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Zircon U-Pb Ages, Petrogenesis And Crustal Evolution Of The Granites In Northeastern Part Of The Great Xing'an Range

Posted on:2008-09-27Degree:DoctorType:Dissertation
Country:ChinaCandidate:Z M SuiFull Text:PDF
GTID:1100360212997804Subject:Mineralogy, petrology, ore deposits
Abstract/Summary:PDF Full Text Request
Granitoids from northeastern Da Hinggan Range are taken as studied object in this thesis. Based on systematic study of field geology, zircon LA-ICPMS U-Pb dating, petrology, geochemistry, Sr-Nd isotope and Hf isotope of these granitoids, their chronology, petrogenesis, tectonic setting and geological implications are discussed, and the following conclusions are obtained.1. Zircon U-Pb dating results by laser ablation inductively coupled plasma mass spectrometer (LA-ICP-MS) indicate that granitoids from northeastern Da Hinggan Range formed mainly in the Early Paleozoic and Jurassic, and some of Late Paleozoic and Triassic. The emplacement ages of Early Paleozoic granitoids including Shibazhan pluton, Neihe pluton, Baiyinna pluton, Chalabanhe pluton and Halapaqi pluton are 499±1 Ma, 500±1 Ma, 460±1 Ma, 465~481 Ma and 461~500 Ma, respectively. Combined with zircon U-Pb dating results of Early Paleozoic granitoids from adjacent regions, the time scale of Early Paleozoic magmatism in the studied area could be determined as 460~500Ma. The Jurassic magmatism of this area could be classified into two stages, i. e. , 188~190Ma and 171~181Ma. The emplacing ages of Hanjiayuanzi pluton and Zhengqicun pluton of the early stage are 188±1 Ma and 190±1 Ma, respectively, while Xinghua-Fanshentun pluton, Jiweidianzi pluton and Heihuashan pluton of the late stage emplaced at 178±1Ma, 176~181 Ma and 171±2 Ma, respectively. Shierzhan granitic pluton of Late Paleozoic formed at 298±2Ma, and Chahayan granitic pluton of 236±1 Ma.2. The Early Paleozoic granitoids in this area mainly consists of granodiorite, monzogranite and syenogranite. The mafic minerals in these rocks are hornblende and biotite, while the accessory mineral is titanite and without Al-rich minerals. These granitoids are metaluminous and weakly peraluminous, belonging to calc- alkaline-high-K calc-alkaline series, enriched in large ion lithophile elements (LILE) (e. g. , Rb, Th, U, K, La, Nd and so on), depleted in high field strength elements (HFSE) such as Nb, Ta, P, Ti, etc. . These lines of evidence indicate they exhibit a geochemical characteristics of postorogenic I-type granite, and should formed at postorogenic or postcollisional stage.3. The Jurassic granitoids in this area are mainly composed of diorite, quartz diorite, granodiorite and monzodiorite, being metaluminous and weakly peralu- minous, belonging to high-K calc-alkaline series. Based on the enrichment and depletion degree of Sr and Y, they can be separated into two types, i. e. , high-Sr and low-Y type (Sr>500μg/g, Y<2μg/g), and low-Sr and high-Y type (Sr<420μg/g, Y>2μg/g). The early stage of Jurassic granite are of high-Sr and low-Y type, such as Zhengqicun pluton, Hanjiayuan pluton, Sandaoka pluton and Wudingshan pluton, while late stage of them are of low-Sr and high-Y type, e. g. , Xinghua-Fanshentun pluton, Jiweidianzi pluton and Heihuashan pluton. The two types of granites possess similar abundance and same evolutional trend of major elements, and similar Sr-Nd and Hf isotope characteristics. Taken together, the two types of granites originated from similar magmatic source of different depth, high-Sr and low-Y type granite being similar to C-type adakites which derived from low crust, while low-Sr and high-Y type granites derived from middles crust of low pressure.The Jurrasic granitoids of this area have similar rock association and geochemical characteristics to granitoids from active continental margin, belonging to NE-NNE distributed granite belt in northeast China, and their origins are related to the subduction of Paleopacific plate.4. Most of the Early Paleozoic granitoids have high 176Hf/177Hf ratios and positiveεHf(t) values, and these spots cluster around the evolutional line of chondrite, implying the source materials of granitoids mainly came from depleted mantle. Some plutons have low 176Hf/177Hf ratios and negativeεHf(t) values (-14.98~-8.19), falling within the evolutional scope of Proterozoic crust, indicating that they should be derived from partial melting of newly accreted crustal materials during Mesoproterozoic- Neoproterozoic, moreover, their magmatic source may possess affinities of enriched mantle. In addition, contamination of old crustal materials may exist during the formation of some rocks.Jurassic granitoids have widely various Hf isotopic composition (εHf(t)= -2.82~+11.64 ), attributing to contribution of accreted crustal materials. Zhengqicun pluton have 176Hf/177Hf ratios of 0.2826~0.2827,εHf(t) values of -2.82~+2.82, and two stage model age of 1.2Ga in average, indicating its magmatic source are accreted crustal materials during Mesoproterozoic-Neoproterozoic. Heihuashan pluton have highest 176Hf/177Hf ratios for zircons whoseεHf(t) values up to +11.64, and its young two stage model age (0.47~0.72Ga) indicating the magmatic source are newly accreted crustal materials from depleted mantle during Neoproterozoic-Phanerozoic. Hanjiayuanzi pluton has 176Hf/177Hf ratios,εHf(t) values (all of them are positive), and two stage model age between that of Zhengqicun pluton and Heihuashan pluton, implying its magmatic source consist of crustal materials mainly accreted during Mesoproterozoic- Neoproterozoic and some of them accreted during Phanerozoic similar to Heihuashan pluton.5. Hf and Nd model ages of granitoids indicate this area have two crust accreted stages, Mesoproterozoic (1.0~1.5Ma) and Neoproterozoic-Phanerozoic (0.4~0.8Ma). Along with age of granitoids from old to young, 176Hf/177Hf ratios andεHf(t) values gradually increased, while model age became young, suggesting decrease of old crustal materials and increase of juvenile accreted crustal materials in the magmatic source, implying the crust growth of Da Xinggan Range is by underplating and remelting of underpalting rocks.6. Nd isotopes and Hf isotopes of zircons for granitoids suggest the line of Humazhen- Shisizhanlinchang should be taken as the boundary of Ergun massif (in the north) and Xing'an massif (in the south), whose crustal growth ages are obviously different. Ergun massif is composed mainly of newly accreted crustal materials in Mesoproterozoic, partly of Neoproterozoic-Phanerozoic and still some of old crustal materials, while Xing'an massif mainly of Neoproterozoic- Phanerozoic. Hence Ergun massif and Xing'an massif have evidently different tectonic characteristics and early evolutional history.
Keywords/Search Tags:granite, Zircon U-Pb ages, Zircon Hf isotope, Crustal evolution, northeastern part of the Great Xing'an Range
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