DIFFERENCE IN THE MECHANISM OF MELT GENERATION BETWEEN SILURIAN-DEVONIAN PLUTONS, ATYPICAL 'ALLEGHANIAN' GRANITES AND ALLEGHANIAN GRANITES IN THE CAROLINA TERRANE, SOUTHERN APPALACHIANS; IMPLICATIONS FOR ACCRETIONARY HISTORY OF THE CAROLINA SUPERTERRANE

The Carolina Terrane in the southern Appalachian is a Peri-Gondwanan arc similar to the Avalon terrane in the northern Appalachians that accreted to the Laurentian margin during the Paleozoic. It hosts a large number of granitic intrusions. Based on age, isotopic ratios and trace element concentrations, these intrusions have been classified into three categories, namely Silurian/Devonian plutons, Alleghanian (320 Ma – 300 Ma) plutons and ‘atypical' Alleghanian plutons. Higher Sm/Nd ratios, lower bulk REE and higher Lu/Hf ratios suggests that the Silurian-Devonian and ‘atypical' Alleghanian plutons were derived from a source with different chemical and isotopic characteristics compared to the granites of known Alleghanian age. The Silurian-Devonian, and ‘atypical' Alleghanian rocks are also distinguished from typical Alleghanian granites in their higher average SiO2 contents, lower average CaO, MgO and Fe-oxide concentrations, higher average orthoclase feldspar concentration and lower average ‘anorthite' concentration of plagioclase feldspar. These differences could have resulted from a combination of different melting temperatures and/or different degrees of partial melting. Such differences would best be explained by fundamental differences in the mechanisms of melt generation between the Alleghanian intrusions and Silurian-Devonian plus ‘atypical' Alleghanian intrusions. Larger degrees of melting, followed by variable differentiation, in a subduction environment might explain the generation of the Silurian-Devonian intrusions and some of the older ‘atypical' Alleghanian plutons. The typical Alleghanian granites may have formed from heat build-up following lithospheric stacking. Such disparity in the mechanism of melt generation of plutons located in the same geographical area can be explained if it is assumed that the Silurian-Devonian plus older ‘atypical' Alleghanian plutons were formed in an arc environment before accretion of the Carolina Terrane with Laurentia. Subsequent accretion of Carolina with Laurentia resulted in an over thickened crust, trapped heat from radioactive decay, and partial melting of a combination of accreted crust plus native Grenvillian Laurentian crust.