干旱区生态环境知识资源中心

Ediacaran and Early Cambrian sedimentary rocks from NW Iberia have been investigated for detrital zircon U-Pb ages. A total of 1,161 concordant U-Pb ages were obtained in zircons separated from four Ediacaran samples (3 from the Cantabrian Zone and one from the Central Iberian zone) and two Lower Cambrian samples (one from the Cantabrian Zone and one from the Central Iberian Zone). Major and trace elements including REE and Sm-Nd isotopes were also analyzed on the same set of samples. The stratigraphically older Ediacaran sequence in the Cantabrian Zone has a maximum sedimentation age of ca. 600 Ma based on detrital zircon content and is intruded by ca. 590-580 Ma granitoids constraining the deposition of this part of the sequence between ca. 600 and 580 Ma. The stratigraphically younger Ediacaran sequence in the Cantabrian Zone has a maximum sedimentation age of ca. 553 Ma. The Ediacaran sample from the Central Iberian Zone has an identical within error maximum sedimentation age of ca. 555 Ma. The detrital zircon U-Pb age patterns are very similar in all the Ediacaran samples from both zones including the main age groups ca. 0.55-0.75 Ga, ca. 0.85-1.15 Ga and minor Paleoproterozoic (ca. 1.9-2.1 Ga) and Archean (ca. 2.4-2.6 Ga) populations. Kolmogorov-Smirnov statistical tests performed on this set of samples indicate that they all were derived from the same parent population (i.e., same source area). The same can be said on the basis of Nd isotopes, REE patterns and trace element concentrations. The two Cambrian samples, however, show contrasting signatures: The sample from the Cantabrian Zone lacks the ca. 0.85-1.15 Ga population and has a high proportion of Paleoproterozoic and Archean zircons (> 60 %) and a more negative epsilon (Nd) and higher T (DM) values than the Ediacaran samples. The Early Cambrian sample from the Central Iberian Zone has the same U-Pb detrital zircon age distribution (based on KS tests) as all the Ediacaran samples but has a significantly more negative epsilon (Nd) value. These data suggest apparently continuous sedimentation in the NW Iberian realm of northern Gondwana between ca. 600 and 550 Ma and changes in the detrital influx around the Ediacaran-Cambrian boundary. The nature and origin of these changes cannot be determined with available data, but they must involve tectonic activity on the margin as evidenced by the angular unconformity separating the Ediacaran and Lower Cambrian strata in the Cantabrian Zone. The absence of this unconformity and the apparent continuity of detrital zircon age distribution between Ediacaran and Cambrian rocks in the Central Iberian Zone suggest that the margin became segmented with significant transport and sedimentation flux changes in relatively short distances. As to the paleoposition of NW Iberia in Ediacaran-Early Cambrian times, comparison of the data presented herein with a wealth of relevant data from the literature both on the European peri-Gondwanan terranes and on the terranes of northern Africa suggests that NW Iberia may have lain closer to the present-day Egypt-Israel-Jordan area and that the potential source of the hitherto enigmatic Tonian-Stenian zircons could be traced to exposed segments of arc terranes such as that described in the Sinai Peninsula (Be’eri-Shlevin et al. in Geology 40:403-406, 2012).