New age determinations for the Banhadão and Itapirapuã complexes in the Ribeira Valley, southern Brazil

ABSTRACT: New age determinations by Ar-Ar and U-Pb SHRIMP for the alkaline complexes of Banhadão and Itapirapuã that occur in the Ribeira Valley are in agreement with the previously radiometric K-Ar ages available in literature. Ar-Ar ages on biotite for Banhadão range in the 106-110 Ma interval, and suggest a minimum age of 106 Ma for the complex. Ar-Ar data on biotite for Itapirapuã are more uniform, indicating an age of 102 Ma for the complex. U-Pb SHRIMP determinations on titanite from Itapirapuã melanite syenitic rocks yielded an age of ~106 Ma. Thus, the age interval of 100 and 110 Ma does exist indeed, suggesting an intermediate magmatic pulse between the already well-defined peaks of 80-90 Ma and 130-140 Ma for the alkaline rocks in southern Brazil, confirming the periodicity of the magmatism.

摘要：对产于里贝拉谷（Ribeira Valley）的班哈当（Banhadão）与伊塔皮拉普阿（Itapirapuã）碱性杂岩开展的氩-氩（Ar-Ar）法与铀-铅高灵敏度离子探针质谱（U-Pb SHRIMP）法新年代学测定结果，与文献中已有的钾-氩（K-Ar）辐射测年数据一致。班哈当杂岩的黑云母氩-氩年龄区间为106~110 Ma，指示该杂岩的最小形成年龄为106 Ma。伊塔皮拉普阿杂岩的黑云母氩-氩数据更为均一，表明其形成年龄为102 Ma。对伊塔皮拉普阿黑榴石正长岩中的榍石开展的铀-铅高灵敏度离子探针质谱测定结果显示其年龄约为106 Ma。由此可见，100~110 Ma的年龄区间确实存在，这表明巴西南部碱性岩的已知80~90 Ma与130~140 Ma两个标志性峰期之间，存在一次过渡性岩浆活动脉冲，证实了该区域岩浆作用的周期性。