PETROGENESIS, TECTONIC SETTING AND FORMATION AGE OF THE METAPERIDOTITES IN THE LAJISHAN OPHIOLITE, NW CHINA

The Lajishan ophiolite is one of the most representative ophiolites in the Central Qilian Block, the Central China Orogenic Belt, NW China. In spite of intensive serpenization of mantle peridotites, detailed microscopic observations and electron probe micro-analyses of unaltered minerals (olive, orthopyroxene and spinel) still demonstrated the metaperidotites might be mantle residue at an ancient mid-ocean ridge setting of the South Qilian Ocean as a product of partial melting and melt extraction of the ocean-lithosphere mantle. Their degrees of partial melting (F) were estimated from 6.41% to 16.21% in terms of Cr# values in the spinels. Such high F values maybe result from multi-episode melting events, which is also illustrated by no linear correlation between Al₂O₃ and ¹⁸⁷Os/¹⁸⁸Os as well as their extremely depleted Re-Os isotopic compositions.

Despite the similarly depleted Re-Os isotopic compositions in most of the metaperidoties, three fertile samples have the suprachondritic isotopes. It reflects an overprint of latter melt/peridotite reaction and melt percolation after an initial mellting depletion. Moreover, the oldest T_RD model age of the depleted metaperidotites (ca. 1.86 Ga) supported that the initial melt-extraction event had occurred earlier than Paleoproterozoic, in response to the contemporaneous crust-growth activities in the Central Qilian Block. Besides, the seven metaperidotites yield an inferred isochron with an apparent age of 494.8±9.2 Ma and initial ¹⁸⁷Os/¹⁸⁸Os of 0.1181±0.0015, implying that the Lajishan ophiolite was eventually emplaced in the period of the late Cambrian. Overall, combined with the previous studies, we proposed an alterative model of the South Qilian Ocean (a part of Paleo-Tethys Ocean) to shed light on its tectonic evolution from the spreading of the ocean to continental collision during the Paleozoic.