西藏班公湖地区富镍硫化物超基性岩的成因  

GENISIS OF THE NICKEL-SULFIDE-ENRICHED ULTRABASIC ROCK IN THE BANGONG LAKE AREA,TIBET

作　　者：罗伟[1,2] 李佑国[1] 彭静[2] 

机构地区：[1]成都理工大学地球科学学院,四川成都610059 [2]四川省地质矿产勘查开发局化探队,四川德阳618000

出　　处：《矿物岩石》2016年第3期29-36,共8页Mineralogy and Petrology

基　　金：中国地质调查局项目西藏班公湖-怒江成矿带找矿远景区评价(编号:1212010818097)

摘　　要：班公湖地区富镍硫化物超基性岩位于藏北班公湖-怒江缝合带西段,呈透镜状产出于侏罗系地层中,岩石已完全蚀变为蛇纹岩,恢复其原岩为方辉橄榄岩。地球化学研究表明：相对于原始地幔,该类超基性岩富w（MgO）（40.96%-42.82%）,贫w（Al_2O_3）（0.19%-1.76%）、w（CaO）（0.11%-0.38%）和TiO_2（0.02%-0.06%）;稀土元素总量低（0.238×10-6-2.044×10-6）,远低于原始地幔值,稀土元素配分型式呈U型;强烈亏损高场强元素Y;岩石的锆石年龄为太古代（2 479 Ma±31 Ma）。以上特征表明其为残余地幔橄榄岩成因。富镍硫化物超基性岩中的镍硫化物呈似球状分布于蛇纹石中,主要为镍黄铁矿,偶见与镍黄铁矿共生的赫硫镍矿。电子探针分析结果表明,镍黄铁矿中w（Ni）为36.77%-42.62%,w（Fe）为18.83%-28.84%,w（S）为31.34%-38.3%。赫硫镍矿中w（Fe）为0.91%-13.36%,w（Ni）为60.5%-70.08%,w（S）为25.25%-27.23%。赫硫铁矿Fe含量高（除两件样品外均〉2%mol）,且与镍黄铁矿的接触边界平直。结合前人的实验资料,认为Fe-Ni硫化物形成于高温阶段（〉300℃）,为贫S的地幔Fe-Ni-S熔体捕虏体成因。The nickel-sulfide-enriched ultrabasic rocks in the western part of Bangong-Nujiang suture belt in the North Tibet occur asenticular shape within the Jurassic strata. These ultraba- sic rocks are completely altered to serpentinite with the original rock as harzburgite. Geochemical research indicates that the nickel-sulfide-enriched ultrabasic rocks, compared with the primitive mantle,have a high w（MgO） （40.96%-42.82%）, low w（Al_2O_3）（0.19%-1.76%）、w（CaO）（0.11%-0.38%）vsTiO_2（0.02%-0.06%） and w（TiO2）（0.02%-0. 06%）. It reveals that the total REE of ultrabasic rocks is very low （0. 238）〈 10-6 -2. 044）〈 10-6） ,far below the value of the primitive mantle,and the chondrite-normalized rare earth element patterns are characteristic of U-type with depletion in high field strength elements-Y. The Zircon U-Pb dating yields a 2 479 Ma＋31 Ma ages and be- longs to Archean igneous rock. These features indicate that the nickel-sulfide-enriched ultrabasic rocks are residual mantle peridotite. In the nickel-sulfide-enriched ultrabasic rocks,the nickel sul- fides are spherical in serpentine and the main minerals are pentlandite, containing a small amount of heazlewoodite symbiotic with pentlandite. Electron microprobe analysis shows that in the pent- landite the contents of w（Ni） are 36.77%-42.62%,w（Fe） are 18.83%-28.84%,w（S） are 31.34%-38.3% ,and in the heazlewoodite,the contents of w（Fe） are 0. 91%-13. 36%,w（Ni） are 60. 5 %-70.08 %, w（S） are 25.25%- 27.23 %. In the heazlewoodite, the contents of Fe are high （more than 2% mol except two samples）, and the boundary contact with＇pentlandite is straight. Based on the study and combined with previous experimental data, it is considered that the Fe-Ni sulfides are likely formed in high temperature phase （more than 300 ℃）, as melted mantle Fe-Ni-S xenolith poor in S.

关 键 词：地幔橄榄岩 岩石地球化学 镍黄铁矿 班公湖地区 西藏 

分 类 号：P571[天文地球—矿物学]