Younger and older zircons from rocks of the oceanic lithosphere in the Central Atlantic and their geotectonic implications
Younger and older zircons from rocks of the oceanic lithosphere in the Central Atlantic and their geotectonic implications
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大西洋中部海洋岩石圈岩石中较年轻和较古老的锆石及其大地构造意义
DOI：
10.1134/s0016852110060038
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发表时间：
2010
期刊：
影响因子：
1.1
通讯作者：
I. S. Ipat’eva
中科院分区：
文献类型：
--
作者：
S. Skolotnev;V. Bel’tenev;E. Lepekhina;I. S. Ipat’eva
对从大西洋中脊 (MAR) 和卡特海山（塞拉利昂海隆）顶部区域的各种岩石中分离出来的锆石进行了局部 U-Pb 定年。原位形成的较年轻的锆石与较老的异种锆石结合在富集的玄武岩、碱性火山岩、辉长岩和斜长花岗岩中。仅在贫化玄武岩和橄榄岩中发现较老的锆石。较老的锆石在大西洋中部年轻的海洋岩石圈中无处不在。 MAR 顶部区域较年轻的锆石的年龄范围为 0.38 至 11.26 Ma，并且从山脊的轴向区域逐渐向后退去。这一事实为海底扩张提供了额外的证据。根据所研究的锆石年龄计算出的半扩散率接近于根据磁异常估计的半扩散率。卡特海山较年轻的锆石的年龄 (58 Ma) 与火山大厦的年龄相对应。老锆石形成了从 53 Ma 到 3200 Ma 的年龄系列。不同年龄的锆石簇显示出约200 Ma的准周期性，大致对应于地球地质演化中的全球构造时代。几个年龄组的较老锆石的颗粒在形态和地球化学上相近：（1）新元古代和显生宙（53-700 Ma）棱柱形颗粒，具有轻微吸收面，保存完好或半透明的振荡分带，以及岩浆锆石固有的地球化学特征； (2) 1811 Ma 的棱柱状颗粒，具有吸收面和边缘、碎片或半透明分带以及岩浆锆石固有的地球化学特征； (3)内部结构和变质地球化学参数混乱的卵圆形和高度吸收的棱柱形颗粒；他们的年龄峰值是1880 Ma。所进行的研究表明，来自 MAR 顶部区域年轻岩石的较古老的异种锆石被熔体捕获或并入可能在软流圈中岩浆生成水平的岩石圈下地幔中的难熔复位岩中。有人认为，锆石可能是地质历史中反复迁移穿过软流圈的熔体结晶而成的，也可能是在冈瓦纳大陆更古老的大陆分裂过程中与解体和分层的大陆岩石圈块一起被软流圈捕获的。即使在单个样本中，较老的锆石年龄的变化也可以被视为由于方向和规模不同的软流圈内对流流动周期性产生和破坏而导致物质活跃搅拌的证据。
Local U-Pb dating of zircons separated from various rocks in the crest zone of the Mid-Atlantic Ridge (MAR) and Carter Seamount (Sierra Leone Rise) is performed. Younger zircons formed in situ in combination with older xenogenic zircons are revealed in enriched basalts, alkaline volcanic rocks, gabbroic rocks, and plagiogranites. Only older zircons are found in depleted basalts and peridotites. Older zircons are ubiquitous in the young oceanic lithosphere of the Central Atlantic. The age of the younger zircons from the crest zone of the MAR ranges from 0.38 to 11.26 Ma and progressively increases receding from the axial zone of the ridge. This fact provides additional evidence for spreading of the oceanic floor. The rate of half-spreading calculated from the age of the studied zircons is close to the rate of half-spreading estimated from magnetic anomalies. The age of the younger zircons from Carter Seamount (58 Ma) corresponds to the age of the volcanic edifice. Older zircons make up an age series from 53 to 3200 Ma. Clusters of zircons differing in age reveal quasiperiodicity of about 200 Ma, which approximately corresponds to the global tectonic epochs in the geological evolution of the Earth. Several age groups of older zircons combine grains close in morphology and geochemistry: (1) Neoproterozoic and Phanerozoic (53–700 Ma) prismatic grains with slightly resorbed faces, well-preserved or translucent oscillatory zoning, and geochemical features inherent to magmatic zircons; (2) prismatic grains dated at 1811 Ma with resorbed faces and edges, fragmentary or translucent zoning, and geochemical features inherent to magmatic zircons; (3) ovoid and highly resorbed prismatic grains with chaotic internal structure and metamorphic geochemical parameters; the peak of their ages is 1880 Ma. The performed study indicates that older xenogenic zircons from young rocks in the crest zone of the MAR were captured by melt or incorporated into refractory restite probably in the sublithospheric mantle at the level of magma generation in the asthenosphere. It is suggested that zircons could have crystallized from the melts repeatedly migrating through the asthenosphere during geological history or were entrapped by the asthenosphere together with blocks of disintegrated and delaminated continental lithosphere in the process of breakup of the continents older than Gondwana. The variability in the age of older zircons even within individual samples may be regarded as evidence for active stirring of matter as a result of periodically arising and destroyed within-asthenospheric convective flows varying in orientation and scale.