浙江衢州新路火山岩盆地西段热点作用及其对铀成矿的控制作用

新路火山岩盆地是浙江省重要的产铀基地。文章通过对该区热点作用的研究,解释了该区铀矿床(点)的形成机制,以期获得该区铀矿找矿的新突破。地球物理资料显示,该区深部40 km处存在相对低速体的塑性体(地幔柱冠状体之上的幔枝构造),在其上部地层存在相对高速体的岩浆侵入体;中生代以来发育4次岩浆旋回;地层结构表现出下部为变质基底,中部为滑脱层,上部为上叠火山岩和白垩纪沉积盆地的三层式结构。热点作用的地质、地球物理特征明显。流体包裹体研究表明,成矿热液的温度为317~420℃,盐度w(NaCleq)为39.76%~49.68%,其成分为CO2及含碱、H2O、卤素、氮和一些金属元素,地幔流体特征明显,与铀矿伴生的微量元素及成矿同期的硫同位素也指示出成矿流体来源于上地幔或下地壳。地球物理和地球化学特征均指示出该区存在热点成矿作用,据此建立了该区的热点活动模式。文章认为,热点对该区铀成矿的控制主要表现为:①当深部基性岩浆运移至上地壳时,使硅铝质岩石发生部分熔融,形成酸性岩浆,喷出地表形成了新路火山岩,侵入时则形成次火山岩体和各类酸性侵入岩体,为成矿准备了有利的赋矿层位或有利围岩;②地幔柱上隆作用,形成或复活了一系列断裂构造,完善或形成了成矿所需的导矿构造和储矿构造;③地幔柱上隆时,提供了成矿所需的大量热能,造成围岩蚀变,从而萃取了围岩中的部分铀源;④随着酸性岩浆的不断分异结晶,岩浆中有用组分浓度不断增高,提供了深部热液中的成矿物质。     

准噶尔盆地中拐地区石炭-二叠纪火山岩特征及构造环境分析

中拐地区位于准噶尔盆地西北缘,是在石炭纪-早二叠世挤压应力场作用下形成的宽缓鼻状古隆起。石炭-二叠纪火山岩是主要的含油层系之一,到目前为止已发现了多个油气藏和出油气点,显示出良好的勘探前景。根据系统、精细的地震资料解释,中拐地区在石炭-二叠纪时期主要存在4期构造运动,分别对应4个不整合面: 1)石炭系与二叠系之间; 2)二叠系佳木河组与下乌尔禾组之间; 3)二叠系下乌尔禾组与上乌尔禾组之间; 4)二叠系上乌尔禾组与三叠系百口泉组之间。通过对中拐地区石炭-二叠纪火山岩的全岩元素分析表明,该区域主要为中-酸性的安山岩、英安岩和流纹岩及火山角砾岩和凝灰岩为主,其中SiO₂的含量普遍较高,平均含量达到了67％,判定中拐地区的石炭-二叠纪火山岩喷发环境应位于岛弧体系的大陆内侧,在喷发过程中经历过陆壳的熔融作用。结合前人对周缘火山岩的研究,认为石炭-二叠纪时期中拐地区由岛弧环境逐渐进入陆内演化阶段,三叠纪开始进入稳定的沉积演化时期。     

Characteristics and genesis of diachronous Carboniferous volcano-sedimentary sequences: insights from geochemistry,petrology and U–Pb dating in the North Junggar basin,China

ABSTRACT

The subduction of oceanic lithosphere during the Carboniferous Period contributed to the formation of widely distributed subduction-related volcanic rocks within the Junggar basin. These volcanic rock associations contain significant clues for understanding the subduction of the Keramaili oceanic lithosphere and the filling of the remnant oceanic basin. Here, we report regional gravity and magnetic data, petrology, geochemistry, and U–Pb dating for Carboniferous volcanic rocks from the North Junggar basin (NJB). Using samples from well Y-1, we distinguish upper and lower volcanic sequences on the basis of selected geochemical data. An isochronous stratigraphic framework of Carboniferous volcano-sedimentary sequences is then constructed and the petrogenesis of these volcanic rocks is discussed. Finally, we propose an explanation for the genesis of these diachronous Carboniferous volcano-sedimentary sequences. The results show that various volcanic rocks are distributed in different areas of the NJB, and mainly consist of calc-alkaline basalt–andesite–dacite assemblages and alkaline basalt–basaltic andesite–andesite assemblages. The geochemical data also demonstrate a binary nature of the Carboniferous volcanic rocks. In the eastern NJB, the lower and upper volcanic sequences are formed during the early and late Carboniferous, respectively. However, all of these volcano-related sequences in the western of the NJB are formed during the late Carboniferous. These volcano-sedimentary sequences exhibit a ‘ladder-style’ of temporospatial evolution from east to west. The geochemical results also indicate that the upper volcanic rocks include island arc components formed in an extensional setting, whereas the lower volcanic rocks were derived from deep crustal cycling metasomatism by various mantle components in a continental arc environment. Earlier closure of the Keramaili oceanic basin and slab roll-back of the Junggar oceanic lithosphere in eastern versus western Junggar basin led to the formation of these diachronous volcano-sedimentary sequences.     

Geochemistry of Early Cretaceous volcanic rocks in the Northeastern Great Xing’an Range,northeast China and implication for geodynamic setting

ABSTRACT

The mechanism that triggered large-scale Late Mesozoic magmatism in the northeastern Great Xing’an Range (NE GXAR) is strongly controversial. In this paper, we present whole rock geochemistry and zircon trace element, U-Pb and Hf isotopic data on the volcanic rocks in the Longjiang and Guanghua formations in the northeastern Xing’an Block. Zircons with ages of 120–119 Ma indicate that these volcanic rocks were formed in the Early Cretaceous. Combined with previous data, it is clear that volcanic rocks in the NE GXAR erupted between 128 and 108 Ma. The andesite samples of the Longjiang Formation show high contents of Al₂O₃, CaO, and MgO, significant negative Nb, Ta, and Ti anomalies; ε_Hf (t) values of zircons from the andesite sample vary from +4.13 to +7.67, indicating an enriched mantle source. The rhyolites of the Guanghua Formation show high SiO₂ and K₂O concentrations, low P₂O₅, MgO, Cr, and Ni contents and Mg# values. The positive ε_Hf (t) values (+5.72 to +10.58) with two-stage Hf model ages ranging from 939 to 701 Ma indicate that the rhyolites are derived from the partial melting of basaltic lower crust. Combined with the regional geological evolution, we conclude that the generation of the Early Cretaceous volcanic rocks in the NE GXAR might be triggered by the dehydration, disintegration, and foundering of the Mongol-Okhotsk Oceanic flat-slab and the subsequent upwelling of the asthenosphere.     

澜沧江南带三叠纪火山岩岩石学、地球化学特征、Ar-Ar年代学研究及其构造意义

滇西三江地区澜沧江南带广泛发育三叠纪火山岩。在北部云县一带,中晚三叠世火山岩出露齐全,自下而上可划分为中三叠统忙怀组(T₂m),上三叠统小定西组(T₃x)和上三叠统芒汇河组(T₃mh)。忙怀组以酸性火山岩为主,为一套流纹岩夹火山碎屑岩组合;小定西组发育为中基性火山熔岩夹火山碎屑岩;芒汇河组具有流纹质火山碎屑岩与玄武岩共存的"双峰式"火山岩特征。地球化学特征表明,南澜沧江带三叠纪火山岩具有弧火山岩与大陆板内火山岩的双重属性,推测其形成环境为过渡型的大陆边缘造山带环境。对南澜沧江带南部景洪附近采集到的石英安山岩样品进行Ar-Ar年龄测试,得到的坪年龄为236.7±2.2Ma,为中三叠世。结合火山岩年代学结果,推测澜沧江洋主碰撞期为早三叠世,中三叠世与晚三叠世早期分别为碰撞后的应力松弛阶段与洋盆继续俯冲期,到晚三叠世末期,俯冲作用结束,澜沧江洋关闭。     

三塘湖盆地马朗凹陷石炭系火山岩系油气成藏主控因素及模式

三塘湖盆地马朗凹陷石炭系主要发育两套烃源岩,分别为哈尔加乌组上段和哈尔加乌组下段,油气藏形成的源控作用十分明显。通过烃源岩和原油地球化学的分析以及油源对比,发现不同类型原油的形成与分布严格受控于对应源岩的分布范围。分析表明,马朗凹陷石炭系火山岩系油气成藏的主控因素是优质烃源岩、强充注油源断裂和有利火山岩相带的合理配置。根据烃源岩与储层的配置关系,石炭系油气藏的形成可以概括为两种模式:一种为风化壳型成藏模式,油气聚集在石炭系火山岩顶部受风化淋漓作用改造的优质储层中,其中的油气来自下部烃源岩,运移通道为与烃源岩相沟通的油源断裂;另一种模式为内幕型成藏模式,储层为流体溶蚀改造的储层,其中聚集的油气来自邻近火山喷发间歇期沉积的炭质泥岩。     

松辽盆地昌德地区营城组火成岩气藏储层特征及其控制因素

松辽盆地昌德地区火山岩储层以酸性岩为主,主要岩石类型为熔结凝灰岩和凝灰岩,与国内其他含油气盆地的火山岩储层明显不同。宏观上岩性较致密,岩心观察仅见构造裂缝和溶蚀孔,局部见溶蚀洞,显微镜下观察储集空间类型多样,发育粒内溶孔、晶内熔孔、晶间微孔、构造微裂缝、炸裂缝和收缩缝等。火山岩厚度普遍较薄,其中芳深9井区火山岩厚度最大,最厚仅为75m。从岩相类型看,芳深9井附近为爆发相,其他地区为溢流相。研究区火山岩为低孔低渗储层,岩性和岩相与物性关系密切,同时控制着原生孔隙的形成。后期的成岩改造,包括构造活动、溶蚀作用等决定了次生孔隙的发育程度,构造运动产生的裂缝是该区储层得以改善的主要因素,而后期充填作用破坏了储层的储集性能。火山口附近爆发相与溢流相的叠合区是天然气勘探的有利目标。     

中国古亚洲域沉积盆地火山岩油气藏储层特征比较及其差异分析

在中国古亚洲域沉积盆地火山岩储层母岩年代和岩性、储集空间类型以及火山岩岩相综合分析的基础上,探讨了火山岩油气藏的储层特征及其差异性。研究表明,中国古亚洲域火山岩储层的母岩发育年代西早东晚,西部的准噶尔、三塘湖和吐哈盆地发育晚古生代海相、海陆交互相的中基性安山岩、玄武岩及火山碎屑岩;东部的松辽、二连和海拉尔盆地发育中生代陆相中酸性流纹岩、安山岩。火山岩原生储集空间包括气孔、孔洞以及冷凝收缩裂缝等;次生储集空间包括各种溶蚀孔及构造裂缝、风化裂缝等。火山岩岩相可分喷出相、火山通道相、次火山相和火山沉积相。其中中基性岩类多以溢流相开始,相序类型为溢流相、爆发相/火山沉积相;中酸性岩类多以爆发相或火山通道相发端,主要相序为爆发相、溢流相/侵出相。溢流相一般发育原生气孔、构造缝;爆发相多为粒间孔,而侵出相以角砾间孔和原生裂缝为主。火山岩储层的差异受多因素影响,包括岩性岩相、喷发环境以及后期构造、成岩作用等。前者奠定火山岩储层形成与分布的基础和储集空间的发育程度;后者则改造储层的储渗性能。     

Mesozoic collision-related magmatism and crust–mantle interaction along the Anhui segment of the Yangtze River Valley,east-central China

The middle segment of the Yangtze River Deep Fault Belt, located in the foreland of the Dabie orogen, contains widely exposed volcanic–intrusive complexes that formed during two episodes of magmatism (post-collisional and post-orogenic), reflecting crust–mantle interactions during the Late Jurassic (J₃) to Early Cretaceous (K₁). This article summarizes research on the Mesozoic igneous suites and xenolith suites in the area along the Yangtze River. ‘Post-collisional magmatism’ occurred during lithospheric extension at ～145–130 Ma. Its beginning and end are marked by gabbroic xenoliths and pyroxene cumulates within intrusions at Tongling, and by alkali-rich magmatic rocks. The association includes peraluminous silicic rocks and metaluminous mafic–felsic igneous suites, ranging from medium-K to high-K calc-alkaline to shoshonitic compositions. Taking the Tongling region as an example, quartz monzodiorite yields a sensitive high resolution ion microprobe (SHRIMP) zircon U–Pb age of 139.5 ± 2.9 Ma, and granodiorite yields an age of 135.5 ± 4.4 Ma. These intrusive rocks contain 52.79–66.46 wt.% SiO₂, 13.12–17.73 wt.% Al₂O₃, 1.37–4.62 wt.% MgO, 3.86–6.84 wt.% FeO^T, and 4.71–7.87 wt.% total alkalis (Na₂O?+?K₂O). ACNK values range from 0.62 to 1.20, and ANK values from 1.45 to 3.48. ‘Post-orogenic magmatism’ occurred during lithospheric delamination at ～130–120 Ma. The start of magmatism was marked by the formation of gabbro containing spinel lherzolite xenoliths in the Nanjing–Wuhu Basin (NWB), and its end was marked by the generation of feldspathoid phenocryst-bearing phonolite in the NWB and the Lujiang–Zongyang Basin (LZB), respectively. The association that formed during this episode ranges from alkaline to peralkaline. Taking the Niangniangshan Formation in the NWB as an example, the Nosite phonolite yields a whole-rock monomineral Rb–Sr isochron age of 120 ± 9 Ma, and contains 49.92–60.09 wt.% SiO₂, 17.67–20.65 wt.% Al₂O₃, 0.08–2.45 wt.% MgO, 1.32–6.62 wt.% FeO^T, and 9.24–13.92 wt.% total alkalis (Na₂O?+?K₂O). ACNK values range from 0.72 to 1.24, and ANK values from 1.03 to 1.35.

The two magmatisms correspond to two episodes of crust–mantle interaction. The first involved intensive interaction between middle–lower crust and underplated basaltic magma derived from the upper mantle lithosphere, whereas the second involved minor interaction between the middle–lower crust and basaltic magma derived from the lower lithospheric mantle.     

Chemical geodynamics of Western Anatolia

We report major and trace element concentrations and Nd–Sr–Pb isotopic data of 10 post-collisional volcanic domains in Western Anatolia, a seismically active part of the Alpine–Himalayan belt in the Aegean extensional province. Our objective is to provide geochemical constraints for tectono-magmatic processes shaping the late Cenozoic geodynamic evolution of Western Anatolia.

Calc-alkaline volcanic rocks occurring to the north of the Izmir–Ankara–Erzincan suture zone show arc-like trace elements and isotopes and were formed by the melting of the metasomatized Neotethyan mantle-wedge; this process was facilitated by asthenospheric upwelling resulting from slab delamination. Calc-alkaline and alkaline volcanic rocks from within the Izmir–Ankara–Erzincan suture zone also show the imprint of subduction fluids in their major and trace elements, but their isotopic compositions indicate derivation from a metasomatized lithospheric mantle followed by assimilation of ancient crust. Volcanics along the N–S-oriented Kirka–Afyon–Isparta trend were derived from the lithospheric mantle that was metasomatized by fluids from the older subduction of the African plate. Golcuk–Isparta volcanic rocks show an asthenospheric imprint; the latter was a consequence of upwelling following a tear in the subducting African lithosphere. Shoshonitic Kula volcanic rocks show very high trace element concentrations, OIB mantle-like trace elements, and Nd–Sr–Pb isotopic signatures, and were formed by partial melting of the upwelling asthenospheric mantle; this event was synchronous with the Aegean extension and possibly also with slab window formation due to ruptures in the African plate.

Inherent in the above chemical geodynamic models are the high ?_Nd(0) values (+6.4) of the end-member volcanic rocks, implying the presence of an asthenospheric source beneath Western Anatolia that is responsible for the currently observed high heat flow, low Pn wave velocities, high seismicity, and tectonic activity.