Dating ultra‐high–pressure (
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
- Award ID(s):
- 1946911
- NSF-PAR ID:
- 10457683
- Publisher / Repository:
- Wiley-Blackwell
- Date Published:
- Journal Name:
- Journal of Metamorphic Geology
- Volume:
- 38
- Issue:
- 3
- ISSN:
- 0263-4929
- Page Range / eLocation ID:
- p. 297-327
- Format(s):
- Medium: X
- 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. -
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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.
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Abstract Gneiss domes are an integral element of many orogenic belts and commonly provide tectonic windows into deep crustal levels. Gneiss domes in the New England segment of the Appalachian orogen have been classically associated with diapirism and fold interference, but alternative models involving ductile flow have been proposed. We evaluate these models in the Gneiss Dome belt of western New England with U‐Th‐Pb monazite, xenotime, zircon, and titanite petrochronology and major and trace element thermobarometry. These data constrain distinct pressure–temperature–time (P‐T‐t) paths for each unit in the gneiss dome belt tectono‐stratigraphy. The structurally lowest units, Laurentia‐derived migmatitic gneisses of the Waterbury dome, document two stages of metamorphism (455–435 and 400–370 Ma) with peak Acadian metamorphic conditions of ~1.0–1.2 GPa at 750–780°C at 391 ± 7 to 386 ± 4 Ma. The next structurally higher unit, the Gondwana‐derived Taine Mountain Formation, records Taconic (peak conditions: 0.6 GPa, 600°C at 441 ± 4 Ma) and Acadian (peak: 0.8–1.0 GPa, 650°C at 377 ± 4 Ma) metamorphism. The overlying Collinsville Formation yielded a 473 ± 5 Ma crystallization age and evidence for metamorphic conditions of 650°C at 436 ± 4 Ma and 1.2–1.0 GPa, 750–775°C at 397 ± 4 to 385 ± 6 Ma. The structurally higher Sweetheart Mountain Member of the Collinsville Formation yielded only Acadian zircon, monazite, and xenotime dates and evidence for high‐pressure granulite facies metamorphism (1.8 GPa, 815°C) at circa 380–375 Ma. Cover rocks of the dome‐mantling The Straits Schist records peak conditions of ~1 GPa, 700°C at 386 ± 6 to 380 ± 4 Ma. Garnet breakdown to monazite and/or xenotime occurred in all units at circa 375–360 and 345–330 Ma. Peak Acadian metamorphic pressures increase systematically from the structurally lowest to highest units (from 1.0 to 1.8 GPa). This inverted metamorphic sequence is incompatible with the diapiric and fold interference models, which predict the highest pressures at the structurally lowest levels. Based upon P‐T‐t and structural data, we prefer a model involving, first, circa 380 Ma thrust stacking followed by syn‐collisional orogen parallel extension, ductile flow, and rise of the domes between 380 and 365 Ma. Garnet breakdown at circa 345–330 Ma is interpreted to reflect further exhumation during collapse of the Acadian orogenic plateau. These results highlight the power of integrating petrologic constraints with paired geochemical and geochronologic data from multiple chronometers to test structural and tectonic models and show that syn‐convergent orogen parallel ductile flow dramatically modified earlier accretion‐related structures in New England. Further, the Gneiss Dome belt documents gneiss dome development in a syn‐collisional, thick crust setting, providing an ancient example of middle to lower crustal processes that may be occurring today in the modern Himalaya and Pamir Range.
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Eclogite bodies exposed across Tibet record a history of subduction-collision events that preceded growth of the Tibetan Plateau. Deciphering the time-space patterns of eclogite generation improves our knowledge of the preconditions for Cenozoic orogeny in Tibet and broader eclogite formation and/or exhumation processes. Here we report the discovery of Permo-Triassic eclogite in northern Tibet. U-Pb zircon dating and thermobarometry suggest eclogite-facies metamorphism at ca. 262–240 Ma at peak pressures of ∼2.5 GPa. Inherited zircons and geochemistry show the eclogite was derived from an upper-plate continental protolith, which must have experienced subduction erosion to transport the protolith mafic bodies to eclogite-forming conditions. The Dabie eclogites to the east experienced a similar history, and we interpret that these two coeval eclogite exposures formed by subduction erosion of the upper plate and deep trench burial along the same ∼3000-km-long north-dipping Permo-Triassic subduction complex. We interpret the synchroneity of eclogitization along the strike length of the subduction zone to have been driven by accelerated plate convergence due to ca. 260 Ma Emeishan plume impingement.more » « less
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Abstract 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 (
P–T ) path even where zircon abundance decreases at the whole‐rock scale.
