Extreme Nd isotope homogeneity in a large rhyolitic province: the Estérel massif, southeast France     

Franck Poitrasson  Christian Pin 《Bulletin of Volcanology》1998,60(3):213-223

 Previous detailed studies of large rhyolite bodies propose that their elemental and isotopic characteristics were largely acquired in shallow crustal magma chambers. This model explains the common chemical and isotopic zonations of large volumes of rhyolites as well as the less common chemical and isotopic homogeneity of such bodies. We report an intermediate situation (the Estérel massif, southeast France) in which chemical variations contrast with Nd-isotope homogeneity. We thus infer that, in this case, large volumes of rhyolite resided for enough time in shallow magma chambers to develop chemical zonations through differentiation, but this process was not accompanied by crustal assimilation. The subordinate amount of mafic rocks cropping out in the Estérel probably evolved from basalt to trachyte through assimilation and fractional crystallization. The relatively radiogenic Nd-isotope signatures of the rhyolite compared with the Hercynian crust show that it cannot have been generated by partial melting of exposed basement rocks. Several geological similarities with large rhyolitic provinces could suggest that the rhyolite was purely mantle derived or, alternatively, generated by partial melting of an ad hoc crustal component. However, mineralogical, geochemical, and geodynamic connections between the Estérel rhyolite and the hypersolvus anorogenic granites of Corsica, as well as the extreme Nd-isotope homogeneity of the rhyolite, lead us to propose that the rhyolite was generated by mixing between mantle-derived magmas and a mafic lower crust. This scenario accounts for the relatively radiogenic Nd-isotope signatures of the rhyolite compared with the Hercynian crust. The good Nd-isotope homogeneity observed in the rhyolite implies that the mixing process, which occurred in the deep crust, was complete and provided a shallow magma chamber with isotopically and probably chemically homogeneous magmas. Received: 5 December 1997 / Accepted: 16 June 1998  相似文献   

大兴安岭北段三根河林场新民组流纹岩锆石U－Pb定年及地球化学特征     

李中会  李阳  李睿杰  李凯 《地质与资源》1992,29(4):310-318

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粤北长塘盆地晚白垩世早期流纹岩的成因:地球化学和Sr-Nd-Pb-Hf-O同位素制约     

劳玉军  巫建华  徐勋胜 《岩矿测试》2016,35(5):558-567

长塘流纹岩为"大量流纹岩-少量玄武岩"构成的不对称双峰式火山岩组合的酸性端元,前人对年代学研究认为是目前华南识别的燕山期最年轻的流纹岩(SHRIMP年龄96.8 Ma),但在岩石地球化学特征及成因方面尚未深入研究。本文通过全岩主微量元素、Sr-Nd-Pb-O和锆石Hf同位素研究得到:流纹岩为亚碱性弱过铝质岩石,稀土元素富集,轻重稀土分馏明显,Eu负异常;具有较小的(~(87)Sr/~(86)Sr)_i、较高的εNd(t)和较低的(~(206)Pb/~(204)Pb)_i、(~(207)Pb/~(204)Pb)_i、(~(208)Pb/~(204)Pb)_i值;锆石的εHf(t)值较高且Hf模式年龄和全岩基本一致,均显示为新生下地壳岩浆;全岩的δ18O值较高。结合区域地质资料,本文认为长塘流纹岩是在晚白垩世早期拉张环境下,源于EMⅡ富集地幔的岩浆混染少量下地壳物质形成新元古代新生下地壳,发生熔融产生的岩浆在上升过程中经历结晶分异演化后喷出地表的产物。晚白垩世早期流纹岩成因对华南陆壳拉张减薄到正常厚度时间节点提供了证据。  相似文献   

The Cougar Point Tuff: Implications for Thermochemical Zonation and Longevity of High-Temperature, Large-Volume Silicic Magmas of the Miocene Yellowstone Hotspot   总被引：6，自引：0，他引：6  

CATHEY  HENRIETTA E.; NASH  BARBARA P. 《Journal of Petrology》2004,45(1):27-58

The 12·7–10·5 Ma Cougar Point Tuff in southernIdaho, USA, consists of 10 large-volume (>10²–10³ km³each), high-temperature (800–1000°C), rhyolitic ash-flowtuffs erupted from the Bruneau–Jarbidge volcanic centerof the Yellowstone hotspot. These tuffs provide evidence forcompositional and thermal zonation in pre-eruptive rhyolitemagma, and suggest the presence of a long-lived reservoir thatwas tapped by numerous large explosive eruptions. Pyroxene compositionsexhibit discrete compositional modes with respect to Fe andMg that define a linear spectrum punctuated by conspicuous gaps.Airfall glass compositions also cluster into modes, and thepresence of multiple modes indicates tapping of different magmavolumes during early phases of eruption. Equilibrium assemblagesof pigeonite and augite are used to reconstruct compositionaland thermal gradients in the pre-eruptive reservoir. The recurrenceof identical compositional modes and of mineral pairs equilibratedat high temperatures in successive eruptive units is consistentwith the persistence of their respective liquids in the magmareservoir. Recurrence intervals of identical modes range from0·3 to 0·9 Myr and suggest possible magma residencetimes of similar duration. Eruption ages, magma temperatures,Nd isotopes, and pyroxene and glass compositions are consistentwith a long-lived, dynamically evolving magma reservoir thatwas chemically and thermally zoned and composed of multiplediscrete magma volumes. KEY WORDS: ash-flow tuff; Bruneau–Jarbidge; rhyolite; Yellowstone hotspot; residence time  相似文献   

澜沧江南带火山岩系侏罗纪火山岩的存在及其地质意义     

唐波浪  刘英超  马旺  岳龙龙  黄世强  庄亮亮  吴志旖  周梦林 《地球学报》2020,41(3):357-370

澜沧江南带火山岩系是古特提斯构造演化的产物,是反演古特提斯演化的利器。本文选择带内南段普洱地区那澜村剖面火山岩进行了系统的年代学和地球化学研究,为古特提斯洋演化过程提供了新的认识。那澜村火山岩为一套发育斑状结构、流纹构造的流纹岩,其斑晶由半自形板柱状钾长石和石英组成,基质为隐晶质长石、石英。主量元素中SiO2含量介于75.59%~77.40%之间,Al2O3含量介于11.69%~12.20%之间,K2O含量(4.13%)大于Na2O(3.72%),属过铝质高钾钙碱性-钙碱性系列火山岩。主量元素CaO/Na2O值为0.01~0.04,Al2O3/(MgO+TFeO)为0.03~0.06,CaO/(MgO+TFeO)为5.51~6.66,微量元素Nb/Ta比值集中在12.27~13.08,放射性同位素组87Sr/86Sr比值为0.714~0.719,207Pb/204Pb为15.733~15.760,206Pb/204Pb为18.903~19.400,表明火山岩源区为重熔的含(变质)泥岩地壳。微量元素标准化配分曲线呈不规则锯齿状且具高度一致性,Rb、Nb、Ta、Y、Yb等数据具富Rb低Nb、Ta、Y、Yb特征,表明澜沧江南带火山岩具有陆缘弧火山岩属性。LA-ICP-MS测定锆石U-Pb年代学表明那澜村火山岩谐和年龄为(200.6±2.8)Ma,和澜沧江南带北段火山岩活动(210–280 Ma)具有一致性,说明澜沧江南带火山活动至少延伸至早侏罗世,也即古特提斯洋的俯冲活动至少持续到早侏罗世。  相似文献   

松辽盆地白垩系营城组流纹岩中石泡构造类型、特征及成因   总被引：1，自引：0，他引：1  

王加强  刘万洙  杨双玲  王璞珺  许中杰 《吉林大学学报(地球科学版)》2007,37(6):1266-1271

通过大比例尺平面地质填图、剖面测量和显微组构分析,九台营城煤矿区流纹岩中石泡构造发育。根据成因、产状、宏观形态和显微特征,石泡构造可分为3类:膨润土中的独立石泡、层状流纹岩表面贴附的石泡和块状流纹岩内部的石泡。独立石泡大小不等,直径由几厘米至几十厘米,表面似葡萄状,平面上呈环带状分布于珍珠岩四周蚀变膨润土中,为富含挥发物质的岩浆遇水淬碎成球体,经结晶分异作用形成。层状流纹岩表面贴附的石泡,直径由几毫米至几厘米,表面形态与独立石泡相似,平面上呈环带状分布于独立石泡带外侧,形成机理与独立石泡相似,由于遇水量减少和岩浆喷发强度增大形成差别。块状流纹岩内部的石泡,直径由几毫米至几厘米,内部具多圈层结构,为含气液混合物的岩浆,气体逸出,液体中的矿物成分随温度下降,由外至内依次结晶形成。3类石泡赋存于喷溢相上部亚相,形成机理为等容降温、冷凝收缩。  相似文献   

冀西北宣化晚三叠世中期流纹斑岩的发现及其地质意义     

张树胜  谢明忠  刘永平  任建帅  周建飞  郑满城  郝星波 《地质论评》2021,67(5):67060017-67060017

冀西北宣化东部地区ZY1井揭露在中元古界下马岭组页岩中,夹有4层流纹斑岩（浅成侵入）,本文测得其LA-ICPMS锆石U-Pb年龄为226.5±1.9 Ma,表明其侵入阶段为晚三叠世中期。本区流纹斑岩不含碱性铁镁矿物,应不属于碱性岩。本区发现的晚三叠世中期流纹斑岩,丰富了华北克拉通北缘中晚三叠世构造体制由碰撞后/造山后伸展向板内伸展转变过程中较少的非碱性岩类及其测年数据,对华北地块印支期活动历史的分析研究具有重要理论意义,对寻找矿产资源具有现实意义。  相似文献   

Petrogenesis of the Swaziland and Northern Natal Rhyolites of the Lebombo Rifted Volcanic Margin, South East Africa   总被引：2，自引：0，他引：2  

Miller  Jodie A.; Harris  Chris 《Journal of Petrology》2007,48(1):185-218

The Jozini and Mbuluzi rhyolites and Oribi Beds of the southernLebombo Monocline, southeastern Africa, have geochemical characteristicsthat indicate they were derived by partial melting of a mixtureof high-Ti/Zr and low-Ti/Zr Sabie River Basalt Formation types.Compositional variations within the different rhyolite typescan largely be explained by subsequent fractional crystallization.The Sr- and Nd-isotope composition of the rhyolites is uniqueamongst Gondwana silicic large igneous provinces, having _Ndvalues close to Bulk Earth (–0·94 to 0·35)and low, but more variable, initial ⁸⁷Sr/⁸⁶Sr ratios (0·7034–0·7080).Quartz phenocryst ¹⁸O values indicate that the rhyolite magmashad ¹⁸O values between 5·3 and 6·7, consistentwith derivation from a basaltic protolith with ¹⁸O values between4·8 and 6·2. The low-¹⁸O rhyolites (< 6·0)come from the same stratigraphic horizon and are overlain andunderlain by rhyolites with more ‘normal’ ¹⁸O magmavalues. These low-¹⁸O rhyolites cannot have been produced byfractional crystallization or partial melting of mantle-derivedbasaltic material. The rhyolites have low water contents, makingit unlikely that the low ¹⁸O values are the result of post-emplacementalteration. Modification of the source by fluid–rock interactionat elevated temperatures is the most plausible mechanism forlowering the ¹⁸O magma value. It is proposed that the low-¹⁸Orhyolites were derived by melting of earlier altered rhyolitein calderas situated to the east, which were not preserved afterGondwana break-up. KEY WORDS: rhyolite; Lebombo; stable and radiogenic isotopes; low-¹⁸O magmas; partial melting  相似文献   

西准噶尔萨吾尔山一带吉木乃组流纹岩LA-ICP-MS锆石U-Pb年代学及地质意义     

支倩  李永军  杨高学  徐倩  宁文涛  罗鑫 《新疆地质》2019,(2):188-193

吉木乃组是西准萨吾尔山地区地质时代久存争议的地层,也是火山岩占地层总厚度比例最高的火山-沉积地层。采用同位素年龄标定火山岩准确的地质时代,是解决本组时代分歧的有效手段。吉木乃组顶部新发现的流纹岩中获得(304.1±2.5)Ma(n=15,MSWD=1.07)的LA-ICP-MS锆石U-Pb年龄,确认成岩时代为晚石炭世晚期(大致相当于卡西莫夫期)。结合前人在本组下部层位采集的植物化石,限定吉木乃组时代为晚石炭世(大致为巴什基尔期卡西莫夫期)。流纹岩同位素定年准确约束了吉木乃组地质时代上限,为区域地层格架的建立和地层对比提供了可靠的时代依据。  相似文献   

滇西南南澜沧江构造岩浆岩带早侏罗世火山岩的发现——兼论云南省境内印支构造旋回的上限   总被引：1，自引：0，他引：1       下载免费PDF全文

吕留彦  李静  曾文涛  俞赛赢  孙载波  王晓峰 《中国地质》2019,46(6):1270-1283

采用锆石激光剥蚀法(LA-ICP-MS)对滇西南南澜沧江火山岩带普洱市江桥—那澜一带的流纹岩进行了年代测定,获得了(196.7±2.3)Ma、(198.1±3.5)Ma的同位素年龄;表明该地区的流纹岩形成于早侏罗世,而非前人划分的晚三叠世,填补了该地区早侏罗世的火山-沉积记录。从岩石的主元素、稀土元素及微量元素资料分析,南澜沧江构造岩浆岩带早侏罗世的火山岩形成于碰撞后的伸展构造背景,可能与西侧昌宁—孟连古特提斯洋盆关闭后,俯冲岩石圈板片或造山带山根的拆沉作用相关,属后碰撞—后造山阶段岩浆活动的产物。具有典型后造山花岗岩或"A"型花岗岩的特征,其指示了印支构造旋回在滇西南地区最终结束的时间上限。  相似文献