Monazite response to ultrahigh-pressure subduction from U–Pb dating by laser ablation split stream

Hacker, Bradley R.; Kylander-Clark, Andrew R.C.; Holder, Robert; Andersen, Torgeir B.; Peterman, Emily M.; Walsh, Emily O.; Munnikhuis, Jonathan Keith

doi:10.1016/j.chemgeo.2015.05.008

Cited by 48 publications

(32 citation statements)

References 72 publications

(95 reference statements)

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“…Roberts & Gee, ; Robinson, ; Terry, Robinson, & Ravna, ). Nevertheless, pre‐Scandian Caledonian ages (460–430 Ma) for metamorphic rocks in the SNC of Jämtland have been scarcely reported for the Blåhø Nappe (Hacker et al., ; Terry, Robinson, Hamilton, et al., ). Thus, the early Caledonian ages obtained in this study can be regarded as a critical age link between the Blåhø Nappe and the SNC of Jämtland, supporting the mapping results.…”

Section: Discussionmentioning

confidence: 99%

“…While the Blåhø Nappe is inferred to be part of the SNC, monazite U‐Th‐Pb dating work on the ‘diamondiferous’ gneiss has thus far mainly yielded Scandian metamorphic ages of c . 420–395 Ma (Hacker et al., ; Terry, Robinson, Hamilton, & Jercinovic, ).…”

Section: Geological Backgroundmentioning

confidence: 99%

“…Among these nappes, the Seve Nappe Complex (SNC), and its equivalents in the Western Gneiss Region (WGR) of Norway termed the Blåhø Nappe (e.g. Terry, Robinson, & Ravna, ), are particularly important as microdiamond has been reported from some metasedimentary rocks within them (Dobrzhinetskaya et al., ; Klonowska et al., ; Majka, Janák, et al., ; Majka, Rosén, et al., ).Various tectonic models have been proposed to interpret the evolutionary history of the Scandinavian Caledonides (e.g., Brueckner, ; Brueckner & van Roermund, ; Hacker et al., ; Hollocher, Robinson, Seaman, & Walsh, ; Majka, Rosén, et al., ; Roberts, ). While discrepancies exist between these tectonic models, it is usually proposed that the SNC subducted as a whole during the early Caledonian orogeny, and that part of the SNC was forced to subduct into the mantle a second time due to later introduction of the Baltica plate into the subduction zone.…”

Section: Introductionmentioning

confidence: 99%

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An anticlockwise P–T–t path at high‐pressure, high‐temperature conditions for a migmatitic gneiss from the island of Fjørtoft, Western Gneiss Region, Norway, indicates two burial events during the Caledonian orogeny

Liu

Massonne

2019

Journal Metamorphic Geology

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The Blåhø Nappe on the island of Fjørtoft, which represents an isolated portion of the Seve Nappe Complex in the Western Gneiss Region, Norway, has been suggested to have experienced two deep burial cycles during the Caledonian orogeny. However, evidence on this multiple burial process by the derivation of a pressure–temperature–time (P–T–t) path has never been given in the literature. In this study, the ‘diamondiferous’ kyanite–garnet gneiss from the Blåhø Nappe on Fjørtoft was revisited to determine if such a process was correct. Two types of garnet, porphyroblastic garnet‐1 and fine‐grained garnet‐2, were recognized in the gneiss. The core of garnet‐1 is poor in Ca and documents P–T conditions of 1.2–1.3 GPa at c. 880°C based on pseudosection modelling. The inner rims of garnet‐1 and the core of garnet‐2 are both richer in Ca, recording P–T conditions of 1.35–1.45 GPa and 770–820°C. Application of conventional geothermobarometry on the outer rim of garnet‐1 and the rim of garnet‐2 yielded retrograde P–T conditions of 0.75–0.90 GPa and 610–685°C. These estimates define an anticlockwise P–T path at pressures below 1.5 GPa. Accessory monazite was dated with the electron microscope. Relicts of detrital monazite in the gneiss point to Sveconorwegian and possibly also Cryogenian provenance for the detritus of the sedimentary protolith. Metamorphic monazite in the gneiss records a wide age range from 460 to 380 Ma, with a peak c. 435 Ma and a shoulder at 395 Ma. These data suggest that the original (Ediacaran?) Baltica margin sediment (gneiss protolith) was transported to the base of an overlying plate during the early Caledonian (pre‐Scandian) orogeny. A long residence time of the metasedimentary rock at this base caused its heating to 880°C and homogenization of the early garnet chemistry. The late Caledonian (Scandian) collision between Baltica and Laurentia led to further burial, during which the studied gneiss was close to the former surface of the downgoing continental plate and, thus, cooled. The reconstructed P–T–t path confirms the multiple burial history of the Blåhø Nappe but contradicts previous ideas of deep burial of the Fjørtoft gneiss to more than 100 km.

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Section: Discussionmentioning

confidence: 99%

Section: Geological Backgroundmentioning

confidence: 99%

Section: Introductionmentioning

confidence: 99%

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An anticlockwise P–T–t path at high‐pressure, high‐temperature conditions for a migmatitic gneiss from the island of Fjørtoft, Western Gneiss Region, Norway, indicates two burial events during the Caledonian orogeny

Liu

Massonne

2019

Journal Metamorphic Geology

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“…Ultrahigh‐pressure domains after Hacker et al (). Eclogite localities from Andersen et al (), Krabbendam and Dewey (), Hacker et al (), Hacker et al (), and Hacker et al () and weakly deformed Baltic Shield basement (BSB) domain after Milnes et al (). Structures of the extensional detachment system are shown in green.…”

Section: Introductionmentioning

confidence: 99%

Deep Crustal Flow Within Postorogenic Metamorphic Core Complexes: Insights From the Southern Western Gneiss Region of Norway

et al. 2019

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Viscous crustal flow can exhume once deeply buried rocks in postorogenic metamorphic core complexes (MCCs). While migmatite domes record the flow dynamics of anatectic crust, the mechanics and kinematics of solid-state flow in the deep crust are poorly constrained. To address this issue, we studied a deeply eroded and particularly well-exposed MCC in the southern Western Gneiss Region of Norway. The Gulen MCC formed during Devonian transtensional collapse of the Caledonian orogeny in the footwall of the Nordfjord-Sogn detachment zone. We developed a semiquantitative mapping scheme for ductile strain to constrain micro-to megascale processes, which brought eclogite-bearing crust from the orogenic root into direct contact with Devonian supradetachment basins. The Gulen MCC comprises different structural levels with distinct metamorphic evolutions. In the high-grade core, amphibolite-facies structures record fluid-controlled eclogite retrogression and coaxial flow involving vast extension-perpendicular shortening. Detachment mylonites formed during ductile-to-brittle noncoaxial deformation and wrap around the core. We present a sequential 3-D reconstruction of MCC formation. In the detachment zone, the combined effects of simple shearing, incision/excision, and erosion thinned the upper crust. Internal necking of the ductile crust was compensated by extension-perpendicular shortening within the deep crust and resulted in differential folding of distinct crustal levels. We identify this differential folding as the main mechanism that can redistribute material within solid-state MCCs. Our interpretation suggests a continuum of processes from migmatite-cored to solid-state MCCs and has implications for postorogenic exhumation of (ultra-)high-pressure rocks.Plain Language Summary The Earth's crust has different layers with contrasting mechanics.Rocks in the upper crust tend to break, while higher temperatures at depth make rocks flow, although very slowly. This contrast is important when continents collide forming mountain belts but also when plates drift apart and mountain ranges collapse. In SW Norway, hundreds of million years of erosion have exposed rocks that once were deep below a large mountain range (the Caledonides). Today, glacier-polished fjords reveal large dome structures that formed when the Caledonides collapsed. Inside such a dome, we find rocks originating from different levels of the crust. Rocks and structures in the core formed at high pressures and temperatures. Wrapped around, we find rocks that deformed while they cooled down, became more resistant to flow, and finally broke apart. Above the dome, we find remnants of the upper crust, which was broken up by faults, eroded, and deposited in sedimentary basins. We reconstruct how mechanical contrasts between crustal layers brought rocks from the root of the mountain belt in contact with sediments deposited at the surface. Understanding this process is important, because it can entirely transform the crust within a-geologically speaking-short period o...

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“…For example, low relative abundances of heavy rare earth elements (HREE) in zircon and monazite are commonly interpreted to represent cogenesis with garnet, and the absence of negative Eu anomalies in normalized REE patterns for zircon and monazite could reflect plagioclase breakdown during eclogite facies metamorphism (e.g. Kylander-Clark et al, 2013;Hacker et al, 2015;Holder et al, 2015). This approach has been successful in elucidating the absolute timing and rates of metamorphic and deformation processes in a variety of tectonic settings.…”

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confidence: 99%

Coupled garnet Lu–Hf and monazite U–Pb geochronology constrain early convergent margin dynamics in the Ross orogen, Antarctica

Hagen‐Peter

Cottle

Smit

et al. 2016

Journal Metamorphic Geology

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The Ross orogen of Antarctica is an extensive (>3000 km-long) belt of deformed and metamorphosed sedimentary rocks and granitoid batholiths, which formed during convergence and subduction of palaeo-Pacific lithosphere beneath East Gondwana in the Neoproterozoic-early Palaeozoic. Despite its prominent role in Gondwanan convergent tectonics, and a well-established magmatic record, relatively little is known about the metamorphic rocks in the Ross orogen. A combination of garnet Lu-Hf and monazite U-Pb (measured by laser-ablation split-stream ICP-MS) geochronology reveals a protracted metamorphic history of metapelites and garnet amphibolites from a major segment of the orogen. Additionally, direct dating of a common rock-forming mineral (garnet) and accessory mineral (monazite) allows us to test assumptions that are commonly used when linking accessory mineral geochronology to rock-forming mineral reactions. Petrography, mineral zoning, thermobarometry and pseudosection modelling reveal a Barrovian-style prograde path, reaching temperatures of~610-680°C. Despite near-complete diffusional resetting of garnet major element zoning, the garnet retains strong rare earth element zoning and preserves Lu-Hf dates that range from c. 616-572 Ma. Conversely, monazite in the rocks was extensively recrystallized, with concordant dates that span from c. 610-500 Ma, and retain only vestigial cores. Monazite cores yield dates that overlap with the garnet Lu-Hf dates and typically have low-Y and heavy rare earth element (HREE) concentrations, corroborating interpretations of low-Y and low-HREE monazite domains as records of synchronous garnet growth. However, ratios of REE concentrations in garnet and monazite do not consistently match previously reported partition coefficients for the REE between these two minerals. High-Y monazite inclusions within pristine, crack-free garnet yield U-Pb dates significantly younger than the Lu-Hf dates for the same samples, indicating recrystallization of monazite within garnet. The recrystallization of high-Y and high-HREE monazite domains over >50 Ma likely records either punctuated thermal pulses or prolonged residence at relatively high temperatures (up to~610-680°C) driving monazite recrystallization. One c. 616 Ma garnet Lu-Hf date and several c. 610-600 Ma monazite U-Pb dates are tentatively interpreted as records of the onset of tectonism metamorphism in the Ross orogeny, with a more robust constraint from the other Lu-Hf dates (c. 588-572 Ma) and numerous c. 590-570 Ma monazite U-Pb dates. The data are consistent with a tectonic model that involves shortening and thickening prior to widespread magmatism in the vicinity of the study area. The early tectonic history of the Ross orogen, recorded in metamorphic rocks, was broadly synchronous with Gondwana-wide collisional Pan-African orogenies.

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Monazite response to ultrahigh-pressure subduction from U–Pb dating by laser ablation split stream

Cited by 48 publications

References 72 publications

An anticlockwise P–T–t path at high‐pressure, high‐temperature conditions for a migmatitic gneiss from the island of Fjørtoft, Western Gneiss Region, Norway, indicates two burial events during the Caledonian orogeny

An anticlockwise P–T–t path at high‐pressure, high‐temperature conditions for a migmatitic gneiss from the island of Fjørtoft, Western Gneiss Region, Norway, indicates two burial events during the Caledonian orogeny

Deep Crustal Flow Within Postorogenic Metamorphic Core Complexes: Insights From the Southern Western Gneiss Region of Norway

Coupled garnet Lu–Hf and monazite U–Pb geochronology constrain early convergent margin dynamics in the Ross orogen, Antarctica

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