Polygenic nature of granite clastites: Communication 2. Secondary and supergene reworking of granitic clastites

Clastogenic rocks spatially associated with granite massifs have been reported in the geological literature from different regions: Caucasus (Leonov, 1974, 1991), Urals (Puchkov, 1968), Kazakhstan (Svarichevskaya and Skublova, 1973), Transbaikal region (Leonov, 2008; Lobanov et al., 1991), Tien Shan (Leonov et al., 2008), North America (Beroush, 1991; Lukin, 1989, 2007; Pippin, 1973). In some places, they represent crushed rocks of indigenous massifs. In other places, they make up accumulations and aprons of clastic products of the granitic composition both on the surface and beneath the sedimentary cover. In the first communication (Leonov et al., 2014) devoted to the origin of granite clastites, we examined specific features of the structure and evolution of granite bodies at the posthumous development stage, i.e., after cooling and introduction into the consolidated layer of the Earth’s crust. It was shown that such rocks are formed at least due to two main processes: supergene1 (chemical and physical weathering) and tectonic (prototectonics and posthumous disintegration). Although the rocks are highly similar in composition, structure, and bedding conditions, they are marked by several specific features described in the first communication that provide insight into their genetic nature. However, the problem of morphostructural characteristics and genetic interpretation of granite clastites cannot be closed here. Reconstruction of the “primary” origin of clastic granitic bodies in some, far from single, cases is complicated by the following fact: the exhumed massifs of tectonically disintegrated granitoids undergo supergene transformations, while sediments in the weathering crust are involved in tectonic reworking. Thus, clastites can be formed in several stages with different successions of events: supergene processes (formation of the weathering crust) can precede the tectonic reworking of rocks or succeed the formation of tectonomixtites. Determination of diagnostic properties of genetically different clastic rocks and stages of their lithostructural alteration is important for solving the issues of regional geology, development of methods for the study of genetically complex sequences, as well as paleogeographic and paleotectonic reconstructions. This problem acquires a specific importance because of two circumstances: first, its solution is at the intersection of two geological disciplines (lithology and tectonics); second, granitic clastite bodies often represent commercial hydrocarbon reservoirs (Areshev et al., 1997; Gavrilov, 2000; Izotov et al., 2003; Lobanov et al., 1991; Lobusev et al., 2002; Lukin, 2007; Martynova, 2002; Pippin, 1973; Sitdikova and Izotov, 2002). Let us discuss two scenarios of the succession of events: scenario 1—“tectonic mixtite” → “supergene reworking”; scenario 2—“weathering crust” → “tectonic reworking”. All other versions are combinations of these two types.     

富铀花岗岩源岩特征

论述了华南陆壳的地层结构、岩性组合及其含铀性,各个构造旋回对铀的活化、迁移和富集作用。指出富铀花岗岩的源岩特征是：具有高硅、富铝、富钾的富铀陆壳。岩石组合主要以陆源泥砂质碎屑岩建造为主,夹中酸性、基性火山碎屑岩建造和碳酸盐岩建造等所组成的岩石。岩石经受了多次构造运动,使其发生不同程度的区域变质作用和混合岩化、花岗岩化,最终形成富铀花岗岩。     

Sedimentology and Chronology of Paleogene Coarse Clastic Rocks in East-Central Tibet and Their Relationship to Early Tectonic Uplift

A systematic sedimentological and chronological study of typical Paleogene basins in eastcentral Tibet suggests that the depositional characteristics of extensively developed huge-bedded, purplish-red coarse clastic rocks formed in a tectonic setting of regional thrusting and strike-slipping represent a typical dry and hot subaerial alluvial fan environment formed in a proximal and rapidaccumulating sediment body in debris flows and a fan-surface braided river. Combining results from basin-fill sequences, sequences of coarse clastic rocks, fauna and sporo-pollen associations and thermochronological data, it is conduded that the coarse clastic rocks formed in the period of 54.2- 24.1 Ma, nearly coeval with the formation of Paleogene basins in the northern （Nangqen-Yushu thrust belt）, middle （Batang-Lijiang fault belt）, and disintegration of large basins in the southern （LanpingSimao fold belt） segments of Tibet. The widespread massive-bedded coarse clastic rocks, fold thrusting and strike-slip, thrust shortening, and igneous activities in the Paleogene basins of eastcentral Tibet indicate that an early diachronous tectonic uplift might have occurred in the Tibetan Plateau from Middle Eocene to Oligocene, related to the initial stage of collision of the Indian and Asian plates.     

The Koshrabad granite massif in Uzbekistan: petrogenesis, metallogeny, and geodynamic setting

The Koshrabad massif, referred to as the Hercynian postcollisional intrusions of the Tien Shan, is composed of two rock series: (1) mafic and quartz monzonites and (2) granites of the main phase. Porphyritic granitoids of the main phase contain ovoids of alkali feldspar, often rimmed with plagioclase. Mafic rocks developed locally in the massif core resulted from the injections of mafic magma into the still unconsolidated rocks of the main phase, which produced hybrid rocks and various dike series. All rocks of the massif are characterized by high f (Fe/(Fe + Mg)) values and contain fayalite, which points to the reducing conditions of their formation. Mafic rocks are the product of fractional crystallization of alkali-basaltic mantle melt, and granitoids of the main phase show signs of crustal-substance contamination. In high f values and HFSE contents the massif rocks are similar to A-type granites. Data on the geochemical evolution of the massif rocks confirm the genetic relationship of the massif gold deposits with magmatic processes and suggest the accumulation of gold in residual acid melts and the rapid formation of ore quartz veins in the same structures that controlled the intrusion of late dikes. The simultaneous intrusion of compositionally different postcollisional granitoids of the North Nuratau Ridge, including the Koshrabad granitoids, is due to the synchronous melting of different crustal protoliths in the zone of transcrustal shear, which was caused by the ascent of the hot asthenospheric matter in the dilatation setting. The resulting circulation of fluids led to the mobilization of ore elements from the crustal rocks and their accumulation in commercial concentrations.     

北秦岭漂池岩体的源区特征及其形成的构造环境

北秦岭漂池花岗质岩基为早古生代岩浆活动的产物，岩石类型主要为二云母花岗岩。通过主要元素，微量元素及Ｎｄ，Ｓｒ，Ｏ同位素特征的分析，其成因类型为Ｓ型花岗岩，地物质来自壳源碎屑物，研究表明，秦岭群片麻岩类是形成漂池岩体的主要源岩，结合区域地质背景分析，岩体并不形成于板块碰撞环境，而形成北秦岭早古生代活动大陆边缘，受板块俯冲作用的动力学影响所诱发的陆缘地壳物质熔融的产物，因此，漂池岩体形成的构造类型是活     

青藏高原东缘古近纪粗碎屑岩沉积学及其构造意义

青藏高原东缘古近纪盆地的填图和沉积学研究表明,在青藏高原东缘区域性走滑-挤压构造背景下形成的古近纪盆地内广泛发育厚层—巨厚层状的紫红色粗碎屑岩系。其沉积特征指示为一种近源快速堆积的泥石流和辫状河道沉积体,形成于干旱炎热气候条件下的典型陆内冲积扇环境。盆地充填序列、粗碎屑岩层序、动植物化石和盆地内岩浆岩~(40)Ar-~(39)Ar年代学等综合研究结果表明,古近纪盆地内粗碎屑岩大约形成于38～29 Ma。该时期与青藏高原东缘北段(横断山地区)古近纪盆地的形成和南段(兰坪—思茅地区)大盆地的裂解时间基本一致,这很可能预示着青藏高原在晚始新世—早渐新世期间曾发生过整体的快速构造隆升。     

原位微区同位素分析在花岗岩成因研究中的应用

花岗岩及其伴生的镁铁质岩石是构成大陆地壳的重要组成部分,是大陆形成演化的标志物。花岗岩的成因研究,包括花岗岩的岩浆源区及成岩过程,蕴含着大陆地壳生长、岩石圈演化等重要信息。随着二次离子质谱仪(SIMS)和激光多接收电感耦合等离子体质谱仪(LA-MC-ICPMS)的问世,原位微区(in-situ)元素和同位素分析方法开发以及应用使花岗岩成因研究,尤其是花岗岩的岩浆源区和成岩过程等方面得到长足的进展。在仔细阅读相关文献的基础上,结合笔者近年来相关研究工作,综述花岗岩成因研究中原位微区同位素源区示踪和成岩过程的最新进展,以期推动我国花岗岩及原位微区同位素分析方法等相关研究。     

Paleoproterozoic Granitoids on Liaodong Peninsula,North China Craton

Paleoproterozoic granitoids are an important constituent of the Jiao–Liao–Ji Belt(JLJB). The spatial-temporal distribution and types of Paleoproterozoic granitoids are closely related to the evolution of the JLJB. In this paper, we review the field occurrence, petrography, geochronology, and geochemistry of Paleoproterozoic granitoids on Liaodong Peninsula, northeast China. The Paleoproterozoic granitoids can be divided into pre-tectonic(～2.15 Ga; peak age=2.18 Ga) and post-tectonic(～1.85 Ga) granitoids. The pre-tectonic granitoids are magnetite and hornblende–biotite monzogranites and granodiorites. Pre-tectonic monzogranites are widespread in the JLJB and have A_2-type affinities. In contrast, pretectonic granodiorites are only present in the Simenzi area and have adakitic affinities. The post-tectonic granitoids consist of porphyritic monzogranite, syenite, diorite, granodiorite, quartz monzonite, monzogranite, and granitic pegmatite, which are adakitic rocks and I-, S-, and A_2-type granitoids. The assemblage of pre-tectonic A_2-type granitoids and adakitic rocks indicates the initial tectonic setting of the JLJB was a continental back-arc basin. The assemblage of post-tectonic adakitic rocks and I-, S-, and A_2-type granitoids indicates a post-collisional setting. The 2.20–2.15 Ga A_2-type granitoids and adakitic rocks were associated with the initial stage of back-arc extension, and the peak of back-arc extension is inferred from the subsequent(2.15–2.10 Ga) mafic intrusive activity. The ～1.90 Ga adakitic rocks mark the beginning of the postcollisional stage, which was followed by the intrusion of low-temperature S-and I-type granitoids. High-to low-pressure granitoids(S-type) were generated during the peak of post-collisional lithospheric delamination and asthenospheric upwelling. The emplacement of later granitic pegmatites occurred during the waning of the orogeny.     

Age and sources of granitoids in the junction zone of the Caledonides and Hercynides in southwestern Mongolia: Geodynamic implications

The paper reports newly obtained geological, geochronological (U-Pb zircon method), Nd isotopic, and geochemical data on Middle and Late Paleozoic granitoids and metamorphic rocks from the southern slope of the Mongolian Altai and Gobi Altai and on granitoids from the Trans-Altai Gobi. Tectonically, the former rocks are hosted in the margin of a Caledonian paleocontinent, and the latter are localized among island-arc and oceanic complexes related to the development of the Hercynian Southern Mongolian Ocean. According to their geological setting, the intrusive complexes are subdivided into two major groups: (i) related to processes of regional metamorphism and (ii) separated from these processes. Geochemical data suggest that the source of most of the granitoids and metamorphic rocks contained island-arc rocks and their erosion products. Nd isotopic evidence indicates that practically all of the allochthonous granitoids, regardless of their composition, age, and structural setting, have positive ?_Nd(T) values [i.e., belong to the ?(+) type] and could not be formed by the melting of metaterrigenous rocks widespread at the modern erosion level. These granitoids in both the Caledonian and the Hercynian structures have practically identical Late Riphean Nd model ages [T_Nd(DM) = 0.97–0.60 Ga], which become slightly younger in the granitoids of the Hercynides. The exception are ultrametamorphic subautochthonous ?(?) granites of the first group localized in the peripheral part of migmatite fields. The sources of these granitoids could be the host metaterrigenous rocks. The results obtained in the course of this research suggest, with regard for preexisting data on granitoids in the isotopic provinces in Central Asia, that the sources of the Paleozoic granitoids were the rocks of the “juvenile” Caledonian and Hercynian island-arc crust and of the older crust of cratonic blocks with a Early Precambrian and Late Riphean basement, respectively. The Late Riphean crustal material in Caledonian and Hercynian structures related to the development of the corresponding oceanic basins most probably consisted of clastic sediments or relatively small fragments of the Late Riphean crust. The occurrence of this crustal material in the sources of the granitoids can be explained by the involvement of sediments in subduction zones and the participation of these sediments and fragments of Late Riphean complexes in the accretionary-collision processes during the closure of the paleoceanic basins. Simultaneously, the subduction zones received juvenile material that could be later involved in the melting processes together with older rocks.     

Fold deformations of the paleozoic basement roof in the Chunkurchak Trough,Kyrgyz Ala-Too Range

A structural–geological study has been performed on the northern slope of the Kyrgyz Ala-Too Range. Deformations of the peneplaned Paleozoic basement surface, structures of granite disintegration, and morphostructural manifestation of Late Cenozoic tectonic movements have been investigated. Based on the location of pre-Paleocene peneplain remnants with the retained weathering mantle partly overlapped by Paleocene–Miocene sedimentary complexes, we have reconstructed the morphology of the folded surface of the Chunkurchak Trough separated from the Chu Basin at the early Miocene. The dome–fold forms, the morphology and arrangement of which are controlled by disintegration of the basement, have been described for the basement surface. It has been established that granites are broken by systems of steeply dipping, fanshaped, and gently dipping faults and fractures. Variously oriented insignificant offsets along slickensides, as well as displacements deduced from the geometry of fracture intersections, are a result of volumetric cataclastic flow of rocks. The tectonic mobility of disintegrated granites, which are abundant in the Paleozoic–Precambrian basement, explains the complexity and scale of tectonic processes initiated by Cenozoic activation. In paleotectonic reconstructions, which take into consideration tectonic flow and the redistribution of basement masses, the estimates of Cenozoic relative rapprochement of the Chu Basin and the Kyrgyz Ala-Too Range decrease substantially to 4–6 km.     

塔里木盆地中部志留-泥盆系沉积构造背景

我国陆上最深的钻井塔参1井位于塔里木盆地中央隆起,井深7200m,在井深3742～4001m钻遇志留泥盆系碎屑岩259m,包括下志留统125m,上泥盆统134m。在该段岩心采样23块(下志留统15块,上泥盆统8块),其全岩和微量元素地球化学分析表明,塔参1井的志留泥盆系碎屑岩均为稳定大陆克拉通内部的沉积,物源可能来自其底部以前震旦纪花岗岩为代表的古老陆壳基底。     

宗务隆构造带西段三叠系隆务河组碎屑锆石特征及其构造意义

宗务隆构造带位于柴北缘构造带与南祁连构造带之间,总体呈北西西向展布。构造带东段丰富的岩浆活动记录了该构造带晚古生代—中生代期间的裂解和闭合过程,而西段岩浆活动的记录较为稀少,对于其东、西段是否具有相同的构造演化尚不清晰。通过分析构造带西段三叠系隆务河组碎屑岩的地球化学特征、碎屑锆石U-Pb年龄及Hf同位素组成,认为隆务河组的碎屑沉积物的源区古风化程度轻微,不具备沉积再循环的特征,原岩主要为长英质岩石,南祁连新元古代花岗质片麻岩和早古生代大陆弧型花岗岩为隆务河组碎屑岩的主要物源;碎屑岩可能沉积于早中三叠世挠曲型盆地中;锆石U-Pb年龄分析表明宗务隆构造带东西段构造演化具有不同的历程,东段发育有限洋盆,而西段并未出现,转换地带可能位于生格至罗根郭勒之间。     

Tectono‐sedimentary evolution of lower to middle Miocene half‐graben basins related to an extensional detachment fault (western Crete,Greece)

Crustal extension in the overriding plate at the Aegean subduction zone, related to the rollback of the subducting African slab in the Miocene, resulted in a detachment fault separating high‐pressure/low‐temperature (HP‐LT) metamorphic lower from non‐metamorphic upper tectonic units on Crete. In western Crete, detachment faulting at deeper crustal levels was accompanied by structural disintegration of the hangingwall leading to the formation of half‐graben‐type sedimentary basins filled by alluvial fan and fan‐delta deposits. The coarse‐grained clastic sediments in these half‐grabens are exclusively derived from the non‐metamorphic units atop the detachment fault. Being in direct tectonic contact with HP‐LT metamorphic rocks of the lower tectonic units today, the basins must have formed in the period between c. 20 and 15 Ma, prior to the exposure of the HP‐LT metamorphic rocks at the surface, and juxtaposed with the latter during ongoing deformation.     

西昆仑造山带中带提热艾力组沉积环境 及物源区构造背景

西昆仑造山带中带上石炭统提热艾力组为一套浅变质碎屑岩沉积,四周被三叠纪岩浆岩所包围,研究其沉积环境及物源特征对揭示该区构造背景具有重要的意义。通过野外详细的剖面测制、沉积相标志的观察,结合室内岩矿鉴定和地球化学等综合分析,揭示该套碎屑岩具典型的鲍马序列结构特征,岩性成分成熟度低,主要为一套近物源的斜坡重力流沉积,其物源区较为复杂,主要为切割岩浆弧、过渡岩浆弧和再旋回造山带,源区构造环境为大陆岛弧及活动大陆边缘,表现出活动大陆边缘弧后盆地的沉积特点。     

阿尔泰造山带喀纳斯群变质碎屑岩地球化学特征及年代学研究

喀纳斯群为一套巨厚的中低压型浅变质碎屑岩系,主要由片岩、片麻岩、变质砂岩等组成,其形成时代未有统一的认识,致使阿尔泰构造带的构造演化过程争议较大。对喀纳斯群变质岩进行原岩恢复,认为该套变质岩为副变质岩,考虑到变质碎屑岩的成岩物质继承母岩特征和变质程度的影响,利用碎屑岩研究方法对元素地球化学特征进行探讨,显示出喀纳斯群变质碎屑岩原岩形成环境以大陆岛弧为主,兼有活动大陆边缘的特征,CIA、ICV指数反应出原岩经历了相对温暖、湿润的风化作用,成熟度较低。锆石U-Pb定年结果表明,最年轻的锆石年龄集中在(500±3.0)Ma,代表喀纳斯群的上限年龄,认为该套地层形成于晚寒武世晚期之前,为一套形成于大陆岛弧或活动大陆边缘的复理石建造。新元古代青白口纪初期基底裂解事件,暗示着阿尔泰构造带存在前寒武纪大陆地壳基底。     

青岛八仙墩海相碎屑岩的岩石地球化学特征及其油气研究意义

青岛八仙墩剖面出露了一套海相陆源碎屑沉积层序。这套碎屑岩受崂山花岗岩侵入已经发生变质,岩性以硬砂岩为主,夹有页岩、含锰细砂岩、长石砂岩,富含白云质。这套地层保留了各种原生的沉积构造,发育有浊积岩的底模构造,同沉积变形,水平层理、微粒序层理。岩石地球化学分析结果表明:碎屑岩的SiO_2含量在59.6%～71.6%之间,含量的多少与岩性无相关性;而黑色页岩的Al_2O_3含量变化于12.31%～16.39%之间,多在15%以上,高于灰白色砂岩的含量(普遍小于14.5%)。镜下特征和Al203含量都表明砂岩的成熟度低。除一个样品外,碎屑岩的∑REE含量高,在146.75×10~(-6)～245.78×10~(-6)之间,La在30.35×10~(-6)～61.39×10~(-6)之间,对球粒陨石标准化的(La/Yb)_N在8.68×10~(-6)～21.89×10~(-6)之间。对PASS标准化碎屑岩的(La/Ce)_N值为1.07～1.18,显示了陆源碎屑物质的主要贡献。该地层弱的Ce/Ce*负异常和富含白云质岩层指示了海相沉积环境。主量元素判别图解和微量元素判别图解表明这套碎屑岩具有活动大陆边缘背景和大陆岛弧的共同特征,但高的∑REE含量、La和Ce丰度支持了活动大陆边缘的沉积环境,但有待于采用更准确的重矿物分析来确定其物源区的大地构造性质。这套碎屑岩的出露指示了南黄海盆地深部应有奥陶统盆地相浊积沉积体系的存在,可能具有良好的浊积砂岩油气储层。     

Geochemical studies of granitoids from Shyok tectonic zone of Khardung-Panamik section,Ladakh, India

The Shyok tectonic zone lies to the north of Ladakh magmatic arc or the Ladakh batholith in the Trans-Himalaya of Ladakh district, J & K. Investigations were carried out on the granitoids exposed along Leh-Siachan highway between Khardung and Panamik villages. The granitoid bodies under study are: Khardung granite (KG), Tirit granite (TG) and Panamik granite (PG) belonging to Ladakh batholith, Shyok ophiolitic mélange and Karakoram batholith respectively. Though the granitoids belong to different litho-tectonic units, yet they have subduction related geochemical characters typical of Andean-type granitoids. Re-melting of crustal rocks of volcanic arc affinity has played an important role for the origin of KG rocks which are more evolved, while the TG and PG rocks represent transitional tectonic environment from primitive to mature arc.     

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。     

Relative contribution of various climatic processes in disintegration of clay-bearing rocks

The climatic processes of heating and cooling, wetting and drying, and freezing and thawing affect the disintegration characteristics of clay-bearing rocks (shales, claystones, mudstones, and siltstones) to varying degrees. Although heating and cooling, wetting and drying, and freezing and thawing are known to be the main processes responsible for physical disintegration of rocks under natural conditions, most of the previous investigators have used methods based only on water content variations (e.g., jar slake, slake index, and slake durability index tests) to assess the disintegration of clay-bearing rocks. Such assessments may not be adequate to explain the field behaviour of clay-bearing rocks subjected to a full range of climatic processes. In order to evaluate the combined effects as well as relative contributions of various climatic processes on the disintegration behaviour, samples of selected clay-bearing rocks, consisting of 5–6 particles, each weighing 85–150 g, were subjected to multiple cycles of heating and cooling, wetting and drying, and freezing and thawing. These treatments resulted in fragmentation of samples with fragments ranging from 50 to 2 mm and finer in dimensions. A new approach, referred to as the disintegration ratio, and defined as the area under the grain size distribution curve of the disintegrated material to the total area encompassing all grain size distribution curves of the samples, was used to account for fragmentation into varying sizes. Statistical analyses were performed to investigate the relationship between fragmentation, mineralogical composition, and physical properties.     

外生铀矿床成矿铀质来源与资源评价若干问题

概述了外生铀矿床的基本特点,即近源、浅成、层控、有机萃取、多阶段成矿、多种成因类型。根据成矿作用和矿床成因,把外生成矿的铀质来源分为:1.同生铀源;2.再造铀源;3.后生铀源等三类。着重讨论了外生铀矿资源评价的若干问题,即铀源岩石的原始铀丰度及活化丢失情况;花岗岩类铀源岩的岩石化学标志;各种铀源地质体形成的大地构造背景;外生铀矿成矿省、成矿带的评价准则。