科研之友
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摘要
It is well established that A-type granites enriched in high field strength elements, such as Zr, Nb and the REE, form in anorogenic tectonic settings. The sources of these elements and the processes controlling their unusual enrichment, however, are still debated. They are addressed here using neodymium and oxygen isotope analyses of samples from the 1.24 Ga Strange Lake pluton in the Paleoproterozoic Core Zone of Quebec-Labrador, an A-type granitic body characterized by hyper-enrichment in the REE, Zr, and Nb. Age-corrected epsilon Nd values for bulk rock samples and sodic amphiboles (mainly arfvedsonite) from the pluton range from -0.6 to -5.7, and -0.3 to -5.3, respectively. The epsilon Nd values for the Napeu Kainiut quartz monzonite, which hosts the pluton, range from -4.8 to -8.1. The Sm-147/Nd-144 ratios of the suite and the host quartz monzonite range from 0.0967 to 0.1659, large variations that can be explained by in situ fractionation of early LREE-minerals (Strange Lake), and late hydrothermal HREE remobilization. Oxygen isotope analyses of quartz of both Strange Lake and the host yielded delta O-18 values between +8.2 and +9.1, which are considerably higher than the mantle value of 5.7 +/- 0.2 parts per thousand. Bulk rock oxygen isotope analyses of biotite-gneisses in the vicinity of the Strange Lake pluton yielded delta O-18 values of 6.3, 8.6 and 9.6 parts per thousand. The negative epsilon Nd values and positive delta O-18 values of the Strange Lake and Napeu Kainiut samples indicate that both magmas experienced considerable crustal contamination. The extent of this contamination was estimated, assuming that the contaminants were sedimentary-derived rocks from the underlying Archean Mistinibi (para-) gneiss complex, which is characterized by low eNd and high delta O-18 values. Mixing of 5-15% of a gneiss, having an eNd value of -15 and a delta O-18 value of +11, with a moderately enriched mantle source (epsilon Nd = +0.9, delta O-18 = +6.3) would produce values similar to those obtained for the Strange Lake granites. Based on analogies between the Nain Plutonic Suite and the Gardar alkaline igneous province (SW-Greenland), we conclude that the Strange Lake pluton and associated REE-mineralized anorogenic bodies formed from a combination of subduction-induced fertilization of the sublithospheric mantle, crustal extension and in situ magma evolution.
- 出版日期2017-7
