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1.
I. M. Derbeko 《Geochemistry International》2009,47(11):1083-1099
Field observations together with petrochemical and geochemical data were used to develop the scheme and sequence of the formation of the Selitkan Zone, the largest volcanoplutonic zone of the Mongol-Okhotsk orogenic belt. Geochemically, this zone is subdivided into essentially andesite and rhyolite sequences associated with plutonic counterparts. All the rocks show negative Nb and Ta anomalies, enrichment in LREE and large ion lithophile elements (Ba, Rb, K, Th, U). They were presumably formed in a subduction setting in three stages: the amagmatic, magmatic andesitic, and magmatic rhyolitic. The entire process of the formation of the Selitkan volcanoplutonic zone lasted for 5 Ma. 相似文献
2.
江西九龙嶂和相山火山-侵入杂岩岩石地球化学的对比研究 总被引：4，自引：2，他引：2
九龙嶂和相山火山-侵入杂岩是中国华南地区与铀矿有关的中生代流纹英安质火山-侵入杂岩。相山火山-侵入杂岩是华南地区最大的火山岩型铀矿赋矿围岩,其主体岩石为流纹质碎斑熔岩,浅成-超浅成侵入岩有英安斑岩、花岗斑岩等;九龙嶂火山-侵入杂岩的铀资源潜力尚待评价,主体岩石为流纹质熔结凝灰岩,浅成-超浅成侵入岩有英安斑岩、石英二长斑岩等。两地区的火山-侵入杂岩的岩石化学成分及其变化特点极为相近,微量元素蛛网图和稀土配分型式也基本一致,它们具有相似的岩浆过程和统一的活动陆缘构造背景;同时,从早期火山岩浆活动到晚期浅成-超浅成岩浆侵入活动,其w(SiO2)和w(Rb)/w(Sr)的值降低,而w(MgO)以及w(K)/w(Rb)、w(Ba)/w(Rb)和w(LREE)/w(HREE)的值增高,晚期浅成-超浅成侵入岩与早期火山岩没有直接的分异演化关系。 相似文献
3.
Analytical data are presented for the following elements: Cs, Rb, Ba, K, Sr, Ca, Na, Fe, Mg, Cu, Co, Ni, Li, Sc, V, Cr, Ga, Al, Si, La, Y, and Zr. Eight samples were analysed by the spark source method for rare earths, Tl, Pb, Hf, Sn, Nb, Mo, Bi, and In. In addition to data on rhyolitic volcanics, a small number of intermediate volcanics and eugeosynclinal sediments were analysed for comparative purposes. The following features are shown by the trace element data:
- The rhyolitic rocks have consistently lower concentrations of most trace and minor elements when compared with recent estimates of average concentrations in granites. None of the criteria for strong fractionation (e.g. low K/Rb, Ba/Rb and K/Cs ratios) are present.
- The data do not indicate any systematic differences between the rhyolitic lavas and ignimbrites, although the very young rhyolitic pumices are consistently more “basic” in their element concentrations compared to the other rhyolitic analyses.
- The residual glasses (and devitrified matrices) are depleted, relative to the total rock compositions, in Fe, Mg, Ca, Sr, V, Sc, and Al, and enriched in Cs, Rb, K, Ba, and Si. Zr is depleted in the residual glasses separated from rhyolites, but not in the andesitic residual matrices.
- The rare earth fractionation patterns of the rhyolitic and andesitic extrusives are very similar, being intermediate between chondritic and sedimentary patterns i.e., there is no evidence of strong fractionation. The rhyolitic patterns also indicate a slight Eu depletion.
- Comparable trace and minor element behaviour (with the possible exception of Zr) seems to exist through the rhyolite-andesite compositional range. This is supported by the whole rock-residual liquid trends for the various elements studied, which broadly coincide with the observed whole rock trends, both through the rhyolitic-andesitic compositonal range, and within the rhyolitic compositional range.
4.
为对内蒙古敖汉地区酸性火山岩进行划分对比,对其进行了锆石U-Pb测年、地球化学分析.流纹岩和流纹质晶屑凝灰岩年龄分别为122.8±2.3 Ma、122.1±1.9 Ma,属于早白垩世义县组.晚侏罗世流纹岩SiO2和全碱含量较高,Al2O3含量较低;稀土总量较低、具有负铕异常;富集Rb、Th、U、K,亏损Ba、Sr、P、Ti.早白垩世流纹岩SiO2、全碱、Al2O3含量较高;稀土总量较高、稀土分馏明显、弱负铕异常;富集K、Rb、Ba、Th、U,亏损Nb、Ta、Sr、P、Ti.将敖汉地区晚侏罗世酸性火山岩进行了解体,首次将其大部分划为早白垩世义县组.晚侏罗世流纹岩可能为高分异流纹岩,形成于相对低的压力环境下,可能起源于中上地壳物质部分熔融.早白垩世流纹岩显示出埃达克质岩石的地球化学特征,可能形成于加厚的下地壳环境. 相似文献
5.
大兴安岭中南段塔尔气地区满克头鄂博组火山岩地球化学特征及地质意义 总被引：1，自引：0，他引：1
塔尔气地区满克头鄂博组火山岩主要为流纹岩、流纹质凝灰熔岩和流纹质晶屑凝灰岩,主量元素分析结果表明该火山岩具有准铝-弱过铝质（Al2O3=11.55%~14.76%,A/CNK=1.01~1.35）、富碱（Na2O+K2O=4.56%~8.79%）、贫钙镁的岩石化学特点.稀土元素分析结果表明火山岩ΣREE含量较高（平均为168.69×10-6）,稀土元素配分模式图均呈右倾型,LREE为轻度富集型,HREE分布相对较为平坦,铕负异常明显（δEu=0.29~0.72）,岩石Rb/Sr=0.3~11.4,微量元素明显富集LILE （Rb、Ba、K）而亏损HFSE （Nb、Ta、Ti）.结合地球化学特征及前人研究资料,认为塔尔气地区满克头鄂博组火山岩具有壳源岩浆的典型特征,岩浆源区有斜长石残留或岩浆经历了斜长石的分离结晶作用;满克头鄂博组火山岩属于A型火山岩,可能是蒙古-鄂霍次克洋闭合后的岩石圈伸展体制下的产物. 相似文献
6.
内蒙古西乌旗色日崩地区早白垩世白音高老组流纹岩为高硅（SiO2=78.32%~86.80%）、富碱（K2O+Na2O=3.84%~7.63%）、高铝（Al2O3=8.59%~13.70%）,低MgO、CaO、TiO2、MnO,稀土元素与微量元素地球化学特征显示其类似于A型花岗岩特征;低Ti/Y、Nb/Y与Ti/Zr比值,高Rb/Nb与Rb/Y比值以及Sr、Eu强烈亏损,表明白音高老组流纹岩为上地壳物质部分熔融成因,形成于早白垩世非造山板内伸展环境. 相似文献
7.
渣尔泰山地区白垩纪酸性火山岩地质特征及成矿作用 总被引：2，自引：2，他引：0
渣尔泰山地区位于内蒙古中西部,大地构造位置处于华北地台北缘狼山-渣尔泰山裂陷槽的东段,根据近几年的找矿实践与研究,在白垩纪的酸性火山岩中已发现银金矿床、金银矿床,主要赋存于下白垩统白女羊盘组酸性火山岩中。该套火山岩具富硅富钾、相对贫钙镁的特点,属于铝过饱和的碱性系列;在球粒陨石标准化图解上岩石化学样品均显示为轻稀土(LREE)相对富集、重稀土(HREE)相对亏损的右倾型配分模式,轻重稀土分异较明显,显示较明显的负铕异常;微量元素显示明显的Rb、Pb正异常和Ba、Sr负异常,不相容元素比值Zr/Nb=2.47~4.87,Zr/Y=18.3~103。属大陆碰撞造山期后作用的产物,来源于地壳部分熔融。其岩石类型主要有:流纹质凝灰角砾岩、流纹质角砾凝灰岩、球粒状流纹岩、珍珠岩、显微嵌晶状流纹岩夹流纹斑岩等。银多金属矿的含矿层主要由复碎裂复硅化流纹质凝灰角砾岩、硅化复碎裂流纹岩、多期次硅化黄铁矿化角砾岩化流纹斑岩等组成。成矿元素Ag、Au、Sb、W显著富集于流纹质凝灰角砾岩、多期次硅化碎裂的流纹岩、流纹斑岩中,其平均含量高于中国流纹岩元素丰度10倍以上,说明该套火山岩提供了成矿物质来源;且元素含量的分异程度较大,指示明显受到后期成矿作用的叠加;有一定的找矿潜力。 相似文献
8.
位于长江中下游的宣城水东地区发育一套酸性火山岩,主要由流纹质角砾岩、流纹岩和珍珠岩组成。本文对该套火山岩进行了详细的锆石U-Pb年代学、主量元素、微量元素以及Nd-Hf同位素研究。LA-ICP MS锆石U-Pb定年结果显示3种岩性的火山岩年龄分别为133.2±0.8、133.4±0.8和131.5±0.9 Ma。主量元素组成上,这套酸性火山岩具高硅(72.51%~81.79%)、富钾(K_2O/Na_2O=2.04~14.93,平均6.72)、贫钙镁(Ca O=0.19%~1.57%,Mg O=0.06%~0.29%)的特征,属于弱过铝质(A/CNK=1.02~1.24)的高钾钙碱性-钾玄岩系列岩石。微量元素方面,轻重稀土元素分馏明显[(La/Yb)_N=5.43~9.17],具明显的负铕异常(Eu/Eu~*=0.44~0.60),富集大离子亲石元素(LILE)Rb、Th、K和Pb等,亏损Ba、Sr、Nb、P和Ti等元素,表现出壳源的特征。全岩Sr-Nd和锆石Hf同位素组成变化范围相对较小,(~(87)Sr/~(86)Sr)_i为0.707 3~0.708 8,εNd(t)值为-7.05~-5.56,εHf(t)为-8.6~-1.3。结合区域地质研究成果,认为宣城水东地区酸性火山岩可能是在约135 Ma古太平洋板块俯冲作用之后的伸展-拉伸环境下,由新元古代早期新生地壳重熔而成。 相似文献
9.
The Island Park-Mount Jackson series in the Yellowstone volcanic field, Wyoming (USA), is a suite of rhyolitic domes and lavas that erupted between the caldera-forming eruptions of the Mesa Fall Tuff (1.3 Ma) and the Lava Creek Tuff (0.6 Ma). Combined zircon U/Pb geochronology, Raman spectroscopy, oxygen isotopic and trace elemental compositions document storage conditions of these magmas between consecutive supereruptions. Based on comparison with co-erupted melt compositions and textural criteria, four zircon compositional groups are identified that record different stages along a continuous magmatic evolution from trace element-poor rhyolite at high temperatures to extremely fractionated rhyolite where zircon trace elements are highly enriched (e.g., >?1000 ppm U). These latter zircon domains are dark in cathodoluminescence images and show broadened Raman peaks relative to near-endmember zircon, indicating that substitution of non-stoichiometric trace elements into zircon leads to distortion of the crystal lattice. Some of these zircon domains contain inclusions of U-Th-REE-phases, likely originating from coupled dissolution–reprecipitation of metastable trace element-rich zircon in the presence of a fluid phase. Rhyolite-MELTS simulations indicate that at the conditions required to produce the observed enrichment in trace elements, a fluid phase is likely present. These findings illustrate that zircons can be assembled from a variety of co-existing magmatic environments in the same magma reservoir, including near-solidus volatile-rich melts close to the magmatic–hydrothermal transition. 相似文献
10.
黑龙江先锋北山金矿床火山岩地球化学特征、LA-ICP-MS锆石U-Pb定年及地质意义 总被引：2，自引：0，他引：2
黑龙江省先锋北山金矿床地处完达山造山带西南段,矿体主要赋存在流纹岩、岩屑晶屑凝灰岩和流纹质角砾岩中。文章系统研究了与成矿密切相关的火山岩的地球化学特征,同时,对其进行了LA-ICP-MS锆石U-Pb测年。与成矿相关的火山岩(流纹岩、岩屑晶屑凝灰岩、流纹质角砾岩)表现出高硅、富碱,贫铝、镁、钙的特征,属碱性系列岩石;具有弱的Eu负异常;富集大离子亲石元素K、Rb、Th等;亏损高场强元素Ta、Nb、P、Ti等。同时,Ba、Sr、Eu等元素亦表现为亏损,具有典型的同碰撞环境的地球化学特征。LA-ICP-MS锆石U-Pb测年显示,流纹岩、流纹质角砾岩和石英闪长玢岩的206Pb/238U谐和年龄分别为(116.98±0.47)Ma、(116.98±0.42)Ma和(108.48±0.99)Ma。结合矿区侵入接触关系,限定先锋北山金矿床的形成时间介于(116.98±0.47)Ma~(108.48±0.99)Ma,即早白垩世晚期。该矿床的形成与太平洋板块的俯冲和佳木斯与完达山地块的拼合有着密切的关系,系古太平洋板块构造体制由挤压变为伸展背景下的产物。 相似文献
11.
本文旨在通过分析苏门答腊岛苏里安斑岩型铜矿至苏利特河矽卡岩型铜矿成矿带中埃达克质侵入岩及矽卡岩的岩石学和地球化学特征,探讨其成因意义。该成矿带是巴东地区辛卡拉克湖以东地球化学异常带的主体部分,其绝大多数侵入岩浆体皆为为Ⅰ-型花岗岩复合体,相当于印支晚期同碰撞火山弧和燕山早期碰撞后岩浆侵入活动的产物。该成矿带内的埃达克质花岗岩和闪长斑岩(早侏罗世和早白垩世)具有低的Y、Yb含量、高的ΣREE含量和Eu正异常,在构造环境判别图上落在活动大陆边缘(ACM)火山弧环境中。埃达克质岩浆来源于俯冲洋壳板片部分熔融。然而,矽卡岩主量元素与岩浆岩侵入体相比,其CaO含量增多,SiO2、Al2O3、K2O以及Na2O含量降,REE普遍表现为Eu负异常。该成矿带内矿化矽卡岩与侵入岩的微量元素模式在蛛网图上显示某些相似性,但是二者的REE配分模式却不尽相同。矽卡岩的微量元素和稀土元素具有低Rb、Ba含量和K、Sr、P、Ti低谷,主要继承了原始岩浆和母岩埃达克质岩的共同特征。矽卡岩化是引起Ti、Yb、Y和REE总量降低的主导控制作用。 相似文献
12.
浙江新昌复合式火成杂岩的岩石学、地球化学及其地质意义 总被引：3，自引：0，他引：3
浙江新昌复合式火成杂岩包括复合火山岩流和复合侵入杂岩。复合火山岩流由玄武岩和流纹岩/流纹质熔结凝灰岩组成,有的含少量安山质-英安质岩流和岩石包体;复合侵入杂岩由辉绿岩和花岗岩复合而成,含闪长质-石英闪长质岩石包体。它们在空间上紧密伴生,同位素年龄为96~113 Ma;地球化学上,它们统属钙碱性岩系,具有轻稀土元素和大离子亲石元素富集、高场强元素亏损等特点,稀土元素、微量元素配分型式基本相同,Sr=0.7069~0.7079,εNd (t ) = -2.3~-5.3,指示它们的微量元素和同位素发生过充分的交换。这些特点与浙闽沿海其他复合岩流和复合侵入杂岩的特点基本一致。它们形成在伸展构造背景,由起源于受消减作用影响的岩石圈地幔部分熔融而产生的玄武岩岩浆底侵,并与深熔的壳源花岗岩浆发生不同程度的岩浆混合而形成。新昌复合式火成杂岩的研究,为更深入研究中国东南部沿海地区晚中生代地球动力学环境变化和构造-岩浆作用提供了一个典型实例。 相似文献
13.
The generalized stratigraphic sequence (20–21.8 m.y.)of the northern flank of the Tweed Volcano is: Beechmont Basalt(base)—Rhyolite (composed of two distinct units, the Springbrookand Binna Burra rhyolites)—Hobwee Basalt. In addition,comendite occurs as a postrhyolite intrusive phase. Chemicallyand mineralogically, the ‘basalts’ are tholeiiticandesites, which are conveniently divided into olivine-normativeand quartz-normative types. Phenocryst mineralogy is olivineand labradorite (microphenocrystic) in the olivine-normativelavas, and plagioclase plus rare augite in the quartz-normativelavas. Rhyolites (which constitute some 7 vol. per cent of the TweedShield volume) are of the potassic two-feldspar type; theseare characterized by highly fractionated trace element patterns,which are most extreme in the Binna Burra rhyolites. The latter,for example, have low K/Rb (<100) and La/Yb, highly depletedEu, Ba, Sr, V, Ni, Cr, and variably enriched Rb, U, Th, Pb,Nb, and Zn. Phenocryst phases are: quartz, oligoclase, sanidine,ilmenite, ferrohypersthene (Springbrook rhyolite), and quartz,sanidine, oligoclase, ilmenite, rare Fe-rich fluor-biotite,and very rare resorbed grains of extremely ulvöspinel-richtitanomagnetite (Binna Burra rhyolite). Phenocryst equilibrationtemperatures are estimated to be in the range 900–1050°C for the Springbrook rhyolite and 800–950 °Cfor the Binna Burra rhyolite, at oxygen fugacities in proximityto the WM buffer. The comendites are characterised by sanidine,quartz, fluor-arfvedsonite, minor acmite, and ilmenite. Pb isotopic compositions indicate at least two distinct groupsof mafic lavas; certain olivine-normative tholeiitic andesiteswith compositions less radiogenic than modern oceanridge basalts(possibly indicative of lower crustal contamination), and asecond more radiogenic group including the remaining isotopicallyanalysed tholeiitic andesites. Sr isotopes reveal small differencesbetween the Beechmont and Hobwee Groups. Pb and Sr isotopiccompositions of the three rhyolitic groups are distinct, andall more radiogenic than the mafic lavas. It is concluded that the tholeiitic andesites represent thefractionation products from an olivine-normative tholeiiticbasalt, and calculations suggest that olivine-plagioclase-aluminousclinopyroxene-Fe-Ti oxides were the likely fractionating mineralphases. The potassic rhyolites are interpreted also in termsof fractionation from a basaltic parent, although the geochemistryof the Binna Burra rhyolite has been further modified by continuedfractionation at the quartz-feldspar minimum. Chemical and mineralogicaldata, however, suggest some modification of the rhyolitic magmasby crustal equilibration (possibly lower crust). Although thecomendite is isotopically distinct from the exposed rhyolites,various least squares mixing calculations suggest that the comenditemay have developed by continued late stage quartz-feldspar ternaryminimum fractionation of rhyolitic magma. 相似文献
14.
大兴安岭地区晚侏罗世—早白垩世流纹岩类广泛分布.根据岩石学和微量元素地球化学特征将其划分为两类, 分别称其为Ⅰ型流纹岩和Ⅱ型流纹岩.稀土和微量元素分析结果显示: (1) Ⅰ型流纹岩呈右倾的稀土分布曲线, 不相容元素以强烈富集Rb, Ba, Th, K和亏损Sr, Ti, P, Nb为特征, 其形成与钙碱性系列玄武岩浆的结晶分异作用有关; (2) Ⅱ型流纹岩具有与大陆裂谷流纹岩一致的稀土和不相容元素分布模式, 以Ba, Sr的强烈亏损与I型流纹岩相区别, 与碱性系列玄武岩类构成双峰式火山岩组合, 其成因与地壳岩石的非理想熔融作用相联系.两类流纹岩的形成与地幔柱上涌导致上覆岩石圈伸展作用有关. 相似文献
15.
A follow-up surface lithogeochemical survey was implemented at North Mammoth Prospect, northeast Victoria, to investigate possible extensions to polymetallic vein mineralization outcropping as a 20-m-wide by 200-m-long gossanous zone in rugged, scree-covered terrain. The en-echelon pyritic veins contain subeconomic Sn, Cu, Pb, Zn, Ag and Au mineralization and occur within locally folded, fractured, brecciated and bleached metasediments adjacent to high-level, altered, quartz feldspar porphyry dykes. The dykes form part of the 3.5-km-long Mammoth Porphyry Complex. An orientation traverse across the mineralized zone and adjacent wall rocks over a 1300 m × 200 m area involved the collection of 27 rock chip samples (0.5–1.0 kg) that were analyzed for 20 major and trace elements by XRF and AAS methods. Acid soluble Sn and cxF were also determined. The results showed that the less mobile elements including Sn (total), Au, As (as the arsenate), Pb and Ba were most effective in outlining the mineralized zone. Major elements (K, Na, Fe, Ca, Mg), more mobile trace elements (Cu, Zn, Rb, Sr, cxF), acid soluble Sn and element ratios (K/Na, Fe/Mg, Rb/Sr, Zr/Y, Ti/Zr) were either difficult to interpret or failed to increase the anomaly size. The expanded lithogeochemical programme (130 samples from a 2.4-km2 area) showed that weakly to strongly anomalous Sn, Pb and As values outline a zone of mineralized and bleached metasediments east of the porphyry complex. A 750 m × 250 m halo of anomalous Sn values includes North Mammoth Prospect as well as possible strike extensions to the mineralized veins. Contrasting dispersions for Au and Ba may indicate a multiphase intrusive/mineralization history for the porphyry complex. Pervasive fracturing infilled by iron oxides in the host rocks has probably facilitated lithogeochemical trace element dispersion in the weathered zone. 相似文献
16.
大兴安岭温布其地区白音高老组火山岩主要由流纹质岩屑晶屑凝灰岩和流纹岩等一系列酸性火山岩构成。LA-ICP-MS锆石U-Pb测年结果显示,研究区流纹岩形成于(131±1.5)Ma,为早白垩世;岩石地球化学研究表明,样品具有高硅SiO_2(72.43%~78.64%)、富铝Al2O3(11.48%~14.24%)、富钾K_2O(4.09%~5.05%)和低镁MgO(0.14%~0.34%)的特点;轻重稀土分馏较明显[(La/Yb)_N=8.22~13.68],Eu负异常明显(δEu=0.39~0.90);微量元素相对富集大离子亲石元素Rb、Ba、K,亏损高场强元素Nb、P、Ti。~(176)Hf/~(177)Hf比值为0.282 830~0.282 983,εHf(t)为6.02~10.19。研究发现,温布其地区白音高老组火山岩岩浆来源于新元古代亏损地幔增生的年轻基性地壳的部分熔融,形成于伸展的构造环境。 相似文献
17.
内蒙古西乌旗地区大石寨组火山岩时代及地球化学特征 总被引：4，自引：1，他引：3
西乌旗地区大石寨组火山岩以流纹岩为主,LA-ICP-MS锆石U-Pb同位素测定结果及全岩主量、微量和稀土元素分析结果表明,本区流纹岩的年龄为280.3Ma±1.4Ma,形成于早二叠世;大石寨组火山岩属高钾过铝质钙碱性火山岩,其中SiO2、K2O、Al2O3含量均较高。稀土元素配分曲线总体向右缓倾斜,轻稀土元素相对于重稀土元素富集,Eu呈明显的负异常,重稀土元素亏损不明显。Rb、Th、U、K等大离子亲石元素富集明显,Ba表现为弱亏损,高场强元素具有明显的Ta、Sr、P及Ti亏损,Zr、Hf相对富集。研究认为,该地区流纹岩可能是由玄武岩、安山岩等中基性火山岩分离结晶作用形成的,且形成的构造环境为大陆边缘弧。 相似文献
18.
Olgeir Sigmarsson Didier Laporte Marion Carpentier Bertrand Devouard Jean-Luc Devidal Joan Marti 《Contributions to Mineralogy and Petrology》2013,165(3):601-622
Phonolite and trachyte are the felsic magmas of the alkaline magma suites, which characterize the Canary Islands. The October 2011 submarine eruption off El Hierro, the westernmost island, nevertheless, produced a small volume of rhyolitic magma. The rhyolite occurred as highly vesicular, white coloured pumices enveloped in and mingled with darker coloured basanitic pumice. The basanitic pumice is relatively crystal poor with a few euhedral olivines (mostly Fo77–79), clinopyroxenes and Fe-rich spinels, whereas very rare olivine of same composition is found together with equally rare Fe-sulphide and FeTi-rich oxides in the rhyolite. The Fe–Mg exchange equilibrium in the oxides permits to calculate an equilibrium temperature of 970–890 °C for the rhyolite, in agreement with quartz-melt equilibrium at ca. 930 °C. A striking mineralogical feature of the rhyolite is the presence of rounded to contorted grains of milky quartz, which are xenocrysts incorporated and partly dissolved into the magma. Analyses of residual volatile concentrations in the glasses show that the rhyolite melt was highly degassed, whereas the basanitic glass still has important halogen concentrations. Trace element patterns of the mafic glasses and their elevated incompatible element concentrations are typical of the western Canary Island basanites. In contrast, the trace element composition of the rhyolite shows surprisingly low concentrations for all elements except the most incompatible ones (e.g. Rb, Ba, K and Th). All other measured LILE, HFSE and REE have significantly lower concentration than the basanitic counterpart that can be explained by fractionation of accessory phases (1 % apatite, 1 % sphene and 0.1 % zircon). Surprisingly, low U concentration is presumably related to elevated oxygen fugacity in the rhyolite, causing U to be in a hexavalent state, and fluxing of F-rich gas leading to volatilization of UF6, known to emanate at low temperature. The results suggest that a gas-rich basanitic melt remobilized a small volume of stagnant rhyolitic melt formed by incorporation of approximately 10 % quartz-rich sediment into a late differentiate of trachytic composition. Sediments at the interface of an old oceanic crust adjacent to a continental shield and younger volcanic island are likely to act as magma traps were sediment assimilation may alter the mantle-derived magma composition. Quartz assimilation thus explains the production of rhyolite magma in a volcanic island characterized by an alkaline magma series from primitive basanites to trachytes. 相似文献
19.
Basaltic lava flows and high-silica rhyolite domes form the Pleistocene part of the Coso volcanic field in southeastern California. The distribution of vents maps the areal zonation inferred for the upper parts of the Coso magmatic system. Subalkalic basalts (<50% SiO2) were erupted well away from the rhyolite field at any given time. Compositional variation among these basalts can be ascribed to crystal fractionation. Erupted volumes of these basalts decrease with increasing differentiation. Mafic lavas containing up to 58% SiO2, erupted adjacent to the rhyolite field, formed by mixing of basaltic and silicic magma. Basaltic magma interacted with crustal rocks to form other SiO2-rich mafic lavas erupted near the Sierra Nevada fault zone.Several rhyolite domes in the Coso volcanic field contain sparse andesitic inclusions (55–61% SiO2). Pillow-like forms, intricate commingling and local diffusive mixing of andesite and rhyolite at contacts, concentric vesicle distribution, and crystal morphologies indicative of undercooling show that inclusions were incorporated in their rhyolitic hosts as blobs of magma. Inclusions were probably dispersed throughout small volumes of rhyolitic magma by convective (mechanical) mixing. Inclusion magma was formed by mixing (hybridization) at the interface between basaltic and rhyolitic magmas that coexisted in vertically zoned igneous systems. Relict phenocrysts and the bulk compositions of inclusions suggest that silicic endmembers were less differentiated than erupted high-silica rhyolite. Changes in inferred endmembers of magma mixtures with time suggest that the steepness of chemical gradients near the silicic/mafic interface in the zoned reservoir may have decreased as the system matured, although a high-silica rhyolitic cap persisted.The Coso example is an extreme case of large thermal and compositional contrast between inclusion and host magmas; lesser differences between intermediate composition magmas and inclusions lead to undercooling phenomena that suggest smaller T. Vertical compositional zonation in magma chambers has been documented through study of products of voluminous pyroclastic eruptions. Magmatic inclusions in volcanic rocks provide evidence for compositional zonation and mixing processes in igneous systems when only lava is erupted. 相似文献
20.
W. D. Birch 《Australian Journal of Earth Sciences》2013,60(1-2):75-87
Thick rhyolite sheets occur within the middle Palaeozoic cauldron subsidence complexes in Victoria. Well‐preserved eutaxitic and vitroclastic textures in the basal regions of the Rubicon, Jemba, and Ryans Creek rhyolites provide evidence of an ash‐flow origin. Subsequent recrystallization during cooling has masked these primary textures towards the central and upper parts of the rhyolitic sheets. All three rhyolites show dense welding throughout and the major zonation observed is in the degree of granophyric recrystallization. Despite the turbulent ash‐flow emplacement and the possibility of post‐eruptive chemical modification, the mineralogical and chemical trends within vertical sequences in the Rubicon and Jemba Rhyolites are essentially continuous and represent the inverted variation within the original magma body. The bulk compositions of the rhyolites suggest that conditions of water saturation were closely approached or attained within the magma chamber. The chemical variation with increasing height and phenocryst content in the rhyolites can be summarized as: increasing K, Ca, Ba and Sr, and decreasing Na and Rb. Whereas this variation is in accord with frac‐tionation theories, it is more likely to have resulted from the partial fusion process believed to have been responsible for the generation of the acid magmas. 相似文献