Journal of Petrology | Volume 43 | Number 11 | Pages 2013-2047 | 2002
© Oxford University Press 2002
The Lithospheric Mantle beneath Continental Margins: Melting and MeltRock Reaction in Canadian Cordillera Xenoliths
1TEXAS CENTER FOR SUPERCONDUCTIVITY, UNIVERSITY OF HOUSTON, HOUSTON, TX 77204, USA
2EARTH AND PLANETARY SCIENCES, McGILL UNIVERSITY, MONTREAL, QUEBEC, H3A 2A7, CANADA
3CRPG-CNRS, 15 RUE NOTRE-DAME DES PAUVRES, B.P. 20, 54501 VANDOEUVRE-LÈS-NANCY, FRANCE
Seven alkali basalt centers in the southern Canadian Cordillera contain mantle xenolith suites that comprise spinel Cr-diopside peridotites, spinel augite-bearing wehrlites and orthopyroxene-poor lherzolites, and minor pyroxenites. The Cr-diopside peridotites appear to be residues of the extraction of Mg-rich basalts by up to 15% partial melting (median 510%) of a pyrolite-like source in the spinel stability field. The xenoliths are similar to other mantle xenolith suites derived from beneath convergent continental margins, but are less depleted, less oxidized, and have lower spinel mg-number than peridotites found in fore-arc settings. Their dominant high field strength element depleted character, however, is typical of arc lavas, and may suggest that fluids or melts circulating through the Canadian Cordillera lithosphere were subduction related. Modeling using MELTS is consistent with the augite-bearing xenoliths being formed by interaction between crystallizing alkaline melts and peridotite. Assimilationfractional crystallization modeling suggests that the trace element patterns of liquids in equilibrium with the augite xenoliths may represent the initial melts that reacted with the peridotite. Moreover, the compositions of these melts are similar to those of some glasses observed in the mantle xenoliths. Meltrock interaction may thus be a viable mechanism for the formation of Si- and alkali-rich glass in peridotites.
KEY WORDS: Canadian Cordillera; mantle xenolith; peridotite; wehrlite; meltrock reaction
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