A petrochemical survey of plutonic intrusions in the Piedmont, Southeastern Appalachians, U.S.A.

Abstract

This paper presents a summary of chemical and mineralogical data on intrusive igneous rocks in a segment of the Southern Appalachian orogenic be]~. The exposed crystalline part of the Southern Appalachians reaches a maximum width of about 300 km in North Carolina and South Carolina and narrows along strike to the northeast and southwest (Fig. I). We have attempted to delineate the major groups of intrusions in this segment. Few detailed studies or discussions of differentiation trends shown by intrusive rocks in this region have been published previously. The purpose of this paper is: (1) To give the general characteristics and distribution of major groups of p]utons. (2) To define the chemical trends in major elements that are exhibited by the groups and (3) to discuss the origin of the intrusive rocks and their role in the evolution of the Southern Appalachians. Major element analyses by rapid methods were made of 198 selected samples. Chemical methods, and precision and accuracy of analyses are the same as described by RAGLA~D et al. (1968). The chemical analyses were performed by a combination of atomic absorption and x-ray spectrometry. All chemically analyzed samples, except for 27 that are very fine-grained, were also modally analyzed. The point-count method of CHAu (1956) was used for fine- or medium-grained rocks. About 1500 points were counted over an area large enough to maintain an analytical error for major minerals of 2 % or less (CHAYES, 1956, p. 82). Where average grain size is greater than about 5 ram, the required number of thin sections becomes prohibitively large; therefore, slabs stained with sodium cobaltinitrite were also used. A slab area at least 100 times greater than the area of the largest mineral grain was used, and 1000 to 1500 points were counted. A point count of two to four thin sections was used to determine the approximate percentage of biotite, sphene, opaque minerals and other minerals in the "dark minerals" category obtained from the slab count. Rock names are used as defined in the descriptive modal classification of PETEI~SON (1961). Modern, large-scale geological maps are available for only a small percentage of total area in the c;ysta]line Southern Appalachians. It will take considerably more mapping, petrographic and chemical studies, and determination of radiometrie ages to decipher the complex intrusive history of the region. We present here a first approximation, which will undoubtedly be modified by later studies.