Olivine fabrics and tectonic evolution of fore‐arc mantles: A natural perspective from the Songshugou dunite and harzburgite in the Qinling orogenic belt, central China

Abstract

To advance our understanding of the deformation characteristics, rheological behaviors, and tectonic evolution of the fore‐arc lithospheric mantle, we analyzed mineral fabrics for a large spinel‐bearing ultramafic massif in the Songshugou area in the Qinling orogenic belt, central China. In the spinel‐poor coarse‐grained dunite, stronger A/D‐type and weaker C‐type‐like fabrics were found, whereas the spinel‐rich coarse‐grained dunite displayed a comparatively stronger B‐type‐like fabric. These olivine fabrics are high‐T fabrics influenced by the presence of melt, in which B and C‐type‐like fabrics are inferred to be produced by melt‐assisted grain boundary sliding during synkinematic high‐T melt‐rock reactions. In contrast, the spinel‐poor porphyroclastic and fine‐grained dunites present weak AG and B‐type‐like fabrics, respectively. Their olivine fabrics (low‐T fabrics) are inferred to transform from A/D‐type fabric in their coarse‐grained counterparts possibly through mylonitization process assisted by low‐T fluid‐rock reactions, during which strain was accommodated by the fluid‐enhanced dislocation slip and/or fluid‐assisted grain boundary sliding processes. Combined with the tectonic results of our previous work, the high‐T olivine fabrics are probably related to a young and warm fore‐arc mantle where intense partial melting and high‐T boninitic melt‐rock reactions prevalently occurred, whereas the low‐T olivine fabrics likely reflect the evolving tectonic settings through the cooling fore‐arc mantle to a continental lower crust in a collisional orogeny where low‐T fluid‐rock reactions were pervasively activated. These low‐T olivine fabrics imply that though cold, the fore‐arc lithospheric mantle may be locally weak (∼20–30 MPa), allowing ductile deformation to occur at a geologically significant strain rate.

Microstructural Evolution of Amphibole Peridotites in Åheim, Norway, and the Implications for Seismic Anisotropy in the Mantle Wedge

“…The other possible mechanism for a fabric change from an A-type to B-type LPO of olivine is an enhancement of dislocation-accommodated grain boundary sliding (DisGBS) [17][18][19]86]. Precigout and Hirth [19] proposed that a fabric transition from an A-type to B-type LPO in olivine that was observed in the Ronda massif, Spain could have resulted from the enhancement of GBS as the grain size decreased.…”

“…For example, the fabric transition of olivine in the mantle wedge from an A-type to a B-type LPO of olivine is proposed as a possible mechanism for the change in the shear wave splitting pattern observed in the subduction zone [3,7,8,[12][13][14]. Many studies have proposed a hypothesis for this change in fabric: a deformation under water-rich conditions [7,8], a deformation under high pressure [10,15,16], an enhancement of grain boundary sliding [17][18][19], a diffusional creep [20], or the presence of partial melt [21,22]. There have been many reports of such fabric transitions recorded in naturally deformed peridotites from various localities: The Bergen Arc, Norway [23], the Ronda massif, Spain [19], Lien, Almklovdalen, Norway [24], the Navajo volcanic field, USA [25], and the Calatrava volcanic field, Spain [26].…”

Crustal Metasomatism at the Slab‐Mantle Interface in a Continental Subduction Channel: Geochemical Evidence From Orogenic Peridotite in the Sulu Orogen

“…This indicates that these dunites originated from the overlying NCC rather than the subducting SCB (Zheng, ). The occurrence of spinels in the Tengjia dunite and Opx porphyroblasts containing Cpx exsolved lamellae, which are similar to the Songshugou peridotite in the Qinling orogen (Cao et al, , ), suggests that the Tengjia dunites originated from the forearc mantle wedge and underwent a high degree of melt extraction under subarc conditions (Song et al, ).…”

Erratum to: Crystal preferred orientations of olivine, orthopyroxene, serpentine, chlorite, and amphibole, and implications for seismic anisotropy in subduction zones: a review

“…Cao et al (2017) reported CPOs of olivine similar to Types C and B in Songshugou peridotites and suggested that those CPOs were formed by a diffusion-accommodated grain boundary sliding (DifGBS) at high temperatures.…”

“…Olivine is the most abundant mineral and is elastically anisotropic (Birch, 1960;Verma, 1960;Abramson et al, 1997) (Table 1). The CPO of olivine is key to understanding the seismic anisotropy and flow pattern of the upper mantle (Hess, 1964;Nicolas and Christensen, 1987;Ben Ismail and Mainprice, 1998;Jung et al, 2006;Karato et al, 2008;Cao et al, 2015;Michibayashi et al, 2016;Cao et al, 2017). In the mantle wedge and at the slab-mantle interface, hydrous minerals such as serpentine, chlorite, and amphibole can form from the fluids generated by the dehydration of hydrous minerals in the subducting slab (Ulmer and Trommsdorff, 1995;Peacock and Hyndman, 1999;Pawley, 2003;Fumagalli and Poli, 2005;van Keken et al, 2011).…”