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西太平洋板块俯冲对华南新生代火山作用的影响
孙普
学位类型博士
导师牛耀龄
2017-05-19
学位授予单位中国科学院大学
学位授予地点北京
学位专业海洋地质
关键词华南新生代玄武岩 盖层效应 地幔交代作用 循环大陆上地壳 岩浆演化
摘要本论文通过分析华南地区新生代玄武岩的主量、微量元素和Sr-Nd-Pb-Hf同位素成分以及通过岩石学、矿物学和岩相学的研究,探究这些玄武岩的地幔源区特征、部分熔融过程和上升过程中的演化。本研究所采集样品主要来自华南两个地区:东南沿海地区和雷州半岛地区。
东南沿海地区新生代玄武岩在空间分布上与平行海岸线的三条断裂带有关。经过对全岩成分中橄榄石微晶的校正以及对结晶分异作用的校正,本论文获得了这些玄武岩原始熔体的成分。它们从内陆向海岸线呈现升高的SiO2、Al2O3但是降低的FeO、MgO、TiO2、P2O5、CaO和CaO/Al2O3。这种主量元素及其比值的空间变化与从内陆向海岸线降低的岩石圈厚度、降低的熔融压力和升高的部分熔融程度一致。这些玄武岩具有不相容元素非常富集的特征,但是却有不同程度的同位素亏损。这种元素和同位素的解耦说明其地幔源区经历了一种近期的、低部分熔融程度的熔体交代。这种熔体富集挥发性组分和不相容元素,但由于交代历史较短,这种地幔交代作用并没有引起放射性成因同位素的积累。另外,Sr和Nd同位素显示出与Nb/Th、Nb/La、Sr/Sr*和Eu/Eu*良好的相关性,说明这些玄武岩的地幔源区有循环大陆上地壳物质的贡献。这些玄武岩的Pb同位素同样具有空间变化,从内陆向海岸线Pb同位素比值逐渐升高。同时,Pb同位素比值与主量元素具有良好的相关性,这说明Pb同位素的空间变化同样是由不同熔融压力和不同部分熔融程度引起的。本论文认为Pb同位素富集组分是亏损的地幔基质,从内陆向海岸线玄武岩中Pb同位素比值的升高与亏损的地幔基质在熔体中贡献的增加一致。
在讨论过东南沿海地区新生代玄武岩的源区特征和地幔熔融过程后,本论文继续对其上升过程中的演化进行了探讨。这些玄武岩中携带有大量的地幔橄榄岩捕掳体。研究表明,携带有地幔捕掳体的碱性玄武岩必须快速上升才能将地幔橄榄岩携带至地壳。因此,人们一般认为携带地幔捕掳体的熔体应该经历过很低程度的结晶分异演化。但是华南沿海地区新生代玄武岩具有不同程度的演化特征,其Mg# = ~ 48-67。这种不同程度的演化特征与其快速的上升速度并不矛盾,而是体现了熔体上升过程中结晶分异作用的必然存在。通过对单斜辉石巨晶的平衡温度和压力的计算,本论文得到了一条寄主熔体上升的P-T轨迹(dT/dP ≈ 7°C/kbar)。这条轨迹比熔融地幔的绝热梯度(4-6°C/kbar)更陡,同时由于熔体温度远高于地幔橄榄岩围岩,因此熔体在上升过程中不可避免会发生热量的损失和结晶分异作用。单斜辉石巨晶的平衡压力计算结果为18-27 kbar,这说明其结晶于岩石圈地幔的条件下,并且熔体在浅层的地壳层位并没有长时间停留,因此没有低压下单斜辉石巨晶的分离结晶。这一推测也与熔体快速上升的认识是一致的。
雷州半岛地区晚新生代火山岩的地球化学特征整体上类似于东南沿海地区玄武岩,即不相容元素富集但Sr-Nd-Pb-Hf同位素呈现不同程度的亏损。但是,雷州半岛火山岩具有变化的微量元素配分模式:来自火炬地区的部分样品具有明显的Nb-Ta的负异常、Pb的正异常和相对富集的Sr-Nd-Pb-Hf同位素成分。而来自九江和田洋地区的火山岩则具有类似于东南沿海地区玄武岩的微量元素配分模式,具有Nb-Ta的正异常、相对弱的Pb的正异常和亏损的Sr-Nd-Pb-Hf同位素特征,但同时具有明显的Sr的正异常。本论文认为,火炬和九江、田洋地区火山岩的地球化学特征代表了两种具有不同元素和Sr-Nd-Pb-Hf同位素特征的物质在软流圈地幔源区的混合。九江、田洋地区火山岩主要来自经历了低部分熔融程度熔体交代的地幔源区。这种熔体富集不相容元素和挥发性组分,可以交代软流圈地幔和上覆的岩石圈地幔底部,并在岩石圈地幔中结晶角闪石等富水矿物,形成一系列的交代脉体。火炬地区火山岩的地幔源区则主要经历了循环大陆上地壳物质的交代,造成了火炬地区火山岩大部分样品的Nb-Ta元素负异常、Pb正异常和更为富集的Sr-Nd-Pb-Hf同位素特征。
其他摘要We analyzed whole-rock major and trace elements and Sr-Nd-Pb-Hf isotopes of the Cenozoic basalts in South China to investigate their mantle source characteristics,melting process and evolution during ascent. Our samples are from two areas, including those from SE China and those from Leizhou Peninsula.
The basalts from SE China are spatially associated with three extensional fault systems parallel to the coast line. After correction for the effect of olivine microlites on bulk-rock compositions and the effect of crystal fractionation, we obtained primitive melt compositions for these samples. These primitive melts show increasing SiO2, Al2O3 but decreasing FeO, MgO, TiO2, P2O5, CaO and CaO/Al2O3 from the interior to the coast. Such spatial variations of major element abundances and ratios are consistent with a combined effect of fertile source compositional variation and increasing extent and decreasing pressure of decompression melting from beneath the thick lithosphere in the interior to beneath the thin lithosphere in the coast. These basalts are characterized by incompatible element enrichment but varying extent of isotopic depletion. This element-isotope decoupling is most consistent with recent mantle source enrichment by means of low-degree melt metasomatism that elevated incompatible element abundances without yet having adequate time for isotopic ingrowth in the mantle source regions. Furthermore, Sr and Nd isotope ratios show significant correlations with Nb/Th, Nb/La, Sr/Sr* and Eu/Eu*, which substantiates the presence of recycled upper continental crustal material(UCC) in the mantle sources of these basalts. Pb isotope ratios also exhibit spatial variation, increasing from the interior to the coastal area. The significant correlations of major element abundances with Pb isotope ratios indicate that the Pb isotope variations also result from varied extent and pressure of decompression melting. We conclude that the elevated Pb isotope ratios from the interior to coast are consistent with increasing extent of decompression melting of the incompatible element depleted mantle matrix, which hosts enriched Pb isotope compositions.
Studies have shown that mantle xenolith-bearing magmas must ascend rapidly to carry mantle xenoliths to the surface. It has thus been inferred inadvertently that such rapid ascending melt must have undergone little crystallization or evolution. However, this inference is apparently inconsistent with the observation that the Cenozoic mantle xenolith-bearing alkali basalts from several locations in SE China have experienced varying degrees of fractional crystallization(Mg# = ~ 48-67). The variably evolved composition of host alkali basalts is not in contradiction with rapid ascent, but rather reflects inevitability of crystallization during ascent. Calculations for equilibrium temperatures and pressures of clinopyroxene(Cpx)megacrysts show an ascent path of the host melt with dT/dP ≈ 7°C/kbar. This path is steeper than the expected adiabat of molten mantle(4-6°C/kbar), which is consistent with the effect of conductive cooling and melt crystallization during ascent because TMelt > TLithosphere. The equilibrium pressure of 18-27 kbar for Cpx megacrysts suggests that the crystallization takes place under lithospheric mantle conditions with limited low-pressure residence in the shallower crust levels. This is in fact consistent with the rapid ascend of the host melt to bring mantle xenoliths to the surface.
The late Cenozoic volcanic rocks from Leizhou Peninsula have geochemical characteristics generally similar to the Cenozoic basalts in SE China, i.e., they are all enriched in incompatible elements but variably depleted in Sr-Nd-Pb-Hf isotopes. However, the volcanic rocks in Leizhou Peninsula show variable trace element patterns. The samples from Huoju have prominent negative Nb-Ta anomalies, positive Pb anomaly and more enriched Sr-Nd-Pb-Hf isotope compositions. In contrast, the samples from Jiujiang and Tianyang show positive Nb-Ta anomaly, moderate positive Pb anomaly, prominent positive Sr anomaly and depleted Sr-Nd-Pb-Hf isotope compositions. The geochemistry characteristrics of these rocks are generally consistent with mixing of two components with different elemental and isotopic characteristics in the mantle source region. The volcanic rocks from Jiujiang and Tianyang largely originated from a mantle source region that experienced a low-degree melt metasomatism. Such melt is enriched in incompatible elements and volatile components, which can re-firtilize the asthenospheric mantle and the overlying lithospheric mantle, cystallize hydrous minerals(e.g. amphibole) and form metasomatic veinlets in the ambient lithospheric mantle. The volcanic rocks from Huoju are derived from a mantle source region that experienced metasomtism of recycled UCC materials. Such UCC material modified mantle source can explain the negative Nb-Ta anomaly, positive Pb anomaly and enriched Sr-Nd-Pb-Hf isotope compositions of Huoju samples.
学科领域地球科学 ; 地质学
语种英语
文献类型学位论文
条目标识符http://ir.qdio.ac.cn/handle/337002/136654
专题海洋地质与环境重点实验室
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孙普. 西太平洋板块俯冲对华南新生代火山作用的影响[D]. 北京. 中国科学院大学,2017.
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