Evolution of calc-alkaline magmas of the Okhotsk-Chukotka volcanic belt

Petrological, geochemical, and isotope geochronological aspects of the evolution of calc-alkaline magmatism were investigated in the Western Okhotsk flank zone, the Okhotsk segment, and the Eastern Chukchi flank zone of the Okhotsk-Chukotka volcanic belt (OCVB). The OCVB is a tectonotype of continental margin volcanic belts comprising much greater volumes of felsic ignimbritic volcanics compared with mature island arcs (MIA, Kuril-Kamchatka and Aleutian) and the Andean continental margin. The volcanic rocks of continental margin volcanic belts (OCVB and Andean belt) are enriched in K, Ti, and P compared with the rocks of MIA and show a trend toward the field of high-potassium calc-alkaline series. Primitive andesite varieties (Mg# > 0.6) were not yet found in the OCVB, but there are relatively calcic varieties unknown in Andean-type structures and a significant fraction of moderately alkaline rocks, which are not typical of MIA. Variations in trace and major element characteristics in the basalts and andesites of the OCVB were interpreted as reflecting the competing processes of assimilation/mixing and fractional crystallization during the evolution of the parental basaltic magma. Significant lateral variations were established in the composition of the mantle sources of calc-alkaline magmas along the OCVB over more than 2500 km. The initial isotopic ratios of Sr, Nd, and Pb in the volcanics of the Okhotsk segment are relatively depleted and fall near the mixing line between PREMA and BSE. The magma source of the Western Okhotsk flank zone is most enriched and approaches EMI, whereas that of the central and eastern Chukchi zones contains an admixture of the EMII component. The geochronological characteristics of all the main stages of OCVB magmatism were comprehensively studied by U-Pb SHRIMP and ID-TIMS zircon dating (86 samples) and ⁴⁰Ar/³⁹Ar analysis (73 samples). In general, a discontinuous character was established for the OCVB magmatism from the middle Albian to the early Campanian (106–77 Ma). The volcanism is laterally asynchronous. There are several peaks of volcanism with modes at approximately 105, 100, 96, 92.5, 87, 82, and 77 Ma. The Coniacian-Santonian peaks correspond to the most extensive stages of the middle and late cycles of felsic volcanism. A decreases and a hiatus in magmatic activity were reconstructed for the end of the Cenomanian and the beginning of the Turonian. The volcanism was terminated by plateau basalts with ages of 76–78 Ma, which mark a change in the geodynamic setting from frontal subduction to the regime of a transform margin with local extension in zones normal to the slip direction. A catastrophic character of eruptions with rather narrow ranges of volcanism (<2 Myr) were established taking into account new reliable age estimates for some individual large calderas. The accumulation rate of volcanic materials in such structures was up to 0.15–0.36 km³/yr and even higher.     

First U-Pb and Sm-Nd isotopic data on granitoids from the Devonian volcanic belt of Kazakhstan

First U-Pb zircon isotopic dates were obtained for rocks from the Devonian volcanic belt in Kazakhstan. The granodiorites of the Zhabden Massif (Karamendinskii Complex) were dated at 391 ± 1 Ma. The Sm-Nd isotopic system of a whole-rock granodiorite sample (?_Nd = 2.5) suggests a high percentage of mantle material in the initial granite melt, which is in good agreement with known data on granitoids in neighboring territories in Kazakhstan. With regard for the isotopic dates obtained for the granodiorites, the material of their source was separated from the mantle at 946 Ma.     

Conditions of the formation of hypabyssal granitoid intrusions using the example of the Sizindzha Massif, the Okhotsk-Chukotka volcanic belt

The Sizindzha hypabyssal intrusive massif located in the Kuidusun volcanic zone of the Okhotsk-Chukotka volcanic belt has been mapped and studied in detail. The complex of textural-petrographic features indicates the in situ formation of granitoids by the successive replacement of the host sedimentary rocks of the Verkhoyansk Complex and the medium-grained diorites (more rarely, gabbro) by later granites. It was established that the oldest fine-grained intermediate rocks observed as xenoliths in all the granitoids are in situ relicts of the molten substrate.     

The Okhotsk-Chukotka volcanic belt: Age of its northern part according to new Ar-Ar and U-Pb geochronological data

Current geochronological data on the Okhotsk-Chukotka volcanic belt (OCVB) and relevant problems are discussed. The belt evolution is suggested to be modeled based on ⁴⁰Ar/³⁹Ar and U-Pb dates more useful in several aspects than common K-Ar or Rb-Sr dates and methods of paleobotanical correlation. Based on new ⁴⁰Ar/³⁹Ar and U-Pb dates obtained for volcanic rocks in the OCVB northern part, the younger (Coniacian) age is established for lower stratigraphic units in the Central Chukotka segment of the belt, and the eastward migration of volcanic activity is shown for terminal stages of this structure evolution.     

松潘－甘孜造山带南段晚三叠世兰尼巴和羊房沟花岗岩岩石学、地球化学特征及成因

对位于松潘-甘孜地体南部的兰尼巴岩体和羊房沟岩体进行了详细的岩石学、地球化学及锆石U-Pb年代学研究,重点讨论了岩体的成因。U-Pb激光剥蚀等离子体质谱(LA-ICPMS)年龄集中在211Ma附近,属晚三叠世,代表了岩体的形成年龄。两岩体具有中等至较高的SiO2含量(58.31%～68.02%)和较高的全碱含量(6.70%～8.80%),具有准铝质(A/CNK=0.68～0.99)的特征,属于高钾钙碱性到橄榄玄粗岩系列。其中,兰尼巴岩体南段富Al2O3(15.85%～16.27%)、K2O(3.29%～3.40%),中等MgO(1.16%～1.47%),并具有高Sr(869×10-6～1032×10-6)、低Y(9.53×10-6～9.85×10-6)特征以及中等至较高的稀土元素分馏[(La/Yb)N>31],并且见有暗色包体,非常类似于下地壳熔融形成的钾质埃达克岩。兰尼巴岩体的北段及羊房沟岩体的主量元素、微量元素特征都很相似,相对兰尼巴岩体南段具有更高的K2O含量(4.08%～5.96%),相对低的Sr(664×10-6～868×10-6),稍高的Y(19.71×10-6～27.76×10-6)和明显较低的Sr/Y比值(29.39～42.05),显示出高钾钙碱性I型花岗岩的特征,可能来自于增厚下地壳的部分熔融。野外地质特征和地球化学特征均显示岩体的形成有幔源物质或新生地壳物质的参与,其中幔源岩浆的加入为松潘-甘孜造山带大量中生代花岗岩的形成提供了热源和部分物源。地幔岩浆的上侵和高钾钙碱性花岗岩体的形成标志着松潘-甘孜造山带至少在晚三叠世就已处于伸展构造环境。     

Evolution of the early devonian bindook volcanic complex,wollondilly basin,eastern lachlan fold belt

The Early Devonian Bindook Volcanic Complex consists of a thick silicic volcanic and associated sedimentary succession filling the extensional Wollondilly Basin in the northeastern Lachlan Fold Belt. The basal part of the succession (Tangerang Formation) is exposed in the central and southeastern Wollondilly Basin where it unconformably overlies Ordovician rocks or conformably overlies the Late Silurian to Early Devonian Bungonia Limestone. Six volcanic members, including three new members, are now recognised in the Tangerang Formation and three major facies have been delineated in the associated sedimentary sequence. The oldest part of the sequence near Windellama consists of a quartz turbidite facies deposited at moderate water depths together with the shallow‐marine shelf Windellama Limestone and Brooklyn Conglomerate Members deposited close to the eastern margin of the basin. Farther north the shelf facies consists of marine shale and sandstone which become progressively more tuffaceous northwards towards Marulan. The Devils Pulpit Member (new unit) is a shallow‐marine volcaniclastic unit marking the first major volcanic eruptions in the region. The overlying shallow‐marine sedimentary facies is tuffaceous in the north, contains a central Ordovician‐derived quartzose (?deltaic) facies and a predominantly mixed facies farther south. The initial volcanism occurred in an undefined area north of Marulan. A period of non‐marine exposure, erosion and later deposition of quartzose rocks marked a considerable break in volcanic activity. Volcanism recommenced with the widespread emplacement of the Kerillon Tuff Member (new unit), a thick, non‐welded rhyolitic ignimbrite followed by dacitic welded ignimbrite and air‐fall tuff produced by a large magnitude eruption leading to caldera collapse in the central part of the Bindook Volcanic Complex, together with an additional small eruptive centre near Lumley Park. The overlying Kerrawarra Dacite Member (new unit) is lava‐like in character but it also has the dimensions of an ignimbrite and covers a large part of the central Bindook Volcanic Complex. The Carne Dacite Member is interpreted as a series of subvolcanic intrusions including laccoliths, cryptodomes and sills. The Tangerang Formation is overlain by the extensive crystal‐rich Joaramin Ignimbrite (new unit) that was erupted from an undefined centre in the central or northern Bindook Volcanic Complex. The volcanic units at Wombeyan and the Kowmung Volcaniclastics in the northwestern part of the complex are probably lateral time‐equivalents of the Tangerang Formation and Joaramin Ignimbrite. All three successions pre‐date the major subaerial volcanic plateau‐forming eruptions represented by the Barrallier Ignimbrite (new unit). The latter post‐dated folding and an extensive erosional phase, and unconformably overlies many of the older units in the Bindook Volcanic Complex. This ignimbrite was probably erupted from a large caldera in the northern part of the complex and probably represents surface expressions of part of the intruding Marulan Batholith. The final volcanic episode is represented by the volcanic units at Yerranderie which formed around a crater at the northern end of the exposed Bindook Volcanic Complex.     

The Late Cretaceous Aarya flora of the northern Okhotsk region and phytostratigraphy of the lower part of the Okhotsk-Chukotka volcanogenic belt section

The Zarya flora comes from volcanogenic sedimentary rocks of the Zarya and Parnyi formations that correspond to the basal part of the section of the Okhotsk-Chukotka volcanogenic belt in the Omsukchan district (Magadan oblast, Russia). The revision of its taxonomic composition resulted in identifying approximately 25 species of horsetails, ferns, cycads, ginkgoalens, czekanowskians, conifers, and angiosperms. The Zarya flora is characterized by a combination of the Early Cretaceous relicts (Hausmannia, Birisia, Sphenobaiera, Phoenicopsis, Nilssonia, Podozamites) and typical Late Cretaceous taxa (Taxodium, Sequoia, Menispermites, Dalembia, Trochodendroides, Cissites, Terechovia, Platanaceae). Among all the paleofloral assemblages of the Okhotsk-Chukotka volcanogenic belt, the Zarya flora is the most similar to the Turonian-Coniacian Arman flora of the Magadan region, which indicates their synchronism and floral unity. The Chingandzha flora of the Omsukchan area, which comes from the same stratigraphic level as Zarya flora, differs substantially from the latter in its taxonomic composition. It is conceivable that the Chingandzha flora was confined to a large river valley which was connected to coastal lowlands. The plant remains of the Arman flora with many mountain relicts could be buried in sediments of intermountain troughs isolated from coastal lowlands. Araucarites ochotensis sp. nov. is described.     

大兴安岭火山岩带中北部中侏罗世中酸性火山岩的厘定及其地质意义

通过对大兴安岭火山岩带中北部地区北岸林场一带中酸性火山岩进行地球化学、锆石U-Pb年代学特征研究,探讨其形成的年代及构造背景。LA-ICP-MS锆石U-Pb同位素测年表明,中酸性火山岩形成时代为(166±1) Ma,属于中侏罗世,岩性为粗面英安岩和粗安岩。在主量元素组成上,SiO_2含量为59.58%～68.34%,(Na_2O+K_2O)为7.78%～8.84%,K_2O/Na_2O值为0.73～0.82,MgO含量为0.75%～2.67%,Mg~#值为25.5～44.4,显示岩石相对富硅、富钠、低钾、碱含量偏高、镁含量偏低的特征。TAS分类图及铝饱和指数A/CNK(0.91～0.97)显示岩石具有准铝质偏碱性的特征。稀土元素总量中等,轻、重稀土分馏明显(La/Yb)_N=6.56～11.79,具有弱的Eu负异常(δEu=0.60～0.81);微量元素相对富集大离子亲石元素Rb、Th、U,贫高场强元素Nb、Ta、Hf、Ti。地质、地球化学分析表明中酸性火山岩为地壳物质部分熔融的产物。岩浆活动可能起源于早侏罗世蒙古—鄂霍茨克洋闭合过程中造山晚期构造环境,但在中侏罗世张性构造环境喷发成岩,提供了俯冲造山环境形成的岩浆在地壳中可能跨时、跨构造环境活动的范例。     

关于造山带花岗岩类填图方法的讨论——以西秦岭天水地区为例

花岗岩类的分类理论决定其填图方法.现行的单元-超单元填图方法是依据花岗岩类的I型、S型分类理论和同源岩浆演化理论而建立的.由于同源岩浆演化理论的不完善性,造山带地壳结构及地质构造特征的复杂性、地质作用过程的多样性,导致了形成花岗岩类源岩组分的复杂性,使得I型、S型分类在造山带具有不确定性,因而单元-超单元填图方法在造山带实践中存在较多问题和矛盾.造山带花岗岩类填图方法应以基本岩石分类方案为依据,以野外可识别、易掌握的客观岩石学、矿物学、组构学为准则, 客观、真实地反映造山带花岗岩类的特色.     

New age data on the magmatic rocks from the western sector of the Okhotsk-Chukotka volcanogenic belt

New age data are reported for the magmatic rocks from the western flank of the Okhotsk-Chukotka volcanogenic belt consisting of the Ul’ya and Kuidusun volcanic zones. Four U-Pb SHRIMP zircon dates within 106-90 Ma were obtained for the upper part of the Emanrin Formation and the lower part of the Ul’ya Group of the Ul’ya volcanic zone. Large Verkhneallakh and Atarbai subvolcanic intrusions from the Kuidusun volcanic zone were dated using the Rb-Sr method at 115–107 Ma and 85 Ma, respectively. U-Pb dating of zircon microsamples from adamellites and diorites of the Sizindzha Massif yielded, respectively, ages of 91 and 90 Ma. New geochronological data indicate that the Selitkan-Sungari and Okhotsk-Chukotka volcanogenic belts are coeval and can be united in a common Okhotsk-Sungari system of volcanic belts and zones (megabelt).     

Types of post-trappean hypabyssal granitoids of the Circum-Siberian belt

Early-Middle Triassic granitoid massifs form the Circum-Siberian belt, which is considered to be an outer zone of a large Siberian igneous province that is represented by Late Permian-Early Triassic basaltoids. Two major types of granitoids are distinguished: (1) granite and leucogranite; (2) mid-alkaline and alkaline granitoids. These massifs are unevenly located in individual segments of the belt. Emplacement of granitoids of the Circum-Siberian belt occurred at the final stages of the evolution of the Siberian plume. The main geochemical and isotope characteristics of granitoids are discussed.     

Geochemistry and geodynamic significance of the dike series of the Aluchin ophiolite complex,Verkhoyansk-Chukotka fold zone,Northeast Russia

The Aluchin ophiolites represent a tectonomagmatic complex, the upper crustal part of which is made up of two dike series. One series includes diabases and gabbrodiabases, which are exposed in the Late Triassic Atamanov Massif (226 Ma) and subdivided into low-potassium, low and moderate-titanium varieties. In terms of rare-earth element (REE) distribution pattern, these rocks correspond to the mid-ocean ridge basalts (N-MORB). At the same time, trace element composition of some samples indicates the enrichment in subduction component (Ba, Th), as well as variable depletion in Ta, Nb and other high-field strength elements (HFSE), which leads us to conclude that these rocks were formed from melts similar to back-arc basin basalts (BABB) at a sufficiently mature stage of back-arc spreading. The diabases of other series form separate dike bodies (dike swarms) that cut across a mantle ultrabasic body in the northern part of the Aluchin Massif. The most part of these rocks reveal prominent island-arc signatures, primarily, REE and trace-element distribution patterns. At the same time, they are characterized by slightly elevated contents of titanium, nickel, and chromium, and low content of aluminum. On the basis of these data, the diabases of the Aluchin Massif can be regarded as BABB basalts with distinct island-arc characteristics, which are usually termed as arc-like member. The joint geochemical evolution of the two diabase series is well consistent with that of the rocks from different structures of the Mariana trough, which, together with compositional data, indicate that the studied dike series mark the initial and mature stages of the opening of the Late Triassic suprasubduction basin.