Genesis For The Sulphide-nickel-bearing Ultrabasic Rocks And Crust Extension Process In Bangong Lake Area, Tibet

The sulphide nickel mineralization of ultrabasic rocks in Bangong lake-Nujiang metallogenic belt, western Tibet Plateau is a new type of deposits discovered in Tibet in recent years. Based on the litho-geochemistry and Sr-Pb isotopic analysis of the sulphide-nickel-bearing ultrabasic rocks (hereinafter referred to as nickel-bearing ultrabasic rocks) in Bangong lake in the west segment of metallogenic belt, we present the characteristics and formation conditions of magma sources for these rocks. And according to the zircon U-Pb LAICPMS dating results, we discuss the basement background of northwest Tibet, and we also point out that these ancient basement were the source of recycling material which had taken account for mid-acid dykes near these ultrabasic rocks.The results indicate that the Ni-bearing ultrabasic rocks,with a moderate content in Rb,Th,K,Ba, are enriched in large-ion incompatible elements (U,Sr,Pb) and high strength field elements(Ta) but depleted in Nb,Ti (HFSE) in the primitive mantle-normalized trace elements patterns.The rare earthe elements, however, exhibit a relative flat REE form in the chrondrite-normalized REE patterns with a very low degree of differentiation from LREE to HREE, a weak positive to weak negative Eu anomalies which may be caused by serpentinization, which appears to derive from a MORB mantle source. The low content of total REE and flat or concave distribution from mid to height REE also suggest that the magma source lack of major REE-rich mineral phases such as garnet and clinopyroxene, and there should be hornblende residue in the source. All these features indicate consistently that the ore-bearing magma derived from an depleted mantle source which had been contaminated by upper crust during rising, and was the mixed result of the Indian Ocean turbidites and Indian Ocean MORB-type mantle with low degrees of partial melting (about 10-15%)at a shallow depth for spinel lherzolite facies. In addition, a population of residual zircon ages of 2.48Ga in the rocks marks a relative simple source of sediments for the mid-Tethys ocean in the Bangong Lake area at that time and it was mainly from the late Archean-early Proterozoic basement. Thus we infer that the turn of Archean and Proterozoic(ca.2.5Ga) might be a rapid growing period for ancient continental crust in the northwestrn Tibet.In the south of the ore-bearing ultrabaisc rocks,a series of NS-and EW-striking dykes, including granite porphyries and diorite porphyrites,may represent a crustal extension event of the Bangong lake arc zone after the closure of Bangong lake mid-Tethys oceanic basin.The granite porphyries occuring only in NS-strike yield a weighted mean age of (79.59±0.32) Ma (MSWD=1.08).Meanwhile, the diorite porphyrites occuring both in NS-and EW-strike yield a weighted mean age of (76.9±1.2)Ma (MSWD=2.8).These results indicate that the crustal extensional process of the Bangong lake arc zone occurring in Late Cretaceous epoch was initiated only in EW-trending and slightly later also in NS-trending.Petrochemically, the two types of the dykes appear arc magmatic features characteristed by enrichment of large iron incompatible elements (Rb,U,Th, K, Pb) and depletion of high field strength elements (Nb and Ti).The granite porphyries display high SiO2 and K2O, low Na2O,CaO,and Fe2O3(T), strong Sr and Ti depletion, low Nb/Ta rations, and strong negative Eu anomalies. All of these signatures consistently indicate that their magma source contain essential plagioclase and amphibole residuals. In other words, they were generated at a shallower depth under amphibolite facies conditions. In comparision, the diorite porphyrites have lower SiO2 contents, largely equivalent K2O, Na20 and CaO contents, increased Fe2O3 (T) contents, and chemically weak positive Sr anomalies, high Nb/Ta ratios, weak Ti depletion and negative Eu anomalies. All of these indicate that their magma source lack of plagioclase and amphibole residuals and they were not generated under amphibolite facies condition but probably under eclogite facies or garnet amphibolite conditions. All these dykes plots into a FOZO domain in the Pb discrimination which suggests that they may come from the lower mantle but metasomatied by the crust components. Sr and Nd isotopics further indicate that these dykes are much like that of those adakitic porphyry copper in Gangdese orogenic belt in Eocene which proved to derive from lower crust that had been metasomatied by the subducted components before melting in the mantle source. Combined with those ancient residual zircon ages from Ni-bearing ultrabasic rocks, we infer that these dykes with Nd model ages mainly from 541.6 Ma to 696.0 Ma were derived from a similar lower crust source, which had undergone partial melting together with a FOZO and EMⅡmagma that had emplaced into it at the end of Neoproterozoic and had been metasomatied by ancient recycling subducted components which might come from those ancient continent basement in 2.48Ga or even older.