OIB-LIKE MANTLE SOURCE FOR CONTINENTAL ALKALINE ROCKS OF THE BALCONES PROVINCE, TEXAS - TRACE-ELEMENT AND ISOTOPIC EVIDENCE

Rocks of the Uvalde portion of the Balcones igneous province of south-central Texas include nephelinite, basanite, alkali basalt, and phonolite. The nephelinites and basanite represent primitive, potentially primary, magmas based on high Mg#, high Ni contents, and the presence of mantle xenoliths. Normalized trace-element patterns and diagnostic elemental ratios for these rocks are very similar to those of ocean-island basalts (OIB). Phonolites display OIB-like patterns strongly overprinted by the effects of fractional crystallization. Trace-element variations in the nephelinites, basanite, and alkali basalt result from variable degrees of partial melting of a grossly homogeneous mantle source. Initial Sr-87/Sr-86, assuming an age of 75 Ma, ranges between 0.70313 and 0.70467 (<0.70369 for all except one sample). Initial epsilon(Nd) is 6.2 +/- 0.4 (1sigma) for the mafic rocks excluding the alkali basalt (5.0); the phonolites have initial epsilon(Nd) = 5.7 +/- 0.1. Initial Pb isotopic ratios for all samples are Pb-206/Pb-204 = 18.87 +/- 0.14 and Pb-207/Pb-204 = 15.56 +/- 0.02. The mafic Uvalde rocks show strongly asthenospheric isotopic and trace-element signatures reflecting a source slightly less depleted than N-MORB and comparable to that of OIB. There is no evidence of significant crustal contamination or a lithospheric mantle signature in any of the mafic Uvalde rocks other than the alkali basalt. Phonolitic rocks probably are derived by fractionation of nephelinitic parental magmas with negligible crustal interaction.