U-Pb zircon,Rb-Sr and Sm-Nd geochronology of high- to very-high-pressure meta-acidic rocks from the western Alps

Three meta-acidic rocks from the western Italian Alps, a magnesiochloritoid-bearing metapelite from the Monte Rosa massif, a coesite-pyrope-quartzite from the Dora Maira massif and the Monte Mucrone granite in the Sesia Zone, have been studied by U-Pb zircon, Rb-Sr on whole-rock, apatite and phengite and Sm-Nd wholerock methods. The mineral parageneses of the investigated rocks indicate high- to very-high-pressure and medium-to-high-temperature metamorphism. This combined isotopic study has enabled us to constrain the ages of magmatic and metamorphic events and also to compare the behaviour of U-Pb zircon systems in three intensely metamorphosed areas of the Pennine domain. The U-Pb zircon data have yielded a magmatic age for the Monte Mucrone granite at 286±2 Ma. This result confirms the occurence of late-Hercynian magmatism in the Sesia Zone, as in other Austro-Alpine units and in other areas of the European crystalline basement. In the Monte Rosa massif, a geologically meaningless lower intercept age of 192±2 Ma has been interpreted as an artefact due to a complex evolution of the U-Pb zircon system. The magmatic shape of the zircons implies a magmatic or volcano-sedimentary protolith for this rock, originally considered as a metasediment. The very-high-pressure metamorphism in the Dora Maira quartzite has produced an opening of the U-Pb zircon system at 121+12–29 Ma. The Rb-Sr data support the occurence of high-grade metamorphism during Cretaceous times. Phengites model ages are slightly younger than the U-Pb zircon lower intercept ages due to cooling phenomena or possible response of the phengites to a later deformation. The Nd model ages from the whole-rock samples, as well as the U-Pb upper intercept ages from zircons of all three investigated rocks, indicate the presence of Proterozoic crustal components inherited from the precursors of these meta-acidic rocks. The studied zircon populations and their U-Pb systems apparently showed open-system behaviour only when affected by extreme metamorphic conditions (700–750° C, > 28 kbar), whereas eclogite-facies conditions of 500–550° C and 14–16 kbar were not enough to disturb significantly the U-Pb zircon evolution. It is also probable that the sedimentary or magmatic origin of the protoliths of these meta-acidic rocks, which involved different characteristics such as grain-size and fluid phase concentration and composition, could be another important factor controlling the U-Pb zircon system behaviour during metamorphic events.     

Understanding the pre-Variscan and Variscan basement components of the central Tauern Window, Eastern Alps (Austria): constraints from single zircon U-Pb geochronology

New single-grain and within-grain U-Pb zircon ages from the central Tauern Window help sorting out the time dimension among the various Variscan and pre-Variscan basement components that were strongly overprinted by Alpine orogeny. Single-grain isotope dilution (ID-TIMS) U-Pb zircon geochronology of three Basisamphibolit samples yield protolith formation ages of 351±2, 349±1 and 343±1 Ma. Laser ablation ICP-MS and ID-TIMS U-Pb detrital zircon dating of the Biotitporphyroblastenschiefer constrained the maximum time of sedimentation to between 362±6 Ma and 368±17 Ma. Paragneisses from the Zwölferzug yield maximum sedimentation ages from 345±5 Ma (ion microprobe data) to 358±10 Ma. Zircons from gabbroic clasts and detrital zircons from a meta-agglomerate from the Habach Phyllite give an upper intercept age of 536±8 Ma and a near-concordant age of 506±9 Ma, respectively. Hence, apart from the Habach Phyllite, the maximum sedimentation ages of the metasediments investigated range from Upper Devonian to Lower Carboniferous. Consequently, the Basisamphibolit, the Biotitporphyroblastenschiefer, and the paragneisses of the Zwölferzug form parts of the Variscan basement series. The Basisamphibolit (351-343 Ma) is distinct both in space and time of formation from the Zwölferzug garnet amphibolite (c. 486 Ma), which forms part of the pre-Variscan basement.     

Magmato-sedimentary Carboniferous to Jurassic evolution of the western Tauern window,Eastern Alps (constraints from U-Pb zircon dating and geochemistry)

New geochronological U-Pb (LA-ICP-MS) zircon data and geochemical analyses from the Variscan orthogneisses and metavolcanic rocks in the western Tauern window are presented and used to reconstruct the pre-Alpine evolution of this area. The late- and post-Variscan stage in the Tauern window was characterised by distinct magmatic pulses accompanied by the formation of volcano-sedimentary basins. The magmatic activity started in the Visean (335.4 ± 1.5 Ma) with the intrusion of a K-rich, durbachitic biotite-granite (protolith of the Ahorn gneiss). Following a period of exhumation and erosion, Westfalian–Stefanian volcanics were deposited (Grierkar meta-rhyodacite: 309.8 ± 1.5 Ma; Venntal meta-rhyolite: 304.0 ± 3.0 Ma). A renewed magmatic pulse occurred in the Early Permian, producing large volumes of tonalites and granodiorites (Tux meta-granodiorite: 292.1 ± 1.9 Ma). The youngest magmatism is characterised by pyroclastic and tuffitic deposits (Pfitsch meta-rhyolite: 280.5 ± 2.6 Ma; Schönach valley meta-andesite: 279.0 ± 4.8 Ma). This volcanism was probably related to crustal extensional faulting within an intra-continental graben and horst setting, asthenospheric upwelling and heat flow increase due to the onset of the Permian rifting. The Permo-Triassic peneplanation and subsidence is documented by shallow marine and evaporitic deposits. Probably in the Middle Jurassic times, the area was flooded and in the Late Jurassic the whole area was covered by limestones, representing post-rift sediments on the southern European continental margin.     

Formation and evolution of the Eastern Kunlun Range,northern Tibet: Evidence from detrital zircon U-Pb geochronology and Hf isotopes

The Eastern Kunlun Range, as a high-elevation and granitoid-rich tectonic element in northern Tibet, records Paleozoic–Mesozoic amalgamation process of the East Asia continent and Cenozoic uplift of the Tibetan Plateau. However, Precambrian evolution of the Eastern Kunlun remains poorly understood and relations between Eastern Kunlun and adjacent terranes (e.g., Qaidam and Qilian) during the Phanerozoic accretion process are still highly controversial. We use detrital zircon U-Pb geochronological and Hf isotopic data of Proterozoic and Paleozoic metasedimentary rocks from the Eastern Kunlun Range, to reconstruct its origin and subsequent evolutionary history. Detrital zircons of the Proterozoic rocks are dominated by early–middle Neoproterozoic ages (700–1000 Ma), with two age peaks at ca. 800 Ma and ca. 920 Ma and εHf(t) values ranging from −10 to 5. The youngest detrital zircon ages (648–788 Ma) demonstrate that these investigated Proterozoic strata, which were previously mapped as Paleoproterozoic to Mesoproterozoic, were most likely deposited in the middle–late Neoproterozoic. Abundant 0.9–1.0 Ga detrital zircon crystals are consistent with those crystalline rocks of similar ages across the Kunlun-Qaidam and Qilian terranes, which are generally interpreted as the product of Grenvillian orogenesis. These findings support the hypothesis that these terranes were probably within a single continental landmass (named as KQQ block) during the Neoproterozoic. The high similarity of detrital zircon ages, Hf isotopes and Neoproterozoic lithostratigraphy between western Yangtze and KQQ blocks, supports a temporary connection of the KQQ block to western Yangtze in Rodinia supercontinent. Detrital zircons of the analyzed Paleozoic rocks are characterized by 390–490 Ma age populations. These results, in combination with published granitoids data of the northern Tibet, favor a scenario in which the Kunlun-Qaidam and Qilian terranes underwent separated subduction and accretion processes during the late Cambrian–Devonian, but together formed an upper plate to northward subduction of the Paleo-Tethys during the Permian–Triassic.     

东准噶尔地区扎河坝——阿尔曼太蛇绿岩多期次岩浆演化特征：来自锆石U-Pb年代学的证据

新疆东准噶尔地区阿尔曼太蛇绿岩被认为是早古生代大洋岩石圈残片,其主要由地幔橄榄岩、辉长岩、玄武岩以及硅质岩组成。通过野外地质观测,发现在扎河坝-阿尔曼太蛇绿岩内侵入了大量的岩体,主要为闪长岩以及闪长玢岩。本文通过LA-ICP-MS 锆石U-Pb 定年,获得蛇绿岩中辉长岩年龄为497.3 ± 4.5 Ma,而后期侵入的闪长玢岩的年龄为424.5 ± 3.1 Ma。前人研究认为这些闪长玢岩具有岛弧岩浆岩特征。结合区域地质背景,本文认为这些后期侵入的闪长玢岩是岛弧岩浆作用的结果, 推测阿尔曼太蛇绿岩为一个岛弧多期次演化的产物。     

冷水坑斑岩型银铅锌矿床含矿岩体锆石SHRIMP U-Pb年代学研究

冷水坑斑岩型银铅锌矿床是中国目前唯一的典型斑岩型银铅锌矿床,对于该矿床的成矿斑岩的形成时代至今仍没有精确的年龄数据。本文通过对冷水坑斑岩型银铅锌矿床成矿斑岩(花岗斑岩)样品中的锆石11个测试点的SHRIMP锆石U-Pb年代学研究,得出206Pb/238U加权平均年龄为(162.0±2)Ma(MSWD=1.4),因此,认为冷水坑花岗斑岩的形成年代为(162.0±2)Ma。冷水坑含矿斑岩成岩年龄与成矿年龄高度一致,成岩年龄与成矿作用开始时间差异不明显。此外,由冷水坑矿床成岩和成矿年龄数据推断出矿化持续时间约27Ma。     

西准噶尔包古图地区地层火山岩锆石LA-ICP-MS U-Pb年代学研究

包古图地区位于西准噶尔东南部,区内出露地层主要为石炭系太勒古拉组、包古图组和希贝库拉斯组,为一套巨厚的半深海相-大陆坡相火山-火山碎屑沉积建造。这套地层的时代归属和地层层序长期以来一直存在争议。本文报导了包古图地区实测地层剖面,并从太勒古拉组玄武岩和包古图组及希贝库拉斯组凝灰岩中分别选出锆石,进行了LA-ICP-MS U-Pb年代学研究,获得²⁰⁶Pb/²³⁸U加权平均年龄分别为357.5±5.4Ma、332.1±3.0Ma和336.3±2.5Ma。由此确定这套地层属于早石炭世的杜内阶到维宪阶,由下到上依次为太勒古拉组、包古图组和希贝库拉斯组。     

金湖凹陷基性岩捕获锆石年龄及其区域构造意义

郯庐断裂带的形成演化是中国东部中生代关键大地构造事件,该断裂带起源机制问题是了解华南与华北板块汇聚过程与方式的基本前提。本文利用郯庐断裂带东侧金湖凹陷内7个新生代基性岩样品394个点的LA-ICP-MS锆石U-Pb年代学数据,在分析岩浆活动时代基础上,利用捕获锆石年龄信息探讨了大别造山带的古生代演化及郯庐断裂带起源机制问题,为认识上述问题提供了一个全新的视角。研究表明,金湖凹陷的基性岩浆活动时代为古近纪早期（66.1~56.5 Ma）,与围岩的沉积时代吻合;基性岩中的捕获锆石来自大别造山带,是印支期板块碰撞造山中楔入郯庐断裂带东侧华南板块的造山带物质,具有华南板块、北淮阳和华北板块的混杂岩石圈信息;捕获锆石中445 Ma（变质锆石）、290 Ma（岩浆锆石）及214 Ma（变质锆石）等峰值年龄信息指示古生代以来北淮阳构造带经历了与北秦岭构造带类似的弧—陆俯冲、碰撞历程,暗示秦岭造山带的二郎坪洋、商丹洋和勉略洋可延伸至大别造山带东端;年代学数据及构造分析支持郯庐断裂带起源于嵌入碰撞背景下的华南板片撕裂边界,其起源形式为陆—陆碰撞下的板块斜向汇聚边界。

     

滇西南云岭锡矿床地质特征与花岗岩锆石U-Pb年代学

滇西南是中国重要的锡矿带之一,前人研究主要集中在白垩纪和新生代花岗岩中的锡矿床,而对印支期花岗岩中的锡矿关注较少.云岭锡矿位于保山地块的东缘,矿体主要以脉状产在黑云母二长花岗岩中,发育云英岩化和电气石化蚀变.矿石矿物为锡石,脉石矿物为石英、白云母、电气石、萤石、方解石,以及黄铁矿、毒砂、黄铜矿、闪锌矿等硫化物.根据矿物...     

新疆博格达山晚古生代地层的形成时代、物源及其演化:碎屑锆石U-Pb年代学证据

新疆博格达山主体由石炭系海相火山—沉积岩系组成, 以发育两期双峰式火山岩, 但不发育花岗岩为特征, 对其晚古生代地层时代的划分和演化争议较大.本文重点对博格达山北部两个晚古生代砂岩进行了碎屑锆石U-Pb年代学分析, 重新标定博格达山地区晚古生代地层的形成时代; 利用物源区的演化, 约束晚古生代构造演化.测年结果显示博格达上亚群砂岩的碎屑锆石表面年龄值分布范围较宽, 主峰年龄为343～284 Ma(80%), 次峰年龄为386～375 Ma(3%)、503～441 Ma(7%)和871～735 Ma(10%); 芦草沟组砂岩的碎屑锆石表面年龄值非常集中, 主峰年龄为358～279 Ma(97%), 次峰年龄为257～251 Ma(约3%).博格达山中部原石炭纪博格达群上亚群与西部和南部下芨芨槽群相当, 应属于早二叠世, 中部—东部的石炭—二叠纪界线应在博格达下亚群—上亚群或居里得能组—沙雷塞尔克组之间的不整合面之中.博格达北部地区晚二叠世以南侧天山物源区供给为主, 反映出晚古生代期间博格达山地区至少存在晚石炭世末和中二叠世两期构造隆升.结合区域火山岩与火山碎屑岩的研究, 认为博格达山地区晚古生代主要经历4个演化阶段: 早石炭世弧后盆地裂解阶段、晚石炭世碰撞拼贴阶段、早二叠世碰撞后伸展阶段、中-晚二叠世再次隆升到稳定阶段.     

大兴安岭西坡额仁陶勒盖银多金属矿区火山岩岩石成因及其地质意义:年代学、地球化学特征

为揭示额仁陶勒盖银矿床火山岩与成矿作用的关系,对矿区内赋矿围岩火山岩展开了系统的年代学和元素地球化学研究。成果揭示:(1)赋矿围岩粗面安山岩的单颗粒锆石U-Pb年龄为(173.4±4.6)Ma;(2)粗面安山岩具有富集大离子亲石元素、亏损高场强元素以及弱的负铕异常的特征;对比洋壳成分,成矿元素呈现强烈亏损V、Cr、Ni、Co、Mo、Cu,富集W、Bi、Pb和相对富集Au、As、Sb、Te以及相似的Zn、Ag、B含量为特征。结合前人的研究成果,认为矿区内火山岩是成矿早期的产物,岩浆起源于流体交代过的大洋性质的岩石圈地幔,岩浆演化过程中有少量斜长石等矿物分离结晶作用和微弱的地壳物质混染,形成的构造环境可能为华北板块、蒙古块体与西伯利亚板块对接碰撞后的初期地壳伸展环境,且该阶段火山作用不具有直接提供成矿流体的属性,提供成矿流体的岩浆作用可能是早白垩世早期石英二长斑岩岩浆(138 Ma)。     

Rb/Sr geochronology in the Eastern Alps

New Rb/Sr data on mineral and whole rock samples from in and around the south-east corner of the Tauern Window are presented. Pennine orthogeneisses yield an Rb/Sr whole rock age of 279±9 m.y., while orthogneiss samples from the Altkristallin Sheet near Innerkrems, Carinthia, yield an age of 381±30 m.y. by the same technique. The apparent mineral age break across the margins of the Tauern Window is investigated in an area of good structural and petrofabric control. The post-Palaeozoic history of the Eastern Alps is then discussed in the context of the available Rb/Sr data. It is argued that the bulk of the Katschberg Phyllites are of pre-Mesozoic age; that the major overthrusting movements of the Austroalpine Units were completed by 60–65 m.y.; and that the Peri-Adriatic intrusives can be little older than middle Tertiary.     

苏南晚二叠世末期双峰次火山岩的发现和单颗粒锆石U－Pb年代学意义

苏南晚二叠世末期双峰次火山岩的发现和单颗粒锆石Ｕ－Ｐｂ年代学意义方中，李惠民，夏邦栋，楚雪君，刘寿和，王学锋（南京大学地球科学系，江苏南京，２１０００８；天津地质矿产研究所，天津，３００１７０）关键词晚二叠世，双峰火山岩，同生锆石，捕虏晶锆石，Ｕ－Ｐ...     

苏南晚二叠世末期双峰次火山岩的发现和单颗粒锆石U－Pb年代学意义

苏南晚二叠世末期双峰次火山岩的发现和单颗粒锆石Ｕ－Ｐｂ年代学意义方中，李惠民，夏邦栋，楚雪君，刘寿和，王学锋（南京大学地球科学系，江苏南京，２１０００８；天津地质矿产研究所，天津，３００１７０）关键词晚二叠世，双峰火山岩，同生锆石，捕虏晶锆石，Ｕ－Ｐ...     

Tertiary metallogenesis in the Eastern Alps: the Waldenstein hematite deposit (Austria)

The specularite deposit at Waldenstein is an epigenetic replacement/vein-type hematite mineralization with distinct alteration zones. It is situated in highly metamorphosed schists and, gneisses of the Koralm Crystalline Complex. Sericitization and chloritization are strictly bound to this alteration and occur within a few meters of the hematite veins. Radiometric dating of the sericitization and the implicity of the mineralization yield a Middle Eocene age. Petrographic evidence proves an early, relatively reduced paragenesis (magnetite, ilmenite) being partly replaced by hematite and chlorite during the main phase of ore formation. Fluid inclusion and stable isotope investigations indicate that formation temperatures were approximately 300°C during the main stage and decreased to 200°C in the latest stage of the hydrothermal event. The H-isotope compositions of chlorites and of fluid inclusions in quartz indicate the influence of seawater. This also explains the high salinities determined by fluid inclusion studies (20–30% NaCl equiv.). According to the genetic model presented in this paper the hydrothermal activity started when the deformation accompanying the uplift of the Koralm Crystalline Complex passed from a ductile to brittle regime (Middle Eocene). The brittle faults acted as channelways for rising, deep fluids, probably of metamorphic origin. Temperature differences between the different uplifting crustal segments supported fluid circulation. Precipitation of the ore occurred at shallow crustal levels where the hydraulic regime was dominated by seawater, and oxidation of the original reducing fluids took place.     

内蒙古甲乌拉铅锌银矿区岩浆岩年代学、地球化学特征及其对成矿的约束

内蒙古满洲里地区的甲乌拉铅锌银矿区的成岩、成矿事件及其相互关系目前仍有很大争议。文中对矿区内与成矿密切相关的石英斑岩、闪长玢岩开展详细研究。锆石SHRIMP U Pb年代学研究表明：矿区致矿侵入体之一石英斑岩形成于(129±4.8) Ma,成矿晚(或后)期的闪长玢岩形成于约(124±2.5) Ma;认为铅锌银等成矿作用应发生于(129±4.8)~(124±2.5) Ma,为早白垩世末期。岩石地球化学特征显示石英斑岩具有高硅、高钾、贫钙等特征,TAS图上属于亚碱性系列,硅铁镁图上属于钙碱系列,硅钾图上属于高钾钙碱系列。轻重稀土分馏明显,具有明显的负铕异常。相对富集大离子亲石元素(LILE),而亏损高场强元素(HFSE),且具有低Sr、高Y等特征。闪长玢岩同样展现出高钾、高铝、贫钙特征,TAS图上属于亚碱性系列,硅铁镁图上属于拉斑系列;轻、重稀土分馏明显,具明显负铕异常,但稀土总量较高;相对富集大离子亲石元素(LILE),亏损高场强元素(HFSE),同具有低Sr、高Y等特征。这些研究结果表明：石英斑岩具有火山弧的特征,主要来源于壳源的熔融;闪长玢岩具有活动大陆边缘弧的特征,主要来源于地幔的熔融,但经历了壳源的混染上述岩浆与甲乌拉的铅锌银矿;推测应为蒙古-鄂霍次克洋由俯冲向陆陆转换阶段产物。     

A new method integrating high-precision U-Pb geochronology with zircon trace element analysis (U-Pb TIMS-TEA)

Increased precision in isotope-dilution thermal ionization mass spectrometry (ID-TIMS) U-Pb geochronology has revealed age complexities in zircon populations that require new tools for understanding how the growth of zircon is related to geologic processes. U and Pb are routinely separated from other elements in dated minerals by ion exchange separation prior to TIMS isotope measurement. We develop a method in which trace elements in the exact same volume of zircon are redissolved and analyzed using solution nebulization inductively coupled plasma sector-field mass spectrometry with matrix-matched external liquid calibration. Using <0.5 ml solution, resulting concentrations are between <1 ppt for elements such as Ti, Nb and Ta and tens of ppb for Zr. By analyzing a series of standard solutions, zircons and procedural blanks, we show that accurate measurements are performed on Zr, Hf, Y, Sc, and the HREE while low-concentration elements can be measured accurately to <5 ppt. We performed combined U-Pb ID-TIMS geochronology with trace element analysis (here called U-Pb TIMS-TEA) on zircons from eight volcanic rocks comprising several volcanic systems and one metamorphic sample. Similar to previous in situ trace element analyses, zircon geochemistry is distinct between different samples and records petrogenetic processes such as fractional crystallization, assimilation and/or magma mixing. Unique from in situ analysis, U-Pb TIMS-TEA can trace geochemical evolution in accessory minerals with adequate age precision to resolve magmatic processes in rocks at least 200 million years old. This provides a means to identify auto-, ante- and xenocrystic zircon and lead to more robust age interpretations in ID-TIMS U-Pb geochronology. One suite of Cretaceous andesitic zircons shows correlations in geochemistry and absolute time that record evolution of a magmatic system over ∼250 ka prior to eruption. Future work will combine U-Pb TIMS-TEA with solution isotopic analysis of Nd, Sr and Hf and will be applied to a host of datable minerals such as monazite, sphene, apatite, rutile, xenotime, and baddeleyite. These combined tools will provide access to an improved understanding of a wide range of igneous and metamorphic processes as a function of time.     

大别山北缘西段双桥中生代火山岩地球化学及锆石U-Pb同位素年代学

首次对大别山北缘西段(河南省境内)中生代火山岩进行了锆石原位U-Pb同位素测年。结果表明,罗山县双桥火山岩集中区石英安山岩锆石LA-ICP-MS U/Pb年龄为133.1±1.5 Ma,与大别东段中生代火山岩主体毛坦厂组的时代一致,属早白垩世,稍早于或与大别山区大规模花岗岩岩基侵位时代相同;与长江中下游地区的宁芜和庐枞盆地火山岩喷发时间同步,可能同属中国东部中生代巨量岩浆活动的重要组成部分,与太平洋板块斜向俯冲引起的大陆岩石圈重力不稳而产生的拆沉导致岩石圈强烈减薄深部动力学过程相联系。大别山西段早白垩世火山岩属高钾钙碱性系列粗安岩-英安岩-流纹岩组合。中酸性火山岩显示轻稀土元素富集、重稀土元素强烈亏损、弱或基本无负Eu异常的稀土元素配分模式以及高Sr低Yb特征,反映岩浆部分熔融源区残余有石榴石,说明当时存在加厚的成熟陆壳。高(87Sr/86Sr)i(0.707 56)、极低的εNd(t)(-20.1)和大离子亲石元素富集、高场强元素Nb、Ta明显亏损,显示岩浆源区的壳源性质。以上特征反映大别山地区该时期岩石圈伸展程度和软流圈上涌规模远不如长江中下游同时期以幔源为主的中基性火山岩发育的宁芜和庐枞地区。     

Oceanic mafic rocks in the Eastern Alps

The Eastern Alps in Austria have been interpreted as a pile of thrust sheets resulting from the collision of two continental masses. The only remains of the ocean-floor which may once have separated these continents could be the highly deformed greenschists, metasediments and serpentinites found in the lower thrust sheets. To test this hypothesis, a total of sixty mafic rocks from the Großglockner, Mooserboden, Fusch, Hochtor, Matrei Zone and Strobl localities have been analysed for the stable trace elements, Ti, Zr, Y, Nb and Cr, and the less stable elements K, Rb, and Sr. Visual and statistical comparison of the stable elements with known magma types reveals that five of the sample groups classify clearly as tholeiitic ocean-floor basalts, while one group, the Fusch locality, classifies as within-plate (probably ocean island) basalts. It is suggested that the tectonic units containing such rocks comprise a mélange of disrupted oceanic crust, upper mantle and seamounts, pelagic sediments and continental margin sediments. The rocks may have formed in a large ocean basin, rather than a marginal basin behind an island arc.