华南东段加里东期花岗岩类形成构造背景探讨

根据近年对区域地质调查取得的认识和前人的研究成果,研究了华南东段加里东期花岗岩类的地质产状、分布规律、形成时代以及Nd、Sr同位素特征;结合研究区超镁铁质-镁铁质岩及火山岩的形成时代、震旦纪—奥陶纪的沉积环境、加里东期构造事件的构造变形特征,对加里东期花岗岩类的形成背景进行了探讨。结果表明,加里东期花岗岩类形成于410～460Ma,大部分岩体属于S型花岗岩,岩浆来源于地壳物质的部分熔融;极少数为I型花岗岩,有幔源物质参与。研究区在早古生代不存在与俯冲有关的弧火山岩,也不存在大面积的早古生代I型花岗岩,震旦纪—奥陶纪地层主要形成于浅海环境。研究提出,加里东期花岗岩类主体形成于板内构造环境。     

金沙江缝合带弯岛湖蛇绿混杂岩带的岩石地球化学特征及其构造背景

弯岛湖蛇绿混杂岩带是金沙江缝合带西段的重要组成部分。蛇绿岩混杂于上三叠统变质碎屑岩夹变质火山岩中,成份主要为镁铁质-超镁铁质杂岩,岩石类型有变质橄榄岩、变质堆晶质辉长岩及其伴生的岛弧型花岗岩系。岩石化学及地球化学特征表明:蛇绿岩主要为低Ti(岛弧-弧后)型、富集型洋中脊(E-MORB)拉斑玄武岩;与之共(伴)生的基性喷出岩、辉绿岩脉属板内洋岛型裂谷型碱性玄武岩及其过渡类型系列。在变质辉长岩获得全岩Sm-Nd等时线年龄值为232±11Ma,代表了镁铁质-超镁铁质杂岩的形成年龄,可能为洋壳初始俯冲变质的时间。在蛇绿岩上覆的硅质岩中发现有中三叠世拉丁晚期至晚三叠世卡宁早期的放射虫化石,表明弯岛湖镁铁质-超镁铁质杂岩可能形成于中三叠世多岛洋盆或弧后盆地构造环境。     

川南平川镁铁质-超镁铁质岩锆石U-Pb年龄及地质意义

为探讨镁铁质-超镁铁质岩的岩石学特征及成岩成矿作用时间序列, 完善区域成岩成矿年代学格架, 对平川地区的镁铁质-超镁铁质岩进行了元素地球化学分析和LA-ICP-MS锆石U-Pb定年.测试结果显示, 黄草坪辉长岩成岩年龄为259.7±1.2 Ma, 其捕获锆石结晶年龄为269.8±2.4 Ma, 辉绿(玢)岩等次火山岩参考成岩年龄为248± Ma, 其变质锆石年龄为67± Ma.研究结果表明, 平川地区的镁铁质-超镁铁质岩产出于大陆裂谷环境, 岩浆源区来自上地幔尖晶石相二辉橄榄岩, 为一套同源异相有利于铁矿化形成的富钠质拉斑玄武质岩石系列.岩浆活动开始孕育时间不晚于269.8±2.4 Ma, 大规模岩浆活动发生于259.7±1.2 Ma, 其约束了岩浆分异型和火山喷发(溢)沉积型矿化的成矿时间, 次火山岩的成岩年龄约束了次火山热液型矿化的成矿时间上限, 变质锆石U-Pb年龄记录了平川地区经历了喜马拉雅期陆内造山作用.平川地区构造岩浆活动具有爆发性、阶段性和成矿专属性等特点, 镁铁质-超镁铁质岩的形成可能与岩石圈的大规模减薄作用有关, 攀西地区海西-印支期的成岩-成矿作用为同源岩浆受区域统一深部地球动力学背景约束演化的异相产物.      

东天山四顶黑山地区545Ma层状镁铁质-超镁铁质岩体的发现及其大地构造学和成矿学意义

在东天山四顶黑山地区元古代深变质地层中发现层状镁铁质-超镁铁质岩体,研究了它的产状、岩石类型、结构构造、矿物组成、稀土微量元素地球化学、年代学等特征及其发现的地质意义。层状镁铁质-超镁铁质岩为后侵位的镁铁质杂岩体和花岗岩所穿切与破坏,其正堆晶结构发育,并可划分出两个结晶与演化旋回。四顶黑山层状钱铁质-超镁铁质岩的稀土微量元素含量和配分结构与该地区镁铁质杂岩、及图拉尔根镁铁质-超镁铁质岩体明显不同,具低∑REE、正δEu 异常、高 Sr 低Rh、高 Nb/U 值、及 Nb、Zr 和 Hf 亏损等地球化学特征,该特征与裂谷成因的攀枝花层状辉长岩和曹家村层状岩体非常相似,这意味着四顶黑山层状镁铁质-超镁铁质岩可能也形成于裂谷构造环境。选择层状镁铁质-超镁铁质岩体下部韵律层顶部辉石角闪石岩、镁铁质杂岩体闪长岩和角闪花岗岩中的角闪石开展~(39)Ar/~(40)Ar 定年,结果显示3种岩体分别形成于545±5Ma、263.8±2.6Ma 和200.8±0.91Ma。四顶黑山545Ma 裂谷成因层状镁铁质-超镁铁质岩体的发育,意味着早寒武世是天山地区镁铁质-超镁铁质岩型 Cu-Ni 矿的成矿期之一,天山地区在元古宙变质岩为基底的古大陆在早寒武世出现了雏形裂谷构造,545Ma可能是古天山洋形成的初始期。     

吉林省夹皮沟花岗岩—绿岩带成因模式探讨

通过对夹皮沟绿岩带层序、岩石成因以及叠加其上的变质变形作用特征的研究,认为绿岩带形成于硅铝壳的扩张环境;英云闪长岩—奥长花岗岩成分的花岗质片麻岩是在大陆或大陆边缘的挤压环境下,由变质镁铁质火山岩经部分熔融侵位的深成花岗质岩石。由此,提出夹皮沟花岗岩—绿岩带的裂谷成因模式。     

江南造山带西南段梵净山地区镁铁质-超镁铁质岩:形成时代、地球化学特征与构造环境

梵净山地区位于江南造山带的西南缘,这里新元古代的镁铁质-超镁铁质岩浆岩广泛发育,岩性包括枕状熔岩、超镁铁质-镁铁质岩床群以及浅成侵入的辉长岩,成分属拉斑玄武岩系列。其中枕状熔岩以富集轻稀土元素和Rb、Ba、Th、U等强不相容元素,亏损高场强元素Nb和Ta,低的εNd(t)值为特征,明显不同于洋脊玄武岩,推测其成因可能与富集型地幔的部分熔融有关,形成于与俯冲有关的弧后小洋盆环境。超镁铁质-镁铁质岩床群主要由辉绿岩和碳酸辉橄岩组成,其中超镁铁质岩床群中出现大量的原生碳酸盐矿物,指示它们形成于拉张(甚至裂谷)的构造环境。辉长岩可能是区内最晚形成的岩浆岩,其SHRIMP锆石U-Pb年龄为821±4Ma。由枕状熔岩经超镁铁质-镁铁质岩床群到辉长岩,高场强元素Nb和Ta的亏损程度减弱、轻稀土元素的富集程度降低、εNd(t)值由负值变为正值,指示随时间的由早到晚,来自亏损地幔的物质不断增加。推测梵净山地区新元古代岩浆作用的顺序大致为:枕状熔岩(～840Ma)→白云母花岗岩(～838Ma)→碳酸超镁铁质岩床群→镁铁质岩床群→辉长岩(～821Ma),构造环境由俯冲-碰撞到拉张-裂谷。     

柴达木盆地北缘鱼卡地区构造混杂岩带解剖填图新进展

针对柴达木盆地北缘构造混杂岩带中不同成因类型镁铁质—超镁铁质和高压—超高压岩的组成、俯冲-碰撞-折返机制及形成演化过程等关键地质问题,在柴北缘鱼卡地区开展了解剖地质填图工作,取得了系列新认识: 查明了深俯冲地质体与原地岩块的接触关系,表现为早期韧性—脆韧性剪切带接触,后期叠加逆冲推覆; 查明了深俯冲地质体中榴辉岩主要呈透镜状、似层状产出,表明榴辉岩原岩在峰期变质之前就发生了分层剪切; 查明了片麻状花岗岩体内部榴辉岩的产出特征,榴辉岩以鱼卡岩群残蚀体产出于片麻状花岗岩体中; 厘定了深俯冲地质体内部变形序列,识别出4期构造变形组合,表现为挤压型变形,指示本区经历了多期挤压汇聚作用; 明确了超基性岩体的就位环境,与深俯冲地质体为侵入接触关系。     

闽中早—中元古代科马提岩

闽中下一中元古界东华组绿片岩原岩主要为一套超镁铁质-镁铁质火山岩。其岩石学特征、岩石化学特征、区域变质特征和国外太古代的科马提岩极为相似；和桂北、湖南益阳中元古代的科马提岩也可对比，但变质程度较高，达低角闪岩相。该绿片岩自下而上分别为辉石质科马提岩、玄武质科马提岩，并伴生有拉斑玄武岩。它和上部卓地组中酸性、酸性火山岩组成一套完整的岩浆演化系列，且具有双模式特征。闽中早一中元古代科马提岩产于拉张盆地的构造环境。     

胶东变质超镁铁—镁铁质岩石的地球化学特征及地幔源区性质

本文以地球化学方法为研究手段,结合野外宏观地质特征,论述了胶东地区前寒武纪变质建造中超镁铁—镁铁质岩石的地球化学性质及其成因。研究结果表明,该区超镁铁—镁铁质岩石为地幔不同程度熔融的产物,超镁铁质岩石相当于科马提岩类;镁铁质岩石为拉斑玄武岩类,并按橄榄质科马提岩→玄武质科马提岩→拉斑玄武岩演化系列的化学组成表现出连续的变化趋势。 拉斑玄武岩中的微量元素(主要是稀土元素)地球化学特征表明,其形成环境与现代板块构造的典型火山(岛)孤或孤后盆地环境相类似,并反映出地幔源区具有“富集型”地幔的特点,且存在着地幔不均一性。     

内蒙古达尔罕茂明安联合旗满都拉地区索伦山蛇绿混杂岩的特征及构造意义

内蒙古中北部满都拉地区出露的蛇绿混杂岩是由层位不全的“破碎的”或“肢解的”蛇绿岩经构造混杂而成的。其基质由受构造剪切的蛇纹质和碎裂凝灰质板岩组成,岩块为大小不等、形态各异的超镁铁质岩、辉长岩、镁铁质火山岩、硅质岩,岩块在基质中占30%～50%。地球化学分析结果表明,变质橄榄岩具有低Cr、Ti、Ca和高Mg的特征,MgO/(MgO FeO鄢)值为0.86～0.89,稀土元素总量极低,LREE明显富集,Eu具正异常;镁铁质火山岩具有洋中脊拉斑玄武岩(MORB)的特征,地球化学特征显示其形成于陆间小洋盆环境。判定其形成时代应当在433.6Ma±3.6Ma之前。     

新疆诺尔特地区岩浆岩形成的构造环境及其意义

诺尔特地区位于阿尔泰北部山区,西伯利亚板块西南缘。花岗岩广泛分布,主要为加里东晚期、华力西中、晚期花岗岩,存在燕山期花岗岩。区内火山岩主要是泥盆纪及石炭纪火山岩,泥盆纪火山岩赋存于上泥盆统忙代恰组中,石炭纪火山岩赋存于下石炭统红山嘴组中,主要为中-酸性火山岩,属钙碱性系列。区内岩浆岩构造环境的研究表明,加里东晚期花岗岩形成于汇聚阶段,属挤压环境;华力西中期花岗岩形成于新陆壳阶段弛张期向活化期过渡的时期,应力体制由拉张向挤压过渡;华力西晚期花岗岩形成于新陆壳阶段活化期,属挤压环境;燕山期花岗岩形成于古生代新陆壳在中生代的发展阶段,属挤压环境;泥盆纪火山岩是新陆壳发展阶段固结期的产物,石炭纪火山岩为弛张期及活化期演化过程中的产物。     

新疆乌伦古河碱性花岗岩的地球化学及其构造意义

在新疆北部准噶尔板块与阿尔泰造山带的缝合带即阿尔曼泰-扎河坝蛇绿混杂岩带附近,沿乌伦古河南岸分布一条碱性花岗岩带。它们形成的时代为292-309Ma,是阿尔泰地区海西期继同碰撞s型花岗岩类、碰撞后抬升Ⅰ型花岗岩类之后的最后一次岩浆活动的产物。这些碱性花岗岩以出现霓石、钠铁闪石、高硅、高碱、低钙、低镁、富集高场强元素为特征,属于典型的A型花岗岩。碱性花岗岩是海西期岩浆旋回的最后产物,活动时间很短暂,在空间上与蛇绿岩带伴生,为后造山A型碱性花岗岩(PA型),是阿尔泰海西期造山运动结束的重要标志。     

The mafic and ultramafic rocks of the Mocksville complex of Central North Carolina: Petrogenetic and tectonic implications

Located in the northwestern part of the Charlotte terrane of the Carolina zone in central North Carolina, the Mocksville complex is a tabular body which trends NE-SW and covers an area of approximately 500?km². It consists of late Proterozoic to early Paleozoic, moderately metamorphosed and variably deformed, mainly plutonic ultramafic, mafic and felsic rocks. The ultramafic rocks are pyroxenites, wehrlites, and hornblendites; the mafic rocks are metagabbros and amphibolites; and, the felsic rocks are granites and diorites. Field, geochemical, and geothermobarometry studies suggest that the igneous and metaigneous rocks of the Mocksville complex are likely to be genetically related, formed by calc-alkaline differentiation of mafic magma, and originated in a moderate pressure environment (~8?kbar). Based mainly on the study of volcanic rocks, the terranes of the Carolina zone have been interpreted as magmatic arc terranes in most tectonic models concerning the evolution of the southern Appalachian orogen. The geochemical features of the mafic and ultramafic plutonic rocks of the Mocksville complex corroborate the arc origin of the Charlotte terrane.     

祁连山地区镁铁——超镁铁岩的多样性

祁连山地区出露有不同类型的镁铁－超镁铁岩，包括蛇绿岩，阿拉斯加型岩体，橄榄岩－闪长岩型岩体以有产于北祁连西部的一种特殊的镁铁－超镁铁岩组合。不同的镁铁－超镁铁岩形成于一同的构造环境，蛇绿岩代表洋壳的残片，阿拉斯加型岩体产生岛弧或活动陆缘环境，橄榄岩一闪长岩体则是板块碰撞之后形成的，根据蛇绿岩与大致同时代的阿拉斯加型岩体产出的位置，推测在北祁连早古生代时存在向南的消减事件。     

Geochemical Features, Age, and Tectonic Significance of the Kekekete Mafic-ultramafic Rocks, East Kunlun Orogen, China

The Kekekete mafic-ultramafic rocks are exposed in the Kekesha-Kekekete-Dawate area,which are in the eastern part of the East Kunlun Orogenic Belt.It outcrops as tectonic slices intruding tectonically in the Paleoproterozoic Baishahe Group and the Paleozoic Nachitai Group.The Kekekete mafic and ultramafic rocks is located near the central fault in East Kunlun and lithologically mainly consists of serpentinite,augite peridotite,and gabbro.The LA-ICP-MS zircon U-Pb age of the gabbro is 501±7 Ma,indicating that Kekekete mafic-ultramafic rocks formed in the Middle Cambrian.This rock assemblage is relatively poor in SiO2 and(Na2 O+K2 O) but rich in MgO and SFeO.The chondrite-normalized REE patterns of the gabbro dip slightly to the right;the primitive mantle and MORBnormalized spidergrams of trace elements show enrichment of large-ion lithophile elements(Cs,Rb,Ba,etc.) and no differentiation of high field strength elements.The general dominance of E-MORB features and the geochemical characteristics of OIB suggest that the Kekekete mafic-ultramafic rocks formed in an initial oceanic basin with slightly enriched mantle being featured by varying degrees of mixing of N-MORB depleted mantle and a similar-OIB-type source.From a comprehensive study of the previous data,the author believes that the tectonic history of the East Kunlun region was controlled by a geodynamic system of rifting and extension in the late stages of the Neoproterozoic to early stages of the Early Paleozoic and this formed the paleo-oceanic basin or rift system now represented by the ophiolites along the central fault in East Kunlun,the Kekekete mafic-ultramafic rocks and Delisitan ophiolite.     

Contributions of basaltic underplating to crustal growth in island arc and extensional tectonic settings in the Chinese Tianshan Orogenic Belt,NW China

A mafic–ultramafic intrusive belt comprising Silurian arc gabbroic rocks and Early Permian mafic–ultramafic intrusions was recently identified in the western part of the East Tianshan, NW China. This paper discusses the petrogenesis of the mafic–ultramafic rocks in this belt and intends to understand Phanerozoic crust growth through basaltic magmatism occurring in an island arc and intraplate extensional tectonic setting in the Chinese Tianshan Orogenic Belt (CTOB). The Silurian gabbroic rocks comprise troctolite, olivine gabbro, and leucogabbro enclosed by Early Permian diorites. SHRIMP II U-Pb zircon dating yields a 427 ± 7.3 Ma age for the Silurian gabbroic rocks and a 280.9 ± 3.1 Ma age for the surrounding diorite. These gabbroic rocks are direct products of mantle basaltic magmas generated by flux melting of the hydrous mantle wedge over subduction zone during Silurian subduction in the CTOB. The arc signature of the basaltic magmas receives support from incompatible trace elements in olivine gabbro and leucogabbro, which display enrichment in large ion lithophile elements and prominent depletion in Nb and Ta with higher U/Th and lower Ce/Pb and Nb/Ta ratios than MORBs and OIBs. The hydrous nature of the arc magmas are corroborated by the Silurian gabbroic rocks with a cumulate texture comprising hornblende cumulates and extremely calcic plagioclase (An up to 99 mol%). Troctolite is a hybrid rock, and its formation is related to the reaction of the hydrous basaltic magmas with a former arc olivine-diallage matrix which suggests multiple arc basaltic magmatism in the Early Paleozoic. The Early Permian mafic–ultramafic intrusions in this belt comprise ultramafic rocks and evolved hornblende gabbro resulting from differentiation of a basaltic magma underplated in an intraplate extensional tectonic setting, and this model would apply to coeval mafic–ultramafic intrusions in the CTOB. Presence of Silurian gabbroic rocks as well as pervasively distributed arc felsic plutons in the CTOB suggest active crust-mantle magmatism in the Silurian, which has contributed to crustal growth by (1) serving as heat sources that remelted former arc crust to generate arc plutons, (2) addition of a mantle component to the arc plutons by magma mixing, and (3) transport of mantle materials to form new lower or middle crust. Mafic–ultramafic intrusions and their spatiotemporal A-type granites during Early Permian to Triassic intraplate extension are intrusive counterparts of the contemporaneous bimodal volcanic rocks in the CTOB. Basaltic underplating in this temporal interval contributed to crustal growth in a vertical form, including adding mantle materials to lower or middle crust by intracrustal differentiation and remelting Early-Paleozoic formed arc crust in the CTOB.     

辽西朱碌科地区海西—印支期花岗质岩石特征及成因

朱碌科地区花岗质岩石侵位时间为二叠-三叠纪.按岩石谱系单位划分原则,由早到晚划分为中官营子和榆树林子两个超单元,前者由西窑单元、二道沟单元组成,后者由李台子、碾盘山和敖包山单元组成.岩浆成因类型为介于"I"型和"S"型花岗岩之间的过渡类型.它们均形成于造山晚期构造环境,与南蒙古复合造山作用有直接关系.     

北秦岭漂池岩体的源区特征及其形成的构造环境

北秦岭漂池花岗质岩基为早古生代岩浆活动的产物，岩石类型主要为二云母花岗岩。通过主要元素，微量元素及Ｎｄ，Ｓｒ，Ｏ同位素特征的分析，其成因类型为Ｓ型花岗岩，地物质来自壳源碎屑物，研究表明，秦岭群片麻岩类是形成漂池岩体的主要源岩，结合区域地质背景分析，岩体并不形成于板块碰撞环境，而形成北秦岭早古生代活动大陆边缘，受板块俯冲作用的动力学影响所诱发的陆缘地壳物质熔融的产物，因此，漂池岩体形成的构造类型是活     

分离的大陆板块边缘镁铁-超镁铁岩系Th/Hf-Ta/Hf判别

本文在论述了 Th/ Hf— Ta/ Hf关系判别岩浆岩源区大地构造环境机理的基础上 ,根据世界上典型大地构造环境区岩浆岩系的 Th、Ta、Hf特征 ,提出了分离的大陆板块边缘镁铁 -超镁铁岩的 Th/ Hf— Ta/ Hf判别标志 :Th/ Ta=1.8～ 4 .0 ,Ta/ Hf>0 .1