ABSTRACT

Massive, boudinaged, rose-coloured layers up to one metre in thickness composed of 60–80 vol% spessartine-rich garnet and 15–40 vol% quartz occur in high-pressure, garnet-mica schist on Margarita Island, Venezuela. In the past, such spessartine-garnet-rich rocks have often, although inaccurately, been called ‘coticule’. Apatite can be significant, reaching 20 vol%. Accessory minerals are allanite, rutile (commonly rimmed by ilmenite) and rare phengite. Within the same mica-schist sequence, intercalated calc-silicate layers from one to two metres in thickness exhibit alternating layers, on a sub-mm- to cm-scale, of quartz and greenish layers rich in zoisite, epidote-supergroup minerals and diopside. Hyalophane and barite are present in small amounts. Calc-silicates are commonly pitted, with yellow to dark-brown, gossanous, botryoidal crusts. The composition of garnet in the garnet-rich layers, the calc-silicate layers and their host metasediments is distinctly different. In the garnet-rich layers, compositions are almandine-dominant (Alm_57-63), followed by spessartine (Sps_18-24), grossular (Grs_2-16), pyrope (Prp_6-11) and andradite (Adr_0-6). Garnet in calc-silicate layers can range from almost pure grossular to Grs_45-49Alm_32-34Sps_13-15, and in the enclosing metasediments is typically Alm_71-73Prp_14-15Grs_3-8Sps_4-7And₂. Chondrite-normalized REE patterns of all three rock types are similar in shape, differing only in absolute contents, so that a common sedimentary precursor is indicated. Major and trace element chemistry of the garnet-rich layers are best explained if their pre-metamorphic protolith originated as a mixture of 60–70% normal detrital input and a precipitate from a distal source of hydrothermal activity. The term meta-exhalite is appropriate. The calc-silicate layers are more enigmatic, but an origin as a mixture of a hydrothermal component with a Ca-rich sediment also appears likely. Numerous lines of evidence indicate that the protoliths of the high-pressure rocks of Margarita Island originated in an Early Cretaceous back-arc basin in NW Colombia, so that an exhalite origin for garnet-rich and calc-silicate rocks is a plausible scenario.

摘要

委内瑞拉玛格丽塔岛的高压石榴石云母片岩中出现了由 60-80 vol% 富含锰铝榴石的石榴石和 15-40 vol% 的石英组成的厚达 1 米的巨大的玫瑰色层。过去，这种富含锰铝榴石的岩石经常被称为“球状体”，尽管不准确。磷灰石可以很重要，达到 20 vol%。辅助矿物是紫蓝石、金红石（通常由钛铁矿镶边）和稀有的白铅矿。在同一云母片岩序列中，厚度从 1 米到 2 米的夹层硅酸钙层显示出交替的层，在亚毫米到厘米尺度上，石英层和绿色层，富含黝帘石、绿帘石超群矿物和透辉石. 少量存在透明质烷和重晶石。钙硅酸盐通常有凹痕，具有黄色至深棕色、棉质、葡萄状外壳。富石榴石层、钙硅酸盐层及其主体变质沉积物中石榴石的成分明显不同。在富含石榴石的层中，成分以铁铝榴石为主（Alm_57-63 )，其次是锰铝榴石 (Sps _18-24 )、钙铝榴石 (Grs _2-16 )、镁铝榴石 (Prp _6-11 ) 和红榴石 (Adr _0-6 )。钙硅酸盐层中的石榴石可以从几乎纯钙质到 Grs _45-49 Alm _32-34 Sps _13-15，并且在封闭的变质沉积物中通常是 Alm _71-73 Prp _14-15 Grs _3-8 Sps _4-7和₂. 所有三种岩石类型的球粒陨石归一化 REE 模式在形状上相似，仅在绝对含量上有所不同，因此表明了一个共同的沉积前体。如果富含石榴石层的主要和微量元素化学起源于 60-70% 的正常碎屑输入和来自远端热液活动源的沉淀物的混合物，则它们的主要和微量元素化学可以得到最好的解释。术语元呼出物是合适的。钙硅酸盐层更神秘，但也可能起源于热液成分与富含钙的沉积物的混合物。大量证据表明，玛格丽塔岛高压岩石的原岩起源于哥伦比亚西北部的早白垩世弧后盆地，因此富含石榴石和钙硅酸盐岩石的喷出物起源是一个合理的假设。