满洲里—额尔古纳地区岩浆作用及其大地构造意义

对满洲里－额尔古纳地区不同时代岩浆岩的地质，地球化学特征进行了初步研究，研究表明晚元古代花岗岩具S型花岗岩或地壳改造型花岗岩特征，形成于同碰撞造山环境，加里东早期花岗岩具I型，科迪勒拉I型或ACG特征,代表活动大陆边缘构造属性，加里东晚期花岗岩具S型或地壳改造型特征，代表陆－陆碰撞造山环境，加里东期花岗岩记录了多宝山－牙克石－伊尔施陆间洋壳消减和闭合的过程，海西晚期花岗岩属富碱低钙钙碱性岩石，具S型花岗岩或CPG，KCG花岗岩特征，形成于碰撞后构造环境，中生代早期岩 岩形成于蒙古－－鄂霍茨克残余洋“剪刀式”闭合所造成的张性似裂谷环境，中生代晚期岩浆岩形成于挤压环境。     

东昆仑祁漫塔格晚志留世—早泥盆世花岗岩： 年代学、地球化学及形成环境

东昆仑祁漫塔格构造—岩浆岩带发育大量与加里东期造山有关的花岗岩,对其进行研究不仅能为祁漫塔格早古生代构造演化提供新信息,而且对探讨整个东昆仑造山带的动力学演化过程具有重要的意义。本文报道祁漫塔格东部那棱郭勒河东黑云母二长花岗岩及祁漫塔格乌兰乌珠尔钾长花岗岩的锆石U Pb年龄和地球化学数据,并对研究区晚志留世—早泥盆世花岗岩成因及其构造意义进行探讨。本区的黑云母二长花岗岩、钾长花岗岩的LA ICP MS锆石U Pb年龄分别为4206 ±26Ma、4212 ±19Ma。祁漫塔格晚志留世—早泥盆世花岗岩类整体属高钾钙碱性准铝—过铝质花岗岩,稀土配分曲线为轻稀土元素相对富集、重稀土元素相对平坦、中等负铕异常, 微量元素表现为大离子亲石元素Rb、Th相对富集,高场强元素Nb、Sr、P、Ti亏损的特征;该时期为后碰撞构造环境,岩石成因类型以高钾钙碱性 I 型花岗岩为主,但同时出露同时期S型花岗岩,推测在同碰撞挤压造山作用向后碰撞区域拉伸构造体制（约420Ma）的转换时期,产生了具有A型花岗岩特征的花岗岩、高钾钙碱性 I 型花岗岩及过铝质花岗岩。     

东昆仑祁漫塔格晚志留世——早泥盆世花岗岩:年代学、地球化学及形成环境

东昆仑祁漫塔格构造—岩浆岩带发育大量与加里东期造山有关的花岗岩,对其进行研究不仅能为祁漫塔格早古生代构造演化提供新信息,而且对探讨整个东昆仑造山带的动力学演化过程具有重要的意义。本文报道祁漫塔格东部那棱郭勒河东黑云母二长花岗岩及祁漫塔格乌兰乌珠尔钾长花岗岩的锆石U-Pb年龄和地球化学数据,并对研究区晚志留世—早泥盆世花岗岩成因及其构造意义进行探讨。本区的黑云母二长花岗岩、钾长花岗岩的LA-ICP-MS锆石U-Pb年龄分别为420.6±2.6Ma、421.2±1.9Ma。祁漫塔格晚志留世—早泥盆世花岗岩类整体属高钾钙碱性准铝—过铝质花岗岩,稀土配分曲线为轻稀土元素相对富集、重稀土元素相对平坦、中等负铕异常,微量元素表现为大离子亲石元素Rb、Th相对富集,高场强元素Nb、Sr、P、Ti亏损的特征;该时期为后碰撞构造环境,岩石成因类型以高钾钙碱性I型花岗岩为主,但同时出露同时期S型花岗岩,推测在同碰撞挤压造山作用向后碰撞区域拉伸构造体制(约420Ma)的转换时期,产生了具有A型花岗岩特征的花岗岩、高钾钙碱性I型花岗岩及过铝质花岗岩。     

中昆仑北部古生代构造岩浆作用及其演化

对地质研究较薄弱的中昆仑北部开展了古生代构造地层格局、构造变形及构造样式、古生代花岗岩区域分布及其地球化学特征、祁漫塔格群火山岩地球化学及其区域对比等研究,认为祁漫塔格群属寒武－早志留世,早古生代祁漫塔格地区可能不存在成熟大洋,而是以大陆裂谷或初始洋盆为特征;早古生代晚期祁漫塔格山北部被动大陆边缘转化为活动大陆边缘,沿鸭子泉－祁漫塔格主脊断裂汇聚闭合;晚古生代,早期以求勉雷克花岗穹隆为核心南北简单剪切滑覆;晚期沿昆中俯冲碰撞,昆中断裂以北地区转化为活动大陆边缘,古特提斯洋闭合。     

新疆北部卡拉麦里富碱花岗岩带的碱性花岗岩与锡矿

卡拉麦里碱性花岗岩形成华力西中晚期、属造山晚期至造山后拉张构造环境下形的Ａ型花岗岩。其岩石学、岩石化学脏球化学特性与国同人外Ａ型花岗岩基本相似。在Ｅｂｙ（１９９２）的Ｅｂ－Ｙ－Ｃｅ民Ｎｂ－Ｙ－３Ｇａ三角图解中，本带碱性花岗岩投影点均落在Ａ２区，表明它是大陆－大陆碰撞作用形成的Ａ２型碱性花岗岩。     

东昆仑祁漫塔格阿确墩地区侵入岩U-Pb年代学、地球化学及其地质意义

祁漫塔格地区岩浆岩的成岩时代和形成环境的确定能对东昆仑造山带加里东期构造演化时限加以约束.对祁漫塔格西北部阿确墩地区石英闪长岩和二长花岗岩进行了年代学和岩石地球化学研究,结果显示,石英闪长岩属准铝质-弱过铝质钙碱性系列岩石;轻重稀土分馏明显,具中等-轻微铕负异常（δEu=0.79~0.90）;相对富集Rb、K、Hf、Zr、Tb、Nd等元素,不同程度地亏损Ba、P、Ti、Nd、Ta、Y;具有I型花岗岩类特征.二长花岗岩属弱过铝质钙碱性系列岩石;轻重稀土分异程度极大,具明显铕负异常（δEu=0.42~0.45）;富集大离子亲石元素（如Rb、K、La、Ce、Nd、Tb等）,亏损高场强元素（P、Ti、Nd、Ta）和Ba、Sr、U等元素;为高分异I型花岗岩.Nd/Th、Nb/Ta、Mg#值等指标显示石英闪长岩为壳源特征且受到幔源岩浆的影响,推测是幔源岩浆底侵地壳物质发生部分熔融形成的;二长花岗岩则是壳源的,可能与幔源岩浆底侵诱发的上地壳物质部分熔融有关,且经历了强烈的结晶分离作用.石英闪长岩和二长花岗岩的LA-ICP-MS锆石U-Pb年龄分别为448.8±3.9 Ma和405.2±3.6 Ma,代表其形成时代.石英闪长岩总体显示出与俯冲消减作用有关的岛弧岩浆岩地球化学特征;二长花岗岩在构造环境图解中显示为碰撞背景,但微量元素与同碰撞花岗岩典型特征不符,综合分析认为形成于后碰撞构造背景下.结合区域构造演化,推测东昆仑祁漫塔格地区在晚奥陶世持续处于俯冲消减环境中,早泥盆世之前进入后碰撞造山阶段.      

东昆仑祁漫塔格地区华力西期花岗岩地质地球化学特征

通过对祁漫塔格地区华力西期花岗岩类型、空间分布、地质地球化学特征的研究表明，该区花岗岩分布最广的是华力西中、晚期花岗岩。华力西中期花岗岩岩石组合为钾长花岗岩、花岗岩、二长花岗岩，为高钾钙碱性系列，稀土元素含量较低，属轻稀土富集型，铕亏损较小，LILE中的Cs、Rb、Ba、K等含量相对富集，而LILE中的Sr、HFSE中Nb、Ti和P相对亏损，形成于碰撞后或造山晚期；华力西晚期花岗岩岩石组合为钾长花岗岩、花岗闪长岩、中粗粒黑云母二长花岗岩、花岗斑岩，为高钾钙碱性系列，稀土元素含量高于华力西中期花岗岩的稀土元素含量，铕亏损较为明显，微量元素以Sr、Ti和P更为亏损为特征，形成于活动大陆边缘到碰撞后花岗岩的过渡环境。     

天山造山带古生代侵入岩地质特征及构造意义

通过对天山造山带古生代侵入岩岩石类型、岩石地球化学特征及年代学研究,初步厘定不同构造带侵入岩浆序列及形成构造环境。天山地区古生代侵入岩主要包括奥陶—志留纪俯冲期钙碱性花岗岩、泥盆纪后碰撞型正长花岗岩、石炭纪中早期俯冲型钙碱性花岗岩、石炭纪晚期后碰撞型正长花岗岩、石炭纪末后碰撞型镁铁-超镁铁质岩、二叠纪早期后造山型碱性花岗岩及洋壳残片等。俯冲期侵入岩主要为花岗闪长岩-石英二长闪长岩-石英闪长岩组合;同碰撞期为花岗闪长岩-二长花岗岩组合;后碰撞期为组成花岗岩-二长花岗岩组合;后造山期为正长花岗岩-碱性花岗岩组合。认为该地区古生代侵入岩与Terskey洋、北天山洋、南天山洋等洋盆演化密切相关,并建立了天山地区古生代构造演化模式图。     

祁漫塔格造山带——青藏高原北部地壳演化窥探

祁漫塔格是东昆仑造山带的一个分支,位于青藏高原中北部,夹持于柴达木盆地和库木库里盆地中间,向西被阿尔金走滑断裂错段。从元古代到早中生代,由于受到多期、多阶段大洋俯冲和关闭影响,导致不同地体间发生碰撞拼贴和大陆增生过程,并由此引发一系列的岩浆事件。祁漫塔格造山带内发育新元古代花岗岩(1000~820 Ma)是对Rodinia超大陆形成的响应。以阿达滩和白干湖逆冲断裂为界,划分为南、北祁漫塔格两地体。北祁漫塔格地体作为活动大陆边缘,发育大量的早古生代与俯冲有关的花岗岩和VA型蛇绿岩;南祁漫塔格地体最初为洋内俯冲形成的原始大洋岛弧,发育早古生代SSZ型蛇绿岩、岛弧拉斑玄武岩和钙碱性火山岩。随着持续俯冲,年轻岛弧伴伴随地壳加厚转变为成熟岛弧。南、北祁漫塔格地体间的碰撞(弧-陆碰撞)可能发生在晚志留世(422Ma),并持续到早泥盆世(398Ma)。在此期间(422~389Ma),南祁漫塔格地体内发育一系列同碰撞型花岗岩;北祁漫塔格地体内发育一系列的大洋岛弧花岗岩。南祁漫塔格作为外来地体,碰撞拼贴对于大陆边缘、大陆增生意义重大。之后,南、北祁漫塔格地体进入后碰撞环境并发育一系列板内花岗岩。此外,伸展导致造山带垮塌,发育中泥盆统磨拉石建造。碰撞使得海沟后退,海沟阻塞导致俯冲减弱甚至停止,因而产生了石炭-二叠纪(357~251 Ma)岩浆活动缺口。古特提斯祁漫塔格洋的最终关闭可能始于晚二叠世,使得库木库里微板块拼贴于大陆边缘;碰撞抬升导致缺失上二叠统-中三叠统地层。早中三叠世(251~237 Ma)由于碰撞,俯冲大洋板片回转,之后断离,软流圈地幔物质沿岩石圈地幔通道上涌,使得新生下地壳部分熔融;到了晚三叠世,大规模岩石圈地幔和下地壳物质拆沉,导致古老地壳物质发生熔融,形成了一系列后碰撞背景下的钙碱性和碱性花岗岩。     

中国东部中一新生代活动大陆边缘构造—岩浆作用演化和发展

本文论述中国东部中－新生代由科迪勒拉型转变为西太平洋活动大陆边缘过程中的构造－岩浆作用及其演化。在印支旋回（２５０～１８５Ｍａ），初始欧亚板块与古太平洋板块强烈挤压俯冲，并伴随大范围改造型花岗岩类的发育。在燕山早期（１８５～１４０Ｍａ），继续俯冲，改造型花岗岩进一步发育，并开始有同熔型花岗岩类的形成。在约１４０Ｍａ两类花岗岩的形成达到高潮。在１４０～１００Ｍａ广泛发育安山－流纹岩系。燕山晚期（１００～７０Ｍａ）由于弧后扩张，配合红色盆地的广泛形成，发育碱性火山岩和碱性花岗岩带．新生代中国东部大陆花岗岩和中－酸性火山岩活动消失，代之以玄武岩活动；边缘海和岛弧逐渐形成，钙碱性火山岩系转入岛弧地带。     

Tracing a late Mesozoic magmatic arc along the Southeast Asian margin from the granitoids drilled from the northern South China Sea

Abstract

The granitoid suites encountered by drilling in the northern South China Sea (SCS) remain important for understanding the evolution of the late Mesozoic Southeast Asian continental margin. They comprise a range of rock types including diorite, tonalite, granodiorite, monzogranite and syenogranite with SiO₂ spanning 56.4–76.8%. Newly acquired secondary ion mass spectrometry (SIMS) U–Pb ages of samples from 14 boreholes indicate two key magmatic episodes: Late Jurassic (161.6–148.2 Ma) and Early Cretaceous (136.5–101.7 Ma). Jurassic magmatism probably began in late Middle Jurassic time, documented by the dates of inherited zircons. The granitoids are dominated by metaluminous to weakly peraluminous I-type granites, are transitional between magnesian and ferroan, and encompass calc-alkaline, high-K calc-alkaline, and shoshonitic series. The geochemical signatures suggest that these granitoids were mostly generated in a normal continental arc environment. Notable features of the I-type samples are well-defined negative Nb–Ta–Ti anomalies typical of arc-related magmas. Taken together, the late Mesozoic arc granites of the SCS, the accretionary wedge of the Palawan terrane to the southeast, and the zone of lithospheric extension north of the SCS throughout Southeast China, define a southeast-to-northwest trench-arc-backarc architecture for the late Mesozoic Southeast Asian continental margin whose geodynamic setting is related to subduction of the Palaeo-Pacific slab beneath the Asian continent. Two key subduction episodes are recognized, one in Late Jurassic and the other in Early Cretaceous time.     

Mesozoic Granitoid Types and Metallogeny of the East China Circum-Pacific Continental Margin

Abstract: This paper synthesizes the geotectonic background, genetic types and metallogenetic relations of the Mesozoic granitoids in the East China continental margin. By the Mesozoic, the Siberia Plate, North China Plate and South China Plate amalgamated together, resulting in formation of a unified Eurasia super–continent. Since the late Triassic to early Jurassic period, the territory of East China gradually became a Cordilleran style active continental margin. During the Jurassic to early Cretaceous (early to middle episodes of Yanshanian orogeny), the Paleo-Pacific plate strongly collided with and subducted under the Eurasia continent, reactivated the consolidated East China continental margin. The granitoids of both transformation series and syntexis series were generated. Many granitoid-related large and giant metal deposits were formed. Furthermore, the W, Sn, Be, Nb, Ta and U mineralizations are mostly associated with the transformation series; while the Fe, Cu, Mo and Au mineralizations are mostly associated with the syntexis series. The late Yanshanian orogeny (late Cretaceous) began a transition to the western Pacific style continental margin. A tensional environment resulted in development of alkaline granitoids and formation of continental red basins. The Cenozoic orogeny was characterized by a backarc spreading and rifting regime in this region.     

Granite-types in the Hohe Tauern (Eastern Alps,Austria) — Some aspects on their correlation to Variscan plate tectonic processes

Abstract

The Zentralgneise in the Hohe Tauern (Penninic Zone, Eastern Alps, Austria) can be interpreted essentially as metamorphosed Late Paleozoic orogenic plutonites with original compositions mainly of granites, granodiorites and tonalites, rarely also of diorites, quartzdiorites, quartzmonzodiorites, quartzmonzonites and quartzsyenites. Most of the granitoids show attributes of “Cordilleran I-type granites” respectively “volcanic arc granites”.

Due to their similarity to subduction derived granitoids from present day active continental margins in the Circum-Pacific area, it is assumed, that the I-type granitoids of the Hohe Tauern reflect the influence of a Variscan destructive plate boundary.

Among the Zentralgneise also minor amounts of granites with features towards the S-type occur, which are more likely related to collision than to subduction.

Seen as a whole, the Late Paleozoic plutonism played probably part in a compressional continental margin and originated during a Variscan subduction-collision scenario, that occurred along the southern flank of Central European Hercynian fold belt.     

广西苍梧社洞钨钼矿床花岗岩类岩石的地球化学特征及其与成矿关系

广西苍梧社洞钨钼矿是与花岗岩类有关的矿床。矿区主要花岗岩类包括加里东期花岗闪长岩、花岗闪长斑岩和燕山晚期花岗斑岩。加里东期花岗闪长岩、花岗闪长斑岩具有低Si、K,富Na、Al和基性组分特征,属于强过铝质的正常钙碱性系列岩石;稀土总量低,轻重稀土分馏明显,弱负Eu异常(δEu=0.62~0.70);Ti、Nb、Ta亏损,Th、U、Pb、Zr、Hf富集,Rb/Sr平均值为0.78,明显富集W、Cu、Mo,属于I型花岗岩,为陆内造山带碰撞早期挤压背景下岩浆活动的产物,表现为对钨、钼、铜的成矿专属性。燕山晚期花岗斑岩具有高Si、K,贫Na、Ca和基性组分,属于强过铝质的高钾-中钾钙碱性系列岩石;稀土总量高,轻重稀土分馏不明显,强负Eu异常(δEu=0.03~0.06);Ti、Ba、K、Eu亏损,Th、U、Sm、Dy、Y、Ho、Yb、Lu富集程度更高,Rb/Sr平均值为7.56,明显富集Sn、Bi,属于燕山晚期岩浆演化程度较高的S型花岗岩,为碰撞后伸展环境的板内花岗岩,表现为对锡、金的成矿专属性。     

南祁连党河南山花岗岩类特征及其构造环境

党河南山地区地处南祁连重要成矿带,区内花岗岩类岩体包括扎子沟岩体、鸡叫沟岩体及贾公台岩体,伴有不同程度的金、铜矿化。扎子沟岩体主要由花岗闪长岩组成,侵位于震旦纪火山岩系。鸡叫沟岩体主要由石英二长闪长岩组成,贾公台岩体主要由斜长花岗岩组成,两者侵位于奥陶纪碎屑岩系。3个岩体Rb-Sr等时线年龄分别为(510.85±14)Ma、(395.06±51)Ma及(355±91)Ma,前者对应于早-中寒武世,后二者对应于志留纪-泥盆纪。3个岩体均属于钙碱性花岗岩系列,其中扎子沟岩体属于该系列的中钾花岗闪长岩系列,鸡叫沟岩体属于高钾二长岩系列,贾公台岩体属于低钾奥长花岗岩系列。综合岩相学、岩石化学及微量元素和稀土元素的地球化学特征,区内3个岩体均形成于I型活动陆缘环境,为中南祁连造山带加里东期构造岩浆活动的产物。     

Neoproterozoic Granitic Magmatism and Evolution of the Eastern Dom Feliciano Belt in Southernmost Brazil: A Tectonic Model

The eastern side of the Dom Feliciano Belt consists of supracrustal rocks and granitic batholiths whose emplacements were controlled by the tectonic evolution of the belt. The evolution of this belt had both a tangential tectonic regime and a transcurrent one. The tangential regime, defined by low angle planar and linear structures with W-NW tectonic transport, was responsible for crustal thickening and tectonic imbrication and controlled the syn-kinematic injections of high-K calc-alkaline granitoid plutons of the Arroio Solidão Intrusive Suite about 800 Ma. The transcurrent regime is defined by high angle deformation sets of planar and linear structures, parallel to the elongation of the belt, and indicative of a N-NE tectonic transport. Its associated magmatism began about 672 Ma with the intrusion of the calc-alkaline granitoids of the Arroio Moinho Intrusive Suite, followed by basic dykes and the 630–617 Ma crustal melt granites of the Cordilheira Intrusive Suite. The transcurrent tectonics generated shear zones hundreds of meters in width, which probably extend deep into the mantle. At the end of this process, an extensional regime was installed and during this transition, late- to post-kinematic calc-alkaline granitoids of the Campinas Intrusive Suite were intruded. This magmatism developed during the extensive regime and is represented by the 585 Ma post-transcurrent calc-alkaline granitoids of the Canguçu Intrusive Suite. Alkaline-metaluminous granitoids of the Encruzilhada Intrusive Suite and a few small bodies of peralkaline granitoids represent the final episodes of the granitic magmatism in this region. The syn-tangential granitoids are high-K calc-alkaline and have similarities to those of a continental collision regime. The syn- to post-transcurrent high-K calc-alkaline granitoids are similar to the late- to post-orogenic injections of other orogenic belts. Peraluminous leucogranites of the Cordilheira Intrusive Suite occur also in this transcurrent tectonic setting. The main characteristics of the calc-alkaline magmatism can be related to the participation of a thick continental crust during its generation similar to the granitic systems of a continental collision. Their negative ɛNd values may reflect a magma source closely connected to the continental crust, with their igneous protoliths of relatively homogeneous composition related to a continental collision phase. This continental collision occurred concurrent with the closing of a passive platform-type basin, after the end of sedimentation and tholeiitic volcanic activities.     

新疆西昆仑造山带北缘中元古代帕什托克闪长岩侵入序列及其地质意义

位于塔里木地块西南缘、西昆仑造山带北缘的帕什托克侵入序列由石英闪长岩-石英二长闪长岩组成,类似TTG组合,形成于中元古代晚期。本文从帕什托克侵入序列的地球化学分析出发,通过对该侵入序列两期侵入岩的主量元素和微量元素的研究,讨论了该侵入序列的成因、构造环境和与其相关的板块间地球动力学模式,认为该侵入序列为I型准铝质高钾钙碱性花岗闪长岩系列,属活动板块边缘碰撞前大陆弧花岗岩类,两期侵入岩为同源岩浆演化,母岩浆属壳幔混合源,且岩浆向酸性演化。根据岩浆演化的物理环境和构造环境,推测早古生代库地洋的完全闭合与库地洋壳向塔里木古陆块俯冲消减有关,是塔里木古陆块和柴达木古陆块在Rodinia超级大陆汇聚过程中的产物。     

南祁连党河南山花岗岩类地球化学及其金铜矿化

南祁连党河南山地区地处南祁连重要成矿带,金铜矿化广布,该矿化与中酸性花岗岩类关系密切.为揭示区域成矿地质背景,查明大地构造的时空演化,并深入研究该地区金铜矿化的成矿规律与控矿因素,对区内主要花岗岩体及其金铜矿化进行了系统的地质地球化学研究.研究表明,区内花岗岩类岩体包括扎子沟岩体、鸡叫沟岩体及贾公台岩体.综合岩相学、岩石化学及微量稀土元素地球化学特征,区内3个岩体均形成于I型活动陆缘环境,为中南祁连造山带加里东期构造岩浆活动的产物.扎子沟岩体与该区金铜矿化不构成物源关系,但其岩浆期后热水溶液可造成火山岩系金、铜组分活化迁移,并在有利构造场所富集成矿,鸡叫沟岩体和贾公台岩体分别是黑刺沟金矿和贾公台金矿的控矿母岩.     

Evolution of the Archean-Palaeoproterozoic Bundelkhand Massif, central India—evidence from granitoid geochemistry

The Bundelkhand massif of Archean-Palaeoproterozoic age is primarily a granite-gneiss complex. Three distinct granitoid suites have been identified within the massif hornblende granitoids, biotite granitoids and leucogranitoids, in order of decreasing age. These granitoids were emplaced in previously deformed basement consisting of gneisses, banded iron formations and other metasediments, mafic to felsic volcanics.
The granitoids exhibit a large compositional range from quartz diorite to syenogranite and show a calc-alkaline trend. They are metaluminous to peraluminous and have I-type characteristics. The SiO₂ content ranges from 49 to 77 wt%. Low K₂O/Na₂O characterizes the granitoids. The oldest hornblende granitoids have low Rb and Yb contents compared to the younger biotite granitoids and leucogranitoids. Rb/Sr values for most of the granitoids are low (< 1). K/Rb ratios range from 95 to 373 which is, in general, comparable with other calc-alkaline suites. Y/Nb ratios of the granitoids are > 1.2 which is a characteristic feature of magmas derived from sources chemically similar to island arc or continental margin basalts.
The features mentioned above coupled with concentrations of Rb, Y, Nb, Yb, Ta and Th indicate a volcanic-arc tectonic setting for the granitoids. It is proposed that the massif represents subduction-related magmatism of an ocean in the southern part of the massif (an Andean plate margin).