Paper No. 4
Presentation Time: 8:00 AM-12:00 PM
ORIGIN, EMPLACEMENT AND FRACTIONATION OF LATE-STAGE DIKES ASSOCIATED WITH THE NAIN ANORTHOSITES, LABRADOR, CANADA
A suite of 1.28 Ga alkaline basaltic dikes crosscut the 1.36-1.295 Ga anorthosites of the Nain Plutonic Suite. These dikes, previously named the LP dikes, have trends that range from NE-SW to E-W and represent the final stages of magmatism associated with the NPS. Many of the dikes retain igneous textures and mineralogies whereas others show minor to moderate alteration and a few are completely metamorphosed to amphibolite facies. Many of the dikes contain amygdules of calcite and analcime that indicate a very shallow level of emplacement. Previous geobarometry gave an emplacement depth of ~17 km for the eastern portion of the Nain Plutonic Suite. These observations and geobarometry data indicate that significant uplift occurred between the emplacement of the anorthosites and the emplacement of the dikes. Major and trace element geochemisty of the least altered dike samples show that they have evolved compositions with MgO contents that range from ~8 to 4 wt%. Major and trace element covariation diagrams show that SiO2, Al2O3, CaO, scandium and vanadium have decreasing trends with decreasing MgO, whereas TiO2, FeO, Na2O and K2O show increasing trends with decreasing MgO. Thermodynamic and empirical modeling of the whole rock data indicate that these trends are best explained by the fractionation of a cotectic assemblage of olivine + clinopyroxene + plagioclase + spinel. Chondrite normalized La/Sm ratios for the more primitive dikes (MgO> 5 wt%) range from 1.0 to 2.4 and the chondrite normalized Dy/Yb ratios range from 1.18 to 1.43. Modeling of rare earth element systematics during adiabatic decompression melting using incremental non-modal batch melting indicates that the dikes formed by low degrees of melting that initiated within the garnet lherzolite stability field within the upper mantle.