Evidence from mantle xenoliths for relatively thin (< 100 km) continental lithosphere below the Phanerozoic crust of southernmost South America

摘要

Garnet peridotite xenoliths in the Quaternary Pali-Aike alkali olivine basalts of southernmost South America are samples of the deeper portion of continental lithosphere formed by accretion along the western margin of Gondwanaland during the Phanerozoic. Core compositions of minerals in garnet peridotites indicate temperatures of 970 to 1160 deg C between 1.9 and 2.4 GPa, constraining a geothermal gradient which suggests a lithospheric thickness of approximately 100 km below this region. Previously, this lithosphere may have been heated and thinned to <= 80 km during the Jurassic break-up of Gondwanaland, when widespread mafic and silicic volcanism occurred in association with extension in southern South America. Subsequent cooling, by up to > 175 deg C, and thickening, by about 20 km, of the lithosphere is reflected in low-temperature ( <970 deg C) spinel peridotites by chemical zonation of pyroxenes involving a rimward decrease in Ca, and in moderate- and high-temperature (>970 deg C) peridotites by textural exidence for the transformation of spinel to garnet. A recent heating event, which probably occurred in conjunction with modal metasomatism related to the genesis of the Pali-Aike alkali olivine basalts, has again thinned the lithosphere to < 100 km. Evidence for this heating is preserved in moderate- and high-temperature (>970 deg C) peridotites as chemical zonation of pyroxenes involving a rimward increase in Ca, and by kelyphitic rims around garne.t The majority of moderate- and high-temperature (>970 deg C) xenoliths are petrochemically similar to the asthenospheric source of mid-oceanic ridge basalts: fertile (> 20percent modal clinopyroxene and garnet), Fe-rich garnet lherzolite with major element conmposition similar to estimates of primitive mantle, but large-ion-lith-ophile and light-ratre-earth element depletion relative to heavy-rare-earth elements, and with Sr, Nd, Pb, Os, and O isotopic compositions similar to MORB. In contrast, infertile, Mg-rich spinel harzburgite is predominant among low-temperature (<970 deg C) xenoliths. This implies a significant chemical gradient and increasing density with depth in the mantle section represented by the xenoliths, and the absence of a deep, low density, olivine-rich root below the southernmost South American crust such as has been inferred below Archean cratons. With respect to both temperature/rheology and chemistry/density, the subcontinental mantle lithsophere below southernmost South America is similar to that below oceanic crust. It is interpreted to have formed by tectonic capture, during the Paleozoic, of a segment of what had previously been oceanic lithosphere generated at a late Proterozoic mid-oceanic spreading ridge.