A 1:1,000,000-scale lithostratigraphic assemblage map of the Lau Basin (southwestern Pacific Ocean) has been created using remote predictive mapping (RPM) techniques developed by geological surveys on land. Formation-level geological units were identified in training sets at scales of 1:100,000–1:200,000 in different parts of the basin and then extrapolated to the areas where geological data are sparse. The final compilation is presented together with a quantitative analysis of assemblage-level crustal growth based on area-age relationships of the assigned units. The data sets used to develop mapping criteria and an internally consistent legend for the compilation included high-resolution ship-based multibeam, satellite- and ship-based gravity, magnetics, seafloor imaging, and sampling data. The correlation of units was informed by published geochronological information and kinematic models of basin opening. The map covers >1,000,000 km2 of the Lau-Tonga arc-backarc system, subdivided into nine assemblage types: forearc crust (9% by area), crust of the active volcanic arc (7%), backarc rifts and spreading centers (20%), transitional arc-backarc crust (13%), relict arc crust (38%), relict backarc crust (8%), and undivided arc-backarc assemblages (<5%), plus oceanic assemblages, intraplate volcanoes, and carbonate platforms. Major differences in the proportions of assemblage types compared to other intraoceanic subduction systems (e.g., Mariana backarc, North Fiji Basin) underscore the complex geological makeup of the Lau Basin. Backarc crust formed and is forming simultaneously at 12 different locations in the basin in response to widely distributed extension, and this is considered to be a dominant pattern of crustal accretion in large arc-backarc systems. Accelerated basin opening and a microplate breakout north of the Peggy Ridge has been accommodated by seven different spreading centers. The result is an intricate mosaic of small intact assemblages in the north of the basin, compared to fewer and larger assemblages in the south. Although the oldest rocks are Eocene (~40 m.y. old basement of the Lau and Tonga Ridges), half of the backarc crust in the map area formed within the last 3 m.y. and therefore represents some of the fastest growing crust on Earth, associated with prolific magmatic and hydro-thermal activity. These observations provide important clues to the geological evolution and makeup of ancient backarc basins and to processes of crustal growth that ultimately lead to the emergence of continents.

Olivine is one of the most important minerals used to reconstruct magmatic processes, yet the rare earth element (REE) systematics of Fe-rich olivine in igneous rocks and ore deposits is poorly understood. As detected by in situ laser ablation–inductively coupled plasma–mass spectrometry (LA-ICP-MS) analysis, cumulate fayalite (Fe2SiO4) in the Paleoproterozoic Vergenoeg F-Fe-REE deposit of the Bushveld large igneous province (LIP) in South Africa contains the highest heavy REE (HREE) contents ever recorded for olivine, with HREE enrichment of as much as 6000× chondritic values. Atom probe tomography maps confirm the incorporation of the HREEs into the fayalite crystal lattice, facilitated by lithium acting as a main charge balancer and by high REE contents in the highly fractionated felsic parental melt that is related to the Bushveld LIP. The high HREE concentrations of fayalite in concert with its high modal abundance (>95 vol%) indicate that the fayalite cumulates are the main host for the HREE mineralization of the Vergenoeg deposit. Fayalites of Vergenoeg demonstrate that Fe-rich olivine may fractionate large amounts of HREEs, and we propose fayalite cumulates as potential future targets for HREE exploration.

Abstract Situated in the centre of the Paleoproterozoic Bushveld Large Igneous Province (LIP) of South Africa the Vergenoeg F–Fe–REE deposit is one of the largest, but at the same time most unusual, fluorite deposits on Earth. In situ major and trace element analyses of fayalite, magnetite, ilmenite, fluorapatite, fluorite and allanite from fayalite-rich rocks are combined with oxygen isotope data for fayalite, magnetite and ilmenite to unravel the complex evolution of the deposit. Textural and compositional characterization of the fayalite-rich rocks supports a magmatic formation as cumulates and an intense late hydrothermal overprint. Fayalite accumulated together with minor Ti-rich magnetite, ilmenite, fluorapatite and allanite from a highly evolved, H2O-poor felsic melt at low oxygen fugacity. Chondrite-normalized rare earth element (REE) patterns of fayalite and the recalculated parental melts, using fayalite–rhyolite partition coefficients, exhibit positive trends with strong enrichment of the heavy REE (HREE) relative to the light REE (LREE). Apart from the LREE depletion the patterns are similar to those of highly fractionated high-silica REE rhyolites that often occur in siliceous LIPs. We attribute the LREE depletion to crystallization of accessory allanite, the main host of the LREE in the cumulates. Chondrite-normalized REE patterns of the parental melt prior to fayalite accumulation, recalculated using allanite–rhyolite partition coefficients, resemble the composition of the rhyolites of the Rooiberg Group and therefore document a petrogenetic link to the Bushveld LIP. High δ18O values of fayalite (up to ≈7·4 ‰) are consistent with its crystallization in a rhyolitic melt that has formed by extensive fractionation from basic melts of the Rustenburg Layer Suite, the mafic member of the Bushveld LIP. Primary fluorite crystallized together with rare quartz, and a second generation of fayalite, magnetite and ilmenite from rare intercumulus melt in interstices between cumulate fayalite. Textural and mineral compositional data, as well as the generally negative δ18O values of magnetite (–2·9 to 0 ‰), are in agreement with the main magnetite–fluorite ore formation in Vergenoeg being related to a hydrothermal overprint, which was responsible for further F and Fe enrichments of the rocks. Fluorine-rich fluids, released from the crystallizing granites of the felsic member of the Bushveld LIP (Lebowa Granite Suite), caused the extensive alteration of fayalite to bowlingite and its replacement by Ti-poor magnetite and quartz. The hydrothermal overprint was associated with the widespread formation of secondary fluorite and minor fluorapatite. Our new petrogenetic model for the Vergenoeg deposit, as constrained from the primary fayalite cumulates, implies that the formation of the Vergenoeg deposit was directly linked to the evolution of the Bushveld LIP.