Geochemistry and geochronology of Alleghanian and 'atypical' Alleghanian granites from south-central Appalachians: Implications for magma evolution and Late Paleozoic terrane accretionary history in the southern Appalachians.

摘要

One of the most important events during the Alleghanian Orogeny in the south-central Appalachians was the emplacement of granites of Late Mississippian-Early Permian age throughout the central and southern Appalachians. These granites represent a very significant volume of magma and hence are ideal rocks for investigating syn-crystallization magma evolution. The mineral apatite can potentially retain finer sub-mineral scale petrogenetic details related to syn-crystallization magma evolution in the form of chemical and isotopic zonations. Hence we selected apatite from a series of Alleghanian plutons to document the evolution of granitic magma. Using a series of thermal annealing and selective chemical dissolution techniques, we measured the isotopic composition ( 87Sr/86Sr and 143Nd/144Nd) of apatite rim and core separately to document the sub-mineral scale differences in the composition. We found that while apatite from some of the granites yielded an isotopically juvenile core (low 87Sr/86Sr and high 143Nd/144Nd) against an evolved rim (high 87Sr/86Sr and low 143Nd/ 144Nd), apatite from other granites yielded exactly the opposite rim-core relation, i.e., evolved core and juvenile rim. These sharp isotopic zonations can only be explained by sharp changes in the magma composition during crystallization.;The Alleghanian granites also have the potential to be used as geochemical probes of the unexposed portions of the lithotectonic units that they intrude. The low negative initial epsilonNd values (-3) of some of the Alleghanian granites suggest the role of isotopically evolved Laurentian crust as a magma source during the Alleghanian magmatism. We tested this hypothesis by modeling the petrogeneses of a series of Alleghanian granitic rocks from Carolina Terrane in terms of whole rock rare earth element (REE) composition, using representative crustal rocks of Carolina Terrane and that of the exposed Grenvillian crust in the southern Appalachians as source rocks. The model results when compared with measured REE patterns of Alleghanian granites, suggested the minor role of Laurentian crust as magma source (∼20%). Independently we also obtained direct evidence of the contribution of Laurentian crust as magma source, in the form of zircon xenocrysts of Grenvillian age (∼1346 Ma and ∼1107 Ma) from selected Alleghanian granites.;We also used the chemical and isotopic composition of the Alleghanian granites to learn about the accretionary history of the Carolina Terrane with the Laurentian margin. On the basis of 147Sm/144Nd ratios we have classified these plutons into the 'atypical' Alleghanian plutons (>0.12) and the typical Alleghanian plutons (0.12). The 'atypical' plutons have a lower Zr/Hf ratio (∼20 to 35), lower bulk REE (84.69--237.33 ppm) and lower CaO wt %, when compared to the typical Alleghanian plutons (SigmaREE = 81.22 to 534.27; Zr/Hf ∼35 to 40) that indicates the formation of these 'atypical' plutons from a fractionated magma. Furthermore, the more felsic 'atypical' plutons have initial epsilon Nd values that suggest that they were formed by anatexis of the Carolina Terrane, while the epsilonNd values of the typical plutons require evolved Laurentian crust as an additional magma source along with the host terrane. These petrogenetic differences between the atypical' and typical Alleghanian granites suggests that the 'atypical' Alleghanian plutons were formed from melt derived by partial melting of Carolina Terrane crust prior to the accretion of Carolina Terrane to the Laurentian margin. Melting during or post accretion may have involved components of native Laurentian crust in addition to terrane curst.