U-Pb Zircon Geochronology, Geochemical And Sr-Nd-Pb Isotopic Compositions Of The Hualong Group In Qilian Mountains: Constraints On Tectonic Evolution Of Crustal Basement

The Qilian Mountains constitutes the western segment of the Central Orogenic System ofChina (Sulu-Dabie-Tongbai-Qinling-Qilian-Kunlun Orogenic Belt). In the middle massif of theQilian Mountains, the Precambrian basement metamorphic series include the Huangyuan Group,the Maxianshan Group, and the Hualong Group and so on. However, the formation time andtectonic affinity of the Qilian Mountains basements are still controversial. This paper carries out astudy of U-Pb dating, geochemical and Sr-Nd-Pb isotopic compositions from the Hualong Groupand foliated granites. The Hualong Group had experienced upper amphibolite faciesmetamorphism, which is the highest grade in the basement metamorphic series of the QilianMountains. The foliated granites intrude into the Hualong Group. In order to do regionalcomparision, U-Pb zircon ages for a paragneiss of the Maxianshan Group in the QilianMountains basement series were also measured.Some conclusions are sumarried as follows:1. Zircons from the paragneiss consist dominantly of detrital magma zircons with roundedor sub-rounded shape. They have ²⁰⁷Pb/²⁰⁶Pb ages mostly ranging from 880 to 900 Ma, with aweighted mean value of 892±8 Ma, which represents the magma crystallization age of itsigneous provenance and can be taken as a lower age limit of the Hualong Group. Magmacrystallization age for the weak-foliated granite is 875±8 Ma, which can be taken as an upperage limit of the Hualong Group. Accordingly, the formation time of the Hualong Group isconstrained to be between 875 and 892 Ma. Similarly, the zircons from the paragneiss of theMaxianshan Group are sub-rounded shapes. All the detrital magma zircon analyses constitute adiscordia with an upper intersect age of 931±19 Ma. Their weighted mean ²⁰⁷Pb/²⁰⁶Pb age is 939±9 Ma, which is in good agreement with the upper intersect age within errors. Therefore, the ageof 939±9 Ma is interpreted as magma crystallization age of igneous provenance. The agedifference of the paragneisses between the Hualong Group and the Maxianshan Group is about40Ma. This indicates that the Neoproterozoic magma events are diachronous. A few zircons fromboth the paragneisses and the weak-foliated granite display old ages of 1000～1700 Ma. 2. The garnet-bearing amphibolites from the Hualong Group have SiO₂= 48.12～53.64%,MgO=3.54～5.94%, CaO=7.21～2.59% and K₂O/Na₂O=0.2～2.96. They have similar REEpatterns with (La/Yb)_N=2.59～5.89 and Eu/Eu^*=0.73～0.90. Trace element compositions show theenrichment of U, Th and the depletion of Nb, Ta, Sr, and P.They have ⁸⁷Sr/⁸⁶Sr ratios (0.710659to 0.731856) and a wide range forε_Nd(0) (-3.17 to -14.13). Their protolith were basaltic. Thegneisses of the Hualong Group, except for sample HL-16 (SiO₂=78.73%, Al₂O₃=7.55%,K₂O+Na₂O=1.57%), display SiO₂=72.69～74.43%, Al₂O₃=11.94～13.24%,K₂O+Na₂O=6.17～8.42%. All the gneiss samples have fractionated REE patterns with(La/Yb)_N=4.76～4.85 and Eu/Eu^*=0.31～0.56. Generally, they have similar trace elementgeochemical characteristics with the depletion of Nb, Ta, Sr, Ti and P.They have high initial Srisotopic ratios (I_Sr) (0.72301 to 0.84504) and negativeε_Nd(t) values (-1.77 to -3.71). The gneissesbelong to paragneiss, which was derived from igneous provenance expect for the sample HL-16.3. The weakly foliated granites display SiO₂ =74.31～76.07% and K₂O/Na₂O=2.28～3.33.They are peraluminous with A/CNK=1.10～1.26 and have similar trace elemental compositions,characterized by the depletion of Nb, Ta, Sr, Ti and P. Except for the sample 0653 displayinglow∑REE abundance and Eu/Eu^*=0.78, all samples have fractionated REE patterns with(La/Yb)_N=6.88～7.33 and Eu/Eu^*=0.23～0.35. They also have high initial Sr isotopic ratios (I_Sr)(0.71939 to 0.81042) and negativeε_Nd(t) values (-2.68 to -5.72). Because of the similarity of thegranite and the paragneiss in their geochemical and Sr-Nd-Pb isotopic compositions, the magmaof the foliated granite was possibly derived from partial melting of the paragneiss. Alternativelythe granites and the original rocks of the paragneiss have the similar compositions of sourceinvolving voluminous hydrous pelite.4. The basement of the Qilian Mountains formed in Jinningian period. Two Neoproterozoicmagmatic events of 940～870 Ma and 790～750 Ma occurred in the Qilian Mountains. Both thebasement rocks and granitoids are characterized by high radiogenic Pb isotopic compositions. Allsamples have ²⁰⁶Pb/²⁰⁴Pb＞18.6, ²⁰⁷Pb/²⁰⁴Pb＞15.6, ²⁰⁸Pb/²⁰⁴Pb＞38.4. The basement rocks andgranites from the Hualong Group have mantle-depleted Nd isotopic model ages (T_DM) rangingfrom 1.65～2.15 Ga, with a peak at～1.90 Ga. Combined with the published Nd isotopic data (T_DMranging mainly from 1.0～2.4 Ga), they show that crustal growth of the Qilian Mountanis mainlyoccurred during the Proterozoic, without significant Archean information. Thus, the formationtime of basement, Neoproterozoic magmatic events, Pb isotopic compositions and T_DM values ofthe basement rocks and granites are similar to those of the Yangtze block but distinct from thoseof the North China block, indicating that the crustal basement of the Qilian Mountains has anaffinity with the Yangtze block.5. The weakly foliated granites are S-type granites, and their petrogenesis was linked tocontinent-continent collision. The abundant detrital magmatic zircons from the Hualong Groupreveal that the provenance of the Hualong Group was dominantly derived from an erodedigneous province. The age difference between the Hualong Group provenance (892±8 Ma) andthe intruded granite (875±8 Ma) is less than 17 Ma. Considering the measurement errors, thebiggest age difference is not over 33 Ma. This indicates that the time-span from erosion of the Hualong Group provenance to transport, then to sedimentation is within 33 Ma, suggesting arapid uplift-erosion process for the Hualong Group provenance and implying an outstandingNeoproterozoic orogenesis in the Qilian Mountains. Moreover, the detrital magmatic zircon agesof 892±8 Ma from the Hualong Group and 939±9 Ma from the Maxianshan Group are close to9～10 Ga tectono-magmatic events obtained from the Qilian, Tarim, Qinling, Qaidam,Oulongbuluke and Longshoushan blocks, suggesting that these west China blocks shared theconvergence of the Rodinia super-continent.