DECODING THE MULTISCALE MAGMATIC CYCLICITY OF MAGMATIC PLUTON CONSTRUCTIONS USING THE PETERSBURG GRANITE, VIRGINIA, USA, AS AN EXAMPLE

The close relationship between crustal magmatism and tectonics leads to the question of their mutual relationships. In some cases a direct genetic link between magmatism and tectonics is obvious, whereas in others the connection is less clear. To explain these observations several authors suggested that the complexity follows directly from the pulsed, incremental assembly of plutons, with pulses having their own spatial, compositional, and temporal characteristics within the tectonic setting during emplacement. To understand the timing we present high-precision ID-TIMS U-Pb geochronology on different magmatic pulses of the Carboniferous Petersburg batholith, and discuss the results in context of the relationship to lithospheric dynamics during the Alleghanian orogeny.

The Petersburg batholith is a large, composite, body located in the eastern Piedmont Province of Virginia, and is well exposed in the James River along the Fall Zone in Richmond. Rocks in the batholith exhibit considerable textural variation (gneissic, porphyritic, weakly foliated, massive); aplite dikes are a late feature. We evaluated three varieties, including: 1) a porphyritic type from near the northwestern margin; 2) the main massive to weakly foliated variety exposed at the Fall Zone; and 3) an aplite dike also from the Fall Zone.

Our ²⁰⁶Pb/²³⁸U zircon dates indicate that construction of the batholith commenced with emplacement of the porphyritic granite at 300.08 ± 0.10 Ma. The more massive, medium grained portion of the batholith was emplaced at 296.33 ± 0.11 Ma. Ages obtained from the aplite dike are within error the same as that of the massive variety. Thus, these dates imply at least two emplacement pulses for the construction of the batholith with a maximum time gap of 4 myrs in-between.

These new dates show that emplacement of the Petersburg batholith occurred in the Late Pennsylvanian during the Alleghanian orogeny, and are ~13-30 Ma younger than previous published dates. In addition, the incremental process of batholith construction shows that at least two tectonic events interacted with the batholith allowing separate magmatic pulses to occur. The overall age interval overlaps with the known age of movement on the western bounding, dextral Hylas mylonite zone, consistent with magma generation in a transpressional tectonic setting.