GEOCHRONOLOGY OF THE ELK HILL COMPLEX AND LAKESIDE FAULT ZONE, CENTRAL VIRGINIA PIEDMONT

New field mapping and geochronologic data from the central Virginia Piedmont constrains the timing of deformation along the Lakeside fault zone (LFZ), which separates the Chopawamsic terrane (CT) from the Elk Hill Complex (EHC). CT west of the LFZ include interlayered amphibolite and felsic gneiss; two samples of CT felsic gneiss yield new SHRIMP U-Pb zircon ages of 460±5 and 471±4 Ma. EHC consists of two-mica quartzofeldspathic paragneiss, amphibolite and granitoid. EHC paragneiss preserves two foliations – an early migmatitic layering (S₁) and penetrative coarse-grained Ms+Bt foliation (S₂) axial planar to tight to isoclinal folds of S₁. S₂ is conformable along and across the sharp contact between CT and EHC rocks. Equally deformed by S₂ is a suite of 442-458 Ma bt-hbl granitoids that intrude both the CT and EHC, which may be syn-kinematic with migmatization in EHC paragneiss.

Migmatitic EHC paragneiss contains zircons of two morphologies that yield different SHRIMP U-Pb zircon ages: cores of rounded grains yield Grenvillian ages of 1000-1200 Ma (n=25); elongate pyramidal grains (aspect ratios up to 8:1) are 442±7 Ma (n=9) and interpreted to reflect time of migmatization; narrow outer rims (n=3) on these elongate grains are ~329 Ma. Carboniferous ages agree with ⁴⁰Ar/³⁹Ar amphibole ages of ~305 Ma from EHC ~12 km to the SW, and published ages of ~330 Ma intrusive rocks of the EHC along the James River ~35 km to the SW. Ordovician ages correspond to intrusion of the Ellisville (444-437 Ma) granodiorite and similar granitoids in the CT and EHC, but are distinctly older than ⁴⁰Ar/³⁹Ar data from muscovite and amphibole (~300 Ma) in the CT east of the Chopawamsic fault.

A narrow (<100 m) belt of fine-grained muscovite-biotite phyllonite marks the trace of the LFZ. Phyllonite preserves dextral S-C (S_2a) fabric overgrown by randomly oriented porphyroblastic muscovite, and intruded by weakly to undeformed muscovite-quartz-K-spar pegmatite. One pegmatite concordant to S_2a foliation yielded a muscovite plateau age of ~295 Ma.

These data suggest that the CT and EHC were juxtaposed by early Alleghanian high-grade deformation (~330 Ma) and that dextral greenschist-facies phyllonitic reactivation occurred ~30 m.y. later, concurrent with significant uplift and rapid cooling through amphibole and muscovite closure.