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在详细形态和内部结构研究基础上,对碧溪岭橄榄岩锆石进行了配套的U-Pb年龄和Hf同位素分析.碧溪岭橄榄岩锆石主体是变质成因的,但有少量岩浆锆石残留.这些锆石主体给出了220~210 Ma的变质生长/重结晶作用年龄;一颗具较高Th/U比值的锆石给出646 Ma的近协和年龄,限定了早期岩浆结晶作用的最小年龄.真正的侵位年龄可以由745 Ma的上交点年龄来限定;210~222 Ma则记录着深俯冲陆壳折返过程中的锆石生长.除少数颗粒的εHf为负值外(-2.9),大部分锆石的εHf是正的(高速+8.1)并具新元古代的亏损地幔模式年龄(0.6~1.0 Ga,平均0.8 Ga).这些结果说明碧溪岭橄榄岩的初始物质是扬子大陆岩石圈内新元古代岩浆堆积作用产物.这样的岩浆堆积物来源于亏损的软流圈地幔,侵入于扬子的深部地壳之中(如底侵)并部分混染了古老地壳组分.它们随扬子向华北之下深俯冲碰撞和造山带折返过程中经历着复杂的变质改造作用. 相似文献
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本文对华北克拉通三个不同地区(河北汉诺坝、内蒙古集宁三义堂、河南鹤壁)新发现的含金云母尖晶石二辉橄榄岩和尖晶石橄榄单斜辉石岩捕虏体进行了详细的矿物组成、单斜辉石的微量元素和Sr-Nd同位素研究.通过与相同地区不含金云母尖晶石二辉橄榄岩捕虏体的系统对比发现通常含金云母的地幔橄榄岩比不含金云母的地幔橄榄岩岩富Al2O3、CaO、NaO、K2O、TiO2,但相对贫镁;其单斜辉石的LREE更为富集,但Sr、Nd同位素组成则相对亏损.这说明地幔交代作用不仅能够造成地幔橄榄岩的玄武质组分和稀土元素的富集,而且亦能够造成全岩和橄榄石Mg#的降低和同位素组成的相对亏损.捕虏体的Rb-Sr等时线年龄暗示地幔交代作用发生在中、新生代;其交代熔体来源于软流圈.同时说明华北新生代岩石圈地幔普遍存在的主、微量元素和同位素组成类似于“大洋型”岩石圈地幔的特征很可能是岩石圈地幔橄榄岩与软流圈来源的熔体的大规模反应的结果,而非真正意义上的新增生的岩石圈地幔. 相似文献
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Mantle xenoliths are common in the Cenozoic basalts of the Changbaishan District, Jilin Province, China. Sulfide assemblages in mantle minerals can be divided into three types: isolated sulfide grains, sulfide-meh inclusions and filling sulfides in fractures. Sulfide-inch inclusions occur as single-phase sulfides, sulfide-silicate melt, and CO2-sulfide-silicate melt inclusions. Isolated sulfide grains are mainly composed of pyrrhotite, but cubanite was found occasionally. Sulfide-inch inclusions are mainly composed of pentlandite and MSS, with small amounts of chalcopyrite and talnakhite. The calculated distribution coefficient KD3 for lherzolite are similar to that of mean experimental value. The bulk sulfides in lherzolite were in equilibrium with the enclosing minerals, indicating immiscible sulfide melts captured in partial melting of upper mantle. Sulfide in fractures has higher Ni/Fe and (Fe + Ni)/S than those of sulfide melt inclusions. They might represent later metasomatizing fluids in the mantle. Ni/Fe and ( Fe + Ni )/S increase from isolated grains, sulfide inclusions to sulfides in fractures. These changes were not only affected by temperature and pressure, but by geochemistry of Ni, Fe and Cu, and sulfur fugacity as well. 相似文献