IS THE SYKESVILLE FORMATION A LARGE IGNIMBRITE-FILLED INTRACALDERA COMPLEX? A REINTERPRETATION OF THE MID-ATLANTIC PIEDMONT'S MOST ENIGMATIC ROCK UNIT

The Sykesville Formation and similar pebble- to boulder-bearing metamorphic rocks called the Laurel and Indian Run Formations occupy a large part of the Piedmont between Baltimore and northern Virginia. The origin of these poorly stratified quartzofeldspathic rocks has been controversial since they were first named more than a century ago. They have variously been called granite, metagraywacke, wildflysch, boulder gneiss, migmatite, metadiamictite, olistostrome, and sedimentary mélange, among others.

A reappraisal of field relations and textural evidence indicates they are best explained as a large stack of ignimbrites (ash-flow tuffs) locally interbedded with rhyodacite lavas, hyaloclastites, and other volcaniclastic units. Where multiple lithologies are present in clean exposures, stratification is recognizable at scales ranging from centimeters to tens of meters. Certain lithic and textural features strongly resemble those seen in young ignimbrites. Ubiquitous inclusions of ‘biotite schist’ and quartzofeldspathic ‘clastic’ rock described by previous workers are interpreted as metamorphosed pumice fragments and felsic volcanic bombs. The former exhibit well-preserved eutaxitic (fiamme) texture and strong primary alignment produced during compaction and welding, whereas the latter commonly exhibit relic flow banding and porphyritic texture. The margins of the Sykesville and Laurel Formations are defined by prominent ‘inclusion-rich’ zones composed in part of large blocks of adjacent rock units. Such zones appear to be mega- and mesobreccias that define the former, normal-faulted walls of one or more collapsed calderas, an interpretation supported by past studies of the cooling ages of various minerals across these zones.

Recognition of a great thickness of silicic pyroclastic rocks, consistent with an intra-caldera setting, answers several longstanding questions about this part of the Taconic Orogen, but raises several others. The caldera complex is intruded by abundant, 450- to 475-ma tonalitic to granitic rocks, which suggests that it may comprise the previously unrecognized extrusive component of a major magmatic arc. Numerous fragments of vein quartz and granitoid lithic clasts in the ignimbrite suggest that this part of the arc may have been nested on continental platform.