甘肃礼县新生代火山喷发碳酸岩的发现及意义被引量：35

Discovery and significance of Cenozoic volcanic carbonatite in Lixian, Gansu province.

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摘要甘肃礼县新生代钾霞橄黄长岩中有大量火山喷发碳酸岩出露,本文对该区各种碳酸岩的地质产状、岩石特征、全岩化学及稀土、微量和C、O同位素地球化学进了研究,结果表明本区碳酸岩属于一种高CaO低碱(Na_2O+K_2O)的火成碳酸岩,其化学成分与东非裂谷乌干达Fort Portal地区的碳酸岩相似。其中碳酸岩的火山砾岩和凝灰岩具有高的SiO_2和Mg/Ca比值,化学成分显示了硅酸盐和碳酸盐混合的特征,可能代表直接由地幔部分熔融形成的原生碳酸盐岩浆。其它类型碳酸岩的成因与原生碳酸盐岩浆或原生钾霞橄黄长岩浆的液态不混溶作用和/或结晶分异作用有关。碳酸岩及共生的高钾火山岩不是典型的大陆裂谷岩浆作用的产物,其起源和成因与软流圈的上涌有关。碳酸岩的发现为查明本区地幔的组成和性质提供了新的岩石学证据。 Volcanic carbonatites are very rare igneous rocks in the earth surface, which close paragenesis with nephelinite, melilitic rocks always outcroped continental rifting area. A lot of Cenozoic volcanic carbonatites in Baihe area of Lixian county, Gansu province outcroped in Cenozoic kamafugite. The carbonatite cantain these rock types of lapilli, tuff and ignimbrite, lava and ball etc. The carbonatites composed by calcites and rare clinopyroxene, nepheline, apatite and phlogopite in mineralogy, and belong to a sovite according to Le Maitre's A Classification of Igneous Rocks and Glossary of Terms. The whole rock chemical composition of carbonatite has a characteristics rich in CaO, but pool in alkaline (Na2O + K2O), and is very like to high-Ca, low-alkali carbonatite from Fort Portal area, Uganda. However, the major element chemistry of each type of carbonatite is different, such as content of SiO2 is decreasing gradually from black lapilli to ball to ignimbrite/tuff to lava to the carbonatite filled in vesicles. The content of TiO2, Al2O3, FeO and Fe2O3, MgO in these rocks have a positive correlation with content of SiO2, but only CaO in the rocks has a negative correlation with SiO2. The carbonatites have a LREE-rich pattern in the figure of chondrite-normalized REE-pattern, but the abundance of LREE, SigmaREE and rare earth elememts(especialy Sr, Ba) in the rocks have a positive correlation with SiO2 like to major elements of the rocks. Carbon and oxygen isotopic compositions of whole-rock samples of the carbonatite lie within ranges of -9.58 to 0.71 for delta(13)C and 12.59 to 20.58 for delta(18)O. These delta(13)C and delta(18)O values of the carbonatite support the conclusion that the carbonatite was products of magmatism and origined from the mantle. However, the isotopic composition of some carbonatite was changed by secondary isotopic exchange. Exposure to alteration under atmospheric conditions and weathering at the surface produced higher delta(18)O and delta(13)C values distinct from primary carbonatite. The researchs show that the carbonatites and high potassic volcanic rocks in this area are not a magmatic products of continental rifting, its origin and genesis may be related to uplifting of asthenosphere. The discovery of the WQL carbonatite provide a new petrological evidence to understand chemical composition and characteristics of the WQL mantle.