松潘-甘孜地体中部晚三叠世安山质增生弧的确定及其意义

本文研究揭示,巴颜喀拉-松潘甘孜地体（简称松潘-甘孜地体）中部三叠系复理杂岩中的安山岩块是增生弧岩浆活动的产物,具有高度不均一均匀的岩石组构、地球化学与同位素组成。特别是这些安山岩显示了与松潘-甘孜三叠系海相浊积岩具有高度的Sr-Nd同位素亲缘性,沿亏损地幔和三叠系海相浊积岩混熔曲线分布,且主要分布于三叠系海相浊积岩端元区。这表明该增生弧安山质岩浆主要由增生楔中的海相浊积岩基质部分熔融形成,并受到了增生楔中大洋岩石圈残片等超镁铁/铁镁质组分的不同程度混染。安山岩时代（226.8±1.9Ma~213.7±0.9Ma）以及同时期广泛的S型花岗岩侵入活动（228±2Ma~204±7Ma）揭示增生弧形成于晚三叠世Norian期。前人工作表明,位于松潘-甘孜地体东北隅的诺尔盖-松潘盆地是一个周缘前陆盆地。因此,松潘-甘孜地体并非单一构造成因的地质体,至少由二叠纪-三叠纪演化的古特提斯大洋和中三叠世拉丁期-晚三叠世诺列期周缘前陆盆地两部分构成。前者是松潘-甘孜地体的主体部分,由松潘-甘孜古特提斯洋在三叠纪时期快速收缩形成的增生楔杂岩组成,其上发育增生弧,局部残存古特提斯大洋盆地及被构造移置的洋壳残片;后者为西秦岭弧（248~234Ma）与扬子地块碰撞的产物。

     

冈底斯岩浆弧东段沉积岩的晚白垩世变质作用及其构造意义

位于青藏高原南部的冈底斯岩浆弧形成于中生代新特提斯洋俯冲过程中,并在印度与欧亚大陆碰撞过程中叠加了新生代岩浆作用和变质作用。冈底斯岩浆弧东段出露的中、新生代变质岩是研究其深部组成与形成演化的理想窗口。本文对冈底斯东段米林田兴村地区的变沉积岩,即石榴夕线黑云片岩、含石榴钙硅酸盐岩、黑云斜长片麻岩和大理岩进行了岩石学和锆石U-Pb年代学研究。研究结果表明,石榴夕线黑云片岩由夕线石、黑云母、石榴石、斜长石、钾长石、石英和钛铁矿组成,经历了中压麻粒岩相变质作用,变质条件为810~820℃和6.4~7.8kbar。锆石年代学研究表明,石榴夕线黑云片岩、含石榴钙硅酸盐岩和大理岩经历了87~83Ma的变质作用。本文和现有研究表明,冈底斯弧东段林芝和米林地区的高压麻粒岩相变质岩分布区代表该岩浆弧的下地壳,而包括本文研究点在内的中压麻粒岩相到角闪岩相变质岩分布区为其中地壳组成部分。本文认为晚白垩世大体积幔源岩浆的注入和随后的新特提斯大洋岩石圈平俯冲,导致了冈底斯岩浆弧地壳的生长、加厚和底冲,使上地壳的沉积岩和岩浆岩运移到中-下地壳,并经历了高温变质与部分熔融,形成了分布在上地壳的花岗岩。这表明岩浆弧的新生地壳在晚白垩世新特提斯洋俯冲过程中发生了再造。以长英质岩石为主的表壳岩进入深地壳很可能是岩浆弧中-下地壳由基性转变成中性成分的重要方式。     

Late Paleozoic–Early Triassic magmatism on the western margin of Gondwana: Collahuasi area, Northern Chile

The basement in the ‘Altiplano’ high plateau of the Andes of northern Chile mostly consists of late Paleozoic to Early Triassic felsic igneous rocks (Collahuasi Group) that were emplaced and extruded along the western margin of the Gondwana supercontinent. This igneous suite crops out in the Collahuasi area and forms the backbone of most of the high Andes from latitude 20° to 22°S. Rocks of the Collahuasi Group and correlative formations form an extensive belt of volcanic and subvolcanic rocks throughout the main Andes of Chile, the Frontal Cordillera of Argentina (Choiyoi Group or Choiyoi Granite-Rhyolite Province), and the Eastern Cordillera of Peru.Thirteen new SHRIMP U–Pb zircon ages from the Collahuasi area document a bimodal timing for magmatism, with a dominant peak at about 300 Ma and a less significant one at 244 Ma. Copper–Mo porphyry mineralization is related to the younger igneous event.Initial Hf isotopic ratios for the ~ 300 Ma zircons range from about − 2 to + 6 indicating that the magmas incorporated components with a significant crustal residence time. The 244 Ma magmas were derived from a less enriched source, with the initial Hf values ranging from + 2 to + 6, suggestive of a mixture with a more depleted component. Limited whole rock ¹⁴⁴Nd/¹⁴³Nd and ⁸⁷Sr/⁸⁶Sr isotopic ratios further support the likelihood that the Collahuasi Group magmatism incorporated significant older crustal components, or at least a mixture of crustal sources with more and less evolved isotopic signatures.     

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.     

The tectonometamorphic and magmatic evolution of the Uppermost Unit in central Crete (Melambes area): constraints on a Late Cretaceous magmatic arc in the Internal Hellenides (Greece)

Crete consists of a nappe pile that formed during Alpine subduction and collision. The lower nappes belong to the External Hellenides, whereas the uppermost nappe is ascribed to the Pelagonian Zone of the Internal Hellenides. The Uppermost Unit consists of several subunits including the Asterousia Crystalline Complex (ACC), which comprises metasedimentary rocks, (meta)granitoids and serpentinite, the protolith age and the tectonometamorphic evolution of which are largely unknown. In the present study, we present new structural, microfabric and geochronological data from the Uppermost Unit in the Melambes area (central Crete). ²⁰⁶Pb/²³⁸U zircon ages (LA-ICP-MS and ID-TIMS) indicate granitic and dioritic intrusions between 71.9 ± 0.6 and 76.9 ± 0.3 Ma. Identical ages have previously been obtained from comparable intrusions in eastern Crete and on Anafi. The composition and geochemical signature suggest an extended magmatic arc along the southern active margin of the Pelagonian-Lycian Block. Post-intrusive shearing transformed granite into orthogneiss, whereas diorite remained free from foliation, because of the lower amount of mechanically weak phases. Deformation microfabrics suggest top-to-the SE shearing under amphibolite facies conditions of the ACC and at greenschist facies conditions of rocks at the base of the ACC referred to as Akoumianos Greenschist. The Akoumianos Greenschist is considered as the northern part of the Pindos realm that was subducted underneath the Pelagonian-Lycian active margin. Based on our new and on published data, the following orogenic stages are suggested to have contributed to the evolution of the Hellenides during the Late Cretaceous to Eocene: (1) pre-middle Campanian collision and subduction of the Pindos lithosphere underneath the southern margin of the Pelagonian-Lycian terrane led to obduction and offscraping of serpentinized ocean floor and stacking of the ACC during amphibolite facies top-to-the SE thrusting, (2) formation of a Campanian magmatic arc along the Pelagonian-Lycian active margin; (3) Maastrichtian collision and stacking of the magmatic arc during top-to-the SE mylonitic shearing; (4) Palaeocene top-to-the SE greenschist-facies shearing of the ACC on top of the Akoumianos Greenschist; (5) Late Eocene thrusting of the Uppermost Unit on top of the Arvi and Pindos units. Thus, top-to the SE was the dominant shear sense in the southern Aegean from at least the mid-Late Cretaceous until the Eocene.     

Generation and metamorphism of new crust in magmatic arcs: a case study from northern Chile

In a deep section of the Jurassic Andean arc, coastal Cordillera of northern Chile, south of Antofagasta, basic magmas from a mantle source accreted large masses of crust in an early stage of arc development. These gabbroic intrusions were isochemically metamorphosed at moderate pressures (≤ 5 kbar) and high temperatures (∼ 800°C) into uniform garnet-absent dino+ortho-pyroxene gneisses (granulites). Subsequently they were partly transformed into amphibolites ( T ≤ 700°C) during tectonically controlled access of fluid without major changes in whole rock chemistry. Locally migmatization occurs in the amphibolites. The metamorphic unit was diversified by enduring magmatic activity in the form of chemically distinct gabbroic and quartz dioritic intrusions. Formation and composition of continental crust might be a typical example for the deeper plutonic sections of this continental magmatic arc.     

Late Permian and Early Triassic evolution of the Northern Indian margin: carbon isotope and sequence stratigraphy

Abstract

The Northern part of Great-India underwent an early rifting phase in the late Paleozoic, just at the end of the large scale Gondwanian glaciation. The beginning of the rifting processes is marked by large hiatus and discontinuities (para- conformities) between the early or middle Paleozoic sedimentary succession and the discontinuous middle-late Permian Traps and transgressive sediments. The Northern Indian passive margin consists of the present High and Lower Himalaya and a small part of the Indian craton and their sedimentary cover. The Permian rift shoulder is located in the Higher Himalaya, with part being in the underthrusted Lower Himalaya. The rim basin (landward of the shoulder) is well developed in the Pottawar- Salt Range area. From the rifting to the beginning of the drifting stages (early late Permian to late early Triassic time), the sedimentary evolution is characterised by three transgressive- regressive (T-R) second order cycles, two in the late Permian and one in the early Triassic. The break-up of the rift occurred during the second cycle (late Dzhulfian).

In the Salt Range area, these three T-R cycles have been subdivided in eight third order sequences, five sequences for the upper Permian and three for the lower Triassic.

At the end of Permian, hiatuses, gaps and local erosion of part of the margin are direct consequences of a first order relative sea-level fall; this is also the time of the largest extinction event of the Phanerozoic that deeply affected the carbonate productivity and the stratal patterns. With the following worldwide sea-level rise, a rapid and large scale transgression occurred in the early Triassic, well dated and recorded on the whole margin. High rate thermal subsidence gave way to generalized pelagic deposits about 2 My after the transgression.

Profiles of whole rock inorganic carbon and oxygen isotopes from Guryul Ravine and Palgham sections in Kashmir, Nammal Gorge and Landu sections in Trans Indus Ranges (Pakistan), Thini Chu section in Kali Gandaki Valley, Central Nepal are presented in connection with the sequence stratigraphic analysis. The upper Permian record of high positive δ¹³C values are closely correlated with the second order T-R cycles and the third order sequences. The results presented in this study confirm the drastic drop of δ¹³C from the high positive values that characterised the upper Permian to lower values in the lower Triassic time. Stratigraphic correlation problems in the lower Triassic using carbon isotope geochemistry are briefly discussed. A positive δ¹³C excursion of 4–5% near the Smithian - Spathian substages boundary is observed for the first time. The δ¹⁸O values of samples from all the sections display major variations suggesting that the oxygen isotope record has been significantly affected by meteoric diagenesis, deep burial diagenesis or/and monsoon signature.     

Late orogenic magmatism and sedimentation within Late Carboniferous to Early Permian basins in the Balkan terrane (Bulgaria): geodynamic implications

The orogenic Balkanid belt, which developed between the Moesian Plate and the Moravian-Rhodopi-Thracian Massifs, was affected by the Late Carboniferous and Early Permian opening of W-E oriented graben structures. The progressive tectonic rejuvenation of the basins is demonstrated by the deposition of repeated regional sedimentary cycles, associated with volcanism that was mostly localised along the tectonic boundaries, in an intramontane setting.The Late Carboniferous volcanism is represented by rhyodacitic explosive products and hyaloclastites, and by andesitic flows. During the Early Permian, subvolcanic rhyodacitic and rhyolitic bodies and the explosive products prevailed in the western sectors, whereas rhyolitic ignimbrites occur to the east.The tectonically active basins are interpreted due to late orogenic collapse, and the alternation of extensional tectonics and minor compressional phases is consistent with the regional transtensional regime, active along the Variscan suture of Pangaea. The volcanic activity associated with the evolution of the basins matches the petrogenetic features and the evolution from early dacitic – andesitic to late rhyolitic activity in the Southern European segment of the Variscan system.These Late Carboniferous-Early Permian sedimentary and tectono-magmatic events in Bulgaria are characterized, and compared with the homologous Permo-Carboniferous sequences along some western European segments of the Variscan belt.     

藏北羌塘盆地早侏罗世-中侏罗世早期沉积构造特征

侏罗纪早期是研究羌塘盆地形成过程、判别盆地性质的重要时期.通过开展早侏罗世-中侏罗世早期的沉积相、古流向、沉积厚度统计等分析,恢复了该时期羌塘盆地的古地理面貌,提出了侏罗纪早期羌塘盆地并非前陆盆地,而是伸展构造背景下形成的裂陷型盆地.该时期盆地以中央隆起带为界,南部为陆缘开阔海盆地,形成滨岸-浅海陆棚相碎屑岩沉积;北部为陆缘近海湖盆,成盆初期(托尔期)发生强烈的裂陷活动,形成一套火山碎屑岩沉积体系,可识别出三个北西向分布的裂陷槽,巴柔期快速下沉,沉积一套厚达2 000 m以上的冲积扇-湖泊三角洲-湖盆相碎屑岩夹少量灰岩和石膏沉积,沉降中心位于湖盆南部,表现出明显的继承性,随后逐步向北东迁移,海水自中央隆起带中段频繁向湖盆浸漫.巴通期中央隆起带进一步下沉,南、北羌塘连成一个统一的拗陷盆地,从此沉积作用进入稳定发展阶段,形成稳定的碳酸盐岩台地沉积体系.     

Late Cretaceous eclogite in the Eastern Rhodopes (Bulgaria): evidence for subduction under the Sredna Gora magmatic arc

International Journal of Earth Sciences - The Rhodopes in Bulgaria and Greece represent a nappe stack of high-grade units with polymetamorphic history. Constraining the time of metamorphism in...     

Widespread relics of high-pressure metamorphism confirm major terrane accretion in Ecuador: a new example from the Northern Andes

Metamorphic rocks from two sections through the North Andean Cordillera Real in Ecuador contain phengites as relics of high-pressure (HP) metamorphism. Their maximum Si contents per formula unit are (1) 3.25 in Al-rich metapelites, (2) 3.35 in ordinary metapelites and garnet-rich metabasites, and (3) 3.45 in garnet-bearing orthogneisses. Detailed pressure–temperature paths were derived by calculating P–T pseudosections. Garnet-bearing metapelite and metabasite yielded a P–T path starting at 1.4 GPa and 525°C. Peak temperature conditions were 560°C at <1.4 GPa. The retrograde path passed through 0.7–0.8 GPa at 500°C. An adjacent metagranitoid experienced a similar P–T evolution at slightly lower temperatures of about 30–50°C. This is the first reported occurrence of HP metamorphism in the Cordillera Real; it is widespread and evidently was the result of continental collision. We hypothesize that a single microcontinent collided with the South American continental margin in Early Cretaceous time. In contrast to our findings, previous models have simply suggested the amalgamation of several terranes in the late Mesozoic or earlier in the region of Ecuador. Therefore, we propose that a widespread search for HP relics should be undertaken. Such relics provide a general criterion for defining subducted terranes and their respective boundaries.     

Diagenesis of clay minerals and K-bentonites in Late Permian/Early Triassic sediments of the Sichuan Basin (Chaotian section,Central China)

Detailed clay mineralogical analyses were carried out on Late Permian/Early Triassic carbonate sediments exposed on the Chaotian section (Sichuan Basin, Central China). The clay assemblages are dominantly composed of illite in platform carbonates and clay seams, and illite–smectite mixed-layers (I/S) in tuff layers (K-bentonites) intercalated in the carbonate succession. Detrital and authigenic volcanogenic clay minerals have been partially replaced through illitisation processes during burial, raising questions about diagenetic effects. The precise determination of I/S occurring in K-bentonites shows that the sediments reached a temperature of about 180 °C, which is consistent with (1) previous estimates based on fluid-inclusion homogenisation temperature analysis, (2) the burial depth of the sedimentary series deduced from the post-Palaeozoic geological history of the Sichuan Basin and (3) the new data (T_max) obtained on organic matter indicating the transition between oil and gas windows. The Wangpo Bed, located close to the Guadalupian–Lopingian Boundary, is interpreted either as a volcanic acidic tuff or as a clastic horizon. This controversial origin probably results from mixed volcanogenic and detrital influences. The Wangpo Bed is therefore interpreted as a reworked bentonite as revealed by the occurrence of I/S similar to those found in tuff layers, together with preserved detrital kaolinite.     

The oldest azhdarchoid pterosaur from the Late Jurassic Solnhofen Limestone (Early Tithonian) of Southern Germany

Based on an almost complete three-dimensionally preserved skeleton, a new genus and species of an azhdarchoid pterosaur Aurorazhdarcho primordius n.gen. n.sp. from the Late Jurassic Solnhofen limestone (Early Tithonian) of the Eichstätt area (Bavaria, Germany) is described. Furthermore, a new family the Protazhdarchidae is proposed. The specimen is attributed to the Azhdarchoidea based on its glenoid fossa level with the sternum, the shovel-like shape of the sternal plate, the wide furca of the coracoid, the metacarpus being longer than radius and ulna, the femur being 1/3 longer than the humerus, the femorotibial ratio, and the hammer-shaped humerus among other diagnostic features. Under UV-light, soft tissue preservation around the external mould of the head is visible. It consists of tiny flakes possibly remnants of skin. The dorsally curved outline of the external mould of the head suggests the presence of a cranial crest. The new species is the oldest record of the azhdarchoid pterosaurs. It supports the Eurasian origin of this group that includes the largest flying animal ever.     

冈底斯岩浆弧东段晚白垩世花岗岩的变质作用：新生地壳生长与再造

青藏高原南部的冈底斯岩浆弧形成于中生代新特提斯大洋岩石圈俯冲过程中。在冈底斯岩浆弧东段出露的中—高级变质岩代表岩浆弧的中- 下地壳组成，是探索大陆地壳形成与演化的窗口。作为这些中—高级变质岩原岩主要组成部分的里龙岩基由晚白垩世辉长岩、闪长岩和花岗岩组成。本文对由里龙岩基上部花岗岩变质形成的片麻岩进行了岩石学与年代学研究，探讨其原岩时代、成因、变质作用条件、时间及构造意义。所研究的片麻岩由斜长石、钾长石、石英、黑云母、绿帘石和白云母组成，含或不含角闪石，SiO2含量为61. 94%~74. 39%，铝饱和指数(A/CNK)为0. 89~1. 03，属于高钾钙碱性、准铝质到弱过铝质岩石。这些岩石具有轻稀土元素富集和重稀土元素亏损的配分模式，并表现为富集大离子亲石元素和相对亏损高场强元素的特征。片麻岩中的锆石由继承的岩浆核和变质边组成，岩浆核获得了92~86 Ma的结晶年龄，变质边获得了81~72 Ma的变质年龄，锆石岩浆核具正的εHf(t)值(+10. 2~+12. 1)。这些片麻岩的变质条件为740~750 ℃和0. 5~0. 6 GPa。本文和现有研究表明，里龙岩基是形成在晚白垩世新特提斯洋俯冲过程中具有亏损地幔地球化学特征的弧岩浆岩，经历了强烈的结晶分异作用，所研究的花岗岩是残余岩浆结晶产物。笔者认为，晚白垩世早期新特提斯洋中脊俯冲过程中巨量幔源岩浆的增生导致了冈底斯弧发生了显著的新生地壳生长；在晚白垩世晚期残余新特提斯洋岩石圈平缓俯冲过程中的弧地壳强烈缩短加厚使里龙岩基被运移到中下地壳，并发生了中—高级变质和部分熔融。冈底斯岩浆弧新生地壳在大洋岩石圈俯冲晚期经历了明显再造。     

Magmatic arc metamorphism: petrology and temperature history of metabasic rocks in the Coastal Cordillera of northern Chile

We investigated the metamorphic cooling history of underplated magmatic rocks at midcrustal depth. Granulites and amphibolites occur within the Jurassic magmatic belt of the Coast Range south of Antofagasta in northern Chile between 23°25' and 24°20' S. The protoliths of the metamorphic rocks are basic intrusions of Early Mesozoic age. They are part of the magmatically formed crust, and the essentially dry magmas were emplaced in an extensional regime. The granulites (clinopyroxene–orthopyroxene–plagioclase) show all stages of fabric development from magmatic to granoblastic fabrics. Pyroxene compositions were reset at temperatures around 800°  C independent of the stage of textural equilibration. The granulites were partially amphibolitized at upper amphibolite facies temperatures of 600–700°  C. Following cooling, a possible reheating to greenschist facies temperatures around 500°  C is indicated by prograde zoning in magnetite–ilmenite pairs. Mineral assemblages are not suitable for barometry, but a conservative estimation of the garnet-in reaction at given whole-rock compositions suggests maximum pressures in the granulite facies of around 5 kbar, and similar pressures are indicated by phengite barometry for the greenschist facies. The P–T  path of granulite–amphibolite metamorphism is one of slow cooling from magmatic temperatures with heterogeneous deformation. The thinning of the pre-Andean (Precambrian–Triassic) crust was apparently compensated by the magmatic underplating and this special tectonomagmatic setting caused the prolonged residence of the accreted rocks at midcrustal levels.     

Jurassic igneous rocks of the central Sanandaj–Sirjan zone (Iran) mark a propagating continental rift,not a magmatic arc

Jurassic igneous bodies of the Sanandaj–Sirjan zone (SaSZ) in SW Iran are generally considered as a magmatic arc but critical evaluation of modern geochronology, geochemistry and radiogenic isotopes challenges this conclusion. There is no evidence for sustained igneous activity along the ~1,200 km long SaSZ, as expected for a convergent plate margin; instead activity was brief at most sites and propagated NW at ~20 mm/a. Jurassic igneous rocks define a bimodal suite of gabbro‐diorite and granite. Chemical and isotopic compositions of mafic rocks indicate subcontinental lithospheric mantle sources that mostly lacked subduction‐related modifications. The arc‐like features of S‐type granites reflect massive involvement of Cadomian crust and younger sediments to generate felsic melts in response to mafic intrusions. We conclude that Jurassic SaSZ igneous activity occurred in a continental rift, not an arc. SaSZ igneous rocks do not indicate that subduction along the SW margin of Eurasia began in Jurassic time.     

The Early Palaeozoic high-grade metamorphism at the active continental margin of West Gondwana in the Andes (NW Argentina/N Chile)

The evolution of the Early Palaeozoic orogen of West Gondwana in the Cambrian to Ordovician basement of the Andes between ~18° and 32° S is investigated for pressure and temperature conditions and age of metamorphism. It is characterized by mid-crust temperatures commonly above the wet granite solidus (~650°C). Widespread felsic migmatite and rare granulite formed at pressures of ca 0.5?C0.7?GPa, locally 1.0?GPa. These rocks represent the deepest exhumed sections of the Early Palaeozoic crust. High pressure?Clow temperature rocks are absent. The crystallization ages, compiled from the literature in combination with new data, for near peak metamorphic conditions of these high-grade metamorphic rocks in NW Argentina and N Chile are ~530?C500?Ma and ~470?C420?Ma. Both age groups are spatially overlapping. Radiogenic isotope signatures (Sr, Nd, Pb) are used to characterize the Early Palaeozoic basement. The Pb and Sr isotope compositions of the Early Palaeozoic basement indicate mixing arrays between pre-Palaeozoic unradiogenic and radiogenic crust. Crustal residence ages (Sm?CNd T_DM) indicate a prominent event of crust formation around ~2?Ga, which is known continent-wide. This material was recycled during Midproterozoic and Early Palaeozoic orogenies without prominent additions of new crust present in the isotope record, i.e. accretion of compositional exotic material is absent.     

南羌塘增生过程的中-晚三叠世岩浆记录:藏北玛依岗日-角木日地区基性岩墙

古特提斯洋向北俯冲形成增生杂岩,它们向南增生构成了南羌塘增生地体,且增生期内发育的岩浆岩可以用来研究南羌塘的增生过程。在南羌塘增生杂岩带内,由北向南,发育有玛依岗日(MG)辉长岩墙、角木日(JM)辉长辉绿岩墙。锆石U-Pb测年结果显示,两者分别形成于237. 1±2. 3Ma和230. 7±1. 8Ma,为南羌塘增生期内岩浆岩。它们的岩石地球化学特征均介于OIB与E-MORB之间,富集Ti、Nb,以及LREE和LILE。Nd-Pb同位素结果显示两者均起源于富集地幔源区。Sm/YbLa/Yb图解显示,两者均起源于尖晶石-石榴石二辉橄榄岩源区,相对于JM辉长辉绿岩MG辉长岩部分熔融程度较高。两者Mg#和Ni、Cr特征表明,MG辉长岩分异程度高于JM辉长辉绿岩。Th/Nb-La/Nb图解显示,相对于JM辉长辉绿岩MG辉长岩经历了一定程度的地壳混染。综合研究认为,MG辉长岩和JM辉长辉绿岩为古特提斯洋壳俯冲过程中软流圈上涌的两次岩浆活动的产物,并受俯冲洋壳和地幔楔影响。MG辉长岩和JM辉长辉绿岩记录了南羌塘的增生过程,它们为相关研究提供了岩浆岩证据。