The nature of the lithospheric mantle near the Tancheng-Lujiang fault, China: an integration of texture, chemistry and O-isotopes
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A unique suite of basalt-borne sheared and granular spinel lherzolites from the Yitong graben provides an insight into the nature of the lithospheric mantle adjacent to the Tancheng-Lujiang fault, northeastern China. Data obtained by ion microprobe (multi-elemental) and laser-fluorination (δ18O ratios) techniques have been integrated with thermobarometric data. Shallow (25-35 km), cool (742-967°C), sheared peridotites contain clinopyroxenes with LREE-depleted profiles and minor enrichments in La. A marked difference in the δ18O ratios of porphyroclast and neoblast olivine in these sheared peridotites may be due to thermal re-equilibration or recent fluid activity. Deeper (50-60 km), hotter (950-1073°C), granular peridotites contain LREE-depleted and enriched clinopyroxenes, the latter depleted in Ti and Zr. Melts in equilibrium with these LREE-enriched and Ti-Zr-depleted pyroxenes are very similar to potassic glasses in wehrlite xenoliths from the Yitong graben. On the basis of these data it is proposed that toward the base of the lithosphere hot, granular ‘refractory’ peridotites predominate and that they have been variably enriched by intergranular migration of small volume potassic melts. It is believed that the cold, sheared, peridotites from near the Moho were derived from a hot, granular, refractory precursor but that movement on the Tancheng-Lujiang fault, a major intracontinental shear zone, generated a ‘sheared’ texture and vertical displacement relocated these mantle rocks in a cooler part of the lithosphere.