DECIPHERING THE SIGNIFICANCE OF COMPOSITIONAL TRENDS IN GRANITOIDS: INSIGHT FROM LATE-SYN TO POST-OROGENIC MESOPROTEROZOIC PLUTONIC ROCKS OF THE VIRGINIA BLUE RIDGE

Granitoids constitute a significant component of nearly all convergent orogens and provide insight into the tectonic evolution of such terranes. Mesoproterozoic rocks of the Blue Ridge, where local orogenesis occurred at 1080-1050 Ma, are dominated by granitoids of 1180-1030 Ma age. Late syn- to post-orogenic rocks of 1078-1030 Ma age include leucogranites, leucocratic biotite monzogranite (Ylgr), low-silica charnockite, and biotite-rich monzogranite (Ybg) with compositions that collectively span 55-79 wt. % silica. Compositions define linear trends in major elements, Rb, Sr, K/Ba, and Rb/Ba that suggest derivation by either igneous fractionation or sequential melting. However, penecontemporaneous 1065-1050 Ma silica-rich leucogranites and ~1045 Ma low-silica charnockites form compositionally distinct plutons that differ in source-related characteristics and are unlikely to constitute a fractionation series. Ylgr shares many compositional characteristics with the leucogranites but is slightly less evolved. Ybg (~1035 Ma) is compositionally transitional between Ylgr and low-silica charnockite, but is demonstrably younger. Nd isotopic compositions, trace-element ratios, and comparison with experimental studies indicate that the leucogranites and Ylgr were produced by partial melting of lower crustal tonalitic sources; whereas the charnockites, which show A-type affinity, were produced through melting of potassic, mafic sources. Although displaying A-type characteristics, compositions of Ybg are not consistent with internal fractionation trends in the charnockite and thus Ybg is unlikely to represent a fractionation product of the latter. We interpret the transitional trace-element characteristics of Ybg to indicate derivation from parental magmas produced through mixing of melts formed from the separate sources that gave rise to the older post-orogenic leucogranitic and charnockitic plutons. Emplacement of compositionally transitional, but distinctly younger, magmas at the end of regional plutonic activity produced linear compositional trends that could be interpreted as, but are not indicative of, an igneous fractionation series. These results underscore the potential pitfalls inherent in interpreting such geochemical variation without geochronologic calibration.