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1.
藏北戈木错渐新世碱性钾质火山岩中单斜辉石斑晶普遍出现正环带、反环带和韵律环带结构,多具有"绿核辉石"的结构特征。通过矿物电子探针测得"绿核辉石"的核部有高Mg#和低Mg#两种不同成分,两者都不同程度发育有筛孔状熔蚀结构,环带结构主要发育在辉石的幔部到边部,与绿色核部存在明显的成分间断,幔部环带结构的成分变化范围相对较窄。"绿核辉石"的成分和结构特征反映了同源碱性钾质岩浆在壳内岩浆房中发生了岩浆补给-混合作用,复杂的环带结构记录了混合岩浆的结晶过程。 相似文献
2.
五大连池钾质火山岩中发现具有反环带结构的单斜辉石斑晶。反环带辉石的核部为次透辉石,与边部相比具有较低 的Mg#值（68~77）、TiO2 （0.23~0.50 wt.%）、Cr2O3 （<0.06 wt.%）和较高的Al2O3 （3.4~5.0 wt.%）、Na2O（0.43~0.78 wt.%）、 FeO（8.8~11.0 wt.%）、MnO含量。核部的稀土元素标准化曲线较为平坦,且具有非常显著的Eu和Sr的负异常（Eu/Eu*= 0.35~0.63,Sr/Sr* = 0.03~0.17）,指示其为曾与斜长石平衡的变质岩辉石。在Mg#-TiO2相关图上,核部总体成分落入华北下 地壳低Mg麻粒岩中单斜辉石的成分范围,因此核部应该是来自下地壳麻粒岩的捕掳晶。反环带辉石边部与正常辉石斑晶成 分一致,具有较高的Mg#值（81~85）、TiO2（0.40~1.65 wt.%）、Cr2O3（0.03~0.25 wt.%）和较低的Al2O3（2.1~3.4 wt.%）、Na2O （0.34~0.63 wt.%）、FeO（4.6~6.6 wt.%）、MnO含量,轻重稀土分馏明显（La/Yb） N = 3.23~7.89,与玄武岩全岩的特征吻合。 利用主量和微量元素的分配系数进行的模拟计算均表明,反环带辉石边部与寄主岩浆已达到成分平衡,说明边部是在熔蚀 核部的基础上再生长而成的岩浆成因辉石。五大连池正常辉石斑晶的Mg#值与TiO2具有负相关性,指示岩浆在地壳深度经历 了一定程度的演化。反环带辉石斑晶边部由里到外Al2O3和Na2O含量逐渐增加的趋势支持岩浆经历演化的结论。核部辉石 的麻粒岩属性表明岩浆演化主要发生在下地壳。温压计计算结果也指示正常辉石斑晶和反环带辉石的边部都结晶于下地壳 深度的系列岩浆房。晚期结晶的辉石斑晶总是比早期结晶的辉石更富集不相容元素,说明分离结晶作用导致五大连池玄武 岩的不相容元素更富集。平衡计算表明,与辉石斑晶平衡的岩浆与玄武岩全岩在微量元素特征上高度相似。考虑到全岩高 度一致的微量元素特征,研究认为五大连池玄武岩的成分变化主要受下地壳岩浆房中的结晶分离作用控制,地壳混染（包 括壳源岩浆混合）可以忽略不计。 相似文献
3.
华南三水盆地古近纪玄武岩（64～43 Ma）记录了南海扩张前大陆裂解过程中的岩浆-构造活动,但其岩浆演化过程及其动力学机制并不清楚。为此,作者对其开展了详细的岩石学和矿物学研究。三水盆地古近纪玄武岩具有OIB型的微量元素组成特征,由软流圈地幔源区部分熔融形成。该玄武岩中单斜辉石斑晶显示复杂的环带结构,包括正环带、反环带及韵律环带结构,指示了复杂的岩浆作用过程。正环带单斜辉石斑晶为核-边结构,其核部圆化或呈不规则形状,边部自形,指示了早期晶体与晚期演化岩浆混合后发生熔蚀-再生长过程。绝大部分反环带单斜辉石斑晶为核-幔-边结构,其核部相对低Mg,并被部分熔蚀呈不规则状或圆化,幔部相对高Mg,边部自形,反映了相对原始岩浆的补给作用。个别反环带单斜辉石为核-边结构,其核部Al2O3、TiO2含量和Mg#值明显偏低,为下地壳捕虏晶,表明岩浆从地幔源区上升汇聚到地壳岩浆房的过程中经历了轻微的地壳同化混染作用。韵律环带结构单斜辉石核部常为不规则状,幔部成分呈韵律状变化,边部自形,记录了岩浆对流及多次岩浆演化-补给... 相似文献
4.
拉萨地块中南部新生代超钾质岩中单斜辉石斑晶的环带成分研究 总被引：1，自引：1，他引：1
在青藏高原拉萨地块中南部的米巴勒、麦嘎乡、赛利普地区分布有新生代的超钾质火山岩,这些火山岩的斑晶中广泛发育单斜辉石正环带,其记录了超钾质岩浆的早期结晶历史.这些单斜辉石的正环带核部到边部Mg*和镁、铬的氧化物含量降低,而铁、钠、铝、钛的氧化物含量升高,它反映了岩浆正常结晶的顺序.米巴勒地区超钾质岩中的单斜辉石还出现反环带和少量的韵律环带,可能与该地区所发现的共生钠质过碱性火山岩有关,韵律环带揭示了其可能是相同成分的超钾质母岩浆从岩浆源区多次补给形成,而反环带则可能是与钠质过碱性火山岩岩浆存在一定程度的混合造成的.尽管三个地区超钾质火山岩的喷发时间稍不同,但是三地区的单斜辉石正环带核部成分基本相同,它们具有相似的Al-Ti值,指示出超钾质岩形成在相同的构造环境并具有相同的岩浆成因. 相似文献
5.
江西广丰早白垩世碱性橄榄玄武岩中单斜辉石矿物化学及其地质意义 总被引：1，自引：0，他引：1
江西广丰早白垩世晚期碱性橄榄玄武岩中含有大、小两类单斜辉石斑晶,它们的主量元素间呈线性相关,且与大斑晶、小斑晶相平衡的岩浆Mg#值均类似于寄主岩石的Mg#值,说明它们与寄主岩石同源。早期的单斜辉石大斑晶粒径达1～1.5cm,SiO2,MgO,TFeO,CaO含量和Mg#值相对较低,Al2O3和TiO2含量相对较高,具有碱性玄武岩中单斜斑晶的矿物学特征;而晚期的单斜辉石小斑晶粒径为0.3～0.5cm,SiO2,MgO,TFeO,CaO含量和Mg#值相对较高,Al2O3和TiO2含量相对较低,也具有碱性玄武岩中斑晶的矿物学特征。单斜辉石大斑晶Wo（硅灰石）相对较高,En（顽辉石）含量相对较低,主要为含钛透辉石、含钛次透辉石,结晶温度为1272～1349℃、压力为1.23～1.93GPa,深度相当于41～64km,可能是原始岩浆绝热底辟上升结晶形成的;单斜辉石小斑晶Wo含量相对较低,En含量相对较高,主要为含钛普通辉石和顽透辉石,结晶温度为1186～1220℃、压力为0.44～0.75GPa,深度相当于15～25km,可能是相对原始的岩浆上升到浅部发生结晶分异作用形成的。 相似文献
6.
江西九瑞矿集区郎君山第三纪玄武岩的成因与岩浆演化:来自辉石和长石的矿物学证据 总被引：1，自引：0，他引：1
近期在江西九瑞矿集区朗君山地区通过钻探获取了新鲜的第三纪火山岩样品,本文对这些样品进行了系统的矿物学研究。结果表明:这些火山岩样品均为玄武岩,可明显分为两个喷发旋回,主要组成矿物为单斜辉石和斜长石,并有少量的磷灰石和铁钛氧化物等矿物。根据其矿物成分特征判断,早期旋回的玄武岩属于钾玄岩系列,晚期旋回的玄武岩属于拉斑玄武岩系列。对比两套玄武岩中单斜辉石的成分可以发现,早期玄武岩岩浆朝着富钙富锰和贫铝贫碱的方向演化,而晚期玄武岩岩浆则朝着富钛富锰和贫铝贫钙贫铬的方向演化。早期旋回玄武岩中斜长石斑晶发育反环带和韵律环带,辉石斑晶中发育正环带、反环带和韵律环带,且基质中斜长石普遍被碱性长石所交代,推测早期喷发的岩浆遭受过多次岩浆混合作用。早期旋回玄武岩中单斜辉石形成的温度为1124~1157℃,压力为1.23~1.42GPa,相应深度为41~47km,表明这套玄武岩可能是由相对原始岩浆上升到较浅部位分异形成的。根据单斜辉石F1-F2图解和Ti O2-Al Z图解判断,该区玄武岩具有明显的与弧有关的构造环境特征,推测应为弧后拉张盆地环境。 相似文献
7.
冀西北姚家庄存在一套晚三叠世的超镁铁岩-正长岩杂岩体,岩体内发育具有环带特征的单斜辉石。辉石的环带有两种:简单环带和复杂环带。简单环带一般为正环带,辉石核部的MgO和Cr2O3含量高,Si O2、Fe O和Na2O含量低;边部的主要氧化物含量与核部刚好相反。简单正环带可以分为两类,其中核边接触带平滑、由核到边化学成分具有渐变特征的正环带辉石可能是岩浆在分离结晶或地壳混染过程中形成。而核边接触带有熔蚀结构、由核到边化学成分突变的正环带辉石可能是早期结晶的辉石颗粒受到晚期低镁岩浆的溶蚀改造而成的。复杂环带具有核-幔-边结构,其中,核部低镁高铁、幔部高镁低铁、边部与核部相似,但其Mg#更低,这些特征暗示了岩浆混合作用的存在,形成辉石核部的母岩浆可能来自富集的岩石圈地幔,幔部高Mg#的特征指示了软流圈地幔物质的贡献,其边部低Mg#的特征则指示了地壳物质的加入。具有韵律环带的复杂辉石是在岩浆多期侵入的过程中形成的。辉石环带的组成特征表明,姚家庄杂岩体是由岩浆多期侵位形成的,后期侵入的岩浆与前期就位的岩浆不断反应,形成了具有多种不同环带特征的辉石,并最终形成了空间上由外到内依次为辉石岩、辉石正长岩和正长岩的环状杂岩体。结合前人的研究成果,推测形成姚家庄岩体的岩浆主要来源于富集的岩石圈地幔,并由少量地壳组分和软流圈物质的贡献。 相似文献
8.
根据矿物结构可以有效的识别开放性体系内的各种岩浆过程,在岩相学研究基础上,对全新世喷发的Ruapehu火山 Whakapapa组安山岩内的辉石斑晶进行了系统的矿物学分析。所研究样品中辉石斑晶以普通辉石及顽火辉石为主,具多样 结构与形态,根据其结构和成分特征可分为3类：（1）骸晶状的辉石（SP型）,具有破碎的、筛孔状外形,核部具有异常高 的FeOT含量,可能为壳源捕掳晶;（2）具均一结构及成分的辉石（HP型）,其内部未显示明显的熔融结构,形成于岩浆房 的内部,基本未受基性岩浆混合的影响;（3）具核-幔-边结构的 “绿核”辉石（GCP型）,其幔部相比核部及边部具有更 高的En、Mg#、CaO、MgO和Cr2O3含量以及更低的FeOT、TiO2含量。且GCP辉石边部相比核部更加富CaO、MgO而贫 FeOT、TiO2。GCP型辉石幔部（边部）与核部形成明显的反环带结构,两者界线清晰,成分上存在突变,是偏基性岩浆注 入岩浆房边界层时,岩浆发生大规模扩散、混合之前形成于脉状岩浆“通道”内;而GCP辉石边部则结晶于混合后的岩浆 之中。辉石的结构及岩相学特征暗示,安山岩在喷发前经历了岩浆混合及地壳物质的混染,混合过程主要发生于地壳浅部 岩浆房的边界层及粥状层内,且这种混合过程可能是诱发火山喷发的重要机制之一。 相似文献
9.
青藏米巴勒地区中新世钠质方沸石(霞石)响岩中单斜辉石环带研究:对岩浆-构造演化的启示 总被引：5，自引：3，他引：2
在拉萨地块中部米巴勒地区产出的中新世钠质方沸石(霞石)响岩中的单斜辉石发育大量环带结构,环带结构包括正环带、反环带和韵律环带。其中正环带结构从核部到边部Mg#、MgO、Cr2O3呈逐渐降低趋势,而FeO、Na2O、Al2O3、TiO2呈逐渐升高的趋势,它反映了岩浆正常结晶的历史。反环带结构中Mg#、MgO、Cr2O3从核部到边部呈升高的趋势,FeO、Na2O、Al2O3的含量总体上呈降低趋势,本文认为响岩中单斜辉石的这种环带结构是受到了和它共存的钾质-超钾质火山岩岩浆混合或混染所致,两种岩石应是各自独立的母岩浆的结晶产物。具韵律环带结构的单斜辉石从核部到边部出现成分规律性波动,Na2O、Al2O3、TiO2、Cr2O3含量的变化在正反环带的范围内,Mg#值的变化范围较小,在正环带斑晶和基质的范围内,它是岩浆多次混合作用的结果。该地区钠质过碱性岩的出现可能指示青藏高原在这套火山岩喷发时(13~12Ma),构造体制发生了显著的改变,进入到一个典型的陆内伸展阶段。 相似文献
10.
岩浆化学混合作用长期通过岩石地球化学进行识别,一直缺乏直接的矿物学证据。针对这一问题,我们对浙江江山碗瑶乡上墅组内薄层状玄武粗安岩夹层中的单斜辉石矿物环带开展了系统的矿物学研究工作,探讨其岩石成因、构造背景和岩浆动力学过程,为江山地区新元古代岩浆构造演化过程提供新的证据。研究表明,单斜辉石以斑晶和基质两种形式产出,斑晶发育核-幔-边结构,其中幔部和边部包含有较多的钛铁矿、单斜辉石和斜长石等矿物包裹体,表明经历了复杂的岩浆活动过程。电子探针分析表明,基质和斑晶辉石都属于普通辉石,斑晶辉石从核部、幔部到边部成分呈现规律性变化,CaO和MgO含量增加、Al2O3和Na2O、K2O含量逐渐降低,但是基质辉石具有相对较低的Ti、Al和Na含量,且与斑晶边缘成分类似。单斜辉石原位微量元素组成表明,斑晶单斜辉石从核部到边部具有相似的特征,呈现Th、La、Nd和Ta等元素相对富集,Nb、K、Ti和Pb等元素相对亏损的特点。温压计算结果显示斑晶和基质的形成温度较低(1040.3～1122.3℃),斑晶核部结晶深度较深... 相似文献
11.
Vladica Cvetkovi? Suzana Eri? Ma?a Radivojevi? Kristina ?ari? 《Mineralogy and Petrology》2012,106(3-4):131-150
The study focuses on clinopyroxene from mantle xenolith-bearing East Serbian basanites and suggests that dissolution of mantle orthopyroxene played an important role in at least some stages of the crystallization of these alkaline magmas. Five compositional types of clinopyroxene are distinguished, some of them having different textural forms: megacrysts (Type-A), green/colourless-cored phenocrysts (Type-B), overgrowths and sieve-textured cores (Type-C), rims and matrix clinopyroxene (Type-D), and clinopyroxene from the reaction rims around orthopyroxene xenocrysts (Type-E). Type-A is high-Al diopside that probably crystallized at near-liquidus conditions either directly from the host basanite or from compositionally similar magmas in previous magmatic episodes. Type-B cores show high VIAl/IVAl≥1 and low Mg# of mostly <75 and are interpreted as typical xenocrysts. Type-C, D and E are interpreted as typical cognate clinopyroxene. Type-D has Mg#<78, Al2O3?=?6–13?wt.%, TiO2?=?1.5–4.5?wt.%, and Na2O?=?0.4–0.8?wt.% and compositionally similar clinopyroxene is calculated by MELTS as a phase in equilibrium with the last 30?% of melt starting from the average host lava composition. Type-C has Mg#?=?72–89, Al2O3?=?4.5–9.5?wt.%, TiO2?=?1–2.5?wt.%, Na2O?=?0.35–1?wt.% and Cr2O3?=?0.1–1.5?wt.%. This clinopyroxene has some compositional similarities to Type-E occurring exclusively around mantle orthopyroxene. Cr/Al vs Al/Ti and Cr/Al vs Na/Ti plots revealed that Type-C clinopyroxene can crystallize from a mixture of the host basanite magma and 2–20?wt.% mantle orthopyroxene. Sieve-textured Type-C crystals show characteristics of experimentally produced skeletal clinopyroxene formed by orthopyroxene dissolution suggesting that crystallization of Type-C was both texturally and compositionally controlled by orthopyroxene breakdown. According to FeO/MgOcpx/melt modelling the first clinopyroxene precipitating from the host basanite was Type-A (T?~?1250?°C, p?~?1.5?GPa). Dissolution of orthopyroxene produced decreasing FeO/MgOmelt and crystallization of Type-E and sieve-textured Type-C clinopyroxene (0.3–0.8?GPa and 1200–1050?°C). The melt composition gradually shifted towards higher FeO/MgOmelt ratios precipitating more evolved Type-C and Type-D approaching near-solidus conditions (<0.3?GPa; ~950?°C). 相似文献
12.
Clinopyroxene phenocrysts in the Kokchetav trachybasalts are variable in composition and textures. Two distinctive cores are recognized: diopside cores and green salite cores. The diopside cores with Mg# of 80–90 are mantled by colorless salite rims with Mg# of 70–80. The green salite cores have especially low Mg# (<70) but high Al and Ti contents. A Mg-rich band (Mg#=82–90) usually occurs between a green salite core and its rim, and/or between a colorless salite mantle and its rim. Dissolution surfaces are observed on all textural variants. Two magma chambers are needed to explain the observed clinopyroxene phenocrysts. A deep chamber at about 120 km in the upper mantle in which diopside cores crystallized, and a shallow chamber at depths of less than 40 km in which diopside cores were resorbed and overgrown by salite rims or mantles. Magma mixing in the shallow chamber is responsible for the formation of dissolution surfaces between the diopside bands and the colorless salite mantles. The dissolution surfaces on the diopside cores formed in the shallow chamber as a result of pressure decrease. This magma evolution scenario is complicated by the occurrence of the crustal-origin green salite cores in diopsides. These green cores likely represent the relics of continental materials, which were captured in the deep chamber and partially re-melted. Our observations indicate that subducted continental materials were returned to the Earth's surface as a result of magmatism. This study therefore provides direct evidence of a link between subducted continental materials (slab) and magmatism in this orogenic belt. 相似文献
13.
阿热岩体呈NNE方向展布于义敦岛弧带中甸弧南端,与相邻格咱断裂方向一致。岩体主要发育有角闪闪长玢岩、辉石闪长玢岩、含辉石角闪闪长玢岩及石英闪长玢岩。本文根据岩体内斑晶分布特征,选择角闪闪长玢岩、辉石闪长玢岩及含辉石角闪闪长玢岩及其析离体中的角闪石和单斜辉石进行岩相学及矿物化学研究,其结果显示岩体中角闪石及单斜辉石均分为高Mg#及低Mg#两类,角闪石及辉石均发育与岩浆混合作用相关的不平衡结构,存在同源异相岩浆混合作用,分异结晶作用不明显;成岩岩浆与三叠纪俯冲的甘孜-理塘洋壳板片有关,俯冲过程中板片脱水形成的流体及部分熔融形成的熔体与地幔楔交代是其岩浆形成的重要机制,其岩浆过程主要受构造应力控制,由不富水或局部富水岩浆向富水岩浆演化,具高MgO及SiO2/Al2O3向低MgO及SiO2/Al2O3演化的特点;角闪石温度压力估算结果显示先形成的低Mg#角闪石结晶压力远大于高Mg#角闪石。根据岩浆侵位的构造、压力环境及其动力学背景,我们认为甘孜-理塘洋壳向西俯冲过程中由于中咱地块西侧的羌塘地块于中三叠时期(~230Ma)开始向东挤压碰撞,在此过程中洋壳板片可能发生断离,导致该区背景构造挤压应力在228~219Ma期间先增大后变小。 相似文献
14.
Mineral chemistry of submarine lavas from Hilo Ridge, Hawaii: implications for magmatic processes within Hawaiian rift zones 总被引：10，自引：0，他引：10
Huai-Jen Yang Frederick A. Frey David A. Clague Michael O. Garcia 《Contributions to Mineralogy and Petrology》1999,135(4):355-372
The crustal history of volcanic rocks can be inferred from the mineralogy and compositions of their phenocrysts which record
episodes of magma mixing as well as the pressures and temperatures when magmas cooled. Submarine lavas erupted on the Hilo
Ridge, a rift zone directly east of Mauna Kea volcano, contain olivine, plagioclase, augite ±orthopyroxene phenocrysts. The
compositions of these phenocryst phases provide constraints on the magmatic processes beneath Hawaiian rift zones. In these
samples, olivine phenocrysts are normally zoned with homogeneous cores ranging from ∼ Fo81 to Fo91. In contrast, plagioclase, augite and orthopyroxene phenocrysts display more than one episode of reverse zoning. Within each
sample, plagioclase, augite and orthopyroxene phenocrysts have similar zoning profiles. However, there are significant differences
between samples. In three samples these phases exhibit large compositional contrasts, e.g., Mg# [100 × Mg/(Mg+Fe+2)] of augite varies from 71 in cores to 82 in rims. Some submarine lavas from the Puna Ridge (Kilauea volcano) contain phenocrysts
with similar reverse zonation.
The compositional variations of these phenocrysts can be explained by mixing of a multiphase (plagioclase, augite and orthopyroxene)
saturated, evolved magma with more mafic magma saturated only with olivine. The differences in the compositional ranges of
plagioclase, augite and orthopyroxene crystals between samples indicate that these samples were derived from isolated magma
chambers which had undergone distinct fractionation and mixing histories. The samples containing plagioclase and pyroxene
with small compositional variations reflect magmas that were buffered near the olivine + melt ⇒Low-Ca pyroxene + augite +
plagioclase reaction point by frequent intrusions of mafic olivine-bearing magmas. Samples containing plagioclase and pyroxene
phenocrysts with large compositional ranges reflect magmas that evolved beyond this reaction point when there was no replenishment
with olivine-saturated magma. Two of these samples contain augite cores with Mg# of ∼71, corresponding to Mg# of 36–40 in
equilibrium melts, and augite in another sample has Mg# of 63–65 which is in equilibrium with a very evolved melt with a Mg#
of ∼30. Such highly evolved magmas also exist beneath the Puna Ridge of Kilauea volcano. They are rarely erupted during the
shield building stage, but may commonly form in ephemeral magma pockets in the rift zones.
The compositions of clinopyroxene phenocryst rims and associated glass rinds indicate that most of the samples were last equilibrated
at 2–3 kbar and 1130–1160 °C. However, in one sample, augite and glass rind compositions reflect crystallization at higher
pressures (4–5 kbar). This sample provides evidence for magma mixing at relatively high pressures and perhaps transport of
magma from the summit conduits to the rift zone along the oceanic crust-mantle boundary.
Received: 8 July 1998 / Accepted: 2 January 1999 相似文献
15.
对青藏高原西南部当惹雍错地区的中新世超钾质粗面岩及共存的富钠质方沸石(霞石)响岩进行了详细的岩石学、矿物学研究,揭示出两套岩石具有不同的矿物学特征.钠质方沸石(霞石)响岩中橄榄石斑晶的熔体包裹体与钾质火山岩的成分相当,应属于早期残余的钾质岩浆,表明了钠质方沸石(霞石)响岩的形成晚于超钾质粗面岩.两类岩石的全岩化学成分、... 相似文献
16.
Takeshi Kuritani Takeyoshi Yoshida Jun-Ichi Kimura Yuka Hirahara Toshiro Takahashi 《Mineralogy and Petrology》2014,108(1):1-11
The water content of low-K tholeiitic basalt magma from Iwate volcano, which is located on the volcanic front of the NE Japan arc, was estimated using multi-component thermodynamic models. The Iwate lavas are moderately porphyritic, consisting of ~8 vol.% olivine and ~20 vol.% plagioclase phenocrysts. The olivine and plagioclase phenocrysts show significant compositional variations, and the Mg# of olivine phenocrysts (Mg#78–85) correlates positively with the An content of coexisting plagioclase phenocrysts (An85–92). The olivine phenocrysts with Mg# > ~82 do not form crystal aggregates with plagioclase phenocrysts. It is inferred from these observations that the phenocrysts with variable compositions were primarily derived from mushy boundary layers along the walls of a magma chamber. By using thermodynamic calculations with the observed petrological features of the lavas, the water content of the Iwate magma was estimated to be 4–5 wt.%. The high water content of the magma supports the recent consensus that frontal-arc magmas are remarkably hydrous. Using the estimated water content of the Iwate magma, the water content and temperature of the source mantle were estimated. Given that the Iwate magma was derived from a primary magma solely by olivine fractionation, the water content and temperature were estimated to be ~0.7 wt.% and ~1,310 °C, respectively. Differentiation mechanisms of low-K frontal-arc basalt magmas were also examined by application of a thermodynamics-based mass balance model to the Iwate magma. It is suggested that magmatic differentiation proceeds primarily through fractionation of crystals from the main molten part of a magma chamber when it is located at <~200 MPa, whereas magma evolves through a convective melt exchange between the main magma and mushy boundary layers when the magma body is located at >~200 MPa. 相似文献
17.
Various petrographic features and geochemical characteristics indicative of disequilibrium are preserved in plagioclase phenocrysts from basaltic to andesitic lavas in East Junggar, northwest China. These characteristics indicate that they crystallized in a magma chamber, which was replenished by less differentiated and high-temperature magmas. The petrographic and geochemical features of the plagioclase phenocrysts are interpreted to record responses to changes in temperature, composition and mechanical effect during magma replenishment. Distinct rare earth element(REE) patterns between cores and rims of the same plagioclase crystal suggest derivation from two end-member magmas. From core to rim, plagioclase phenocrysts commonly display sharp fluctuations of anorthite(An) content up to 20, which either correspond to reverse zoning associated with ovoidal cores and resorption surface(PI), or normal zoning with euhedral form and no resorption surface(P2). Plagioclase crystals with diverse textures and remarkably different An content coexist on the scale of a thin-section. Cores of these plagioclases in each sample display a bimodal distribution of An content. From core to rim in PI, concentrations o f FeOT and Sr increase remarkably as An content increases. During magma replenishment, pre-existing plagioclase phenocrysts in the andesitic magma, which were immersed into hotter and less differentiated magmas, were heated and resorbed to form ovoidal cores, and then were overgrown by a thin rim with much higher contents of An, FeO~T and Sr. However, pre-existing plagioclase phenocrysts in the basaltic magma were injected into cooler and more evolved magmas, and were remained as euhedral cores, which were later enclosed by oscillatory zoned rims with much lower contents of An, Sr and Ba. 相似文献