青海上庄含碳酸岩杂岩体的地球化学特征及其地质意义

Geochemical Characteristics and Geological Significance of the Shangzhuang Carbonate Complex, Qinghai

中文关键词:  上庄  杂岩体  碳酸岩  地球化学特征  地质意义

英文关键词:Shangzhuang  Complex  Carbonatite  Geochemical characteristics  Geological significance

中文摘要:

      青海上庄杂岩体为磷、铁、稀土矿化岩体,且磷矿石是磷肥生产的重要材料。上庄杂岩体包括超镁铁质岩、碱性正长岩和碳酸岩,碳酸岩主要为方解石碳酸岩,次为白云石碳酸岩,前人对辉石岩进行了详细的研究,然而,辉石岩-正长岩-碳酸岩整体的岩石成因和矿化机制还不清楚。本文开展了岩相学、全岩主微量元素、C-O-Sr-Nd同位素的研究。岩相学研究表明,岩石形成顺序为辉石岩-正长岩-碳酸岩。方解石碳酸岩的Sr、Ba含量分别为10928.13ppm、14293.18ppm,δ₁₃^C值为-4.72‰,表明其为火成碳酸岩。辉石岩-正长岩-碳酸岩杂岩体富集轻稀土元素(LREE)和大离子亲石元素(LILE)Rb、Sr、Ba,高场强元素(HFSE)Nb、Ta、Zr、Hf亏损,相似的微量元素配分型式及Sr-Nd同位素特征表明杂岩体具共同的地幔源区,该地幔源区曾被俯冲物质交代而富集。研究表明,上庄偏碱性辉石岩-碱性正长岩-碳酸岩杂岩体形成于俯冲向伸展环境转变的构造体制,幔源偏碱性碳酸质基性岩浆由于镁铁质矿物及磷灰石结晶形成Fe-P-REE富集的单斜辉石岩,残余熔体成分向长英质端元分异,形成正长岩,随后碳酸岩熔体可能由不混溶作用产生,形成碳酸岩,碳酸岩出溶的流体对正长岩交代可能是交代正长岩具较高稀土含量的原因。

英文摘要:

      The Shangzhuang complex in Qinghai is a mineralized rock body of phosphorus, iron and rare earth, and phosphorus ore is an important material for phosphate fertilizer production. The Shangzhuang complex consists of ultramafic rocks, alkaline syenites and carbonatites. The carbonatites are mainly calcite carbonatites, others are dolomite carbonatite. The predecessors have studied the pyroxenite in detail. However, the petrogenesis and mineralization mechanism of the pyroxenite syenite carbonatite as a whole are still unclear. In this paper, petrography, major and trace elements of the whole rock, and C-O-Sr-Nd isotopes have been studied. Petrographic study shows that the sequence of rock formation is pyroxenite-syenite-carbonatite. The Sr and Ba contents of calcite carbonatite are 10928.13ppm and 14293.18ppm respectively, The δ¹³C value is -4.72 ‰, indicating that it is igneous carbonate rock. The pyroxenite syenite carbonatite complex is rich in light rare earth elements (LREE) and large ion lithophile elements (LILE) Rb, Sr, Ba, and depleted in high field strength elements (HFSE) Nb, Ta, Zr, Hf. Similar trace element distribution patterns and Sr Nd isotope characteristics indicate that the complex has a common mantle source area, which was enriched by subduction materials. The study shows that the Shangzhuang subalkaline pyroxenite alkali syenite carbonatite complex was formed in the tectonic system of subduction to extension. The mantle derived subalkaline carbonatite basic magma formed Fe-P-REE enriched clinopyroxenite due to the crystallization of mafic minerals and apatite. The composition of the residual melt differentiated towards felsic end elements, forming syenite. Subsequently, the carbonatite melt may be produced by immiscibility, forming carbonatite, The metasomatism of syenite by the exsolution fluid of carbonatite may be the reason why the metasomatic syenite has high REE content.