Temporal and petrological constraints of ultrahigh-pressure metamorphism and exhumation of crustal material from mantle depths to Earth's surface: Insights from a large and small ultrahigh-pressure terrane
AuthorDesOrmeau, Joel William
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Ultrahigh-pressure (UHP) terranes expose continental material that has subducted to mantle depths and then returned to Earth’s surface. These terranes typically consist mainly of migmatitic host ortho- and paragneiss, with minor (~5%) layers and lenses of UHP eclogite; at least twenty terranes have been described and identified by the preservation of UHP minerals (i.e., coesite and diamond) mainly within eclogite. These terranes fall into two broad categories: 1) large (30,000 km2) coherent terranes characterized by slow (>20 Myr) subduction–exhumation histories, and 2) small (~4,000 km2) terranes that have undergone rapid (<10 Myr) subduction–exhumation histories. To better understand the geodynamic processes involved in deep continental subduction and subsequent exhumation of buoyant crustal material within these two types of UHP terranes, it is important to constrain the timing and conditions of: 1) peak UHP metamorphism recorded in the eclogites; 2) the subsequent eclogite retrograde metamorphism; and 3) the host-rock migmatization which likely occurs during exhumation from the mantle to the upper crust.This study investigates the Western Gneiss Region (WGR) of Norway, a giant UHP terrane, and the gneiss domes exposed in the D’Entrecasteaux Islands in eastern Papua New Guinea (PNG), a small UHP terrane, in order to understand the similarities and differences among the subduction and exhumation of these end-member terranes. To better understand the maximum pressure and temperature the rocks reached within the mantle, thermobarometry and phase-diagram modeling are applied to the PNG (U)HP eclogites, as previous work suggested a wide range of results, many of which were not at UHP. In order to understand the timing and rates of subduction and exhumation events, high-precision zircon U-Pb isotope dilution–thermal ionization mass spectrometry geochronology and trace-element analyses (ID-TIMS-TEA) of the (U)HP rocks exposed in the two terranes are used, as these tools can decipher tectonic events that occur on a sub-million year timescale. In the WGR, UHP rocks are exposed within three domains that have been interpreted to have undergone a similar tectonic history, with a long duration of (U)HP metamorphism from ca. 425–400 Ma associated with the Scandian-phase of the Caledonian orogeny. In order to test if UHP metamorphism was a single ~25 Myr event, eclogite was collected from two of the three UHP domains for high-precision ID-TIMS-TEA analysis. Zircon was extracted from the bulk rock, mounted in an epoxy mount, and screened for Scandian ages using high-spatial resolution laser ablation split-stream inductively coupled plasma mass spectrometry (LASS-ICP-MS). These Scandian-aged zircons were subsequently analyzed by ID-TIMS-TEA. The LASS analyses reveal a spread in results from both samples, with ages between 414 and 397 Ma. In comparison, the ID-TIMS analyses from the exact same zircons analyzed by LASS reveal two age populations of ca. 409 Ma and ca. 402 Ma from a garnet–quartz layer within the Saltaneset eclogite of the southern UHP domain. In comparison, the Ulsteinvik eclogite collected from the central UHP domain also yields two ID-TIMS age populations: ca. 409–407 Ma and ca. 402 Ma. Thus, two eclogites from different regions within the giant WGR UHP terrane reveal the same two age populations. Zircon trace-element data collected via laser-ablation and solution analyses for the two-age populations yield depleted heavy rare earth element (HREE) patterns and flat-to-positive Eu anomalies. The combined age and trace-element data indicate that the WGR terrane underwent two distinct eclogite-facies zircon (re)crystallization events at ca. 409–407 Ma and ca. 402 Ma at localities ~40 km apart. These results support the interpretation that the UHP terrane was subducted and exhumed as a large (30,000 km2) coherent slab of crustal material, but that as this terrane was subducted, there were two different events that affected this giant terrane while it was at eclogite-facies conditions. The Pliocene PNG UHP terrane exposes a series of east–west gneiss domes, Normanby, Oiatabu, Mailolo, and Goodenough that contain eclogites within mainly highly migmatitic quartzofeldspathic gneiss. This UHP terrane is unique in that it is the only one on Earth that is actively exhuming, in this case within the Woodlark Rift. To better understand the pressure-temperature-time-deformation path taken by this young UHP terrane, a suite of fresh to nearly-completely retrogressed eclogites were sampled from Oiatabu, Mailolo, and Goodenough Domes for thermobarometry, pseudosection modeling, and high-precision ID-TIMS-TEA zircon analyses. A kyanite-phengite eclogite from Oiatabu Dome records equilibration at UHP conditions of ~30–31 kbar and ~635–660 °C, whereas a fresh phengite eclogite from the central Mailolo Dome yields peak conditions of ~27–30 kbar and ~510–560 °C. Zircons extracted from the Mailolo Dome eclogite reveal UHP metamorphism occurred from ca. 6.0 to 5.2 Ma, based on zircon that contain inclusions of the peak metamorphic assemblage. Following UHP recrystallization, the crustal material ascended along a near-isothermal decompression path, accompanied by partial melting and retrogression, to the base of the crust. The host ortho- and paragneiss from the eastern Normanby and Oiatabu Domes record the early stages of this retrogression, with ID-TIMS zircon dates that document retrogression-related metamorphism in the structurally higher portions of the domes at ca. 5.7–4.5 Ma. Zircons from retrogressed eclogites collected within the Oiatabu and Mailolo Domes, also record initial exhumation at ca. 4.6–4.3 Ma; the zircon from these samples are associated with garnet- and omphacite-breakdown reactions. Continued exhumation and near-complete retrogression in the lower crust occurred at ca. 2.8–2.6 Ma, based on zircons from heavily retrogressed eclogites collected in the westernmost Goodenough Dome. Zircon trace-element data from all the eclogites show depleted HREE and absent negative Eu anomalies suggesting eclogite-facies zircon (re)crystallization, although some grains are clearly in textural equilibrium with lower-pressure phases (i.e., amphibole and plagioclase).To further track the exhumation history of the PNG UHP terrane, samples representing different melt generations (e.g., strongly-deformed leucosomes versus nondeformed dikes) that formed during exhumation are used to record different parts of the deformation history. The crystallization of strongly-deformed sills and leucosomes likely associated with cooling and amphibolite-facies retrogression suggests the terrane reached neutral buoyancy near the base of the crust first in the east by ca. 4.1 Ma in Normanby Dome, by ca. 3.5–3.0 in the central Mailolo Dome, and by ca. 3.9–2.8 in the far west in Goodenough Dome. Further exhumation and continued amphibolite-facies metamorphism within the mid-crust are marked by weakly deformed dikes recording melt crystallization at ca. 3.0–2.9 Ma in Oiatabu Dome and ca. 2.4–2.3 Ma in Mailolo and Goodenough Domes. Final extension-related exhumation of the entire terrane within the upper crust occurred by ca. 1.8 Ma, as recorded by the crystallization of non-deformed plutons, pegmatites, and dikes. Taking into account all of the petrologic, structural, geophysical, and geochronologic constraints, a model for the exhumation of the PNG UHP terrane involves (re)crystallization of the previously subducted continental material during a flux of hot asthenospheric fluids related to westward seafloor spreading in the Woodlark Rift from ca. 6.0 to 5.2 Ma. Subsequently, the partially-molten (U)HP rocks rose as diapirs that underwent rapid near-vertical exhumation to the base of the crust, resulting in the generation of abundant partial melting and the initial retrogression of the crustal material. Upon reaching neutral buoyancy at the base of the crust, the (U)HP body laterally flowed, but also cooled, allowing the crystallization of the strongly deformed leucosomes. Further exhumation to the upper crust was assisted by enhanced buoyancy due to later injection of partial melt and concurrent extension within the active Woodlark Rift. Eclogites from the WGR and eastern PNG were both subjected to upper mantle depths, but they preserve different metamorphic and exhumation histories. The ID-TIMS zircon results from the small PNG UHP terrane document rapid (≥1.5 cm/yr) exhumation from peak metamorphism in the upper mantle at ca. 6.0–5.2 Ma to emplacement within the brittle upper crust in ~3 Myr, some of the fastest rates of UHP exhumation documented on Earth. In comparison, studies based on multiple geochronological techniques have suggested that the WGR likely resided at mantle depths for tens of millions of years, from ca. 425–400 Ma. However, the new results from ID-TIMS U-Pb zircon dates suggest eclogite-facies metamorphism occurred during at least two distinct (re)crystallization events at ca. 409–407 Ma and ca. 402 Ma, at the youngest end of the previously proposed timescale of UHP metamorphism. Thus, these new results suggest that the interpretation that giant UHP terranes undergo a long continuous duration of UHP metamorphism may need to be reevaluated, as there may be distinct events hidden within this metamorphic window that can only be deciphered via a high-precision geochronometric technique. This is supported by the results from the PNG UHP terrain, where mantle to crustal exhumation occurs in only ~3 Myr. The high-precision results from the two UHP terranes that differ in size, age, and in their exhumation history provide important constraints on the timing and duration of UHP metamorphism and exhumation to the upper crust. Understanding tectonic events on such short timescales has drastic implications for geodynamic models attempting to characterize the switch from subduction to exhumation and the transfer of continental material through the lower crust to Earth’s surface via coupled buoyancy-driven and extension-related exhumation processes.