Young Basalts Research Articles

Chemistry of deep (3500–5600 m) Pacific MORB—Why is the Pacific anomalous?

In contrast with the Atlantic and Indian Oceans, the Pacific shows the global correlations of axial depth and basalt chemistry only weakly or not at all for unaveraged data. In the past, this observation has been partly explained by the limited depth window of available Pacific samples. We report on new, very young basalt samples collected by submersible from Pacific ridges at ∼3500–5600 m, nearly doubling the depth interval of Pacific samples. These samples, from the Pito Deep, Hess Deep, and the Garrett transform, do not conform to the global correlation of depth vs. chemistry and show the local trend of chemical variation. The Pacific as a whole is thus unusual compared to the Atlantic and Indian Oceans. We propose that the anomalous behavior of the Pacific is due mostly to its relatively rapid spreading rate and tectonic shifts leading to an absence of a dynamic equilibrium between mantle temperature and seafloor depth observed elsewhere.

Late Quaternary rhyolitic eruptions from the Acigöl Complex, central Turkey

Two major rhyolitic tephra units (Lower and Upper Acigöl Tuffs) have been erupted from the Acigöl Complex. The Lower Acigöl Tuff (>13km 3 uncompacted volume) was erupted more than c. 180 ka ago, forming an extensive Plinian pumice-fall deposit with associated ashfall and ignimbrite. Well-developed ignimbrite veneer facies are preserved where the pyroclastic flows passed over hilly topography. Following the eruption, a thick rhyolite coulée (Bogazköy obsidian) was extruded. The Upper Acigöl Tuff eruption (age 70 < t< 150 ka) generated a second major Plinian deposit, an ignimbrite preserved up to 24 km from the vent, and an 80 m thick proximal succession of surge and fallout deposits; it resulted in collapse of the 6×5 km Acigöl caldera. The two major tuffs are chemically similar but can be distinguished on the basis of lithic populations. Subsequent rhyolitic activity formed the c. 0.25 km 3 dome of Kocadag Tepe (c.70ka) and five dome/maar clusters (c. 20 ka). Late Quaternary and/or Holocene basalts and andesites surround the Acigöl caldera, which developed in a zone of sinistral shear associated with approximate N-S compression. Vent distribution patterns, the eruption of young basalts, and the crystal-poor nature of the c. 20 ka rhyolites are consistent with basaltic stoking of a sizeable silicic magma chamber beneath the Acigöl Complex.

Partial melting of melt metasomatized subcontinental mantle and the magma source potential of the lower lithosphere

Decompression melting of peridotitic mantle is unlikely to produce the large volumes of magma erupted during the early stages of extension in continental extensional provinces such as the Great Basin of western North America. First, a substantial amount of extension is required before asthenospheric mantle ascends to depths shallow enough to begin melting, even if it is unusually hot. The lithospheric mantle is too cold to undergo melting, even after large amounts of extension. Second, decompression melting of peridotite produces progressively greater amounts of magma during extension; in the Great Basin silicic magmatism was voluminous during the early stages of extension but diminished in volume as extension progressed. Comparison of empirical melting relations with modeled pressure and temperature conditions in the lithosphere during extension indicates that large volumes of magma may be produced by partial melting of melt metasomatized subcontinental mantle (SCM) during the earliest stages of extension. Melt‐metasomatized SCM is expected to contain components less mafic than peridotite, emplaced during older magmatic episodes. Mafic components within the SCM will begin to melt during the earliest stages of extension if the base of the lithosphere is ∼1300°C or warmer and at depths greater than 75 km. Interaction of these melts with crustal rocks, fractionation processes, and crustal anatexis driven by the heat contained in the ascending mantle melts can produce the silicic to intermediate compositions observed in Great Basin Middle Tertiary magmas. A suite of models comparing different parameters show that the potential for the lower portions of the SCM to produce melt during the early stages of extension depends most strongly on the pre‐extension thickness of the lithosphere. If the lithosphere is thicker than about 150 km, the base of the SCM will lie well within the subsolidus field for mafic rocks and a substantial amount of extension is required before it begins to melt. At pressures less than about 2.0 GPa, ascent paths in the SCM are nearly parallel to the basalt solidus, and so little melting occurs if the lithosphere is thinner than about 60 km. Optimal conditions for producing large volumes of melt during the early stages of extension require an initial lithosphere ∼100–150 km thick. Under these conditions the lower 25 km of the SCM may undergo partial melting during extension. Predicted magma flux rates match the space‐time pattern of Middle Tertiary silicic magmatism in the Great Basin; the model predicts a high rate of melt production at the onset of extension which peaks within the first 10–20 m.y. and then decreases rapidly with continuing extension. Melt production within the SCM is consistent with trace element and isotope characteristics of mid‐Miocene and younger basalts in the Great Basin, which suggest a transition at 5 Ma from old, geochemically evolved magma sources within the lithosphere to later asthenospheric sources. The later asthenospheric melts are compatible with decompression melting of peridotitic mantle after ≥50% extension.

Spatial migration and compositional changes of Miocene‐Quaternary magmatism in the Western Grand Canyon

Lithospheric extension appears to be responsible for erosional and tectonic evolution of the western margin of the Colorado Plateau. Late Cenozoic (20 Ma to 10,000 years) basaltic flows, scoria cones, necks, dikes, and sills exposed in the Western Grand Canyon region offer a unique opportunity to place age constraints on the landscape evolution. New K‐Ar ages show that these basaltic rocks become progressively younger to the northeast from the Hualapai and Shivwits Plateaus onto the Uinkaret Plateau, a distance of about 100 km. Magmatism appears to have migrated at a rate of about 0.45 cm/yr until sometime between 13.5 and 9.2 Ma, when the rate changed to about 1.15 cm/yr. Recent regional geologic mapping indicates that extensional faulting, with ubiquitous downdrop to the west, accompanied this magmatism; the Hurricane Fault offsets lavas as young as 0.29 Ma. Young fault scarps in sedimentary rocks, and historic seismic activity along these faults, indicate that the region is still undergoing faulting. The basalts form three relatively distinct groups based on their age, chemistry, and geographic distribution: (1) 20–11 Ma, (2) 11–4.5 Ma, and (3) ≤4.5 Ma. The older basalts (>11 Ma) are petrographically and compositionally distinct from younger basalts; their olivine and augite are more Mg rich with lower TiO2 and higher Ni, Cr, and incompatible element contents. Group 1 basalts have more radiogenic Sr and less radiogenic Nd isotopic compositions than younger group 2 and 3 basalts. Overall, present‐day Sr isotopic compositions range from 0.70344 to 0.70505, 143Nd/144Nd from 0.51291 to 0.51227 (εNd = +5.4 to −7.1), and 206Pb/204Pb from 17.951 to 19.091. Group 1 basalts were probably derived from asthenospheric mantle melts that underwent crustal contamination. These basalts appear to have been generated originally from more primitive melts than groups 2 and 3, which were probably also derived from asthenospheric mantle‐derived magmas but underwent only minor crustal contaminations. Group 2 basalts appear to be transitional between groups 1 and 3. The intervals 13–9 Ma and 4.6–4.3 Ma temporally separate the three chemically distinct groups of basalts in the Western Grand Canyon. These periods correspond closely with major Miocene and Pliocene plate reorganizations in the western United States at about 12.5–10 Ma and 3.9–3.4 Ma. These changes in plate motion were reflected in changes in the rate of magmatism migration in the Western Grand Canyon. Recent movements on the Hurricane and Toroweap Faults appear directly related to the <4.5 Ma volcanism and are probably a result of recent plate reorganization.

Spatial and temporal relationships between mid‐Tertiary magmatism and extension in southwestern Arizona

Cenozoic magmatism in southwestern Arizona, which is within the Basin and Range tectonic province, occurred almost entirely between 15 and 25 Ma. Volcanic rocks typically consist, in ascending order, of (1) a thin sequence of mafic to intermediate lava flows, (2) voluminous felsic lava flows and pyroclastic rocks with minor to moderate amounts of intermediate to mafic lava flows, and (3) basalt and andesite. Volcanic rock sequences rest disconformably on pre‐Tertiary bedrock in most areas but locally overlie substantial coarse clastic debris that was deposited immediately before and during earliest magmatism. Prevolcanic clastic debris is interpreted as a consequence of local early normal faulting. In most regions, tilting related to extension began later and occurred during or after eruption of felsic volcanic rocks and before the end of younger mafic volcanism. Extension generally ended before about 17 Ma except in a northwest trending belt adjacent to the relatively unfaulted and topographically elevated Transition Zone tectonic province which is adjacent to the Colorado Plateau. Rapid cooling of metamorphic core complexes and tilting of young basalts and coarse clastic rocks continued in this belt until as recently as 11 Ma. Extension was extreme in this belt, whereas it was generally moderate to slight in other parts of southwestern Arizona. Large‐magnitude extension was not associated with areas of greatest igneous activity, and rapid cooling and exhumation of core complexes postdated local magmatism. These relationships are inconsistent with theories that relate genesis of metamorphic core complexes to magma intrusion in the upper crust. Except for young extension in this northwest trending belt, there are no apparent regional migration trends for either magmatism or extension within southwestern Arizona. Lack of substantial extension before magmatism and general lack of magmatism during youngest extension are inconsistent with the hypothesis that magmatism was the product of decompression melting during lithospheric extension. The long duration and large magnitude of extension adjacent to the Transition Zone tectonic province and within an area of earlier crustal thickening are consistent with the hypothesis that extension was driven by the gravitational potential energy of elevated land mass and crustal roots. Regional magmatic heating apparently weakened the lithosphere and triggered extension but did not control extension locally.

Investigations of the East Greenland continental margin between 70° and 72° N by deep seismic sounding and gravity studies

Results are presented from a deep seismic sounding experiment with the research vessel POLARSTERN in the Scoresby Sund area, East Greenland. For this continental margin study 9 seismic recording landstations were placed in Scoresby Sund and at the southeast end of Kong Oscars Fjord, and ocean bottom seismographs (OBS) were deployed at 26 positions in and out of Scoresby Sund offshore East Greenland between 70° and 72° N and on the west flank of the Kolbeinsey Ridge. The landstations were established using helicopters from RV POLARSTERN. Explosives, a 321 airgun and 81 airguns were used as seismic sources in the open sea. Gravity data were recorded in addition to the seismic measurements. A free-air gravity map is presented. The sea operations — shooting and OBS recording — were strongly influenced by varying ice conditions. Crustal structure 2-D models have been calculated from the deep seismic sounding results. Free-air gravity anomalies have been calculated from these models and compared to the observed gravity. In the inner Scoresby Sund — the Caledonian fold belt region — the crustal thickness is about 35 km, and thins seaward to 10 km. Sediments more than 10 km thick on Jameson Land are of mainly Mesozoic age. In the outer shelf region and deep sea a ‘Moho’ cannot clearly be identified by our data. There are only weak indications for the existence of a ‘Moho’ west of the Kolbeinsey Ridge. Inside and offshore Scoresby Sund there is clear evidence for a lower crust refractor characterised byp-velocities of 6.8–7.3 km s−1 at depths between 6 and 10 km. We believe these velocities are related to magmatic processes of rifting and first drifting controlled by different scale mantle updoming during Paleocene to Eocene and Late Oligocene to Miocene times: the separation of Greenland/Norway and the separation of the Jan Mayen Ridge/Greenland, respectively. A thin igneous upper crust, interpreted to be of oceanic origin, begins about 50 km seaward of the Liverpool Land Escarpment and thickens oceanward. In the escarpment zone the crustal composition is not clear. Probably it is stretched and attenuated continental crust interspersed with basaltic intrusions. The great depth of the basement (about 5000 m) points to a high subsidence rate of about 0.25 mm yr−1 due to sediment loading and cooling of the crust and upper mantle, mainly since Miocene time. The igneous upper crust thickens eastward under the Kolbeinsey Ridge to about 2.5 km; the thickening is likely caused by higher production of extrusives. The basementp-velocity of 5.8–6.0 km s−1 is rather high. Such velocities are associated with young basalts and may also be caused by a higher percentage of dykes. Tertiary to recent sediments, about 5000 m thick, form most of the shelf east of Scoresby Sund, Liverpool Land and Kong Oscars Fjord. This points to a high sedimentation rate mainly since the Miocene. The deeper sediments have a rather high meanp-velocity of 4.5 km s−1, perhaps due to pre-Cambrian to Caledonian deposits of continental origin. The upper sediments offshore Scoresby Sund are thick and have a rather low velocity. They are interpreted as eroded material transported from inside the Sund into the shelf region. Offshore Kong Oscars Fjord the upper sediments, likely Jurassic to Devonian deposits, are thin in the shelf region but thicken to more than 3000 m in the slope area. The crust and upper mantle structure in the ocean-continent transition zone is interpreted to be the result of the superposition of the activities of three rifting phases related to mantle plumes of different dimensions: During period 2 and 3 only a few masses of extrusives were produced, but large volumes of intrusives were emplaced. So the margin between Scoresby Sund and Jan Mayen Fracture Zone is interpreted to be a stretched margin with low volcanic activity.

Magnetic properties of zero‐age oceanic crust; A new submarine lava flow on the Juan de Fuca Ridge

A volcanic eruption that occurred in October, 1993 on the Juan de Fuca Ridge shows unusually high magnetization when compared with older samples from the off‐axis Juan de Fuca Ridge and other Pacific sites. These data, plus data from other young oceanic basalts, indicate that oceanic crustal magnetization may decay rapidly, by 10% in the first year. The considerably slower decrease in magnetization for rocks from older, off‐axis sites agrees with previous models of progressive low temperature oxidation of the mineral titanomagnetite. The magnetization for newly erupted submarine basalts, however, is not consistent with the exponential decay predicted from older sites, indicating that oxidation may not be the sole process responsible for the short‐term decay. Similarly, weak field magnetic susceptibility of the younger rocks does not show an obvious corresponding decrease with increasing age, as expected for the oxidation of titanomagnetite to titanomaghemite. These preliminary results indicate that decay of magnetization of oceanic crustal rocks with time may have two distinct stages, with only the later stage being due to the progressive oxidation of the magnetic minerals.

Reversal of the palaeodrainage system in the Sea of Galilee area as an indicator of the formation timing of the Dead Sea Rift valley base level in northern Israel

The 'Amud Stream in eastern Galilee, Israel, drains southeastwards to the Sea of Galilee base level, which is part of the Dead Sea Rift. Conglomerates of Neogene age, exposed close to the stream, show a WNW palaeodrainage direction. Young basalt flows, 2.5-2.2 Ma (±0.2 Ma) old, along the 'Amud Stream slope northwards towards a younger conglomerate which fills the 'Amud valley. This conglomerate, which also slopes to the WNW, contains basalt pebbles derived from the basalt flows [yielding a KAr age of 2.7-2.3 Ma (±0.5 Ma)]. Based on the relationship between the different conglomerates, the basalt flows and the physiography of the 'Amud Stream, it is concluded that at least until 2 m.y. ago this region, which is close to the present eastern base level, was drained to the WNW towards the Mediterranean. Thus, the Sea of Galilee area did not constitute a base level, at least for the 'Amud Stream catchment area, prior to at the most 2 m.y. ago.

Basaltic volcanism and extension near the intersection of the Sierra Madre volcanic province and the Mexican Volcanic Belt

Three groups (Miocene, Pliocene, and Pleistocene) off oceanic-type basalts associated with normal faulting have been identified in an area surrounding the city of Guadalajara, Mexico. Although most basalts within these groups have compositional characteristics of an asthenospheric source, each group is also associated with lavas that have subduction-related traits. The first group, the San Cristobal plateau basalts, has a minimum volume of 1,800 km 3 of predominantly alkali olivine basalt and lesser basaltic andesite and fills a pre-existing extensional basin of unknown configuration ∼10 m.y. ago. The relatively rapid eruption of these basalts along with their volume, structural association, and chemistry suggest an upwelling mantle plume beneath the Guadalajara region. This region may be the earliest indication of the separation of the Jalisco block from North America. The second basalt group, the Guadalajara basalts, consists of small volumes of porphyritic basalts and basaltic andesites, 3.3-5.0 Ma in age, that outcrop near the northern outskirts of Guadalajara and near the town of Hostotip-aquillo to the northwest. The youngest basalts in the Guadalajara area are the 0.4-1.4 Ma Santa Rosa suite of basalts-hawaiites-mugearites close to the contemporary (>0.2 Ma) andesitic central volcano, V. Tequila; these either flowed from nearby cinder cones toward the Rio Santiago canyon or erupted in the canyon to dam the river. The flat-lying Tertiary (23-27 Ma) ash flows of the Sierra Madre volcanic province that lie to the north of Guadalajara are faulted (north-northeast) into horsts and grabens that in one case can be dated as being older than 21.8 Ma. This fault trend has been reactivated closer to Guadalajara, as normal faults (200- to 300-m displacement) cut the Miocene plateau basalts but are hidden by a cover of younger volcanic rocks close to the city. Also hidden by the young volcanic succession there is the north-west-southeast fault zone that extends to the Gulf of California as the Tepic-Zacoalco graben system. Faulting of this zone is well displayed in the region of the Santa Rosa dam in the Santiago canyon, northwest of Guadalajara. Two styles of faulting are found there: an older (>1 Ma, 3 , has been approximately north-northeast for at least 4 m.y., characteristic of the Mexican Basin and Range province.

Off-axis volcanism at the East Pacific Rise detected by uranium-series dating of basalts

RECENT detailed surveys of the East Pacific Rise have revealed the complexity of the volcanic and magmatic processes occurring along and across fast-spreading ocean ridge crests1–7. In parallel with geological and geochemical investigations, it is now possible to investigate the temporal and spatial pattern of volcanism at ocean ridges by dating young basalts using mass spectrometric uranium-series disequilibria methods8–11. Here we use 238U-230Th and235U-231Pa ages for basalts to quantify the spatial extent of young volcanism and crustal accretion at 9°31′ N on the East Pacific Rise. Most of the ages are younger than would be expected based on off-axis distance and spreading rate. We infer from these anomalously young ages that most of the dated basalts on the crestal plateau were erupted 0.5–2 km outside the axial summit caldera, with some volcanism occurring as far as 4 km off-axis. Melts erupted outside the axial summit caldera can have crustal residence times and magmatic supply systems that differ from those of axial lavas.

Occurrence of siderite and ankerite in young basalts from the Galapagos spreading center (DSDP Holes 506G and 507B)

Occurrence of siderite and ankerite in young basalts from the Galapagos spreading center (DSDP Holes 506G and 507B)

The petrogenesis of 30?20 Ma basic and intermediate volcanics from the Mogollon-Datil Volcanic Field, New Mexico, USA

In the western USA calcalkaline magmas were generated hundreds of kilometres from the nearest destructive plate margin, and in some areas during regional extension several Ma after the cessation of subduction. The Mogollon-Datil Volcanic Field (MDVF) in southern New Mexico was a centre of active magmatism in the mid- to late-Tertiary, and a detailed field, petrographic and geochemical study has been undertaken to evaluate the relations between extensional tectonics and calcalkaline magmatism in the period 30–20 Ma. The rocks comprise alkalic to high-K calcalkaline lavas, ranging from basalt to high silica andesitc. Most of the basaltic rocks have relatively low HFSE abundances, elevated 87Sr/86Sr and low 143Nd/144Nd, similar to many Tertiary basalts across the western USA, and they are inferred to have been derived from the continental mantle lithosphere. Two differentiation trends are recognised, with the older magmas having evolved to more calcalkaline compositions by magma mixing between alkalic basaltic andesites and silicic crustal melts, and the younger rocks having undergone 30–40% fractional crystallisation to more alkalic derivatives. The younger basalts also exhibit a shift to relatively higher HSFE abundances, with lower 87Sr/86Sr and higher 143Nd/144Nd, and these have been modelled as mixtures between an average post-5 Ma Basin and Range basalt and the older MDVF lithosphere-derived basalts. It is argued that the presence of subduction-related geochemical signatures and the development of calcalkaline andesites in the 30–20 Ma lavas from the MDVF are not related to the magmatic effects of Tertiary subduction. Rather, basic magmas were generated by partial melting of the lithospheric mantle which had been modified during a previous subduction event. Since these basalts were generated at the time of maximum extension in the upper crust it is inferred that magma generation was in response to lithospheric extension. The association of the 30–20 Ma calcalkaline andesites with the apparently ‘anorogenic’ tectonism of late mid-Tertiary extension, is the result of crustal contamination, in that fractionated, mildly alkaline, basaltic andesite magmas were mixed with silicic crustal melts, generating hybrid andesite lavas with calcalkaline affinities.

Occurrence of siderite and ankerite in young basalts from the Galápagos Spreading Center (DSDP Holes 506G and 507B)

As soon as they are emplaced on the sea floor, oceanic basalts go through a low-temperature alteration process which produces black halos concentrical with exposed surfaces and cracks, whereas the grey internal parts of the basaltic pieces apparently remain unaltered. This paper reports for the first time the occurrence of authigenic siderite and ankerite in oceanic basalts and more particularly in the grey internal parts of the latter. Small (8–50 μm) crystals of zoned siderite and ankerite have been observed in ten vesicles of two samples recovered from DSDP Holes 506G and 507B drilled south of the Galápagos Spreading Center (GSC). These Fe-carbonates show a large range of chemical composition (FeCO 3=47−88%; CaCO 3=5−40%; MgCO 3=1−20%; MnCO 3=0−11%). Most of them are Ca-richer than siderite reported in the literature. The chemical composition of the carbonate clearly reflects the fluctuation of the fluid chemical composition during crystallization. Mn and at least part of the Fe are thought to be hydrothermal in origin, whereas Mg and probably Ca were provided by seawater. It is proposed that siderite and ankerite formed at relatively low temperature (<85°C) and is metastable. The alteration of the GSC basalts seems to have proceeded in two stages: during the first, reducing stage, pyrite precipitated from hydrothermal fluids. A little further in the rock, siderite precipitated from the fluid which had already been modified by the formation of pyrite, and thus in a microenvironment where particular conditions prevailed (high P CO2, increasing pS 2− or increasing pH or increasing or decreasing pe). During the second, oxidizing, stage of alteration, a seawater-dominated fluid allowed crystallization of mixtures of Fe-rich smectites and micas, and Fe-hydroxides forming the black halos in the external portion of the basalt pieces and locally oxidizing pyrite and siderite in their innermost part. It is shown in this paper that, even at its earliest stage, and at low temperature, alteration of the upper oceanic crust (lavas) involves fluids enriched in Fe and Mn, interpreted to be of hydrothermal origin.

Paleomagnetic results from Thailand and Myanmar: implications for the interpretation of tectonic rotations in Southeast Asia

In Myanmar, preliminary paleomagnetic results from young basalt flows collected from the Central Volcanic Arc at Monwya are unrotated. Data collected from the Mesozoic Kalaw redbeds of the Shan Hills in Myanmar show 40° of clockwise rotation. These sites pass a fold test, indicating that the magnetization is pre-Late Cretaceous in age. This direction gives a virtual geomagnetic pole (VGP) similar to those from the Khorat plateau. The pole is rotated clockwise with respect to the stable Eurasia Mesozoic poles. Thus, the Shan Hills of Myanmar, like the Khorat plateau of Thailand have rotated clockwise with respect to Eurasia, either in Late Mesozoic times or since. Paleomagnetic data collected from the Krabi, Phetchaburi, and Mae Moh intermontane basins of Thailand display rotations of both senses. We interpret this inconsistency to indicate that the observed Tertiary deflections in these basins are controlled primarily by local rotations in discrete fault zones. The sense and amount of motion along that fault controls the sense and amount of rotation. The Mesozoic data, however, indicate that coherent patterns, likely reflecting plate wide motions, are preserved outside of these discrete fault zones.

Disequilibrium partial melting model and its implications for trace element fractionations during mantle melting

A model to describe trace element evolutions during partial melting is proposed by assuming surface equilibrium instead of complete equilibrium between mineral grains and melt and by considering the following factors: (1) chemical diffusion in minerals; (2) melting and melt extraction. For mantle melting rates of ∼ 10−7 yr−1, inferred from mid-ocean ridge spreading rates, elements with diffusivities (D) in minerals of > 10−15 cm2/s be at equilibrium between minerals of 0.5 cm in diameter and melt during mantle melting. However, the anomalous radioactive excess of 236Ra over 230Th (up to 300%) in young basalts which cannot be explained by equilibrium melting because of the highly incompatible nature of both elements can be accounted for by the present model only when melting rate is in the orders of 10−6 to 10−5 yr−1. The disequilibrium effect cannot be ignored for elements with D < 10−13 cm2/s if the melting rate is 10−5 yr−1. Disequilibrium melting lowers the incompatibility of an incompatible element; the high the incompatibility, the more obvious this effect is.Fractionation of trace elements during disequilibrium melting depends both on their equilibrium partition coefficients (k) and their diffusivities. This has two consequences: (1) fractionation of two highly incompatible elements (k ⩽ 0.01) by more than 20% is still possible at melting degree as high as 15% if their diffusivities differ by a factor of 5 or more, as compared to the requirement for less than 5% partial melting for the same extent of fractionation by equilibrium melting; and (2) the incompatibility order of two incompatible elements can be reversed under certain circumstances (i.e., k1 >k2, but D1 >D2). The Pb paradox, namely, that the U/Pb ratio in mantle sources inferred from Pb isotopes has increased during the differentiation of the Earth even though Pb is less incompatible than U, is explainable if the diffusivity of Pb is higher than that of U (e.g., by a factor of 10) and if the melting rate is in the order of 10−5 yr−1.

Remelting mechanisms for shallow source regions of mare basalts

Two mechanisms are studied which could induce remelting at shallow depths within the Moon: a radioactive heating model in which remelting is the result of lateral variations of the heat source concentration emplaced by the end of the initial differentiation event, and a thermal insulation model in which remelting is induced by lateral variations of near-surface conductivity as a result of a high-porosity ejecta blanket created by basin-forming impacts. The effects of these perturbations are studied by evaluating the thermal evolution of an otherwise radially symmetric lunar model. The radioactive heating models predict extensive pre-basin volcanism, which would probably remove from the source region many of the heat-producing elements potentially responsible for melting. Therefore, localized high concentrations of heat-producing elements emplaced early would most probably not be heat sources for the basin-filling mare basalts; however, they are plausible heat sources for pre-basin mare basalts. The thermal insulation model predicts a delay of about 200 m.y. between impact and volcanism. Thermal insulation may also allow for the persistence of source regions for young basalts at shallow depths. If greater basin ages are correct, from 4.2 to 3.9 Ga, and most volcanism occurred between 3.8 and 3.6 Ga, then the thermal insulation model may explain the delay between impact and basin filling. If the basins formed during a terminal cataclysmic event at about 3.9 Ga, then the earliest stages of basin filling at about 3.8 Ga may have been a result of melting beneath the basin as a result of mantle uplift. Basin filling may have been characterized by a migration of the source region from beneath the basin to beneath the adjacent highlands. Cooling of the top 200 km beneath the basin after 200 m.y. because of its high surface conductivity is consistent with the mascon support requirement.

Are the alteration halos of massive sulfide deposits syngenetic? Evidence from U-Pb dating of hydrothermal rutile at the Kidd volcanic center, Abitibi subprovince, Canada

Research Article| June 01, 1990 Are the alteration halos of massive sulfide deposits syngenetic? Evidence from U-Pb dating of hydrothermal rutile at the Kidd volcanic center, Abitibi subprovince, Canada E. S. Schandl; E. S. Schandl 1Department of Geology, Royal Ontario Museum, 100 Queen's Park, Toronto, Ontario M5S 2C6, Canada Search for other works by this author on: GSW Google Scholar D. W. Davis; D. W. Davis 1Department of Geology, Royal Ontario Museum, 100 Queen's Park, Toronto, Ontario M5S 2C6, Canada Search for other works by this author on: GSW Google Scholar T. E. Krogh T. E. Krogh 1Department of Geology, Royal Ontario Museum, 100 Queen's Park, Toronto, Ontario M5S 2C6, Canada Search for other works by this author on: GSW Google Scholar Author and Article Information E. S. Schandl 1Department of Geology, Royal Ontario Museum, 100 Queen's Park, Toronto, Ontario M5S 2C6, Canada D. W. Davis 1Department of Geology, Royal Ontario Museum, 100 Queen's Park, Toronto, Ontario M5S 2C6, Canada T. E. Krogh 1Department of Geology, Royal Ontario Museum, 100 Queen's Park, Toronto, Ontario M5S 2C6, Canada Publisher: Geological Society of America First Online: 02 Jun 2017 Online ISSN: 1943-2682 Print ISSN: 0091-7613 Geological Society of America Geology (1990) 18 (6): 505–508. https://doi.org/10.1130/0091-7613(1990)018<0505:ATAHOM>2.3.CO;2 Article history First Online: 02 Jun 2017 Cite View This Citation Add to Citation Manager Share Icon Share Facebook Twitter LinkedIn MailTo Tools Icon Tools Get Permissions Search Site Citation E. S. Schandl, D. W. Davis, T. E. Krogh; Are the alteration halos of massive sulfide deposits syngenetic? Evidence from U-Pb dating of hydrothermal rutile at the Kidd volcanic center, Abitibi subprovince, Canada. Geology 1990;; 18 (6): 505–508. doi: https://doi.org/10.1130/0091-7613(1990)018<0505:ATAHOM>2.3.CO;2 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 Kidd volcanic complex is composed of felsic volcanic and subvolcanic rocks of Archean age. The felsic rocks are intercalated with sedimentary rocks, and are in contact with komatiitic ultramafic rocks. Large diorite plutons were emplaced into the rhyolite, and the complex is overlain by younger basalts. Metasomatic events affecting the lithology of the Kidd volcanic complex include silicification, extensive CO2 metasomatism (carbonate), K-metasomatism (sericite-fuchsite), and chlorite and minor carbonate alterations. Petrographic evidence, supported by stable isotope and fluid inclusion studies, suggests that silicification and early carbonate alteration were synvolcanic, and therefore related to ore deposition. During subsequent extensive K-metasomatism, sericite precipitated in the rhyolite, and fuchsite precipitated in the ultramafic rocks. Although chlorite post-dates K-metasomatism, the micas and chlorite are both found in anastomosing microfissures, commonly occupying the same set of fractures.Hydrothermal rutile formed by the breakdown of magnetiteilmenite during K-metasomatism and chlorite alteration gives an age of 2624 ±62 Ma (95% confidence level) by Pb-Pb isotopic measurements. It is therefore approximately 100 m.y. younger than syngenetic massive sulfide mineralization (2717 ±2 Ma). Sulfide stringers within sericite and chlorite veins suggest some remobilization of the ores during these later events. This alteration assemblage, generally thought to be intimately associated with mineralization, is identical to that found associated with many lode-gold deposits in the Superior province. Recent dating of micas and rutile associated with gold deposits in the Abitibi subprovince gives comparable ages to the rutile in the Kidd volcanic complex, which must therefore record a widespread, late hydrothermal event affecting mineralized rocks. This content is PDF only. Please click on the PDF icon to access. First Page Preview Close Modal You do not have access to this content, please speak to your institutional administrator if you feel you should have access.

Crustal extension, crustal density, and the evolution of Cenozoic magmatism in the basin and range of the western United States

During the Cenozoic Era, volcanism in the Basin and Range Province of the western United States changed from predominantly intermediate (andesitic and dacitic) to predominantly basaltic. Accompanying this shift was a general decrease in the amount of contamination suffered by basalts en route to the surface. Existing models relate these progressive changes to plate tectonic events at the continental margin of western North America, and assume that extensional tectonism favors eruption of basalt. However, in many areas, synextensional rocks are predominantly intermediate or silicic in composition. Here we propose that the nature of volcanism in an area undergoing extension is a strong function of the density contrast between a magma and the overlying crust. Subtle variations in this density difference will determine the level of neutral buoyancy for a magma, and the height to which a column of magma may rise by hydrostatic pressure. Increases in crustal density favor eruption of dense magmas. Contamination of stagnated basalts by crustal material is more efficient than olivine fractionation in reducing the density of residual liquids, and a stagnated magma may ascend buoyantly after relatively small amounts of contamination. A progressive increase in mean crustal density, caused by crustal extension and consequent thinning of the low‐density upper crust coupled with emplacement of mafic plutons into the crust, may account for the observations that (1) the proportion of basalt erupted in the Basin and Range in the Cenozoic has increased with time, (2) younger basalts are generally less contaminated than older basalts, (3) major young silicic centers are apparently restricted to areas of low‐density crust, and (4) midcrustal magma bodies are generally located at prominent P wave velocity discontinuities.

Excess radiogenio argen in young continenial basalt from North China

Excess radiogenio argen in young continenial basalt from North China

Late Precambrian to Late Devonian mafic magmatism in the Antigonish Highlands of Nova Scotia: multistage melting of a hydrated mantle

Five suites of alkalic basalt ranging in age from Late Precambrian to Late Devonian are found in the Antigonish Highlands of Nova Scotia. In contrast, on neighbouring Cape Breton Island, alkalic basalts are rare even in suites that are contemporaneous with those in the Antigonish Highlands. Late Precambrian alkalic basalts in the Antigonish Highlands are genetically associated with calc-alkalic rocks and are probably subduction related, whereas the younger suites are continental, rift related, and within plate. Major and compatible trace-element abundances can be explained by crystal fractionation of olivine ± clinopyroxene ± orthopyroxene ± spinel ± garnet. However, incompatible trace-element concentrations are strongly influenced by mantle metasomatism that occurred prior to, or synchronously with, the oldest alkalic rocks. The metasomatic event enriched the mantle in Fe, Ti, P, Zr, and light rare-earth elements. The trace-element composition of the younger suites is similar to that of the oldest alkalic rocks and may have been strongly influenced by the Late Precambrian metasomatic event. The anomalously low Nb/Y ratio (generally less than 1 in all suites) and application of phase-equilibria studies indicate that the metasomatic fluid was probably rich in H2O. This fluid may have been derived from dehydration of the subducting slab in Late Precambrian time, resulting in metasomatism of the overlying mantle wedge in the Late Precambrian. It is proposed that the younger suites obtained their fluids by dehydration of the previously metasomatized mantle associated with the generation of local pull-apart basins. Thus, the metasomatic fluid was exotic with respect to the oldest basalts but indigenous with respect to the younger basalts. In the younger basalts, the indigenous fluid was probably focussed at the site of melting by structural events (i.e., rifting). In situations in which the chemistry of mafic magmas is predetermined by earlier metasomatic events, caution is advised in using trace-element criteria to evaluate the tectonic setting.