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A microanalytical investigation of feldspars in the Skaergaard intrusion, East Greenland : ternary feldspar compositional relations and lead isotopic geochemistry Cho, June On Yee


The Eocene Skaergaard intrusion, part of the East Greenland flood basalt province, is the type example of closed-system differentiation of basaltic magma. Studies of this remarkable layered intrusion have profoundly influenced our understanding of fundamental petrologic concepts. In this study, two complementary microanalytical applications of feldspar geochemistry are used to compare and evaluate (1) differentiation processes preserved in primocryst and interstitial feldspar, (2) the validity of the closed-system model for crystallization of the Skaergaard magma, and (3) the genetic link between the Skaergaard intrusion and the age-specific interval of the contemporaneous flood basalts. The spatially-controlled analyses include measurements of major and minor element oxide contents by electron microprobe and of lead (Pb) isotope ratios by laser ablation-inductively coupled plasma-mass spectrometry (LA-ICP-MS). The major element results (>2600 analyses) span nearly the entire range of permissible ternary feldspar compositions, revealing the diverse range of processes that operated in the crystal mush. These compositions include calcic plagioclase in reactive symplectites, intermediate-composition plagioclase primocrysts with different zoning patterns that track fractionation of the resident magma, exsolved alkali feldpsars in interstitial granophyres produced during silicate liquid immiscibility, and replacement albite formed as a result of meteoric fluid circulation and subsequent alteration. The Pb isotopic results (>750 analyses) demonstrate that most of the Skaergaard intrusion has the same Pb isotope composition within analytical uncertainty and confirm that crystallization occurred under essentially closed-system conditions. Only minor incorporation (<1%) of crustal materials is detectable in select hybrid rocks from the outermost margins. The Pb isotope signature of the Skaergaard parental magma, represented by the average Layered Series, is more radiogenic than most of the flood basalts. This indicates prior assimilation of granulitic gneisses at depth and links the Skaergaard intrusion to the eruption of the Milne Land Formation, the lowermost and earliest of the voluminous and widespread 56-54 Ma flood basalts. This integrated microanalytical study of feldspar from the Skaergaard intrusion provides new tools and avenues for petrologic research that can be applied to investigating the origin and evolution of mafic layered intrusions and other feldspar-bearing plutonic rocks in the Earth’s crust.

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