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1.
Khromykh  S. V.  Semenova  D. V.  Kotler  P. D.  Gurova  A. V.  Mikheev  E. I.  Perfilova  A. A. 《Geotectonics》2020,54(4):510-528

Studies of volcanic rocks in orogenic troughs of Eastern Kazakhstan were carried out. The troughs were formed at late-orogenic stages of evolution of Hercynian Altai collision system. Volcanic rocks are represented by basalts, andesites, dacites and rhyolites. Based on geochemical and isotopic data, the basalts and andesites derived from mafic magmas that formed as a result of partial melting of garnet peridotites in the upper mantle under the orogen. U–Pb zircon data prove two volcanic stages: more-scaled Middle Carboniferous (~311 Ma) and less-scaled Early Permian (297–290 Ma). Basalts and andesites in lower parts of the orogenic troughs and independent dacite-rhyolite structures were formed at the Middle Carboniferous stage. Parental mafic magmas were formed as a result of partial melting of mantle substrates in local transtensional zones along large shear faults. The formation of dacites and rhyolites could have been caused by partial melting of crustal substrates under effect of mafic magmas. Transtensional movements in the lithosphere of orogenic belts may indicate the beginning of collapse of orogens. A smaller volume of basalts and andesites formed at the Early Permian stage. Geochemical data prove the independent episode of partial melting in upper mantle. Synchronous basalts and andesites also appeared at wide territory in Tian Shan, Central Kazakhstan, and Central and Southern Mongolia. Early Permian volcanism indicates general extension of the lithosphere at the postorogenic stages. Large-scaled Early Permian mafic and granitoid magmatism in Central Asia has been interpreted in recent years as the Tarim Large Igneous Province caused by Tarim mantle plume activity. Thus, the extension of the lithosphere and associated volcanism in the Early Permian can be an indicator of the onset of the plume–lithosphere interaction process.

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2.
The meta-basic volcanic rocks in the Tengtiaohe Zone yield zircon U–Pb ages of 258.8±2.5 Ma and 259.2±1.8 Ma, respectively which agree with the ages of flood basalts of ELIP and are similar to the basaltic rocks and komatiites from the Song Da Zone in northern Vietnam. The results suggest that the age of meta-basic volcanic rocks is Late Permian, rather than the Early Permian or Early Carboniferous ages as previously inferred. Most meta-basic volcanic rocks are strongly enriched in LREEs relative to HREEs and display trace element patterns similar to the ELIP high-Ti basalts, and are enriched in LILEs with negative Sr anomalies. Their initial ~(87)Sr/~(86) Sr ratios range from 0.705974 to 0.706188 and εNd(t) from-0.82 to-2.11. Their magmas were derived from an enriched and deep mantle source without significant crustal contamination. These meta-basic volcanic rocks formed in ELIP. Therefore, the Tengtiaohe Zone is not an ophiolite zone and can link to the Song Da Zone in northern Vietnam.  相似文献   

3.
Several Paleozoic sutures in Southwestern China provide a record of the history of the Paleo-Tethys Ocean, whose birth and final closure are associated with the breakup and assembly of Gondwanaland. Recent studies indicate that there are widespread OIB-type mafic volcanic rocks within these suture zones and intervening terranes. This paper examines the geology and geochemistry of volcanic rocks in the Xiaruo-Tuoding area, a remnant passive margin succession of the Jinshajiang Paleo-Tethyan Ocean. The sedimentary and volcanic stratigraphy of this area is interpreted as a seaward dipping margin with a few continentward dipping normal faults. The available geochemistry of these volcanic rocks suggest that they are OIB-like basalts, characterised by SiO2 = 42.78–50.46 wt.%, high TiO2 contents (TiO2 = 2.2–3.55 wt.%), moderate MgO = 4.15–6.49 wt.%, Mg# = 0.37–0.50, high Ti/Y ratios (mostly > 450), large ion lithosphere elements enrichment, high strength field elements and rare earth elements, with La/Nb = 1.04–1.39, Ce/Yb = 18.38–30, Sm/Yb = 2.16–3.52, (87Sr/86Sr)i = 0.705350–0.707867, and Nd(t) = − 1.43–1.90. These geochemical and isotopic signatures are generally similar to those of the Emeishan flood basalts, which together with stratigraphic constraints, demonstrate that these volcanics were formed in a volcanic rifted margin, probably associated with a mantle plume. A new model is proposed to interpret the evolution of the Jinshajiang Paleo-Tethyan Ocean and its possible relationship to the Emeishan mantle plume. In this model, we argue that the opening of the Jinshajiang Paleo-Tethyan Ocean in the Carboniferous was caused by a mantle plume. The mantle plume was active to the east along the western margin of the Yangtze Craton between 300 and 260 Ma, from which the voluminous Emeishan flood basalts were erupted at 260 Ma. The closure of the Jinshajiang Ocean occurred since the Middle Permian. Continuous westward subduction generated the Jiangda-Weixi magmatic arc to the west of the Jinshajiang suture. This subduction also partly destroyed and/or tectonically sliced the volcanic rifted margin. Some seaward dipping volcanic-sedimentary sequences on the east flank of the Jinshajiang Ocean were preserved, but are strongly deformed.  相似文献   

4.
黔西南晚二叠世龙潭组物源分析及区域沉积古地理重建   总被引：1，自引：0，他引：1  
于鑫  杨江海  刘建中  杜远生  柴嵘 《地质学报》2017,91(6):1374-1385
黔西南上二叠统龙潭组为一套以海陆交互相细屑岩为主的沉积地层,与下伏茅口组平行不整合接触。龙潭组砂岩碎屑组分以板片状—微晶状的玄武—安山质火山岩岩屑为主,并含少量霏细晶—隐晶质的酸性火山岩岩屑。泥质岩全岩主量元素以低SiO_2(28.3%~51.9%)、高TiO_2(1.0%~4.1%)和低Al_2O_3/TiO_2比值(3.9~6.9)为特征,与峨眉山高Ti玄武岩的Al_2O_3/TiO_2比值(集中在2.5~5.0之间)较为接近。龙潭组的碎屑锆石显示峰期年龄为~260Ma,与峨眉山大火成岩省的火山喷发期(258~263Ma)相吻合。锆石微量元素指示其与板内/非造山环境的岩浆活动有关,结合同时期北部陆相和南部深水盆地相的沉积物源研究,该结果表明龙潭组碎屑岩的源岩为峨眉山玄武质和长英质火山岩。基于此,本文对晚二叠世滇黔桂地区峨眉山源区—河流—滨岸—右江深水盆地的沉积古地理格局进行重建。  相似文献   

5.
Located in the Cordillera Oriental of the Dominican Republic, the Early Cretaceous Los Ranchos Fm (LRF) comprises a > 3-km thick sequence of volcanic and volcaniclastic rocks with variable geochemical characteristics, which is intruded by tonalite batholiths, minor gabbro/diorite plutons and mafic dykes. From top to bottom, three main stratigraphic units have been mapped: upper basaltic, intermediate rhyodacitic and lower basaltic. Combined detailed mapping, stratigraphy, geochemistry, Rb–Sr/Sm–Nd isotopic studies and U–Pb/Ar–Ar geochronology show that the mafic rocks of the LRF include boninites and LREE-depleted island arc tholeiites (IAT) in the lower unit, both which appear genetically related, whereas normal IAT occur in the upper unit. The source for these rocks is thought to reflect variably depleted mantle, overprinted by a subduction zone component. Contemporaneous Aptian U–Pb zircon ages were obtained for a rhyodacite from the intermediate unit (116.0 ± 0.8 Ma) and a tonalite of the Zambrana batholith (115.5 ± 0.3 Ma) that intrudes the LRF. The similarity of trace element signatures in both units argues for genetic link between the felsic volcanics of the LRF and the tonalite plutonism. Low-K rhyolites and tonalite batholiths are interpreted as products of secondary melting at the base of thickened early arc crust. 40Ar/39Ar plateau ages of hornblende in most tonalites are Albian (109–106 Ma) and interpreted as final cooling ages, prior to unroofing and growth of unconformable overlying reef limestones of the Hatillo Fm (112–100 Ma). The LREE-depleted IAT and boninites of lower basaltic unit are interpreted to have formed during subduction zone initiation in the Caribbean Island arc, and the normal IAT of the upper unit are thought to represent the subsequent establishment of the volcanic front.  相似文献   

6.
The Rooiberg Group is a 6-km-thick sequence of mostly volcanic rocks, which represent the first phase of magmatic activity associated with the Bushveld Complex. These strata include, in ascending stratigraphic order, the Dullstroom, Damwal, Kwaggasnek, and Schrikkloof Formations. Units of the lower Dullstroom Formation range from basalts to andesites and comprise two compositional suites: high Ti and low Ti. Compositional data indicate that melts represented by the overlying, more siliceous volcanic rocks, which include dacites and rhyolites, were derived from low Ti melts by fractional crystallization and assimilation of crustal material (AFC processes).

Rb–Sr isotopic data (28 samples) for units of the Dullstroom and Damwal Formations loosely constrain a crystallization age of 2071+94/−65 Ma (these errors and those below: 95% confidence limits), which agrees with previously reported age data. These data suggest an initial value for 87Sr/86Sr of 0.70655+0.00087/−0.00051 for the Rooiberg Group. In contrast, Rb–Sr isotopic compositions of six samples of the Kwaggasnek Formation indicate post-crystallization alteration, which was probably associated with the Lebowa Granite Suite of the Bushveld Complex. Sm–Nd isotopic data (29 samples) for volcanic units of the Rooiberg Group provide a poorly constrained age of 1837+360/−320 Ma with an initial value for 143Nd/144Nd of 0.50976+0.00026/−0.00035. These Rb–Sr and Sm–Nd isotopic compositions are similar to those indicated for melts that crystallized to form the Rustenburg Layered Suite (RLS) of the Bushveld Complex. Extruded and intruded melts probably were derived from the same or similar sources and may have resided in the same magma chambers before emplacement.  相似文献   


7.
We present zircon U–Pb dating, whole-rock geochemistry, and Sr–Nd isotope results for the Upper Permian–Upper Triassic volcanic rocks to constrain the timing of the final closure of the eastern segment of the Palaeo-Asian Ocean. The volcanic rocks were mainly collected from the Yanbian area in eastern Jilin Province, northeastern China. The zircon U–Pb dating results indicate that the samples can be classified as Upper Permian–Lower Triassic basalts (ca. 262–244 Ma) and Upper Triassic dacites (ca. 216 Ma). The whole-rock geochemical results indicate that the rocks predominately belong to the medium-K and high-K calc-alkaline series. The basalts are enriched in large ion lithophile elements (LILEs, e.g. Ba and K) and depleted in high field strength elements (HFSEs, e.g. Nb and Ta), with weak positive Eu anomalies. The dacites are enriched in LILEs (e.g. Rb, Ba, Th, and K) and light rare earth elements (LREEs) and marked depletion in some HFSEs (e.g. Nb, Ta, and Ti), with significant negative Sr, P, and Eu anomalies. Moreover, the Upper Permian–Lower Triassic basalts have low initial 87Sr/86Sr ratios (0.7037–0.7048) and high εNd values (4.4–5.4). In contrast, the Upper Triassic dacites possess relatively high initial 87Sr/86Sr ratios (0.7052) compared with their low εNd values (1.4). The basaltic magma likely originated from the partial melting of a depleted mantle wedge metasomatized by subduction-related fluids, and the felsic magmas likely originated from the partial melting of a dominantly juvenile source with a minor component of ancient crust. Taken together, the Upper Permian–Lower Triassic basalts (ca. 262–244 Ma) are arc basalts that formed in an active continental margin setting, and the Upper Triassic dacites (ca. 216 Ma) are A-type granitic rocks that formed in an extensional setting. Therefore, the final closure of the Palaeo-Asian Ocean occurred during the Middle–Late Triassic.  相似文献   

8.
Charlotte M. Allen 《Lithos》2000,51(4):331-349
A swarm of felsic and mafic dikes cuts a Late Carboniferous–Permian batholith called the Urannah Suite in central coastal Queensland. Late Permian–Triassic westward thrusting (Hunter–Bowen Orogeny) exposed this mid-crustal Suite and the crosscutting dikes, thus enabling examination of dikes that range in age from syn- to post-batholithic. Although both mafic and felsic dikes have the same dominant northerly strike, field, geochronologic and geochemical examination reveal that the swarm is composite; felsic dikes are older (285 Ma) and geochemically and isotopically distinct from mafic dikes (273–229 Ma). Dike compositions are compared to those of the host plutonic rocks, and to volcanic rocks the same age as the dikes. Whereas the felsic (older) dikes are compositionally similar to their host granites (initial 87Sr/86Sr>0.7045), the mafic (younger) dikes are isotopically (Sr, Nd, Pb) less radiogenic. Moreover, several different types of mafic dikes are evident based on geochemistry; most of these have mixed characteristics in terms of tectonic classification. All but two dikes of basalt and basaltic andesite composition classify as ‘with-in plate' on Ti–Zr–Y tectonic classification plots. Many of the basalts have high TiO2 contents (1.5–3.0 wt.%). Most of these have REE and spider diagram patterns like calc-alkaline tholeiites, the exceptions being a few alkali basalts recognized by their alkali content, and high Ti, Ce, Nb and Zr contents. When put into the context of all plutonic rocks in the area (late Paleozoic and Mesozoic), these dikes record a transition at 280 Ma, after which time, all magmatism in the region is less isotopically evolved (initial 87Sr/86Sr=0.7033–0.7044). A model of slab retreat and hinge movement to the east in the latest Permian explains the change of geochemical signature from arc-front to backarc at about 280 Ma.  相似文献   

9.
A suite of continental flood basalts sampled over a vast exposure and stratigraphic thickness in the Emeishan large igneous province (LIP), SW China was investigated for laser microprobe 40Ar/39Ar dating. There are two 40Ar/39Ar age groups for these basalts, corresponding to 259-246 Ma and 177-137 Ma, respectively. A well-defined isochron gives an eruption age of huge quantities of mafic magmas at 258.9±3.4 Ma, which is identical to previous dating and paleontological data. Much younger 40Ar/39Ar ages for some basalts with low-greenschist metamorphic facies probably recorded a late thermo-tectonic event caused by collision between the Yangtze and Qiangtang continental blocks during the Mesozoic, which resulted in the reset of argon isotope system. The 40Ar/39Ar age data, we present here, combined with previous dating and paleontological data, suggest relatively short duration (about 3 Ma) of mafic volcanism, which have important implication on mantle plume genesis of the Emeishan continental flood basalts in the LIP.  相似文献   

10.
The Neogene Yamadağ volcanics occupy a vast area between Sivas and Malatya in eastern Anatolia, Turkey. These volcanic rocks are characterized by pyroclastics comprising agglomerates, tuffs and some small outcrops of basaltic–andesitic–dacitic rocks, overlain upward by basaltic and dacitic rocks, and finally by basaltic lava flows in the Arapkir area, northern Malatya Province. The basaltic lava flows in the Arapkir area yield a 40Ar/39Ar age of 15.8 ± 0.2 Ma, whereas the dacitic lava flows give 40Ar/39Ar ages ranging from 17.6 through 14.7 ± 0.1 to 12.2 ± 0.2 Ma, corresponding to the Middle Miocene. These volcanic rocks have subalkaline basaltic, basaltic andesitic; alkaline basaltic trachyandesitic and dacitic chemical compositions. Some special textures, such as spongy-cellular, sieve and embayed textures; oscillatory zoning and glass inclusions in plagioclase phenocrysts; ghost amphiboles and fresh biotite flakes are attributable to disequilibrium crystallization related to magma mixing between coeval magmas. The main solidification processes consist of fractional crystallization and magma mixing which were operative during the soldification of these volcanic rocks. The dacitic rocks are enriched in LILE, LREE and Th, U type HFSE relative to the basaltic rocks. The basaltic rocks also show some marked differences in terms of trace-element and REE geochemistry; namely, the alkaline basaltic trachyandesites have pronounced higher HFSE, MREE and HREE contents relative to the subalkaline basalts. Trace and REE geochemical data reveal the existence of three distinct magma sources – one subalkaline basaltic trachyandesitic, one alkaline basaltic and one dacitic – in the genesis of the Yamadağ volcanics in the Arapkir region. The subalkaline basaltic and alkaline basaltic trachyandesitic magmas were derived from an E-MORB type enriched mantle source with a relatively high- and low-degree partial melting, respectively. The magmatic melt of dacitic rocks seem to be derived from an OIB-type enriched lithospheric mantle with a low proportion of partial melting. The enriched lithospheric mantle source reflect the metasomatism induced by earlier subduction-derived fluids. All these coeval magmas were generated in a post-collisional extensional geodynamic setting in Eastern Anatolia, Turkey.  相似文献   

11.
A new geochronological and geochemical study of the volcanic rocks of the Ponza and Palmarola Islands, Pontine Archipelago, has been carried out. This archipelago is located along the boundary between the Italian continental shelf and the opening Tyrrhenian basin. It is a key area to study volcanism related to the opening of the Tyrrhenian Sea. Ponza is the oldest felsic magmatic manifestation in the central Tyrrhenian area. Previous studies suggested that Ponza volcanic activity began before 5 Ma. Twenty-five new K–Ar ages constrain the volcanic activity (rhyolitic hyaloclastites and dykes) to the last 4.2 Ma, with two episodes of quiescence between 3.7 and 3.2 Ma and between 2.9 and 1.0 Ma. A new volcanic episode dated at 3.2–2.9 Ma has been identified on the central and southern Ponza, with emplacement of pyroclastic units. At 1.0 Ma, a trachytic episode ended the volcanic activity. The near island of Palmarola exhibits rhyolitic hyaloclastites and domes dated between 1.6 and 1.5 Ma, indicating that the island was entirely built during the Early Pleistocene in a short span of time of ca. 120 ka. Although only 6–8 km apart, the two islands display significantly different geochemical signatures. Ponza rhyolites show major and trace element compositions representative of orogenic magmas of subduction/collision zones: high-K calc-alkaline and metaluminous rhyolites (Agpaitic Index [AI] and Alumina Saturation Index [ASI] <1), high LILE/HFSE (Th/Ta=16–21) and LREE/HFSE ratios (La/Nb>3), and Nb–Ta negative anomalies. In Palmarola, the orogenic character is also present, but much less marked than in Ponza: rhyolites have a peralkaline character (AI>1), lower LILE/HFSE (Th/Ta=11–15), low LREE/HFSE ratios (La/Nb=1–2) close to those of anorogenic lavas, and the Nb–Ta negative anomalies are almost absent. Y/Nb ratios indicate different magmatic sources, one similar to island-arc or active continental margin basalts for Ponza rhyolites, and the others probably involving an OIB type component for Palmarola rhyolites and Ponza trachytes. Palmarola volcanics represent a transitional magmatism: although a preserved collisional geochemical imprint, they show geochemical features approaching those of anorogenic lavas erupted in a within-plate context. The change of magmatism evidenced in this study can be related to the tectonic evolution of the area. Indeed, Hf, Ta and Rb contents suggest that the oldest Pliocene rhyolites of Ponza would emplace in a syn- to late-collisional setting, while the younger Pleistocene rhyolites of Palmarola would be emplaced in a post-collisional setting in which the orogenic character (Th/Ta) decreases and mantle influence (Nb/Ta) increases. Geochemical modeling strongly suggests that the Palmarola rhyolites represent the waning stages of a subduction-related magmatism. The K–Ar datings allow us to estimate precisely the transition of magmatism to last less than 1.3 Ma. The transitional magmas may be the result of the upwelling of asthenospheric mantle inducing melting of a metasomatized lithospheric mantle and the mixing between these two sources. This upwelling could occur during the extension of the Tyrrhenian basin, caused by the slab retreat and steepening, or during a process of slab break-off starting in the Pliocene.  相似文献   

12.
在滇东南建水地区发现产于峨眉山玄武岩中的高镁火山岩包体,这对于地幔柱的形成演化具有重要研究意义.对这些包体进行了锆石U-Pb年代学、地球化学和岩矿分析.高镁火山岩包体具斑状结构,致密块状构造,斑晶主要为贵橄榄石和透辉石.13颗锆石U-Pb LA-ICP-MS加权平均年龄为259±2Ma(MSWD=1.9),显示与寄主岩石同期形成.包体岩石具有高镁(Mg~#=68～75)、低硅(SiO_2=45.11%～45.93%)特征,轻稀土元素(LREE)、高场强元素(HFSE)富集而重稀土元素(HREE)亏损,属于亚碱性、拉斑玄武岩系列,具有板内玄武岩(IPB)特征.火山岩包体的原始岩浆起源于石榴子石二辉橄榄岩低程度部分熔融的产物,岩浆演化过程中发生了橄榄石和单斜辉石的分离结晶作用,在侵位上升过程中未受明显的地壳混染作用.该高镁火山岩的存在,显示地幔柱除了垂直上升运动外,在地球深部不同的边界还有多次侧向扩展移动,表明滇东南晚二叠世存在峨眉山地幔柱的一个分支-地幔枝活动.  相似文献   

13.
峨眉山大火成岩省在形成后遭受了强烈的风化剥蚀,与右江盆地构成了独特的源-汇沉积体系,相关的碎屑沉积记录是重建峨眉山大火成岩省晚期火山序列、揭示大火成岩省岩浆演化的关键。本文对黔西南地区右江盆地北缘下三叠统下部飞仙关组开展了详细的沉积物源研究。飞仙关组泥岩由长石、石英和黏矿物等组成,具有低SiO2、高TiO2含量和低Al2O3/TiO2值的特征,具Eu的弱负异常,微弱的Nb、Ta负异常。基于Al2O3/TiO2和La/Sm物源图解分析,研究样品的主要物源由峨眉山大火成岩省的高-Ti玄武岩和少量酸性火山岩构成,这与右江盆地晚二叠世的碎屑沉积物源组成一致。碎屑锆石具有～260 Ma的年龄峰值,对应锆石具有较低的Th/Nb和较高的Nb/Hf值,均支持峨眉山大火成岩省作为主要的沉积物源区,而同时存在的>400 Ma的碎屑锆石则对应于峨眉山玄武质岩浆上升时所捕获的华南地壳物质。与上二叠统的碎屑锆石U-Pb年龄组成和锆石...  相似文献   

14.
北山古生代火山岩尤其是石炭纪-二叠纪火山岩的形成环境及成因备受学者关注且长期以来存在争议。本文收集了近年来发表的关于北山石炭纪-二叠纪火山岩研究的地球化学数据,岩石地球化学特征显示北山石炭纪玄武岩主要为安山玄武岩,属拉斑系列,二叠纪火山岩主要为安山玄武岩和亚碱性玄武岩,落入拉斑系列及过渡区;石炭纪玄武岩和二叠纪玄武岩均具有LREE富集的球粒陨石标准化稀土元素配分模式,轻重稀土元素分馏程度均较低。在微量元素蛛网图上,石炭纪-二叠纪遭受地壳混染的玄武岩呈现出明显的Nb-Ta亏损和微弱的Ti亏损特征,而未遭受地壳混染作用的绝大多数石炭纪-二叠纪玄武岩主要呈现出与OIB相似的"隆起"状不相容元素标准化配分模式。岩石成因分析认为,石炭纪-二叠纪玄武质岩浆可能主要来源于地幔柱,部分石炭纪-二叠纪玄武岩在形成演化过程中遭受了明显的大陆地壳混染作用,导致其出现十分相似于岛弧或活动大陆边缘的地球化学特征。结合区域构造演化分析及构造环境判别,认为石炭纪-二叠纪玄武岩均形成于大陆板内环境。  相似文献   

15.
The Eocene and Miocene volcanic rocks between the cities of Trabzon and Giresun in the Eastern Pontides (NE Turkey) erupted as mildly and moderately alkaline magmas ranging from silica-saturated to silica-undersaturated types. 40Ar-39Ar dating and petrochemical data reveal that the studied volcanic rocks are discriminated in two: Lutetian (Middle Eocene) mildly alkaline, (basaltic rocks: 45.31 ± 0.18 to 43.86 ± 0.19 Ma; trachytic rocks: 44.87 ± 0.22 to 41.32 ± 0.12 Ma), and Messinian (Late Miocene) moderately alkaline volcanic rocks (tephrytic rocks: 6.05 ± 0.06 and 5.65 ± 0.06 Ma). The trace and the rare earth element systematic, characterised by moderate light earth element (LREE)/heavy rare earth element (HREE) ratios in the Eocene basaltic and trachytic rocks, high LREE/HREE ratios in the Miocene tephrytic rocks, and different degrees of depletion in Nb, Ta, Ti coupled with high Th/Yb ratios, show that the parental magmas of the volcanic rocks were derived from mantle sources previously enriched by slab-derived fluids and subducted sediments. The Sr, Nd and Pb isotopic composition of the Eocene and Miocene volcanic rocks support the presence of subduction-modified subcontinental lithospheric mantle. During the magma ascent in the crust, parental magmas of both the Eocene and Miocene volcanic rocks were mostly affected by fractional crystallisation rather than assimilation coupled with fractional crystallisation and mixing. The silica-undersaturated character of the Miocene tephrytic rocks could be attributed to assimilation of carbonate rocks within shallow-level magma chambers. The parental magmas of the Eocene volcanic rocks resulted from a relatively high melting degree of a net veined mantle and surrounding peridotites in the spinel stability field due to an increase in temperature, resulting from asthenospheric upwelling related to the extension of lithosphere subsequent to delamination. The parental magmas for the Miocene volcanic rocks resulted from a relatively low melting degree of a net veined mantle domain previously modified by metasomatic melts derived from a garnet peridotite source after decompression due to extensional tectonics, combined with strike-slip movement at a regional scale related to ongoing delamination.  相似文献   

16.
The rate of lithospheric extension has previously been suggested as the most important factor governing the compositions of magmas generated in the Cenozoic Ethiopian volcanic province (CEVP). However, the distribution and chemistry of volcanic rocks extending from the western plateau margin at Addis Ababa to the rift floor in Nazret, northern sector of the main Ethiopian rift (MER), suggest that transitional magmatism in the region may have been triggered by an increase in the amount of lithospheric extension in the Early Pliocene. The rocks occur across an area of variable crustal thickness and show a general age progression from Upper Miocene (≤9 Ma) to Recent toward the rift. Alkalic basalts are extensive in the western part of the rift and along its margin but are found only locally within the rift, whereas transitional basalts are found within the rift only. Both types of basalts appear to have been derived from a common mantle source. In contrast, alkalic and transitional basalts on the Ethiopian plateau are mutually exclusive in terms of their spatial distribution, but exhibit a compositional contiguity which suggests that transitional magmas on the plateau formed at the expense of alkalic magmas, i.e. by equilibration of alkalic magmas at relatively shallow depth. The alkalic basalts bear clear record of a decrease in the degree of partial melting with time, suggesting that magmatism on the plateau was possibly triggered by a transient thermal anomaly.  相似文献   

17.
杜世俊  徐兴旺  杨列坤  符超  苏捷  崔敏利 《岩石学报》2009,25(12):3251-3258
山东临朐-昌乐地区新生代岩浆活动强烈,以形成含大量地幔包体的玄武岩为特征.作者在考察与研究昌乐北岩古火山口玄武岩中地幔包体的过程中,发现一些包体中发育有地幔钾交代成因的金云母细脉.鉴于金云母的封闭温度远低于玄武岩浆喷发时的温度以及金云母可以记录喷发的玄武岩浆冷却通过金云母封闭温度的时间,本文尝试通过金云母所记录的~(40)Ar/~(39)Ar年龄来推断玄武岩浆喷溢的结束时间.该火山口玄武岩不同部位的三个地幔岩包体其金云母的~(40)Ar/~(39)Ar坪年龄分别为18.42±0.21Ma、18.65±0.27Ma和18.39±0.36Ma,年龄结果具有很好的一致性,充分说明了该定年手段的有效性.因此可以确定该火山口玄武岩浆喷溢活动约在18.5Ma前结束.由此推测山旺盆地中不整合发育在源于该火山口喷溢的玄武岩之上的化石群的形成时代下限约为18.5Ma.用玄武岩地幔包体金云母的~(40)Ar/~(39)Ar年龄来确定玄武岩年龄是玄武岩定年一个有效的新方法.  相似文献   

18.
The Carboniferous–Early Permian rift-related volcanic successions, covering large areas in the Chinese Tianshan and its adjacent areas, make up an important phanerozoic large igneous province in the word, which can be further divided into two sub-provinces: Tianshan and Tarim. The Early Permian volcanic rocks have been considered to be the products of an intraplate volcanism by most researchers. However, there is still strong controversy about the nature and geological setting of the Carboniferous volcanic rocks. The regional angular unconformity of Lower Carboniferous upon basement or pre-Carboniferous rocks, the ages (360–351 Ma) of the youngest ophiolite and the peak of subduction metamorphism of high pressure-low temperature metamorphic belt and the occurrence of Ni-Cu-bearing mafic-ultramafic intrusion with age of ~352 Ma and A-type granite with age of ~358 Ma reveal that the final closure of the Paleo-Asian Ocean might take place in the Early Mississippian. Our studies reveal that although contamination by continental crust or lithosphere can impart subduction-like signature (e.g., low Nb, low Ta and low Ti) and lead to misidentification of contaminated Carboniferous basaltic lavas from the Chinese Tianshan and its neighboring areas as arc related, there are still some essential differences between the Carboniferous basaltic lavas and arc related ones; such as: uncontaminated Carboniferous basaltic lavas have higher Nb concentrations (9–22 ppm), Nb/La > 1, “hump-shaped” OIB-like trace element patterns and moderate positive ɛNd values that distinguish them from the arc related ones; whereas, the contaminated Carboniferous basaltic lavas are characterized by pronounced negative Nb, Ta and Ti anomalies, but, their concentrations of incompatible trace elements are conspicuously higher than those of subduction-zone basalts that also distinguishes them from the arc related ones. Our summation suggests that the Carboniferous volcanic successions did indeed erupt in an intracontinental rift setting and their generation is likely confined to mantle plume.  相似文献   

19.
Volcanic rocks of the Afyon province (eastern part of western Anatolia) make up a multistage potassic and ultrapotassic alkaline series dated from 14 to 12 Ma. The early-stage Si-oversaturated volcanic rocks around the Afyon city and further southward are trachyandesitic volcanic activity (14.23 ± 0.09 Ma). Late-stage Si-undersaturated volcanism in the southernmost part of the Afyon volcanic province took place in three episodes inferred from their stratigraphic relationships and ages. Melilite–leucitites (11.50 ± 0.03 Ma), spotted trachyandesites, tephryphonolites and lamproites (11.91 ± 0.13 Ma) formed in the first episode; trachyandesites in the second episode and finally phonotephrites, phonolite, basaltic trachyandesites and nosean-bearing trachyandesites during the last episode. The parameter Q [normative q-(ne + lc + kls + ol)] of western Anatolia volcanism clearly decreased southward with time becoming zero in the time interval 10–15 Ma. The magmatism experienced a sudden change in the extent of Si saturation after 14 Ma, during late-stage volcanic activity of Afyon volcanic province at around 12 Ma, though there was some coexistence of Si-oversaturated and Si-undersaturated magmas during the whole life of Afyon volcanic province.  相似文献   

20.
The Atesina Volcanic District, the Monte Luco volcanics, and the Cima d'Asta, Bressanone-Chiusa, Ivigna, Monte Croce and Monte Sabion intrusions, in the central-eastern Southern Alps, form a wide calc-alkaline association of Permian age (ca. 280–260 Ma). The magmatism originated during a period of post-orogenic extensional/transtensional faulting which controlled the magma ascent and emplacement. The magmatic products are represented by a continuum spectrum of rock types ranging from basaltic andesites to rhyolites, and from gabbros to monzogranites, with preponderance of the acidic terms. They constitute a metaluminous to weakly peraluminous series showing mineralogical, petrographic and chemical characteristics distinctive of the high-K calc-alkaline suites. In the MORB-normalized trace element diagrams, the most primitive volcanic and plutonic rocks (basaltic andesites and gabbros with Mg No.=66 to 70; Ni=25 to 83 ppm; Cr=248 to 679 ppm) show LILE and LREE enriched patterns with troughs at Nb–Ta and Ti, a distinctive feature of subduction-related magmas. Field, petrographic, geochemical and isotopic evidence (initial 87Sr/86Sr ratios from 0.7057 to 0.7114; εNd values from −2.7 to −7.4; ∂18O values between 7.6 and 9.5‰) support a hybrid nature for both volcanic and plutonic rocks, originating through complex interactions between mantle-derived magmas and crustal materials. Only the scanty andalusite–cordierite and orthopyroxene–cordierite bearing peraluminous granites in the Cima d'Asta and Bressanone-Chiusa intrusive complexes can be interpreted as purely crustal melts (initial 87Sr/86Sr=0.7143–0.7167; initial εNd values between −7.9 and −9.6, close to average composition of the granulitic metasedimentary crust from the Ivrea Zone in the western Southern Alps). Although the Permian magmatism shows geochemical characteristics similar to those of arc-related suites, palaeogeographic restorations, and geological and tectonic evidence, seem not to support any spatial and/or temporal connection with subduction processes. The magmatism is post-collisional and post-orogenic, and originated in a regime of lithospheric extension and attenuation affecting the whole domain of the European Hercynian belt. A change in the convergence direction between Gondwana and Laurasia, combined with the effects of gravitational collapse of the Hercynian chain, could have been the driving mechanism for lithosphere extension and thinning, as well as for upwelling of hot asthenosphere that caused thermal perturbation and magma generation. In the above context, the calc-alkaline affinity and the orogenic-like signature of the Permian magmatism might result from extensive contamination of basaltic magmas, likely derived from enriched lithospheric mantle source(s), with felsic crustal melts.  相似文献   

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