Oscillatory Sr isotopic signature in plagioclase megacrysts from the Damiao anorthosite complex, North China: Implication for petrogenesis of massif-type anorthosite

Abstract

Formation of Proterozoic massif-type anorthosites is known to be related to polybaric process involving early high-pressure crystallization of plagioclase and high-Al orthopyroxene megacrysts at mantle–crust boundary, followed by emplacement of plagioclase-dominated mushes to shallow-level crust. Therefore, the plagioclase megacrysts record important information about the magmatic sources and crystallization process of anorthosites. The ~1.74-Ga Damiao complex, North China, comprises >90vol.% anorthosites and leuconorites containing abundant plagioclase megacrysts associated with minor high-Al orthopyroxene megacrysts (HAOMs). The plagioclase megacrysts are generally euhedral to subhedral (mostly 1 to 30cm in diameter), and some of them contain very fine lamellae of orthoclase and undefined Fe–Ti-rich minerals (1–5μm). The HAOMs occur as subhedral grains or as angular grains with an intercumulus relationship to the plagioclase megacrysts. They contain abundant thin, regular lamellae of exsolved plagioclase (15–20vol.%), indicative of originally high-Al features (originally 6.0–7.3wt.% Al2O3). Based on the Al-in-orthopyroxene geobarometry, the HAOMs and associated plagioclase megacrysts are constrained to be crystallized together at pressures of 9.4–11.2kbar (33–36km), indicative of their crystallization at lower crust depths or crust–mantle boundary. In contrast, orthopyroxene grains from late differentiation phases such as oxide-apatite gabbronorites do not contain plagioclase lamellae, and have much lower Al2O3 contents (1.5 to 1.7wt.%), indicating final crystallization at <5kbar (<15km). Four plagioclase megacrysts analyzed in this study have slightly different compositions, but collectively they all display comparable, oscillatory variations of An values (43–55), Sr (1100–1800ppm), Ba (800–1400ppm), La (2 to <7ppm) and 87Sr/86Sr ratios (0.70283–0.70466) from center to rim. Although Sr content is also likely related to pressures, our work suggests that the oscillatory chemical and Sr isotopic signature of the plagioclase megacrysts was mainly controlled by the compositions of the magmas, which temporally changed in the deep magma chamber. A mixing-assimilation modeling, based on Sr contents and initial 87Sr/86Sr ratios of synchronous mantle-derived mafic dykes and ancient lower crustal xenoliths in North China, suggests that the parental magma was initially a depleted mantle-derived basaltic magma that assimilated with ~30% lower crustal materials (Al2O3=15–24wt.%; Sr=800–2000ppm) or partial melts of the lower crust when ponding at the base of lower crust. We consider that an increasing of Al2O3 in the basaltic magma due to assimilation may have triggered saturation of plagioclase, because the oscillatory chemical and isotopic patterns are consistent from center to rim in all plagioclase megacrysts, indicative of initiation of the assimilation processes at the very beginning of plagioclase crystallization. Our study supports the model that the parental magmas were derived from partial melting of the depleted mantle combined with all-important crustal contamination in deep magma chambers or during rising of crystal mushes, and this may also account for variable isotopic signature for different phases in many anorthosite suites.