广州地化所在华北克拉通新生代玄武岩成因及深部动力学机制研究上取得新进展

  

  克拉通是地球表层重要组成部分,约占陆地面积的50%。其作为地球上相对稳定的构造单元,主要形成于前寒武纪。克拉通具有厚度较大、密度和热流值较低的特点,因而“漂浮”于软流圈地幔之上而不易遭受破坏。华北克拉通自18亿年前形成后至中生代一直保持相对稳定,然而自中生代以来其发生了大规模的构造变形和岩浆活动,致使克拉通遭受了强烈的破坏。这是对有关古老克拉通岩石圈稳定性的经典理论提出了挑战。因而,揭示克拉通破坏的原因-深部动力学机制,是地学界的一个重大科学问题。 

  华北克拉通广泛发育新生代玄武岩。尽管前人对这些玄武岩做了大量的研究,但对其地幔源区性质及其成因、深部动力学机制仍存在激烈争论。 

  中科院广州地化所同位素地球化学国家重点实验室的硕士生钱生平(现博士1年级)在任钟元研究员的指导下,对汉诺坝新生代玄武岩橄榄石斑晶中的熔体包裹体的化学和Pb同位素组成进行了深入的研究。结果表明汉诺坝玄武岩的地幔源区存在富集的EM1型地幔组分,其组分的来源与再循环古老洋壳和沉积物有关。另外汉诺坝玄武岩源区还存在FOZO组分(一般认为FOZO组分来自下地幔物质)。地球物理资料显示在华北克拉通中部下方存在从下地幔到上地幔的低速异常,可能指示了一个从下地幔上升的地幔柱(An et al., 2009; Lei et al., 2013)。根据汉诺坝地幔源区存在EM1FOZO组分,以及华北克拉通新生代玄武岩中发现高Fo*的橄榄石,结合前人地球物理的研究结果,我们认为汉诺坝玄武岩以及华北克拉通其他地区的新生代玄武岩的成因与地幔柱活动有关。

   

汉诺垻玄武岩(碱性和拉斑玄武岩)和玄武岩中的熔体包裹体的原位Pb同位素组成 

   

Lei et al. (2013, Phy. Earth Planet. Inter.) 

  Lei (2012) collected a large number of high-quality arrival-time data recorded at provincial seismic networks in the North China Craton and manually picked from original seismograms (Zheng et al., 2009, 2010) of teleseismic events. His tomographic model shows an obviously Y-shaped low-V anomaly under the Datong volcano and Bohai Sea, and this low-V anomaly extends down to the lower mantle (Figs. 11 and 12)(Lei et al., 2013; Phy. Earth Planet. Inter.), suggesting that the Datong volcano might originate from the lower mantle. It is also found that the high-V anomalies representing the stagnant Pacific slab in the mantle transition zone show a clear gap (Fig. 11g and h), which may correspond to the upwelling of the hot material from the lower mantle (Lei et al., 2013). 

 

       研究成果已发表在国际地球化学权威期刊Chemical geology上,文献信息如下:

       Qian Sheng-Ping,Ren Zhong-Yuan*, Zhang Le, Hong Lu-Bing, Liu Jian-Qiang,2015. Chemical and Pb isotope composition of olivine-hosted melt inclusions from the Hannuoba basalts, North China Craton: Implications for petrogenesis and mantle source. Chemical geology 401, 111-125)。 

  链接:http://www.sciencedirect.com/science/article/pii/S0009254115000662     

  Abstract 

  Melt inclusions and their host olivine provide unique information about the nature, distribution and scale of mantle source heterogeneity. We present the first analyses of the chemical and Pb isotope compositions of melt inclusions and their host olivine from the Cenozoic Hannuoba basalts, North China Craton, which contains coexisting suites of alkali and tholeiitic basalts. There is limited variation in major and trace element composition, but significant Pb isotopic variation, in the tholeiitic samples. This contrasts with the substantial variation in major and trace elements, but limited Pb isotopic variation, in the alkali basalt samples. Based on the results of the major- and trace-element modeling, the compositional variation of the alkali basalts may be primarily attributed to garnet pyroxenite melting, with only a small input from garnet peridotite melts. The garnet–pyroxenite component involved in the genesis of the alkali basalts, of metasomatic origin, may have formed in the lithosphere. The wide range of isotopic composition of the tholeiitic basalts and remarkably limited variability of major and trace element composition is argued to be characteristic of the source region, which is a mixture of peridotite with small amounts of pyroxenite transformed from recycled ancient oceanic crust and sediment. The presence of recycled oceanic crust in the mantle source and the low–velocity anomaly observed beneath the Taihang Mountains, located 50 km to the southeast of the Hannuoba region, supports a plume model for their origin.   

  (注:橄榄石的Fo*为橄榄石中镁橄榄石的摩尔百分数。Fo = Mg/(Mg+Fe)*100%,其中MgFe为橄榄石中MgFe离子数。

    

  同位素地球化学国家重点实验室和广州地化所科技处供稿 

 
 

  

  

  

  

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