Ophiolitic and metamorphic rocks in the Percy Isles and Shoalwater Bay region,New England Fold Belt,central Queensland

Ophiolitic and metamorphic rocks of the eastern part of the New England Fold Belt in the Shoalwater Bay region and the Percy Isles are grouped in the Marlborough and Shoalwater terranes, respectively. Marlborough terrane units occur on South Island (Percy Isles) and comprise the Northumberland Serpentinite, antigorite serpentinite with rodingite and more silicic dykes and mafic inclusions, the Chase Point Metabasalt, some 800+ metres of pillow lava, and the intervening South Island Shear Zone containing fault‐bounded slices of mafic and ultramafic igneous rocks, schist, and volcaniclastic sedimentary rocks, and zones of mélange. The Shoalwater terrane, an ancient subduction complex, consists of the Shoalwater Formation greenschist facies metamorphosed quartz sandstone and mudstone on North East Island and on the mainland at Arthur Point, the Townshend Formation, amphibolite‐grade quartzite, schist and metabasalt on Townshend Island, and the Broome Head Metamorphics on the western side of Shoalwater Bay, upper amphibolite facies quartz‐rich gneiss. With the exception of a sliver emplaced onto the western Yarrol terrane, possibly by gravity sliding, Shoalwater terrane rocks show the effects of Late Permian polyphase deformation. The Shacks Mylonite Zone along the northwest edge of the Broome Head Metamorphics marks a zone of oblique thrusting and is part of the major Stanage Fault Zone. The latter is a northeast‐striking oblique‐slip dextral tear fault active during Late Permian west‐directed thrusting that emplaced large ultramafic sheets farther south. Marlborough terrane rocks were emplaced along the Stanage Fault Zone, probably from the arc basement on which rocks of the Yarrol terrane were deposited. Structural trends and the distribution of rock units in the Shoalwater Bay‐Percy Isles region are oblique to the overall structural trend of the northern New England Fold Belt, probably due to the presence of a promontory in the convergent margin active in this region in Devonian and Carboniferous time.     

Structure of the Soresat Metamorphic Complex,North Sanandaj–Sirjan Zone,northwest Iran

The metasedimentary and granitoid rocks of the Soresat Metamorphic Complex occur along the northern margin of the Sanandaj–Sirjan Zone in northwest Iran. Four different deformational events (D1–D4) are recorded in the Soresat Metamorphic Complex. The D1 and D2 progressive deformation events resulted from north-northeast–south-southwest regional horizontal shortening due to the subduction of Neo-Tethys oceanic lithosphere beneath the Sanandaj–Sirjan Zone. Post-suturing convergence between Arabia and Iran, which resulted in a right lateral-reverse displacement along the suture caused the north-northwest–south-southeast horizontal shortening of D3. D4 is recorded by normal faulting. Andalusite, cordierite and sillimanite (fibrolite) record the thermal peak (with a geothermal gradient >30°C/km). Field and microscopic studies of intruded granitoid rocks in the Soresat Metamorphic Complex divide them into three major groups: (i) syn-deformation (syn-D2) granitic gneiss; (ii) late- to post-deformation (late- to post-D2) granites and granodiorites; and (iii) post-deformation (post-D2) alkali granites.     

青海沱沱河乌石峰与德尔尼蛇绿岩玄武岩地球化学特征对比浅析

对乌石峰和德尔尼蛇绿岩玄武岩单元地球化学特征的对比分析,发现其形成环境存在某些共性又有区别。通过稀土配分图和构造环境判别图,初步认为乌石峰蛇绿岩代表的是不成熟洋壳环境,而德尔尼蛇绿岩代表的是典型的快速扩张的洋中脊环境,两蛇绿岩带同属古特提斯多岛洋构造体系,但其发育程度有一定差别。     

冈瓦纳大陆北缘早期的洋壳信息──来自青藏高原羌塘中部早古生代蛇绿岩的依据

冈瓦纳大陆北缘裂解普遍认为发生在泥盆纪,形成了古特提斯洋并持续演化到晚三叠世。最近在羌塘中部的桃形湖—果干加年山一带发现了完整的蛇绿岩组合,蛇绿岩中的堆晶辉长岩具有洋中脊玄武岩的地球化学特征,在堆晶辉长岩中获得467 Ma～431Ma的锆石SHRIMP U-Pb年龄,这是龙木错-双湖缝合带首次发现早古生代蛇绿岩,应记录了冈瓦纳北缘早期的洋壳演化信息,冈瓦纳大陆北缘的裂解可能发生于早古生代。     

新疆达拉布特超镁铁岩成因——来自铬尖晶石的证据

通过研究西准噶尔达拉布特蛇绿混杂岩中方辉橄榄岩和橄榄辉石岩的岩石学特征,分析方辉橄榄岩广泛发育的铬尖晶石和斜方辉石构成的蠕虫状共生连晶结构的成因,得出结论认为:这种共生连晶结构不是前人所认为的文象结构或者石榴石的后成合晶,而是原始地幔岩熔融形成富铬岩浆的演化产物。这种富铬岩浆高度分异形成铬铁矿块体(即萨尔托海铬铁矿矿床)后,熔体进入地幔岩中结晶形成铬尖晶石和斜方辉石的蠕虫状共生连晶结构。因此,铬尖晶石与辉石的共生连晶结构可以作为豆荚状铬铁矿的重要找矿标志。方辉橄榄岩中的斜方辉石发育铬尖晶石出溶结构,出溶棒的成分特点表明,该结构是达拉布特蛇绿岩在快速就位过程中环境氧逸度突然升高诱发变质反应的结果。     

青海德尔尼铜(锌钴)矿床Re-Os年龄及地质意义

东昆仑德尔尼大型铜(锌钴)矿是赋存在蛇绿岩中的块状硫化物矿床,8件块状硫化物矿石的Re-Os同位素等时线年龄为(295.5±7.2) Ma,略小于矿区玄武质熔岩的锆石U-Pb平均年龄(308.2±4.9) Ma,矿床形成于晚石炭世-早二叠世,属于阿尼玛卿洋盆扩张期。成矿时代早于矿区北部印支期花岗岩,证明德尔尼矿床的成因与印支期花岗岩无直接关系。矿石组构、矿物组合和岩石地球化学研究指示,矿床为海底喷流沉积-热液叠加成矿,具有类似于塞浦路斯型块状硫化物矿床的构造背景和矿物组合特征。矿区及外围填图发现,德尔尼铜矿体主要产出在角砾状蛇纹岩中,部分位于蛇纹岩与含碳铁硅质岩之间,这种特殊的赋矿层位可能与晚古生代特提斯洋的快速扩张、岩浆补给少、变质橄榄岩迅速抬升有关。     

西藏日喀则蛇绿岩镁铁质岩石Re-Os同位素特征及意义

通过对日喀则蛇绿岩的镁铁质岩石进行岩石学和地球化学研究,探讨其岩石成因及源区性质,同时根据Re-Os同位素的地球化学约束来探索雅鲁藏布蛇绿岩的形成机制。研究表明,日喀则蛇绿岩镁铁质岩石微量元素的标准化配分型式与洋中脊玄武岩类似,又具有岛弧玄武岩的地球化学特征。结合微量元素和同位素特征,均表明岩石的形成与俯冲作用有关。日喀则蛇绿岩产出于远离大陆地壳的SSZ环境,其形成过程未受到陆壳的混染;同时,Re-Os同位素体系受蚀变作用的影响也不明显。日喀则蛇绿岩镁铁质岩石的Re、Os含量低,¹⁸⁷Os/¹⁸⁸Os同位素比值较高,主要是源区性质和俯冲作用影响的结果。特提斯洋早期发生的多次俯冲作用造成地幔源区不均一。新特提斯洋壳俯冲过程中,上述不均一地幔发生部分熔融产生的镁铁质岩浆上升,经过遭受了早期熔体/岩石作用的纯橄岩通道,发生强烈的Re-Os同位素分馏,使熔体与地幔残余Os同位素组成表现出明显的解耦现象,进而形成现今的日喀则蛇绿岩。     

青海拉脊山东段峡门蛇绿混杂岩带的物质确定及其构造意义

在拉脊山东段青海省民和县峡门镇以南地区新识别出一条蛇绿混杂岩带, 其物质组成复杂, 构造岩块主要由蛇绿岩岩块、外来岩块、火山岩岩块以及变沉积岩岩块组成. 基性火山熔岩和辉绿岩具轻稀土富集的"右倾"配分模式, 大离子亲石元素Ba、Th、Sr相对富集, 高场强元素P、Ti、Nb相对亏损, 总体岩石地球化学特征与岛弧拉斑玄武岩类似. 岩石学、岩石地球化学、同位素年代学特征及区域资料综合研究认为, 峡门蛇绿岩属与俯冲有关的SSZ型蛇绿岩, 很可能是早古生代原特提斯洋俯冲消减的弧后小洋盆, 属于弧后盆地扩张型蛇绿岩.     

新疆西准噶尔北部谢米斯台山南坡蛇绿岩带 的发现及其意义

新疆西准噶尔和什托洛盖谷地以北,沿谢米斯台山南坡出露一套蛇绿混杂岩(查干陶勒盖蛇绿岩),岩石组合自下而上包括蛇纹石化橄榄岩、变质辉长岩、玄武岩、细碧岩及伴生的硅质岩、同源火山碎屑岩等,基质为蛇纹石化橄榄岩和绿片岩相变质玄武岩。蛇绿岩中辉长岩的LA-ICP-MS锆石U-Pb定年结果为517Ma±3Ma和519Ma±3Ma,代表了蛇绿岩的形成年龄。蛇绿岩中不同组分(橄榄岩、辉长岩和细碧岩)的稀土元素和微量元素特征,以及细碧岩的Nb/Yb-Th/Yb图解均显示该蛇绿岩为洋中脊(MOR)型。查干陶勒盖蛇绿岩与西准噶尔北部的塔尔巴哈台(库吉拜)蛇绿岩、洪古勒楞蛇绿岩,邻区东哈克斯坦北部的扎乌尔-塔金蛇绿岩和巴尔克别克蛇绿岩可以很好地进行对比连接,为成吉斯-塔尔巴哈台构造带在西准境内的时空特征提供更多的约束。     

班公湖-怒江缝合带西段拉果错蛇绿岩中斜长岩成因及其对中特提斯洋演化的制约

拉果错蛇绿岩位于青藏高原北部的改则地区,是狮泉河-永珠带内保存最好、岩石组合最完整的蛇绿岩之一。为进一步约束拉果错蛇绿岩成因及其构造属性,对其中斜长岩开展了详细的岩石学、锆石U-Pb年代学和地球化学研究。结果显示,斜长岩主要呈不规则脉体或透镜体侵入到辉长岩中,锆石U-Pb定年结果表明斜长岩形成于162 Ma,略晚于蛇绿岩中基性岩形成时代。这些锆石均具有明显正的εHf（t）值（+15.8~+19.7）,暗示其岩浆源自亏损的地幔源区。斜长岩具有明显低SiO2、TiO2和高CaO的特征,具有与正常型大洋中脊玄武岩（N-MORB）类似的稀土元素配分形式和低的微量元素含量,富集Rb、Ba、Th和Sr,亏损Nb、Ta、Zr等微量元素。此外,斜长岩显示出高的Nb/La值和低的Th/Nb、Ba/Nb值,暗示其源自富集地幔源区高程度的部分熔融,形成过程还受到俯冲组分的影响。综合上述特征,推测拉果错斜长岩是地幔柱影响下再富集的亏损地幔源区高程度部分熔融的产物。俯冲过程中,再富集地幔源区（地幔楔）产生的原始岩浆快速上涌过程,并伴随着富Mg-Fe矿物的结晶分异,残余富Ca、Al岩浆。残余岩浆侵入到俯冲带上盘洋壳中（辉长岩）,由于快速减压而形成斜长岩。结合区域研究成果,表明拉果错蛇绿岩形成于与早侏罗世—晚侏罗世早期俯冲相关的构造背景。