MAGMATIC RECORD OF GRENVILLE-AGE OROGENIC AND POST-OROGENIC PROCESSES, BLUE RIDGE PROVINCE, VIRGINIA

The magmatic records of Earth’s major orogenic zones reflect complex interrelationships of tectonic environment and source characteristics that are manifested by petrologic, geochemical, and isotopic signatures of the rocks produced. U-Pb SHRIMP isotopic analyses of zircon and monazite indicate that the largely granitic Grenville-age rocks of the northern Virginia Blue Ridge were formed episodically over a period of about 150 m.y. These rocks include suites of both (1) pre-orogenic (1.18–1.15 and circa 1.11 Ga) and (2) syn- to post-orogenic (1.08–1.03 Ga) origin. Contemporaneity of opx-bearing charnockites and opx-free granitoids in both suites suggests that production of charnockites, a noteworthy feature of many Grenvillian terranes, is controlled by intrinsic magmatic characteristics such as pH2O and Fe/Mg, and is not necessarily a reflection of metamorphic conditions. Most rocks are compositionally transitional between I- and A-types and derived from chemically heterogeneous sources that are consistent with an evolving contractional orogen. However, a subset of post-orogenic granitoids display distinctive A-type compositions characterized by high Ga/Al and elevated concentrations of high-field-strength elements such as Ce and Zr. The presence of such A-type granitic rocks indicates a change in both tectonic environment (possible extension following contractional orogenesis) and source characteristics (less chemically evolved) that is preserved despite subsequent metamorphism at 1.02–0.98 Ga. The age of these post-kinematic rocks, combined with observed crosscutting field relationships with older foliated lithologies, limits the time of major regional deformation to 1.08–1.06 Ga which broadly correlates with the timing of orogenic activity in other parts of the Grenville province in North America. The geochemical record of Blue Ridge magmatism is thus a sensitive indicator of both ancient tectonic processes and magmatic source characteristics associated with local development of the Grenville orogen. Results from this study underscore the importance of multi-disciplinary efforts involving field mapping, petrologic and geochemical investigations, and high-resolution U-Pb geochronology in deciphering the geological evolution of complex orogenic regions.