Rift-related Magmatism Research Articles

Rift-related magmatism of the Central Atlantic magmatic province in Algarve, Southern Portugal

Lower Jurassic volcanic-sedimentary successions of the Algarve Basin (∼ 198 Ma) are associated with the Central Atlantic Magmatic Province (CAMP), that was one aspect of Pangean intracontinental rifting. Volcanic products of the Algarve CAMP include subaerial lava flows, pyroclastic deposits and peperites, and contemporaneous sedimentation is dominated by mudstones and conglomerates, often containing volcanic fragments, intercalated in the volcanic sequence. These lithological characteristics and associations are compatible with a facies model typical of continental basaltic magmatism. Volcanic rocks are fractionated low-Ti tholeiites enriched in large ion lithophile elements relative to high field strength elements. The compositional spectrum (e.g., Ba/Nb–Zr/Nb) is consistent with derivation of primary magmas by partial melting of heterogeneous sources, initially continental lithospheric mantle progressively replaced by an asthenospheric source. Fractional crystallization processes occurred in magma chambers located at depths close to the crust/mantle boundary from density and geobarometric constraints, reflecting magma underplating during the initial stages of continental rifting. Sporadic occurrence of crustal xenoliths, widespread evidence for basement carbonate assimilation, as well as Sr (( 87Sr/ 86Sr) 0 > 0.7050) and oxygen ( δ 18O > + 7.40‰) isotope ratios that are higher than those typical of continental lithospheric mantle, indicate that crustal contamination also contributed to the observed geochemical variations. The Carnian–Norian (228.0 to 203.6 Ma) or older age estimated for the basal Silves Sandstones suggests that the inception of rift related sedimentation preceded volcanism by 20 to 30Ma. These results lead us to propose a passive rifting model for the Algarve sector of CAMP magmatism, the orientation of which was conditioned by strain localization along pre-existing lithospheric mechanical anisotropies.

Crustal structure of the French Guiana margin, West Equatorial Atlantic

SUMMARY Geophysical data from the Amazon Cone Experiment are used to determine the structure and evolution of the French Guiana and Northeast Brazil continental margin, and to better understand the origin and development of along-margin segmentation. A 427-km-long combined multichannel reflection and wide-angle refraction seismic profile acquired across the southern French Guiana margin is interpreted, where plate reconstructions suggest a rift-type setting. The resulting model shows a crustal structure in which 35‐37-km-thick pre-rift continental crust is thinned by a factor of 6.4 over a distance of ∼70 km associated with continental breakup and the initiation and establishment of seafloor spreading. The ocean‐continent boundary is a transition zone up to 45 km in width, in which the two-layered oceanic-type crustal structure develops. Although relatively thin at 3.5‐5.0 km, such thin oceanic crust appears characteristic of the margin as a whole. There is no evidence of rift-related magmatism, either as seaward-dipping sequences in the reflection data or as a high velocity region in the lower crust in the P-wave velocity model, and as a such the margin is identified as non-volcanic in type. However, there is also no evidence of the rotated fault block and graben structures characteristic of rifted margins. Consequently, the thin oceanic crust, the rapidity of continental crustal thinning and the absence of characteristic riftrelated structures leads to the conclusion that the southern French Guiana margin has instead developed in an oblique rift setting, in which transform motion also played a significant role in the evolution of the resulting crustal structure and along-margin segmentation in structural style.

Detrital zircon geochronology and provenance of the southeastern Yukon–Tanana terrane

Two samples of late Paleozoic grit and Late Mississippian quartzite–chert conglomerate collected from southeastern Yukon–Tanana terrane (YTT) — a composite thrust sheet resting structurally above North American parautochthonous strata and intervening imbricate sheets of the late Paleozoic oceanic Slide Mountain terrane — yielded, respectively, 89 and 74 concordant or nearly concordant (<20% discordant) U–Pb ages on single detrital zircons. They provide constraints on the provenance of this allochthonous pericratonic terrane. Zircons in the grit range from 1770 to 2854 Ma, with a well-defined Early Proterozoic peak between 1800 and 2100 Ma. Precambrian zircons in the conglomerate also show a dominant peak between 1800 and 2100 Ma and smaller peaks between 2200 and 3200 Ma, with a few older grains, and younger grains with ages of 998, 1219, 1255, 1256, and 1417 Ma. The conglomerate also yielded three Devonian grains, with ages of 366 ± 23, 373 ± 12, and 379 ± 23 Ma. Their ages are approximately coeval with the oldest felsic to intermediate arc- and rift-related magmatism in the YTT. The age spectra from southeastern YTT units compare closely with those from Mississippian and older pericratonic units in the Coast Mountains, confirming correlations previously made on lithologic grounds. They also strongly resemble detrital zircon populations from craton-derived Paleozoic units of the northern North American autochthon. This robust U–Pb data set lends support to the idea that YTT once formed part of the outer, active margin of the North American continent, prior to Mississippian rifting and marginal ocean basin development.

Permian exhumation of the Buffalo Pitts orogenic peridotite massif, northern Cordillera, Yukon

We report the results of a geochemical and U–Pb zircon geochronological study aimed at constraining the timing and tectonic setting of the exhumation of an orogenic peridotitic mantle massif in central Yukon within northern Canadian Cordillera. The Buffalo Pitts orogenic massif is inferred to have been exhumed into continental metasedimentary rocks within the pericratonic Yukon–Tanana terrane. Structurally admixed with the peridotite were boudins of metaleucogabbro and metatroctolite. A metamorphic aureole, defined by migmatite with abundant leucosome, characterizes the metasedi mentary wall rocks to the massif. Whole-rock chemical analyses indicate significant light rare-earth element enrichment of the leucogabbro and the metatroctolite, characteristics commonly ascribed to within-plate or rift settings. Crystallization of the leucogabbro occurred at 261.5 ± 2.3 Ma. The metatroctolite yields a similar crystallization age. These ages are coeval with metamorphism of the wall rocks to the orogenic massif, as indicated by leucosome crystallization at 262.3 ± 0.43 Ma. These geochemical and geochronological data are consistent with the orogenic massif having been exhumed within a continental rift at about 262 Ma, giving rise to metamorphism of the upper crustal rocks into which the massif was exhumed, and coeval with rift-related magmatism. Regional considerations suggest that rifting occurred within the back arc of a northeast-facing magmatic arc, represented by the Klondike schist. Coeval eclogite and blueschist along the northeast margin of the Yukon–Tanana terrane may mark the paleo-trench, along which a southwest-dipping slab is assumed to have subducted beneath Yukon–Tanana terrane.

Signature of Precambrian extension events in the southern Siberian craton

We investigate extension events in the southern Siberian craton between 1.8 and 0.7 Ga. Signature of Late Paleoproterozoic within-plate extension in the Northern Baikal region is found in 167 4± 29 Ma dike swarms. A Mesoproterozoic extension event was associated with intrusion of the 1535 ± 14 Ma Chernaya Zima granitoids into the Urik-Iya graben deposits. Neoproterozoic extension recorded in the Sayan-Baikal dike belt (740-780 Ma dike complexes) was concurrent with the breakup of the Rodinia supercontinent and the initiation of the Paleoasian passive margin along the southern edge of the Siberian craton. The scale of rifting-related magmatism and the features of the coeval sedimentary complexes in the southern Siberian craton indicate that Late Paleoproterozoic and Early Mesoproterozoic extension did not cause ocean opening, and the Paleoasian Ocean opened as a result of Neoproterozoic rifting.

Continental mafic magmatism of different ages in the same terrane: Constraints on the evolution of an enriched mantle source

Research Article| April 01, 2007 Continental mafic magmatism of different ages in the same terrane: Constraints on the evolution of an enriched mantle source J. Brendan Murphy; J. Brendan Murphy 1Department of Earth Sciences, St. Francis Xavier University, Antigonish, Nova Scotia B2G 2W5, Canada Search for other works by this author on: GSW Google Scholar Jaroslav Dostal Jaroslav Dostal 2Department of Geology, St. Mary's University, Halifax, Nova Scotia B3H 3C3, Canada Search for other works by this author on: GSW Google Scholar Author and Article Information J. Brendan Murphy 1Department of Earth Sciences, St. Francis Xavier University, Antigonish, Nova Scotia B2G 2W5, Canada Jaroslav Dostal 2Department of Geology, St. Mary's University, Halifax, Nova Scotia B3H 3C3, Canada Publisher: Geological Society of America Received: 07 Jun 2006 Revision Received: 23 Nov 2006 Accepted: 27 Nov 2006 First Online: 09 Mar 2017 Online ISSN: 1943-2682 Print ISSN: 0091-7613 Geological Society of America Geology (2007) 35 (4): 335–338. https://doi.org/10.1130/G23072A.1 Article history Received: 07 Jun 2006 Revision Received: 23 Nov 2006 Accepted: 27 Nov 2006 First Online: 09 Mar 2017 Cite View This Citation Add to Citation Manager Share Icon Share Facebook Twitter LinkedIn MailTo Tools Icon Tools Get Permissions Search Site Citation J. Brendan Murphy, Jaroslav Dostal; Continental mafic magmatism of different ages in the same terrane: Constraints on the evolution of an enriched mantle source. Geology 2007;; 35 (4): 335–338. doi: https://doi.org/10.1130/G23072A.1 Download citation file: Ris (Zotero) Refmanager EasyBib Bookends Mendeley Papers EndNote RefWorks BibTex toolbar search Search Dropdown Menu toolbar search search input Search input auto suggest filter your search All ContentBy SocietyGeology Search Advanced Search Abstract The Antigonish Highlands, Nova Scotia, are within the Avalon terrane of the northern Appalachians and contain four distinct episodes of rift-related magmatism: Neoproterozoic, Cambrian, Middle Ordovician, and Late Devonian. All four magmatic suites are composed of basalts and subordinate crustally derived felsic rocks. The mafic rocks of these suites, which do not appear to be significantly contaminated by continental crust, display similar geochemical and Sm-Nd isotopic characteristics consistent with an enriched mantle source that was metasomatically enriched prior to the oldest rifting event, probably between 0.8 and 1.1 Ga. These data also imply that the Avalonian crust and its subcontinental lithospheric mantle remained coupled during four magmatic events. Because the earliest phases of magmatism occurred when Avalonia was located along the Gondwanan margin, and the latest phases after Avalonia accreted to Laurentia, the coupling of crust and mantle in the Antigonish Highlands suggests that the migration of Avalonia did not completely detach its lithospheric mantle from its crustal cover. This study demonstrates the importance of comparing magmatism of different ages in the same terrane. You do not have access to this content, please speak to your institutional administrator if you feel you should have access.

Geology, Geochronology, and Geodynamics of the Khan Bogd alkali granite pluton in southern Mongolia

The Khan Bogd alkali granite pluton, one of the world’s largest, is situated in the southern Gobi Desert, being localized in the core of the Late Paleozoic Syncline, where island-arc calc-alkaline differentiated volcanics (of variable alkalinity) give way to the rift-related bimodal basalt-comendite-alkali granite association. The tectonic setting of the Khan Bogd pluton is controlled by intersection of the near-latitudinal Gobi-Tien Shan Rift Zone with an oblique transverse fault, which, as the rift zone, controls bimodal magmatism. The pluton consists of the eastern and the western ring bodies and comes into sharp intrusive contact with rocks of the island-arc complex and tectonic contact with rocks of the bimodal complex. The inner ring structure is particularly typical of the western body and accentuated by ring dikes and roof pendants of the country island-arc complex. According to preliminary gravity measurements, the pluton is a flattened intrusive body (laccolith) with its base subsiding in stepwise manner northwestward. Reliable geochronologic data have been obtained for both plutonic and country rocks: the U-Pb zircon age of alkali granite belonging to the main intrusive phase is 290 ± 1 Ma, the 40Ar/39Ar ages of amphibole and polylithionite are 283 ± 4 and 285 ± 7 Ma, and the Rb-Sr isochron yields 287 ± 3 Ma; i.e., all these estimates are close to 290 Ma. Furthermore, the U-Pb zircon age of red normal biotite granite (290 ± 1 Ma) and the Rb-Sr age of the bimodal complex in the southern framework of the pluton are the same. The older igneous rocks of the island-arc complex in the framework and roof pendants of the pluton are dated at 330 Ma. The geodynamic model of the Khan Bogd pluton formation suggests collision of the Hercynian continent with a hot spot in the paleoocean; two variants of this model are proposed. According to the first variant, the mantle plume, after collision with the margin of the North Asian paleocontinent, reworked the subducted lithosphere and formed a structure similar to an asthenospheric window, which served as a source of rift-related magmatism and the Khan Bogd pluton proper. In compliance with the second variant, the emergence of hot mantle plume resulted in flattening of the subducted plate; cessation of the island-arc magmatism; and probably in origin of a local convective system in the asthenosphere of the mantle wedge, which gave rise to the formation of a magma source. The huge body of the Khan Bogd alkali granite pluton and related volcanic rocks, as well as its ring structure, resulted from the caldera mechanism of the emplacement and evolution of magmatic melts.

Petrology, geochemistry and magnetic properties of Sadara sill: Evidence of rift related magmatism from Kutch basin, northwest India

Mafic volcanic rocks of the Mesozoic Kutch basin represent the earliest phase of Deccan volcanic activity. An olivine-clinopyroxene-plagioclase-phyric undersaturated basalt occurs as a sill near Sadara in the Pachham upland, Northern Kutch. The Sadara sill is deformed and emplaced along faults. The sill is alkaline in character and is transitional between basalt and basanite. Compared to primitive mantle, the Sadara sill is enriched in Sr, Ba, Pb and LREE but depleted in Nb, Cr, Y, Cs and Lu. Fractional crystallization of olivine and clinopyroxene from an alkaline mafic melt generated by low degree partial melting of mantle peridotite can explain the observed chemical variation in the sill. IRM and L-F test experiments and mineral analyses show titano-magnetite as the major remanence carrying magnetic mineral. AF and thermal demagnetizations of the Sadara sill yielded a mean ChRM direction as D=315.6°, I=−43.0° ( α 95=9.78; k=25.38) and the corresponding VGP at 25°S; 114.6°E (d p/d m=6.58°/11.6°). The Sadara sill pole is significantly different from those of the Deccan (65 Ma) and the Rajmahal Traps (118 Ma) but is close to the Cretaceous poles of 85–91 Ma rock units from southern India. This suggests a pre-Deccan age for the sill.

Inherited arc signature in Ediacaran and Early Cambrian basins of the Ossa-Morena Zone (Iberian Massif, Portugal): Paleogeographic link with European and North African Cadomian correlatives

Geochemical data from clastic rocks of the Ossa-Morena Zone (Iberian Massif) show that the main source for the Ediacaran and the Early Cambrian sediments was a recycled Cadomian magmatic arc along the northern Gondwana margin. The geodynamic scenario for this segment of the Avalonian-Cadomian active margin is considered in terms of three main stages: (1) The 570–540 Ma evolution of an active continental margin evolving oblique collision with accretion of oceanic crust, a continental magmatic arc and the development of related marginal basins; (2) the Ediacaran–Early Cambrian transition (540–520 Ma) coeval with important orogenic magmatism and the formation of transtensional basins with detritus derived from remnants of the magmatic arc; and (3) Gondwana fragmentation with the formation of Early Cambrian (520–510 Ma) shallow-water platforms in transtensional grabens accompanied by rift-related magmatism. These processes are comparable to similar Cadomian successions in other regions of Gondwanan Europe and Northwest Africa. Ediacaran and Early Cambrian basins preserved in the Ossa-Morena Zone (Portugal and Spain), the North Armorican Cadomian Belt (France), the Saxo-Thuringian Zone (Germany), the Western Meseta and the Western High-Atlas (Morocco) share a similar geotectonic evolution, probably situated in the same paleogeographic West African peri-Gondwanan region of the Avalonian-Cadomian active margin.

Late Palaeozoic intra- and pericratonic basins on the East European Craton and its margins

Abstract The (Mid-) Late Devonian to Early Carboniferous was a time of widespread rifting on the East European Craton (EEC) and its margins. The most prominent basin among these and, accordingly, the best documented is the Dniepr-Donets Basin (DDB) in Ukraine and southern Russia. The DDB is associated with voluminous rift-related magmatism and broad basement uplift. Two other large, extensional, basin systems developed along the margins of the EEC at the same time: the East Barents Basin (EEB) and its onshore prolongation the Timan-Pechora Basin (TPB), and the Peri-Caspian Basin (PCB). Rifting, associated magmatism, and possible domal basement uplift are also reported elsewhere within the EEC, suggesting a common, 'active', rifting process, involving a cluster of thermal instabilities (or generalized thermal instability) at the base of the lithosphere beneath widely separated parts of the EEC by Mid-Late Devonian times. The DDB is an intracratonic rift basin, cutting across the Archaean-Palaeoproterozoic structural grain of its basement and, as such, differs from the EBB -TPB and PCB, which are pericratonic rift basins developed on reworked and juvenile crystalline basement accreted to the EEC during the Neoproterozoic. The DDB opened into a deep basin, possibly having oceanic lithospheric affinity, to the SE, in the area where it adjoins the southern PCB, suggesting the possibility that rifting led to (limited?) continental break-up in this area at this time. Post-rift compressional tectonic reactivations and basin inversion in the DDB, leading to the formation of its prominent Donbas Foldbelt segment, are related to Tethyan events (Cimmerian and Alpine orogenies) occurring on the nearby southern margin of the EEC. Post-rift compressional inversions in the PCB and TPB, which lie closer to the Urals margin of the EEC, are related to Uralian tectonics.

Mid-Cretaceous extension-related magmatism in the eastern Colombian Andes

Cretaceous magmatism in the Eastern Cordillera of Colombia is related to lithospheric stretching during the late Early–early Late Cretaceous. The small amount of preserved igneous material is represented by small mafic intrusions. This study focuses on three localities, from east to west: Pajarito, Pacho, and Cáceres. The investigated igneous bodies are classified as gabbros, pyroxene–hornblende–gabbros, and pyroxene–hornblendites mainly composed of plagioclase, clinopyroxene, and/or amphibole. Although their timing of emplacement and geodynamic position seem similar, significant differences in their geochemical and petrological characteristics rule out simple models of melt genesis. Clinopyroxene and bulk chemistry indicate increasing alkalinity from west to east. Trace element concentrations point to melt sources that range from a slightly enriched mantle in the west to a highly enriched one in the east. In addition, the data reflect a decreasing degree of partial melting from west to east and the decreasing importance of residual garnet in the mantle source. Probable mantle metasomatism in the source region by slab-derived fluids, as displayed by high Ba/Nb and moderate Sr n /P n , is clear in the west and very slight to the east. Mantle metasomatism and melt generation probably are processes of different epochs. The lack of large volumes of igneous rocks and the absence of tectonically controlled unconformities in the investigated areas indicate that a mantle plume did not affect the regional tectonics and magmatism. We favor a model of rift-related magmatism in which melt composition is modified from east to west from a highly enriched to a less enriched mantle region, the latter metasomatized by fluids derived from an older subduction phase.

Interaction between Permo-Triassic rifting, magmatism and initiation of the Adriatic-Dinaridic carbonate platform (ADCP)

Permo-Triassic rift-related magmatism in the Dinarides produced an intrusive gabbro-diorite-syenite-granite formation and an extrusive basalt-andesite-dacite formation with abundant pyroclastic rocks. They are spatially and genetically related to Late Permian to Norian rift-related sedimentary formations of the Adriatic-Dinaridic carbonate platform (ADCP). The volcanic and pyroclastic rocks are interlayered with fossiliferous sediments that range between the Late Permian and Middle Norian; the plutonic rocks, which are intrusive into Late Paleozoic and Scythian-Anisian sediments, have radiometric ages ranging between 262 and 212 Ma. Based on major and trace element contents, rocks of the Permo-Triassic magmatic association originated by fractional crystallization from primitive alkalic basalt to olivine tholeiite melts. Volcanic rocks were affected by strong ocean-hydrothermal metamorphism. Initial 87Sr/86Sr ratio of 0.703 and d18O of 5.6‰ of the most primitive rocks indicate an upper mantle origin. Most ...

A review of the Pan-African, Neoproterozoic Rosh Pinah Zn-Pb deposit, southwestern Namibia

Research Article| March 01, 2005 A review of the Pan-African, Neoproterozoic Rosh Pinah Zn-Pb deposit, southwestern Namibia D.J. Alchin; D.J. Alchin Kumba Resources, P.O. Box 1224, Sishen South, Postmasburg, South Africa, e-mail: dennis.alchin@kumbaresources.com Search for other works by this author on: GSW Google Scholar J.M. Moore J.M. Moore Geology Department, Rhodes University, Grahamstown, South Africa, e-mail: J.Moore@ru.ac.za Search for other works by this author on: GSW Google Scholar Author and Article Information D.J. Alchin Kumba Resources, P.O. Box 1224, Sishen South, Postmasburg, South Africa, e-mail: dennis.alchin@kumbaresources.com J.M. Moore Geology Department, Rhodes University, Grahamstown, South Africa, e-mail: J.Moore@ru.ac.za Publisher: Geological Society of South Africa First Online: 07 Mar 2017 Online ISSN: 1996-8590 Print ISSN: 1012-0750 © 2005 Geological Society of South Africa South African Journal of Geology (2005) 108 (1): 71–86. https://doi.org/10.2113/108.1.71 Article history First Online: 07 Mar 2017 Cite View This Citation Add to Citation Manager Share Icon Share Facebook Twitter LinkedIn MailTo Tools Icon Tools Get Permissions Search Site Citation D.J. Alchin, J.M. Moore; A review of the Pan-African, Neoproterozoic Rosh Pinah Zn-Pb deposit, southwestern Namibia. South African Journal of Geology 2005;; 108 (1): 71–86. doi: https://doi.org/10.2113/108.1.71 Download citation file: Ris (Zotero) Refmanager EasyBib Bookends Mendeley Papers EndNote RefWorks BibTex toolbar search Search Dropdown Menu toolbar search search input Search input auto suggest filter your search All ContentBy SocietySouth African Journal of Geology Search Advanced Search Abstract This paper provides a synopsis of our latest understanding of the regional geological setting, mineralisation and ore genesis of the Neoproterozoic Rosh Pinah Zn-Pb deposit in southwestern Namibia. During continental rifting at ~740 Ma, an intracratonic rift (Rosh Pinah Graben) became detached from the main Gariep depository by a basement high. The rift subsequently failed and was filled mainly by a volcanoclastic rift-fill sequence, containing predominantly siliciclastic and subordinate calcareous rocks, and was accompanied by contemporaneous bimodal but dominantly felsic volcanism. Volcanism may have provided the heat engine for contemporaneous sedimentary-exhalative and hydrothermal replacement base metal deposits, which formed within anoxic, carbonaceous mudstone horizons in the deep rift. Mineralisation consists mainly of sphalerite, pyrite, galena and chalcopyrite within mudstone, with interbedded, dolomitic and Ba-rich carbonate lenses, and lesser bornite, tennantite-tetrahedrite, stromeyerite, and native gold. Pervasive hydrothermal alteration has affected the surrounding host rocks, and whole-rock lithogeochemical trends indicate that as the ore body is approached, increases in the elements Mn, Fe, Cu, Zn, Ba and Pb are apparent, with barium being the most extensively dispersed, followed by copper and zinc. Evidence for the presence and locality of a potential hydrothermal vent zone is provided by preliminary metal-zoning studies, which is confirmed by the spatial distribution of dominant brecciation and silicification, and the position of peripheral barium-rich carbonate ore bodies. Vertical zoning is indicated by elevated copper contents in the footwall breccia zone, and increases in lead and zinc towards the massive sulphide zones occurring near or at the hanging wall contact. Fluid inclusion studies reveal the presence of two chemically distinct fluid populations – a pre-orogenic, rift-related Rosh Pinah ore-forming fluid dominated by MgCl2- and CaCl2-rich fluids, probably representing hypersaline brines induced by rift-related magmatism, and a general orogenic fluid dominated by a NaCl-CaCl2-H2O (with minor MgCl2) system. The association of the Rosh Pinah deposit with basin-margin rifting, second-order basins, anoxic sediments, and the finely banded/laminated nature of the ore are indicators of possible sedimentary exhalative (SEDEX) origins for the mineralisation. The close association of the sulphides with silicification and brecciation may also indicate significant post-depositional replacement and/or impregnation. You do not have access to this content, please speak to your institutional administrator if you feel you should have access.

A case for mantle plumes

The existence of at least several plumes in the Earth’s mantle can be inferred with few assumptions from wellestablished observations. As well, thermal mantle plumes can be predicted from wellestablished and quantified fluid dynamics and a plausible assumption about the Earth’s early thermal state. Some additional important observations, especially of flood basalts and rift-related magmatism, have been shown to be plausibly consistent with the physical theory. Recent claims to have detected plumes using seismic tomography may comprise the most direct evidence for plumes, but plume tails are likely to be difficult to resolve definitively and the claims need to be well tested. Although significant questions remain about its viability, the plume hypothesis thus seems to be well worth continued investigation. Nevertheless there are many non-plate-related magmatic phenomena whose association with plumes is unclear or unlikely. Compositional buoyancy has recently been shown potentially to substantially complicate the dynamics of plumes, and this may lead to explanations for a wider range of phenomena, including “headless” hotspot tracks, than purely thermal plumes.

U–Pb zircon geochronology and geochemistry of Neoproterozoic volcanic rocks in the Tarim Block of northwest China: implications for the breakup of Rodinia supercontinent and Neoproterozoic glaciations

As the lowest volcanics-bearing unit of the Neoproterozoic succession, the Beiyixi Formation is the key to understanding the early response to the breakup of the Roninia supercontinent in the Tarim Block. The SHRIMP analyses of zircons from the volcanic rocks at the bottom of the Beiyixi Formation yield a weighted mean 206Pb/ 238U age of 755 ± 15 Ma. This is interpreted as the eruption age of the Beiyixi volcanic rocks. The Beiyixi volcanic rocks consist of bimodal basalt and dacite-rhyolite with a SiO 2 gap between 55% and 65%. The mafic rocks display negative ɛ Nd (755 Ma) values (−9.9 to −10.8), moderate enrichment in LILE and variable depletion in Nb, Ta and P, resembling those of the tholeiitic basalts in continental rift. Geochemical and Nd isotopic characteristics suggest that the mafic rocks were derived from partial melting of an enriched lithospheric mantle reservoir. The felsic rocks show negative ɛ Nd (755 Ma) values (−7.9 to −9.2), negative Nb, Ta, P and Ti anomalies, very high La N/Yb N (62–92) ratios and LILE abundances, and may be generated by melting of eclogites or garnet amphibolites in the lower crust, as a result of basalt emplacement into continental crust during continental rifting. The age of 755 ± 15 Ma indicates that the Beiyixi glaciation took place later than 755 Ma and it could be correlated with the Chang’an glaciation in the Yangtze Block and the Sturtian–Rapitan glaciation in other Rodinia Blocks. The geochemical characteristics of the Beiyixi volcanic rocks resemble those of the rift-related magmatism in other Rodinia Blocks, suggesting that the Beiyixi volcanism was a part of global magmatism during the breakup of Rodinia supercontinent. The age and geochemical features of the Beiyixi volcanic rocks also reveal that the mantle plume activity spread to the northwestern margin of the Rodinia supercontinent and probably resulted in the breakup between Australia and Tarim Blocks.

Grenvillian and Caledonian tectono-magmatic activity in northeasternmost Svalbard

Abstract The Nordaustlandet Terrane of NE Svalbard forms an exposed part of the Barentsia microcontinent. Augen gneisses, migmatites, granites and gabbros dominate the scattered outcrops along the northeastern coast of Nordaustlandet, and on the smaller islands to the north and east, as far as Kvitøya. These outcrops probably represent the deepest exposed crustal levels within the folded Caledonian basement of the Nordaustlandet Terrane. In the present study, a variety of rock types have been analysed by different U-Pb dating techniques (conventional, Pb-evaporation and ion microprobe) on zircon, titanite and monazite The major and trace element compositions and Sm-Nd isotope geochemistry of these rocks have also been investigated. The augen gneisses yield U-Pb ages of c. 950 Ma, indicating that they are deformed late Grenvillian granites, similar to the Grenville-age granites and augen gneisses of northwestern and central Nordaustlandet. Migmatites, grey granites, aplitic dykes and a syenite (boulder) yield U-Pb ages mainly falling in the 430–450 Ma range, slightly older than the 410–420 Ma late-tectonic Caledonian granites further west. Both the Grenvillian and Caledonian granites are of crustal anatectic origin, and the Caledonian granites and migmatites may have formed largely by remelting of Grenvillian crust. The ages of the mafic rocks are uncertain, but Sm-Nd data indicate a possible emplacement age of c. 700 Ma for two of the gabbros, suggesting that they may be the result of rift-related magmatism in connection with the opening of the Iapetus Ocean. A few enigmatic inherited zircons of similar late Neoproterozoic age found in younger granites may possibly be related to this event. No evidence for late Neoproterozoic orogenic activity, similar to that in the Timanides of northern Russia, is seen in eastern Svalbard. At this time, eastern Svalbard (Barentsia) was probably part of the Laurentian margin, and probably located far away from northern Baltica.

Fluid–mantle interaction in an intra-oceanic arc: constraints from high-precision Pb isotopes

We present new isotopic and trace element data, including high-precision double-spike Pb isotope measurements, for back-arc lavas from the Izu–Bonin arc. Systematic along-arc isotopic variation of lavas has been identified in the Izu–Bonin arc that is coherent between the volcanic front and back-arc. The Sr isotopes are more radiogenic in the north, while Pb isotopes are less radiogenic in the north compared to the central part of the arc. This is particularly apparent in the back-arc seamounts. Decoupled variation of Pb and Sr isotopes cannot be explained by changes in the amount of a single subduction component. Almost parallel but distinct trends on Pb–Pb isotope plots imply differing mantle sources in the northern and central parts of the arc. The decoupling of Sr and Pb isotopic variation for both volcanic front and back-arc can be explained by the presence of two mantle components: a MORB source observed in the back-arc basins of the Philippine Sea Plate and a Pacific MORB-like source. An internally consistent model which explains along- and across-arc isotopic trends can be obtained by assuming mixing between the two mantle sources, but a lesser contribution of Pacific MORB-type source in the northern part of the arc. The source mantle of the rifting-related volcanism is distinct from the back-arc seamount chain volcanism and has a much stronger signature of Pacific MORB. A strong correlation between Sr isotopes and fluid-mobile element enrichment in the volcanic front lavas imply a significant contribution of slab-derived fluid to the source of volcanic front. In contrast, back-arc seamounts and rifting-related volcanism show a more limited influence of a fluid phase contribution to their source. Instead, high Δ7/4 and low 143Nd/ 144Nd associated with high Th/Ce imply that subducted bulk sediment is an important component in the back-arc. By assuming the regional mantle end-members, the relative contribution of a subduction-related component can be estimated. The magmatism in the back-arc seamounts is estimated to have a 0.2–0.3% bulk sediment addition to the source for both the northern and central parts of the arc. Minimal addition of fluid from altered oceanic crust is also predicted. For the rifting-related magmatism, a lower sediment contribution is predicted. Volcanic front magmatism is compatible with having no direct bulk sediment input, but is likely to involve the contribution of 2–2.5% of a fluid derived from altered oceanic crust and sediment in a mixing ratio of about 99:1.

Age, geochemistry, and tectonic significance of Neoproterozoic alkaline granitoids in the northwestern margin of the Gyeonggi massif, South Korea

Alkaline meta-granitoids, ranging in composition from syenite to alkali granite, occur in the northwestern Gyeonggi massif. Ion microprobe U–Pb zircon analyses indicate that the granitoids were emplaced at 742±13 Ma, and are corroborated by a Rb–Sr whole rock age of 770 ± 40 Ma. Major and trace element characteristics, together with Sr and Nd isotopic data, suggest that the granitoid magma was derived from ancient (TDM = 2.6–2.2 Ga) continental crust with addition of juvenile mantle-derived basaltic magma. The generation of the alkaline granitoid is attributed to crustal thinning induced by deep-seated thermal activity such as mantle upwelling or mafic magma influx. Alkaline igneous activity at 742 Ma is coeval with Neoproterozoic rift-related magmatism prevalent in South Korea and the South China Block but lacking in the North China Block. Thus, we suggest that the Gyeonggi massif is correlative with the South China Block and has experienced a rifting event during Rodinia breakup. © 2002 Elsevier Science B.V. All rights reserved.

Reavaliação da evolução geológica em terrenos pré-cambrianos brasileiros com base em novos dados U-Pb SHRIMP, parte II: Orógeno Araçuaí, Cinturão Mineiro e Cráton São Francisco Meridional

This paper discusses new zircon SHRIMP (Sensitive High Resolution Ion Microprobe) U-Pb geochronological data for 19 key-exposures of several geological units exposed at the eastern border of the Southern Sao Francisco Craton and at the adjacent Proterozoic Mineiro and Aracuai orogens. Samples were collected along several E-W transects, aiming at tracing the precise limit of the Sao Francisco Craton Archean basement, as well as assessing the extension of the successive proterozoic orogenic collages. Due to the complex geologic history and/or high grade metamorphism which most of the rock units investigated have undergone, zircon morphology and the U-Pb analytical data exhibit very complex patterns. These are characterized by a combination of inheritance, partial resetting and new zircon growth during high-grade metamorphism. As a consequence, very careful and detailed analyses of cathodoluminescence imagery were required to allow distinction between inheritance, newly melt-precipitated zircon and partially reset zircons, as well as between the ages of magmatic and metamorphic events. In the southeastern border of the craton 5 units yielded Archean crystallization ages ranging from ca . 3000-2700 Ma, with poorly constrained metamorphic ages ranging from ca . 2850 to 550 Ma. The TTG gneissic complex exposed to the east and south of the Quadrilatero Ferrifero, formerly ascribed to the Archean basement, have crystallization ages from ca . 2210 Ma to 2050 Ma, and can now be interpreted as representing pre- to syn-collisional magmatic phases of the Mineiro Belt. Metamorphic ages of ca . 2100 Ma and 560 Ma are also well constrained in zircon populations from these gneisses. The crystallization age of ca 1740 Ma observed for an alkaline granite of the Borrachudos Suite (intrusive into the Archean basement east of the Southern Espinhaco Range) confirmed previous conventional U-Pb data for this Paleoproterozoic rift-related magmatism. One of the major basement inliers within the Aracuai Orogen, the Caparao charnockitic granulite, revealed magmatic crystallization age of ca . 2195 Ma. Zircons from an amphibolite of the Pocrane Complex, also a basement inlier within the Aracuai Orogen, yielded an age of ca . 1500 Ma. Both inliers furnished well-constrained neoproterozoic metamorphic overprint at ca . 590 Ma. The subalkaline gneissic granite from the Salto da Divisa Suite yielded a magmatic crystallization age of ca . 870 Ma, close to U-Pb ages of the continental rift magmatism found in the African counterpart of the Aracuai Orogen. Four gneissic granites from the Aracuai Orogen (three of them previously interpreted as part of the Archean/Paleoproterozoic basement) were dated in the range of ca . 560-570 Ma, and were correlated to the g2 2 syn-collisional magmatism. Borders of some zircon grains from the older ( ca. 570 Ma) dated unit (Nanuque Granite) yielded ages around 500 Ma, similar to U-Pb ages obtained from titanite, monazite and zircon in other localities from the Aracuai Orogen. This youngest ages could be related to heating effects caused by melting of country rocks and ascent of magmas formed during the collapse of the orogen.

Relationship of Epithermal Gold Deposits to Large-Scale Fractures in Northern Nevada

Geophysical maps of northern Nevada reveal at least three and possibly six large-scale arcuate features, one of which corresponds to the northern Nevada rift that possibly extends more than 1,000 km from the OregonIdaho border to southern Nevada. These features may reflect deep discontinuities within the earth’s crust, possibly related to the impact of the Yellowstone hot spot. Because mid-Miocene epithermal gold deposits have been shown to correlate with the northern Nevada rift, we investigate the association of other epithermal gold deposits to other similar arcuate features in northern Nevada. Mid-Miocene and younger epithermal goldsilver deposits also occur along two prominent aeromagnetic anomalies west of the northern Nevada rift. Here, we speculate that mid-Miocene deposits formed along deep fractures in association with mid-Miocene riftrelated magmatism and that younger deposits preferentially followed these preexisting features. Statistical analysis of the proximity of epithermal gold deposits to these features suggests that epithermal gold deposits in northern Nevada are spatially associated with large-scale crustal features interpreted from geophysical data.