渤海盆地演化的年代学研究

作为亚洲大陆边缘海最年轻的部分,渤海陆架是研究亚洲大陆边缘海早期演化、海陆相互作用、区域环境变化等问题的重要窗口。本文回顾了过去渤海陆架演化研究的重要成果,重点探讨了不同年代学方法（包括14C测年、光释光测年、磁性地层学以及天文调谐等）在限定重要地质、环境事件发生时代的优势与挑战。在此基础上,对于晚第四纪3次主要海侵事件发生的年龄,本文认为第二海侵层始于MIS 5期,在MIS 3期尚有残余沉积保留的可能,而第三海侵层始于MIS 7期,可能持续至MIS 6期。对于海侵发生之前的区域环境过程,依据环渤海岩石地层和年代地层对比,本文认为上新世以来的渤海陆架演化表现为三阶段模式:1）约3.7Ma之前的盆地快速沉降;2）3.7~0.3Ma的“渤海古湖”发育与庙岛古隆起相对地势较高;3）约0.3Ma以来“渤海古湖”消失与渤海陆架形成。     

粤东北长田地区南雄群沉积岩锆石年代学及其地质意义

粤东北长田盆地是广东省重要的能源盆地之一. 为详细了解该区地质结构、沉积物质特征、示踪砂岩物源等信息,在野外地质调查基础上,利用显微鉴定、电子探针分析（EPMA）和激光剥蚀电感耦合等离子体质谱仪（LA-ICP-MS）等方法,对长田盆地南雄群典型砂岩开展系统研究. 结果表明：研究区白垩系上统南雄群上亚群主要为（含炭质）岩屑石英砂岩,其次为钙质砂岩、（泥质）粉砂岩、细砂岩、砾岩及杂砂岩,普遍发育明显的次生变化和金属矿化现象. 南雄群碎屑锆石研究显示,样品中大部分碎屑锆石具有较好的振荡环带且Th/U值常大于0.4,指示锆石主要为岩浆结晶成因,有少量复杂成因变质锆石. U-Pb谐和年龄可大致分为2506~1666 Ma、1815~941 Ma、510~434 Ma、308~234 Ma、172~99 Ma五组,可与粤东北已知大地构造运动事件相对应,表明研究区的构造-岩浆活动主要受中国东部岩浆构造活动控制,并具有阶段性幕式发展演化的特点. 通过对南雄群碎屑岩岩相学、锆石U-Pb年龄、稀土元素特征的系统分析,并与可能物源区进行对比研究,认为南雄群碎屑物质主要来自长田盆地西缘的中生代岩浆岩侵入体.     

云南清水河早古生代过铝质花岗岩的发现及其对保山地块岩石圈地幔拆沉的限定

保山地块早古生代过铝质花岗岩的岩石成因尚存在争议, 为进一步探讨岩石成因, 对云南清水河花岗岩开展了锆石SHRIMP U-Pb定年及岩石地球化学研究。结果表明, 清水河花岗岩主要由二云母花岗岩、黑云母花岗岩组成。二云母花岗岩岩浆结晶年龄为473~462 Ma, 黑云母花岗岩岩浆结晶年龄为479 Ma, 侵位时代都为早古生代。含较高的SiO2(67.48%~74.03%)、(Na2O+K2O)-Ca2O(4.47%~6.46%), A/CNK＞1.1, 为钙碱性强过铝质花岗岩; 均富集Pb、大离子亲石元素Rb、K等, 相对亏损高场强元素, 显示明显的负Eu异常(δEu=0.31~0.54)。Sr-Nd-Pb同位素及地球化学研究表明, 清水河花岗岩均显示S-型花岗岩特征, 其源岩物质为古老陆壳富粘土的泥质岩, 受到部分幔源物质的混入。清水河花岗岩与双脉地花岗岩存在可对比性, 可能由460~479 Ma岩石圈地幔拆沉作用形成。     

新疆富蕴县蒙库铁矿床赋矿浅粒岩锆石U-Pb年代学研究

蒙库铁矿床位于阿尔泰山南缘成矿带的麦兹火山-沉积盆地内,矿床主矿体成因类型为与火山作用有关的热水喷流沉积矿床.矿体主要赋存于泥盆系康布铁堡组地层中,主要岩性有浅粒岩、变粒岩及斜长角闪岩,原岩恢复显示为海相火山岩,主要为细碧岩-角斑岩-石英角斑岩,次为流纹岩-凝灰岩,沉积岩为钙质凝灰质砂岩夹结晶灰岩.通过对蒙库铁矿床浅粒岩锆石LA-ICP-MS UPb定年,确定了矿床的形成时代及康布铁堡组地层的形成时代.锆石中Th含量分别为29.5×10-6~82.7×10-6和32.1×10-6~109.3×10-6,U含量分别为35.7×10-6~132.1×10-6和47.6×10-6~140.6×10-6,Th/U比值为0.41~1.19和0.51~0.85,为典型岩浆锆石特征.蒙库铁矿浅粒岩的U-Pb年龄分别为397.5±2.5 Ma和389.0±4.7 Ma.结合区域年代学资料,确定康布铁堡组形成于早泥盆世,康布铁堡组下亚组上限为389±4.7 Ma,而蒙库铁矿床主矿体形成时代为397.5±2.5 Ma.     

俄罗斯远东地区晚中生代花岗岩类的时空分布及其地质意义

在俄罗斯远东地区晚中生代花岗岩类年龄和相关地球化学数据的基础上,初步建立了该区晚中生代花岗岩类的年代学格架：大致以145Ma为界,分为侏罗纪(178~151Ma)和早白垩世(142~122Ma)2期。侏罗纪的花岗岩类主要为花岗岩－花岗闪长岩－石英二长岩组合,总体上为准铝质—强过铝质高钾钙碱性系列;早白垩世的花岗岩类主要为花岗岩－石英闪长岩－石英二长岩组合,主要为过铝质钙碱性—高钾钙碱性系列—钾玄岩系列。2期花岗岩稀土元素配分曲线均呈右倾型,重稀土元素曲线较平坦,都富集大离子亲石元素(如U、K)和轻稀土元素。与中国东北地区晚中生代花岗岩类对比,中国东北地区总体以兴安岭为中心,中间为早白垩世的花岗岩类,两侧为侏罗纪花岗岩类对称分布。境内外的侏罗纪花岗岩类构造背景不同,其分布与鄂霍次克洋和太平洋板块的俯冲有关,早白垩世花岗岩类可能形成于鄂霍次克带挤压造山后的伸展垮塌和太平洋板块的俯冲弧后伸展阶段。     

Modification of the Sm–Nd isotopic system in garnet induced by retrogressive fluids

Garnet, as a major constitutive mineral of eclogite, is important for Sm–Nd dating of eclogite due to its high Sm/Nd ratio and its stability during retrogression. However, a comprehensive study of the petrography, mineral chemistry, garnet water content, and Sm–Nd isotopic composition of eclogites from the Bixiling massif, Central Dabie Zone (CDZ), reveals significant modification of the Sm–Nd isotopic system in garnet as a result of retrogression. This problem constitutes a challenge for Sm–Nd dating of the Bixiling eclogites, with the Sm–Nd isochron ages of 218 ± 4 to 210 ± 9 Ma reported in the literature being younger than 226 ± 3 Ma, which is the generally accepted peak metamorphic age of the CDZ. Petrographic analysis reveals heterogeneity in colour within single fractured garnet grains. There are light‐pink garnet (Grt‐P) and red garnet (Grt‐R) types that possess distinct chemical compositions. Compared to Grt‐P, Grt‐R has higher Fe and andradrite contents but lower Al and grossular contents. Grt‐P also has lower water contents (15–35 ppm) than Grt‐R (34–65 ppm), which, together with the spatial association between Grt‐R and fractures, suggests that the colour change is related to fluid alteration. Grt‐P is an ultra‐high‐pressure (UHP) mineral, and Grt‐R is the product of the interaction between Grt‐P and a fluid during retrogression. Moreover, Grt‐R features lower Sm and Nd contents but higher Sm/Nd ratios than Grt‐P. The Sm–Nd isochrons defined by UHP minerals (Grt‐P+Omp+Rt or Grt‐P+Cpx+WR) from three eclogite samples yield consistent ages of 226.0 ± 3.8 Ma, 225.0 ± 3.9 Ma and 226.2 ± 6.9 Ma, which are identical to the peak metamorphic age of 226 ± 3 Ma for the CDZ. The retrogressed garnet (i.e., Grt‐R), omphacite and rutile, together define a pseudoisochron with younger ages of 218.9 ± 5.9 to 202.8 ± 4.8 Ma, which are geologically meaningless. The increase in the Sm/Nd ratio with constant or lower ¹⁴³Nd/¹⁴⁴Nd ratios during the transformation of Grt‐P to Grt‐R was probably the cause of these younger ages.     

The geological evolution of the Gangpur Schist Belt,eastern India: Constraints on the formation of the Greater Indian Landmass in the Proterozoic

The Central Indian Tectonic Zone (CITZ) is a Proterozoic suture along which the Northern and Southern Indian Blocks are inferred to have amalgamated forming the Greater Indian Landmass. In this study, we use the metamorphic and geochronological evolution of the Gangpur Schist Belt (GSB) and neighbouring crustal units to constrain crustal accretion processes associated with the amalgamation of the Northern and Southern Indian Blocks. The GSB sandwiched between the Bonai Granite pluton of the Singhbhum craton and granite gneisses of the Chhotanagpur Gneiss Complex (CGC) links the CITZ and the North Singhbhum Mobile Belt. New zircon age data constrain the emplacement of the Bonai Granite at 3,370 ± 10 Ma, while the magmatic protoliths of the Chhotanagpur gneisses were emplaced at c. 1.65 Ga. The sediments in the southern part of the Gangpur basin were derived from the Singhbhum craton, whereas those in the northern part were derived dominantly from the CGC. Sedimentation is estimated to have taken place between c. 1.65 and c. 1.45 Ga. The Upper Bonai/Darjing Group rocks of the basin underwent major metamorphic episodes at c. 1.56 and c. 1.45 Ga, while the Gangpur Group of rocks were metamorphosed at c. 1.45 and c. 0.97 Ga. Based on thermobarometric studies and zircon–monazite geochronology, we infer that the geological history of the GSB is similar to that of the North Singhbhum Mobile Belt with the Upper Bonai/Darjing and the Gangpur Groups being the westward extensions of the southern and northern domains of the North Singhbhum Mobile Belt respectively. We propose a three‐stage model of crustal accretion across the Singhbhum craton—GSB/North Singhbhum Mobile Belt—CGC contact. The magmatic protoliths of the Chhotanagpur Gneisses were emplaced at c. 1.65 Ga in an arc setting. The earliest accretion event at c. 1.56 Ga involved northward subduction and amalgamation of the Upper Bonai Group with the Singhbhum craton followed by accretion of the Gangpur Group with the Singhbhum craton–Upper Bonai Group composite at c. 1.45 Ga. Finally, continent–continent collision at c. 0.96 Ga led to the accretion of the CGC with the Singhbhum craton–Upper Bonai Group–Gangpur Group crustal units, synchronous with emplacement of pegmatitic granites. The geological events recorded in the GSB and other units of the CITZ only partially overlap with those in the Trans North China Orogen and the Capricorn Orogen of Western Australia, indicating that these suture zones are not correlatable.     

Integrated garnet and zircon–titanite geochronology constrains the evolution of ultra‐high–pressure terranes: An example from the Sulu orogen

Dating ultra‐high–pressure (UHP) metamorphic rocks provides important timing constraints on deep subduction zone processes. Eclogites, deeply subducted rocks now exposed at the surface, undergo a wide range of metamorphic conditions (i.e. deep subduction and exhumation) and their mineralogy can preserve a detailed record of chronologic information of these dynamic processes. Here, we present an approach that integrates multiple radiogenic isotope systems in the same sample to provide a more complete timeline for the subduction–collision–exhumation processes, based on eclogites from the Dabie–Sulu orogenic belt in eastern China, one of the largest UHP terranes on Earth. In this study, we integrate garnet Lu–Hf and Sm–Nd ages with zircon and titanite U–Pb ages for three eclogite samples from the Sulu UHP terrane. We combine this age information with Zr‐in‐rutile temperature estimates, and relate these multiple chronometers to different P–T conditions. Two types of rutile, one present as inclusions in garnet and the other in the matrix, record the temperatures of UHP conditions and a hotter stage, subsequent to the peak pressure (‘hot exhumation') respectively. Garnet Lu–Hf ages (c. 238–235 Ma) record the initial prograde growth of garnet, while coupled Sm–Nd ages (c. 219–213 Ma) reflect cooling following hot exhumation. The maximum duration of UHP conditions is constrained by the age difference of these two systems in garnet (c. 235–220 Ma). Complementary zircon and titanite U–Pb ages of c. 235–230 Ma and c. 216–206 Ma provide further constraints on the timing of prograde metamorphism and the ‘cold exhumation' respectively. We demonstrate that timing of various metamorphic stages can thus be determined by employing complementary chronometers from the same samples. These age results, combined with published data from adjacent areas, show lateral diachroneity in the Dabie–Sulu orogeny. Three sub‐blocks are thus defined by progressively younger garnet ages: western Dabie (243–238 Ma), eastern Dabie–northern Sulu (238–235 Ma) and southern Sulu terranes (225–220 Ma), which possibly correlate to different crustal slices in the recently proposed subduction channel model. These observed lateral chronologic variations in a large UHP terrane can possibly be extended to other suture zones.     

Coupled garnet Lu–Hf and monazite U–Pb geochronology constrain early convergent margin dynamics in the Ross orogen,Antarctica

The Ross orogen of Antarctica is an extensive (>3000 km‐long) belt of deformed and metamorphosed sedimentary rocks and granitoid batholiths, which formed during convergence and subduction of palaeo‐Pacific lithosphere beneath East Gondwana in the Neoproterozoic–early Palaeozoic. Despite its prominent role in Gondwanan convergent tectonics, and a well‐established magmatic record, relatively little is known about the metamorphic rocks in the Ross orogen. A combination of garnet Lu–Hf and monazite U–Pb (measured by laser‐ablation split‐stream ICP‐MS) geochronology reveals a protracted metamorphic history of metapelites and garnet amphibolites from a major segment of the orogen. Additionally, direct dating of a common rock‐forming mineral (garnet) and accessory mineral (monazite) allows us to test assumptions that are commonly used when linking accessory mineral geochronology to rock‐forming mineral reactions. Petrography, mineral zoning, thermobarometry and pseudosection modelling reveal a Barrovian‐style prograde path, reaching temperatures of ~610–680 °C. Despite near‐complete diffusional resetting of garnet major element zoning, the garnet retains strong rare earth element zoning and preserves Lu–Hf dates that range from c. 616–572 Ma. Conversely, monazite in the rocks was extensively recrystallized, with concordant dates that span from c. 610–500 Ma, and retain only vestigial cores. Monazite cores yield dates that overlap with the garnet Lu–Hf dates and typically have low‐Y and heavy rare earth element (HREE) concentrations, corroborating interpretations of low‐Y and low‐HREE monazite domains as records of synchronous garnet growth. However, ratios of REE concentrations in garnet and monazite do not consistently match previously reported partition coefficients for the REE between these two minerals. High‐Y monazite inclusions within pristine, crack‐free garnet yield U–Pb dates significantly younger than the Lu–Hf dates for the same samples, indicating recrystallization of monazite within garnet. The recrystallization of high‐Y and high‐HREE monazite domains over >50 Ma likely records either punctuated thermal pulses or prolonged residence at relatively high temperatures (up to ~610–680 °C) driving monazite recrystallization. One c. 616 Ma garnet Lu–Hf date and several c. 610–600 Ma monazite U–Pb dates are tentatively interpreted as records of the onset of tectonism metamorphism in the Ross orogeny, with a more robust constraint from the other Lu–Hf dates (c. 588–572 Ma) and numerous c. 590–570 Ma monazite U–Pb dates. The data are consistent with a tectonic model that involves shortening and thickening prior to widespread magmatism in the vicinity of the study area. The early tectonic history of the Ross orogen, recorded in metamorphic rocks, was broadly synchronous with Gondwana‐wide collisional Pan‐African orogenies.     

GGR Biennial Critical Review: Analytical Developments Since 2010

Advances in the chemical and isotopic characterisation of geological and environmental materials can often be ascribed to technological improvements in analytical hardware. Equally, the creation of novel methods of data acquisition and interpretation, including access to better reference materials, can also be crucial components enabling important breakthroughs. This biennial review highlights key advances in either instrumentation or data acquisition and treatment, which have appeared since January 2010. This review is based on the assessments by scientists prominent in each of the given analytical fields; it is not intended as an exhaustive summary, but rather provides insight from experts of the most significant advances and trends in their given field of expertise. In contrast to earlier reviews, this presentation has been formulated into a unified work, providing a single source covering a broad spectrum of geoanalytical techniques. Additionally, some themes that were not previously emphasised, in particular thermal ionisation mass spectrometry, accelerator‐based methods and vibrational spectroscopy, are also presented in detail.