Geochemistry of cumulates from the Bjerkreim-Sokndal layered intrusion (S. Norway). Part I. Constraints from major elements on the mechanism of cumulate formation and on the jotunite liquid line of descent

摘要

Whole-rock major element compositions are investigated in 99 cumulates from the Proterozoic Bjerkreim-Sokndal layered intrusion (Rogaland Anorthosite Province, SW Norway), which results from the crystallization of a jotunite (Fe-Ti-P-rich hypersthene monzodiorite) parental magma. The scattering of cumulate compositions covers three types of cumulates: (1) ilmenite-leuconorite with plagioclase, ilmenite and Ca-poor pyroxene as cumulus minerals, (2) magnetite-leuconorite with the same minerals plus magnetite, and (3) gabbronorite made up of plagioclase, Ca-poor and Ca-rich pyroxenes, ilmenite, Ti-magnetite and apatite. Each type of cumulate displays a linear trend in variation diagrams. One pole of the linear trends is represented by plagioclase, and the other by a mixture of the mafic minerals in constant proportion. The mafic minerals were not sorted during cumulate formation though they display large density differences. This suggests that crystal settling did not operate during cumulate formation, and that in situ crystallization with variable nucleation rate for plagioclase was the dominant formation mechanism. The trapped liquid fraction of the cumulate plays a negligible role for the cumulate major element composition. Each linear trend is a locus for the cotectic composition of the cumulates. This property permits reconstruction by graphical mass balance calculation of the first two stages of the liquid line of descent, starting from a primitive jotunite, the Tjorn, parental magma. Another type of cumulate, called jotunite cumulate and defined by the mineral association from the Transition Zone of the intrusion, has to be subtracted to simulate the most evolved part of the liquid line of descent. The proposed model demonstrates that average cumulate compositions represent cotectic compositions when the number of samples is large (> 40). The model, however, does not account for the K2O evolution, suggesting that the system was open to contamination by roof melts. The liquid line of descent corresponding to the Bjerkreim-Sokndal cumulates differs slightly from that obtained for jotunitic dykes in that the most Ti-, P- and Fe-rich melts (evolved jotunite) are lacking. The constant composition of the mafic poles during intervals where cryptic layering is conspicuous is explained by a compositional balance between the Fe-Ti oxide minerals. which decrease in Fe content in favour of Ti. and the pyroxenes which increase in Fe. (c) 2005 Elsevier B.V. All rights reserved.