Deciphering mantle sources and petrogenetic history of the Oka carbonatite complex, Quebec, Canada.

摘要

This thesis reports detailed chemical, isotopic, geochronological and melt inclusion data for individual mineral phases from both carbonatites and silicate rocks associated with the Oka carbonatite complex. The latter represents the most westerly intrusion of the Monteregian Igneous Province (MIP), Quebec, Canada. The main goals are to decipher in detail the petrogenetic history and mantle sources for this complex. Large major and trace element variations recorded by the constituent minerals (e.g., apatite, calcite, niocalite, and perovskite) are not consistent with closed system fractional crystallization, but rather suggest the involvement of more than one parental melt and ensuing magma mixing. In-situ U-Pb ages for apatite, perovskite and niocalite define a ∼15 Myr protracted history of magmatic activity. The overall distribution of ages indicates that alnoite was intruded first, followed by okaite and carbonatite, whereas ijolite defines a bimodal emplacement history. The combined and variable radiogenic (Sr, Nd and Pb) and stable (C and O) isotope data clearly demonstrate the involvement of at least three mantle components (HIMU, EM1 and ADM) in the petrogenetic history of Oka, and these are inconsistent with crustal contamination. The melt inclusion data reveal that the parental carbonatite melt composition is similar to that of the natrocarbonatite lavas presently erupting from Earth's sole active carbonatite volcano, Oldoinyo Lengai (Tanzania). Thus, this result provides some direct evidence for the more pervasive occurrence of Na- and K-rich carbonatite liquids in the geological past and in different continental areas. Overall, the multi-analytical, high spatial resolution approach adopted here demonstrates quite clearly its advantages over "classic" investigations focused solely on using bulk rock methods.