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.

Sinian hydrocarbon accumulation conditions and exploration potential at the northwest margin of the Yangtze region, China

Based on outcrop, drilling, logging and seismic data, the reservoir forming conditions, reservoir forming model and exploration potential of the ultra-deep Sinian Dengying Formation at the northwest margin of Yangtze craton region were examined.(1) This area is in craton rifting stage from Sinian to Early Cambrian, characterized by syn-sedimentary faults and rapid subsidence, significant sedimentary differences, and development of Dengying Formation platform margins on both sides of the rift.(2) The Sinian–Cambrian in this area has two sets of high-quality source rocks, Doushantuo Formation and Maidiping-Qiongzhusi Formation;of which, the latter has a thickness of 150–600 m and hydrocarbon generation intensity of(100-200)×10;m;/km;.(3) The mounds and shoals in the platform margin of Sinian Dengying Formation controlled by faults are thick and distributed in rows and zones;they are reformed by contemporaneous–quasi-contemporaneous and supergene karstification jointly, forming pore-type reservoirs with a thickness of 200-400 m.(4) The two sets of source rocks enter oil generation windows from Permian to Early Triassic, and the oil migrates a short distance to the lithologic traps of mounds and shoals to form a huge scale paleo-oil reservoir group;from Late Triassic to Jurassic, the oil in the paleo-oil reservoirs is cracked into gas, laying the foundation of present natural gas reservoirs.(5) The mound-shoal body at the platform margin of Dengying Formation and the two sets of high-quality source rocks combine into several types of favorable source-reservoir combinations, which, with the advantage of near-source and high-efficiency reservoir formation, and can form large lithologic gas reservoirs. The Mianyang-Jiange area is a potential large gas field with trillion cubic meters of reserves. According to seismic prediction, the Laoguanmiao structure in this area has the Deng-2 Member mound-shoal reservoir of about 1300 km^(2), making it a ultra-deep target worthy of exploration in the near future.

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).

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 magmafic origin. The weighted average 206pb/238U 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‰（1 or, 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 magmafic 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.

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.

扬子板块西南缘早前寒武纪地质演化:来自撮科元古宙变沉积岩碎屑锆石U-Pb年龄、Hf-O同位素的制约

扬子板块的太古宙-古元古代基底出露十分有限,制约了对扬子早前寒武纪地质演化的认识。本次研究对扬子板块西南缘撮科地区的大红山群和昆阳群变沉积岩开展了碎屑锆石原位U-Pb年龄、Hf-O同位素及微量元素研究。测年结果显示,大红山群谐和锆石年龄分布于3092~1786Ma之间,最年轻碎屑锆石年龄为1786Ma,限定大红山群最大沉积时代为古元古代晚期。昆阳群谐和锆石年龄分布于2874~1031Ma之间,最年轻碎屑锆石年龄为1031Ma,表明昆阳群最大沉积时代为中元古代晚期。除少量较年轻锆石(<1.75Ga)外,昆阳群和大红山群具有相似的年龄分布特征,两个主要的年龄峰为2.0~1.85Ga和2.4~2.2Ga。尽管古元古代碎屑锆石的年龄峰与撮科地区已报道的古元古代岩浆活动期次一致,但仅有少部分碎屑锆石的Hf-O同位素特征与撮科古元古代岩浆岩相似,大多数碎屑锆石Hf-O同位素组成与加拿大Rae克拉通古元古代变沉积岩中碎屑锆石相似,暗示古元古代碎屑物质可能仅少部分来自撮科地区已识别的古元古代岩浆岩,而大部分可能来自与Rae克拉通发育的古元古岩浆岩相似的源区。大红山群和昆阳群中1.9~1.03Ga的碎屑锆石可能来自扬子西南缘发育的古元古代晚期-中元古代晚期岩浆岩。结合前人的资料,我们认为我国撮科和越南北部的Phan Si Pan带经历了与加拿大Rae克拉通相似的古元古代演化过程,支持2.4~2.3Ga扬子西南缘卷入Arrowsmith造山事件,并与Rae克拉通一起参与哥伦比亚超大陆聚合过程的认识。

扬子地块西北缘侏罗纪重磁化及其大地构造意义

造山过程通常伴随着重磁化的广泛发生,因而造山带的重磁化研究有助于厘定其构造定型的时间.为了确定扬子地块西北缘逆冲推覆构造的定型时间,本文对大巴山弧形逆冲带西北端的西乡地区和龙门山逆冲带北段的宁强至广元地区成冰纪—寒武纪多个岩石单元开展了古地磁学、岩石磁学和岩相学研究,揭示出广泛的侏罗纪重磁化作用.样品的退磁特征、岩石磁学和扫描电镜观察结果显示,该重磁化分量为化学剩磁,载磁矿物为自生磁铁矿和赤铁矿,其形成与造山作用引发的流体迁移有关.结合研究区已发表的侏罗纪重磁化数据,大巴山弧形逆冲带和西乡—宁强地区记录了一致的侏罗纪重磁化方向,表明这一带逆冲构造格架的形成不晚于侏罗纪,而广元地区青林乡和陈家坝剖面的重磁化方向相对发生了~60°的逆时针旋转,表明该地区逆冲构造格架的形成晚于侏罗纪,但该区灌县—安县断层以东的近四川盆地区域未受影响,没有发生旋转.

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。

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。

扬子北缘大洪山地区新元古代辉长辉绿岩中单斜辉石成因及其构造意义

为进一步解析扬子地块北缘新元古代造山作用过程,本文对扬子地块北缘大洪山造山带~820Ma辉长辉绿岩中的单斜辉石进行了岩相学和原位主微量元素分析,共识别出三类单斜辉石:Ⅰ类单斜辉石核部和边部结晶温度分别为1159~1192℃、1162~1187℃,结晶压力分别为0.16~0.44GPa、0.31~0.52GPa;Ⅱ类单斜辉石结晶温度为1145~1179℃,结晶压力为0.27~0.53GPa;Ⅲ类单斜辉石结晶温度为1118~1150℃,结晶压力为0.09~0.35GPa。三类单斜辉石反演获得的母岩浆含水量分别为1.6%~2.4%、1.7%~2.2%、0.9%~2.1%。三类单斜辉石结晶于岩浆演化的不同阶段:早期基性岩浆形成了高Mg#值的Ⅰ类单斜辉石核部;随着岩浆结晶演化,形成低Mg#值的Ⅰ类单斜辉石边部和Ⅱ类单斜辉石;岩浆演化晚期伴随地壳混染,形成Ⅲ类单斜辉石。综合研究表明扬子地块北缘大洪山辉长辉绿岩形成于弧后盆地背景。

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))。

西大别地区吕王-高桥混杂岩带石墨赋矿岩系碎屑锆石U-Pb年龄:对中元古代海相沉积事件的约束

【研究目的】扬子陆块北缘大别山地区古元古代—中元古代的物质记录有限,制约了前寒武纪地质构造演化认识。大别山核部吕王—高桥混杂岩带内“石墨片岩–石英岩夹大理岩”沉积岩系的年代学研究可为大别山地区中元古代古洋(海)盆演化提供新约束,对区域前寒武纪成矿与找矿提供科学指导。【研究方法】在开展1∶10000地质测量查明混杂岩带岩石组合特征的基础上,本文重点利用激光剥蚀电感耦合等离子体质谱方法,对赋石墨矿变沉积岩系的石英岩开展锆石原位U–Pb同位素测年。【研究结果】大别造山带核部发育中元古代“石墨片岩–石英岩夹大理岩”变沉积岩系,为一套浅海相沉积、成熟度高的砂岩。石英岩原岩的碎屑锆石年龄峰值主要为2.55 Ga、2.06 Ga、1.86 Ga及1.43 Ga,最年轻峰值年龄1.43 Ga约束了原岩沉积时代下限。这套变沉积岩原岩沉积时代为中元古代晚期,其沉积物源可能主要来自扬子陆块基底。【结论】吕王—高桥混杂岩带保留了大别山核部较早的海相沉积记录,并为区域石墨成矿提供了物质基础。该变沉积岩系原岩可能形成于中元古代哥伦比亚超大陆裂解期的扬子陆块边缘海盆地。

鄂尔多斯块体西北缘层析成像研究

基于内蒙古测震台网记录的数字地震观测报告,使用3398个M≥1.0地震的Pg波和Sg波数据,采用双差层析成像方法对鄂尔多斯块体西北缘(37°~43°N,104°~110°E)地下结构进行研究,并对地震进行重定位,同时对该区上地壳速度结构进行详细分析。结果表明,从Pg波速度结构来看,5~10 km深度从东北到西南对角线存在不规则的连续高速体,15~20 km深度高速体不再连续,而呈散状分布;从Sg波速度结构来看,5 km和15 km处基本与Pg波速度结构一致,10 km和20 km处低速区分布较多且地震多发生在高低速体交界处和断裂周围,表明该区地壳的脆弱性。

二维初至波层析成像揭示的兰坪盆地—扬子西缘浅层地壳结构

兰坪盆地—扬子西缘位于青藏高原东南缘,在地质构造上属特提斯-喜马拉雅强烈挤压、碰撞造山带的东部,兼跨华南板块与古冈瓦纳板块两大构造单元。其浅层地壳是记录地壳形变、岩浆作用与成矿作用的重要载体。本文利用兰坪盆地—扬子西缘220km长的深反射地震剖面的初至波(Pg震相)数据,通过层析成像反演方法,获得了测线下方4km以浅的上地壳浅层P波速度结构。成像结果显示,兰坪盆地沉积厚度从西到东逐渐减薄,扬子块体西缘东部表现出“两侧厚中间薄”的特点;兰坪盆地内有多组逆冲断裂,金沙江断裂与程海断裂近乎垂直;金顶铅锌矿的形成可能与底部热隆有关,北衙金矿下方结晶基底向上突起,推测由岩浆上涌所致。

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。