Dating ultra‐high–pressure (
Continental subduction and collision are recorded by ultrahigh‐pressure (UHP) terranes; UHP terranes that form at early stages of an orogeny tend to be small and experience short residence at eclogite‐facies depths, whereas terranes that form at mature stages of an orogeny tend to be larger and experience longer residence at these depths, but accurately determining eclogite‐facies residence time requires a large geochronologic dataset tied to metamorphic conditions (via trace elements and/or inclusions). In the Dulan area, North Qaidam UHP terrane, China, it remains unclear whether the terrane experienced a long residence at eclogite‐facies depths, marking the mature stage of an orogeny or two distinct (ultra)high pressure ([U]HP) events (with short residence times), interpreted as the transition from oceanic subduction to continental collision, where one (U)HP event is related to the former and second (U)HP event to the latter. To address this issue, we report new zircon U–Pb ages and trace‐element data from eclogite and host paragneiss from the Dulan area and show that this terrane records ~42 Myr of eclogite‐facies metamorphism at (U)HP conditions, similar to other large UHP terranes. Zircon from 11 eclogite and 2 gneiss samples yields weighted mean ages of 463–425 Ma, flat heavy rare earth element (HREE) patterns without negative Eu anomalies, and eclogitic mineral inclusions, indicating eclogite‐facies conditions. Paragneiss metamorphic ages overlap with ages from eclogite but are generally younger, suggesting that a lack of internally generated fluids may have inhibited zircon growth and/or recrystallization until early decompression and white mica consumption in felsic gneiss generated fluids; thus, we interpret that these felsic rocks record the later stages of continental collision. Dataset patterns from all new and previously published analyses for the Dulan area (34 eclogite and 14 gneiss) suggest that metamorphic zircon in eclogite records prograde, peak and possibly early retrograde conditions, in contrast to the prediction from mass balance models that metamorphic zircon should only grow during exhumation and cooling. We reconcile our observations with these model predictions by recognizing that differential solubility can lead to grain‐scale zircon growth or recrystallization over a large segment of the pressure–temperature (
- NSF-PAR ID:
- 10391147
- Publisher / Repository:
- Wiley-Blackwell
- Date Published:
- Journal Name:
- Journal of Metamorphic Geology
- Volume:
- 41
- Issue:
- 4
- ISSN:
- 0263-4929
- Format(s):
- Medium: X Size: p. 557-581
- Size(s):
- ["p. 557-581"]
- Sponsoring Org:
- National Science Foundation
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Abstract UHP ) metamorphic rocks provides important timing constraints on deep subduction zone processes. Eclogites, deeply subducted rocks now exposed at the surface, undergo a wide range of metamorphic conditions (i.e. deep subduction and exhumation) and their mineralogy can preserve a detailed record of chronologic information of these dynamic processes. Here, we present an approach that integrates multiple radiogenic isotope systems in the same sample to provide a more complete timeline for the subduction–collision–exhumation processes, based on eclogites from the Dabie–Sulu orogenic belt in eastern China, one of the largestUHP terranes on Earth. In this study, we integrate garnet Lu–Hf and Sm–Nd ages with zircon and titanite U–Pb ages for three eclogite samples from the SuluUHP terrane. We combine this age information with Zr‐in‐rutile temperature estimates, and relate these multiple chronometers to differentP–T conditions. Two types of rutile, one present as inclusions in garnet and the other in the matrix, record the temperatures ofUHP conditions and a hotter stage, subsequent to the peak pressure (‘hot exhumation') respectively. Garnet Lu–Hf ages (c . 238–235 Ma) record the initial prograde growth of garnet, while coupled Sm–Nd ages (c . 219–213 Ma) reflect cooling following hot exhumation. The maximum duration ofUHP conditions is constrained by the age difference of these two systems in garnet (c . 235–220 Ma). Complementary zircon and titanite U–Pb ages ofc . 235–230 Ma andc . 216–206 Ma provide further constraints on the timing of prograde metamorphism and the ‘cold exhumation' respectively. We demonstrate that timing of various metamorphic stages can thus be determined by employing complementary chronometers from the same samples. These age results, combined with published data from adjacent areas, show lateral diachroneity in the Dabie–Sulu orogeny. Three sub‐blocks are thus defined by progressively younger garnet ages: western Dabie (243–238 Ma), eastern Dabie–northern Sulu (238–235 Ma) and southern Sulu terranes (225–220 Ma), which possibly correlate to different crustal slices in the recently proposed subduction channel model. These observed lateral chronologic variations in a largeUHP terrane can possibly be extended to other suture zones. -
Mica- and garnet-rich selvages are often developed around eclogitized mafic blocks within felsic gneiss in HP to UHP metamorphic terranes. The development of these metasomatic features ranges from readily identified reaction zones between the eclogite and host gneiss to shear zones where the spatial relationships between eclogite blocks and host gneiss are completely obscured. Block-selvage relationships within the Luliang Shan HP/UHP belt (North Qaidam, China) and the Tso Morari UHP terrane (NW Himalaya, India) approximate end members of the selvage preservation process. Here we apply whole-rock and incompatible trace element compositions coupled with B and O isotopic data in white mica to constrain the relationship of metasomatism vs. deformation during selvage formation. Within the Luliang Shan, extensive fluid flow formed thick, compositionally hybridized phengite- and garnet-bearing selvages between eclogite (SiO2 ~ 50%) and quartzofeldspathic gneiss (SiO2 ~ 80%). The Luliang Shan HP selvages have intermediate SiO2 and range from 5-10 m in thickness as "halos" around spheroidal eclogite blocks. Volatile enrichment at near-UHP conditions in the selvage is indicated by enrichment of Li, Cs, Ba, Ar, and δ18O and very light δ11B values in phengite. The retrograde muscovite from the host gneiss is low in Li, Cs, Rb, and Sr but possess remarkably high B concentrations (up to 3000 ppm) and positive δ11B values that are best explained by interaction with fluids devolatilized from accreted sediments within cooler regions of the subduction zone. Alternatively, the Tso Morari UHP terrane features boudinaged discoids of eclogite encased within highly strained quartzofeldspathic gneiss. Whole rock major element sampling performed normal to the foliation reveal consistently high SiO2 (78-80%). Highly variable degrees of metasomatic recrystallization occur within the phengite-rich rocks spatially associated with eclogite. The selvage rocks exhibit heterogeneous degrees of enrichment in Li, Be, B, and Ba and yield δ11B values of -4 to -6‰ typical of undevolatilized oceanic and continental crust. We conclude that fluid-mediated metasomatic reaction between eclogite and gneiss at Tso Morari is sheared out into lenses that are incorporated into, and heterogeneously distributed throughout, the host gneiss.more » « less
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Rock recycling within the forearcs of subduction zones involves subduction of sediments and hydrated lithosphere into the upper mantle, exhumation of rocks to the surface, and erosion to form new sediment. The compositions of, and inclusions within detrital minerals revealed by electron microprobe analysis and Raman spectroscopy preserve petrogenetic clues that can be related to transit through the rock cycle. We report the discovery of the ultrahigh-pressure (UHP) indicator mineral coesite as inclusions in detrital garnet from a modern placer deposit in the actively exhuming Late Miocene–Recent high- and ultrahigh-pressure ((U)HP) metamorphic terrane of eastern Papua New Guinea. Garnet compositions indicate the coesite-bearing detrital garnets are sourced from felsic protoliths. Carbonate, graphite, and CO2inclusions also provide observational constraints for geochemical cycling of carbon and volatiles during subduction. Additional discoveries include polyphase inclusions of metastable polymorphs of SiO2(cristobalite) and K-feldspar (kokchetavite) that we interpret as rapidly cooled former melt inclusions. Application of elastic thermobarometry on coexisting quartz and zircon inclusions in six detrital garnets indicates elastic equilibration during exhumation at granulite and amphibolite facies conditions. The garnet placer deposit preserves a record of the complete rock cycle, operative on <10-My geologic timescales, including subduction of sedimentary protoliths to UHP conditions, rapid exhumation, surface uplift, and erosion. Detrital garnet geochemistry and inclusion suites from both modern sediments and stratigraphic sections can be used to decipher the petrologic evolution of plate boundary zones and reveal recycling processes throughout Earth’s history.
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Abstract 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 atT ~ 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 atc. 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 atc. 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. -
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Abstract The Pamir gneiss domes represent the most extensive exposure of mid to lower crustal rocks in the Himalayan‐Tibetan orogen north of the India‐Asia suture zone. Unlike other domes in the Central and Southern Pamir, the Muztaghata dome stands out due to its higher metamorphic grade, more complex structural elements, and variable timing of metamorphism. In order to unravel the P‐T‐t history of the Muztaghata dome and better constrain the timing of peak metamorphism, we applied petrologic modeling in concert with geochronology to samples from the structure. The Muztaghata gneiss dome is composed of a structurally higher metapelite‐dominated terrane in the west and a structurally lower orthogneiss terrane in the east. Our results from the western terrane indicate high‐pressure eclogite facies peak conditions of ~800°C/22 kbar at ~25–20 Ma. Zircon grains from metapelitic samples from the western terrane also yield Early Jurassic metamorphic U‐Pb ages with REE signals that indicate coeval garnet growth. Our results from the eastern terrane record high‐pressure amphibolite facies peak conditions of ~650°C/14 kbar at ~24–20 Ma, noticeably lower than the structurally higher western terrane indicating structural juxtaposition during Miocene exhumation. Peak metamorphic conditions from the eastern terrane indicate depths below the current Moho, supporting the interpretation that the Early Miocene Pamir crust was thicker than present. This was followed by rapid exhumation from depths of ~75–80 km and partial westward collapse of the Pamir after 20 Ma, possibly driven in part by regional lithospheric delamination.
