西藏拉萨地块MORB型榴辉岩的岩石地球化学特征

MORB-type eclogites in the Lhasa block, Tibet, China:Petrochemical evidence.

松多榴辉岩的SiO2、Al2O3含量分别为45.3%～50.6%、12.93%～14.27%,K2O+Na2O为1.3%～2.8%,TFeO/MgO为1.11～2.02,CaO为10.4%～14.47%,K2O<0.3%,Na2O为0.97%～2.47%。总体上具有玄武质或辉长质岩石的成分特征,属于钙碱性系列,具有MORB的特征,表明其原岩来自大洋环境。松多榴辉岩大多数样品的稀土元素配分模式显示轻稀土元素亏损,无明显的Eu异常,重稀土元素平坦。微量元素蛛网图均显示从Zr到Yb属于平坦型,基本上未发生分馏。松多榴辉岩的LaN/YbN比值为0.19～0.56,LaN/CeN为0.75～0.81,LaN/SmN为0.37～0.81,三者均小于1;Hf/Th除了石英榴辉岩中的06Y-344样品为6.81外,其他样品为11.10～33.97,均大于8;Ce/Nb为2.72～19.02,均大于2;Th/Yb为0.01～0.10,均小于或等于0.1,也说明其原岩具有典型MORB的特征。锆石SHRIMPU-Pb定年获得松多榴辉岩的变质年龄为261Ma±5.3Ma,推测其原岩可能为古特提斯洋盆的残留。

The Sumdo eclogite occurs in the Lhasa block, over 100 km northeast of the city of Lhasa. Most of the eclogites are massive, coarse-grained and fresh, and three distinct types are recognized： phengite eclogite, quartz eclogite and eclogite. All of the rocks have whole-rock chemical compositions similar to those of tholeiitic MORB with 45.3-50.6 wt% SiO2, 12.93-14.27 wt% Al2O3 and 1.3-2.8 wt% total alkalis （K2O＋Na2O）. Generally, it has the compositional features of basaltic or gabbroic rocks, belonging to the calc-alkaline suite, and shows the features of MORB, indicating that its protolith came from the oceanic environment. The chondritenormalized REE patterns of most samples show depletion in LREE and no pronounced Eu anomaly, with flat HREE patterns. The MORB-normalized trace element spidergrams show depletion in LREE, with flat patterns from Zr to Yb, and no fractionation of the fi-action. The LaN/YbN, LaN/CeN and LaN/SmN ratios are 0.19-0.56, 0.75-0.81 and 0.37-0.81 respectively, the three ratios being all 〈 1. The Hf/Th ratios ofaU samples are 11.10-33.97, being 〉8, except sample 06Y-344 from quartz eclogite, which is 6.81. The Ce/ Nb ratios are 2.72-19.02, all being 〉2. The Th/Yb ratios are 0.01-0.10, all being ≤ 1. This also suggests that its protolith has the features of typical MORB. SHRIMP U-Pb dating of zircons yields a metamorphic age of 261.7±5.3 Ma for the Sumdo eclogite, suggesting that its protolith was probably remnants of the Paleo-Tethys ocean basin and that the eclogite formed by subduction of oceanic crust.