Thermal state and structure of the lithosphere beneath eastern China: A synthesis on basalt-borne xenoliths

Abstract

Application of reliable thermobarometer on garnet-bearing mantle xenoliths and granulite xenoliths entrained by Cenozoic basalts in eastern China reveals two main types of geotherm. The first type, as exampled by Hannuoba (汉诺坝), Mingxi (明溪) and probably Northeast China, is characterized by constant slope of data in the P-T space. The second type, as exampled by the high geotherms of Nüshan (女山) and probably Xinchang (新昌), is characterized by variable slopes, with the samples with pressure <2 MPa defining a slow slope, whereas the samples with pressure >2 MPa define a virtually vertical slope. The different slopes in the second type of geotherm may correspond to different heat transfer mechanisms, with conductive transfer for the shallow upper mantle and advective transfer for the deep mantle. This observed transition in thermal transfer mechanism is consistent with theoretical modeling. The two types of geotherm are not mutually exclusive, because the second type may characterize the thermal state of whole lithospheric section including both mechanical boundary layer (MBL) and thermal boundary layer (TBL), while the first type may only depict the MBL. The variable geotherms for different regions are indicative of a heterogeneous lithospheric structure in eastern China. (a) Eastern North China craton (NCC) is characterized by a second-type geotherm, corresponding to a thin lithosphere (~70 km). Comparison of the equilibrium temperatures of spinel peridotites with this geotherm constrains the depth to Moho in eastern North China craton to be 30 km. In contrast, western NCC (Hannuoba: the first-type geotherm) possesses a relatively low thermal gradient, indicative of a thick lithosphere (>90-100 km) and a thick crust-mantle transition zone. The dramatic change in crustal and mantle structure across the DTGL (Daxing'anling (大兴安岭)-Taihangshan (太行山) gravity lineament) is consistent with recent seismic studies. (b) There is a decrease in thermal gradient and in lithospheric thickness from the coast (Xinchang: the second-type geotherm) to the inland (Mingxi: the first-type geotherm) in South China (from ~80 km to >90 km), which is collaborated with westward variation in basalt geochemistry. (c) The weak convex-upward pattern of the geotherm in Qilin (麒麟) and Leizhou (雷州) Peninsula is peculiar, probably reflecting a transiHuang Xiaolong and Xu Yigang 712 tional feature between conductive and advective heat transfer. It may result from impregnation of mantle plume on the base of the lithosphere. These new results not only provide a basic framework for the ongoing 4-D lithosphere mapping project in eastern China, but also yield important implications for deep processes that operated over the past.

Microscale effects of melt infiltration into the lithospheric mantle: Peridotite xenoliths from Xilong, South China

“…The 'ghost Pl' signature suggests refertilization by a melt that preserved the signature of an original Pl-rich source, perhaps recycled crust (Bonadiman et al, 2008;Marchesi et al, 2013;Sobolev et al, 2000). Thus, we consider that the K 2 O-rich melts could be derived b a from the partial melting of pre-existing enriched lithospheric mantle (metasomatized by subduction-related fluids/melts), due to the rising geotherm as the asthenosphere ascends (Huang and Xu, 2010).…”

Recycling of oceanic crust from a stagnant slab in the mantle transition zone: Evidence from Cenozoic continental basalts in Zhejiang Province, SE China

“…A geotherm of Cenozoic lithosphere beneath the Xinchang area ( Fig. 12) has been inferred by the xenoliths from surrounding areas (Huang and Xu, 2010). The area has a high heat flow of 70-80 mW/m 2 , which might reflect lithospheric extension and thinning, probably related to the opening of the South China Sea (Ho et al, 2003;Xu et al, 1995).…”

“…9a, the Re-Os contents of our studied samples fall in the same range as the Emeishan basalts of South China, which are thought to have formed from a plume-like deep mantle, further supporting our conclusion that the Zhejiang basalts were not generated in the SCLM. Huang and Xu (2010). The shield and oceanic geotherms are after O' Reilly and Griffin (1996).…”

The Xinchang peridotite xenoliths reveal mantle replacement and accretion in southeastern China

“…1), which provide opportunity to study the nature and evolution of the lithospheric mantle beneath the SE China. A number of petrological and geochemical studies have been undertaken on mantle xenoliths trapped by the Cenozoic basalts from SE China (Fan et al, 2000;Huang and Xu, 2010;Lin et al, 1995;Qi et al, 1995;Xu et al, 1995Xu et al, , 2000Xu et al, , 2002Xu et al, , 2008Yu et al, 2003Yu et al, , 2006Zheng et al, 2004). Previous studies have shown that most of the peridotites from SE China are fertile and represent residues of low degrees (i.e., b5%) of partial melting (Qi et al, 1995;Xu et al, 2000), except some spinel peridotites from Mingxi (Fujian) and Qilin (Guangdong) with higher degrees of melt extraction (Qi et al, 1995;Xu et al, 2000Xu et al, , 2001.…”