Journal of Petrology, Volume 39, Issue 3: March 1 1998.

Texture-Temperature-Geochemistry Relationships in the Upper Mantle as Revealed from Spinel Peridotite Xenoliths from Wangqing, NE China

YIGANG XU^1,2, MARTIN A. MENZIES¹, PIETER VROON¹, JEAN-CLAUDE MERCIER³ AND CHUANYONG LIN⁴

¹DEPARTMENT OF GEOLOGY, ROYAL HOLLOWAY UNIVERSITY OF LONDON, EGHAM HILL, EGHAM,SURREY TW20 0EX, UK ²GUANGZHOU INSTITUTE OF GEOCHEMISTRY, CHINESE ACADEMY OF SCIENCES, 510640 WUSHAN, GUANGZHOU, PEOPLE'S REPUBLIC OF CHINA ³DÉPARTEMENT DES SCIENCES DE LA TERRE, UNIVERSITÉ DE LA ROCHELLE, AVENUE MARILLAC, F-17000 LA ROCHELLE, FRANCE ⁴ INSTITUTE OF GEOLOGY, STATE SEISMOLOGICAL BUREAU, 100029 BEIJING, PEOPLE'S REPUBLIC OF CHINA

Spinel peridotite xenoliths from Wangqing, NE China, exhibit correlated variations in texture, temperature, geochemistry and radiogenic isotopes. Protogranular and transitional peridotites are less refractory than equigranular samples, which are predominantly harzburgites and clinopyroxene-poor lherzolites. Deformed harzburgites and lherzolites are variably enriched in light rare earth elements (LREE), and Ce/Yb-MgO variations indicate that these olivine-rich rocks have been preferentially infiltrated by metasomatic agents. Clinopyroxenes from the fertile protogranular and transitional lherzolites have high ¹⁴³Nd/¹⁴⁴Nd ([epsilon]_Nd = 10-18) and low ⁸⁷Sr/⁸⁶Sr (0·7020-0·7036) consistent with long-term time-integrated depletions (>1·34 Gy). By contrast, clinopyroxenes in the majority of the equigranular samples have higher ⁸⁷Sr/⁸⁶Sr (>0·7035) and less radiogenic ¹⁴³Nd/¹⁴⁴Nd ([epsilon]_Nd <4·5) values. Mineral chemistry shows intra-grain and inter-grain heterogeneity, and the degree of heterogeneity decreases from protogranular to equigranular peridotites. Thermometric calculation further reveals a cooling event from >1200°C to 835-930°C at which all samples last equilibrated. Integration of these data suggests that the Wangqing xenoliths may represent fragments of the lithosphere that has been isolated from the asthenosphere for a long time (>1·34 Gy). This aged lithosphere was locally altered by asthenosphere-derived fluids, which in turn enhanced grain boundary migration and recrystallization, giving rise to coupled textural and geochemical variations. During the thermal erosion of the lithosphere that took place since the late Mesozoic, the base of lithosphere represented by the precursor rocks of the Wangqing peridotites became rheologically similar to the thermal boundary layer such that it was able to intrude with the asthenosphere diapirically the rigid uppermost lithospheric mantle at a time just before the host basalt eruption.

Keywords: petrography;trace element; isotope compositions; peridotite xenoliths; Eastern China

Pages 469-493