OIB-LIKE MANTLE SOURCE FOR CONTINENTAL ALKALINE ROCKS OF THE BALCONESPROVINCE, 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 phonol ite. The nephelinites and basanite represent primitive, potentially pr imary, magmas based on high Mg#, high Ni contents, and the presence of mantle xenoliths. Normalized trace-element patterns and diagnostic el emental ratios for these rocks are very similar to those of ocean-isla nd basalts (OIB). Phonolites display OIB-like patterns strongly overpr inted by the effects of fractional crystallization. Trace-element vari ations in the nephelinites, basanite, and alkali basalt result from va riable degrees of partial melting of a grossly homogeneous mantle sour ce. Initial Sr-87/Sr-86, assuming an age of 75 Ma, ranges between 0.70 313 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 b asalt (5.0); the phonolites have initial epsilon(Nd) = 5.7 +/- 0.1. In itial 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 reflecti ng 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 fract ionation of nephelinitic parental magmas with negligible crustal inter action.