J. Robert Butler

Mesozoic igneous provinces of New England and the opening of the North Atlantic Ocean

Mesozoic igneous rocks in New England and adjacent areas can be divided into four provinces, on the basis of their ages, distribution, physical aspects, and petrology. These provinces represent intraplate magmatism that is correlated with successive stages in the opening of the Atlantic Ocean. The Coastal New England province includes three alkalic complexes and possibly some olivine dolerite dikes, Late Permian to Early Triassic in age, that are located in southern Maine and coastal New Hampshire and that were formed during uplift and early rifting in the Gulf of Maine region. Early Jurassic dolerite dikes occur in most of New England and are part of the Eastern North America province that extends along the eastern Appalachians from Alabama to Newfoundland, correlated with maximum rift-basin formation and continental breakup. Plutons of the classic White Mountains Magma Series are mainly Early Jurassic syenitoids and granitoids in central and northern New Hampshire. This elongate north-south province may have formed along an ancient transform or rift fault in the New England basement crust. After a relatively nonmagmatic period during the Late Jurassic, the Early Cretaceous New England–Quebec province of alkalic syenite-gabbro plutons and lamprophyre dikes formed in a large area of northern New England and adjacent Quebec. Similar magmas created seamount chains southeast of the New England coast, possibly along an extension of transverse Appalachian fracture zones. The Mesozoic New England igneous intrusions and tectonic activity represent incipient rifting in the continental platform, related to mantle convection and plate motions in the North Atlantic basin.

Petrochemical and geochronological studies of plutonic rocks in the southern Appalachians: III. Leucocratic adamellites of the Charlotte belt near Salisbury, North Carolina

The Salisbury, Yadkin, Southmont, Gold Hill, and Kannapolis plutons and related smaller bodies in the Charlotte belt of central North Carolina are leucocratic albite adamellite. The Rb-Sr whole-rock ages for these plutons are 413 to 386 m.y., and the Salisbury-group granites generally have low initial Sr 87 /Sr 86 ratios near 0.703. A mica date of 368 m.y. is an upper bracket for metamorphism. The Gold Hill fault truncates the Gold Hill pluton; major movement probably occurred between about 400 and 368 m.y. ago. Gold mineralization apparently is younger than major deformation. The gold deposits are strongly localized in shear zones, although some gold-bearing veins cut the Gold Hill and Salisbury plutons. Acadian deformation in this part of the southern Appalachians is mainly restricted to shear zones trending about 25° east of north and was accompanied or followed by lower–greenschist-facies metamorphism that overprinted an earlier greenschist- to amphibolite-facies metamorphism.

Geology and Gravity of the Lilesville Granite Batholith, North Carolina

The Lilesville Granite is the southernmost granite intrusion of the Eastern Piedmont in North Carolina. It is a sheet or tongue-shaped concordant mass extending to a maximum depth of about 1.75 mi. It is surrounded by a contact aureole of metasedimentary mica schist and mica gneiss and has an areal extent of 125 sq mi. The granite is characterized by a porphyritic rapakivi texture with a medium- to coarse-grained matrix of plagioclase, quartz, and biotite. The intrusion is compositionally zoned and consists of adamellite, granodiorite, and tonalite. The granite is considered to have been emplaced as a crystal mush and to have crystallized in place. The modal variations and textural features of the granite may be due to crystal settling, contamination, and rapid late-stage crystallization (sudden escape of water pressure). A large positive Bouguer anomaly associated with the granite is attributed to two major features: (1) a gabbro body which intrudes the granite near its eastern margin, and (2) a mica gneiss unit which underlies the batholith. The proposed sequence of geologic events is: (1) deposition of felsic volcanic rocks and argillites, (2) formation of the proposed anticline and low-rank regional metamorphism, (3) intrusion and crystallization of the Lilesville Granite and the formation of the mica gneiss and mica schist units by thermal metamorphism, (4) intrusion of gabbro body into the Lilesville Granite, (5) Triassic faulting and sedimentation, (6) intrusion of Triassic or Jurassic dikes, and (7) deposition of Cretaceous and Tertiary sands and gravels.

Petrochemical and geochronologic studies of plutonic rocks in the southern Appalachians: II, The Sparta granite complex, Georgia

Samples from near the center of a granite complex in Hancock County, Georgia, have a Rb-Sr isochron age of 295 ± 2 m.y. and an initial Sr 87 /Sr 86 ratio of 0.7035 ± 0.0004 (λ Rb 87 = 1.39 × 10 −11 yr −1 ). This low initial ratio makes it highly unlikely that the magma was significantly affected by anatexis or assimilation of sialic crustal material. The age, initial ratio, mineralogy, and texture of these rocks are very similar to other 300-m.y.-old granite masses in the eastern Piedmont of North and South Carolina. Twenty-one samples from the same granitic complex but with different textures than the above five samples also plot on a series of approximately 300-m.y. isochrons. The initial ratios vary with sample location, but most range from 0.729 to 0.741. Scatter on a CaO-K 2 O-Na 2 O diagram suggests Na and K migration; the values group according to sample location and texture. The linearity and distribution of data points on isochron diagrams argue against significant contamination of magma with low initial Sr 87 /Sr 86 ratios by older country rock. Intrusion of magma with a low initial Sr 87 /Sr 86 ratio apparently was accompanied by Na and K migration, Sr-isotopic re-equilibration, and recrystallization of pre-existing granitic rocks. If this model is correct, Rb/Sr ratios suggest that the older rock is at least 530 m.y. old.