ISSN:
1432-0967
Source:
Springer Online Journal Archives 1860-2000
Topics:
Geosciences
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.
Type of Medium:
Electronic Resource
URL:
http://dx.doi.org/10.1007/BF00373584
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