Very high-pressure orogenic garnet peridotites

Abstract

Mantle-derived garnet peridotites are a minor component in many very high-pressure metamorphic terranes that formed during continental subduction and collision. Some of these mantle rocks contain trace amounts of zircon and micrometer-sized inclusions. The constituent minerals exhibit pre-and postsubduction microstructures, including polymorphic transformation and mineral exsolution. Experimental, mineralogical, petrochemical, and geochronological characterizations using novel techniques with high spatial, temporal, and energy resolutions are resulting in unexpected discoveries of new phases, providing better constraints on deep mantle processes.D ata on the composition of the subcontinental lithospheric mantle are essential for erecting realistic large-scale models of the Earth's geochemical and tectonic evolution (1). Our knowledge of mantle composition and petrochemical processes has been derived mainly from studies of xenoliths and xenocrysts in kimberlites, mantle-derived volcanic rocks, and experimental very highpressure (VHP) phase equilibria, and from the interpretation of seismic tomographic images. Recent studies of orogenic peridotites provide additional insights regarding upper mantle processes at convergent lithospheric plate boundaries. It was found that many orogenic peridotites were derived from a depleted, metasomatized mantle or crustal cumulate, and later were subjected to subduction-zone VHP metamorphism (e.g., refs. 2-6). Some peridotites preserve a record of ultradeep origin revealed by mineral exsolution and the persistence of VHP polymorphs (6-14), and several peridotites contain dense hydrous magnesian silicates (DHMS) that are stable only at mantle depths (15,16). It was also found that some garnet peridotites, and their host continental crust, underwent coeval subduction-zone VHP metamorphism under pressure-temperature (P-T) conditions characterized by low thermal gradients (Յ5°C/km), based on sensitive high-resolution ion microprobe (SHRIMP) U-Pb ages of zircon separates from both rock types (e.g., refs. 17-20). Furthermore, VHP experiments have revealed that numerous hydrous phases and nominally anhydrous minerals containing substantial amounts of H 2 O are stable under such conditions. Therefore, cold subduction zones are the principal sites of H 2 O recycling back into the mantle (for reviews, see refs. 21 and 22). Such findings have advanced our knowledge of the thermal structure of subduction zones and of the recycling of volatiles into the mantle.These petrochemical findings lead to new challenges posed by critical tectonic questions: How were deep-seated (Ͼ200 km) mantle rocks transported to shallow depths? How were such peridotites incorporated into subduction-zone orogens? How can we distinguish the petrochemical/geochronological processes taking place in a mantle wedge setting from those affecting deeply subducted ultramafic rocks of the continental lithosphere?In the spirit of synergy of 21st century science and technology, this article presents an overview of VHP metamorp...

Yin and yang of continental crust creation and destruction by plate tectonic processes

“…UHP nappes may be exhumed because of regional mantle upwelling, subduction channel backflow, or underplating combined with extensional collapse . Exhumation of majoritic (garnet-pyroxene solid solution) peridotites indicates that material subducted to >200 km depth may be returned to the surface (Liou et al 2007;Scambelluri et al 2008). The return of deeply subducted material to the surface often does not occur, as shown by the fact that no UHP terranes have been identified in North America despite many collisions over geologic time and an abundance of metamorphic petrologists hungry to make the discovery.…”

Eight good reasons why the uppermost mantle could be magnetic

“…In continental areas, the lithospheric mantle (consisting of plagioclase-and spinel-peridotites) is below the Curie temperature (T c ) 1 and can contribute to magnetic anomalies, whereas garnet-lherzolites, present at greater depths, are too hot to carry a magnetic remanence. Mantle peridotites are found at the Earth's surface either as ophiolite (e.g., Nicolas, 1986), as Alpine-type peridotites (e.g., Liou et al, 2007), or as 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60 61 62 63 64 65 5/39 xenoliths (e.g., Nixon, 1987). Exposed mantle rocks alter through serpentinization, a process that forms abundant magnetite (e.g., Toft et al, 1990;Borradaile and Lagroix, 2001;Alt and Shanck, 2003;Frost et al, 2013).…”

Experimental constraints on formation of hematite in olivine at high pressures and temperatures

“…Our knowledge of mantle composition has been mainly derived from xenoliths and xenocrysts in kimberlite, mantle-derived volcanic rocks and laboratory experiments. In continental collision zones, garnet peridotite occurrence is generally widespread and studies of such provide insights to the tectonic evolution of both the subducted plate and the upper mantle (Liou et al 2007). …”

“…Mantle-derived garnet peridotites are an important component of the continental collision zones and are derived from depleted, metasomatized mantle or crustal cumulates, subjected to metamorphism during subduction (Zhang et al 2004;Spengler et al 2006). The constituent minerals tend to exhibit pre-and post-subduction microstructures, including polymorphic transformation and mineral exsolution (Liou et al 2007). …”