Petrogenesis of an 800 m lava sequence in eastern Uruguay: insights into magma chamber processes beneath the Parana flood basalt province

Major and trace element along with representative Sr, Nd and Pb isotope data are presented for drill core samples which intersect an 800 m lava pile in eastern Uruguay. The lavas form part of the Parana flood basalt province, are low-Ti in composition but distinct from the low-Ti Gramado magma type, and have been termed the Treinte Y Tres magma type. The lava pile overlies a large positive gravity anomaly inferred to reflect an east-west trending, mid-crustal mafic intrusive body with a calculated volume of 35,000 km(3). Smooth up-section compositional variations in the basalts are interpreted to record magma evolution within this mid-crustal magma chamber. Sr-87/Sr-86 and (206)pb/(204)pb increase throughout the sequence yet Mg# remains relatively constant in the lower 200 m of the sequence, suggesting a role for magma chamber recharge. Above this the lavas show a regular, up-section decrease in Mg# coupled with increasing 87Sr/86Sr and 206pb/204pb and this is interpreted to reflect crystal fractionation combined with crustal contamination. The data provide further evidence that contamination of flood basalt magmas in crustal magma chambers is a common phenomenon and calculations suggest that the amount of crustal addition may be as high as 60-70%. Nevertheless, the effects of this crustal contamination do not appear able to account for the discrepancy between key incompatible trace element ratios and isotope ratios of the lavas and those of any putative mantle plume. In fact, La/Ta decreases with decreasing Mg# and increasing Sr-87/Sr-86 indicating that the effects of crustal contamination were actually to reduce La/Ta and implying that the parental magmas had very high La/Ta (similar to 90). These constraints are clearly inconsistent with an asthenospheric origin for the parental magmas and so, consistent with mass balance calculations, it is inferred that they were derived from the lithospheric mantle. (C) 1999 Elsevier Science Ltd. All rights reserved.