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Although it is widely recognized that mantle peridotite is compositionally heterogeneous, primitive mantle-like peridotite is commonly considered the potential peridotite source when identifying the source lithology of oceanic and continental basalts. Here, a simple equation that compatible with thermodynamic result (Ol(wt%)=144.27ln((Fe+Mg+Ca+2Na)/Si), mol ratio) is introduced to distinguish peridotitic and pyroxenitic rocks. Meanwhile, the basic assumptions, alternative interpretations and uncertainties of many of the literature markers for pyroxenite contributions in basalts are briefly summarized, and some detailed comments on these markers are presented using literature data on olivines and basalts from Eastern China. Both high-CaO (>0.1wt%) and low-CaO (<0.1wt%) olivines in these continental basalts are likely generated by a xenocryst diffusive re-equilibration process, suggesting that the olivine chemistry method may not be suitable for the source lithology identification of the basalts. For the Cenozoic and Mesozoic continental basalts in Eastern China, nearly all of the markers for pyroxenite sources based on whole-rock major and/or trace elements might be compromised by refertilized/metasomatized peridotite sources. These uncertainties also exist for many other oceanic and continental basalts. However, the FC3MS (FeO T /CaO-3*MgO/SiO2, all in wt%) value, combined with those of Na2O+K2O, Mg # , MgO and La/Yb, suggests that Cenozoic OIB-type basalts in Eastern China are mostly primary pyroxenite-derived products, and caution should be paid when interpreting the petrological and geological implications of these basalts, particularly that of the low-MgO pyroxenite-derived basalts. At the normal mantle potential temperature (~1,350-1,400°C), the pyroxenite source can produce melts that explain all of the compositional characteristics of the Cenozoic basalts, suggesting that the origin temperature and pressure of the Cenozoic basalts determined by the traditional peridotite model are overestimated. For the Mesozoic basalts, compositionally diverse peridotite sources can account for their petrogenesis, but the pyroxenite source cannot be excluded. We acknowledge that the pyroxenite source may represent crustal materials (continental or oceanic crust) recycling and/or mantle metasomatism, which have important geodynamic implications, but further work is needed to determine the origin of the pyroxenite.

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The basaltic pillow lavas in the Liuyuan region of NW China, considered to be part of an ophiolitic suite, have been central to the models on tectonic setting, evolution and timing of the final closure of the Palaeo-Asian Ocean. New field evidence on the sedimentary units associated with the basalts reveals comparable sequences in the northern and southern flanks of the Liuyuan Volcanic Belt with coarse to fine sediments from periphery to the centre. The dacites and rhyolites formed coevally with the pillow basalts. The pillow basalts are interlayered with lacustrine sandstone, claystone and clayey lake deposits. Detrital zircons from these sediments yield zircon U–Pb ages of 291–285 Ma. Andesites, dacites and rhyolites from the basaltic sequence yield U–Pb ages of 280–277 Ma, similar to the 282–280 Ma ages of gabbros that intrude the pillow lavas. All these rocks cover the 460–440 Ma granite and greenschist basement and have been intruded by gabbros of c. 272 Ma age, with subsequent (230–227 Ma) north–south contractional thrusting and folding. The data from our study are incompatible with the existing models that consider the basalts as part of an ophiolitic suite. Along the northern continental margin of China from west to east, the Tarim, Dunhuang-Alxa and North China cratonic areas all show evidence for regional extension through rifting during early–middle Permian time. These rift features and basaltic eruptions occurred coevally with the assembly of various microcontinental blocks against the Siberian craton at c. 300–250 Ma, synchronous with amalgamation of the Central Asian Orogenic Belt (CAOB) on the northern side of the Liuyuan Rift. These events were also broadly synchronous with formation of the global supercontinent Pangea.

Abstract.The precursory swarm, three mainshocks (M 7.2,6.7, 7.2), and aftershocks of the Songpan earthquakes have been reanalyzed using both local and teleseismic data. The three mainshocks of this sequence occurred on the Huya fault over a 7-day period.Relocations of the aftershocks using local arrival times show that three fault strands were activated during this sequence.Each mainshock occurred on a separate strand, each one south of the strand activated in the previous mainshock, and the aftershock zones of each mainshock appear to abut rather than overlap.Fault plane solutions determined by matching teleseismic P waveforms at World-Wide Standard Seismograph Network stations with synthetic seismograms are consistent with the observed aftershock zones 19The first a& third mainshocks (Mo = 1.3xl0 and 8.4xl0 N m, respectively) showed almost identical senses of motion, a combination of reverse and left-lateral strike-slip motion, on parallel strands, striking Nl5°W, that were separated by a large rightstepping en yghelon offset. The second mainshock (Mo = 4.0xl0 N m), occurred in this offset on a fault at a steep angle (-125°) to the other two strands and showed almost pure reverse motion. Differences in the orientations of the slip vectors of the three mainshocks show that the first mainshock increased the normal and shear stresses on the fault segment that moved in the second mainshock and that the second mainshock decreased the normal stress on the fault segment activated by the third mainshock. These changes in normal stresses may have given rise to the longer time between the first and second events ( 5 days) as compared with the time between the second and third events (30 hours).A precursory swarm that preceded the Songpan sequence by 3 years occurred in a volume that surrounded the northernmost part of the planar aftershock zone.The time between the start of the swarm and the mainshocks and the magnitude of the largest event in the swarm are similar to those seen for precursory swarms in Soviet Central