Fluid driven processes in the crust – the formation of anorthositic dykes in the Troodos ophiolite (Cyprus)

Abstract

Anorthositic dykes related with the Upper Cretaceous Troodos ophiolite are described for the first time. An entire anorthosite dyke swarm appears within the Limassol Forest Complex, crosscutting the supply channels of late Troodos extrusives. As inferred from crosscutting relations, the intrusion of dykes must be related to a very late magmatic stage of oceanic crust formation.<br /> The appearance of anorthosites as dykes, imply that their composition had to be liquid at the time of intrusion. Melting experiments of such compositions show that even under H₂O saturated conditions, temperatures required for whole-sale melting are well above 1250 °C at 500 MPa, which is unrealistic for a derivative melt.<br /> Pivotal aspect of the present work is the development of a genetic model, based on petrological, geochemical and experimental observations. Complex, multi-stage, open system phenomena are unrevealed, involving differentiation and unmixing of magmatic fluids and melts such as the enrolment of hydrothermal fluids.<br /> Anorthositic dykes are interpreted to represent precipitates of a magmatic liquid that intruded the upper crust as a crystal mush. The anorthosite liquid is derived from unmixing of a highly evolved melt at depth. The fluid thereby is considered as a viscous gel, highly enriched in silicate solute, high field strength elements and rare earth elements. The solubility of the different species is granted by the saline, probably chloride rich composition of the liquid.<br /> Normative bulk compositions of dykes range 80 - 100 wt.% plagioclase such that they are classified as anorthosites. Primary magmatic plagioclase approaches pure end member anorthite composition (X_An = 0.76 - 1.0). Anorthite, calcic pyroxene and accessory phases (titanite, rutile and zircon) are inferred as liquidus phases. The observed mineral textures such as the trace element- and isotope composition confirms the intrusion of anorthosites to be related with a highly viscous magmatic fluid having a mutual source in boninites. A perfect correlation between lutetium content and the ¹⁷⁶Lu/¹⁷⁷Hf isotope ratio is observed in anorthosite rocks but does not appear in other Troodos extrusives emphasizing its unique generation history. Liquidus experiments in the hydrous anorthosite system reveal melting temperatures of the bulk assembly exceeding 1150 °C; and anorthite (X_An = 1) as liquidus phase. Phase relation experiments in the H₂O saturated anorthite - diopside - forsterite system aim to duplicate and quantify the process responsible for the enrichment of the plagioclase component in the melt. It is demonstrated that the presence of a H₂O saturated melt coexisting with a H₂O-dominated fluid phase largely suppresses the crystallisation of anorthite from the fluid saturated melt. Simple system experiments further reveal the separation of a melt saturated fluid from the fluid saturated melt at conditions below the second critical point of water.<br /> Decompression recrystallisation experiments with hydrous tholeiite compositions reconcile that fractionation of a mafic melt prior to unmixing of a fluid phase results in accumulation of the anorthite component and hence could represent the parental magma composition.