K-RICH GLASS-BEARING WEHRLITE XENOLITHS FROM YITONG, NORTHEASTERN CHINA - PETROLOGICAL AND CHEMICAL EVIDENCE FOR MANTLE METASOMATISM

Ultramafic xenoliths in Cenozoic alkali basalts from Yitong, northeast China comprise three types in terms of their modal mineralogy: lherzo lite, pyroxenite and wehrlite. The wehrlite suite always contains inte rstitial pale/brown glass which occupies several per cent by volume of the whole rock. The texture of the wehrlites is porphyroclastic with some large strained grains of olivine (0.5-1 mm) scattered in a very f ine grained matrix (0.1 mm), implying a metamorphic origin for the pro tolith rather than an igneous origin. The host minerals are compositio nally zoned, showing evidence of reaction with a melt. Petrological ev idence for resorption of spinel (lherzolite) and orthopyroxene (wehrli te) by infiltrating melt further supports the hypothesis that the wehr lites result from interaction between a partial melting residue and a melt, which preferentially replaced primary spinel, Cr-diopside and en statite to produce secondary clinopyroxene (cpx) + olivine (ol) +/- ch romite +/- feldspar (fd). The composition of the mineral phases suppor ts this inference and, further indicates that, prior to melt impregnat ion, the protoliths of these wehrlites must have been subjected to at least one earlier Fe-enrichment event. This explanation is consistent with the restricted occurrence of glasses in the wehrlite suite. The g lass is generally associated with fine-grained (0.1 mm) minerals (cpx + ol + chromite +/- fd). Electron microprobe analyses of these glasses show them to have high SiO2 content (54-60 wt%), a high content of al kalis (Na2O, 5.6-8.0%; K2O, 6.3-9.0%), high Al2O3 (20-24%), and a depl etion in CaO (0.13-2.83%), FeO (0.89-4.42%) and MgO (0.29-1.18%). Ion probe analyses reveal a light rare earth element-enrichment in these g lasses with chondrite normalised (La)(n) = 268-480. The high K2O conte nts in these glasses and their mode of occurrence argue against an ori gin by in-situ melting of pre-existent phases. Petrographic characteri stics and trace element data also exclude the possibility of percolati on of host-basalt related melts for the origin of these glasses. Thus the glasses must have resulted from local penetration of mantle metaso matic melts which may have been produced by partial melting of peridot ites with involvement of deep-seated fluids. Such melts may have been significantly modified by subsequent fractional crystallization of ol, cpx and sp, extensive reaction with the mantle conduit and the xenoli th transport process.