Considerations about the plate tectonic model, volcanism and continental crust in the southern part of the Central Andes

Mesozoic and Cenozoic volcanism in the Central Andes occurs near the boundary of convergent oceanic and continental lithospheric plates. The rocks show a chemical and temporal zonation with respect to the present Chile-Peru trench. It is inferred that the geometry of the subduction zone has not changed in the last 190 m.y.; the continental crust during Jurassic time in northern Chile may have been between 15 and 27 km thick.     

东昆仑阿尼玛卿地区古特提斯火山作用和板块构造体系

东昆仑阿尼玛卿蛇绿岩带标志古特提斯洋关闭后的板块缝合带。在该缝合带及周围识别出4套与洋盆扩张和俯冲作用有关的火山岩,由南至北,分别为洋底玄武岩、岛弧火山岩、弧后盆地玄武岩和后碰撞火山岩。板块构造体系说明洋壳俯冲极性从南向北。已有年代学证据表明:阿尼玛卿洋盆的开启时代至少可以早到晚石炭世(308Ma),洋盆关闭可能在早三叠世;岛弧火山岩的时代为晚二叠世(260Ma);弧后盆地火山岩的时代为早中三叠世;后碰撞火山岩的时代为晚三叠世。三叠纪沿缝合带及其北部形成了一系列巨大的左旋走滑断裂系,包括东昆仑南缘左旋走滑断裂(200~220Ma)、阿尔金断裂的早期走滑剪切断裂系(220~230Ma)以及南祁连南缘巨型左旋走滑断裂(240~250Ma)。认为它们形成于阿尼玛卿古特提斯洋的关闭和斜向碰撞作用,但主要在俯冲板块折返阶段或逆冲岩片的抬升阶段,其时也是后碰撞岩浆活动和火山喷发阶段。     

Morphology of the subducted Indian plate in the Indo-Burmese convergence zone

Earthquake hypocenters and travel time residuals have been analysed to constrain the geometry and physical state of the subducted Indian plate in the Indo-Burmese convergence zone. A critical analysis of earthquake hypocenters reveals the existence of a non-uniform Benioff zone, progressively shortening from north to south. The deepest level of seismicity is observed beneath the Naga hills (160 km) followed by that under the Chin hills (120 km) and Arakan-Yoma ranges (80 km). The region seems to be devoid of moderate sized shallow (< 40 km) earthquakes. Differential travel time residuals from pairs of shallow and intermediate depth earthquakes recorded at teleseismic distances show significantly faster travel time (up to l.2s) in the north-northeast and south-southwest azimuths, whilst slower arrivals (1.2 to 1.5 s) are recorded in the transverse direction. This observation points to the presence of a high velocity slab possibly linked to the subduction of the Indian oceanic lithosphere.     

Cenozoic volcanism and tectonic evolution of the Tibetan plateau

Cenozoic volcanism on the Tibetan plateau, which shows systematic variations in space and time, is the volcanic response to the India–Asia continental collision. The volcanism gradually changed from Na-rich + K-rich to potassic–ultrapotassic + adakitic compositions along with the India–Asia collision shifting from contact-collision (i.e. “soft collision” or “syn-collision”) to all-sided collision (i.e. “hard collision”). The sodium-rich and potasium-rich lavas with ages of 65–40 Ma distribute mainly in the Lhasa terrane of southern Tibet and subordinately in the Qiangtang terrane of central Tibet. The widespread potassic–ultrapotassic lavas and subordinate adakites were generated from ~ 45 to 26 Ma in the Qiangtang terrane of central Tibet. Subsequent post-collisional volcanism migrated southwards, producing ultrapotassic and adakitic lavas coevally between ~ 26 and 8 Ma in the Lhasa terrane. Then potassic and minor adakitic volcanism was renewed to the north and has become extensive and semicontinuous since ~ 20 Ma in the western Qiangtang and Songpan–Ganze terranes. Such spatial–temporal variations provide important constraints on the geodynamic processes that evolved at depth to form the Tibetan plateau. These processes involve roll-back and break-off of the subducted Neo-Tethyan slab followed by removal of the thickened Lhasa lithospheric root, and consequently northward underthrusting of the Indian lithosphere. The Tibetan plateau is suggested to have risen diachronously from south to north. Whereas the southern part of the plateau may have been created and maintained since the late-Oligocene, the northern plateau would have not attained its present-day elevation and size until the mid-Miocene when the lower part of the western Qiangtang and Songpan–Ganze lithospheres began to founder and detach owing to the persistently northward push of the underthrust Indian lithosphere.     

西藏班公湖-怒江缝合带中段侏罗纪高镁安山质岩石对中特提斯洋演化的制约

班公湖-怒江缝合带中段地区南北向分布了三条分支蛇绿岩亚带,它们记录了该地区中特提斯洋复杂的构造演化过程。目前对于该地区洋盆俯冲消减动力学过程一直缺乏有效制约。为探讨这一问题,本文对班公湖-怒江缝合带中段新近厘定的安山岩和闪长岩开展了系统的野外、岩石地球化学和锆石U-Pb年代学研究。安山岩主要呈不整合覆盖于晚三叠世沉积地层之上,或与侏罗纪俯冲增生杂岩和橄榄岩以断层接触,闪长岩主要呈岩脉体侵入于橄榄岩中。锆石U-Pb定年表明,安山岩和闪长岩均形成于中晚侏罗世(165～161Ma)。安山岩和闪长岩地球化学组成类似,它们大都具有高的MgO含量和Mg#值,这与高镁安山岩相类似。稀土和微量元素组成显示出典型的岛弧岩浆岩特征,富集轻稀土(LREE)和Rb、Th、U、Pb等元素,亏损Ba和高场强元素(HFSE; Nb,Ta和Ti)。同时,样品还显示出较低的Ba/Th和较高的(La/Sm)N比值,以及负的锆石εHf(t)值和古老的锆石Hf模式年龄。这些特征表明这些高镁安山岩和闪长岩是大洋板片俯冲沉积物部分熔融的熔体交代地幔楔的产物。结合区域地质和前人研究,认为这些岩石可能形成于靠近海沟的大陆边缘环境,是班公湖-怒江中特提斯洋中段北拉-拉弄分支洋盆初始俯冲消减的产物,该初始俯冲作用可能与安多微陆块和南羌塘地块碰撞导致的俯冲南向跃迁有关。     

Late Jurassic-Neocomian volcanism of the northern Okhotsk region: Geology,tectonic settings,and mineralization

The paper deals with new data on the geology, geochronometry, and petrogeochemistry of the Upper Jurassic-Neocomian basaltic andesites constituting the base of the Cretaceous volcanogenic section in the northern Okhotsk region. The geochronometric data confirm the Late Jurassic-Neocomian age of these rocks (147–138 Ma, K-Ar method). Their characteristic features provide grounds for considering these rocks as fragments of the Uda volcanic belt. The comparison of the petrogeochemical characteristics with similar parameters of other different-age tectonic structures of the region under consideration that were formed in different geodynamic environments (intracontiental and continental marginal volcanogenic belts, island arcs) revealed similarity between the conditions of the formation of the Late Jurassic-Neocomian (Uda belt) and Albian-Cenomanian (Okhotsk-Chukotka belt) volcanics: both characterize continental marginal environments. It is assumed that the Late Jurassic-Neocomian volcanism associates with Au-Ag mineralization, which is also expected in other similar although poorly known volcanic structures.     

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。

     

甘肃西秦岭新生代钾霞橄黄长岩火山作用及其构造含义

钾霞橄黄长岩是一种产于大洋和/或大陆环境,尤以大陆裂谷最为多见的超钾质火山岩,也是世界上出露最少的火成岩之一,目前世界只有少数几个国家和地区有这类岩石的报道.甘肃西秦岭新生代火山岩是我国目前唯一有报道的钾霞橄黄长岩产区.本文详细介绍了西秦岭新生代钾霞橄黄长岩及深源包体的岩石学特征,提供了该区13个岩体30个具代表性的全岩化学和主要矿物的化学成分分析结果,6个具代表性的Sr,Nd,Pb同位素分析结果,以及2个火山岩全岩Ar/Ar同位素定年结果.研究表明西秦岭新生代钾霞橄黄长岩主要是中新世火山作用的产物.火山岩的化学成分贫SiO2和Al2O3,富MgO,CaO,TiO2和(K2O+Na2O),以及具有高的[MgO]值(平均大于68)和NNiO 含量(平均在0.3%以上).火山岩中含有丰富的深源包体,岩石的结晶程度很差.所有这些表明西秦岭存在原生钾霞橄黄长岩浆.而且本区钾霞橄黄长岩与碳酸岩密切共生,显示了典型的大陆区钾霞橄黄长岩的特征.火山岩的∑REE在365×10-6～649×10-6之间,其中轻稀土总量(∑LREE)达319×10-6～569×10,尤以强烈富集大离子轻石元素(K、Rb、Sr、Ba)和相对亏损高场强元素(Nb、Ta等)为特征.火山岩具有相对高的143Nd/144Nd值(0.512768～0.512911),相对低的87Sr/86Sr值(0.70412～0.70525)和206Pb/204Pb(18.418～18.625).εNd介于+1. 8209～+2.1002之间,207Pb/204Pb和208Pb/204Pb比值分别为15.476～15.551和38.618～38.792.本区火山岩的Sr,Nd和Pb同位素与东非裂谷,意大利,德国等地的钾霞橄黄长岩不同,也与青藏高原周边其它地区的钾质火山岩不同.结合火山岩中深源包体温压计算结果,以及火山岩的形成时代,大地构造背景及地球物理资料的分析,对火山岩产出的构造环境及起源和成因进行了初步讨论.提出本区钾霞橄黄长岩火山作用是印度-欧亚板块碰撞后青藏高原强烈隆升在周边地区的反映,本区原生钾霞橄黄长岩岩浆直接起源于地幔,其成因与岩石圈伸展的条件下,岩石圈底部热边界层(TBL)的低度部分熔融有关.这种热边界层(TBL)在发生岩浆作用之前是被来自软流圈深部的流体和/或熔体交代富集过的地幔.     

Neogene volcanism and Holocene earthquakes in the Tanlu fault zone, eastern China

In this paper, we study the relationship between Neogene volcanism and Holocene earthquakes in the Tanlu fault, eastern China. We find that fault segments through which Neogene and Quaternary magma have extruded do not show Holocene slip because they are covered by unfaulted basalts. In contrast, fault branches that are away from the Quaternary volcanic centers display Quaternary faulting and are responsible for earthquakes as recorded both historically and geomorphologically. Therefore, magma intrusion appears to modify fault activity in two different ways: (1) within the faults, the cooling magma serves as a cohesion or barrier, welding the faults so that they become stronger in resisting slip; (2) beneath the faults, the upwelling magma promotes slip of faults above the magma body, and hence generation of earthquakes. Physically, the first case results from contraction of the cooling magma, which causes a relative increase in fault-normal stress so that the fault failure resistance is enhanced. The second case results from the upward dynamic force and the heat brought in by the magma body, both of which cause the effective fault-normal stress to decrease so that the fault failure resistance is reduced. That could explain why earthquakes occur on faults bypassing the volcanic centers as typified in the Yishu fault zone and in the regions where heat flows are relatively high as shown in the Bohai Bay area.     

Metamorphism and tectonic evolution of the Shangdan fault zone, Shaanxi, China

Abstract The Shangdan fault in the Qinling Orogenic Belt of China is an important boundary between the Caledonian North Qinling Fold Belt and the Hercynian South Qinling Fold Belt. In the Danfeng area, the fault zone strikes WNW–ESE and comprises four strongly deformed zones and three weakly deformed domains parallel to each other. The fault zone has a complex history of multiple deformation and each domain has a different tectonic style that was formed at different stages of the deformation.
The rocks exposed in the weakly deformed domains belong to the Qinling, Danfeng and Liuling Groups. In this paper, the mineral chemistry and mineral assemblages are used to infer the metamorphic conditions and the P–T paths of these units. The metamorphic units in and near the fault zone have different metamorphic conditions and histories that are correlated with the tectonic evolution of the fault zone. Caledonian–Hercynian uplift and southward thrusting of the Proterozoic Qinling Group, over the Danfeng and the Liuling Groups, produced the main metamorphic and tectonic features of the fault zone. Folding of both the Liuling Group and the thrust faults during the Hercynian–Indosinian was accompanied by northward thrusting.     

祁连山新元古代中—晚期至早古生代火山作用与构造演化

祁连山地区的新元古代中—晚期至早古生代火山作用显示系统地时、空变化,其乃是祁连山构造演化的火山响应。随着祁连山构造演化从Rodinia超大陆裂谷化—裂解,经早古生代大洋打开、扩张、洋壳俯冲和弧后伸展,直至洋盆闭合、弧-陆碰撞和陆-陆碰撞,火山作用也逐渐从裂谷和大陆溢流玄武质喷发,经大洋中脊型、岛弧和弧后盆地火山活动,转变为碰撞后裂谷式喷发。850~604 Ma的大陆裂谷和大陆溢流熔岩主要分布于祁连和柴达木陆块。从大约550 Ma至446 Ma,在北祁连和南祁连洋-沟-弧-盆系中广泛发育大洋中脊型、岛弧和弧后盆地型熔岩。与此同时,在祁连陆块中部,发育约522~442 Ma的陆内裂谷火山作用。早古生代洋盆于奥陶纪末(约446 Ma)闭合。随后,从约445 Ma至约428 Ma,于祁连陆块北缘发育碰撞后火山活动。此种时-空变异对形成祁连山的深部地球动力学过程提供了重要约束。该过程包括:(1)地幔柱或超级地幔柱上涌,导致Rodinia超大陆发生裂谷化、裂解、早古生代大洋打开、扩张、俯冲,并伴随岛弧形成;(2)俯冲的大洋板片回转,致使弧后伸展,进而形成弧后盆地;(3)洋盆闭合、板片断离,继而发生软流圈上涌,诱发碰撞后火山活动。晚志留世至早泥盆世(420~400 Ma),先期俯冲的地壳物质折返,发生强烈的造山活动。400 Ma后,山体垮塌、岩石圈伸展,相应发生碰撞后花岗质侵入活动。     

Depleted arc volcanism in the Alboran Sea and shoshonitic volcanism in Morocco: geochemical and isotopic constraints on Neogene tectonic processes

Samples of volcanic rocks from Alborán Island, the Alboran Sea floor and from the Gourougou volcanic centre in northern Morocco have been analyzed for major and trace elements and Sr–Nd isotopes to test current theories on the tectonic geodynamic evolution of the Alboran Sea. The Alborán Island samples are low-K tholeiitic basaltic andesites whose depleted contents of HFS elements (0.5×N-MORB), especially Nb (0.2×N-MORB), show marked geochemical parallels with volcanics from immature intra-oceanic arcs and back-arc basins. Several of the submarine samples have similar compositions, one showing low-Ca boninite affinity. ¹⁴³Nd/¹⁴⁴Nd ratios fall in the same range as many island-arc and back-arc basin samples, whereas ⁸⁷Sr/⁸⁶Sr ratios (on leached samples) are somewhat more radiogenic. Our data point to active subduction taking place beneath the Alboran region in Miocene times, and imply the presence of an associated back-arc spreading centre. Our sea floor suite includes a few more evolved dacite and rhyolite samples with (⁸⁷Sr/⁸⁶Sr)₀ up to 0.717 that probably represent varying degrees of crustal melting. The shoshonite and high-K basaltic andesite lavas from Gourougou have comparable normalized incompatible-element enrichment diagrams and Ce/Y ratios to shoshonitic volcanics from oceanic island arcs, though they have less pronounced Nb deficits. They are much less LIL- and LREE-enriched than continental arc analogues and post-collisional shoshonites from Tibet. The magmas probably originated by melting in subcontinental lithospheric mantle that had experienced negligible subduction input. Sr–Nd isotope compositions point to significant crustal contamination which appears to account for the small Nb anomalies.

The unmistakable supra-subduction zone (SSZ) signature shown by our Alboran basalts and basaltic andesite samples refutes geodynamic models that attribute all Neogene volcanism in the Alboran domain to decompression melting of upwelling asthenosphere arising from convective thinning of over-thickened lithosphere. Our data support recent models in which subsidence is caused by westward rollback of an eastward-dipping subduction zone beneath the westernmost Mediterranean. Moreover, severance of the lithosphere at the edges of the rolling-back slab provides opportunities for locally melting lithospheric mantle, providing a possible explanation for the shoshonitic volcanism seen in northern Morocco and more sporadically in SE Spain.     

中国东北东部中生代火山活动与泛太平洋板块

长白山脉中生代火山岩带与东侧之洋陆边界平行展布。火山岩浆作用可划分为三大构造阶段(2 30~ 190Ma ,178~ 12 5Ma ,118~ 6 5Ma) ,四大旋回 ,11个旋回和 2 4个火山事件。由安粗岩系和钙碱性岩系组成的一个新的火山岩浆作用构造类型 ,称之为走滑大陆边缘—拉分张裂性火山岩系。洋陆板块互动方式的更迭及速率的变化是火山岩浆作用地球动力学深部发展过程的主控因素。     

内蒙古察右后旗三道沟一带韧性剪切带磁组构特征及其构造意义

变形岩石的磁组构参数K1、K2、K3、P、T、F、L、E等可以用来定量地分析和了解构造变形的性状。笔者对分布于察哈尔右翼后旗三道沟一带的构造岩进行磁组构特征研究发现:E值多大于1,或接近于1;T值大于0的明显多于小于0的;P值均大于1,而以F和L的对数值为轴所作的变型弗林图解中数据主要集中在剪切区和压扁区。上述结果表明磁化率椭球体主要表现为磁面理较磁线理发育的压扁型椭球体,三道沟一带韧性剪切带的构造岩石经历了力学机制为挤压剪切的强烈构造活动。这种由磁组构揭示的应力应变特征得到了野外露头及显微构造的佐证。     

西藏拉轨岗日构造带孔木韧性剪切带特征

在拉轨岗日构造带佩枯错新发现了一条韧性剪切带,至少存在3期变形。韧性剪切作用可能与应变能转化的热能有关,应变能常沿断裂集中,形成方解石脉;然后,经由北往南的逆冲推覆,形成(叠瓦状)逆冲推覆构造;最后,由于山体重力势能的作用,伸展跨塌。该剪切带成为藏南拆离系的组成部分。     

西藏拉轨岗日构造带孔木韧性剪切带特征

在拉轨岗日构造带佩枯错新发现了一条韧性剪切带,至少存在3期变形。韧性剪切作用可能与应变能转化的热能有关,应变能常沿断裂集中,形成方解石脉;然后,经由北往南的逆冲推覆,形成(叠瓦状)逆冲推覆构造;最后,由于山体重力势能的作用,伸展跨塌。该剪切带成为藏南拆离系的组成部分。     

西藏拉轨岗日构造带孔木韧性剪切带特征

在拉轨岗日构造带佩枯错新发现了一条韧性剪切带，至少存在3期变形。韧性剪切作用可能与应变能转化的热能有关，应变能常沿断裂集中，形成方解石脉；然后，经由北往南的逆冲推覆，形成(叠瓦状)逆冲推覆构造；最后，由于山体重力势能的作用，伸展跨塌。该剪切带成为藏南拆离系的组成部分。