Discovery and Sedimentary Sequences of a Special Paleoseamount within the Mazongshan Subduction Accretionary Complex in Beishan Orogen, Gansu Province
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摘要: 古海山是缝合带的关键组成部分,中亚造山带西段的天山造山带内已发现多处古海山,而北山地区却鲜有报道,古海山的发现可以弥补该地区海山研究的不足.通过在北山中部野马泉开展地质调查、测制剖面,发现了一套原始层序完整、以玄武岩、玄武质凝灰岩和大理岩为主的地层,具有火山岩基座和碳酸盐岩顶盖的结构,二者原始接触关系为整合接触,符合海山的沉积特征.依据地层中的岩石组合和沉积构造,确定其形成于海山斜坡相.野马泉古海山残骸呈NWW-SEE向延伸,东南部更接近海山顶.该海山中的玄武岩富集大离子亲石元素、亏损高场强元素,具有岛弧玄武岩的特征.该海山为洋内弧型海山,野马泉一带位于海山斜坡,其被构造肢解后呈NWW-SEE向分布.Abstract: Paleoseamounts are key components of suture zones. In the Tianshan orogen located in the southwestern segment of the Central Asian Orogenic Belt, many paleoseamounts have been reported, while few were discovered in the Beishan orogen. Discovery of a paleoseamount can compensate for the lack of studies on seamounts in the Beishan orogen. After geological survey on the Yemaquan area in central Beishan orogen and measuring a profile, strata composed of basalts, basaltic tuffites and marbles with primitive sequences were discovered. The strata contain volcanic basement and carbonate cap, and the two segments primitively conformably contacted, indicating that the strata are fragments of a paleoseamount. The sedimentary structures and rock associations of the paleoseamount in the Yemaquan area indicate that they were deposited in a seamount slope. The seamount fragment in this study trends NWW-SEE and the southeastern part approaches the seamount top. The basalts in the seamount are enriched in large iron lithophile elements and depleted in high field strength elements, showing characteristics of island arc basalts. The seamount in this study is an oceanic arc type seamount. The rocks in the Yemaquan area were deposited in seamount slope facies and trended NWW-SEE after they were dismembered.
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图 1 (a) 北山地区构造位置示意图;(b)北山造山带构造单元划分图
a.据Yu et al.(2016);b.据Xiao et al. (2010); 张克信(2015)修改,蛇绿岩年龄据周国庆等(2000); 任秉琛等(2001); 于福生等(2006); 张元元和郭召杰(2008); Ao et al. (2012); 武鹏等(2012);Tian et al. (2014); 胡新茁等(2015); Wang et al. (2018)
Fig. 1. (a)Tectonic map show the locations of the Beishan orogen; (b) Map showing the tectonic units of the Beishan orogen
图 2 野马泉地区地质图(据Wang et al., 2018修改)
Fig. 2. Geological map of the Yemaquan area (modified after Wang et al., 2018)
图 7 (a) 野马泉海山西北侧角砾岩;(b)海山北侧玄武岩-凝灰岩-大理岩的沉积序列;(c)海山西南侧角砾大理岩;(d)海山的边界;(e)海山东南侧凝灰岩-大理岩-砾岩的沉积序列
Fig. 7. (a) Breccia in the northwestern seamount; (b) Sedimentary sequences composed of basalts, tuffites and marbles in the northern seamounts; (c) Calcirudytes in the southwestern seamount; (d) Boundary of the seamount; (e) Sedimentary sequences composed of tuffites, marbles and conglomerates
图 8 (a) 火山岩SiO2-(K2O+Na2O)分类命名图解; (b)原始地幔标准化微量元素蛛网图; (c)球粒陨石标准化稀土元素配分图
a.据Peccerillo and Taylor(1976); c.球粒陨石、原始地幔、OIB、E-MORB、N-MORB值据Sun and McDonough(1989); IAB值据杨婧等(2016)
Fig. 8. (a) SiO2 versus (K2O+Na2O) discrimination diagram; (b) Primitive mantle normalized trace element diagrams; (c) Chondrite normalized REE diagrams
表 1 玄武岩主量、微量元素含量表
Table 1. Major and trace elements concentrations of basalts in this study
样品号 SiO2 TiO2 Al2O3 TFe2O3 MnO MgO CaO Na2O K2O P2O5 LOI PM21-7-1H 52.35 0.87 16.54 7.35 0.56 6.10 9.57 2.36 1.62 0.22 1.94 PM21-8-1H 53.80 0.80 15.90 9.61 0.23 2.88 12.09 1.14 1.88 0.15 1.35 PM21-9-1H 54.22 0.84 17.78 8.60 0.12 2.69 9.73 2.46 1.25 0.23 1.88 样品号 Rb Ba Th U Nb Ta Pb Sr Zr Hf Cs Pb Y PM21-7-1H 51.6 809 2.46 0.82 3.31 0.23 7.13 341 98.8 2.66 1.41 7.13 23.0 PM21-8-1H 42.5 625 3.61 1.15 6.64 0.47 10.40 239 108.0 2.84 4.33 10.40 33.6 PM21-9-1H 28.8 419 2.16 0.65 2.21 0.15 9.33 268 79.1 2.12 2.41 9.33 19.5 样品号 La Ce Pr Nd Sm Eu Gd Tb Dy Ho Er Tm Yb Lu PM21-7-1H 13.1 31.9 4.45 19.1 4.56 1.46 4.40 0.69 4.04 0.77 2.16 0.32 2.05 0.31 PM21-8-1H 10.6 25.5 3.34 14.6 3.88 1.09 4.90 0.86 5.54 1.10 2.98 0.43 2.64 0.38 PM21-9-1H 11.7 25.9 3.64 15.8 3.69 1.19 3.88 0.57 3.27 0.71 1.91 0.28 1.78 0.26 注:主量元素单位为%, 微量元素单位为10-6. -
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