Duration Of Magmatism Research Articles

40Ar/ 39Ar dating of intrusive magmatism in the Angara-Taseevskaya syncline and its implication for duration of magmatism of the Siberian traps

Dolerite sills are the dominant type of mafic intrusions within the Siberian traps province. Their total volume is comparable to the volume of effusive magmas. Six large dolerite sills are known on basis of geological survey data within the Angara-Taseevskaya syncline at the southeastern part of the Siberian traps. We obtained 40Ar/ 39Ar ‘plateau’ ages of 240.1 ± 1.0/2.5/5.4 Ma and 241.6 ± 1.3/2.6/5.5 Ma for Tulunskii and Padunskii sills, respectively (errors are reported as x/ y/ z, where x, y and z are analytical, internal and external errors, respectively). The obtained ages are indistinguishable from each other at level of analytical errors and suggest emplacement of the sills during the same Middle Triassic pulse of magmatism. Samples from Tolstomysovskii and Chuna-Biryusinskii sills did not yield statistically acceptable plateau ages, though their 40Ar/ 39Ar spectra and K/Ca versus apparent age diagrams suggest that they were emplaced in the Middle Triassic too. The dolerites are characterized by uniform trace element and platinum group element patterns typical for low-Ti tholeiites of the Noril’sk area, although the Noril’sk low-Ti tholeiites were erupted at Permo-Triassic boundary about 9 Ma earlier. The low-Ti tholeiites make up the greatest volume of the flood basalts and possess a distinctive chemical signature. Our data suggests that low-Ti tholeiitic magma was emplaced at more than one locality in the Siberian traps province, and in more than one temporal pulse.

Zircon SHRIMP U–Pb ages of the Gangdese Batholith and implications for Neotethyan subduction in southern Tibet

The Trans-Himalayan magmatism, which occurred extensively in the Lhasa terrane of southern Tibet, has long been related to the Neotethyan subduction before the India–Asia collision. To better delineate the magmatic duration, we report a geochronological study with 25 SHRIMP zircon U–Pb ages from the Gangdese Batholith that represents the largest Trans-Himalayan plutonic complex. The results suggest two distinct stages of plutonism in the Late Cretaceous (ca. 103–80 Ma) and early Paleogene (ca. 65–46 Ma), respectively. Our new data confirm if not refine the notion that a Gangdese magmatic gap or quiescent period existed between ca. 80 and 70 Ma. It is furthermore identified that the early stage ended with adakitic intrusion and the latter stage is marked by a peak activity at ca. 50 Ma. We attribute the cessation of the early stage, and following magmatic gap, to a flattening of the northward Neotethyan subduction, and the initiation of the latter stage to rollback of the subducted slab. The proposed scenarios can also account for the southward migration and intensification of Cretaceous to Paleogene volcanism in the Lhasa terrane that demonstrates a coeval, eruptive “flare-up” event around 50 Ma, interpreted as the result of detaching the Neotethyan oceanic slab from the adherent, more buoyant Indian continental lithosphere owing to the India–Asia collision. Our model is, moreover, in general accord with sedimentary and structural geologic records from southern Tibet where subduction-related orogenesis appears to have evolved through time before India started colliding Asia.

Subduction of a segmented ridge along a curved continental margin: Variations between the western and eastern Sanak–Baranof belt, southern Alaska

Between the western and the eastern parts of the southern Alaskan Sanak–Baranof belt there exist variations in pluton spacing, geochronology, geochemistry and metamorphism. All such characteristics can be explained by increase in heat flux towards the eastern parts of the belt. Despite such differences, the initiation of magmatism propagated eastward at a constant rate of 19 cm/year throughout the entire belt. The average spacing of magmatic centers changes from 165 ± 88 km in the west to 67 ± 41 km in the east. The duration of magmatism as measured by U/Pb zircon and monazite ages increases from 1–2 Ma in the west to 4–5 Ma in the east. The duration of Ar–Ar biotite cooling ages also increases to the east. In agreement with the extended cooling times, large regions of greenschist and amphibolite facies metamorphism are present in the eastern segments, whereas outside of narrow pluton aureoles high-temperature metamorphism is not present in the western Sanak–Baranof belt. Finally, intrusions with adakitic characteristics such as high Sr/Y ratios are present in the east, but not in the west suggesting that some degree of slab melting occurred in the eastern Sanak–Baranof belt. We suggest that the above east–west thermal and magmatic variations can be explained by a decreasing rate of subduction and a persistent slab window in the eastern Sanak–Baranof belt caused by the oblique subduction of a segmented spreading ridge along a curved continental margin, and that this model is compatible with the Resurrection plate hypothesis.

U–Pb zircon and Re–Os molybdenite geochronology from La Caridad porphyry copper deposit: insights for the duration of magmatism and mineralization in the Nacozari District, Sonora, Mexico

Uranium-lead zircon (laser ablation multi-collector ICP-MS spot analysis) ages from La Caridad porphyry copper deposit in the Nacozari District, Northeastern Sonora, Mexico, suggest a short period of magmatism, between 55.5 and 52.6 Ma. Two U-Pb ages from the mineralized quartz monzonite unit, showing different textural characteristics, yielded indistinguishable crystallization ages (~54 Ma), and indicate that the intrusion responsible for the mineralization occurred as a single large complex unit, instead of multiple pulses of magmatism. Some zircons analyzed also show inherited ages in cores recording dates of 112-124 Ma, 141-166 Ma and 1.4 Ga. The Re-Os molybdenite ages from the potassic and phyllic hydrothermal alteration veins yielded identical ages within error, 53.6±0.3 Ma and 53.8±0.3 Ma, respectively (weighted average of 53.7±0.21 Ma), supporting a restricted period for the mineralization. The geochronological data thus indicate a short-lived magmatic and hydrothermal system. The inherited zircons of Precambrian and Late Jurassic-Mid Cretaceous age found in the intrusive rocks of La Caridad deposit, can be explained considering two possible scenarios within the tectonic/magmatic evolution of the area. The first scenario considers the presence of a Precambrian anorogenic granitic basement that is intruded by Mesozoic (Jurassic-Cretaceous) units present beneath the La Caridad deposit. The second scenario suggests that the Mesozoic Glance Conglomerate Formation of Arizona underlies the Paleocene volcanic-igneous pile in the La Caridad area.

Karoo large igneous province: Brevity, origin, and relation to mass extinction questioned by new 40Ar/39Ar age data

The peak activities of continental flood basalts are currently considered as huge and brief (∼1 m.y.) magmatic events, with strong implications for geodynamics and biotic turnover. New 4 0 Ar/ 3 9 Ar dates on the Karoo flood basalts (southern Africa) show a longer duration of magmatism (∼8 m.y., with 6 m.y. for the main volume) with an apparent south-to-north migration, along with briefer distinctive pulses inside the province. This suggests that the Karoo province does not fit the general plume model invoked for most continental flood basalts (including the Karoo) and may explain the absence of a major contemporaneous mass extinction.

Rapid formation of the Small Isles Tertiary centre constrained by precise 40Ar/ 39Ar and U–Pb ages

The relative chronology of magmatic and tectonic events is key to an understanding of the influence of the Iceland plume on the North Atlantic. In particular, the location and duration of magmatism is of fundamental importance. Initial widespread flood basalt formation occurred in Baffin Island, Greenland, and Britain before complete plate break up at 56 Ma after which time magmatism became concentrated in the active rift zone. Historically the British Tertiary Igneous Province (BTIP) has been instrumental in advancing many concepts of igneous petrology. However, the absolute age and duration of the province remains unresolved. Here, we present new internally consistent 40Ar/ 39Ar ages that help to constrain the volcanic activity in the Small Isles centre to within 2 my. This short duration has implications for the onset of magmatism in the larger North Atlantic province, the rapid unroofing of the Rum volcano, and more significantly, some of the evidence used to propose pulsing of the Iceland plume.

Origin of adakitic intrusives generated during mid-Miocene east–west extension in southern Tibet

Adakite is an intermediate to felsic rock with low K, high Al, Na and Sr, and depleted in Y and HREE, usually occurring in arc settings related to subduction of an oceanic slab. Here we report the occurrence of potassic adakites from south Tibet in an orogenic belt produced by the Indo–Asian continent collision. These adakitic intrusives, as a product of Neogene east–west extension, occur in a Miocene Cu-bearing porphyry belt, which developed along the Gangdese arc paralleling the Yarlung–Zangbo suture, but is locally controlled by NS-striking normal faulting systems. Available age data define a duration of magmatism of 10–18 Ma for the adakitic intrusives and related extrusive analogues in south Tibet, which occur in a post-collisional extensional setting. Geochemical data indicate that these adakitic intrusives are shoshonitic and exhibit calc-alkaline composition with high K, and high Sr/Y and La/Y coupled with low Y and HREE, similar to adakites derived from slab melting. However, a wide range for ϵNd(t) (−6.18 to +5.52), initial 87Sr/86Sr (0.7049–0.7079), 207Pb/204Pb (15.502–15.626), and 208Pb/204Pb (38.389–38.960), as well as high K2O contents (2.6–8.6 wt%) and relatively high Mg# values (0.32–0.74) indicate that these adakitic magmas were formed by a complex mechanism involving partial melting of mafic materials in a thickened lower crust with input of enriched mantle and/or upper crust components. Absence of a negative Eu anomaly, extreme depletion in Y, Nb and Ti, and variable high Sr/Y and La/Yb ratios suggest that the lower crustal source is probably a hydrous amphibole eclogite or garnet amphibolite, as exhumed in the western and eastern Himalayan syntaxes on the Tibetan plateau. Partial melting of the lower crust was most likely triggered by mantle-derived ultra-potassic magmatism (17–25 Ma) formed by slab breakoff or mantle thinning. During the formation and migration of pristine adakitic melts, additional input of ultra-potassic magmas and upper crustal materials could account for the observed ϵNd–ϵSr signatures and high Rb/Sr, K and Mg# characteristics for most of the adakitic intrusives in south Tibet.

Timing and duration of syn-magmatic deformation in the Mabel Downs Tonalite, northern Australia

Detailed outcrop mapping combined with microstructural and U–Pb SHRIMP zircon data indicate that emplacement of the Mabel Downs Tonalite spanned progressive regional D 3 deformation in the Palaeoproterozoic Halls Creek Orogen of northern Australia, and that the duration of magmatism exceeded the crystallisation time of the pluton had its entire volume been emplaced instantaneously (∼10 5 y). The pluton comprises several compositionally distinct phases, which show (i) a regional solid-state S 3a foliation-forming event, predated by a strongly deformed porphyritic monzogranite with a U–Pb SHRIMP zircon age of 1837.3±6.0 Ma (95% confidence level); and (ii) overprinting by the localised S 3b Ord River Shear Zone, which crosscuts a 1831.9±3.3 Ma foliated granodiorite and contains 1826.6±7.3 Ma undeformed felsic veins, providing a younger age limit for D 3 deformation. The protracted nature of deformation and magmatism during regional D 3 deformation is significant in the context of the evolution of Halls Creek Orogen, which is characterised by a prolonged thermal event spanning three regional deformation events ( D 2, D 3 and D 4) within a 30–40 million-year interval. The accumulated finite strain is more probably the product of relatively long-lived events (of the order of several millions of years) with low average crustal strain rates, rather than high crustal strain rates during short-lived deformation episodes (=10 5 y). Thus the partitioning of strain accumulation into discrete deformation events during the rapid development of the Halls Creek Orogen was probably not as pronounced as in orogenic belts characterised by higher accumulated strain or longer intervals between deformation events.

3-D, 40Ar 39Ar geochronology in the Paraná continental flood basalt province

New 40Ar 39Ar analyses of borehole samples from the Paraná-Etendeka continental flood basalt province provide critical evidence of eruption rates in unexposed regions. When combined with surface samples, the new data clarify the duration of this major volcanic province. New ages from both surface and borehole samples confirm a 10–12 million year duration of magmatism, which post-dates the Tithonian mass extinction, and provide a unique 3-dimensional picture of the spatial and temporal eruption of the lava pile. Chemically defined magma types are diachronous and the onset of magmatism occurred 500–1000 km inland, migrating southeast towards the incipient South Atlantic ocean. Calculated eruption rates increased from 0.03 km 3 yr −1 between 138 and 135 Ma, through 0.13 km 3 yr −1 between 135 and 133 Ma, to 0.21 km 3 yr −1 between 133 and 131 Ma, supporting the notion that the rate of magmatism increased with time and proximity to the developing South Atlantic. After 131 Ma the continental eruption rates dropped dramatically to 0.01 km 3 yr −1, presumably because rifting had occurred, and eruption centred in the oceanic Rio Grande Rise with eruption rates of around 0.3–0.5 km 3 yr −1. Given the available constraints on the potential temperature of the plume, thickness of the continental lithosphere and degrees of extension through time, predicted melt production rates within the plume seriously underestimate the calculated eruption rates on the continent. However, the eruption rates can be reconciled if melting occurred by conductive heating of volatile-enriched mantle. The diachronous nature of the magma types supports the idea that melting occurred over a wide area and that the different magma types reflect different source regions rather than the temporal evolution of magmas from a single source. Given that 10 million years is appropriate to the timescales for conductive heating above the mantle plume, it is suggested that these distinct source regions were located within the lithospheric mantle, consistent with geochemical studies.

Relation of peralkaline magmatism to heterogeneous extension during the middle Miocene, southeastern Nevada

Volcanism migrated southward in the northern Basin and Range province in the Oligocene and early Miocene to produce voluminous calcalkaline silicic ash flow tuffs. Alkaline volcanism became dominant by middle Miocene (17–14 Ma) as smaller volumes of rhyolite‐trachyte‐basalt suites were erupted from the relatively small Kane Springs Wash caldera complex including the Narrow Canyon, Boulder Canyon, and Kane Springs Wash calderas in southeastern Nevada. Only minor extension affected the Kane Wash area before the end of calcalkaline activity, but extension expressed by rate of progressive stratal tilt peaked (15–13.5 Ma) with peralkaline magmatism (14.7–14.4 Ma). Variations in distribution, degree, style, and timing of deformation demonstrate heterogeneous extension in the Kane Wash area. Only minor extension and tilting persisted post‐middle Miocene (<12 Ma). All major eruptive sources overlap domains of rapid extension. Most of the eruptive volumes from the two oldest calderas of the complex apparently pooled within their calderas, creating outflow deficits. Denudation faulting associated with magmatic tumescence may have followed preexisting active extensional fault systems to unload magma chambers, thus triggering eruptions into structural depressions. Evolution of alkaline magmas is demonstrated by progressive increases in peralkalinity and high field strength elements such as Zr, Y, and Nb. Nd, Pb, and Sr isotopic compositions provide evidence that significantly less crustal interaction affected middle Miocene peralkaline magmas than pre‐middle Miocene calcalkaline magmas. εNd values are −5 to −7 for peralkaline magmas and −7 to −11 for calcalkaline magmas; 208Pb/204Pb ratios are 38.2–38.6 for peralkaline magmas and 38.5–38.9 for calcalkaline magmas. Regional cooling, short duration of magmatism, small volumes of magma, and local extension caused less crustal interaction in peralkaline Kane Wash magmas than in earlier magmas. North of the Kane Wash area, older more voluminous calcalkaline magmas intruded hotter crust for a longer period and thus interacted with the crust to a greater degree in spite of synvolcanic extension.

Excess 40Ar in biotite and hornblende from the Noril'sk 1 intrusion, Siberia: implications for the age of the Siberian Traps

The Siberian Traps represent the largest known Phanerozoic manifestation of continental flood volcanism. The general agreement is now that the volcanism was of short duration (∼ 1 m.y.) and was synchronous with the Permian-Triassic boundary. Discrepancies between different geochronologic results prevent rigorous evaluation of the age and duration of magmatism. Several alkalic mafic intrusions associated with the Siberian Traps represent subvolcanic feeders for the Trap lavas. The Noril'sk 1 intrusion is intruded into the earliest flows and is chemically and isotopically related to stratigraphically overlying flows. Biotites from the Noril'sk 1 intrusion have previously yielded 40Ar 39Ar plateau dates (254-251 Ma) that are significantly older than seemingly reliable 40Ar 39Ar plateau dates (250-247 Ma) on plagioclase and whole-rock samples from Trap basalts that are geologically constrained to be older than the intrusion. Possible explanations of this discrepancy include: (1) plagioclase and whole-rock samples are anomalously young due to 40Ar loss via alteration or episodic outgassing, and/or (2) the biotites contain excess 40Ar. Detailed laser heating 40Ar 39Ar analysis of six individual grains of biotite in a total of 214 steps yield data that permit characterization of multiple components of Ar through isotope correlation patterns. An isochron age of 250.1 ± 1.5 Ma (2σ) and an excess Ar component with 40Ar 36Ar = 565 ± 39 , are obtained by regression of the 115 highest temperature steps. Step heating of a bulk hornblende sample also reveals excess Ar, with 40Ar 36Ar = 327 ± 10 and an isochron age of 249.3 ± 1.6 Ma. Neglecting uncertainties in the ages of neutron fluence monitors, the two dates just overlap at 2σ, but are distinctly younger than the previous mean plateau date (253.9 ± 1.6 Ma) for biotite. The different initial 40Ar 36Ar for the two phases is interpreted as reflecting the evolution of an initial lower mantle composition towards increasing crustal contamination.

Geochronological constraints on the emplacement history of an anorthosite ? rapakivi granite suite: U?Pb zircon and baddeleyite study of the Korosten complex, Ukraine

U−Pb zircon/baddeleyite ages obtained for the Korosten anorthosite-rapakivi granite complex, Ukrainian shield, suggest that different magmatic phases were emplaced during a period of ca. 30 million years as a series of distinct igneous episodes. The earliest 1789.1±2.0 Ma anorthosites were followed by 1781.3±3.2 Ma dykes of plagiogranite porphyries. The emplacement of a major rapakivi granite phase took place at 1767.4±2.2 Ma, and was followed by emplacement of layered intrusions of anorthosites, gabbronorites, diabases and ultrabasic rocks between 1761 and 1758 Ma. The minimum duration of magmatism of about 30 million years, the 6–15 million years interval between igneous pulses, and alternation of discrete episodes of basic and felsie magmatism are common features of major anorthositemangerite-charnockite-rapakivi granite complexes. Temporal distribution of igneous activity in the Korosten complex shows that the gabbro-anorthosites and the granites are not comagmatic, although they are possibly cogenetic, and that at least four portions of granitic and basic magmas were generated during a relatively long period of at least 30 million years. The time gap of about 20–25 million years between early basic and later and more voluminous granitic magmatism, characteristic of the Korosten pluton, Wiborg and Salmi batholiths, probably reflects the duration of extensional processes before the generation of large volumes of magma in the lower crust.

Rhenium-Osmium Evidence for Regional Mineralization in Southwestern North America

More than 40 base metal porphyry ore deposits in southwestern North America are associated with the Laramide orogeny (about 90 million to 50 million years ago). Rhenium-osmium dates on molybdenite, a rhenium-enriched sulfide common in many of the deposits, reveal that in individual deposits mineralization occurs near the final stages of magmatic activity irrespective of the time of inception, magnitude, or duration of magmatism. Deposits that differ widely in location and in the extent and timing of magmatism have nearly identical ages for mineralization. Rhenium-osmium-ages suggest that mineralization occurred during two distinct intervals from about 74 million to 70 million years ago and from 60 million to 55 million years ago. Most deposits that formed in the oldest interval are within the older Precambrian basement of northwestern Arizona, whereas the younger deposits are restricted to the younger Precambrian basement in southern Arizona and northern Mexico. Synchronous, widespread mineralization indicates that similar crust-mantle interaction occurred on a regional scale for ore deposits once thought to be the product of localized processes.

Evidence of regional structural controls on vent distribution: Springerville Volcanic Field, Arizona

Quantitative analysis of the geographic distribution of vents and comparison with regional structural, petrologic, and vent age data provide insight into the processes governing the emplacement of vents in the Springerville volcanic field, Arizona. A total of 409 vents in the Springerville volcanic field (SVF) have a mean distance to nearest neighbor vents of 955 m, a much closer spacing than is common in some platform‐type volcanic fields. Based upon a cluster analysis search radius parameter of 4500 m, these vents comprise seven geographic clusters, with only five outlying vents occurring in the entire field. Cinder cone clusters in the western portion of the field are significantly older than clusters in the eastern portion of the field (p value of <0.001), and there is a tendency for cluster age to decrease to the east. This is particularly evident when mean cluster ages are calculated for tholeiite, alkaline olivine basalt, and evolved alkaline rock types independently. Application of the two‐point azimuth and Hough transform methods demonstrates that regional cinder cone alignments transect these clusters. The most prominent of these alignments trend ENE in the eastern portion of the field and WNW in the western portion of the field, creating an overall arcuate pattern that is subparallel to the trend of the Mogollon Rim and the Colorado Plateau/Transition Zone boundary. These observations suggest that vents (and clusters) migrated from west to east in response to plate motion, but the general pattern of vent migration was complicated by regional structures, which enhanced the volume and duration of magmatism in some areas. The fractures or faults implied by vent alignments indicate that Shmin is oriented radial to the Colorado Plateau in the SVF. Preferred vent alignment orientations may be related to extension resulting from plateau uplift, and to a much smaller degree from a minor Basin and Range imprint. While regional in extent, the implied structures appear to differ significantly from some of those in several other plateau‐marginal fields in that they cannot be related to major reactivated Precambrian structures. Our vent alignment data differ from those seen by other workers in the Zuni‐Bandera and Mount Taylor fields, suggesting the stress field for the SVF is different from other fields in the proposed Jemez lineament. The stress field implied by vent alignment data, combined with structural data, suggests that the southwestern tectonic boundary of the Colorado Plateau of Brumbaugh (1987) should be extended southeastward to include the SVF at the plateau's southern boundary.

Northern Trans-Pecos magmatic province: Introduction and comparison with the Kenya rift

The Trans-Pecos magmatic province of West Texas and southern New Mexico is a more eroded analogue of the Kenya (Gregory) rift in East Africa. Trans-Pecos alkalic rocks are similar to the basalt-phonolite-trachyte-rhyolite assemblage from fissure and multicenter eruptions, and from some central volcanoes, in the Kenya rift. In both provinces, quartz-normative and nepheline-normative mafic rocks occur, providing likely parents for the entire observed range of silica saturation. Thus, no special mechanism is needed for deriving silica-undersaturated and silica-oversaturated rocks from one another, because both groups evolved independently. The Kenya rocks tend to be more mafic and, at the silicic end of their compositional range, more peralkalic than the Trans-Pecos rocks. Similarities in tectonic style, extent and duration of magmatism, and igneous rock compositions in the two provinces suggest that the alkalic rocks in both regions were generated by differentiation from quartz-and nepheline-normative parents in the upper parts of elongate mafic or ultramafic intrusions, probably diapiric welts that caused doming and normal faulting.

A Summary of Radiometric Ages of Igneous Rocks in the Oquirrh Mountains, North-Central Utah

Interpretation of duration of magmatism, relationships between occurrence of volcanism and plutonism

Thermal evolution of the moon

The thermal history and current state of the lunar interior are investigated using constraints imposed by recent geological and physical data. Theoretical temperature models are computed taking into account different initial conditions, heat sources, differentiation and simulated convection. To account for the early formation of the lunar highlands, the time duration of magmatism and presentday temperatures estimated from lunar electrical conductivity profiles, it is necessary to restrict initial temperatures and abundances of radioactivie elements. Successful models require that the outer half of the Moon initially heated to melting temperatures, probably due to rapid accretion. Differentiation of radioactive heat sources toward the lunar surface occurred during the first 1.6 billion years. Temperatures in the outer 500 km are currently low, while the deep interior (radius less than 700 to 1000 km) is warmer than 1000°C, and is of primordial material. In some models there is a partially melted core. The calculated surface heat flux is between 25 and 30 erg/cm2 s.