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1.
In the Cuthbert Lake region of north-central Manitoba, northeasterly trending ultramafic-mafic dikes, part of the Molson dike swarm, show a range of composition from gabbro to olivine-hornblende pyroxenite to hornblende peridotite. The major dike which is ultramafic in composition is 60 m thick. Olivine and chromian spinel were the earliest cumulus phases formed in a subcrustal magma chamber before the emplacement of the dikes. Orthopyroxene and clinopyroxene were formed following emplacement at about 1120° C. Plagioclase and hornblende were the latest phases to crystallize from the intercumulus melt. Mineralogical and chemical variations across the major dike are interpreted to have resulted from flow differentiation of multiple injections of magma carrying suspended olivine crystals. Olivine phenocrysts changed their compositions from about Fo87 to values ranging from Fo80to Fo73 as a function of the amount of intercumulus melt. The composition of this melt is estimated to have been basaltic. A mafic dike, about 10 m thick and occurring about 20 m away from the main ultramafic dike, is believed to have been formed from magmas that were tapped from an upper layer overlying the olivine-rich zone in a subcrustal magma chamber. Separation must have occurred when clinopyroxene and plagioclase appeared on the liquidus.Geological Survey of Canada Contribution 36486  相似文献   

2.
The NNW-trending Nova Lacerda tholeiitic dike swarm in Mato Grosso State, Central Brazil, intrudes the Nova Lacerda granite (1.46 Ga) and the Jauru granite-greenstone terrain (ca. 1.79–1.77 Ga). The swarm comprises diabases I and II and amphibolites emplaced at ca. 1.38 Ga. Geochemical data indicate that these are evolved tholeiites characterized by high LILE/HSFE and LREE/HSFE ratios. Isotopic modelling yields positive ?Nd(T) values (+0.86 to?+2.65), whereas values for ?Sr(T) range from positive to negative (+1.96 to -5.56). Crustal contamination did not play a significant petrogenetic role, as indicated by a comparison of isotopic data (Sr–Nd) from both dikes and country rocks, and by the relationship between isotopic and geochemical parameters (SiO2, K2O, Rb/Sr, and La/Yb) of the dikes. We attribute the origin of these tholeiites to fractional crystallization of evolved melts derived from a heterogeneous mantle source. Comparison of the geochemical and isotopic data of the studied swarm and other tholeiitic Mesoproterozoic mafic intrusions of the SW Amazonian Craton – the Serra da Providência, Colorado, and Nova Brasilândia bimodal suites – indicates that parental melts of the Nova Lacerda swarm were derived from the most enriched mantle source. This enrichment was probably caused by the stronger influence of the EMI component on the DMM end-member. These data, coupled with trace element bulk-rock geochemistry of the country rocks, and comparisons with the Colorado Complex of similar age, suggest a continental-margin arc setting for the emplacement of the Nova Lacerda dikes.  相似文献   

3.
The Roshtkhar area is located in the Khaf-Kashmar-Bardaskan volcano-plutonic belt to the northeastern Iran along the regional E–W trending Dorouneh Fault, northeastern of the Lut Block. There are several outcrops of subvolcanic rocks occurring mainly as dikes in the area, which intruded into Cenozoic intrusive rocks. We present U–Pb dating of zircons from a diabase dike and syenite rock using LA-ICP-MS that yielded an age of 1778 ± 10 Ma for the dike, indicating this Cenozoic dike has zircon xenocrysts inherited from deeper sources; and 38.0 ± 0.5 Ma, indicating an Late Eocene crystallization age for the syenite. Geochemically, the dikes typical of high-K calc-alkaline to shoshonitic magmas. Petrographic observations and major and trace element variations suggest that diabase melts underwent variable fractionation of clinopyroxene, olivine, and Fe-Ti oxides and minor crustal contamination during the differentiation process. Primitive mantle-normalized multi-element diagrams display enrichment in LILE, such as Rb, Ba, Th, U, and Sr compared to HFSE, as well as negative anomalies of Nb, Ta, P, and Ti, suggesting derivation from subduction-modified mantle. Chondrite-normalized REE plots show moderately LREE enriched patterns (<3.83 LaN/YbN <8.27), and no significant Eu anomalies. Geochemical modelling using Sm/Yb versus La/Yb and La/Sm ratios suggests a low-degree of batch melting (~1–3%) of a phlogopite-spinel peridotite source to generate the mafic dikes. The geochemical signatures suggest that the Roshtkhar mafic dikes cannot be related directly to subduction and likely resulted from melting of upper mantle in an extensional setting where the heat flow was provided from deeper levels. These dikes presumably derived the zircon xenocrysts from the assimilation of upper crust of Gondwanian basement. Processes responsible for partial melting of metasomatized lithospheric mantle and post-collision magmatism in NE Iran was triggered by heating due to asthenospheric upwelling in an extensional setting.  相似文献   

4.
The thermal and mechanical structures of the southern part of the Precambrian Indian Shield have been estimated using available heat flow data and shear stress profiles from olivine rheology. These and other geological, geochronological and geophysical data including deep seismic studies (DSS) profiles of Proterozoic Cuddapah basin on South Indian Shield, are utilized to examine thermal models for the evolution of Precambrian intracratonic, platform basins on the Archean lithosphere of Indian Shield. Evidence of mantle perturbations and cycles of thermal events are documented to be important in the Cuddapah basin's evolution. Haxby et al.'s (1976) thermal model has been shown to explain the Cuddapah basin's flexuring and magnitude of subsidence.  相似文献   

5.
Mesozoic mafic dikes in the Gan-Hang tectonic belt (GHTB) provide an opportunity to explore both the nature of their mantle source(s) and the secular evolution of the underlying Mesozoic lithospheric mantle in the region. The geochronology and primary geochemical and Sr–Nd–Pb isotopic compositions of Group 1 (middle section of GHTB) and Group 2 (the rest of the section) dolerite dikes spanning the GHTB were investigated. K–Ar ages indicate that dikes of both groups were emplaced during the Cretaceous (131–69 Ma). The dikes are doleritic in composition and are enriched in both large ion lithophile elements (LILEs; e.g. Rb, Ba, and Pb) and light rare earth elements (LREEs), with a wide range of Eu anomalies, but are depleted in high field strength elements (HFSEs; e.g. Nb, Ta, and Ti) and heavy rare earth elements (HREEs). Dikes sampled in the middle section of the GHTB (Group 1) show more pronounced REE differentiation and a greater contribution from crustal material than those from the east and west sections (Group 2) and are similar to GHTB volcanic rocks in exhibiting a slight enrichment in LREEs. The dolerites are further characterized by a wide range in 87Sr/86Sr i ?=?0.7041–0.7110, 143Nd/144Nd i ?=?0.511951–0.512758, ?Nd t ?=?–10.4 to?+5.6, and Pb isotopic ratios (206Pb/204Pb i ?=?18.1–18.3, 207Pb/204Pb i ≈ 15.6, and 208Pb/204Pb i ?=?38.2–38.7). The dikes have undergone fractional crystallization of olivine, clinopyroxene, plagioclase, and Ti-bearing phases, except for dikes from the Anding area, which possibly experienced fractionation of plagioclase. Geochemically, all the dike samples originated from mantle sources ranging in composition from depleted to enriched that contained a component of foundered lower crust; crustal contamination during the ascent of these magmas was negligible. In the context of the late Mesozoic lithospheric extension across South China, mafic dike magmatism was likely triggered by the reactivation of deep faults, which promoted foundering of the lower crust and subsequent mantle upwelling in the GHTB.  相似文献   

6.
The early Permian Xiaomiao mafic dike swarm in the East Kunlun orogenic belt (EKOB) provides an excellent opportunity to study the petrogenesis of such swarms developed in supra-subduction zone environments, and to investigate the early plate tectonic history of the Palaeo-Tethyan Ocean. Hornblende 40Ar–39Ar dating results indicate that the mafic dikes formed in the early Permian (277.76 ± 2.72 Ma). The Xiaomiao mafic hypabyssals have the following compositional range: SiO2 = 46.55–55.75%, MgO = 2.80–7.38%, Mg# = 36–61, and (Na2O + K2O) = 2.87–4.95%. Chemically, they display calc-alkali affinities, ranging in composition from gabbro to gabbroic diorite. All analysed dikes are enriched in light rare earth elements and large-ion lithophile elements (e.g. Rb and Ba), but are depleted in heavy rare earth elements and high field strength elements (e.g. Nb, Ta, and Ti). Their ISr and ?Nd(t) values range from 0.707 to 0.715 and –2.60 to +2.91, respectively. They are geochemically similar to subduction-related basaltic rocks (e.g. island arc basalt), but differ from E-MORB and N-MORB. Petrographic and major element data reveal that fractional crystallizations of clinopyroxene, olivine, hornblende, and Fe–Ti oxides may have occurred during magma evolution, but that crustal contamination was minor. Based on geochemical and Sr–Nd isotopic bulk-rock compositions, we suggest that the mafic dikes were likely generated by 10–20% partial melting of a spinel + minor garnet lherzolite mantle source metasomatized by subducted, slab-derived fluids, and minor sediments. Based on our results, we propose that the early evolution of the Palaeo-Tethyan Ocean involved the spreading and initial subduction of the Carboniferous to early Permian ocean basin followed by late Permian subduction, which generated the magmatic arc.  相似文献   

7.
The oldest igneous rocks in the Paleoproterozoic (~1.88–1.85 Ga) North Baikal postcollisional volcanoplutonic belt of the Siberian craton are the basaltoids of the Malaya Kosa Formation (Akitkan Group). The youngest are the composite (dolerite–rhyolite) and doleritic dikes cutting the granitoids of the Irel’ complex and the felsic volcanic rocks of the Khibelen Formation (Akitkan Group). The position of Malaya Kosa basaltoids in the Akitkan Group section and published geochronological data on the felsic volcanic rocks overlying Malaya Kosa rocks suggest that their age is ~1878 Ma. The rhyolites from the center of a composite dike were dated by the U–Pb zircon method at 1844 ± 11 Ma, and the dolerites in the dikes are assumed to be coeval with them. Malaya Kosa basaltoids correspond to high-Mg tholeiites and calc-alkaline andesites, whereas the dolerites in the dikes correspond to high-Fe tholeiites. Geochemically, these basaltoids and dolerites are both similar and different. As compared with the dolerites, the basaltoids are poorer in TiO2 (an average of 0.89 vs. 1.94 wt.%), Fe2O31 (9.54 vs. 14.71 wt.%), and P2O5 (0.25 vs. 0.41 wt.%). However, these rocks are both poor in Nb but rich in Th and LREE, εNd(T) being negative. According to petrographic and geochemical data, they derived from compositionally different sources. It is assumed that the basaltoids originated from subduction-enriched lithospheric mantle, whereas the dolerites originated from refractory lithospheric mantle metasomatized by subduction fluids. The isotopic and geochemical features of mafic rocks in the North Baikal belt are well explained by their formation during crustal extension which followed subduction and collision in the region. The early stages of postcollisional extension evidenced the melting of subduction-enriched lithospheric mantle with the formation of parent melts for Malaya Kosa basaltoids. At the final stages of the formation of the North Baikal belt, during the maximum crustal extension, Fe-enriched melts rose to the surface and generated the dolerites of the dikes.  相似文献   

8.
Based on K-Ar isotope analyses, Mesozoic mafic (and alkali ultramafic) dikes from western and eastern Shandong Province, China, are dated at 88.2±1.70 Ma to 169.5±3.7 Ma with the majority of ages ranging from 90 Ma to 140 Ma. The emplacement of the dikes suggests a major Yanshanian (Cretaceous) crustal extension in Shandong province. Together with other available age data, this study suggests four periods of crustal extension at about 80 Ma, 100 Ma, 120 Ma and 140 Ma, respectively. Besides the effect of collapse of the Yanshanian orogenic belt on the emplacement of the mafic dikes in Shandong in the Cretaceous, the mantle plume and the extensive left-lateral advection and extension of the Tanlu fault also have controlled the crustal extension and the emplacement of the mafic dikes in eastern Shandong and western Shandong, respectively.  相似文献   

9.
《International Geology Review》2012,54(12):1389-1400
Post-orogenic mafic dikes are widespread across eastern Shandong Province, North China Craton, eastern China. We here report new U–Pb zircon ages and bulk-rock geochemical and Sr–Nd–Pb isotopic data for representative samples of these rocks. LA-ICP-MS U–Pb zircon analysis of two mafic dike samples yields consistent ages of 118.7 ± 0.25 million years and 122.4 ± 0.21 million years. These Mesozoic mafic dikes are characterized by high (87Sr/86Sr) i ranging from 0.7082 to 0.7087, low ?Nd(t) values from??17.0 to??17.5, 206Pb/204Pb from 17.14 to 17.18, 207Pb/204Pb from 15.44 to 15.55, and 208Pb/204Pb from 37.47 to 38.20. Our results suggest that the parental magmas of these dikes were derived from an ancient, enriched lithospheric mantle source that was metasomatized by foundered lower crustal eclogitic materials prior to magma generation. The mafic dikes underwent minor fractionation during ascent and negligible crustal contamination. Combined with previous studies, these findings provide additional evidence that intense lithospheric thinning beneath eastern Shandong occurred at ～120 Ma, and that this condition was caused by the removal/foundering of the lithospheric mantle and lower crust.  相似文献   

10.
陕西安康的瓦房店、紫阳、岚皋、镇坪和湖北竹溪一带的早古生代地层中广泛出露具双峰式岩浆岩组合的岩墙群。本文通过紫阳高滩辉长岩锆石LA-ICP-MS U-Pb年代学研究,获得其侵位年龄为410±4.5 Ma。高滩辉长岩具有类似OIB的稀土元素和微量元素分配特征,Th/Nb比值小于0.11,Nb/Zr大于0.15,显示其形成于与地幔柱活动相关的富集地幔源区。Nb/U、Ce/Pb比值及U、Th、Pb和Ni含量特征显示岩浆演化早期经历橄榄石和辉石的弱分离结晶作用,岩浆上升过程中几乎未受到地壳物质混染作用。高滩辉长岩年代学、微量元素地球化学特征及区域地质特征表明,早古生代晚期扬子陆块北缘地幔柱作用使得该区软流圈地幔上涌,导致扬子板块北缘陆内裂解,引发紫阳、岚皋一带早古生代晚期的基性岩浆作用。由于地幔深部热动力条件制约,这次大陆裂解事件未能持续。高滩辉长岩锆石年龄是对扬子板块北缘陆内裂解事件的响应。  相似文献   

11.
Ten dolerite dikes intruded into Triassic fault troughs in the Piedmont area of North Carolina have been analyzed for the contents of major elements plus selected trace elements. The average composition of the initial magma, as indicated by four chill margins for major elements and three for trace elements, is: SiO2, 48.6%; Al2O3, 16.9%; TiO2, 0.57%; Fe2O3, 3.30%; FeO, 6.72%; MgO, 10.59; CaO, 10.42%; Na2O, 2.03%; K2O, 0.20%; MnO, 0.20%; Rb, 2.6 ppm; Sr, 133 ppm; Zr, 46 ppm; Th, 0.4 ppm; and U, below detection limit of approximately 0.1 ppm. One large dike (BP) exhibits a Palisades-type of differentiation by crystal settling of olivine, and the comparatively thick JY dike shows development of micropegmatite toward the center; the smaller dikes, however, are relatively homogeneous across their width. Study of the relationship between SiO2 content and the ratio FeO+Fe2O3/MgO+ FeO+Fe2O3 indicates that most dikes crystallize under conditions of decreasing oxygen pressure, but the differentiation trend of the JY, RD, and RS dikes indicates either constant or increasing oxygen pressure during their evolution.Statistical comparison of the composition of the initial dolerite magmas with a variety of basalt types around the world suggests that the North Carolina dolerites are far more similar to oceanic or oceanic margin tholeiites than to continental tholeiites. The North Carolina rocks are distinctly different from plateau basalts but are similar to the chill zones of the Precambrian Bushveld and Stillwater lopoliths. The comparatively low contents of Th, U, and Sr, plus the relatively high K/Rb ratio all support the possibility that the magmas for the North Carolina dolerites evolved in a dominantly oceanic environment. It seems distinctly possible that continental-type crust and mantle did not exist in the Appalachian Piedmont area in Triassic time, even after major orogeny and the concurrent formation of granitic intrusions.  相似文献   

12.
Sven Dahlgren 《Lithos》1994,31(3-4):141-154
Ultramafic dikes with carbonatitic affinities (“damtjernites”) in southern Norway were generated during two magmatic events separated by about 275 Ma. The older event is late Proterozoic and the younger is mid Carboniferous.

More than 50 satellitic damtjernite intrusions occur within a 1500 km2 large region surrounding the Fen Central Complex. Phlogopite macrocrysts from 10 of these satellites yield a Rb---Sr isochron age of 578±24 Ma (2σ expanded errors). This demonstrates that the late Proterozoic carbonatitic magmatism centered at the Fen Central Complex occurred on a regional scale. This region is termed “the Fen Province”. The emplacement of the magmas in the Fen Province most likely occurred in connection with minor extensional tectonic activity on the Baltic platform during the drift-phase after the Proto-Atlantic opening.

Sr-isotopic data also show that a dike mineralogically and chemically similar to the Fen damtjernites was emplaced at 324±4 Ma (mid Carboniferous). This dike very likely dates the initiation of magmatism in the Oslo rift. Consequently very similar carbonate-bearing ultramafic magmas were generated within the south Norwegian mantle during the relatively minor Fen event and in the initial extensional period when the magma production in the Oslo rift was still low.  相似文献   


13.
The Zedong ophiolite is the largest ophiolite massif east of Dazhuqu in the Yarlung Zangbo Suture Zone in the southern Tibetan Plateau. However, its age, geodynamic setting and relationship to the Xigaze ophiolite remain controversial. New zircon U–Pb ages, whole-rock geochemical and Nd–Pb isotopic data from ophiolitic units provide constraints on the geodynamic and tectonic evolution of the Zedong ophiolite. U–Pb zircon geochronology of dolerite lavas and late gabbro–diabase dikes yield weighted mean ages of 153.9 ± 2.5 Ma and 149.2 ± 5.1 Ma, respectively. Strong positive εNd(t) and positive Δ7/4Pb and Δ8/4Pb values indicate derivation from a highly depleted mantle source with an isotopic composition similar to that of the Indian MORB-type mantle. The geochemistry of ophiolitic lavas and early dikes are analogous to typical island arc tholeiites whereas late dikes are similar to boninites. The geochemistry of these rock types suggests multi-stage partial melting of the mantle and gradually enhanced subduction influences to the mantle source through time. Combined with the MORB-like 162.9 ± 2.8 Ma Luobusha ophiolitic lavas, we suggest that the Luobusha lavas, Zedong lavas and early dikes originated in an infant proto-arc setting whereas late dikes in the Zedong ophiolite originated in a forearc setting. Together, they represent a Neo-Tethyan subduction initiation sequence. The Late Jurassic intra-oceanic proto-arc to forearc setting of the Zedong ophiolite contrasts with the continental margin forearc setting for the Xigaze ophiolite, which suggests a laterally complex geodynamic setting for ophiolites along the Yarlung Zangbo Suture Zone.  相似文献   

14.
Summary The investigated mantle section of the Leka ophiolite complex extends 1.4 km from and 1.1 km along the exposed Moho. The foliated peridotite contains numerous tabular and elongated dunite bodies, orthopyroxenite dikes, websterite veins, and dikes. The foliation of the peridotite is inclined by about 45° to the Moho. The dunite bodies and the dikes cut the foliation at low angles. The dunite bodies vary in width from 0.1 to 50 m and in length from 10 m to more than 1 km. Wider dunite bodies are commonly surrounded by 0 to 1.0 m wide margins of dunitized peridotite. Websterite veins may be present outside these margins. Apart from sporadic chromite layers the dunite is very homogenous. The dunite bodies are considered to have formed by deposition of olivine along the walls of dikes originally containing tholeiitic melt. The tholeiitic melt at first heated the peridotitic sidewalls so that they became partially molten and dunitized. The ascending magma then eroded the sidewalls and removed olivine as xenocrysts. When the ascent rate decreased, the temperature of the sidewalls decreased, so that olivine (Fo89–92) began to crystallize along the dike walls. There is also evidence for percolative melt migration along foliation planes, however, the largest proportion of the melts intruded along dikes. The websterite dikes are mostly 1 to 4 cm wide and 3 to 20 m long and dispersed with mutual distances of 20–50 m. The websterite veins and dikes probably originated from melts that were generated along the heated sidewalls of the dunite bodies. The 0.02 to 10 m wide orthopyroxenite dikes have exceptionally high MgO contents for their SiO2 contents; about 36 wt.% MgO and 50 wt.% SiO2. They may have formed as segregates from a SiO2-rich magma, although the parent magma does not appear to have been boninitic. The parent magma may instead have formed by second stage partial melting of depleted lherzolite.  相似文献   

15.
The dike swarm of lamprophyres at Pishan on the SW margin of the Tarim terrane is composed of ultra-potassic lamprophyres that have intruded into the Proterozoic hornblende schists. 40Ar/39Ar dating data suggest that the isochron ages for phlogopite and whole rock separates of the lamprophyres are 231.7±0.3 Ma and 228.5±0.3 Ma, respectively. Geochemical data indicate that the dikes were derived from a subduction-related or metasomatized mantle. During the Middle-Late Triassic the western Kunlun area entered the post-orogeny period, and thus the occurrence of the dike swarm in the study area may have been controlled by underplating.  相似文献   

16.
Study on the Tectonic Setting for the Ophiolites in Xigaze, Tibet   总被引：4，自引：0，他引：4  
The Xigaze ophiolite is located in the middle section of the Yarlung Zangbo River ophiolite belt and includes a well-preserved sequence section of seven ophiolite blocks. The relatively complete ophiolitic sequence sections are represented by Jiding, Dejixiang, Baigang, and Dazhuqu ophiolites and consist of three–four units. The complete ophiolite sequence in order from the bottom to top consists of mantle peridotite, cumulates, sheeted sill dike swarms, and basic lavas±radiolarian chert. These cumulates are absent in the remaining blocks of Dejixiang and Luqu. The age of radiolaria in the radiolarian chert is Late Jurassic–Cretaceous. The basalt and ultramafic rock of the ophiolite also are overlaid by Tertiary Liuqu conglomerate, which contains numerous pebble components of ophiolite, indicating that the Tethys Ocean began to close at the end of Cretaceous Period. The isotopic data of gabbro, diabase, and albite granite in the Xigaze ophiolite are approximately 126–139 Ma, which indicates that the ophiolite formed in the Early Cretaceous. The K–Ar age of amphibole in garnet amphibolite in the ophiolite mélange is 81 Ma, indicating that tectonic ophiolite emplacement occurred at the end of Late Cretaceous.  相似文献   

17.
WIEBE  R.A. 《Journal of Petrology》1986,27(6):1253-1275
Nodules and xenocrysts dominated by high-A1 orthopyroxene occurin Proterozoic basaltic dikes that cut the Nain anorthositecomplex, Labrador. This pyroxene (En73–68, Al2O3 = 6.5–4.5)lacks exsolution and occurs both as anhedral xenocrysts up to10 cm in diameter and with euhedral plagioclase (An55) in ophiticnodules. Rarely, olivine (Fo70) occurs with orthopyroxene andAl-spinel with plagioclase. Scarce, more Fe-rich nodules containtwo pyroxenes (orthopyroxene + pigeonite and pigeonite+augite)and coarse intergrowths of ilmenite and Ti-rich magnetite. Pyroxenepairs yield temperatures of 1250? to 1170 ?C; coexisting oxidelamellae yield temperatures between 1145? and 1120 ?C. The highsubsolidus temperatures of the nodules contrasts with the lowtemperature of the host anorthosite at the time of dike emplacementand indicates a deep source for the nodules. Coexisting olivine(Fo70) and plagioclase (An54) suggest a maximum pressure ofabout 11 kb.The dominant orthopyroxene in these nodules is nearlyidentical in composition to the high-Al orthopyroxene megacrystswith exsolved plagioclase (HAOM) found in most Proterozoic anorthosites,and the ophitic nodules have textures similar to ophitic occurrencesof HAOM in anorthosite. Rafting of cotectic nodules from thelower crust can explain occurrences of HAOM in shallow levelanorthosites.The nodules and xenocrysts have compositions consistentwith crystallization from magmas that were parental to the anorthosites.They lend support to models which derive anorthosites by fractionalcrystallization of basaltic magma near the base of the crust.  相似文献   

18.
A large mafic dike swarm is radially distributed in southern Qiangtang. Three typical samples were selected for geochronology, geochemistry, and Hf isotopic analysis. Zircon U–Pb dating indicates that the three dikes formed at 291 ± 2, 292 ± 3, and 300 ± 2 Ma. Whole-rock compositions show that the southern Qiangtang mafic dikes are alkaline, Fe + Ti rich, and exhibit relative enrichment in light rare-earth elements. The ratios of incompatible elements are similar to those of oceanic island and Emeishan basalts. Geochemical diagrams show that the dikes erupted in an intraplate environment. Zircon Hf isotopic data suggest that magma that produced the mafic dikes was derived from a depleted mantle source. The geochemical characteristics of the dikes approximate that of eruption products of a brief period of mantle plume activity (300–280 Ma). According to eight geologic maps of Qiangtang, the mafic dikes crop out over an area of 150 km from north to south and 500 km from east to west, radiating outward from Mayigangri. We conclude that the mafic dikes in southern Qiangtang are related to the combined effect of Permian plate motions and mantle plume activity, and the Mayigangri area overlies the hot spot. Furthermore, the mantle plume in southern Qiangtang may have propelled the closing of the Palaeo-Tethys Ocean.  相似文献   

19.
 Ultramafic xenoliths are found in Kishyuku Lava, Fukue-jima, Southwest Japan. These include spinel lherzolite, harzburgite and dunite, as well as pyroxenite. The compositions of the constituent minerals of the peridotite xenoliths are in the range of upper mantle peridotites. Variable Cr/(Cr+Al) ratios (0.1–0.5) of spinel, together with a limited range in olivine composition (Fo90–Fo92), indicate that the xenoliths are derived from slightly to highly depleted residual mantle. The combination of previously published clinopyroxene-olivine geothermobarometry and clinopyroxene-orthopyroxene geothermometry applied to the xenoliths yields a high geotherm of 1070° C at 1.0 GPa up to 1200° C at 2.2 GPa. Existence of such depleted upper mantle is compatible with the existing model of asthenospheric injection during the rifting of the Northeast China and the Japan Sea. The high geotherm is caused by thermal perturbation due to the injection of the hot asthenosphere and/or post-rifting uprise of mantle diapirs since 11 Ma. Received: 15 May 1995 / Accepted: 3 January 1996  相似文献   

20.
陕西西乡县广泛出露的晋宁期岩浆岩是研究和理解扬子克拉通基底地质演化的一个窗口。通过对西乡县龙王塘地区的辉长岩体、花岗岩脉和钠长斑岩脉进行锆石U-Pb定年和原位Lu-Hf同位素分析,确定了岩浆侵位时代,揭示了岩浆源区特征。年代学研究结果表明,辉长岩体和花岗岩脉的侵位时间分别为（863.6±4.4）Ma和（863.6±3.8）Ma,是新元古代岩浆作用的产物。在两条钠长斑岩脉中识别出（905.2±8.2）Ma、（807.0±8.9）Ma和（762.5±4.2）Ma三组锆石年龄,第一组为继承锆石的年龄,后两组年龄分别代表两条岩脉的侵位时代。综上可知,龙王塘地区存在~864 Ma、~807 Ma和~763 Ma三期岩浆作用,以及早期（~905 Ma）构造热事件,岩浆作用具有长期性和多阶段性特征。Lu-Hf同位素分析结果表明,辉长岩体和脉岩的εHf（t）平均值介于+7.1~+10.0之间。对已有地质资料进行综合分析发现,汉南地区的绝大多数新元古代岩体的εHf（t）为正值,具有随时间偏离亏损地幔线,接近球粒陨石线的基本演化趋势。上述锆石U-Pb年龄和原位Lu-Hf同位素结果表明,龙王塘地区出露的新元古代岩浆岩起源于亏损地幔或新生地壳的部分熔融,形成于活动型大陆边缘环境。   相似文献   

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