Lithospheric mantle provinces and crust-mantle decoupling beneath northeastern China: Insights from peridotite xenoliths.

The origin and evolution of the subcontinental lithospheric mantle (SCLM) underlying orogenic belts bordering cratons, as either newly added domains or strongly reworked older cratonic lithosphere, remains controversial. This limits our understanding of deep lithospheric behavior and processes during plate convergence. Here, we use detailed petrology, whole-rock and mineral compositions, and in-situ Sr isotopic compositions for spinel-facies peridotite xenoliths from Jiaohe (northeastern China) in the southeastern Central Asian Orogenic Belt (CAOB), which is pinched between two NE-striking Mesozoic to Cenozoic giant trans-lithospheric fault systems. These data are combined with whole-rock Re-Os isotope and platinum-group element compositions and literature data, to explore the physicochemical evolution of the regional SCLM and examine crust-SCLM relationships for this vast swath of continental lithosphere. Lherzolites predominate at Jiaohe, and have higher whole-rock Al2O3 contents than harzburgites (2.27-3.46 wt% versus 0.84-1.02 wt%), but exhibit FeO enrichment similar to harzburgite (bulk FeO up to 9.54 wt%). The lherzolitic clinopyroxenes (Cpx) have higher heavy rare earth element (HREE) levels (2.79-5.11 ppm) and Ti/Eu (3882-6864), coupled with wider variation of 87Sr/86Sr (0.7021-0.7038) and lower average ƒO2 (oxygen fugacity relative to the fayalitemagnetite- quartz buffer; FMQ-1.92 ± 0.55) compared to the harzburgites (HREE: 0.94-2.11 ppm; Ti/Eu: 163-2044; 87Sr/86Sr: 0.7032-0.7036; ƒO2 = FMQ-1.25 ± 0.20). All these observations suggest that the lherzolites were produced by adding Cpx ± spinel to the protoliths from reducing silicate melts, whereas the harzburgites may have originated from a remnant, highly depleted mantle domain that variably interacted with silica-undersaturated oxidizing melts. Bulk-rock and mineral compositional relationships as well as ƒO2 for peridotite samples from the southeastern CAOB (including Jiaohe) are similar to those of the northeastern NCC (including Huinan). Their characteristics contrast with those from the northwestern CAOB, suggesting that they belong to distinct lithospheric provinces with respect to both composition and oxidation state. Jiaohe harzburgites yield Paleoproterozoic Re depletion Os model ages (TRD up to 1.76 Ga), similar to Huinan peridotites (up to 1.92 Ga), just 170 km away, and may also represent strongly reworked cratonic mantle. By analogy with evidence from recent continental collision zones, the Jiaohe SCLM may have originated as cratonic SCLM that migrated northeastward during plate convergence along giant translithospheric fault systems. [ABSTRACT FROM AUTHOR]