Identification And Interpretation Of Eclogite Protoliths Using Immobile Element Geochemistry: Some New Methodologies

Methodologies for fingerprinting metabasalts have been applied to eclogites with mixed success. Some, including Alpine examples famously studied by Gary Ernst >30 years ago, have been successfully assigned to tectonic settings and the results used to understand the now-disappeared ocean and its margins. Others, however, present two particular, well-documented problems: 1) many are cumulates rather than lavas and so have very low abundances of some elements as well as non-liquid compositions; 2) the subduction and exhumation processes can lead to infiltration of the protolith by subduction- and crustally-derived fluids/melts before and after eclogite-facies metamorphism and so impart apparent subduction or continental character even when none existed. Here we demonstrate new methodologies for dealing with these issues, taking as an example the eclogites from the Chinese Continental Scientific Drilling (CCSD) Deep Borehole. We adopt a set of immobile element proxies that highlight the presence and behaviour of particular cumulate phases (e.g. Cr for chromite, Ni for olivine, Sc,V for clinopyroxene, Ti,V for oxide, Ga for plagioclase, P and Zr for apatite and zircon, Nb for interstitial melt). Using a training data set from well-studied cumulate sequences such as Bushveld and Skaergaard, we can assign protolith rock names on the basis of these proxy elements. Variation diagrams enable us identify the crystallization sequence of the plutonic protolith, itself a function of the original tectonic setting. For example, the Borehole contains a thick, eclogite-facies cumulate sequence which we can reconstruct in detail as a layered complex containing cumulate dunite and peridotite, mela-troctolite, troctolite, gabbro, ferrogabbro, ferrodiorite, quartz-diorite and tonalite. The iron enrichment and inferred saturation sequence of chr+ol-plag-cpx-mt-ap-zr are characteristic of low-oxygen fugacity, tholeiitic MORB and continental margin intrusions. In this, and more easily in other, smaller sill- and dyke-like bodies, we can use these proxy diagrams to identify rocks representative of liquid (non-cumulate) compositions, equivalent to the chilled facies of classic intrusions. Although these also follow fluid- and melt-infiltration vectors, especially around silica-rich rocks which reached melting temperatures, a small number of unmodified liquid compositions can be identified. These allow the setting to be determined using Th-Nb-Ti-Yb systematics. In this case, the data mostly plot in the centre of the MORB-OIB array on a Th/Yb-Nb-Yb plot indicating undepleted mantle. Some samples plot to higher Th/Yb indicating limited crustal assimilation. The samples plot in the MORB, rather than OIB, field on a Ti/Yb-Nb/Yb plot indicating shallow-melting indicative of thin lithosphere. Thus, an origin at a volcanic-rifted continental margin may be inferred. This procedure can also be applied to the debate over the origin and settings of eclogite inclusions in kimberlites.