秦岭造山带晚三叠世糜署岭岩体的岩石磁学及磁组构可靠性约束

近年来,针对秦岭造山带晚三叠世花岗岩体侵位机制的巨大争议,一些研究采用磁组构方法分析了岩体的内部组构特征及其与区域构造的关系,提出了具有重要意义的新认识.然而,目前这些研究均缺乏对岩体磁组构本质意义的分析,利用该方法约束岩体内部组构的可靠性并不十分清晰.针对这一问题,本文以秦岭造山带内具典型代表性意义的晚三叠世糜署岭花岗岩体为例,开展了该岩体的磁组构、岩石磁学、矿物形态组构和显微构造的综合研究.结果表明,糜署岭岩体的磁化率总体较低,属钛铁矿系列花岗岩.绝大部分样品的磁化率受控于顺磁性的黑云母等铁镁硅酸盐矿物,部分高磁化率样品包含了少量多畴磁铁矿等铁磁性组分的贡献,且随磁化率增大,铁磁性组分的贡献更为明显.样品的磁组构也主要是黑云母组构或由黑云母与磁铁矿的亚组构复合而成.由于样品中磁铁矿含量较低且与黑云母密切共生,磁组构与黑云母形态组构基本一致,因此,黑云母与磁铁矿的亚组构基本共轴.糜署岭岩体的磁组构本质上等同于黑云母组构,反映了黑云母等页硅酸盐矿物在岩体中的分布,可以有效的指示岩体的内部构造特征.宏观和显微构造观察还显示,糜署岭岩体的内部组构形成于岩浆侵位的晚期阶段,叠加了同岩浆期区域构造的关键信息,是从岩体构造角度开展区域构造演化的良好载体.

Magnetic mineralogy and the reliability of AMS in the Late Triassic Mishuling pluton,Qinling orogen

The emplacement mechanism of the Late Triassic granite plutons in the Qinling orogen remains controversial to date. Several recent studies had placed some new constraints on this problem based on the magnetic fabrics of these granite plutons. These studies have successfully elucidated the correlation between regional tectonics and granite emplacement. However, in these studies, the nature and reliability of the magnetic fabrics are not specifically studied. We thus conducted a combined magnetic fabric and rock magnetism study of the Mishuling granite pluton, one of the typical Late Triassic plutons in the Qinling orogen, to address this issue.#br#Several methods including magnetic fabric, rock magnetism, mineral shape preferred orientation (SPO of biotite) and microstructures were used to identify the magnetic susceptibility carrier and its AMS of the Mishuling pluton. The magnetic fabrics were measured under low applied filed at room temperature. Rock magnetism studies include the temperature-dependent susceptibility curve (χ-T curve), magnetic hysteresis loops, stepwise acquisition curve of the isothermal remanent magnetization (IRM) and stepwise back-field demagnetization curve of the saturation IRM (SIRM). Further mineral identification was conducted by the microscopic observations of thin sections and electron probe microanalysis (EPMA). SPO of biotites in the pluton were measured to determine whether differences exist between magnetic fabric and mineral fabric. Microstructural characteristics of the pluton were observed to assess when the magnetic fabrics were acquired during the magma emplacement.#br#Magnetic fabrics of the Mishuling pluton are well grouped. The contours of magnetic foliations in stereonets show a small circle girdle pattern, indicating that magnetic foliations within the pluton are concentric. Dip angles of the foliations are moderate to steep. Magnetic lineations form two subgroups that strike NW-SE and NE-SW respectively with low to moderate dip angle. Several sites show N-S striking lineations with shallow plunge angles. The mean magnetic susceptibility (κ_m) of most samples from the Mishuling pluton is lower than 500 μSI, which indicates this pluton is ilmenite-series granite. Corrected anisotropy degree (P_J) values of Mishuling pluton samples vary from 1.004 to 1.403, with an average of 1.042. Most samples have P_J values lower than 1.08, predominantly between 1.02 to 1.06, and only four sites have values above 1.10. Shape parameter T varies greatly, and all three ellipsoid types (prolate, neutral and oblate) can be observed. The oblate ellipsoids are predominant. However, no reliable correlations can be established among κ_m, P_J and T in the pluton. Rock magnetism studies show that both the paramagnetic and ferromagnetic minerals contribute to the magnetic susceptibility and AMS of the Mishuling pluton. Magnetic susceptibility carrier of most samples is biotite, whereas few samples of high κ_m have multi domain (MD) magnetite carriers. The contributions of magnetite to the magnetic susceptibility increase along with the κ_m value. SPO measurement of biotites shows that there is no difference of orientation between mineral (biotite) and magnetic fabrics. The magnetic fabrics are mainly biotite fabric or composite fabrics of biotite and magnetite subfabrics. Because i) the magnetites are rare in the samples, ii) magnetite occurs mainly mimetically along biotite cleavage planes, iii) the AMS result closely approximates the biotite preferred orientations, the biotite and magnetite subfabrics should therefore be coaxial. Field and microstructural observations identify two types of fabric, magmatic and submagmatic flow, within the Mishuling pluton, indicating that the internal fabrics in Mishuling pluton are acquired at the late stage of magma emplacement, and recorded the influence of regional tectonics, which can be confidently employed to discuss regional tectonic evolution.#br#The carrier of magnetic susceptibility and AMS of the Mishuling pluton is mainly biotite. For some samples with high κ_m values, contribution from the magnetite increases. The biotite and magnetite subfabrics of the Mishuling pluton are coaxial; magnetite generally occurs mimetically along the biotite cleavage planes. The magnetic fabric of the Mishuling pluton can therefore be regarded as the biotite fabric, and can be confidently used to delineate the internal fabrics of the Mishuling pluton. Moreover, microstructural characteristics show that the magnetic fabrics of the pluton were acquired during the late stage of magma emplacement.