New tectonic model and division of the Ubendian-Usagaran Belt, Tanzania: A review and in-situ dating of eclogites

ABSTRACT Records of high-pressure/low-temperature (HP-LT) metamorphic interfaces are not common in Precambrian orogens. It should be noted that the association of HP-LT metamorphic interfaces and strongly deformed ocean plate stratigraphy that form accretionary prisms between trenches and magmatic arcs are recognized as hallmark signatures of modern plate tectonics. In East Africa (Tanzania), the Paleoproterozoic Ubendian-Usagaran Belt records a HP-LT metamorphic interface that we consider as a centerpiece in reviewing the description of tectonic units of the Ubendian-Usagaran Belt and defining a new tectonic model. Our new U-Pb zircon age and the interpretations from existing data reveal an age between 1920 and 1890 Ma from the kyanite bearing eclogites. This establishment adds to the information of already known HP-LT metamorphic events at 2000 Ma, 1890–1860 Ma, and 590–520 Ma from the Ubendian-Usagaran Belt. Arc–back-arc signatures from eclogites imply that their mafic protoliths were probably eroded from arc basalt above a subduction zone and were channeled into a subduction zone as mélanges and got metamorphosed. The Ubendian-Usagaran events also record rifting, arc and back-arc magmatism, collisional, and hydrothermal events that preceded or followed HP-LT tectonic events. Our new tectonic subdivision of the Ubendian Belt is described as: (1) the western Ubendian Corridor, mainly composed of two Proterozoic suture zones (subduction at 2000, 1920–1890, Ma and 590–500 Ma) in the Ufipa and Nyika Terranes; (2) the central Ubendian Corridor, predominated by metamorphosed mafic-ultramafic rocks in the Ubende, Mbozi, and Upangwa Terranes that include the 1890–1860 Ma eclogites with mid-ocean ridge basalt affinity in the Ubende Terrane; and (3) the eastern Ubendian Corridor (the Katuma and Lupa Terranes), characterized by reworked Archean crust.

Cr-spinel records metasomatism not petrogenesis of mantle rocks

Nature Communications ◽

10.1038/s41467-019-13117-1 ◽

2019 ◽

Vol 10 (1) ◽

Cited By ~ 8

Author(s):

Hamed Gamal El Dien ◽

Shoji Arai ◽

Luc-Serge Doucet ◽

Zheng-Xiang Li ◽

Youngwoo Kil ◽

...

Keyword(s):

Subduction Zone ◽

Plate Tectonics ◽

Mantle Metasomatism ◽

Mantle Melting ◽

Ultramafic Rocks ◽

Chromian Spinel ◽

Core Composition ◽

Mid Ocean Ridge ◽

Ocean Ridge ◽

Petrogenetic Indicator

Abstract Mantle melts provide a window on processes related to global plate tectonics. The composition of chromian spinel (Cr-spinel) from mafic-ultramafic rocks has been widely used for tracing the geotectonic environments, the degree of mantle melting and the rate of mid-ocean ridge spreading. The assumption is that Cr-spinel’s core composition (Cr# = Cr/(Cr + Al)) is homogenous, insensitive to post-formation modification and therefore a robust petrogenetic indicator. However, we demonstrate that the composition of Cr-spinel can be modified by fluid/melt-rock interactions in both sub-arc and sub-mid oceanic mantle. Metasomatism can produce Al-Cr heterogeneity in Cr-spinel that lowers the Cr/Al ratio, and therefore modifies the Cr#, making Cr# ineffective as a geotectonic and mantle melting indicator. Our analysis also demonstrates that Cr-spinel is a potential sink for fluid-mobile elements, especially in subduction zone environments. The heterogeneity of Cr# in Cr-spinel can, therefore, be used as an excellent tracer for metasomatic processes.

A subduction influence on ocean ridge basalts outside the Pacific subduction shield

Nature Communications ◽

10.1038/s41467-021-25027-2 ◽

2021 ◽

Vol 12 (1) ◽

Author(s):

A. Y. Yang ◽

C. H. Langmuir ◽

Y. Cai ◽

P. Michael ◽

S. L. Goldstein ◽

...

Keyword(s):

Upper Mantle ◽

The Arctic ◽

Mantle Flow ◽

Mantle Heterogeneity ◽

Gakkel Ridge ◽

The Pacific ◽

Mid Ocean Ridge ◽

The Pacific Ocean ◽

Back Arc ◽

AbstractThe plate tectonic cycle produces chemically distinct mid-ocean ridge basalts and arc volcanics, with the latter enriched in elements such as Ba, Rb, Th, Sr and Pb and depleted in Nb owing to the water-rich flux from the subducted slab. Basalts from back-arc basins, with intermediate compositions, show that such a slab flux can be transported behind the volcanic front of the arc and incorporated into mantle flow. Hence it is puzzling why melts of subduction-modified mantle have rarely been recognized in mid-ocean ridge basalts. Here we report the first mid-ocean ridge basalt samples with distinct arc signatures, akin to back-arc basin basalts, from the Arctic Gakkel Ridge. A new high precision dataset for 576 Gakkel samples suggests a pervasive subduction influence in this region. This influence can also be identified in Atlantic and Indian mid-ocean ridge basalts but is nearly absent in Pacific mid-ocean ridge basalts. Such a hemispheric-scale upper mantle heterogeneity reflects subduction modification of the asthenospheric mantle which is incorporated into mantle flow, and whose geographical distribution is controlled dominantly by a “subduction shield” that has surrounded the Pacific Ocean for 180 Myr. Simple modeling suggests that a slab flux equivalent to ~13% of the output at arcs is incorporated into the convecting upper mantle.

Tholeiitic volcanic rocks of the late Archean Blake River Group, southern Abitibi greenstone belt: origin and geodynamic implications

10.1139/e92-116 ◽

1992 ◽

Vol 29 (7) ◽

pp. 1448-1458 ◽

Cited By ~ 60

Author(s):

M. R. Laflèche ◽

C. Dupuy ◽

J. Dostal

Keyword(s):

Greenstone Belt ◽

Volcanic Rocks ◽

Incompatible Trace Element ◽

Progressive Change ◽

Abitibi Greenstone Belt ◽

Mid Ocean Ridge ◽

Compositional Variations ◽

Arc Setting ◽

Back Arc ◽

The late Archean Blake River Group volcanic sequence forms the uppermost part of the southern Abitibi greenstone belt in Quebec. The group is mainly composed of mid-ocean-ridge basalt (MORB)-like tholeiites that show a progressive change of several incompatible trace element ratios (e.g., Nb/Th, Nb/Ta, La/Yb, and Zr/Y) during differentiation. The compositional variations are inferred to be the result of fractional crystallization coupled with mixing–contamination of tholeiites by calc-alkaline magma which produced the mafic–intermediate lavas intercalated with the tholeiites in the uppermost part of the sequence. The MORB-like tholeiites were probably emplaced in a back-arc setting.

Origin of the Indus ophiolite linked to the mantle transition zone (410–660 km)

10.1130/2021.2552(02) ◽

2021 ◽

Author(s):

Souvik Das ◽

Asish R. Basu

Keyword(s):

Transition Zone ◽

Plate Tectonics ◽

Tectonic Setting ◽

Ultrahigh Pressure ◽

Historical Contingency ◽

Mantle Transition Zone ◽

Contingency Model ◽

Mid Ocean Ridge ◽

Consistent Change ◽

ABSTRACT The southeast Ladakh (India) area displays one of the best-preserved ophiolite sections in this planet, in places up to 10 km thick, along the southern bank of the Indus River. Recently, in situ, ultrahigh-pressure (UHP) mineralogical evidence from the mantle transition zone (MTZ; ∼410–660 km) with diamond and reduced fluids were discovered from two peridotite bodies in the basal mantle part of this Indus ophiolite. Ultrahigh-pressure phases were also found by early workers from podiform chromitites of another coeval Neo-Tethyan ophiolite in southern Tibet. However, the MTZ phases in the Indus ophiolite are found in silicate peridotites, but not in metallic chromitites, and the peridotitic UHP phases show systematic and contiguous phase transitions from the MTZ to shallower depth, unlike the discrete UHP inclusions, all in Tibetan chromitites. We observe consistent change in oxygen fugacity (fO2) and fluid composition from (C-H + H2) to (CO2 + H2O) in the upwelling peridotitic mantle, causing melting to produce mid-ocean-ridge basalt (MORB). At shallow depths (<100 km) the free water stabilizes into hydrous phases, such as pargasitic amphibole, capable of storing water and preventing melting. Our discoveries provide unique insights into deep sub-oceanic-mantle processes, and link deep-mantle upwelling and MORB genesis. Moreover, the tectonic setting of Neo-Tethyan ophiolites has been a difficult problem since the birth of the plate-tectonics concept. This problem for the origin of ophiolites in mid-ocean-ridge versus supra-subduction zone settings clearly confused the findings from Indus ophiolites. However, in this contribution, we provide arguments in favor of mid-ocean-ridge origin for Indus ophiolite. In addition, we venture to revisit the “historical contingency” model of E.M. Moores and others for Neo-Tethyan ophiolite genesis based on the available evidence and have found that our new results strongly support the “historical contingency” model.

Differentiation and source of the Nipissing Diabase intrusions, Ontario, Canada

10.1139/e93-095 ◽

1993 ◽

Vol 30 (6) ◽

pp. 1123-1140 ◽

Cited By ~ 20

Author(s):

P. C. Lightfoot ◽

H. de Souza ◽

W. Doherty

Keyword(s):

Trace Element ◽

Crustal Contamination ◽

Primitive Mantle ◽

Magma Source ◽

Magma Type ◽

Ni Content ◽

Chemical Signatures ◽

Mid Ocean Ridge ◽

Major and trace element data are presented for 2.2 Ga Proterozoic diabase sills from across the Nipissing magmatic province of Ontario. In situ differentiation of the magma coupled with assimilation of Huronian Supergroup roof sediments is responsible for the variation in composition between quartz diabase and granophyric diabase seen within many of the differentiated intrusions. Uniform trace element and isotope ratio signatures, such as La/Sm (2.8 – 3.7) and εNdCHUR (−2.7 to −5.9) characterize chilled margins and undifferentiated quartz diabases. These chemical signatures suggest the existence of a single magma source that was parental to intrusions throughout the magmatic province; this magma has higher La/Sm and lower Ti/Y than primitive mantle and is displaced towards the composition of shales. Most chilled diabases and quartz diabases have a similar Mg# (0.64 and 0.60) and Ni content (98 and 127 ppm), and it is argued that the magma differentiated at depth and was emplaced as a uniform low-Mg magma. The Wanapitei intrusion and Kukagami Lake sill are an exception in that although the quartz diabase has La/Sm similar to the Nipissing magma type, which suggests that they came from the same source, the Mg# (0.68–0.71) and Ni content (130–141 ppm) are higher, which may suggest that they are either slightly more primitive examples of the normal Nipissing magma or that cumulus hypersthene has been resorbed. The light rare earth element enriched signature of the Nipissing magmas was perhaps introduced from the continental crust as the magma migrated from the mantle to the surface, but a remarkably constant and large amount (>20%) of crustal contamination would be required. An addition of 1 –3% shale to the source of a transitional mid-ocean ridge basalt type magma can broadly reproduce the compositional features of the Nipissing magma type. The source characteristics were perhaps imparted during subduction accompanying the terminal Kenoran orogeny.

Geochronology and geochemistry of Precambrian gneisses, metabasites, and pegmatite from the Tobacco Root Mountains, northwestern Wyoming craton, MontanaThis article is one of a series of papers published in this Special Issue on the theme of Geochronology in honour of Tom Krogh.T.E. Krogh deceased April 2008.

10.1139/e10-095 ◽

2011 ◽

Vol 48 (2) ◽

pp. 161-185 ◽

Cited By ~ 10

Author(s):

Thomas E. Krogh ◽

Sandra L. Kamo ◽

Thomas B. Hanley ◽

David F. Hess ◽

Peter S. Dahl ◽

...

Keyword(s):

Special Issue ◽

Zircon Age ◽

Mafic Rocks ◽

Wyoming Craton ◽

Fault Block ◽

Mid Ocean Ridge ◽

Tonalitic Gneiss ◽

Back Arc ◽

Ocean Ridge ◽

Tobacco Root Mountains

The Middle Mountain Metamorphic Domain of the Montana Metasedimentary Terrane, northwestern Wyoming Craton, within the northwestern Tobacco Root Mountains, mainly comprises migmatized tonalitic gneiss interlayered with amphibolitic (hornblende) gneiss, both of which are cut by metamorphosed mafic rocks. Together, these gneisses are defined as Middle Mountain Gneiss. Archean tonalitic gneiss from west of, and amphibolitic gneiss from east of, the Bismark Fault give, from chemically and air-abraded zircon grains, U–Pb ID–TIMS ages of 3325.5 ± 1.7 and 3340 Ma, respectively. These results reflect primary magmatic ages and show that the Middle Mountain Gneiss extends into the northern area of the Central Fault Block, between the Bismark and Mammoth faults. Older crustal processes in the tonalitic gneiss are evidenced by inherited grains, the oldest of which is >3460 Ma. A metabasite hosted in tonalitic gneiss in the Bismark Fault selvage zone yields a zircon age of 2468 Ma, which is interpreted as the time of metamorphism. This date and other ca. 2470 Ma dates known in the region reflect a series of thermotectonic events designated here as the Beaverhead – Tobacco Root Orogeny. Geochemical evidence in the Central Fault Block metabasites suggests that their >2470 Ma precursors evolved in a back-arc – arc-rift setting, whereas their equivalents west of the Bismark Fault were largely mid-ocean ridge basalt-related tholeiites and east of the Central Fault Block were back-arc tholeiites showing some continental affinity. The metabasite was metamorphosed, deformed, and intruded by pegmatite at 1756 Ma during the Big Sky Orogeny. This orogenic event also produced new zircon growth in Archean tonalitic gneiss. Monazite with an age of 75 Ma, found at one location, reflects nearby intrusion of the Cretaceous Tobacco Root Batholith.

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

10.1139/e95-166 ◽

1995 ◽

Vol 32 (12) ◽

pp. 2128-2146 ◽

Cited By ~ 13

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.

Geology and geochemistry of an Archean mafic dike complex in the Chan Formation: basis for a revised plate-tectonic model of the Yellowknife greenstone belt

10.1139/e95-052 ◽

1995 ◽

Vol 32 (5) ◽

pp. 614-630 ◽

Cited By ~ 31

Author(s):

Kate Maclachlan ◽

Herb Helmstaedt

Keyword(s):

Greenstone Belt ◽

Volcanic Rocks ◽

Mafic Dike ◽

Tectonic Model ◽

Dike Complex ◽

Metavolcanic Rocks ◽

Depleted Mantle ◽

Mid Ocean Ridge ◽

A Minor ◽

An Archean mafic dike complex in the Chan Formation at the base of the mafic volcanic section of the Yellowknife greenstone belt consists of multiple metagabbro dikes and sills separated by screens of pillowed mafic volcanic rocks, which are cut by a younger postvolcanic metadiabase dike swarm. Field relationships and geochemical characteristics are compatible with a comagmatic origin for the metagabbro and metavolcanic rocks that were fed through and deposited on an older, rift-related, supracrustal sequence of the Dwyer Group. The synvolcanic metagabbro dikes have extended the strike length of the volcanic section by at least 100%. The mafic rocks of the Chan Formation are geochemically similar to mid-ocean ridge basalt, possibly with a minor subduction-zone component. Preliminary εNd values for metagabbroic rocks are consistent with the derivation of magmas predominantly from a normal, depleted-mantle source. The Chan Formation is interpreted to have formed in a marginal basin-like setting, adjacent to a previously rifted and attenuated protocontinental margin.

Investigating influences on the Pb pseudo-isochron using three-dimensional mantle convection models with a continental reservoir

10.31223/x5ss5c ◽

2021 ◽

Author(s):

James Panton ◽

J. Davies ◽

Tim Elliott ◽

Morten Andersen ◽

Donald Porcelli ◽

...

Keyword(s):

Mantle Convection ◽

Plate Tectonics ◽

Three Dimensional ◽

Isotope Ratios ◽

Pb Isotope ◽

Ocean Island Basalts ◽

Mid Ocean Ridge ◽

Subducted Oceanic Crust ◽

Ocean Ridge ◽

Relative Change

For mid-ocean ridge basalts (MORBs) and ocean island basalts (OIBs), measurements of Pb isotope ratios show broad linear correlations with a certain degree of scatter. In 207Pb/204Pb - 206Pb/204Pb space, the best fit line defines a pseudo-isochron age (τPb) of ~1.9 Gyr.Previous modelling suggests a relative change in the behaviours of U and Pb between 2.25-2.5 Ga, resulting in net recycling of HIMU (high U/Pb) material in the latter part of Earth's history, to explain the observed τPb. However, simulations in which fractionation is controlled by a single set of partition coefficients throughout the model runs fail to reproduce τPb and the observed scatter in Pb isotope ratios. We build on these models with 3D mantle convection simulations including parameterisations for melting, U recycling from the continents and preferential removal of Pb from subducted oceanic crust.We find that both U recycling after the great oxygenation event (GOE) and Pb extraction after the onset of plate tectonics, are required in order to fit the observed gradient and scatter of both the 207Pb/204Pb - 206Pb/204Pb and 208Pb/204Pb - 206Pb/204Pb arrays. Unlike much previous work, our model does not require accumulations of subducted oceanic crust to persist at the CMB for long periods of time in order to match geochemical observations.

Chemical Geodynamics of Asthenospheric Outflow in the western Pacific: Philippine Sea Back-arc Basin Mantle Source of the Yap Trench Forearc Lavas

10.5194/egusphere-egu2020-1727 ◽

2020 ◽

Author(s):

Limei Tang ◽

Ling Chen

Keyword(s):

Trace Element ◽

Western Pacific ◽

Philippine Sea Plate ◽

Philippine Sea ◽

Trace Element Chemistry ◽

Lithospheric Extension ◽

Mid Ocean Ridge ◽

Back Arc ◽

Ocean Ridge ◽

The Western Pacific

We present new major and trace element chemistry and Sr, Nd, and Pb isotope data from basalts, recovered from the forearc setting of the Yap Trench in the western Pacific, and discuss their melt evolution and petrogenesis within the framework of the geodynamic interactions among the Caroline Plate, the Caroline ridge, and the Philippine Sea plate. These rocks have mid-ocean ridge basalt (MORB)-like geochemical features, including medium Fe contents, tholeiitic affinity, high TiO2 values at a given Fe2O3/MgO ratio, Ti/V, Nb/Y, Ba/Yb, and Ba/Th ratios similar to those of back-arc basin basalts (BABB), and trace element patterns commonly displayed by MORB and BABB lavas. However, these basalts are characterized by highly radiogenic Sr and Pb contents, reminiscent of western Pacific sediments. We suggest that forearc magmatism was responsible for the origin and petrogenesis of these rocks. Forearc magmatism was induced by the shrinking of the Philippine Sea plate, which squeezed out the underlying back-arc basin asthenosphere with Indian&#8211;type ambient mantle characteristics to invade the forearc mantle of the Yap Trench and causes lithospheric extension. Upwelling and decompression melting of this mantle produced MORB-like lavas in the narrow forearc setting. An apparent slab tear or gap in the subducting plate facilitate the penetration of the mantle outflow. The collision of the Caroline Ridge subducted more sediments into the mantle wedge. Melting of the subducted sediments and the invasion of the Indian-type asthenosphere into the forearc account for the highly radioactive Sr and Pb isotopes of the MORB-like lavas.