新疆北准噶尔乔夏哈拉——老山口苦橄岩建造及其构造意义

PICRITE CONSTRUCT AND IT’S TECTONIC SIGNIFICANCE IN QIAOXIAHALA-LAOSHANKOU,NORTH JUNGGAR, XINJIANG, CHINA

近年在新疆北准噶尔构造带的乔夏哈拉-老山口地区发现有苦橄岩存在.苦橄岩与苦橄质玄武岩、玄武玢岩、辉绿岩、安粗岩、粗面岩以及凝灰岩、碳酸盐岩组成洋底苦橄岩建造,总厚大于1780 m,为新的岩层,建议采用'乔夏哈拉组'组名.苦橄岩有橄榄石苦橄岩、橄榄石-辉石苦橄岩和辉石苦橄岩3种.苦橄岩ω(SiO2)39.9%～46.78%;ω(MgO)16.4%～36.67%;ω(Na2O+K2O)0.17%～1.47%,多数小于0.5%;ω(TiO2)0.28%～0.62%,个别1.05%.总体具低硅、低铝、低钛、低碱、高镁特征.稀土元素总量＜45×10-6,轻稀土略富集,Eu低正异常.微量元素Li、Be、U、Rb、Sr、Ba、V、F、Bi、Pb等元素明显富集,而Ni、Cr、Co和S明显亏损.表明经过一定程度的分异作用,并有上地壳成分的污染,铂族元素含量接近世界其它地区苦橄岩,具有Pt/Pd＜1和贫Os、Ir的特征.

Picrite was discovered in the Qiaoxiahala-Laoshankou region in the north Junggar tectonic zone, Xinjiang, China in recent years. Picrite, picritic basalt, basalt-porphyrite, diabase, latite, trachyte, tuff and carbonate rocks comprise the ocean-floor picrite construct which is more than 1780m thick. It appears to be a new formation and the name “Qiaoxiahala Formation” is proposed. Picrite includes three types： olivine picrite, olivine-pyroxene picrite and pyroxene picrite. As a whole, picrite is low in silicon, ahiminium, titanium, alkali, but high in magnesium, with the following contents： w （SiO2） 39.9-46.78%; w （MgO） 16.4-36.67%; w （Na2O＋K2O） 0.17-1.47%, the majority less than 0.5%; w （TiO2） 0.28～0.62%, some up to 1.05%. The gross total of rare earth elements （REE） is less than 45 ×10^-6, light REE is enriched slightly, and the europium content is a low positive anomaly. Microelements, like Li, Be, U, Rb, Sr, Ba, V, F, Bi and Pb, are obviously enriched, while Ni, Cr, Co and S are in the extreme depletion. This suggests that they have been differentiated to a certain degree and contaminated by upper crustal compositions. The platinum group elements （PGE） content is similar to those of other picrites in the world, with the character of Pt/Pd〈1 and low contents of Os and Ir.