EVIDENCES OF SYN-CRYSTALLIZATION MAGMA EVOLUTION IN TYPICAL AND ‘ATYPICAL' ALLEGHANIAN PLUTONS FROM SOUTHERN APPALACHIANS: IMPLICATIONS FOR GRANITE PETROGENESIS AND TERRANE ACCRETIONARY HISTORY

The Carolina Terrane in the southern Appalachian hosts a large number of granitic intrusions, majority of which intruded during the Alleghanian Orogeny (330 Ma to 300 Ma). Based on isotopic and trace element composition, we have classified these intrusions into typical Alleghanian plutons and ‘atypical’ Alleghanian plutons. The ‘atypical’ Alleghanian plutons have higher ¹⁴⁷Sm/¹⁴⁴Nd (> 0.12) and lower bulk REE concentration (ΣREE 84.69 – 237.33 ppm) compared to the typical Alleghanian granites (¹⁴⁷Sm/¹⁴⁴Nd < 0.12, ΣREE = 81.22 to 534.27). The ‘atypical’ plutons also have lower Zr/Hf ratios (~20 to 35) and higher Rb/Sr (0.81 to 7.38) ratios compared to the typical Alleghanian plutons (Zr/Hf ~30 to 45, Rb/Sr = 0.08 to 2.79). These compositional differences suggest that the atypical’ plutons underwent significant syn-crystallization accessory mineral (zircon, apatite and monazite) and anorthite fractionation and hence are petrogenetically different from the typical plutons. Furthermore, the differences in the ε_Nd(T) values of Alleghanian (+3.0 to -6.7) and ‘atypical’ plutons (+2.1 to -3.8) suggest that the ‘atypical’ plutons were formed solely by partial melting of Carolina Terrane crust prior to the accretion of Carolina to the Laurentian margin. Melting during or post accretion involving components of native Laurentian crust in addition to terrane crust gave rise to the typical Alleghanian plutons. Hence, evidences of magma evolution in the ‘atypical’ plutons coupled with the differences in the possible magma sources might hold clues to the accretionary history of the Carolina Terrane. In addition, apatite from selected Alleghanian plutons shows remarkable intracrystalline Sr isotopic zonations. ⁸⁷Sr/⁸⁶Sr ratios of apatite cores from the Harbison granite (~0.70473) were lower than that obtained from the rims (~ 0.70490). In contrast, apatite from the Danburg pluton yielded cores with ⁸⁷Sr/⁸⁶Sr higher (~0.7059) than those from apatite rims (~0.7056). We suggest that such intracrystalline compositional variation reflect syn-crystallization magma evolution. Hence, the study of syn-crystallization magma evolution of the ‘atypical’ and typical Alleghanian granites can provide valuable information related to their petrogenesis as well as terrane accretionary history of the southern Appalachian.