豆荚状铬铁矿:古大洋岩石圈残片的重要证据  

PODIFORM CHROMITITES: A KEY TO IDENTIFY THE ANCIENT OCEANIC LITHOSPHERIC RELICTS

作　　者：李江海[1] 牛向龙[1] 黄雄南[1] 冯军[1] 

机构地区：[1]北京大学地球与空间科学学院地质学系,北京100871

出　　处：《地学前缘》2002年第4期235-246,共12页Earth Science Frontiers

基　　金：国家自然科学基金资助项目 (4 983 2 0 3 0 );北京大学985资助项目

摘　　要：豆荚状铬铁矿为蛇绿岩的特征性矿产 ,保留了上地幔岩浆构造作用、高温变形以及岩石成因的重要信息。它们常见于方辉橄榄岩内 ,位于大洋岩石圈莫霍面下 1～ 2km的古深度范围内。豆荚状铬铁矿常被纯橄岩薄壳围限 ,保留特征的豆状、豆壳状等构造。豆荚状铬铁矿的TiO2 含量较低 ,铂族元素 (PGE)的分布模式显示特征的负斜率。普遍认为 ,豆荚状铬铁矿形成于部分熔融条件下 ,涉及原始地幔熔体与亏损地幔橄榄岩的相互作用 ,伴随复杂的岩浆混合及结晶过程。狭窄的上地幔岩浆通道或孔穴为豆荚状铬铁矿理想的堆积部位。超俯冲带 (弧后盆地、岛弧、弧前 )、大洋中脊、转换断层均可能是豆荚状铬铁矿形成的理想环境。其中 ,洋脊扩张模式及大洋上俯冲带模式较好地解释了豆荚状铬铁矿成因。对于经历高级变质及多期变形的华北大陆基底 ,豆荚状铬铁矿是研究古老蛇绿岩最直接而有效的地质标志 ,对于研究古大洋岩石圈增生过程 ,上地幔演化 ,探索早期板块构造意义重大。Chromitites are commonly classified into podiform and stratiform types. Podiform orebodies are very different from the stratiform ones that occur within the layered igneous intrusions. The pods are generally irregular in shape, exhibit nodular or orbicular textures, and show distinct textures of high-temperature deformation, such as pull-apart textures and strong mineral lineation, all of which are unique to podiform chromitites. Furthermore, podiform chromites commonly developed dunite envelope. Many investigations show that podiform chromite deposits are the characteristic of ophiolites. They generally occur within harzburgites of oceanic mantle sequences or the mantle-crust transition zone of ophiolitic complexes, situated 1 to 2 km immediately below the petrolographic Moho. They preserve valuable information about the evolution, the magmatism, the petrologic genesis and the structural deformation of upper mantle with ophiolites. In contrast to stratiform chromites, podiform chromitites commonly have lower TiO 2 concentration, and their chondrite-normalized platinum-group element(PGE) patterns show negative slope. It is widely believed that the podiform chromitites are generated under the condition of partial melting,involving the interaction between primitive mantle melt and depleted mantle peridotite, leading to complicate magma mingling and the precipitation of chromite. An active magma flow in narrow dykes within upper mantle and an active convection inside the cavity are suitable for the accmulation of podiform chromite. Supra-subduction zone (back-arc basin, island arc, and fore-arc basin), mid-oceanic ridge, transform fault are possibly favorable settings for podiform chromitite formation. They may well explain the origin and structural evolution of podiform chromite. As the Archean basement of North China is intensely metamorphosed and deformed, the documentation of podiform chromite will provide a crucial and effective marker for identification of ancient ophiolites. The various textures with

关 键 词：蛇绿岩 豆荚状铬铁矿 构造类型 地质背景 板块构造 华北克拉通 

分 类 号：P578.4[天文地球—矿物学] P571[天文地球—地质学]