Cenozoic paleo-environmental evolution of the Pamir–Tien Shan convergence zone

The retreat of the Tethys Sea and the uplift of the Tibetan Plateau play the critical roles in driving Asian climatic changes during the Cenozoic. In the Pamir–Tien Shan convergence zone, over 3000 m of Cenozoic successions, consisting of marine deposits in the lower, continental clay and fine sand in the middle, and molasse in the upper part, record the evolution of the Tethys Sea, the Asian aridification, and the deformation of the Pamir. In this work, the existing biostratigraphic subdivisions and new electronic spinning resonance dating results were used to assign ages to formations within the Ulugqat section. Sedimentary facies analysis and multi-proxy indices were used to reconstruct the paleo-environmental evolution. The results show: (1) the Pamir–Tien Shan convergence zone has undergone progressive environmental changes from shallow marine before ∼34 Ma to arid land at ∼23 Ma and finally to inter-mountain basin by ∼5.3 Ma; (2) the overall increase in mean size of grains, decrease in redness, in magnetic susceptibility, and in proportion of the ultrafine component of the sediments studied revealed a long-term strengthening in potential energy to transporting medium, cooling, and enhanced continental aridity, respectively; (3) the easternmost edge of the Tethys Sea prevailed in the western Tarim Basin from late Cretaceous to early Cenozoic, and finally retreated from this region around the Eocene–Oligocene transition, which in turn strengthened the Asian aridification; (4) accumulation of molasse with an upper age of ∼1 Ma suggests that the deformation front of the Pamir migrated to this area at or before that time.     

东疆红山Cu-Au矿床氧化带硫酸盐矿物穆斯堡尔谱特征及其意义

穆斯堡尔谱对确定铁离子占位、核外环境及氧化态方面有着独特的优势。在红山铜金矿床氧化带硫酸盐矿物的XRD、TA、湿法化学分析和红外光谱测试的基础上,测定了板铁矾、针绿矾等8种硫酸盐矿物的室温57Fe穆斯堡尔谱,并根据常温下硫酸盐矿物穆斯堡尔谱参数和其晶体结构中Fe3+和Fe2+的占据位置对其谱峰进行了指派。结果表明本矿床氧化带硫酸盐矿物的穆斯堡尔谱的同质异能位移较小、四级矩分裂值分布范围较大、无磁超精细分裂等特征,且硫酸盐矿物结构中存在着共价键。通过与青海锡铁山铅锌矿氧化带硫酸盐矿物的穆斯堡尔谱相比较,两者在近地表风化及氧化过程中所处的物理化学条件基本相同,酸性和氧化性的环境为硫酸盐矿物的产生和保存提供了良好条件,但红山矿床更为干旱少雨,导致两者硫酸盐的穆斯堡尔谱参数略有不同。     

《北京西山地质志》中、英文本比较研究

依据文本对勘的方法,从图片和文字两个方面对《北京西山地质志》中、英文本进行比较研究。图片方面,英文本作了大量的增补和解说,内容更为详实准确。文字方面,中、英文本目录存在明显差异。英文本序言和注释有其独到的价值和意义。中文本正文的细节增补,应是吸收了当时的最新研究成果。中、英文本比较研究为认识《北京西山地质志》的价值与地位提供了准确可靠的学术依据。     

西天山卡特巴阿苏金矿床成矿年代学及其地质意义-来自绢云母~(40)Ar-~(39)Ar同位素年龄证据

新疆卡特巴阿苏金矿床是近年来在西天山地区新发现的一个大型金矿床。该矿床大地构造位置处于塔里木板块和哈萨克斯坦-准噶尔板块结合部位的那拉提构造-岩浆岩带,其金矿体主要赋存于蚀变二长花岗岩内。为了确定金矿化的形成时代,对该矿床主成矿阶段与金矿化密切相关的蚀变绢云母开展了40Ar-39Ar同位素定年测试。绢云母40Ar-39Ar坪年龄为(268.56±1.8)Ma,正等时线年龄为(268.38±2.2)Ma,反等时线年龄为(268.52±2.2 Ma),三者在误差范围内一致,测试结果可信,可代表卡特巴阿苏金矿化的形成年龄,这也是首次对该矿床成矿年龄的精确限定。对比中亚造山带典型金矿床,卡特巴阿苏金矿床是西天山二叠纪后碰撞构造环境下岩浆流体活动的产物,矿床成因属与中温岩浆热液有关的构造蚀变岩型矿床。     

龙门山晚新生代地表剥蚀量的定量估算

龙门山是青藏高原周边山脉中地形梯度变化最大的山脉.利用数字高程模型(digital elevation models, DEM),采用三维残余面法恢复龙门山晚新生代古残余面DEM,并与现代地形面做差值运算,得到研究区域的剥蚀量地形,进而定量估算青衣江、岷江、沱江和涪江主要水系流域晚新生代的地表剥蚀量.结果表明:龙门山晚新生代地表剥蚀总量为80 500~92 800 km3;岷江流域对龙门山地区剥蚀量贡献率约33.9%~37.1%,其次为涪江(33.6%~38.4%)、青衣江(24.1%~31.9%),沱江流域贡献率为0.4%~0.6%;类似2008年“5·12”汶川地震的次生灾害引发的地表快速剥蚀,是青藏高原东缘龙门山造山带晚新生代地表剥蚀的主要原因.      

断裂熔融作用中单质铁的形成及其指示的孕震环境

目前缺乏来自于地震断裂带的假玄武玻璃的原始特征研究,制约着我们对地震孕震环境和地震发生机制的认识。本文以龙门山断裂带汶川科钻2号钻孔岩心的碎裂岩及花岗闪长岩为研究对象,通过氩气环境下的高温加热实验(最高温度达1750℃)、显微结构观察、地球化学分析和岩石磁学测试,探讨断裂熔融作用中单质铁的形成及其指示意义。花岗闪长岩和碎裂岩在1100℃时发生部分熔融作用,形成了非晶质和微晶;在≥1300℃时形成了大量单质铁组成的微球粒,可能是含铁矿物发生了以碳物质为还原剂的高温还原反应。高温实验后样品的磁化率值高于碎裂岩的磁化率值,新生成的磁铁矿是≤1100℃的样品高磁化率值的主要原因;熔融作用中形成的单质铁和少量的磁铁矿是≥1300℃样品高磁化率值的重要原因。结合前人对于快速摩擦实验熔体、地幔物质及陨石的研究,我们认为粘土矿物、硅酸盐矿物含量较多的断层泥在同震滑移中因热增压机制很难形成单质铁;而花岗岩、安山岩、辉长岩、闪长岩等岩浆岩和糜棱岩在流体作用弱和硫化物含量低的环境中易在大地震中形成单质铁。龙门山断裂带的假玄武玻璃的熔融温度≥1300℃,硫元素含量较低并发现有过剩铁元素。因此,表明龙门山断裂带的假玄武玻璃中可能形成了单质铁,其和磁铁矿是假玄武玻璃高磁化率值异常的重要原因,并指示了深部流体作用弱、硫化物含量低、还原性的孕震环境。     

Tectonometamorphic evolution of the Atbashi high‐P units (Kyrgyz CAOB,Tien Shan): Implications for the closure of the Turkestan Ocean and continental subduction–exhumation of the South Kazakh continental margin

The South Tien Shan (STS) belt results from the last collision event in the western Central Asian Orogenic Belt (CAOB). Understanding its formation is of prime importance in the general framework of the CAOB. The Atbashi Range preserves high‐P (HP) rocks along the STS suture, but still, its global metamorphic evolution remains poorly constrained. Several HP units have been identified: (a) a HP tectonic mélange including boudins of mafic eclogites in a sedimentary matrix, (b) a large (>100 km long) high‐P metasedimentary unit (HPMU) and (c) a lower blueschist facies accretionary prism. Raman Spectroscopy on carbonaceous material combined with phengite and chlorite multiequilibria and isochemical phase diagram modelling indicates that the HPMU recorded homogeneous P–T conditions of 23–25 kbar and 560–570°C along the whole unit. ⁴⁰Ar/³⁹Ar dating on phengite from the HPMU ranges between 328 and 319 Ma at regional scale. These ages are interpreted as (re‐) crystallization ages of phengite during T_max conditions at a pressure range of 20–25 kbar. Thermobarometry on samples from the HP tectonic mélange provides similar metamorphic peak conditions. Thermobarometry on the blueschist to lower greenschist facies accretionary prism indicates that it underwent P–T conditions of 5–6 kbar and 290–340°C, highlighting a 17–20 kbar pressure gap between the HPMU‐tectonic mélange units and the accretionary prism. Comparison with available geochronological data suggests a very short time span between the prograde path (340 Ma), HP metamorphic peak (330 Ma), the T_max (328–319 Ma) and the final exhumation of the HPMU (303–295 Ma). Extrusion of the HPMU, accommodated by a basal thrust and an upper detachment, was driven by buoyant forces from 70–75 km up to 60 km depth, which directly followed continental subduction and detachment of the HPMU. At crustal depths, extrusion was controlled by collisional tectonics up to shallow levels. Lithological homogeneity of the HPMU and its continental‐derived character from the North Tien Shan suggest this unit corresponds to the hyper‐extended continental margin of the Kazakh continent, subducted southward below the north continental active margin of the Tarim craton. Integration of the available geological data allows us to propose a general geodynamic scenario for Tien Shan during the Carboniferous with a combination of (a) N‐dipping subduction below the Kazakh margin of Middle Tien Shan until 390–340 Ma and (b) S‐dipping subduction of remaining Turkestan marginal basins between 340 and 320 Ma.     

Influence of dissolution/reprecipitation reactions on metamorphic greenschist to amphibolite facies mica 40Ar/39Ar ages in the Longmen Shan (eastern Tibet)

Linking ages to metamorphic stages in rocks that have experienced low‐ to medium‐grade metamorphism can be particularly tricky due to the rarity of index minerals and the preservation of mineral or compositional relicts. The timing of metamorphism and the Mesozoic exhumation of the metasedimentary units and crystalline basement that form the internal part of the Longmen Shan (eastern Tibet, Sichuan, China), are, for these reasons, still largely unconstrained, but crucial for understanding the regional tectonic evolution of eastern Tibet. In situ core‐rim ⁴⁰Ar/³⁹Ar biotite and U–Th/Pb allanite data show that amphibolite facies conditions (~10–11 kbar, 530°C to 6–7 kbar, 580°C) were reached at 210–180 Ma and that biotite records crystallization, rather than cooling, ages. These conditions are mainly recorded in the metasedimentary cover. The ⁴⁰Ar/³⁹Ar ages obtained from matrix muscovite that partially re‐equilibrated during the post peak‐P metamorphic history comprise a mixture of ages between that of early prograde muscovite relicts and the timing of late muscovite recrystallization at c. 140–120 Ma. This event marks a previously poorly documented greenschist facies metamorphic overprint. This latest stage is also recorded in the crystalline basement, and defines the timing of the greenschist overprint (7 ± 1 kbar, 370 ± 35°C). Numerical models of Ar diffusion show that the difference between ⁴⁰Ar/³⁹Ar biotite and muscovite ages cannot be explained by a slow and protracted cooling in an open system. The model and petrological results rather suggest that biotite and muscovite experienced different Ar retention and resetting histories. The Ar record in mica of the studied low‐ to medium‐grade rocks seems to be mainly controlled by dissolution–reprecipitation processes rather than by diffusive loss, and by different microstructural positions in the sample. Together, our data show that the metasedimentary cover was thickened and cooled independently from the basement prior to c. 140 Ma (with a relatively fast cooling at 4.5 ± 0.5°C/Ma between 185 and 140 Ma). Since the Lower Cretaceous, the metasedimentary cover and the crystalline basement experienced a coherent history during which both were partially exhumed. The Mesozoic history of the Eastern border of the Tibetan plateau is therefore complex and polyphase, and the basement was actively involved at least since the Early Cretaceous, changing our perspective on the contribution of the Cenozoic geology.     

走廊南山河流纵剖面高海拔裂点的成因

河流水力侵蚀物理模型表明基岩河道纵剖面在均衡状态时表现为平滑上凹的形态,其特征反映了构造、基岩抗侵蚀能力和气候的作用;然而自然界河道纵剖面多呈现以裂点为特征的不均衡形态,不均衡的剖面形态以及裂点的研究同样可以对外力作用的变化起到很好的指示作用。位于北祁连的走廊南山高海拔河道纵剖面普遍呈现不均衡形式且发育海拔较高的裂点。通过对裂点成因分析发现,这些裂点并不主要受控于岩性、气候、构造等因素,而反映了冰川作用遗留地形与河流地形的分界。这一结果说明在对河道纵剖面高海拔裂点进行分析时要考虑到古冰川遗留地形也会对现代河道纵剖面产生重要影响,为进一步认识和理解造山带地貌演化以及控制因素提供了思路。     

双峰山金矿床地质特征及成因探讨

双峰山金矿产于双峰山背斜核部下石炭统巴塔玛依内山组第二亚组陆相火山岩内。即金矿化赋存于次生石英岩及蚀变安山岩中。金矿化与硅化、黄铁矿化关系密切,矿床具有Ａｕ、Ａｇ、Ａｓ、Ｓｂ、Ｈｇ等元素组合异常,表现出浅成低温热液矿化蚀变特征。提出了双峰山金矿为浅成低温热液型冰长石—绢云母金矿亚型矿床的成因认识。