Thrust-related metamorphism beneath the Shetland Islands oceanic fragment, northeast Scotland

1988 ◽  
Vol 25 (11) ◽  
pp. 1760-1776 ◽  
Author(s):  
John G. Spray
Keyword(s):  
High Pressures ◽  
Hanging Wall ◽  
Crust And Upper Mantle ◽  
Thermal Inversion ◽  
Shetland Islands ◽  
Suprasubduction Zone ◽  
Mid Ocean Ridge ◽  
Close Proximity ◽  
Ocean Lithosphere ◽  
Ocean Ridge

A ≤400 m thick metamorphic sequence showing thermal inversion is present beneath a dismembered ultrabasic–basic complex in the Shetland Islands of northeast Scotland. The metamorphic grade changes from upper amphibolite facies in metabasites at the top of the sequence to low greenschist facies in metasediments at the base. Garnet–clinopyroxene thermometry yields temperatures of ~ 750 °C (at 300 MPa) for the highest grade assemblage. There is no evidence for high pressures of metamorphism, and maximum overburden may never have exceeded the original thickness of the overlying ultrabasic–basic complex, which is estimated to have been ~ 10 km.The internal structure and field relations of the ultrabasic–basic complex reveal that it is a displaced fragment of oceanic crust and upper mantle of Ordovician age. The chemistry of its basic lithologies suggests low-K tholeiite, suprasubduction zone, pre-arc affinities. In contrast, the underlying meteamorphic sequence possesses a mid-ocean ridge basalt (MORB) signature.Four K–Ar age determinations from amphibole mineral separates of the metamorphic sequence range from 479 ± 6 to 465 ± 6 Ma. The highest age is interpreted as the date of the onset of metamorphic sole formation and the initial tectonic displacement of the oceanic fragment.It is concluded that the metamorphic sequence was generated during intraoceanic thrusting during the destruction of a young, marginal oceanic basin located between a continental margin and the ocean lithosphère of Iapetus. Certain MORB lithologies were metamorphosed and transferred to the marginal basin hanging wall during the subduction of Iapetus. Apparent thermal inversion was caused during overthrusting by the gradual underplating of the hanging wall in close proximity to a suprasubduction zone spreading centre.

Mid-ocean-ridge seismicity reveals extreme types of ocean lithosphere

Nature ◽  
2016 ◽  
Vol 535 (7611) ◽  
pp. 276-279 ◽  
Author(s):  
Vera Schlindwein ◽  
Florian Schmid
Keyword(s):  
Mid Ocean Ridge ◽  
Ocean Lithosphere ◽  
Ocean Ridge

scholarly journals Evidence of Late Ediacaran Hyperextension of the Laurentian Iapetan Margin in the Birchy Complex, Baie Verte Peninsula, Northwest Newfoundland: Implications for the Opening of Iapetus, Formation of Peri-Laurentian Microcontinents and Taconic – Grampian Orogenesis

2013 ◽  
Vol 40 (2) ◽  
pp. 94 ◽  
Author(s):  
Cees R. Van Staal ◽  
Dave M. Chew ◽  
Alexandre Zagorevski ◽  
Vicki McNicoll ◽  
James Hibbard ◽  
...  
Keyword(s):  
Hanging Wall ◽  
Ultramafic Rocks ◽  
Nous Proposons ◽  
Mafic Rocks ◽  
Metasedimentary Rocks ◽  
Iapetus Ocean ◽  
Icp Ms ◽  
Mid Ocean Ridge ◽  
Continental Lithospheric Mantle ◽  
Ocean Ridge

The Birchy Complex of the Baie Verte Peninsula, northwestern Newfoundland, comprises an assemblage of mafic schist, ultramafic rocks, and metasedimentary rocks that are structurally sandwiched between overlying ca. 490 Ma ophiolite massifs of the Baie Verte oceanic tract and underlying metasedimentary rocks of the Fleur de Lys Supergroup of the Appalachian Humber margin. Birchy Complex gabbro yielded a Late Ediacaran U–Pb zircon ID–TIMS age of 558.3 ± 0.7 Ma, whereas gabbro and an intermediate tuffaceous schist yielded LA–ICPMS concordia zircon ages of 564 ± 7.5 Ma and 556 ± 4 Ma, respectively. These ages overlap the last phase of rift-related magmatism observed along the Humber margin of the northern Appalachians (565–550 Ma). The associated ultramafic rocks were exhumed by the Late Ediacaran and shed detritus into the interleaved sedimentary rocks. Psammite in the overlying Flat Point Formation yielded a detrital zircon population typical of the Laurentian Humber margin in the northern Appalachians. Age relationships and characteristics of the Birchy Complex and adjacent Rattling Brook Group suggest that the ultramafic rocks represent slices of continental lithospheric mantle exhumed onto the seafloor shortly before or coeval with magmatic accretion of mid-ocean ridge basalt-like mafic rocks. Hence, they represent the remnants of an ocean – continent transition zone formed during hyperextension of the Humber margin prior to establishment of a mid-ocean ridge farther outboard in the Iapetus Ocean. We propose that microcontinents such as Dashwoods and the Rattling Brook Group formed as a hanging wall block and an extensional crustal allochthon, respectively, analogous to the isolation of the Briançonnais block during the opening of the Alpine Ligurian–Piemonte and Valais oceanic seaways.SOMMAIRELe complexe de Birchy de la péninsule de Baie Verte, dans le nord-ouest de Terre-Neuve, est constitué d’un assemblage de schistes mafiques, de roches ultramafiques et de métasédiments qui sont coincés entre des massifs ophiolitiques d’ascendance océanique de la Baie Verte au-dessus, et des métasédiments du Supergroupe de Fleur de Lys de la marge de Humber des Appalaches en-dessous. Le complexe de gabbro de Birchy a donné une datation U-Pb sur zircon ID-TIMS correspondant à la fin de l’Édiacarien, soit 558,3 ± 0,7 Ma, alors qu’un gabbro et un schiste tufacé intermédiaire montrent une datation LA-ICP-MS Concordia sur zircon de 564 ± 7,5 Ma et 556 ± 4 Ma, respectivement. Ces datations chevauchent la dernière phase de magmatisme de rift observée le long de la marge Humber des Appalaches du Nord (565-550 Ma). Les roches ultramafiques associées ont été exhumées vers la fin de l’Édiacarien et leurs débris ont été imbriqués dans des roches sédimentaires. Les psammites de la Formation de Flat Point susjacente ont donné une population de zircons détritiques typique de la marge laurentienne de Humber des Appalaches du Nord. Les relations chronologiques et les caractéristiques du complexe de Birchy et du groupe de Rattling Brook adjacent, permettent de penser que ces roches ultramafiques pourraient être des écailles de manteau lithosphérique continental qui auraient été exhumées sur le plancher océanique peu avant ou en même temps que l’accrétion magmatique de roches mafiques basaltiques de type dorsale médio-océanique. Par conséquent, elles seraient des vestiges d’une zone de transition océan-continent formée au cours de l’hyper-extension de la marge de Humber avant l’apparition d’une dorsale médio-océanique plus loin au large dans l’océan Iapétus. Nous proposons que des microcontinents comme de Dashwoods et du groupe de Rattling Brook ont constitués respectivement un bloc de toit et un allochtone crustal d’extension, de la même manière que le bloc Briançonnais a été isolé lors de l’ouverture des bras océaniques alpins de Ligurie-Piémont et de Valais.

Boninitic and tholeiitic dykes in the Lewis Hills mantle section of the Bay of Islands ophiolite: implications for magmatism adjacent to a fracture zone in a back-arc spreading environment

1995 ◽  
Vol 32 (12) ◽  
pp. 2128-2146 ◽  
Author(s):  
Stephen J. Edwards
Keyword(s):  
Partial Melting ◽  
Fracture Zone ◽  
Tholeiitic Magma ◽  
Suprasubduction Zone ◽  
Geochemical Study ◽  
Mid Ocean Ridge ◽  
Hill Area ◽  
Back Arc ◽  
Ocean Ridge ◽  
Mantle Section

A detailed, integrated field, petrographic, and geochemical study of the Springers Hill area of the Bay of Islands ophiolite exposed in the Lewis Hills was undertaken to explain the anomalously high abundance of veins and dykes of chromitite, orthopyroxenite, and clinopyroxenite, and their associated dunites, hosted by a refractory harzburgite–dunite mixture. A geodynamic situation is presented, which is constrained by previous studies requiring formation of the Springers Hill mantle section at a ridge–fracture zone intersection, and the whole of the Bay of Islands ophiolite within a back-arc spreading environment. The veins and dykes formed during magmatism at the ridge–fracture zone intersection and along the fracture zone, as progressively hotter, more fertile (richer in clinopyroxene) asthenosphere ascended and was channelled up and along the fracture zone wall. Shallow melting of refractory harzburgite in the presence of subduction-derived hydrous fluids produced light rare earth element (LREE)-enriched boninitic magma from which crystallized chromitites, some of their associated dunites, and orthopyroxenites. This melting event dehydrated much of the mantle in and around the zone of partial melting. Continued rise and shallow partial melting of hotter, more fertile mantle under conditions of variable hydration generated LREE-depleted, low-Ti tholeiitic magma. This magma crystallized olivine clinopyroxenite, some associated dunite, and clinopyroxenite. The final magmatic event may have involved partial melting of mid-ocean-ridge basalt-bearing mantle at depth, ascent of the magma, and formation of massive wehrlite–lherzolite bodies at the ridge–fracture zone intersection and along the fracture zone. Ridge–fracture zone intersections in suprasubduction-zone environments are sites of boninitic and tholeiitic magmatism because refractory asthenospheric mantle may melt as it is channelled with subduction-derived fluids to shallow depths by the old, cold lithospheric wall of the fracture zone. Heat for melting is provided by the ascent of hotter, more fertile mantle. Extremely refractory magmas do not occur along "normal" oceanic fracture zones because volumes of highly refractory mantle are much less, subduction-derived hydrous fluids are not present, and fracture zone walls extend to shallower depths.

Bay of Islands and Little Port complexes, revisited: age, geochemical and isotopic evidence confirm suprasubduction-zone origin

1991 ◽  
Vol 28 (10) ◽  
pp. 1635-1652 ◽  
Author(s):  
G. A. Jenner ◽  
G. R. Dunning ◽  
J. Malpas ◽  
M. Brown ◽  
T. Brace
Keyword(s):  
Transform Fault ◽  
Isotopic Data ◽  
Isotopic Study ◽  
Zircon Age ◽  
Suprasubduction Zone ◽  
Mid Ocean Ridge ◽  
Appalachian Orogen ◽  
Ocean Ridge ◽  
Source Materials ◽  
Oceanic Domain

The Bay of Islands Complex of the Humber Arm allochthon, west Newfoundland, contains the best-exposed ophiolite in the Appalachian Orogen. Associated structural slices, the Little Port and Skinner Cove complexes, also contain rocks formed in an oceanic domain, although their relationship to the Bay of Islands Complex remains controversial.To constrain the origin of the complexes and obtain a better understanding of the geology of the Humber Arm allochthon, we have undertaken an integrated geochronological, geochemical, and isotopic study. A U/Pb zircon age of [Formula: see text] Ma for the Little Port Complex and a zircon and baddeleyite age of 484 ± 5 Ma for the Bay of Islands Complex have been obtained. Geochemical and isotopic data on trondhjemitic rocks from the two complexes indicate that petrogenetic models for these rocks must account for fundamental differences in source materials and mineralogy during differentiation. The Little Port Complex trondhjemites are characterized by initial εNd of −1 to +1, whereas those in the Bay of Islands have εNd of +6.5. Geochemical signatures in mafic and felsic volcanics of the complexes are diverse, and show a complete gradation between arc and non-arc.The Bay of Islands and Little Port complexes are not related by any form of a major mid-ocean-ridge – transform-fault model. An alternative model to explain the relationships between the two complexes interprets the Little Port as arc-related and the Bay of Islands as a suprasubduction-zone ophiolite.

On the provenance of mid-Cretaceous turbidites of the Pindos zone (Greece): implications from heavy mineral distribution, detrital zircon ages and chrome spinel chemistry

2006 ◽  
Vol 143 (3) ◽  
pp. 329-342 ◽  
Author(s):  
P. FAUPL ◽  
A. PAVLOPOULOS ◽  
U. KLÖTZLI ◽  
K. PETRAKAKIS
Keyword(s):  
Detrital Zircon ◽  
Heavy Mineral ◽  
Heavy Minerals ◽  
Chrome Spinel ◽  
Internal Source ◽  
Suprasubduction Zone ◽  
Mid Ocean Ridge ◽  
Detrital Zircon Ages ◽  
Back Arc ◽  
Ocean Ridge

Two heavy mineral populations characterize the siliciclastic material of the mid-Cretaceous turbidites of the Katafito Formation (‘First Flysch’) of the Pindos zone: a stable, zircon-rich group and an ophiolite-derived, chrome spinel-rich one. U/Pb and Pb/Pb dating on magmatic zircons from the stable heavy mineral group clearly illustrate the existence of Variscan magmatic complexes in the source terrain, but also provide evidence for magmatism as old as Precambrian. Based on microprobe analyses, the chrome spinel detritus was predominantly supplied from peridotites of mid-ocean ridge as well as suprasubduction zone origin. A small volcanic spinel population was mainly derived from MORB and back-arc basin basalts. The lithological variability of the mid-Cretaceous ophiolite bodies, based on spinel chemistry, is much broader than that of ophiolite complexes presently exposed in the Hellenides. The chrome spinel detritus compares closely with that from the Outer and Inner Dinarides. The source terrain of the ophiolite-derived heavy minerals was situated in a more internal palaeogeographic position than that of the Pindos zone. The zircon-rich heavy mineral group could have had either an external and/or an internal source, but the chrome spinel constantly accompanying the stable mineral detritus seems to be more indicative of an internal source terrain.

scholarly journals Diamond and Other Exotic Mineral-Bearing Ophiolites on the Globe: A Key to Understand the Discovery of New Minerals and Formation of Ophiolitic Podiform Chromitite

Crystals ◽  
2021 ◽  
Vol 11 (11) ◽  
pp. 1362
Author(s):  
Fei Liu ◽  
Dongyang Lian ◽  
Weiwei Wu ◽  
Jingsui Yang
Keyword(s):  
Mineral Inclusion ◽  
Carbon And Nitrogen ◽  
Crustal Recycling ◽  
Suprasubduction Zone ◽  
Deep Mantle ◽  
Mid Ocean Ridge ◽  
Deep Earth ◽  
Podiform Chromitites ◽  
Ocean Ridge ◽  
Podiform Chromitite

Ophiolite-hosted diamond from peridotites and podiform chromitites significantly differs from those of kimberlitic diamond and ultra-high pressure (UHP) metamorphic diamond in terms of occurrence, mineral inclusion, as well as carbon and nitrogen isotopic composition. In this review, we briefly summarize the global distribution of twenty-five diamond-bearing ophiolites in different suture zones and outline the bulk-rock compositions, mineral and particular Re-Os isotopic systematics of these ophiolitic chromitites and host peridotites. These data indicate that the subcontinental lithospheric mantle is likely involved in the formation of podiform chromitite. We also provide an overview of the UHP textures and unusual mineral assemblages, including diamonds, other UHP minerals (e.g., moissanite, coesite) and crustal minerals, which robustly offer evidence of crustal recycling in the deep mantle along the suprasubduction zone (SSZ) and then being transported to shallow mantle depths by asthenospheric mantle upwelling in mid-ocean-ridge and SSZ settings. A systematic comparison between four main genetic models provides insights into our understanding of the origin of ophiolite-hosted diamond and the formation of podiform chromitite. Diamond-bearing peridotites and chromitites in ophiolites are important objects to discover new minerals from the deep earth and provide clues on the chemical composition and the physical condition of the deep mantle.

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