Melting of metasomatically enriched lithospheric mantle – Constraints from Pan-African monzonites (Damara Orogen, Namibia)
Melting of metasomatically enriched lithospheric mantle – Constraints from Pan-African monzonites (Damara Orogen, Namibia)
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DOI：
10.1016/j.lithos.2021.106332
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发表时间：
2021-10
期刊：
影响因子：
3.5
通讯作者：
R. Pontow;Stefan Jung;F. Hauff;J. Berndt
中科院分区：
文献类型：
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作者：
R. Pontow;Stefan Jung;F. Hauff;J. Berndt
560.8±2.4 Ma古Okamutambo岩体是纳米比亚达马拉带合成碰撞的Otjimbingwe碱性杂岩（OAC）的最东端侵入物，由基性-中碱性岩石组成，属于二黄长岩-正长岩系列。所研究的样品是钾质（K2O/Na2O），具有中等高的镁（MgO: 5.7-2.6 wt%）、镍（Ni: 66-26 ppm）和铬（Cr: 223-62 ppm）浓度。LILE （Ba: 2425 ~ 1243 ppm, Sr: 1359 ~ 941 ppm）和HFSE （Zr: 447 ~ 202 ppm, Nb: 32.8 ~ 16.4 ppm, Hf: 4.7 ~ 9.4 ppm, Ta: 1.2 ~ 2.0 ppm）含量也很高。锶和Nd同位素数据显示存在两个岩浆套，表明多源起源。第1组二长岩-石英二长岩具有中等演化的Sr和Nd同位素组成（初始87sr /86Sr: 0.7066 ~ 0.7073，初始εNd:−3.5 ~−5.0）和放射性成因Pb同位素比值（206Pb/204Pb: 17.65 ~ 18.02, 207Pb/204Pb: 15.62 ~ 15.67, 208Pb/204Pb: 38.19 ~ 38.32）。二组二辉长岩-正长岩的Sr和Nd同位素组成更为演化（初始87sr /86Sr: 0.7088 ~ 0.7090，初始εNd: - 6.7 ~ - 7.1）， Pb同位素比例相似（206Pb/204Pb: 17.63 ~ 17.82, 207Pb/204Pb: 15.64 ~ 15.66, 208Pb/204Pb: 38.21 ~ 38.37）。第1组二长岩-石英二长岩的分化涉及AFC过程，而第2组二长岩-正长岩的分化则受分异结晶作用的影响。尽管二级过程起作用，但最原始样品的高总碱含量、不相容的微量元素浓度超过总地壳值，以及演化的同位素组成都是源控制的，为母岩浆的起源提供了线索。根据文献实验资料推断，Okamutambo碱性岩代表由富集岩石圈地幔（辉绿岩-辉橄榄岩）熔融而形成的演化熔体。原始地幔归一化微量元素模式的Nb-Ta和Ti负异常和Pb正异常与地幔源中含有循环地壳成分一致。同位素组成表明，地幔富集是一个古老的特征，可能与元古代的俯冲有关。561 Ma时，达马拉造山带的地球动力学特征主要是刚果克拉通和卡拉哈里克拉通在辐合和大陆碰撞过程中的挤压作用，由于碱性岩浆活动通常与伸展构造机制有关，因此很难与OAC的形成相协调。然而，OAC与一个主要缝合带有关，该缝合带可能参与了斜向扁平俯冲期间的局部张拉构造，从而使幔源碱性熔体的产生和上升成为可能。
The 560.8 ± 2.4 Ma old Okamutambo Pluton is the easternmost intrusion of thesyn-collisional Otjimbingwe Alkaline Complex (OAC; Damara Belt, Namibia) and consists of mafic to intermediate alkaline rocks that belong to the monzodiorite-syenite series. The studied samples are potassic (K2O/Na2O > 1) and have moderately high magnesium (MgO: 5.7–2.6 wt%), nickel (Ni: 66–26 ppm), and chromium (Cr: 223–62 ppm) concentrations. LILE (Ba: 2425–1243 ppm; Sr: 1359–941 ppm) and HFSE (Zr: 447–202 ppm, Nb: 32.8–16.4 ppm, Hf: 4.7–9.4 ppm, Ta: 1.2–2.0 ppm) contents are also high. Strontium and Nd isotope data reveal the existence of two magmatic suites indicating a multi-source origin. Group 1 monzonites-quartz monzonites have moderately evolved Sr and Nd isotopic compositions (initial87Sr/86Sr: 0.7066 to 0.7073; initial εNd: −3.5 to −5.0) and radiogenic Pb isotope ratios (206Pb/204Pb: 17.65–18.02;207Pb/204Pb: 15.62–15.67;208Pb/204Pb: 38.19–38.32). In contrast, group 2 monzodiorites-syenites display more evolved Sr and Nd isotopic compositions (initial87Sr/86Sr: 0.7088 to 0.7090; initial εNd: −6.7 to −7.1) but similar Pb isotope ratios (206Pb/204Pb: 17.63–17.82;207Pb/204Pb: 15.64–15.66;208Pb/204Pb: 38.21–38.37). Differentiation involved AFC processes in group 1 monzonites-quartz monzonites whereas group 2 monzodiorites-syenites were modified by fractional crystallization. Although second-order processes were operative, high total alkali contents, incompatible trace element concentrations in excess of bulk crustal values, and evolved isotopic compositions of the most primitive samples are source-controlled and provide insight into the origin of the parental magmas. With reference to experimental data from the literature, it is inferred that the Okamutambo alkaline rocks represent evolved melts that were generated through melting of enriched lithospheric mantle (phlogopite-lherzolite). The observed negative Nb-Ta and Ti anomalies and positive Pb anomalies in primitive mantle-normalized trace element patterns are in line with a mantle source that contains a recycled crustal component. Isotopic compositions indicate that mantle enrichment is an ancient feature that might be linked to Proterozoic subduction. At 561 Ma, the geodynamic regime in the Damara Orogen was mainly characterized by compression during convergence and continental collision between the Congo and Kalahari cratons which is difficult to reconcile with the generation of the OAC as alkaline magmatism is commonly associated with extensional tectonic regimes. The OAC is, however, associated with a major suture zone that may have been involved in localized transtensional tectonics during oblique flat subduction and thus enabled the generation and ascent of mantle-derived alkaline melts.