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1.
The Leiqiong area, which includes the Leizhou Peninsula and the northern part of the Hainan Island, is the largest province of exposed basalts in southern China. Ar–Ar and K–Ar dating indicates that incipient volcanism in the Leiqiong area may have taken place in late Oligocene time and gradually increased in tempo toward the Miocene and Pliocene Epoch. Volcanic activities were most extensive during Pleistocene, and declined and ended in Holocene. Based on radiometric age dating and geographic distribution, Pliocene and Quaternary volcanism in Hainan Island can be grouped into two stages and six eruptive regions. The early volcanism is dominated by flood type fissure eruption of quartz tholeiites and olivine tholeiites whereas the later phase is dominated by central type eruption of alkali olivine basalts and olivine tholeiites. The systematic decrease of MgO, ΣFeO and TiO2 with increasing SiO2 content for basalts from Hainan Island indicates that fractional crystallization of olivine, clinopyroxene and Ti-bearing opaques may have occurred during magmatic evolution. From coexisting Fe–Ti oxide minerals, it is estimated that the equilibrium temperatures range from 895–986°C and oxygen fugacities range from 10−13.4 to 10−10.7 atmospheres in the basaltic magmas. The incompatible element ratios and the chondrite-normalized REE patterns of basalts from the Leiqiong area are generally similar to OIB. The Nb/U ratios (less than 37) in most of the tholeiitic rocks and the negative Nb anomaly observed in the spidergram of some basalts indicated that the influence of a paleo-subduction zone derived component can not be excluded in considering the genesis of the basalts from the Leiqiong area. The tholeiites in the Leiqiong area may have mixed with a more enriched lithospheric mantle component as well as undergone relatively larger percentages of partial melting than the alkali basalts.  相似文献   

2.
The historical site of the Monte Mario lower Pleistocene succession (Rome, Italy) is an important marker of the Pliocene/Pleistocene boundary. Recently, the Monte Mario site was excavated and restudied. A spectacular angular unconformity characterizes the contact between the Monte Vaticano and the Monte Mario formations, which marks the Pliocene/Pleistocene boundary. Biostratigraphical analyses carried out on ostracod, foraminifer, and calcareous nannofossil assemblages indicate an Early Pliocene age (topmost Zanclean, 3.81–3.70 Ma) for the underlying Monte Vaticano Formation, whereas the Monte Mario Formation has been dated as early Pleistocene (Santernian, 1.66–1.59 Ma). Palaeomagnetic analyses point to C2Ar and C1r2r polarity chrons for the Monte Vaticano and the Monte Mario formations, respectively. The Monte Mario Formation consists of two obliquity-forced depositional sequences (MM1 and MM2) characterized by transgressive systems tracts of littoral marine environments at depths, respectively, of 40–80 m and 15–20 m. The data obtained from foraminifer and ostracod assemblages allow us to reconstruct early Pleistocene relative sea-level changes near Rome. At the Plio/Pleistocene transition, a relative sea-level drop of at least 260 m occurred, as a result of both tectonic uplift of the central Tyrrhenian margin and glacio-eustatic changes linked to early Pleistocene glaciation (Marine Isotope Stage 58).  相似文献   

3.
Sequence stratigraphy in marine foredeep and thrust-top basins is controlled by the conventional variations in eustatic sea-level and sedimentation rate together with tectonics. Vertical motions reflect combinations of subsidence due to regional flexure and uplift on local thrust anticlines which act to modify the volume and shape of accommodation space together with syn-depositional slopes. Plio-Pleistocene successions on Sicily were deposited in thrust-top and foredeep basins, above and ahead of evolving structures of the Maghrebian fold and thrust belt. Collectively the sediments represent a single megasequence defined at its base by a maximum flooding surface of earliest Pliocene age following reconnection with global sea-level at the end of the Messinian. The internal stratigraphy of this megasequence consists of Trubi chalks, blue marls and a coastal calcarenite package with subordinate silciclastic sand. Plankton biostratigraphy allows these facies to be placed in a chronostratigraphic framework. Regionally the upper assemblage progrades away from the orogenic hinterland, recording a tectonically forced regression in response to regional uplift from late Pliocene times. This uplift may be associated with isostatic unloading in the orogenic hinterland due to tectonic collapse of the more internal thrust sheets. Prior to this, flexure from orogenic loading is inferred to have been sufficient for regional subsidence locally to outstrip uplift associated with the growth of some thrust structures. For shallow-water facies the competition between thrust-related uplift and flexural subsidence can be investigated from the stacking patterns of parasequence sets. For structures developed at greater palaeobathymetries receiving fine-grained pelagic sediment, active tectonics may be recognized from depositional hiatuses.  相似文献   

4.
Ambitle Volcano (new name) is the most recently active of four eruptive centres that make up the mainly Pliocene–Pleistocene Tabar–Lihir–Tanga–Feni (TLTF) alkalic volcanic province, located in the New Ireland Basin, Papua New Guinea. Ambitle Volcano is a submarine and subaerial stratovolcano occupying all of Ambitle Island. The volcano rises 2500 m above the surrounding sea floor to sea level and, with a maximum elevation of 479 m above sea level, indicates a structure nearly 3000 m high. Volcanic deposits rest unconformably on Oligocene basement rocks of the New Ireland Basin. The cone of Ambitle Volcano is constructed mainly of lavas and pyroclastic and epiclastic rocks; lavas are commonly vesiculated. These lavas are strongly undersaturated and intermediate in composition (phonolitic tephrite and tephritic phonolite) with alkali basalt, tephrite and basanite and trachybasalt and trachyandesite also present. Syenite porphyry and monzonite stocks intrude the cone-forming mafic–intermediate sequence at Kabang–Matangkaka and in the upper Nanum River. The central part of the Ambitle Volcano is now modified as a prominent semi-circular topographic rim around the Nanum Valley. The Nanum Valley Crater (new name) is the product of large-scale summit failure of the SW flanks of the summit of the Ambitle Volcano. This event is dated no younger than 0.68–0.49 Ma. The Ambitle Crater (new name), the product of Late Quaternary resurgence of volcanism following sector collapse of Ambitle Volcano, is located in the NE portion of the Nanum Valley Crater. The crater is elongated NNE and measures 900 m × 550 m at its widest development. The strong NNE–SSW linearity of the western rim of Ambitle Crater is structurally controlled by the Kabang Fault. Tephra was erupted from the Ambitle Crater at 2300 ± 100 a and is widely dispersed throughout the Nanum Valley Crater and beyond. This is the youngest volcanic event in the TLTF volcanic province. The Niffin graben is a major NW–SE-trending structural corridor that transects Ambitle Island. The structural corridor is parallel to the NW–SE strike of the TLTF volcanic province suggesting it has been an important control on magmatism and volcanism. Presently active geothermal systems are located along Niffin graben structures in the western valleys of the island and in the Nanum Valley Crater. The volcanic rocks of Ambitle Volcano host porphyry Cu–Au style mineralisation and epithermal Ladolam-type Au mineralisation. Extensive exploration including surface sampling and subsurface drilling completed since 1983 on many prospects has not defined an economic resource.  相似文献   

5.
Study of the geochemical fingerprints of four geologically distinct suites of volcanic rocks on Cyprus are used to sketch a tectonic history of the island. Lavas from the Mamonia complex resemble alkalic within-plate basalts; lower pillow lavas and diabases of the Troodos Massif have features both of ocean-floor and island-arc tholeiites and could have been erupted in an interarc basin; the upper pillow lavas of the Troodos Massif resemble primitive tholeiitic basalts from island arcs; lavas from the Kyrenia range resemble transitional to alkalic within-plate basalts. The low TiO2 concentrations from the Troodos Massif may indicate a slow spreading rate. The Sr concentrations in the upper pillow lavas indicate an eruption at a maximum distance of 80 km above a Benioff zone. The results suggest formation of the Troodos Massif in the Campanian by spreading in an interarc basin followed by eruption of island-arc tholeiites. Obduction of continental material and ocean islands may have taken place in the Maestrichtian and Middle Miocene.  相似文献   

6.
The frequency and periodicity of preserved graded turbidite cycles in submarine fans in the Coral Sea and Sea of Japan are correlated to times of tectonic uplift in response to major collisions in the Owen-Stanley Range of Papua and the Hida Range of Japan, respectively. Large frequencies and shorter-term periodicities of turbidites at DSDP Site 210 were coeval with early Pliocene maximum tectonic-uplift rates which occurred in the Owen-Stanley Range in response to obduction. Similarly, large frequencies and shorter-term periodicities of turbidites at Site 299 (Toyama Submarine Fan) were coeval with the late Pleistocene uplift in the Hida Range; this uplift of 1000 to 1500 m occurred in response to collision tectonics. In both cases, trends of increasing frequencies and towards shorter-term periodicities of preserved turbidite depositional events correlate to trends of increasing rates of tectonic uplift.The role of sea-level fluctuations on changing denudation rates in these two collision zones is secondary. At Site 210, larger frequencies and short-term periodicities of preserved turbidites were coeval with early Pliocene high stands of sea level, whereas at Site 299, Pleistocene sea-level fluctuations are considered minor because at low stands of sea level, both relief and denudation rates were increased by about ten to 14%. At Site 286 (New Hebrides Basin), Eocene turbidite deposition is coeval with high stands of sea level, whereas at Site 297 (Northern Shikoku Basin), turbidite deposition was coeval with both rising and falling sea level.Analysis of both frequency and periodicity of turbidites by fan subenvironment at Site 299 indicates a record of continuous deposition, and maintainance of frequency and periodicity trends controlled by tectonic uplift. Late Pleistocene channel and overbank deposits showed periodicity differences of less than 28% of an order of magnitude, whereas Miocene-Pliocene overbank and distal turbidite periodicities differed by a 19% order of magnitude. Greater differences in magnitude occurred between distal turbidites or early Pleistocene age and Pliocene age than between Miocene-Pliocene overbank and distal turbidite deposition with a magnitude difference of 860%. These findings suggest that shifting depocenters and differences in sedimentation history in subenvironments of submarine fans are secondary to the role of tectonic uplift in this particular case.The minimal rate of tectonic uplift required to generate deep-sea fan turbidities appears to be approximately 400 m/million years. This figure is tentative and is based on very few observation points.Frequency and periodicity of preserved turbidite cycles in submarine fans in active continental margins and ancient counterparts should provide an independent measurement of rates and timing of tectonic uplift, particularly in collision terrains. Because this sediment parameter is a record of a single process from a single source and a record of “event stratigraphy”, its usage is preferable over standard and bulk sediment accumulation rates determined from age depth curves.  相似文献   

7.
基于TM遥感图像解译和野外调研，分析了攀西地区大渡河、安宁河深切河谷地貌特征和断裂带构造变形特征，建立了安宁河断裂带晚新生代5阶段变形历史。研究表明，中新世晚期—上新世早期，安宁河断裂以挤压走滑活动为主；上新世晚期至早更新世时期，断裂以斜张走滑活动为主，活动强度较弱；早中更新世之间发生的元谋运动使昔格达组湖相地层褶皱变形；中晚更新世时期发生断陷作用，形成安宁河两堑夹—垒的构造格局；晚更新世—全新世时期又以左旋走滑活动为主。综合安宁河、大渡河河谷地貌和晚新生代地层记录和变形特征，提出了攀西高原晚新生代4阶段隆升模式：中新世早中期(12Ma之前)以缓慢隆升与区域夷平化作用为主，中新世晚期—上新世早期(12～3．4Ma)是高原快速隆升与河流强烈下切的时期，上新世晚期—早更新世(3．4～1.1Ma)为昔格达湖盆发育时期，中晚更新世—全新世(1．1Ma以来)是高原快速隆升与河谷阶地发育时期。最后指出，至上新世晚期(3．4Ma以前)，攀西高原海拔高度可能超过了3000m。  相似文献   

8.
Marine ostracodes from 50 localities were studied to determine the age and elevation of Pleistocene sea levels in the Atlantic coastal plain from Maryland to northern Florida. Using ostracode taxon and concurrent ranges, published planktic biostratigraphic, paleomagnetic, and radiometric data, ostracode assemblage zones representing early (1.8-1.0 my), middle (0.7-0.4 my), and late (0.3-0.01 my) Pleistocene deposition were recognized and used as a basis for correlation. Ostracode biofacies signifying lagoonal, oyster bank, estuarine, open sound, and inner sublittoral environments provided estimated ranges of paleodepths for each locality. From these data the following minimum and maximum Pleistocene sea-level estimates were determined for the southeastern coastal plain: late Pleistocene, 2–10 m from Maryland to northern Florida; middle Pleistocene, 6–15 m in northern South Carolina; early Pleistocene, 4–22 m in central North Carolina, 13–35 m in southern North Carolina, and 6–27 m in South Carolina. Climatically induced glacio-eustatic sea-level fluctuations adequately account for the late Pleistocene sea-level data, but other factors, possibly differential crustal uplift, may have complicated the early Pleistocene record.  相似文献   

9.
A geochronological framework based on amino acid racemisation (AAR) and constrained by previously reported optically stimulated luminescence (OSL) ages is presented for the evolution and paleosea-level record of the Pleistocene Bridgewater Formation of the Mount Gambier region, of southern Australia. Within the study area, the Bridgewater Formation is represented by late early Pleistocene [Marine Isotope Stage (MIS) 23 at 933 ka] to Holocene barrier shoreline successions deposited during sea-level highstands. Regional monotonic uplift (0.13 mm yr–1) and pervasive calcrete development during the Pleistocene have preserved the sequence of calcarenite (mixed quartz-skeletal carbonate sand) shoreline complexes from denudation. AAR analyses confirm that the barriers generally increase in age landwards and correlate with sea-level highstands associated with interglacials as defined by the marine oxygen isotope record. AAR analyses on the benthic foraminifer Elphidium crispum have proved more reliable than the whole-rock method in extending the age range of AAR dating of these relict shoreline successions. Paleosea-levels from the coastal plain are as follows: MIS 7, –9 ± 2 m; MIS 9, 4 ± 1 m; and a minimum sea-level of 2 ± 2 m is derived for MIS 11. Paleosea-level could not be determined for MIS 15, 19 or 23 as diagnostic sea-level indicators were not identified within these sedimentary successions. Dismal Range, dated at 933 ± 145 ka (MIS 23), represents a correlative feature to the East Naracoorte Range but is some 25 km seaward of the Kanawinka Fault compared with the same barrier at Naracoorte. Mingbool Range (788 ± 18 ka) is of similar age to the West Naracoorte Range (MIS 19) and formed as an arcuate shoreline complex that became attached to the higher relief of the area represented by the Mount Burr Volcanic Province. The higher topographical relief resulted from crustal doming of the Oligo-Miocene Gambier Limestone caused by the intrusion of magma associated with the volcanic province. The AAR age of 788 ± 118 ka for Mingbool Range indicates that the Mount Burr volcanics predate the deposition of this shoreline complex.  相似文献   

10.
Minimal and maximal models of Late Pleistocene Glaciation on the Tibetan Plateau are considered. The large ice sheet models indicate that disintegration of the ice sheet could have contributed up to 7 mm/yr of present vertical uplift and 2 mm/yr of horizontal extension. The former value can account for more than 50% of the observed uplift in central Tibet. The peak free-air gravity anomaly arising from the deglaciation would be around −5.4 mGal. In contrast, the smaller ice sheet models do not contribute significantly to the signals of present uplift and gravity anomalies. Modern geodetic measurements therefore have the potential to constrain the Late Pleistocene glaciation of the Tibetan Plateau. Assuming a large ice sheet over the Tibetan Plateau, the disintegration can contribute up to 6 m of eustatic sea-level rise.  相似文献   

11.
刘欣宇  李永祥 《沉积学报》2021,39(5):1171-1184
发生于白垩纪中期塞诺曼期—土伦期之交的大洋缺氧事件2（OAE2）被认为是研究大洋缺氧事件形成与其他地质过程异常的重要窗口。重建晚塞诺曼期OAE2事件发生之前的古海洋环境对于理解OAE2的成因机制至关重要。因而选择沉积速率快且有高分辨率年代标尺的西藏定日地区OAE2剖面事件层位之下冷青热组地层开展了详细的岩石磁学研究,旨在为重建研究区晚塞诺曼期古海洋环境演化提供新约束。对剖面-5.20 m 至30 m的地层以10 cm间隔采样352个,并测得这些样品的磁化率,重点对-5.20 m至0 m样品测量其非磁滞剩磁,饱和等温剩磁等岩石磁学参数。结合已有的0 m 至37.2 m的岩石磁学数据,获得了-5.20 m至37.2 m的完整的岩石磁学记录,并将其年龄限定为（95.58±0.15） Ma至（94.55±0.15） Ma。岩石磁学结果显示1）95.58~95.10 Ma期间磁性矿物含量增多,反映沉积区物源供给逐渐增多。这很可能是由于晚塞诺曼期全球海平面下降（KCe4）所导致,且（95.10±0.15） Ma时海平面下降至最低。2）磁性矿物种类变化反映定日地区古海洋环境在~94.7 Ma发生了较显著变化,从95.10~94.70 Ma期间亚氧化为主的环境逐渐演变为~94.7 Ma后趋于缺氧的海洋环境。这很可能是由于95.10 Ma海平面上升及相伴的低氧带（OMZ）扩张至研究区所致。研究限定的（95.10±0.15） Ma作为全球海平面上升的初始时间可为研究其他OAE2剖面海平面上升对OAE2形成的影响提供重要的年代约束。结合晚塞诺曼期活跃的火山活动,我们认为晚塞诺曼期全球性海平面上升和火山活动共同作用导致了OAE2事件的发生。  相似文献   

12.
《Tectonophysics》2001,330(1-2):25-43
A detailed gravimetric study has been integrated with the most recent stratigraphic data in the area comprised between the Arno river and the foothills of the Northern Apennines, in northern Tuscany (central Italy). A Plio–Pleistocene basin lies in this area; its sedimentary succession can be subdivided from the bottom, in five allostratigraphic units: (1) Lower–Middle Pliocene shallow marine deposits; (2) Late Pliocene (?)–Early Pleistocene fluvio-lacustrine deposits; (3) late–Early Pleistocene–Middle Pleistocene alluvial to fluvial red conglomerates (Montecarlo Formation); (4) Middle Pleistocene alluvial to fluvial red conglomerates (Cerbaie and Casa Poggio ai Lecci Formations); (5) alluvial to fluvial deposits of Late Pleistocene age. The Bouguer anomaly map displays a strong minimum in the northeastern sector of the basin, and a gentle gradient from west to east. The map of the horizontal gradients permits to recognise three major fault zones, two of which along the southwestern and northeastern margins of the basin, and one along the southeastern edge of the Pisani Mountains. A 2.5D gravimetric modelling along a SW–NE section across the basin displays a thick wedge of sediments of density 2.25 g/cm3 (about 1700 m in the depocenter) overlying a layer of density 2.55 g/cm3, 1000 m thick, which rests on a basement of 2.72 g/cm3. The most of the sediment wedge is here referred to Upper Pliocene (?)–Lower Pleistocene, because borehole data show Pliocene marine deposits thinning northward close to the southern margin of the area. The layer below is referred to Ligurids and upper Tuscan Nappe units; the densest layer is interpreted as composed of Triassic evaporites, quartzites and Palaeozoic basement. According to Carmignani low-angle extensional tectonics began between Serravallian and early Messinian, thinning the Apennine nappe stack. At the end of Middle Pliocene, syn-rift deposition ceased in the Viareggio Basin (west of the investigated area) as demonstrated by Argnani and co-workers, and high-angle extensional tectonics migrated eastward up to the Monte Albano Ridge. A syn-rift continental sedimentary wedge developed in Late Pliocene–Early Pleistocene, until its hanging wall block was dismembered, during late Early Pleistocene, by NE-dipping faults, causing the uplift of its western portion (the Pisani Mountains). This breakup caused exhumation and erosion of Triassic units whose clastics where shed into the surrounding palaeo-Arno Valley in alluvial–fluvial deposits unconformably overlying the Lower Pleistocene syn-rift deposits. In the late Pleistocene SW–NE-trending fault systems created the steep southeastern edge of the Pisani Mountains and the resulting throw is recorded in Middle Pleistocene deposits across the present Arno Valley. This tectonic phase probably continues at present, offshore Livorno, as evidenced by the epicentres of earthquakes.  相似文献   

13.
We report major and trace element X-ray fluorescence (XRF) datafor mafic volcanics covering the 15-Ma evolution of Gran Canaria,Canary Islands. The Miocene (12–15 Ma) and Pliocene-Quaternary(0–6 Ma) mafic volcanics on Gran Canaria include picrites,tholeiites, alkali basalts, basanites, nephelinites, and melilitenephelinites. Olivineclinopyroxene are the major fractionatingor accumulating phases in the basalts. Plagioclase, Fe–Tioxide, and apatite fractionation or accumulation may play aminor role in the derivation of the most evolved mafic volcanics.The crystallization of clinopyroxene after olivine and the absenceof phenocrystic plagioclase in the Miocene tholeiites and inthe Pliocene and Quaternary alkali basalts and basanites withMgO>6 suggests that fractionation occurred at moderate pressure,probably within the upper mantle. The presence of plagioclasephenocrysts and chemical evidence for plagioclase fractionationin the Miocene basalts with MgO<6 and in the Pliocene tholeiitesis consistent with cooling and fractionation at shallow depth,probably during storage in lower-crustal reservoirs. Magma generationat pressures in excess of 3•0–3•5 GPa is suggestedby (a) the inferred presence of residual garnet and phlogopiteand (b) comparison of FeO1 cation mole percentages and the CIPWnormative compositions of the mafic volcanics with results fromhigh-pressure melting experiments. The Gran Canaria mafic magmaswere probably formed by decompression melting in an upwellingcolumn of asthenospheric material, which encountered a mechanicalboundary layer at {small tilde}100-km depth.  相似文献   

14.
罗布泊东部阿奇克谷地第四纪古地理   总被引：5，自引：0，他引：5  
通过对罗布泊东部阿奇克谷地中部AK1孔及露头剖面第四纪沉积特征的综合研究,初步分析了阿奇克谷地第四纪以来的沉积环境与古地理演化。结果表明,阿奇克谷地第四纪古地理受上新世以来区域构造控制,谷地两侧北山及阿尔金山的隆升,一方面为其提供了丰富的物质来源,同时也控制了其沉积环境的演化。其中第四纪以来阿奇克谷地沉积环境有两次明显变化,早更新世中期谷地中开始出现湖相沉积;中更新世晚期湖相沉积范围扩大,为罗布泊湖的大发展期。晚更新世谷地两侧普遍出现砾石层,与中更新世呈不整合接触,表明构造抬升造成湖泊退缩。晚更新世以来湖泊沉积环境波动变化加快。  相似文献   

15.
In this paper we show that the development of the sediment architecture at the leeward toe-of-slope of Great Bahama Bank (Ocean Drilling Project Leg 166, Bahama Transect) during the last 6 Ma is not only a response to sea-level fluctuations, but also to major paleo-oceanographic and climatic changes. A major sequence boundary close to the Miocene/Pliocene boundary (dated at 5.6-5.4 Ma) is interpreted to reflect a major sea-level drop that was followed by a sea-level rise, which led to the re-flooding of the Mediterranean Sea at the end of the Messinian and increasing sea-surface temperatures at Great Bahama Bank. Distinct erosional horizons occurred during the Pliocene (dated at 4.6 and 3.3-3.6 Ma) related to sea-level change and the intensification of the Gulf Stream when the emergence of the Isthmus of Panama reached a critical threshold. The Gulf Stream brings warm, saline and nutrient-poor waters to the Bahamas. Starting at the Early-Late Pliocene boundary at 3.6 Ma this paleo-oceanographic reorganization in combination with enhanced sea-level fluctuations associated with the Late Pliocene main intensification in Northern Hemisphere Glaciation (since 3.2 Ma) led to (1) a gradual change from a ramp-type to a flat-topped type morphology, and (2) a change from a skeletal to a non-skeletal-dominated sedimentary system (mainly peloidal). Increased sea-level fluctuations during the second half of the Pleistocene led to an intensified high stand-shedding depositional pattern within the surrounding basins.  相似文献   

16.
A multidisciplinary analysis of intraplate volcanic complexes interbedded with shallow and deeper marine sediments of a Late Miocene carbonate platform (Iblean Plateau, Sicily) has allowed a detailed paleo-environmental reconstruction. Our approach includes sedimentology, physical volcanology, stratigraphy, geochemistry/mineralogy, paleontology and 40Ar/39Ar dating. Four volcanic complexes are distinguished from each other. Two comprise an eastern shallow water platform (diatreme field and Carlentini complex) and two a western deeper water environment representing a seamount belt on the carbonate ramp (Valle Guffari seamount and Mineo complex). The late Miocene volcanism was not time-equivalent: episodic eruptions took place from the Late Tortonian (ca. 9.38 Ma at Mt. Carrubba) to Early Messinian (ca. 6.46 Ma at Valle Guffari). Explosive volcanism of the diatreme field may be related geodynamically to the period of periodic sea-level oscillations at the onset of the Messinian Salinity Crisis. Marine diatomites preserved in the crater areas of two diatremes are the only remnants of Early Messinian diatomites in the eastern Iblean Mountains.  相似文献   

17.
太行山中段左权羊角镇发育新生代玄武岩, 记录了太行山新生代以来的构造隆升事件。在详细的野外调查和研究的基础上, 通过与玄武岩发育相关的地貌面及其上的地层特征分析, 初步确定该玄武岩是上新世末期到早更新世初期的喷发产物, 初步揭示了太行山中段区域上晚上新世以来地貌发育历史, 主要存在6次构造隆升与剥蚀期: 在唐县期宽谷面形成的基础上, 于上新世晚期存在一次隆升和一次稳定侵蚀期, 并侵蚀形成“U”形谷; 早更新世初, 玄武岩开始间歇性喷发, 同时发生以西武家坪为中心的地区上拱, “U”形谷为玄武岩充填, 之后经剥蚀堆积形成第四级阶地面; 早更新世末, 该区再次发生隆升, 并形成第四级阶地; 中更新世末, 该区发生隆升, 形成第三级阶地; 晚更新世以来, 太行山中段又连续发生两次抬升, 从而在玄武岩体上形成了4级阶地, 形成太行山现今地貌。研究同时表明, 太行山中段上新世晚期以来的隆升主要发生于上新世末到早更新世时期。这一认识为探讨太行山中段晚上新世以来的构造隆升提供了具体证据。  相似文献   

18.
Geochemistry of tholeiites from Lanai,Hawaii   总被引：3，自引：0，他引：3  
Lanai is the third smallest of the fifteen principal subaerial shield volcanoes of the Hawaiian hotspot. This volcano apparently became extinct during the shield-building stage of volcanism, as shown by the absence of both alkalic cap and post-erosional lavas. Major and trace element analyses of 22 new samples collected primarily from 3 stratigraphic sections show that Lanai tholeiites span a large range in composition. Some Lanai lavas are unique geochemically among Hawaiian tholeiites in having the lowest abundances of incompatible trace elements of any Hawaiian lavas and well-developed positive Eu anomalies. The geochemical characteristics of these low-abundance Lanai tholeiites are not the result of alteration, differences in mantle source modal mineralogy, the presence of residual accessory mantle phases or fractional crystallization of such phases, assimilation of depleted [MORB] wall-rock, or accumulation/resorption of phenocrysts or xenocrysts. Incompatible trace element ratios (e.g., Nb/La, Nb/Th, La/Th, La/Hf, Ce/Pb) in Lanai tholeiites span considerable ranges and form coherent trends with each other and with absolute abundances of these elements. Large variations in La/Sm, La/Yb, and absolute REE abundances at constant MgO suggest that Lanai tholeiites formed by variable amounts of partial melting. However, large ranges in incompatible element ratios cannot be explained solely by variations in partial melting of a geochemically homogeneous source, but must reflect geochemical heterogeneities in the Lanai source. Partial melting modeling indicates that the mixed Lanai source is probably LREE-enriched [i.e., (La/Yb)CN>1]. One component in the Lanai source, exemplified by the low-abundance tholeiites, has markedly lower REE/HFSE, Th/HFSE, alkali/HFSE, and Ce/Pb ratios than other Lanai or Hawaiian tholeiites and may indicate the presence of recycled residual subduction zone materials in the Hawaiian plume source. The positive Eu anomalies that characterize the low-abundance Lanai tholeiites are not the result of plagioclase accumulation or assimilation but are a feature of this source component. Progressive temporal geochemical variations in Lanai tholeiites from 2 stratigraphic sections indicate that the source composition of these lavas probably evolved over time. This change could have resulted from a progressive decrease in the extent of partial melting of the Lanai source. The compositional variability of Lanai tholeiites suggests that geochemical heterogeneities in their source are larger than the scale of partial melting. Lanai tholeiites could not have formed by smaller degrees of partial melting of plume material than did the larger-volume Hawaiian shields. Therefore, volume differences between Hawaiian shields must be controlled primarily by differences in the volume of supplied plume material rather than by differences in the degree of partial melting. The premature cessation of eruptive activity at Lanai may be attributed to relatively large degrees of partial melting of a small plume.  相似文献   

19.
《International Geology Review》2012,54(10):1295-1313
The environment where the Permian Emeishan large igneous province (LIP) of Southwest China erupted remains controversial, especially regarding whether it was terrestrial, involving a 1 km scale domal uplift, or submarine. Slightly younger Daqiao conglomerate and Binchuan pillow lavas suggest that the Emeishan LIP erupted in a submarine environment. We show that at Binchuan, sandstone and rhyolite lie beneath the pillow lavas. In the Daqiao cross-section, there is an eastwards-verging syncline that reverses the succession of basalt and conglomerate. The conglomerate is not a basal conglomerate, and it does not contain any magmatic hydrovolcanic deposits. The basalt underlying the conglomerate is not the first of the LIP eruptions; that first eruption is found ~420 m below, on top of the Permian Maokou limestone. All together, these observations show that the deposits, including the conglomerate and pillow lava, do not represent the environment at the very start of the LIP volcanic eruptions, but represent conditions that existed before and possibly during the Emeishan LIP eruptions. Based on field investigations, the petrology of the rocks, and structural features, we conclude that submarine sedimentation and subaerial basalt eruptions coexisted in time and space in the region during or prior to the Emeishan LIP basalt eruptions.  相似文献   

20.
藏北改则地区鱼鳞山组火山岩同位素年代学   总被引：8，自引：7，他引：8  
李才  朱志勇等 《地质通报》2002,21(11):732-734
鱼鳞山组火山岩是青藏高原隆升过程中一次重要的碱性钾质火山活动的产物，自发现之日起时代一直定为上新世末至早更新世，对比范围跨羌塘南部到冈底斯地区。通过对鱼鳞山组火山岩同位素测年研究，鱼鳞山组白榴石响岩^40Ar-^39Ar积分年龄为27．8Ma、K－Ar法年龄为30－18Ma,确定鱼鳞山组时代为渐新世至中新世，其分布仅限于班公湖－怒江缝合带以北地区。  相似文献   

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