Abstract
A ca. 1450 Ma, east-west oriented suite of Proterozoic mafic intrusions in eastern Canada, located between Labrador City and coastal Labrador near Rigolet, has been divided into two groups: the Shabogamo Gabbro that is exposed west of Churchill Falls and the Michael Gabbro that outcrops east of Churchill Falls. Both suites of gabbro are within-plate tholeiites that range from sub-alkaline basalt to basaltic-andesite and are enriched in P2O5 and K2O. Collectively, the two suites are better interpreted as a single event, termed the Michael-Shabogamo Gabbro (MSG), with ages ranging from 1425-1450 Ma, based on new U-Pb ID-TIMS baddeleyite ages of 1449 ± 3 Ma, 1437 ± 4 Ma and 1435 ± 10 Ma, along with previously published ages.
Within the MSG, two geochemical groups are evident. Most notably, each is characterized by differing P2O5/Zr (P/Zr) ratios: 1) the high P/Zr group - ca. 1425-1437 Ma; and 2) the low P/Zr group – ca. 1435-1450 Ma. The positive slope of the P2O5 vs Zr data arrays for each group indicates that melting of apatite was pivotal in influencing the rare earth concentrations of the MSG magmas and their secular changes in geochemistry. K2O is also enriched in the MSG relative to enriched mid-ocean ridge basalts and is correlated with P2O5, Ba and Rb.
Multiple origins for the enrichment of P2O5 and K2O in the MSG are considered and the available evidence is most consistent with melting of a metasomatized subcontinental lithospheric mantle (M-SCLM) source interpreted to be derived from subduction of a phosphate-rich sediment. A slab window is proposed to have facilitated lithospheric melting at ca. 1425-1450 Ma.