Table of Content

20 October 2020, Volume 26 Issue 5

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Petrogenesis of the Miocene Adakitic Rocks from Chabu of Xietongmen County in the Middle Segment of the Gangdese Belt, and Its Geological Significances

HONG Yufei，QIU Jiansheng*，WANG Ruiqiang，WEN Dingjun，XU Hang，LI Yanfang

2020, 26(5):  481-496.  DOI: 10.16108/j.issn1006-7493.2019109

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The Chuabu composite granitic pluton, with an outcrop area of about 400 km2, is located in the middle segment of the Gangdese belt. This paper presents comprehensive zircon U-Pb age, elemental geochemistry and Sr-Nd-Hf isotopic compositions for the hypabyssal biotite monzogranite porphyry and subvolcanic dacite porphyry within the pluton, aiming to elucidate their origin and to discuss their geological significances. Zircon LA-ICP-MS U-Pb dating yields crystallization ages of about 13-16 Ma for these rocks, indicating that they were generated during Miocene. Geochemically, these rocks are high-K calc-alkaline, metaluminous-weakly peraluminous, enriched in light rare earth elements and large ion lithophile elements (LILEs, e.g., Rb, Th and U), and depleted in high field strength elements (HFSEs, e. g., Nb, Ta and Ti). They also have low Cr, Ni contents, and show indistinct Eu negative anomalies. These rocks have high Sr, low Y and Yb concentrations, with high Sr/Y and (La/Yb)N ratios, suggesting that they can be grouped into adakitic rocks. Isotopically, these rocks have higher (87Sr/86Sr)i ratios (=0.706347-0.707584), negative εNd(t) values (=-4.15- -7.33), and with wide range of zircon εHf(t) values varying from negative to positive (=-5.99-7.78). The integrated petrology, and elemental and isotopic compositions suggest that during Miocene, the Indian ancient continental crust had been subducted underneath the Lhasa terrane. The Chabu adakitic rocks were formed under a post-collisional extensional setting, and were likely produced by mixing between magmas from partial melting of the thickened mafic juvenile crust and components from the subducted ancient Indian continental crust.

Studies on the Volcanic-Plutonic Connection

WANG Shuo，WANG Xiaolei，DU Dehong

2020, 26(5):  497-505.  DOI: 10.16108/j.issn1006-7493.2020014

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The relationship between volcanic and plutonic rocks has been debated for over a century. The controversy has been focused on the petrogenesis of intermediate to felsic rocks, with two main opinions proposed on the formation of silicic volcanic rocks: (1) they crystallized from the melt extracted from the “crystal mush” of granitic magma; and (2) they don’t have relations with the coeval plutonic rocks but have independent magma formation process. In recent years, with the rapid improvements of high-precision dating methods and the combination of other research methods such as geophysics, people have some new understandings on the genetic relationship between siliceous volcanic and plutonic rocks. The models of magma mush and transcrustal magmatic system have received much attention, but they also resulted in new contradictions and problems. This paper systematically summarizes the volcanic-plutonic connection in terms of different viewpoints, research history, and contradictions, and proposes some research areas for future research on the volcanic-intrusive connection. Generally, the overall geochemical similarities between siliceous volcanic and plutonic rocks displayed by big data do not rule out the existence of geochemical
differences that confined to “crystal mush” model in a specific area. At the end of the manuscript, it is pointed out that deep exploration, in-situ detailed geochemical analysis experiments, and modeling are expected to make break throughs in the future study of volcanic-plutonic connection. We suggest that the combinations with continental evolution and environmental changes may be important areas for future research on the volcanic-plutonic connection.

Genesis of Diorite Porphyry in the Shaxi Porphyry Cu (Au) Orefield

YUE Na，LIU Xin，ZHANG Qianming，TU Wenchuan，CUI Xiangjie，LU Xiancai

2020, 26(5):  506-519.  DOI: 10.16108/j.issn1006-7493.2019057

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The Shaxi Cu (Au) deposit is the largest porphyry copper deposit with proved resource reserve in the Middle-Lower Yangtze River Valley metallogenic belt till present. This paper discusses the genesis of the diorite porphyry by integrating the analysis of chronology, whole-rock chemical composition and isotopic geochemistry. The LA-ICP-MS zircon U-Pb dating results reveal that the diorite porphyry was the product of early cretaceous magmatic activities, with a weighted mean 206Pb/238Pb age of 128.3±1.5 Ma. The diorite porphyry is rich in alkali (ALK=6.12~7.53 wt%), K and magnesium (Mg#=38.99~51.53), and present metaluminous characteristic (A/CNK=0.90~0.99), which can be grouped into calc-alkalic-high-K calc-alkalic rock series. Light rare earth elements (LREEs) are enriched, heavy rare earth elements (HREEs) are depleted (LaN/YbN=12.63~17.63), and almost no Eu anormaly (δEu=0.84~1.14) is shown. In addition, an enrichment of large ion lithophile elements (LILEs) (e.g., Ba, Sr) as well as Pb, and a depletion of high field strength elements (HFSEs) (e.g., Nb, Ta, Ti) are observed. The diorite porphyry have (87Sr/86Sr)t values of 0.7051~0.7056, and εNd(t) values range of -6.09~-3.42; (206Pb/204Pb)t values are 17.51~18.16, (207Pb/204Pb)t values are 15.49~15.63, and (208Pb/204Pb)t values are 37.57~38.45, which indicate the rock-forming materials were derived from melted crust with involvement of mantle materials. Around 130 Ma, the stretching of the translational Tanlu fault caused large-scale magmatism. The magma from the deep intruded along the Tanlu fault belt and experienced contamination of considerable amount of crustal materials, finally the magma emplaced at relative shallow depth and resulted in the formation of the Shaxi diorite porphyry and quartz diorite.

On the Origin of Dolomite in Laustrine Sediments of the Borehole SG-1 in Western Qaidam Basin

WANG Xiaoxiao，HAN Zuozhen，LI Minghui，FANG Xiaomin，ZHAO Yanyang

2020, 26(5):  520-529.  DOI: 10.16108/j.issn1006-7493.2019061

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Since dolomite cannot be synthetized in inorganic conditions, the cause of formation of dolomite has long been a disputed topic. A 938 m-long borehole, located in Western Qaidam Basin, contains plenty of dolomite and ankerite, which were found in the intervals of 938-438 m and 938-520 m, respectively. This study is focused on investigating the formation of dolomite and ankerite based on the evolution of the paleolake. As an evaporative mineral, dolomite cannot be synthetized without microbial activities. Therefore, evaporation and microbial catalysis are necessary for the formation of dolomite. Evaporation makes Mg2+ become highly concentrated and microbial catalysis helps Mg2+ enter into the structure of CaCO3 to form dolomite crystals. As carbonate minerals are generally precipitated in the early stage of a lake’s evolution, the contents of dolomite in the Core SG-1 in brackish conditions are higher than those in salty conditions. However, ankerite, which is a type of dolomite, has different forming conditions. Fe2+ can be from deep hydrothermal fluid and was released during the transformation process between clays. Fe2+ is participated into the structure of dolomite to form ankerite, which is mainly finished in high temperatures and inorganic conditions with minor or without microbial activities.

Heavy Mineral Assemblages of Jurassic-Paleogene Sandstones in Southern Tibet: Implications for Provenance Interpretations of Magmatic Arc and Continental Block

2020, 26(5):  530-539.  DOI: 10.16108/j.issn1006-7493.2019074

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Provenance analysis is the key of paleogeographic reconstruction and basin analysis, whose typical end-members include “magmatic arc provenance”, “continental block probvenance”, and “recycled orogen provenance”. Kinds of heavy minerals contain a wealth of parent rock information, which are widely used for provenance analysis. Modern sand researches have shown there is a link between detrital heavy mineral assemblages and their geodynamic backgrounds. Unfortunately, this law cannot be directly applied to ancient sandstone because of the diagenesis. We know little about the heavy mineral assemblages of sandstones sourced from magmatic arc and continental block. The Jurassic-Paleogene Xigaze forearc basin and Tethys Himalayan sandstones in Tibet are either from Gangdese arc of the Asian continent or from Indian continental block, which is an excellent lab to study the heavy mineral assemblages of ancient sandstones derived from magmatic arc and continental block. The quantitative analyses result of 16 sandstones shows that: 1) heavy minerals are seriously modified by diagenesis, unstable minerals (e.g. amphibole, pyroxene) are disappeared while authigenic growth of new minerals such as epidote is common; 2) sandstones derived from magmatic arc are mainly composed of epidote or apatite, with ZTR<40; while sandstones derived from continental block are mainly composed of zircon, tourmaline and rutile, with ZTR>75. The sandstones sourced from magmatic arc and continental block yield distinctive heavy mineral assemblages, which make sense for provenance interpretation.

Separation and Detection Methods of Organic Acids with Different Properties in Source Rocks

DU Jiazong，HE Bei，CAI Jingong，LEI Tianzhu，LIU Qing，XIE Zhonghuai

2020, 26(5):  540-548.  DOI: 10.16108/j.issn1006-7493.2019075

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Organic acids in source rocks are derived from organic matters. The different occurrence forms of organic matter and various properties of organic acid can affect the relationship between organic acids and organic matters, and further affect the separation and detection methods of organic acid. On the basis of summarizing the occurrence of organic matter and the properties of organic acid, it is proposed that organic acids in source rocks seldom exist in a single form, but have three forms, i.e., watersoluble, fat-soluble and insoluble. The free organic matters contain water-soluble and fat-soluble organic acids, while in mineralcombined organic matters, there are water-soluble, fat-soluble and insoluble organic acids. Thus, it is necessary to take stepwise separation methods to separate organic acids of different properties in free organic matters and mineral-combined organic matters. The appropriate detection methods for organic acids were selected according to their properties. For water-soluble organic acids, ion chromatography is a proper method. The fat-soluble organic acids, which were obtained by Soxhlet extraction using dichloromethane, were detected by gas chromatography-mass spectrometry. The insoluble organic acids were obtained by
acidolysis of the extraction residue and detected using infrared spectrum. The types and contents of organic acids with different properties can be comprehensively and systematically understood using these detection methods. The successful separation and detection of different organic acids are of great significance to further investigate the characteristics and evolution of various types of organic acids in source rocks, and deepen the understanding of mechanisms of hydrocarbon generation and organic-inorganic interactions in source rocks.

Sequence Stratigraphy of Evaporites-bearing Ordovician Majiagou Formation in the Northern Shaanxi Saline Basin and Its Geological Significance

WANG Tianyun，ZHANG Yongsheng, ZHANG Jianjun,SUN Jiachuan，DING Qingxiang，LI Tao，HOU Yan，GUI Baoling

2020, 26(5):  549-558.  DOI: 10.16108/j.issn1006-7493.2019062

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The middle and deep evaporites-bearing strata in the Ordos Basin are highlighted for exploration and development of continental salt resources in China. On the basis of previous studies, the sequence stratigraphy of the middle Ordovician Majiagou Formation was studied by using seismic data, well logging data and Vail classical sequence stratigraphy theory. Tectonic movement and sea level change, and well seismic combination technology were adopted in the study. The development controls of the evaporite series stratigraphy were further explored. Results show that the Majiagou Formation is divided into 1 second-order sequence, 3 third-order sequences and 8 fourth-order sequences. Based on the study of regional tectonic features and sedimentary facies, and in combination with the distribution of evaportie strata of the Majiagou Formation, it is considered that the second order sequence (structural sequence) and the third order sequence (sedimentary sequence) jointly affect the planar and vertical development of evaporite strata. Sedimentation (third order sequence) controls the formation of the original salt bodies. Tectonism (second order sequence) controls the formation of the original salt basin, and destorys, transforms and regulates the formation of the salt strata. The study of the sequence stratigraphy development and geological significance of evaporites-bearing strata can provide direction and basis for further exploration and development of middle and deep salt deposits in marine basins.

Inversion of the Dynamic Gas Content of the Coal Reservoir during Stable Production in the Dafosi Area Based on “Constant Volume Method”

LIU Qiaoni，LIU Yuhui

2020, 26(5):  559-568.  DOI: 10.16108/j.issn1006-7493.2019083

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The gas content of coal reservoir is one of the key factors affecting gas production, which cannot be achieved directly for most gas wells, and the gas content of low-rank coal measured by desorption method is not accurate. The purpose of this study is to determine the dynamic variation of gas content of the coal reservoir in the Dafosi coal field by synchronizing feedback of coalbed methane (CBM) well drainage and production data to coal reservoir parameter dynamics and by studying the real-time gas content inversion through “constant volume method” using the production data of CBM wells. The results show that (1) for multiple original gas contents, qA decreases linearly with time and the descending trends are the same. Also, the real-time gas content changes with the same linear slope: gas production and gas consumption are synchronized, regardless of the production time interval. (2) Analyzing different time steps of 1 d, 3 d, and 5 d, and setting original gas content of 2 m3/t, 3 m3/t, 4 m3/t, 5 m3/t, 6 m3/t, and 8 m3/t respectively, the real-time gas content change relationship of coal reservoir is highly consistent. It is considered that CBM wells follow the law of “constant volume” gas production, that is, there is no formation and expansion of pressure drop funnel.（3）In the continuous drainage stage, the real-time gas content decreases linearly with the drainage time. The linear reduction rates of real-time gas content in coal reservoirs are different before and after the interruption of drainage and production: 0.00546 and -0.00435, respectively; the slope of real-time gas content change in the second stage is smaller than that in the first stage, which is interpreted to result from coal powder that was produced in the process of drainage and mining, blocking the desorption of lump coal, causing reservoir damage and reducing the volume of coal seam to be desorbed.

Geological Features and Tectonic Implications of the Paleozoic Granitoids in the Southern Yili Block

ZHOU Yong，WANG Bo，ZHONG Linglin，ZHU Xiaoyan

2020, 26(5):  569-584.  DOI: 10.16108/j.issn1006-7493.2019049

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Granitoids are well developed in the southern Yili Block intruding the Proterozoic metamorphic rocks of the Tekes Group. Their ages and tectonic settings are poorly constrained. Our field investigations indicate that the granitoids are composed of weakly foliated biotite granites, strongly foliated two mica granites and undeformed biotite granites. Whole-rock geochemical and zircon U-Pb geochronological analyses revealed that the weakly foliated biotite granites and strongly foliated two mica granites belong to peraluminous calc-alkaline series and formed during 438 Ma and 426 Ma, respectively. Whereas, the undeformed biotite
granites emplaced during 400-380 Ma and have geochemical features similar to typical calc-alkaline series, showing enrichment in the large ion lithophile elements and depletion of the high field strength elements. Combined with regional geological data and previous results, we propose that the Paleozoic granitoids in the study area resulted from two episodes of magmatism related to different tectonic settings, namely, the Early Paleozoic peraluminous calc-alkaline, weakly foliated biotite granites and strongly foliated two mica granites were generated in the post-orogenic setting during the amalgamation of the Kazakhstan microcontinent, and the mid-late Paleozoic calc-alkaline, undeformed biotite granites were produced in an active margin related to subduction of
the Junggar Ocean.

Physical Modelling of Cretaceous Salt Rafts in the Lower Congo Basin

CHENG Peng，LI Jianghai，ZHANG Yu，WANG Dianju，LIU zhiqiang

2020, 26(5):  585-591.  DOI: 10.16108/j.issn1006-7493.2019067

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As an extreme structural form of thin-skined extension under gravitational gliding, salt rafts are widely developed in the salt-bearing basins on a passive margin, which exerts important impacts on the evolution of the basin after the rifting period. Based on physical modelling, the combined effects of basement angle and syn-kinematic deposition rate are studied. In this study, the initial model setting and actual geological conditions are emphasized in addition to considering the single factor effects of salt layer thickness, base angle, and thickness of pre-kinematic sedimentary layer, making the results more meaningful. The results show
that the Cretaceous salt raft activity in the Congo Basin was caused by gravity gliding rather than differential loading; the most critical controlling factors of the salt raft structures are the basement angle and the syn-kinematic deposition rate; the basal angle controls the fracture distribution and rafts gliding rates; while the syn-kinematic deposition rate controls the pattern of deposition between the rafts during the formation process. The relative speed of the two controls the structural styles of the salt rafts. The Cretaceous salt rafts in the Lower Congo Basin was a result of the relatively fast depositional conditions.

Climate Studies since 6.2-1.3 cal ka BP Recorded by Palynological Records of Lacustrine-Marsh Facies Sediments in the Toushe Basin, Taiwan

2020, 26(5):  592-600.  DOI: 10.16108/j.issn1006-7493.2019063

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Toushe Basin is located in the central part of Taiwan Province. It is the forefront of the East Asian monsoon area in China. It is very sensitive to the response of the East Asian monsoon. It is of great scientific significance to study the paleovegetation and reconstruct the paleoclimate since the Mid-Late Holocene, and to explore the relationship between the climate and the East Asian summer monsoon. Based on the AMS14C dating and volumetric magnetic susceptibility measurements of peat-lake sediments in Toushe Basin, the vegetation changes since the Mid-Late Holocene were reconstructed and paleoclimate was reconstructed on the basis of Palynologyrecords. The results show that there are six stages since the middle and late Holocene: 6.2-6.0 cal ka BP, the climate was cool and dry, the vegetation type was subtropical evergreen broad-leaved forest; 6.0-4.0 cal ka BP, the climate was warm and humid, the vegetation type was subtropical evergreen broad-leaved forest with more tropical components; 4.0-2.2 cal ka BP, the climate was relatively cool and dry, and the vegetation type turns to subtropical evergreen broad-leaved forest; 2.2-1.9 cal ka BP, climate was warm and humid; 1.9-1.7 cal ka BP, climate was drying rapidly, forest degradation; 1.7-1.3cal ka BP, climate is warm and humid again, vegetation type is similar to modern marsh grassland. As a whole, the climate change of 6.2 to 2.2 cal ka BP is a trend from warm and humid to warm and cool and dry. This is caused by the decrease of solar radiation since activities the middle and late Holocene. The decrease of solar radiation leads to the southward of ITCZ, which leads to the weakening of the East Asian summer monsoon. After 2.2 cal ka BP, the climate fluctuates greatly, possibly caused by weakening of the summer monsoon, coupling with ENSO activities and human activities were  trengthened.