中国东南新生代玄武岩地球化学研究：源区富集组分的来源

发布时间：2018-03-24 14:23

  本文选题：中国　切入点：东南　出处：《中国科学技术大学》2017年博士论文

【摘要】：板内玄武岩的起源和成因已经持续争论了数十年,至今仍在不断引起地球化学家讨论与关注。板内玄武岩具有的富集地球化学特征往往被地球化学家和地幔源区中来自地球浅部圈层再循环物质联系起来。中国东部地区新生代广泛发育板内玄武岩,为研究再循环物质组分在大陆板内玄武岩源区的贡献,以及俯冲活动对板内玄武岩的影响提供了可能。当前已有的研究表明中国东部新生代玄武岩源区中富集组分的可能来自于:(1)俯冲板块中的再循环洋壳或者大洋沉积物;(2)中生代拆沉的加厚大陆下地壳/岩石圈地幔;(3)具有富集特征大陆岩石圈地幔。中国东部地区自中生代以来经历了复杂的地质过程。中国东部地区自中生代以来经历了多期次的俯冲事件,地球物理地震图像表明俯冲的太平洋板块至今还滞留在地幔过渡带(MTZ)中,同时,地球物理地震图像证据也表明在长白山和海南岛地区可能存在有地幔柱结构。中国东部地区的复杂地质历史背景为研究不同地质背景下的玄武岩源区的差别提供了可能。中国东南沿海新生代板内玄武岩下部的三种不同地幔结构特征:地球物理地震图像表明中国东南沿海地区北部地区(浙江地区与江西地区)下的地幔过渡带里有滞留的太平洋俯冲板块存在;中部地区(福建与广东北部)的MTZ深度的地震波异常逐渐减弱直至消失;海南岛与雷州半岛有地幔柱结构存在。水和Ce在岩浆和地幔矿物中的分配行为相近似,其比值在岩浆演化过程(部分熔融和分离结晶)中不会发生明显改变,同时,不同种类的再循环物质组分具有不同的H20/Ce比值,并且区别于DMM的H20/Ce比值。如果将玄武岩的H20/Ce与Ba/Th、Nb/La、Ce/Pb等微量元素比值相结合,可以判断其地幔源区是否有再循环物质存在,以及源区再循环物质组的来源。同时,地表再循环物质大多经历过浅部的低温反应,通常具有与普通地幔迥异的氧同位素比值(δ18O),以鉴别再循环物质存在与否。Liu et al.(2015 a和b)采用H20/Ce和δ18O联合示踪的方法提示出华北克拉通的新生代玄武岩中再循环物质组份的存在。为了确定中国东南沿海地区新生代玄武岩中是否存在再循环物质及其来源,本文分析了采自中国东南沿海地区具有不同地质背景的新生代玄武岩的主微量元素组成、Sr-Nd-Pb放射性成因同位素组成、单斜辉石斑晶的水含量、主量元素组成、氧和Li同位素组成。随后,依据O'Leary etal.(2010)提出的单斜辉石斑晶成分与水分配系数关系的经验方程,由单斜辉石斑晶主量元素组成计算出水在单斜辉石斑晶与玄武岩熔体中的分配系数,进而反演出玄武岩浆的"原始"水含量,为中国东南沿海地区新生代玄武岩源区中富集组分的来源提出了新线索和制约:(1)浙江新生代玄武岩水含量分布范围为1.3-2.6%,与岛弧玄武岩和弧后盆地玄武岩的水含量一致。浙江新生代玄武岩主要喷发于两个时代:20-27 Ma以及晚于11Ma。两个时期喷发的玄武岩具有迥异的地球化学特征。偏老的玄武岩采自于西垄(XL)地区,具有更高的碱含量、微量元素含量、La/Yb、Ce/Pb和Nb/La比值,更低的硅铝含量,以及H20/Ce和Ba/Th比值。西垄玄武岩可以在蛛网图上的观察到强烈的K、Pb、Zr、Hf和Ti负异常,以及Nb(Ta)正异常。较年轻的新昌玄武岩(包括高坪(GP)和双彩(SC)两个地区)具有比西垄玄武岩低的碱含量、微量元素含量、La/Yb、Ce/Pb和Nb/La比值,更高的硅铝含量,以及更高的 H2O/Ce 和 Ba/Th 比值。Ba/La、H20/Ce、Nb/La、Ce/Pb 与 Ba/Th 之间协同的变化关系表明西垄新生代玄武岩源区除了亏损地幔以外还有再循环洋壳组分存在,新昌新生代玄武岩的源区除了 DMM与再循环洋壳组分以外,还含有再循环大洋沉积物组分。再循环物质组分在浙江新生代玄武岩源区的连续变化表明在20-11Ma之间有新的再循环物质的持续供给与加入。华南自中生代以来只受西向的太平洋俯冲的影响,因此浙江新生代玄武岩源区连续加入的再循环物质组分只可能来源于滞留在地幔过渡带内的太平洋俯冲板块。(2)福建新生代玄武质岩石喷发或侵位于17Ma之后,其微量元素组成与GP和SC的浙江玄武岩相类似。明溪(MX)和石衡(SH)玄武岩在蛛网图上具有明显的Th、U、Pb、K、Zr、Hf和Ti负异常,而白琳(BL)辉绿岩在蛛网图中不具有明显的K、Zr、Hf和Ti异常。与浙江玄武岩都具有高水含量特征不同,福建地区三个采样点的玄武质岩石水含量差异较大:SH玄武岩的水含量为1.3-2.4%;MX玄武岩的水含量为0.3-0.5%;BL地区的辉绿岩的由于单斜辉石中的H经历过扩散,其表观水含量为1.9%-2.1%。明溪新生代玄武岩的水含量明显低于其他浙江和福建地区的新生代玄武岩,表明水在上地幔的分布具有明显的不均一性。福建新生代玄武质岩石中的单斜辉石斑晶具有高于普通地幔的δ18O值,表明其源区有再循环物质组分的存在。最高的δ18O值出现在具有中等Si/Al和Ca/Al比值的单斜辉石斑晶中,表明多种含有不同比例再循环物质组分的岩浆后混合。福建玄武质岩石的H2O/Ce、Ba/La、Nb/La、Ce/Pb和Ba/Th之间协变关系,表明福建地区新生代玄武质岩石的源区同样存在再循环的大洋物质组分。尽管现存的地球物理学证据表明,MX和SH下并没有与滞留于于MTZ的俯冲太平洋板片,但是这些再循环组分可能来自于已经下沉到地幔的俯冲大洋板片在上幔的零星残留。如果考虑到福建玄武质岩石的放射成因同位素并非过于富集,这些"消失"的俯冲大洋板片的年龄不可能太老,太平洋俯冲的可能性依然最大。(3)相对于浙江和福建新生代玄武岩,海南新生代玄武岩在微量元素蛛网图上表现出多种类型的OIB-like特征,包括类似HIMU和类似EM的特征,表明海南玄武岩的源区相对于浙江和福建新生代玄岩源区具有更大的不均一性和复杂性。相对MORB而言,海南新生代玄武岩具有更富集的Sr-Nd-Pb放射成因同位素特征和不同于典型地幔值(+5.7‰)的δ18O值,表明其源区存在有再循环物质。雷虎岭(LH)新生代玄武岩单斜辉石斑晶的Li同位素剖面分析结果表明其在结晶后经历了 Li扩散,其与马鞍岭(ML)以及Sanjiaoyuan(SJY)近乎完全"干"的水含量特征并不是其源区的原始特征,而可能是单斜辉石斑晶结晶后H丢失的结果。在去掉受H丢失影响的样品后,LH,大洋(DY)和福基田(FJT)玄武岩水含量分别为2.7%、1.3%以及2.6%。LH玄武岩具有类似HIMU的微量元素特征、高δ18O、低Ba/Th与H20/Ce比值表明其地幔源区存在再循环上部洋壳组分,少量的低δ18O单斜辉石斑晶表明除再循环部洋壳组分外,其源区可能还含有少量的再循环大洋下地壳的辉长岩组分。DY和FJT玄武岩的高Ba/Th、δ18O和H2O/Ce比值特征则表明其源区可能含有再循环的大洋洋壳和沉积物组分。总的看来,具有不同地幔结构的中国东南沿海新生代玄武岩源区都有再循环的洋壳物质组分存在。中国东南沿海新生代玄武岩通常都具有与岛弧玄武岩和弧后盆地玄武岩相类似的高水含量,即使这些玄武岩具有不同的放射性成因Sr-Nd-Pb同位素比值和微量元素特征。放射性成因Sr-Nd-Pb同位素比值和微量元素的变化表明中国东南沿海不同地区的玄武岩的地幔源区中的再循环大洋物质组分组成不同,最有可能是受到了西向的俯冲的太平洋板块的影响。
[Abstract]:The origin and genesis of intraplate basalt has been debated for decades, still continue to cause geochemist discussion and attention. With the enrichment of intraplate basalt geochemistry are Geochemists and mantle sources in shallow layers of material recycling from Earth linked. The eastern region of Cenozoic China widely developed for Inouchi Xuantakeiwa. Study on the recycling of material components in the continental intraplate basalt source contribution, provides the possibility and influence of subduction activities on intraplate basalt. The current research shows that Chinese Eastern Cenozoic basalt source region in the enriched components may come from: (1) the subducting plate of recycled oceanic crust or oceanic sediments; (2) the Mesozoic lower crust, and thickened sink / lithospheric mantle; (3) with enrichment characteristics of continental lithospheric mantle in the eastern region of China. Since the Mesozoic in experience The complicated geological process. In the eastern region of China experienced many times since Mesozoic subduction events, geophysical seismic images show that the subduction of the Pacific plate still stranded in the mantle transition zone (MTZ), at the same time, geophysical seismic image evidence also shows that there might be a mantle plume structure in Changbai Mountain and Hainan Island area. Provide the difference may be complex geological history background of Eastern China as research under different geological background of basalt source area. Three different mantle structure Chinese along the southeast coast of Cenozoic Basalt in the lower part of the plate: geophysical seismic images show that the northern coastal area of Southeast China (Zhejiang area and Jiangxi area) of the mantle transition zone. There are stranded in the subduction of the Pacific plate; the central region (Fujian and Northern Guangdong) seismic wave MTZ depth anomaly gradually weaken until disappeared; the sea The South Island and the Leizhou Peninsula exist mantle plume structure. The distribution behavior of water and Ce in magma and mantle mineral phase in the approximation, the ratio in the process of magmatic evolution (partial melting and fractional crystallization) will not change obviously, at the same time, different kinds of material recycling groups with different H20/Ce ratio, H20/Ce ratio and difference in DMM. If the H20/Ce and Ba/Th basalts of Nb/La, Ce/Pb and other trace element ratio combination, can determine whether the mantle source region have recycled material existence, and the source area of recycled substances group. At the same time, the surface of recycled substances have experienced low temperature shallow, usually with oxygen isotope ratios and ordinary different mantle (delta 18O), to identify the presence and absence of recycled substances (2015 a.Liu et al. and b) by H20/Ce and delta 18O combined tracer indication of the North China Craton Cenozoic basalts Recycle components. In order to determine the existence of recycled substances and new sources of the southeast coastal area of Chinese on behalf of Xuan Wuyan, this paper analyzes the main trace elements collected from the coastal area of Southeast China with different geological background of the new generation of Xuan Wuyan's composition, Sr-Nd-Pb radiogenic isotopic composition, water content of clinopyroxene phenocrysts, major elements composition, oxygen and Li isotope composition. Then, according to O'Leary etal. (2010) proposed empirical equations clinopyroxene phenocryst composition and water distribution coefficient, by Dan Xiehui phenocrysts major element composition distribution coefficient calculation of water in clinopyroxene phenocrysts and Xuan Wuyan in the melt, to reverse the "primitive basaltic magma" water content, put forward new clues and control for the source of the new enrichment group in southeast coastal areas of China generation Xuan Wuyan source region: (1) the new generation of Zhejiang Xuan Wuyan The distribution range of water content is 1.3-2.6%, and the water content of consistent basin basalts and arc Shima Xuantakeiwa. Zhejiang Cenozoic basalts mainly erupted in two times: the geochemical characteristics of 20-27 Ma and two 11Ma. later than the period of eruption of the basalt is different. The older basalts collected from the west ridge (XL) area, has alkali content is higher, the content of trace elements La/Yb, Ce/Pb, and Nb/La ratio, silicon aluminum content lower, and H20/Ce and Ba/Th ratio. The west ridge basalt in the spider diagram of the observed strong K, Pb, Zr, Hf and Ti negative anomalies, and Nb (Ta) is abnormal. The younger the basalt of Xinchang (including Gaoping (GP) and Shuangcai (SC) two area) with west ridge basalts with low alkali content, La/Yb content of trace elements, Ce/Pb, Nb/La and the ratio of silicon, aluminum content higher, and higher H2O/Ce and Ba/Th ratios of.Ba/La, H20/Ce, Nb/La, Ce/Pb and B The relationship between a/Th showed synergistic ridge Cenozoic source region of West Xuan Wuyan in addition to depleted mantle besides recycled oceanic crust components exist, the source area of Xinchang new generation of Xuan Wuyan in addition to the DMM and recycled oceanic crust components, also containing recycled oceanic sediment components. The recycling material group changes continuously in Zhejiang new generation Xuan Wuyan source the show at 20-11Ma between the continuous supply of new material recycling and join. Effects of Southern China since Mesozoic only by westward subduction of the Pacific, so the recycle group of Zhejiang new generation Xuan Wuyan source area continuous addition only may be derived from the mantle transition zone in the stranded in the subduction of the Pacific plate. (2) after the eruption of basaltic rocks or invade Fujian Cenozoic is located in 17Ma, the trace element composition similar to GP and SC in Zhejiang. Mingxi basalt (MX) and Shi Heng (SH) Xuan Wuyan in the spider net With Th, Pb, K map U, Zr, Hf and Ti negative anomalies, and Bai Lin (BL) - has no obvious K in the spider diagram of Zr, Hf and Ti anomalies. And Zhejiang basalt are characterized by high water content in different regions of Fujian, the three sampling points of basaltic rock water content is quite different: the water content of SH basalt is 1.3-2.4%; water content MX basalt is 0.3-0.5%; BL region because of diabase in clinopyroxene H through diffusion, its apparent water content water content 1.9%-2.1%. Mingxi Cenozoic basalts from Zhejiang and Fujian was significantly lower than that in other regions of the Cenozoic basalts, shows that water inhomogeneity in the distribution of upper mantle. The Fujian Cenozoic basaltic rocks in clinopyroxene phenocrysts with delta 18O is higher than that of normal mantle values showed that the source region of recycle material composition. 8 18O the highest value appeared in medium and Si/Al The ratio of the grouper clinopyroxene crystal Ca/Al, showed that many components with different proportion of recycled magma mixing. Fujian basaltic rock H2O/Ce, Ba/La, Nb/La, Ce/Pb and Ba/Th between the covariant relations, indicating that the oceanic material formation in Fujian area of Cenozoic basaltic rocks in the source region are also recycled. Although the physics of the earth the existing evidence suggests that MX and SH were not associated with the subduction of the Pacific plate held in MTZ, but these recycling components may come from the already sinking to the subducted oceanic slab mantle in the mantle of the scattered residue. If taking into account the radiogenic isotope basaltic rocks in Fujian are not too rich, these "dive the ocean plate disappeared" film age may not be too old, the possibility of the subduction of the Pacific is still the largest. (3) compared with the Zhejiang and Fujian Cenozoic basalts, Hainan Cenozoic basaltic rocks in the trace Element spider diagram showing OIB-like characteristics of various types, including features like HIMU and similar EM, showed that the source region of Hainan basalts relative to Zhejiang and the Fujian Cenozoic dolerite source region is uneven and greater complexity. Compared with MORB, the Hainan Cenozoic basaltic rocks have isotopic characteristics of Sr-Nd-Pb radiation causes more enrichment and different from the typical mantle value (+5.7%) of the 18O values, show that the source region has recycled material. Lei Huling (LH) Li isotope profile analysis results of Cenozoic basalts indicate that the clinopyroxene Shi Banjing experienced Li diffusion after crystallization, and the Saddle Ridge (ML) and Sanjiaoyuan (SJY) the original features almost completely "dry" water content is not in the source area, and may be clinopyroxene phenocryst after crystallization. In the result of the loss of H removed by H lost samples, LH, ocean (DY) and Fuji (FJT) Tian Xuan Wu rock water content were 2.7%, 1.3% and 2.6%.LH characteristics of trace elements in basalt is similar to HIMU, high Ba/Th and low 18O value, H20/Ce ratio showed that the recycling of the upper oceanic crust component on the mantle source region, a low delta 18O clinopyroxene phenocrysts showed that except the shell group were divided into.DY ocean recirculation, and FJT basalts the high Ba/Th group gabbro in the source area may also contain a small amount of recycled oceanic crust, Delta 18O and H2O/Ce ratios show that the source region containing recycled oceanic crust and sediment components. In general, the new oceanic crust group in southeast coastal Chinese generation basalt source areas with different structures of the mantle the recycle. China along the southeast coast of the Cenozoic basalts are usually with high water content and Shima Xuantakeiwa back arc basin basalt is similar, even if these basalts have different radioactivity into Because of the Sr-Nd-Pb isotope and trace element characteristics. Indicating that the change of radiogenic Sr-Nd-Pb isotope and trace elements of recycled oceanic material group in different areas of southeast coastal China basalt mantle source region of different components, most likely to be affected by the west to the Pacific plate subduction.

【学位授予单位】：中国科学技术大学
【学位级别】：博士
【学位授予年份】：2017
【分类号】：P588.145
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本文编号：1658687