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1.
距今约276 Ma的皮羌层状岩体位于塔里木板块西缘,是二叠纪塔里木大火成岩省的组成部分。岩体主要由辉长岩组成并赋存大型钒钛磁铁矿矿床。矿物结构和磁铁矿、钛铁矿和单斜辉石成分变化特征表明,矿物结晶顺序为单斜辉石+斜长石→磁铁矿+钛铁矿。磁铁矿中V2O3含量变化于0.49%~0.97%,说明岩浆演化过程氧逸度较低(相似文献
2.
距今约276 Ma的皮羌层状岩体位于塔里木板块西缘,是二叠纪塔里木大火成岩省的组成部分。岩体主要由辉长岩组成并赋存大型钒钛磁铁矿矿床。矿物结构和磁铁矿、钛铁矿和单斜辉石成分变化特征表明,矿物结晶顺序为单斜辉石+斜长石→磁铁矿+钛铁矿。磁铁矿中V2O3含量变化于0.49%~0.97%,说明岩浆演化过程氧逸度较低(相似文献
3.
攀枝花镁铁质层状岩体磷灰石中的熔融包裹体：岩浆不混熔的证据 总被引：2，自引：2，他引：0
攀西(攀枝花-西昌)地区出露一系列赋含大型钒钛磁铁矿矿床的镁铁-超镁铁质层状岩体(例如:攀枝花、红格、白马和太和岩体),它们是~260Ma峨眉山大火成岩省的重要组成部分。显微岩相学研究显示,攀枝花岩体中部带浅色辉长岩磷灰石中存在深色和浅色两类熔融包裹体:深色熔融包裹体为深棕色或黑色,大小约10~60μm,外形呈圆形或磷灰石负晶形,主要由单斜辉石、角闪石、斜长石、磁铁矿等子矿物以及微粒结晶物质组成;浅色熔融包裹体为白色或淡绿色,大小约5~60μm,外形呈圆形或磷灰石负晶形,由斜长石、角闪石、单斜辉石、少量磁铁矿和石英等子矿物以及微粒结晶物质组成。我们将熔融包裹体加热至1080~1200℃,浅色熔融包裹体完全均一,其平均成分为69.7% SiO2、0.24% TiO2、14.5% Al2O3、2.76% FeO、0.64% MgO、5.14% CaO、2.82% Na2O、2.26% K2O和0.25% P2O5,为富Si相;深色熔融包裹体未完全均一,结合电子探针分析以及质量平衡估算,可得其平均成分为42.4% SiO2、1.43% TiO2、8.83% Al2O3、20.5% FeO、5.46% MgO、16.3% CaO、1.11% Na2O、0.30% K2O 和1.41% P2O5,为富Fe相。这一结果说明,在攀枝花层状岩体形成过程中,岩浆房中可能同时存在富Fe和富Si两种熔体,暗示演化的玄武质岩浆曾发生了不混熔作用。岩浆不混熔及重力分异共同作用导致了岩浆房硅酸盐熔体成分分层,下部的富Fe硅酸盐岩浆层演化形成了岩体下部巨厚的钒钛磁铁矿矿体和暗色辉长岩,上部的富Si硅酸盐岩浆层演化形成了岩体上部的长英质岩脉、透镜体和浅色辉长岩。 相似文献
4.
攀西白马岩体的矿物结晶顺序与钒钛磁铁矿成因 总被引：1，自引：0，他引：1
距今约260 Ma的白马岩体位于上扬子板块西缘的攀西裂谷中,是一个大型的含钒钛磁铁矿镁铁质-超镁铁质杂岩体,是峨眉山大火成岩省的重要组成部分。含矿岩体主要由磁铁橄长岩和橄榄辉长岩组成,主要工业矿体赋存在下部的橄长岩岩相带中。显微镜下显示橄榄石和角闪石均存在2种不同的结构状态,岩浆具有多次脉动的侵位特点。矿物结构特点及磁铁矿、钛铁矿、橄榄石、角闪石及斜长石等矿物电子探针成分测定显示,矿物的结晶顺序大致为斜长石+橄榄石+辉石→角闪石+磁铁矿+钛铁矿→角闪石。根据角闪石和斜长石成分计算角闪石最低结晶温度为1090℃,斜长石的最高结晶温度是1120℃,推测磁铁矿的结晶温度介于1090~1120℃之间。橄榄石的Fo值由下部的磁铁橄长岩向上部的橄榄辉长岩呈逐渐降低的变化趋势,表明随着岩浆的结晶分异进程,系统的氧逸度是逐步变化的,暗示整个结晶分异过程系统处于封闭状态。磁铁矿中w(V2O3)变化于0.72%~1.37%之间,可近似看成是岩浆演化过程氧逸度较低的量化标志(FMQ+0.5),这种低氧逸度条件下硅酸盐矿物的结晶,会导致粒间熔体氧逸度逐步升高且成分向着富Fe的方向演化。岩浆的这种成分演化特点,是晚期形成不混溶熔浆及富Fe-Ti矿浆的主要原因。 相似文献
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阿翁错地区早白垩世花岗闪长岩位于班公湖-怒江成矿带西段,广泛发育暗色微粒包体,是研究岩浆混合作用的理想对象。本文从地质学、岩相学和矿物化学等方面对花岗闪长岩开展了详细研究。包体形态多样,与寄主岩呈渐变或截然型接触关系,包体与寄主岩之间相互穿插,接触带发育冷凝边,包体从寄主岩中捕获了大量捕虏晶矿物,包体和寄主岩中均见针状磷灰石。电子探针结果显示:具正环带特征的寄主岩斜长石An值变化范围不大,在48.16～59.05之间,具致密韵律环带特征,可能代表镁铁质岩浆注入长英质岩浆房前结晶的斜长石;具反环带特征斜长石的An值变化范围较大,核部An值为29.86,往外An值陡增至51.09,整体呈逐渐增大趋势,暗示存在富钙基性岩浆的加入;具正-反环带特征的寄主岩斜长石核部至中间区域An值变化相对复杂,具稀疏韵律环带结构特征,反映斜长石结晶过程中因岩浆混合作用岩浆房环境发生突变,随着混合作用持续进行,岩浆逐渐冷却,斜长石在静态环境下继续结晶形成干净的正环带边部;包体斜长石虽然存在正环带,但其An值变化范围较大,在28.63～62.40之间,核部An值高,边部An值骤然降低,可能是镁铁质岩浆与长英质岩浆... 相似文献
6.
斜长石和橄榄石成分对四川攀枝花钒钛磁铁矿床成因的指示意义 总被引：8，自引：5，他引：3
攀枝花岩体下部和中部岩相带各旋回中磁铁辉长岩和辉长岩的岩相结构特征表明,钛铁氧化物和斜长石、橄榄石的结晶发生在相近的温度区间内,这为我们利用斜长石和橄榄石的成分探讨磁铁矿形成时温度、氧逸度和岩浆成分的变化提供了可能.电子探针分析结果表明,下部和中部岩相带中斜长石An牌号自下向上有规律地逐渐降低,而在每一个旋回内部,橄榄石的F(o)值总是由磁铁辉长岩向辉长岩表现出强烈降低的趋势.这些特征说明攀枝花岩体经历了多次富铁钛的岩浆的补充.斜长石An牌号小幅度的规律性降低说明岩浆氧逸度和Fe3/Fe2+比值变化对斜长石成分影响很小,因此,我们可以根据斜长石成分估计钛铁氧化物结晶过程中温度变化.然而,同一旋回中橄榄石Fo值变化较大说明橄榄石成分在很大程度上取决于岩浆中的Fe3+/Fe2+和Fe2 +/Mg含量,因此,可以根据橄榄石成分分析磁铁辉长岩与辉长岩形成过程中氧逸度和Fe3+/Fe2比值的相对变化.计算得到下部和中部岩相带中斜长石的结晶温度介于1079～1121℃之间,认为钛铁氧化物的结晶也大致发生在此温度区间;根据同一旋回中磁铁辉长岩与邻近辉长岩中橄榄石Fo值的差异,发现每次新补充的岩浆分离结晶过程中氧逸度总是逐渐降低,这与前人对封闭体系岩浆结晶分异过程中氧逸度变化规律的认识一致. 相似文献
7.
浙江普陀花岗杂岩体中的石英闪长质包体：斜长石内部复杂环带研究与岩浆混合史记录 总被引：11，自引：7，他引：11
浙江普陀花岗杂岩体包含若干石英闪长质包体,该类包体中存在三种不同类型的斜长石:正常环带的斜长石、筛孔构造的斜长石和酸性斜长石为核的“反环带”斜长石。根据斜长石的环带构造特征和成分分析,认为本区的岩浆演化过程大致如下:下部基性岩浆注入到上覆酸性岩浆中并进行混合作用,酸性岩浆中已结晶的富钠质斜长石晶体进入偏基性的混合岩浆中,部分熔融形成筛孔构造;随着端员岩浆的进一步混合,富钠质斜长石晶体与中性混合熔体仅形成粗糙的边界,而保留原先构造特征;同时混合岩浆可以直接结晶出正常环带斜长石,呈单颗粒或以膜的形式包围其它环带构造的长石。本文还通过与平潭甬闪辉长岩杂岩体内筛孔斜长石的对比,认为斜长石的环带构造和成分可以反映岩浆源区特征和岩浆演化历史。 相似文献
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9.
峨眉山大火成岩省内带黑谷田含钒钛磁铁矿层状岩体成因 总被引：2，自引：2，他引：0
黑谷田岩体产于峨眉山大火成岩省内带,是一个小型含钒钛磁铁矿辉长岩体。与区内其它典型大型基性-超基性层状岩体具有多个旋回岩相的特征不同,黑谷田层状岩体分为下部、上部两个岩相带:下部岩相带从底到顶依次为橄榄辉石岩、磁铁辉长岩、含磷灰石辉长岩和中粒辉长岩,上部岩相带为细粒辉长岩,二者呈突变接触关系。黑谷田岩体的锆石SHRIMP U-Pb年龄为263±5Ma,表明其是~260Ma峨眉山地幔柱岩浆主活动期的产物。岩石的矿物组合(主要为单斜辉石、斜长石,磁铁矿,少量橄榄石等)、元素地球化学(富Fe2O3、TiO2、P2O5,高Sm/Yb及低La/Sm)及低的初始87Sr/86Sr值和亏损的εNd(t)值特征一致指示黑谷田岩体与峨眉山高Ti玄武岩具有密切的内在成因联系。岩体的岩相学及地球化学特征暗示下部岩相带是富FeTi岩浆侵入发生橄榄石、单斜辉石、磁铁矿、斜长石、磷灰石等矿物分离结晶、堆积固结的产物,而上部岩相带是另一期岩浆上侵较为快速冷却固结的结果,矿物堆晶作用不显著,但是二者起源于相同的母岩浆。下部岩相带比上部岩相带具有相对低的初始87Sr/86Sr值(分别为0.7041~0.7051和0.7050~0.7056)和略高的εNd(t)值(分别为2.1~4.4和0.6~1.3),表明后者比前者经历了稍微强烈的地壳物质同化混染。下部岩相带仅有橄榄辉石岩及辉长岩而缺少正长岩和花岗岩、以及较厚的氧化物矿体赋存在岩体底部下凹部位说明黑谷田钒钛磁铁矿形成于岩浆通道系统中,磁铁矿在流动过程中由于重力作用堆积成矿。黑谷田含钒钛磁铁矿岩体的发现表明小型层状岩体也具有重要的Fe-Ti氧化物成矿潜力,在勘探找矿中不容忽视。 相似文献
10.
攀枝花层状辉长质岩体的微量元素和锶钕铅氧同位素系统:地幔源区和矿床成因的证据 总被引：8，自引：0，他引：8
攀枝花岩体是攀西地区一个典型含钒钛磁铁矿的层状辉长质岩体,是峨眉山大火成岩省的组成部分。其Sr、Nd、Pb同位素组成变化范围较窄,落在峨眉山玄武岩的范围内,同时也落在洋岛玄武岩范围内,说明攀枝花岩体与峨眉山玄武岩有着成因联系,均与地幔柱作用有关。其低的δ18O值(<6‰)说明没有地壳物质的混染,其高的La/Nb比值(>1)和微量元素原始地幔标准化图解出现的负Nb异常则表明源区有岩石圈地幔物质的混染。因而攀枝花岩体是地幔柱和岩石圈地幔部分熔融的产物,岩浆在岩浆房中没有经历地壳物质的混染作用。Mg#与P2O5的正相关以及P2O5与FeO含量的负相关,表明P不是导致氧化物熔体不混熔的主要原因;而Mg#与Fe2O3/FeO的负相关则说明,岩浆演化过程中氧逸度的升高可能是导致氧化物熔体不混熔的主要原因。 相似文献
11.
The Xinjie layered intrusion in the Panxi region,SW China,hosts both Fe-Ti oxide and platinum-group element（PGE） sulfide mineralization.The intrusion can be divided,from the base upward,into UnitsⅠ,ⅡandⅢ,in terms of mineral assemblages.UnitsⅠandⅡare mainly composed of wehrlite and clino-pyroxenite, whereas UnitⅢis mainly composed of gabbro.PGE sulfide-rich layers mainly occur in Unit I, whereas thick Fe-Ti oxide-rich layers mainly occur in UnitⅢ.An ilmenite-rich layer occurs at the top of UnitⅠ.Fe-Ti oxides include magnetite and ilmenite.Small amounts of cumulus and intercumulus magnetite occur in UnitsⅠandⅡ.Cumulus magnetite grains are commonly euhedral and enclosed within olivine and clinopyroxene.They have high Cr2O3 contents ranging from 6.02 to 22.5 wt.%,indicating that they are likely an early crystallized phase from magmas.Intercumulus magnetite that usually displays ilmenite exsolution occupies the interstices between cumulus olivine crystals and coexists with interstitial clinopyroxene and plagioclase.Intercumulus magnetite has Cr2O3 ranging from 1.65 to 6.18 wt.%, lower than cumulus magnetite.The intercumulus magnetite may have crystallized from the trapped liquid.Large amounts of magnetite in UnitⅢcontains Cr2O3（<0.28 wt.%） much lower than magnetite in UnitsⅠandⅡ.The magnetite in UnitⅢis proposed to be accumulated from a Fe-Ti-rich melt.The Fe-Ti-rich melt is estimated to contain 35.9 wt.%of SiO2,26.9 wt.%of FeOt,8.2 wt.%of TiO2,13.2 wt.%of CaO, 8.3 wt.%of MgO,5.5 wt.%of Al2O3 and 1.0 wt.%of P2O5.The composition is comparable with the Fe-rich melts in the Skaergaard and Sept Iles intrusions.Paired non-reactive microstructures,granophyre pockets and ilmenite-rich intergrowths,are representative of Si-rich melt and Fe-Ti-rich melt,and are the direct evidence for the existence of an immiscible Fe-Ti-rich melt that formed from an evolved ferro-basaltic magma. 相似文献
12.
The ～?2-km-thick Panzhihua gabbroic-layered intrusion in SW China is unusual because it hosts a giant Fe–Ti oxide deposit in its lower zone. The deposit consists of laterally extensive net-textured and massive Fe–Ti oxide ore layers, the thickest of which is ～?60 m. To examine the magmatic processes that resulted in the Fe enrichment of parental high-Ti basaltic magma and the formation of thick, Fe–Ti oxide ore layers, we carried out a detailed study of melt inclusions in apatite from a ～?500-m-thick profile of apatite-bearing leucogabbro in the middle zone of the intrusion. The apatite-hosted melt inclusions are light to dark brown in color and appear as polygonal, rounded, oval and negative crystal shapes, which range from ～?5 to ～?50 µm in width and from ～?5 to ～?100 µm in length. They have highly variable compositions and show a large and continuous range of SiO2 and FeOt with contrasting end-members; one end-member being Fe-rich and Si-poor (40.2 wt% FeOt and 17.7 wt% SiO2) and the other being Si-rich and Fe-poor (74.0 wt% SiO2 and 1.20 wt% FeOt). This range in composition may be attributed to entrapment of the melt inclusions over a range of temperature and may reflect the presence of µm-scale and immiscible Fe-rich and Si-rich components in different proportions. Simulating results for the motion of Si-rich droplets within a crystal mush indicate that Si-rich droplets would be separated from Fe-rich melt and migrate upward due to density differences in the interstitial liquid when the magma unmixed. Migration of the Si-rich, immiscible liquid component from the interstitial liquid caused the remaining Fe-rich melt in the lower part to react with plagioclase primocrysts (An59–60), as evidenced by fine-grained lamellar intergrowth of An-rich plagioclase (An79–84)?+?clinopyroxene in the oxide gabbro of the lower zone. Therefore, magma unmixing within a crystal mush, combined with gravitationally driven loss of the Si-rich component, resulted in the formation of Fe-rich, melagabbro and major Fe–Ti oxide ores in the lower part and Si-rich, leucogabbro in the upper part of the intrusion. 相似文献
13.
The Adamello gabbro exposed on the summit of Cornone di Blumone, Western Alps, Italy, has been fused by lightning strikes to form magnetite-rich fulgurites produced by melting of magnetite, hornblende, calcic plagioclase and minor clinopyroxene. The composition of quench magnetite in the fulgurite is 44.4 Fe3O4; 27.5 MgFe2O4; 15.1 FeAl2O4; 7.9 Fe2TiO4; 2.5 Fe2SiO4; 1.9 CaFe2O4; 0.8 MnFe2O4 and is inferred to have crystallized from a low-Si, Fe-rich melt under high oxidation conditions of about 1 log unit below the log10?O2 of hematite–magnetite. The low Si, Fe-rich melt is considered to have been produced from fusion of magnetite + hornblende-rich areas of the host gabbro and/or possible separation of an immiscible high Fe2O3/FeO Fe-rich, low-Si melt from a more siliceous glass during superheating. Skeletal-dendritic morphologies of magnetite in the fulgurite indicate crystallization under conditions of extreme supercooling. Juxtaposition of areas exhibiting different growth habits and crystal sizes of magnetite may reflect compositionally different local melt domains and/or small differences in the delicate balance between nucleation and growth in domains that had slightly different, although ultrafast, cooling rates. 相似文献
14.
The study of coronitic textures in ferrogabbros and data on rhythmic layering of the Elet’ozero Massif supports the existence of a specific low-temperature Fe-rich liquid in nature. This liquid was formed during solidification of intrusion owing to the local multiple accumulation of Fe and Ti contents in a parental Fe-rich Fe–Ti basaltic melt. According to obtained data, this occurred on micro- and macroscale: 1) in the interglanular (intercumulus) space of the crystallization zone where intercumulus melt becomes rich in Fe and Ti owing to the crystallization of cumulus silicate minerals and is transformed into Fe-rich liquid, which concentrates residual components of an intergranular melt; 2) during formation of rhythmic layering when Fe-rich residual melt is accumulated before the upper part of the moving front of solidification; when Fe content reaches a certain limit, the melt is also transformed in a separate Fe-rich liquid, the interlayers of which form the upper (lowest temperature) members of rhythms. It was concluded that the emergence of a Fe-rich melt is related to its specific structure, which is formed when the Fe content reaches certain critical values in a liquid. Thus, this liquid is not a product of immiscible splitting of a melt, but represents a peculiar phenomenon. The preservation of primary textures and structures of the rocks is supposedly related to the lyophobic properties of surfaces, i.e., “repulsion” of nonwetting liquid by facets of cumulus crystals, especially plagioclase. Owing to this, the drops and even horizons of heavy Fe-rich liquid are retained in situ of their formation. 相似文献
15.
A critical study of 311 published WR chemical analyses, isotopic and mineral chemistry of anorthosites and associated rocks
from eight Proterozoic massif anorthosite complexes of India, North America and Norway indicates marked similarities in mineralogy
and chemistry among similar rock types. The anorthosite and mafic-leucomafic rocks (e.g., leuconorite, leucogabbro, leucotroctolite,
anorthositic gabbro, gabbroic anorthosite, etc.) constituting the major part of the massifs are characterized by higher Na2O + K2O, Al2O3, SiO2, Mg# and Sr contents, low in plagioclase incompatible elements and REE with positive Eu anomalies. Their δ
18O‰ (5.7–7.5), initial 87Sr/86Sr (0.7034–0.7066) and ɛ
Nd values (+1.14 to +5.5) suggest a depleted mantle origin. The Fe-rich dioritic rocks occurring at the margin of massifs have
isotopic, chemical and mineral composition more close to anorthosite-mafic-leucomafic rocks. However, there is a gradual decrease
in plagioclase content, An content of plagioclase and XMg of orthopyroxene, and an increase in mafic silicates, oxide minerals content, plagioclase incompatible elements and REE from
anorthosite-mafic-leucomafic rocks to Fe-rich dioritic rocks. The Fe-rich dioritic rocks are interpreted as residual melt
from mantle derived high-Al gabbro melt, which produced the anorthosite and mafic-leucomafic rocks. Mineralogically and chemically,
the K-rich felsic rocks are distinct from anorthosite-mafic-leucomafic-Fe-rich dioritic suite. They have higher δ
18O values (6.8–10.8‰) and initial 87Sr/86Sr (0.7067–0.7104). By contrast, the K-rich felsic suites are products of melting of crustal precursors. 相似文献
16.
本文报道了攀枝花钒钛磁铁矿含矿岩体边缘岩相带中的苦橄玢岩和岩体中淡色辉长岩的锆石微量元素特征。结果表明二者所含锆石都具有明显的Ce正异常和Eu负异常,以及轻稀土元素亏损和重稀土元素富集的特征,其Th/U比值为0.35～3.23,都属于典型的岩浆锆石。本次研究利用最新实验标定的锆石氧逸度计对苦橄玢岩和淡色辉长岩的氧逸度进行了估算。估算结果表明苦橄玢岩和淡色辉长岩均具较高的氧逸度,分别为QFM+0.3～QFM+2.5和QFM+0.7～QFM+3(QFM为石英-铁橄榄石-磁铁矿缓冲剂)。苦橄玢岩作为来自深部岩浆房侵入到攀枝花主岩体的富橄榄石"晶粥体",其高氧逸度的特征反映出攀枝花岩体的原生岩浆以及地幔源区是相对氧化的,而导致这一结果的原因很可能与古老俯冲事件导致的地幔交代作用有关。通过地幔柱-岩石圈相互作用,在较高氧逸度下发生部分熔融形成了铁质苦橄岩及其堆晶作用产物苦橄玢岩。此外,淡色辉长岩的氧逸度也显示出较高的特征,这说明这种氧化的特征很可能是贯穿了整个成岩过程的,对钒钛磁铁矿成矿,特别是导致铁钛氧化物早期结晶起到了不可忽视的作用。 相似文献
17.
J. G. Brophy Michael J. Dorais Julie Donnelly-Nolan Bradley S. Singer 《Contributions to Mineralogy and Petrology》1996,126(1-2):121-136
The rhyolite of Little Glass Mountain (73–74% SiO2) is a single eruptive unit that contains inclusions of quenched andesite liquid (54–61% SiO2) and partially crystalline cumulate hornblende gabbro (53–55% SiO2). Based on previous studies, the quenched andesite inclusions and host rhyolite lava are related to one another through fractional crystallization and represent an example of a fractionation-generated composition gap. The hornblende gabbros represent the cumulate residue associated with the rhyolite-producing and composition gap-forming fractionation event. This study combines textural (Nomarski Differential Interference Contrast, NDIC, imaging), major element (An content) and trace element (Mg, Fe, Sr, K, Ti, Ba) data on the style of zonation of plagioclase crystals from representative andesite and gabbro inclusions, to assess the physical environment in which the fractionation event and composition gap formation took place. The andesite inclusions (54–61% SiO2) are sparsely phyric with phenocrysts of plagioclase, augite and Fe-oxide±olivine, +/–orthopyroxene, +/–hornblende set within a glassy to crystalline matrix. The gabbro cumulates (53–55% SiO2) consist of an interconnected framework of plagioclase, augite, olivine, orthopyroxene, hornblende and Fe-oxide along with highly vesicular interstitial glass (70–74% SiO2). The gabbros record a two-stage crystallization history of plagioclase+olivine+augite (Stage I) followed by plagioclase+orthopyroxene+ hornblende+Fe-oxide (Stage II). Texturally, the plagioclase crystals in the andesite inclusions are characterized by complex, fine-scale oscillatory zonation and abundant dissolution surfaces. Compositionally (An content) the crystals are essentially unzoned from core-to-rim. These features indicate growth within a dynamic (convecting?), reservoir of andesite magma. In contrast, the plagioclase crystals in the gabbros are texturally smooth and featureless with strong normal zonation from An74 at the core to around An30. K, and Ba abundances increase and Mg abundances decrease steadily towards the rim. Ti, Fe, and Sr abundances increase and then decrease towards the rim. The trace element variations are fully consistent with the two-stage crystallization sequence inferred from the gabbro mineralogy. These results indicate progressive closed-system in situ crystallization in a quiescent magmatic boundary layer environment located along the margins of the andesite magma body. The fractional crystallization that generated the host rhyolite lava is one of inward solidification of a crystallizing boundary layer followed by melt extraction and accumulation of highly evolved interstitial liquid. This mechanism explains the formation of the composition gap between parental andesite and rhyolite magma compositions. 相似文献
18.
Patricia J. Loferski Richard J. Arculus 《Contributions to Mineralogy and Petrology》1993,114(1):63-78
Multiphase inclusions, consisting of clinopyroxene+ilmenite+apatite, occur within cumulus plagioclase grains from anorthosites in the Stillwater Complex, Montana, and in other rocks from the Middle Banded series of the intrusion. The textures and constant modal mineralogy of the inclusions indicate that they were incorporated in the plagioclase as liquid droplets that later crystallized rather than as solid aggregates. Their unusual assemblage, including a distinctive manganiferous ilmenite and the presence of baddeleyite (ZrO2), indicates formation from an unusual liquid. A process involving silicater liquid immiscibility is proposed, whereby small globules of a liquid enriched in Mg, Fe, Ca, Ti, P, REE, Zr and Mn exsolved from the main liquid that gave rise to the anorthosites, became trapped in the plagioclase, and later crystallized to form the inclusions. The immiscibility could have occurred locally within compositional boundaries around crystallizing plagioclase grains or it could have occurred pervasively throughout the liquid. It is proposed that the two immiscible liquids were analogous, n terms of their melt structures, to immiscible liquid pairs reported in the literature both in experiments and in natural basalts. For the previously reported pairs, immiscibility is between a highly polymerized liquid, typically granitic in composition, and a depolymerized liquid, typically ferrobasaltic in composition. In the case of the anorthosites, the depolymerized liquid is represented by the inclusions, and the other liquid was a highly polymerized aluminosilicate melt with a high normative plagioclase content from which the bulk of the anorthosites crystallized. Crystallization of the anorthosites from this highly polymerized liquid accounts for various distinctive textural and chemical features of the anorthosites compared to other rocks in the Stillwater Complex. A lack of correlation between P contents and chondrite-normalized rare earth element (REE) ratios of plagioclase separates indicates that the amount of apatite in the inclusions is too low to affect the REE signature of the plagioclase separates. Nevertheless, workers should use caution when attempting REE modelling studies of cumulates having low REE contents, because apatite-bearing inclusions can potentially cause problems. 相似文献
19.
The Potato River intrusion is a Keweenawan (1100 Ma) mafic plutonemplaced in Keweenawan volcanics and earlier Proterozoic metasedimentaryrocks along the southeastern flank of the Lake Superior syncline.It comprises the following lithostratigraphic zones: a thinto absent Border zone of altered olivine gabbro; a Lower zoneof olivine gabbro; a Picritic zone of picrite and troctolite;a Middle zone of olivine gabbro and leucogabbro; an Upper zoneof quartz leucogabbro and ferrogabbro; and a Roof zone of granophyricand granitic rocks. Fractional crystallization is evident fromcompositional changes in the rocks and cumulus minerals withstratigraphic height. Elements concentrated in the cumulus mineralsolivine and plagioclase (Mg, Fe2+, Al, Ca, Ni, Co, Cr, Sr) decreasewith height; elements concentrated in the trapped liquid (Na,K, La, Y, Zr, Nb, Rb, Ba) increase with height; and other elements(Ti, Fe3+, P, Ga, V, Sc, Cu, Zn) show complicated behavior relatedto the appearance of additional cumulus phases such as clinopyroxene,Fe-Ti oxides, and apatite. Lower zone rocks contain some sulfide,probably from sulfur derived from the country rock, and theUpper zone has sulfides probably precipitated from an immisciblesulfide liquid. The sulfide-bearing rocks have similaritiesto those of other intrusions, such as Bushveld, Stillwater,and Skaergaard. The picritic and troctolitic rocks of the Picritic zone indicatethat the intrusion was open to additional injections of maficmagma. Roof zone granophyric rocks are residual liquids intrudedalong the upper margin of the intrusion during regional tilting,but Roof zone granitic rocks are probably melted country rock.An attempt is made to estimate by reverse stratigraphic summationthe compositional path of the magma that solidified above thePicritic zone. The first compositions are highly aluminous,which suggests that the upper part of the intrusion has beenenriched in plagioclase by convection-aided crystal sorting.A complementary unit of mafic rocks is not exposed, but it couldbe present down dip. Some of the later compositions are similarto typical Keweenawan high-Al tholeiites. The magma did notundergo extreme iron enrichment, probably because of oxygenfugacity buffering. 相似文献