哀牢山构造带晚渐新世地壳深熔与熔/流体迁移:锆石U-Pb年龄与微量元素证据

哀牢山构造带是青藏高原东南缘重要的边界构造带,其内出露的深变质岩系一直被认为是古老的变质基底岩石。利用LA-ICP-MS原位微区分析技术对哀牢山深变质岩系锆石进行U-Pb年龄、微量元素分析。结果表明深变质岩系的原岩有728±8Ma、727±3Ma、231±4Ma的花岗质岩石和其它年龄的碎屑岩,变质时代为27.8~23.7Ma。综合野外地质特征和分析结果,我们认为哀牢山深构造带在晚渐新世27.8~23.7Ma发生了大规模的地壳深熔作用,现今所见深变质岩系是由不同时代、不同岩性的原岩在晚渐新世(27.8~23.7Ma)变质形成,不全是古老的变质基底岩石。深熔过程中熔/流体发生了明显的迁移。哀牢山变质带具有混合岩化特征的岩石很可能是峰期变质作用后减压熔融的产物。晚渐新世地壳深熔作用与左行走滑剪切是哀牢山深变质带折返过程中近似同时发生的两种不同变质表现形式,两者相互影响、相互制约。     

广西云开地区元古代不整合事件的确定及其构造意义

通过详细的地质填图,在云开地区原定为“云开群”的变质岩系中识别出古老的不整合事件,表现为中浅变质岩系不整合覆盖在中深变质岩系之上,二者在岩石组合、原岩沉积环境以及变形变质特征上均存在较大的区别,同位素年龄及变质作用P-T-t轨迹的研究证明上覆中浅变质岩系明显晚于下伏中深变质岩系形成。利用SHRIMP法测得下伏岩系石榴辉石岩锆石的谐和207Pb/206Pb年龄为(1894.4±17.36)M a和(1846.8±58.58)Ma,并获得3个分析结果的207Pb/206Pb加权平均年龄(1817±36)Ma,其形成时代应为古元古代。上覆岩系中一些微古植物化石保存完好,微古植物化石组合的时代为长城纪—蓟县纪,部分为青白口纪;获得其变质基性火山岩锆石SHRIMP法同位素年龄为(1462±28)Ma,表明上覆岩系的时代应为中—新元古代。分析结果表明,二者之间存在一明显的巨大灾变事件,其应为古元古代末吕梁期造山作用形成的产物。笔者把下伏岩系从“云开群”中分离出来,并命名为“天堂山岩群”。     

Distribution and characteristics of metamorphic belts in the south-eastern Alaska part of the North American Cordillera

The Cordilleran orogen in south-eastern Alaska includes 14 distinct metamorphic belts that make up three major metamorphic complexes, from east to west: the Coast plutonic–metamorphic complex in the Coast Mountains; the Glacier Bay–Chichagof plutonic–metamorphic complex in the central part of the Alexander Archipelago; and the Chugach plutonic–metamorphic complex in the northern outer islands. Each of these complexes is related to a major subduction event. The metamorphic history of the Coast plutonic–metamorphic complex is lengthy and is related to the Late Cretaceous collision of the Alexander and Wrangellia terranes and the Gravina overlap assemblage to the west against the Stikine terrane to the east. The metamorphic history of the Glacier Bay–Chichagof plutonic–metamorphic complex is relatively simple and is related to the roots of a Late Jurassic to late Early Cretaceous island arc. The metamorphic history of the Chugach plutonic–metamorphic complex is complicated and developed during and after the Late Cretaceous collision of the Chugach terrane with the Wrangellia and Alexander terranes. The Coast plutonic–metamorphic complex records both dynamothermal and regional contact metamorphic events related to widespread plutonism within several juxtaposed terranes. Widespread moderate-P/T dynamothermal metamorphism affected most of this complex during the early Late Cretaceous, and local high-P/T metamorphism affected some parts during the middle Late Cretaceous. These events were contemporaneous with low- to moderate-P, high-T metamorphism elsewhere in the complex. Finally, widespread high-P–T conditions affected most of the western part of the complex in a culminating late Late Cretaceous event. The eastern part of the complex contains an older, pre-Late Triassic metamorphic belt that has been locally overprinted by a widespread middle Tertiary thermal event. The Glacier Bay–Chichagof plutonic–metamorphic complex records dominantly regional contact-metamorphic events that affected rocks of the Alexander and Wrangellia terranes. Widespread low-P, high-T assemblages occur adjacent to regionally extensive foliated granitic, dioritic and gabbroic rocks. Two closely related plutonic events are recognized, one of Late Jurassic age and another of late Early and early Late Cretaceous age; the associated metamorphic events are indistinguishable. A small Late Devonian or Early Mississippian dynamothermal belt occurs just north-east of the complex. Two older low-grade regional metamorphic belts on strike with the complex to the south are related to a Cambrian to Ordovician orogeny and to a widespread Middle Silurian to Early Devonian orogeny. The Chugach plutonic–metamorphic complex records a widespread late Late Cretaceous low- to medium/high-P, moderate- T metamorphic event and a local transitional or superposed early Tertiary low-P, high-T regional metamorphic event associated with mesozonal granitic intrusions that affected regionally deformed and metamorphosed rocks of the Chugach terrane. The Chugach complex also includes a post-Late Triassic to pre-Late Jurassic belt with uncertain relations to the younger belts.     

华北克拉通中部造山带早前寒武纪变质演化历史评述

根据变质作用程度不同,华北中部造山带早前寒武纪基底可以进一步分为高级区和花岗-绿岩带。前者变质程度可达高角闪岩相-麻粒岩相,包括太华、吕梁、阜平、恒山、怀安、宣化等杂岩,花岗-绿岩带的变质程度较低,多为绿片岩相-角闪岩相,包括登封、赞皇、五台等杂岩。已有变质演化研究表明,高级区恒山、怀安和宣化杂岩中的麻粒岩和(或)退变榴辉岩记录的峰期变质压力最高,恒山杂岩、阜平杂岩和宣化杂岩中的麻粒岩记录的峰期变质温度最高;花岗-绿岩带中的赞皇杂岩和五台杂岩出露高压斜长角闪岩和高压变泥质岩。中部带各变质杂岩中可识别出早期进变质、峰期、峰后快速降压和晚期冷却等变质阶段,拥有顺时针近等温降压型的变质作用P-T轨迹,与华北克拉通中部的俯冲碰撞有关。大量变质年代学数据显示,中部带各变质杂岩中至少记录了~1.85Ga、~1.95Ga和~2.5Ga三组变质年龄,其中,~1.85Ga的变质年龄占据了主导地位,大致与区域片麻理形成的时间一致,代表变质高峰期时代;~1.95Ga的变质年龄代表了峰期前的某个进变质片段;~2.5Ga的变质年龄则指示了更早一期的变质事件,推测与古老陆块~2.5Ga所遭受的大量幔源岩浆的侵入和底垫作用有关。然而,变质年龄与变质阶段的对应关系尚不明确。     

变质核杂岩研究进展

变质核杂岩是大陆流变伸展的重要表现形式。在近20年的国际大陆动力学研究中,变质核杂岩与伸展构造是一个方兴未艾的热点课题。在大量文献资料调研和作者科研实践的基础上,对变质核杂岩与伸展构造的研究现状进行了综述,分析了变质核杂岩的几何学、运动学、年代学特征、拆离断层特征、形成的大地构造背景与成矿关系等,对变质核杂岩基本特征及判别标志进行了归纳总结。同时,对变质核杂岩的热点争论问题,如变质核杂岩是否一定存在巨厚地壳柱被切失的问题、变质核杂岩形成过程中岩浆作用与伸展作用的主从关系问题、低角度拆离断层的成因等进行了讨论,并提出自己的认识。     

高级变质地体中多期变质事件的甄别:以东南极埃默里地区为例

在多期高级变质地体中确定每期变质事件的性质、时代和P-T轨迹是一项非常困难的研究工作,也是变质地质学研究领域的前沿课题。将精细的地质年代学研究与详细的构造解析和岩石学观察相结合,这是建立同位素年龄与变质事件之间有机联系的最好方法。使用这种方法来甄别东南极埃默里地区格林维尔期和泛非期高级变质事件获得了初步的成功,基本上查明了两期变质事件的影响范围、变质时代和P-T演化,但也存在许多尚未解决的科学问题。一般而言,在同一变质地体中发育两期/多期具有不同构造指向的挤压-伸展变形、保存不同类型(如顺时针和逆时针)或者不连续的P-T演化轨迹以及含有两组/多组变质年龄等可视为存在多期变质事件的标志。当前对多期高级变质作用叠加的研究中还存在两个不清楚的岩石学问题,其一是早期变质作用对晚期变质重结晶施加了什么样的影响?其二是晚期变质作用如何叠加在早期变质矿物组合之上?所以,探索多期高级变质事件的叠加机理、变质行为及控制因素仍是未来岩石学家的艰巨任务。     

内蒙古红山子地区新太古代变质侵入岩年代学及地球化学特征

华北克拉通西部陆块阴山地块花岗-绿岩带中出露的花岗岩类型比较复杂,研究其类型、成因及其与绿岩带之间的联系,对于探讨花岗-绿岩带的形成和演化有重要意义。本文对内蒙古三合明铁矿西侧红山子地区的花岗-绿岩带进行了研究。其不仅包含有大量的变质科马提岩、科马提质玄武岩和拉斑玄武岩等绿岩带的表壳岩,而且发育着多种变质侵入岩。研究表明,区内与绿岩带相关的侵入岩主要有3种,即变质英云闪长岩、变质花岗闪长岩、变质花岗岩,本文对其中的变质英云闪长岩和变质花岗岩两种岩石进行了地球化学和锆石测年的研究。变质英云闪长岩是一套富铝、低钾、贫铁镁和稀土元素强烈分异的太古宙高铝型TTG岩,SHRIMP锆石U-Pb测年为(2 534±7)Ma(MSWD=1.3),与研究区南部的赞岐岩和TTG的年龄相同;而变质花岗岩具有高硅、富碱,低铁、镁、钛、锰的特点,是一套钙碱性的过铝质岩石,SHRIMP锆石U-Pb测年为(2 509±7)Ma(MSWD=1.5),野外地质现象表明变质花岗岩明显地侵入到区内的绿岩地层和变质英云闪长岩之中。研究发现变质英云闪长岩形成于板块俯冲时TTG岩系岩浆弧构造环境,而变质花岗岩成因则可能与加厚地壳中变沉积岩的熔融有关。     

Early Jurassic metamorphism of the eastern segment of the Lhasa terrane in South Tibet and its tectonic significance

The metamorphic belt in the Dongjiu area is located in the eastern segment of the Lhasa terrane in South Tibet. The Dongjiu metamorphic rocks are primarily composed of schist and gneiss, with minor amounts of marble, and the protoliths are sedimentary rocks with Precambrian and early Palaeozoic zircons probably deposited during the Palaeozoic or late Neoproterozoic. On the basis of petrology and phase equilibria modelling, this study shows that the Dongjiu metamorphic belt has experienced a kyanite-grade metamorphism, which is characterized by a decompressional vector with slight cooling from a peak of 9.6 kbar and 745°C to medium-pressure amphibolite-facies metamorphic overprinting at 5–6 kbar and 600–630°C. This P–T path was well recorded and recovered by garnet zoning profiles. Laser ablation inductively coupled plasma mass spectrometry in situ U–Pb analyses on metamorphic zircons and zircon rims yielded concordant ²⁰⁶Pb/²³⁸U ages of c. 194–192 Ma, suggesting that the Dongjiu metamorphic rocks were formed during the Early Jurassic. Therefore, the Dongjiu metamorphic belt, together with the western Nyainqentanglha, Basongco, and Zhala metamorphic belts, constitutes a nearly continuous tectonic unit with an E–W extension of at least 500 km between the northern and southern Lhasa terranes. The metamorphic ages of these belts, ranging from 230 to 192 Ma, show a younger trend from west to east, indicating that the central segment of the Lhasa terrane experienced an eastward asynchronous collisional orogeny during the Late Triassic to Early Jurassic.     

变质地质学研究中的一些困难问题

变质地质学研究中,首先需要准确判明变质过程是属于多期或单期、多峰或单峰变质作用。研究中应该对同一研究区不同类型的变质岩石开展综合研究,查明各种变质反应结构,以期对变质反应结构的解释尽可能准确,并对变质反应进行定量论证。同一变质岩内前、后两个世代变质矿物组合之间,并不存在所谓的热力学局部平衡。在应用方面,矿物温度计与压力计属于反演方法,热力学视剖面图模拟属于正演方法。本文指出了它们之间的相同之处和各种区别。众所周知,目前反演出的变质作用压力并无方向性,差异应力对总应力的贡献尚不明朗。最好根据同一视域或同一块变质岩石,来反演变质作用P-T-t轨迹。实际上,P-T-t轨迹一般不应该是圆滑的曲线,其构造意义的解释也还存在不确定性。同一地区变质作用PT条件的突变、P-T-t轨迹的明显差异,表明地质体之间存在断层接触关系。变形作用与变质作用的耦合研究需要加强,变质作用的定年也不能仅仅局限于变质副矿物。特别值得注意的是,同一造山带内范围有限的地域,也未必属于同一变质相系。     

医巫闾山变质核杂岩构造特征

经过野外宏观观测、室内显微分析及同位素年龄测定,基本厘定医巫闾山是一个白垩纪时形成的变质核杂岩。变质核杂岩中心为晚燕山期的医巫闾山二长花岗岩体,周围是由代表地壳深部变形特点的太古宇变质岩组成的变质核。变质核北面和东面以拆离断层与盖层下部中新元古界相接触,并为中新元古界组成的韧性流变的中间层所环绕。变质核西面以阜新盆地东南缘边界正断层( 孙家湾- 稍户营子正断层) 与盖层上部以脆性变形为特征的白垩纪碎屑岩相邻。变质核杂岩的变形片理、线理及运动指向说明该变质核杂岩为对称型变质核杂岩。     

Nature and age of metamorphic rocks from the basement of the West Siberian megabasin (according to U-Pb isotopic dates)

The SHRIMP-II zircon U-Pb dates for metamorphic rocks from the West Siberian basement are determined for the first time. It is established that the major protolith of the metamorphic strata from the Shaimsk-Kuznetsovsk meganticlinorium is composed of sedimentary Late- and Middle-Devonian rocks (395–398 Ma). It is likely that the greywackes, whose strata were mainly formed under erosion of ophiolitic rocks, served as a substrate for the metamorphic rocks. The metamorphic transformations of the rocks occurred under conditions of greenschist and occasionally lower amphibolite facies of metamorphism during the Late Carboniferous-Early Permian period.     

秦岭勉(县)略(阳)缝合带及南秦岭地块的变质动力学研究

将变质地质学与构造地质学的研究相结合,将造山过程与变质演化相结合,系统研究了勉(县)略(阳)缝合带的康县—高川段变质动力学特征及其与佛坪递增变质带的关系,认为该区变质作用时空演化特征表现为:不同构造环境同时出现不同类型的变质(如俯冲、拉张等),同一地带不同时期或阶段变质类型的叠加(如俯冲与碰撞),不同地段具差异抬升的历史等特征,从而导致变质类型在空间上的复杂性。这一结果也表明勉略带是一复杂的蛇绿构造混杂岩带,其内部不同的一系列构造岩片早期曾有不同的变质演化历史,后期经历了相似的变质叠加,最后在构造作用下差异抬升。该区变质作用演化可细分为4个阶段:主体发生于海西期并延续至印支早期的拉张变质作用阶段,晚海西—印支期俯冲变质作用阶段,印支晚期碰撞变质作用阶段和印支末期的热接触变质作用和动力退变质作用阶段。勉略缝合带康县—高川段不同岩片及佛坪递增变质带的PTt轨迹研究,揭示了它们不同的变质动力学演化及俯冲、碰撞、拆沉的构造演化过程和机制,也反映了缝合带是由不同层次、起源和环境的岩石经构造作用混杂而成,并经历了后期差异隆升剥蚀历史。     

Geochronological and geochemical study of gneiss–schist complexes and associated granitoids,Beishan Orogen,southern Altaids

The tectonic nature of metamorphic terranes and their role in orogenesis are problematic. Here we present new U–Pb ages and geochemical data for widespread metamorphic rocks and associated granitoids from Northwest China. Orthogneisses from the metamorphic complexes have crystallization ages of ～457, ～452, and ～526 Ma. One paragneiss (schist) has a maximum depositional age of 312 ± 7 Ma. Three foliated granites were emplaced at ～450, ～349, and ～410 Ma, and all lack inherited Precambrian ages. The metamorphic terranes may have undergone multiple petrotectonic events as revealed by the metamorphic ages. Both the orthogneisses and granitoids show enrichment in large ion lithophile elements (LILEs) and light rare Earth elements (LREEs), and depletion in high field strength elements (HFSEs), which indicate that they formed in a subduction-generated accretionary arc setting. Our study demonstrates that the metamorphic terranes in the Beishan area, originally considered as Precambrian basement with suspected Neoarchaean to Palaeoproterozoic ages, are actually parts of early Palaeozoic arcs. The protoliths were probably metamorphosed arc plutonic and sedimentary rocks. Combined with other studies, we speculate that the Beishan Orogen formed by progressive arc accretion during the latest Neoproterozoic to early Palaeozoic time. This new interpretation has implications for other high-grade metamorphic terranes within orogens that have been assumed to represent ancient or pre-existing micro-continental blocks. If so, the importance of collision as a mechanism of mountain building has been overestimated, and the accretionary process as a mechanism of continental growth has been underestimated.     

Variations in crustal level and geothermal gradient during the evolution of the Lewisian complex of northwest Scotland

The metamorphic reactions observed in the polycyclic Precambrian gneisses of parts of western Sutherland and the Outer Hebrides are discussed in the light of other evidence bearing on the tectonic and metamorphic evolution of the Lewisian complex. The evidence that early granulite facies assemblages were transformed into successively lower-pressure assemblages seems consistent with the views expressed by several authors that the complex remained continuously at deep crustal levels for nearly a thousand million years after the earliest dated metamorphic event and that it began to rise towards the earth's surface during the Laxfordian metamorphic stage. The Scourie dyke swarm appears to have been emplaced under deep-seated conditions. The Laxfordian tectonic patterns record the effects of regional uplift and the associated metamorphic assemblages suggest local steepening of the geothermal gradient.     

The Higo metamorphic complex in Kyushu, Japan as the fragment of Permo–Triassic metamorphic complexes in East Asia

The Higo terrane in west-central Kyushu Island, southwest Japan consists from north to south of the Manotani, Higo and Ryuhozan metamorphic complexes, which are intruded by the Higo plutonic complex (Miyanohara tonalite and Shiraishino granodiorite).The Higo and Manotani metamorphic complexes indicate an imbricate crustal section in which a sequence of metamorphic rocks with increasing metamorphic grade from high (northern part) to low (southern part) structural levels is exposed. The metamorphic rocks in these complexes can be divided into five metamorphic zones (zone A to zone E) from top to base (i.e., from north to south) on the basis of mineral parageneses of pelitic rocks. Greenschist-facies mineral assemblages in zone A (the Manotani metamorphic complex) give way to amphibolite-facies assemblages in zones B, C and D, which in turn are replaced by granulite-facies assemblages in zone E of the Higo metamorphic complex. The highest-grade part of the complex (zone E) indicates peak P–T conditions of ca. 720 MPa and ca. 870 °C. In addition highly aluminous Spr-bearing granulites and related high-temperature metamorphic rocks occur as blocks in peridotite intrusions and show UHT-metamorphic conditions of ca. 900 MPa and ca. 950 °C. The prograde and retrograde P–T evolution paths of the Higo and Manotani metamorphic complexes are estimated using reaction textures, mineral inclusion analyses and mineral chemistries, especially in zones A and D, which show a clockwise P–T path from Lws-including Pmp–Act field to Act–Chl–Epi field in zone A and St–Ky field to And field through Sil field in zone D.The Higo metamorphic complex has been traditionally considered to be the western-end of the Ryoke metamorphic belt in the Japanese Islands or part of the Kurosegawa–Paleo Ryoke terrane in south-west Japan. However, recent detailed studies including Permo–Triassic age (ca. 250 Ma) determinations from this complex indicate a close relationship with the high-grade metamorphic terranes in eastern-most Asia (e.g., north Dabie terrane) with similar metamorphic and igneous characteristics, protolith assembly, and metamorphic and igneous ages. The north Dabie high-grade terrane as a collisional metamorphic zone between the North China and the South China cratons could be extended to the N-NE along the transcurrent fault (Tan-Lu Fault) as the Sulu belt in Shandong Peninsula and the Imjingang belt in Korean Peninsula. The Higo and Manotani metamorphic complexes as well as the Hida–Oki terrane in Japan would also have belonged to this type of collisional terrane and then experienced a top-to-the-south displacement with forming a regional nappe structure before the intrusion of younger Shiraishino granodiorite (ca. 120 Ma).     

Sm–Nd garnet dating of polyphase metamorphism: northern Coast Mountains, south-eastern Alaska, USA

Sm–Nd ages of garnet from the northern Coast Mountains of south-eastern Alaska, USA, constrain the timing of thermal events in polyphase metamorphic rocks of the western metamorphic belt and provide new data on the spatial extent of Cretaceous regional metamorphism. Bulk garnet–whole-rock Sm–Nd ages for a sillimanite-zone amphibolite (Taku Inlet) and a biotite-zone metapelite (Tracy Arm) are 77±17 Ma and 59±12 Ma, respectively. Garnet core–whole-rock (80±9 Ma), core–matrix (84±9 Ma), rim–whole-rock (59±4 Ma) and rim–matrix (62±4 Ma) ages were obtained from a sample collected 200  m west of a Palaeocene Coast plutonic–metamorphic complex sill-like pluton that separates medium-grade metamorphic rocks from high-grade metamorphic rocks and voluminous Tertiary plutons in the core of the orogen. The garnet core ages of c. 80 Ma indicate that the regional metamorphic grade reached garnet zone prior to the intrusion of the plutons and high-grade metamorphism of rocks to the east. Similar ages for the younger plutons, the youngest garnets and the rim of a multistage garnet ( c. 59 Ma) indicate a later episode of contact metamorphic garnet growth. Documentation of pre-71 Ma garnet-zone metamorphism along the western edge of the Coast plutonic–metamorphic complex confirms that Albian to Late Cretaceous metamorphism associated with crustal thickening affected this part of the orogen. The similarity of garnet Sm–Nd ages to independent age estimates for metamorphic events confirms that this technique provides useful estimates for the timing of Late Cretaceous to Tertiary thermal events. The c. 20  Myr difference between garnet core and rim ages suggests that the Sm–Nd isotope systematics of a single garnet grain can be used for distinguishing between multiple metamorphic events.     

部分熔融与高级变质岩流变机制——以内蒙古大青山高级变质岩为例

部分熔融作用与高级变质岩变形作用是相互制约,变形作用能够提高岩石部分熔融程度,降低熔融温度。熔体存在影响和制约岩石强度和变形机制。大青山高级岩经历了下部地壳构造层次变质变形和深熔作用改造,形成了复杂构造要素组合。宏观与微观构造特点表明:高级变质岩变形机制主要为熔体增强颗粒边界扩散和颗粒流动,使岩石发生大规模的塑性流动。在宏观上形成了不对称流动组构、熔融线理、岩石和矿物条带、层内底辟褶皱和大型穹窿构造。但是,在微观上矿物颗粒变形不明显,晶内变形组构不发育,表现为三边平衡结构,与静态结晶变质岩结构相似,形成了地壳深部构造层次上变质构造岩-构造片麻岩。     

西藏八宿吉利地区新发现寒武纪变质花岗岩锆石U-Pb年龄、地球化学特征及其地质意义

八宿吉利地区寒武纪变质花岗岩位于曲扎湖-提卡一带,主要由变质二长花岗岩和变质花岗闪长岩组成。这一新发现对于认识和恢复原特提斯构造历史演化具有重要意义。锆石CL图像显示变质花岗岩锆石为岩浆成因。锆石LA-ICP-MS测年得出片理化变质二长花岗岩年龄为503.7±4.7Ma、变质花岗闪长岩年龄为494.7±3.4Ma,表明该岩体形成时代属于寒武纪。通过岩石地球化学分析,变质二长花岗岩SiO2含量介于69.87%~79.89%之间;变质花岗闪长岩SiO2含量介于66.63%~70.15%之间。前者Al2O3含量变化于12.36%~14.82%,Na2O含量为2.54%~7.16%,K2O含量为0.15%~5.95%,K2O/Na2O=0.02~2.34;后者Al2O3含量变化于14.66%~15.41%,Na2O含量为3.60%~5.63%,K2O含量为0.77%~2.78%,K2O/Na2O=0.14~0.77,属于钙碱性-碱性过铝质花岗岩。在侵入岩构造环境Rb-(Y+Nb)判别图解、Rb-(Yb+Ta)判别图解中,样品均落入“火山弧花岗岩”区域中,表明其形成于大陆边缘火山弧环境。结合锆石测年结果及区域地质背景分析,认为吉利地区变质花岗岩形成于冈瓦纳大陆裂离卡穷微陆块阶段,同时表明原特提斯洋形成最早时限可追溯至寒武纪。     

羌塘中部高压变质带的退变质作用及其构造侵位

羌塘中部的高压变质带主要由榴辉岩、石榴石白云母片岩和蓝片岩等组成,它们在遭受高压变质作用之后折返,构造侵位于晚古生代展金组地层中,二者以韧性变形带为接触边界.本文以高压变质带中的榴辉岩和韧性变形带为研究对象,讨论了高压变质带折返过程中的退变质作用特征及折返时代.研究表明,榴辉岩在高峰期变质作用之后的折返过程中经历了由榴辉岩相→蓝片岩相→绿帘角闪岩相的退变质作用演化过程;在高压变质带构造侵位过程形成的韧性变形带中,白云母石英片岩的白云母40Ar-39Ar坪年龄为219±2Ma.高压变质带在219Ma左右构造侵位于展金组地层中,并于214Ma之前最终抬升出露地表.     

Sub-ophiolite metamorphic rocks from NW Anatolia, Turkey

The metamorphic rocks from near Kütahya in north-west Anatolia record different stages in the history of closure of the Neo-Tethyan İzmir–Ankara–Erzincan ocean. Sub-ophiolite metamorphic rocks within the Tavşanlı zone are a tectonically composite sequence of quartz–mica schists, amphibole schists, amphibolites and garnet amphibolites. They show increasing metamorphic grade towards the base of the ophiolite. A first metamorphic event, typical of sub-ophiolite metamorphic sole rocks, was characterized by high-grade assemblages, and followed by retrograde metamorphism. A second event was marked by a medium-to high-pressure overprint of the first-stage metamorphic assemblages with assemblages indicating a transition between the blueschist and greenschist facies. The chemistry of the sub-ophiolite metamorphic rocks indicates an ocean island basalt origin, and Ar–Ar dating indicates a high temperature metamorphic event at 93±2 Ma. Counter-clockwise P–T–t paths recorded by the sub-ophiolite metamorphic rocks are interpreted to result from intra-oceanic thrusting during the closure of the İzmir– Ankara–Erzincan ocean, initiating subduction, which formed the high-temperature assemblages. Further subduction then produced the widespread blueschists of the Tavşanlı zone during the Late Cretaceous. Later cold thrusting obducted the ophiolite (with the metamorphic sole welded to its base), ophiolitic melanges and blueschists onto the Anatolide passive margin in the latest Cretaceous. All these events pre-date the final Anatolide–Pontide continent–continent collision.