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在甘肃北山西三羊井东一带产有一种特殊类型的碳酸盐质岩体。该岩体呈不规则带状、小岩瘤状产出,岩体与围岩之间具明显的港湾状、枝状侵入接触、岩石不具层理、岩石表现出的细粒结晶等粒结构、岩体内见围岩捕掳体(碳酸岩内部发育的部分捕掳体与围岩成分一致)及与其伴生的超镁铁质岩(橄榄岩)等特征均表明其为火成碳酸岩岩体。岩石地球化学显示,岩石具富CaO(54.41%~54.52%)、CO_2(43.36%~43.39%),贫SiO_2、TiO_2、MgO、MnO、Fe_2O_3、FeO、Na_2O、K_2O的特点,CaO/(CaO+MgO+FeO+Fe_2O_3+MnO)比值为0.99,为方解石碳酸岩;岩石稀土总量低(ΣREE=11.20×10~(-6)~12.00×10~(-6))、铕负异常(δEu=0.30~0.33)明显,轻重稀土略有分馏(LREE/HREE为4.11~4.85,(La/Yb)N为9.30~9.70),明显不同于幔源成因的碳酸岩,也不同于沉积成因的碳酸盐岩,而与壳源成因的碳酸岩类似。根据上述特征并结合区域地质背景,认为该碳酸岩为沉积碳酸盐岩在内侵幔源岩浆作用下发生熔融而成的火成碳酸岩。
There is a special type of carbonate rock mass in the west of Sheyang, Gansu Beishan. The rock mass is irregular belt-shaped, small rock-like output, obvious bay-like, dendritic invaded contact between the rock mass and the surrounding rock, the rock is not stratified, and the rock shows fine grain crystal grain structure, rock In vivo, the entrapment of the surrounding rocks (part of the entrapped body developed in the interior of the carbonate rock in line with the composition of the surrounding rock) and its associated ultramafic rocks (peridotites) are all characterized as igneous rocks. Lithogeochemistry shows that the rocks are characterized by rich CaO (54.41% ~ 54.52%), CO_2 (43.36% ~ 43.39%), poor SiO_2, TiO_2, MgO, MnO, Fe_2O_3, FeO, Na_2O and K_2O. (ΣEV = 11.20 × 10 -6 ~ 12.00 × 10 -6)), negative Eu (δEu = (LREE / HREE: 4.11-4.85, (La / Yb) N: 9.30-9.70), which is obviously different from those of mantle-derived carbonates and carbonates of sedimentary origin Salt rock, similar to the carbonate source rocks. According to the above characteristics and the regional geological background, it is considered that the carbonate rocks are pyrocarbons formed by the melting of sedimentary carbonate rocks under mantle-derived magma.