白云鄂博碳酸岩的方解石-白云石地质温度计

Calcite-dolomite geothermometry of Bayan Obo carbonatites

利用方解石-白云石地质温度计对白云鄂博地区碳酸岩的平衡温度进行了测定。出露于东矿下盘的白云岩质火山岩和出露于尖山的方解石-白云石型火山岩获得了较高的温度,分别为681℃和648℃。这些样品中的方解石呈二十微米左右晶形较完整的小片,被稍大粒度的白云石颗粒包裹,未受交代作用影响,推测这种碳酸岩在快速冷却的情况下保存下了其岩浆侵位时的成分特点,从而指示出接近碳酸岩浆侵位时的温度。但本区多数碳酸岩的平衡温度在400～500℃之间,有下列三种情况:(1)具有自形-半自形中粗粒粒状变晶结构的碳酸岩最后的平衡温度为415～496℃;(2)产自东矿的其余样品(火山岩),所测最后平衡温度为431～485℃,在测温的微区范围内可见极细粒白云石方解石与稀土等矿物共生的现象;(3)为交代重结晶结构的碳酸岩明显受到后期热液流体的交代,在流体的作用下共生方解石和白云石在成分上达到新的平衡,平衡温度为432～507℃。本文所分析的样品多数结果(371～507℃)与用白云石(方解石)和磁铁矿氧同位素温度计对白云鄂博碳酸岩的计算结果(360～546℃)十分一致。虽然有研究者对方解石-白云石温度计用于火成碳酸岩表示过质疑,但本文资料表明火成碳酸岩最后的平衡温度是可以运用方解石-白云石温度计法来计算的。

Calcite-dolomite geothermometry has been used to determine the equilibrium temperature of the Bayan Obo carbonatite. The dolomite at East ore body and the calcite-dolomite dyke at Jianshan yeild higher temperatures of 681℃and 648℃respectively,in which the dolomite grains host the small patches of calcite which seem to be less influenced by metasomatism and preserving the igneous signature related to the carbonatite magma.Therefore,these temperatures are more closing to that of the original carbonatite magmas. The calcite-dolomite thermometry of some medium-coarse-grained euhedral-subhedral granular carbonatites have the temperatures of 415～496℃.Some carbonatite volcanics of the East ore body are comprised of some very fine-grained carbonate minerals associated with REE minerals,the calculated equilibrium temperatures are of 431～485℃.Some carbonatites with metasomatized-relict texture have the temperatures of 432～507℃,indicating that which may be affected by epigenetic hydrothermal solution which reacting with carbonate minerals change their composition.Most of equilibrium temperatures(371～507℃) obtained in this study are consistent with the temperatures(360～546℃) obtained by carbonate-magnetite thermometric pairs of the Bayan Obo carbonatites suggesting that the calcite-dolomite geothermometry can be used for the carbonatite.