RbSr and oxygen isotopic study of alkalic rocks from the Trans-Pecos magmatic province, Texas: Implications for the petrogenesis and hydrothermal alteration of continental alkalic rocks
Rb-Sr and O isotopic data for mid-Tertiary alkalic rocks from the Trans-Pecos magmatic province of west Texas demonstrate that hydrothermal alteration and fluid/rock (cation exchange) interactions have affected the isotope geochemistry of these rocks. Strontium and O isotopic data for late-stage minerals in an alkali basalt (hawaiite) sill record two episodes of fluid/rock interactions. Firstly, Sr isotopic data for calcite precipitated in open spaces suggest that most of the Sr incorporated in this mineral was derived from the surrounding silicate minerals; however, O was derived from heated formation fluids ( δ18O = +13; t< 100° C) in isotopic equilibrium with surrounding Cretaceous marine limestones. Secondly, Sr isotopic data for analcime and natrolite lie along mixing lines between a magmatic 87Sr/ 86Sr and a Cretaceous marine 87Sr/ 86Sr. Oxygen isotopic data for natrolite suggest fluids in equilibrium with this mineral were isotopically light ( δ18O = -8), very similar to present-day geothermal waters of the Trans-Pecos region ( δ18O = -6 to -9). These data suggest that later meteoric fluids introduced Sr with a Cretaceous marine 87Sr/ 86Sr ratio into minerals with significant cation exchange capacity. Dilute HCl leaching experiments demonstrate the removal of this labile or exchangeable Sr from the alkali basalt. Rb-Sr isotopic data for the leached alkali basalt and handpicked calcite record a crystallization age of 42 Ma, consistent with K-Ar data for an unaltered alkali basalt (hawaiite) dike from the same area (42.6 ± 1.3 Ma). Leaching experiments on one phonolite suggest the Sr isotopic variability in unleached phonolite and nepheline trachyte samples may be attributed to Sr in secondary calcite and zeolites, which have an upper Cretaceous marine 87Sr/ 86Sr ratio. RbSr isotopic data for leached phonolite and sanidine separate yield an age of 36.5 ± 0.8 Ma, within analytical uncertainty of a K-Ar biotite age (36.0 ± 1.1 Ma) of another phonolite. These leaching experiments demonstrate that the Rb-Sr isotopic systematics of hydrothermally-altered continental alkalic rocks may be significantly improved, providing more reliable geochronologic and isotopic tracer information necessary in constructing precise models of mantle sources.