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The Lithospheric Mantle beneath Continental Margins: Melting and Melt-Rock Reaction in Canadian Cordillera Xenoliths |
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| Authors: | PESLIER ANNE H; FRANCIS DON; LUDDEN JOHN |
| Institution: | 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 |
| Abstract: | Seven alkali basalt centers in the southern Canadian Cordilleracontain mantle xenolith suites that comprise spinel Cr-diopsideperidotites, spinel augite-bearing wehrlites and orthopyroxene-poorlherzolites, and minor pyroxenites. The Cr-diopside peridotitesappear to be residues of the extraction of Mg-rich basalts byup to 15% partial melting (median 5–10%) of a pyrolite-likesource in the spinel stability field. The xenoliths are similarto other mantle xenolith suites derived from beneath convergentcontinental margins, but are less depleted, less oxidized, andhave lower spinel mg-number than peridotites found in fore-arcsettings. Their dominant high field strength element depletedcharacter, however, is typical of arc lavas, and may suggestthat fluids or melts circulating through the Canadian Cordilleralithosphere were subduction related. Modeling using MELTS isconsistent with the augite-bearing xenoliths being formed byinteraction between crystallizing alkaline melts and peridotite.Assimilation–fractional crystallization modeling suggeststhat the trace element patterns of liquids in equilibrium withthe augite xenoliths may represent the initial melts that reactedwith the peridotite. Moreover, the compositions of these meltsare similar to those of some glasses observed in the mantlexenoliths. Melt–rock interaction may thus be a viablemechanism for the formation of Si- and alkali-rich glass inperidotites. KEY WORDS: Canadian Cordillera; mantle xenolith; peridotite; wehrlite; melt–rock reaction |
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