桂北融水地区钾镁煌斑岩Ar- Ar年代学、地球化学特征及其构造意义

桂北地区发育较多的钾镁煌斑岩脉，可能为该区近源金刚石重砂矿的母岩，对于其形成时代一直是长期争议悬而未决的重要地质问题。本文研究表明，融水地区金云钾镁煌斑岩的金云母(斑晶、基质) 40 Ar- 39 Ar冷却封闭年龄为203~204 Ma，表明其形成于晚三叠世。岩石SiO 2为55. 68%~56. 85%，高钾(K 2O/Na 2O=1. 87~3. 06)、高Mg # (71. 90~73. 20)、轻稀土富集、重稀土相对亏损，轻重稀土分异明显，Eu略亏损，富集大离子亲石元素，亏损高场强元素，地球化学特征表明其来源于俯冲流体交代的富集地幔特征。上述研究表明，扬子克拉通东南缘桂北地区中生代岩石圈地幔为经历俯冲流体交代的富集地幔，并于晚三叠世经历了重要的岩石圈伸展减薄构造事件，其形成可能与古太平洋板块平板式俯冲有关。

Geochemistry and Ar- Ar geochronology of the lamproites from the Rongshui area, North Guangxi province and their tectonic implications

Many lamprophyre dykes occurring in north Guangxi province are probably the source rock for proximal diamond placer deposit, but their formation age has long been a pending geological issue. This study shows that phlogopites (phenocryst and matrix) in lamproite in the Rongshui area have a 40 Ar- 39 Ar cooling age of 203~204 Ma, indicating that they formed in late Triassic. Analytical results show that the dykes are characterized by SiO 2 content of 55. 68%~56. 85%, high K (K 2O/Na 2O=1. 87~3. 06) and high Mg# (71. 90~73. 20), enrichment of LREE and relative depletion of HREE with distinct differentiation between LREE and HREE, slightly Eu anomaly, enrichment of large ion lithophile elements (LILE) and depletion of high field strength elements (HFSE), geochemically suggesting that the dykes sourced from the enriched lithospheric mantle metasomatized by subduction- related fluids. This study shows that the Mesozoic lithospheric mantle under the southwestern margin of the Yangtze Carton is the an enriched mantle which experienced metasomatization of subduction fluid and thinning event due to lithospheric extension in the late Triassic, and its formation was probably related to the NW- trending subduction of the paleo- Pacific plate.