南极乔治王岛晚白垩世地幔源火山岩Nd、Sr同位素和微量元素证据

对乔治王岛上白垩统半三角组火山岩样品进行了 Rb- Sr同位素等时线年龄测定 ,所获结果为 71 .3 3± 0 .3 Ma。通过 εND(T) - 14 7Sm/14 4Nd,14 3 Nd/14 4Nd- 87Sr/86 Sr,87Sr/86 Sr- Sr和 87Sr/86 Sr- K2 O/(K2 O+Na2 O)的相关性研究表明 ,该火山岩物质主要来自亏损地幔 (DMM)源区 ,基本上未受到地壳物质混染。其中 6个样品的平均 Sm- Nd模式年龄 (TNd DM)为 443 .3± 2 0 .6 Ma,可能反映了在研究地区的岩浆源区化学分异事件的时代。微量元素的特点表明 ,半三角组样品属典型的钙碱性火山岩系列 ,与长城站地区第三系火山岩相类似 ,同属于岛弧火山活动的产物。

EVIDENCES OF SM-ND AND RB-SR ISOTOPES AND TRACE ELEMENT FOR LATE CRETACEOUS VOLCANIC ROCKS FROM DEPLETED MANTLE SOURCE IN KING GEORGE ISLAND,ANTARCTICA

Twelve tuffite samples dealt with in the present paper were collected by the 4th Chinese Antarctic Expedition(1987-1988) from Upper Cretaceous Half Point Formation on the southeast edge of Fildes Peninsula,King George Island(Shen,1991,1992)(Fig.1). The samples were decomposed in Teflon bombs.Rb, Sr and REE were separated from major elements with cation column. Total chemical blanks for Nd and Sr are <5-7×10 -11 g/g and < 2×10 -9 g/g respectively. The isotopes of Rb, Sm and Nd were measured through the VG354 mass specrtometer in Modern Analytical Center. Nd and Sr isotopic ratios take 146 Nd/ 142 Nd=0.636151 and 86 Sr/ 88 Sr =0.1194 as the normalized values respectively. The 87 Sr/ 86 Sr ratio determined in the center is 0.71022±4(2δ). 143 Nd/ 144 Nd ration determined by USGSBCR-1 is 0.51187. 87 Sr/ 86 Sr ratio determined by NBS 607 is 1.20014+10(2δ). The isotopic analysis of Rb, Sr, Sm and Nd leads to the results as shown in Table 1,2, The Rb-Sr isochron of 12 whole rock samples gives t=71.3±0.3 Ma, I=0.70314 and R=0.99998(Fig.2).The ε Sr (T) and ε Nd (T) values range from -17.9 to -18.2 and +6.04 to +6.66 respectively; the f Rb/Sr and f Sm/Nd from-0.76 to +9.59 and -0.265 to -0.303; the ε Sr (0) and ε Nd (0) from -6.64 to -18.9 and +5.53 to +6.15 respectively. The Sm-Nd model age (T Nd DM ) range from 409.3Ma to 471.1Ma. A good linearity (R) of Rb-Sr isotopic isochron revealed as figure 2 and the identity of ( 87 Sr/ 86 Sr) i values shown in table 3 of these sampkes reflect the homogenization of Sr isotope of the source region. Thus, the value of 71.33±0.3Ma by Rb-Sr whoke-rock isochron is adopted as the crystallization age of the rocks or as the age of the volcanic eruption. As shown in the figure 6,the samples have higher 143 Nd/ 144 Nd (0.512922-0.512954),lower 87 Sr/ 86 Sr (0.70318-0.703320) and slight negative 143 Nd/ 144 Nd- 87 Sr/ 86 Sr relations which fall in the typical range of oceanic island basalts or in the vicinity of the boundary between MORB and other oceanic island basalts. Covariance of Al 2O 3/ TFe 2O 3 ratio, TiO 2(wt%) content and Th(ppm) content with 87 Sr/ 86 Sr suggest that these volcanic rocks were contaminated by a litte or no crustal components.The low ( 87 Sr/ 86 Sr) i ration are not attributed to the result of seawater rock interaction and indicate that the samples belong to the continental origin tather than the marine facies ones. The average T Nd DM of the samples is 444.3±20.6Ma, obviously older than Rb-Sr age, which reflects crust-mantle mixed or the crustal preserved age and indicated the possibilities of the existence of lower Palaeozoic basement.The high content of the Al 2O 3 in the rocks may be closely related to the degrees of lipuation of the mantle. MgO has a positive correlation with some incmpatible elements and has as Pb,Sr,Li,Nb and Hf a negative correlation with Y, Th, Ta, U, Zr and Br reflecting the relation of the continued evolution of the magmas. The different geochemical features and the diagrams to determine the structural environment indicat that the samples consist mainly of calc-alkaline basalts (CAB) which belong to the volcanic are basalts erupting from the convergent plate margin. Therefore, the volcanic rocks of the Half Three Point Formation were formed from the mantle-derived magmas during the island are volcanic activities.