Two A—type Syenite—Granite Belts in Anhui

There exist two parallel A-type syenite-granite belts on both banks of the Yangtze River in Anhui Province: one is the Dalongshan-Chengshan-Huangmeijian belt on the north bank and the other is the Huayuangong-Maotan-Banshiling belt on the south bank. Both of them consist of syenite, quartz-syenite and alkali feldspar granite, which were formed at about 125 Ma ago and are enriched in alkalies and high-field-strength elements but depleted in water, with the total alkali accounting for 9–12%. and H₂O⁺ only for 0.50%. The Ga× 10⁻⁴/Al ratio is as high as 2.7 to 3.8 for granite. They were formed in the extension stage of ancient rifting. So they are considered to be the A-type syenite-granite belts. Granites in the two belts resulted from a syenite magma by an AFC mechanism while the magma was derived from alkali basalt magma through the fractional-crystallization-dominated AFC mechanism. Financially supported by the National Eighth-Five-Year Science and Technology Breakthrough Project No. 85-901-03-04.     

Geology and Genesis of Peraluminous Granites in East Tianshan Upper Paleozoic Island Arc Belt

A peraluminous granite belt occurs along the connecting zone between the Turpan-Hami Precambrian block and the Upper Paleozoic island arc belt. Muscovite granite and twomica granite are the essential lithological components of that belt. All the potassium feldspars in these granites are microcline. Heavy minerals are dominated by magnetite. SiO₂ contents of these granites are greater than 73% with most of the A/NKC values greater than 1.1, normative corundum values greater than 1. Plots of CIPW norms in the (Al-K-Na)-Ca-(Fe²⁺+ Mg) diagram are mostly situated in the plagioclase-cordierite-muscovite region. The rocks are characterized by very low contents of minor elements and ΣREE with strong Eu depletion, δ¹⁸O values between 6.6‰ and 7.0‰, Rb-Sr isochron age of 260.2 ± 6.2 Ma and an initial⁸⁷Sr/⁸⁶ Sr ratio of 0.7052. These granites might have been produced by partial melting of moderately acidic volcanites and low-maturity sediments in the basement sequences and could be genetically connected with the southward A-type subduction of the Turpan-Hami block following the closure of the Middle Carboniferous back-arc basin.     

Petrogenesis of the Paleoproterozoic Guandishan Granitoids in Shanxi Province： Constraints from Geochemistry and Nd Isotopes

The Guandishan granitoids consist mainly of various granitoid intrusions with different scales, including the Huijiazhuang intrusion, Shizhuang intrusion and Hengjian intrusion, which were formed between 1906 Ma and 1848 Ma. On the basis of geological and petrological characteristics, these granitoids can be classified into two groups： the earlier gneissic granodiorites and monzogranites, and the later massive leuco-monzogranites. Their geochemical and Nd isotopic features indicate that they could be derived from complicated partial melting of supracrustal rocks with an affinity of continental arc materials, such as sandy shale and pelite, and with garnet, pyroxene, hornblende and plagioclase as residual phases. Biotite, feldspar and other minerals were most likely fractionated during the magma evolution. Their source may have an affinity with continental arcs, and the granitoids could be derived from the main syn-collisional to late-orogenic tectonic environment, which may be related to the final amalgamation between the Eastern and Western continental blocks in the North China Craton.     

Geochemistry of the Baishitouquan Amazonite—and Topaz—bearing Granite in the Mid—Tianshan Belt,Xinjiang,China

The Baishitouquan amazonite and topaz-bearing granite is one of the typical high-rubidium and high-fluorine granites in the eastern part of the Mid-Tianshan belt. This intrusion is in sharp contact with Mid-Proterozoic schists, gneisses and marbles, and is composed of four zones transitional from the bottom upwards: leucogranite, amazonite granite, topaz-bearing amazonite granite and topaz quartz albitite. The Baishitouquan granite contains highly ordered K-feldspar, Li-rich mica, Mn-rich garnet, α-quartz and low temperature zircon and is chemically high in Si, K, Na, Al, Li, Rb, Cs and F, and low in Ti, Fe, Ca, Mg, P, Co, Ni, Cr, V, Sr and Ba, with Na₂O<K₂O. Amazonite from the amazonite granite zone contains 1867 ppm Rb. The F contents of bulk rocks are 3040 and 4597 for the amazonite granite and topaz-bearing amazonite granite zones, respectively. These two zones have δ¹⁸O values of 8.97–9.85‰ (SMOW) and show flat REE distribution patterns with strong Eu depletion. K-Ar and Rb-Sr ages of this intrusion are 226. 6 Ma and 209. 6 Ma respectively, with an initial⁸⁷Sr/⁸⁶Sr ratio of 0.987±0.213. The Baishitouquan granite is the product of crystallization of a low temperature, and water, rubidium and fluorine-rich magma, which may have been derived from partial melting of muscovite-rich crustal rocks. Consolidation of this granite involved two contrasting and successive stages: melt crystallization and hydrothermal metasomatism and precipitation. Various geological features of this granite were formed during the transition from the magmatic to the hydrothermal stage.     

Speciation and Precipitation of Uranium Complexes in Hydrothermal Solutions Related to Granite—type Uranium Deposits

Uranium-bearing hydrothermal solutions during the stage of ore deposition are weakly alkaline and of the Ca^2 -Na^ /HCO3^- -F^- type.UO2(CO3)2^2- and UO2F4^-, are dominant in the hydrothermal solutions with respect to their activity.Wall-rock hydrothermal alterations ,temperature and pressure drop and the reducing capability of rock assemblage (Δeh) led to a decrease in Eh of the hydrothermal solutions and an increase in Eh at which uranium began precipitating.Therefore,the mechanism of uranium precipitation is essentially the reduction of uranium complexes.The granite-type uranium deposits are the most important type of uranium resources in China.Discussions will be made in this paper concerning the hydrothermal speciation and precipitation mech-anisms of uranium complexes in the light of fluid inclusion and geological data from some major de-posits of this type in South China.     

Confirmation and Preliminary Study of Aluminous A—Type Granites in the Wuyi Mountain Area,Eastern Jiangxi Province,China

Widespread in the Wuyi Mountain area of eastern Jiangxi are petrochemically peraluminous granites and they are characterized as being high in silica (SiO₂ > 75% ) and highly alkaline (ALK=6.9% -7.5%) with K₂O > Na₂O and rather high ratios of FeO^T/MgO (11.3-17.9). The rocks have low contents of CaO, MgO, TiO₂ and P₂ O₅. The granites are enriched in REE (ΣREE =210.3 - 496. 8 μg/g) with remarkable negative Eu anomalies, but depleted in Eu, Ba, Sr, V, Co, and Ni, with 10000 x Ga/Al ratios, varying from 6. 1 to 9. 8. It is clear that these granites are obviously different from the I- and S-type granites, but are quite similar to those typical A-type granites such as aluminous A-type granites in the coastal areas of Fujian Province. State Geological Survey Project: supported by the Regional Geological Survey Project (No: 20001300002091 ) on the basis of the maps (scale 1: 250000) of Jingdezheng City, Nanchang City and Shangrao City.     

Important Geological Factors Controlling the Formation of Gold Deposits in East China

The major gold deposits in east China are characterized by their occurrence in pre-Cambrian host metamorphic rocks,restriction in some specific stratigraphic-structural terranes,tendentious localization in ductile or ductile-brittle shear zones,association with the syntexis type granitoids,and significant remobilization,superimposition and enrichment by the later stage Mesozoic tectono-magmatic activities.     

Environmental Geochemistry of Fluorine in the Rock—Soil—Water System in the Karst Region of Central Guizhou Province

Systematic determination of and adsorption experiment on fluorine in the carbonate rock-soil-water system in the karst region studied in Guzhou Province,in conjunction with the mineral surface and soil chemistry data,have revealed the geochemical characteristics of F and the mechanisms of its transport and entrichment in the rock-soil-water-system of the karst region central guizhou province,Deep-seated underground waters(-100m or lower)and soil layers in the karst region of central Guizhou are characterized by high-F anomalies whereas shallow-level underground and surface waters by low contents of F(mostly lower than 05mg/L).Fluorine in soil and water in the region studied comes largely from Triassic marine strata dominated by gypsum-bearing carbonate rocks.The special adsorption and desorption of F on the surface of geothite in soil layers are the important mechanisms of its transport and enrichment in the rock-soil-water system of the karst region studied.     

Geological and Geochemical Characteristics and Petrogenesis of the Anqing—Lujiang Quartz Syenite Rock Belt,East China

The Anqing-Lujiang quartz syenite rock belt consists of the Huangmeijian, Chengshan and Dalongshan composite batholiths which intruded into the Mesozoic strata. The country rocks were subjected to thermal contact metamorphism with little sign of folding and regional metamorphism. The rock belt is Late Yenshanian in age with a Rb-Sr isochron age of 135 Ma. Major rock types are quartz syenite and, to a lesser extent, syenite porphyry and alkali feldspar granite. Rock-forming minerals are dominantly potash feldspar (more than 50%) and lesser amounts of plagioclase and quartz. Mafic minerals, mostly Mg-biotite with lesser amounts of amphibole and pyroxene, are rare. Occasionally, alkalic mafic minerals (aegirine, riebeckite) are found. Characteristic accessary minerals of the earlier intrusives are magnetite, sphene and apatite and those of the later intrusives are ilmenite and zircon. Typologic distribution and evolutionary trend of zircon population are very similar to those of granites of mantle origin as suggested by Pupin, J. P. Petrochemically, the rock belt is poor in Ca but rich in alkali and Al with Na₂O+K₂O > 10%. It belongs to the K-Na transitional series, with a high alkalinity ratio (A. R. = 3–7) and a K /Na (atom) ratio close to unity. Rocks in the belt are rich in REE which tends to decrease from the early to the late stage, belonging to LREE type. The initial Sr ratio is 0.7078–0.7064. The rock belt is the Mesozoic anorogenic product of alkalic magmatic activity from a deep-seated source. According to a mantle-crust mixing model for Sr and Pb isotopes, it is estimated that 60.2–53.8% of the materials has been derived from the mantle. Additionally, its rich alkali, poor water content and anorogenic characteristics suggest that the belt is similar to the A-type granites.     

The Middle Devonian Bimodal Association of Volcanic Rocks in the Northern Area of East Junggar, Xinjiang

The Middle Devonian volcanic rocks in the northern area of East Junggar, located between the Ertix andUlungur rivers of northern Xinjiang, may be divided into basic and acid ones. It is evident that a compositionalgap exists between the two groups so that the volcanic rocks are not in line with a calc-alkaline series becausethe intermediate rocks are absent in the area. The fact shows that the volcanic rocks are a typical bimodal asso-ciation. The formation of the bimodal association of volcanic rocks in the area was closely related to continen-tal rifting or continental extension in the Middle Devonian. In such a tectonic setting, magmas were first pro-duced by partial melting of the mantle. Where crustal thinning was greater, the magmas ascended and eruptedon the surface directly so that the basic volcanic rocks formed, but olivine and/or partial pyroxenefractionation occurred in the magmas during their ascent through the thinning crust. On the other hand, wherecrustal thinning was less, ascending mantle-derived magmas reached the lower crust and accumulated there, re-sulting in partial melting of the lower crust and thus giving rise to the contaminated magma which was consoli-dated as acid volcanic rocks on the surface.     

辽北-吉西南地区早白垩世富碱花岗岩地质特征

辽北-吉西南地区早白垩世富碱质花岗岩主要由碱长花岗岩、石英二长岩组成,同位素年龄为125 Ma左右,岩石化学以富碱、富硅为特征,具A型花岗岩特点,但铝质偏高,又具过铝质花岗岩的特点,应属于后造山期之PA型花岗岩。其构造背景为晚印支期-早燕山期滨太平洋造山带造山晚期坍塌阶段的产物。     

磐石西部地区三叠纪印支期花岗岩的基本特征

以往本区花岗岩多被确定为华力西晚期花岗岩或燕山期花岗岩、印支期花岗岩比较少见。本文以同位素年龄为依据。将本区以往确定的华力西晚期的板石花岗岩确定为晚印支期花岗岩,并详述了其基本特征和侵入期次。初步探讨了成因类型。     

东昆仑南坡埃坑北花岗斑岩的成岩年龄及其地质意义

东昆仑南坡埃坑北花岗斑岩位于青海省都兰县诺木洪镇南八宝滩以南、马尔争以北的埃坑德勒斯特沟中游一带埃坑铜钼矿带的北矿区,其大地构造位置属于东昆仑南坡俯冲增生杂岩带中生代弧后前陆盆地,是古特提斯北部活动陆缘的南部边缘区域,也是广义的东昆仑岩浆弧构造带的组成部分。该斑岩体是埃坑铜钼矿的围岩和成矿母岩,花岗斑岩体的LA-ICP-MS锆石U-Pb成岩年龄为242±1.2Ma,推测斑岩型铜钼矿成矿时代应为中三叠世—晚三叠世早期。这一成果对该带斑岩型铜钼矿的找矿勘查工作有一定指导意义。     

Postcambrian stratified rocks of the northern part of the Baikal-Vitim fold area

The Ordovician stratified rocks previously referred to Riphean or Vendian are widespread in the northern part of the Baikal-Vitim fold area. They include volcanosedimentary rocks which accumulated in continental riftlike troughs after Vendian-Cambrian platform regime and were accompanied by subvolcanic bodies. The coarse-clastic facies, frequently olistostromes with olistoplaques, are abundant in the marginal parts of the troughs. The sandy-silty-clay rocks dominate in the centre. The volcanic rocks represent the contrast rhyolite-basaltic series and compose paleovolcanoes of the central type, locally, complicated by calderas. Tuffaceous material is nearly almost present in synchronous sedimentary rocks. In the Ordovician, the northern part of the Baikal-Vitim fold area was an uplift of island arc type complicated by the riftlike troughs. It occupied the central part of the Baikal-Vitim arc. The location of the troughs was subjected to its structural plan. They were formed successively after the Riphean-Vendian Baikal-Muya volcano-plutonic belt. The uplift existed within a shallow basin occupying nearly the entire Siberian Platform and Baikal-Vitim fold area. The data obtained confirm that marine sedimentary basin with islands sometimes existed within the Baikal-Vitim fold area and was a part of the basin occupying the adjacent part of the Siberian Platform. The effect of pulsating plume in the northern part of the Baikal-Vitim fold area periodically, including Ordovician, led to the origination of the uplift and to renewal of tectonic and volcano-plutonic activity.     

东昆仑五龙沟花岗岩特征及其构造背景

根据岩石学、地球化学等特征及MC-ICP-MS锆石U-Pb定年,对东昆仑五龙沟地区花岗岩进行了构造环境研究。黄龙沟中粒正长花岗岩具有高硅、高碱、相对富铝、高FeOt/MgO和104Ga/Al值、富集轻稀土、明显的Eu负异常、相对原始地幔明显富集Zr、Ga、Y和Hf等高场强元素并强烈亏损Ba、Sr、P和Ti元素的特征,为A型花岗岩。锆石U-Pb法获得206Pb/238U值加权平均年龄为416.9±1.3 Ma,是早古生代晚志留世造山旋回末后造山碰撞产物,标志晚志留世东昆仑造山旋回进入造山后碰撞阶段。岩金沟矿区东花岗闪长岩为典型准铝质钙碱性花岗岩,具有I型花岗岩稀土元素特征,为I型花岗岩,形成年龄为258.9±2.1 Ma,属于昆南断裂结束后、于晚二叠世开始南区巴颜喀拉地体与昆中区南昆仑地体间发生强烈的壳幔剪切作用的产物。     

扬子北缘晚造山阶段弧形构造特征与盆地演化

扬子北缘晚造山阶段(即晚侏罗世—晚白垩世)发育以弧形构造为特征的前陆薄皮逆冲—褶皱构造,包括了沿秦岭—大别造山带发育的北西向的大洪山和大巴山弧形带,以及沿江南—雪峰造山带发育的北东向的川东—湘鄂西弧形带。详细的构造解析、盆地沉积及物源特征综合分析表明,弧形构造不仅将早期的前陆序列卷入变形,并且控制了晚侏罗世—晚白垩世的盆地演化和古地理格局。总结扬子北缘晚造山阶段的盆山演化特征,可以将其划分为3个阶段:(1)晚侏罗世—早白垩世早期,大洪山和大巴山弧形带的发育控制了四川盆地东北部及秭归盆地上侏罗统蓬莱镇组的沉积,川东—湘鄂西弧形带限制了盆地的东南边界,加之位于四川盆地西部的龙门山逆冲带,三面围限构成具前渊沉降的克拉通内盆地或称为“墙围盆地”(walled sedimentary basin);(2)早白垩世中期—早白垩世晚期,大洪山和大巴山弧形带的逆冲构造变形逐渐减弱,而川东—湘鄂西弧形带继续向北西扩展,构造线呈北东向展布,在弧形带前缘的宜昌地区形成沉积中心,并覆盖了现今的黄陵背斜;(3)晚白垩世,川东—湘鄂西弧形带继续向北西推进,构造线呈北北东向展布,弧形带北翼的黄陵背斜初始隆起,沉积中心分别位于北翼宜昌地区及南翼习水地区。与此同时,在弧形带内部薄皮构造的向斜部位形成楔顶沉积,发育如恩施盆地、黔江盆地、来凤盆地等一系列规模较小的背驼式盆地。     

中蒙边界地区成矿区带划分

中蒙边界地区成矿区带划分是近年来该区成矿规律研究中的重要科学问题之一。以板块构造-地球动力学理论为指导,在中蒙合作完成的该区1∶100万建造-构造图和成矿规律图的基础上,对中蒙边界地区成矿区带进行了统一厘定和划分。该区Ⅰ级成矿域隶属于古亚洲成矿域和滨太平洋成矿域。古亚洲成矿域可划分为阿尔泰、准噶尔-南蒙古和塔里木3个成矿省、12个成矿带和21个成矿亚带。滨太平洋成矿域划分出大兴安岭和华北陆块2个成矿省、4个成矿带和15个成矿亚带。研究提出,蒙古的欧玉陶勒盖-查干苏布尔嘎大型-超大型斑岩型矿床成矿亚带向西与中国的东天山-北山成矿亚带相连,为该区寻找同类型斑岩型矿床指明了方向。     

原地-半原地深熔花岗岩特征: 以华北克拉通北缘包头地区石榴花岗岩为例

在华北克拉通北缘大青山地区,广泛的深熔作用导致新太古代晚期石榴花岗岩发育。石榴花岗岩空间上与新太古代晚期大青山表壳岩（主要为石榴黑云母片麻岩）共生,渐变过渡。宏观上岩性具有不均一性,在包头哈德门沟一条实测地质剖面上可以观察到石榴混合闪长岩、石榴混合石英闪长岩和石榴混合花岗闪长岩等不同岩石类型。岩相学研究表明,石榴花岗岩与源岩矿物组合相近,但含量却大不相同。石榴花岗岩和源岩中的石榴石特征十分相似。石榴石周围存在放射状黑云母,是典型的逆反应结构。研究表明深熔作用可能发生在具有顺时针P-T轨迹的变质作用演化峰期及峰期后近等温降压阶段,主要发生钾长石熔融和黑云母熔融。地球化学特征研究表明石榴花岗岩与石榴黑云母片麻岩具有亲缘关系,均表现出高Al₂O₃、CaO含量,K₂O/Na₂O比值存在较大变化,富集轻稀土和大离子亲石元素,亏损Nb、Ta、P和Ti。但是,石榴花岗岩存在组成变化,Eu富集型和亏损型稀土模式为原地-半原地深熔花岗岩的标志性特征。SHRIMP锆石年代学研究表明深熔作用发生在2.43Ga之前,与区内新太古代晚期水平顺层滑脱变形有关,揭示华北克拉通北缘新太古代晚期构造热事件。

     

Early Proterozoic syn-and postcollision granites in the northern part of the Baikal fold area

Early Proterozoic granitoids are of a limited occurrence in the Baikal fold area being confined here exclusively to an arcuate belt delineating the outer contour of Baikalides, where rocks of the Early Precambrian basement are exposed. Geochronological and geochemical study of the Kevakta granite massif and Nichatka complex showed that their origin was related with different stages of geological evolution of the Baikal fold area that progressed in diverse geodynamic environments. The Nichatka complex of syncollision granites was emplaced 1908 ± 5 Ma ago, when the Aldan-Olekma microplate collided with the Nechera terrane. Granites of the Kevakta massif (1846 ± 8 Ma) belong to the South Siberian postcollision magmatic belt that developed since ～1.9 Ga during successive accretion of microplates, continental blocks and island arcs to the Siberian craton. In age and other characteristics, these granites sharply differ from granitoids of the Chuya complex they have been formerly attributed to. Accordingly, it is suggested to divide the former association of granitoids into the Chuya complex proper of diorite-granodiorite association ～2.02 Ga old (Neymark et al., 1998) with geochemical characteristics of island-arc granitoids and the Chuya-Kodar complex of postcollision S-type granitoids 1.85 Ga old. The Early Proterozoic evolution of the Baikal fold area and junction zone with Aldan shield lasted about 170 m.y. that is comparable with development periods of analogous structures in other regions of the world.