| 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 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.;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.;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, 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. (Abstract shortened by UMI.). |
