Multiple growth of garnet,sillimanite/kyanite and monazite during amphibolite facies metamorphism: implications for the P–T–t and tectonic evolution of the western Altai Range,Mongolia

Four amphibolite facies pelitic gneisses from the western Mongolian Altai Range exhibit multistage aluminosilicate formation and various chemical‐zoning patterns in garnet. Two of them contain kyanite in the matrix and sillimanite inclusions in garnet, and the others have kyanite inclusions in garnet with sillimanite or kyanite in the matrix. The Ca‐zoning patterns of the garnet are different in each rock type. U–Th–Pb monazite geochronology revealed that all rock units experienced a c. 360 Ma event, and three of them were also affected by a c. 260 Ma event. The variations in the microstructures and garnet‐zoning profiles are caused by the differences in the (i) whole‐rock chemistry, (ii) pressure conditions during garnet growth at c. 360 Ma and (iii) equilibrium temperatures at c. 260 Ma. The garnet with sillimanite inclusions records an increase in pressure at low‐P (~5.2–7.2 kbar) and moderate temperature conditions (~620–660 °C) at c. 360 Ma. The garnet with kyanite inclusions in the other rock types was also formed during an increase in pressure but at higher pressure conditions (~7.0–8.9 kbar at ~600–640 °C). The detrital zircon provenance of all the rock types is similar and is consistent with that from the sedimentary rocks in the Altai Range, suggesting that the provenance of all the rock types was a surrounding accretionary wedge. One possible scenario for the different thermal gradient is Devonian ridge subduction beneath the Altai Range, as proposed by several researchers. The subducting ridge could have supplied heat to the accretionary wedge and elevated the geotherm at c. 360 Ma. The differences in the thermal gradients that resulted in varying prograde P–T paths might be due to variations in the thermal regimes in the upper plate that were generated by the subducting ridge. The c. 260 Ma event is characterized by a relatively high‐T/P gradient (~25 °C km^?1) and may be due to collision‐related granitic activity and re‐equilibrium at middle crustal depths, which caused the variations in the aluminosilicates in the matrix between the rock units.     

扬子地块北缘晚奥陶世赫南特期岩相古地理

扬子地块北缘与秦岭接壤,早古生代经历了上扬子克拉通盆地(∈-O2)和隆后盆地(O3-S)两个阶段。区内发育上奥陶统观音桥组或南郑组地层,为研究赫南特期的岩相古地理,探讨赫南特期冰川事件的沉积响应奠定了坚实的基础。根据沉积特征,结合古生物组合,扬子地块北缘观音桥组可划分为滨岸相和陆棚相。按照岩性等,陆棚相可进一步划分为陆缘碎屑陆棚相、混积陆棚相。滨岸相主要由石英砂岩、钙质砂岩和含砾砂岩组成,发育少量生物,以腕足为主;而在陆棚相中,则是以含粉砂泥岩、生屑灰岩、泥灰岩夹钙质泥岩和含钙泥岩为主,富含生物,以三叶虫(Dalmanitina)、腕足以及标志性冷水、浅水Hirnantia动物群为主。在沉积相详细研究的基础上,分析了该时期的岩相古地理面貌及其空间分布。在北部和西部,继续早期的格局,持续存在汉南隆起,围绕隆起分布的是滨岸相;往南至大两会-桥亭一线,为陆缘碎屑浅海陆棚,大两会-桥亭一线往南区域分布为混积陆棚。由于在研究区,无论在滨岸地带还是浅海地区,均发育赫南特贝化石,说明在赫南特期,由于受南冈瓦纳冰川事件的影响,冰水侵进扬子北缘全区。这次冰川事件导致海平面下降,致使扬子北缘水体变浅,早期的深水陆棚变为浅水陆棚,生物也由浮游相笔石迅速变为壳相赫南特贝,早期隆起范围进一步扩大,并出现地层的缺失。     

冀北新杖子地区水系沉积物地球化学特征及找矿预测

冀北新杖子地区位于阜新—集宁成矿带的张家口—承德成矿区东北部,具有较好的找矿前景。在全区1∶50000水系沉积物测量基础上,对区内地球化学特征进行了初步分析。本文综合研究元素地球化学特征参数、元素R聚类相关性、单元素异常特征及因子分析表明,Au、Ag、Cu、Zn、Pb等元素具有一定的找矿潜力;依据水系沉积物异常圈定、筛选、评序及主要异常解释推断的成果,结合区内成矿地质构造背景,进一步圈定出石湖金矿、潘家店金矿、路通沟多金属矿和荣家庄多金属矿4个远景区。     

青海省柴达木南北缘岩浆熔离型镍矿的找矿——以夏日哈木镍矿为例

夏日哈木铜镍矿是在东昆仑造山带首次被发现的岩浆熔离型矿床,该类型矿床是青海省铜镍矿勘查研究方向之一.笔者通过夏日哈木镍矿成矿特征研究认为:碰撞伸展环境是夏日哈木超大型镍-钴硫化物矿形成的基本条件,成岩成矿年龄集中于早泥盆世,岩体侵位于古元古代金水口白沙河组,参与后期混染成矿作用.对柴达木南北缘岩浆熔离型镍矿的成矿条件分析发现:泥盆纪热事件显示柴达木盆地南、北缘在该期均发生了碰撞伸展作用和成矿作用,镁铁质-超镁铁质岩均侵入古老地层,地层硫参与成矿作用;成矿镁铁质-超镁铁质岩体与物探磁异常,Ni、Co、Cu组合异常相对应.综合分析,柴达木南北缘岩浆熔离型镍矿的找矿潜力大,提出了今后柴达木南北缘岩浆熔离型镍矿勘查重点是柴达木南缘的昆中岩浆弧带、柴达木北缘高压变质岩带、欧龙布鲁克陆块.     

芬兰北部金矿床产出环境、地质特征和成矿作用

芬兰北部的金矿床数量众多,大多数金矿床(点)产出于古元古界中央拉普兰绿岩带(CLGB)和库萨莫—帕拉波加片岩带内,主要分为3种类型:造山型金矿、铁氧化物铜-金(IOCG型)矿床及砂金矿。其中造山型金矿绝大部分分布于中央拉普兰绿岩带中部及库萨默地区和帕拉波加地区,最常见的矿石矿物为黄铁矿、毒砂、黄铜矿和磁黄铁矿,其次为方铅矿、闪锌矿、黝铜矿等。围岩蚀变包括钠长石化、绢云母化、黑云母化、碳酸盐化及绿泥石化。造山型金矿床主要控制因素包括构造、岩性和造山期区域变质。IOCG型矿床主要分布在CLGB的西部克拉瑞地区,容矿围岩是钠长石-直闪石石棉型岩石,矿石中富含Fe-Cu-Au±Ag、Bi、Ba、Co、Mo、Sb、Se、Te、Th、U和轻稀土元素。围岩蚀变强烈,并且有明显的分带现象。IOCG型金矿床的形成受岩浆热液活动和构造的影响较大。砂金矿床包括古砂金矿床和现代砂金矿床,主要分布在CLGB东北部。芬兰北部金矿床的研究对北极圈及邻区的找矿勘查工作具有重要意义,也为中国胶东半岛、华北克拉通北缘、小秦岭和秦岭地区及新疆北部地区进一步金矿勘查提供参考。     

南海北部典型区块岩石圈有效弹性厚度分析

有效弹性厚度(Te)是刻画岩石圈弹性性质的重要参数,通过分析有效弹性厚度值,可以获得与区域构造相关的信息,为后续解释工作提供一些依据。基于均衡响应函数法,选取南海北部区域4个典型区块的数据,计算均衡响应函数值,并根据公式推算得到各区块的Te值,通过分析Te值对研究区地质构造有所了解。     

柴达木盆地北缘晚海西—印支期古构造应力分析

为了更好认识柴北缘晚海西—印支期古构造应力特征与演化过程,在天峻快日玛乡、城墙沟、石灰沟等地区详细野外观测的基础上,利用地层恢复和古构造应力反演技术对柴北缘及邻区石炭系—新生界构造观测数据进行分析。结果表明晚二叠世—中三叠世,柴北缘在南—北向挤压作用下向北俯冲碰撞,盆地内部上二叠—中三叠统缺失。晚三叠世开始,西秦岭斜向碰撞柴达木盆地,柴北缘进入造山阶段,所受挤压作用方向转变为北西—南东向,盆地内大范围缺失上三叠统。晚海西—印支期柴北缘内部二叠纪—三叠纪地层大面积剥蚀加之该期强烈挤压作用造成尕海南山石炭系与白垩系、旺尕秀煤矿石炭系与侏罗系之间的角度不整合,意味着下伏石炭系可能遭受强烈破坏。     

江南造山带东段含钨岩体地球化学特征

江南造山带东段燕山晚期含钨的东源岩体与非含钨的旌德岩体、桃岭岩体,其地球化学特征具有埃达克质岩的亲缘性,是先前(新元古代)交代的岩石圈地幔发生部分熔融底侵到壳幔过渡带附近,导致加厚下地壳发生部分熔融的产物,可能与少量的幔源岩浆发生岩浆混合作用。含矿岩体与非含矿岩体存在不同的Sr-Nd同位素组成,表明含矿与非含矿岩体具有不同的源区。锆石LA-ICPMS定年显示含矿的东源岩体侵位时间147 Ma,非含矿岩体侵位时间140 Ma。含矿岩体的副矿物组合以独居石+辉钼矿+白钨矿+白钨矿+电气石为特征。岩体中富含成矿元素、富挥发分、富钾,黑云母中富F、低Fe和fO2,斜长石以钠长石为主,均可作为含矿花岗(斑)岩体的重要地球化学参数。     

Formation of the Jurassic Changboshan‐Xieniqishan highly fractionated I‐type granites,northeastern China: implication for the partial melting of juvenile crust induced by asthenospheric mantle upwelling

Zircon U–Pb ages, major and trace elements, and Sr, Nd and Hf isotope compositions of the Changboshan‐Xieniqishan (CX) intrusion from the Great Xing'an Range (GXAR), northeastern China, were studied to investigate its derivation, evolution and geodynamic significance. Laser ablation inductively coupled plasma mass spectrometry (LA‐ICP‐MS) zircon U–Pb dating yields an emplacement age of 161 ± 2 Ma for the CX intrusion. Bulk‐rock analyses show that this intrusion is characterized by high SiO₂, Na₂O and K₂O, but low MgO, CaO and P₂O₅. They are enriched in large‐ion lithophile elements and light rare earth elements, with marked Eu anomalies (mostly from 0.36 to 0.65), and depleted in heavy rare earth elements and high field strength elements. Most samples have relatively low (⁸⁷Sr/⁸⁶Sr)_i values (0.70423–0.70457), with ε_Nd(t) fluctuating between −0.4 and 2.3. The ε_Hf(t) for zircons varies from 5.4 to 8.7. Sr–Nd isotope modelling results, in combination with young Nd and Hf model ages (760–986 and 549–728 Ma, respectively) and the presence of relict zircons, indicate that the CX intrusion may originate from the partial melting of juvenile crust, with minor contamination of recycled crustal components, and then underwent extensive fractional crystallization of K‐feldspar, plagioclase, biotite, sphene, apatite, zircon and allanite. Considering the widespread presence of granitoids with coeval volcanic rocks, we contend that the CX intrusion formed in an extensional environment related to the upwelling of asthenospheric mantle induced by the subduction of the Palaeo‐Pacific plate, rather than a lithospheric delamination model. Copyright © 2013 John Wiley & Sons, Ltd.