Emplacement and deformation of late syn-orogenic, Grenville-age granites in the Llano Uplift, central Texas




Reed, Robert M., 1963-

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During the waning stages of Grenvillian orogenesis, voluminous, 1126 to 1070 Ma granitic plutons intruded Mesoproterozoic rocks of the Llano Uplift, central Texas. Previously the granites were believed to be "anorogenic," but this study has demonstrated that they are late syn- to early post-tectonic intrusions. These granites can be divided into two categories on the basis of intrusion morphology, intrusion mechanism, relation to deformation, and their ages. Intrusions of the first category are elongate to irregular in shape, have passive intrusion mechanisms related to folding or shearing, are commonly deformed, and apparently are older. Intrusions studied in detail include the Wolf Mountain intrusion, the Grape Creek pluton, and the Midway sill. The Wolf Mountain intrusion is a chevron-shaped granitic body that occupies the hinge of a large SE-plunging syncline. This sill-like body intruded and cooled to subsolidus conditions during folding. One foliation is magmatic and folded, whereas another is axial planar to the fold and has a solid-state component. Folding while the intrusion was not completely crystallized has resulted in complex internal structures in the hinge.