Serpentinites of different tectonic origin captured in an HP/LT terrane:
the case for New Caledonia (SW Pacific)
Abstract
Serpentinites play a vital role in subduction zone processes: as one of
the dominant carriers of water in downgoing plates, they are critical
constituents in the global volatile cycle and add complexity to the bulk
rheology and density of downgoing and overlying plates, with strong
implications for seismicity and mechanical mixing. These rocks can form
in a number of tectonic settings where water reacts with peridotite
under certain conditions, including at or near mid-ocean ridges and in
mantle overlying dehydrating subducted plates. In the HP/LT terrane
preserved on the island of New Caledonia, serpentinites outcrop as
meter-scale resistant blocks and as highly deformed “matrix” that
hosts a range of metasedimentary, metamafic, and meta-ultramafic lenses.
The origin of these HP serpentinites has been debated, with competing
hypotheses linking protolith to nearby obducted ophiolite or to the
subducted oceanic plate. We analyzed 30 serpentinites and associated
hybrid rocks from across the HP terrane to discern the tectonic origin
of their protoliths and to better understand their reaction history
through subduction and exhumation. Whole rock major and trace element
and stable isotope geochemistry reveal the existence of at least two
distinct types of serpentinites in the HP terrane. Serpentinites in the
far NE exhibit elevated HREE’s, δ18O values of 6-10‰, and scatter in
major element concentrations. In the SE, a ~1 km2
ultramafic massif contains serpentinites with relatively more depleted
REE, δ18O values of 5-7‰, and a more restricted range of major element
concentrations, with distinctly higher MgO and lower Al2O3 than samples
from the NE. We compare these data to a global geochemical compilation
of serpentinites from various tectonic settings. Ongoing Raman
spectroscopy work will determine serpentine polymorph(s) and electron
probe microanalysis will target isolated relict pyroxene grains and
oxide minerals to retrieve additional protolith information. The
recognition of multiple types of serpentinites in this HP/LT terrane
adds important information to debate about the origin of the ultramafic
material, and may speak to complex interactions between the downgoing
plate and overlying mantle or to systematic spatial differences in
protolith composition or degree of metamorphism or deformation.