Anorthosites produced by water-fluxed anatexis of deep arc gabbros, Gangdese batholith, Tibet

Abstract

During arc growth, the remelting and cannibalization of older arc rocks may add to the complexity of the arc system, as shown by compositional variations that deviate from the normal compositional trends of arc magmas. While a range of arc rocks is ultimately produced, anorthosites are seldom described in arcs. Based on the study of gabbro migmatites and anorthosites from the eastern Gangdese arc in Tibet, supported by thermodynamic modelling, we demonstrate that anorthosites can form by anatexis of arc gabbros in deep crustal sections fluxed by aqueous fluids enriched in Na and Sr. Anorthosites in the eastern Gangdese arc are associated with the Lilong pluton, which comprises rocks ranging from ultramafic cumulates to gabbros and quartz diorites. The anorthosites appear as leucosomes in gabbro migmatites associated with residual melanosomes, dominated by amphibole, garnet and epidote. They appear also as dykes with variable contents of euhedral poikilitic amphibole, with or without garnet. These anorthosites exhibit distinctive compositions, with elevated Na₂O (> 5 wt.%), Al₂O₃ (> 20 wt.%) and Sr (> 1000 ppm), as well as reduced rare earth element and high field strength element contents. These features set them apart from typical mantle-derived anorthosites, such as those in layered-intrusions. The anatectic rocks (magmatic and residual rocks) yield ages from 86 Ma to 75 Ma, recording >10 Myr of anatexis. The start of anatexis overlaps the youngest ages in the protolith gabbros (97 Ma to 84 Ma) suggesting the possibility of anatexis starting before the end of complete protolith crystallization. Phase equilibrium calculations demonstrate that anorthositic melts can be produced by the melting of gabbroic rocks under granulite-facies at ∼ 1.2 GPa and ∼ 950-830 °C in the presence of significant H₂O influx with added Na. Melt generation produces a residue enriched in garnet, amphibole and epidote, consistent with field observations and previous experimental studies. Composition of zircons in the residue and in the anatectic anorthosites tell complementary stories about the melting process. Significantly, anorthosite zircons are characterized by high Eu/Eu* and low Th concentrations, indicative of plagioclase breakdown, and apatite and epidote remaining in the source. This signature of arc anorthosite zircons can be used to search for anorthosite contribution elsewhere. The results highlight the role of Na-Sr aqueous fluids in the arc root in generating unusual magmas that result in compositional diversity of arc magmas.