也谈铙钹寨超镁铁岩体的成因和构造类型的归属问题  

Origin and structural nature of Raobozhai ultamafic rock:A discussion.Acta Petrologica

作　　者：王希斌[1] 杨经绥[1] 陈松永[1] 史仁灯[1] 

机构地区：[1]中国地质科学院地质研究所,北京100037

出　　处：《岩石学报》2005年第6期1593-1608,共16页Acta Petrologica Sinica

基　　金：国家重点基础研究发展规划(973)项目(2003CB716503);国家自然科学基金重大项目(40399143);中国地质调查局地质大调查项目(编号121201056606)的联合资助

摘　　要：铙钹寨岩体是一个遭受角闪岩相-麻粒岩相变质作用改造的超镁铁岩体.由于其变质程度的不均一性,一部分仍保留着原地幔橄榄岩的基本特征(如粗粒,具三联点结构和熔融残余结构的方辉橄榄岩和纯橄岩).另一部分则表现明显的重结现象,原地幔橄榄岩类的基本特征已部分或全部消失,形成以出现不同数量的新生矿物组合为特征的变橄榄岩类(以细粒变纯橄岩和似斑状角闪方辉橄榄岩为代表).恢复其原岩,该岩体由纯橄岩(极少)、方辉橄榄岩(为主)和二辉橄榄岩组成.但在纯橄岩-方辉橄榄岩组合与二辉橄榄岩组合两者之间在成分上显示一个明显的间断.前者以贫Al(Al2O3含量＜1%)、Ca(CaO含量＜1%)、Ti(TiO2含量＜0.05%)和富Mg(Mg#=92～93)为特征.其主要矿物Ol和Opx均属富镁橄榄石(Fo92～93)和顽火辉石(En92～93).后者则相对富Al(Al2O3含量由3.36%～3.75%)、Ca(CaO含量由2.07%～3.15%)、Ti(TiO2 0.1%～0.12%)和相对低镁(Mg#=89～90)为特征.其主要造岩矿物Ol和Opx属相对低Mg的镁橄榄石(Fo90)和顽火辉石(En89～90).副矿物尖晶石具有多期熔融的复杂成因,但其成分区及趋势显示蛇绿岩地幔橄榄岩所常见的双频分布(bimodal distribution)特征.综合研究表明该两套地幔岩原岩组合代表了不同亏损程度和不同构造环境下多期熔融的地幔残余.以方辉橄榄岩(和少量纯橄岩)为代表,并伴有豆荚状铬铁矿化及其特征的瘤状矿石的产出背景,标志其遭受较高的部分熔融程度,可能代表产于洋内俯冲带之上(Supra-Subduction zone)亲弧环境的上地幔残余,带有SSZ型蛇绿岩的某些印记.二辉橄榄岩可能代表来自MOR环境下,弱亏损的上地幔残余岩片.故该岩体超镁铁岩不具有大陆地幔橄榄岩的成因性质.The Raobazhai rock body is an uhramafic body that has undergone amphibolite-granulite facies metamorphism. Due to the heterogeneity of its metamorphic grade, the basic features of original mantle peridotites （ e. g coarse-grained harzburgites and dunites with triple-junction and melting residual textures） have been still preserved in a part of it; whereas the other part exhibits a distinctive recrystallization and the basic features of the original mantle peridotites have partially or all disappeared, forming metaperidotites （represented by fine-grained metadunites and porphyroclostic hornblende harzburgites） marked by the appearance of different quantities of neogenic mineral assemblages. The reconstructed protoliths of the rock body consist of dunites （rare） , harzburgites （dominant） and lherzolites, However, there is a marked compositional discontlinuity between the dunite-harzburgite unit and lherzolites unit. The former is poor in AI（Al2O3 〈1%）, Ca（CaO 〈1%） and Ti（TiO 〈0.05%） and rich in Mg （Mg^# =92 -93） and its dominant minerals olivine and orthopyroxene belong to magnesium-rich forsterite （Fo92 -93 ） and enstatite （En92 -93 ） respectively, while the latter is relatively rich in Al （ Al2O3 3. 36 - 3.75% ） , Ca （ CaO 2.0 - 3.15% ） and Ti （ TiO2 0.1 - 0.12% ） and relatively poor in Mg （Mg^#= 89 -90 ） and its dominant minerals olivine and orthopyroxene belong to relatively low-magnesium forsterite （Fo90） and enstatite （En89 - 90） respectively, The accessory mineral spinel has a complex origin of polyphase melting, but its compositional field and trend show the bimodal distribution characteristics that are commonly seen in mantle peridotites of ophiolites. Integrated study indicates that the two protolith suites of mantle peridotites represent remnants after polyphase melting of differently depleted mantle in different tectonic environments. The setting of occurrence of the rock suite represented by harzburgites （ and minor dunites） and

关 键 词：铙钹寨岩体 超镁铁岩 蛇绿岩 地幔橄榄岩 变质作用的不均一性 原岩恢复 

分 类 号：P588.125[天文地球—岩石学]