Steven A. Goldberg

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.

Anomalous strontium and lead isotope signatures in the off-rift Öræfajökull central volcano in south-east Iceland: Evidence for enriched endmember(s) of the Iceland mantle plume?

Abstract The currently active off-rift central volcano Oraefajokull in south-east Iceland sits unconformably on much older (∼10–12 Ma) and eroded crust. The composition of recent volcanics ranges from basalt to rhyolite, but the series is more sodic alkaline than the common rift zone tholeiitic suites. In this study we present Sr, Nd, Pb and O isotopic data for a suite of Oraefajokull samples. The complete suite shows typical mantle values for oxygen isotopes. The 87Sr/86Sr ratios (average of 15 samples=0.703702) of the modern Oraefajokull rocks (basalts as well as rhyolites) are much higher than observed so far for any other Icelandic rocks. The 143Nd/144Nd ratios (average=0.512947; n=15) are lower than for rift rocks, but similar to rocks of the off-rift Snaefellsnes volcanic zone. Furthermore, the Oraefajokull rocks are enriched in the 207Pb/204Pb and 208Pb/204Pb isotope ratios compared to Icelandic rift basalts. The enriched nature of the suite indicates that Oraefajokull samples a source component that has characteristics common with EM2 type mantle. Furthermore, it is concluded that the silicic rocks of Oraefajokull formed by fractional crystallization from mafic melts rather than by partial melting of older crust.

Alleghanian development of the Goat Rock fault zone, southernmost Appalachians: Temporal compatibility with the master decollement

The Goat Rock and associated Bartletts Ferry fault zones, which mark the eastern margin of the Pine Mountain Grenville basement massif, are controversial due to the suggestion that they are rare exposed segments of the late Paleozoic southern Appalachian master decollement. The controversy in part stems from reported middle Paleozoic (Acadian) radiometric dates postulated as the time of movement along these fault zones. Ultramylonite samples from the type area at Goat Rock Dam yield a 287 [plus minus] 15 Ma Rb-Sr isochron interpreted as the time of Sr isotopic rehomgenization during mylonitization. This date is corroborated by Late Pennsylvanian-Early Permian [sup 40]Ar/[sup 39]Ar mineral ages on hornblende (297-288 Ma) and muscovite (285-278 Ma) from neomineralized and dynamically recrystallized rocks within and straddling the fault zone. These Late Pennsylvanian-Early Permian dates indicate the time of right-slip movement (Alleghenian) along the Goat Rock fault zone, which is compatible with the timing suggested by COCORP for thrusting along the southern Appalachian master decollement.

Petrogenesis of the volcanic suite of Bouvet ya (Bouvet Island), South Atlantic

Whole rock and mineral compositions of volcanic rocks collected during the Norwegian Polarsirkel expedition (1978/79) to the volcanic island of Bovetoya (close to the Bouvet Triple Junction) are discussed and compared with previously published data from the island. The rock types, hawaiite, benmoreite, and peralkaline trachyte and rhyolite (comendite) are related to each other by crystal fractionation processes. The trace element and radiogenic isotope signatures displayed by the Bouvetoya rocks are those of a moderately enriched oceanic island suite. On several isotope plots Bouvetoya rocks fall on or close to mixing lines between the enriched EM-1 and HIMU mantle components. Mixing between depleted morb mantle (DMM) and enriched components is not likely. Thus, Bouvetoya displays a typical plume signature.