| Mesozoic mafic dyke swarms are well developed in the coastal southeastern (SE) China, intruding widespread Mesozoic granitoids and volcanic rocks. Previous studies of the dyke swarms have mainly focused on petrological, geochronological and geochemical analyses of the origin of the dykes, the nature of the magma source and the tectonic setting. However, little attention has been given to emplacement processes, emplacement tectonic setting and the potential magma source locations of these dyke swarms. In this paper, we present a detailed study of rock magnetic properties and AMS (Anisotropy of Magnetic Susceptibility) of Mesozoic mafic dyke swarms in coastal SE China to reconstruct magma emplacement process, to locate possible feeder magma chambers and to better understand the tectonic setting which controlled the dyke swarms emplacement.Magma flow directions for 7 Mesozoic mafic dyke swarms (Shengsi dyke swarms (SSDS), Dongji dyke swarms (DJDS), Tiantai dyke swarm (TTDS), Wencheng dyke (WCD), Changle dyke swarms (CLDS), Meizhoudao dyke swarms (MZDS), and Jinjiang dyke swarms (ZJDS)) exposed in coastal SE China were analyzed using rock magnetism, anisotropy of magnetic susceptibility (AMS) and field evidence. K-T curves, IRM curves, hysteresis loops and coercive force spectrums suggested the main contributors of AMS in the dykes are magnetite and titanomagnetite. FORC (First-order reversal curve) diagrams showed that there were three types of magnetic particles in the specimens, consisting of SD (Single domain), PSD (Pseudo-single domain) and MD (Multidomain) particles. In addition, magma flow directions for Wanning dyke swarms (WNDS) exposed at southeast of Hainan province were analyzed.At least one dyke with a different strike was sampled at every locality. Five types of magnetic fabric (normal, inverse, intermediate, abnormal and random) were detected. The predominant normal fabric is related to magma flow. AMS of the dyke swarms originates mainly from the distribution anisotropy of intersertal magnetite or titanomagnetite that crystallized during late stage of magma flow or after magma cooled.At SSDS, the NE-trending dyke was fed by horizontal or sub-horizontal flow from southwest to northeast, the EW-trending dyke was laterally injected from west to east, the NW-trending dyke was laterally injected from southeast to northwest. This emplacement mode suggested that a mafic magma source may exist in the southwestern area far away from Shengsi Island. At DJDS, NW-trending dykes were emplaced from northwest to southeast at moderate to high angles. In contrast, magma flow in dyke MZH-7 was from the southwest to northeast while in dyke QB-2 it was from the northeast to southwest, both ascending at moderate to high angles. The opposite sub-vertical magma flow in NE-trending dykes on two islands probably resulted from multiple multi-directional magma impulses from a magma chamber below Dongji Town. At TTDS, magma flow of dyke TT-1D was sub-vertical from west to east and the magma reservoir was located to the west of Tiantai. At WCD, MZDS and JJDS, vertical magma flows indicate that the magma reservoirs were located below the swarms but with no definite flow directions inferred. At WNDS, diabase dykes were emplaced from the NNW to the SSE at low to moderate angles and the magma chamber was located in the area far to the north of Wanning.The low Pj (the anisotropy degree of AMS fabric) values, different orientations of principal AMS axes, and parallel magnetic foliations of normal fabric oblique to dyke walls indicate the syntectonic emplacements of DJDS, TTDS, WCD, MZDS and JJDS, under an extensional environment caused by subduction of the Palaeo-Pacific plate during the Late Mesozoic. Emplacement of MZDS and JJDS was controlled by Changle-Nan’ao fault, which was a sinistral strike-slip shear zone before 90±2 Ma and then transformed to dextral strike-slip shear zone. |
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