Postorogenic carbonatites at Eden Lake,Trans-Hudson Orogen(northern Manitoba,Canada):Geological setting,mineralogy and geochemistry

摘要

The Eden Lake pluton in the Trans-Hudson Orogen is the first known occurrence of carbonatites in Manitoba.The pluton is largely made up of modally and geochemically diverse syenitic rocks derived from postorogenic magma(s)of shoshonitic affinity.Their diversity can be accounted for by a combination of crystal fractionation and fluid release in the final evolutionary stage(crystallization of quartz alkali-feldspar syenite).At Eden Lake,carbonatites,represented predominantly by coarse-grained massive to foliated sovite,occur as branching veins and lenticular bodies up to 4 m in thickness showing crosscutting relations with respect to all of the syenitic units.The host rocks are intensely fenitized at the contact,and there is also abundant mineralogical and textural evidence for assimilation of silicate material by carbonatitic magma through wallrock reaction and xenolith fragmentation and digestion.The bulk of the carbonatites are composed of(in order of crystallization):Sr-REE-rich fluorapatite,aegirine-augite,and coarse calcite crystals surrounded by fine-grained calcite(on average,~ 90 vol.% of the rock).Noteworthy accessory constituents are celestine,bastnasite-(Ce)(both as primary inclusions in calcite),Nb-Zr-rich titanite,low-Hf zircon,allanite-(Ce)and andradite.The calcite is chemically uniform(Sr-rich,Mg-Mn-Fe-poor and low in ~(13)C),but shows clear evidence of ductile deformation and syndeformational cataclasis.Geochemically,the carbonatites are enriched in Sr,Ba,light rare-earth elements,Th and U,but depleted in high-field-strength elements(particularly,Ti,Nb and Ta).The stable-isotope composition of coarse-and fine-grained calcite from the carbonatites and interstitial calcite from syenites is remarkably uniform:ca.-8.16±0.27 per thousand delta ~(13)C(PDB)and+8.04±0.19 per thousand delta ~(18)O(SMOW).The available textural and geochemical evidence indicates that the Eden Lake carbonatites are not consanguineous with the associated syenites and may have been derived from a Nb-Ti-retentive and ~(13)C-depleted source such as the subducted crustal material underlying the Eden Lake deformation corridor.