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1.
LA-ICP-MS zircon U–Pb ages and geochemical data are presented for the Mesozoic volcanic rocks in northeast China, with the aim of determining the tectonic settings of the volcanism and constraining the timing of the overprinting and transformations between the Paleo-Asian Ocean, Mongol–Okhotsk, and circum-Pacific tectonic regimes. The new ages, together with other available age data from the literature, indicate that Mesozoic volcanism in NE China can be subdivided into six episodes: Late Triassic (228–201 Ma), Early–Middle Jurassic (190–173 Ma), Middle–Late Jurassic (166–155 Ma), early Early Cretaceous (145–138 Ma), late Early Cretaceous (133–106 Ma), and Late Cretaceous (97–88 Ma). The Late Triassic volcanic rocks occur in the Lesser Xing’an–Zhangguangcai Ranges, where the volcanic rocks are bimodal, and in the eastern Heilongjiang–Jilin provinces where the volcanics are A-type rhyolites, implying that they formed in an extensional environment after the final closure of the Paleo-Asian Ocean. The Early–Middle Jurassic (190–173 Ma) volcanic rocks, both in the Erguna Massif and the eastern Heilongjiang–Jilin provinces, belong chemically to the calc-alkaline series, implying an active continental margin setting. The volcanics in the Erguna Massif are related to the subduction of the Mongol–Okhotsk oceanic plate beneath the Massif, and those in the eastern Jilin–Heilongjiang provinces are related to the subduction of the Paleo-Pacific Plate beneath the Eurasian continent. The coeval bimodal volcanic rocks in the Lesser Xing’an–Zhangguangcai Ranges were probably formed under an extensional environment similar to a backarc setting of double-direction subduction. Volcanic rocks of Middle–Late Jurassic (155–166 Ma) and early Early Cretaceous (145–138 Ma) age only occur in the Great Xing’an Range and the northern Hebei and western Liaoning provinces (limited to the west of the Songliao Basin), and they belong chemically to high-K calc-alkaline series and A-type rhyolites, respectively. Combined with the regional unconformity and thrust structures in the northern Hebei and western Liaoning provinces, we conclude that these volcanics formed during a collapse or delamination of a thickened continental crust related to the evolution of the Mongol–Okhotsk suture belt. The late Early Cretaceous volcanic rocks, widely distributed in NE China, belong chemically to a low- to medium-K calc-alkaline series in the eastern Heilongjiang–Jilin provinces (i.e., the Eurasian continental margin), and to a bimodal volcanic rock association within both the Songliao Basin and the Great Xing’an Range. The volcanics in the eastern Heilongjiang–Jilin provinces formed in an active continental margin setting related to the subduction of the Paleo-Pacific Plate beneath the Eurasian continent, and the bimodal volcanics formed under an extensional environment related either to a backarc setting or to delamination of a thickened crust, or both. Late Cretaceous volcanics, limited to the eastern Heilongjiang–Jilin provinces and the eastern North China Craton (NCC), consist of calc-alkaline rocks in the eastern Heilongjiang–Jilin provinces and alkaline basalts in the eastern NCC, suggesting that the former originated during subduction of the Paleo-Pacific Plate beneath the Eurasian continent, whereas the latter formed in an extensional environment similar to a backarc setting. Taking all this into account, we conclude that (1) the transformation from the Paleo-Asian Ocean regime to the circum-Pacific tectonic regime happened during the Late Triassic to Early Jurassic; (2) the effect of the Mongol–Okhotsk suture belt on NE China was mainly in the Early Jurassic, Middle–Late Jurassic, and early Early Cretaceous; and (3) the late Early Cretaceous and Late Cretaceous volcanics can be attributed to the subduction of the Paleo-Pacific Plate beneath the Eurasian continent.  相似文献   

2.
The 40Ar/39Ar geochronological method was applied to date magmatic and hydrothermal alteration events in the Mantos Blancos mining district in the Coastal Cordillera of northern Chile, allowing the distinction of two separate mineralization events. The Late Jurassic Mantos Blancos orebody, hosted in Jurassic volcanic rocks, is a magmatic-hydrothermal breccia-style Cu deposit. Two superimposed mineralization events have been recently proposed. The first event is accompanied by a phyllic hydrothermal alteration affecting a rhyolitic dome. The second mineralization event is related to the intrusion of bimodal stocks and sills inside the deposit. Because of the superposition of several magmatic and hydrothermal events, the obtained 40Ar/39Ar age data are complex; however, with a careful interpretation of the age spectra, it is possible to detect complex histories of successive emplacement, alteration, mineralization, and thermal resetting. The extrusion of Jurassic basic to intermediate volcanic rocks of the La Negra Formation is dated at 156.3 ± 1.4 Ma (2σ) using plagioclase from an andesitic lava flow. The first mineralization event and associated phyllic alteration affecting the rhyolitic dome occurred around 155–156 Ma. A younger bimodal intrusive event, supposed to be equivalent to the bimodal stock and sill system inside the deposit, is probably responsible for the second mineralization event dated at ca. 142 Ma. Other low-temperature alteration events have been dated on sericitized plagioclase at ca. 145–146, 125, and 101 Ma. This is the first time that two distinct mineralization events have been documented from radiometric data for a copper deposit in the metallogenic belt of the Coastal Cordillera of northern Chile. Electronic supplementary material The online version of this article (doi:) contains supplementary material, which is available to authorized users.  相似文献   

3.
Several major volcanic zones are distributed across the eastern North China Craton, from northwest to southeast: the Greater Xing’an Range, Jibei-Liaoxi, Xishan, and Songliao Basins, and the Yanji, Huanghua, and Ludong volcanic zones. The Huanghua depression within the Bohai Bay Basin was filled by middle Late Mesozoic volcanic rocks and abundant Cenozoic alkaline basalts. Zircon LA-ICP-MS and SHRIMP U–Pb dating show that basic–intermediate volcanic rocks were extruded in the Early Cretaceous of 118.8 ± 1.0 Ma (weighted mean 206Pb/238U age), before Late Cretaceous acid lavas at 71.5 ± 2.6 Ma. An inherited zircon from andesite has a Paleoprotoerozoic core crystallization age of 2,424 ± 22 Ma (206Pb/207Pb age) indicating that the basement of the Bohai Bay Basin is part of the North China Craton. Early Cretaceous basic and intermediate lavas are characterized by strong enrichments in LREE and LILE and depletions in HREE and HFSE, indicating a volcanic arc origin related to oceanic subduction. Depletion in Zr only occurs in basic and intermediate volcanic rocks, while depletions in Sr and Ti exist only in acid samples, indicating that the acid series is not genetically related to the basic–intermediate series. Formation ages and geochemical features indicate that the Late Cretaceous acid lavas are products of crustal remelting in an extensional regime. Combined information from all these volcanic zones shows that subduction-related volcanic rocks were generated in the Jibei-Liaoxi and Xishan volcanic zones during the Early Jurassic, about 60 Ma earlier than their analogues extruded in the Huanghua and Ludong volcanic zones during the Early Cretaceous. This younging trend also exists in the youngest extension-related volcanism in each of these zones: Early Cretaceous asthenosphere-derived alkaline basalts in the northwest and Late Cretaceous in the southeast. A tectonic model of northwestward subduction and continuous oceanward retreat of the Paleo-Pacific Plate is proposed to explain the migration pattern of both arc-related and post-subduction extension-related volcanic rocks. As the subduction zone continuously migrated, active continental margin and backarc regimes successively played their roles in different parts of North China during the Late Mesozoic (J1–K2).  相似文献   

4.
The Rodna Mountains afford the most internal structural window into the crystalline units of the Eastern Carpathians in Romania. The Rodna Mountains consist of Variscan metamorphic nappes that were restacked in the Alpine phase of Carpathian development forming the Subbucovinian and Infrabucovinian nappes. In order to evaluate age of deformation, ten samples were taken from the zone of greenschist facies mylonitic schist that marks the Alpine tectonic boundary between the Subbucovinian and Infrabucovinian nappes and 40Ar/39Ar laser single-grain ages determined for schistosity-forming muscovite. Microstructural assessment of quartz and muscovite distinguished two deformation events. Single-grain ages from the microstructurally most strongly reworked samples (four samples) give a tight clustering of ages at ca. 95 Ma. The least reworked schists have a broader clustering of ages spanning ca. 200–280 Ma with a late Permian peak and some samples showing outlier ages in the range 200–100 Ma. The relative development of the outliers, which correlates with evidence for increased microstructural reworking, is interpreted to mark progressive isotopic resetting. The ca. 95 Ma ages for the most reworked schists are estimates for the age of the Alpine nappe stacking. The ca. 200–280 Ma ages are similar to those of magmatism, metamorphism, and sedimentation thought to mark post-Variscan-pre-Alpine rifting and ocean basin formation in parts of the Alps and may be the thermal imprint of a related event in the Eastern Carpathians.  相似文献   

5.
Lower Cretaceous volcanic lithic arenites, widely distributed in the Tethyan Himalaya, provide insights into the continental breakup of Eastern Gondwana. In southern Tibet they are represented by the Wölong Volcaniclastics. The volcanic rocks that contributed clastic material to the lower parts of this unit were predominantly alkali basalts, whereas rhyolitic/dacitic volcanism becomes the predominant source of the upper strata. Geochemical analyses of basaltic grains and of detrital Cr-spinels from the Wölong Volcaniclastics demonstrate the alkaline character of the volcanism and suggest “within-plate” tectonic setting for the volcanism. Zircon U–Pb ages confirm that this volcanism continued from ～ 140 Ma to ～ 119 Ma. Hf-isotope data on these Early Cretaceous zircons indicate that their parental magmas were mantle-derived, but in the later stage of magmatic activity mantle-derived magmas were mixed with partial melts derived from the continental crust.The Lower Cretaceous volcaniclastics occur along a broad belt paralleling the northern margin of Greater India. The onset of volcaniclastic deposition in the Himalayas appears to become progressively younger toward the west, but it ended synchronously during the Late Albian (～ 102 Ma). The low volume of volcanic rocks and their intra-plate tectonic setting suggest that they are the result of decompressional melting along extensional deep-seated fractures cross-cutting the continental crust, and reflect changes in regional intra-plate tectonic stresses when Greater India began to separate from the Australia–Antarctica supercontinent.  相似文献   

6.
《International Geology Review》2012,54(11):1417-1442
ABSTRACT

The Ordos Basin, situated in the western part of the North China Craton, preserves the 150-million-year history of North China Craton disruption. Those sedimentary sources from Late Triassic to early Middle Jurassic are controlled by the southern Qinling orogenic belt and northern Yinshan orogenic belt. The Middle and Late Jurassic deposits are received from south, north, east, and west of the Ordos Basin. The Cretaceous deposits are composed of aeolian deposits, probably derived from the plateau to the east. The Ordos Basin records four stages of volcanism in the Mesozoic–Late Triassic (230–220 Ma), Early Jurassic (176 Ma), Middle Jurassic (161 Ma), and Early Cretaceous (132 Ma). Late Triassic and Early Jurassic tuff develop in the southern part of the Ordos Basin, Middle Jurassic in the northeastern part, while Early Cretaceous volcanic rocks have a banding distribution along the eastern part. Mesozoic tectonic evolution can be divided into five stages according to sedimentary and volcanic records: Late Triassic extension in a N–S direction (230–220 Ma), Late Triassic compression in a N–S direction (220–210 Ma), Late Triassic–Early Jurassic–Middle Jurassic extension in a N–S direction (210–168 Ma), Late Jurassic–Early Cretaceous compression in both N–S and E–W directions (168–136 Ma), and Early Cretaceous extension in a NE–SW direction (136–132 Ma).  相似文献   

7.
The waning stage(s) of the Tethyan ocean(s) in the Balkans are not well understood. Controversy centres on the origin and life‐span of the Cretaceous Sava Zone, which is allegedly a remnant of the last oceanic domain in the Balkan Peninsula, defining the youngest suture between Eurasia‐ and Adria‐derived plates. In order to investigate to what extent Late‐Cretaceous volcanism within the Sava Zone is consistent with this model we present new age data together with trace‐element and Sr–Nd–Pb isotope data for the Klepa basaltic lavas from the central Balkan Peninsula. Our new geochemical data show marked differences between the Cretaceous Klepa basalts (Sava Zone) and the rocks of other volcanic sequences from the Jurassic ophiolites of the Balkans. The Klepa basalts mostly have Sr–Nd–Pb isotopic and trace‐element signatures that resemble enriched within‐plate basalts substantially different from Jurassic ophiolite basalts with MORB, BAB and IAV affinities. Trace‐element modelling of the Klepa rocks indicates 2%–20% polybaric melting of a relatively homogeneously metasomatised mantle source that ranges in composition from garnet lherzolite to ilmenite+apatite bearing spinel–amphibole lherzolite. Thus, the residual mineralogy is characteristic of a continental rather than oceanic lithospheric mantle source, suggesting an intracontinental within‐plate origin for the Klepa basalts. Two alternative geodynamic models are internally consistent with our new findings: (1) if the Sava Zone represents remnants of the youngest Neotethyan Ocean, magmatism along this zone would be situated within the forearc region and triggered by ridge subduction; (2) if the Sava Zone delimits a diffuse tectonic boundary between Adria and Europe which had already collided in the Late Jurassic, the Klepa basalts together with a number of other magmatic centres represent volcanism related to transtensional tectonics.  相似文献   

8.
More than 100 volcanic necks in central Scania (southern Sweden) are the product of Jurassic continental rift-related mafic alkaline magmatism at the southwest margin of the Baltic Shield. They are mainly basanites, with rarer melanephelinites. Both rock groups display overlapping primitive Mg-numbers, Cr and Ni contents, steep chondrite-normalized rare earth element patterns (LaN /YbN = 17–27) and an overall enrichment in incompatible elements. However, the melanephelinites are more alkaline and have stronger high field strength element enrichment than the basanites. The existence of distinct primary magmas is also indicated by heterogeneity in highly incompatible element ratios (e.g. Zr/Nb, La/Nb). Trace element modelling indicates that the magmas were generated by comparably low degrees of melting of a heterogeneous mantle source. Such a source can best be explained by a metasomatic overprint of the mantle lithosphere by percolating evolved melts. The former existence of such alkaline trace element-enriched melts can be demonstrated by inversion of the trace element content of green-core clinopyroxenes and anorthoclase which occur as xenocrysts in the melanephelinites and are interpreted as being derived from crystallization of evolved mantle melts. Jurassic magmatic activity in Scania was coeval with the generation of nephelinites in the nearby Egersund Basin (Norwegian North Sea). Both Scanian and North Sea alkaline magmas share similar trace element characteristics. Mantle enrichment processes at the southwest margin of the Baltic Shield and the North Sea Basin generated trace element signatures similar to those of ocean island basalts (e.g. low Zr/Nb and La/Nb) but there are no indications of plume activity during the Mesozoic in this area. On the contrary, the short duration of rifting, absence of extensive lithospheric thinning, and low magma volumes argue against a Mesozoic mantle plume. It seems likely that the metasomatic imprint resulted from the earlier Permo-Carboniferous rifting episode which affected the entire study area and clearly was accompanied by plume activity (Ernst and Buchan in American Geophysical Union, pp 297–337, 1997). Renewed rifting in Jurassic times triggered decompression melting in the volatile-enriched lithospheric mantle and the alkaline melts generated inherited the earlier stored plume signature.This revised version was published online September 2004 with a correction to the footnote of the sample list.  相似文献   

9.
Mafic rocks from the Bamenda volcanic province along the Cameroon Volcanic Line have been dated from 17 to 0 Ma. Associated with some trachytes and rhyolites, this volcanism covers a period of more than 25 Ma. The studied rocks are basalts to mugearites. Most of them have been contaminated by continental crust during their transit to the surface. The oldest rocks are the most contaminated. One group of samples shows high Eu, Sr and Ba contents. This characteristic is not due to crustal contamination process, but has a mantle source origin. We argue that these characteristics have been acquired by mixing of melts formed by partial melting of mantle pyroxenites with melts formed in mantle peridotites. Such pyroxenites have been observed as mantle xenoliths in the Adamaoua province, and their chemical and isotopic compositions are consistent with such a model.  相似文献   

10.
Mineralization and alteration events at ten Miocene porphyry Cu and porphyry-related epithermal mineral deposits in southern, central, and northern Ecuador were dated by means of molybdenite Re-Os, biotite and alunite 40Ar/39Ar, and titanite U-Pb geochronology. Most of these hydrothermal events show a spatio-temporal correlation with porphyry intrusion emplacement as constrained by zircon U-Pb ages. The total age range for these events spans the 23.5–6.1 Ma period, without displaying systematic along- or across-arc age distribution trends. While epithermal deposits tend to be spatially associated with volcanic rocks of a similar age, porphyry Cu deposits in Ecuador are frequently spatially associated with deeper-seated basement units and batholith-scale precursor intrusive systems assembled over ≥5 m.y. time periods. In most cases, formation of the porphyry Cu deposits is related to the youngest magmatic (-hydrothermal) event in a given area, postdating batholith construction at a regional scale. The majority of Miocene deposits occurs in southern Ecuador where areally extensive, post-mineralization (late Miocene to recent) volcanic sequences with the potential to conceal mineralization at depth are lacking. Only few Miocene deposits occur in northern-central Ecuador, where they mainly crop out in the Western Cordillera, west of the productive present-day volcanic arc. The surface distribution of post-mineralization arc volcanism reflects along-arc variations in subducting slab geometry. Porphyry Cu and epithermal deposits in Ecuador define a Miocene metallogenic belt broadly continuous with its coeval counterpart in northern-central Peru. Although both belt segments were formed in an overall similar tectonomagmatic and metallogenic setting, their respective metal endowments differ significantly.  相似文献   

11.
Laser-probe dating of mylonite whole-rock samples from the North Tianshan—Main Tianshan fault zone that cross-cuts the North Tianshan domain’s southern margin yielded 40Ar/39Ar spectra with 255–285 Ma ages. Biotite from an undeformed, Early Carboniferous granite, which cuts the steep mylonitic foliation in the Proterozoic basement of the Yili arcs’s southern margin, gave a 263.4 ± 0.6 Ma plateau age (1σ). Pre-Carboniferous metasediments overlying this basement yielded plateau ages (1σ) of 253.3 ± 0.3 (muscovite) and 252.3 ± 0.3 (biotite) Ma. The Permian ages of mylonites date movement on these ductile, dextral strike-slip shear zones, whereas the mica ages are interpreted by recrystallisation as a result of fluid flow around such transcurrent faults. We propose that the Tianshan’s Permian syn-tectonic bimodal magmatism was created in a non-plume-related Yellowstone-like extensional–transtensional tectonic regime. Gold mineralisation, tracing aqueous flow in the crust, peaked in Permian time and continued locally into the Triassic. The picture is emerging that a convective fluid system partly driven by magmatic heat, existed in a strongly fractured and weakened crust with an elevated heat flow, leading to regional-scale isotope resetting. We suggest that surprisingly young isotopic ages in the literature for early orogenic (ultra)high-pressure metamorphism are similarly due to fluid-mediated recrystallisation.  相似文献   

12.
Kyushu Island, Japan, is located at the junction of the Southwest Japan arc and the Ryukyu arc. There are two major late Cenozoic epithermal gold-silver provinces in Kyushu, which are termed the Northern and Southern provinces. The provinces are characterized by: 1) Pliocene volcanism dominated by calc-alkaline andesite, followed by Quaternary volcanism including extrusion of both calc-alkaline and tholeiitic magmas; 2) formation of extensional grabens; 3) Pliocene to Pleistocene mineralization, which was dominated by abundant low sulfidation (LS) epithermal deposits with a few high sulfidation (HS) examples. The two epithermal gold-silver provinces have evolved differently since about 5 Ma; the Northern province has exhibited diminished hydrothermal activity from the Pliocene to Pleistocene, whereas the Southern province has witnessed increased hydrothermal activity mainly in easterly and northerly directions. Changes of tectonic setting from the Pliocene to Pleistocene account for the variable trends in epithermal gold deposit formation. Westward oblique subduction of the Philippine Sea plate beneath the Southwest Japan arc caused development of the Hohi graben and arc-related volcanism at about 6 Ma. This was associated with widespread LS mineralization in and surrounding the Hohi graben, as is represented by the Bajo and Taio deposits. The subduction of the relatively buoyant Kyushu-Palau ridge during the early Pliocene strengthened the coupling between the slab and overriding Ryukyu arc, leading to polygenetic andesite volcanism with associated HS (Kasuga, Iwato, and Akeshi) and LS (Kushikino) mineral deposits forming in the Southern province. A change of the subduction direction of the Philippine Sea plate, from west to north-northwest in the early Pliocene, increased the orthogonal convergence rate between the Southwest Japan arc and the Philippine Sea plate, resulting in a decrease of volcanic and hydrothermal activity in the Hohi graben of the Northern province. The more northerly subduction of the Philippine Sea plate shifted the locus of the Kyushu-Palau ridge subduction northward, resulting in underplating of the older (85–60 Ma), negatively buoyant Amami basin oceanic slab in the Southern province, rather than continued subduction of the young (27–15 Ma), buoyant Shikoku basin slab. This replacement caused steepening of the slab angle and slab-rollback in the Southern province, which was associated with regional extension, an eastward shift of the Ryukyu volcanic front, and development of the Kagoshima and Shimabara grabens, as well as the Okinawa trough. Rhyolite and basalt volcanism, in addition to andesite volcanism, have occurred since 2 Ma in the area of the Ryukyu back arc; coincident LS mineralization at Hishikari and Ohkuchi was affiliated with the rhyolite volcanism. Another change of the subduction direction of the Philippine Sea plate to the northwest occurred at 2–1 Ma. The forearc sliver of the Southwest Japan arc shifted westward, in association with right-lateral strike-slip faulting along the Median tectonic line, due to the increase of the westward convergence rate. This shift resulted in shortening and cessation of graben development in the Hohi area, restricting the subsequent volcanism and related hydrothermal activity to the central part of the graben.  相似文献   

13.
中国东南部晚中生代-新生代玄武岩与壳幔作用   总被引：46，自引：2，他引：44  
中国东南部的火山活动在早中生代时期仅有很小规模,晚中生代最早的、较大规模的岩浆活动始于中侏罗世早期,至早白垩世是火山岩浆活动的鼎盛期,在近100个Ma的时间内形成了大面积分布的晚中生代火山-侵入岩,而在新生代则以面积较小的玄武岩浆喷出活动为主,局限分布于沿海一带。以晚中生代湘南、赣南和闽西南的近EW向火山岩带和浙、闽沿海地区的近NNE向火山岩带,以及新生代近NNE向火山岩带为研究对象,对这些火山岩的地球化学特征对比研究,结合时空分布,讨论了它们的起源及其与壳幔相互作用的关系,以及它们形成的构造环境,其结果显示,EW向晚中生代火山岩带（180～170Ma）的西段玄武岩独立产出,且明显属碱性系列;而中段和东段玄武岩和流纹岩伴生,其中的玄武岩均为亚碱性系列的拉斑玄武岩。它们形成于板内拉张构造环境,是中国东南部特提斯构造域向太平洋构造域转换、晚中生代大规模岩浆作用的序幕。研究表明,该火山岩带自西向东表现出不同程度的壳幔相互作用,玄武岩在成岩过程中有少量陆壳组分加入。NNE向晚中生代火山岩带（130～90Ma）主要为流纹质岩石,安山岩和玄武岩很少。即使是双峰式火山岩也以酸性岩为主,玄武岩仅占全部火山岩体积的30％以下。其中的玄武岩主要属钙碱性系列,少数属拉斑系列。它们形成于火山弧构造环境,是中国东南部受太平洋构造域影响发生大规模火山岩浆作用的主旋律。其中玄武岩岩浆成分受到了较高程度的陆壳物质混染,同时代的中性火山岩是由底侵的玄武岩岩浆和陆壳物质来源的酸性岩浆发生岩浆混合作用而形成的,反映了强烈的壳幔相互作用。NNE向新生代火山岩带,分布在浙闽沿海,以碱性系列玄武岩为主,均含幔源包体,并受NNE向大陆边缘断裂构造的控制。它们形成于板内裂谷环境,是中国东南沿海由晚中生代火山弧构造环境转换为新生代板内裂谷环境的标志,起源于软流圈地幔,并有EMII岩石圈地幔的混合组分,但基本没有受到陆壳物质的混染。  相似文献   

14.
Geotectonically the Fengyang and Zhangbaling regions belong to the North China craton and the Dabie-Sulu oragene, respectively. Neo-Archean gneiss and amphibolite and metamor-phosed sea-facies sodic volcanic rocks axe the main outcrops in the two regions, respectively. The Zhangbaling terrane strike-skipped along the Tancheng-Lujiang fault zone in Mesozoic and Cenozo-ic eras and got close to the Fengyang terrane. Mesozoic Yanshanian intrusions occur broadly in thetwo regions. Gold-beating quartz veins occur in the metamorphic rocks in the Fengyang region and in the granodiorite and metamorphosed sea-facies sodic volcanic rocks in the Zhanghaling region.Generally, the formation of the auriferous quartz veins involved three stages. At the first stage,gold-poor sulfide quartz veins were formed; at the second stage gold-rich quartz sulfide veins wereformed; and at the third stage gold-poor barite and/or carbonate veins were formed. The 40^Ar/29^Ar step-heating plateau ages of the first-stage and the second-stage quartz aggregates from the Zhuding, Maoshan and Shangeheng gold deposits range between 116.1 0.6 Ma and 118.3 0.5 Ma and are pretty close to their least apparent ages and isoehronal ages, respectively. All plat-eau, least apparent and isoehronal ages range between 113.4 0.4 Ma and 118.3 0.5 Ma,which are considered as the formation age range of the quartz. It is reasonable and reliable to takethe 40^Ar/39^Ar age range of the quartz as the formation age range of gold-bearing quartz veins onthe basis of spatial relationship between gold-bearing quartz veins and their country rocks. Thegold deposits in the two regions were formed in Aptian, Cretaceous, when the Tancheng-Lujiangfault zone moved as a normal fault with slightly right-lateral strike-skip, was extensional and expe-rienced very strong magnmtic process. It is shown that the magnmtic hydrothermal fluid is a veryimportant part of the gold ore-forming hydrothermal fluid in the Fengyang and Zhanghaling re-gions. The formation of the gold ore deposits in the Fengyang and Zhanghaling regions had genetic relations with the extensional movement of the Tancheng-Lujiang fault zone and magmatic activities and took place under the extensional dynamic condition in Late Cretaceous. Therefore, the exten-sional movement of the Tancheng-Lujiang fault zone presented the energy and space for magmatic and gold ore-forming processes.  相似文献   

15.
歧口凹陷侏罗系火成岩主要以中-酸性的流纹岩、粗面岩和粗面安山岩为主,白垩系主要为中-基性的粗面玄武岩和玄武质粗面安山岩,第三系火成岩以粗面玄武岩、碱性玄武岩及辉绿岩为主。同位素定年结果获得白垩系玄武岩的年龄为133±20 Ma,安山岩和粗安岩年龄分别为111.8±0.9 Ma和122.1±3.1 Ma,第三系辉绿岩年龄为16.57±0.23 Ma。地球化学特征显示,侏罗系火成岩相对低钛、高碱和钠、低铁镁,轻重稀土元素强烈分馏、微量元素相对富集LILE、贫化HFSE、亏损Nb和Ti、P,部分高Sr、低Yb,总体指示为挤压环境下加厚地壳物质的熔融产物,白垩系火成岩中等钛和碱的含量,高钠、铁和镁,稀土和微量元素显示其来自于伸展背景的富集地幔源,但受到陆壳物质的混染,第三系火成岩高钛含量、中等碱含量但变化大、高钠、富铁镁,轻重稀土元素分馏弱到中等、微量元素富集LILE和HFSE,同样来自板内伸展环境的富集地幔源的产物。表明歧口凹陷构造转换发生在晚侏罗世和早白垩世之间。  相似文献   

16.
孟凡超  刘嘉麒  崔岩  高金亮  刘祥  童英 《岩石学报》2014,30(12):3569-3586
东北地区中生代经历了蒙古-鄂霍茨克构造体系向太平洋构造体系的转换,形成了不同期次火山活动。本文归纳总结了露头区与覆盖区中生代火山岩的年代学、空间分布、岩石组合以及地球化学特征,揭示了两个构造域的时空分布范围。该区火山岩锆石U-Pb年龄统计结果表明中生代存在五期火山活动:早-中侏罗世(190~160Ma)、晚侏罗世(160~145Ma)、早白垩世早期(145~120Ma)、早白垩世晚期(120~100Ma)、晚白垩世早期(100~90Ma)。早-中侏罗世火山岩分布较少,火山岩仅分布在大兴安岭西部满洲里地区和东部张广才岭以及南侧辽宁北票-朝阳地区,火山岩属于高钾钙碱性系列,为蒙古-鄂霍茨克海闭合和法拉隆板块双俯冲作用的产物。晚侏罗世东北地区火山活动明显增强,主要分布在大兴安岭地区,张广才岭以及小兴安岭也有少量分布。西部大兴安岭地区以粗面安山岩、粗面岩为主,属于同碰撞造山成因,为蒙古-鄂霍茨克海闭合造山环境产物。东部以中酸性、酸性岩为主,为法拉隆板块背离欧亚大陆,岩石圈伸展引起的壳源物质熔融产物。早白垩世早期火山活动最为强烈,火山岩主要分布在大兴安岭地区。岩性以高钾钙碱性系列的粗面玄武安山岩、粗面安山岩、安山岩、粗面岩为主,为蒙古-鄂霍茨克海闭合造山后伸展环境产物。早白垩世晚期火山岩主要分布在松辽盆地内部。火山岩以中酸性岩为主,属于中钾-高钾钙碱性系列,为伊泽奈崎板块俯冲引起的弧后拉张,软流圈上涌导致年轻地壳熔融的产物。晚白垩世早期火山岩仅分布在小兴安岭及吉林、黑龙江省东部地区。火山岩为一套玄武岩、玄武安山岩、安山岩和英安岩组合,属于中钾钙碱性系列,是伊泽奈崎-库拉板块高角度俯冲的大陆边缘岩浆活动产物。东北地区中生代不同期次火山岩记录了蒙古-鄂霍茨克构造域向太平洋构造域转换过程及其时空影响范围。  相似文献   

17.
The coastal Changle-Nan’ao tectonic zone of SE China contains important geological records of the Late Mesozoic orogeny and post-orogenic extension in this part of the Asian continent. The folded and metamorphosed T3–J1 sedimentary rocks are unconformably overlain by Early Cretaceous volcanic rocks or occur as amphibolite facies enclaves in late Jurassic to early Cretaceous gneissic granites. Moreover, all the metamorphic and/or deformed rocks are intruded by Cretaceous fine-grained granitic plutons or dykes. In order to understand the orogenic development, we undertook a comprehensive zircon U–Pb geochronology on a variety of rock types, including paragneiss, migmatitic gneiss, gneissic granite, leucogranite, and fine-grained granitoids. Zircon U–Pb dating on gneissic granites, migmatitic gneisses, and leucogranite dyke yielded a similar age range of 147–135 Ma. Meanwhile, protoliths of some gneissic granites and migmatitic gneisses are found to be late Jurassic magmatic rocks (ca. 165–150 Ma). The little deformed and unmetamorphosed Cretaceous plutons or dykes were dated at 132–117 Ma. These new age data indicate that the orogeny lasted from late Jurassic (ca. 165 Ma) to early Cretaceous (ca. 135 Ma). The tectonic transition from the syn-kinematic magmatism and migmatization (147–136 Ma) to the post-kinematic plutonism (132–117 Ma) occurred at 136–132 Ma.  相似文献   

18.
The temporal evolution of North American kimberlites   总被引：1，自引：0，他引：1  
North American kimberlite magmatism spans a period of time in excess of 1 billion years from Mesoproterozoic kimberlites in the Lake Superior and James Bay Lowlands region of Ontario to Eocene kimberlites in the Lac de Gras field, N.W.T. Based on a compilation of more than 150 robust radiometric age determinations, several distinct kimberlite emplacement patterns are recognized. In general, the temporal pattern of kimberlite emplacement in North America can be broadly subdivided into five domains: (1) a Mesoproterozoic kimberlite province in central Ontario, (2) an Eocambrian/Cambrian Labrador Sea Province in northern Québec and Labrador, (3) an eastern Jurassic Province, (4) a central Cretaceous corridor and (5) a western mixed domain that includes two Type-3 kimberlite provinces (i.e. multiple periods of kimberlite emplacement preserved in the Slave and Wyoming cratons). For some provinces the origin of kimberlite magmatism can be linked to known mantle heat sources such as mantle plume hotspots and upwelling asthenosphere attendant with continental rifting. For example, the timing and location of Mesoproterozoic kimberlites in North America coincides with and slightly precedes the timing of 1.1 Ga intracontinental rifting that culminated in the Midcontinent Rift centered in the Lake Superior region. Many of the kimberlites in the Eocambrian/Cambrian Labrador Sea province were emplaced soon after the opening of the Iapetus Ocean at about 615 Ma and may also be linked to mantle upwelling associated with continental rifting. The eastern Jurassic kimberlites record an age progression where magmatism youngs in a southeast direction from the 200 Ma Rankin Inlet kimberlites to the 155–126 Ma Timiskaming kimberlites. The location of several kimberlite fields and clusters in Ontario and Québec lie along a continental extension of the Great Meteor hotspot track and represents one of the best examples in the world of kimberlite magmatism triggered by mantle plumes. The central Cretaceous (103–94 Ma) corridor extends for more than 4000 km from Somerset Island in northern Canada through the Fort à la Corne field in Saskatchewan to the kimberlites in central USA. This is the first recognized corridor of kimberlite magmatism of this magnitude. The possible westward younging of Cretaceous to Eocene corridors of kimberlite magmatism could reflect major changes in plate geometry during subduction of the Kula–Farallon plate.  相似文献   

19.
The Shihu gold deposit is situated in the central shear zone of the Meso-Cenozoic Fuping metamorphic core complex in the middle-northern part of the Taihang Mountains. This kind of gold deposits named after ‘Shihu type’ widely occurs in this region. Gold-bearing quartz veins are their most important industrial orebodies. Detailed laser 40Ar/39Ar geochronology of the mineralized quartz veins in the study area reveals details of its tectono-magmatic history. The 40Ar/39Ar ages for the quartz are between 134.4 Ma and 155.9 Ma with a mean age of 141±4 Ma on the isochron line and 139±7 Ma on the inverse isochron line, respectively, which is interpreted to be the best estimate of the crystalline age of the quartz veins. The gold mineralization was most likely related to an underplating event that took place in the northern part of the Taihang Mountain at ca. 140 Ma. The timing of gold mineralization in the area is similar to those observed in other major gold-producing provinces in the NCC. This episode is simultaneous with those in the eastern NCC, indicative of a widespread late Yanshanian metallogenic event that was a response either to the subduction of the Izanagi-Pacific plate beneath eastern China or to the removal of the Early Cretaceous lithosphere in the eastern NCC. The ore-forming material sources may be the Early Precambrian metamorphic basement, instead of intermediate-acid rock bodies and dykes.  相似文献   

20.
New 40Ar/39Ar ages, based on incremental heating techniques for groundmass separates of 25 samples, are presented for the Harrat Al-Madinah volcanic field, part of Harrat Rahat in the north western part of the Arabian plate. This area is an active volcanic field characterized by the occurrence of two historical eruptions approximately in 641 and 1256 AD. Field investigations of the main volcanic landforms indicate dominantly monogenetic strombolian eruptions, in addition to local more explosive eruptions. The lavas consist mainly of olivine basalt and hawaiite flows with minor evolved rocks of mugearite, benmoreite, and trachyte that occur mainly as domes, tuff cones and occasionally as lava flows. Previous K/Ar dating shows that the Harrat Al-Madinah lava flows and associated domes comprise seven units spanning an age range of ca. 1.7 Ma–Recent. The new 40Ar/39Ar age determinations confirm, to a great extent, the previously obtained K/Ar ages in the sense that no major systematic biases were found in the general stratigraphy of the different flow units. However, the 40Ar/39Ar plateau ages show that volcanism in this area began in the Neogene (∼10 Ma) and continued to Recent, with the most voluminous eruptions occurring in the Quaternary. Neogene volcanism occurred in at least three pulses around 10, 5 and 2 Ma, whereas Quaternary volcanism produced at least seven units reflecting lava flow emplacement in the time period of 1.90 Ma–Recent. Thus, the whole duration of volcanic activity in the Harrat Al-Madinah (10 Ma–Recent) appears much longer than that previously identified. The longevity of volcanism in the same part of the moving Arabian plate and absence of evidence for uni-directional migration of volcanic activity indicate that there is no fixed plume beneath this region. The NNW-trending distribution of the volcanic vents is parallel to the Red Sea, and suggests their origin is related to periodic extensional episodes along the reactivated Red Sea fault system.  相似文献   

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