Age Of Dikes Research Articles

40Ar/39Ar and U-Pb SIMS zircon ages of Ediacaran dikes from the Arabian-Nubian Shield of south Jordan

A spectacular feature of the Arabian-Nubian Shield (ANS) is the abundance of well-exposed and extensive Neoproterozoic dike swarms of variable compositions. Most of these dikes are late to post-orogenic with respect to the East African Orogen (EAO) and are unmetamorphosed. We dated a composite dike with latite margins and a rhyolite core (607 ± 6 Ma, U-Pb), a biotite rhyolite dike (600 ± 4 Ma, 40Ar/39Ar age of biotite), an andesite dike (594 ± 3, 40Ar/39Ar age of amphibole) and a dolerite dike (~579 Ma, 40Ar/39Ar whole rock total gas age). We propose that the first three dikes represent one generation that was emplaced at different episodes extending between 607 and 590 Ma. Time and composition equivalent dikes are common in the northern ANS. The dikes crosscut late collisional granitoids and geochemically display a subduction-related character as evidenced by a negative Nb-Ta anomaly. These dikes are absent in the alkali feldspar A-type Humrat Syenogranite dated at 586 ± 5 Ma in Jordan and equivalent rocks in the northern ANS, which are crosscut only by the (~579 Ma) dolerite dikes. The within-plate character of the dolerite dikes is supported by the absence of the Nb-Ta anomaly and high field strength element geochemistry. We propose that the dolerite dikes are a generation, distinct from the ~607–590 Ma dikes, that reflects a change in tectonic regime and represents the last magmatic activity of the Neoproterozoic in the northern ANS. The ages of the dikes dated in this study agree with the published age range of the transitional stage from late orogenic calc-alkaline to extensional alkaline tectono-magmatic setting for the ANS. We propose that the magmatic activity was terminated ~50 m.y. before the age of the Cambrian unconformity at ~530 Ma. Correlation with ages of dikes and magmatic rocks in the northern ANS favors this supposition. The dike geochemistry and geochronology are compatible with a tectonic model that involves mantle lithosphere delamination from below the northern ANS after a significant crust-mantle thickening caused by the EAO, followed by thermal relaxation, subsidence and gradual denudation until the age of the unconformity at ~530 Ma.

Succession of soil macro-faunal biodiversity in forests converted from croplands after long-term coastal reclamation

The long-term succession of the soil macro-faunal community after reclamation at coastal areas is not well understood, especially at the time scale of centuries. The objective of this work was to analyze soil development by measuring its principal physico-chemical properties and study the macro-faunal biodiversity succession along the successional trajectory created by intermittent reclamation. We hypothesized that soil macro-faunal taxonomic and functional diversity, richness, and density would increase after long-term reclamation, and that macro-faunal succession was related to dike age. In July 2014, sites of varying dike age (30, 50, 100, and 200 years) were used to represent a successional trajectory of Fluvisol soils at a reclaimed coast at Yancheng, China. The selected habitats were planted with 14–17 years old poplar (Populus euramericana) or dawn redwood (Metasequoia glyptostroboides) trees. With increasing dike age, soil nutrient status improved over the successional trajectory, as indicated by the increased soil organic carbon (SOC) concentration, total nitrogen (TN), and total phosphorous (TP). Soil salt concentration (SSC), pH, and carbon-nitrogen (C/N) ratio continuously decreased, as did soil bulk density (BD), the latter of which had a smaller coefficient of variation than the other properties did. Over the first 100 years of macro-faunal succession, taxonomic richness, abundance, Margalef’s richness index, and the Shannon-Weaver diversity index increased significantly, but did not differ significantly between the habitats with dike ages of 100 and 200 years. Significant differences in the measured communities’ characters were detected between forests planted with different trees when the dike age was 50 years but not when it was 30 years. Differences in macro-faunal community structure were primarily related to SOC and SSC, which were considerably related to dike age. The taxonomic richness and abundance of functional groups varied significantly among habitats and was related to changes in food resources in the soil environment. The abundance and taxonomic richness of phytophagous and saprophagous species increased with increasing dike age; no significant differences were noted in taxonomic richness of omnivorous species and predator abundance. The results thus supported our hypotheses. In conclusion, long-term macro-faunal community succession was affected by dike age, land use, and soil development after reclamation, and the soil macro-faunal biodiversity tended to increase with increasing dike age.

Chemical and Sr-Nd compositions and 40Ar/39Ar ages of NW-trending dolerite dikes of Burkina Faso: Evidence for a Mesoproterozoic magmatism in the West African Craton

The Paleoproterozoic basement of the northeastern part of the Leo-Man craton is intruded by generally NW-trending dikes. These regional scale dikes extend over 1000 km in Burkina Faso, Mali and Niger. We present chemical and Sr-Nd isotope compositions, as well as 40Ar/39Ar ages of these dikes with the following strikes N98°–N112°, and N114°–N124° in NE Burkina Faso. Field relationships show that the dikes are posterior to all other rock types dated between 2.26 Ga and 2.0 Ga. Chemical data indicate that the dikes are continental flood basalts and composed of low-Ti (TiO2 ≤ 2 wt.%) sub-alkaline basalts and andesites. They exhibit a minor negative Europium anomaly (0.86–0.99) and slightly fractionated REE patterns ((La/Yb)N = 2.5–9.1; YbN = 9.5–19.9). The ratios of Th/Ta (1.3–11.4) and Ce/Pb (5.2–58.5) suggest a varying crustal assimilation of the dike magmas during ascent in the continental crust for all studied samples. Calculated P-T conditions indicate that the magma reached temperatures of 1285 °C (calculated from olivine compositions) and pressures of 6.9 kbar (calculated for pyroxene minerals). Calculated initial 87Sr/86Sr (0.70040–0.70260) and ɛNd(t) = +2.1 to −3.5 at 1575 Ma, also point to a crustal contamination with the most primitive samples showing TDM values of 1946 Ma and 2154 Ma. The low values of La/Ba (<0.2) and Nb/La (<1.0), contrast with the low Th/Nb (<0.9), and suggest a lithospheric mantle or subduction-modified mantle as possible source for the dikes. Sr-Nd data, Mg# and Nb-Ta-Zr-Y-Th-Tb-Yb compositions further suggest that the most primitive samples were emplaced in a none orogenic setting and their magmas were subjected to variable crustal contaminations. Literature and the present whole rock 40Ar/39Ar age determinations show that the dikes were emplaced during a widespread Mesoproterozoic magmatism between 1.6 Ga and 1.2 Ga, and were affected by a thermal event causing the argon systematics resetting, best constrained by the date of sample KK1 (1236 ± 20 Ma, 40Ar/36Ar = 294 ± 13, MSWD = 2.2). Contemporaneous 1590–1570 Ma extensive magmatism is reported in other crustal blocks in Baltica (Sveconorw-Goth, svecofennian) NW Laurentia (Slave craton, Yukon), and Australia (Gawler craton), and together with the 1575 studied dikes, are related to the breakup of the supercontinent Nuna.

Petrogenesis of composite dikes in granitoids of western Transbaikalia

The performed studies have revealed two varieties of composite dikes differing in morphology and internal structure depending on (1) the proportions of salic and basic components and (2) the rheologic state of the host environment. The latter can be both a solid substrate with open fractures and a melt at different stages of crystallization. The evaluated isotopic age of dikes in the Shaluta massif, 290.8 ± 2.7 Ma and 283.4 ± 3.4 Ma, is correlated with the time of the pluton formation. The age of the composite dike breaking through the metamorphic deposits on the eastern shore of Lake Baikal is 284.10 ± 0.96 Ma. The mass formation of composite dikes in western Transbaikalia is correlated with the Late Paleozoic magmatism, which resulted in one of the Earth’s largest granitoid provinces. The intrusion of dikes was not a single-stage event; it lasted at least 10-12 Myr and was apparently related to the repeated intrusion of large volumes of salic magmas and the formation of granitoid plutons. Early Mesozoic composite dikes associated with alkali-granitoid plutons of the Late Kunalei igneous complex (230-210 Ma) are much scarcer. Basic magmas of the composite dikes were generated at depths greater than 75 km as a result of the melting of a modified (enriched in crustal components) mantle source. Salic components of the dikes, independently of their geologic position, are generally similar in composition to the granitoids of the plutonic facies, and the differences are apparently due to hybridization proceeding at great depths.© 2018, V.S. Sobolev IGM, Siberian Branch of the RAS. Published by Elsevier B.V. All rights reserved.

Multi-stage gold mineralization in the Taldybulak Levoberezhny deposit, Tien Shan, Kyrgyzstan: Reply to comment by Boris Trifonov on “Re–Os pyrite and U–Pb zircon geochronology from the Taldybulak Levoberezhny gold deposit: Insight for Cambrian metallogeny of the Kyrgyz northern Tien Shan”

Our recent geochronological study on the Taldybulak Levoberezhny gold deposit in Kyrgyz Northern Tien Shan (KNTS) received critical comments by Boris Trifonov (2016) regarding the age of dikes and its relationship with gold mineralization at the deposit. In this contribution, based on a re-evaluation of our published data, together with new geochronological, geochemical and Sr–Nd–Hf isotopic data, the age of gold mineralization, magmatic and tectonic events and the relationship between dikes and mineralization at the deposit, it is proposed that the Taldybulak Levoberezhny gold deposit is a product of long-lived magmatic-hydrothermal system with multiple stages of gold mineralization from Cambrian to late Carboniferous–early Permian.

Abrupt spatial and geochemical changes in lamprophyre magmatism related to Gondwana fragmentation prior, during and after opening of the Tasman Sea

High-precision 40Ar/39Ar dating of lamprophyre dike swarms in the Western Province of New Zealand reveals that these dikes were emplaced into continental crust prior to, during and after opening of the Tasman Sea between Australia and New Zealand. Dike ages form distinct clusters concentrated in different areas. The oldest magmatism, 102–100Ma, is concentrated in the South Westland region that represents the furthest inboard portion of New Zealand in a Gondwana setting. A later pulse of magmatism from ~92Ma to ~84Ma, concentrated in North Westland, ended when the first oceanic crust formed at the inception of opening of the Tasman Sea. Magmatic quiescence followed until ~72–68Ma, when another swarm of dikes was emplaced. The composition of the dikes reveals a dramatic change in primary melt sources while continental extension and lithospheric thinning were ongoing. The 102–100Ma South Westland dikes represent the last mafic calc-alkaline magmatism associated with a long-lived history of the area as Gondwana's active margin. The 92–84Ma North and 72–68Ma Central Westland dike swarms on the other hand have strongly alkaline compositions interpreted as melts from an intraplate source. These dikes represent the oldest Western Province representatives of alkaline magmatism in the greater New Zealand region that peaked in activity during the Cenozoic and has remained active up to the present day. Cretaceous alkaline dikes were emplaced parallel to predicted normal faults associated with dextral shear along the Alpine Fault. Furthermore, they temporally correspond to polyphase Cretaceous metamorphism of the once distal Alpine Schist. Dike emplacement and distal metamorphism could have been linked by a precursor to the Alpine Fault. Dike emplacement in the Western Province coupled to metamorphism of the Alpine Schist at 72–68Ma indicates a period of possible reactivation of this proto Alpine Fault before it served as a zone of weakness during the opening of the oceanic Emerald Basin (at ~45Ma) and eventually the formation of the present-day plate boundary (~25Ma–recent).

Impact of dike age on biodiversity and functional composition of soilmacrofaunal communities in poplar forests in a reclaimed coastal area

Four patches of poplar forest with different dike histories (about 30 to 200 years) were selected from the coastal area of Yancheng, China, for detecting the effect of dike age on soil macrofauna. A total of 825 individuals belonging to 21 taxonomic groups were collected. Omnivorous, phytophagous, predaceous, and saprophagous groups were classified on the basis of diet. Higher biodiversity values were observed in the forests with longer dike histories (100 and 200 years) than in those with shorter dike histories (30 and 50 years). By principal coordinate analysis, the soil macrofaunal communities from the forests were distinguished as three groups with shorter or longer dike histories (30 years, 50 years, and above 100 years). One-way ANOSIM analysis revealed significant differences among soil macrofaunal communities (P < 0.050), except for those with longer dike histories (100 and 200 years, P = 0.217). The groups of omnivorous and predaceous macrofauna showed no significant differences in taxonomic richness and abundance among habitats, while significant differences were observed between the other functional groups. A significant difference in functional composition between the soil macrofaunal communities in the forests with dike ages of 30 and 100 years was found with the chi-square test (P = 0.027). The biodiversity and functional composition of soil macrofauna was significantly affected by dike age in the reclaimed coast.

Geochemical and Sr–Nd isotopic constraints on the mantle source of Neoproterozoic mafic dikes of the rifted eastern Laurentian margin, north-central Appalachians, USA

Abundant and widely distributed unmetamorphosed mafic dikes intrude Mesoproterozoic rocks of the New Jersey Highlands. The age of the dikes is imprecisely known but interpreted to fall between 615 and 576 Ma, which is consistent with the range of ages of mafic dikes from Labrador and Nova Scotia south to Pennsylvania that were emplaced along the rifted eastern Laurentian margin. New Jersey Highlands dikes are a few cm to 18 m wide and have lengths of as much as several km. They have sharp, largely discordant contacts against enclosing Mesoproterozoic rocks and aphanitic chilled margins that grade into coarser grained interiors. Columnar joints are present locally and suggest emplacement at a shallow crustal level. Geochemical compositions of the dikes range from alkalic to less common tholeiitic basalt having generally high TiO 2 , P 2 O 5 , Zr, Nb, Y, and La/Yb, and low MgO, Cr, and Ni. TiO 2 contents define high-Ti and low-Ti dikes that differ in high field strength elements (HFSE) and light rare earth elements (LREE) but overlap in abundances of most other elements. Dike magma evolved in an ascending mantle plume of OIB-like asthenosphere from enriched higher TiO 2 compositions to more depleted lower TiO 2 compositions. Subtle differences in the dike compositions are due to variations in the amount of partial melting within the plume and the depth of melt segregation. Sr–Nd isotope values of both dike compositions overlap and are characterized by ε Nd (T) of + 1.5 to + 3.8 and initial 87 Sr/ 86 Sr ranging from 0.7032 to 0.7077. Higher Sr isotope ratios are interpreted as resulting from local interaction of the dike magma with heterogeneous, high 87 Sr/ 86 Sr lithospheric mantle having EMI or EMII-like geochemical characteristics. Dikes form tabular structures that have long segments striking an average of N44°E and short segments striking about east–west. Their regional geometries form right-stepping, rhomb-shaped patterns due to emplacement into rift-related dilational fractures likely formed through a combination of southeast-directed extension and strike-slip shear stresses. Geochemical compositions of the dikes are the same regardless of their structural trend or location implying they formed during a single magmatic event. They, along with other mafic dikes in the north-central Appalachians, were emplaced in a within-plate tectonic setting along the rifted margin of eastern Laurentia, prior to opening of the Iapetus Ocean. • Mantle sources of Neoproterozoic mafic dikes were modeled from Sr–Nd isotopes and geochemistry. • Dikes formed from partial melting of a garnet-free, enriched OIB-like source. • Geochemical similarity to other regional Neoproterozoic dikes along the eastern Laurentian margin • Dikes emplaced from single magmatic event during rifting that formed the Iapetus Ocean

Late Paleoproterozoic basic dikes in the Ulkan-Uchur district, eastern Aldan-Stanovoi Shield: Structural position, composition, and paleogeodynamic setting

Late Paleoproterozoic dikes of the Maimakan Complex were studied in the Ulkan-Uchur district at the eastern margin of the Aldan-Stanovoi Shield. The dikes are parallel or arranged en echelon in the Uchur-Uyan, South Uchur, and Ukikan fields of dike swarms. The spatial distribution of the dike swarms pertaining to the Maimakan Complex in the Ulkan Trough and its framework shows that the area of their intersection is located in the center of the Ulkan granitoid batholith. The basic dikes, which are distinguished by elevated contents of alkali metals, Fe, Ti, and P in combination with a low Mg content, are defined as moderately alkaline rocks transitional from tholeiitic to alkaline series similar in composition to within-plate basalts and E-MORB. The REE pattern is comparable to that of tholeiitic and subalkaline series in extensional settings. Along with the geological data, this indicates that the complex was formed under conditions of intracontinental extension. As follows from geological relationships, the age of dikes is estimated as 1670–1715 Ma.

Age of dikes in ancient tonalites of the Vodlozero terrane as the key to Archean evolution of basic magmatism of the Fennoscandian shield

Age of dikes in ancient tonalites of the Vodlozero terrane as the key to Archean evolution of basic magmatism of the Fennoscandian shield

Earliest Eocene (53 Ma) convergence in the Southwest Pacific: evidence from pre‐obduction dikes in the ophiolite of New Caledonia

AbstractUncertainty about the timing and location of the initiation of convergence in the western and south‐western Pacific greatly hinders accurate plate tectonic reconstructions of subduction systems in that area. The chemistry and age of dikes intruding mantle peridotite in the ophiolite of New Caledonia infer that subduction‐related magmatism began before 53 Ma. These new results infer that obduction in the south‐west Pacific is unrelated to the reorientation of the Pacific plate motion that occurred c. 43 Ma and confirm new interpretations showing that changes in mantle flow, hotspot and plate motion may have occurred as soon as late Paleocene or early Eocene.

K-Ar age of high magnesium andesite dikes at the Toki River in Gifu Prefecture, central Japan.

A high-magnesium andesite dike swarm intruding into the Jurassic mélange of the Mino-Tamba Belt and the Cretaceous Toki Granite is exposed along the Toki River in central Japan. At the time of intrusion, the direction of the maximum horizontal compressive stress was E-W in this area, as indicated by the predominant direction of the dike swarm. New K-Ar age dating suggests that the intrusive age of the dike swarm is 60-70 Ma. We point out of possibility that high-magnesium andesite magmatism occurred at 60-70 Ma in the eastern part of Inner Zone of Southwest Japan.

K–Ar and 40Ar/ 39Ar ages of dikes emplaced in the onshore basement of the Santos Basin, Resende area, SE Brazil: implications for the south Atlantic opening and Tertiary reactivation

New K–Ar and 40Ar/ 39Ar data of tholeiitic and alkaline dike swarms from the onshore basement of the Santos Basin (SE Brazil) reveal Mesozoic and Tertiary magmatic pulses. The tholeiitic rocks (basalt, dolerite, and microgabbro) display high TiO 2 contents (average 3.65 wt%) and comprise two magmatic groups. The NW-oriented samples of Group A have (La/Yb) N ratios between 15 and 32.3 and range in age from 192.9±2.2 to 160.9±1.9 Ma. The NNW-NNE Group B samples, with (La/Yb) N ratios between 7 and 16, range from 148.3±3 to 133.9±0.5 Ma. The alkaline rocks (syenite, trachyte, phonolite, alkaline basalts, and lamprophyre) display intermediate–K contents and comprise dikes, plugs, and stocks. Ages of approximately 82 Ma were obtained for the lamprophyre dikes, 70 Ma for the syenite plutons, and 64–59 Ma for felsic dikes. Because Jurassic–Early Cretaceous basic dikes have not been reported in SE Brazil, we might speculate that, during the emplacement of Group A dikes, extensional stresses were active in the region before the opening of the south Atlantic Ocean and coeval with the Karoo magmatism described in South Africa. Group B dikes yield ages compatible with those obtained for Serra Geral and Ponta Grossa magmatism in the Paraná Basin and are directly related to the breakup of western Gondwana. Alkaline magmatism is associated with several tectonic episodes that postdate the opening of the Atlantic Ocean and related to the upwelling of the Trindade plume and the generation of Tertiary basins southeast of Brazil. In the studied region, alkaline magmatism can be subdivided in two episodes: the first one represented by lamprophyre dykes of approximately 82 Ma and the second comprised of felsic alkaline stocks of approximately 70 Ma and associated dikes ranging from 64 to 59 Ma.

Dike swarms on Seward Peninsula, Alaska, and their implications for the kinematics of Cretaceous extension in the Bering Strait region

Late Cretaceous dike swarms on Seward Peninsula, northwestern Alaska, represent the youngest local manifestation of a ~115–75 Ma magmatic event in the Bering Strait region. Magmatism accompanied and followed high-grade metamorphism and ductile deformation. A Late Cretaceous extensional tectonic setting for the region is suggested by the thickness and seismic-reflection characteristics of the crust, regional basin development, formation of high-strain tectonites with subhorizontal foliations, bimodal magmatism, and dike swarms. The orientation of the dike swarms is used to address the kinematics of extension. A diabase dike swarm in the Kigluaik Mountains consists of dikes that strike northeast (040°) and dip steeply. Phenocrysts include plagioclase, clinopyroxene, orthopyroxene, and hornblende. Geochemical data indicate that SiO2 ranges from 48% to 56%, and K2O from 1.2% to 4.0%. The dikes are geochemically similar to the mafic to intermediate root of the 90 Ma Kigluaik pluton. Sr- and Nd-isotope data show that initial 87Sr/86Sr ranges from 0.7070 to 0.7077 and initial εNd ranges from –0.85 to –2.90. Field relations and 40Ar/39Ar geochronology bracket the dike ages between 90 and 84 Ma. Diabase dikes in the York Mountains are associated with normal faults that strike east–west to east-northeast. Dikes in the Bendeleben Mountains are both mafic and felsic, but their orientations are unknown. Alkalic dikes in the Darby Mountains strike 030°–050°, similar to those in the Kigluaik Mountains. Regional relationships including the orientation of dikes, normal faults, mineral stretching lineations, and other shear-sense indicators suggest that between 110 and 90 Ma extension on Seward Peninsula was generally oriented north–south to north-northwest–south-southeast.

Cone sheet formation and intrusive growth of an oceanic island—The Miocene Tejeda complex on Gran Canaria (Canary Islands)

Research Article| March 01, 1999 Cone sheet formation and intrusive growth of an oceanic island—The Miocene Tejeda complex on Gran Canaria (Canary Islands) Carsten Schirnick; Carsten Schirnick 1GEOMAR Forschungszentrum, Wischhofstrasse 1-3, D-24148 Kiel, Germany Search for other works by this author on: GSW Google Scholar Paul van den Bogaard; Paul van den Bogaard 1GEOMAR Forschungszentrum, Wischhofstrasse 1-3, D-24148 Kiel, Germany Search for other works by this author on: GSW Google Scholar Hans-Ulrich Schmincke Hans-Ulrich Schmincke 1GEOMAR Forschungszentrum, Wischhofstrasse 1-3, D-24148 Kiel, Germany Search for other works by this author on: GSW Google Scholar Author and Article Information Carsten Schirnick 1GEOMAR Forschungszentrum, Wischhofstrasse 1-3, D-24148 Kiel, Germany Paul van den Bogaard 1GEOMAR Forschungszentrum, Wischhofstrasse 1-3, D-24148 Kiel, Germany Hans-Ulrich Schmincke 1GEOMAR Forschungszentrum, Wischhofstrasse 1-3, D-24148 Kiel, Germany Publisher: Geological Society of America First Online: 02 Jun 2017 Online ISSN: 1943-2682 Print ISSN: 0091-7613 Geological Society of America Geology (1999) 27 (3): 207–210. https://doi.org/10.1130/0091-7613(1999)027<0207:CSFAIG>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 Carsten Schirnick, Paul van den Bogaard, Hans-Ulrich Schmincke; Cone sheet formation and intrusive growth of an oceanic island—The Miocene Tejeda complex on Gran Canaria (Canary Islands). Geology 1999;; 27 (3): 207–210. doi: https://doi.org/10.1130/0091-7613(1999)027<0207:CSFAIG>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 More than 500 trachytic to phonolitic cone sheet dikes, hypabyssal syenite stocks, and subordinate radial dikes form a 20-km-diameter intrusive complex in the volcaniclastic fill of the Miocene Tejeda caldera (20 × 35 km) on Gran Canaria (Canary Islands). The dikes intruded concentrically around a central axis of radial symmetry and dip uniformly an average of ∼41° toward the center. Single-crystal 40Ar/39Ar ages of dikes and syenite stocks ranging from 12.3 ± 0.1 to 7.32 ± 0.05 Ma suggest more or less continuous intrusive activity during both volcanically active and inactive phases. Intrusions were emplaced at average intervals of ∼5–10 k.y., while explosive eruptions took place at >50 k.y. intervals. The estimated aggregate volume of evolved cone sheet magma added at shallow level (<2000 m below sea level) amounts to ∼250 km3 compared to ⋙500 km3 of evolved extrusive volcanics erupted during the same period.Formation of the Tejeda cone sheets most likely resulted from deformation processes due to resurgent doming, initiated by the recurrent replenishment of a flat, laccolith-like shallow magma chamber. Magma supply exceeding the volume that could be compensated for by updoming of the overlying caldera fill resulted in the formation of cone-shaped fractures. 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.

仙台市，黒鼻山玄武岩の層準 後期中新世〜前期鮮新世に噴出したピジョン輝石玄武岩

Detailed stratigraphical investigation revealed that pigeonite-phyric basalts from Kurohana-Yama, Sendai, Japan erupted at late Miocene or early Pliocene, with the age of the eruption is younger than 8 Ma. It is because K-Ar ages of dikes which are stratigraphicaly older than Kurohana-Yama lavas are about 8 Ma (Tsunakawa et al., 1983). The pigeonite basalts from Kurohana-Yama are chemically devided into two types; type A basalts contain higher MgO (7 wt%) and lower Al2O3 (17-18 wt%) than type B basalts (MgO 5.2-5.5 wt% and Al2O3 19-20 wt%). All these basalts contain plagioclase and olivine phenocrysts .Assemblages of phenocryst pyroxenes are various and most of the basalts contain large pigeonite phenocrysts.

K‐Ar ages of basanitic dikes, Awatere Valley, Marlborough, New Zealand

Abstract Twenty‐one basanites from a regional dike swarm in the Awatere River, Winterton River and Gladstone Stream, Awatere Valley area, Marlborough, have been analysed (major and trace elements) and dated by the K‐Ar method. K‐Ar ages of the dikes range between 100 and 60 Ma. The younger age extends the previously determined age range of igneous activity in the Awatere Valley (fossil age of volcanic activity; K‐Ar and fission‐track ages of volcanic and plutonic rocks) of 103–90 Ma for another 30 Ma into early Tertiary time. Considering only phenocryst‐free or phenocryst‐poor dikes, SiO2, total alkalis, Ba, Ce, Y, Zr, and Differentiation Index (D.I.) increase with decreasing age, and MgO, CaO, Cr, Cu, Ni, Sc, and Mg number decrease. This variation reflects a time‐dependent evolution of magma composition controlled by early‐stage olivine/pyroxene fractionation, gradually increasing sodic enrichment of plagioclase, and later crystallisation of K‐feldspar.

K-Ar ages of carbonate- and mantle nodule-bearing lamprophyre dikes from Shingu, central Shikoku, Southwest Japan.

K-Ar age determinations were carried out on two lamprophyre dikes intruded into the Sambagawa metamorphic rocks at Shingu in the central part of Shikoku Island, Southwest Japan. These dikes are very distinctive among Neogene igneous rocks in Shikoku because of their abundant mantle xenoliths and carbonate minerals in the fresh groundmass, and of their unique chemical compositions. In order to eliminate the effect of extraneous argon from xenoliths and carbonates, removal of large crystals and acid leaching experiments were done for three grain size fractions. Apparent ages ranging from 18.6±0.7Ma to 21.6 ±2.0Ma were obtained for both rocks. However, positive correlation between apparent ages and atm. 40Ar% suggests that samples contain initial argon which is not atmospheric. The well defined isochron age of 17.7±0.5Ma is obtained from all samples with the initial 40Ar/36Ar ratio of 310±5 in the 40Ar/36Ar-40K/36Ar diagram. As the initial 40Ar/36Ar ratio is higher than that of atmosphere, apparent ages do not show any geologic meaning, and the isochron age of 17.7±0.5Ma gives the intrusive age of dikes. The Shingu lamprophyres are much older than other Miocene igneous rocks in Shikoku ranging in age from 14 to 12Ma, and both magmatisms could be genetically independent. As the Shikoku Basin kept opening in a northeast-southwest direction at 18Ma, it could not subduct significantly beneath Southwest Japan from the south when the Shingu lamprophyres intruded into the Sambagawa metamorphic belt. Petrogenesis of the Shingu magma could be related to the rifting of the back arc basin, the Japan Sea.

K-Ar ages of dikes in Northeast Japan.

Thirteen dike swarms in Northeast Japan were dated by the K-Ar method. The K-Ar ages in this study give some crucial constraints to the volcanostratigraphy and imply that the geological situation of some dike swarms estimated in the other studies should be reexaminned while other dike swarms showed the K-Ar ages concordant with the geological ages. Available K-Ar ages of the volcanic rocks from the present and other studies indicate that the volcanic front in Northeast Japan had a temporal position about 22–23 Ma ago to the east of that in the Quaternary and that the volcanism appears to have been inactive from 17 to 21 Ma ago.

K-Ar ages of dikes in Southwest Japan.

Eleven samples from six dike swarms in Southwest Japan were dated by the K-Ar method. Most K-Ar ages obtained for these samples indicate that a volcanism as dike intrusions occurred about 15 Ma ago at some areas in the inner zone of Southwest Japan. The previous geochronological studies by other workers have already revealed the occurrence of granitic intrusions and their related effusive rocks at about 14 Ma ago in the outer zone of Southwest Japan. Hence, it is inferred that the igneous activity was intensive in the relatively wide regions of Southwest Japan about 15 Ma ago.