Origin and evolution of post-collisional magmatism: Coeval Neoproterozoic calc-alkaline and alkaline suites of the Sinai Peninsula

Two Late Neoproterozoic post-collisional igneous suites, calc-alkaline (CA) and alkaline–peralkaline (Alk), widely occur in the northernmost part of the Arabian–Nubian Shield. In Sinai (Egypt) and southern Israel they occupy up to 80% of the exposed basement. Recently published U–Pb zircon geochronology indicates a prolonged and partially overlapping CA and Alk magmatism at 635–590 Ma and 608–580 Ma, respectively. Nevertheless in each particular locality CA granitoids always preceded Alk plutons. CA and Alk igneous rocks have distinct chemical compositions, but felsic and mafic rocks in general and granitoids from the two suites in particular cannot be distinguished by their Nd, Sr and O isotope ratios. Both suites are characterized by positive ε_Nd(T) values, from + 1.5 to + 6.0 (150 samples, 28 of them are new analyses), but predominance of juvenile crust in the region prevents unambiguous petrogenetic interpretation of the isotope data. Comparison of geochemical traits of felsic and mafic rocks in each suite suggests a significant contribution of mantle-derived components to the silicic magmas. Model calculation shows that the alkaline granite magma could have been produced by partial (~ 20%) melting of rocks corresponding to K-rich basalts. Material balance further suggests that granodiorite and quartz monzonite magmas of the CA suite could form by mixing of the granite and gabbro end-members at proportions of 85/15. In the Alk suite, alkali feldspar and peralkaline granites have evolved mainly by fractional crystallization of feldspars and a small amount of mafic minerals from a parental syenogranite melt. Thus the protracted, 20 m.y. long, contemporaneous CA and Alk magmatism in the northern ANS requires concurrent tapping of two distinct mantle sources. Coeval emplacement of CA and Alk intrusive suites was described in a number of regions throughout the world.     

Late Paleozoic granitoids in western Transbaikalia: sequence of formation,sources of magmas,and geodynamics

The evolution of Late Paleozoic granitoid magmatism in Transbaikalia shows a general tendency for an increase in the alkalinity of successively forming intrusive complexes: from high-K calc-alkaline granites of the Barguzin complex (Angara–Vitim batholith) at the early stage through transitional from calc-alkaline to alkaline granites and quartz syenites (Zaza complex) at the intermediate stage to peralkaline granitoids (Early Kunalei complex) at the last stage. This evolution trend is complicated by the synchronous development of granitoid complexes with different sets and geochemical compositions of rocks. The compositional changes were accompanied by the decrease in the scales of granitoid magmatism occurrence with time. Crustal metaterrigenous protoliths, possibly of different compositions and ages, were the source of granitoids of the Angara–Vitim batholith. The isotopic composition of all following granitoid complexes points to their mixed mantle–crustal genesis. The mechanisms of granitoid formation are different. Some granitoids formed through the mixing of mantle and crustal magmas; others resulted from the fractional crystallization of hybrid melts; and the rest originated from the fractional crystallization of mantle products or the melting of metabasic sources with the varying but subordinate contribution of crustal protoliths. Synplutonic basic intrusions, combined dikes, and mafic inclusions, specific for the post-Barguzin granitoids, are direct geologic evidence for the synchronous occurrence of crustal and mantle magmatism. The geodynamic setting of the Late Paleozoic magmatism in the Baikal folded area is still debatable. Three possible models are proposed: (1) mantle plume impact, (2) active continental margin, and (3) postcollisional rifting. The latter model agrees with the absence of mafic rocks from the Angara–Vitim batholith structure and with the post-Barguzin age of peralkaline rocks of the Vitim province.     

Late Paleozoic granitoids of western Transbaikalia: magma sources and stages of formation

Geochemical and geochronological studies of the main types of granitoids of the Angara-Vitim batholith (AVB) and granites of the Zaza complex in western Transbaikalia were carried out. U-Pb (SHRIMP-II) and Rb-Sr dating yielded the age of autochthonous gneiss-granites of the Zelenaya Griva massif (325.3±2.8 Ma), quartz syenites of the Khangintui pluton (302.3±3.7 Ma) and intruding leucogranites of the Zaza complex (294.4±1 Ma), monzonites of the Khasurta massif (283.7±5.3 Ma), and quartz monzonites of the Romanovka massif (278.5±2.4 Ma). The U-Pb and Rb-Sr dates show that the Late Paleozoic magmatism in western Transbaikalia proceeded in two stages: (1) 340–320 Ma, when predominantly mesocratic granites of the Barguzin complex, including autochthonous ones, formed, and (2) 310–270 Ma, when most AVB granitoids formed. We suggest that at the early stage, crustal peraluminous granites formed in collision geodynamic setting. At the late (main) stage, magmatism occurred in postorogenic-extension setting and was accompanied by the formation of several geochemical types of granitoids: (1) typical intrusive mesocratic granites of the Barguzin complex, similar to those produced at the first stage; (2) melanocratic granitoids (monzonitoids, quartz syenites), which were earlier dated to the early stage of the AVB evolution; (3) leucocratic medium-alkali (peraluminous) granites of the Zaza intrusive complex; and (4) some alkali-granite and syenite intrusions accompanied by alkaline mafic rocks. The diversity of granitoids that formed at the late stage of magmatism was due to the heterogeneous composition of crust protoliths and different degrees of mantle-magma participation in their formation.     

Alkali-calcic and alkaline post-orogenic (PO) granite magmatism: petrologic constraints and geodynamic settings

The end of an orogenic Wilson cycle corresponds to amalgamation of terranes into a Pangaea and is marked by widespread magmatism dominated by granitoids. The post-collision event starts with magmatic processes still influenced by subducted crustal materials. The dominantly calc-alkaline suites show a shift from normal to high-K to very high-K associations. Source regions are composed of depleted and later enriched orogenic subcontinental lithospheric mantle, affected by dehydration melting and generating more and more K- and LILE-rich magmas. In the vicinity of intra-crustal magma chambers, anatexis by incongruent melting of hydrous minerals may generate peraluminous granitoids bearing mafic enclaves. The post-collision event ends with emplacement of bimodal post-orogenic (PO) suites along transcurrent fault zones. Two suites are defined, (i) the alkali-calcic monzonite–monzogranite–syenogranite–alkali feldspar granite association characterised by [biotite+plagioclase] fractionation and moderate [LILE+HFSE] enrichments and (ii) the alkaline monzonite–syenite–alkali feldspar granite association characterised by [amphibole+alkali feldspar] fractionation and displaying two evolutionary trends, one peralkaline with sodic mafic mineralogy and higher enrichments in HFSE than in LILE, and the other aluminous biotite-bearing marked by HFSE depletion relative to LILE due to accessory mineral precipitation. Alkali-calcic and alkaline suites differ essentially in the amounts of water present within intra-crustal magma chambers, promoting crystallisation of various mineral assemblages. The ultimate enriched and not depleted mantle source is identical for the two PO suites. The more primitive LILE and HFSE-rich source rapidly replaces the older orogenic mantle source during lithosphere delamination and becomes progressively the thermal boundary layer of the new lithosphere. Present rock compositions are a mixture of major mantle contribution and various crustal components carried by F-rich aqueous fluids circulating within convective cells created around magma chambers. In favourable areas, PO suites pre-date a new orogenic Wilson cycle.     

青海祁漫塔格中晚三叠世花岗岩:年代学、地球化学及成矿意义

地处柴达木盆地西南缘的青海祁漫塔格地区不仅是一个特征显著的构造-岩浆岩带,而且也是重要的多金属成矿带。本文对该区中晚三叠世花岗岩开展了详细的年代学、岩石地球化学及Sr-Nd-Pb同位素组成研究,并探讨了成矿意义。结果表明,本区中晚三叠世花岗岩均系准铝质到弱过铝质高钾钙碱性花岗岩类,晚三叠世花岗岩具有更高的K2O/Na2O比值,富集大离子亲石元素(LILE)和轻稀土元素(LREE),明显亏损高场强元素(HFSE),中等初始锶比值和偏负的εNd(t)值表明它们主要源于古老地壳物质的深熔或重熔,并可能有幔源物质的加入;发育闪长质暗色微粒包体的中三叠世花岗岩锆石U-Pb年龄为230～237Ma,大多具斑状或似斑状结构的晚三叠世高分异富钾花岗岩形成于204～228Ma,表明大约240Ma祁漫塔格主造山已由挤压转入伸展并伴有幔源岩浆活动,晚三叠世后演化到后碰撞阶段;中晚三叠世花岗岩与本区密集产出的矽卡岩型和斑岩型多金属矿床的时空与成因关系密切,具有重要找矿指示意义。     

Granitoid evolution in Sinai,Egypt, based on precise SHRIMP U–Pb zircon geochronology

Late-stage Pan-African granitoids, including monzogranite, syenogranite and alkali granite, were collected from four separate localities in Sinai. They were selected to represent both the calc-alkaline and alkaline suites that have been viewed as forming separate magmatic episodes in the Eastern Desert of Egypt, with the transition to alkali granite at ~ 610 Ma taken to mark the onset of crustal extension. Although intrusive relations were observed in the field, the emplacement ages of the granitoids cannot be distinguished within analytical uncertainty and they all formed within a restricted time span from 579 to 594 Ma. This indicates that the two suites are coeval and that some calc-alkaline rocks were also likely generated during the late extensional phase. These ages are identical to those recently obtained from similar rocks in the North-Eastern Desert, confirming that Sinai is the northern extension of the Eastern Desert Pan-African terrane of Egypt. Rare inherited zircons with ages of ~ 1790 and ~ 740 Ma are present in syenogranite from northeastern Sinai and indicate that older material is present within the basement. A few zircons record younger ages and, although some may reflect later disturbance of the main zircon population, those with ages of ~ 570 and 535 Ma probably reflect thermal events associated with the extensive emplacement of mafic and felsic dykes in both northeastern and southern Sinai.     

Post-collisional magmatism in the northern Arabian-Nubian Shield: The geotectonic evolution of the alkaline suite at Gebel Tarbush area, south Sinai, Egypt

Post-collisional alkaline magmatism (∼610–580 Ma) is widely distributed in the northern part of the Neoproterozoic Arabian-Nubian Shield (ANS), i.e. the northern part of the Egyptian Eastern Desert and Sinai. Alkaline rocks of G. Tarbush constitute the western limb of the Katharina ring complex (∼593 ± 16 Ma) in southern Sinai. This suite commenced with the extrusion of peralkaline volcanics and quartz syenite subvolcanics intruded by syenogranite and alkali feldspar granite. The mineralogy and geochemistry of these rocks indicate an alkaline/peralkaline within-plate affinity. Quartz syenite is relatively enriched in TiO₂, Fe₂O₃, MgO, CaO, Sr, Ba and depleted in SiO₂, Nb, Y, and Rb. The G. Tarbush alkaline suite most likely evolved via fractionation of mainly feldspar and minor mafic phases (hornblende, aegirine) from a common quartz syenite parental magma, which formed via partial melting of middle crustal rocks of ANS juvenile crust. Mantle melts could have provided the heat required for the middle crustal melting. The upper mantle melting was likely promoted by erosional decompression subsequent to lithospheric delamination and crustal uplift during the late-collisional stage of the ANS. Such an explanation could explain the absence or scarce occurrence of mafic and intermediate lithologies in the abundant late- to post-collisional calc-alkaline and alkaline suites in the northern ANS. Moreover, erosion related to crustal uplift during the late-collision stage could account for the lack or infrequent occurrence of older lithologies, i.e. island arc metavolcanics and marginal basin ophiolites, from the northern part of the ANS.     

Timing of formation and geological setting of low-sulphidation epithermal gold deposits in the continental margin of NE China

The margin of NE China, a part of the West Pacific metallogenic belt, contains innumerable low-sulphidation mineral deposits. Gold deposits in this region can be classified into three distinct types based on geology and ore mineral paragenesis: (1) low-sulphidation epithermal silver–gold deposits, (2) low-sulphidation tellurium–gold deposits, and (3) low-sulphidation epithermal tellurium–gold deposits. Ores formed during the late Early Cretaceous and the early Late Cretaceous reflect three distinct metallogenic periods: the Fuxin Stage at 115.98 ± 0.89 Ma, the Quantou Stage at 107.2 ± 0.6 Ma or <103 Ma, and the Qingshankou or Yaojiajie Stage at < 97 Ma and 88.2 ± 1.4 Ma. The Fuxin Stage is dominated by trachyandesitic magmatism, with magmas emplaced at hypabyssal depths. In comparison, the Quantou Stage is characterized by high-K calc-alkaline, calc-alkaline, and sodic andesitic, dacitic, and rhyolitic magmatism of three different suites. The first of these is a high-K calc-alkaline andesitic magmatic suite that was accompanied by the emplacement of a calc-alkaline sodic dacite during the formation of the Ciweigou and Wufeng ore deposits. The second suite is dominated by calc-alkaline sodic rhyolite and high-K calc-alkaline sodic dacite magmatism associated with the formation of the Sipingshan ore deposit. The third suite is typified by high-K calc-alkaline andesitic magmatism associated with the emplacement of calc-alkaline hypabyssal granitoid complexes accompanying the formation of the Dong'an and Tuanjiegou ore deposits. The Qingshankou or Yaojia Stage is characterized by calc-alkaline sodic dacite magmatism associated with the formation of the Wuxing ore deposit. Metallogenesis during the Fuxin Stage characterized by trachytic magmatism is closely related to the formation of a deep-seated fault within a magmatic arc or the back-arc region of an immature continental margin and is associated with the Early Cretaceous subduction of the Pacific plate beneath Eurasia. Ore deposits that formed during the Fuxin Stage were generally related to magmato-hydrothermal fluids associated with mantle-derived magmas. In contrast, metallogenesis during the Quantou and Qingshankou or Yaojiajie stages was closely related to the formation of a mature high-K calc-alkaline magmatic arc within a continental margin setting again associated with the westward subduction of the Pacific plate. This metallogenic event was a product of magmato-hydrothermal systems derived from crust–mantle interaction and mixing of magmas derived from partial melting of different sections of the continental crust.     

Circum-Caribbean Granitoids: Characteristics and Origin

Caribbean granitoids occur among a series of widespread magmatic arcs that developed during a period of major oceanic plate convergence and subduction that began in Late Cretaceous time. Evaluation of Caribbean granitoids reveals that two main suites of granitoids are widespread. Low-K granitoids, including gabbro, diorite, quartz diorite, tonalite, and trondhjemite, comprise one of these suites. The second main type is distinctly more potassic and consists primarily of quartz monzodiorite and granodiorite, but also includes monzodiorite, quartz monzonite, and granite. Both groups contain rocks that are transitional to the other group. The granitoids are part of an extensive Caribbean calc-alkaline assemblage that includes low-K, medium-K, and high-K rocks. Island-arc tholeiitic and normal-K calc-alkalic compositions reflect geochemical continua within the orogenic granitoids. The granitoids (including low-K rocks) lie within the calc-alkaline field on FeO^T/MgO and AFM diagrams. Alkali-lime indices generally correlate with potassium content, the low-K varieties being calcic (tholeiitic) and the higher K rocks being calc-alkalic.

Rare-earth-element concentrations range from fairly primitive compositions having flat patterns and abundances of ～10 to 30 times chondrites to more evolved types that show mild enrichment in the light rare earths and typically are normal calc-alkaline. Mean initial ⁸⁷Sr/⁸⁶Sr ratios are commonly between 0.703 and 0.704. These low ratios are consistent with a mantle source in which assimilation of old sediments is not a factor. Limited isotopic and trace-element data suggest, instead, that the granitoids formed by partial melting in the mantle wedge and that at least some of the melting was accompanied by the addition of an aqueous fluid phase that was derived from the subducted oceanic plate.

Normative An-Ab-Or compositions are consistent with the presence of two groups of Caribbean granitoids. The group that is low in normative Or follows fractional crystallization vectors of parent magmas that typically are depleted in incompatible elements. The second group has higher normative Or and fractionates toward more enriched incompatible-element compositions. Normative compositions also indicate that there are temporal relations between the intrusive and extrusive rocks of Puerto Rico. Granitoids and lavas of Middle Cretaceous (125 to 100 Ma) and Early Tertiary (65 to 35 Ma) age are geochemically primitive types that display only limited degrees of mantle enrichment. In contrast, intrusive and extrusive rocks of Late Cretaceous age (100 to 65 Ma) show a wide range of incompatible-element concentrations, reflecting varying degrees of mantle enrichment.     

俄罗斯远东地区晚中生代花岗岩类的时空分布 及其地质意义

在俄罗斯远东地区晚中生代花岗岩类年龄和相关地球化学数据的基础上,初步建立了该区晚中生代花岗岩类的年代学格架：大致以145Ma为界,分为侏罗纪(178~151Ma)和早白垩世(142~122Ma)2期。侏罗纪的花岗岩类主要为花岗岩－花岗闪长岩－石英二长岩组合,总体上为准铝质—强过铝质高钾钙碱性系列;早白垩世的花岗岩类主要为花岗岩－石英闪长岩－石英二长岩组合,主要为过铝质钙碱性—高钾钙碱性系列—钾玄岩系列。2期花岗岩稀土元素配分曲线均呈右倾型,重稀土元素曲线较平坦,都富集大离子亲石元素(如U、K)和轻稀土元素。与中国东北地区晚中生代花岗岩类对比,中国东北地区总体以兴安岭为中心,中间为早白垩世的花岗岩类,两侧为侏罗纪花岗岩类对称分布。境内外的侏罗纪花岗岩类构造背景不同,其分布与鄂霍次克洋和太平洋板块的俯冲有关,早白垩世花岗岩类可能形成于鄂霍次克带挤压造山后的伸展垮塌和太平洋板块的俯冲弧后伸展阶段。     

水泉沟杂岩体的地质学和岩石学研究

水泉沟岩体为岩浆成因,岩体的侵位深度2~3km,侵位时代为印支期,划分为4种岩石组合.水泉沟岩体是一个钾质-高钾质的钙碱性、弱碱性和碱性岩石共存的杂岩体.岩石化学成分变异图解证明岩浆存在两种演化趋势,一是酸性演化趋势:辉石闪长岩→角闪二长岩(包括二长岩)→石英碱长正长岩→碱长花岗岩;二是碱性演化趋势:正长岩→霓辉正长岩→碱长正长岩.同时也探讨了这两种演化趋势的内在机理.     

Geochronology and geochemistry of early Mesozoic magmatism in the northeastern North China Craton: Implications for tectonic evolution

This paper reports geochronological, geochemical, zircon U–Pb and Hf–O isotopic data of the Late Triassic and Early Jurassic intrusive rocks in the northeastern North China Craton (NCC), with the aim of reconstructing the tectonic evolution and constraining the spatial–temporal extent of multiple tectonic regimes during the early Mesozoic. Zircon U–Pb ages indicate that the early Mesozoic magmatism in the northeastern NCC can be subdivided into two stages: Late Triassic (221–219 Ma) and Early Jurassic (180–177 Ma). Late Triassic magmatism produced mainly granodiorite and monzogranite, which occur as a NE–SW-trending belt parallel to the Sulu–Jingji Belt. Geochemically, they are classified as high-K calc-alkaline and metaluminous to weakly peraluminous granitoids, and are enriched in large-ion lithophile elements (LILEs) and light rare earth elements (LREEs), and depleted in high-field-strength elements (HFSEs; e.g., Nb, Ta, Ti, and P) and heavy rare earth elements (HREEs), indicating an affinity to adakite. Combined with their ε_Hf(t) values (−17.9 to −3.2) and two-stage model ages (2387–1459 Ma), we conclude that the Late Triassic granitoid magma in the northeastern NCC was derived from partial melting of the thickened lower crust of the NCC and was related to deep subduction and collision between the NCC and the Yangtze Craton (YC). The Early Jurassic magmatism is composed mainly of monzogranites, which are classified as metaluminous, high-K calc-alkaline, and I-type granite. Their ε_Hf(t) values and two-stage model ages are −16.7 to −4.2 and 2282–1491 Ma, respectively. Compared with the Late Triassic granitoids, the Early Jurassic granitoids have relatively high HREE contents, similar to calc-alkaline igneous rocks in an active continental margin setting. These Early Jurassic granitoids, together with the coeval calc-alkaline volcanic rocks and gabbro–diorite–granodiorite association in the northeastern (NE) Asian continental margin, comprise a NNE–SSW-trending belt parallel to the NE Asian continental margin, indicative of the onset of Paleo-Pacific Plate subduction beneath Eurasia.     

Geochemical constraints on the genesis of the Sarıcakaya intrusive rocks,Turkey: Late Paleozoic crustal melting in the central Sakarya Zone

The Sakarya Zone is intruded by several Late Paleozoic granitoids, of which the Sar?cakaya intrusive rocks in the central Sakarya (Eski?ehir) region, is the least-studied. The Sar?cakaya intrusive rocks consist mainly of quartz diorite-granodiorite, granodiorite and granite. They are, geochemically, divided into two groups: diorites and granites. The former is medium-K and calc-alkaline (mainly I-type), whereas the latter is high-K to shoshonite and calcic (I-type). Typical minerals for both rock types are plagioclase, K-feldspar, quartz, biotite, hornblende and Fe–Ti oxides. Chondrite-normalized REE patterns for the Sar?cakaya intrusive rocks are moderately fractionated and have small negative Eu anomalies. They are enriched in LILE and LREE relative to HFSE showing characteristics of arc-related granitoids. Geochemical characteristics of the Sar?cakaya intrusive rocks indicate a hybrid origin through partial melting of lower crustal source rocks.     

中蒙边境中段花岗岩时空分布特征及构造和找矿意义

本文精确地厘定了中蒙边境中段白乃庙片麻状石英闪长岩(459-454 Ma)、锡林浩特代托吉卡山中粒晶洞正长花岗岩(268±6.9 Ma)、镶黄旗巴音察汗灰白色中细粒角闪黑云母花岗闪长岩(261.7±6.1 Ma)、镶黄旗二长花岗岩(262.7±6.0 Ma)、镶黄旗哈达庙黑云母石英闪长岩(277.2±2.9 Ma)、锡林浩特白音锡勒中细粒正长花岗岩(231.1±7.6 Ma)、苏尼特左旗(东苏)二长花岗岩(216.9±5.4 Ma)、苏尼特左旗沙尔塔拉碱长花岗岩(152.1±2.5 Ma)的时代。并在前人工作的基础上, 总结了本地区花岗岩的时空分布规律: 区内花岗岩空间上呈3条近东西向的条带分布, 时间上可划分为5个重要期次:早-中古生代(490-387 Ma)、石炭纪(342-302 Ma)、二叠纪(282-257 Ma)、三叠纪(249-204 Ma)和晚中生代(152-118 Ma), 峰期间隔约为40 Ma, 并存在两条巨型碱性花岗岩带(东乌珠穆沁旗一带的二叠纪碱性岩带、华北板块北缘的晚三叠世碱性正长岩带), 岩浆活动呈现不对称性。结合区域地质、岩石、年代学等证据说明, 北部贺根山蛇绿岩与南部索伦山-西拉木伦蛇绿岩代表两个洋盆体系, 贺根山洋闭合早于中二叠世, 而索伦山-西拉木伦缝合带所代表残留古亚洲洋关闭, 中朝板块与西伯利亚板块最终碰撞拼合的时代应在晚二叠世-早三叠世。同时, 区内与花岗岩有关的矿产发育, 包括铜、钨、锌等矿种, 多产于造山后伸展或岩石圈拆沉, 区域大规模伸展环境。     

Post-collisional potassic granitoids from the southern and northwestern parts of the Late Neoproterozoic East African Orogen: a review

Potassic metaluminous granitoids with enrichments of HFS elements constitute part of widespread post-collisional magmatism related to the Late Neoproterozoic Pan-African orogeny in northeastern Africa (Sudan, Ethiopia, Somalia) and Madagascar. The plutons were emplaced between 580 and 470 Ma and comprise both subsolvus and hypersolvus biotite–granite, biotite–hornblende–granite, quartz–monzonite and quartz–syenite. Pyroxene-bearing granitoids are subordinate. Basic dikes and enclaves of monzodioritic composition are locally associated with the granitoid plutons. Granitoids emplaced in pre-Neoproterozoic crust have Sr_i-ratios between 0.7060 and 0.7236 and _Nd(t) values between −15.8 and −5.6 while those emplaced in, or close to the contact with, juvenile Neoproterozoic crust have lower Sr_i-ratios (0.7036–0.7075) and positive _Nd(t) values (4.6). However, it is unlikely that the potassic granitoids represent products of crustal melting alone. The association with basic magmas derived from subduction-modified enriched mantle sources strongly suggests that the granitoids represent hybrid magmas produced by interaction and mixing of mantle and crust derived melts in the lower crust. The most intense period of this potassic granitoid magmatism occurred between 585 and 540 Ma, largely coeval with HT granulite facies metamorphism in Madagascar and with amphibolite facies retrogression in northeastern Africa (Somalia, Sudan). Granitoid magmatism and high-grade metamorphism are probably both related to post-collisional lithospheric thinning, magmatic underplating and crustal relaxation. However, the emplacement of potassic granites continued until about 470 Ma and implies several magmatic pulses associated with different phases of crustal uplift and cooling. The potassic metaluminous granites are temporally and spatially associated with post-collisional high-K calc-alkaline granites with which they share many petrographical, geochemical and isotopical similarities, except the incompatible element enrichments. The resemblance indicates a strongly related petrogenesis of both granite associations.     

Granitoids in the Dalat zone,southern Vietnam: age constraints on magmatism and regional geological implications

The Dalat zone in southern Vietnam comprises a Cretaceous Andean-type magmatic arc with voluminous granitoids and contemporary volcanic rocks. On the basis of petrographical and mineralogical studies, the granitoids were subdivided into three suites: Dinhquan, Deoca and Cana. Rocks of the Dinhquan suite are hornblende–biotite diorites, granodiorites and minor granites. The Cana suite encompasses mainly leucocratic biotite-bearing granites with scarce hornblende. The Deoca suite is made up of granodiorites, monzogranites and granites. Geochemically, the granitoids are of subalkaline affinity, belong to the high-K, calc-alkaline series, and most of them display typical features of I-type granites. This paper presents the new Rb–Sr mineral and U–Pb zircon and titanite age data for the granitoids, which establish the ages of the plutonic suites as: the Dinhquan at ~112–100 Ma, Cana at ~96–93 Ma and Deoca at ~92–88 Ma. These ages are significantly different from earlier publications, and indicate that the earliest magmatism in the Dalat zone began at ~112 Ma ago, that is ~30–50 Ma later than previously thought. Our geochronological data are also support the continuation of an Andean-type arc running from SE China via southern Vietnam to SW Borneo.     

Petrology and geochemistry of Zijinshan alkaline intrusive complex in Shanxi Province, western North China Craton: Implication for magma mixing of different sources in an extensional regime

In situ zircon U–Pb ages and Hf isotopic compositions and whole rock geochemical and Sr–Nd–Pb isotopic data are presented for the Zijinshan alkaline intrusive complex from the Shanxi Province, western North China Craton. Salic rocks dominate the complex with the monzonite occurring in the outermost and pseudoleucite phonolitic breccia in the center. The intrusion took place 127 Ma ago with the earliest emplacement of monzonite and the termination of cryptoexplosive pseudoleucite phonolitic breccia. All rocks from this complex show LREE enrichment and HFSE depletion and exhibit enriched to depleted Sr–Nd isotopic features. The presence of inherited zircons and enriched Hf isotopic compositions in zircon rims, along with the enriched whole rock Sr–Nd isotopic compositions, indicate that the monzonite was formed through the mixing of lithospheric mantle-derived magma with lower crust-derived melts. The diopside syenite and nepheline-bearing diopside syenite are more depleted than the monzonite in terms of the Sr and Nd isotopes, together with their very high concentrations of LILE, we proposed that they originated from a mixed mantle source of enriched lithospheric mantle and depleted asthenosphere. The nepheline syenite has very low concentrations of MgO, Ni, Cr, suggesting that the magma underwent significant crystal fractionation. The most depleted Sr and Nd isotopic compositions ((⁸⁷Sr/⁸⁶Sr)_i = 0.7036–0.7042, ε_Nd(t) = − 0.2–0.3) among all rock types indicate a great contribution of asthenosphere to the nepheline syenite. The Zijinshan complex and its related crust-mantle interaction occurred in an extensional environment which resulted in continuously asthenospheric upwelling. Such an extensional environment might have been developed during the post-orogenic stage of the Late Paleozoic amalgamation of North China Craton with Mongolian continents and subsequent Mongol–Okhotsk ocean closure.     

Alkaline orogenic plutonism in the Karakorum batholith: the Upper Cretaceous Koz Sar complex (Karambar valley,N. Pakistan)

Abstract

The Karakorum is located north of the India/Kohistan-Ladakh/Eurasia sutures. Along the Karambar valley, its axial batholith comprises four plutonic complexes. (1) The largest one represents the westerly continuation of the huge mid-Cretaceous calc-alkaline Hunza plutonic unit. This unit here displays a remarkable reverse zoning that would result from a differentiation at depth followed by multipulse intrusions. (2) A stock of subalkaline (i.e. intermediate between alkaline and calc-alkaline) granitoids (Warghut porphyritic granite). (3) A composite group of fine-grained granitoids. (4) The so-called Koz Sar alkaline complex (KSAC), a unique example of this composition of plutonism so far reported in the batholith. In addition, leucogranite dykes and rare alkaline mafic ones occur.

The KSAC is a heterogeneous and more or less deformed body, ca. 5 km wide and possibly 20 km long, comprising two coeval groups of rocks. (1) Medium- to coarse-grained rocks are the most representative members of the complex. They consist of metaluminous to slightly peralkaline monzonite, quartz monzonite, granite and leucogranite, with iron-rich mafic silicates and Fe-Ti oxide. These subsolvus granitoids define a strongly ferriferous alkaline series. Five monzonite and quartz monzonite samples yield an isochron Rb-Sr age of 88 ± 4 Ma (⁸⁷Sr/⁸⁶Sr_i = 0.70440 ± 7; MSWD = 1.7). (2) Fine-grained rocks (monzogabbro to quartz syenite) are compositionally comparable to the dark-coloured members of the preceding group.

The KSAC was emplaced into a post-collisional environment resulting from the accretion, maybe at least since Aptian times, of the Kohistan island arc to the Karakorum. Its alkaline character testifies to the development of extensional tectonics, a process compatible with an oblique collision and/or with the decrease, at the time of collision, of the convergence velocity between the two colliding terrenes. Available data suggest that this alkaline complex (1) is late-orogenic, (2) is genetically-related to the nearby subalkaline granitoids and originates from the same mantle-source with a small crustal contribution, and (3) represents the ultimate member of the mid-Cretaceous subduction-related plutonism emplaced into the Karakorum continental margin.     

祁连山地区花岗质岩浆作用及构造演化

祁连山在构造上是一条经历了多期构造旋回叠加的早古生代复合型造山带,花岗质岩浆作用研究对揭示其构造演化具有重要意义。锆石U-Pb年代学统计结果表明,祁连地区花岗质岩浆活动可以分为7个大的阶段,包括古元古代早期(2 470~2 348 Ma)、古元古代晚期(1 778~1 763 Ma)、中元古代晚期-新元古代早期(1 192~888 Ma)、新元古代中期(853~736 Ma)、中寒武世-志留纪(516~419 Ma),泥盆纪-早石炭世(418~350 Ma)以及中二叠世-晚三叠世(271~211 Ma)。其中古元古代早期发育强过铝质高钾钙碱性S型和准铝质低钾拉斑-高钾钙碱性I型花岗岩,记录了早期的陆壳增生及改造事件。古元古代晚期为准铝质-弱过铝质高钾钙碱性-钾玄质A型花岗岩,是Columbia超大陆裂解事件的产物。中元古代晚期-新元古代早期以过铝质-强过铝质钙碱性-钾玄质S型花岗岩为主,新元古代中期以准铝质-强过铝质钙碱性-高钾钙碱性A型花岗岩为主,分别对应Rodinia超大陆的汇聚和裂解事件。中寒武世-志留纪花岗岩是洋陆转换过程中的产物,约440 Ma加厚基性下地壳部分熔融形成的低Mg埃达克岩的广泛出现指示祁连地区全面进入碰撞造山阶段。泥盆纪-早石炭世花岗岩代表后碰撞伸展阶段岩浆岩组合,发育准铝质-强过铝质低钾拉斑-钾玄质等一系列花岗岩。中二叠世-晚三叠世花岗岩以准铝质-弱过铝质钙碱性-高钾钙碱性I型花岗岩为主,有少量弱过铝质高钾钙碱性A型花岗岩,是宗务隆洋俯冲消减以及碰撞后伸展过程的产物。     

东昆仑东段益克郭勒侵入岩LA-ICP-MS锆石U-Pb定年及其地质意义

东昆仑东段益克郭勒侵入体主要由闪长岩、石英碱长正长岩、英云闪长斑岩及花岗闪长岩组成.锆石LA-ICP-MS U-Pb定年结果显示,闪长岩、石英碱长正长岩和英云闪长斑岩的年龄分别为(223.4±1.1)Ma、(222.9±1.3)Ma和(230.9±1.1)Ma,表明益克郭勒侵入体形成于中三叠世.益克郭勒中三叠世侵入岩属...