新疆黄山东岩体Hf-Nd同位素特征及其地质意义

黄山东镁铁质-超镁铁质岩体位于东天山北部的土墩-黄山-镜儿泉-图拉尔根镁铁-超镁铁质岩带中段,侵位于元古宙变质岩中.主要岩石类型有二辉橄榄岩、角闪橄榄岩、苏长岩、辉长苏长岩、辉长岩、角闪辉长岩和闪长岩,是一个由3次侵入形成的复式岩体.黄山东岩体单颗粒锆石的176Hf/177Hf值为0.283008～0.283077,平均值为0.283038;176Yb/177Hf值为0.013483～0.050167,平均值为0.031085;176Lu/177Hf值为0.000602～0.002129,平均值为0.001338;εHf(t)(t=274Ma)值为14.20～16.60,平均值为15.20;εHf(O)值为8.30～10.80,平均值为9.40.εNd(t)值为6.6～8.3,初始87Sr/86Sr值为0.7031～0.7038.锆石单颗粒Hf同位素和全岩Sr-Nd同位素组成特征表明,岩体的岩浆源区具有亏损型地幔特征,可能与MORB有关,岩浆在上升过程中只有少量的(小于2%)地壳物质的加入.

Hf-Nd Isotoptic Characteristics of the Huangshandong Mafic-ultramafic Intrusion, Eastern Xinjiang, and Their Geological Implications

The Huangshandong mafic-ultramafic intrusion,is located at the centre segment of Tudun-Huangshan-Jing'erquan-Tulaergen mafic-ultramafic rock belt,the northern of the East Tianshan,controlled by the Kangurtag-Huangshan ductile sheer zone,and it is a multiple intrusion formed by at least three pulses of magmas with different compositions,and composes of lherzolite,olivine gabbro,gabbro norite,and gabbro diorite.Sulfide mineralization is associated with the second (gabbronorite) and third (plagioclase-bearing lherzolite-wehrlite) intrusive phases that are volumetrically minor in the intrusion.The 176Hf/177Hf ratios of the Huangshandong zircons range from 0.283 037 to 0.283 165 with an average of 0.283 098; 176Yb/177Hf ratios range from 0.013 590 to 0.046 774 with an average of 0.030 491,that these ratios are all less than 0.05 indicates that little radiogenic Hf has accumulated in these zircons; 176Lu/177Hf ratios range from 0.000 700 to 0.002 094 and average 0.001 318; the εHf(t) (t=274 Ma) values range from 15.38 to 19.91 and average 17.57; εHf(O) values range from 9.50 to 14.30 with an average of 11.78.The calculated εNd(t) values and initial 87Sr/86Sr values (t=274 Ma) of the Huangshandong intrusive rocks between 6.6 and 8.3,and between 0.703 1 and 0.703 8,respectively.The Hf and Sr-Nd isotopic compositions of the Huangshandong intrusion are significantly different from that of the Tarim basalts and coeval mafic-ultramafic dykes in the Tarim basin,similarly with the volcanic arc basalts and consistent with the mid-ocean basalts (MORB).The results of mixing calculations using Sr-Nd isotopes show that crustal contamination is 2% in the Huangshandong parental magmas.