Magnetic fabric data on interactive syntectonic magmas of contrasting composition in composite dikes from south Brazil

Abstract

The composite dikes of the Zimbros Bay in the Porto Belo-Bombinhas region, southern Brazil, are emplaced in the late stages of post-collisional magmatism, at ca. 590 Ma as NE-striking, subvertical tabular bodies intrusive in basement orthogneisses. The composite dikes are formed by a felsic core containing a swarm of mafic microgranular enclaves, bordered on both sides by mafic material of irregular thickness, and individual mafic dikes parallel to the main intrusion. The felsic and mafic rocks contain a well-developed magmatic foliation with solid-state deformation near the contacts. Stretching lineation is horizontal to sub-horizontal and better developed in the felsic rocks. The absence of a well-developed linear fabric in the mafic rocks hinders investigation of their emplacement based on conventional structural data. In order to investigate/explore the relationship between contemporaneous mafic and felsic rocks, as well as to understand the emplacement of these intrusions in relation to the transcurrent tectonics, a study on the magnetic fabric was made using anisotropy of magnetic susceptibility (AMS) and rock magnetism techniques for composite and individual mafic dikes. Magnetic mineralogy of both dikes is predominantly controlled by ferromagnetic minerals (e.g. magnetite and Ti-poor tinanomagnetite), with little contribution of paramagnetic minerals. The rock magnetism study shows that the magnetite grains have varied sizes. Individual mafic dikes have small grain sizes, whilst in the mafic borders, sizes vary from small to large. Felsic cores have large grain sizes. SEM data show oxyexsolution textures in the felsic core samples and in samples of individual mafic dikes. Only Ilmenite is found in the mafic border samples, either as anhedral grains or as thin lamellae, interpreted to result from intense oxyexsolution of magnetite. Increasing oxyexsolution degrees observed in samples from individual mafic dikes to felsic cores and mafic borders suggest different degrees of interaction between the original magmas. The studied intrusions show normal magnetic fabrics, with subvertical magnetic foliation (k1-k2 plane) and subhorizontal (k1) lineation. Magnetic data are concordant with structural field measurements and indicate sub-horizontal magma flow during emplacement. The much larger volume and size of enclaves found in the southern portion of the dike is compatible with its northward directed emplacement. The obtained magnetic data are also in agreement with the syntectonic emplacement of the composite dikes relative to the transcurrent movement of the Major Gercino Shear Zone.