Geological Character And Genesis Of Niubiziliang Mafic-ultramafic Layered Intrusion In The Northwest Margin Of Qaidam Block, Qinghai, P.R. China

Niubiziliang intrusion is located at Eboliang area in the northwest margin of Qaidam block.As a mafic-ultramafic layered intrusion, it is a typical example in the324intrusions in thenorthwestern of Qaidam block and Altun fault belt. Therefore, this paper chooses it as studyobject. Petrology, mineragraphy, crystal chemistry, major and trace elements, rare earthelements, Nd-Sr-Pb-O-S isotopes and geochronology are presented to provide information onmagmatic properties, the nature of magma source and mineralization processes. Based onthese researches, the following understanding could be obtained:1. As a mafic-ultramafic layered intrusion with cumulate bedding and multiplies cumulaterhythms and cycles, the body is consist of peridotite, pyroxenite and gabbro. The largenumber of quartz diorite and tonalite demonstrates that the magma has experienceddifferentiation and evolution processes which is dominated by fractional crystallization. Theparental magma is full of H2O or volaties, so a lot of brown hormblendes crystallize intohornblendite. The weighted average age and concordant age of zircons U-Pb are essentiallyapproximate. It presentes that the intrusion intruded and crystallized at367.0±2.0Ma.2. The m/f values of all ultramafic rocks range from3.79to4.98, so Niubiziliang is aferruginous ultrabasic intrusion. The Mg#values of all samples have a large range between55and83. It is from15.67×10-6to83.77×10-6that the concentration of rare earth elements andtrace elements ranges. The (La/Yb)Nvalues of all of rocks changes in2.65-9.85， the(La/Sm)Nin1.47-4.01, and the (Gd/Yb)Nin1.61-2.57. The rocks show LREE enrichedpattern in chondirite-normalized REE diagram and primary mantle-normalized trace elementsdiagram with LILE abundance, HFSE depletion and lack of obvious europium negative orpositive variation. It is clearly that the Nb and Ta show negative anomaly in all kinds of rocks.3. Both field investigation and microscope observation show that the intrusion has stronglyassimilated and contaminated with country rocks. The analytical data of Nd, Sr, Pb and Oisotopes are all agree with geological observation. As a result, there are different contaminateddegrees among the whole kinds of rocks. The degree rises gradually from peridotite tograbbro with the evolution of magma, then it turns to deduce.4. The content of Na2O+K2O has a large range between0.58%and6.5%, parental magmabelongs to calc-alkali basalt series. However, magma property will change from tholeiiticbasalt series to calc-alkali basalt series by large degree assimilation and contamination, so wecan’t certify that the primary magma is not tholeiitic basalt. Base on the principle of olivine-melt equilibrium, the calculated primary magma contains about11.56%MgO. Thisindicates that the primary magma is high MgO tholeiitic basalt, thus we can’t get rid of thepossibility that the primary magma is picritic.5. The peridotite, by contrast, is not easy to be contaminated, so sample like that shouldrepresent magma source. The Nd and Sr isotope composition is located in the oceanic islandbasalt area; therefore, the primary magma could be generated from depletion mantle sourcewith Nd and Sr isotope composition like OIB.6. The intrusion includes two ore bodys in different layers. One is at an altitude of3100m±, the other is at an altitude of2800~2900m. At the western, there is a hydrothermal ore bodyin the fault zone which has experienced activation and migration of faults. The structures ofore are mainly disseminated, sponge iron meteorite and massive. Mineralization processes canbe divided into three metallogenic epochs and four metalizing phases. Multi-disciplinarystudies show that free differentiation evolution and contamination jointly lead to sulfideliquation, and the mineralization process is a multi-epoch and multi-phase process rather thana continuous process.