Abstract The early-syntectonic 563.7±6.1 Ma old Oamikaub diorite (Damara orogen, Namibia) consists of metaluminous, magnesian, calc-alkalic to calcic diorites, granodiorites and granites. Associated gabbro-diorites and gabbros belong to the Neikhoes metagabbro. Linear major and trace element variations imply that the rock suite evolved through fractional crystallization processes involving amphibole, biotite, Fe-Ti oxides, zircon and apatite. Initial Sr (87Sr/86Sr: 0.7058-0.7123) and Nd (e Nd: -2.1 to -18.8) isotopic compositions are highly variable and negatively correlated indicating that assimilation of crustal components occurred. Unradiogenic initial 206Pb/204Pb (16.23-17.23) and 207Pb/204Pb ratios (15.50-15.57) suggest derivation from or interaction with ancient crust with low U/Pb. Two gabbro-diorites have MgO, Ni and Cr abundances that are compatible with derivation of these rocks from upper mantle lithologies. Their initial e Nd values (-2.1 and – 7.4) and 87Sr/86Sr ratios (0.7058 and 0.7076) imply derivation from an aged metasomatized lithospheric mantle. Other mafic samples have MgO abundances and compatible element concentrations that exceed the values commonly accepted for primary mafic melts implying some accumulation of clinopyroxene and amphibole. The granodiorites form a homogenous group in which the isotope data (initial e Nd: -12.4 to -14.1; initial 87Sr/86Sr: 0.7083-0.7096) imply a lower crustal source. The granites are also magnesian and calc-alkaline but two of them are strongly peraluminous. Their isotope data (initial e Nd: -13.2 to -18.8; initial 87Sr/86Sr: 0.7099 to 0.7123) imply derivation from more ancient sources, alternatively these samples gained their isotope systematics through extensive AFC processes from parental granodiorites. A common subduction zone environment as suggested from negative Nb-Ta anomalies in multi-element diagrams seems unlikely for all samples because of a lack of isotopically depleted signatures. The data from the Oamikaub diorite and other mafic complexes are better explained by a “flat” subduction model involving mainly continental mantle lithosphere and crust with limited, if any, melting of asthenospheric mantle.

中文翻译：

---

同碰撞岩浆作用过程中的地壳-地幔相互作用——来自 Oamikaub 闪长岩和 Neikhoes metagabbro（纳米比亚达马拉造山带）的证据

摘要 早构造构造563.7±6.1 Ma老Oamikaub闪长岩（纳米比亚达马拉造山带）由金属铝质、镁质、钙碱性至钙质闪长岩、花岗闪长岩和花岗岩组成。伴生辉长闪长岩和辉长岩属于 Neikhoes 变长岩。线性主要和微量元素变化意味着岩石套件是通过涉及角闪石、黑云母、Fe-Ti 氧化物、锆石和磷灰石的分级结晶过程演化而来的。初始 Sr (87Sr/86Sr: 0.7058-0.7123) 和 Nd (e Nd: -2.1 至 -18.8) 同位素组成高度可变且呈负相关，表明发生了地壳成分的同化。非放射性初始 206Pb/204Pb (16.23-17.23) 和 207Pb/204Pb 比值 (15.50-15.57) 表明源自低 U/Pb 的古地壳或与其相互作用。两个辉长闪长岩含有 MgO，Ni 和 Cr 丰度与从上地幔岩性推导这些岩石相一致。它们的初始 e Nd 值（-2.1 和 – 7.4）和 87Sr/86Sr 比值（0.7058 和 0.7076）意味着源自老化的交代岩石圈地幔。其他镁铁质样品的 MgO 丰度和相容元素浓度超过了初级镁铁质熔体通常接受的值，这意味着单斜辉石和角闪石的一些积累。花岗闪长岩形成一个同质群，其中同位素数据（初始 e Nd：-12.4 至 -14.1；初始 87Sr/86Sr：0.7083-0.7096）暗示一个下地壳源。花岗岩也是镁质和钙碱性的，但其中两种是强过铝质的。它们的同位素数据（初始 e Nd：-13.2 至 -18.8；初始 87Sr/86Sr：0.7099 至 0.7123）暗示来自更古老的来源，或者，这些样品通过广泛的 AFC 过程从母体花岗闪长岩中获得了它们的同位素系统。由于缺乏同位素耗尽特征，多元素图中的负 Nb-Ta 异常表明所有样品都不太可能出现常见的俯冲带环境。来自 Oamikaub 闪长岩和其他基性复合体的数据可以通过“平坦”俯冲模型更好地解释，该模型主要涉及大陆地幔岩石圈和地壳，软流圈地幔的熔化（如果有的话）有限。