Rift Magmatism Research Articles

Central Afar: An analogue for oceanic plateau development

The structure, composition, and evolution of oceanic plateaus are poorly understood and strongly debated. Here, we compared the magmatic history and crustal structure of Afar with the Greenland−Iceland−Faroe Ridge and other oceanic plateaus. Key similarities indicate that Central Afar represents the early stage of development of a specific type of oceanic plateau: a rifted oceanic magmatic plateau (ROMP). These features begin their formation before continental rifting and develop into wide magmatic rift systems capable of isolating slivers of continental crust within the new igneous crust. Importantly, the anomalous magmatism continues through breakup and for several tens of millions of years afterward. The recognition of Central Afar as a precursor of this type of oceanic plateau allows us to better understand their formation. Increased melt production causes early and voluminous magmatism, ultrathick igneous crust, and repeated reorganization of the extension locus during rift/ridge jumps, which delay the onset of oceanization and Penrose-style crustal production. These factors differentiate ROMPs from many magma-rich rifted continental margins and from other types of oceanic plateaus, highlighting that Central Afar and other ROMPs should neither be considered as conventional magma-rich margins nor be considered as normal oceanic crust.

Intraplate alkali basalts related to end-Cretaceous Deccan magmatism: implications for tectonomagmatic processes

This study aims to investigate the earliest imprint of Deccan rift magmatism as preserved in alkali basalts from the northwestern Indian shield. The alkali basalts are petrographically classified as nephelinites and basanites. They are silica undersaturated and their high Mg numbers, CaO/Al 2 O 3 , Cr and Ni indicate their primitive character. Geochemically, they are similar to global ocean island basalts; their bulk rock trace elements and Sr–Nd–Hf isotopic signatures suggest their derivation from a garnet-bearing peridotite field. However, their elevated values of Sr/Sm, Sm/Hf, Zr/Hf, Nb/Ta; positive Ba, Sr and negative Zr, Hf spikes suggest that the magma source represents a mixture of garnet peridotites and carbonated melts. Estimated primary melt compositions closely follow the trajectory defined by the high-pressure experimental partial melting trend of a low-carbonated peridotite source. The melting environment approximates to a high mantle potential temperature. A low 87 Sr/ 86 Sr ratio and a negative correlation between 176 Hf/ 177 Hf and 143 Nd/ 144 Nd of the alkali basalts suggest that the mantle source is a mixture of a depleted Indian MORB-type mantle and an enriched mantle type 2. We correlate this event with the melting of the leading edge of the Réunion plume head during Gondwana break-up in a relatively short span of the Cretaceous/Paleogene boundary.

Simultaneous rift-scale inflation of a deep crustal sill network in Afar, East Africa

Decades of studies at divergent plate margins have revealed networks of magmatic sills at the crust-mantle boundary. However, a lack of direct observations of deep magma motion limits our understanding of magma inflow from the mantle into the lower crust and the mechanism of sill formation. Here, satellite geodesy reveals rift-scale deformation caused by magma inflow in the deep crust in the Afar rift (East Africa). Simultaneous inflation of four sills, laterally separated by 10s of km and at depths ranging 9–28 km, caused uplift across a ~ 100-km-wide zone, suggesting the sills are linked to a common mantle source. Our results show the supply of magma into the lower crust is temporally episodic, occurring across a network of sills. This process reflects inherent instability of melt migration through porous mantle flow and may be the fundamental process that builds the thick igneous crust beneath magmatic rifts and rifted margins globally.

End-Mesoproterozoic (ca. 1.08 Ga) epeiric seaway of the Nonesuch Formation, Wisconsin and Michigan, USA

Abstract The Nonesuch Formation and related sedimentary units of the Oronto Group, southern Lake Superior region, midwestern United States, are commonly held to have been deposited in a lacustrine rift basin within interior continental Laurentia. Here, we present new sedimentologic and stratigraphic evidence that shows a marine influence on deposition. Tidally influenced shallow-marine sandstone and evaporitic, sandy and muddy tidal flat facies pass upward into fine-grained estuarine and sandy turbidite deposits, which are sharply overlain by mixed sandy and muddy tidal flat and floodplain deposits. These observations are evidence that the lower Oronto Group was deposited in an epeiric seaway, one of several such seaways that developed during the final amalgamation of Rodinia at a time of globally high sea level. Retrogradational-aggradational-progradational-degradational stratal architecture records changes in the relative balance between generation of accommodation space and sedimentation rates, which we interpret to reflect the combined influence of Grenvillian Ottawan phase tectonic subsidence and thermal subsidence from earlier Midcontinent Rift magmatism. We use this revised stratigraphic framework to show that the geochemical proxies of the Nonesuch Formation are tied closely to sedimentary facies and reflect intrabasinal redox heterogeneity rather than global anoxia at the end-Mesoproterozoic. Further, our sedimentology shows that the microfossils recovered from the Nonesuch rocks are primarily associated with tidal flat facies. The combined influence of marine and local nonmarine conditions must be considered when invoking the Nonesuch Formation, or similar marine-influenced interior basin deposits, as global analogues.

Early Eocene Arctic volcanism from carbonate-metasomatized mantle

Melilitite, nephelinite, basanite, and alkali basalt, along with phonolite differentiates, form the Freemans Cove Complex (FCC) in the south-eastern extremity of Bathurst Island (Nunavut, Canada). New 40Ar/39Ar chronology indicates their emplacement between ~ 56 and ~ 54 million years ago within a localized extensional structure. Melilitites and nephelinites, along with phonolite differentiates, likely relate to the beginning and end phases of extension, whereas alkali basalts were emplaced during a main extensional episode at ~ 55 Ma. The melilitites, nephelinites, and alkali basalts show no strong evidence for significant assimilation of crust, in contrast to some phonolites. Partial melting occurred within both the garnet- and spinel-facies mantle and sampled sources with He, O, Nd, Hf, and Os isotope characteristics indicative of peridotite with two distinct components. The first, expressed in higher degree partial melts, represents a relatively depleted component (“A”; 3He/4He ~ 8 RA, εNdi ~ + 3 εHfi ~ + 7, γOsi ~ 0). The second was an enriched component (“B” 3He/4He < 3 RA, εNdi < – 1 εHfi < + 3, γOsi > + 70) sampled by the lowest degree partial melts and represents carbonate-metasomatized peridotite. Magmatism in the FCC shows that rifting extended from the Labrador Sea to Bathurst Island and reached a zenith at ~ 55 Ma, during the Eurekan orogeny. The incompatible trace-element abundances and isotopic signatures of FCC rocks indicate melt generation occurred at the base of relatively thin lithosphere at the margin of a thick craton, with no mantle plume influence. FCC melt compositions are distinct from other continental rift magmatic provinces worldwide, and their metasomatized mantle source was plausibly formed synchronously with emplacement of Cretaceous kimberlites. The FCC illustrates that the range of isotopic compositions preserved in continental rift magmas are likely to be dominated by temporal changes in the extent of partial melting, as well as by the timing and degree of metasomatism recorded in the underlying continental lithosphere.

The Charnwood Terrane revisited: an integrated petrogenetic and petrophysical model for crustal structure in southern Britain

We based an integrated petrological and petrophysical model for the Ediacaran crust of southern Britain on a review of the c. 570–550 Ma Charnian volcano-sedimentary complex. The latter was emplaced in a magmatic rift wedge within the juvenile continental crust of the c. 720–600 Ma Marches Terrane, a subduction magmatic domain formed at the margin of the Gondwana palaeocontinent. The Charnian magmatic arc is characterized by primitive island arc tholeiite to more evolved calc-alkaline compositions. The inversion of aeromagnetic potential field data and petrophysical modelling reveals details of the internal structure of the Charnian Domain, including a median rift, superimposed annular structures and partitioning lineaments. The modelling suggests that the arc foundation could incorporate magnetite-rich cumulates, which may explain the anomalous geophysical properties, including crustal thickness, rigidity and buoyancy. There is no evidence for significant tectonic displacement between the Charnian Domain and its Marches Terrane host. Instead, the domain likely occupies a wedge-shaped arc/marginal rift basin complex, propagated from a neighbouring ocean into the Gondwana margin. Contemporaneous volcanic rift successions in the Welsh Borderland and Wales of the 570–550 Ma Charnian magmatic phase developed in coeval ensialic rifts within less strongly extended Marches Terrane lithosphere. A comparable diversity of subduction-related magmatism is found in the Neogene–recent Hikurangi destructive margin of New Zealand, providing a plausible analogue for Charnian magmatism. Supplementary material: Supplementary Publication 1 (borehole geophysical log correlation; petrophysical data table; unannotated geophysical maps and sections) and Supplementary Publication 2 (geochemistry analytical conditions and data table) are available at https://doi.org/10.6084/m9.figshare.c.6805248

Subsurface structural control of geothermal resources in a magmatic rift: gravity and magnetic study of the Tulu Moye geothermal prospect, Main Ethiopian Rift

Since the Quaternary, extension and magmatism in the Main Ethiopian Rift (MER) have been mainly focused into narrow magmatic segments that have numerous volcanic centers and caldera collapses that offer favorable conditions for the occurrence of geothermal resources. However, the subsurface structure of the volcanic systems (0–10 km) and their link to the distribution of shallow geothermal resources remain unclear. To investigate the role of subsurface structures on the occurrence of these resources, we conducted gravity and magnetic studies combined with geological constraints within the Tulu Moye Geothermal Prospect (TMGP), one of the current geothermal prospects in the central MER associated with caldera collapses. Gravity data from the Global Gravity Model plus (GGMplus 2013) and ground magnetic data transformed into residual and derivative maps reveal that shallow magmatic intrusions occur under the volcanic centers (Tulu Moye, Bora, and Bericha). Our interpretation along with recent magnetotelluric model suggests that only the intrusion beneath Tulu Moye is currently magmatically active and includes partial melt, consistent with it being a primary heat source for the geothermal system. A new caldera formation model is proposed where the TMGP hosts an older large caldera (about 25 km diameter) within which there are several smaller nested caldera systems associated with the Bora, Bericha, and Tulu Moye volcanoes. Along with existing geologic, seismic, and magnetotelluric studies, our gravity and magnetic analysis indicate the interaction between NNE-SSW (rift-parallel) and NW-SE (cross-rift) trending faults, along with shallow magmatic intrusions and caldera systems, suggesting that such a large geothermal system is possible under these conditions.

Focused Mid‐Crustal Magma Intrusion During Continental Break‐Up in Ethiopia

AbstractSignificant volumes of magma can be intruded into the crust during continental break‐up, influencing rift evolution by altering the thermo‐mechanical structure of the crust and its response to extensional stresses. Rift magmas additionally feed surface volcanic activity and can be globally significant sources of tectonic CO2 emissions. Understanding how magmatism may affect rift development requires knowledge on magma intrusion depths in the crust. Here, using data from olivine‐hosted melt inclusions, we investigate magma dynamics for basaltic intrusions in the Main Ethiopian Rift (MER). We find evidence for a spatially focused zone of magma intrusion at the MER upper‐lower crustal boundary (10–15 km depth), consistent with geophysical datasets. We propose that ascending melts in the MER are intruded over this depth range as discrete sills, likely creating a mechanically weak mid‐crustal layer. Our results have important implications for how magma addition can influence crustal rheology in a maturing continental rift.

The Magmatic Rifting of Santos Basin: Aeromagnetic Mapping of Dykes, Terranes and Marginal Structures and the Interplay Between Tectonism and Volcanism

AbstractWe present a new magnetic map integrating continental and oceanic features at the Santos Basin, in order to investigate the connection between onshore and offshore tectonics and magmatism at the early stages of rifting. The magnetic and seismic data evidence the offshore continuity of the Florianópolis and Serra do Mar dyke swarms, while the Ponta Grossa Dyke Swarm is interrupted by the marginal structures. This structural relationship brings to evidence a different timing of formation of the Ponta Grossa Dyke Swarm relative to the early opening of the margin. A very high amplitude magnetic lineament ‐ the Santos Marginal Anomaly (SAMA) reaches up to 1,500 nT and trends NE‐SW, with local E‐W inflections forming the Cananéia and Guaratiba Arcuate anomalies. Our modeling suggests that the SAMA can be associated with dykes and syn‐rift extrusive rocks linked at depth to larger magmatic bodies at the proximal and necking domains. Based on the increasing width and amplitude of the magnetic anomalies oceanwards, we propose a mechanism whereby the igneous accretion (i.e., the amount of magmatism) augmented as rifting evolved from continent to ocean, creating a volcanic rift axis. This rift was segmented by crustal discontinuities that probably nucleated on preexisting rheological contrasts, forming tectonomagmatic segments during the early extensional stages. The onshore‐offshore relationships highlight that where there was an agreement between basement fabric and dyke orientation, inheritance may have influenced their emplacement, although it was not a limiting factor for their formation. The regional geodynamics of the Gondwana opening was the first order control on the emplacements of the dyke swarms, with deviations of the magmatically influenced rift by tectonic inheritance.

The last Neoproterozoic rift magmatism on the western margin of Yangtze block, South China: New insights of Marinoan onset from low-δ18O magmatic events

The geological record preserves evidence of “Snowball Earth” glaciations during the Cryogenian Period, the Sturtian onset at ca. 720 Ma and the Marinoan onset at ca. 650 Ma. Compared to the Sturtian, the trigger for the Marinoan is still unclear due to the absence of large igneous provinces (LIPs). This study first discovered low-δ18O Luoluobuzi (LLBZ) granite which emplaced on the eve of the Marinoan at the Yangtze Block in South China. The granite with U-Pb age of 651.2 ± 2.1 Ma and has εHf (t) values between + 6.8 to + 13.3, and low-δ18O values between + 2.74 to + 5.34 ‰. Granite samples show Ba, Nb, Ta, Sr, Eu, and Ti depletion and enrichment in Rb, U, Th, and Pb, modeling calculations to suggest they originated from partial melting of nearby Neoproterozoic mafic rocks and formed in a continental rift. We systematically compiled data and found an interglacial period between the Sturtian-Marinoan glaciations is absent of magmatic activities. Extensional magmatic activity similar to the LLBZ granite started at ca. 651 Ma, which within errorsoverlapped the onset of the Marinoan. Such temporal ruling also existed in other worldwide rift-related magmatic activities. After combining chemical index of alteration (CIA) and palaeomagnetic results, we found the last episode of Rodinia breakup played an important role in the onset of the Marinoan.

The development of multiple phases of superposed rifting in the Turkana Depression, East Africa: Evidence from receiver functions

The Turkana Depression in Eastern Africa separates the elevated plateaus of East Africa to the south and Ethiopia-Yemen to the north. It remains unclear whether the Depression lacks dynamic mantle support, or if the entire East Africa region is dynamically supported and the Depression compensated isostatically by thinned crust. Also poorly understood is how Miocene-Recent extension has developed across the Depression, connecting spatially separated magmatic rift zones in Ethiopia and Kenya. Receiver function analysis is used to constrain Moho depth and bulk-crustal VP/VS ratio below new seismograph networks in the Depression, and on the northern Tanzania craton. Crustal thickness is ∼40km below northern Uganda and 30–35km below southern Ethiopia, but 20–30km below most of the Depression, where mass-balance calculations reveal low elevations can be explained adequately by crustal thinning alone. Despite the fact that magmatism has occurred for 45Ma across the Depression, more than 15Ma before East African Rift (EAR) extension initiated, bulk crustal VP/VS across southern Ethiopia and the Turkana Depression (∼1.74) is similar to that observed in areas unaffected by Cenozoic rifting and magmatism. Evidence for voluminous lower crustal intrusions and/or melt, widespread below the Ethiopian rift and Ethiopian plateau to the north, is therefore lacking. These observations, when reviewed in light of high stretching factors (β≤2.11), suggest Cenozoic extension has been dominated until recently by faulting and plate stretching, rather than magma intrusion, which is likely an incipient process, operating directly below seismically-active Lake Turkana. Early-stage EAR basins to the west of Lake Turkana, with associated stretching factors of β≈2, formed in crust only moderately thinned during earlier rifting episodes. Conversely, ∼23km-thick crust beneath the Kino Sogo Fault Belt (KSFB) has small offset faults and thin sedimentary strata, suggesting almost all of the observed stretching occurred in Mesozoic times. Despite the KSFB marking the shortest path between focused extensional zones to the north and south, seismicity and GPS data show that modern extension is localized below Lake Turkana to the west. Failed Mesozoic rift zones, now characterized by thinned crust and relatively refractory mantle lithosphere, are being circumnavigated, not exploited by EAR rifting.

Time-space variations in the East African Rift magmatism: the role of different mantle domains

The East African Rift System (EARS) is the classic example of an active continental rift where extensional tectonics and lithospheric thinning have been closely associated to the generation of large volumes of magmas and represents the environment with the largest range of erupted magma types all over the world. The geochemical signature of erupted magmas testifies the involvement of different mantle domains and depths (i.e., subcontinental lithosphere, asthenosphere and deeper mantle sources). Our aim is to investigate the variable contribution of different mantle domains in the genesis of the EARS magmas through space and time, considering not only the geochemical signature of erupted magmas but also the geochemical message of mantle xenoliths. The main goal is to provide a large-scale view of the common process driving the origin of magmas in the EARS beyond the local peculiarities linked to specific settings. To this aim, we screened an exhaustive geochemical database of basalts and mantle xenoliths from the EARS, and we report original trace element and Sr-Nd isotope data of new samples collected from the Main Ethiopian Rift and Turkana depression. The data were subdivided according to spatial and temporal criteria. From a spatial point of view, the samples were ascribed to five groups, namely: Afar, Ethiopia, Turkana, Eastern Branch, and Western Branch; from a temporal point of view, the magmatic activity of the EARS was subdivided into three main temporal intervals: 45-25 Ma, 25-10 Ma and 10-0 Ma. The geochemical and radiogenic isotope (Sr, Nd, Pb) signature of the selected basalts denotes the variable contributions of a mantle plume, a more depleted asthenospheric mantle (DMM), and different SubContinental Lithospheric Mantle (SCLM) domains, depending on their temporal and spatial distribution. The geochemistry of the selected basalts shows a marked correspondence with the compositional heterogeneity of mantle xenoliths, whose isotopic systematics (Sm-Nd, Re-Os) indicates the formation of the local SCLM in the Archean and during the Pan-African orogeny. Both SCLM domains contributed significantly to magma genesis in the Western Branch (whose signature points towards a contribution of the Pan-African lithosphere) and Eastern Branch (which is also affected by Archean SCLM domains) magmas. The contribution of the SCLM generally increases with time, possibly related to an increase of the geothermal gradient in response to the arrival and flattening of the plume head at the base of the lithosphere and later extension, thinning and shallower melting. Our interpretation supports a pivotal role of the different SCLM domains in magma genesis that is able to fully explain the large compositional heterogeneity of the EARS basalts and represents a reasonable alternative to the putative presence of multiple mantle plumes or a heterogeneous mantle upwelling.

A possible sea-level fall trigger for the youngest rejuvenated volcanism in Hawaiʻi

Many intraplate oceanic islands undergo “rejuvenated” volcanism following the main edifice-building stage. Honolulu features Hawaiʻi’s most recent rejuvenated volcanism. K-Ar dating of Honolulu volcanism suggests that it started at ca. 750 ka and ended at &amp;lt;100 ka. Here, we present new 40Ar/39Ar ages and olivine diffusion modeling from Koko Rift lavas to resolve when the most recent Honolulu eruptions occurred and to evaluate possible mechanisms of rejuvenated volcanism and volcanic hazards. Diffusion modeling of olivine zoning profiles in Koko Rift basalts suggests that magmas were stored in the crust for many months prior to eruption. Six new 40Ar/39Ar ages cluster at 67 ± 2 ka (2σ), which demonstrates that Koko Rift is Hawaiʻi’s youngest known area of rejuvenated volcanism. The timing of Koko Rift eruptions coincides with the pronounced drop in global sea level (∼100 m) during Marine Isotope Stage 4. This major sea-level fall may have triggered the eruptions of Koko Rift magmas that were stored in the crust for months to years at &amp;lt; 15 km depth. The proposed mechanism is similar to that at other volcanic islands, which suggests that changes in global sea level may have significant control on the magnitude and frequency of eruptions at ocean island volcanoes.

Oxidation and decomposition of stratiform SedEx sulfidic mineralization in the epidote-amphibolite facies producing cassiterite, V-rich micas, In-Sn-Ag-Sb-Pb-Bi-Zn-Fe-As-Cu-Ni-Co sulfides and Fe-Ca-Pb carbonates in situ (Bystrý potok locality, Gemeric unit, W. Carpathians)

The studied area of Bystrý potok locality (Gemeric unit, W. Carpathians) is built of lenses of lydites and limestones, being a part of the graphite bearing phyllites of Silurian Holec Beds. The Holec Beds represent the bedrock of the albite-quartz bearing keratophyre (trachyte sensu IUGS classif.) sequence with stratiform SedEx (sedimentary-exhalation) sulfidic mineralization in the Gemeric Lower Paleozoic stratigraphy. This SedEx mineralization was contemporaneous with Late Silurian–Devonian keratophyre/basalt magmatism and originated at 280–340 °C from the seafloor exhalates in Lower Paleozoic rift magmatism. The Silurian-Devonian rock sequences were metamorphosed in Permian. The limestone lenses were altered to Permian skarn in the epidote-amphibolite facies (526–546 °C, 3–6 kbar) and in related chlorite-apatite zone (420–540 °C) they consisted of garnet Grs41.4-60.2Sps19.8-32.8Alm16.7-22.2Adr0-8.5, hedenbergite Wo44.5-50.1Fs31.6-38.2En12.7-20.7, epidote, actinolite, fluorapatite, titanite, chlorite, ankerite and siderite. The disseminated original millerite was partially replaced by hauchecornite, hauchecornite-(Sb) and cobaltite was formed in the silicate matrix of the skarn. The Permian metamorphism of the epidote-amphibolite facies and the chlorite-apatite zone has also released a fluid phase rich in O2, H2O, CO2, H3PO4, H2S, HF and V from the organic matter-bearing rock (Holec Beds) into the overlying bed with keratophyres and primary stratiform SedEx sulfidic mineralization where V-rich micas, V-rich chlorite and schreyerite formed. The primary stratiform SedEx sulfidic mineralization composed predominantly of pyrite 1 less pyrrhotite, chalcopyrite, sphalerite 1, galena 1, arsenopyrite, ferrokesterite, stephanite, gudmundite, bismuthinite 1, PbBiSb-rich sulphide (A2B2S5-type) and kobellite has been oxidized and decomposed by this fluid phase to form a new metamorphic minerals in situ. The pyrrhotite decomposed to form szomolnokite, pyrite 2, goethite and siderite. The galena 1 was partly oxidized and decomposed to anglesite and cerussite. The In-rich ferrokesterite, Sn-rich sakuraiite and In-rich sphalerite 1 oxidized, decomposed, and reacted with the fluid phase to form cassiterite, chalcopyrite, In-rich sakuraiite and sphalerite 2. The PbSbBi-rich sulfide (A2B3X6-type), wittite, bismuthinite 2, native Bi and Se-rich galena 2 are new minerals, formed by the decomposition of original PbBiSb-rich sufhide (A2B2X5-type), kobellite and bismuthinite 1. Tetrahedrites 1–3 ranging from tetrahedrite-(Fe), kenoargentotetrahedrite-(Fe) to rozhdestvenskayaite-(Fe) show a gradual ordering in three separate zones controlled by immiscibility gaps in the Cu-Ag substitution of tetrahedrite group. These tetrahedrites were formed by the decomposition of the original SedEx ferrokesterite, stephanite and gudmundite. The gudmundite was also commonly oxidized to form valentinite and the decomposition of ferrokesterite, galena 1 and gudmundite also produced bournonite and plumosite-like. Altogether 22 reactions illustrate the relationship between the source SedEx mineralization and the new metamorphic minerals, which have formed at the expense of the original stratiform SedEx sulfidic mineralization in situ. The metamorphogenic fluid phase, released from the Holec Beds, has been enriched with additional elements from the stratigraphic horizon of the stratiform SedEx sulfidic mineralization and is genetically part of the Permian metamorphic-magmatic-hydrothermal (MMH) cycle (281–256 Ma).

The Zircon Isotope Dating of the Jurassic Basalts from the Ghoithsk Volcanic Area of the Western Greater Caucasus (Russia)

The paper presents the results of the U-Th-Pb isotope system study of the accessory zircon from the basalts of the Ghoithsk volcanic region (GVO) of the Western Caucasus. The sample for isotope dating was taken from basalt porphyrites of the Chataltapa volcanic complex, in the Tuapse River basin. It was shown by using of the ion microprobe isotope dating method of the zircon, that the effusion of basalts of the Chataltapa volcanic complex of the GVO occurred in the Jurassic at the boundary of the Aalenian and Bajocian age (169 Ma), during the tectonic transformation of the Greater Caucasus riftogenic basin. A low Th/U ratio was obtained for zircons from basalts, which is more typical for rocks of acidic composition. This fact indirectly confirms that the evolution of the Jurassic rift magmatism in the Caucasus occurred due to the assimilation of the continental type crust.

A Jurassic volcanic passive margin in Iran and Turkey

AbstractBroadly similar Early to Middle Jurassic stratigraphic sequences including bimodal igneous rocks of the Sanandaj–Sirjan Zone of Iran and the Sakarya Zone of Turkey suggest that these formed in a common tectonic setting in an extensional basin that evolved from a terrestrial magmatic rift to a marine shelf and passive continental margin. Whole‐rock chemistry and Sr–Nd isotope signatures indicate derivation of mafic melts from partial melting of the subcontinental lithosphere. Decompression associated with extension led to 5%–30% partial melting of spinel–garnet lherzolite with minor involvement of continental crust, producing tholeiitic to transitional basaltic magma. Extensional basins inverted during the Mid‐Late Jurassic. These relationships suggest the Early to Middle Jurassic formation of a volcanic rifted margin on the SW Eurasian margin, similar to that of offshore Norway.

Late Jurassic syn-rift deposition in the Flemish Pass basin, offshore Newfoundland: Evidence for Tithonian magmatism and Appalachian-Variscan sediment sources from quantitative mineral and detrital zircon U–Pb-Hf isotope studies of Mizzen discovery strata

Quantitative mineral and detrital zircon U–Pb-Hf isotope studies of syn-rift sandstone units in the Mizzen F-09 delineation well, northern Flemish Pass basin, were conducted to test competing models for Grand Banks paleogeography and Late Jurassic development of the Newfoundland-Iberia magma-poor rift system. Bodhrán formation (informal nomenclature) strata yield ca. 145–155 Ma detrital zircon grain populations that indicate a late Tithonian (146 ± 1 Ma) maximum depositional age for syn-rift fluvial to tide-modulated fluvial strata in Mizzen F-09 and evidence for Grand Banks rift magmatism to have occurred throughout the Late Jurassic. Late Tithonian tectonic subsidence and magmatism in the northern Flemish Pass basin region were coincident with the onset of necking processes and extreme crustal thinning within the Newfoundland-Iberia rift system. The chondritic to superchondritic Hf isotope compositions of Late Jurassic detrital zircon grains indicate repeated partial melting of lithospheric mantle during rift development, perhaps along master detachment faults or transfer zone systems. Bodhrán formation strata have chemically unstable minerals (plagioclase, potassium feldspar, garnet, muscovite, rutile, staurolite) and late Paleozoic to Paleoarchean detrital zircon age populations that together show provenance from Appalachian-Variscan basement and cover assemblages. A positive correlation between the abundance of metamorphic minerals and unique-aged, mid-Permian (ca. 271–280 Ma) detrital zircon grains from post-Variscan igneous rocks indicate Iberian provenance contributions to Bodhrán formation strata. The new results from Mizzen F-09 are consistent with models for WNW-directed fluvial discharge into the northern Flemish Pass basin and sediment derivation from upper Paleozoic foreland basin or cover assemblages located west of Variscan front, some which may have been stripped off Beothuk Knoll, Flemish Cap, or adjacent highlands.

The Interconnected Magmatic Plumbing System of the Natron Rift

AbstractUnderstanding the magmatic plumbing system of rift volcanoes is essential when examining the interplay between magmatic and tectonic forces. Recent seismicity, volcanic activity, magma emplacement, and volatile release make the Natron basin the ideal location to study these processes in the East African Rift System. Here, we present the first high‐resolution tomographic imaging of Oldoinyo Lengai volcano and surrounding volcanic systems using attenuation mapping. High scattering and absorption features reveal fluid‐filled fracture networks below regions of magmatic volatile release at the surface and a close spatial association between carbonatite volcanism and deeply penetrating, fluid‐filled faults. High‐absorption features appear sensitive to fluids and thermal gradients, revealing a central sill complex and connected plumbing system down to the mid‐crust, which links volcanoes and rift segments across the developing magmatic rift.

North America's Midcontinent Rift magma volume: A coincidental rendezvous of a plume with a rift

Abstract The Midcontinent Rift of North America is a ca. 1.1 Ga, 3000-km-long failed rift that nearly split the Precambrian continent of Laurentia. Unlike most continental rifts, which are filled with a mixture of volcanic rocks and sediments, the Midcontinent Rift contains a large volume of flood basalts that were emplaced during both syn- and post-rift stages. Consequently, the Midcontinent Rift, which comprises the Keweenaw large igneous province, is the most significant positive anomaly on gravity maps of central North America. We investigated the mantle conditions required to produce this large volume of flood basalt and the observed two main stages of emplacement. To explore whether these magma volumes required a plume or, instead, could have resulted from the increased ambient mantle temperatures expected for the Neoproterozoic, we used a geodynamic model for a range of ambient mantle and plume temperatures under different scenarios of lithospheric extension. The most favorable scenario for the generation of both syn-rift and post-rift lavas combines a plume with excess temperatures between 175 and 225 °C introduced during the syn-rift phase and ambient mantle potential temperatures between 1393 and 1443 °C, with an initial lithospheric thickness not exceeding 150 km for 3 mm/yr extension rates.

Cambrian rift magmatism recorded in subvolcanic sills of the Ediacaran-Cambrian La Ciénega Formation, NW Mexico

Mafic-ultramafic subvolcanic metabasalt sills intrude Ediacaran and Cambrian strata of the La Ciénega Formation of Sonora, Mexico. Sills are typically 1–7.5 m thick, with minor associated thin (∼10 cm thick) intrusions. Sills are typically bed parallel with upper and/or lower tempered (quenched) borders and porphyritic textures characterized by abundant olivine and pyroxene pseudomorphs that have been transformed to clay and carbonate minerals. Metamorphic halos in adjacent dolostones and quartzites occur at the bottom and top contacts, and metabasalt sills contain carbonate veins typical of hydrothermal alteration and mobilization. Unlike surface flows of the overlying Cerro Rajón Formation, all sills in the La Ciénega Formation lack evidence of crystal settling, yet evidence for assimilation of country rocks is present. Assimilation is more typical of sills rather than flows and is diagnostic of sills in the El Arpa Formation, a stratigraphically lower unit in the succession. Actinolite-chlorite-albite metamorphic mineral paragenesis suggests low-grade greenschist facies metamorphism for these metabasaltic rocks. Although their primary mineralogy is obscured due to alteration, olivine and clinopyroxene pseudomorphs are present and their major and trace element geochemistry suggest they were derived from alkaline basaltic magmas with low (normalized) SiO2N (22.7–43.1 wt%), high MgON (7.25–16.6 wt%), and high TiO2N (4.04–6.27 wt%). Immobile trace elements suggest these sills were originally alkaline rift-related basalts with OIB-type signatures. Tectonic discrimination diagrams suggest continental rift magmatism with low geochemical evolution ratios and relatively low mantle melting rates when compared to contemporaneous rift related magmatism. Sr-Hf-Nd-Pb isotopes suggest the metabasalts originated from an Enriched Mantle I (EMI) reservoir and melting occurred during adiabatic decompression. These geochemical characteristics are remarkably similar to volcanic rocks of the Cerro Rajón Formation. The inferred magmatic composition, mantle melting rates, petrography and field relations of the La Ciénega Formation metabasalt are consistent with a low viscosity feeder system like that envisaged to have sourced volcanic rocks of the overlying Cambrian (Terreneuvian) Cerro Rajón Formation. Together, evidence from these two volcanic units is consistent with a Cambrian continental rift event close to the onset of the Sauk transgression.