Modification of an ancient subcontinental lithospheric mantle by continental subduction: Insight from the Maowu garnet peridotites in the Dabie UHP belt, eastern China

Abstract

Orogenic mantle-derived peridotites commonly originate from the subcontinental lithospheric mantle (SCLM) and thus provide a key target to investigate the modification of the SCLM by a subducting slab. The Maowu ultramafic rocks from the Dabie ultrahigh-pressure (UHP) metamorphic belt have formerly been debated as representing cumulates or mantle-derived peridotites. Detailed petrological and geochemical data presented in this study provide new constraints on the origin and formation of the peridotites involving melt depletion in the ancient SCLM and deep crustal metasomatism. The Maowu garnet dunites have refractory bulk compositions characterized by high Mg# (91.9-92.0) and Ni (2537-2892 ppm) values and low Al 2 O 3 (0.26-0.76 wt.%), CaO (0.05-0.32 wt.%), TiO 2 (<0.03 wt.%), Pd/Ir (0.40-0.46) and 187 Os/ 188 Os (minimum 0.11461) values. The Paleoproterozoic model ages (T RD = 2.1 Ga, T MA = 2.3 Ga) of the most refractory dunites represent minimum estimates for the age of the initial melt extraction. The extremely depleted nature, high olivine Fo (92.7-93.9), high Cr# (82-87) of spinel, and Re-Os isotopic data suggest that the Maowu garnet dunites are the residues of ~40% partial melting and represent a Paleoproterozoic fragment of the SCLM beneath the southeastern margin of the North China craton. Many garnet orthopyroxenite veins crosscutting the Maowu dunites preserve abundant metasomatic textures and show variable enrichment in incompatible elements. Mineral and whole-rock chemistry indicate that these veins represent metasomatic products between the wall dunites and silica-rich hydrous melts under UHP conditions. The veins show large variations in platinum-group element (PGE) signatures and Re-Os isotopes. The garnet-poor orthopyroxenite veins are characterized by low Al 2 O 3 (<2 wt.%) and S (<31 ppm) contents and have PGE patterns and 187 Os/ 188 Os ratios similar to the wall dunites, whereas the garnet-rich orthopyroxenite veins have high Al 2 O 3 (>6 wt.%) and S (99-306 ppm) contents and show melt-like PGE patterns and high 187 Os/ 188 Os ratios (up to 0.36910). These features, combined with the occurrence of interstitial sulfides in the garnet-rich orthopyroxenite veins, suggest that crust-derived sulfur-saturated silicate melts may have significantly modified the PGE signature and destroyed the Re-Os systematics the SCLM. However, when the crust-derived silicate melts became sulfur-depleted, such melts would not significantly modify the PGE patterns, radiogenic Os-isotope compositions or the Re-depletion model ages of the SCLM. Consequently, deep crust-mantle interactions in continental subduction zones could induce high degrees of Os isotopic heterogeneity in the SCLM wedge.

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

“…A Re‐Os isotope analysis was performed for dunites from Rongcheng, which have a similar composition to those from Tengjia, and the result suggests that their formation occurred in the Paleoproterozoic (Su, Chen, Guo, Chu, et al, ). Other Re‐Os isotope analyses of the M‐type orogenic peridotites from the Dabie‐Sulu orogenic belt also yield Paleoproterozoic ages (Chen et al, , and references therein). Such ages are consistent with the Paleoproterozoic age of the subcontinental lithospheric mantle (SCLM) in the periphery of the NCC but are much older than the Neoproterozoic age of the SCLM in the subducted continental lithosphere of the SCB (Zhang & Zheng, ).…”

Garnetite and Pyroxenite in the Mantle Wedge Formed by Slab‐Mantle Interactions at Different Melt/Rock Ratios

“…The presence of phlogopite and Ti‐clinohumite in the two veins provides robust evidence of metasomatic origin. Moreover, the texture of resorbed olivine inclusions in pyroxenes (Figures c and f) has been widely viewed as evidence for the formation of secondary pyroxene at the expense of olivine (Chen et al, ; Chen, Ye, Guo, et al, ). This interpretation is verified by the similar depleted compositions between the olivine inclusions in the veins and the matrix olivines in the host peridotites.…”

A multivariate discrimination scheme of detrital garnet chemistry for use in sedimentary provenance analysis

“…With prior 'equal-M' three out of four examples are still classified correctly by 78-100%, but one example (D-b) is assigned to granulite-facies rocks. This is due to comparatively low Cr 2 O 3 content and rather high Al 2 O 3 /SiO 2 ratio for these garnets, which derive from orthopyroxenite veins representing metasomatic products between the wall dunites (see example D-a that is correctly classified; Table 7) and silica-rich hydrous melts under UHP metamorphic conditions (Chen et al, 2017). Using the prior 'global' all ultramafic examples failed, which is obviously due to the very low prior probability for D. Together with the discussion of priors in Section 5.2 these examples clearly suggest that the prior 'global' is inappropriate when ultramafic rocks are considered as possible source of the investigated garnets.…”