Isotopic characteristics of potassic rocks: evidence for the involvement of subducted sediments in magma genesis

Abstract

The potassic igneous rock suite (with molar K 2O/Na 2O > 1) can be divided into an “orogenic” subgroup that occur in subduction-related tectonic settings and an “anorogenic” sub-group that are confined to stable continental settings. Representatives of both sub-groups possess trace element and isotopic features consistent with the contamination of their magma sources by incompatible element rich and isotopically evolved “metasomatic” components. It is argued here that these metasomatic components are principally derived from subducted lithosphere, including subducted sediments. Most examples of orogenic potassic magmatism (e.g. Italian potassic rocks, Spanish lamproites, Sunda arc leucitites) have trace-element and Sr, Nd and Pb isotopic characteristics consistent with the contamination of their mantle sources by a component derived from marine sediments. Anorogenic sub-group potassic magmas have generally similar incompatible trace element and Sr and Nd isotopic characteristics to those of orogenic potassic magmas, but many examples have unusual Pb isotopic compositions with unradiogenic 206Pb/ 204Pb. Modern marine sediments characteristically have low U/Pb ratios and the unradiogenic 206Pb/ 204Pb of anorogenic potassic magmas may have evolved during long-term storage of subducted sediments (or components derived from them) within the subcontinental lithosphere. These unusual Pb isotopic compositions require substantial time periods (> 1 Ga) to have elapsed between the fractionation events lowering the U/Pb ratio (i.e. erosion and sedimentation at the Earth's surface) and subsequent potassic magmatism and it is therefore not surprising that most examples of anorogenic potassic magmatism are not associated with recent subduction processes. Although the eruption of potassic magmas is commonly related to rifting or hotspot activity, these processes do not necessarily play an important role in the genesis of the unusual sources from which potassic magmas are derived.