Thermal state and structure of lithospheric mantle beneath the Xing'an Massif, northeast China: Constraints from mantle xenoliths entrained by Cenozoic basalts

The lithospheric mantle beneath the Xing'an Massif (XM), northeast China, was formed at the Paleoproterozoic or even earlier, but its thermal state and structure remain poorly understood. To address this issue, we calculate pressure-temperature (P-T) arrays for mantle xenoliths entrained by Cenozoic basalts from the XM, using major-element-based and rare earth element (REE)-based two-pyroxene thermometry as well as single clinopyroxene barometry. Samples (include spinel or garnet lherzolites and harzburgites, dunite and pyroxenite) come from the Keluo, Nuomin, and Aershan regions, which are located on either side of the North-South Gravity Lineament (NSGL). To the west of the NSGL, in the Nuomin and Aershan regions, mantle xenoliths yield consistent REE-based and major-element-based temperatures, suggesting that these samples were derived from stable thermal conditions. Garnet peridotites from these regions yield pressures of 2.37-3.01 GPa and temperatures of 1066-1303 degrees C, revealing a high geothermal gradient (corresponding to a surface heat flow of 70-80 mW/m(2)) and a crustal thicknesses of similar to 37 km. The lithosphere-asthenosphere boundary determined from the intersection of this geotherm with the mantle adiabat occurs at a depth of similar to 100 km, which is much shallower than the lithospheric thickness calculated by geophysical analyses (140-160 km). This discrepancy may have resulted from locally upwelling hot asthenosphere. Low-temperature (800-900 degrees C) peridotites from the Nuomin and Aershan are chemically fertile with low olivine Mg values and high modal clinopyroxene contents. By contrast, high-temperature (1050-1350 degrees C) peridotites consist mainly of refractory harzburgites or clinopyroxene-poor lherzolites. These facts reveal that the lithospheric mantle beneath the Nuomin and Aershan regions is characterized by juvenile mantle dominating shallow levels and ancient mantle occurring at greater depths. To the east of the NSGL, some Keluo mantle xenoliths yield higher REE-based temperatures (839-863 degrees C) than major-element-based temperatures (<800 degrees C), indicating cooling by thermal relaxation. This cooling process produced a low geothermal gradient (similar to 45 mW/m(2)) in the lithosphere beneath the Keluo region. The mantle xenoliths comprise both fertile lherzolites and refractory harzburgites without correlation between mineral compositions and temperatures, suggesting that the lithospheric mantle beneath the Keluo region is characterized by juvenile mantle mixed with ancient mantle at all depths. The results show contrasting thermal state and structure of the lithospheric mantle on either side of the NSGL within the XM, which are attributed to a previous cooling process in the Keluo region and local upwelling of hot asthenosphere beneath the Aershan and Nuomin regions.