Role of titanite in the redistribution of key trace elements during partial melting of meta-mafic rocks: An example from Namche Barwa migmatite

Abstract

南迦巴瓦地区广泛出露的中下地壳变基性岩部分熔融形成的层状混合岩和淡色花岗岩，为研究部分熔融过程中榍石的地球化学行为对熔体的微量元素组成的影响提供了良好的机会。相对于源岩或熔融残留体，淡色体亏损Ti、V、REE、Y、Nb、Ta、U等元素，与混合岩中榍石的微量元素特征互补。混合岩、淡色体和榍石微量元素特征表明南迦巴瓦角闪岩部分熔融形成的淡色体的微量元素特征主要受控于榍石的地球化学行为。角闪岩脱水部分熔融过程中，由于长英质熔体的低Ti溶解度，榍石以未熔残留体形式存在于暗色体中，导致熔体亏损Ti、REE、Nb、Ta、V、U等元素和Sr/Y比值相对升高。关键元素在榍石和熔体之间的配分系数受熔体成分影响明显。角闪岩中变质榍石D_Nb/Ta<1，因此变质榍石残留导致熔体Nb/Ta相对于源岩升高；而高Si-Al花岗质熔体中榍石D_Nb/Ta>1，因此与高Si-Al熔体平衡的榍石的分离（转熔或结晶分异）将导致熔体Nb/Ta比值相对源岩降低。榍石在部分熔融过程中的微量元素效应为理解变基性岩部分熔融产生熔体的地球化学特征提供新的认识。;Layered migmatite formed by partial melting of amphibolites from mid to lower crust levels is widespread in the Namche Barwa. Titanite, as a key metamorphic residual and magmatic phase, occurs in the migmatite as well as in the leucosome, which provides a good opportunity to investigate the role of titanite in regulating the trace element compositions in the amphibolite-derived melts. Compared with migmatites, the leucosomes are depleted in elements such as Ti, V, REE, Y, Nb, Ta and U, which are enriched in titanite. Such characteristics indicate that the trace element compositions in the anatectic melts are mainly controlled by the geochemical behavior of titanite. During partial melting of amphibolite, due to low solubility of TiO₂ in felsic melts, titanite grains commonly retain in the residue and in turn result in the depletion of those key elements compatible in titanite structure. The trace element partition coefficients between titanite and silicic melts are significantly affected by the melt's compositions. Residual metamorphic titanite with D_Nb/Ta<1 will result in higher Nb/Ta in melt than that in the protolith. However, when equilibrating with high Si-Al melts, magmatic titanite preferred Nb over Ta with D_Nb/Ta>1. Separation of such titanite grains will decrease the Nb/Ta ratio in the residual melts. Data and results presented in this study demonstrate that titanite, similar to other accessory phases, could strongly affect the geochemical characteristics of melts produced by melting mafic rocks.