| 1. |
- Barnes, Christopher J., et al.
(författare)
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Zircon and monazite reveal late Cambrian/early Ordovician partial melting of the Central Seve Nappe Complex, Scandinavian Caledonides
- 2022
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Ingår i: Contributions to Mineralogy and Petrology. - : Springer. - 0010-7999 .- 1432-0967. ; 177:9
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Tidskriftsartikel (refereegranskat)abstract
- The Seve Nappe Complex (SNC) comprises continental rocks of Baltica that were subducted and exhumed during the Caledonian orogeny prior to collision with Laurentia. The tectonic history of the central SNC is investigated by applying in-situ zircon and monazite (Th-)U-Pb geochronology and trace element analysis to (ultra-)high pressure (UHP) paragneisses in the Avardo and Marsfjallet gneisses. Zircons in the Avardo Gneiss exposed at Sippmikk creek exhibit xenocrystic cores with metamorphic rims. Cores show typical igneous REE profiles and were affected by partial Pb-loss. The rims have flat HREE profiles and are interpreted to have crystallized at 482.5 +/- 3.7 Ma during biotite-dehydration melting and peritectic garnet growth. Monazites in the paragneiss are chemically homogeneous and record metamorphism at 420.6 +/- 2.0 Ma. In the Marsfjallet Gneiss exposed near Kittelfjall, monazites exhibit complex zoning with cores enveloped by mantles and rims. The cores are interpreted to have crystallized at 481.6 +/- 2.1 Ma, possibly during garnet resorption. The mantles and rims provide a dispersion of dates and are interpreted to have formed by melt-driven dissolution-reprecipitation of pre-existing monazites until 463.1 +/- 1.8 Ma. Depletion of Y, HREE, and U in the mantles and rims compared to the cores record peritectic garnet and zircon growth. Altogether, the Avardo and Marsfjallet gneisses show evidence of late Cambrian/early Ordovician partial melting (possibly in (U)HP conditions), Middle Ordovician (U)HP metamorphism, and late Silurian tectonism. These results indicate that the SNC underwent south-to-north oblique subduction in late Cambrian time, followed by progressive north-to-south exhumation to crustal levels prior to late Silurian continental collision.
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| 2. |
- Callegaro, S., et al.
(författare)
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Geochemistry of deep Tunguska Basin sills, Siberian Traps : correlations and potential implications for the end-Permian environmental crisis
- 2021
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Ingår i: Contributions to Mineralogy and Petrology. - : Springer Nature. - 0010-7999 .- 1432-0967. ; 176:7
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Tidskriftsartikel (refereegranskat)abstract
- A vast portion of the plumbing system of the Siberian Traps Large Igneous Province (STLIP) is emplaced in the Tunguska Basin, where borehole data reveal ubiquitous and abundant sills with great lateral extension. These intrusions intersect Cambrian-Ordovician evaporite, carbonate and siliciclastic series, and locally coal-bearing Permian host rocks, with a high potential for thermogenic gas generation. Here we present new geochemical data from 71 magmatic and 4 sedimentary rock samples from the Tunguska Basin center and periphery, recovered from 15 deep sills intercepted by boreholes. The studied samples are all low-Ti basalt and basaltic andesites, confirming absence of high-Ti and alkaline STLIP magmatism in the Tunguska Basin. The sills derive from picritic parental melts produced by extensive melting of a mantle source with recycled crustal components below a thinned lithosphere (50-60 km), within the spinel stability field. The mantle source was dominantly peridotitic, with enriched pyroxenitic domains formed by recycled lower crust, in agreement with previous models for the main tholeiitic STLIP phase. Limited amounts (up to 5%) of highly radiogenic granitoids or moderately radiogenic metapelites were assimilated in upper crustal magma reservoirs. After emplacement, sills intruded in Cambrian evaporites assimilated marlstones and interacted with the evaporitic host rocks, probably via fluids and brines. This is the first time that such process is described in subvolcanic rocks from all across the volcanic basin. The sills are correlated geochemically with the established chemostratigraphy for the on-craton STLIP lava piles and intrusions (Norilsk region). Sills correlated with the Morongovsky-Mokulaevsky Fm. and the Norilsk-type intrusions are the most voluminous, present all across the central Tunguska Basin, and bear the strongest evidence of interaction with evaporites. Massive discharge of thermogenic volatiles is suggested by explosive pipes and hydrothermal vent structures throughout the Tunguska Basin. We propose that this voluminous pulse of magmatism is a good candidate for the hitherto unidentified early intrusive phase of the STLIP, and may link the deep Tunguska basin sills to the end-Permian environmental crisis.
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| 3. |
- Cruz-Uribe, A. M., et al.
(författare)
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Trace element and isotopic zoning of garnetite veins in amphibolitized eclogite, Franciscan Complex, California, USA
- 2021
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Ingår i: Contributions to Mineralogy and Petrology. - : Springer Science and Business Media LLC. - 0010-7999 .- 1432-0967. ; 176:5
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Tidskriftsartikel (refereegranskat)abstract
- Here we present major element, trace element, and oxygen isotope data for garnet from an amphibolitized eclogite block from Ring Mountain, Franciscan Complex, California, USA. Garnetite veins 1-5 cm thick are laterally continuous up to 10 m within an Mg-rich blackwall zone of the eclogite block. Complex major and trace element zoning patterns reveal multiple stages of garnet growth in both the matrix and garnetite veins. Similarities in major and trace element zoning between matrix and vein garnet suggest that crystallization of the garnetite veins began toward the end of matrix garnet core growth, and continued throughout the garnet growth history of the rock. Oscillatory zoning in rare-earth elements suggests garnet growth in pulses, with matrix-diffusion-limited growth in between pulses. Oxygen isotope analyses of matrix and vein garnet have a range in delta O-18 values of 5.3-11.1 parts per thousand. Differences in delta O-18 values of up to similar to 4 parts per thousand between garnet core and rim are observed in both the matrix and vein; garnet cores range from 9.8 to 11.1 parts per thousand (median 10.4 parts per thousand), garnet mantles range from 8.3 to 10.0 parts per thousand (median 9.7 parts per thousand), and garnet rims range from 5.8 to 7.8 parts per thousand (median 6.7 parts per thousand). Late-stage vein crystallization appears as a garnet "cement" that fills in a network of small (typically 5-50 mu m) garnet cores, and likely crystallized from an amorphous phase. The low delta O-18 values of this latest stage of garnet growth are consistent with interaction with serpentinites, and likely represent the physical incorporation of the eclogite block into the serpentinite matrix melange.
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| 4. |
- Day, James M. D., et al.
(författare)
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Early Eocene Arctic volcanism from carbonate-metasomatized mantle
- 2023
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Ingår i: Contributions to Mineralogy and Petrology. - : Springer Nature. - 0010-7999 .- 1432-0967. ; 178:12
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Tidskriftsartikel (refereegranskat)abstract
- Melilitite, nephelinite, basanite, and alkali basalt, along with phonolite differentiates, form the Freemans Cove Complex (FCC) in the south-eastern extremity of Bathurst Island (Nunavut, Canada). New Ar-40/Ar-39 chronology indicates their emplacement between similar to 56 and similar to 54 million years ago within a localized extensional structure. Melilitites and nephelinites, along with phonolite differentiates, likely relate to the beginning and end phases of extension, whereas alkali basalts were emplaced during a main extensional episode at similar to 55 Ma. The melilitites, nephelinites, and alkali basalts show no strong evidence for significant assimilation of crust, in contrast to some phonolites. Partial melting occurred within both the garnet- and spinel-facies mantle and sampled sources with He, O, Nd, Hf, and Os isotope characteristics indicative of peridotite with two distinct components. The first, expressed in higher degree partial melts, represents a relatively depleted component ("A"; He-3/He-4 similar to 8 R-A, epsilon(i)(Nd) similar to + 3 epsilon(Hf)i similar to + 7, gamma(Os)i similar to 0). The second was an enriched component ("B" He-3/He-4 < 3 R-A, epsilon(Nd)i < - 1 epsilon(Hf)i < + 3, gamma(Os)i > + 70) sampled by the lowest degree partial melts and represents carbonate-metasomatized peridotite. Magmatism in the FCC shows that rifting extended from the Labrador Sea to Bathurst Island and reached a zenith at similar to 55 Ma, during the Eurekan orogeny. The incompatible trace-element abundances and isotopic signatures of FCC rocks indicate melt generation occurred at the base of relatively thin lithosphere at the margin of a thick craton, with no mantle plume influence. FCC melt compositions are distinct from other continental rift magmatic provinces worldwide, and their metasomatized mantle source was plausibly formed synchronously with emplacement of Cretaceous kimberlites. The FCC illustrates that the range of isotopic compositions preserved in continental rift magmas are likely to be dominated by temporal changes in the extent of partial melting, as well as by the timing and degree of metasomatism recorded in the underlying continental lithosphere.
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| 5. |
- Halama, R., et al.
(författare)
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Boron isotope record of peak metamorphic ultrahigh-pressure and retrograde fluid-rock interaction in white mica (Lago di Cignana, Western Alps)
- 2020
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Ingår i: Contributions to Mineralogy and Petrology. - : Springer Science and Business Media LLC. - 0010-7999 .- 1432-0967. ; 175:3
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Tidskriftsartikel (refereegranskat)abstract
- This study presents boron (B) concentration and isotope data for white mica from (ultra)high-pressure (UHP), subduction-related metamorphic rocks from Lago di Cignana (Western Alps, Italy). These rocks are of specific geological interest, because they comprise the most deeply subducted rocks of oceanic origin worldwide. Boron geochemistry can track fluid-rock interaction during their metamorphic evolution and provide important insights into mass transfer processes in subduction zones. The highest B contents (up to 345 mu g/g B) occur in peak metamorphic phengite from a garnet-phengite quartzite. The B isotopic composition is variable (delta B-11 = - 10.3 to - 3.6%) and correlates positively with B concentrations. Based on similar textures and major element mica composition, neither textural differences, prograde growth zoning, diffusion nor a retrograde overprint can explain this correlation. Modelling shows that B devolatilization during metamorphism can explain the general trend, but fails to account for the wide compositional and isotopic variability in a single, well-equilibrated sample. We, therefore, argue that this trend represents fluid-rock interaction during peak metamorphic conditions. This interpretation is supported by fluid-rock interaction modelling of boron leaching and boron addition that can successfully reproduce the observed spread in delta B-11 and [B]. Taking into account the local availability of serpentinites as potential source rocks of the fluids, the temperatures reached during peak metamorphism that allow for serpentine dehydration, and the high positive delta B-11 values (delta B-11 = 20 +/- 5) modelled for the fluids, an influx of serpentinite-derived fluid appears likely. Paragonite in lawsonite pseudomorphs in an eclogite and phengite from a retrogressed metabasite have B contents between 12 and 68 mu g/g and delta B-11 values that cluster around 0% (delta B-11 = - 5.0 to + 3.5). White mica in both samples is related to distinct stages of retrograde metamorphism during exhumation of the rocks. The variable B geochemistry can be successfully modelled as fluid-rock interaction with low-to-moderate (< 3) fluid/rock ratios, where mica equilibrates with a fluid into which B preferentially partitions, causing leaching of B from the rock. The metamorphic rocks from Lago di Cignana show variable retention of B in white mica during subduction-related metamorphism and exhumation. The variability in the B geochemical signature in white mica is significant and enhances our understanding of metamorphic processes and their role in element transfer in subduction zones.
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| 6. |
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| 7. |
- Jaranowski, Maciej, et al.
(författare)
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U-Pb and trace element zircon and apatite petrochronology of eclogites from the Scandinavian Caledonides
- 2023
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Ingår i: Contributions to Mineralogy and Petrology. - : Springer. - 0010-7999 .- 1432-0967. ; 178
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Tidskriftsartikel (refereegranskat)abstract
- The petrochronological records of eclogites in the Scandinavian Caledonides are investigated using EPMA and LA-ICPMS of zircon and apatite for U-Pb geochronology, combined with major and trace element characteristics. Metamorphic zircon from two eclogites from the Lofoten-Vesteralen Complex (Lofoten Archipelago region) collectively yielded a Concordia age 427.8 & PLUSMN; 5.7 Ma and an upper intercept U-Pb age 425 & PLUSMN; 30 Ma. Apatites from the same eclogites provided U-Pb lower intercepts at 322 & PLUSMN; 28 Ma and 354 & PLUSMN; 33 Ma, with the latter also yielding a younger age of 227 & PLUSMN; 24 Ma. Two eclogites from the Lower Seve Nappe (Northern Jamtland) demonstrate different zircon and apatite age records. Metamorphic zircon provided Concordia ages of 467.2 & PLUSMN; 5.9 Ma and 444.5 & PLUSMN; 5.5 Ma, which resolve the age of prograde metamorphism and zircon growth during retrogression, respectively. The lower intercept U-Pb ages of apatites from the same eclogites are 436 & PLUSMN; 18 and 415 & PLUSMN; 25 Ma, respectively. In combination with their geochemical characteristics, they suggest two separate stages of exhumation of eclogite bodies in the Lower Seve Nappe. Zircons from an eclogite from the Blaho Nappe (Nordoyane Archipelago) yielded a continuum of concordant U-Pb dates from ca. 435 to 395 Ma, which suggests several cycles of HT metamorphism within short intervals. Distinctive trace element characteristics of apatites from the Blaho Nappe eclogite suggest formation coeval with zircon and garnet during HT metamorphism, but Pb diffusion behaved as an open system until cooling during exhumation of the nappe at 390 & PLUSMN; 12 Ma (lower intercept U-Pb age of apatite). To summarize, this study presents the high potential of coupled zircon and apatite petrochronology of eclogites in resolving their metamorphic evolution, particularly with respect to using trace element characteristics of apatites to constrain the records of their growth, alterations and the meaning of their U-Pb age record.
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| 8. |
- Kosminska, Karolina, et al.
(författare)
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Monazite in the eclogite and blueschist of the Svalbard Caledonides : its origin and forming-reactions
- 2023
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Ingår i: Contributions to Mineralogy and Petrology. - : Springer. - 0010-7999 .- 1432-0967. ; 178
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Tidskriftsartikel (refereegranskat)abstract
- High-pressure low-temperature rocks from Svalbard are an excellent target for studying metamorphic reactions in Phanerozoic subduction zones. This study reveals the presence of monazite in an eclogite and a blueschist from the Vestgotabreen Complex, southwestern Svalbard. In order to investigate the monazite-forming reaction, we obtained pressure-temperature estimates coupled with U-Pb and Lu-Hf dating. Combined geothermobarometry allows to constrain three evolutionary stages of garnet growth in the eclogite: nucleation (1.6 & PLUSMN; 0.3 GPa at 460 & PLUSMN; 60 & DEG;C), peak-pressure (2.3 & PLUSMN; 0.3 GPa at 507 & PLUSMN; 60 & DEG;C), and peak-temperature (2.1 & PLUSMN; 0.3 GPa at 553 & PLUSMN; 60 & DEG;C). A zircon age of 482 & PLUSMN; 10 Ma is interpreted to belong to the prograde part of the pressure-temperature path. Monazite forms inclusions within garnet rims, or it is surrounded by allanite and apatite, altogether forming pseudomorphs of a tabular shape in the matrix. Textures, geothermobarometry and geochronology support the conclusion the monazite formed under high-pressure conditions at 471 & PLUSMN; 6 Ma. We propose that the monazite crystallization in the eclogite happened due to a decomposition of accessory phases during the decompression after peak-pressure of the metamorphic cycle. Monazite in the blueschist occurs as inclusions in garnet cores and gives an indicative age of 486 & PLUSMN; 6 Ma, which is interpreted to reflect the prograde growth of the garnet. Lu-Hf garnet dating resolves an age of peak-pressure metamorphism in the blueschist at 471.1 & PLUSMN; 4 Ma under conditions of 2.0 & PLUSMN; 0.03 GPa and 500 & PLUSMN; 30 & DEG;C. The Vestgotabreen Complex provides evidence for an early Ordovician modern-style subduction system in the proximity of the Baltica margin. Hence, this study also supports the tectonic models that favour a mixed Baltican and Laurentian provenance of south-western Svalbard.
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| 9. |
- Manzotti, Paola, et al.
(författare)
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A journey towards the forbidden zone : a new, cold, UHP unit in the Dora-Maira Massif (Western Alps)
- 2022
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Ingår i: Contributions to Mineralogy and Petrology. - : Springer Science and Business Media LLC. - 0010-7999 .- 1432-0967. ; 177:6
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Tidskriftsartikel (refereegranskat)abstract
- The distribution of ultrahigh-pressure metamorphism (UHP) at the scale of a mountain belt is of prime importance for deciphering its past subduction history. In the Western Alps, coesite has been recognized in the southern Dora-Maira Massif, in the lens-shaped Brossasco-Isasca Unit, but has not been found up to now in the other parts of the massif. We report the discovery of a new UHP unit in the northern Dora-Maira Massif (Western Alps), named Chasteiran Unit. It is only a few tens of metres thick and consists of graphite-rich, garnet–chloritoid micaschists, whose protoliths may be black shales of Silurian age. Garnet inclusions (chloritoid, rutile) and its growth zoning allow to precisely model the P–T evolution. Coesite crystals, which are pristine or partially transformed to palisade quartz occur as inclusions in the garnet outer cores. According to thermodynamic modelling, garnet displays a continuous record of growth during the prograde increase in P and T (25–27 kbar 470–500 °C) (stage 1), up to the coesite stability field (27–28 kbar 510–530 °C) (stage 2), as well as sub-isothermal decompression of about 10 kbar (down to 15 kbar 500–515 °C) (stage 3). The main regional, composite, foliation, marked by chloritoid and rutile, began to develop during this stage, and was then overprinted by chlorite–ilmenite (stage 4). The Chasteiran Unit is discontinuously exposed in the immediate hangingwall of the Pinerolo Unit, and it is located far away from, and without physical links to the classic UHP Brossasco-Isasca Unit. Moreover, it records a different, much colder, P–T evolution, showing that different slices were detached from the downgoing subduction slab. The Chasteiran Unit is the fourth and the coldest Alpine UHP unit known so far in the entire Alpine belt. Its P–T conditions are comparable to the ones of the Tian Shan coesite–chloritoid-bearing rocks.
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| 10. |
- Manzotti, Paola, et al.
(författare)
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Missing lawsonite and aragonite found : P-T and fluid composition in meta-marls from the Combin Zone (Western Alps)
- 2021
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Ingår i: Contributions to Mineralogy and Petrology. - : Springer Science and Business Media LLC. - 0010-7999 .- 1432-0967. ; 176:8
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Tidskriftsartikel (refereegranskat)abstract
- We report the first findings of several occurrences of lawsonite and metamorphic aragonite in the meta-sediments from the Combin Zone (Piemonte-Liguria ocean, Western Alps), where the early blueschist-facies episode is poorly documented. New field and metamorphic data (thermodynamic modelling and Raman spectroscopy on carbonaceous material) are used to elucidate the P-T evolution and fluid composition of the Combin Zone and investigate the lawsonite growth and breakdown reactions. Two tectonometamorphic units have been identified within the Combin Zone with distinct geometry, lithological content and P-T conditions. In the higher grade unit, metamorphic aragonite occurs as inclusions in titanite. Lawsonite and garnet were stable at peak P-T conditions (similar to 16-17 kbar and 460-480 degrees C) at very low X(CO2) values. Lawsonite is systematically pseudomorphed, but preserves hourglass zoning or internal fabric associated with the prograde ductile deformation. The lower grade unit (similar to 8 +1 kbar similar to 370-400 degrees C) is discontinuously exposed along the western base of the continental Dent Blanche nappe and records P-T conditions similar to those recorded by the Dent Blanche nappe. A metamorphic discontinuity is, therefore, documented between the largest part of the Combin Zone on the one hand, and the Dent Blanche nappe on the other hand. The discovery of lawsonite and metamorphic aragonite allows a better understanding of the large-scale metamorphic structure of the Western Alps.
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| 11. |
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| 12. |
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| 13. |
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| 14. |
- Tamblyn, R., et al.
(författare)
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Resubduction of lawsonite eclogite within a serpentinite-filled subduction channel
- 2020
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Ingår i: Contributions to Mineralogy and Petrology. - : Springer Science and Business Media LLC. - 0010-7999 .- 1432-0967. ; 175:8
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Tidskriftsartikel (refereegranskat)abstract
- Translating burial and exhumation histories from the petrological and geochronological record of high-pressure assemblages in subduction channels is key to understanding subduction channel processes. Convective return flow, either serpentinite or sediment hosted, has been suggested as a potential mechanism to retrieve rocks from significant depths and exhume them. Numerical modelling predicts that during convective flow, subducted material can be cycled within a serpentinite-filled subduction channel. Geochronological and petrological evidences for such cycling during subduction are preserved in lawsonite eclogite from serpentinite melange in the Southern New England Orogen, eastern Australia. Ar-Ar, Rb-Sr phengite and U-Pb titanite geochronology, supported by phase equilibrium forward modelling and mineral zoning, suggest Cambro-Ordovician eclogite underwent two stages of burial separated by a stage of partial exhumation. The initial subduction of the eclogite at ca. 490 Ma formed porphyroblastic prograde-zoned garnet and lawsonite at approximateP-Tconditions of at least 2.9 GPa and 600 degrees C. Partial exhumation to at least 2.0 GPa and 500 degrees C is recorded by garnet dissolution. Reburial of the eclogite resulted in growth of new Mg-rich garnet rims, growth of new prograde-zoned phengite and recrystallization of titanite atP-Tconditions of approximately 2.7 GPa and 590 degrees C. U-Pb titanite, and Ar-Ar and Rb-Sr phengite ages constrain the timing of reburial to ca. 450 Ma. This was followed by a second exhumation event at approximately 1.9 GPa and 520 degrees C. These conditions fall along a cold approximate geotherm of 230 degrees C/GPa. The inferred changes in pressure suggest the lawsonite eclogite underwent depth cycling within the subduction channel. Geochronological data indicate that partial exhumation and reburial occurred over ca. 50 M y., providing some estimation on the timescales of material convective cycling in the subduction channel.
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| 15. |
- Troll, Valentin R., et al.
(författare)
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Petrogenesis of the Loch Bà ring-dyke and Centre 3 granites, Isle of Mull, Scotland
- 2021
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Ingår i: Contributions to Mineralogy and Petrology. - : Springer Science and Business Media LLC. - 0010-7999 .- 1432-0967. ; 176:2
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Tidskriftsartikel (refereegranskat)abstract
- The Loch Bà ring-dyke and the associated Centre 3 granites represent the main events of the final phase of activity at the Palaeogene Mull igneous complex. The Loch Bà ring-dyke is one of the best exposed ring-intrusions in the world and records intense interaction between rhyolitic and basaltic magma. To reconstruct the evolutionary history of the Centre 3 magmas, we present new major- and trace-element, and new Sr isotope data as well as the first Nd and Pb isotope data for the felsic and mafic components of the Loch Bà intrusion and associated Centre 3 granites. We also report new Sr, Nd and Pb isotope data for the various crustal compositions from the region, including Moine and Dalradian metasedimentary rocks, Lewisian gneiss, and Iona Group metasediments. Isotope data for the Loch Bà rhyolite (87Sr/86Sri = 0.716) imply a considerable contribution of local Moine-type metasedimentary crust (87Sr/86Sr = 0.717–0.736), whereas Loch Bà mafic inclusions (87Sr/86Sri = 0.704–0.707) are closer to established mantle values, implying that felsic melts of dominantly crustal origin mixed with newly arriving basalt. The Centre 3 microgranites (87Sr/86Sri = 0.709–0.716), are less intensely affected by crustal assimilation relative to the Loch Bá rhyolite. Pb-isotope data confirm incorporation of Moine metasediments within the Centre 3 granites. Remarkably, the combined Sr–Nd–Pb data indicate that Centre 3 magmas record no detectable interaction with underlying deep Lewisian gneiss basement, in contrast to Centre 1 and 2 lithologies. This implies that Centre 3 magmas ascended through previously depleted or insulated feeding channels into upper-crustal reservoirs where they resided within and interacted with fertile Moine-type upper crust prior to eruption or final emplacement.
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| 16. |
- Hyde, William R., et al.
(författare)
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Microstructural and isotopic analysis of shocked monazite from the Hiawatha impact structure : development of porosity and its utility in dating impact craters
- 2024
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Ingår i: Contributions to Mineralogy and Petrology. - 0010-7999. ; 179:3
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Tidskriftsartikel (refereegranskat)abstract
- U–Pb geochronology of shocked monazite can be used to date hypervelocity impact events. Impact-induced recrystallisation and formation of mechanical twins in monazite have been shown to result in radiogenic Pb loss and thus constrain impact ages. However, little is known about the effect of porosity on the U–Pb system in shocked monazite. Here we investigate monazite in two impact melt rocks from the Hiawatha impact structure, Greenland by means of nano- and micrometre-scale techniques. Microstructural characterisation by scanning electron and transmission electron microscopy imaging and electron backscatter diffraction reveals shock recrystallisation, microtwins and the development of widespread micrometre- to nanometre-scale porosity. For the first time in shocked monazite, nanophases identified as cubic Pb, Pb3O4, and cerussite (PbCO3) were observed. We also find evidence for interaction with impact melt and fluids, with the formation of micrometre-scale melt-bearing channels, and the precipitation of the Pb-rich nanophases by dissolution–precipitation reactions involving pre-existing Pb-rich high-density clusters. To shed light on the response of monazite to shock metamorphism, high-spatial-resolution U–Pb dating by secondary ion mass spectrometry was completed. Recrystallised grains show the most advanced Pb loss, and together with porous grains yield concordia intercept ages within uncertainty of the previously established zircon U–Pb impact age attributed to the Hiawatha impact structure. Although porous grains alone yielded a less precise age, they are demonstrably useful in constraining impact ages. Observed relatively old apparent ages can be explained by significant retention of radiogenic lead in the form of widespread Pb nanophases. Lastly, we demonstrate that porous monazite is a valuable microtexture to search for when attempting to date poorly constrained impact structures, especially when shocked zircon or recrystallised monazite grains are not present.
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