The tholeiitic dyke swarm of the Arraial do Cabo peninsula (SE Brazil): 39Ar/40Ar ages, petrogenesis, and regional significance

We present 40Ar/39Ar age determinations and chemical and Sr-Nd isotopic data for the tholeiitic dyke swarm cropping out at Arraial do Cabo peninsula at the eastern end of the Rio de Janeiro coastline. The age determinations indicate that the swarm emplaced approximately 55 Ma. ago and thus is similar in age to the K-rich alkaline dykes and plugs also found in the peninsula. The dykes are basalts and basaltic andesites that belong to low Ti tholeiitic series. One dyke group is basaltic in composition, has relatively low Zr/Nb (8.9 +/- 1.8), roughly flat mantle-normalized incompatible element patterns, relatively low initial Sr-87/Sr-86 (0.70434-0.70426), and high epsilonNd((55)) (+2.3 to +3.8). These dykes chemically resemble enriched midocean ridge basalts (MORB). A second dyke group is formed by more evolved basaltic andesites with hi-her Zr/Nb ratios (12.7 +/- 2.1), negative Nb anomalies in the mantle-normalized patterns, higher initial Sr-87/Sr-86 (0.70548-0.70613), and lower epsilonNd((35)) (-1.8 to -2.2). Chemical and isotopic data exclude the possibility that the two dyke types are comagmatic or related to each other through crustal contamination processes. The genesis of the Arraial do Cabo tholeiites was likely caused by extension-related melting of a largely incompatible element-depleted mantle, with no trace of the enriched component in the roughly coeval, K-rich mafic alkaline magmas of the Serra do Mar province. (C) 2003 Elsevier Science Ltd. All rights reserved.