Late Pleistocene and Holocene activity of the Atacazo–Ninahuilca Volcanic Complex (Ecuador)     

Silvana Hidalgo  Michel Monzier  Eduardo Almeida  Gilles Chazot  Jean-Philippe Eissen  Johannes van der Plicht  Minard L. Hall 《Journal of Volcanology and Geothermal Research》2008

The Atacazo–Ninahuilca Volcanic Complex (ANVC) is located in the Western Cordillera of Ecuador, 10 km southwest of Quito. At least six periods of Pleistocene to Holocene activity (N1 to N6) have been preserved in the geologic record as tephra fallouts and pyroclastic flow deposits. New field data, including petrographic and whole-rock geochemical analyses of over forty soil and tephra sections, 100 pumice and lithic samples, and 10 new ¹⁴C ages allow us to constrain: (1) the tephra fall isopachs and detailed characteristics of the last two events (N5–N6) including volume estimates of the tephra and pyroclastic flow deposits and the corresponding volcanic explosivity index (VEI); (2) the petrographical and geochemical correlations between domes, tephras, and pyroclastic flow deposits; and, (3) the timing of the last 4 eruptive events and a period of quiescence that endured a few thousand years (1000–4000).  相似文献   

Paleoproterozoic granitic gneisses of the Dinggye and LhagoiKangri areas from the higher and northern Himalaya, Tibet: Geochronology and implications   总被引：2，自引：0，他引：2  

Liao QunAn  Li DeWei  Lu Lian  Yuan YieMing  Chu LingLin 《中国科学D辑(英文版)》2008,51(2):240-248

Granitic gneisses have been widely found in crystalline rocks in the Dinggye area of the Higher Himalaya (HHM) and the LhagoiKangri area of the North Himalaya (NHM), Tibet. In the HHM, the gneisses intruded in the granulite-amphibolite facies metamorphosed sedimentary rocks, known as Nyalam group. In the NHM, the gneisses intruded in the amphibolite facies metamorphosed ones, known as LhagoiKangri group. These granitic gneisses are peraluminous monzonitic granites in terms of their mineral assemblage, and are considered as being derived from metamorphosed sedimentary rocks by anatexis based on the transitional relationship of the gneisses with their migmatitized wall rocks. Zircons are similar in crystal shape and interior structure from both gneisses. Most of them are euhedral or subhedral elongated prism-shaped transparent crystals, with fine oscillatory zoning, showing the magmatic genesis. Some of them are short prism-shaped and with relict core inherited from magma source and oscillatory zoning mantle crystallized from magma. SHRIMP U-Pb dating of zicons shows that both the granitic gneisses in the HHM and NHM are Paleoproterozoic (1811.6±2.9 Ma and 1811.7±7.2 Ma, respectively). These ages are similar to those (1815 to 2120 Ma) from granitic gneiss which is widely distributed in the Lesser Himalaya (LHM). The ages of inherited zircons (>2493.9±7.0 Ma, 2095.8± 8.8 Ma, 1874±29 Ma) exhibit the possible presence of several thermal events in Paleoproterozoic. All of the ages suggest the same India basement beneath the different units in Himalaya area, and do not support the idea that the HHM and NHM are accretionary terranes in Pan-Africa orogenic event. The fact that the basement in HHM is as old as or even younger than LHM is inconsistent with the presently prevalent orogenic models such as either extrusion of low-viscosity mid-crust or orogenic channel.  相似文献   

Cordilleran-type orogeny and episodic growth of continents: Insights from the circum-Pacific continental margins     

Takahiro Tagami  & Noriko Hasebe 《Island Arc》1999,8(2):206-217

A new material migration hypothesis for the plate subduction zone orogeny, so-called ‘the cordilleran-type orogeny’, is proposed on the basis of geological constraints as well as mechanics of accretionary wedges. The major tectonic processes of the hypothesis comprise: (i) episodic, extensive magmatism along the margin of an overriding plate; (ii) supply of voluminous igneous and eroded materials through forearc to trench, with an increase in the net sediment influx into trench; (iii) accelerated accretion of sediments beneath an overriding older accretionary wedge; and (iv) upward material migration within the wedge and resultant exhumation of high-P/T metamorphic rocks near the inland margin of the wedge. This hypothesis was validated by the test using available geo- and thermo-chronological data from two classical types of subduction-related orogens in Southwest Japan and California. The hypothesis, coupled with the thermochronologic point of view, requires the reconsideration of coevality of paired metamorphism. The temporal pairing is to be observed between the beginning of the regional high-T/P metamorphism and that of the uplift and exhumation of high-P/T metamorphism, with some time lag needed for material migration. Where the temporal pairing is examined therefore, the formation age of igneous rocks and related high-T/P metamorphic rocks should be compared to the exhumation age of high-P/T metamorphic rocks. The episodic, extensive magmatism that triggers the cordilleran-type orogeny shows a temporal correlation in the mid-Cretaceous for most circum-Pacific continental margins. The resultant widespread formation of accretionary complexes is also observed in the western part of the circum-Pacific margins. The deduced mid-Cretaceous circum-Pacific orogeny accompanied a gross increase in the continental crust production rate, and was approximately coeval with the Pangea breakup and the Central Pacific superplume episode, implying the orogeny as a part of the mid-Cretaceous pulsation of the Earth.  相似文献   

Geology and thermochronometry of the east edge of the Median Batholith (Median Tectonic Zone): a new perspective on Permian to Cretaceous crustal growth of New Zealand   总被引：2，自引：0，他引：2  

Nick Mortimer  Phil Gans  y Calvert and  Nick Walker 《Island Arc》1999,8(3):404-425

Abstract We report new field, petrological and isotopic data and interpretations from one of New Zealand’s major basement geological boundaries, the contact between the east side of the Median Batholith (formerly Median Tectonic Zone) and the allochthonous Mesozoic terranes of the Eastern Province. In the Nelson and Hollyford–Eglinton areas this contact is a Cenozoic fault, the Median Tectonic Line of earlier workers. However, in the Longwood Range, unfaulted pre-Cenozoic geological relations are preserved intact. Our new Ar–Ar, U–Pb and isotopic data show that the Median Batholith in the Longwood Range consists of two suites. (i) Eastern, isotopically primitive (⁸⁷Sr/⁸⁶Sr_i = 0.702 to 0.703; ?_NdT = + 7 to + 8) trondhjemite and gabbroic rocks of Permian age that we believe are part of the intraoceanic Brook Street arc of the Eastern Province. (ii) Western, isotopically more evolved (⁸⁷Sr/⁸⁶Sr_i = 0.703 to 0.704; ?_NdT = + 3 to + 5) quartz diorites, quartz monzodiorites and rare granites of Middle Triassic to Early Jurassic age that we correlate with a pulse of magmatism elsewhere in the Median Batholith. Field observations in the Longwood Range indicate intrusive, not faulted, contacts between these units and constrain accretion of the Brook Street Terrane to Gondwana to have occurred 230–245 Ma. Intra-batholith shear zones (T ~ 600°C and P ~ 0.2–0.3 GPa) were active at approximately 220 Ma. Modelling of K-feldspar Ar incremental heating ages indicate that most of the Longwood Range had cooled below 175°C by the Middle Jurassic and experienced no subsequent reheating. Significant additional post-accretionary Early Cretaceous and Cenozoic thermotectonic activity in Median Batholith in the Hollyford-Eglinton area is indicated by a new 140 ± 2 Ma U-Pb zircon date on a Largs ignimbrite, as well as by Cenozoic K-feldspar Ar–Ar ages in the Middle Triassic Mistake Diorite.  相似文献   

张宣金矿密集区基底及成矿作用年代学研究进展   总被引：3，自引：0，他引：3  

李怀坤  陆松年  李惠民  杨春亮 《地球学报》1999,20(2):169-176

张宣地区是华北地台北缘著名的金矿密集区之一,分布着规模不等的金矿床（点）百余处。区内前寒武纪基底呈双层结构模式：其下层结构是由厘定后的桑干杂岩组成,主体是太古宙晚期－元古宙初期的深成花岗质片麻岩,含规模不等和时代更老的各类表壳岩包体;上层结构是时代上可能和邻区集宁岩群相当的艾家沟岩组孔兹岩系。和基底有关的金矿主要与下层结构关系密切。Ａｕ丰度１０倍于地壳克拉克值的初始地壳,是该区形成金矿密集区的物质基础。精确的单颗粒锆石Ｕ－Ｐｂ年代学研究表明,区内重要金矿床小营盘金矿在吕梁运动期１８００Ｍａ左右的退变质作用过程中有一期重要矿化作用;而著名的东坪金矿则形成于其赋矿围岩石英二长岩就位之后约６０Ｍａ的３５０Ｍａ的海西早期。  相似文献   

Cosmogenic10Be: A critical view on its widespread dominion in geosciences     

D. Lal 《Journal of Earth System Science》2000,109(1):181-186

The radionuclide¹⁰Be (half-life = 1.5my), produced naturally in the Earth’s atmosphere by nuclear interactions of cosmic rays, was sought in ocean sediments in the late fifties, considering its potential usefulness as a radiotracer for dating sediments.¹⁰Be was discovered independently by two groups, one in India and the other in the USA, and used only for dating marine sediments and manganese nodules until the seventies. Subsequently, as a result of a technical advance resulting in the improvement in the sensitivity of measurement of¹⁰Be by about a factor of 10⁶, there was a global rush to measure this nuclide in most materials participating in the physical, chemical and biological processes in the dynamic geosphere. This paper outlines the reasons for this “isotope rush”, and the lessons learned from these studies. I also present my personal views of the special attractive features of this nuclide on the one hand, and on the other, the pitfalls or the wrong message this nuclide could convey!  相似文献   

LASER PROBE ~(40)Ar/ ~(39)Ar DATING OF MICAS ON THE DEFORMED ROCKS FROM ALTYN FAULT AND ITS TECTONIC IMPLICATIONS,WESTERN CHINA     

Liu Yongjiang    Ye Huiwen    Ge Xiaohong    Liu Junlai    Pan Hongxun    Chen Wen  WT  ”BX 《地学前缘》2000,(Z1)

LASER PROBE ~(40)Ar/ ~(39)Ar DATING OF MICAS ON THE DEFORMED ROCKS FROM ALTYN FAULT AND ITS TECTONIC IMPLICATIONS,WESTERN CHINAtheNationalNaturalScienceFundCommittee (NO .4 9772 157)  相似文献   

Sr–Nd isotope ratios of gabbroic and dioritic rocks in a Cretaceous-Paleogene granite terrain, Southwest Japan     

Shigeru Iizumi  Teruyoshi Imaoka and  Hiroo Kagami 《Island Arc》2000,9(1):113-127

Abstract Whole‐rock chemical and Sr and Nd isotope data are presented for gabbroic and dioritic rocks from a Cretaceous‐Paleogene granitic terrain in Southwest Japan. Age data indicate that they were emplaced in the late Cretaceous during the early stages of a voluminous intermediate‐felsic magmatic episode in Southwest Japan. Although these gabbroic and dioritic rocks have similar major and trace element chemistry, they show regional variations in terms of initial Sr and Nd isotope ratios. Samples from the South Zone have high initial ⁸⁷Sr/⁸⁶Sr (0.7063–0.7076) and low initial Nd isotope ratios (?_Nd, ?2.5 to ?5.3); whereas those from the North Zone have lower initial ⁸⁷Sr/⁸⁶Sr (usually less than 0.7060) and higher Nd isotope ratios (?_Nd, ?0.8 to + 3.3). Regional variations in Sr and Nd isotope ratios are similar to those observed in granitic rocks, although gabbroic and dioritic rocks tend to have slightly lower Sr and higher Nd isotope ratios than granitic rocks in the respective zones. Limited variations in Sr and Nd isotope ratios among samples from individual zones may be attributed partly to a combination of upper crustal contamination and heterogeneity of the magma source. Contamination of magmas by upper crustal material cannot, however, explain the observed Sr and Nd isotope variations between samples from the North and South Zones. Between‐zone variations would reflect geochemical difference in magma sources. The gabbroic and dioritic rocks are enriched in large ion lithophile elements (LILE) and depleted in high field strength elements (HFSE), showing similar normal‐type mid‐ocean ridge basalt (N‐MORB) normalized patterns to arc magmas. Geochronological and isotopic data may suggest that some gabbroic and dioritic rocks are genetically related to high magnesian andesite. Alternatively, mantle‐derived mafic or intermediate rocks which were underplated beneath the crust may be also plausible sources for gabbroic and dioritic rocks. The magma sources (the mantle wedge and lower crust) were isotopically more enriched beneath the South Zone than the North Zone during the Cretaceous‐Paleogene. Sr and Nd isotope ratios of the lower crustal source of the granitic rocks was isotopically affected by mantle‐derived magmas, resulting in similar initial Sr and Nd isotope ratios for gabbroic, dioritic and granitic rocks in each zone.  相似文献   

New K–Ar ages of shield lavas from Waianae Volcano, Oahu, Hawaiian Archipelago     

H. Guillou  J. Sinton  C. Laj  C. Kissel  N. Szeremeta 《Journal of Volcanology and Geothermal Research》2000,96(3-4)

A geochronological study utilized the unspiked potassium–argon (K–Ar) technique to obtain ages from the two main volcanic members of the shield stage of the Waianae Volcano, HI. These new dates are further constrained using a combination of stratigraphic relationships, magnetostratigraphy and major element geochemistry. Exposed shield lavas encompass 0.85 Ma, with reliably dated tholeiitic lavas from the main shield ranging from 3.93±0.08 to 3.54±0.04 Ma, and a later shield stage ranging in age from 3.57±0.04 to 3.08±0.04 Ma. These data suggest that the total extent of Waianae shield activity was significantly more than 1 Ma. The age of faulting in two flank zones is constrained to be about 3.4 Ma. Preliminary estimates of lava accumulation rates vary from about 0.3 to 2.0 mm/a; calculated rates show no systematic variation with location in the volcano or with time.  相似文献   

粤西阳春中生代钾玄质侵入岩及其构造意义:Ⅰ.岩石学和同位素地质年代学   总被引：45，自引：7，他引：38  

李献华  周汉文  刘颖  李寄嵎  陈正宏  于津生  桂训唐 《地球化学》2000,29(6):513-520

粤西阳春地区马山二长岩的岩石化学组成特征为典型的钾玄质岩石,岗尾、轮水石英二长岩和花岗闪长岩属酸性一偏中性的钾玄质系列岩石(少数样品为高钾钙碱性岩石),而石菉花岗闪长岩则为典型的钙碱性系列岩石,这些岩体在岩石化学演化趋势上存在显著的差异,不属于同一个岩石成因系列.40Ar-39Ar和Rb-Sr同位素定年结果表明,马山和岗尾岩体的形成年龄分别为(164±2)Ma和(154±1)Ma.阳春地区从中侏罗世到早白垩世期间发生了3次主要的岩浆活动,从早到晚岩浆的K富集程度逐渐降低,岩浆岩成分从钾玄质系列过渡到钙碱系列.  相似文献