In orogens worldwide and throughout geologic time, large volumes of deep continental crust have been exhumed in domal structures. Extension‐driven ascent of bodies of deep, hot crust is a very efficient mechanism for rapid heat and mass transfer from deep to shallow crustal levels and is therefore an important mechanism in the evolution of continents. The dominant rock type in exhumed domes is quartzofeldspathic gneiss (typically migmatitic) that does not record its former high‐pressure (HP) conditions in its equilibrium mineral assemblage; rather, it records the conditions of emplacement and cooling in the mid/shallow crust. Mafic rocks included in gneiss may, however, contain a fragmentary record of a HP history, and are evidence that their host rocks were also deeply sourced. An excellent example of exhumed deep crust that retains a partial HP record is in the Montagne Noire dome, French Massif Central, which contains well‐preserved eclogite (garnet+omphacite+rutile+quartz) in migmatite in two locations: one in the dome core and the other at the dome margin. Both eclogites record P ~ 1.5 ± 0.2 GPa at T ~ 700 ± 20°C, but differ from each other in whole‐rock and mineral composition, deformation features (shape and crystallographic preferred orientation, CPO), extent of record of prograde metamorphism in garnet and zircon, and degree of preservation of inherited zircon. Rim ages of zircon in both eclogites overlap with the oldest crystallization ages of host gneiss at c. 310 Ma, interpreted based on zircon rare earth element abundance in eclogite zircon as the age of HP metamorphism. Dome‐margin eclogite zircon retains a widespread record of protolith age (c. 470–450 Ma, the same as host gneiss protolith age), whereas dome‐core eclogite zircon has more scarce preservation of inherited zircon. Possible explanations for differences in the two eclogites relate to differences in the protolith mafic magma composition and history and/or the duration of metamorphic heating and extent of interaction with aqueous fluid, affecting zircon crystallization. Differences in HP deformation fabrics may relate to the position of the eclogite facies rocks relative to zones of transpression and transtension at an early stage of dome development. Regardless of differences, both eclogites experienced HP metamorphism and deformation in the deep crust at c. 310 Ma and were exhumed by lithospheric extension—with their host migmatite—near the end of the Variscan orogeny. The deep crust in this region was rapidly exhumed from ~50 to <10 km, where it equilibrated under low‐P/high‐T conditions, leaving a sparse but compelling record of the deep origin of most of the crust now exposed in the dome.
<p>Oriented rutile needles in garnet commonly occur in high-temperature / high-pressure rocks such as high-pressure granulites, ultrahigh-pressure rocks, and mantle peridotite. Faceted inclusions of plagioclase and quartz in garnet are also indicators of high &#8211; and possibly very high &#8211; grade conditions. Both inclusion textures are spectacularly displayed in garnets in sillimanite-bearing gneiss of the North Cascade Range (USA) in rocks that record peak P-T conditions of 1 GPa and 725&#176;C; i.e.&#160; at significantly lower pressure than most other occurrences of rutile needle-bearing garnets and at the low end of the temperature range relative to most other occurrences of faceted (negative crystal) inclusions.</p><p>The most dramatic example of faceted inclusions is in sillimanite gneiss containing ~1-2 mm garnets that contain kyanite inclusions and abundant negative crystals of plagioclase. Matrix plagioclase (Pl) is unzoned, but Pl inclusions in garnet are strongly zoned: anorthite content increases by up to 24 mol% from core to rim. Zoned inclusions are surrounded by depletion haloes in Ca and Mg in garnet, documenting inclusion-garnet reaction. Zoning in garnet is most pronounced near Pl inclusions with visible fractures that connect to the garnet rim/matrix. Reaction involving Grt and Pl must involve other phases, such as Qz and kyanite/sillimanite, indicating that inclusions were not completely armored. Inclusion faceting and Grt/Pl zoning indicate that Grt interiors experienced significant modification after entrapment of the inclusions.</p><p>Some quartz inclusions are slightly faceted to rounded and are surrounded by Ca-poor regions of garnet. A recent study that applied Qz-in-Grt barometry to isolated, rounded inclusions in these rocks determined lower P (~0.6-0.7 GPa) than previous conventional-barometry results at similar T. These lower-P results are inconsistent with the presence of Ky inclusions in Grt and may reflect the modification of Qz inclusions that is apparent in garnet zoning around Qz and Pl inclusions.</p><p>Possible explanations for these observations are that: (1) the estimated P and/or T conditions are significantly lower than the actual conditions and the gneiss therefore experienced previously-unrecognized high-P granulite and/or eclogite facies metamorphism, or (2) rutile needles and faceted inclusions in Grt can form during metamorphism at upper amphibolite facies conditions; in this case, possibly the nature of the P-T-t path and/or role of fluids were important. The first possibility has significant implications for the tectono-metamorphic evolution of the orogen and perhaps other continental arc-related orogens, and the second is important for understanding the metamorphic processes that produce these inclusion textures in garnet. Using element maps and other methods for evaluating garnet and inclusion textures and compositions, we discuss these interpretations and implications.</p>
<p>Migmatite domes are common structures in orogens, and in some cases are comprised of deeply-sourced crust that experienced lateral and subsequent vertical flow, with ultimate emplacement in the mid/upper crust. The record of the deep-crustal history survives in layers and lenses of refractory rock types within the dominant quartzofeldspathic gneiss. These deep-crustal relics are typically the best archives of pressure-temperature-time-deformation conditions of crustal flow, although it can be difficult to extract information about the duration of deep-crustal residence &#8211; such as might accompany lateral flow of deep-crust &#8211; because intracrystalline diffusion at protracted high temperatures may erase much of the history and/or minerals may record only the timing of final emplacement and cooling. One possible indicator of deep-crustal history is the extent of recrystallization of zircon that experienced eclogite-facies conditions; the conditions of zircon growth/recrystallization are indicated by REE abundance and results of Ti-in-zircon thermometry. For example, in the eclogite-bearing Montagne Noire migmatite dome of the southern French Massif Central, zircon in eclogite from the core of the dome has been extensively recrystallized under eclogite-facies conditions. In contrast, zircon in eclogite from the margin of the dome experienced very little recrystallization and largely consists of inherited (magmatic) cores with very thin (<20 um) eclogite-facies rims. The two eclogites, which both contain garnet + omphacite + rutile + quartz, record the same age of protolith crystallization (~450 Ma) and high-P metamorphism (~315 Ma), and similar metamorphic conditions (700 &#177; 20&#176;C, 1.4 &#177;0.1 GPa). Differences in extent of recrystallization of zircon in the two eclogites may relate to duration at high T and/or extent of interaction with aqueous fluid (ongoing work to obtain in situ oxygen isotope data for zircon and garnet will evaluate the latter for each eclogite). Deformation may have been involved in recrystallization of zircon, but is not the primary factor accounting for the differences in extent of recrystallization; both eclogites were deformed during eclogite-facies metamorphism, as indicated by crystallographic-preferred orientation of omphacite and shape-preferred orientation of rutile. Other variables that are also unlikely to explain differences in these eclogite zircons are differences in host rock chemistry, availability of Zr from decompression reactions involving Zr-bearing minerals, extent of radiation damage, and original crystal size. The two most likely explanations for variations in zircon recrystallization are duration at high-T and extent of fluid-rock interaction. In the case of the former, dome-margin eclogite may have had a shorter residence time in the deep crust and was more directly exhumed from a proximal source, whereas the dome-core eclogite may have had a more extended transit in the deep-crust before being exhumed in the steep, median high-strain zone of the migmatite dome.</p>
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