scholarly journals Mantle rock exposures at oceanic core complexes along mid-ocean ridges

Geologos ◽  
2015 ◽  
Vol 21 (4) ◽  
pp. 207-231 ◽  
Author(s):  
Jakub Ciazela ◽  
Juergen Koepke ◽  
Henry J.B. Dick ◽  
Andrzej Muszynski
Keyword(s):  
Partial Melting ◽  
Geophysical Methods ◽  
Ex Situ ◽  
Crustal Rocks ◽  
New Class ◽  
Ocean Ridges ◽  
Common Structure ◽  
Spreading Ridges ◽  
Slow Spreading ◽  
Lower Crustal

Abstract The mantle is the most voluminous part of the Earth. However, mantle petrologists usually have to rely on indirect geophysical methods or on material found ex situ. In this review paper, we point out the in-situ existence of oceanic core complexes (OCCs), which provide large exposures of mantle and lower crustal rocks on the seafloor on detachment fault footwalls at slow-spreading ridges. OCCs are a common structure in oceanic crust architecture of slow-spreading ridges. At least 172 OCCs have been identified so far and we can expect to discover hundreds of new OCCs as more detailed mapping takes place. Thirty-two of the thirty-nine OCCs that have been sampled to date contain peridotites. Moreover, peridotites dominate in the plutonic footwall of 77% of OCCs. Massive OCC peridotites come from the very top of the melting column beneath ocean ridges. They are typically spinel harzburgites and show 11.3–18.3% partial melting, generally representing a maximum degree of melting along a segment. Another key feature is the lower frequency of plagioclase-bearing peridotites in the mantle rocks and the lower abundance of plagioclase in the plagioclase-bearing peridotites in comparison to transform peridotites. The presence of plagioclase is usually linked to impregnation with late-stage melt. Based on the above, OCC peridotites away from segment ends and transforms can be treated as a new class of abyssal peridotites that differ from transform peridotites by a higher degree of partial melting and lower interaction with subsequent transient melt.

Zircon U–Pb ages, major and trace elements, and Hf isotope characteristics of the Tiantangzhai granites in the North Dabie orogen, Central China: tectonic implications

2013 ◽  
Vol 151 (5) ◽  
pp. 916-937 ◽  
Author(s):  
XIN DENG ◽  
KUNGUANG YANG ◽  
ALI POLAT ◽  
TIMOTHY M. KUSKY ◽  
KAIBIN WU
Keyword(s):  
Partial Melting ◽  
Eastern China ◽  
Major And Trace Elements ◽  
Central China ◽  
Dabie Orogen ◽  
Crustal Rocks ◽  
The North ◽  
Granitic Magmatism ◽  
Two Peaks ◽  
Lower Crustal

AbstractCretaceous granites are widespread in the North Dabie orogen, Central China, but their emplacement sequence and mechanism are poorly known. The Tiantangzhai Complex in the North Dabie Complex is the largest Cretaceous granitic suite consisting of six individual intrusions. In this study, zircon U–Pb ages are used to constrain the crystallization and protolith ages of these intrusions. The Shigujian granite is a syn-tectonic intrusion with an age of 141 Ma. This granite was emplaced under a compressional regime. Oscillatory rims of zircons have yielded two peaks at 137±1 Ma and 125±1 Ma. The 137±1 Ma peak represents the beginning of orogenic extension and tectonic collapse, whereas the 125±1 Ma peak represents widespread granitic magmatism. Zircon cores have yielded concordant ages between 812 and 804 Ma, which indicate a crystallization age for the protolith. The Tiantangzhai granites show relatively high Sr contents and high La/Yb and Sr/Y ratios. The Shigujian granite has positive Eu anomalies resulting from partial melting of a plagioclase-rich source in an over-thickened crust. Correspondingly, in situ Lu–Hf analyses from zircons yield high negative εHf(t) values from −24.8 to −26.6, with two-stage Hf model ages from 2748±34 to 2864±40 Ma, suggesting that the magmas were dominantly derived from partial melting of middle to lower crustal rocks. The Dabie orogen underwent pervasive NW–SE extension at the beginning of the early Cretaceous associated with subduction of the Palaeo-Pacific plate beneath eastern China.

scholarly journals Present structure of the Near-Bug area: conditions of formation and history of development

2021 ◽  
Vol 43 (2) ◽  
pp. 96-115
Author(s):  
O.V. Usenko
Keyword(s):  
Partial Melting ◽  
Main Part ◽  
Solidus Temperature ◽  
Age Determination ◽  
Granulite Facies ◽  
Geological Structure ◽  
General Sequence ◽  
Crustal Rocks ◽  
The Moment ◽  
Lower Crustal

General sequence establishment of geological Precambrian events and associating formations, which were created in them, to the results of isotope age definition, is the task, which has no single valued solution for southwestern part of the Ukrainian Shield. Important is to create a general development model, which will describe the modern geological structure of an area, structural and textural rocks features, accounting PT-conditions in the Earth's crust during the Archean—Paleoproterozoic. Isotopic age determination demonstrates, that from the moment of protolith creation (not later than 3.75 billion years ago, up to 1.9 billion years ago), intrusion of mantle melts and partial melting of the lower crustal rocks, occurred many times over. Pobuzhie formation cannot be imagined, as a single process of accumulation, plunge, crumpling into folds and sedimentary strata metamorphism. It is necessary, to take into account, the plume (mantle) component of the general geodynamic process. In the structure of the Bug megablock and Golovanevskaya suture zone, two main structural plans are displayed. The main part of the territory displays a region of areal distribution of Archean enderbites (generated 2.8 billion years ago) and Proterozoic granites (generated 2.03 billion years ago). The paper compares the temperature distribution with depth, corresponding to the thermal model of the metamorphic temperatures found in the samples, and the solidus temperatures of the basic rocks. It is shown that at the time of the metamorphism development, 2.0 billion years ago, the rocks were at a depth of more than 20 km, and before that — at an even greater depth. During the Archean and Paleoproterozoic, the center of partial melting was repeatedly renewed here, since the temperatures were higher than the solidus temperature of gabbro. Metamorphic changes (and more often migmatization, partial melting and following crystallization in the granulite facies conditions) happened after the presence of the thermal asthenosphere on the core—mantle border, and were accompanied by bringing the substance from it. Therefore the main part of modern surface is folded by palingenic granites. In Archean and Paleoproterozoic the composition of substances were different. After 2.0 billion years ago the level of modern surface was located higher. The second structural plan is presented with vertical structures, building of which often close to concentrically zonal or linear monoclinal. They are confined to fault zones and nodes of their intersections. These structures contain rock complexes, which did not occur until 2.0 billion years ago on any craton in the world.

Unveiling the volcanic evolution of the Mohns Ridge

2021 ◽  
Author(s):  
Håvard Stubseid ◽  
Anders Bjerga ◽  
Haflidi Haflidason ◽  
Rolf Birger Pedersen
Keyword(s):  
Autonomous Underwater Vehicles ◽  
Volcanic Eruptions ◽  
Integrated Approach ◽  
Hydrothermal Systems ◽  
Ocean Ridges ◽  
Spreading Ridges ◽  
Geological Evolution ◽  
Fast Spreading ◽  
Slow Spreading ◽  
Resolution Dataset

<p>Volcanic eruptions are far less common along slow-spreading ridges compared to fast-spreading ridges. Consequently, knowledge of the volcanic rejuvenation along close to 1/3 of the global mid-ocean ridges is poorly constrained. To determine the temporal evolution of the rift valley of one of the slowest spreading-ridges in the world, the Mohns Ridge in the Norwegian-Greenland Sea, we have interpreted more than 3000 km of sub-bottom profiles. Sedimentation rates derived from several core locations along the ridge are used to calculate the age of the underlying volcanic crust. Here we present a framework for understanding the geological evolution of rift valleys of slow-spreading ridges using an integrated approach combining geological and geophysical data. The high-resolution dataset acquired using autonomous underwater vehicles, cover more than 50% of the 575 km long Mohns Ridge. The results unravel large variation in sediment thickness inside the central rift area, from exposed basalts to several meters of sediments, within only a few hundreds of meters. Studied sub-bottom profiles reveal active volcanism in the deepest parts of the ridge, areas thought to be inactive, surrounded by significantly older crust covered in meters of sediments. We find that all axial volcanic ridge systems (AVRs) in our area completely renewed their surface within the last 30-50 ka. Detailed volcanological investigation of the central parts of an AVR reveal at least 72 individual eruptions during the last 20 ka ranging in size from 1.2x10<sup>3 </sup>m<sup>2</sup> - 2.6 x10<sup>5</sup> m<sup>2</sup>. These estimates have been verified with visual observations and sampling using an ROV. Our estimates indicate that more than 230 eruptions are required to renew the surface of an average AVR. Based on the acquired age assessments a volcanic eruption is anticipated to occur approximately every 200 years. Volcanic renewal is a first order control on the lifetime of magmatically driven hydrothermal systems.</p>

scholarly journals Constraining the maximum depth of brittle deformation at slow- and ultraslow-spreading ridges using microseismicity

Geology ◽  
2019 ◽  
Vol 47 (11) ◽  
pp. 1069-1073 ◽  
Author(s):  
Ingo Grevemeyer ◽  
Nicholas W. Hayman ◽  
Dietrich Lange ◽  
Christine Peirce ◽  
Cord Papenberg ◽  
...  
Keyword(s):  
Spreading Rate ◽  
Maximum Depth ◽  
Southwest Indian Ridge ◽  
Spreading Center ◽  
Ocean Ridges ◽  
Spreading Ridges ◽  
Brittle Layer ◽  
Full Rate ◽  
Slow Spreading ◽  
The Caribbean

Abstract The depth of earthquakes along mid-ocean ridges is restricted by the relatively thin brittle lithosphere that overlies a hot, upwelling mantle. With decreasing spreading rate, earthquakes may occur deeper in the lithosphere, accommodating strain within a thicker brittle layer. New data from the ultraslow-spreading Mid-Cayman Spreading Center (MCSC) in the Caribbean Sea illustrate that earthquakes occur to 10 km depth below seafloor and, hence, occur deeper than along most other slow-spreading ridges. The MCSC spreads at 15 mm/yr full rate, while a similarly well-studied obliquely opening portion of the Southwest Indian Ridge (SWIR) spreads at an even slower rate of ∼8 mm/yr if the obliquity of spreading is considered. The SWIR has previously been proposed to have earthquakes occurring as deep as 32 km, but no shallower than 5 km. These characteristics have been attributed to the combined effect of stable deformation of serpentinized mantle and an extremely deep thermal boundary layer. In the context of our MCSC results, we reanalyze the SWIR data and find a maximum depth of seismicity of 17 km, consistent with compilations of spreading-rate dependence derived from slow- and ultraslow-spreading ridges. Together, the new MCSC data and SWIR reanalysis presented here support the hypothesis that depth-seismicity relationships at mid-ocean ridges are a function of their thermal-mechanical structure as reflected in their spreading rate.

The Xigaze ophiolite: fossil ultraslow-spreading ocean lithosphere in the Tibetan Plateau

2020 ◽  
pp. jgs2020-208
Author(s):  
Tong Liu ◽  
Chuan-Zhou Liu ◽  
Fu-Yuan Wu ◽  
Henry J.B. Dick ◽  
Wen-Bin Ji ◽  
...  
Keyword(s):  
Tibetan Plateau ◽  
Spreading Rate ◽  
Southwest Indian Ridge ◽  
The Tibetan Plateau ◽  
Crustal Accretion ◽  
Ocean Ridges ◽  
Magma Supply ◽  
Spreading Ridges ◽  
Ocean Lithosphere ◽  
Lower Crustal

The crust and mantle in both ophiolites (fossil ocean lithosphere) and in modern oceans are enormously diverse. Along-axis morphology and lower crustal accretion at ultraslow-spreading ocean ridges are fundamentally different from those at faster-spreading ridges, and are key to understanding how crustal accretion varies with spreading rate and magma supply. Ultraslow-spreading ridges provide analogs for ophiolites, to identify those that may have formed under similar conditions. Parallel studies of modern ocean lithosphere and ophiolites therefore can uniquely inform the origin and genesis of both. Here we report the results of structural and petrological studies on the Xigaze ophiolite in the Tibetan Plateau, and compare it to the morphology and deep drilling results at the ultraslow-spreading Southwest Indian Ridge. The Xigaze ophiolite has a complete but laterally discontinuous crust, with discrete diabase dikes/sills cutting both mantle and lower crust. The gabbro units are thin (∼350 m) and show upward cyclic chemical variations, supporting for an episodic and intermittent magma supply. These features are comparable to the highly focused magmatism and low magma budget at modern ultraslow-spreading ridges. Thus we suggest that the Xigaze ophiolite represents an on-land analog of ultraslow-spreading ocean lithosphere.

scholarly journals 13 million years of seafloor spreading throughout the Red Sea Basin

2021 ◽  
Vol 12 (1) ◽  
Author(s):  
Nico Augustin ◽  
Froukje M. van der Zwan ◽  
Colin W. Devey ◽  
Bryndís Brandsdóttir
Keyword(s):  
Red Sea ◽  
Gravity Data ◽  
Ocean Basin ◽  
Gravity Gradient ◽  
Ocean Ridges ◽  
Spreading Ridges ◽  
Vertical Gravity Gradient ◽  
Slow Spreading ◽  
Vertical Gravity ◽  
High Resolution Bathymetry

AbstractThe crustal and tectonic structure of the Red Sea and especially the maximum northward extent of the (ultra)slow Red Sea spreading centre has been debated—mainly due to a lack of detailed data. Here, we use a compilation of earthquake and vertical gravity gradient data together with high-resolution bathymetry to show that ocean spreading is occurring throughout the entire basin and is similar in style to that at other (ultra)slow spreading mid-ocean ridges globally, with only one first-order offset along the axis. Off-axis traces of axial volcanic highs, typical features of (ultra)slow-spreading ridges, are clearly visible in gravity data although buried under thick salt and sediments. This allows us to define a minimum off-axis extent of oceanic crust of <55 km off the coast along the complete basin. Hence, the Red Sea is a mature ocean basin in which spreading began along its entire length 13 Ma ago.

scholarly journals Evolution of the volcanic plumbing systemof Alicudi (Aeolian Islands - Italy): evidence from fluid and melt inclusionsin quartz xenoliths

2009 ◽  
Vol 47 (4) ◽  
Author(s):  
R. Bonelli ◽  
M. L. Frezzotti ◽  
V. Zanon ◽  
A. Peccerillo
Keyword(s):  
Partial Melting ◽  
Fluid Inclusions ◽  
Type I ◽  
Type Ii ◽  
Early Type ◽  
Late Type ◽  
Crustal Rocks ◽  
Melt And Fluid Inclusions ◽  
Two Generations ◽  
Lower Crustal

Quartz-rich xenoliths in lavas (basalts to andesites; 90-30 ka) from Alicudi contain abundant melt and fluid inclusions. Two generations of CO2-rich fluid inclusions are present in quartz-rich xenolith grains: early (Type I) inclusions related to partial melting of the host xenoliths, and late Type II inclusions related to the fluid trapping during xenolith ascent. Homogenisation temperatures of fluid inclusions correspond to two density intervals: 0.93-0.68 g/cm3 (Type I) and 0.47-0.26 g/cm3 (Type II). Early Type I fluid inclusions indicate trapping pressures around 6 kbar, which are representative for the levels of partial melting of crustal rocks and xenolith formation. Late Type II fluid inclusions show lower trapping pressures, between 1.7 kbar and 0.2 kbar, indicative for shallow magma rest and accumulation during ascent to the surface. Data suggest the presence of two magma reservoirs: the first is located at lower crustal depths (about 24 km), site of fractional crystallization, mixing with source derived magma, and various degrees of crustal assimilation. The second magma reservoir is located at shallow crustal depths (about 6 km), the site where magma rested for a short time before erupting.

Magmatic and tectonic emplacement of the Pukaskwa batholith, Superior Province, Ontario, CanadaThis article is one of a series of papers published in this Special Issue on the theme of Geochronology in honour of Tom Krogh.

2011 ◽  
Vol 48 (2) ◽  
pp. 187-204 ◽  
Author(s):  
Gary P. Beakhouse ◽  
Shoufa Lin ◽  
Sandra L. Kamo
Keyword(s):  
Partial Melting ◽  
Abrupt Change ◽  
Superior Province ◽  
Density Inversion ◽  
Tectonic Processes ◽  
Slab Breakoff ◽  
Greenstone Belts ◽  
Lower Crustal ◽  
Basaltic Crust ◽  
Structural Dome

The Neoarchean Pukaskwa batholith consists of pre-, syn-, and post-tectonic phases emplaced over an interval of 50 million years. Pre-tectonic phases are broadly synvolcanic and have a high-Al tonalite–trondhjemite–granodiorite (TTG) affinity interpreted to reflect derivation by partial melting of basaltic crust at lower crustal or upper mantle depths. Minor syn-tectonic phases slightly post-date volcanism and have geochemical characteristics suggesting some involvement or interaction with an ultramafic (mantle) source component. Magmatic emplacement of pre- and syn-tectonic phases occurred in the midcrust at paleopressures of 550–600 MPa and these components of the batholith are thought to be representative of the midcrust underlying greenstone belts during their development. Subsequent to emplacement of the syntectonic phases, and likely at approximately 2680 Ma, the Pukaskwa batholith was uplifted as a structural dome relative to flanking greenstone belts synchronously with ongoing regional sinistral transpressive deformation. The driving force for vertical tectonism is interpreted to be density inversion (Rayleigh–Taylor-type instabilities) involving denser greenstone belts and underlying felsic plutonic crust. The trigger for initiation of this process is interpreted to be an abrupt change in the rheology of the midcrust attributed to introduction of heat from the mantle attendant with slab breakoff or lithospheric delamination following the cessation of subduction. This process also led to partial melting of the intermediate to felsic midcrust generating post-tectonic granitic phases at approximately 2667 Ma. We propose that late density inversion-driven vertical tectonics is an inevitable consequence of horizontal (plate) tectonic processes associated with greenstone belt development within the Superior Province.

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