MANTLE XENOLITH SAMPLES FROM THE BALCONES IGNEOUS PROVINCE, CENTRAL TEXAS: MESOPROTEROZOIC STABILIZATION OF LITHOSPHERIC MANTLE AND SUBSEQUENT SUBDUCTION- FLUID MODIFICATION DURING THE OUACHITA OROGENY

The Knippa quarry in central Texas, 70 miles west of San Antonio, exposes mantle xenoliths entrained in Late Cretaceous (80 – 82 Ma) basanites from the Balcones Igneous Province. Elemental and isotopic data reveal two events experienced by the mantle beneath Knippa: Mesoproterozoic (1.3 to 1.1 Ga) melt extraction and subsequent Late Paleozoic metasomatism by subduction-related fluids. Evidence for the melt depletion event is recorded in both major and trace element compositions. Lherzolites are residues of less than 9 percent and harzburgite of up to 15 percent melting of a fertile mantle source. Modeling of the elemental compositions is consistent with fractional rather than batch extraction of melt. Melt depletion is overprinted by enrichment in fluid-mobile elements, indicating subduction-related metasomatism. The timing of these events is revealed by Os and Hf isotopes. Os isotopes from sulfide grains fall into 2 populations, a Mesoproterozoic group which represents the age of melt extraction and a Paleozoic-aged group. Hf isotopic signatures yield an age of approximately 379 Ma (i.e. the Ouachita collisional orogeny). Nd isotopes were measured, but infiltration of the samples during xenolith entrainment masks chronologic information. Instead, the Nd isotopes show that the mantle source for Balcones magmatism was HIMU, with similarities to Indian Ocean-type OIB. This same mantle source signature is found in Jurassic (160 Ma) mafic samples entrained in salt diapirs from southern Louisiana formed during opening of the Gulf of Mexico. This study shows that xenolith studies faithfully record lithospheric evolution for melt and/or fluid events. The timing and nature of these events from the southern margin of Laurentia are distinct from xenoliths in Cenozoic lavas from western North America, which share the Proterozoic melt depletion event, but lack the Ouachita metasomatic event.