Geophysical and geologic studies in southern Mecklenburg County and vicinity, North Carolina and South Carolina

Abstract

Geophysical methods consisting of gravity, aeromagnetics and aeroradioactivity have been applied to part of the Charlotte and Carolina slate belts in southern Mecklenburg County and vicinity to help interpret geology, lithology and structure. High aeroradioactivity is associated with potassium-rich granitic plutons, muscovite-rich gneisses, schists, and metavolcanic rocks; positive gravity and magnetic anomalies are associated with gabbro plutons; and negative gravity anomalies are associated with granitic plutons. At the west side of the slate belt, the Tillery phyllite is interpreted as having undergone progressive metamorphism. The underlying Uwharrie Formation extends into the Charlotte belt where it is mapped as metavolcanic rocks. Gravity models of the Carolina slate belt indicate that it is a synform containing a wedge of metasedimentary and volcanoclastic rock on plutonic basement. The basement is exposed in the adjacent Charlotte belt antiform. The northern Charlotte belt contains mainly plutonic rocks which have been divided into 3 supergroups of plutons based upon chemistry, mineralogy, texture, and age. They are: 1. Old Plutonic supergroup - plutons 545-490 m.y. that are medium to coarse-grained tonalite, quartz diorite, and granodiorites. 2. Concord-Salisbury supergroup -- plutons 426-350 m.y. which form sheet-like intrusions of differentiated gabbro; local volcanic centers with ring complexes 13 km in diameter that suggest magma chambers 0 - 8 km deep; smaller bodies of diorite, monzonite, and syenite; and small Salisbury type granodiorites. 3. Landis supergroup -- plutons 350-280 m.y. that are usually very coarse-grained, porphyritic, 'big feldspar,' potassium-rich granites. The Mecklenburg-Weddington gabbro complex of the Concord-Salisbury supergroup, the largest feature in the study area, contains three large gabbro plutons. The gabbro intruded old Plutonic complex rocks and could-have produced the metamorphic reaction K-feldspar + sillimanite quartz + muscovite reflected in the mineral assemblage of adjacent felsic metavolcanic rocks. Gravity models indicate a lopolith 3.5- to 4.5-km thick with a 2 km sill extending to the northeast. Positive magnetic and gravity anomalies suggest the lopolith is. connected with the Concord gabbro complex to the northeast. The sheet-like intrusions of Concord-Salisbury group gabbros, forming the core of the composite 5atholith, have medium-grained Salisbury type granodiorite above, and coarser-grained Landis granite below. The position of the supergroups as presently exposed may be a function of level of erosion versus level of emplacement. The plutons in the composite batholith span 200 m.y. according to current age data and are arranged with the oldest at the top and the youngest at the bottom. However, Rb-Sr and K-At ages in the Piedmont are more likely to reflect age of crustal uplift than the age of metamorphism or intrusion. The Charlotte belt composite batholith, therefore, may very well be the result of a shorter single tectonic event or process.