Abstract We present detailed rock magnetic and paleomagnetic results for a late Neoproterozoic mafic-ultramafic layered intrusion of Gebel Dahanib, South Eastern Desert, Egypt. Gebel Dahanib intrusion (GDI) is an elliptical body with saucer or funnel shape, and was intruded into deformed country rocks at ∼710 Ma. It consists of two main undeformed and unmetamorphosed sequences: I) a lower ultramafic sequence composed mainly of peridotite (dunite and lherzolite cumulate) and pyroxenite (olivine websterite and websterite cumulates). Dunite cumulate occupies the basal part, which grades upward to lherzolite and pyroxenite cumulates at the top of the ultramafic sequence; and II) an upper mafic sequence that comprises olivine gabbronorite, gabbronorite, norite, and a chilled margin of fine-grained gabbronorite. An intrusive contact was observed between ultramafic and mafic sequences, which suggests an asynchronous emplacement history. To understand the magmatic evolution of GDI and to make a new contribution to the small African Neoproterozoic paleomagnetic database, 260 oriented samples were collected from four rock units (peridotite, pyroxenite, gabbro, and fine-grained gabbro). Rock magnetic experiments combined with Fe–Ti microscopic observations and demagnetization procedures indicate that both sequences have different magnetic mineral contents and paleomagnetic components. A stable component with northwesterly declination and shallow negative inclination carried by magnetite and pyrrhotite was isolated from the ultramafic sequence (Dec = 326.6°, Inc = −28.2°, α95 = 5.5°, and k = 58.72), and the corresponding paleomagnetic pole lies at lat = 39.0°N, long = 261.5°E, A95 = 4.6°, and K = 83.13. Paleomagnetic analyses on the mafic sequence yielded northeast-directed declinations, with intermediate positive inclinations (Dec = 11.3°, Inc = 47.6°, α95 = 3.9°, and k = 103.19), and a paleomagnetic pole lat = 78.7°N, long = 96.2°E, A95 = 4.4°, and K = 81.49. We postulate that the Dahanib intrusion was emplaced in two pulses: the first includes the ultramafic assemblages at the base of the intrusion; the second magma pulse makes up the upper mafic sequence. Assessment of Egyptian paleomagnetic poles from a similar time period (∼710-540 Ma) indicates that only one pole (out of 21) can be considered reliable, and also from the African dataset only five of 43 poles are of high quality. These indicate that new high-quality paleomagnetic poles are needed to refine the late Neoproterozoic paleomagnetic database for Egypt and the African continent.

中文翻译：

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东南沙漠（埃及）Gebel Dahanib 晚新元古代基性-超镁铁质层状侵入体的古地磁：对岩浆演化和磁化获取时间的影响

摘要 我们提供了埃及东南沙漠 Gebel Dahanib 晚新元古代基性-超镁铁质层状侵入体的详细岩石磁性和古地磁结果。Gebel Dahanib 侵入体（GDI）是一个椭圆体，呈碟状或漏斗状，在 710 Ma 时侵入变形的围岩。它由两个主要的未变形和未变质层序组成：I) 一个下部超镁铁质层序，主要由橄榄岩（纯英岩和橄榄石堆积体）和辉石岩（橄榄石橄榄石和橄榄石堆积体）组成。单晶堆积体占据基底部分，向上分级为锂辉石，辉石堆积在超镁铁质层序的顶部；和 II) 上部基性岩层序，包括橄榄石辉长岩、辉长岩、紫长岩和细粒辉长岩的冷边缘。在超镁铁质和镁铁质序列之间观察到侵入性接触，这表明存在不同步的侵位历史。为了了解GDI的岩浆演化并为小型的非洲新元古代古地磁数据库做出新的贡献，从四个岩石单元（橄榄岩、辉石岩、辉长岩和细粒辉长岩）采集了260个定向样品。岩石磁性实验结合 Fe-Ti 显微观察和退磁程序表明两个层序具有不同的磁性矿物含量和古地磁成分。从超镁铁质层序（Dec = 326.6°，Inc = -28.2°，α95 = 5.5°，k = 58.72）中分离出一个由磁铁矿和磁黄铁矿携带的西北偏向和浅负倾角的稳定成分，对应的古磁极位于在纬度 = 39.0°N，长 = 261.5°E，A95 = 4.6°，K = 83.13。对基性岩层序的古地磁分析产生了东北方向的偏角，中间为正倾角（Dec = 11.3°，Inc = 47.6°，α95 = 3.9°，k = 103.19），古磁极纬度 = 78.7°N，长 = 96.2°E，A95 = 4.4°，K = 81.49。我们假设 Dahanib 侵入发生在两个脉冲中：第一个包括侵入底部的超镁铁质组合；第二个岩浆脉冲构成了上部基性岩层序。对类似时期（~710-540 Ma）的埃及古地磁极的评估表明，只有一个极（21 个中）可以被认为是可靠的，而且从非洲数据集来看，43 个极中只有 5 个是高质量的。