内蒙古贺根山蛇绿岩地幔属性:来自方辉橄榄岩元素地球化学和Re-Os同位素的制约

Mantle property of Hegenshan ophiolite in Inner Mongolia:Constrains from element geochemistry and Re-Os isotopes of harzburgite

贺根山蛇绿岩带是中亚造山带东部出露面积最大的晚古生代蛇绿岩带,然而研究者对它是大洋岩石圈的残余还是陆内岩石圈地幔伸展的产物仍存在较大争议,主要归因于一直缺少橄榄岩Re-Os同位素的有效制约。基于此,本文对贺根山蛇绿岩中方辉橄榄岩进行了系统的岩石学、地球化学和Re-Os同位素研究。方辉橄榄岩主要由橄榄石、斜方辉石、粗粒的铬尖晶石和少量单斜辉石组成。贺根山方辉橄榄岩具有难熔地球化学特征,显示出高的MgO(45.1%~48.1%)含量,低的CaO(0.09%~0.26%)和Al 2O 3(0.58%~0.90%)含量及Tb/Yb比值,低于原始上地幔的^(187)Os/^(188)Os(0.1236~0.1263)同位素组成;橄榄石具有较高的Fo值(91.0~91.7)和NiO含量(0.26%~0.48%),而且单斜辉石具有低的重稀土含量,暗示了贺根山方辉橄榄岩是尖晶石相陆下岩石圈地幔高程度(20%~30%)部分熔融的难熔残余。另外,方辉橄榄岩相对富集LREE和大离子亲石元素,亏损高场强元素(例如Nb、Ta),且单斜辉石具有高Ti/Eu值和低的La/Yb,揭示了地幔源区受到硅酸盐熔体交代作用的改造。另一方面,方辉橄榄岩显示出相对年轻的Re亏损模式年龄,t RD为454~825Ma,表明贺根山地区岩石圈地幔形成的时代应该在0.8Ga之前,而后经历过多期熔体抽离和交代,与区域上大规模地壳增生时代吻合,可能是罗迪尼亚超大陆裂解的响应。结合蛇绿岩中大量深源矿物(金刚石等)和捕获锆石的存在、南北两侧一致的地质记录,本文认为贺根山方辉橄榄岩可能是陆内伸展环境下深部地幔物质上涌减压熔融的产物。

The Hegenshan ophiolite is the largest Late Paleozoic one exposed in the eastern Central Asian Orogenic Belt.However,it is controversy about whether it was a remnant of an oceanic lithosphere or an intracontinental lithospheric mantle resulted from extension,mainly due to the lack of effective geochemical and Re-Os isotopic constraints on the peridotites.This paper reports the petrology,geochemistry and Re-Os isotopes of the harzburgites in the Hegenshan ophiolite,which is mainly composed of spinel,olivine and orthopyroxene with minor clinopyroxene.The Hegenshan harzburgites have refractory geochemical characteristics,with high MgO(45.1%~48.1%),low CaO(0.09%~0.26%)and Al 2O 3(0.58%~0.90%)contents and Tb/Yb ratios.They have lower ^(187)Os/^(188)Os ratios(0.1236~0.1263)than the primitive upper mantle.The olivines in the harzburgites have high Fo(91.0~91.7)and NiO(0.26%~0.48%)contents,while the clinopyroxenes have relatively low heavy rare earth contents.All of the geochemical features suggest that the Hegenshan harzburgites were the refractory residue of high-degree(20%~30%)partial melting of a subcontinental lithospheric mantle of spinel facies.However,the harzburgites are relatively enriched in LREE and large ion lithophile elements,and depleted in high field strength elements(such as Nb,Ta).The clinopyroxenes exhibite high Ti/Eu but low La/Yb values,indicating that the mantle source had been modified by metasomatism of silicate melts.On the other hand,the harzburgites show relatively young Re-depletion model ages(454~825Ma),indicating that the lithospheric mantle beneath the Hegenshan area was formed before 0.8Ga,and then it experienced a multi-stage melt extraction.It is consistent with the significant growth of continental crust that might be correspondent to the breakup of the Rodinia supercontinent.Combined with the existence of a large number of deep mantle-derived minerals(diamond,etc.)and captured zircons in the ophiolite,and the similar geological records of the north and south sides of the ophiolite,we suggest that the Hegenshan harzburgite may be the product of the upwelling and decompression melting of deep mantle materials in an intracontinental extensional environment.