ALBERT

Notes: Abstract High-silica rhyolites of the Sierra La Primavera, a late Pleistocene center near Guadalajara, are extremely Sr-poor (0.3–1.3 ppm), yet (with one exception) values of 87Sr/86Sri are relatively low at 0.7041–0.7048. Values of 143Nd/144Nd for all the rhyolites are (within errors) identical to a basalt at 0.5129. These surprisingly primitive values, along with feldspar ∂18O of +6.6‰, are consistent with an origin by fractional crystallization of mantle-derived basalt. However, absence of the large volume of associated intermediate rocks that would be expected if the 40 km3 of erupted rhyolite were produced mainly by fractional crystallization suggests alternative processes involving partial melting of Mesozoic or Tertiary mafic intrusive rocks (or lower-crustal metamorphic equivalents). The latter interpretation is preferred, especially in light of comparative data for other North American, Cenozoic, high-silica rhyolites. Isotopic compositions correlate with basement age, but generally lie between values for associated basalts and the underlying crust. Nearly all can be interpreted as containing both a young mantle-derived component and a crustal component, probably derived by partial melting at intermediate to deep levels of the crust. No matter what the proportions of mantle- and crust-derived material in parental magmas, the extremely low concentrations of Sr and Ba in the high-silica rhyolites require extensive fractional crystallization of feldspar-rich assemblages after parental liquids attain rhyolitic compositions. At La Primavera, contamination by shallow roof rocks probably led to the 0.708 87Sr/86Sri ratio of the earliest postcaldera lava dome, which is thought to have erupted through the same vent as the caldera-forming pyroclastic flows. Contamination associated with collapse apparently affected only a small volume of magma in contact with brecciated wall rocks close to the vent, as nearby lavas that erupted during the same episode about 95 ky ago are unaffected. No identifiable lowering of ∂18O took place on caldera collapse. Rhyolitic lavas that erupted 75, 60, and 30 ky ago document postcaldera chemical recovery of the chamber to progressively more evolved compositions in its upper reaches, but show little variation in ∂18O, 87Sr/ 86Sri, or 143Nd/144Nd with time, suggesting that the bulk of the rhyolitic magma within the chamber was isolated from significant wall-rock contamination. Most of the small range of 87Sr/86Sri among the rhyolites can be attributed to pre-eruptive, in situ decay of 87Rb, resulting in a measurable secular increase of 87Sr/86Sr in these Sr-poor magmas. The 87Sr/86Sri of the youngest rhyolite, however, is somewhat lower than predicted, suggesting that the silicic magma chamber was at times open to interaction with more-mafic magmas from below.

Notes: Abstract Concentric zoning in the Criffell pluton takes the form of a discontinuous outer margin of metaluminous hornblende granodiorite and an inner core of increasingly peraluminous muscovite granite. Previous investigations using major and selected trace elements have shown the variation to consist of both smooth and abrupt trends. This study of 15 samples for the rare earth elements shows patterns which strongly correlate with Sr and O isotope data. The principal feature of these data is a progressive decrease in total rare earths with approach to the geochemical centre of the pluton, and evolution to more radiogenic Sr and more silicic and peraluminous compositions. No significant europium anomaly is developed. The slope of light to heavy rare earths using La/Yb ratios varies in a complex manner showing no significant correlation with any of the main indices of bulk composition, but with peak values occurring within the inner part of the outer portion of the pluton. A map of Ce/Y variation based on 172 Ce and Y determinations is essentially identical. These data are considered in terms of various petrogenetic models and it is concluded that the data can only be interpreted in terms of a major and progressive involvement of crustally-derived anatectic magma towards the pluton interior. Trace element modelling favours processes of the assimilation-fractional crystallisation (AFC) type for the generation of this example of I-type to S-type granitoid zonation.