Petrology of volcanic products younger than 42 ka on the Lipari–Vulcano complex (Aeolian Islands, Italy): an example of volcanism controlled by tectonics

Over the last 42 ka, volcanic activity at Lipari Island (Aeolian Arc, Italy) produced lava domes, flows and pyroclastic deposits with rhyolitic composition, showing in many cases evidence of magma mixing such as latitic enclaves and banding. In this same period, on nearby Vulcano Island, similar rhyolitic lava domes, pyroclastic products and lava flows, ranging in composition from shoshonite to rhyolite, were erupted. As a whole, the post-42 ka products of Lipari and Vulcano show geochemical variations with time, which are well correlated between the two islands and may correspond to a modification of the primary magmas. The rhyolitic products are similar to each other in their major elements composition, but differ in their trace element abundances (e.g. La ranging from 40 to 78 ppm for SiO₂ close to 75 wt%). Their isotopic composition is variable, too. The ⁸⁷Sr/⁸⁶Sr (0.704723–0.705992) and ¹⁴³Nd/¹⁴⁴Nd (0.512575–0.512526) ranges partially overlap those of the more mafic products (latites), having ⁸⁷Sr/⁸⁶Sr from 0.7044 to 0.7047 and ¹⁴³Nd/¹⁴⁴Nd from 0.512672 to 0.512615. ²⁰⁶Pb/²⁰⁴Pb is 19.390–19.450 in latites and 19.350–19.380 in rhyolites. Crystal fractionation and crustal assimilation processes of andesitic to latitic melts, showing an increasing content in incompatible elements in time, may explain the genesis of the different rhyolitic magmas. The rocks of the local crustal basement assimilated may correspond to lithotypes present in the Calabrian Arc. Mixing and mingling processes between latitic and rhyolitic magmas that are not genetically related occur during most of the eruptions. The alignment of vents related to the volcanic activity of the last 40 ka corresponds to the NNW–SSE Tindari–Letojanni strike-slip fault and to the correlated N–S extensional fault system. The mafic magmas erupted along these different directions display evidence of an evolution at different P_H2O conditions. This suggests that the Tindari–Letojanni fault played a relevant role in the ascent, storage and diversification of magmas during the recent volcanic activity.     

甘肃白银厂矿田流纹岩的成因及其构造意义

白银厂矿田早-中寒武世流纹岩被认为是大陆裂谷环境或洋内弧岩浆作用的产物。地球化学分析结果显示：流纹岩具有Ti，Nb，Ta明显负异常，Sr和P中等负异常，LREE略富集到富集，相对亏损HFSE，La／Nb值为1～2，La/Th值为1.1～2.5,Th/Ta值高（6～12），部分样品表现为MREE负异常的稀土元素配分特征 ，δu=0.5～1.1,(^87Sr/^86Sr)t=0.7026～0.7048,ENd(t)= 5.60～ 6.14，综合其地球化学特征，推测流纹岩形成应是在大陆地壳基础上，由于与俯冲作用有关的玄武岩浆底侵作用导致镁铁质下地壳发生部分熔融所致。     

博格达裂谷闭合和区域隆起的同位素年代学证据及地质意义

博格达造山带位于准噶尔和吐鲁番-哈密两个前寒武纪地块之间，是晚古生代的大陆裂谷。经研究，博格达大石头-色皮口地区的流纹岩Rb-Sr同位素年龄为307.1±1.3 Ma，表明博格达地区火山活动由早石炭世持续到晚石炭世。该年龄可视为博格达裂谷闭合和区域隆什之后造山阶段产物的形成年龄。该区流纹岩其源区物质主要来自于亏损地幔或是由玄武岩浆演化经分离结晶作用形成的流纹岩这一特征是十分罕见的。     

大兴安岭中南段塔尔气地区满克头鄂博组流纹岩年代学和地球化学研究

对出露在大兴安岭中南段塔尔气地区的满克头鄂博组的流纹岩进行了详细的年代学和地球化学研究。LA-ICP-MS锆石U-Pb测年结果表明,流纹岩形成于143～141 Ma,时代为早白垩世早期,而不是以前认为的晚侏罗世。地球化学特征表明该时期流纹岩属高钾钙碱性过铝质岩石,球粒陨石标准化稀土配分曲线呈轻稀土富集的右倾形式,(La/Yb)N=2.55～12.82,具有明显负Eu异常。原始地幔标准化图解显示大离子亲石元素(LILE)Rb、Th、U、K、Pb的富集及高场强元素(HFSE)Nb、Ta、Ti、P的强烈亏损,这是壳源岩浆的特征。流纹岩锆石εHf(t)=+4.6～+10.9,二阶段Hf模式年龄为501～905 Ma,反应其源区物质主要为早古生代—新元古代时期增生的年轻地壳物质。满克头鄂博组流纹岩形成于造山后伸展环境,可能与蒙古—鄂霍茨克洋闭合后的岩石圈伸展作用相关。     

Can crystallization of olivine tholeiite give rise to potassic rhyolites?—an experimental investigation

Experiments were conducted to determine whether the rhyolites and basalts of the intraplate silica-saturated potassic suites could be genetically related through crystallization. Extreme crystallization (96–97%) of a high-MgO (10.62 wt%) olivine tholeiite from the Snake River Plain with an initial bulk water content of 0.4 wt% at a mid-crustal pressure of 4.3 kbar generated potassic rhyolitic liquids similar in major element chemistry to those found in the Quaternary rhyolite domes of the Snake River Plain and their plutonic equivalents in the Proterozoic Laramie Anorthosite Complex. Residual liquids comparable in composition to the bulk rock compositions of intermediate rocks found at the Craters of the Moon and Cedar Butte eruptive centers in the Snake River Plain are also generated along this crystallization path. This paper constitutes part of a special issue dedicated to Bill Bonnichsen on the petrogenesis and volcanology of anorogenic rhyolites.     

Volatile emission during the eruption of Baitoushan Volcano (China/North Korea) ca. 969 AD

3 [magma volume (DRE): 24 ± 5 km³]. The main phase (ca. 95 vol.%) is represented by comenditic tephra deposited dominantly as widespread fallout blankets and proximal ignimbrites. The eruption column is estimated to have reached ca. 25 km and thus entered the stratosphere. A late phase (5 vol.%) is represented by trachyte emplaced chiefly as moderately welded ignimbrites. The comendites contain  ∼ 3, and the trachytes 10–20 vol.% phenocrysts, mainly anorthoclase, hedenbergite, and fayalite. Primary glassy melt inclusions with no signs of leakage were found only in phenocrysts in the comenditic tephra, whereas those in phenocrysts in the trachytes are devitrified. The comendite magma is interpreted to have been generated by fractional crystallization from a trachyte magma represented by melt inclusions in the phenocrysts in the comendite tephra. The mass of volatiles emitted to the atmosphere during the eruption was estimated using the petrologic method. The average H₂O concentration of the comenditic matrix glass is 1.5 wt.% (probably largely secondary) and of the corresponding melt inclusions  ∼ 5.2 wt.%. Melt inclusions in feldspar and quartz present the highest halogen concentrations with a calculated average for chlorine of 4762 ppm and for fluorine of 4294 ppm. The comenditic matrix glasses are represented by a fluorine-rich (3992 ppm F) and fluorine-poor group (2431 ppm F), averaging 3853 ppm for chlorine. Only 20% of all sulfur analyses of the comenditic matrix glasses and melt inclusions are above the detection limit of  ≥ 250 ppm S. The difference between pre- and post-eruptive concentration of H₂O is at least 3.7 ± 0.6 wt.% H₂O taking into consideration re-hydration of the matrix glass and possible leakage of melt inclusions. The difference between pre- and post-eruptive concentrations of the halogens amounts to 909 ± 90 ppm Cl, and 1863 ± 280 ppm and 302 ± 40 ppm F. The difference for S was estimated based on the average of the maximum S concentrations in the melt inclusions (455 ppm S) and the detection limit, resulting in 205 ± 40 ppm S. The calculated mass of volatiles injected into the atmosphere, based on the erupted magma volume and volatile data, is 1796 ± 453 megatons for H₂O, 45 ± 10 megatons for chlorine, 42 ± 11 megatons for fluorine, and 2 ± 0.6 megatons for sulfur. The 969 ± 20 AD eruption of Baitoushan Volcano, one of the largest eruptions of the past 2000 years, is thought to have had a substantial but possibly short-lived effect on climate. Received: 25 July 1998 / Accepted: 8 September 1999     

Precambrian Acid Volcanism in Central India - Geochemistry and Origin

The Early Proterozoic Bijli rhyolites (2180 ± 25 Ma) constitute an important part of the Nandgaon group in the Central Indian craton, a tectonically active block sandwiched between the Dharwar craton in the south and the Bundelkhand craton in the north. The rhyolites are chemically heterogeneous. They have high SiO₂ (74.4%), K₂O (4.41%), Ba (834 ppm), and low MgO (0.22%), CaO (1.0%), iron (2.8%) V, Cr and Ni (5, 6 and 1 ppm respectively). They contain moderate Sr (88 ppm), Rb (140 ppm), Th (27 ppm) and U (6 ppm). Total REE ranges from 223 ppm to 453 ppm with high LREE (ΣLREE = 307) and all the rhyolites have low to high negative europium anomalies (Eu/Eu* 0.03 to 0.77). The overall chemical signatures support a crustal anatectic origin for the rhyolites from a source of intermediate composition. Basalt pooling below the crust and associated rifting most likely initiated anatexis.     

新疆阿尔泰南缘康布铁堡组钾-钠质流纹岩锆石U-Pb年龄和地球化学

阿尔泰南缘分布着大量的晚古生代康布铁堡组火山岩系,是许多铁矿、铜矿以及铅锌矿的赋矿围岩。阿尔泰南缘麦兹和克朗火山-沉积盆地内的钾-钠质流纹岩的年龄分别为396.7±1.4Ma和394.0±6.0Ma,结合近期研究成果,进一步表明阿尔泰南缘火山岩主要形成于晚古生代早期,锆石U-Pb年龄峰期在400Ma左右。钾-钠质流纹岩具有高硅(SiO₂的含量范围为73%~82%)、高碱(总碱含量介于4%~7%)和过铝质(高A/CNK值>1)的特征,并见有白云母和黑云母的矿物组合,属于高硅高碱过铝质的钙碱性火山岩。此外,它们的Sr和Nd同位素分别为⁸⁷Sr/⁸⁶Sr=0.7074~0.7144,¹⁴³Nd/¹⁴⁴Nd=0.512072~0.512252,具有上地壳来源的特征,说明其岩石成因与初生地壳的部分熔融作用有着密切关系。结合区域地质背景分析,它们都产在与俯冲消减作用有关的陆缘岛弧的地质环境中。因此,我们推断本区钾-钠质流纹岩的原始岩浆为高硅高碱的花岗质岩浆,是由进入陆壳的高侵位玄武岩浆的底侵作用导致其上部地壳近固相线的低程度部分熔融的产物。     

内蒙古扎鲁特盆地坤都地区满克头鄂博组流纹岩锆石U-Pb年龄及岩石成因

为研究内蒙古扎鲁特盆地坤都地区晚侏罗世火山岩的构造属性,对研究区内流纹岩开展了锆石U-Pb定年和原位Lu-Hf同位素研究。锆石U-Pb定年结果显示,流纹岩形成于151.2±1.2Ma,属于晚侏罗世。岩石地球化学研究表明,火山岩具高硅富碱、贫镁钙的特征,稀土元素总量为110.38×10~(-6)~138.88×10~(-6),轻、重稀土元素分馏中等,弱负Eu异常(δEu=0.72~0.98),微量元素特征为富集Cs、Rb、Ba和轻稀土元素,强烈亏损Sr、P、Ti,相对亏损Nb、Ta,ε_(Hf)(t)值为-10.1~4.9,对应的地壳模式年龄(T~C_(DM))为1192~3639Ma,反映其岩浆来源于元古宙、太古宙地壳物质的部分熔融,之后经历矿物分离结晶作用。通过对比大兴安岭地区同时代岩浆-构造活动,研究区满克头鄂博组流纹岩形成于后造山伸展背景,与蒙古-鄂霍茨克造山后伸展作用有关。     

川西义敦岛弧中部图姆沟组流纹岩年代学、地球化学特征及其对岩浆成因的限定

【研究目的】 义敦岛弧是三江特提斯复合造山带的重要组成部分。区域现有的研究较多集中在弧花岗质侵入岩及其成矿作用上,而对区内流纹岩的研究则相对匮乏。【研究方法】 本文在详细野外填图的基础上,对图姆沟组流纹岩进行岩石学、地球化学和同位素年代学研究。【研究结果】 流纹岩锆石U-Pb成岩年龄216.5 Ma,为晚三叠世。岩石SiO₂含量为73.24%~74.72%,全碱含量为5.26%~6.27%,为钙碱性系列,富集大离子亲石元素Rb、Th、U、K和轻稀土元素,亏损高场强元素Nb、Ta、Ti、P和重稀土元素,具典型的岛弧火山岩特征。【结论】 图姆沟组流纹岩是印支期甘孜—理塘洋壳向西俯冲环境下地壳部分熔融的产物。