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 dat a are presented for drill core samples which intersect an 800 m lava pile i n 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 rel atively 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 dec rease in Mg# coupled with increasing 87Sr/86Sr and 206pb/204pb and this is interpreted to reflect crystal fractionation combined with crustal contamin ation. The data provide further evidence that contamination of flood basalt magmas in crustal magma chambers is a common phenomenon and calculations s uggest that the amount of crustal addition may be as high as 60-70%. Nevert heless, the effects of this crustal contamination do not appear able to acc ount 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 an d implying that the parental magmas had very high La/Ta (similar to 90). Th ese constraints are clearly inconsistent with an asthenospheric origin for the parental magmas and so, consistent with mass balance calculations, it i s inferred that they were derived from the lithospheric mantle. (C) 1999 El sevier Science Ltd. All rights reserved.