Protoconodont fossils for refining the Cambrian bottom and the contribution to shale gas formation along the southwest margin of Yangtze Block

It has been an intense debate on the exact boundary between Ediacaran and Cambrian in the southwest Yangtze Block.The calibration of this critical boundary has a remarkable influence on the further investigation of the break-up of the Rodinia Supercontinent,the early life evolution,and the mechanism of the phosphorite deposit.Ediacaran and Cambrian strata and fossils are widely distributed in Anning,Yunnan Province in China.In recent years,the Xiaowaitoushan Member from the Lower Yuhucun Formation has been studied.Through this interval with continuous collections,the first appearance datums(FADs)of the protoconodont(Fomitchella cf.inchoate Yang et He,Protohertzina cf.anabarica Missarzhevsky)and globular embryos fossil(Olivooides sp.)earlier than these in the Lower Cambrian strata of the Meishucun Formation were discovered.This discovery indicates that the Xiaowaitoushan Member has included more FADs than the previously discovered single FAD of Anabarites primitivus Qian et Jiang,and the Ediacaran–Cambrian boundary in southwest China should be replaced below the Point“A”of the Meishucun Formation in Yunnan Province.The Point“B”of the Meishucun Formation is younger than the suggested age 541 Ma of the Ediacaran–Cambrian boundary and can no longer reference the Global Boundary Stratotype Section and Point(GSSP)correlation in southwest China.It can be suggested based on the previous stratigraphy and palaeontology studies from northern Sichuan and southern Shaanxi and the FAD of the globular embryos fossils that the Ediacaran‒Cambrian boundary in the southwest Yangtze Block should be placed at the base of the Xiaowaitoushan Member;other phosphorite strata refer to Xiaowaitoushan Member.The discovery of the FADs of the shelly fossils in the Xiaowaitoushan Member provides new evidence for the global correlation of the Ediacaran–Cambrian boundary in the southwest Yangtze Block.The conodont discoloration index(CAI)of the specimens in Anning is between 2 and 3,which indicates that the organic matter in Xiaowaitoushan Member is matured and has high potential to form a shale gas reservoir.

Ca. 1.7 Ga Magmatism on Southwestern Margin of the Yangtze Block: Response to the Breakup of Columbia

This paper presents some data of the Jiaopingdu gabbro and Caiyuanzi granite at the southwestern margin of the Yangtze Block,on the geochemical compositions,zircon LA-ICP-MS U-Pb ages and Hf isotopic data.The Jiaopingdu gabbro gives the age of 1721±5 Ma,the Caiyuanzi granite 1732±6 Ma and 1735±4 Ma,and the Wenjiacun porphyry granite 1713±4 Ma,suggesting nearly contemporaneous formation time of the gabbro and granite.The bimodal feature is demonstrated by the gabbro Si O2 content of 44.64-46.87 wt%and granite 73.81-77.03 wt%.In addition,the granite has high content of Si O2 and Na2 O+K2 O,low content of Al2 O3 and Ca O,enriched in REEs(except Eu)and Zr,Nb,Ga and Y,depleted in Sr,implying it belongs to A-type granite geochemistry and origin of within-plate environment.The zirconε(Hf)(t)of the granite and gabbro is at the range of 2-6,which is near the 2.0 Ga evolution line of the crust,implying the parent magma of the gabbro being derived from the depleted mantle and a small amount of crustal material,and the parent magma of the granite from partial melting of the juvenile crust and some ancient crustal material at the same time.Compared with 1.8-1.7 Ga magmatism during breakup of other cratons in the world,we can deduce that the Columbia has initially broken since ca.1.8 Ga,and some continental marginal or intra-continental rifts occurred at ca.1.73 Ga.

DISCUSSION ON RELATIONSHIP BETWEEN WESTERN MARGIN OF YANGTZE BLOCK AND SOUTHWESTERN SANJIANG REGION IN PROTEROZOIC

The western margin of Yangtze block and southwestern Sanjiang region absorbed much attention from geologists. It has been proved that there occurred a series of plate subduction, collision, assembly, rifting and breakup processes between them since Palaeozoic and the tectonic evolutionary relationship between them is clear. But in Proterozoic this kind of links between them became unclear. Did they undergo the assembly and breakup processes of the Rodinia super continent? This paper will take a primary discussion on this question on the basis of basement component, structure characteristics and magmatic activities.1\ Basement features\;(1) In western margin of Yangtze block its basement is composed of crystalline basement and folded basement, a so\|called double\|layer structure. The crystalline basement is made up of Kangding group, Pudeng Formation and Dibadu Formation, among them Kangding group is a representative and composed mainly of migmatite, compositing gneiss, hornblende schist and granulitite. The isotopic age of crystalline basement is older than 1900Ma, so its geological time is late Archaean to early Proterozoic. The folded basement is composed of Dahongshan group, Hekou group, Kunyang group, Huili group and Yanbian group. Their rock associations are made up mainly of spilite\|keratophyre formation, carbonate formation, clastic rock and clastic rock formation with some basic volcanic rocks. The folded basement is assigned to be early and middle Proterozoic (1000～1700M a).

The Type and Evolution of Neoproterozoic Sedimentary Basin in the Dahongshan Region,Northern Margin of the Yangtze Block:An Insight from Sedimentary Characteristics of the Huashan Group

Objective The Huashan group（composed of the lower Hongshansi Formation and the upper Liufangzui Formation）is an important Neoproterozoic stratigraphic unit along the northern margin of the Yangtze Block.Previous documents have focused on the geochronological and geochemical aspects of the Neoproterozoic sedimentary basin in the Dahongshan region.However.

扬子陆块西缘寒武系砂岩的物源分析:对古地理位置重建和构造背景的指示

扬子陆块西缘寒武系主要为一套碎屑岩-碳酸盐岩的岩石组合,前人研究多认为形成于相对稳定的克拉通盆地。但同时期出现的大陆岩浆作用显然与前期认定的克拉通盆地性质不符,需要借助扬子西缘的物质来源探讨构造背景。基于野外露头等资料,本文通过对扬子陆块西缘会泽和会东附近寒武系3件砂岩样品进行重矿物分析、电气石电子探针和碎屑锆石U-Pb测年分析,确定扬子西缘寒武纪沉积物的源区;并结合沉积序列等综合探讨扬子陆块西缘寒武纪的构造背景。沉积序列表明,扬子西缘寒武系沧浪铺组、西王庙组和二道水组主要由砂岩和白云岩等组成,沉积环境为滨岸—潮坪。细—粗砂岩碎屑颗粒为次棱角状—次圆状,分选较差;碎屑组分主要为石英,岩屑几乎全部为燧石,长石含量较少。测试分析结果表明:重矿物分析指示扬子西缘寒武系砂岩重矿物主要由锆石、赤—褐铁矿、电气石、钛铁矿、金红石、磷灰石等组成,重矿物组合指示岩浆岩为其主要母岩;电气石电子探针分析结果表明,物源主要来自于贫锂花岗岩和变砂岩、变泥岩;碎屑锆石测年分析表明物源区母岩主要为983~540 Ma岩浆岩。碎屑锆石年龄对比等综合分析表明,寒武系沉积物部分源自康滇古陆983~708 Ma的岩浆岩和变沉积岩,部分源自冈瓦纳大陆东非造山带663~540 Ma的岩石,物源区岩石经历再旋回产物作用。扬子西缘寒武系的沉积序列、碎屑锆石年龄谱图和碎屑组成等特征综合分析表明,扬子陆块西缘寒武系形成于前陆盆地。

Zircon U/Pb dating and Hf-O isotopes of the Zhouan ultramafic intrusion in the northern margin of the Yangtze Block,SW China:Constraints on the nature of mantle source and timing of the supercontinent Rodinia breakup

The Zhouan ultramafic intrusion in the northern margin of the Yangtze Block is mainly composed of lherzolite.Zircon grains selected from lherzolite are irregular in shape with distinct oscillatory and sector zoning and have Th/U ratios ranging from 0.8 to 10.6,indicating a magmatic origin.The weighted average 206 Pb/238 U age is 637±4 Ma (2σ,n=15),which can be considered as the crystallization age of the Zhouan intrusion.Zircon grains have δ18O values ranging from 5.2‰ to 7.0‰,with an averaged value of 5.8±0.4‰ (1σ,n=33),similar to the mantle δ18O value of zircon.Their 176Hf/177Hf(t) ratios range from 0.282410 to 0.282594 with εHf(t) values ranging from 1.3 to 7.6,lower than the corresponding value of the depleted mantle (～15),indicating an enriched mantle source.The enriched mantle source may have generated from a metasomatized lithospheric mantle with subducted slab.A number of ～635 Ma mafic-ultramafic intrusions in the Suizao basin are associated with coeval bimodal volcanics of the Yaolinghe Formation,indicating a continental rift setting.The ～635 Ma magmatic event in this region may represent the product of the last breakup of the Rodinia supercontinent in the northern margin of the Yangtze Block at Neoproterozoic.

扬子陆块西北缘早新元古代俯冲增生过程的岩浆记录

华南早新元古代俯冲相关岩浆作用记录为揭示罗迪尼亚超大陆边缘陆块的聚合及增生过程提供了重要制约。本文聚焦华南扬子陆块西北缘出露的早新元古代岩浆作用记录,总结梳理了其年代学框架、地球化学特征以及同位素特征,查明了其源区性质和岩石学成因、并探讨了不同阶段岩浆记录所对应的构造环境。结果表明,扬子陆块西北缘约1.0~0.9 Ga岩石具有与新生岛弧岩浆类似的微量元素特征,强不相容元素的含量略低于大陆弧,并且具有亏损的Sr-Nd-Hf同位素组成和略低于地幔值的锆石δ~(18)O值,这些特征指示约1.0~0.9 Ga岩浆岩最有可能形成于洋内弧环境。相比之下,约0.9~0.83 Ga岩浆岩具有与平均上地壳类似的微量元素特征,富集强不相容元素和轻稀土元素,亏损高场强元素,并且具有富集的SrNd-Hf同位素组成,锆石δ^(18)O值与地幔值相当或略高,指示其可能形成于大陆弧环境。此外,镁铁质岩石全岩Nd和锆石Hf同位素随年龄的长期变化趋势揭示了地幔源区性质周期性地富集和亏损,这可能是由于俯冲带间歇性地前进和后撤引起的挤压–伸展构造体制的不断交替所致。本文结果为扬子陆块西北缘新元古代早期的构造演化历史和俯冲增生动力学机制提供了制约。

沉积矿床中重要(变价)金属元素与古海洋环境关系综述--以新元古代-早古生代扬子板块周缘为例

对氧化—还原敏感元素和生命必需元素地球化学性质和相关沉积矿产形成机制及其时空分布特征的分析、总结和归纳,有助于对古海洋环境演化与相关沉积矿产形成关系的深入认识,这对沉积矿床学理论的发展具有重要意义。新元古代南华纪—早古生代早志留世,扬子板块周缘黑色岩系中依次沉积了举世瞩目的南华纪锰矿、震旦纪—早寒武世沉积磷块岩、早寒武世重晶石矿、镍—钼—钴多金属矿、钒矿和早志留世铀矿的完整沉积矿床系列,不同时代地层、同一时代不同沉积环境地层赋存不同类型的沉积矿产,有必要对其成矿机理进行系统研究、总结和归纳。笔者等在对氧化—还原敏感金属元素、生命必需元素和海底热液元素等元素地球化学性质综述的基础上,兼论了它们在结合黄铁矿矿物学对氧化—还原环境、生物生产力和盆地限制性分析方面具体作用;进一步结合新元古代—早古生代扬子板块古环境演化特征,对扬子板块周缘特定时代中特定沉积矿产的分布、成因机理进行分析、总结和归纳;最终得出扬子板块周缘相关沉积矿产,是在全球氧化事件、冰期—间冰期、Rodina大陆裂解大背景下,海洋中成矿金属元素得到长期积累,在特定时代区域性海水—海底热液—生物作用(根据不同矿种各有侧重)影响下,在相关重要元素地球化学性质的控制下、局部区域特殊氧化—还原环境的控制下形成的特定矿产的初步认识。最后,结合相关沉积矿床研究中存在的问题,提出下一步解决方案与展望。

荥经-峨眉地区上三叠统物源变化及对扬子板块西缘构造活动的启示

碎屑岩物源研究是当今地质学研究的热点,利用多种物源研究方法可以全面揭示碎屑岩的源岩信息,揭示源区构造古地理格局的变化。三叠纪期间,扬子板块西缘由被动大陆边缘转变为挤压构造带,引起了显著的古地理格局和沉积体系的变化,但目前对这一转变过程仍缺乏研究。本文综合利用砂岩颗粒成分统计、电子探针测试重矿物成分和碎屑锆石U-Pb年龄方法,研究了荥经-峨眉地区上三叠统碎屑岩物源转变时间,并结合其他地区物源研究探讨了扬子板块西南缘的物源变化过程。结果显示,上三叠统马鞍塘组的砂岩主要由石英和长石组成;电气石主要来自贫锂的花岗岩类和相关伟晶岩、细晶岩,其次来自富铝贫钙变质碎屑岩;铬尖晶石来自具有洋岛和大火成岩省背景的基性-超基性火山岩;碎屑锆石U-Pb年龄的主要分布在730~850Ma范围内。上三叠统须家河组的砂岩由石英、岩屑和长石组成,具有再旋回造山带物源;电气石大部分来自富铝贫钙、贫铝贫钙的变质碎屑岩,小部分来自贫锂的花岗岩类和相关伟晶岩、细晶岩;铬尖晶石主要来自具岛弧背景的橄榄岩;碎屑锆石U-Pb年龄主要分布在200~300Ma、400~500Ma、650~850Ma、1750~1950Ma和2400~2600Ma年龄段。综合分析表明,马鞍塘组的物源为康滇构造带的新元古代花岗质结晶基底,并包含少量新元古代中酸性火山岩、变质碎屑岩和二叠纪基性-超基性火山岩;须家河组的物源包括松潘-甘孜褶皱带的三叠系浊积岩、花岗岩类以及龙门山断裂带的上古生界碎屑岩与新元古界基底。马鞍塘组与须家河组之间的物源转变,表明扬子板块西缘的松潘-甘孜褶皱带和龙门山断裂带取代康滇构造带成为新的地貌高部位和主要剥蚀区,这一古地理格局的转变发生于马鞍塘组沉积之后,即晚三叠世的卡尼期与诺利期之间或诺利期期间。

扬子地块北缘镇巴穿心店地区南华系碎屑锆石U-Pb年龄及其物源示踪

扬子地块北缘镇巴穿心店地区发育有完整的南华系—震旦系沉积地层,记录了研究区及邻区南华纪沉积特征,对于研究扬子地块北缘新元古代晚期构造演化历史和Rodinia超大陆演化过程具有重要的地质意义。研究区南华系由下至上可划分为4套岩石地层单位,分别为长安组、古城组、大塘坡组和南沱组。南华系砂岩地球化学特征表明,研究区南华系化学风化作用较弱,经历了较低程度的沉积物再旋回作用。砂岩的CIA、CIW、PIA、Rb/Sr等古气候指标显示,研究区南华系具有由寒冷干燥—有波动的寒冷干燥—趋于稳定的温暖湿润—寒冷干燥的古气候演化趋势。在Dickinson三角图解中清晰地反映了南华系各组砂岩具有再旋回造山带和切割岩浆弧物源的特征,同时砂岩岩石地球化学特征也指示南华系的物源区主体应由中酸性岩浆岩组成。南华系长安组、古城组和南沱组3件碎屑锆石U-Pb年龄样品的年龄分布特征类似,主要年龄区间为新元古代晚期(约780~约641 Ma),并显示出约750~约740 Ma的显著峰值。结合区域地质资料,研究区南华系沉积时代应为约720~约635 Ma,物源应主要来自位于现今研究区西侧的扬子地块北缘汉南—米仓山微地块的新元古代岩浆岩,其沉积事件对应于新元古代中晚期扬子地块北缘伸展-裂解阶段,为Rodinia超大陆裂解过程的沉积响应。

扬子地块西缘峨山新元古代高分异Ⅰ型花岗岩地球化学特征及岩石成因

峨山高分异Ⅰ型花岗岩位于扬子地块西缘,是扬子地块新元古代岩浆岩带的重要组成部分,对其源区、成因及其构造背景进行系统研究能够揭示新元古代地壳演化历史。本文对峨山高分异Ⅰ型花岗岩开展了LA-MC-ICP-MS锆石U-Pb定年及岩石地球化学研究。研究结果表明,峨山花岗岩主要由肉红色中粗粒花岗岩和灰白色中粗粒花岗岩组成,它们的形成年龄分别为(746±34)Ma(MSWD=4.2)和(732±30)Ma(MSWD=3.3)。全岩地球化学显示峨山花岗岩具有高SiO_(2)(70.32%~78.41%)、Na_(2)O(3.09%~3.94%)、K_(2)O(5.13%~7.35%)含量,低CaO(0.52%~0.90%)、TiO 2(0.001%~0.025%)、P_(2)O_(5)(0.061%~0.097%)含量,富集K、Rb、Th等元素,亏损Nb、P、Ti等元素的特征,与高分异Ⅰ型花岗岩特征一致。全岩Sr-Nd同位素结果显示,ε_(Nd)(t)=-10.8~-7.5,两阶段Nd模式年龄(T DM2)为2.3~2.0 Ga。结合区域地质研究,本次研究认为峨山高分异Ⅰ型花岗岩是在伸展的构造背景下,地幔或年轻下地壳熔融形成的岩浆底侵由中高钾玄武质岩和黑云母片麻岩组成的古元古代上地壳,经部分熔融产生母岩浆,后经高程度的分离结晶作用而形成的。

扬子西缘东炉房Cu-Mo矿床侵入岩年代学、地球化学特征及其成矿意义

东炉房Cu-Mo矿床位于扬子陆块西缘,矿床发育与石英二长斑岩相关的斑岩型Mo(-Cu)矿和接触带矽卡岩型Cu矿。前人开展的研究工作主要集中在斑岩Mo矿化的成因,但是对矽卡岩型Cu矿化的研究较为薄弱。本文基于系统整理项目组对该矿床的勘查成果,对矽卡岩型Cu矿化有关的石英闪长玢岩开展锆石U-Pb年代学和岩石地球化学分析。结果表明,石英闪长玢岩锆石U-Pb加权平均年龄为82 Ma,表明其形成于晚白垩世,与格咱弧晚白垩世W-Mo(Cu)成矿作用时代一致。石英闪长玢岩具有高硅(SiO_(2)=60.74%~63.36%)、富碱(Na_(2)O+K_(2)O=7.03%~9.29%)特征,属准铝质(A/CNK=0.63~0.86)、钾玄岩系列I型花岗岩。岩石具有轻、重稀土分异明显(右倾型)和典型的埃达克质岩亲和性特征,可能为幔源岩浆注入中-基性下地壳部分熔融的产物。岩浆岩源区性质、围岩类型等条件决定了东炉房矿床的矿化分带性,深部石英二长斑岩的钾化带内发育斑岩型Mo(-Cu)矿化,浅部石英闪长玢岩和碳酸盐岩接触带发育矽卡岩型Cu(-Mo)矿化,外围大理岩局部发育脉状Pb-Zn-Au-Ag矿化。矿床由深到浅、由内到外构成了Mo(-Cu)→Cu(-Mo)→Pb、Zn、Au、Ag的斑岩成矿系统。通过区域综合对比认为格咱弧晚白垩世岩浆岩及相关成矿作用向南延伸至扬子陆块西缘,形成于碰撞后伸展环境,且由北往南岩浆分异程度逐渐降低,幔源岩浆贡献逐渐增多,导致成矿作用差异明显(休瓦促W-Mo→热林Mo-W→红山、铜厂沟Mo(-Cu)→东炉房Cu(-Mo))。

Geophysical evidence for thrusting of crustal materials from orogenic belts over both sides of the Yangtze Block and its geological significance

Based on deep geophysical detections, we have reconstructed the crustal structure from the eastern margin of the Tibetan Plateau to the Jiangnan-Xuefeng orogenic belt. The results suggest that the Yangtze Block was overthrusted by crustal materials in its NW direction from the eastern Tibetan Plateau but in its SE direction from the Jiangnan orogen. These overthrusting effects control the crustal structure from the western Sichuan to the western area of the Jiangnan orogen-Xuefeng orogenic belt. The eastward extruded materials from the eastern Tibetan Plateau were blocked by the rigid basement in the Sichuan Basin, where upper-middle crust was overthrusted whereas the lower crust was underthrusted beneath the Sichuan Basin. The underthrusted unit was absorbed by crustal folding, shortening and thickening in the Yangtze Block, forming the Xiongpo and Longquan Mountains tectonic belts and resulting in the NW-directed thrusting of the Pujiang-Chengdu-Deyang fault, and the western hillsiden fault in the Longquan Mountain. These results provide resolution to the controversy where the eastward extrusion material from the Qinghai-Tibet Plateau had gone. Overall, that Yangtze Block was subjected to thrusting of the crustal materials from the orogenic belts over its both sides. This finding has implications for the study of the intracontinental orogenic mechanism in South China, the reconstruction of tectonic evolutionary history and the kinematics processes during the lateral extrusion of the Tibet Plateau.

U-Pb Zircon Age,Geochemical and Sr-Nd-Hf Isotopic Compositions of Neoproterozoic Granitoids in Northwestern Margin of Yangtze Block (South China):Implications for Neoproterozoic Tectonic Evolution

ABSTRACT： The widespread Neoproterozoic magmatism along the Yangtze block carries critical in- formation for understanding the Neoproterozoic evolution of the Yangtze block. In the northwestern margin of the Yangtze block, the Hannan （汉南） intrusive complex includes the Wudumen （五堵门）, Erliba （二里坝） and Zushidian （祖师殿） granitoids. Using LA-ICP-MS U-Pb zircon dating method, the Wudumen and Erliba granitoids yielded magma crystallization ages of 785±4 and 778±3 Ma, respectively. Samples from these three granitoids show variable SiO2 contents ranging from 58.8% to 72.6%. They are characterized by enrichment of Al2O3（14.97%-17.87%）, Na2O（3.80%-5.33%） and Sr （504ppm-741 ppm）, and depletion of Y （〈19 ppm） and HREE （e.g., Yb〈1.6 ppm）, resulting in high Sr/Y （29-161） and （La/Yb）N （7.3-27.8） ratios. The geochemical features of the granitoids are comparable with those of adakite. The granitoids have zircon εHdt） values of ＋3.65 to ＋10.05, whole-rock εNd（t） values of -0.09 to ＋2.98 and whole-rock initial ^87Sr/^86Sr ratios of 0.7034-0.7039, indicating that their magma was derived from a juvenile crustal source. Together with geochemical and Hf-Sr-Nd isotopic compositions, it is suggested that the granitoids formed in island-arc setting and originated from partial melting of a subducted oceanic slab. The results support a model that the Yangtze block was surrounded by ocean and arc magmatism in its northern and northwestern margins in Neoproterozoic.

扬子地块北缘毕机沟层状岩体亲铜元素地球化学特征及其对岩浆演化和硫化物熔离过程的制约

毕机沟层状岩体是扬子地块北缘汉南杂岩中最重要的镁铁-超镁铁质侵入体之一,主要由下部带超镁铁质岩、中部带辉长岩和上部带闪长岩组成。本文对该岩体下部带橄长岩、橄榄辉长岩和橄榄辉长苏长岩以及中部带粗粒辉长岩、磁铁辉长岩和角闪辉长岩进行全岩主微量元素及亲铜元素分析,全岩(La/Sm)_(N )(0.43~2.89)和(Tb/Yb)_N(1.08~1.52)比值以及La/Yb-Sm/Yb图解均表明毕机沟层状岩体源区属于尖晶石二辉橄榄岩。Ni/Cu-Pd/Ir图解显示毕机沟层状岩体母岩浆主要为高镁玄武岩,模拟计算及高的S/Se比值显示其母岩浆经历了~5%的地壳混染并引进了外界硫,导致毕机沟母岩浆在深部发生早期硫化物熔离,造成其极高的Cu/Pd比值(5.21×10~3~1.67×10^(6))和低的PGE含量。毕机沟母岩浆侵位到浅部岩浆房后,下部带极少量的硫化物熔离进一步导致残余岩浆亏损PGE,但S和Cu含量相对升高;中部带从下部带残余岩浆中结晶分异,具有更低的PGE含量和相对较高的S以及Cu含量。根据毕机沟层状岩体岩浆演化及硫化物熔离过程,推测岩体深部及附近具有寻找Cu-Ni-PGE矿床的潜力,查明其岩浆通道系统及岩浆运移方向对找矿勘查具有重要意义。

扬子陆块北缘构造演化新认识:来自原花山群年代学和地球化学的制约

原花山群分布于紧邻南秦岭的扬子陆块北缘大洪山地区,出露于重要的构造部位,是研究其形成时期扬子陆块构造演化及其与南秦岭关系的重要载体,其物质组成、形成时代和构造属性长期存在争论。本文将原花山群解体为花山构造混杂岩和正常的火山—沉积地层(本文所指花山群)两部分来讨论。笔者重新厘定了花山群的沉积时限,有针对性地对有构造背景争议的花山群进行玄武岩地球化学研究,对有时代争议的混杂岩进行锆石U-Pb年代学研究。年代学、地球化学和沉积学综合研究表明,花山群的沉积时限为ca.830 Ma至ca.800 Ma,形成于与Rodinia超大陆裂解有关的陆内裂谷盆地。花山构造混杂岩带可能不只是晋宁期的缝合带,而是具有多期物质组成、经历了多期构造叠加的复合型缝合带。结合他人成果,我们提出了扬子陆块与南秦岭从新元古代到早古生代的构造演化新模式。

扬子地块西南缘红泥坡铜矿床成矿流体特征与铜矿化机制——来自流体包裹体和原位S同位素的证据

扬子地块西南缘红泥坡矿床是近年来新发现的大型铜矿床。本文通过流体包裹体岩相学、显微测温、激光拉曼成分分析和硫化物原位S同位素分析,揭示了红泥坡铜矿床的成矿物质来源、成矿流体来源及矿质沉淀机制。该矿床成矿过程分为火山-沉积期和热液成矿期。热液成矿期石英+方解石+硫化物阶段(Ⅰ)的石英中发育纯CO_(2)包裹体、水溶液-CO_(2)包裹体、含固相的水溶液-CO_(2)包裹体、含固相的水溶液包裹体以及富液相两相水溶液包裹体。各类包裹体成群分布,均一温度(106~500℃)和盐度(8.8%~59.8%)变化大。激光拉曼分析表明成矿流体中挥发分成分为H_(2)O、CO_(2)和少量CH_(4)。火山-沉积期黄铁矿原位δ^(34)S值为9.18‰~9.34‰,为海水硫和岩浆硫的混合硫;热液成矿期硫化物的原位δ^(34)S值为4.42‰~5.26‰,为岩浆硫和少量地层硫的混合硫。综合矿床成矿时代、流体包裹体及S同位素组成特征,认为古元古代火山-沉积作用形成含Fe和Cu的矿源层;新元古代随着罗迪尼亚超大陆的裂解,来自深部的岩浆流体携带的大量成矿物质在向上运移过程中与中低盐度、富CO_(2)的变质流体发生混合,触发了成矿物质的饱和沉淀,形成了红泥坡矿床的工业铜矿体。

扬子北缘大洪山造山带中新元古代基性岩和浊积岩的年代学和地球化学研究:对弧后盆地演化的指示

扬子北缘~820Ma可能记录了从挤压环境向伸展环境的重要转换,是解决扬子新元古代演化争议的关键时间节点。本研究在扬子北缘大洪山造山带圆潭地区识别出一套基性岩和浊积岩组合。其中基性岩全岩Sm-Nd等时线年龄为845Ma±81Ma(MSWD=1.5),浊积岩碎屑锆石年龄谱包括四个主要峰值年龄(~835Ma、~1597Ma、~2002Ma、~2482Ma)和两个次要峰值年龄(~2680Ma、~2934Ma),综合前人研究成果将浊积岩的形成时代约束在835~820Ma。浊积岩与花山群的形成时代一致,应代表花山群的北向延伸。浊积岩的物源来自扬子北缘桐柏-大别地区,不同于前人对花山群的分析结果,指示花山群可能具有多个物源。以上地层碎屑锆石年龄谱和地层层序特征与弧后盆地相似。本研究基性岩样品具有低的SiO_(2)(45.41%~50.29%)、K 2O(0.03%~1.49%),高的TiO_(2)(1.42%~3.03%),和低的U/Th(0.15~0.29)、Th/Zr(0.003~0.011)和Nb/Y(0.059~0.147)以及正的εNd(t)(+1.18~+1.57),类似N-MORB地球化学特征。基性岩来自于软流圈,经历<3%部分熔融而生成,并在上升过程中受到了一定程度的地壳混染。综合以上分析,本文认为这些基性岩和浊积岩共同构成一套弧后背景环境岩石组合。

扬子陆块西缘中元古代晚期元谋花岗岩的时代、成因及构造意义

扬子陆块西缘晚中元古代地质演化史一直存在较大争议,本文选择以扬子西缘元谋杂岩中一套二长花岗岩为研究对象,开展岩相学、锆石U-Pb年代学、全岩地球化学等综合研究,为认识和理解扬子西缘晚中元古代地质演化提供支撑。两件元谋二长花岗岩样品的LA-ICP-MS锆石U-Pb年龄分别为1086±10 Ma(MSWD=1.4,n=50)和1099±10 Ma(MSWD=1.8,n=58)。所有样品具有高硅(SiO2为69.44%~73.98%)、富碱(Na_(2)O+K_(2)O为6.11%~7.72%)、贫钙(CaO为0.39%~1.46%)、贫镁(MgO为0.52%~0.76%)、低钛(TiO_(2)为0.30%~0.59%)的特点,同时表现出强过铝质(A/CNK=1.19~1.35)及中钾钙碱性–钾玄岩系列特征。它们具有高的稀土元素总量(ΣREE=211.60×10^(−6)~349.01×10^(−6)),呈现轻稀土元素富集和重稀土元素亏损((La/Yb)N=4.32~7.36);富集Rb、U、Th等大离子亲石元素和Zr、Th、Hf等,亏损Nb、Ta、Ba等元素,并具有明显的负Eu异常(δEu=0.46~0.59),锆石饱和温度介于827~912℃之间,展示了A型花岗岩的属性。这些二长花岗岩可能是通过中上地壳的中酸性火成岩的部分熔融形成,结合前人的研究成果,它们最可能形成于弧后的伸展环境,综合扬子陆块周缘晚中元古代的岩浆记录,元谋杂岩中1.09 Ga二长花岗岩的形成应与扬子陆块开始参与Rodinia超大陆聚合有关。

扬子板块北缘镇巴地区吴家坪组富Li-REE-Nb-V黏土岩地球化学特征及其地质找矿意义

沉积型锂矿主要产于黏土岩中,黏土岩的主微量元素可有效识别沉积物源及沉积环境。本文首次报道扬子板块北缘镇巴地区二叠系吴家坪组黏土岩锂、稀土、铌、钒等元素超异常富集的特征,基于详细的地球化学研究和区域地质资料,分析吴家坪组黏土岩的形成环境及沉积物源,并探讨扬子板块北缘锂、稀土找矿方向。研究表明,镇巴地区吴家坪组黏土岩产于晚二叠世,发育于吴家坪组与阳新组的平行不整合界面之上,代表了古风化壳沉积,黏土岩的Li2O最高品位0.39%,ΣREO(稀土氧化物总量)最高品位0.142%,V_(2)O_(5)最高品位0.26%,Nb_(2)O_(5)最高品位0.022%。主、微量元素地球化学特征指示黏土岩形成于咸化-半咸化海水相,氧化-还原过渡、碱性的沉积环境,对应于黏土岩化阶段。综合矿物学、地球化学、区域地质资料,认为研究区吴家坪组黏土岩来源于峨眉山大火成岩省的玄武岩,锂元素的富集与火山岩受流体交代-淋滤有关。扬子板块北缘广泛发育吴家坪组黏土岩,规模较大,锂、稀土、铌、钒综合成矿效果较好,找矿潜力巨大,应重点关注与沉积间断面有关的关键金属矿产,助力新一轮找矿突破。