The lujavrite and khibinite bodies in the Poços de Caldas alkaline massif, southeastern Brazil: a structural and petrographic study
The lujavrite and khibinite bodies in the Poços de Caldas alkaline massif, southeastern Brazil: a structural and petrographic study
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巴西东南部 Poços de Caldas 碱性地块中的 lujavrite 和 khibinite 体：结构和岩相研究
DOI：
10.5327/rbg.v30i4.885
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发表时间：
2000
期刊：
影响因子：
--
通讯作者：
M. Ulbrich
中科院分区：
文献类型：
--
作者：
H. Ulbrich;M. Ulbrich
在巴西东南部碱性Pocos de Caldas地块的北缘，出露了两个小型的榴辉石-千宾岩体，一个东部，一个西部。详细的填图显示，在暴露较好的西部身体的中心，有一个粗粒、中级、片麻状的真透辉石霞石正长岩(绿榴石)，具有强烈的次水平面理(Lu I)，在接触处显示出更细的边界褪色(Lu II)。两个粗面霞石正长岩(Nes I和Nes II)作为中央绿榴石的包裹体出现，紧随其后的是粗粒真透析岩(Nes III或khibinite)的外壳。两者的内部结构都是一个碟形结构，由鲁一-鲁二-北东向二组成连续的叶状贝壳(东部部分不存在)，在不同相间的接触处有较陡峭的倾角，都被外北东向三号面包围。围岩为锡语言岩和灰色北东向(北东向四)。Tguaite是最早的岩石(大约76-80 Ma前侵入)，其次是较粗的岩石，Nes IV侵位在褐铁矿及其伴生类型之前。榴辉石岩体侵位是一个连续的过程，始于强无牌岩浆的强烈亚水平侵位。岩浆上涌挤压了初始岩浆批次，通过压实和液体提取形成了具有片状亚水平结构(Lu I)和外壳(Lu II)的绿榴石盖层。同时，侵入作用迫使岩浆室发生横向扩张。在这个地块中，埃派提斯岩浆总是以晚期侵入的形式出现。与其他产状(伊利莫萨克、洛沃零、皮兰斯贝格、德克萨斯跨佩科斯省)的对比表明，在强有力的条件下侵位可能是形成褐铁矿最终组构的控制因素。
Two small lujavrite-khibinite bodies, an eastern and a western one, are exposed at the northern edge of the alkaline Pocos de Caldas massif, southeastern Brazil. Detailed mapping reveals at the center of the better exposed western body a coarse-grained, mesocratic, gneissic-looking eudialyte nepheline syenite (lujavrite) with a strong subhorizontal foliation (Lu I), showing at the contacts a finer-grained border fades (Lu II). Two trachytoid nepheline syenites (NeS I and II) occur as an envelope to the central lujavrites, followed by an outer shell of coarsegrained eudialyte NeS (NeS III or khibinite). The internal structure of both bodies is that of a saucer, with successive foliated shells (in part absent in the eastern body) constituted by Lu I - Lu II- NeS I-NeS II, with rather steep dips at the contacts between the different fades; all are surrounded by the outer NeS III. Country rocks are tinguaites and a grey NeS (NeS IV). Tinguaites were the earliest rocks (intruded some 76-80 Ma ago), followed by the coarser rocks, with NeS IV emplaced before the lujavrites and associated types. The emplacement of the lujavrite bodies was a continuous process, initiated with forceful subhorizontal intrusion of strongly agpaitie magmas. Magma upwelling compressed the initial magma batch, generating the lujavrite caps by compaction and liquid extraction, with a foliated subhorizontal structure (Lu I) and an outer shell (Lu II). The intrusion forced at the same time a lateral expansion of the magma chamber. In this massif, agpaitie magmas appear always as late intrusions. A compareson with other occurrences (Ilimaussaq, Lovozero, Pilansberg, the Texan Trans-Pecos province) suggests that emplacement under forceful conditions may be a controlling factor in shaping the final fabric of lujavrites.