Abstract
Abstract The nature of metamorphism, magma compositions, the spatial distribution of plutons, and foreland sediments reflect, in part, the character and thickness of continental crust. We utilized metamorphic pressure-temperature-time (P-T-t) paths, garnet Sm-Nd ages, zircon U-Pb ages, and pluton compositions to estimate paleocrustal thickness and temporal changes in crustal magma sources in the Blue Ridge of the southernmost Appalachians. Garnet Sm-Nd ages for amphibolite-facies metamorphic rocks range from 331 ± 4 to 320 ± 3 Ma. Low- and high-Sr/Y plutons that intruded these metamorphic rocks have zircon U-Pb ages of 390 ± 1 to 365 ± 1 Ma and 349 ± 2 to 335 ± 1 Ma, respectively. Therefore, garnet growth began during regional metamorphism synchronous with or shortly after intrusion of the youngest high-Sr/Y trondhjemite plutons. Phase diagram sections and thermobarometry indicate that garnet growth initiated at ∼5.8 kbar and 540 °C and grew during temperature increases of 60–100 °C and pressure increases of 2–3 kbar. The older, low-Sr/Y magmas are inferred to have been sourced in the crust at depths <∼30 km, insufficient for garnet to be stable. However, the younger, high-Sr/Y magmas are inferred to have been sourced at >30 km depths where garnet was stable. Hafnium isotopic compositions for all the plutons, but one, exhibit a range from negative initial εHf(i) to weakly positive initial εHf(i), indicating incomplete mixing of dominantly crustal sources. Our data require minimum crustal thicknesses of ∼33 km at 331 Ma; however, Alleghanian crustal thicknesses must have locally reached 39 km, based on crustal reconstruction adding the Alleghanian thrust sheet beneath the eastern Blue Ridge. We infer the presence of hot, tectonically thickened crust during intrusion of the early Alleghanian high-Sr/Y plutons and conclude that garnet growth and plutonism reflect a progressive increase in crustal thickness and depth of magma generation. The crustal thickening was synchronous with deposition of Mississippian to early Pennsylvanian sediments in the foreland basin of the Appalachian orogen between 350 and 320 Ma. This crustal thickening may have preceded emplacement of the Alleghanian thrust sheets onto the North American craton.
Highlights
Many collisional orogenic belts are intruded by paired magmatic belts that likely reflect spatial and temporal variation in magmatic sources; the processes by which and the tectonic settings in which these belts form are poorly understood (e.g., Tulloch and Kimbrough, 2003)
Paired high- and low-Sr/Y magmatic belts occur in the Tibetan Himalayas, Fiordland (South Island, New Zealand), the western U.S Cordillera (Klamath Mountains, Blue Mountains Province), Peninsular Ranges, and the southern Appalachian orogen (Tulloch and Kimbrough, 2003; Chung et al, 2005, 2009; Xu et al, 2010; Schwartz et al, 2011; Zeng et al, 2011)
We found that Neoacadian contractional deformation locally resulted in crustal thickening of the eastern Blue Ridge to >35 km
Summary
Many collisional orogenic belts are intruded by paired magmatic belts that likely reflect spatial and temporal variation in magmatic sources; the processes by which and the tectonic settings in which these belts form are poorly understood (e.g., Tulloch and Kimbrough, 2003). They are typified by geochemically distinct suites of (1) mafic to felsic magmas with low Na, Al, and Sr, and high Y values (so-called “low-Sr/Y magmas”), and (2) less common, intermediate to felsic magmas with high Na, Al, and Sr, and low Y values (so called “high-Sr/Y magmas”). Unraveling the interplay among magmatism, deformation, and crustal thickening in the development of paired magmatic belts is important in understanding how magmatism and melt generation operate and change in evolving orogenic belts
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