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1.
下扬子天目山盆地火山岩锆石LA-ICP-MS定年及地质意义   总被引：1，自引：0，他引：1  
天目山盆地是下扬子江南隆起带保存较完整的中生代火山盆地,中生代火山岩系岩性自下而上主要为流纹岩-英安岩-安山岩。对盆地内黄尖组下段流纹岩和英安岩分别进行了锆石 LA-ICP MS定年,分别获得了133．6±1．5 Ma（MSWD＝0．73）和135．0±2．1 Ma（MSWD＝0．78）的锆石U-Pb年龄,指示天目山盆地黄尖组火山岩时代为早白垩世。天目山盆地火山活动起始时间和长江中下游地区晚中生代火山活动基本一致,说明江南隆起带和长江中下游地区在早白垩世均处于强烈拉张环境。  相似文献   

2.
腾冲火山活动的时代和岩浆来源问题   总被引：35，自引：0，他引：35       下载免费PDF全文
47个腾冲火山岩样品的K-Ar年龄值域在0.09和17.84Ma之间。4条火山岩的40Ar/36Ar-40K/36Ar等时线年龄分别为2.93、0.81、0.31和0.13Ma。火山喷发的时代从中新世到更新世,喷发的高潮在晚更新世。腾冲火山目前还不是死火山,而腾冲及其邻区的热事件（侵入-热变质-喷发）又是连续发生的。20个样品的Rb和Sr含量、稳定Sr同位素初始比（0.70578-0.71437）以及其它地球化学资料还表明,这些火山岩是属于板块碰撞带生成的高钾钙碱性岩浆系列。火山岩的母岩浆来源于地幔的玄武岩浆,但在上升过程中受到过富含放射性成因Sr的地壳物质的强烈渐进混染。  相似文献   

3.
Mount Hasan is a double-peaked stratovolcano, located in Central Anatolia, Turkey. The magmas erupted from this multi-caldera complex range from basalt to rhyolite, but are dominated by andesite and dacite. Two terminal cones (Big Mt. Hasan and Small Mt. Hasan) culminate at 3253 m and 3069 m respectively. There are four evolutionary stages in the history of the volcanic complex (stage 1: Kecikalesi volcano, 13 Ma, stage 2: Palaeovolcano, 7 Ma, stage 3: Mesovolcano and stage 4: Neovolcano). The eruptive products consist of lava flows, lava domes, and pyroclastic rocks. The later include ignimbrites, phreatomagmatic intrusive breccias and nuées ardentes, sometimes reworked as lahars. The total volume is estimated to be 354 km3, the area extent 760 km2. Textural and mineralogical data suggest that both magma mixing and fractional crystallization were involved in the generation of the andesites and dacites. The magmas erupted from the central volcanoes show a transition with time from tholeite to calc-alkaline. Three generations of basaltic strombolian cones and lava flows were emplaced contemporaneously with the central volcanoes. The corresponding lavas are alkaline with a sodic tendency.  相似文献   

4.
The eruptive history of the Tequila volcanic field (1600 km2) in the western Trans-Mexican Volcanic Belt is based on 40Ar/39Ar chronology and volume estimates for eruptive units younger than 1 Ma. Ages are reported for 49 volcanic units, including Volcán Tequila (an andesitic stratovolcano) and peripheral domes, flows, and scoria cones. Volumes of volcanic units 1 Ma were obtained with the aid of field mapping, ortho aerial photographs, digital elevation models (DEMs), and ArcGIS software. Between 1120 and 200 kyrs ago, a bimodal distribution of rhyolite (~35 km3) and high-Ti basalt (~39 km3) dominated the volcanic field. Between 685 and 225 kyrs ago, less than 3 km3 of andesite and dacite erupted from more than 15 isolated vents; these lavas are crystal-poor and show little evidence of storage in an upper crustal chamber. Approximately 200 kyr ago, ~31 km3 of andesite erupted to form the stratocone of Volcán Tequila. The phenocryst assemblage of these lavas suggests storage within a chamber at ~2–3 km depth. After a hiatus of ~110 kyrs, ~15 km3 of andesite erupted along the W and SE flanks of Volcán Tequila at ~90 ka, most likely from a second, discrete magma chamber located at ~5–6 km depth. The youngest volcanic feature (~60 ka) is the small andesitic volcano Cerro Tomasillo (~2 km3). Over the last 1 Myr, a total of 128±22 km3 of lava erupted in the Tequila volcanic field, leading to an average eruption rate of ~0.13 km3/kyr. This volume erupted over ~1600 km2, leading to an average lava accumulation rate of ~8 cm/kyr. The relative proportions of lava types are ~22–43% basalt, ~0.4–1% basaltic andesite, ~29–54% andesite, ~2–3% dacite, and ~18–40% rhyolite. On the basis of eruptive sequence, proportions of lava types, phenocryst assemblages, textures, and chemical composition, the lavas do not reflect the differentiation of a single (or only a few) parental liquids in a long-lived magma chamber. The rhyolites are geochemically diverse and were likely formed by episodic partial melting of upper crustal rocks in response to emplacement of basalts. There are no examples of mingled rhyolitic and basaltic magmas. Whatever mechanism is invoked to explain the generation of andesite at the Tequila volcanic field, it must be consistent with a dominantly bimodal distribution of high-Ti basalt and rhyolite for an 800 kyr interval beginning ~1 Ma, which abruptly switched to punctuated bursts of predominantly andesitic volcanism over the last 200 kyrs.Electronic Supplementary Material Supplementary material is available in the online version of this article at Editorial responsility: J. Donnelly-NolanThis revised version was published online in January 2005 with corrections to Tables 1 and 3.An erratum to this article can be found at  相似文献   

5.
Rubini  Soeria-Atmadja  Dardji  Noeradi 《Island Arc》2005,14(4):679-686
Abstract   The evolution of volcanism in Sumatra and Java during Tertiary and Quaternary time can be divided into three phases: (i) lava flows of the Early Tertiary event (43–33 Ma) consisting of island arc tholeiites; followed by (ii) eruption of tholeiitic pillow basalt at the beginning of the Late Tertiary (11 Ma); and succeeded by (iii) medium-K calc-alkaline magmatism in the Pliocene and Quaternary. The present available field data on the occurrence of Paleogene volcanic rocks and subsurface data in south Sumatra and northern west Java indicate a much larger area of distribution of the volcanic rocks than previously recognized. Because the eastward continuation of the northern west Java volcanic rocks had not been found, early investigators were inclined to assume that they continued to south Kalimantan. In contrast, the early Tertiary volcanic rocks that occupy the south coast of Java can be traced further east as far as Flores. The occurrence of Paleogene volcanics in south Sumatra and northern west Java can be interpreted as a Paleogene volcanic arc that was presumably related to the late Cretaceous–Paleogene trench parallel to Sumatra and west Java due to subduction of the Indian Plate toward the northeast (Meratus trend).  相似文献   

6.
The Chiang Khong segment of the Chiang Khong–Lampang–Tak Volcanic Belt is composed of three broadly meridional sub‐belts of mafic to felsic volcanic, volcaniclastic, and associated intrusive rocks. Associated sedimentary rocks are largely non‐marine red beds and conglomerates. Three representative Chiang Khong lavas have Late Triassic (223–220 Ma) laser ablation inductively coupled mass‐spectroscopy U–Pb zircon ages. Felsic‐dominated sequences in the Chiang Khong Western and Central Sub‐belts are high‐K calc–alkaline rocks that range from basaltic to dominant felsic lavas with rare mafic dykes. The Western Sub‐belt lavas have slightly lower high field strength element contents at all fractionation levels than equivalent rocks from the Central Sub‐belt. In contrast, the Eastern Sub‐belt is dominated by mafic lavas and dykes with compositions transitional between E‐mid‐oceanic ridge basalt and back‐arc basin basalts. The Eastern Sub‐belt rocks have higher FeO* and TiO2 and less light rare earth element enrichment than basalts in the high‐K sequences. Basaltic and doleritic dykes in the Western and Central sub‐belts match the composition of the Eastern Sub‐belt lavas and dykes. A recent geochemical study of the Chiang Khong rocks concluded that they were erupted in a continental margin volcanic arc setting. However, based on the dominance of felsic lavas and the mainly non‐marine associated sediments, we propose an alternative origin, in a post‐collisional extensional setting. A major late Middle to early Late Triassic collisional orogenic event is well documented in northern Thailand and Yunnan. We believe that the paucity of radiometric dates for arc‐like lavas in the Chiang Khong–Lampang–Tak Volcanic Belt that precede this orogenic event, coupled with the geochemistry of the Chiang Khong rocks, and strong compositional analogies with other post‐collisional magmatic suites, are features that are more typical of volcanic belts formed in a rapidly evolving post‐collisional, basin‐and range‐type extensional setting.  相似文献   

7.
Contrary to general belief, not all large igneous provinces (LIPs) are characterised by rocks of basaltic composition. Silicic-dominated LIPs, such as the Whitsunday Volcanic Province of NE Australia, are being increasingly recognised in the rock record. These silicic LIPs are consistent in being: (1) volumetrically dominated by ignimbrite; (2) active over prolonged periods (40–50 m.y.), based on available age data; and (3) spatially and temporally associated with plate break-up. This silicic-dominated LIP, related to the break-up of eastern continental Gondwana, is also significant for being the source of >1.4×106 km3 of coeval volcanogenic sediment preserved in adjacent sedimentary basins of eastern Australia.The Whitsunday Volcanic Province is volumetrically dominated by medium- to high-grade, dacitic to rhyolitic lithic ignimbrites. Individual ignimbrite units are commonly between 10 and 100 m thick, and the ignimbrite-dominated sequences exceed 1 km in thickness. Coarse lithic lag breccias containing clasts up to 6 m diameter are associated with the ignimbrites in proximal sections. Pyroclastic surge and fallout deposits, subordinate basaltic to rhyolitic lavas, phreatomagmatic deposits, and locally significant thicknesses of coarse-grained volcanogenic conglomerate and sandstone are interbedded with the ignimbrites. The volcanic sequences are intruded by gabbro/dolerite to rhyolite dykes (up to 50 m in width), sills and comagmatic granite. Dyke orientations are primarily from NW to NNE.The volcanic sequences are characterised by the interstratification of proximal/near-vent lithofacies such as rhyolite domes and lavas, and basaltic agglomerate, with medial to distal facies of ignimbrite. The burial of these near-vent lithofacies by ignimbrites, coupled with the paucity of mass wastage products such as debris-flow deposits indicates a low-relief depositional environment. Furthermore, the volcanic succession records a temporal change in: (1) eruptive styles; (2) the nature of source vents; and (3) erupted compositions. An early explosive dacitic pyroclastic phase was succeeded by a later mixed pyroclastic-effusive phase producing an essentially bimodal suite of lavas and rhyolitic ignimbrite. From the nature and distribution of volcanic lithofacies, the volcanic sequences are interpreted to record the evolution of a multiple vent, low-relief volcanic region, dominated by several large caldera centres.  相似文献   

8.
Abstract Temporal–spatial variations in Late Cenozoic volcanic activity in the Chugoku area, southwest Japan, have been examined based on 108 newly obtained K–Ar ages. Lava samples were collected from eight Quaternary volcanic provinces (Daisen, Hiruzen, Yokota, Daikonjima, Sambe, Ooe–Takayama, Abu and Oki) and a Tertiary volcanic cluster (Kibi Province) to cover almost all geological units in the province. Including published age data, a total of 442 Cenozoic radiometric ages are now available. Across‐arc volcanic activity in an area approximately 500 km long and 150 km wide can be examined over 26 million years. The period corresponds to syn‐ and post‐back‐arc basin opening stages of the island arc. Volcanic activity began in the central part of the rear‐arc ca 26 Ma. This was followed by arc‐wide expansion at 20 Ma by eruption at two rear‐arc centers located at the eastern and western ends. Expansion to the fore‐arc occurred between 20 and 12 Ma. This Tertiary volcanic arc was maintained until 4 Ma with predominant alkali basalt centers. The foremost‐arc zone activity ceased at 4 Ma, followed by quiescence over the whole arc between 4 and 3 Ma. Volcanic activity resumed at 3 Ma, covering the entire rear‐arc area, and continued until the present to form a Quaternary volcanic arc. Adakitic dacite first occurred at 1.7 Ma in the middle of the arc, and spread out in the center part of the Quaternary volcanic arc. Alkali basalt activities ceased in the area where adakite volcanism occurred. Fore‐arc expansion of the volcanic arc could be related to the upwelling and expansion of the asthenosphere, which caused opening of the Japan Sea. Narrowing of the volcanic zone could have been caused by progressive Philippine Sea Plate subduction. Deeper penetration could have caused melting of the slab and resulted in adakites. Volcanic history in the Late Cenozoic was probably controlled by the history of evolution of the upper mantle structure, coinciding with back‐arc basin opening and subsequent reinitiation of subduction.  相似文献   

9.
Taupo volcanic centre is one of two active rhyolite centres in the Taupo Volcanic Zone (TVZ), and has been sporadically active over the past ca. 300 ka. At least four large-scale ignimbrites have erupted from the centre, including the well documented 26.5 ka Oruanui ignimbrite and 1.8 ka Taupo ignimbrite. Because stratigraphy of earlier ignimbrites and their sources are masked by later volcanism, disrupted by regional tectonics and obscured by poor exposure, indirect methods must be applied in order to determine their source regions. In this paper detailed componentry, density and petrology of lithic fragments from three ignimbrites (Rangatira Point, Oruanui, Taupo) are used to reveal aspects of the sub-Taupo caldera geology, including the evolution of the Taupo volcanic centre, to assist in ignimbrite correlation and to evaluate structures within the Taupo caldera complex. Lithic fragments identify a complex subsurface geology. The Rangatira Point ignimbrite sampled dominantly rhyolite lavas, plus a variety of welded ignimbrites, rare high-silica dacites and a single dolerite. Most lithic fragments in the Oruanui ignimbrite are andesite with minor rhyolite, welded ignimbrite, dacite and rounded greywacke, while in the Taupo ignimbrite, rhyolite is again the dominant lithic component with subordinate welded ignimbrites, andesite, and greywacke. The densities of lithic fragments indicate similar ranges of values for all lava types, and thus density is a poor indicator of lithology. Care must, therefore, be taken before interpreting subcrustal stratigraphy using density as the sole criterion. The petrography and geochemistry of lithic types are more specific, and the variation can be used to identify sources for the ignimbrites. Both pumice chemistry and rhyolite lithic fragments from the Rangatira Point ignimbrite are comparable to domes exposed at the southern end of the Western Dome Complex and, combined with limited outcrop information, suggest the most likely source for this unit is in the northern part of the Taupo caldera complex. The dominance of andesite lithic fragments in the Oruanui ignimbrite suggests a major andesite cone existed beneath the source area, and the different lithic suites between Oruanui and Taupo ignimbrites suggest these ignimbrites came, at least in part, from mutually exclusive collapse structures. We believe that the Oruanui caldera is sited principally in the northwestern part of present-day Lake Taupo and the Taupo caldera in the northeastern part. Identification of abundant ignimbrite lithics in the Taupo ignimbrite, which are considered to represent an intracaldera facies of an earlier ignimbrite, that is not exposed at the surface, suggest there was a further (pre-Oruanui) ignimbrite caldera in the Taupo ignimbrite eruptive vent region.  相似文献   

10.
Mount Drum is one of the youngest volcanoes in the subduction-related Wrangell volcanic field (80×200 km) of southcentral Alaska. It lies at the northwest end of a series of large, andesite-dominated shield volcanoes that show a northwesterly progression of age from 26 Ma near the Alaska-Yukon border to about 0.2 Ma at Mount Drum. The volcano was constructed between 750 and 250 ka during at least two cycles of cone building and ring-dome emplacement and was partially destroyed by violent explosive activity probably after 250 ka. Cone lavas range from basaltic andesite to dacite in composition; ring-domes are dacite to rhyolite. The last constructional activity occurred in the vicinity of Snider Peak, on the south flank of the volcano, where extensive dacite flows and a dacite dome erupted at about 250 ka. The climactic explosive eruption, that destroyed the top and a part of the south flank of the volcano, produced more than 7 km3 of proximal hot and cold avalanche deposits and distal mudflows. The Mount Drum rocks have medium-K, calc-alkaline affinities and are generally plagioclase phyric. Silica contents range from 55.8 to 74.0 wt%, with a compositional gap between 66.8 and 72.8 wt%. All the rocks are enriched in alkali elements and depleted in Ta relative to the LREE, typical of volcanic arc rocks, but have higher MgO contents at a given SiO2, than typical orogenic medium-K andesites. Strontium-isotope ratios vary from 0.70292 to 0.70353. The compositional range of Mount Drum lavas is best explained by a combination of diverse parental magmas, magma mixing, and fractionation. The small, but significant, range in 87Sr/86Sr ratios in the basaltic andesites and the wide range of incompatible-element ratios exhibited by the basaltic andesites and andesites suggests the presence of compositionally diverse parent magmas. The lavas show abundant petrographic evidence of magma mixing, such as bimodal phenocryst size, resorbed phenocrysts, reaction rims, and disequilibrium mineral assemblages. In addition, some dacites and andesites contain Mg and Ni-rich olivines and/or have high MgO, Cr, Ni, Co, and Sc contents that are not in equilibrium with the host rock and indicate mixing between basalt or cumulate material and more evolved magmas. Incompatible element variations suggest that fractionation is responsible for some of the compositional range between basaltic andesite and dacite, but the rhyolites have K, Ba, Th, and Rb contents that are too low for the magmas to be generated by fractionation of the intermediate rocks. Limited Sr-isotope data support the possibility that the rhyolites may be partial melts of underlying volcanic rocks. Received March 13, 1993/Accepted September 10, 1993  相似文献   

11.
The Izu–Ogasawara arc contains, from east to west, a volcanic front, a back-arc extensional zone (back-arc knolls zone), and a series of across-arc seamount chains that cross the extensional zone in an east-northeast and west-southwest direction and extend into the Shikoku Basin. K–Ar ages of dredged volcanic rocks from these across-arc seamount chains and extension-related edifices in the back-arc region of the Izu–Ogasawara arc were measured to constrain the volcanic and tectonic history of the arc since the termination of spreading in the Shikoku Basin. K–Ar ages range between 12.5 and 1 Ma. Andesitic to dacitic rocks of 12.5–2.9 Ma occur mainly on the western part of the chains. The western part of the chains are the locus of volcanism behind the front which erupted mainly calc-alkaline andesitic lavas. The youngest rocks (< 2.8 Ma), characterized by cpx-ol basalt, occur along the western margin of the back-arc knolls zone. Basaltic rocks of 12.5–2.9 Ma have relatively high concentrations of Na2O (> 2.0 wt%), Zr (> 50 p.p.m.) and Y (> 20 p.p.m.) and low CaO (< 12 wt%). On the other hand, basalts of 2.8–1 Ma have lower Na2O (< 1.8 wt%), Zr (< 50 p.p.m.) and Y (< 20 p.p.m.), but significantly higher CaO (> 12 wt%). The age inferred for the initiation of back-arc rifting (∼ 2.35–2.9 Ma: Taylor 1992 ) behind the current volcanic arc coincides with the time that basalt chemistry changed drastically (eruption of the low-Na2O and high-CaO basalt). This implies that post-2.8 Ma volcanism in the back-arc knolls zone is associated with rifting. Similarly, the change in chemical composition might be explained by a different type of source mantle following rift initiation. Volcanism in the western seamounts ceased after the onset of rifting at ∼ 2.8 Ma.  相似文献   

12.
The Setouchi volcanic rocks include high-Mg andesites (HMAs) and garnet-bearing dacite–rhyolite, and are sporadically distributed along the Median Tectonic Line, Japan. New U–Pb zircon ages and geological and geochemical data are presented for those rocks in the Western Setouchi region (W-Setouchi). Previous studies referred to the altered andesite in the W-Setouchi as “pre-Setouchi volcanic rocks.” However, on the basis of the new U–Pb age (14.4 Ma ± 0.3 Ma) and geochemical characteristics, we redefine it as the Jikamuro Formation, part of the Setouchi volcanic rocks. Incompatible elements are more enriched in the Jikamuro Formation rocks than in the Setouchi HMAs. The characteristic element compositions may be explained by mixing of compositionally different magmas, including subducted sediment melts, plus a contribution from crustal contamination. A stress-inversion technique with Bingham distribution method was applied to the orientations of felsic and mafic dikes within the Setouchi volcanic rocks, and indicates paleo-stress conditions during the period of Setouchi volcanism in the W-Setouchi. The analysis reveals NNW-extensional stresses and a strike-slip stress. We infer that the former represents extensional conditions during the main period of volcanism and the latter represents a stress transition during the most recent period of volcanism (after 12 Ma).  相似文献   

13.
Achieving an understanding of the nature of monogenetic volcanic fields depends on identification of the spatial and temporal patterns of volcanism in these fields, and their relationships to structures mapped in the shallow crust and inferred in the deep crust and mantle through interpretation of geophysical data. We investigate the spatial and temporal distributions of volcanism in the Abu Monogenetic Volcano Group, Southwest Japan, and compare these distributions to fault and seismic data in the brittle crust, and P-wave tomography of the crust and upper mantle. Essential characteristics of the volcano distribution are extracted by a nonparametric kernel method using an algorithm to estimate anisotropic bandwidth. Overall, E-W elongate smooth modes in spatial density are identified that are consistent with the spatial extent of P-wave velocity anomalies in the lower crust and upper mantle, supporting the idea that the spatial density map of volcanic vents reflects the geometry of a mantle diapir. While the number of basalt eruptions decreased after 0.2 Ma, andesite eruptions increased and overall volume eruption rate is approximately steady-state. Estimated basalt supply to the lower crust is also constant. This observation and the spatial distribution of volcanic vents suggest stability of magma productivity and essentially constant two-dimensional size of the source mantle diapir since 0.46 Ma.  相似文献   

14.
Two fundamentally different types of silicic volcanic rocks formed during the Cenozoic of the western Cordillera of the United States. Large volumes of dacite and rhyolite, mostly ignimbrites, erupted in the Oligocene in what is now the Great Basin and contrast with rhyolites erupted along the Snake River Plain during the Late Cenozoic. The Great Basin dacites and rhyolites are generally calc-alkaline, magnesian, oxidized, wet, cool (<850°C), Sr-and Al-rich, and Fe-poor. These silicic rocks are interpreted to have been derived from mafic parent magmas generated by dehydration of oceanic lithosphere and melting in the mantle wedge above a subduction zone. Plagioclase fractionation was minimized by the high water fugacity and oxide precipitation was enhanced by high oxygen fugacity. This resulted in the formation of Si-, Al-, and Sr-rich differentiates with low Fe/Mg ratios, relatively low temperatures, and declining densities. Magma mixing, large proportions of crustal assimilation, and polybaric crystal fractionation were all important processes in generating this Oligocene suite. In contrast, most of the rhyolites of the Snake River Plain are alkaline to calc-alkaline, ferroan, reduced, dry, hot (830–1,050°C), Sr-and Al-poor, and Nb-and Fe-rich. They are part of a distinctly bimodal sequence with tholeiitic basalt. These characteristics were largely imposed by their derivation from parental basalt (with low fH2O and low fO2) which formed by partial melting in or above a mantle plume. The differences in intensive parameters caused early precipitation of plagioclase and retarded crystallization of Fe–Ti oxides. Fractionation led to higher density magmas and mid-crustal entrapment. Renewed intrusion of mafic magma caused partial melting of the intrusive complex. Varying degrees of partial melting, fractionation, and minor assimilation of older crust led to the array of rhyolite compositions. Only very small volumes of distinctive rhyolite were derived by fractional crystallization of Fe-rich intermediate magmas like those of the Craters of the Moon-Cedar Butte trend. Electronic supplementary material The online version of this article (doi:) contains supplementary material, which is available to authorized users.  相似文献   

15.
A great deal of practical data in recent years have proved that the East Kunlun orogenic belt and even the China central orogenic belt are complex orogenic belts that underwent polycycle orogenic evolvement[1―7]. Each orogenic cycle has left a compositional print, the multi-period ophiolites[4―6] and various types of tec-tono-magmatic production in the same orogenic belt. There is a suite of shallow metamorphic volcanic rocks in the Nuomuhong area in the east part of the East Kunlun orogen…  相似文献   

16.
Volcanism related to subduction of the Philippine Sea (PHS) plate began in Central Kyushu at 5 Ma, after a pause of igneous activity lasting about 10 m.y. It formed a large volcano-tectonic depression, the Hohi volcanic zone (HVZ), and has continued to the present at a decreasing eruption rate. The products are largely andesite and dacite, which became enriched in K with time. The proportion of tholeiitic to calc alkalic rocks also increases with time. Calc-alkalic high-Mg basaltic andesites (YbBs) were erupted in the early stage of the HVZ activity (5–3 Ma), and high-alumina basalts (KjBs) were erupted in the later stage (2–0 Ma). In contrast to the basalts in the HVZ, Northwest Kyushu basalts (NWKBs) have been erupted on the backarc side of the HVZ since 11 Ma, and hence are not related to the PHS plate subduction. They are mainly high-alkali tholeiitic to alkali basalt that shows no notable chemical change with time. NWKB, YbB, and KjB have MORB-normalized incompatible-element spectra that differ from each other, as is well expressed in both Nb and Sr anomalies. The patterns of KjB and NWKB are typical of those for island-arc basalt (IAB) and ocean-island basalt (OIB), respectively. YbB shows a pattern intermediate between the two. We suggest that the magma source beneath the HVZ changed in composition from an OIB-type mantle to an IAB-type mantle as the subduction of PHS plate advanced. However, the magma source remained fertile under Northwest Kyushu. In order to explain the temporal change of source mantle beneath the HVZ, we propose a model for progressive contamination of the mantle wedge, in which three processes (contamination by a slab-derived component, subtraction of magma from the mantle, and mixing of the mantle residue and slab-derived component) are repeated as subduction continues. As long as the progressive contamination of mantle wedge proceeds, its trace-element composition converges at a steady-state value for a short period. This value does not depend on the initial composition of the mantle wedge but instead on the composition of the slab-derived component. The trace-element composition of the magma produced in such a mantle wedge approaches that of the slab-derived component with time, but the major-element composition is determined by the phase relations of mantle peridotite. The slab-derived component may be basaltic liquid that is partially melted from rutile-bearing eclogite.  相似文献   

17.
There are wide spread Cenozoic volcanic rocks in Tengchong (CVRT), Yunnan province, SW China. These rocks comprise three rock types: basalt, andesite (dominant type) and dacite. Most samples are sub‐alkaline, and among the sub‐alkaline rocks, most are high‐K calc‐alkaline. These rocks have a SiO2 range of 49.1 wt.% to 66.9 wt.%. TiO2 contents are not high and have a variation of 0.7 wt.%–1.6 wt.%. Trace element concentrations and element ratios (such as Nb/U, Ce/Pb, Nb/La, etc.) of these rocks have a large variation. 87Sr/86Sr values fall in the range of 0.7057–0.7093 and 143Nd/144Nd values change from 0.5120 to 0.5125. 206Pb/204Pb, 207Pb/204Pb, and 208Pb/204Pb ratios are in the range of 17.936–19.039, 15.614–15.810, and 38.894–39.735, respectively. These geochemical characteristics of CVRT make them resemble island‐arc volcanic rocks. We suggest that the magmas were generated in the lithospheric mantle that had already been metasomatized by previous subduction processes. By the study of the uplift history of the Tibetan Plateau, we found that the beginning of the geotectonic processes to the eruption of CVRT was coeval with one uplift event. Therefore, we propose that the uplift of the Tibetan Plateau caused collapse of the collisional orogeny in Tengchong, which further triggered the generation and eruption of the CVRT magmas.  相似文献   

18.
In this study, new geochemical, zircon U–Pb, and Lu–Hf isotopic data are presented for volcanics from the Hadataolegai Formation of the central Great Xing'an Range (GXR) in Northeast China. These new data offer insights into the petrogenesis of the volcanics of the Hadataolegai Formation and the tectonic evolution of the Paleo–Asian Ocean (PAO) and Mongol–Okhotsk Ocean (MOO). These volcanics of the Hadataolegai Formation are divided into andesite‐trachyandesites and dacite‐trachydacites. Zircon U–Pb ages show that the volcanics of the Hadataolegai Formation erupted between 230 Ma and 228 Ma during the Late Triassic, which agrees with recently obtained data. The volcanic rocks in this study have low Y (9.9–21.1 ppm) and Yb (0.78–2.02 ppm) contents, high Sr (444–954 ppm) contents, and slight Eu anomalies (δEu = 0.82 to 0.94), similar to ‘adakite‐like’ rocks. The dacites were formed by fractional crystallization of coeval andesitic magmas. The zircons within the andesite and trachyandesite yield higher positive εHf(t) values (+6.3 to +12.0) and model ages (TDM2) between 860 Ma and 453 Ma, which indicates that the magmas were generated by a newly accreted continental crustal source. Moreover, some of the volcanics are relatively high in MgO contents. These characteristics indicate that the volcanic magmas were derived from the partial melting of delaminated lower crust and mixing with mantle materials. Combining these data with previous studies, we suggest that the magmatism in the central GXR was governed by extension due to the closure of the PAO and the back‐arc extension associated with the southward subduction of the MOO plate (western GXR, near the Erguna Block).  相似文献   

19.
Early Yanshanian magmatic suites predominate absolutely in the Nanling granite belt. They consist mainly of monzogranite and K-feldspar granite. There occur associations of early Yanshanian A-type granitoids (176 Ma-178 Ma) and bimodal volcanic rocks (158 Ma-179 Ma) in southern Jiangxi and southwestern Fujian in the eastern sector of the granite belt and early Yanshanian basalts (177 Ma-178 Ma) in southern Hunan in the central sector of the belt. Both the acid end-member rhyolite in the bimodal volcanic rock association and A-type granitoids in southern Jiangxi have the geochemical characteristics of intraplate granitic rocks and the basic end-member basalt of the association is intraplate tholeiite, while the basaltic rocks in southern Hunan include not only intraplate tholeiite but also intraplate alkali basalt. Therefore the early Yanshanian magmatic suites in the Nanling region are undoubtedly typical post-orogenic rock associations. Post-orogenic suites mark the end of a post-collision or late orogenic event and the initiation of Pangaea break-up, indicating that a new orogenic Wilson cycle is about to start. Therefore it may be considered that the early Yanshanian geodynamic settings in the Nanling region should be related to post-orogenic continental break-up after the Indosinian orogeny and the break-up did not begin in the Cretaceous.  相似文献   

20.
Early Yanshanian magmatic suites predominate absolutely in the Nanling granite belt.They consist mainly of monzogranite and K-feldspar granite.There occur associations of early Yanshanian A-type granitoids(176 Ma-178 Ma) and bimodal volcanic rocks(158 Ma-179 Ma) in southern Jiangxi and southwestern Fujian in the eastern sector of the granite belt and early Yanshanian basalts(177 Ma-178 Ma) in southern Hunan in the central sector of the belt.Both the acid end-member rhyolite in the bimodal volcanic rock association and A-type granitoids in southern Jiangxi have the geochemical characteristics of intraplate granitic rocks and the basic end-member basalt of the association is intraplate tholeiite,while the basaltic rocks in southern Hunan include not only intraplate tholeiite but also intraplate alkali basalt.Therefore the early Yanshanian magmatic suites in the Nanling region are undoubtedly typical post-orogenic rock associations.Post-orogenic suites mark the end of a post-collision or late orogenic event and the initiation of Pangaea break-up,indicating that a new orogenic Wilson cycle is about to start.Therefore it may be considered that the early Yanshanian geodynamic settings in the Nanling region should be related to post-orogenic continental break-up after the Indosinian orogeny and the break-up did not begin in the Cretaceous.  相似文献   

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