Geochemistry of the nain massif anorthosite, labrador: Magma diversity in five intrusions

Abstract

The petrography and major- and trace-element concentrations of the anorthositic Bird Lake Massif (BLM), Susie Brook Slab (SBS), Lister Massif (LM), Port Man vers Run (PMR), and Kikkertavak area (KIK) in the Nain Plutonic Suite (NPS), Labrador, delineate the mineralogical and composition ranges of these large intrusions. From these, a variety of magmas and staging histories can be inferred. The BLM and SBS are mainly noritic anorthosites of the western region exposed along Tikkoatokhakh Bay (TIK). Tilting and stretching deformation distinguish SBS from BLM, but the two units show no mineralogical or chemical differences. The KIK and PMR intrusions are mainly troctolitic to noritic anorthosites of the eastern region, and they contain more mafic components than the TIK bodies. Chemically, all the anorthositic rocks are high in Ba, Sr, and the ratio K Rb , and low in Zr, Rb, and Rb Sr . The three plutonic bodies, BLM, SBS, and LM, along TIK are similar and can be clearly distinguished from KIK and PMR intrusions by element concentrations and ratios. Diagrams of element pairs show that each intrusion or group has its own evolutionary trend and variable style, indicating that they represent independent magma batches rather than fractionally related volumes of the same magma. REE patterns show light REE enrichment and heavy REE depletion, and strong positive Eu anomalies, correlated with cumulus plagioclase. In chemical comparison with adjacent plutons, one PMR-chilled leucotroctolite, after subtraction of 18% cumulus plagioclase, is similar to the Kiglapait magma (KI); one melatroctolite in KIK is close to the comb-layered melatroctolite of the Snowflake zone in the Hettasch Intrusion (HI); and the average leucotroctolite and leuconorite in KIK are similar to the leucotroctolite unit and the upper leuconorite, respectively, at Paul Island (PI). Therefore, the magmas from PMR and KIK are similar to those of wellknown troctolitic bodies and their feldspar-rich cumulates. KIK differs from PMR in having somewhat higher An and Xmg values. All these lines of evidence show that at least three types of magma existed in the study area: sodic noritic magma accumulating plagioclase in TIK, troctolitic magma in KIK, and a magma similar to but more evolved than KIK in PMR. The methods used also illustrate the feasibility of characterizing magma compositions from diverse types of cumulates. Regional differences in modal and chemical composition occur in the NPS. Noritic compositions with pale plagioclase are located in the western and southern regions, and troctolitic compositions with darker plagioclase are located in the eastern region. A reasonably accurate boundary can now be drawn between the two regions, which differ in the nature of their transport from the mantle. In comparison with a primitive mantle composition, high Ba, Pb, K, Sr, Ti, and low Rb, Nb, Zr in the Nain anorthositic rocks are distinctive. These anomalies and the isotopic results of other investigators are consistent with mantle source enrichment followed by variable contamination during magma ponding at the base of the depleted crust.