O. Don Hermes

Sr isotopic evidence for a multi-source origin of the potassic magmas in the neapolitan area (S. Italy)

New Sr isotopic data on lavas and xenoliths from Somma-Vesuvius and other nearby volcanic areas (Phlegrean Fields and Ischia) are presented. Chemical and isotopic evidences show that not all the Phlegrean Fields rocks belong to the low K series, but some of them may be interpreted as low pressure differentiates of Somma magmas, i.e. as a part of the high K series. Two rock groups are defined in the Ischia low K series, which are well identified both in time and in chemical and isotopic features, and cannot be derived from the same magma source. The low K series in the studied area generally has lower Sr isotopic values than the high K series.Historical Vesuvian lavas show two distinct linear trends with negative slopes when87Sr/86Sr ratios are plotted against their ages of eruption. Such trends are interpreted to result from mixing of magmas in two separate reservoirs. Evidence from the Vesuvian ejecta shows that Somma-Vesuvius magmas underwent high or low pressure fractionation, in connection with different events of the Vesuvian activity. Distinct magma reservoirs developed episodically at different depths. Isotopic and geochemical evidences do not favour large scale assimilation of crustal materials by Somma-Vesuvius magmas, but instead appear to reflect mantle characteristics.A minimum of three different (inhomogeneous) source regions is necessary to account for the isotopic features of the studied rocks.

Quenched crystal mush and associated magma compositions as indicated by intercumulus glasses from Mt. Vesuvius, Italy

Abstract The presence of xenolithic accumulate rocks that contain combinations of clinopyroxene, olivine, biotitic phlogopite, and apatite in the ejecta of Somma-Vesuvius volcano verify that crystal fractionation was an operative process in the pretrogenesis of the leucite-bearing lavas. Many of the cumulates contain small quantities of pervasive interstitial glass that leads us to interpret these rocks as fragments of ejected crystal mush which was quenched upon rapid upward transport and ejection. Petrographic and chemical data indicate that the glasses represent liquids in equilibrium with the associated crystallizing phases and probably are representative of larger volumes of magma. Despite the fact that our glasses are in cumulate rocks mainly from the 1944 eruption, the compositions generally overlap the entire compositional range exhibited by lava compositions and tephra reported for the 2000-year life span of Vesuvius activity. Generally similar glass compositions also are exhibited by three samples that stratigraphically occur between two lava flows thought to be erupted, respectively, in 1440 and 1631, but the limited number of samples studied for this eruption does not allow us to determine if the range of compositions are as diverse as the 1944 suite. For the glasses (intercumulus liquids) of single eruptive episodes to exhibit such compositional ranges is strong evidence that the Vesuvius magma reservoir was heterogeneous. It may have existed as either a chemically zoned magma chamber, or a series of local magma pockets that evolved independently to different degrees, but were simultaneously traversed or tapped during these two eruptive episodes.

Late Proterozoic and Devonian plutonic terrane within the Avalon zone of Rhode Island

The U-Th-Pb radiometric age of zircons demonstrates that much of Rhode Island consists of a late Proterozoic plutonic complex that subsequently was intruded by a large Devonian alkalic to subalkalic igneous complex. Two groups of late Proterozoic rocks can be recognized: (1) the Esmond Granite and related plutonic rocks in northern Rhode Island, ranging in composition from gabbro to granite and yielding an upper concordia intercept of 621 ± 8 m.y., and (2) quartz-rich, recrystallized gneissic rocks (Ten Rod Granite Gneiss and Hope Valley Alaskite Gneiss) in southern Rhode Island, giving an upper concordia intercept age of 601 ± 5 m.y. The lower intercept of the zircon discordia for the gneissic rocks indicates the isotopic systems were disturbed during the late Paleozoic Alleghanian Orogeny, whereas zircons from the Esmond Granite and related plutonic rocks have been subjected mainly to loss of lead attributable to recent dilatancy. Rocks determined to be of Devonian age include the Scituate Granite and plutonic rocks of the East Greenwich Plutonic Suite. The Quincy Granite of Quinn (1971) in northeastern Rhode Island may be of a similar Devonian age, or it may be slightly younger. Zircon data from these rocks fall upon a chord that gives an upper concordia intercept of 370 ± 7 m.y. and a lower intercept close to the origin. Collectively, these rocks form a composite pluton with an area >700 km 2 . The Devonian rocks exhibit petrologic characteristics distinct from slightly older Acadian orogenic rocks that occur in lithostratigraphic zones to the west and are more comparable to anorogenic Ordovician to Devonian alkalic granitoids that trend north-northeast across Massachusetts and the Gulf of Maine. The new age determinations confirm that much of Rhode Island contains plutonic rocks intruded during the Avalonian Orogeny but that a hertofore unrecognized major Devonian plutonic episode also occurred. The distinctly different ages probably had gone unrecognized because of generally similar lithologic and textural features exhibited by the late Proterozoic and Devonian rocks. We present new petrologic and geochemical criteria that should assist in distinguishing the rocks of different ages as additional mapping and geologic study proceeds.

Fractional crystallization in granites of the Sierra Nevada: How important is it?

Although compositional variation in zoned calc-alkalic plutons is often ascribed to crystal fractionation, diagnostic large-scale field evidence of crystal accumulation in these slowly cooled bodies is generally missing. In many plutons, however, small-scale crystal cumulates have been preserved as layered schlieren and in microcosm may allow an assessment of the importance of crystal fractionation in their host pluton9s development. Small, widely separated patches of schlieren in the Tuolumne Intrusive Series, Yosemite National Park, California, formed as cumulates. Their darkest layers show high concentrations of magnetite, sphene, biotite, horn-blende, and zircon, and have unusually fractionated major and trace element compositions (FeO >33%; Al 2 O 3 in chondrites ∼750; Zr ∼2000 ppm). The layers define smooth trends on major and trace element Al 2 O 3 variation diagrams that diverge strongly from patterns for the main-sequence rocks of the Tuolumne Series and granitoids throughout the Sierra Nevada. Removal of such cumulates from any main-sequence magma would produce Al-rich evolved rocks, not the Al- poor felsic rocks of the pluton. The findings suggest that fractional crystallization did not produce the dominant chemical patterns seen in the Tuolumne and similar Sierra Nevada granites.

Archean inheritance in zircon from late Paleozoic granites from the Avalon zone of southeastern New England: an African connection

Abstract In southeastern New England the Narragansett Pier Granite locally intrudes Carboniferous metasedimentary rocks of the Narragansett basin, and yields a monazite U Pb Permian emplacement age of 273 ± 2Ma. Zircon from the Narragansett Pier Granite contains a minor but detectable amount of an older, inherited component, and shows modern loss of lead. Zircon from the late-stage, aplitic Westerly Granite exhibits a more pronounced lead inheritance —permitting the inherited component to be identified as Late Archean. Such old relict zircon has not been previously recognized in Proterozoic to Paleozoic igneous rocks in New England, and may be restricted to late Paleozoic rocks of the Avalon zone. We suggest that the Archean crustal component reflects an African connection, in which old Archean crust was underplated to the Avalon zone microplate in the late Paleozoic during collision of Gondwanaland with Avalonia.

Late Proterozoic and Silurian alkaline plutons within the southeastern New England Avalon zone

Distinct pulses of quartz-bearing, alkaline plutonism and volcanism are known to have occurred in the Avalon zone of southeastern New England during the Late Ordovician, Early Silurian, Devonian, and Carboniferous. Zircon separates from the Franklin and Dartmouth plutons demonstrate that two additional, previously unrecognized periods of alkaline magmatism occurred. The Franklin pluton yields an age of

Application of petrographic and geochemical methods to sourcing felsitic archaeological materials in Southeastern New England

417 \pm 6 Ma

A transitional alkalic dolerite dike suite of Mesozoic age in Southeastern New England

(Late Silurian), whereas the Dartmouth pluton is Late Proterozoic (