Paul D. Fullagar

Mid-Cretaceous strontium-isotope stratigraphy of deep-sea sections.

Large variations exist between published mid-Cretaceous (late Barremian to early Turonian stages) seawater Sr-isotope stratigraphies; this has resulted in disparate interpretations of crustal production rates. We report on a detailed investigation of seawater Sr-isotope stratigraphy based on foraminifers and, where available, on inoceramid bivalves from 12 mid-Cretaceous Deep Sea Drilling Project and Ocean Drilling Program sections. The effects of diagenesis are assessed using scanning electron microscope observations and traceelemental analyses, but are best distinguished by comparing the 87 Sr/ 86 Sr values of similarage samples from different sites. Strontiumisotope analyses compiled from 9 of 12 sites that have detailed age control define one band of common values. This band is used as a composite curve, which presumably represents seawater 87 Sr/ 86 Sr values. The composite curve shows a “trough” of markedly lower 87 Sr/ 86 Sr values in the Aptian and early Albian stages, higher but constant values for the middle Albian-Cenomanian stages, followed by a decrease in 87 Sr/ 86 Sr values in the early Turonian.

Age and Origin of Plutonic Intrusions in the Piedmont of the Southeastern Appalachians

Samples from 25 plutonic intrusions in the Piedmont, primarily of North and South Carolina, were analyzed to determine Rb-Sr whole-rock ages and initial Sr 87 /Sr 86 ratios. The 50-b.y. half-life for Rb 87 is used, and initial ratios are given relative to a Sr 87 /Sr 86 ratio of 0.7077 for the E and A SrCO 3 . The 14 whole-rock isochron ages occur in three groups: (1) 595 to 520 m.y., (2) 415 to 385 m.y., and (3) approximately 300 m.y. The initial Sr 87 /Sr 86 ratios for 14 granitic plutons range from 0.7023 to 0.7050; one granitic pluton has an initial ratio of 0.7062. Seven gabbrodiorite plutons have initial ratios of 0.7035 to 0.7042. The very low initial Sr 87 /Sr 86 ratios and their limited range indicate that the granitic magmas were derived from the lower crust; these magmas were not contaminated by or produced from older upper crustal rocks of sialic composition. The initial ratios suggest that the lower crust beneath the Piedmont 600 to 300 m.y. ago had a lower Rb/Sr ratio than did the lower crust beneath other continental areas during the same period of time. Whole-rock ages and petrographic studies indicate that the most recent significant metamorphic episode to affect the Piedmont occurred prior to 300 m.y. ago. The thermal peak of this last significant metamorphic event may have occurred between 380 to 420m.y. ago. The intensity and time of the metamorphic peak varied from place to place within the Piedmont. Many of the Piedmont mica ages probably have been affected by uplift and cooling of deep-seated rocks and thus have limited geologic significance.

Plutonism in Pan-African belts and the geologic evolution of northeastern Africa

Abstract Plutonic igneous rocks of Pan-African belts (500–600 m.y. age) in Africa can be described in terms of three types of assemblages: (1) migmatites formed largely by remobilization of pre-existing sialic crust during Pan-African time; (2) calcalkaline batholithic suites formed in association with subduction of oceanic crust; and (3) alkali-rich, post-tectonic granites. Many suites cannot be placed precisely in one of these categories, either because of intermediate (gradational) characteristics or because of lack of adequate information. Distinction of rocks formed during Pan-African time from older ones whose radiometric clocks were reset at that time is also difficult. The rock suites are very unevenly distributed geographically. Migmatites formed by ensialic crustal remobilization occur mostly in southern and central Africa. Calcalkaline suites occur in north Africa, particularly the northeast. Alkali-rich granites are virtually restricted to the northeastern portion of the continent, the one major part of Africa not occupied by an Archean shield. Evidence for subduction of oceanic crust is also restricted to northeastern Africa. The geologic history proposed for northeastern Africa is as follows: During, and immediately preceding, Pan-African time an ocean basin of uncertain size and shape occurred between the West African and Nubian-Arabian shields. Subduction zones were active on the margins and within the basin. During, and at the end of, Pan-African time extensive development of alkali-rich granites was associated with cratonization of the basin. Paleozoic sediments were then deposited under platform conditions on the newly formed craton, which showed mild epeirogenic activity throughout the Phanerozoic.

Precise UPb zircon ages of Neoproterozoic plutons in the southern Appalachian Blue Ridge and their implications for the initial rifting of Laurentia

Late Precambrian plutonic rocks of the Crossnore Complex in the North Carolina Blue Ridge are considered to have formed in response to rifting of the Laurentian continent. Previously published ages are equivocal, suggesting only that these rocks crystallized 690–820 Ma. New UPb data indicate that the Crossnore Complex formed at 741 ± 3 Ma. This age is 10–40 Ma older than has been obtained by other investigators for similar plutonic rocks in the central Appalachian Blue Ridge of Virginia; our age for the Crossnore Complex is 5–20 Ma younger than the UPb zircon age reported for the Mount Rogers volcanics in Virginia. Thus, our new data, plus published data, suggest that Laurentian intra-continental rifting in the southern and central Appalachians was initiated at different times in different places. Available high-precision UPb analyses of zircon indicate that this rift-related magmatism occurred over a span of at least 30–40 Ma. Core-bearing zircon from the Beech metagranite of the Crossnore Complex has a UPb concordia upper-intercept age of 1424 ± 29 Ma, which is interpreted as the age of at least some of the source rocks for the Crossnore Complex. The existence of Mesoproterozoic crust in this region increases the known extent of rocks of this age. Other Crossnore-type plutons exhibit less inheritance of old zircon, possibly the result of enhanced dissolution of old zircon due to magma temperature differences, or due to a more reactive chemical environment.

Ion microprobe age and geochemistry of southern appalachian basement, with implications for Proterozoic and Paleozoic reconstructions

Abstract Ion microprobe U–Pb analyses of zircons from basement units in the southern Appalachians, combined with supporting isotopic compositions and major and trace element geochemistry, have delineated a granitic magmatic pulse ∼1165–1150 Ma. The pulse is manifested by the Watauga River Gneiss (western Blue Ridge), Toxaway, Wiley, and Sutton Creek gneisses (eastern Blue Ridge), Pilot Mountain and Grassy Creek gneisses (Sauratown Mountains window), and possibly the Forbush gneiss (∼1140 Ma, Inner Piedmont or Sauratown Mountains window) and Cranberry-Mine Layered Gneiss (∼1190 Ma, western Blue Ridge). Additional samples analyzed include the Blowing Rock Gneiss (∼1080 Ma, Grandfather Mountain window), and the Carvers Gap Granulite Gneiss (∼1.8 Ga) and Cloudland Granulite Gneiss (detrital cores ∼1.2–1.8 Ga) from the Mars Hill terrane. Age data were evaluated by calculating concordia ages and concordia probability plots using 206 Pb*/ 238 U and 207 Pb*/ 206 Pb* data simultaneously. Rocks in the main magmatic pulse are granitic (63–72 wt.% SiO 2 ), but elevated in K and incompatible trace elements compared to typical subduction-related magmas, and initial Nd ratios cluster tightly near CHUR. Mars Hill terrane samples are distinct in age, geochemistry (poorer in K and incompatible elements), and isotopic compositions (e Nd −7.6 and −5 at 1.0 Ga). Zircons from almost all samples have metamorphic rims that yield ages ∼1030 Ma, with the exception of the Blowing Rock Gneiss. Ages of Grenvillian magmatism and metamorphism are similar to reported ages from the northern Blue Ridge of Virginia, the Adirondack Highlands, the Central Metasedimentary Belt (Canadian Grenville Province), and the Llano uplift of Texas. This suggests the entire southeastern margin of Laurentia has a similar history ∼1.2–1.0 Ga. Although consistent with known ages in Laurentia, the presence of ∼1.8 Ga rocks and T DM ages commonly >1.6 Ga is inconsistent with the inferred 1.6 Ga margin of Laurentia. This suggests either that the 1.4–1.5 Ga mid-continent terrane separates older portions of Laurentia, that this region was exotic, or that it was a rifted fragment of Laurentia reattached during Grenville orogeny. Surprisingly few Paleozoic metamorphic zircon rims have been identified, but the few analyses obtained from samples in the eastern Blue Ridge yield late Acadian ages (∼350 Ma). The similarity of basement units across the southern Appalachians suggests a relationship between these provinces and that the Piedmont terrane is not exotic to Laurentia during the Appalachian orogenic cycle.

Inside the “African Cattle Complex”: Animal Burials in the Holocene Central Sahara

Cattle pastoralism is an important trait of African cultures. Ethnographic studies describe the central role played by domestic cattle within many societies, highlighting its social and ideological value well beyond its mere function as ‘walking larder’. Historical depth of this African legacy has been repeatedly assessed in an archaeological perspective, mostly emphasizing a continental vision. Nevertheless, in-depth site-specific studies, with a few exceptions, are lacking. Despite the long tradition of a multi-disciplinary approach to the analysis of pastoral systems in Africa, rarely do early and middle Holocene archaeological contexts feature in the same area the combination of settlement, ceremonial and rock art features so as to be multi-dimensionally explored: the Messak plateau in the Libyan central Sahara represents an outstanding exception. Known for its rich Pleistocene occupation and abundant Holocene rock art, the region, through our research, has also shown to preserve the material evidence of a complex ritual dated to the Middle Pastoral (6080–5120 BP or 5200–3800 BC). This was centred on the frequent deposition in stone monuments of disarticulated animal remains, mostly cattle. Animal burials are known also from other African contexts, but regional extent of the phenomenon, state of preservation of monuments, and associated rock art make the Messak case unique. GIS analysis, excavation data, radiocarbon dating, zooarchaeological and isotopic (Sr, C, O) analyses of animal remains, and botanical information are used to explore this highly formalized ritual and the lifeways of a pastoral community in the Holocene Sahara.

Whole-rock Pb and Sm-Nd isotopic constraints on the growth of southeastern Laurentia during Grenvillian orogenesis

The conventional view that the basement of the southern and central Appalachians represents juvenile Mesoproterozoic crust, the final stage of growth of Laurentia prior to Grenville collision, has recently been challenged. New whole-rock Pb and Sm‑Nd isotopic data are presented from Mesoproterozoic basement in the southern and central Appalachians and the Granite-Rhyolite province, as well as one new U-Pb zircon age from the Granite-Rhyolite province. These data, combined with existing data from Mesoproterozoic terranes throughout southeastern Laurentia, further substantiate recent suggestions that the southern and central Appalachian basement is exotic with respect to Laurentia. Sm-Nd isotopic compositions of most rocks from the southern and central Appalachian basement are consistent with progressive growth through reworking of the adjacent Granite-Rhyolite province. However, Pb isotopic data, including new analyses from important regions not sampled in previous studies, do not correspond with Pb isotopic compositions of any adjacent crust. The most distinct ages and isotopic compositions in the southern and central Appalachian basement come from the Roan Mountain area, eastern Tennessee–western North Carolina. The data set indicates U-Pb zircon ages up to 1.8 Ga for igneous rocks, inherited and detrital zircon ages >2.0 Ga, Sm-Nd depleted mantle model (T DM ) ages >2.0 Ga, and the most elevated 207 Pb/ 204 Pb observed in southeastern Laurentia. The combined U-Pb geochronologic and Sm-Nd and Pb isotopic data preclude derivation of southern and central Appalachian basement from any nearby crustal material and demonstrate that Grenville age crust in southeastern Laurentia is exotic and probably was transferred during collision and assembly of Rodinia. These new data better define the boundary between the exotic southern and central Appalachian basement and adjacent Laurentian Granite-Rhyolite province.

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.

Alleghenian Regional Diagenesis: A Response to the Migration of Modified Metamorphic Fluids Derived from beneath the Blue Ridge-Piedmont Thrust Sheet

To examine the nature and origin of fluids that caused widespread diagenetic alteration and remagnetization of the Appalachian foreland during the late Paleozoic, we first examined the evidence for metamorphic fluids originating from beneath the Blue Ridge and Piedmont, causing extensive alteration along the Linville Falls fault in the Grandfather Mountain window, North Carolina. These fluids were hot (310° to 400°C), enriched in radiogenic strontium, and had