北山南部二叠纪海相玄武岩地球化学特征及其构造意义

Geochronology and petrogenesis of the Permian marine basalt in the southern Beishan region and their tectonic implications

基于北山南部晚古生代构造演化争议较大,本文对该区4条下—中二叠统代表剖面中的玄武岩进行了岩相学、元素地球化学、及全岩Sr- Nd同位素等研究;并对地层中的中酸性火山岩进行了锆石U- Pb定年。获得英安岩与安山岩的锆石U- Pb年龄分别为289. 5±2. 3Ma和267. 8±3. 1Ma,结合已发表年龄数据及古生物资料,明确了火山岩的喷发时代介于早二叠世亚丁斯克期—中二叠世沃德期。该套玄武岩产于海相地层中,主体为拉斑玄武岩系列;氧化物含量变化较大,TiO 2 含量中等(0. 9%~2. 53%);轻重稀土分馏不明显[(La/Yb) N=1. 6~6. 2],且无显著Eu异常(δ Eu为0. 79~1. 19);不同程度亏损Nb、Ta、Ti、P等元素,富集La、Ce、Nd、Zr等元素;ε Nd ( t )值介于1. 4~10. 9,( 87 Sr/ 86 Sr) i 值为0. 703365~0. 707285。玄武岩源区为受俯冲物质改造的软流圈地幔,经不同程度熔融所形成;既与岛弧或陆缘弧所产生的火山岩有所差别,又不同于地幔柱或热点所产生的玄武岩,在各判别图解中,大多投入板内玄武岩(WPB)或洋脊玄武岩(MORB)。结合该区域其他地质资料,推测该套玄武岩的产生与北山南部加厚岩石圈的拆沉作用有关。

In order to clarify the tectonic evolution of the southern Beishan orogen during the Late Paleozoic, a comprehensive study was carried out on the Permian basalt, andesite, and dacite collected from the southern Beishan area, using whole rock geochemistry, Sr- Nd isotopes and zircon U- Pb age. Zircon LA- ICP MS U- Pb dating yields the ages of 289. 5±2. 3 Ma and 267. 8±3. 1 Ma for dacite and andesite, respectively. Combining with the isotopic ages of volcanic rocks and fossils in the profiles,we suggest that the eruptive age of volcanic rocks ranges from Arinskin to Wordian (290~265Ma). This set of basaltic rocks occurring in marine sequence belongs to tholeiitic series with changeable contents of major elements and moderate concentrations of TiO 2 (0. 9%~2. 53), and show no distinct differentiation of LREE and HREE ((La/Yb) N=1. 6~6. 2) with unobvious Eu anomalies (δ Eu=0. 79~1. 19). They exhibit similar geochemical characteristics including varying degrees of enrichment in La, Ce, Nd, Zr, and depleted in Nb, Ta, Ti, P, etc. The ε Nd ( t ) and ( 87 Sr/ 86 Sr)i values for these basalts vary from +1. 4 to +10. 9,and from 0. 703365 to 0. 707285 respectively. The trace elements characteristics and Sr- Nd isotopic data suggest that the Permian basalts from southern Baishan area derived from asthenosphere hybridized by subduction material with different degree partial melting of garnet- bearing spinel Iherzolite. Discrimination diagram shows that the basalts formed neither in arc (island arc and continental arc) nor in plume- related settings, are similar to WPB (within- plate basalt) and MORB (mid ocean ridge basalt) in discrimination diagrams. Combined with other geological evidence, it can be suggested that the basalts were triggered by delamination of thickened lithosphere after collision.