From my dissertation area, the Wolf Mountain Intrusion (Phacolith), Proterozoic Llano Uplift, central Texas. This is an autolith of biotite-rich, enclave bearing, porphyritic granite in another more phenocrystic granite. This clearly establishes the timing between the two major phases of porphyritic granite in my field area. The fact that the earlier, darker granite was competent enough to break while the second phase was fluid enough to incorporate an autolith is interesting. The presence of elongated mafic enclaves in the xenolith is of interest to working out the timing of deformation within the intrusion. The elongation of the enclaves is parallel to the elongation of the autolith and the foliation with the granite. A less obvious foliation in present in the surrounding granite has the same orientation. The question is whether the enclaves were flattened before, after, or before and after they were incorporated in the autolith. Comments welcome.
235-trend is parallel to short-axis of the image and up. Foliations are parallel to the axial plane of the phacolith. Photo by RMR