K-rich glass-bearing wehrlite xenoliths from Yitong,Northeastern China: petrological and chemical evidence for mantle metasomatism |
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| Authors: | Y Xu J-C C Mercier Chuanyong Lin Lanbin Shi M A Menzies J V Ross B Harte |
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| Institution: | (1) Laboratoire de pétrologie physique, Université Paris 7 and I.P.G.P., 2 Place Jussieu, F-75252, Paris, France, FR;(2) Department of Geology, Royal Holloway University of London, Egham, Surrey, TW20 OEX, UK, GB;(3) Guangzhou Institute of Geochemistry, Chinese Academy of Sciences, 510640 Guangzhou, P.R. China, CN;(4) Department of Geoscience, University of British Columbia, Vancouver, Canada VGT 1Z4, CA;(5) Department of Geology and Geophysics, University of Edinburgh, Edinburgh, EH9 3JW, UK, GB;(6) Institute of Geology, State Seismological Bureau, 100029 Beijing, P.R. China, CN |
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| Abstract: | Ultramafic xenoliths in Cenozoic alkali basalts from Yitong, northeast China comprise three types in terms of their modal
mineralogy: lherzolite, pyroxenite and wehrlite. The wehrlite suite always contains interstitial 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 fine grained matrix (0.1 mm), implying a metamorphic origin for the protolith rather
than an igneous origin. The host minerals are compositionally zoned, showing evidence of reaction with a melt. Petrological
evidence for resorption of spinel (lherzolite) and orthopyroxene (wehrlite) by infiltrating melt further supports the hypothesis
that the wehrlites result from interaction between a partial melting residue and a melt, which preferentially replaced primary
spinel, Cr-diopside and enstatite to produce secondary clinopyroxene (cpx) + olivine (ol) ± chromite ± feldspar (fd). The
composition of the mineral phases supports this inference and, further indicates that, prior to melt impregnation, 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 glass 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 alkalis (Na2O, 5.6–8.0%; K2O, 6.3–9.0%), high Al2O3 (20–24%), and a depletion 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 glasses with chondrite normalised (La)n = 268–480. The high K2O contents in these glasses and their mode of occurrence argue against an origin by in-situ melting of pre-existent phases. Petrographic characteristics and trace element data also exclude the possibility of percolation
of host-basalt related melts for the origin of these glasses. Thus the glasses must have resulted from local penetration of
mantle metasomatic melts which may have been produced by partial melting of peridotites 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 xenolith transport process.
Received: 1 August 1995 / Accepted: 19 June 1996 |
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