块状榴辉岩920℃开放体系脱水部分熔融过程及熔体成分研究

选取江苏省东海青龙山含钠云母的石英榴辉岩块状样品,在0.1 GPa、920 ℃恒温加热4 h的条件下,进行了开放体系的脱水部分熔融实验.样品中含有钠云母、蓝闪石和绿辉石退变形成的后成合晶中的角闪石等含水矿物.该榴辉岩的熔融可以划分为3个阶段:含水矿物的脱水暗化、部分熔融和几乎全部熔融阶段.熔融从含水矿物的脱水暗化开始,玻璃质熔体首先出现在含水矿物边界.在不同的局部熔融体系内,熔体成分从基性到酸性,受到局部熔融体系内部物质组成的控制,与全岩化学成分无关.     

大别-苏鲁超高压变质带榴辉岩部分熔融的证据——微量元素和铅同位素

俯冲玄武质洋壳的部分熔融作用是一种重要的大陆生长作用，但是陆壳物质在俯冲过程中能否发生部分熔融缺少直接的证据。笔者利用已有的微量元素分配系数，讨论了岛弧玄武岩在脱水和部分熔融2种过程中微量元素行为和铅同位素演化的差异。Pb在2种过程中都属于不相容元素，但在脱水过程中其不相容性明显高于Rb、Ba、Th、U，而在熔融过程中则低于上述元素，在ω(Rb)/ω(Pb)-ω(Rb)、ω（Ba)/ω(Pb)-ω(Ba)、ω（Th)/ω(Pb)-ω(Th)、ω（U）／ω(Pb)-ω(U)相关图上2种过程的演化方向差别明显。此外，Pb和Th、U的相容性的差异也使得石榴石和绿辉石的单矿物铅同位素在2种过程中明显不同。在此基础上，结合大别－苏鲁榴辉岩的实测数据，认为大别－苏鲁榴辉岩可能部分是岛弧玄武岩部分熔融后的残余体，部分为单纯脱水的产物。大陆玄武质岩石在俯冲过程中也可能发生部分熔融作用。     

榴辉岩部分熔融过程中钒分配系数的实验测定

钒(V)是常见的变价元素,其在矿物-熔体间的分配行为主要受氧逸度控制。近年来, V的分配行为与氧逸度之间的关系常被用于揭示地幔的氧化还原状态。板块俯冲过程中, V能否迁移是理解俯冲带V地球化学行为的关键环节,也是探讨俯冲带地幔楔氧逸度的重要前提。本文使用活塞圆筒装置模拟含水榴辉岩部分熔融过程,测定石榴子石、单斜辉石和金红石与熔体之间V的分配系数。实验压力为2.5GPa、温度介于900～1125℃之间,实验产物计算的氧逸度ΔFMQ介于-5.24～+0.74之间,实验结果表明V在石榴子石、单斜辉石和金红石中为相容元素(矿物/熔体分配系数D>1.0),并且DV(金红石)>DV(单斜辉石)>DV(石榴子石); V的分配系数也是氧逸度、温度和熔体聚合度的函数,随氧逸度和温度的增加,石榴子石、单斜辉石和金红石的V分配系数都减小,而随着熔体聚合程度的增加, V分配系数增大。模拟结果表明在榴辉岩部分熔融过程中熔体将显著亏损V,因此, V在榴辉岩(俯冲带洋中脊玄武岩)部分熔融过程中是不运移的;而板块俯冲过程中,榴辉岩部分熔融产生的熔体交代地幔楔不能明显改变地幔的V含量, V在地幔矿物和...     

1.5GPa、950℃下块状斜长角闪岩部分熔融微量元素的分配特征

在1.5GPa,950℃,恒温101h条件下对华北北缘太古宙地体中的斜长角闪岩块状样品进行了脱水部分熔融实验,实验产物组合为Hb+Cpx+Gt+Gl,获得的熔体为花岗闪长质成分。利用LA-ICP-MS测试了矿物和熔体的微量元素,获得该体系内各矿物/熔体的微量元素分配系数。角闪石、单斜辉石和石榴石的分配系数与前人在类似条件下的实验结果基本一致。这说明无论实验的初始物质是粉末状还是块状,对元素的分配没有太大的影响。各矿物的REE分配系数对离子半径的拟合曲线很好地符合晶格应变弹性模型。整体特征上,角闪石、单斜辉石和石榴石的LILE,LREE分配系数较低,而HREE的分配系数较高,石榴石具有强烈富集HREE的特征。由此,造成实验熔体表现出LILE、LREE富集而HREE亏损的特征。残留相中无金红石,使得熔体中没有明显的Nb、Ta负异常。熔体的主-微量元素特征符合华北北缘中生代埃达克质岩石的基本特征,进一步支持了该类岩石"可能起源于古老下地壳的部分熔融"的成因模式。     

大别山北坡尖晶石橄榄岩中的高温石榴辉石岩包体：兼论上地幔部分熔融

出露于饶拔寨一带的含尖晶石橄榄岩体是大别山区最大的一个上地幔残片,固态侵位于北大别深成片麻岩中,呈无根侵入体的形式产出。岩体内的石榴辉石岩和榴辉岩包体,具高温成因的特点。属于上地幔部分熔融的产物,在Pl-CIPW值对Al_2O_3图解上落于拉斑玄武岩区;榴辉岩、石榴辉石岩和含尖晶石橄榄岩的REE配分形式相似,均属平坦型,只是尖晶石橄榄岩的∑REE较低。在Ni-Co-Sc三组分图解上,石榴辉石岩投绘于深位玄武岩熔体的趋势线附近,而尖晶石橄榄岩的投点则接近超镁铁质残余的趋势,清楚揭示出地幔部分熔融的迹象。尖晶石的Cr~#=12～21,而共生橄榄石Fo=92～93,说明部分熔融程度不高,估计约15％。尖晶石贫TiO_2暗示熔融过程氧逸度低。饶拔赛含尖晶石橄榄岩的出现表明;伴随超高压岩石单元的折返和隆升,会有大陆地幔残片被携带上来。软流圈的上涌或板片的断离使侵出的岩石圈板片得以维持较高的温度,这也就是石榴辉石岩早期深位退变质为麻粒岩相的原因,是后续的进一步抬升,才出现以韭角闪石 斜长石组合为特征的高角闪岩相的退变质作用。     

2.0GPa,850～1070℃条件下多硅白云母榴辉岩脱水熔融研究——对大别早白垩富K埃达克岩成因的约束

以产出于大别新店高压变质杂岩带的含角闪石多硅白云母榴辉岩(XD963)为实验源岩,使用YJ-3000t紧装式六面顶压机装置,在2.0GPa,850～1070℃条件下,进行了脱水部分熔融实验研究。结果显示,在850～950℃下,多硅白云母通过不一致脱水熔融反应产生了≤12%的过铝质(铝饱和指数ASI>1.2)的高K钙碱性(K2O/Na2O=1.2～1.4)花岗质熔体,与其共存的残留相为石榴子石+绿辉石+石英+多硅白云母+角闪石+Fe-Ti氧化物+蓝晶石±金红石。1000～1070℃时,熔体比例陡然增大(>35%),成分变为准铝质的钙碱性花岗闪长质,残留相组合以石榴子石+绿辉石+普通辉石为主。实验熔体与早白垩世富K埃达克岩的主微量元素对比结果表明:除ASI以及Eu/Eu*值特征外,低温熔体(900～950℃)与富K埃达克岩具有很好的相似性。此外,本次实验源岩在相对低温(900和950℃)条件下形成的熔体具备富K埃达克岩的两个主要特征—高K钙碱性和花岗质。因此可认为形成高K钙碱性的花岗质岩浆不需要过高的压力以及高度富K的源岩。     

950℃、1.0～3.5GPa下斜长角闪岩脱水熔融——压力和时间对结构的影响

利用YJ-3000t和JL-3600t多顶砧压力机,以哀牢山造山带南部红河县大白能—乐育剖面上的天然块状斜长角闪岩为初始样品,在950℃、1.0～3.5GPa、恒温20～300h条件下进行了两个系列的斜长角闪岩块状样品脱水部分熔融实验:(1)保持温度T=950℃,加热时间t=100h不变,改变压力(1.0～3.5GPa)的实验;(2)保持温度T=950℃,压力p=3.0GPa不变,改变加热时间(20～300h)的实验。结果表明,1.0～3.5GPa、950℃、恒温100h的条件下,随压力升高,斜长角闪岩中依次生成了单斜辉石+石榴石+熔体的矿物组合(1.0～1.5GPa)和单斜辉石+石榴石+熔体+硬玉+SiO2矿物+蓝晶石(2.0～3.5GPa)的矿物组合。3.0GPa、950℃条件下,随加热时间增加,实验产物中依次生成了单斜辉石+石榴石+熔体+硬玉+SiO2矿物+蓝晶石的矿物组合(20～100h)和单斜辉石+石榴石+熔体的矿物组合(150～300h)。斜长角闪岩的原岩结构决定了实验产物中新生矿物和熔体的分布。依据实验产物的矿物组合和新生矿物的分布特征,讨论了950℃、1.0～3.5GPa、恒温(20～300h)条件下,斜长角闪岩部分熔融过程的结构变化、变质反应以及石榴石冠状体的成因。     

秦岭造山带若干岩石高温高压脱水熔融的特征及其意义

对秦岭造山带和邻区块状岩石样品进行了高温（1 100~1 300 ℃）和高压（1.2~1.8 GPa）脱水熔融实验。通过对实验产物综合分析发现许多样品中出现了熔融玻璃和雏晶。玻璃代表的熔体成分为基性和中性,部分接近于超基性范围,熔体与原来岩石全岩成分比较,更偏基性。熔体出现的空间和成分都显示了局部熔融体系的特征,即含水矿物（角闪石或黑云母）和浅色矿物（石英或斜长石）控制了熔融的发生并且决定了熔体的成分。脱水熔融产生了比原岩更偏基性的熔体,这意味着熔融后残留部分将愈偏酸性。如果这种机制存在于大陆中-下地壳,将对探讨大陆地壳的结构和物质组成,解释某些地区的长英质中下地壳的成因等具有重要的意义。     

大别山东部榴辉岩同位素体系和变质作用的关系

大别山东部榴辉岩同位素地质年代学研究反映出三方面的问题:(1)Sm-Nd两点等时线的不确定性;(2)部分高压-超高压变质年龄实际是片麻岩的分析结果;(3)首批发表的"C型榴辉岩"Sm-Nd年龄,测定岩石不是榴辉岩.大别山东部榴辉岩年龄反映的是加里东期高压-超高压变质的年代学信息.进一步研究应着重于榴辉岩锆石成因及Sm-Nd体系适用性.岩相学是研究榴辉岩锆石成因的重要方法.     

压力对辉长岩矿物反应和部分熔融影响的实验研究

本文在高温高压条件下,开展了辉长岩矿物反应与部分熔融实验,利用偏光显微镜与扫描电镜对实验样品微观结构观察,研究实验中的新生矿物与熔体的分布;通过电子探针分析熔体成分特征。实验结果表明,在低压(300MPa)条件下,静压和塑性变形实验样品中,单斜辉石以固体反应方式生成橄榄石,在高压(1300MPa)塑性实验中所有实验样品都没有发现新生矿物颗粒,这与相图中低压条件下斜长石与橄榄石稳定共存,而高压下斜长石-辉石稳定共存相吻合。高压塑性变形条件下,单斜辉石和黑云母首先发生部分熔融,随着温度增高,斜长石逐渐参与熔融,熔体呈薄膜状分布在矿物颗粒边界,熔体成分依赖于参与熔融的矿物成分,表明出现的熔体为非平衡熔融结果。     

Experimental Investigation on Low-Degree Dehydration Partial Melting of Biotite Gneiss and Phengite-Bearing Eclogite at 2 GPa

The ultrahigh-pressure(UHP) eclogite and gneiss from the Dabie(大别)-Sulu(苏鲁) orogen experienced variable degrees of partial melting during exhumation.We report here dehydration partial melting experiments of biotite gneiss and phengite-bearing eclogite at 2 GPa and 800-950 ℃.Our results show that the partial melting of gneiss is associated with the breakdown of biotite into almandine-rich garnet starting at 900 ℃.About 10% granitic melt can be produced at 950 ℃.In contrast,the partial melting of phengite-bea...     

黑云母片麻岩—H2O系统在0.1—0.2GPa压力下的熔融实验

本文提出了冀东黑云母片麻岩—H2O系统在0.1一0.2GPa压力下熔融买验的相关系。其固相线温度分別为0.1GPa时762℃,0.2GPa时712℃。黑云母消失的温度分別为0.1GPa时787℃,0.2GPa时737℃,石英消失的温度分别为0.1GPa时837.℃,0.2GPa时787.℃。采用Burn-ham模型计算的在液相线温度下岩浆饱和水的含量分別为0.1GPa时3.8%与0.2GPa时5.8%。根据实验结果以及早前寒武纪时冀东陆壳的古地温可知,该区早前寒武纪角阿岩相岩石分布的地区广泛出现的混合岩化作用应主要归因于陆壳岩石(黑云母片麻岩等)的局部熔融作用。由实验结果以及现代冀东陆壳的地温可推知,壳内低速层可能不是由岩石局部熔融所引起,而是由岩石中含有隙间水流体引起。     

Immiscible Transition from Carbonate-rich to Silicate-rich Melts in the 3 GPa Melting Interval of Eclogite + CO2 and Genesis of Silica-undersaturated Ocean Island Lavas

We explore the partial melting behavior of a carbonated silica-deficienteclogite (SLEC1; 5 wt % CO₂) from experiments at 3 GPa and comparethe compositions of partial melts with those of alkalic andhighly alkalic oceanic island basalts (OIBs). The solidus islocated at 1050–1075 °C and the liquidus at 1415 °C.The sub-solidus assemblage consists of clinopyroxene, garnet,ilmenite, and calcio-dolomitic solid solution and the near solidusmelt is carbonatitic (<2 wt % SiO₂, <1 wt % Al₂O₃, and<0·1 wt % TiO₂). Beginning at 1225 °C, a stronglysilica-undersaturated silicate melt (34–43 wt % SiO₂)with high TiO₂ (up to 19 wt %) coexists with carbonate-richmelt (<5 wt % SiO₂). The first appearance of carbonated silicatemelt is 100 °C cooler than the expected solidus of CO₂-freeeclogite. In contrast to the continuous transition from carbonateto silicate melts observed experimentally in peridotite + CO₂systems, carbonate and silicate melt coexist over a wide temperatureinterval for partial melting of SLEC1 carbonated eclogite at3 GPa. Silicate melts generated from SLEC1, especially at highmelt fraction (>20 wt %), may be plausible sources or contributingcomponents to melilitites and melilititic nephelinites fromoceanic provinces, as they have strong compositional similaritiesincluding their SiO₂, FeO^*, MgO, CaO, TiO₂ and Na₂O contents,and CaO/Al₂O₃ ratios. Carbonated silicate partial melts fromeclogite may also contribute to less extreme alkalic OIB, asthese lavas have a number of compositional attributes, suchas high TiO₂ and FeO^* and low Al₂O₃, that have not been observedfrom partial melting of peridotite ± CO₂. In upwellingmantle, formation of carbonatite and silicate melts from eclogiteand peridotite source lithologies occurs over a wide range ofdepths, producing significant opportunities for metasomatictransfer and implantation of melts. KEY WORDS: carbonated eclogite; experimental phase equilibria; partial melting; liquid immiscibility; ocean island basalts     

Partial Melting Experiments of Peridotite + CO2 at 3 GPa and Genesis of Alkalic Ocean Island Basalts

We document compositions of minerals and melts from 3 GPa partialmelting experiments on two carbonate-bearing natural lherzolitebulk compositions (PERC: MixKLB-1 + 2·5 wt% CO₂; PERC3:MixKLB-1 + 1 wt% CO₂) and discuss the compositions of partialmelts in relation to the genesis of alkalic to highly alkalicocean island basalts (OIB). Near-solidus (PERC: 1075–1105°C;PERC3: 1050°C) carbonatitic partial melts with <10 wt%SiO₂ and 40 wt% CO₂ evolve continuously to carbonated silicatemelts with >25 wt% SiO₂ and <25 wt% CO₂ between 1325 and1350°C in the presence of residual olivine, orthopyroxene,clinopyroxene, and garnet. The first appearance of CO₂-bearingsilicate melt at 3 GPa is 150°C cooler than the solidusof CO₂-free peridotite. The compositions of carbonated silicatepartial melts between 1350 and 1600°C vary in the rangeof 28–46 wt% SiO₂, 1·6–0·5 wt% TiO₂,12–10 wt% FeO*, and 19–29 wt% MgO for PERC, and42–48 wt% SiO₂, 1·9–0·5 wt% TiO₂,10·5–8·4 wt% FeO*, and 15–26 wt% MgOfor PERC3. The CaO/Al₂O₃ weight ratio of silicate melts rangesfrom 2·7 to 1·1 for PERC and from 1·7 to1·0 for PERC3. The SiO₂ contents of carbonated silicatemelts in equilibrium with residual peridotite diminish significantlywith increasing dissolved CO₂ in the melt, whereas the CaO contentsincrease markedly. Equilibrium constants for Fe*–Mg exchangebetween carbonated silicate liquid and olivine span a rangesimilar to those for CO₂-free liquids at 3 GPa, but diminishslightly with increasing dissolved CO₂ in the melt. The carbonatedsilicate partial melts of PERC3 at <20% melting and partialmelts of PERC at 15–33% melting have SiO₂ and Al₂O₃ contents,and CaO/Al₂O₃ values, similar to those of melilititic to basaniticalkali OIB, but compared with the natural lavas they are moreenriched in CaO and they lack the strong enrichments in TiO₂characteristic of highly alkalic OIB. If a primitive mantlesource is assumed, the TiO₂ contents of alkalic OIB, combinedwith bulk peridotite/melt partition coefficients of TiO₂ determinedin this study and in volatile-free studies of peridotite partialmelting, can be used to estimate that melilitites, nephelinites,and basanites from oceanic islands are produced from 0–6%partial melting. The SiO₂ and CaO contents of such small-degreepartial melts of peridotite with small amounts of total CO₂can be estimated from the SiO₂–CO₂ and CaO–CO₂ correlationsobserved in our higher-degree partial melting experiments. Thesesuggest that many compositional features of highly alkalic OIBmay be produced by 1–5% partial melting of a fertile peridotitesource with 0·1–0·25 wt% CO₂. Owing to verydeep solidi of carbonated mantle lithologies, generation ofcarbonated silicate melts in OIB source regions probably happensby reaction between peridotite and/or eclogite and migratingcarbonatitic melts produced at greater depths. KEY WORDS: alkali basalts; carbonated peridotite; experimental petrology; ocean island basalts; partial melting     

The Formation of Yanshanian Granitic Magma in Dabie Mountains: Dehydration or Aquifer Melting -with Tiantangzhai and Jiuzihe Granites as Examples

l. IntroductionFluid, especially water is very important to thefOrmation of I--type and S-type granitic magmas,evel1 under high temperature (for example, l000 "C).Some researches suggest that dehydration melting ofhydrous minerals (muscovite, biotite and amphiboIe)under high temperature is the main mechanism ofdeep crustal anatexis (Clemens et al., l987, Skierlie,l993, WoIf et al., l995, Johannes, l996), and themain modeIs of the formation of granitic magmas inthe orogenic belt. Howevef, t…     

Anhydrous Partial Melting Experiments on MORB-like Eclogite: Phase Relations, Phase Compositions and Mineral-Melt Partitioning of Major Elements at 2-3 GPa

We present melt and mineral compositions from nominally anhydrouspartial melting experiments at 2–3 GPa on a quartz eclogitecomposition (G2) similar to average oceanic crust. Near-soliduspartial melts at 3 GPa, determined with melt traps of vitreouscarbon spheres, have 55–57 wt % SiO₂, rather less silicathan the dacitic compositions that are generally assumed fornear-solidus eclogite partial melts. At 2 GPa, equivalent near-soliduspartial melts are less silicic (