哈密坳陷火山岩SHRIMP 锆石U-Pb年龄分析

哈密坳陷因未有钻井揭示,石炭系是否发育,存在较大的不确定性。本文根据哈密坳陷内新钻探井红1井情况,进行火山岩SHRIMP锆石U-Pb测年,确定红1井3 536~3 549 m安山岩的锆石年龄为(298.2±3.5)Ma,处于晚石炭世格舍尔阶与早二叠世阿瑟尔阶的界限附近。综合岩石岩性组合、常规测井及成像测井等资料,将红1井段3 497~4 352 m确定为石炭系,对哈密坳陷石炭系的油气勘探具有重大的意义。     

内蒙古中部宝力高庙组长英质火山岩U-Pb-Hf同位素特征及其地质意义

锆石SHRIMP U-Pb定年结果显示: 内蒙古中部白音乌拉地区原宝力高庙组的流纹岩形成时代为300.0±2.9 Ma, 属晚石炭世; 青格勒宝拉格地区原宝力高庙组的凝灰岩结晶年龄为159.6±1.4 Ma, 并获得 3颗捕获锆石的年龄分别为291.8±3.4, 304.0±3.3和734.7±9.2 Ma, 应属于晚侏罗世满克头鄂博组。锆石LA-MC-ICP-MS Hf同位素分析显示: 流纹岩锆石ε_Hf(t) 值为+10.5~+12.9, T_DM^C值为493~645 Ma; 凝灰岩岩浆锆石ε_Hf(t)值为+10.1~+13.1, T_DM^C值为369~563 Ma。研究结果表明, 流纹岩源于晚古生代新生地壳的重熔并混入少量老地壳物质, 凝灰岩源于晚古生代地壳的熔融。Hf同位素特征显示晚古生代流纹岩和中生代凝灰岩源于相似的源区, 揭示了晚古生代的一次重要的增生事件, 并且在约160 Ma时期发生过地壳的再造。结合前人的研究成果表明, 兴蒙造山带在约300 Ma时处于古亚洲洋演化过程中岛弧向碰撞后伸展环境的转换时期, 在约160 Ma受到蒙古?鄂霍茨克构造域的影响。     

Ductile deformation within Upper Himalaya Crystalline Sequence and geological implications, in Nyalam area, Southern Tibet

The South Tibet Detachment System(STDS) is a flat normal fault that separates the Upper Himalaya Crystalline Sequence(UHCS) below from the Tethyan Sedimentary Sequence(TSS) above.Timing of deformations related to the STDS is critical to understand the mechanism and evolution of the Himalaya collision zone.The Nyalam detachment(ND)(～86°E) locates in the middle portion of STDS(81°-89°E).Dating of deformed leucocratic dykes that are most probably syntectonic at different depth beneath the ND,allow us to constrain the timing of deformation.(1) Dyke T11N37 located ～3500 m structurally below the ND emplaced at 27.4± 0.2 Ma;(2) Dyke T11N32 located ～1400 m structurally below the ND emplaced at 22.0±0.3 Ma;(3) T11N25 located within the top to the north STD shear zone,～150 m structurally below the ND,emplaced at 17.1±0.2 Ma.Combining ND footwall cooling history and T11N25 deformation temperature,we indicate a probable onset of top to the north deformation at ～16 Ma at this location.These results show an upward younging of the probable timing of onset of the deformation at different structural distance below the ND.We then propose a new model for deformation migration below the ND with deformation starting by pure shear deformation at depth prior to ～27.5 Ma that migrates upward at a rate of ～ 0.3 mm/a until ～18 Ma when deformation switches to top to the north shearing in the South Tibet Detachment shear zone(STDsz).As deformation on the ND stops at 14-13 Ma this would imply that significant top to the North motion would be limited to less than 5 Ma and would jeopardize the importance of lower channel flow.     

川东L2井三叠系飞仙关组碳酸盐样品的锶同位素年龄标定

测试了四川盆地东部L2井三叠系飞仙关组碳酸盐样品87Sr/86Sr比值,以及相应的Mg,Sr和Mn的含量.在对样品进行成岩蚀变性评价的基础上,根据锶同位素地层学的原理,尝试对这些样品进行了年龄标定.4个样品分别位于L2井深度为3 297.6 m,3 290.0 m,3 268.8 m和3 201.6 m处;其87Sr/86Sr比值分别为0.707 330,0.707 340,0.707 348和0.707 383;年龄的标定结果分别为245.3 Ma,245.2 Ma,245.1 Ma和245.0 Ma.除深度3 201.6 m的样品因成岩蚀变,其年龄值的可靠程度较低以外,其余3个样品的年龄值都具有较高的可靠程度.     

赣南苗云岩体U-Pb年代学、地球化学特征及其成因

位于武夷—云开造山带中部的广东始兴—江西全南一带,是研究华南早古生代构造属性的重要区域;以盆地内苗云岩体二云母花岗岩为研究对象,通过系统的年代学和地球化学研究,分析其成因,对早古生代赣南地区构造属性特征进行补充。结果表明:苗云岩体二云母花岗岩具有高硅、高碱、富铝等特征,且相对富钾贫钠,属于高钾钙碱性系列过铝质S型花岗岩类;岩石富集大离子亲石元素Rb、K和Th,亏损Ba、Nb、Ta、Sr等元素,具有岛弧岩浆岩的特征;稀土元素总量较低,富集轻稀土,轻重稀土分馏明显,具有中等Eu负异常,暗示其主要来自于地壳熔融;构造判别图显示其形成于同碰撞的挤压环境,表明苗云岩体可能形成于陆陆碰撞所导致的陆壳加厚环境。SHRIMP锆石U-Pb年龄分析结果显示,其加权平均年龄为(442. 7±3. 2) Ma,为晚奥陶末期—早志留初期岩浆产物;结合区域地质特征,表明赣南地区在晚奥陶末期—早志留初期已由俯冲环境转换到陆陆碰撞环境。     

Garnet Sm-Nd and U-Pb systems: A case study of a granulite from the European Variscan belt

This study presents zircon and garnet ages of a mafic granulite from the high-grade Variscan basement of the Black Forest, Germany and discuss isotope closure temperature of garnet Sm-Nd and U-Pb systems. Zircon grains yield 207Pb/206Pb ages between ～340 and ～414 Ma by the U-Pb and evaporation methods. In contract, garnet dating gives Sm-Nd and Pb-Pb isochron ages of (398±3) Ma and (411±14) Ma, respectively, which are older than most of zircon ages. These data imply that most of zircons lost radiogenic Pb, probably due to metamictization or recrystallisation during the granulite-facies metamorphism (～800℃) at ～340 Ma. Garnet Sm-Nd and U-Pb systems preserve chronological information of pro-grade metamorphism, probably profiting from a fluid-absence metamorphic environment. These results demonstrate that garnet mineral can be a better candidate than zircon mineral to date high-grade metamorphism by the U-Pb and Sm-Nd methods in some cases.     

K-Ar dating of late Mesozoic volcanism and geochemistry of volcanic gravels in the North Huaiyang Belt, Dabie orogen: Constraints on the stratigraphic framework and exhumation of the northern Dabie orthogneiss complex

Two eruption episodes are identified through systematic field investigations and K-Ar dating of the lateMesozoic volcanic rocks in the North Huaiyang belt (NHB),Dabie orogenic belt, of which the earlier volcanic suitetermed Maotanchang Fm. (Fm.) occurring at Jinzhai,Xianhualing and Maotanchang, etc., was erupted from 149Ma to 138 Ma. The other named Xiaotian Fm. mainly dis-tributed at Xiaotian, Shucheng, etc., was formed between132 Ma and 116 Ma. During the eruption gap of the two vol-canic suites deposited a volcano-sedimentary conglomeratelayer, which are composed of the multi-compositional gravels, including the North Dabie orthogneiss complex (NDOC),volcanic gravels, etc. These volcanic gravels in the con-glomerate layer show identical geochemical and isotopic compositions (87Sr/86Sr(t) =0.7084-0.7092, (Nd (t) = 21.8-24.4) to the Maotanchang Fm. volcanic rocks (87Sr/86Sr = 0.7086-0.7102, (Nd = 19.2-24.4), but significantly distinct from those of Xiaotian Fm. (87Sr/86Sr = 0.7076-0.7084, (Nd = 17.2 - 19.2). K-Ar dating results of its underlying andoverlying volcanic sequences indicate that the conglomerate layers were deposite d at ~135 Ma. This suggests that the NDOC was rapidly exhumed to the surface dur ing or shortly before ~135 Ma and became the important provenance of the late Me sozoic volcano-sedimentary basins in the NHB. In combination with the regional v olcano-sedimentary correlation, we divided the Mesozoic stratigraphic sequence i n the NHB from base to top into Fanghushan Fm. (>160 Ma), Yuantongshan Fm. (/mid dle- lower segment of Sanjianpu Fm.) (160-149 Ma), Maotanchang Fm. (/Zhougongsh an Fm./upper segment of Sanjianpu Fm./Fenghuangtai Fm.) (149-135 Ma) and Xiaoti an Fm. (/Baidafan Fm./Heshidu Fm.) (135-116 Ma).     

Sm-Nd dating and Nd-Sr isotopic characteristics of the Shimian ophiolite suite, Sichuan Province

By measuring the Sm-Nd and Rb-Sr isotopic compositions of harzburgite and gabbro from Shimian ophiolite suite, we got the whole rock Sm-Nd isochron age of (938±30) Ma (2?), and the ??Nd of 7.6±0.8 (2?), which shows that the ophiolite was formed at the Early Neoproterozoic. The obvious change (0.70209-0.70708) of ISr values of the ophiolite is caused by the meteoric hydrothermal alteration. The high ? Nd values indicate that the primitive magma was derived from the intense depleted mantle reservoir. It is suggested that this area was in a back-arc basin environment during the Early Neoproterozic.