Fractionation and Liquid Immiscibility in an Anorthositic Pluton of the Nain Complex, Labrador |
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| Authors: | WIEBE R A |
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| Institution: | Department of Geology, Franklin and Marshall College Lancaster, Pennsylvania 17604 |
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| Abstract: | Fine-grained anorthositic dikes are associated with a massiveleuconorite pluton (Cl = 15) which is exposed over an area ofabout 200 km2. Internally, the pluton shows little compositionalvariation; average plagioclase composition ranges from An52to An48. The dikes are nearly uniform in composition and similarto the estimated bulk composition of the pluton (55 per centSiO2). They therefore appear to represent the parental magmaof the leuconorite pluton. A small body of granite (10 km2) was emplaced within and priorto the complete solidification of the leuconorite. The graniticintrusion caused local deformation of the leuconorite and filter-pressingof its late stage interstitial liquids. These liquids occurin the younger hydrous granite as very finegrained, chilledpillows of nearly anhydrous Fe-rich diorite and granite. Mostof the pillows are diorites with approximately 55 per cent SiO2.On oxide plots these lie approximately on a plagioclase controlline passing through the composition of the leuconorite dikes.The entire group of chilled pillows ranges in composition from45 to 71 per cent SiO2 with a gap between 57 and 63 per centSiO2. On oxide plots they produce a smooth trend which is obliqueto and truncates the plagioclase control line. Variation inthe pillows can best be explained by late-stage liquid immiscibility. Fractionation in the interstitial magma was controlled earlyby crystallization of plagioclase and later by plagioclase pluspyroxene. Very late stage differentiation was controlled mainlyby liquid immiscibility and produced FeO- and SiO2-rich liquids. |
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