The possible synglaciogenic Ediacaran hematitic banded iron salt formation (BISF) at Hormuz Island,southern Iran: Implications for a new style of exhalative hydrothermal iron-salt system

The Ediacaran BISF at Hormuz Island is a newly identified glaciogenic iron-salt deposit in the Tethyan margin of Gondwana. The BISF was formed by synchronous riftogenic A-type submarine felsic volcanism and evaporate deposition. The mineralization occurs in a proximal felsic tuff cone and jaspilitic distal zones and contains 1 million tonne of hematite-rich ore with an average grade of 58% Fe. The ore structure shows cyclicity of macrobandings, mesobandings and microbandings of anhydrite, halite, hematite and chert, which marks a new record in BIFs geohistory. The alteration minerals in the proximal and distal zones are actinolite, ripidolite, epidote, sericite, tourmaline, clinochlore, anhydrite and clay minerals. The occurrence of metamorphosed polygenetic bullet-shape dropstones in BISF attests that there was probably a continuous process of ice melting, episodic submarine volcanism and exhalative hydrothermal banded iron salt formation during the Late Ediacaran time. The non-metamorphosed Neoproterozoic stratigraphy, the presence of genus Collenia, U-Pb dating (558 ± 7 Ma) and the marked negative δ¹³C excursion in cap carbonates are representative of Late Ediacaran glaciation, which has been identified worldwide. The REE+Y display light REE enrichment, unusually strong Tb-Tm anomaly, a weak positive Y anomaly, but no distinguished Eu and Ce anomalies, reflecting the glaciogenic nature of the BISF. The contents of Zr, Hf, Nb, Ta, Th, La, Ce and Y in BISF, dropstones, halite and cap carbonates are similar to those of the Neoproterozoic glaciogenic BIFs. Also, the Ni/Fe, P/Fe ratios and Fe/Ti – Al/Al + Fe + Mn + Ca + Na + K diagram suggest an exhalative hydrothermal Ediacaran-type BISF. The absence of brecciated magnetite in the ore association and the low contents of copper (9–493 ppm) and gold (<5–8 ppb) are not in favor of the IOCG – Kiruna-type iron oxide ores. The co-paragenesis of hematite with several alteration minerals, in particular actinolite, tourmaline and anhydrite, indicates that the exhalative hydrothermal fluids were generated by the interaction of seawater with the felsic rocks and sediments at about 200–500 °C. The interaction of seawater with felsic magma and sediments led to the formation of Mg-rich alteration minerals, leaching Si, Fe, Mn and other elements and forming the potential ore fluids. It is highlighted that the A-type alkaline submarine felsic volcanism could be considered as an exploration target for BISF.     

内蒙古阿巴嘎旗白音图嘎地区晚石炭世花岗岩地球化学特征、锆石U-Pb定年及其地质意义

内蒙古白音图嘎地区位于西伯利亚板块东南缘晚古生代陆缘增生带,地处二连—贺根山板块对接带西北侧,该区发育大量晚古生代花岗岩,主要岩石类型为斑状二长花岗岩和碱长花岗岩。本文对该区花岗岩进行了LA-ICP-MS锆石U-Pb测年和岩石地球化学测试。结果表明:斑状二长花岗岩和碱长花岗岩锆石U-Pb年龄分别为302.8±1.3Ma(MSWD=1.4)和301.1±0.6 Ma(MSWD=0.84),属于晚石炭世;岩石地球化学数据显示,岩体属钾玄岩系,具有高硅(SiO_2=70.83%~75.01%),富碱(Na_2O+K_2O=11.58%~13.20%),贫钙(CaO=0.50%~1.22%)和镁(MgO=0.11%~0.33%)特征。铝饱和指数A/CNK=0.81~1.01,为准铝—弱过铝质花岗岩。岩体整体富集Rb、Th、U等大离子亲石元素(LILEs),亏损Ba、Sr以及高场强元素(HFSEs)Nb、Ta、P、Ti;LERR富集,HERR相对亏损,具有明显的负Eu异常(δEu=0.11~0.43)。初始岩浆温度(759.33℃~801.81℃)和岩石地球化学分析表明,白音图嘎花岗岩具有A型花岗岩特征。同时,低的Nb/Ta(7.38~19.50)、Zr/Hf(29.61~35.26)和Sm/Nd(0.22~0.32)比值指示其源岩为壳源。综合研究表明,白音图嘎花岗岩为陆壳变杂砂岩部分熔融的产物,形成于晚石炭世早期西伯利亚板块和华北板块后碰撞作用的板内伸展环境,为兴蒙造山带晚古生代构造岩浆演化及古亚洲洋闭合的时限提供了新的约束和佐证实例。     

西天山阿吾拉勒成矿带群吉A型花岗岩成因、地质意义及成矿潜力评价

群吉钠长斑岩具有富Si O2(65.3%~76.9%)、Na_2O+K_2O(6.02%~9.66%)、FeOT/Mg O(6.01~21.4),中等的A/CNK(0.90~1.04),低Al2O3(9.82%~15.64%)、Ca O(0.47%~1.84%)、MgO(0.13%~0.45%)的主量元素特征,同时富集Th、U、Ta、Zr、Hf等HFSE,轻稀土富集、重稀土相对平坦分布。高的Zr(468×10~(-6)~707×10~(-6))、Y(20.7×10~(-6)~91.4×10~(-6))、Nb(21.3×10~(-6)~57.7×10~(-6))、Ga(11.2×10~(-6)~19.7×10~(-6))以及Ce(36.7×10~(-6)~98.2×10~(-6))和Zr饱和温度(880℃)特征表明岩体具有A型花岗岩的特征。群吉钠长斑岩的ISr值较低(0.70203~0.70549),εNd(t=303Ma)为正值(+4.1~+5.2),同时全岩Pb同位素落于地幔和下地壳之间的区域,表明形成该岩体的源岩可能为下地壳玄武质岩石。地壳的伸展引起了软流圈地幔底侵,在异常地温梯度下被底侵的玄武质下地壳发生部分熔融,形成该地区晚古生代A型花岗岩岩体。群吉地区A型花岗岩的发现,表明在晚石炭纪西天山阿吾拉勒地区为伸展的构造背景,同时岩石圈的拆沉、下地壳的加厚在晚石炭纪就已发生。群吉钠长斑岩中有局部或全部的铜矿化,该岩体是阿吾拉勒成矿带主要的含矿岩体。钠长斑岩基质中含有自形-半自形的黄铁矿及斑岩中锆石较低的Ce4+/Ce3+(19.5~93.0,平均为39.6),反映了岩浆低氧逸度的成岩条件,这种条件使得S从高价态变为了低价态,有利于成矿。岩浆中Zr的含量与岩浆中的水分含量成反比,H2O的加入也可以使岩浆具有高的氧逸度,因此岩浆中高的Zr含量及低的氧逸度,说明岩浆的源区为"干"体系,H2O的加入很少,而较"干"的体系对形成大型斑岩矿床不利。     

滇西腾冲地块高黎贡山群早志留世变质花岗岩体的年代学、地球化学特征及意义

西南特提斯构造带广泛发育的早古生代岩浆岩是冈瓦纳大陆边缘原特提斯洋增生造山作用的产物,目前报导的岩浆岩侵位时代在536~448Ma。本文通过LA-ICP-MS锆石U-Pb定年,在腾冲地块东缘高黎贡山群中首次发现了年轻至~437Ma的片麻状花岗质岩体,并结合其锆石Hf同位素和全岩主微量地球化学特征,进一步制约原特提斯洋构造演化过程。样品主量元素显示此片麻状花岗岩体具有高硅(SiO_2=72.78%~73.69%)、富碱(K_2O+Na_2O=7.23%~8.70%)的过铝质(A/CNK=1.08~1.12)特征,微量元素显示此岩体相对富集轻稀土元素、大离子亲石元素(K、Rb)和Pb,亏损高场强元素(Nb、Ta、P、Zr、Ti)以及Ba、Sr、Eu。综合岩石样品的矿物组合特征和地球化学特征,判断该岩体为S型花岗岩,源于以砂屑岩为主的沉积岩类的部分熔融,且源区有斜长石的残留。锆石εHf(t)值(-9.8~-6.2)和二阶段模式年龄tDM2(2.0~1.8Ga)也表明其源于古老地壳沉积物,且无幔源物质加入。根据全岩锆饱和温度计和锆石Ti温度计得出其岩浆从源区发生部分熔融到固结的过程中,温度从794℃左右下降到约754℃。熔浆温度较高,推测源区部分熔融过程中有地幔热的供给。综合前人研究成果,冈瓦纳大陆边缘在早古生代依次经历了原特提斯洋板片俯冲(ca.530~510Ma)、地块群增生与洋板片断离(ca.510~490Ma)、岩石圈挤压增厚(ca.490~475Ma)和岩石圈地幔拆沉(ca.470~460Ma)。岩石圈地幔拆沉将导致软流圈上涌及随后大陆岩石圈的持续伸展。腾冲地块侵位于~437Ma的花岗质岩体系该拆沉构造后的伸展环境中,以砂屑岩为主的古老地壳沉积岩在地幔热的供给下发生部分熔融的产物。     

Geological and Geochemical Constraints on the Newly Discovered Yangchongli Gold Deposit in Tongling Region, Lower Yangtze Metallogenic Belt

The newly discovered Yangchongli gold deposit is a unique independent gold deposit in the Tongling ore-cluster region controlled by the tectonic alteration firstly discovered in the Lower Yangtze Metallogenic Belt(LYMB). The host magmatic rocks mainly consist of monzodiorite and K-feldspar granite. The LA-ICP-MS U-Pb zircons dating yielded weighted mean ~(206)Pb/~(238)U ages of 140.7 ± 1.8 Ma and 126.4 ± 1.2 Ma for the monzodiorite and K-feldspar granite, respectively. Monzodiorites are enriched in Sr, Ba, Rb, and depleted in Y, Yb with high Sr/Y and La/Yb ratios, similar to the geochemical features of adakite, considered as products of differentiation of mafic magmas originating from lithospheric mantle melt/fluids caused by metasomatism during paleo-Pacific Plate subduction in the Mesozic. In contrast, the compositions of K-feldspar granites are A-type granites, indicating an extensional tectonic background. Gold ores hosted in the fracture zone occurred as quartz vein within cataclastic rock. Sulfur and lead isotopes from pyrites show crust-mantle mixing characteristics. Metal components from strata also took part in the gold mineralization, and resulted from two episodes of magmatism that were probably related to tectonic transition from a compressive to an extensional setting between 140–126 Ma, which led to the Mesozoic large-scale polymetallic mineralization events in eastern China.     

阿尔金断裂带东段古生代花岗岩浆作用及其大陆动力学意义

造山带花岗岩浆作用一直是地学的重要研究方向, 它记录了地球动力学深部过程的信息, 开展深入的研究工作可以更好的了解板块汇聚环境的陆壳生长和再造以及壳幔之间的相互作用。北祁连造山带是一典型的早古生代造山带, 先后经历了洋盆的打开到闭合, 敦煌地块则是主要由前寒武纪TTG片麻岩和变质表壳岩组成。北祁连造山带和敦煌地块分别位于阿尔金断裂带东段的东南侧和西北侧, 且均出露有大面积的古生代花岗岩体。本文以阿尔金主断裂两侧产出的花岗岩类为研究对象, 涉及北祁连造山带中的赵家庄二长花岗岩, 石包城复式岩体(花岗岩、正长花岗岩和花岗闪长岩)和红柳河花岗岩, 敦煌地块中的党河水库花岗闪长岩、沙枣园二长花岗岩、安盆沟复式岩体(正长花岗岩和花岗岩)以及小草湖似斑状花岗岩。通过对上述花岗岩体的岩相学、锆石U-Pb年代学、地球化学和锆石Hf同位素的研究, 取得了新的认识:     

东准噶尔贝勒库都克铝质A型花岗岩微量元素地球化学特征及地质意义

通过对新疆东准噶尔卡拉麦里地区贝勒库都克岩体的岩石地球化学特征的研究,结果表明,在贝勒库都克黑云母花岗岩中Rb、K和Th等大离子亲石元素明显富集,相对富集Zr、Hf等高场强元素,相对亏损Ba、Sr、Nb和Eu等元素,稀土元素含量相对较高,Eu的负异常极强,稀土元素配分模式呈平坦的"V"字型,属于典型的铝质A型花岗岩。该花岗岩在成因上属于A2型,形成于后碰撞的张性环境,其来源可能与洋壳和岛弧建造组成的年轻地壳有关。花岗岩微量元素构造判别图显示它是一种后碰撞花岗岩,标志卡拉麦里地区在晚石炭世造山作用的结束和板内构造演化的开始。该岩体锡质量分数普遍都比较高(15.50 10-6),为锡的成矿物质来源和锡矿矿床学的深入探索提供重要参考。     

内蒙古扎鲁特旗三叠纪黑云母二长花岗岩特征

内蒙古扎鲁特旗北部黑云母二长花岗岩岩体位于兴蒙造山带东段,形成于晚三叠世.黑云母二长花岗岩岩石地球化学表现为富SiO₂、Al₂O₃和碱质,Na₂O/K₂O比值小于1,A/CNK比值介于1.021~1.084之间,轻重稀土元素分馏明显,轻稀土元素富集,具明显负铕异常.微量元素中亏损Ba、Sr、Ti、P,稀土元素中亏损Eu、Yb,具有A型花岗岩的地球化学特征.结合区域地质资料和前人相关研究,推测黑云母二长花岗岩可能形成于板块内部造山后期地壳伸展减薄的环境.     

西天山的增生造山过程

西天山位于中亚造山带的西南缘,经历了复杂的增生造山过程。它也是标志塔里木地块北部被动陆缘与西伯利亚地块南侧宽阔活动陆缘最后拼合的构造带。根据近年来的研究进展,将西天山划分为北天山弧增生体、伊犁地块北缘活动陆缘、伊犁地块、伊犁地块南缘活动陆缘、中天山复合弧地体、西天山（高压）增生楔和塔里木北部被动大陆边缘。同时综述了西天山蛇绿岩、高压变质岩、花岗岩类的年代学新资料,讨论了其增生造山的过程。西天山增生造山与早古生代帖尔斯克依古洋、早古生代晚期—晚古生代南天山洋和晚古生代北天山洋3个代表洋盆的演化相关,增生造山结束的时间可能是早石炭世末。二叠纪时期,西天山至整个中亚地区进入后碰撞演化阶段。现有资料证实西天山为晚古生代增生造山带,并非三叠纪碰撞造山带。     

西天山特克斯达坂一带晚古生代花岗岩类的地球化学特征及其构造意义

依据地质学、岩石学和年代学特征,西天山特克斯达坂一带花岗岩类被解体为早石炭世库勒萨依序列和早二叠世其那尔萨依序列,其同位素年龄分别为347Ma、291～292Ma。通过对该花岗岩类地球化学特征的研究得出,库勒萨依序列A/CNK平均值为1,K2O/Na2O平均值为0.96,（Na2O+K2O）/Al2O3平均值为0.66,里特曼指数δ平均值为3.2,为准铝质钙碱性花岗岩;其那尔萨依序列A/CNK平均值为1.05,K2O/Na2O平均值为1.17,富钾,为过铝质碱性花岗岩。库勒萨依序列为形成于岛弧环境的钙碱性花岗岩,是古亚洲洋向北俯冲的产物;其那尔萨依序列为形成于后造山期松弛阶段的碱性花岗岩。