Subcrop Gondwana of Bengal basin and their reservoir characteristics

In Bengal basin the subcrop Gondwana sediments occur in N-S trending elongated grabens originated largely by the graben forming tectonisms of the Gondwanaland which overlie the crystalline basement in the sub-surface in an intra-cratonic setup. So far five wells in shelf part of Bengal Basin have penetrated Gondwana sediments, out of which three wells i.e.,G1, G3 and G2 have been drilled up to Precambrian basement. In subcrop Gondwana graben, glacial to glacio-fluvial Talchir Formation of Early Permian age was deposited above the basement. Early Permian Barakar Formation overlies Talchir Formaion. Flood basin model of deposition is postulated for this coal rich unit. In the absence of Barren Measure Formation, coal bearing Raniganj Formation overlies Barakar Formation. The fluvial set up changed over to arid environment during deposition of Panchet/Supra Panchet Formation (undifferentiated). The deposition of Panchet Formation is followed by eruption of doleritic rocks in both subcrop and outcrop Bengal Gondwana whereas lamprophyres are absent in subcrop of Bengal Gondwana. Rifting as well as pull-apart basin model due to transtensional movement is postulated for Gondwana basins. Detailed lithostratigraphic analysis of the core / cutting samples of the Gondwana sediments reveal that the sediments are mainly fine to coarse grained, poorly sorted sandstone. These sediments are characterized as poor reservoir and needs some treatment for permeability enhancement for hydrocarbon production.     

Palynological evidence for Jurassic strata in the Panagarh area, West Bengal, India

The Rajmahal Traps were discovered in the Panagarh area, West Bengal, during the exploration for coal resources. A Gondwana succession was found beneath the traps, consisting of the Early Cretaceous Intratrappean Rajmahal Formation, the Early Triassic Panchet Formation and the Late Permian coal-bearing Raniganj Formation. The present palynological study was aimed at confirming the age of the Panchet Formation. As a result of this study it has been found that Jurassic sediments are also included in the Panchet Formation. The study has revealed that the Panchet Formation, defined on a lithological basis, is a time-transgressive unit extending from the Early Triassic to the Late Jurassic, with a phase of non-deposition between the Middle Triassic and Middle Jurassic.     

Low-temperature thermochronology of the flanks of the southern Upper Rhine Graben

The Upper Rhine Graben (URG) is the most perceptible part of the European Cenozoic Rift System. Uplifted Variscan basement of the Black Forest and the Vosges forms the flanks of the southern part of the graben. Apatite and zircon fission-track (FT) analyses indicate a complex low-temperature thermal history of the basement that was deciphered by inverse modelling of FT parameters. The models were tested against the observed data and independent geological constraints. The zircon FT ages of 28 outcrop samples taken along an E–W trending transect across the Black Forest and the Vosges range from 136 to 312 Ma, the apatite FT ages from 20 to 83 Ma. The frequency distributions of confined track lengths are broad and often bimodal in shape indicating a complex thermal history. Cooling below 120°C in the Early Cretaceous to Palaeogene was followed by a discrete heating episode during the late Eocene and subsequent cooling to surface temperature. The modelled time–temperature (t–T) paths point to a total denudation of the flanks of URG in the range of 1.0–1.7 km for a paleogeothermal gradient of 60°C/km, and 1.3–2.2 km for a paleogeothermal gradient of 45°C/km since the late Eocene.     

Gondwana biostratigraphy of the Purnea Basin (eastern Bihar,India), and its correlation with Rajmahal and Bengal Gondwana basins

Integrated biostratigraphic studies are undertaken on the newly discovered Gondwana successions of Purnea Basin which have been recognized in the subsurface below the Neogene Siwalik sediments. The four exploratory wells, so far drilled in Purnea Basin, indicated the presence of thick Gondwana sussession (± 2450m) with varied lithological features. However, precise age of different Gondwanic lithounits of this basin and their correlation with standard Gondwana lithounits is poorly understood due to inadequate biostratigraphic data.Present biostratigraphic studies on the Gondwana successions in the exploratory wells of PRN-A, RSG-A, LHL-A and KRD-A enable recognition of fifteen Gondwanic palynological zones ranging in age from Early Permian (Asselian-Sakmarian) to Late Triassic (Carnian-Norian). Precise age for the Gondwanic palynological zones, recognized in the Purnea Basin and already established in other Indian Gondwana basins, are provided in the milieu of additional palynological data obtained from the Gondwana successions of this basin.The Lower Gondwana (Permian) palynofloras of Purnea Basin recorded from the Karandighi, Salmari, Katihar and Dinajpur formations resemble the palynological assemblages earlier recorded from the Talchir, Karharbari, Barakar and Raniganj formations respectively, and suggests the full development of lower Gondwana succession in this basin. The Upper Gondwana (Triassic) succession of this basin is marked by the Early and Middle to Late Triassic palynofloras that resemble Panchet and Supra-Panchet (Dubrajpur/Maleri Formation) palynological assemblages, and indicates the occurrence of complete Upper Gondwana succession also in the Purnea Basin.The lithological and biostratigraphic attributes of Gondwana sediments from Purnea, Rajmahal and western parts of Bengal Basin (Galsi Basin) are almost similar and provides strong evidences about the existence of a distinct N-S trending Gondwana Graben, referred as the Purnea-Rajmahal-Galsi Gondwana Graben. Newly acquired biostratigraphic data from the Gondwana sediments of CHK-A, MNG-A and PLS-A wells from central part of Bengal Basin and Bouguer anomaly data suggest that these wells fall in a separate NE-SW trending graben of “Chandkuri-Palasi-Bogra Gondwana Graben”. Although, the post-Gondwana latest Jurassic-Early Cretaceous Rajmahal Traps and and intertrappean beds succeed the Upper Gondwana successions in Rajmahal, Galsi and Chandkuri-Palasi Gondwana basins, but not recorded in the drilled wells of Purnea Basin, instead succeeded by the Neogene Siwalik sediments.     

Provenance Ages of Cretaceous Strata in the Songliao Basin,Northeast China Inferred from Fission Track Data

The Songliao Basin in northeast China is an intra-continental fossil rift basin mainly developed from the latest Jurassic and throughout the Cretaceous. The Daqing oil field in the basin is one of the largest hydrocarbon exploitation areas in China. A fission track (FT) study has been carried out on sandstone well samples in the Songliao Basin to constrain the depositional ages of Cretaceous strata and to ascertain their source region. FT dating also has been carried out on welded tuffs in the Dahinganling Mountains to determine their eruption ages. The detrital zircon FT age suggests that the depositional age of the middle Quantou Formation is younger than 109 Ma. Zircon FT ages for Cretaceous strata (from the Nenjiang Formation to the Quantou Formation) suggest that the source region has a fairly homogeneous cooling history at ～120-100 Ma. Eruption ages of welded tuffs in the Dahinganling Mountains show an age range from 110 to 92 Ma. The regional geology and FT data suggest that volcanic rocks in the Dahinganling Mountains were eroded soon after their deposition and were reworked toward the Songliao basin.     

碎屑矿物年代对佳木斯隆起隆升时间的制约

对佳木斯隆起周缘的勃利盆地和鸡西盆地中的早白垩世穆棱组和滴道组砂岩样品进行了系统的碎屑锆石U-Pb同位素年代学和碎屑白云母40Ar/39Ar同位素年代学研究。结果表明,滴道组和穆棱组沉积时期,没有500Ma或大于500Ma的来自佳木斯地块基底的年龄纪录,暗示佳木斯隆起在早白垩世穆棱组沉积时期尚未隆起。黑龙江东部盆地群为统一的原型盆地,结合对该区白垩纪砂岩的矿物组成和砾石统计的研究成果,认为早白垩世末东山组沉积以后佳木斯隆起开始隆升,猴石沟组沉积时期佳木斯隆起已经隆升到一定高度并为其周缘盆地提供物源,同时随着佳木斯隆起的隆升,原来早白垩世的统一盆地遭到分割破坏,形成诸多相对独立的盆地。     

柴达木盆地古近系下干柴沟组上段碎屑锆石U- Pb测年及盆山耦合探讨

柴达木盆地被祁连山、阿尔金山及昆仑山所环绕，盆地古近系下干柴沟组(E23)地层独特的岩性和沉积格局指示了复杂盆山体系和源区多样成因。本文选取了盆地内部不同构造带5口钻井下干柴沟组的中粗砂岩样品，利用碎屑锆石U- Pb定年等分析方法对柴达木盆地进行了构造、物源系统分析。研究结果表明，位于祁连山前的XX- 1井样品下干柴沟组锆石年龄介于2692~156 Ma之间，主要峰值年龄为448 Ma和249 Ma，L6- 1井样品锆石年龄介于2693~220 Ma之间，主要峰值年龄为499 Ma和415 Ma；位于盆地西部沉降中心内部的YIT- 1井样品下干柴沟组锆石年龄介于2796~266 Ma之间，主要峰值年龄为423 Ma和255 Ma；阿尔金山前的N- 105井样品下干柴沟组锆石年龄介于2481~242 Ma之间，主要峰值年龄为422 Ma和259 Ma，N- 109井样品锆石年龄介于2638~228 Ma之间，主要峰值年龄为444 Ma和246 Ma。通过与主要源区年龄对比可知，盆地不同构造带下干柴沟组的物源差异较大，靠近祁连山前的XX- 1井、L6- 1井的物源主要来自于祁连山内部；位于阿尔金山前相邻的N- 105、N- 109井物源主体来自于阿尔金山内部，但N- 109井存在祁连山物源贡献；柴西坳陷内部YIT- 1井物源受祁连山及东昆仑共同控制。物源分析结果表明祁连山在古近纪已大规模隆升，并且作为青藏高原北部边界为柴达木盆地持续提供物源；阿尔金山及昆仑山在下干柴沟组沉积时期已经形成雏形，但并未大规模隆升，造成了山前带复杂的物源体系。     

The denudation history of the Argentera Alpine External Crystalline Massif (Western Alps,France-Italy): an overview from the analysis of fission tracks in apatites and zircons

Apatite/zircon fission track (FT) records of the Argentera external crystalline massif (Western Alps) show three tectonic pulses, respectively at 22 Ma (zircons), 6 and 3.5 Ma (apatites). The first pulse is consistent with the basement exhumation and initiation of the major deformation recorded in the foreland of the belt from Middle to early Upper Miocene. The two others might be respectively local expressions of the syncollisional extension mainly controlled by a westward sedimentary cover detachment and a Plio-Quaternary uplift acceleration. Zircon ages of 50-80 Ma in a limited NW area and evidence of an uplift elsewhere show that in a large fraction of the massif, temperatures in post-Variscan times never reached 320°C. Finally, FT data show that the Argentera massif did not behave as a single block during its denudation. First, in the NW of the massif, a small fault-limited block was already separated since the Cretaceous and later on recorded the 6 Ma denudation event, the 22 Ma pulse being recorded only in the remaining part of the massif. Second, less than 3.5 Ma ago, the northeastern part of the massif overthrust the southwestern block along the Bersézio-Veillos fault zone.     

塔里木西北缘阿克苏地区震旦系苏盖特布拉克组沉积物源分析：碎屑锆石年代学证据

阿克苏地区位于新疆塔里木克拉通西北缘,是研究塔里木克拉通前寒武纪演化的关键地区。该区出露有较好的前寒武系变质基底和沉积盖层,包括新元古界阿克苏群,南华系巧恩布拉克组和尤尔美那克组以及震旦系苏盖特布拉克组和奇格布拉克组。震旦系苏盖特布拉克组主要为一套红色河流相,湖相砂岩—粉砂岩序列,底部发育有砾岩和玄武岩夹层。本文对苏盖特布拉克组顶底两个沉积岩样品进行了LA-ICP-MS碎屑锆石UPb定年,旨在揭示苏盖特布拉克组的源区特征以及区域构造演化历史。碎屑锆石年龄谱指示出600~680Ma、730~900Ma、1800~2050Ma和2300~2600Ma四期构造-岩浆活动,与塔里木克拉通广泛发育的前寒武系岩体时代相匹配,说明塔里木克拉通可以为苏盖特布拉克组提供直接的物源。这四期构造-岩浆活动分别反映了塔里木克拉通对泛非造山、Rodinia超大陆、Columbia超大陆和新太古代—古元古代早期全球陆壳生长事件的响应。苏盖特布拉克组碎屑锆石记录落入碰撞背景盆地范围。结合区域地质资料,苏盖特布拉克组沉积可能代表了该区由会聚背景向伸展背景的转换。     

南秦岭中生代东河群碎屑锆石U-Pb年龄及其地质意义

南秦岭微陆块是秦岭造山带的重要构造单元,为了探讨南秦岭中生代断陷盆地的物质来源,本文选取凤太地区下白垩统东河群田家坝组复成分砾岩、周家湾组凝灰质砂岩为研究对象,对其中的碎屑锆石进行了LA-ICP-MS锆石U-Pb同位素测年,获得2组各40个有效年龄。结果明显可以分为5个年龄峰,范围分别是:2 595~2 771 Ma、1 215~1 916 Ma、749~1 142 Ma、424~576 Ma、218~306 Ma,分别对应Kenor、Columbia、Rodinia、Gondwana、Pangaea等5次超大陆事件。通过对测年数据进行分析,结合区域地质资料,认为北秦岭增生造山带和华北板块为南秦岭中生代盆地提供了大部分的物源,扬子板块次之,北祁连造山带和西秦岭北缘构造带也有一定的贡献。     

The low-temperature thermochronological record of sedimentary rocks from the central Rovuma Basin (N Mozambique) — Constraints on provenance and thermal history

Fission track (FT) dating of detrital titanites, zircons and apatites combined with sandstone petrography from rocks of the Rovuma Basin was used to constrain the basin's provenance and its post depositional thermal history. A comparison of metamorphic basement and sedimentary titanite and zircon FT data indicates that erosion was localized along a zone which followed the margin of the Rovuma Basin and was the source for the late Jurassic to Cretaceous sandstones. Time–temperature models of apatite FT data show, that after the deposition, the sandstones were heated up to temperatures of ca. 60–110 °C most likely due to a combination of intensified regional heat flux and burial heating caused by fast sedimentation in a transtensional pull-apart setting and intensified by regionally elevated heat flux. The utter western part of the basin was inverted between ca. 60 and 40 Ma, concordant with the drop in the global eustatic sea level which led to a rearrangement of the source-to-sink system. Some reworked zircons were deposited in the Cenozoic strata in the eastern part of the basin.     

唐王陵昭陵组砾岩碎屑锆石U-Pb年代学分析

唐王陵砾岩的时代归属及其沉积物源环境一直是鄂尔多斯盆地南缘新元古界-下古生界沉积地层学研究和油气地质勘探备受关注且长期存有争议的问题。采用LA-ICP-MS锆石U-Pb年代学方法,定量分析探讨了唐王陵昭陵组砾岩的时代归属和物质来源。结果表明:昭陵组砾岩的碎屑锆石U-Pb测年数据主要分布在744~943 Ma(n=6)、1 005~1 412 Ma (n=15)、1 449~2 209 Ma(n=255)和2 274~2 696 Ma (n=49)四个年龄区间,相应的峰值年龄集中在815 Ma、1 182 Ma、1 811 Ma、2 454 Ma,最年轻的单颗粒锆石年龄为744 Ma。它与盆地西缘贺兰山地区震旦纪正目观组的碎屑锆石U-Pb年龄谱非常相似,但明显有别于盆地南缘晚奥陶世平凉组,尤其是缺少平凉组碎屑锆石主要集中在454 Ma的峰值年龄,由此限定昭陵组砾岩的沉积时代主要发生在晚前寒武纪或震旦纪。锆石U-Pb年龄谱的区域对比揭示,昭陵组砾岩的沉积物源主要来自于华北(鄂尔多斯)地块的古元古代变质基底岩系和其南缘北秦岭构造带的晚前寒武纪岩浆岩-变质杂岩,呈现稳定地块与活动带双向混合物源特征。     

Ordovician sedimentation and bimodal volcanism in the Southern Qiangtang terrane of northern Tibet: Implications for the evolution of the northern Gondwana margin

This article reports our new interpretations of the depositional environment and provenance of the Dawashan Formation in the Longmuco–Shuanghu–Lancangjiang suture zone (LSLSZ), in the Southern Qiangtang terrane of northern Tibet, in order to gain a better understanding of the Ordovician tectonic evolution of the northern margin of Gondwana. The Dawashan Formation is dominated by greywacke and shale, with interlayered bimodal volcanic rocks that were deposited in a bathyal to abyssal marine basin. The detrital zircons in the greywacke of the Dawashan Formation have peak ages of 550, 988, 1640, and 2500 Ma, indicating a northern Gondwana margin provenance. The bimodal metavolcanic rocks from the Dawashan Formation are dominated by metarhyolite with subordinate metabasalt. The results of zircon LA-ICP-MS U–Pb dating indicate that the metarhyolite formed between 470 and 455 Ma. The metavolcanic samples are bimodal (SiO₂ = 45.27–55.05 and 66.09–74.59 wt.%). In comparison, the metabasalt has a wide range of MgO concentrations and Mg^# values, contains variable Cr and low Ni concentrations, is depleted in Rb, Ba, and Sr, and is enriched in TiO₂, Th, U, Nb, and Ta. Geochemical diagrams show that the metabasalt erupted in an intra-plate environment. The metarhyolites have high SiO₂, Th, and U concentrations, low concentrations of MgO, P₂O₅, Nb, Sr, and Ti, and negative Eu anomalies. The metarhyolites yield negative zircon ε_Hf(t) values (–2.08 to – 4.50) and T^C_DM model ages of 1436–1567 Ma. The metarhyolites formed from magma derived from the partial melting of old continental crust. These data indicate that the Dawashan Formation records Middle–Upper Ordovician bathyal to abyssal turbidite deposition in a deep-water rift basin at the northern margin of Gondwana.     

安徽繁昌盆地中生代火山岩锆石LA-ICPMS U-Pb年龄及其意义

繁昌盆地是长江中下游地区重要的陆相火山断陷盆地之一。本文对繁昌火山岩盆地中分村组粗安岩、赤沙组黑云母粗安斑岩、蝌蚪山组流纹岩、三梁山组黑云母粗面岩进行了锆石LA-ICPMSU-Pb测年,获得其年龄分别为134.4±2.9、131.3±1.8、130.8±2.2、128.1±3.1Ma,显示盆地内火山岩浆活动的时限约在135～128Ma之间,火山岩均为早白垩世岩浆活动的产物。同时在火山岩中测得了多颗继承或捕获锆石的古老年龄,其中最大的为2894Ma,可能对应了2.7Ga峰值年龄的全球性岩浆活动和大陆生长,一定程度上为繁昌盆地可能存在太古宙基底提供了证据。对比显示,繁昌盆地中生代火山岩与长江中下游地区其它断陷盆地火山岩几乎形成于同一时期,与庐枞、宁芜盆地的4个旋回火山活动可以完全对应,而135Ma可以作为长江中下游地区陆相火山断陷盆地早白垩世火山岩浆活动全面展开的时间。盆地内中生代火山活动期(135～128Ma)处于中国东部岩石圈减薄的高峰期,火山岩正是这一地球动力背景下深源岩浆活动的产物。     

Provenance and tectonic setting transition as recorded in the Neoproterozoic strata,western Jiangnan Orogen:Implications for South China within Rodinia

The Neoproterozoic Tonian strata(ca.870-725 Ma)in the western Jiangnan Orogen archive the records of sedimentary provenance and tectonic setting which can be used to understand the geological evolution of the South China Continent.These strata are separated into the basement and cover sequences by a regional angular unconformity.The basement sequence can be subdivided into the lower and the upper parts by the widespread interbedded ca.840 Ma basalt with pillow structure.In the present work,234 concordant detrital zircon analyses are obtained from three Tonian sandstone samples in the Fanjingshan district,Guizhou Province.Combined with previous results,a total of 1736 analyses of detrital zircon U-Pb ages derived from 12 formations of Tonian strata in the western Jiangnan Orogen are used to decipher the integrated sedimentary and tectonic histories.The zircons from the lowermost part of the basement sequence(the Yujiagou Formation)show oval morphology and display two Paleoproterozoic age peaks at 2325 Ma and 1845 Ma which are similar with the detrital zircon age peaks from the Late Paleoproterozoic to Early Mesoproterozoic Dongchuan/Dahongshan/Hekou groups,suggesting a passive margin basin in which the sediments were mainly sourced from the southwestern Yangtze Block.However,the zircon age population of the lower part of the basement sequence(the Xiaojiahe,Huixiangping formations and their equivalents)indicates the sedimentary derivation from bidirectional sources(the ca.870 Ma arc materials in the south and the old detritus from the southwestern Yangtze Block)which is consistent with a back arc setting for the deposition of the sediments.Zircons from the upper part of the basement sequence(the Duyantang Formation and its equivalent)show euhedral and subangular morphology and display a unimodal age peak at ca.835 Ma.This sequence was possibly deposited in a convergent setting and the detritus were came from the locally distributed syn-collisional igneous rocks.The lower part of the cover sequence(the Xinzhai and Wuye formations and their equivalents)shows a distinct zircon age peak at 815—809 Ma and two subordinate peaks at 2485 Ma and 2018 Ma,suggesting that the basin had gradually transformed into a continental rift basin and received the detritus from the ca.815 Ma post-collisional magmatic rocks as well as from different Paleoproterozoic source rocks in the northern Yangtze Block.We propose a tectonic evolution model that envisages eruption of ca.840 Ma basalt in a back arc basin that existed during ca.870-835 Ma,an angular unconformity was formed during amalgamation of the Yangtze Block and the Cathaysia Block at ca.835-820 Ma and the rifting of the South China Continent was initiated at ca.800 Ma.Our study concludes that the South China Continent was formed on the periphery of the Rodinia supercontinent.     

库鲁克塔格地区新元古代沉积物源分析:来自碎屑锆石年代学的证据

库鲁克塔格位于南天山和塔里木盆地接合部,保留相对较完整的地层记录。新元古代贝义西组主要为一套火山-沉积组合序列,育肯沟组为浊流成因的砂岩和粉砂岩组合。对取自两个地层的沉积岩样品,分别进行了碎屑锆石LA-ICP-MS U-Pb测年,并分别获得了93组和71组U-Pb有效年龄。其中贝义西组锆石U-Pb年龄主峰值为821Ma、次峰值为861Ma的碎屑锆石(761~847Ma)具有显著优势,育肯沟组年龄值为768Ma(次峰值为800Ma和741Ma)的碎屑锆石比较集中,说明库鲁克塔格地区在741Ma、768Ma、800Ma、821Ma和861Ma有大规模的岩浆活动。10个太古代碎屑锆石年龄以及综合已有的研究成果,确认库鲁克塔格存在中-新太古代基底。贝义西组砂岩主要源自761~847Ma、851~972Ma和1808~2498Ma岩石。育肯沟组主要来自734~845Ma、858~963Ma和1708~2486Ma的岩石。     

安徽巢湖北部地区早石炭世碎屑锆石年代及其沉积意义分析

安徽巢湖北部地区石炭系出露完整,层序清楚,沉积类型多样,是下扬子地区研究石炭系的重要地区之一。本研究在分析石炭系剖面的基础上,结合U-Pb碎屑锆石年代学分析,探讨了巢湖北部地区早石炭世物源特征。本研究共对两组岩石样品进行了LA-ICP-MS U-Pb锆石年代学分析,第一组样品为采集于高骊山组中段的砂岩,第二组样品为采集于和州组顶部的砂屑生屑灰岩。高骊山组样品锆石最晚年龄为404.4±10.2Ma,最早年龄为3194.1±40.7Ma,获得了两个峰值年龄435±6Ma与846±14Ma。和州组样品锆石最晚年龄为410±10.8Ma,最早年龄为2780±46.0Ma,获得了两个峰值年龄448±20Ma与849±12Ma。早石炭世沉积特征及年龄数据表明,巢湖北部地区早石炭世的陆源碎屑沉积主要来源于扬子海周围晋宁期及加里东期形成的古陆,部分沉积物可能来自其他古陆。     

Regional thermal history of the Lennard shelf,Canning Basin,from apatite fission track analysis: Implications for the formation of Pb‐Zn ore deposits

Zinc mineralization in Devonian carbonates of the Lennard Shelf, northern Canning Basin is similar in many respects to that of the Mississippi Valley‐type including estimated minimum temperatures of sulphide precipitation between 70 and 110°C. Apparent apatite fission track ages for Precambrian granitic basement and for detrital apatites in Devonian carbonates in and near Pb‐Zn mineralization generally range between 260 and 340 Ma, with Precambrian samples tending to have slightly older apatite fission track ages than the Devonian carbonates. These apparent ages are younger than the stratigraphic age of the material analysed, indicating that appreciable annealing of fission tracks in apatite has occurred in post‐Devonian times. Mean horizontal confined track lengths are 12–13 μm for most samples and preclude attaching any ‘event’ significance to the fission track ages. Studies of well sequences (Grevillea 1 and Kennedia 1) indicate a period of rapid uplift in the area during the Late Triassic/Early Jurassic. Assuming a constant geothermal gradient of 30°C/km, approximately 1.5 km of uplift and erosion is estimated. Immediate thermal effects related to Miocene lamproite intrusion into Precambrian basement appear to be restricted to within 200 m of the contact zone.

For outcropping Devonian carbonates, a thermal history is proposed involving burial in the Late Palaeozoic/Early Mesozoic, followed by uplift and cooling from peak temperatures around 70–80°C in mid‐Mesozoic times. With reference to this period of burial, Pb‐Zn occurrences represent thermal anomalies when reported fluid inclusion homogenization temperatures are compared with the estimated peak temperatures. However the possibility of a phase of higher temperatures during the Late Devonian/ Early Carboniferous is suggested by the apatite fission track results, in which case sulphide mineralization may reflect ambient regional temperatures if it formed at that time. The absence of enhanced annealing effects in detrital apatites proximal to Pb‐Zn deposits suggests that either sulphide mineralization preceded or accompanied peak regional temperatures suspected during the Late Devonian/Early Carboniferous, or that the mineralizing episodes were of too short a duration to significantly anneal fission tracks in apatite.     

Sedimentary Facies,Provenance and Geochronology of the Heshangzhen Group: Implications for the Tectonic Evolution of the Eastern Jiangnan Orogen,South China

The Neoproterozoic Jiangnan orogen plays an important role in the study of the Precambrian tectonic evolution of South China. The tectonic nature of the Neoproterozoic sedimentary basins is still controversial, due to poor understanding of the sedimentary sequences and the lack of geochronological data. Here, we present sedimentological, provenance and geochronological data from the Heshangzhen Group in the eastern Jiangnan orogen. Sedimentological analysis shows that the Luojiamen Formation was deposited in a submarine fan, and the overlying Hongchicun Formation was deposited in front of a fan delta. The youngest detrital zircons constrain the lower Luojiamen and Hongchicun formations with ages of 827.3 ± 8.4 Ma and 825 ± 12 Ma, respectively. The sandstones of the Luojiamen Formation are characterized by a large number of intermediate to felsic volcanic grains, suggesting a volcanic arc source. In contrast, quartz and sedimentary lithic grains increase in the Hongchicun Formation, showing a new input from a collisional orogenic source. Detrital zircon from six sandstone samples in the Luojiamen and Hongchicun formations yield similar age spectra of 930–820 Ma with a peak at ca. 845–860 Ma, with one main cluster at 930–820 Ma. Detrital zircons of 930–845 Ma show a positive value of εHf(t)(+2.4 to +11, mean +7.6), which is similar to the volcanic arc of the nearby Shuangxiwu Group. There are a minor group of zircons with U-Pb ages ranging from 820 Ma to 845 Ma from the middle part of the Luojiamen Formation and Hongchicun Formation, with εHf(t) values between-20 to +2.4, which are consistent with the characteristics of the Shuangqiaoshan Group. within light of the bidirectional paleocurrents in the Luojiamen Formation, it is speculated that the zircons of 820–845 Ma were recycled from the Shuangqiaoshan Group, which is derived from a continental arc to the northwest. Our data suggests that the Luojiamen Formation was formed in an inter-arc basin, while the Hongchicun Formation was formed in an accretionary wedge-top basin. When juxtaposed with the conglomeratic characteristics at the bottom of the Luojiamen Formation, it is believed that the unconformity represented by the ‘Shen Gong Movement' reflects the rapid erosion and accumulation process of island arc volcanic material. The disconformity between the Luojiamen and Hongchicun formations is the imprint of transition from inter-arc basin to accretionary wedge-top basin,which represents the collision between the Shuangxiwu arc and the Yangtze Plate.