PROGRESS IN NATURAL SCIENCE Vol .15 , No .8 , August 2005 Zhang-Xuan thermal uplift and its genesis＊ SHAO Ji' an1 ＊＊ , NIU Shuyin2 , ZHANG Lǜqiao3 and LI Hongyang2 (1 .School of Earth and S pace S ciences, Peking University , Beijing 100871 , China;2 .S hijiazhuang University of Economics, S hiji- azhuang 050031 , C hina;3.Geological Bu reau of Inner Mongolia, Huhhot 010020 , China) Received November 15 , 2004 ;revised December 14 , 2004 Abstract Based on the researches of the Mesozoic thermal evolution history , the deformed structures, and fission track analysis of the Zhangjiakou-Xuanhua area, we obtained the follow ing know ledge:the area experienced in 140 Ma an underplating w hich is marked by emplacement of mantle derived magma ;during 130 —110 Ma a series of eruptions and intrusions happened , and during 80 —10 Ma a rising p rocess occurred w hich is synchronous with the adjacent Taihang and Yanshan Mountains and w as accompanied by a series of structures dow n slipping from the just formed Zhang-Xuan uplift tow ard its periphery.Combining the Zhang-Xuan thermal lift w ith the large detach- ment structure in Baoding depression of the North C hina Basin in the Late Mesozoic confirms that an even larger extensional tectonic back- ground existed in North C hina during this period . Keywords: Zhang-Xuan area, underplating, thermal uplift, fission track, extensional structure. Various forms of crust defo rmation accompany- into tw o parts .The northern part is categorized as ing the lithosphere thinning in North China since the the Archean Dantazi g roup , w hich is a parametamor- M esozoic have draw n attention from many geologists . phic rock system used to be named Hongqiyingzi The records of deformation and thermal evolutionary g roup .The southern part is the Archean Sang gan history in different periods are quite complete for two Com plex , mainly an o rthometamorphite system w ith uplift belts , w hich located respectively at the near E- basic granuly te at the bo ttom and charnockite and W striking Yanshan Mt .intersecting w ith the NN E trondjemite-tonalite-g ranodiorite (TTG)suite at the striking Taihang M t .and Da Hig gan M t .rising in top , in which goldfields were distributed around Xi- the M esozoic , now the Zhang jiakou-Xuanhua and aoyingpan region[ 1] .The center of the metamorphic Chifeng-Harqin areas.The former area has been a fo- basement w as intruded by the Shuiquangou alkaline cus of attention for its g oldfield[ 1] , and its tectonic complex at first and then by Late M esozoic settings have been studied from a view-point of crust- Honghualiang g ranite .The southern and eastern mantle interaction[ 2 , 3] .We are interested in the gen- parts of the metamorphic basement are M iddle Pro- esis of Zhangjiakou-Xuanhua uplift , w hich has a cen- terozoic strata .The periphery of Zhang-Xuan uplift is ter located in Chong li County and will be called there- surrounded by three Zhang jiakou formation volcanic after Zhang-Xuan uplift o r Zhang-Xuan therm al up- basins of the Early Cretaceous (Fig .1). lift , and the relationships of ex tensio nal deformation On the periphery of Zhang-Xuan uplift exist a in its crustal derm al and the deep thermal uplift . series of outw ards declining brittle or ductile shear Through this study w e try to discuss the deformation sliding planes (Fig .1), including some low-angle de- process and related regional tectonic backg round . tachment-thrust faults .For exam ple , at Dalingpu , Chicheng County in the eastern part of Zhang-Xuan 1 Composition and deformation characteris- uplift , there are a series of scaly detachment sheets. tics of Zhang-Xuan uplift Among those sheets , the autochthonous sy stem con- sisting of Tuanshanzi fo rmation dolomitite of the The main body of Zhang-Xuan uplift is Pre- Changcheng system is overlaid by the Chuanlinggou Cambrian metamorphic basement .Its central section fo rmation sheets of the Changcheng sy stem and the is the Archean hypometamorphic g ranuly te-gneiss latter is covered by the Chang zhougou formation area , w hich is divided by the Chongli-Chicheng fault sheets , w hich in turn is overlaid by the Archean stra- ＊ Supported by National Natural Science Foundation of China (Grant Nos.40072073 , 40372103 and 40272088)and Chinese Academy of S ciences (Grant No .KZCX-07) ＊＊ To w hom correspondence should be addressed .E-mail:w [email protected] .cn this 726 w ww .tandf.co .uk/journals Prog ress in Natural Science Vol .15 No .8 2005 ta , and all the strata have w ell developed structures Cretaceous.Such brittle or ductile shear sliding (Fig .2).Bedded iron o res of Chuanlinggou formation planes exist no t only in the Pre-Cambrian strata , but have been discovered in “the thrust outliers”[ 4] .Fo r also in the volcanic rocks of Zhangjiakou formation as ano ther ex ample , at Shengligou Village northw est in well as in some syenite (Fig .1 , Table 1).This fact the Zhang-Xuan uplift it can be seen that Archean indicates an impo rtant uplift-sliding event occurring sheets slipped over the Zhangjiakou formation of Low after Zhang-Xuan uplift formation . Fig .1 . The geological map of Zhang-Xuan area.1 , Archaeozoic;2 , Middle-Upper Proterozoic;3 , Upper Jurassic;4 , Low er Cretaceous;5 , syenite;6 , granite;7 , py roxene diorite ;8 , fault ;9 , slip-sheet.Distribution of sketch diagram corresponds to the position of (a)─(h)in the map .B, bedding or gneissosity ;F, fault;S , slip sheet;C , closely spaced joint slip sheet ;L, extensional lineation . Fig .2 . Detachment sheets at Dalingpu of C hicheng County .1 , Archean gneiss;2 , quartzite of C hangzhougou formation ;3 , shale of Chuanling- gou formation ;4 , argillaceous limestone of Tuanshanzi formation ;5 , quartz sandstone of Dahongyu formation ;6 , dolomitite of Gaoyuzhuang for- mation ;7 , bedded iron ores;8 , sliding planes. Prog ress in Natural Science Vol.15 No .8 2005 w ww .tandf.co .uk/journals 727 Table 1 . Attitudes of sliding plane around the Zhang-Xuan uplift Attitudes of Glide Stratigraphic Location Lithlogy Developed degree sliding plane direction attitudes Sou th of Xiasiganqi Hybridization gneiss (Ar) 325°∠70°-54°-30° 290°∠50° 6/ 5 m 350°∠45°, (a) North of Xiasiganqi Gneiss(Ar) 330°∠40° 290°∠30° Fault, (b) Shizhuizi Gneiss(Ar) 330°∠30°-40° North of Sanchakou Rhyolite (K 1) 280°∠10° 5-2/ 1 m East of Beiw opu Garnet-amphibole Gneiss(Ar) 295°∠10° 300°∠15° 4/ 1 m 40°∠60°, (c) Xiaw opu Rhyolite(K1) 300°∠15° Closely cleavage 50°∠30°, (d) East of Xiw angshan Hybridization gneiss (Ar) 240°∠20° 12/ 1 m 185°∠70° Sou th of S huangmiao Gneiss(Ar) 185°∠80° 350°∠70°-80° Xiaoyingpan gold-mining area Quartz vein 250°∠20° 220°∠36° Sou th of Xiaoyingpan gold-min- K-feldspar vein 200°∠35° 170° Closely cleavage 170°∠20°, (e) ing area East of Xiaoyingpan gold-mining Felsic mylonite 145°∠14° 170°-220° area Jianziling Sericite-schist 158°∠26° 145°∠42°,(h) Dalingpu Quartzite(Pt 2-3) 140°∠45° 145°∠42° 14/ 1 m 133°∠34°, (g) Womaken Sericite-schist 30°∠35° faule Huangtuliang of Zhengningpu Syenite 60°∠49° Closely cleavage fault , (f) Sou th of Nanshanyao Amphibole (Ar) 40°∠25° 50°∠30° Sou th of Baiqi Amphibolite (Ar) 10°∠30° (a)—(h)correspond to those positions in Fig .1 2 Record of the Zhang-Xuan uplift ing points for better fission track analysis . Fission track of apatite is used in this work to deter- 2 .2 Results of fission track analysis mine the uplift time and rate of Zhang-Xuan uplift . The results of sam ples Z-1 , Z-5 , and Z-6 show n 2 .1 Selection of sampling location in Table 2 and Fig .3 demonstrate that cooling of the rock body from 110 ℃ to 60 ℃ was a relatively slow The Hong hualiang bio tite alkali feldspar g ranite process during the period of 80 Ma to 10 Ma , with a located at 12 km southeast to Chongli County w as se- cooling rate of 0 .7 ℃/M a and a mountain mass un- lected fo r sampling in fission track analysis because of roofing rate of 0 .025 km/M a if the average geother- [ 1] its young rock body (133 —115 M a) and its loca- mic gradient is estimated as 30 ℃/km .The cooling tion at the center of Zhang-Xuan uplift .The detach- process from 60 ℃ to 20 ℃ since 10M a has been rela- ment sheets discovered on the periphery of Zhang-Xu- tively fast with a cooling rate of 4 ℃/M a and an un- an uplift are all over the 138 M a Zhangjiakou form a- roofing rate of 0 .13 km/Ma . tion strata , indicating that the Zhang-Xuan thermal uplift should appear after 138 M a .Selection of the Sample Z-4 is an exception .Its results do not 133 —115 Ma old Honghualiang rock body could ex- show a comparatively rapid cooling stage since 10M a , clude possible effects of other events .Its sufficiently instead it reveals a comparatively uniform cooling pro- large outcrop area and topog raphic relief are beneficial cess since 80 Ma with a cooling rate of 1 .1 ℃/Ma and to the enlargement of the height limits of the measur- an unroofing rate of 0 .04 km/M a . Table 2 . Result of fission track analysis on apatite＊ Grain ρ(105/ cm) ρ(105/ cm) ρ(105/ cm) Sample Level(m) s i d P(%) T(±1σ)(Ma) L(μm)(N) Number (n) Number (Ns) Number (Ni) Number (Ni) Z-1 1660 16 0 .705 1 .666 10 .180 93 .9 76 .6 12 .2 ±1 .9 (135) (319) (5090) ±8 .1 (86) Z-4 1580 30 0 .697 1 .965 10 .180 100 64 .3 12 .7 ±1 .8 (148) (417) (5090) ±6 .3 (101) Z-5 1520 30 0 .550 2 .443 10 .180 100 40 .9 11 .8 ±2 .0 (120) (533) (5090) ±4 .2 (74) Z-6 1515 14 0 .899 2 .746 10 .180 100 59 .3 12 .2 ±2 .0 (55) (168) (5090) ±9 .3 (87) ＊ Dr.Yuan Wanmin at the Institute of High Energy Physics, Chinese Academy of S ciences, preformed the fission track analysis 728 w ww .tandf.co .uk/journals Prog ress in Natural Science Vol .15 No .8 2005 Fig.3 .