Shear zone-related syenites in the Damara belt (Namibia): the role of crustal contamination and source composition
Shear zone-related syenites in the Damara belt (Namibia): the role of crustal contamination and source composition
复制标题
达马拉带（纳米比亚）与剪切带相关的正长岩：地壳污染和来源成分的作用
DOI：
10.1007/s00410-004-0589-8
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发表时间：
2004
影响因子：
3.5
通讯作者：
S. Hoernes
中科院分区：
文献类型：
--
作者：
Stefan Jung;K. Mezger;S. Hoernes
地球化学和Nd-Sr-Pb-O同位素数据的一套同碰撞（约。与元古宙达马拉造山带（纳米比亚）的主要剪切带有关的正长岩限制了它们的来源和岩石成因。剪切带内外的主要岩石类型从高钾霞石正长岩到石英正长岩，主要由幔源碱性岩浆的分离结晶作用产生。即使是最原始的样品显示显着的亏损在Nb，Ti，Sr和P的原始地幔标准化图，表明参与了再循环的地壳成分的来源。将剪切带内正长岩的Sr-Nd-Pb-O同位素组成外推回假设的含10wt% MgO的母岩熔体，表明其来源于中等富集的岩石圈上地幔（87 Sr/86 Sr：0.705，ε Nd：−2，δ 18 O：6‰，206 Pb/204 Pb：19.40，207 Pb/204 Pb：15.82）。演化程度较高的石英正长岩，其87 Sr/86 Sr比值增大，δ 18 O值增大，但随着MgO的减少，放射成因ε Nd值和Pb同位素减少，表明Ca的同化作用。10%为太古代-元古宙局部下地壳，ε Nd非放射成因，87 Sr/86 Sr高，U/Pb低。对于剪切带外的样品，假设的含10wt%MgO的母体熔体具有明显更多的放射成因Sr，但较少的放射成因Nd同位素组成（87 Sr/86 Sr：0.712，ε Nd：−13），具有强烈的非放射成因Pb同位素比值（206 Pb/204 Pb：17.40，207 Pb/204 Pb：15.50），表明这些岩石具有另一个强烈富集的岩石圈地幔源区。剪切带外的分异正长岩表现出降低的87 Sr/86 Sr、升高的δ 18 O值、更高的放射成因ε Nd值和Pb同位素比值，MgO降低，表明与低Rb/Sr、高Sm/Nd和U/Pb的岩石圈组分发生了相互作用。
Geochemical and Nd-Sr-Pb-O isotope data for a suite of syn-collisional (ca. 520 Ma) syenites associated with a major shear zone in the Proterozoic Damara orogen (Namibia) constrain their sources and petrogenesis. Major rock types from within and outside the shear zone range from highly potassic nepheline syenites to quartz syenites and were primarily generated by fractional crystallization from a mantle-derived alkaline magma. Even the most primitive samples show pronounced depletion in Nb, Ti, Sr and P on a primitive mantle-normalized diagram, indicating the involvement of a recycled crustal component in the source. Extrapolation of the Sr-Nd-Pb-O isotope composition of the syenites from within the shear zone back to a hypothetical parental melt with 10 wt% MgO suggests derivation from a moderately enriched lithospheric upper mantle (87Sr/86Sr: 0.705, ε Nd: −2, δ18O: 6‰, 206Pb/204Pb: 19.40, 207Pb/204Pb: 15.82). More evolved quartz syenites show increasing 87Sr/86Sr ratios, increasing δ18O values but less radiogenic ε Nd values and Pb isotopes with decreasing MgO, indicating assimilation of ca. 10% Archaean to Proterozoic local lower crust with unradiogenic ε Nd, high 87Sr/86Sr and low U/Pb. For samples from outside the shear zone a hypothetical parental melt with 10 wt% MgO has distinctly more radiogenic Sr but less radiogenic Nd isotopic composition (87Sr/86Sr: 0.712, ε Nd: −13), with strongly unradiogenic Pb isotope ratios (206Pb/204Pb: 17.40, 207Pb/204Pb: 15.50), suggesting another strongly enriched lithospheric mantle source for these rocks. Differentiated syenites from outside the shear zone show decreasing 87Sr/86Sr, increasing δ18O values, more radiogenic ε Nd values and Pb isotope ratios with decreasing MgO indicating interaction with a lithospheric component with low Rb/Sr but high Sm/Nd and U/Pb.