【作者】 潘少逵；

【导师】 郑建平；

【作者基本信息】 中国地质大学 ， 矿物学、岩石学、矿床学， 2013， 博士

【摘要】 大陆岩石圈地幔的性质,形成和演化一直是现今固体地球科学界研究的热点问题之一。最近十几年里,对于华北克拉通和我国东部岩石圈地幔的演化和改造的研究取得了长足的进展。然而,对位于中亚造山带东部,我国著名重力梯度线稍偏西位置的大兴安岭岩石圈地幔的研究却一直未能一起地质学家足够的重视。与克拉通相比,造山带岩石圈地幔往往记录了板片俯冲交代,壳幔相互作用等更为丰富的信息。因此,我们选取大兴安岭南部锡林浩特地区橄榄岩捕虏体与大兴安岭中部阿尔山橄榄岩捕虏体为研究对象,在结合前人研究资料基础上,拟通过详细的岩相学和地球化学分析,采用综合对比分析等手段,确定大兴安岭岩石圈地幔的性质,分析其所经历的熔融交代过程,并进一步探讨大兴安岭岩石圈地幔的形成和演化。大兴安岭南部锡林浩特地区橄榄岩捕虏体以二辉橄榄岩为主,有少量的方辉橄榄岩(Cpx<5%）。与锡林浩特橄榄岩捕虏体相比,大兴安岭中部阿尔山地区橄榄岩捕虏体方辉橄榄岩数量更多。这两个地方的捕虏体均为尖晶石相橄榄岩,岩石组合为橄榄石+斜方辉石±单斜辉石±尖晶石。手标本和镜下观察未见石榴石的出现。阿尔山橄榄岩部分样品含有少量显性交代矿物：角闪石和锂辉石。大兴安岭南部锡林浩特捕虏体按照单斜辉石稀土元素配分曲线的形态可以分为三组。主量元素方面,锡林浩特第一组捕虏体为方辉橄榄岩或贫单斜辉石二辉橄榄岩,单斜辉石含量小于7vol%。该组捕虏体具有较高的橄榄石Mg#值,尖晶石Cr#值,以及较低的全岩玄武质组分(CaO, TiO2, Al2O3)含量,经模拟计算其部分熔融程度为14-18%,显示了适度难熔(难熔程度小于鹤壁但远大于山旺)的地球化学特点。第二组捕虏体均为二辉橄榄岩,单斜辉石含量大于9vol%。该组捕虏体具有较低的橄榄石Mg#值,尖晶石Cr#值,以及较高的全岩CaO, TiO2, Al2O3含量,经模拟计算其部分熔融程度小于10%,显示了较为饱满(略比山旺难熔)的特征。第三组捕虏体既有方辉橄榄岩也有二辉橄榄岩,单斜辉石含量介于3-14vol%之间。该组捕虏体具有较大的主量元素变化范围,基本上横跨了第一组与第二组橄榄岩捕虏体,部分熔融程度为5-18%。微量元素方面,锡林浩特第一组捕虏体具有右倾的单斜辉石稀土元素配分曲线,富集大离子亲石元素,亏损高场强元素,但相对于部分熔融程度具有更高的Nb颔联,显示了碳酸盐熔体交代作用的痕迹。第三组橄榄岩捕虏体具有正弦曲线式单斜辉石稀土元素配分曲线,适度富集大离子亲石元素和轻稀土元素,相对于其部分熔融程度具有更高的Nb, Zr, Ti含量,但Ti/Eu比值变化较大,显示了多期次的交代作用的痕迹。而第二组捕虏体具有左倾的单斜辉石稀土元素配分曲线,没有明显的受后期交代作用的痕迹。锡林浩特第三组捕虏体全岩稀土元素配分曲线为轻稀土富集的右倾型,与单斜辉石不同,可能为矿物粒间存在富集轻稀土元素的熔流体导致。大兴安岭中部阿尔山地区橄榄岩捕虏体按照单斜辉石稀土元素配分曲线也可分为三组。与锡林浩特捕虏体相似,阿尔山地区第一组橄榄岩捕虏体均为方辉橄榄岩,单斜辉石含量不大于3vo1%,该组捕虏体具有较高的橄榄石Mg#值,尖晶石Cr#值,以及较低的全岩CaO,TiO2, Al2O3含量,经模拟计算其部分熔融程度为14-20%,也显示了适度难熔(难熔程度小于鹤壁但远大于山旺)的地球化学特点。第二组捕虏体为二辉橄榄岩,单斜辉石含量为12vol%。该组捕虏体具有较低的橄榄石Mg#值,尖晶石Cr#值,以及较高的全岩CaO, TiO2, Al2O3含量,经模拟计算其部分熔融程度约为1%,显示了非常饱满的特征。第三组捕虏体既有方辉橄榄岩也有二辉橄榄岩,单斜辉石含量不大于7vo1%。该组捕虏体具有较大的主量元素变化范围,部分熔融程度为5-20%。对阿尔山第三组捕虏体进行石榴石相部分熔融模拟计算,部分第三组橄榄岩源区可能来自含有石榴石,但石榴石含量应该不会太高。微量元素方面,阿尔山第一组橄榄岩捕虏体单斜辉石稀土元素配分曲线比较平坦,但微量元素蛛网图上具有明显Nb, Ta, Zr, Hf, Ti等高场强元素负异常,无Sr正异常,经模拟计算相对于其部分熔融程度基本没有Nb的再富集,岩相学上观察到了角闪石的出现,显示了含水流体交代作用的痕迹。阿尔山第三部捕虏体具有正弦曲线型单斜辉石稀土元素配分曲线形态,部分第三组捕虏体无Sr正异常,但有显著的Ti负异常,可能反映了多期次的不同交代介质作用的特点,其余第三组捕虏体无高场强元素负异常,显示了硅酸盐熔体的交代特征。而阿尔山第二组捕虏体单斜辉石具有左倾的轻稀土元素亏损的配分曲线形态,没有表现出明显的交代特征。此外,锡林浩特和阿尔山第三组捕虏体还可能受到了某种程度的熔体／橄榄岩反应的影响,斜方辉石含量较高,单斜辉石Cr与尖晶石Ti的含量明显偏高,在全岩主量元素组成上也显示了Ti的富集与轻微的Fe富集作用。反应过程可能溶解了单斜辉石,生成斜方辉石和／或橄榄石。将锡林浩特和阿尔山捕虏体与北部的五大连池-科洛地区捕虏体和华北克拉通东部的鹤壁和山旺捕虏体相对比,大兴安岭橄榄岩捕虏体整体上体现了从适度难熔到饱满的特点。锡林浩特和阿尔山第一组捕虏体的平衡温度都高于第二组捕虏体的平衡温度。但结合最近的研究资料显示,锡林浩特和阿尔山地区橄榄岩捕虏体代表难熔程度的尖晶石Cr#值并没有随平衡温度的不同而变化。以阿尔山,锡林浩特和五大连池-科洛橄榄岩捕虏体为代表的大兴安岭岩石圈地幔的Sr-Nd同位素显示了较为亏损的特征,187Os/188Os值与平衡温度无相关关系。大兴安岭捕虏体与中国东北其它地区橄榄岩捕虏体具有相似的Sr-Nd, Re-Os同位素组成。整体上体现了以新生饱满岩石圈地幔为主,局部零星存在古老难熔的地幔残余。我们认为中亚造山带是一个由多种构造单元(洋岛,岛弧,增生楔,前寒武纪微陆块)组成的复杂拼合体,大兴安岭地区位于中亚造山带东部,在古生代和中-新生代分别收到了古亚洲洋构造域和古太平洋构造域的影响,长期的俯冲作用和岩浆活动使岩石圈维持较高的热与正浮力状态,随着俯冲作用和岩浆活动的结束,岩石圈逐渐冷却并在重力上失稳,从而发生岩石圈的拆沉,软流圈物质上涌冷却,最终形成了新增生的岩石圈地幔,而古老陆块残片下的岩石圈地幔由于重力上的相对稳定性可能得以(部分)保存。从而形成了现今大兴安岭乃至中国东北以新生饱满的岩石圈地幔为主,局部零星存在古老难熔的岩石圈地幔残余的特点。但是对于五大连池-科洛地区的岩石圈地幔来说,前人结合区域上钾质玄岩地球化学资料进行分析,认为该区岩石圈地幔具有上新下老的特征,可能为外来岩石圈地幔就位。五大连池-科洛地区岩石圈地幔这一特殊的现象仍须进一步研究和探索。更多还原

【Abstract】 The nature and evolutionary process of the subcontinental lithospheric mantle has long been one of the hottest issues of solid Earth science community. In the last twenty years, the studies on the evolution and modification of the lithospheric mantle underneath the eastern North China Craton (NCC) and eastern China have got considerable achievements. However, the lithosperic mantle beneath the Great Xing’an Range, which is located at the eastern Central Asian Orogenic Belt-a little west to the famous China Gravity Gradient Stabilization Line, has not been paid enough attentions. Compared with the lithopheric mantle beneath cratons, the orogenic lithospheric mantle may record informatons such as subduction and crust-mantle interaction. Therefore, we collected the peridotite xenoliths from the Xilinhot (XLHT) region and Ar Shan (AS) region, which is located in the southern and middle part of the Great Xing’an Range respectively. Based on the detailed petrologe and geochemistry studies, combined with previous studies and using comparison method, we tend to determine the nature of the lithospheric mantle beneath the Great Xing’an Range, discuss the mantle process such as partial melting and metasomatism and further explore the formation and evolution of the lithospheric mantle beneath the Great Xing’an Range.The XLHT peridotite xenoliths (in the southern part of the Great Xing’an Range) are mainly lherzolite with minor harzburgite (Cpx<5%). Compared with peridotite xenoliths in the XLHT region, peridotite xenoliths in the AS region contain more harzburgite. Xenoliths in the two locations are all spinel peridotite xenoliths, with mineral assemblage of olivine+orthopyroxene stone±clinopyroxene±spinel. No garnet was observed in the hand specimen and thin sections. Some peridotite xenoliths in the AS region contain small amount of modal metasomatic minerals such as amphibole and spondumene.Peridotite xenoliths from the XLHT region can be divided into three groups according to their clinopyroxene REE patterns. The Group1peridotite xenoliths consist of harzburgite or clinopyroxene-poor lherzolite, with clinopyroxene content less than7vol%. Group1xenoliths have high olivine Mg#, spinel Cr#values and low whole-rock basaltic components (CaO, TiO2, A12O3) level. The partial melting degrees of Group1xenoliths are14-18%through model calculating, showing the moderately refractory (less refractory than the Hebi but much more refractory than Shanwang) geochemical characteristics. The Group2xenoliths are lherzolites, with the clinopyroxene content more than9vol%. The Group2xenoliths have low olivine Mg#, spinel Cr#values and high CaO, TiO2, Al2O3bulk contents. The partial melting degrees of Group2xenoliths are no more than10%. The xenoliths of Group2show fertile (slightly more refractory than Shanwang) geochemical characteristics. The Group3xenoliths are composed of both harzburgites and lherzolites. Xenoliths of this Group contain clinopyroxene contents ranging between3-14vol%. The Group3xenoliths have large variations of major-elements, basically overlapping the Group1and Group2peridotite xenoliths. The partial melting degree of Group3xenoliths is5-18%. The Group1xenoliths from the XLHT region show LREE enrichment REE patterns. From the extended trace-element diagrams, the clinopyroxene of the Group1xenoliths have high large ion lithophile elements (LILE) levels and strong negative anomalies in high field strength elements (HFSE). However, relative to the degree of melt extraction, the clinopyroxenes of the Group1xenoliths have higher Nb contents, displaying clues of carbonate metasomatism. The clinopyroxenes in xenoliths of the Group3have sinusoid REE patterns with peak at Sm or Nd. They are moderately enriched in LILE and LREE content. Relative to the degree of partial melting, they have higher Nb, Zr, Ti concentrations and variable Ti/Eu values, displaying clues for the multi-stage metasomatism. The clinopyroxenes in the Group2xenoliths have LREE depleted patterns. There are no apparent characteristics for mantle metasomatism. For the Group3xenoliths, whole-rock REE patterns differ from the clinopyroxene REE patterns, possibly due to the melt/fluid in the mineral grain boudaries which are enriched in LREE.The peridotite xenoliths from AS region can be subdivided into three groups, according to their clinopyroxene REE distribution curves. Similar to the XLHT xenoliths, the Group1xenoliths from the AS region are all harzburgites, with clinopyroxene mode content not more than3vol%. They have high olivine Mg#, spinel Cr#values, and have low whole-rock CaO, TiO2, Al2O3levels. Through model calculating, the degree of partial melting of Group1xenoliths are14-20%, showing moderate refractory (less refractory than Hebi but more refractory than Shanwang) geochemical characteristics. The Group2xenoliths are Iherzolite, with clinopyroxene mode content of12vol%. The Group2xenoliths have low olivine Mg#, spinel Cr#values and high whole-rock CaO, TiO2, Al2O3levels. Its melt extraction degree is about1%, showing very fertile features. The Group3xenoliths contain both harzburgite and lherzolite, with clinopyroxene mode contents no more than7vol%. The Group3xenoliths have a large range of major-elements, with the partial melting degrees from5%to20%. Part of the Group3xenoliths from AS region may contain garnet. However, after model calculating, the garnet contents of the Group3xenoliths, if have, can not be very high. The clinopyroxene in the Group1xenoliths from AS region have flat REE patterns. But they has significantly negative (Nb, Ta, Zr, Hf, Ti) HFSE anomalies, positive Sr anomaly from the trace-elements extended diagrams. They aslo show no enrichment of Nb contents relative to its degree of partial melting. Combined with thin section observations (the presence of hornblende), the Group1xenoliths show characteristics of aqueous fluid metasomatism. The Group3xenoliths have sinusoid clinopyroxene REE patterns. Some of the Group3xenoliths have Sr positive anomaly and significantly Ti anomalies, reflecting the features of multi-stage metasomatism. No HFSE negative anormalies of the rest Group3xenoliths shows the characteristics of silicate metasomatism. The Group2xenoliths from the AS region have LREE depleted patterns, showing no apparent metasomatic features. The Group3xenoliths from the XLHT and AS region experienced melt/rock reaction by some degree. They have higher levels of orthopyroxene modes, higher clinopyroxene Cr and spinel Ti contents, ande whole-rock Ti enrichment and slight Fe enrichment. The reactions may dissolve clinopyroxenes and precipitate orthopyroxenes and/or olivines.Compared with the North China Craton xenoliths (Hebi and Shanwang), peritotite xenoliths from the Great Xing’an Range show moderately refractory to fertile features. The Group1xenoliths in the XLHT and AS region have higher equilibrium temperatures than the Group2xenoliths. However, taking account for recent geochemical data, the peridotite xenoliths from the XLHT and AS region show no regular variations between spinel Cr#values and equilibrium temperatures. For the XLHT and AS xenoliths, the Sr-Nd data and187Os/188Os values also show no regular change with equilibrium temperatures. However, for the Wudalianchi-Keluo (WEK) xenoliths, the187Os/188Os values seem to decrease with increasing of the equilibrium temperatures. The xenoliths from the Great Xing’an Range show similar Sr-Nd, Re-Os isotopic compositions with other places in the northeast China peridotite xenoliths. The lithospheric mantle beneath the northeast China is mainly fertile and new, with local presence of the remnants of ancient refractory mantle. We interpreted that the Central Asian orogenic belt is composed of a variety of tectonic units (oceanic island, island arcs, accretionary wedge and Precambrian microcontinental). The Great Xing’an Range is located in the eastern Central Asian Orogenic Belt. During the the Paleozoic and Mesozoic, the Great Xing’an Range was governed by Paleo-Asian Ocean and Paleo-Pacific ocean domain. Long term slab subduction and magmatic activity maintain a high heat budge and positive buoyancy of the lithosphere relative to the asthenosphere. After the end of the subduction and magmatism, new accreted lithospheric gradually cooled and became instability on gravity. Finally, the new accreted lithosphere delaminated and the upwelled asthenosphere cooled to form newly accreted lithospheric mantle. Lithospheric mantle underneath the ancient continent fragments may be (partly) preserved due to theric gravity stability. Today, the lithospheric mantle beneath the Great Xing’an Range, even northeastern China are mainly new accreted and fertile, with local presence of ancient refractory lithospheric mantle relict. For the lithospheric mantle beneath the WEK region, previous studies on the host potassic basalts suggested that the garnet-face lithospheric mantle is formed before Mesoproterozoic. Thus the lithospheric mantle beneath the WEK region is featured by younger mantle at shallow levels and older mantle at deep levels. The lithospheric mantle beneath the WEK region need to be further investigated.更多还原