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Oxygen isotopes are a valuable tool for determining the role of supracrustal rocks in the genesis of magmas and help constrain the source regions of plutonism in the northern Appalachian mountains of North America. Zircons from 35 igneous or metaigneous rocks mainly from southern New England, USA, were analyzed for [sup.18]O/[sup.16]O to locate and investigate the origin of anomalous [δ.sup.18]O magmas and to evaluate the regional meteoric water-rock interaction. [δ.sup.18]O values of the zircons obtained using the laser fluorination technique vary between 2.3 [per thousand] and 11.0‰, relative to Vienna Standard Mean Ocean Water (VSMOW). Zircons from two plutons in the Avalon terrane reveal the presence of anomalously low- [δ.sup.18]O ( Les isotopes de l'oxygene sontun outil important pour determiner le role des roches supracrustales dans lagenese des magmas et ils aident a delimiter les regions sources du plutonisme dans le nord des montagnes appalachiennes de l'Amerique du Nord. Des zircons provenant de 35 roches ignees ou meta-ignees, principalement du sud de la NouvelleAngleterre, ont ete analyses pour [sup.18]O/[sup.16]O afin de localiser et d'etudier l'origine des magmas a anomalie [δ.sup.18]O et d'evaluer les interactions regionales entre l'eau meteorique et les roches. Les valeurs [δ.sup.18]O des zircons, obtenues au moyen de la technique de fluoration au laser, varient entre 2,3 ‰ et 11,0 ‰ VSMOW (eau oceanique moyenne normalisee de Vienne). Les zircons provenant de deux plutons du terrane d'Avalon revelent la presence de zircons a teneur [δ.sup.18]O anormalement basse (> (Ordovicien tardif a Silurien precoce) du nord du Connecticut (zircon [δ.sup.18]O en vrac = 7,56 ‰) indiquent l'incorporation de quantites importantes de composantes sedimentaires a teneur elevee [δ.sup.18]Oa partir des regions sources plus anciennes dans ces magmas. Les terranes d'Avalon et de Gander du sud-est de la Nouvelle-Angleterre et la bordure est de Laurentia ont subi de longs historiques d'enfouissement ou de subduction, de fusion et d'assimilation de roches provenant de la surface au cours du Neoproterozoique et du milieu du Paleozoique. [Traduit par lar Redaction], Introduction Compared with other rock-forming minerals such as quartz and feldspar, zircon is highly resistant to oxygen isotopic exchange during post-crystallization hydrothermal alteration (e.g., Valley 2003) and thus can be [...]

Geochemical data are reported for samples from the flanks and floor of the southern Kenya Rift Valley in the Lake Magadi area, and from two central volcanoes located within the rift valley. Rift lavas include samples of Singaraini and Ol Tepesi basalts on the eastern flank, Kirikiti basalts from the western flank, and plateau trachytes from the rift valley floor. Central volcano samples are from Ol Esayeiti and Lenderut located on the eastern flank. The rift basalts are mildly ne-normative, moderately evolved (Mg# = 0.39-0.62) alkali basalts and show an overall range in differentiation. Incompatible trace element abundances are moderately elevated (Nb = 17-51; Zr = 93-274; La = 17-55 ppm) and show strongly coherent variations and constant inter-element ratios (e.g. Zr/Nb = 4.2-5.5; Nb/Ta = 17.5 [+ or -] 0.4; [(La/Sm).sub.n] = 7.3 [+ or -] 1.1); isotope ratios are restricted in range ([sup.87]Sr/[sup.86]Sr = 0.70393-0.70436; [sup.143]Nd/[sup.144]Nd = 0.51272-0.51280; [sup.206]Pb/[sup.204]Pb = 19.87-19.92; [sup.207]Pb/[sup.204]Pb = 15.68-15.70; [sup.208]Pb/[sup.204]Pb = 39.56-39.71). Central volcano lavas are more alkaline in character and include basanite (Ol Esayeiti; Mg# > 60) and hawaiite to benmoreite (Lenderut; Mg# = 0.48-0.38). Incompatible element ratio are similar to those of the rift basalts, although the chondrite normalised REE patterns are steeper [(La/Sm).sub.n] = 17.4 [+ or -] 1.2). [sup.87]Sr/[sup.86]Sr (0.70358, 0.70391), [sup.143]Nd/[sup.144]Nd (0.51280, 0.51267), [sup.206]Pb/[sup.204]Pb (19.96,20.17), [sup.207]Pb/[sup.204]Pb (15.66,15.76) and [sup.208]Pb/[sup.204]Pb (39.80,40.00) ratios of Ol Esayeiti basanites are similar to the rift basalts, whereas the Lenderut lavas have unusually low[sup.143]Nd/[sup.144]Nd (0.512388-0.512453) ratios for their [sup.87]Sr/[sup.86]Sr (0.70370-0.70481) ratios, and distinctly less radiogenic and variable Pb isotope compositions ([sup.206]Pb/[sup.204]Pb = 17.93-19.01; [sup.207]Pb/[sup.204]Pb = 15.43-15.58; [sup.208]Pb/[sup.204]Pb = 37.91-39.14). An integrated model is developed in which the geochemical signature of the lavas is attributed to variable degrees of melting to depths within the garnet stability field, and in the presence of residual amphibole. The stability fields of these phases in P-T space indicates that the lavas must have formed within the sub-continental lithosphere rather than within the underlying ambient asthenosphere or a rising mantle plume. The subcontinental lithospheric mantle must therefore extend to a depth of at least 75 km beneath the Lake Magadi area, which contrasts with recent gravity models for the area, which infer that lithospheric mantle is absent beneath this section of the southern Kenya Rift.

The rifting of the Laurentian continent which formed Iapetus Ocean continued for 200 million years and involved two significant pulses of extrusive ingenious activity. This is indicated by U-pb ages of zircon from rhyolites of the Catoctin and mount Rogers Formations which are dated at about 564 Ma and 758 Ma respectively. The first pulse involving Mount Rogers Formations was confined to the south of Potomac river and did not cause continental separation. The Catoctin formation was due to later pulses and caused the opening of the Iapetus Ocean.

The Skagit Gneiss Complex forms a more-or-less continuous terrane within the northern, more deeply eroded part of the North Cascade Range. The complex comprises abundant plutons intruded at mid-crustal depths into a variety of metamorphosed supracrustal rocks of both oceanic and volcanic-arc origin. A plethora of syntectonic pegmatite, small plutons, and granitic dikes gives the complex a migmatitic aspect. U-Pb zircon ages from gneissic plutons within and near the Skagit Gneiss Complex indicate magmatic crystallization between 75 and 60 Ma. Deformation, recrystallization, and migmatization in part postdate intrusion of the 75-60 Ma plutons. This latest Cretaceous and earliest Tertiary plutonism and migmatization may reflect thermal relaxation following early Late Cretaceous orogeny documented elsewhere in the North Cascades. The complex was ductilely extended northwest-southeast shortly after intrusion of granite dikes at 45 Ma, but before emplacement of the earliest ( 34 Ma) plutons of the Cascade arc. Outcrops of Late Cretaceous and earliest Tertiary plutons, migmatites of the Skagit Gneiss Complex, and rocks with young ductile deformation are roughly coextensive, all apparently marking a region of greater middle Eocene unroofing. Unroofing was apparently contemporaneous with east-west extension in the Okanogan region to the east and north-south and northwest-southeast strike-slip faulting within the North Cascades.

Atmospheric oxygen has evolved from negligible levels in the Archean to the current level of about 21% through 2 major step rises: The Great Oxidation Event (GOE) in the early Proterozoic and the Neoproterozoic Oxygenation Event (NOE) during the late Proterozoic. However, most previous methods for constraining the time of atmospheric oxygenation have relied on evidence from sedimentary rocks. Here, we investigate the temporal variations of the Th/U of arc igneous rocks since 3.0 billion y ago (Ga) and show that 2 major Th/U decreases are recorded at ca. 2.35 Ga and ca. 0.75 Ga, coincident with the beginning of the GOE and NOE. The decoupling of U from Th is predominantly caused by the significant rise of atmospheric oxygen. Under an increasingly oxidized atmosphere condition, more uranium in the surface environment became oxidized from the water-insoluble U4+ to the water-soluble U6+ valance and incorporated in the sea water and altered oceanic crust. Eventually, the subduction of this altered oceanic crust produced the low-Th/U signature of arc igneous rocks. Therefore, the sharp decrease of Th/U in global arc igneous rocks may provide strong evidence for the rise of atmospheric oxygen. We suggest that the secular Th/U evolution of arc igneous rocks could be an effective geochemical indicator recording the global-scale atmospheric oxygen variation. [ABSTRACT FROM AUTHOR]

The Yulong porphyry Cu (Au) deposit occurs on the eastern margin of the Tibetan plateau and is one of the largest porphyry Cu (Au) deposits in China, containing more than 6 million tons of Cu metal. Similar to other porplayry Cu deposits in the world, the Yulong porphyry is highly oxidized. Sulfate is the dominant sulfur species in fluid inclusions hosted by magmatic quartz phenocrysts, and no sulfide is observed, indicating an oxygen fugacity above the SSO buffer during magmatic processes. The sulfur species changed from sulfate dominant during magmatic processes to sulfide dominant during the main mineralization processes, with sulfate and sulfide mineral assemblages observed in fluid inclusions in mineralized quartz veinlets and only sulfides in the orebodies, corresponding to a decrease in redox potential from the porphyry to the hydrothermal fluid. Magnetite crystallization was coincident with the onset of major sulfate reduction, indicating that magnetite isolated trivalent iron, reducing sulfate, and consequently leading to the formation of porphyry Cu-Au mineralization. Most porphyry Cu-Au ore deposits are spatially associated with, and genetically related to, oxidized felsic magmas. Our results indicate that sulfate reduction promoted by magnetite crystallization is essential for the final precipitation of Cu-Au-bearing sulfides, i.e., decreasing redox potential of the fluid is the key to, and direct cause of, ore formation in Yulong. Initial high redox potential is a prerequisite for and may be an indirect indicator of most mineralization of this kind, because it enables efficient transportation of Cu-Au. Redox potential fluctuations can be well preserved in zircon, a resistant accessory mineral commonly found in porphyries, which might be developed as a handy and reliable exploration tool for porphyry Cu deposits associated with magnetite. [ABSTRACT FROM AUTHOR]