Abstract

The Pinckneyville quartz diorite complex underlies an area in eastern Alabama extending from the Coosa River in northwest Elmore County northeast through Coosa and Tallapoosa counties into Clay County.

Dark-gray, coarse-grained quartz diorite gneiss constitutes the major part of the complex, but there are smaller amounts of granodiorite and granite gneiss. Hornblende-biotite gneisses also occur and are considered to represent metamorphosed basic sills and dikes intruded before the quartz diorite complex. Schist septa lie within the complex and are usually parallel to the regional strike. The sequence of intrusion as interpreted from inclusions and petrographic data is (1) quartz diorite, (2) granodiorite, (3) granite. Small but persistent amounts of allanite and epidote are characteristic of the rocks of the complex. The texture is subporphyritic and shows evidence of protoclastic deformation.

A northeast-southwest trend of inclusions, foliation, lineation, and contacts dominates. Primary flow structures are shown in the orientation of inclusions and micas. Secondary structures were superposed on the original flow structures shortly after intrusion and prior to complete consolidation. A prominent secondary structure is a lineation in b. Stretching in b is indicated by boudinage structures, the lineation, and small displacements of dikes. The intensity of the planar structures in the gneiss grades from strong to indistinct or lacking from east to west. Structures in schist inclusions indicate that some folding had taken place prior to intrusion.

Thin pegmatite, granodiorite, and granite dikes are common. Most of them are essentially parallel to the regional strike.

The petrographic boundary between gneiss and schist is always sharp. On a large scale, the gneiss structures are essentially parallel to the contacts, but slight to marked discordant relations occur at many localities. The schist structures usually parallel the contacts.

Petrofabric analysis confirms the structural field observations. The mica fabrics are ac-girdles and are most complete where the foliation is weakest. Microscopic s-planes do not always coincide with the observed foliation. The quartz fabrics are also ac-girdles but are not so well developed as the mica girdles. Quartz orientations cannot always be related to known s-planes.

It is concluded that the Pinckneyville quartz diorite complex represents a syntectonic intrusion—that is, that intrusion took place while external or regional forces were still active. The shape of the intrusion was controlled largely by the wall-rock structures which had developed earlier. The rocks show both igneous and metamorphic characteristics because the intrusion took place under regional metamorphic conditions.

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