Geochemical and Nd-Hf isotopic constraints on the origin of the ~174-Ga Damiao anorthosite complex, North China Craton

摘要

The ~1.74-Ga Damiao complex in the North China Craton consists of anorthosite (85%), norite (10%), mangerite (4%) and minor troctolite (<1%), all of which are cut by gabbroic and ferrodioritic dikes. The complex hosts abundant Fe-Ti-P oxide ores and has an emplacement sequence from anorthosite and norite to Fe-Ti-P oxide ores to mangerite. All of the different lithologies in the complex have similar light rare earth element (LREE)-enriched patterns and Nd isotopic compositions with ε_(Nd)(t) values mostly within the range of- 5.0 to -4.0. Average ε_(Hf){t) values of zircon are -4.7 for mangerite and - 5.9 for norite. Plagiodase and clinopyroxene show a continuous range of composition from anorthosite and norite to Fe-Ti-P oxide ores to mangerite, suggesting that these lithologies formed by differentiation from a common parental magma. Gabbroic dikes in the Damiao complex are characterized by high Al2O3 (14.5-17.1 wt%), Sr (～1000 ppm) and Mg#s [100 Mg/(Mg + Fe~(2+))] = 56-73), similar to the high-Al gabbros parental to the Harp Lake and Laramie anorthosite complexes in North America. The high-Al gabbroic dikes of the Damiao complex may reflect the parental melt from which the complex formed, whereas the ferrodioritic dikes may represent residual melts after anorthosite crystallization from the high-Al gabbroic magma. The high-Al gabbros have incompatible element ratios (Zr/Nb = 14-24; La/Nb = 3.6-4.8), which differ significantly from those of nearby coeval, mantle-derived, mafic dike swarms and volcanic rocks but are similar to those of the lower crust Their zircon ε_(Hf)(f) and whole-rock ε_(Nd)(t) compositions plot along the evolution lines of the ~2.5 Ga Archean rocks in the North China Craton, suggesting that the Damiao complex was derived from an ancient lower crust Exceptionally high temperatures and high-degrees of melting (>75%) would have been required to form the parental high-Al gabbroic magma from the lower crust. We propose that the crustal rocks were dragged into the upper mantle during amalgamation of the North China Craton, where the ambient temperature was high enough (>1271 °C at 12 kbar) to cause extensive melting, producing a deep-seated magma chamber. The melt then evolved through polybaric crystallization, producing anorthositic magmas with abundant suspended plagiodase that were finally injected at mid-crustal depth to form the Damiao complex.