| As a fragment of oceanic crust and lithospheric mantle,ophiolite is usually considered as reliable evidence of the presence of ancient ocean,which can provide abundant information on the formation and evolution history of the ocean.Nevertheless,ultramafic rocks in ophioltes are poorly studied,making it difficult to boost the further studies on ophiolite,oceanic evolution as well as orogensis.That is because most scholars preferred to focus on the mafic suites of ophiolites,due to the lack of analytical methods that can be applied well in the ultramafic rocks.Qilian Orogenic Belt?QOB?is an important component of the famous Qinling-Qilian-Kunlun Orgenic Belt in central China.In the QOB,ophiolites are widely distributed as several NW-SE-trending bands and characterized by their large quantities,many controversies as well as good preservation.In this study,the ultramafic rocks from four typical ophiolites in the QOB?namely Aoyougou,Lajishan,Minhe and Bianmagou?were selected for electron probe micro-analyses?EPMA?,whole-rock geochemical and Re-Os isotopic measurements.Petrographic observations suggested that almost all ultramafic rocks were intensively affected by serpentinization.Even so,undulating extinction,kink band,embayed texture as well as EPMA data of the spinels are strong evidence for a mantle-residue origin of these ultramafic rocks,as remnants of the ocean-lithospheric mantle after variable partial melting.Mineral chemistry of the spinels,olivines and orthopyroxenes further indicated that the Lajishan metaperidotites and Bianmagou serpentinites were emplaced in the mid-ocean ridges as similar to abyssal peroditites,while the Aoyougou and Minhe serpentinites had formed in the forearc settings.Meanwhile,their degrees of partial melting were calculated by spinel Cr#values,which varied from 6%to 19%,supporting that the ultramafic rocks had commonly experienced multiple-episode melting and extraction events.Many geochemical fingerprints?e.g.,LREE enrichments,U-shape REE pattern and positive Eu anomalies?and whole-rock Re-Os compositions implied that the Aoyougou forearc serpentinitites underwent mantle metasomatism induced by slab-derived melts,including melt/rock reaction,melt percolation and refertilization;however,the Lajishan metaperidotites experienced the MORB melt/peridotite reactions and melt percolation.Not only that,all of the ultramafic rocks are significantly marked by variable enrichments of some certain fluid-mobile elements?e.g.,U,Pb,Sr?,as a consequence of the late-stage fluid-dominated serpentinization.The Lajishan metaperidotites defined an inferred isochron with an apparent age of 495±9 Ma and initial 187Os/188Os of 0.1181±0.0015,implying that this ophiolite was most likely emplaced during the late Cambrian.More importantly,the Aoyougou serpentinites and Lajishan metaperidotites also yielded the Paleoproterozoic Re-depletion ages(TRD),at 2.28Ga and 1.86 Ga respectively.The former is well consistent with their Yb-isochron age??2.4Ga?.All these results demonstrated that the ancient melt depletion had happened earlier than the Paleproterozoic,closely associated with the contemporary crust-growth events in the QOB.Based on our data and previous studies,a tectonic model might be a reasonable interpretation for the evolution of the QOB during the early Paleozoic:The breakup of Rodinia supracontinent resulted in the opening and spreading of the North Qilian Ocean and South Qilian Ocean since the Neoproterozoic.In the Cambrian,these two oceans successively experienced the northward intra-ocean and ocean-continent subduction,and were eventually closed since the early Silurian,which was accompanied by continental collision and orogenesis.After that,the QOB entered into the post-collision stage of collapse and extension during the late Devonian. |
PDF Full Text Request