What caused the Early Silurian change from mafic to silicic (S-type) magmatism in the eastern Lachlan Fold Belt?
نویسنده
چکیده
One of the characteristic features of Ordovician tectonism in the east Lachlan Fold Belt is the occurrence of calcalkaline to shoshonitic basalts, which have been recognised as part of an intraoceanic arc environment (Glen et al. 1998), consistent with early tectonic interpretations (Oversby 1971; Scheibner 1973). Aside from typical traceelement abundances of intraoceanic arcs, critical evidence is the isotopic composition of the volcanics: Nd values range from 6.5 to 8.0 and from 4.2 to 6.7 for the Early and Late Ordovician basalts, respectively, indicating minimal crustal contamination (Wyborn & Sun 1993; Watkins in Glen et al. 1998). Very low 207/204Pb and 208/204Pb isotopic ratios for the mafic rocks also indicate an exclusive mantle origin (Carr et al. 1995), which is consistent with an intraoceanic arc environment. What caused the Early Silurian change from mafic to silicic (S-type) magmatism in the eastern Lachlan Fold Belt?
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