中亚造山带北山南部柳园洋晚古生代两阶段古地理演化

甘蒙北山地区位于中亚造山带中段,其间的柳园洋是古亚洲洋的重要分支之一,关于该洋盆的演化过程存在两阶段裂谷盆地和单阶段长期俯冲两种认识。柳园洋位于北山南部石板山地块和双鹰山地块之间,石板山地块的晚古生代沉积序列和物源变化完整记录了洋盆的古地理演化过程,对于重建区域构造演化有重要意义。此次研究聚焦石板山地块独山地区的上古生界火山- 沉积序列,通过砂岩碎屑颗粒组分和碎屑锆石U- Pb- Hf同位素分析,提供柳园洋古地理演化的约束证据。本文数据表明：独山地区下—中泥盆统碎屑锆石年龄为早古生代单峰式分布（~415 Ma）,来自于柳园洋向南俯冲在石板山地块边缘而形成的岩浆弧;上石炭统—下二叠统样品中出现了明显的中元古代碎屑锆石年龄（~1426 Ma）,来自于北侧的双鹰山地块和中天山地块,指示洋盆闭合事件;下二叠统碎屑锆石年龄主要为晚石炭世—早二叠世单峰式分布（301~290 Ma）,来自于裂谷火山活动。此外,本次研究通过对北山南部已发表物源学数据的统计计算,进一步证明并完善了古生代柳园洋两阶段演化模型：中奥陶世—中泥盆世,柳园洋双向俯冲至敦煌- 石板山地块和双鹰山地块之下;晚泥盆世,柳园洋闭合,直至石炭纪晚期,敦煌- 石板山- 双鹰山地块形成了统一的陆缘环境;早—中二叠世,柳园裂谷盆地逐步发育至原洋盆地。     

Paleozoic subduction of the southern Dunhuang Orogenic Belt,northwest China: metamorphism and geochronology of the Shuixiakou area

Amphibolites in the Shuixiakou area of the southern Dunhuang Orogenic Belt, southernmost Central Asian Orogenic Belt (CAOB), occur as lenses within hornblende-biotite-plagioclase gneiss or pelitic schist, exhibiting block-in-matrix feature of tectonic mélange. Three generations of metamorphic mineral assemblages (M1, M2, and M3) have been recognized in the garnet-bearing amphibolite lenses. The metamorphic prograde assemblage (M1) is documented with inclusion trails (hornblende + plagioclase + quartz) within garnet porphyroblasts, and are estimated to be formed under 610–690 °C and 6.5–10.2 kbar. The metamorphic peak assemblage (M2) consists of garnet + hornblende + clinopyroxene + plagioclase + quartz in the matrix and records metamorphic peak P-T conditions of 720–750 °C and 13.4–14.7 kbar. The retrograde assemblage (M3) is represented by the symplectic assemblage (hornblende + plagioclase + quartz ± biotite ± magnetite) rimming the garnet porphyroblast, formed in the decompression stage under P-T conditions of 630–730 °C and 3.8–7.2 kbar. The derived metamorphic P-T paths show similar tight clockwise loops including nearly isothermal decompression processes, typical of orogenic metamorphism. SIMS dating of metamorphic zircons from the amphibolites confirm that the high-pressure metamorphism (M2) occurred at ca. 438–398 Ma.     

Metamorphism and geochronology of garnet amphibolite from the Beishan Orogen,southern Central Asian Orogenic Belt:Constraints from P-T path and zircon U-Pb dating

Numerous lenses of garnet amphibolite occur in the garnet-bearing biotite-plagioclase gneiss belt in the Baishan area of the Beishan Orogen,which connects the Tianshan Orogen to the west and the Mongolia-Xing'anling Orogen to the east.The study of metamorphism in Beishan area is of great significance to explain the tectonic evolution of Beishan orogen.According to the microstructures,mineral relationships,and geothermobarometry,we identified four stages of mineral assemblages from the garnet amphibolite sample:(1) a pre-peak stage,which is recorded by the cores of garnet together with core-inclusions of plagioclase(Pl_1);(2) a peak stage,which is recorded by the mantles of garnet together with mantle-inclusions of plagioclase(Pl_2)+amphibole(Amp_1)+Ilmenite(Ilm_1)+biotite(Bt_1),developed at temperature-pressure(P-T) conditions of 818.9-836.5℃ and7.3-9.2 kbar;(3) a retrograde stage,which is recorded by garnet rims + plagioclase(Pl_3)+amphibole(Amp_2)+orthopyroxene(Opx_1)+biotite(Bt_2)+Ilmenite(Ilm_2),developed at P-T conditions of 796.1-836.9℃ and5.6-7.5 kbar;(4) a symplectitic stage,which is recorded by plagioclase(Pl_4)+orthopyroxene(Opx_2)+amphibole(Amp_3)+biotite(Bt_3) symplectites,developed at P-T conditions of 732 ±59.6℃ and 6.1 ±0.6 kbar.Moreover,the U-Pb dating of the Beishan garnet amphibolite indicates an age of 301.9 ±4.7 Ma for the protolith and 281.4±8.5 Ma for the peak metamorphic age.Therefore,the mineral assemblage,P-T conditions,and zircon U-Pb ages of the Beishan garnet amphibolite define a near-isothermal decompression of a clockwise P-T-t(Pressure-Temperature-time) path,indicating the presence of over thickened continental crust in the Huaniushan arc until the Early Permian,then the southern Beishan area underwent a process of thinning of the continental crust.     

中亚造山带东段古生代山弯构造

中亚造山带东段位于西伯利亚和华北克拉通之间，经历了多构造体系叠加和多旋回洋陆转换的复杂演化过程，目前大量研究均以构造带为核心来限定区域构造格局，但一直争议较大。本文以构造单元的构造属性及其形成过程为主线，结合区域构造带演化，重新厘定了中国东北地区基本构造格局，建立了中国东北山弯构造演化模型。研究表明，古生代时期中国东北地区的主要构造单元由两个具前寒武纪基底的古老地块——额尔古纳地块和佳木斯地块及其张广才岭陆缘弧与两个古生代增生地体——兴安增生地体和松辽增生地体组成，其间由古亚洲洋分支新林- 喜桂图洋、贺根山- 嫩江洋、龙凤山洋和索伦洋分割。早古生代，西部额尔古纳地块东南部为西太平洋型活动陆缘，发育有嘎仙- 吉峰- 环宇洋内弧和头道桥等洋岛，~500 Ma随着新林- 喜桂图洋的关闭，这些洋内弧和洋岛拼贴增生至额尔古纳地块东南缘。随后贺根山- 嫩江洋的俯冲和后撤形成了一系列沟- 弧- 盆体系，持续的俯冲导致弧陆碰撞和陆缘增生，形成兴安增生地体的主体。同时，东部佳木斯地块西侧发育有龙凤山洋的安第斯型俯冲活动陆缘，形成了张广才岭陆缘弧。伴随着各大洋的俯冲和陆缘增生，额尔古纳地块和佳木斯地块以及它们的陆缘增生带构成了一个早古生代近东西向展布的地块链。南部以锡林浩特- 龙江微地块为核心发生陆缘俯冲，形成松辽增生地体雏形。索伦洋发生双向俯冲，并通过弧陆碰撞产生陆缘增生。晚古生代，伴随着古亚洲洋的北向俯冲和后撤，早期形成的地块链逐渐发生向南弯曲。二叠纪末期—中三叠世古亚洲洋俯冲消减闭合以及西北部蒙古- 鄂霍茨克洋和东部泛大洋的俯冲挤压，导致地块链进一步弯曲，同时，早期的古老地块、增生地体、弧岩浆岩、沉积建造等发生汇聚，最终形成一个以额尔古纳地块和兴安增生地体为西翼，佳木斯地块和张广才岭陆缘弧为东翼，松辽增生地体为核心的大规模山弯构造——中国东北山弯构造。     

Late Carboniferous-Early Permian Mafic-Ultramafic Complexes in Beishan,Southwestern Central Asian Orogenic Belt and Their Significance

正Numeral Permian mafic-ultramafic complexes occur in the Beishan terrane atthe northeastern margin of the Tarim craton,southwestern Central Asian Orogenic Belt,including the Pobei,Cantoushan,Bijiashan,Hongshishan,Xuanwoling,Zhongposhan and Luodong etc.,intrusions (Qin et al.,2011;Zhang et al.,2017;Xue et al.,2018).These Beishan mafic-ultramafic complexes are composed of peridotite,pyroxenite,troctolite and gabbro with significant cumulate rhythmic layers of magmatic     

Early Paleozoic tectonic evolution of the North Qinling orogenic belt: Evidence from geochemistry,phase equilibrium modeling and geochronology of metamorphosed mafic rocks from the Songshugou ophiolite

The Shangdan suture zone (SSZ) is the main collisional boundary between the North China Craton and the South China Craton, along which discontinuous Paleozoic ophiolites and subduction–accretion related volcanic arc assemblages occur. Here we report the petrology, geochemistry, geochronology and phase equilibria modeling of garnet amphibolite from the Songshugou ophiolite which is one of the largest ophiolite outcrops in the northern side of the SSZ. From petrological studies, we identify: (1) prograde stage, defined by garnet + clinopyroxene + calcic amphibole + ilmenite + rutile + epidote + plagioclase + quartz; (2) peak stage with garnet + clinopyroxene + ilmenite + rutile + quartz; and (3) retrograde stage with amphibole + plagioclase + titanite + ilmenite. Our pseudosection analysis defines stability of the peak assemblage at 750–850 °C, 15–19 kbar and traces a clockwise P–T path in the system Na₂O–CaO–FeO–MgO–Al₂O₃–SiO₂–H₂O–TiO₂–Fe₂O₃ (NCFMASHTO), suggesting high pressure (HP) metamorphism. Subsequently, the rocks experienced rapid decompression and cooling. LA-ICP-MS U-Pb analyses of zircons from the garnet amphibolite yield a weighted mean ²⁰⁶Pb/²³⁸U age of 515 ± 12 Ma. This Early Paleozoic metamorphic age represents the emplacement time of the Songshugou ophiolite, and suggests that the HP metamorphism is possibly related to the northward deep subduction of the Shangdan oceanic crust in Early Paleozoic.     

新疆谢米斯台地区晚古生代俯冲作用: 来自塞勒肯特岩体的岩石学、年代学及地球化学证据

塞勒肯特岩体出露于谢米斯台中东部,岩性主要为二长花岗岩、石英闪长岩及花岗闪长岩。本文通过锆石U-Pb年代学、岩石地球化学和Sr-Nd-Hf同位素等研究,探讨其形成的构造环境及岩石成因。结果表明,二长花岗岩(400.9±4.3Ma)和石英闪长岩(398.1±4.5Ma)形成于早泥盆世,花岗闪长岩(381.7±2.9Ma)形成于晚泥盆世。岩体整体富碱,属于准铝质-弱过铝质高钾钙碱性花岗岩类。轻重稀土分馏较明显且富集轻稀土((La/Yb)_N=5.09~9.22),Eu异常不明显,相对富集Rb、Th、U、K等元素,亏损Nb、Ta、Ti等高场强元素;二长花岗岩和石英闪长岩具有低的(⁸⁷Sr/⁸⁶Sr)_i值(0.7040~0.7043),正ε_Nd(t)值(+4.85~+6.18),年轻的t_DM1(Nd)年龄(663~732Ma),二长花岗岩锆石ε_Hf(t)值为+7.94~+12.12,t_DM2(Hf)=648~889Ma;花岗闪长岩也具有低的(⁸⁷Sr/⁸⁶Sr)_i值(0.7045~0.7046),正ε_Nd(t)值(+4.61~+4.80),年轻的t_DM1(Nd)年龄(731~749Ma),花岗闪长岩锆石ε_Hf(t)值为+4.26~+11.69,t_DM2(Hf)=631~1103Ma。综合研究表明,塞勒肯特岩体形成于俯冲背景下的大陆边缘弧环境,可能是俯冲板片脱水交代地幔楔产生的玄武质岩浆上涌,导致新生下地壳发生部分熔融。二长花岗岩及石英闪长岩均来源于新生下地壳的部分熔融;花岗闪长岩主要来源于新生下地壳的部分熔融,并有少量幔源物质的加入,花岗闪长岩中的暗色微粒包体可能是幔源物质与新生下地壳部分熔融的岩浆未发生完全混合,最终冷凝结晶的产物。谢米斯台地区与俯冲相关的中酸性岩浆活动至少从晚奥陶世一直延续至晚泥盆世。

     

Geochemistry,geochronology and Hf isotope of granitoids in the northern Alxa region:Implications for the Late Paleozoic tectonic evolution of the Central Asian Orogenic Belt

The tectonic setting of the northern Alxa region during the Late Paleozoic is highly controversial.The key to resolve this controversy is to recognize the Late Paleozoic magmatic processes in the northern Alxa.In this paper,we present new zircon U-Pb ages,Hf-isotopic compositions and whole-rock geochemical data of four granitoids along the Zhusileng-Hangwula Tectonic Belt in the northern Alxa region that could provide critical information about the tectonic evolution of this region.The zircon U-Pb data could be grouped as two phases:Late Devonian granite and diorite(ca.373-360 Ma),and Late Carboniferous granodiorite(ca.318 Ma).The Late Devonian granites and diorites are metaluminous to slightly peraluminous,with A/CNK and A/NK ratios of 0.90-1.11 and0.95-2.19,respectively.The Late Devonian diorites are characterized by high MgO,Cr and Ni contents and MgO#values,together with variable ε_(Hf)(t) values from-1.0 to+1.3 and old T_(DM2) ages varied from 1283 Ma to 1426 Ma,indicating the primary magma was potentially derived from magma mixing of depleted mantle with Mesoproterozoic continental crust.Even though the Late Devonian granites yielded most positive and minor negative e_(Hf)(t) values between-1.1 to+5.7(three grains are negative) with two-stage model ages(T_(DM2)) of 1003-1438 Ma,they display low MgO,Cr and Ni contents and MgO#values,suggesting that they were mainly derived from juvenile crustal materials,mixed with a small amount of ancient crust.The Late Carboniferous granitoids are metaluminous and medium-K calc-alkaline series,with A/CNK and A/NK ratios ranging from 0.88 to 0.95 and1.75 to 1.90,respectively.These rocks were potentially derived from juvenile crustal materials mixed with depleted mantle,as evidenced by their high ε_(Hf)(t) values(+11.6 to+14.1) and young TDM2 ages(427 Ma to 586 Ma),as well as high Mg#values,and MgO,Ni and Cr contents.Our data,along with available sedimentary evidence and previous researches,indicate that the Late Devonian and Late Carboniferous rocks are arc-related granitoids under the subduction setting.The identification of arc-related granitoids in the northern Alxa region not only reveals the Late Paleozoic magmatic process in response to the subduction of Paleo Asian Ocean,but also provide significant constrains to the tectonic evolution of the Central Asian Orogenic Belt.     

内蒙古北山造山带北带晚古生代地层时空格架的建立

内蒙古北山造山带是研究中亚造山带时空演化的重要地区,也是天山造山带与兴蒙造山带对接的关键地区。长期以来,由于工作条件的制约,该地区研究程度相对薄弱,晚古生代地层格架混乱且矛盾。根据近年来在该地区完成的1：5万区调成果,通过晚古生代地层的区域分布、物质组成、时代归属等方面的系统梳理,并结合构造-岩浆配置和造山带结构的认识,重新建立了内蒙古北山地区黑鹰山地层分区晚古生代地层格架。重新划分和厘定出志留纪—泥盆纪雀儿山群红尖山组、碎石山组、清河沟组和圆锥山组,石炭纪绿条山组、白山组及二叠纪双堡塘组、金塔组,明确了雀儿山群为雀儿山陆缘弧环境形成的火山-沉积组合,包括晚志留世—中泥盆世红尖山组岛弧火山岩和碎石山组-清河沟组弧内盆地碎屑岩组合,以及晚泥盆世圆锥山组岛弧火山岩组合;石炭纪绿条山组、白山组形成于沟-弧-盆体系中,以红石山-百合山结合带为界南北具有明显的差异性：绿条山组北区为低成熟度的弧间盆地北缘滨浅海碎屑岩沉积,南区为成熟度较高的弧间盆地南缘滨浅海-半深海碎屑岩沉积;白山组北区为晚石炭世继承岛弧火山组合,南区为石炭纪陆缘弧火山组合。双堡塘组和金塔组为后碰撞－碰撞后的裂谷盆地沉积,具有下部碎屑岩、上部火山岩的二元结构,化石组合指示其时代为早—中二叠世。     

Petrogenesis and tectonic implications of early Paleozoic granitoids in East Kunlun belt: Evidences from geochronology,geochemistry and isotopes

The East Kunlun Orogenic Belt(EKOB) provides an important link to reconstruct the evolution of the Proto-Tethys and Paleo-Tethys realm. The EKOB is marked by widespread Early Paleozoic magmatism.Here we report the petrology, bulk geochemistry, zircon Ue Pb dating and, Lue Hf and SreN d isotopic data of the Early Paleozoic granitic rocks in Zhiyu area of the southern EKOB. Based on the zircon U-Pb dating, these granitoids, consisting of diorite, granodiorite and monzogranite, were formed during 450 -430 Ma the Late Ordovician to Middle Silurian. The diorite and granodiorite are high Sr/Y ratio as adakitic affinities, and the monzogranite belongs to highly fractionated I-type. Their(~(87)Sr/~(86)Sr)ivalues range from 0.7059 to 0.7085, εNd(t) values from -1.6 to -6.0 and the zircon εHf(t) values show large variations from +9.1 to -8.6 with Hf model ages(T_(DM2)) about 848 Ma and 1970 Ma. The large variations of whole-rock Nd and zircon Hf isotopes demonstrate strong isotopic heterogeneity of the source regions which probably resulted from multi-phase underplating of mantle-derived magmas. Geochemical and isotopic studies proved that the diorite and granodiorite had been derived from partial melting of heterogeneous crustal source with variable contributions from ancient continental crust and juvenile components, and the monzogranites were representing fractional crystallization and crustal contamination for arc magma. The Early Paleozoic adakitic rocks and high-K calc-alkaline granitoids in the southern EKOB were likely emplaced in a continental marginal arc setting possibly linked to the southwards subduction of the Paleo Kunlun Ocean and the magma generation is linked to partial melting of thickened continental crust induced by underplating of mantle-derived magmas.     

Middle Paleozoic convergent orogenic belts in western Inner Mongolia (China): framework,kinematics, geochronology and implications for tectonic evolution of the Central Asian Orogenic Belt

Based mainly on field geological observation and geochronologic data, six tectonic units have been recognized in western Inner Mongolia (China), including, from south to north: North China Craton (NCC), Southern Orogenic Belt (SOB), Hunshandake Block (HB), Northern Orogenic Belt (NOB), South Mongolia microcontinent (SMM), and Southern margin of Ergun Block (SME), suggesting that the tectonic framework of the CAOB in western Inner Mongolia is characterized by an accretion of different blocks and orogenic belts. The SOB includes, from north to south, fold belt, mélange, arc-pluton belt, and retroarc foreland basin, representing a southern subduction–collision system between the NCC and HB blocks during 500–440 Ma. The NOB consists also of four units: arc-pluton belt, mélange, foreland molasse basin, and fold belt, from north to south, representing a northern subduction–collision system between the HB and SMM blocks during 500–380 Ma. From the early Paleozoic, the Paleo-Asian oceanic domains subducted to the north and the south, resulting in the forming of the SOB and the NOB in 410 Ma and 380 Ma, respectively. This convergent orogenic system, therefore, constrained the consumption process of the Paleo-Asian Ocean in western Inner Mongolia. A double subduction–collision accretionary process is the dominant geodynamic feature for the eastern part of the CAOB during the early to middle Paleozoic.     

Paleozoic tectonic evolution of the eastern Central Asian Orogenic Belt in NE China

The eastern Central Asian Orogenic Belt (CAOB) in NE China is a key area for investigating continental growth. However, the complexity of its Paleozoic geological history has meant that the tectonic development of this belt is not fully understood. NE China is composed of the Erguna and Jiamusi blocks in the northern and eastern parts and the Xing’an and Songliao-Xilinhot accretionary terranes in the central and southern parts. The Erguna and Jiamusi blocks have Precambrian basements with Siberia and Gondwana affinities, respectively. In contrast, the Xing ’an and Songliao-Xilinhot accretionary terranes were formed via subduction and collision processes. These blocks and terranes were separated by the Xinlin-Xiguitu, Heilongjiang, Nenjiang, and Solonker oceans from north to south, and these oceans closed during the Cambrian (ca. 500 Ma), Late Silurian (ca. 420 Ma), early Late Carboniferous (ca. 320 Ma), and Late Permian to Middle Triassic (260 –240 Ma), respectively, forming the Xinlin-Xiguitu, Mudanjiang-Yilan, Hegenshan-Heihe, Solonker-Linxi, and Changchun-Yanji suture zones. Two oceanic tectonic cycles took place in the eastern Paleo-Asian Ocean (PAO), namely, the Early Paleozoic cycle involving the Xinlin-Xiguitu and Heilongjiang oceans and the late Paleozoic cycle involving the Nenjiang-Solonker oceans. The Paleozoic tectonic pattern of the eastern CAOB generally shows structural features that trend east-west. The timing of accretion and collision events of the eastern CAOB during the Paleozoic youngs progressively from north to south. The branch ocean basins of the eastern PAO closed from west to east in a scissor-like manner. A bi-directional subduction regime dominated during the narrowing and closure process of the eastern PAO, which led to “soft collision” of tectonic units on each side, forming huge accretionary orogenic belts in central Asia.©2022 China Geology Editorial Office.     

新疆博格达造山带晚古生代构造-岩浆演化过程:火山岩组合及地球化学证据

最近,笔者在博格达山东段北部的西地-伊齐-小红柳峡一带的地质调查中发现,该区发育有大量晚石炭世柳树沟组双峰式火山岩及早二叠世卡拉岗组酸性火山岩建造,但双峰式火山岩性质及成因有别于其南侧七角井早石炭世双峰式火山岩。玄武岩富钠贫钾(K_2O=0.18%～0.45%,Na_2O=2.24%～3.63%),属拉斑系列;TiO_2=1.6%～1.7%,略高于MORB,较高的Al(Al_2O_3=16.2%～16.7%)、高Mg(MgO=8.12%～9.54%,Mg~#=61～64),以及低K_2O/TiO_2和K_2O/P_2O_5比值(分别为0.1～0.27、0.63～1.68),反映了在岩浆演化过程中分离结晶作用不明显;Rb/Sr比值0.01～0.02,Zr/Nb=21.6～39.7,Zr/Y=5.38～7.47,以及不相容元素Ba、Zr、Hf相对略富集、Nb-Ta和Th相对亏损,显示岩石具有板内玄武岩的特点;稀土元素球粒陨石标准化配分图上整体接近于平坦型,(La/Yb)_N=1.8～1.9,Eu无异常至轻微正异常(δEu=1.07～1.12),正ε_(Nd)(t)值(+5.63～+5.89),(~(143)Nd/~(144)Nd)_I=0.512927～0.512944,Th/Yb0.2,Ta/Yb=0.1,表明玄武岩浆源于亏损软流圈地幔,且在演化过程中不曾发生过斜长石的分离结晶作用,并暗示当时的大陆地壳可能由于拉张而变得较薄,玄武岩浆形成后快速上升至地表喷发。双峰式火山岩中的流纹岩Rb-Sr等时线年龄为296±2Ma(1σ),具高Si(SiO_2=76%～80%),富钾贫钠(K_2O=5.1%～5.7%,Na_2O=0.94%～2.03%);低Al(Al_2O_3=7.9%～10.4%);低Ti、Ca和P含量,属高钾钙碱性系列;微量元素Rb、Th、Zr、Hf、K相对富集,Ba、Sr、P、Ti、Nb、Ta为显著亏损;轻稀土元素适度富集且轻、重稀土分馏程度低,(La/Yb)_N=5.1～7.1,(La/Sm)_N=2、(Gd/Yb)_N=1.6～2.2,以及强烈的负Eu异常(δEu=0.17～0.2),(~(87)Sr/~(86)Sr)_I=0.7051～0.7052,δ~(18)O=11.6‰,指示岩石源于地壳物质的部分熔融,源区存在有斜长石残留,形成于大陆裂谷环境。早二叠世末流纹岩(Rb-sr等时线年龄为278±2Ma)具高Si(SiO_2=74%),富钾贫钠(K_2O/Na_2O2),低Al(Al_2O_3=11.0%)以及较低的Ti和P含量的特征,岩石为高钾钙碱性系列;微量元素PM标准化图解上表现为Rb、Ba、Th、U、K、La、Ce不相容元素相对富集,高场强元素Nb、Ta、P、Ti以及Sr为明显的负异常;轻稀土轻度富集,(La/Yb)_N=5～6,(La/Sm)_N=3,(Gd/Yb)_N=1.3～1.4,以及强烈的负Eu异常(δEu=0.31～0.39),(~(87)Sr/~(86)Sr)_I为0.7069,δ~(18)O=11.97‰,指示源于地壳物质部分熔融的产物,形成于伸展垮塌的构造环境。综合研究结果表明,博格达山前身裂谷岩浆作用始于早石炭世,结束于晚石炭世末期,早二叠世末进入后造山伸展的演化阶段。     

中亚造山带东段晚古生代伸展构造环境的证据:内蒙古双井地区哲斯组沉积学及年代学研究

本研究从沉积学与年代学角度切入,聚焦于中亚造山带东段晚古生代末期的演化历史。以内蒙古双井地区中二叠统哲斯组陆源粗碎屑沉积岩为研究对象,利用沉积剖面实测、岩相古地理分析及锆石年代学测试等方法,厘定其沉积时限与沉积构造环境,以探讨晚古生代末期中亚造山带东段的构造演化历史。来自哲斯组的2个砂岩样品的最年轻谐和锆石组的谐和年龄分别258. 1±1. 9Ma与261. 6±1. 7Ma,在误差范围内相等,限定了研究区内哲斯组地层的最大沉积年龄约为260Ma,处于中二叠世末的Capitanian期;其主要物质来源包括华北板块太古代至早元古代的结晶基底、华北板块北缘早古生代陆缘弧及中-晚泥盆世西别河组。碎屑锆石年代学与沉积学研究表明,研究区内哲斯组所处沉积盆地类型为伸展构造环境下的裂谷盆地,早期具有短时间内快速充填巨厚层粗碎屑沉积的补偿盆地特征,并伴有活跃的同沉积火山与构造活动,发育近岸水下扇相沉积;晚期逐渐转变为过补偿盆地,发育陆相干旱型冲积扇相沉积;表现出沉积环境由海相变为陆相、碎屑物供应充足且近源低成熟度碎屑增多的盆地充填过程。根据沉积相组合及展布、沉积年龄与碎屑锆石主要结晶年龄间隔等特征,结合研究区存在双峰式火山岩及碱性岩带等证据,哲斯组应代表晚古生代末期的裂谷盆地沉积。上述研究提供了中亚造山带东段二叠纪发生陆内伸展过程的证据。     

北山造山带古生代蛇绿混杂岩研究现状及进展

蛇绿混杂岩带作为造山带最重要的组成部分之一,其形成和就位过程与区域造山事件密切相关.通过系统梳理北山造山带蛇绿混杂岩的研究现状及存在问题,认为在北山造山带内卷入的4条蛇绿混杂岩带中,红柳河-牛圈子-洗肠井蛇绿混杂岩带为形成时代最古老的蛇绿岩,代表了北山洋盆最早于早寒武世即已开启,北山地区发育的洋盆最晚可能于二叠纪闭合,...     

中亚造山带北山南部下泥盆统火山—沉积地层的岩相、时代及古地理意义

中亚造山带北山南部的火山—沉积地层对探讨古地理演化有重要意义。本文通过北山南部双鹰山地块5条剖面实测,将该区火山—沉积地层分为4种相组合19种岩相,并通过LA-ICP-MS锆石U-Pb测年获得酸性火山岩夹层年龄(418.9±1.2Ma、417.8±1.2Ma、417.5±1.4Ma),证明各剖面均形成于早泥盆世洛赫科夫期(Lochkovian)。双鹰山地块南缘的下泥盆统由共生火山岩相(coherent facies)和原生火山岩相组合(primary volcanic facies)构成,是熔岩溢流、爆发坠落、火山碎屑流和岩浆侵出的产物,代表陆上隆升的普林尼型(Plinian)古火山作用。双鹰山地块中部墩墩山盆地充填序列自下而上由原生火山岩相组合、含火山碎屑沉积岩相组合(volcanogenic sedimentary facies)和正常沉积的河流相组合构成,代表火山活动逐步减弱和水体深度逐步加深的沉积过程。墩墩山盆地火山—沉积层序指示伸展构造背景,与同造山期磨拉石建造有显著区别。     

Geochemistry,geochronology and Hf isotope of granitoids in the Chinese Altai: Implications for Paleozoic tectonic evolution of the Central Asian Orogenic Belt

The Chinese Altai in northwestern Xinjiang has numerous outcrops of granitoids which provide critical information on accretionary orogenic processes and crustal growth of the Central Asian Orogenic Belt.Zircon U-Pb ages, Hf-isotopic compositions and whole-rock geochemistry of monzogranite and granodiorites in the Qinghe County are employed to elucidate Paleozoic tectonics of the Chinese Altai. Granodiorites have crystallization ages of 424.6 ± 3.1 Ma(MSWD = 0.23) and 404.0 ± 3.4 Ma(MSWD = 0.18);monzogranite was emplaced in the early Permian with a crystallization age of 293.7 ± 4.6 Ma(MSWD = 1.06). Both granodiorites and monzogranite are I-type granites with A/CNK ratios of 0.92 -0.97 and 1.03 -1.06, respectively. They also show similar geochemical features of high HREE and Y contents, low Sr contents and Sr/Y ratios, as well as enrichment of Cs, Rb, Th and U, and depletion of Nb, Ta, P and Ti.These geochemical features indicate that the monzogranite and granodiorites were formed in an arc setting related to subduction. The gneissic monzogranites display high SiO_2 and K_2 O contents, and belong to the high-K calc-alkaline series. In the chondrite normalized REE distribution pattern, the monzogranite samples exhibit enrichment of LREE with strong negative Eu anomalies(σE u =0.44 -0.53), zircon εHf(t) values from +7.24 to +12.63 and two-stage Hf model ages of 463 -740 Ma. This suggests that the monzogranite was generated from the mixing of pelitic and mantle material. The granodiorite samples are calc-alkaline granites with lower contents of Si O_2 and Na_2 O + K_2 O, higher contents of TiO_2, Fe_2O_3~t, MgO and CaO compared to the monzogranite samples. They also show enrichment of LREE and moderate negative Eu anomalies(σE u= 0.54 =0.81), as well as slightly higher differentiation of LREE than that of HREE. The425 Ma granodiorite has zircon εHf(t) values from -0.51 to +1.98 and two-stage Hf model ages of 1133 -1240 Ma, whereas the 404 Ma granodiorite displays those of +2.52 to +7.50 and 816 -1071 Ma.Geochemistry and zircon Hf isotopic compositions indicate that granodiorites were formed by partial melting of juvenile lower crust. Together with regional geology and previous data, the geochemical and geochronological data of the monzogranite and granodiorites from this study suggest long-lived subduction and accretion along the Altai Orogen during ca. 425 -294 Ma.     

兴蒙造山带中部晚古生代构造格局:来自晚泥盆-早石炭世色日巴彦敖包组碎屑锆石和火山岩岩浆锆石年代学的制约

内蒙古苏尼特左旗地区位于兴蒙造山带中段,是研究古生代俯冲-增生造山作用和地壳生长的关键地区.在苏尼特左旗南部,晚泥盆-早石炭世色日巴彦敖包组角度不整合在早古生代增生楔之上,已有研究对于其沉积环境、盆地属性及区域构造意义一直存在较大分歧.本文对色日巴彦敖包组敖木根呼都格剖面和阿拉塔特剖面碎屑岩和火山岩夹层开展锆石U-Pb...     

Harzburgite found in the Hegenshan ophiolite,southeastern Central Asian Orogenic Belt: Petrogenesis and geological implications

This paper reports detailed studies on harzburgite and serpentinite in the Hegenshan ophiolitic mélange. Harzburgite consists mainly of olivine and orthopyroxene with trace amounts of clinopyroxene and chromian spinel. Clinopyroxene occurs as isolated crystals or in the intergrowth of chromian spinel–clinopyroxene–orthopyroxene. Harzburgite is moderately to highly depleted, displaying high Fo contents in olivine (90.8–92.2), moderate Al₂O₃ contents in orthopyroxene (1.59–2.79 wt%), low heavy REE abundances in clinopyroxene, and moderate Cr# values of spinel (0.50–0.62). The modal proportions of olivine and orthopyroxene pseudomorph grains imply that the parent of the Hegenshan serpentinite should be harzburgite. Whole-rock compositions of the harzburgite and serpentinite samples are characterized by depletions in Al₂O₃ and CaO and enrichments in light REE, Sr, and U. Geochemical modeling suggests that the Hegenshan harzburgite represents residues after 17–18% partial melting of the primitive mantle. The melt in equilibrium with clinopyroxene is more depleted than typical forearc basalt and boninite. Various pyroxene thermobarometers yield equilibrated temperatures of 945–1067 °C and pressures of 4.8–8.0 kbar for the Hegenshan harzburgite. The oxygen barometer yields results of +0.4 to +1.7 log units above the fayalite–magnetite–quartz buffer for the Hegenshan harzburgite. These petrological and geochemical characteristics, as well as the estimated P–T–fO₂ conditions support a back-arc setting for the Hegenshan ophiolitic mélange.     

兴蒙造山带西段乌珠新乌苏花岗岩岩石成因和构造背景:地球化学、U-Pb年代学和Sr-Nd-Hf同位素约束

兴蒙造山带的构造演化及古亚洲洋的闭合时限尚存在较大争议,沿缝合带出露的花岗岩的侵位时代和成因对于约束古亚洲洋的演化具有重要意义。乌珠新乌苏花岗岩位于兴蒙造山带西段的索伦缝合带内,其Na_2O+K_2O含量为7.14%～9.36%,Al_2O_3含量为13.18%～13.49%,K_2O/Na_2O为1.10～1.52,属于高钾钙碱性系列岩石。稀土总量富集(174.8×10~(-6)～213.7×10~(-6)),相对富集轻稀土元素,轻重稀土分异中等[(La/Yb)_N=3.68～5.41],Eu负异常明显。乌珠新乌苏花岗岩富集Rb、Th、Hf,亏损Nb、Sr、P、Ti等元素,符合典型高分异A型花岗岩的低Sr、高Yb、Eu负异常的特点,显示后碰撞花岗岩的特征,形成于造山后地壳减薄阶段。乌珠新乌苏花岗岩锆石LA-ICP-MS U-Pb定年显示其~(206)Pb/~(238)U加权平均年龄为279±2.7Ma(MSWD=1.6)和276±1.9Ma(MSWD=0.69)。正的全岩ε_(Nd)(t)值(2.1～2.7)和锆石ε_(Hf)(t)值(7.7～10.2)以及相对年轻的亏损地幔模式年龄(Nd的t_(DM2)为826～874Ma;Hf的t_(DM2)为655～931Ma),表明乌珠新乌苏花岗岩母岩浆可能来源于新元古代新生下地壳的部分熔融。综合花岗岩的地质地球化学、年代学及同位素特征,认为兴蒙造山带西段的乌珠新乌苏地区在早二叠世处于后碰撞的伸展环境。