Petrogenesis, tectonic setting and formation age of the metaperidotites in the Lajishan ophiolite, Central Qilian Block, NW China

摘要

The Lajishan ophiolite is one of the most representative ophiolites in the Central Qilian Block (CQB), NW China. Detailed microscopic observation and electron probe micro-analysis of minerals demonstrate that the metaperidotites of the Lajishan ophiolite were mantle residues formed during the spreading mid-ocean ridge of South Qilian Ocean and had been produced by partial melting and melt extraction of the lithospheric mantle. Their degrees of partial melting (F) were estimated to be varying from 6.41% to 16.21% in terms of the spinel Cr# values. Such high F values might result from multi-episode partial melting events, evidenced by the lacking of linear relationship between Al2O3 and Os-187/ Os-188, as well as their significantly depleted Re-Os isotopic compositions. Although the whole-rock Re-Os isotopes of the metaperidotites are marked by subchondritic Re-187/Os-188 (0.0352-0.2443) and Os-187/Os-188 (0.1163-0.1206) ratios, three fertile metapyrox- enites have the Re-Os isotopic compositions (Re-187/Os-188 = 172.6997-1811.4094, Os-187/Os-188 = 2.6447-15.1121) akin to suprachondrite. The suprachondrite-like compositions reflect an overprint of melt/peridotite reaction and melt percolation after an initial melting depletion. The oldest T an model age of the depleted metaperidotites (ca. 1.86 Ga) suggest that the initial melt-extraction event had occurred earlier than the Paleoproterozoic, in response to contemporaneous crust-growth activities in the CQB. Moreover, the seven samples yield an inferred isochron with an apparent age of 495 +/- 9 Ma and initial Os-187/Os-188 of 0.1181 +/- 0.0015, implying that the Lajishan ophiolite was eventually emplaced after this age and most likely during the late Cambrian.