【作者】 卢俊生；

【导师】 倪志耀；

【作者基本信息】 成都理工大学 ， 矿物学、岩石学、矿床学， 2010， 硕士

【摘要】 冀北赤城红旗营子群中变质橄榄岩呈透镜状或团块状产于黑云斜长片麻岩之中。在空间上,变质橄榄岩往往与退变榴辉岩紧密相伴。通过电子探针分析,结果显示变质橄榄岩中残留橄榄石的化学成分比较稳定,而且MgO含量比较高(48.67-52.37%),FeO、CaO和MnO含量比较低,NiO平均含量为0.376%,与地幔橄榄岩中橄榄石的NiO含量(平均为0.4%;Sato ,1977)接近;变质橄榄岩中的斜方辉石化学成分比较稳定,而且MgO含量比较高(35.76-37.11%),FeO和CaO含量比较低;变质橄榄岩中铬尖晶石的Cr#值介于0.647-0.865之间,明显高于深海橄榄岩中铬铁矿的相应值,相当于Dick和Bullen(1984)的Ⅲ类橄榄岩中尖晶石,其成分特征符合亲弧环境或大洋型地壳的过渡带。在铬尖晶石的Cr#-Mg#相关图中,随Cr#的增加而Mg#相应降低,与世界其它地区蛇绿岩中变质橄榄岩相似(Irvine et al., 1972)。以上结果表明,研究区的变质橄榄岩属于蛇绿岩型变质橄榄岩,应来自高亏损的地幔环境,与岛弧有一定的亲缘性。变质橄榄岩为强烈蛇纹石化的方辉橄榄岩,富MgO,贫CaO,Al2O3和FeO*。低TiO2(0.01%-0.04%,平均值分别为0.02),低于大洋中脊地幔的TiO2含量(TiO2>0.1%;Robinson等,1995),而与消减带之上的蛇绿岩中相应岩石的TiO2含量相当(Pearce et al.,1984)。在TiO2-Cr相关图中,位于SSZ型地幔橄榄岩的成分区域。岩石的REE含量低于原始地幔,过渡金属元素表现为不对称的“W”型配分型式,在Ti和Cu处形成明显的负异常“谷”。上述特征意味着它们可能来自消减带之下上地幔,并且是高度部分熔融的残余物。在空间上,与变质橄榄岩紧密相伴的退变榴辉岩的原岩主要为基性岩类,其SiO2含量介于44.57%-54.03%之间。退变榴辉岩样品具有较高的MgO含量(6.15%-11.31%),Mg#值介于48.68-65.34之间。退变榴辉岩的TiO2含量介于0.92%-1.70%之间,平均为1.25%,位于典型的洋中脊拉斑玄武岩(1.50%)与活动大陆边缘及岛弧拉斑玄武岩(0.83%)之间。在TiO2-MnO-P2O5和FeO*/MgO-TiO2图解中,退变榴辉岩样品的投影点位于MORB或IAT区。另外,随岩石中TiO2含量的升高,Al2O3/TiO2和CaO/TiO2相应降低,两者的线性相关性比较明显,与大洋中脊玄武岩的演化趋势相一致。退变榴辉岩具有三种不同的REE配分型式。第一种为LREE亏损型,与典型N-MORB的稀土元素特征类似;第二种具平坦型的REE配分型式,与T-MORB的稀土元素特征比较相似,但总量徧高；第三种为LREE富集型,明显不同于N-MORB、E-MORB和OIB的REE特征。由于其HREE为平坦型配分型式且总量为球粒陨石的25-30倍左右,因而极有可能是陆壳物质的加入使得岩石的LREE发生富集之故。退变榴辉岩的高场强相容元素与MORB相近,岩石的fSm/Nd为0.08-0.39(平均0.23),εNd(t= 438 Ma)为1.37-9.94(平均6.34)。退变榴辉岩的地球化学特征显示,它们来自于亏损地幔且其原岩为兼具洋中脊和岛弧属性的拉斑玄武岩。研究区变质橄榄岩与退变榴辉岩在空间上紧密共生,它们都呈构造岩块的形式存在于红旗营子群黑云斜长片麻岩之中,具有一定的成因联系。变质橄榄岩属于蛇绿岩型变质橄榄岩,来自于消减带之下的上地幔。而与变质橄榄岩有成因联系的退变榴辉岩则形成于岛弧基础上发展起来的弧后盆地。因此,变质橄榄岩和退变榴辉岩的原岩拉斑玄武岩应该是形成于岛弧环境下的蛇绿岩组合,它们早期曾经历过俯冲-消减作用,后来因岩块的构造侵位而就位于现今红旗营子群之中。更多还原

【Abstract】 Metamorphic peridotites from the Chicheng, North Hebei Province, China occur as separate tectonic lenses or lumps within the biotite-plagioclase gneisses of the Late Paleozoic Hongqiyingzi Group. In space, the metamorphic peridotites are often closely associated with retrograded eclogites. The olivines’components are relative constantly. They contain more w(MgO), but less w(FeO), w(CaO), w(MnO). Their average w(NiO2) is near to the mantle peridotites’(0.4%;Sato ,1977). En’components are also relative constantly, and w(MgO) in En is higher while w(FeO) and w(CaO) are lower. Cr/(Cr+Al) in the metamorphic peridotites’chromites ranges from 0.647 to 0.865, which is significantly higher than deep sea peridotite corresponding value in the chromite, equivalent of Dick and Bullen (1984) classⅢin the spinel peridotite and the composition features are compatible with the pro-arc or oceanic crust transition zone. In Cr/(Cr+Al) vs. Mg/(Mg+Fe2+) digram, with Cr/(Cr+Al) increased Mg/(Mg+Fe2+) reduced accordingly, and this trend is similar to the rest of the world ophiolitic metamorphic. These results suggest that the study area metamorphic peridotites are from ophiolite metamorphic peridotite and they should come from the highly depleted mantle, and has a certain affinity to the island arc.The metamorphic peridotites of harzburgitic composition, contain more MgO, but less CaO, Al2O3 and FeO*, and their average w(TiO2) is 0.02 %, which is lower than the mid-ocean ridge mantle TiO2 content, but is corresponding to Subduction zone ophiolite rock TiO2 content Their total REE is less than the primitive pyrolite and the PM (primitive mantle)-normalized TME (Transitional metal element) distribution patterns are anisomerous W-type, and have remarkably negative Ti and Cu anomaly. Geochemical characteristics of metamorphic peridotites show they are similar to SSZ-type pyrolite that came from upper mantle under subduction zone and might represent the depleted pyrolite which underwent higher partial melting.Retrograded eclogites’protolith belongs to the basic rock, their w(SiO2) content ranges from 44.57% to 54.03%. Retrograded eclogites contain more w(MgO) (6.15%-11.31%) and Mg# ranges from 48.68 to 65.34. Their w(TiO2) ranges from 0.92% to 1.70%, the average is 1.25%, which between in a typical mid-ocean ridge tholeiite (1.50%) and active continental margin of island arc tholeiite (0.83%). In TiO2-MnO-P2O5 vs. FeO*/MgO-TiO2 diagram, samples fall into the MORB or IAT field. With TiO2 increased Al2O3/TiO2 and CaO/TiO2 reduced accordingly, and this trend is consistent to the evolution of the ocean ridge basalt. In this areas , retrograded eclogites with three different REE distribution patterns. The first one is LREE-depleted, which is MORB-like ophiolitic basalts; The second one is flat REE pattern, which is similar to T-type Middle Ocean Ridge Basalts; The last one is LREE enriched pattern, which is distinguish from N-type MORB、E-type MORB and Ocean Island Basalts. But the last pattern can’t represent retrograded eclogites’protolith nature, because it must be contaminate by continental crust. Compatible high field strength elements (HFSE) in retrograded eclogites are similar to MORB, their fSm/Nd ranges from 0.08 to 0.39, the average is 0.23 andεNd(when t= 438 Ma) ranges from1.37 to 9.94, the average is 6.34. Geochemistry of retrograded eclogites shows that, they come from depleted mantle and their protolith have the properties of both mid-ocean ridge basalts and island arc tholeiite.In this area, metamorphic peridotites and retrograded eclogites in close intergrow in space, they have some genetic connection. Metamorphic peridotites belong to ophiolitic peridotites and they formed in an island arc environment, from the subduction zone under the mantle. Retrograded eclogites formed in a back-arc basin, which developed on the basis of island arc. So, metamorphic peridotites and retrograded eclogites’protolith-tholeiite are the highly dismembered ophiolite, which should be formed in an island arc environment. They once underwent the subduction then emplaced into the Hongqiyingzi Group by the faulting mainly.更多还原