Petrography, geochemistry, and geochronology of the Rocky Mount batholith, northeastern North Carolina Piedmont / by Alfred M. Moncla, III.
| Author/creator | Moncla, Alfred M. author. |
| Other author | Spruill, Richard K., degree supervisor. |
| Other author | East Carolina University. Department of Geology. |
| Format | Theses and dissertations |
| Production | 1990. |
| Description | [vi], 61 leaves : illustrations ; 28 cm |
| Supplemental Content | Access via ScholarShip |
| Subjects |
| Summary | The post-metamorphic Rocky Mount batholith (478 km2) is located along the Fall Line in the northeastern North Carolina Piedmont. Two intrusions comprise the batholith. One intrusion (gray granitoids) with a color index of 5 to 25 is represented by medium-grained microcline megacrystic hornblende biotite granodiorite to biotite granite and rare biotite tonalite and varieties of quartz monzodiorite. Leucocratic (color indexes less than 5), medium-grained equigranular monzogranites (white granites) comprise a second intrusion. Textures within the white granites adjacent to contacts with gray granitoids indicate that the white granite intruded the gray granitoids. Contacts between the white granites and the gray granitoids are straight to irregular. Mineral changes at some of the contacts may indicate some reaction between the two magmas. A third lithology, hornblende biotite tonalite, is found in the northern portions of the batholith. However, relations of these rocks with the others is unknown. These are the most mafic rocks of the batholith and may be a part of the gray granitoids or may represent a third intrusion. Rocks of the batholith are peraluminous [ Al/(Ca+Na+K) ranges from 1.06 to 1.28] and exhibit typical calcalkaline differentiation trends on an AFM plot and a plot of K20-Na20-CaO. The batholith is generally more mafic than the late Paleozoic post-metamorphic granitoids defined by Speer and others ( 1980). Petrographic data and linear trends exhibited by the gray granitoids on Barker plots indicate that these rocks evolved predominantly by fractional crystallization, possibly in combination with magma mixing or unmixing processes as well. Chemical data indicate that the less siliceous hornblende biotite tonalite from the northern portions of the batholith may be related to the gray granitoids. The white monzogranites are generally homogenous but have great modal, chemical, and textural variability near contacts with the gray granitoids. This variability may be due to both mechanical differentiation and late-stage, water rich crystallization. A subsolidus mineral assemblage consisting of chlorite, phengite, and epidote indicates greenschist facies conditions. However, the occurrence of these minerals is not pervasive, and the mineral assemblage may have formed during cooling of the batholith and not during greenschist-grade metamorphism. Strained quartz is the only pervasive deformational texture. A crystallization age of 345+/-1 Ma is interpreted for the batholith based on Rb-Sr whole-rock data of samples from all three lithologies. The low initial 87Sr/86Sr ratio of 0.70444 indicates a common source of either upper mantle/lower crustal material or rapidly recycled upper crustal material for the batholith. Rb-Sr mineral ages (biotite, 318+/-3 Ma) and K-Ar mineral ages(hornblende, 355+/-8 Ma; biotite, 337+/-8 Ma; and microcline, 202+/-5 Ma) indicate rapid cooling of the batholith. The Rocky Mount batholith was produced by the approximately contemporaneous crystallization of two (or three) distinct magmas generated from similar sources. Early genetic relationships between the two magmas cannot be determined due to complex evolutionary histories of both intrusions, although chemical data indicate some relationships may exist. The Rocky Mount batholith is older and, based on mineralogical as well as chemical data, more mafic than the late Paleozoic post-metamorphic granitoids (Speer and others, 1980) and may represent the earliest magmatic pulse of the Alleghanian orogeny. The lack of pervasive deformation and metamorphism of the batholith and the relatively old mineral ages suggest that the East Carolina Slate Belt east of the Hollister mylonite zone was not affected by the late Paleozoic tectono-thermal event which conspicuously affected the Raleigh belt.. |
| General note | Submitted to the faculty of the Department of Geology. |
| General note | Advisor: Richard K. Spruill |
| Dissertation note | M.S. East Carolina University 1990 |
| Bibliography note | Includes bibliographical references (leaves 26-29). |
| Genre/form | Academic theses. |
| Genre/form | Academic theses. |
| Genre/form | Thèses et écrits académiques. |