafic–ultramafic intrusions by ion-microprobing contact-melt zircon: examples from SW Sweden

Zircons from anatectic melts of the country rocks of three Proterozoic mafic–ultramafic intrusions from the Sveconorwegian Province in SW Sweden were microanalyzed for U–Th–Pb and rare earth elements. Melting and interaction of the wall rocks with the intrusions gave rise to new magmas that crystallized zircon as new grains and overgrowths on xenocrysts. The ages of the intrusions can be determined by dating this newly crystallized zircon. The method is applied to three intrusions that present different degrees of complexity, related to age differences between intrusion and country rocks, and the effects of post-intrusive metamorphism. By careful study of cathodoluminescent images and selection of ion probe spots in zircon grains, we show that this approach is a powerful tool for obtaining accurate and precise ages. In the contact melts around the 916?±?11?Ma Hakefjorden Complex, Pb-loss occurred in some U-rich parts of xenocrystic zircon due to the heat from the intrusion. In back-veins of the 1624?±?6?Ma Olstorp intrusion we succeeded in geochemically distinguishing new magmatic from xenocrystic zircon despite small age differences. At Borås the mafic intrusion mixed with country rock granite to form a tonalite in which new zircon grew at 1674?±?8?Ma. Reworking of zircon occurred during 930+33/–34?Ma upper amphibolite facies Sveconorwegian metamorphism. Pb-loss was the result of re-equilibration with metamorphic fluids. REE-profiles show consistent differences between xenocrystic, magmatic, and metamorphic zircon in all cases. They typically differ in Lu/La_N, Ce/Ce*, and Eu/Eu*, and igneous zircon with marked positive Ce/Ce* and negative Eu/Eu* lost its anomalies during metamorphism.     

Enhanced diffusion of Uranium and Thorium linked to crystal plasticity in zircon

The effects of crystal-plasticity on the U-Th-Pb system in zircon is studied by quantitative microstructural and microchemical analysis of a large zircon grain collected from pyroxenite of the Lewisian Complex, Scotland. Electron backscatter diffraction (EBSD) mapping reveals a c.18° variation in crystallographic orientation that comprises both a gradual change in orientation and a series of discrete low-angle (<4°) boundaries. These microstructural data are consistent with crystal-plastic deformation of zircon associated with the formation and migration of dislocations. A heterogeneous pattern of dark cathodoluminescence, with the darkest domains coinciding with low-angle boundaries, mimics the deformation microstructure identified by EBSD. Geochemical data collected using the Sensitive High Resolution Ion MicroProbe (SHRIMP) shows a positive correlation between concentrations of the elements U, Th and Pb (ranging from 20–60 ppm, 30–110 ppm, and 14–36 ppm, respectively) and Th/U ratio (1.13 – 1.8) with the deformation microstructure. The highest measured concentrations and Th/U coincide with low-angle boundaries. This enrichment is interpreted to reflect enhanced bulk diffusion of U and Th due to the formation and migration of high-diffusivity dislocations. ²⁰⁷Pb/²⁰⁶Pb ages for individual analyses show no significant variation across the grain, and define a concordant, combined mean age of 2451 ± 14 Ma. This indicates that the grain was deformed shortly after initial crystallization, most probably during retrograde Inverian metamorphism at amphibolite facies conditions. The elevated Th over U and consistent ²⁰⁷Pb/²⁰⁶Pb ages indicates that deformation most likely occurred in the presence of a late-stage magmatic fluid that drove an increase in the Th/U during deformation. The relative enrichment of Th over U implies that Th/U ratio may not always be a robust indicator of crystallization environment. This study provides the first evidence of deformation-related modification of the U-Th system in zircon and has fundamental implications for the application and interpretation of zircon trace element data.     

Magma evolution beneath Bequia,Lesser Antilles,deduced from petrology of lavas and plutonic xenoliths

Extrusive and intrusive igneous rocks represent different parts of a magmatic system and ultimately provide complementary information about the processes operating beneath volcanoes. To shed light on such processes, we have examined and quantified the textures and mineral compositions of plutonic and cumulate xenoliths and lavas from Bequia, Lesser Antilles arc. Both suites contain assemblages of iddingsitized olivine, plagioclase, clinopyroxene and spinel with rare orthopyroxene and ilmenite. Mineral zoning is widespread, but more protracted in lavas than xenoliths. Plagioclase cores and olivine have high anorthite (An?≤?98) and low forsterite (Fo?≤?84) compositions respectively, implying crystallisation from a hydrous mafic melt that was already fractionated. Xenolith textures range from adcumulate to orthocumulate with variable mineral crystallisation sequences. Textural criteria are used to organize the xenoliths into six groups. Amphibole, notably absent from lavas, is a common feature of xenoliths, together with minor biotite and apatite. Bulk compositions of xenoliths deviate from the liquid line of descent of lavas supporting a cumulate origin with varying degrees of reactive infiltration by evolved hydrous melts, preserved as melt inclusions in xenolith crystals. Volatile saturation pressures in melt inclusions indicate cumulate crystallization over a 162–571 MPa pressure range under conditions of high dissolved water contents (up to 7.8 wt% H₂O), consistent with a variety of other thermobarometric estimates. Phase assemblages of xenoliths are consistent with published experimental data on volatile-saturated low-magnesium and high-alumina basalts and basaltic andesite from the Lesser Antilles at pressures of 200–1000 MPa, temperatures of 950–1050 °C and dissolved H₂O contents of 4–7 wt%. Once extracted from mid-crustal mushes, residual melts ascend to higher levels and undergo H₂O-saturated crystallization in shallow, pre-eruptive reservoirs to form phenocrysts and glomerocrysts. The absence of amphibole from lavas reflects instability at low pressures, whereas its abundance in xenoliths testifies to its importance in mid-crustal differentiation processes. A complex, vertically extensive (6 to at least 21 km depth) magmatic system is inferred beneath Bequia. Xenoliths represent fragments of the mush incorporated into ascending magmas. The widespread occurrence of evolved melts in the mush, but the absence of erupted evolved magmas, in contrast to islands in the northern Lesser Antilles, may reflect the relative immaturity of the Bequia magmatic system.     

How deceptive are microstructures in granitic rocks? Answers from integrated physical theory,phase equilibrium,and direct observations

In this contribution, we address the vexed question of the extent to which microstructures in granitic rocks reflect their igneous histories or have been masked by later events. The previous works have tended to address the problem either using theoretical or modelling considerations, or by interpretation of observed microstructures. Here, we use an approach that integrates the theory of microstructural development and the results of experimental phase-equilibrium studies with direct observation of natural examples on a variety of scales. We show that the predictions of the theoretical and experimental approaches agree perfectly with the mesoscopic and microscopic evidence from granitic rocks themselves. Our conclusion is that although, in many cases, granitic rock microstructures have been modified by near-solidus reactions and crystallisation, in the absence of tectonic deformation the fundamental elements of their igneous heritage remain intact. This means that it is perfectly in order to infer aspects of crystallisation sequences, magmatic reactions, and magma flow through careful microstructural observations. Thus, our answer to the question of how deceptive granitic textures are is, in most instances, ‘not very’. However, some undeformed plutons have undergone fluid-driven alteration, and others have been affected by contact metamorphism. Thus, each case should be examined on its own merits.     

Ion probe dating of a migmatite in SW Sweden: the fate of zircon in crustal processes

A migmatitic orthogneiss in the Western Segment in the Sveconorwegian Province of the Baltic Shield was dated using the ion-probe U–Pb method on zircon grains, which were also analysed for rare earth elements. Mesosome zircons have 1.605±0.010 Ga magmatic cores, which places the gneiss protolith in the same 1.61–1.59 Ga time bracket as continental arc-related gneisses, abundant in this part of the Sveconorwegian Province. These cores show REE profiles with strong HREE enrichment, positive Ce- and negative Eu-anomalies, typical of magmatic zircon. Migmatite leucosomes are folded and parallel with or slightly discordant to the fabric. They contain a small population of zircon with cores and metamorphic rims, which are interpreted as xenocrysts incorporated in the leucosome during melting of the mesosome. CL-bright metamorphic embayments and rims on xenocrysts reflect 1.01±0.05 Ga Sveconorwegian metamorphic reworking. Ce-anomalies are nearly absent and Eu-anomalies are reduced relative to igneous spots. This is probably a feature of fluid controlled environments where Ce and Eu oxidation states are buffered by the metamorphic fluid. From this and discordant rims from the mesosome we also conclude that the rims formed by reworking of the older zircon where the Pb-loss was also fluid induced. In the leucosome veins, magmatic acicular zircon gives 0.92±0.01 Ga, ascribed to the crystallisation of the veins. They originated by local melting, probably augmented by magma that formed at a deeper level. Widespread granitic and noritic late-Sveconorwegian magmatism close to 0.92 Ga in other parts of the Western Segment has equivalents in the Norwegian sectors of the Sveconorwegian Province. Leucosome formation was therefore part of a regional event related to exhumation of the Sveconorwegian Eastern Segment. We also provide the first unequivocal evidence for ductile deformation related to late-Sveconorwegian magmatism.     

Hafnium isotope homogenization during metamorphic zircon growth in amphibolite-facies rocks: Examples from the Shackleton Range (Antarctica)

Hafnium isotope analyses of a large number of metamorphic zircon grains of two garnet-kyanite-staurolite schist samples from the Shackleton Range yielded ¹⁷⁶Hf/¹⁷⁷Hf of 0.28160 ± 0.00003 and 0.28142 ± 0.00003, respectively. The variations of these analyses are less than ±1.2 epsilon units and indicate that all metamorphic zircon grains in the two rocks formed in environments with nearly homogenous Hf isotopic composition. The metamorphic origin of the zircon grains is constrained by textures as well as by their low Th/U (<0.2), ¹⁷⁶Lu/¹⁷⁷Hf (<0.0003), and ¹⁷⁶Yb/¹⁷⁷Hf ratios (<0.009), indicating that they grew in the presence of garnet. Furthermore, the grains yield Pb-Pb ages of c. 1.7 Ga, which is the time of amphibolite-facies metamorphism. In combination with petrological results, it is suggested that the observed ¹⁷⁶Hf/¹⁷⁷Hf homogeneity was caused by a fluid- and deformation-assisted dissolution of detrital zircon grains, followed by new zircon re-precipitation that was accompanied by Hf transport on at least a hand-specimen scale. This interpretation is supported by results obtained from an additional paragneiss sample that contains zoned zircon grains with xenocrystic cores formed at 2.6-1.8 Ga and metamorphic rims with a U-Pb age of 1.7 Ga. The ¹⁷⁶Hf/¹⁷⁷Hf variation of the zircon rims is mostly at ±0.0003, which is much less than that of the magmatic cores (±0.0019). The metamorphic fluid for the dissolution-homogenization-re-precipitation process most likely resulted from prograde reactions among the minerals chlorite-muscovite-biotite-garnet-staurolite-apatite, in agreement with thin section observations and P-T pseudosection calculations.     

What is underneath the juvenile Ordovician Macquarie Arc (eastern Australia)? A question resolved using Silurian intrusions to sample the lower crust

The Ordovician intra-oceanic Macquarie Arc of eastern Australia collided with the eastern Gondwanan margin at ~440 Ma. However, the deep crustal architecture resulting from this assembly is poorly known. This is addressed here by a zircon U-Pb-Hf study of the post-assembly Silurian Browns Creek Intrusive Complex and Davies Creek Granite dykes that intrude into the arc, and not adjacent Gondwanan sedimentary sequences. Zircon UPb dating integrated with CL imagery indicate two igneous phases at 430–437 Ma and 420–426 Ma and a zircon recrystallisation phase at 395–396 Ma attributed to a late thermal event. The magmatic zircon initial ɛHf values vary from −5.1 to +4.7. This signature indicates the source of these granitic rocks is strongly influenced by typical pre-Silurian Gondwanan material. Granitic rock and zircon compositions demonstrate that at the likely temperature of the Silurian granitic magma, especially the Davies Creek Granite dykes, inherited source zircons were mostly dissolved, explaining the absence of pre-Ordovician xenocrysts within the zircon population. The unradiogenic Hf isotopic signatures preserved in the Silurian magmatic zircons demonstrate the contribution of Gondwanan crustal material to the magma source region. These results support the interpretation of the Macquarie Arc as an intra-Panthalassa ocean allochthon, emplaced and resting over the edge of Gondwanan crystalline basement, possibly including the continent-derived sedimentary rocks of the Adaminaby Group.     

A new method integrating high-precision U-Pb geochronology with zircon trace element analysis (U-Pb TIMS-TEA)

Increased precision in isotope-dilution thermal ionization mass spectrometry (ID-TIMS) U-Pb geochronology has revealed age complexities in zircon populations that require new tools for understanding how the growth of zircon is related to geologic processes. U and Pb are routinely separated from other elements in dated minerals by ion exchange separation prior to TIMS isotope measurement. We develop a method in which trace elements in the exact same volume of zircon are redissolved and analyzed using solution nebulization inductively coupled plasma sector-field mass spectrometry with matrix-matched external liquid calibration. Using <0.5 ml solution, resulting concentrations are between <1 ppt for elements such as Ti, Nb and Ta and tens of ppb for Zr. By analyzing a series of standard solutions, zircons and procedural blanks, we show that accurate measurements are performed on Zr, Hf, Y, Sc, and the HREE while low-concentration elements can be measured accurately to <5 ppt. We performed combined U-Pb ID-TIMS geochronology with trace element analysis (here called U-Pb TIMS-TEA) on zircons from eight volcanic rocks comprising several volcanic systems and one metamorphic sample. Similar to previous in situ trace element analyses, zircon geochemistry is distinct between different samples and records petrogenetic processes such as fractional crystallization, assimilation and/or magma mixing. Unique from in situ analysis, U-Pb TIMS-TEA can trace geochemical evolution in accessory minerals with adequate age precision to resolve magmatic processes in rocks at least 200 million years old. This provides a means to identify auto-, ante- and xenocrystic zircon and lead to more robust age interpretations in ID-TIMS U-Pb geochronology. One suite of Cretaceous andesitic zircons shows correlations in geochemistry and absolute time that record evolution of a magmatic system over ∼250 ka prior to eruption. Future work will combine U-Pb TIMS-TEA with solution isotopic analysis of Nd, Sr and Hf and will be applied to a host of datable minerals such as monazite, sphene, apatite, rutile, xenotime, and baddeleyite. These combined tools will provide access to an improved understanding of a wide range of igneous and metamorphic processes as a function of time.     

Precambrian crystalline basement in southern Mongolia as revealed by SHRIMP zircon dating

Single zircon ages determined by ion microprobe (SHRIMP II) for granitoid gneisses from the southern slope of the Baga Bogd massif (Gobi-Altai, southern Mongolia) reveal several episodes of zircon growth, ranging from late Palaeoproterozoic to late Cambrian. The oldest events are documented by a zircon crystallization age for a gneiss protolith at 1519 ± 11 Ma and by a xenocrystic zircon from a dark grey augen-gneiss yielding an age of c. 1701 Ma. Discrete igneous events are recorded in granite-gneisses with protolith emplacement ages of 983 ± 6, 956 ± 3 and 954 ± 8 Ma. These ages provide the first record of early Neoproterozoic magmatic activity in this region. A much younger and discrete magmatic event is recorded by several dioritic to granitic orthogneisses which are tectonically interlayered with the older gneisses and have protolith emplacement ages between 502 and 498 Ma. These late Cambrian granitoids of calc-alkaline affinity are likely to have been emplaced along an active continental margin and suggest that the Baga Bogd Precambrian crustal fragment was either docked against the southward (present-day coordinates) growing margin of the CAOB or was a large enough crustal entity to develop an arc along its margin. We speculate that the Precambrian gneisses of this massif may be part of a crustal fragment rifted off the Tarim Craton.     

The field and microstructural signatures of deformation‐assisted melt transfer: Insights from magmatic arc lower crust,New Zealand

Melt must transfer through the lower crust, yet the field signatures and mechanisms involved in such transfer zones (excluding dykes) are still poorly understood. We report field and microstructural evidence of a deformation‐assisted melt transfer zone that developed in the lower crustal magmatic arc environment of Fiordland, New Zealand. A 30–40 m wide hornblende‐rich body comprising hornblende ± clinozoisite and/or garnet exhibits 'igneous‐like' features and is hosted within a metamorphic, two‐pyroxene–pargasite gabbroic gneiss (GG). Previous studies have interpreted the hornblende‐rich body as an igneous cumulate or a mass transfer zone. We present field and microstructural characteristics supporting the later and indicating the body has formed by deformation‐assisted, channelized, reactive porous melt flow. The host granulite facies GG contains distinctive rectilinear dykes and garnet reaction zones (GRZ) from earlier in the geological history; these form important reaction and strain markers. Field observations show that the mineral assemblages and microstructures of the GG and GRZ are progressively modified with proximity to the hornblende‐rich body. At the same time, GRZ bend systematically into the hornblende‐rich body on each side of the unit, showing apparent sinistral shearing. Within the hornblende‐rich body itself, microstructures and electron back‐scatter diffraction mapping show evidence of the former presence of melt including observations consistent with melt crystallization within pore spaces, elongate pseudomorphs of melt films along grain boundaries, minerals with low dihedral angles as small as <10° and up to <60°, and interconnected 3D melt pseudomorph networks. Reaction microstructures with highly irregular contact boundaries are observed at the field and thin‐section scale in remnant islands of original rock and replaced grains, respectively. We infer that the hornblende‐rich body was formed by modification of the host GG in situ due to reaction between an externally derived, reactive, hydrous gabbroic to intermediate melt percolating via porous melt flow through an actively deforming zone. Extensive melt–rock interaction and metasomatism occurred via coupled dissolution–precipitation, triggered by chemical disequilibrium between the host rock and the fluxing melt. As a result, the host plagioclase and pyroxene became unstable and were reacted and dissolved into the melt, while hornblende and to a lesser extent clinozoisite and garnet grew replacing the unstable phases. Our study shows that hornblendite rocks commonly observed within deep crustal sections, and attributed to cumulate fractionation processes, may instead delineate areas of deformation‐assisted, channelized reactive porous melt flow formed by melt‐mediated coupled dissolution–precipitation replacement reactions.     

Systematics of zircon crystallisation in the Cretaceous Separation Point Suite, New Zealand, using U/Pb isotopes, REE and Ti geothermometry

Plutonic zircons from the Cretaceous Separation Point Suite (SPS) were analysed by LA-ICPMS for U–Pb isotope ratios and trace element concentrations. Pooled ²⁰⁶Pb/²³⁸U ages range from 112 to 124 Ma. Cathodoluminescence imaging reveals minor inheritence and textural evidence of repeated dissolution and re-precipitation of zircon. Core and rim spot analyses, however, document zircon growth during extended periods of time (>2 myr). Protracted crystallisation histories for simple plutonic systems are inconsistent with generalised thermal constraints, which predict cooling below the solidus within <1 myr. Consequently, we conclude that the SPS granitoids sampled in this study were not emplaced rapidly but incrementally over extended time periods. Zircon Th/U and Zr/Hf ratios are positively correlated with crystallisation temperatures, consistent with crystallisation from evolving melts. However, highly variable trace element concentrations, along with temperature reversals are indicative of complex crystallisation histories involving continuous fractional crystallisation repeatedly punctuated by hotter, more mafic magma recharge. Normalised abundances of the redox-sensitive elements Eu and Ce in zircon vary systematically with degrees of whole rock differentiation, pointing to evolutionary trends in magmatic oxidation states coupled with feldspar crystallisation.     

Young porphyries,old zircons: new constraints on the timing of deformation and gold mineralisation in the Eastern Goldfields from SHRIMP U–Pb zircon dating at the Kanowna Belle Gold Mine,Western Australia

The Kanowna Belle Gold Mine is a Late Archaean orogenic lode-gold deposit hosted by felsic volcaniclastic and intrusive rocks (porphyries) of the Kalgoorlie Terrane, Western Australia. Rare gold occurs in fragments of veins and alteration that form clasts within the Black Flag Group volcaniclastic rocks at the Kanowna Belle mine, indicating that epithermal gold mineralisation accompanied Black Flag Group volcanism. The SHRIMP U–Pb zircon age of the volcaniclastic unit is 2668±9 Ma, and xenocrystic zircons with ∼2.68, 2.70 and 2.71 Ga age groupings are common. The Black Flag Group rocks are faulted by a D1 thrust, and ∼2670 Ma is thus an older limit for regional D1 deformation. Although SHRIMP U–Pb zircon ages of felsic porphyries commonly give the best constraints on the timing of deformation and structurally controlled gold mineralisation, the data are complex and dates from single samples can be ambiguous. Four Porphyry samples from the Kanowna Belle Gold Mine were analysed. Backscattered electron and cathodoluminescence imaging show that most magmatic zircon in the porphyries is either high-U and metamict, or restricted to rims on older xenocrysts that are too narrow to be dated by SHRIMP. Some porphyries appear to have been saturated with zircon at source and contain only xenocrystic zircons. Zircons that are interpreted to be magmatic in a sample of the mineralised Kanowna Belle Porphyry gives a mean age of 2655±6 Ma. The Kanowna Belle Porphyry is cross cut by regional D2 fabrics and ∼2655 Ma is thus the maximum age for regional D2 deformation. This is a maximum age for epigenetic lode-gold mineralisation. The age of resetting of high-U zircon grains (2.63 Ga) and the age of ore-related Pb–Pb galenas (2.63 Ga) serves as an approximate date for lode-gold mineralisation. If the complex zircon history of the felsic porphyries at Kanowna Belle is typical of this suite throughout the Eastern Goldfields Province, it is clear that existing single zircon dates from this Province require reevaluation, backed up by careful backscattered and cathodoluminescence imaging and textural studies.     

Microstructural evidence for the transition from dislocation creep to dislocation-accommodated grain boundary sliding in naturally deformed plagioclase

We use quantitative microstructural analysis including misorientation analysis based on electron backscatter diffraction (EBSD) data to investigate deformation mechanisms of naturally deformed plagioclase in an amphibolite gabbro mylonite. The sample is from lower oceanic crust exposed near the Southwest Indian Ridge, and it has a high ratio of recrystallized matrix grains to porphyroclasts. Microstructures preserved in porphyroclasts suggest that early deformation was achieved principally by dislocation creep with subgrain rotation recrystallization; recrystallized grain (average diameter ∼8 μm) microstructures indicate that subsequent grain boundary sliding (GBS) was active in the continued deformation of the recrystallized matrix. The recrystallized matrix shows four-grain junctions, randomized misorientation axes, and a shift towards higher angles for neighbor-pair misorientations, all indicative of GBS. The matrix grains also exhibit a shape preferred orientation, a weak lattice preferred orientation consistent with slip on multiple slip systems, and intragrain microstructures indicative of dislocation movement. The combination of these microstructures suggest deformation by dislocation-accommodated GBS (DisGBS). Strain localization within the recrystallized matrix was promoted by a transition from grain size insensitive dislocation creep to grain size sensitive GBS, and sustained by the maintenance of a small grain size during superplasticity.     

岩石显微构造分析现代技术——EBSD技术及应用

EBSD技术的发展,为岩石显微构造分析开辟了一个全新的领域。它与现代扫描电子显微镜和能谱分析等设备配合,可以同时对块状样品进行晶体结构与成分分析,从而使显微构造、微区成分与结晶学数据分析有机结合起来。 EBSD技术可以精确、快速定量标定包括各种晶系晶体颗粒的晶格方位和描述晶体颗粒的边界、形态等特征,对于具有低角度边界的晶体颗粒提供精确数据,为阐述岩石变形机制提供重要约束,并为高级晶族和不透明矿物结晶学组构与变形机制研究提供了有效的手段。EBSD尤其使获取微米级甚至纳米级尺度上颗粒（亚颗粒）或相之间的定向差别（达到20 nm的空间分辨率和0.3度角度分辨率）成为可能。EBSD技术在矿物相鉴定、亚微域内的应变分析、矿物出溶作用等方面的应用,进一步证明了这一新技术在显微构造分析及相关领域的应用前景。其广泛应用必将带来岩石显微构造研究的新突破,也将成为未来岩石变形机制与岩石圈流变学研究取得飞速发展不可或缺的技术手段。     

The petrological significance of misorientations between grains

Misorientation analysis quantifies microstructural features in tectonites, metamorphic and igneous rocks, and allows hypotheses on their formation to be tested. The misorientation between two lattices can be expressed by a rotation axis and rotation angle. For lattices with symmetry, it is conventional to take the minimum angle that enables one lattice to be rotated into the other. For a group of lattice measurements two types of misorientation distribution can be calculated. Selecting random pairs of grains gives the random-pair misorientation distribution. Selecting neighbouring pairs gives the neighbour-pair misorientation distribution. The forms of both distributions are visualised using histograms or cumulative frequency diagrams. They are strongly influenced by any overall crystallographic preferred orientation and by intrinsic crystal symmetry. In many rocks, the random-pair misorientation distribution and neighbour-pair misorientation distribution are statistically significantly different (quantified using the Kolmogorov-Smirnov test). Differences between the random-pair misorientation distribution and neighbour-pair misorientation distribution imply that adjacent grains have physically interacted or are inherited from a precursor microstructure. Interactions include (1) reduction in surface energy by lattice alignment. We show this may have occurred in garnet clusters in schist, and olivine in a cumulate. It is well-known in metals and may be a common geological process. (2) Nucleation, where those nuclei have influenced the orientation of adjacent nuclei. (3) Mechanical rotations of facetted grains in compacting crystal mushes, so that faces become parallel. (4) Growth twinning. Inheritance includes (1) subgrain rotation recrystallisation in tectonites deforming by crystal plastic processes. (2) Mechanical and transformation-related twinning. (3) Domainal microstructures, e.g. where grains have formed from a few large original grains, may give rise to spurious correlations when the orientation data cover more than one domain. With this proviso, misorientation analysis can be used to investigate many important microstructural processes.