SYN-TECTONIC, METEORIC WATER-DERIVED CARBONATION OF THE NEW CALEDONIA PERIDOTITE NAPPE

Ultramafic rocks, originated in the mantle, are sometimes carried upon continents through the process of obduction. The weathering of such rocks under warm and wet climates results in the formation of laterites associated with nickel ore deposits. A typical example is provided by the peridotite nappe of New Caledonia, SW Pacific. Carbonation of the ultramafic rocks is also indicated by abundant veins of magnesite (MgCO₃) found along the serpentine sole of the nappe. In this study (Quesnel et al., 2013), exceptional outcrops recently exposed in the Koniambo Massif allow to document (1) the meteoric origin of the fluids from which the veins have been formed, by means of stable isotopic analyses (C, O), (2) the syn-kinematic character of many magnesite veins with respect to pervasive shear deformation of the sole. This indicates that carbonation and laterization represent complementary records of meteoric water infiltration and provides the first known example of syn-tectonic carbonation of an ultramafic nappe.Taken together, these results suggest a scenario where syn-laterization tectonic activity has enhanced water infiltration, favoring the exportation of leached elements like Mg, which led to widespread carbonation of the serpentine sole. This calls for renewed examination of other magnesite-bearing ultramafic nappes worldwide in order to establish wether active tectonics is commonly a major agent for carbonation.

Reference: Quesnel et al. (2013), Syn-tectonic, meteoric water-derived carbonation of the New Caledonia Peridotite Nappe. Geology, October issue, in press.