An Unusual Garnet Pyroxenite from the Granulitgebirge, Germany: Origin in the Transition Zone (>400 km depths) or in a Shallower Upper Mantle Region?

Very high-pressure orogenic garnet peridotites

“…Exsolved needles of pyroxene, rutile, and apatite along garnet (111) planes from the Yangkou eclogite layer within peridotite (11,45,51) are based on optical and SEM observations. Similar exsolution lamellae, suggesting that a majoritic garnet precursor formed at depths Ͼ200 km, have also been reported in the Erzgebirge massif of Germany, another VHP terrane (13). Despite numerous reports of majoritic garnets, however, formation of VHP solid-solution phases (e.g., majorite) may have taken place either in the deep upper mantle, and then later been sequestered in the mantle wedge such as in Norway, or in a subduction zone such as in the Kokchetav of northern Kazakstan (see below).…”

“…[2][3][4][5][6]. Some peridotites preserve a record of ultradeep origin revealed by mineral exsolution and the persistence of VHP polymorphs (6)(7)(8)(9)(10)(11)(12)(13)(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.…”

Orogenic, Ophiolitic, and Abyssal Peridotites

“…These rocks have attracted much attention recently as a potential source of information on relatively deep mantle processes such as accretion of lithospheric roots, transport, reaction and storage of fluids/melts in suprasubduction mantle wedges, and subduction/exhumation mechanisms Brueckner and Medaris, 2000;Brueckner et al, 2004;Ernst and Liou, 2008;Green et al, 1997;Green et al, 2010;Katayama et al, 2003Katayama et al, , 2005Kotkova et al, 2011;Liou et al, 2002aLiou et al, ,b, 2007Liou et al, , 2009Liu et al, 2007Liu et al, , 2009MacKenzie et al, 2005;Malaspina et al, 2009Malaspina et al, , 2010Paquin et al, 2004;Sapienza et al, 2009;Scambelluri et al, 2006Scambelluri et al, , 2008Scambelluri et al, , 2010Song et al, 2004;van Roermund et al, 2001;Yoshida et al, 2004;Zhang and Liou, 1998;Zhang et al, 2005;Zhang et al, 2003Zhang et al, , 2004aZheng et al, 2003Zheng et al, , 2011. Numerous studies were published, mostly in the last decade, providing data on the petrology, geochronology, and geochemistry of HP/UHP garnet peridotites and related rocks from four principal regions: Liati and Gebauer, 2009;Medaris et al, 1995Medaris et al, , 2005Medaris et al, , 2006Massonne and Bautsch, 2002;Massonne and Neuser, 2005;…”

Exsolution microstructures in ultrahigh-pressure rocks: Progress, controversies and challenges

“…The reconstructed precursor supersilicic garnet composition suggested for the first time subduction and exhumation of continental crust rocks to depths greater than 200 km [9] . Since then, similar exsolution microstructures were reported from various UHP terraines one after the other including Erzgebirge massif [44] of German and the northern margin of Caidam [17] and Altyn [45][46][47] of western China. Dobrzhinetskaya et al [48] successfully simulated the exsolution of clinopyroxene from supersilicic garnet by a series of decompression experiments (pressure decreasing to 12 GPa from 14 GPa at constant temperature of 1400℃), demonstrating that the exsolved clinopyroxene "blebs" mainly occur at the sites of triple conjunction of polycrystalline aggregates of supersilicic garnet.…”