PETROLOGY AND GEOCHEMISTRY OF THE NEOPROTEROZOIC LAWSONITE-BEARING AND LAWSONITE-ABSENT BLUESCHISTS IN ANGLESEY (WALES, UK)

In the Neoproterozoic, the Earth experienced significant changes in its thermal regime, tectonic styles, and geodynamics that are preserved in the metamorphic rock record. Prior to the Neoproterozoic, the metamorphic rock record is dominated by granulites and amphibolites, which record high-temperature/low-pressure thermal gradients. However, in the Neoproterozoic, high-pressure/low-temperature metamorphic rocks such as blueschists appear, signaling a transition from a warmer Earth to a cooler Earth with modern-style plate tectonics and subduction. Blueschists from the Neoproterozoic are thus unique archives of the conditions and processes that occurred during this important transition in Earth’s history. Of the known Neoproterozoic blueschist localities, the Anglesey (Wales, UK) blueschists are of particular importance because they contain the mineral lawsonite and are thought to be the oldest known lawsonite-bearing rocks in the geologic record.

To evaluate the conditions that led to the formation and preservation of the oldest known lawsonite in the geologic record, we are conducting a comparative petrologic and geochemical study of the lawsonite-bearing and lawsonite-absent blueschists in Anglesey, Wales. Preliminary petrologic studies of an epidote-bearing blueschist from Anglesey shows that it consists of glaucophane, epidote, quartz, and chlorite, with accessory titanite and oxides. The main trace element hosts are titanite and epidote. Titanite is enriched in the heavy rare earth elements (HREEs) relative to the light and middle rare earth elements (LREEs and MREEs), and limited Zr-in-sphene geothermometry results suggest metamorphic temperatures greater than 500 degrees Celsius. Epidote generally yields flat chondrite-normalized REE patterns, however, some epidote grains show trace element zoning with core-to-rim variations in the sign of the Eu anomaly and the concentrations of REEs and Fe. Bulk-rock and mineral major and trace element compositional analyses, as well as Zr-in-sphene geothermometry, will be used to understand the formation of lawsonite-bearing versus lawsonite-absent blueschists in Anglesey, which may help explain the present-day linear distribution of lawsonite-bearing rocks in the blueschist belt.