Trace element partitioning between plagioclase feldspar and melt: A study of experiments and applications to magmatic evolution.

摘要

Among common igneous minerals, plagioclase is noted for its sluggish diffusion and high closure temperatures. Consequently, it has a great potential to reveal the composition of the parental primary and evolving magma. The dissertation: (1) derives plagioclase/melt trace element partition coefficients (D{dollar}sb{lcub}iota{rcub}){dollar} for plagioclases of different anorthite content (X{dollar}sb{lcub}rm An{rcub}){dollar} at natural concentration levels in experimental charges; 2) shows that calcic plagioclase phenocryst cores are capable of surviving plutonic cooling and mixing, and retain information about preexisting magma composition; and (3) uses these homogeneous calcic cores in the Transbaikalian anorogenic province in order to reconstruct parental basic magma trace element evolution during pluton-scale and a province-scale magmatic evolution accompanying cratonization.; The partition coefficients are based on ion microprobe analyses of plagioclase-glass pairs in 17 experimental products of Drake and Weill (1975). Analyses are tabulated for the following undoped trace elements: Li, Be, B, F, Mg, P, Cl, K, Ti, V, Cr, Fe, Co, Rb, Sr, Y, Zr, Nb, Ba, La, Ce, Pr, Nd, Sm, Eu, and Pb, as well as for doped Sr, Ba, Y and REE. The main result is a linear relationship of the form RT ln(D{dollar}sb{lcub}iota{rcub}{dollar}) = a X{dollar}sb{lcub}rm An{rcub}{dollar} + b, which is useful in many petrological and geochemical applications involving plagioclases of varying composition and crystallizing at different temperatures.; Ion microprobe analyses of the trace elemental compositions of near-liquidus plagioclase, together with established D{dollar}sb{lcub}iota{rcub}{dollar}'s, reveal the trace elemental composition of the parental magma as well as its fractionation and mixing history during the evolution of the Devonian Shaluta plutonic complex in Transbaikalia.; Trace element concentrations of selected relict calcic homogeneous cores of plagioclase megacrysts in basic rocks belonging to a single cratonic lithospheric block in Transbaikalia and arranged in a 400 Ma time sequence were used to track down chemical trends reflecting that of their near-liquidus basic magmas. The established trends include an increase in high field strength elements, K{dollar}sb2{dollar}O and decrease in lithophile, hydrophile elements and {dollar}deltasp{lcub}18{rcub}{dollar}O through time and change of magmatic affinity from calc-alkaline to within plate-type. These trends reflect similar and fundamental evolution in this and other anorogenic provinces. This study contributes to the understanding of the cratonization process and provides a near-liquidus plagioclase megacryst tool to unravel pristine chemical features of parental basic magmas.