Pegmatite emplacement during post-orogenic cooling and extension: Constraints from multi-sensor high resolution remote sensing and structural analysis of one of the largest pegmatite swarms on Earth

Pegmatites form typically in swarms of tens to thousands of sheeted intrusions that may display regional fractionation trends that indicate zones of pegmatite mineralization or a potential parental granite. To substantiate these and other spatial relationships concerning pegmatite formation, studies that address the structural–tectonic relationships of an entire swarm are necessary. In the Seridó Pegmatite Province (NE Brazil), we employed remote sensing multispectral images with subsequent field validation, and identified more than 10,000 pegmatite bodies, with length and orientation precision, over the southeastern 20% of the Seridó Belt. Structural criteria, from distribution of pegmatite-host rock discordance to power law length scaling indicate the Seridó Pegmatite Province was formed mainly after regional transcurrent shearing and without control of bounding shear zones, by a combination of forceful migration through intact rock, and injection of pegmatite-forming melts in pre-existing fracture networks. Orientation distributions of discordant pegmatites are compatible with a near-hydrostatic extensional stress setting developed in an episode of renewed regional cooling, and match the NNE trend of deep-seated modeled elongate dome structures. Due to the 65 Myr hiatus separating pegmatite emplacement from the peak of HT-LP metamorphism, and the lack of a parental granite, we estimate that the ca. 3 , 1 6 0 × 1 ⁢ 0 6 m 3 of pegmatite in over 10,000 bodies could potentially constitute the largest pegmatite swarm on Earth. It was likely formed after orogeny by release of residual melts fractionated from deep, slowly cooled plutons generated by melting of diverse rock types in the orogenic crust. This new survey may help guide future critical mineral exploration efforts and testing pegmatite petrogenesis models.