Belt Of Rocks Research Articles

Geochronology, geochemistry and oxidation state of the Dongyuan biotite granite in the Jiangnan tungsten ore belt: Implications for the I-type granite-related W mineralization

The Dongyuan deposit, located in the southern Anhui Province, China, is a large porphyry W–(Mo) deposit in the Jiangnan tungsten ore belt. Ore minerals are dominated by scheelite and molybdenite, which occur as disseminated or veined in biotite granite. Zircon U–Pb dating of biotite granite is 143.5 ± 0.7 Ma, which is consistent with formation age of regional tungsten mineralization. Chemical compositions of biotites from biotite granite plot near or below Ni–NiO buffer on a Fe3+–Fe2+–Mg2+diagram, suggesting that the Dongyuan biotite granite has a relatively low oxygen fugacity. The biotite granite samples are characterized by high SiO2 (68.78–70.98 wt%), Al2O3 (14.60–15.32 wt%), K2O (3.45–4.82 wt%) contents and K2O/Na2O ratios (0.98–1.59), with A/CNK ranging from 0.97 to 1.02, belonging to I-type granite. They show adakite-like geochemical features, with high Sr/Y (35.98–57.90) and (La/Yb)N (21.41–38.78) ratios but low Y (5.54–6.77 ppm) and Yb (0.44–0.55 ppm) contents, which have low MgO (0.60–0.77 wt%), Cr (15.39–28.52 ppm) and Ni (3.68–5.66 ppm) contents and Mg# (31–35) values. These samples have high initial 87Sr/86Sr ratios (0.7144–0.7160) and negative ƐNd(t) values (−9.1 to −5.5), with two-stage Nd model ages (TDM2) of 1379–1695 Ma, while the zircon ƐHf(t) values and two-stage Hf model ages (TC DM) are −6.4 to −0.7 and 1243 to 1601 Ma, respectively. These data suggest that the Dongyuan biotite granite was derived from partial melting of a thickened crustal source which had been previously produced by Neoproterozoic magmatic underplating. Combined with previously published data, we propose that the W mineralization of the Dongyuan deposit was controlled by metal-rich source region, fractional crystallization, low oxygen fugacity condition, and enrichment of fluorine volatiles, which also influenced other I-type granite-related W deposits in the Jiangnan tungsten ore belt. Furthermore, the W-related granitic rocks in this tungsten ore belt developed in a compression–extension transitional setting that was triggered by the subducted Paleo-Pacific slab roll-back.

Himalayan Miocene adakitic rocks, a case study of the Mayum pluton: Insights into geodynamic processes within the subducted Indian continental lithosphere and Himalayan mid-Miocene tectonic regime transition

AbstractThe Himalayan Orogen is a typical continental collision orogenic belt that formed during India-Asia collision. The orogeny involved a transition in tectonic regime, which led to E-W–trending extension during the middle Miocene. At the same time, widespread post-collisional adakitic magmatism occurred in southern Tibet, particularly in the Lhasa Terrane, and these rocks provide a valuable record of the collisional geodynamic processes within the lithosphere. Few studies have focused on the middle Miocene adakitic rocks of the Himalayan Orogen, so further research is required to constrain their origin and geodynamics. This study presents new geochronological and geochemical data, including the whole rock Sr-Nd and zircon Hf isotopic compositions from the Mayum pluton in the Mayum-Gong Tso area, Northern Himalaya. Zircon U-Pb ages show that the Mayum granodiorite porphyries crystallized at 16.67 ± 0.14 Ma (mean square weighted deviation [MSWD] = 0.82, n = 29) and 16.68 ± 0.16 Ma (MSWD = 1.13, n = 28), indicating that they formed during the middle Miocene. The major and trace element characteristics are as follows: SiO2 = 65.79–67.31 wt%, Al2O3 = 15.28–16.00 wt%, MgO = 1.77–1.89 wt%, Y = 12.0–13.5 ppm, Yb = 0.11–0.99 ppm, Sr = 719–822 ppm, (La/Yb)N = 21.89–27.02, Sr/Y = 56.1–65.5, and the rocks have weak negative Eu anomalies (Eu/Eu* = 0.76–0.85), indicating that they are adakitic rocks and show high Sr-Ba granite affinity. The Mayum granodiorite porphyries have high K2O contents (3.42–3.65 wt%), Na2O (3.76–4.04 wt%), and K2O/Na2O ratios of 0.91–0.95, indicating a high-K calc-alkaline affinity. All samples are enriched in large-ion lithophile elements and depleted in high field strength elements. The initial 87Sr/86Sr ratios are 0.709262–0.709327 and εNd(t) values are −6.36 to −7.07, which correspond to two-stage Nd model ages [TDM2(Nd)] of 1405–1348 Ma. In situ zircon Lu-Hf isotopic compositions are variable, with 176Hf/177Hf ratios of 0.2823845–0.282824, εHf(t) values of −13.37 to +2.17, and two-stage Hf model ages [TDM2(Hf)] of 1704–841 Ma. These geochemical and Sr-Nd-Hf isotopic characteristics indicate that the Mayum granodiorite porphyries may be derived from partial melting of the subducted Indian thickened ancient mafic lower crust (∼92%) mixed with depleted mid-ocean ridge basalt mantle material input (∼8%) slightly. Crust-mantle interaction was induced by asthenospheric upwelling followed by the underplating of the Himalayan lower crust beneath Southern Tibet during the middle Miocene in response to significant changes in the geodynamics of the India-Asia collisional orogen. These deep geodynamic processes reflect the break-off or rollback of the subducted Indian continental slab, which caused asthenospheric upwelling, the input of juvenile heat, and the addition of depleted mantle material. This study indicates that the middle Miocene Himalayan adakitic rocks, which include the Miocene Yardoi two-mica granite, Gyangzê granite porphyry dike, Bendui two-mica granite, Langkazi enclave, Kuday dacitic dike, Lasa pluton, and Mayum pluton, form a belt of adakitic rocks in the Northern Himalaya to the south of the Indus-Tsangpo Suture Zone. These adakitic rocks have similar temporal and spatial distributions, geochemical features, and Sr-Nd isotopic compositions, indicating that their petrogenesis and geodynamic settings were similar. At that time there was widespread initiation of N-S–trending rifting, exhumation of central Himalayan eclogites, and large changes in the compositions of Himalayan leucogranites in the Himalayan Orogen. These can be attributed to significant changes in the characteristics of the subducted Indian continental lithosphere during the middle Miocene. The middle Miocene Himalayan adakitic rocks provide valuable insights into this transition in the tectonic regime and deep geodynamic processes.

High-resolution lithospheric structures of the Qinling-Dabie orogenic belt: Implications for deep subduction and delamination of continental lithosphere

The Qinling-Dabie orogenic belt was formed during the collision between the North China Craton and the Yangtze Block. A large volume of the high-pressure (HP) and ultra-high-pressure (UHP) metamorphic rocks are exposed in the eastern Dabie Mountains. In order to better understand the collision processes and the exhumation mechanism of the HP/UHP rocks, we process ambient noise data recorded at stations in the Qinling-Dabie orogenic belt and surrounding areas to extract broadband Rayleigh wave dispersion curves at 8–95 s periods. Using these dispersion curves, we construct a high-resolution 3-D shear wave velocity model of the Qinling-Dabie orogenic belt and surrounding regions. Our 3-D model reveals high velocities in the uppermost mantle beneath the western Dabie Mountains and also beneath the western Qinling Mountains, suggesting the presence of remnant continental lithosphere from the past continental subduction. Low velocities in the lower crust and uppermost mantle beneath the eastern Qinling and eastern Dabie are imaged, suggesting the continental lithosphere delaminated in the past. Our 3-D model supports a two-stage exhumation model of the HP/UHP rocks in the eastern Dabie orogenic belt, in which HP/UHP rocks were first exhumed to the mid/lower crust in the first stage and were then further exhumed to the surface under the driving force of the upper mantle upwelling induced by the lithosphere delamination.

Petrogenesis and tectonic setting of Mid‐Triassic volcanic rocks in the East Kunlun orogenic belt, NW China: Insights from geochemistry, zircon U–Pb dating, and Hf isotopes

The Elashan magmatic belt is located at the eastern margin of the East Kunlun orogenic belt (E‐KOB) in northwestern China, where voluminous magmatism occurred during the Late Permian to Late Triassic. Mid‐Triassic volcanism produced the Xilikete Formation, which shows a geochemistry similar to that of highly fractionated I‐type (HFI) volcanic rocks. These rocks are characterized by high SiO2 (73.82–74.97 wt%), differentiation index (DI; 91.01–94.69), and alkalis (e.g., high K2O + Na2O, with K2O/Na2O ratios greater than 1.0) and by enrichment in some large‐ ion lithophile elements (LILEs; e.g., Rb, K, and Pb) and depletion in other LILEs (e.g., Ba and Sr) as well as some high‐field‐strength elements (HFSEs; e.g., Nb, Ta, and Ti); these features confirm that they are HFI volcanic rocks. Zircon U–Pb ages indicate that the volcanic rocks were emplaced at ~239 Ma. These zircons have εHf(t) values ranging from −2.26 to +0.36, with two‐stage model ages (TDM2) of 1,441–1,221 Ma, indicating a magma source that involved partial melting of Mesoproterozoic lower crust accompanied by a minor juvenile mantle component. The ~240 Ma magmatism in the E‐KOB was probably developed in response to subduction of the Palaeo‐Tethys oceanic plate. Previous studies indicate that a tectonic transformation from subduction to continental collision occurred at ~240 Ma. Combining these data with regional geological observations, we conclude that the Daheba volcanic rocks formed in an active continental margin setting related to the subduction of the Palaeo‐Tethys oceanic plate beneath the E‐KOB.

Early–Middle Jurassic magmatic rocks along the coastal region of southeastern China: Petrogenesis and implications for Paleo-Pacific plate subduction

• The 200–160 Ma arc magmatic belt is identified along the coastal region of SE China. • This study firstly reports the ancient basement-derived granitoids in SE China. • The Paleo-Pacific Plate subduction underneath southeast China started at ca. 200 Ma. This study presents new zircon U–Pb ages and whole-rock geochemical data for the Fanyindong (FYD) and Jincheng (JC) granitoid intrusions in coastal Zhejiang and Fujian Provinces, respectively, southeast China, to investigate their petrogenesis and provide new constraints on the regional tectonic evolution and crustal growth. Zircon U–Pb ages indicate that the FYD and JC intrusions were emplaced at c. 170 Ma and 193 Ma, respectively. Both granitoids belong to the high-K calc-alkaline series, and display arc-like trace element patterns, with enrichment of large ion lithophile elements and Nb-Ta depletion. The FYD granitoids show adakitic geochemical characteristics, with high radiogenic Sr isotope ratios, and the most enriched bulk-rock Nd and zircon Hf isotopic compositions (crustal Nd–Hf model ages = ca. 2.35–2.75 Ga) ever reported in the Mesozoic granitoids within southeast China. Geochemical characteristics suggest that the FYD granitoids were generated by partial melting of mostly Paleoproterozoic basement rocks in a thickened lower crust. However, whole-rock Sr–Nd and zircon Hf isotopes of the JC granites show depleted source signatures. Geochemical signatures suggest the JC granites originated from the partial melting of a mixed source, consisting of the pre-existing, ancient, lower crust and juvenile, mantle-derived, basaltic rocks. The data from this study, in conjunction with previously published data, indicates that there exists a NE-trending, Early–Middle Jurassic, arc-related, magmatic rock belt along the southeast coast of China, and the Paleo-Pacific Plate subduction most likely started at c. 200 Ma. The early stage subduction of the Paleo-Pacific plate and the induced reactivation and extension of pre-existing E–W striking faults in the Nanling region of southeastern China gave rise to the generation of the JC granites. Ongoing subduction of the Paleo-Pacific plate resulted in a thickening of the continental margin, and subsequent crustal anatexis of basement materials produced the FYD granitoids.

Characteristic of Fault Structures in the south coastal Zone of Taizhou Based on High-Precision Aeromagnetic Data

The south coastal zone of Taizhou lies on the magmatic rock belt of southeast coast of China, which has complex regional geological structure, intense tectonic movement and frequent magmatic activities. Based on the latest high-precision aeromagnetic data, integrated interpretation was completed, combining with regional geology, regional gravity and magnetic susceptibility information. According to the block features in different zones of the RTP aeromagnetic data, the magnetic field characteristics and the relationship with the structure division were described in detail. It showed that the different characteristics of magnetic field is the concentrated reflection such as tectonic movement, magmatic activity and stratigraphic distribution, and the fault structure was inferred and studied, especially the deep and large fault structures. The fault structures were mainly distributed in NE, NNE and NW direction, with approximately equal spacing distribution. The magnetic anomaly is mainly characterized by the boundary, gradient zones and beaded anomalies in different magnetic field. The faults act not only as the important tectonic boundaries in this region, but also the important tectonic belts, controlling the distribution of mineralization. Under the interaction of these faults, they form the basic structural pattern of East-West zone and North-South block; Among them, the NE faults have the largest scale and obviously control over different magnetic field and magmatic activity. The results presented in the paper can provide reference for further study on the distribution and activity characteristics of magmatic rocks in the coastal zone.

Provenance and paleogeography of Archean Fig Tree siliciclastic rocks in the East-Central Barberton Greenstone Belt, South Africa

The 3.6–3.2 Ga Barberton Greenstone Belt, South Africa, is a complex terrain divided into multiple structural blocks. The structural deformation and poor geochronological constraints often make correlation among blocks difficult. To overcome structural complexities, provenance proxies including sandstone petrography, shale geochemistry, and detrital zircon geochronology are here used to compare source terrain signatures among multiple structural blocks in the East-Central Domain of the Barberton Greenstone Belt. This will expand our understanding of Paleoarchean paleogeography and the nature of crustal uplift in the southeastern part of the Barberton Greenstone Belt.The Manzimnyama and Mlumati Synclines are composed of 600 m-thick sections of the Fig Tree Group, Mapepe Formation. These synclines have high sedimentary and volcanic lithic contents in sandstones, an upward transition from felsic to mafic signatures in shales, and a detrital zircon age distribution transition from a basal member with a main peak at 3.28 Ga to members with a peak at about 3.45 Ga. There are three other East-Central Domain, fault-bounded blocks made up mostly of Mapepe Formation strata. The Eastern Barite Valley lies northwest of the Manzimnyama Syncline. From older to younger strata, the Eastern Barite Valley progresses from high lithics to more quartz and feldspar in sandstones, from more mafic to felsic signatures in shales, and from a unimodal 3.45 Ga detrital zircon signature at the base to complexly mixed age distributions with a strong 3.24 Ga peak at the top. The Paulus Syncline, southeast of the Manzimnyama Syncline, has a high chert content, mafic to ultramafic geochemistry signatures, and a main detrital zircon peak at 3.28–3.30 Ga with a minor peak at 3.45 Ga. The Emlembe Syncline, southeast of the Paulus Syncline, has a high monocrystalline quartz content, mixed geochemistry signatures, and unimodal 3.28–3.30 Ga detrital zircon ages. The proximity and sedimentology of these structural blocks could suggest deposition in a single basin; however, contrasts in provenance suggests sediments were deposited in different basins, at different times, or in one basin with provenance partitioning. The blocks likely had locally different sources, implying the sequences were not part of a single large source, transport, and depositional fairway.

Petrogenetic Evolution of Early Paleozoic Trachytic Rocks in the South Qinling Belt, Centra China: Insights from Mineralogy, Geochemistry, and Thermodynamic Modeling

Petrogenetic Evolution of Early Paleozoic Trachytic Rocks in the South Qinling Belt, Centra China: Insights from Mineralogy, Geochemistry, and Thermodynamic Modeling

Geological characteristics, geodynamic setting of magmatism and metallogeny of Early Cretaceous Sn (W) deposits in southeastern coastal belt of China, and their implication for exploration

我国华南是世界上最重要的锡矿资源省之一，其锡矿成矿时代从新元古代、古生代、早中生代至晚中生代均有发育，其中以晚中生代锡矿成矿作用最为重要，成矿集中在中晚侏罗世（160~150Ma）和晚白垩世（100~80Ma）。然而，最近在东南沿海成矿带新识别出一期早白垩世锡（钨）（145~130Ma）成矿事件，改变了以往华南锡（钨）矿床时空分布格局。本文在详细介绍东南沿海早白垩世锡（钨）矿床地质特征基础上，系统总结了其成岩成矿时空分布，并与南岭板内锡（钨）成矿作用进行对比，浅析了早白垩世锡（钨）矿的成岩成矿背景和找矿潜力。研究显示，早白垩世锡（钨）成矿主要发生在东南沿海成矿带的粤东和赣州会昌地区，矿床类型以斑岩型和锡石硫化物型为主。与成矿有关的岩石为黑云母花岗岩和花岗斑岩，成因上属于高分异I型或A型，与南岭板内锡（钨）矿相比，具有较高的锆石ε_Hf（t）值和较低δ¹⁸O值，以及较高成岩温度，表明成岩过程中有较多新生地壳或地幔物质加入。结合区内发育同时期基性岩脉，认为华南这期早白垩世锡（钨）矿形成于岩石圈伸展背景，与古太平洋板块俯冲后撤有关。最后指出覆盖大面积晚白垩世火山岩的东南沿海地区，火山岩盖层下部具有寻找该期锡（钨）矿床的找矿潜力。;South China is one of the most important tin provinces in the world, and the tin mineralization formed from Neoproterozoic, Early Paleozoic, to Early and Late Mesozoic are well developed in this area. In particular, the Late Mesozoic tin mineralization is the most important one, happened at two age peaks of the Middle-Late Jurassic (160~150Ma) and Late Cretaceous (100~80Ma), respectively. However, an Early-Cretaceous (145~130Ma) tin (tungsten) metallogenic event has also been newly recognized in southeastern coastal belt of China. This makes a change in temporal-spatial framework of tin (tungsten) deposits. In this paper, based on detailed observations of geological characteristics of the Early Cretaceous tin (tungsten) deposits in southeastern coastal belt, we summarize the temporal-spatial distribution of these deposits and their associated magmatisms, and compared them with those in the Nanling Range, in order to identify their metallogenic geodynamic setting and implications for metallogenic potential. The Early Cretaceous tin (tungsten) deposits are mainly developed in the coastal eastern Guangdong Province and Huichang district of the southern Jiangxi Province. Genetically, the tin mineralization mainly comprises porphyry and cassiterite-sulfide-vein types, and associated granitoids include biotite granite and granitic porphyry characterized by high-fractionated I- and A-type affinities. Compared with those in the Nanling Range, these granites display higher zircon ε_Hf(t) and δ¹⁸O values, as well as higher magma temperatures, indicating a contribution of more mantle or juvenile crustal components. Given the coeval mafic dikes in the region, we suggest that the Early Cretaceous tin (tungsten) deposits were formed in the setting of lithospheric extension, triggered by the retreat of the subducting paleo-Pacific plate. Finally, we suggest that there is potential for tin (tungsten) exploration under the large-scale cover of Late Cretaceous volcanic rocks in southeastern coastal belt of China.

Slab‐fluid metasomatism in the Early Paleozoic forearc mantle deduced from the Motai serpentinites, South Kitakami Belt, northeast Japan

AbstractThe present study examines the petrology and geochemistry of the Early Paleozoic Motai serpentinites, the South Kitakami Belt, northeast Japan, to reveal the subduction processes and tectonics in the convergent margin of the Early Paleozoic proto‐East Asian continent. Protoliths of the serpentinites are estimated to be harzburgite to dunite based on the observed amounts of bastite (orthopyroxene pseudomorph). Relic chromian spinel Cr# [=Cr/(Cr + Al)] increases with decreasing amount of bastite. The compositional range of chromian spinel is similar to that found in the Mariana forearc serpentinites. This fact suggests that the protoliths of the serpentinites are depleted mantle peridotites developed beneath the forearc regions of a subduction zone. The Motai serpentinites are divided into two types, namely, Types 1 and 2 serpentinites; the former are characterized by fine‐grained antigorite and lack of olivine, and the latter have coarse‐grained antigorite and inclusion‐rich olivine. Ca‐amphibole occurs as isolated crystals or vein‐like aggregates in the Type 1 serpentinites and as needle‐shaped minerals in the Type 2 serpentinites. Ca‐amphibole of the Type 1 serpentinites is more enriched in LILEs and LREEs, suggesting the influence of hydrous fluids derived from slabs. By contrast, the mineral assemblage, mineral chemistry, and field distribution of the Type 2 serpentinites reflect the thermal effect of contact metamorphism by Cretaceous granite. The Ca‐amphibole of the Type 1 serpentinites is different from that of the Hayachine–Miyamori Ophiolite in terms of origin; the latter was formed by the infiltration of melts produced in an Early Paleozoic arc–backarc system. Chemical characteristics of the Ca‐amphibole in the ultramafic rocks in the South Kitakami Belt reflect the tectonics of an Early Paleozoic mantle wedge, and the formation of the Motai metamorphic rocks in the forearc region of the Hayachine–Miyamori subduction zone system, which occurred at the Early Paleozoic proto‐East Asian continental margin.

Formation Mechanism and Stability Analysis of the Hejia Landslide

The existing research data show that, after reservoir impoundment, due to the repeated rise and fall of water level and water‐rock interaction, the mechanical parameters of landslide are reduced, which will have an adverse effect on the stability of landslide. Therefore, sufficient attention must be paid to the stability of slope after reservoir impoundment. Hejia landslide is the largest landslide near the bank of Miaojiaba hydropower station, and its stability plays an important role in the normal operation of the hydropower station. Through field investigation and analysis of regional geological conditions, it is concluded that Hejia landslide is a large‐scale landslide, through long‐term sliding‐bending deformation; it is generated from the external hard rock with thick layers and sliding zone for layered soft rock; the formation mechanism of landslide is as follows: (1) high‐steep and hard‐soft layered slope is the slope structure condition that caused the large landslide; (2) the existence of thick soft rock belt provides material conditions for the formation of slip surface; (3) certain air conditions provide displacement space for the separation and disintegration of the sliding body, and the landslide is stable at present. Numerical analysis results show that reservoir impoundment will adversely affect the stability of landslide. In order to ensure the normal operation of power station, certain engineering measures must be taken to treat Hejia landslide. After taking measures, years of monitoring data show that the deformation of Hejia landslide tends to be stable, and the current operation is normal, indicating that the engineering treatment measures are reasonable and feasible.

Characteristics and significance of aegirine and arfvedsonite in Boziguoer Nb-Ta-Zr-Rb-REE deposit related to alkaline granite, Xinjiang

碱性造岩矿物能够记录碱性岩源区特征、岩浆演化以及晚期成矿的重要信息，是开展碱性岩成岩成矿研究的有效手段。波孜果尔碱性花岗岩型铌-钽-锆-铷-稀土矿床位于塔里木北缘-中亚南天山晚古生代造山带，是塔里木地块北缘铌成矿带中典型的碱性岩型矿床。本文通过对含矿岩体中的霓石和钠铁闪石开展矿物学研究，结合全岩成分揭示波孜果尔稀有-稀土金属矿床含矿岩体的岩石类型、演化特征、构造背景及成矿条件。研究发现，含矿碱性岩体由霓石钠铁闪石碱长花岗岩、霓石钠铁闪石石英碱长正长岩、霓石钠铁闪石碱长正长岩组成。不同岩相岩石均表现出相似的稀土和微量元素配分模式，以富集轻稀土元素和高场强元素，亏损大离子亲石元素和重稀土元素为特征，具有显著的负Eu异常，指示三种岩石类型是同源岩浆演化的产物。含矿岩体中的辉石为霓石-霓辉石，角闪石为钠铁闪石。霓石和钠铁闪石稀土和微量元素配分模式相似且含量都较低，富集重稀土、亏损轻稀土，具有显著的负Eu异常，并富集Zr、Hf等高场强元素，亏损Ba、Sr等大离子亲石元素。霓石和钠铁闪石微量元素特征指示三个岩相单元的演化程度由低到高依次为霓石钠铁闪石碱长花岗岩→霓石钠铁闪石石英碱长正长岩→霓石钠铁闪石碱长正长岩，且结晶分异作用控制了岩体的形成。同时，早期霓石钠铁闪石碱长花岗岩的钠铁闪石呈现Ce正异常，晚期岩相中则无异常，指示演化的早期阶段氧逸度较高，随着磁铁矿等氧化物的结晶熔体趋于还原。波孜果尔含矿岩体成矿元素除Rb外分布并不均匀，表现为演化早期的岩相富集Nb，而演化晚期的岩相更富集稀土和Zr。进一步研究发现，塔里木北缘碱性岩带发育的铌、稀土、钽、锆、铷、铀等稀有稀土金属矿化与地幔柱引起的幔源岩浆底侵有关，其构造背景可能为地幔柱对造山带的叠置。;Alkaline minerals can record the important information of source area characteristics, magmatic evolution and late mineralization of alkaline rocks, which is an effective means for the study of diagenesis and mineralization of alkaline rocks. Boziguoer Nb-Ta-Zr-Rb-REE deposit is a typical deposit related to alkaline rocks, which is located in the Late Paleozoic orogenic belt of the southern Tianshan-the northern margin of Tarim block. Based on mineralogical study of aegirine and arfvedsonite in the ore-bearing intrusion, combining with its whole rock compositions, this paper reveals the rock types, evolution characteristics, tectonic setting and metallogenic conditions of the intrusion in the Boziguer rare-earth metal deposit. The rock types of ore-bearing intrusion are aegirine arfvedsonite alkali feldspar granite, aegirine arfvedsonite quartz alkali feldspar syenite and aegirine arfvedsonite alkali feldspar syenite. All of the three types of rocks show similar REE and trace element distributions, with enriched LREE and HFSE, depleted HREE and LILE, and a significant negative Eu anomaly, indicating that they are the products of homologous magmatic evolution. The pyroxene group minerals are aegirine, and the amphibole group minerals are arfvedsonite. Aegirine and arfvedsonite show similar distribution patterns of trace and rare earth elements, which are characterized by enriched HREE and depleted LREE with a significant Eu negative anomaly. They all show enrichment in HFSE (Zr, Hf), depletion in LILE, such as Ba and Sr. Trace element and REE distributions of these minerals indicate that the degree of their evolution from low to high is: aegirine arfvedsonite alkali feldspar granite→aegirine arfvedsonite quartz alkali feldspar syenite→aegirine arfvedsonite alkali feldspar syenite, showing the formation of the intrusion is controlled by crystallization and differentiation. The arfvedsonite of aegirine arfvedsonite alkali feldspar granite shows a positive anomaly of Ce, indicating a high oxygen fugacity environment at the early stages of its evolution. The crystallization of magnetite in the early lithofacies make the evolution environment of the melt tend to be reduced. The Rb in all rock types is equally distributed, and the rocks of early evolution stage is more enriched in Nb, while that of the late-stage more enriched in REE and Zr. The mineralization of rare earth metals such as Nb, REE, Ta, Zr, Rb and U developed in the alkaline rock belt in the northern margin of Tarim basin is related to the mantle magma underplating caused by mantle plume, and its tectonic setting may be the superposition of mantle plume to orogenic belt.

Zircon U-Pb geochronology and Sr–Nd–Pb–Hf isotope geochemistry for Permian–Early Triassic arc-related magmatism in Pohang, Jangsari, and Yeongdeok, southeastern Korean Peninsula

Granitoids in the southern Korean Peninsula range in age from Precambrian to Cenozoic, but were emplaced mostly during the Jurassic and Cretaceous periods. The late Permian–Early Triassic granitoids are minor in volume and restricted to the northeastern part of the Cretaceous Gyeongsang Basin (Yeongdeok and Jangsari). Here we report on Permian granites from the Cenozoic Pohang Basin of southeastern Korea. The samples were obtained during the drilling of injection wells in the Pohang enhanced geothermal system (4219 m depth for the drill core; 3658–4174 m for the drill cuttings). We also analyzed outcrop samples from Yeongdeok and Jangsari. We present zircon U-Pb ages and Hf isotope, whole-rock geochemical, and Sr–Nd–Pb isotope data for these Phanerozoic granitoids. The zircons yielded early–late Permian ages (ca. 283–264 Ma) for the Pohang granites, late Permian ages (ca. 261–256 Ma) for the Jangsari granitoids, and late Permian–Early Triassic ages (ca. 253–249 Ma) for the Yeongdeok granitoids. The granitoids are enriched in large-ion lithophile elements and depleted in high-field-strength elements, indicating formation in an arc-related tectonic setting. The Permian Pohang and Jangsari granitoids have a non-adakitic arc-like geochemistry, whereas the Yeongdeok intrusions are adakite-like and show high Sr and low Y-Yb contents. The granitoids have low (87Sr/86Sr)i (0.7038–0.7049) and positive εNd(t) values (+3.7 to +4.7 for the Pohang, +4.4 to +4.9 for the Jangsari, and + 2.4 to +4.1 for the Yeongdeok plutons), and are distinct from the basement of the Yeongnam Massif. The Pb isotope ratios of the studied samples plot in the field for Indian mid-ocean ridge basalts. Zircons from the plutons have positive εHf(t) values: +9.8 to +17.6 for the Pohang, +8.8 to +17.7 for the Jangsari, and + 8.5 to +18.3 for the Yeongdeok plutons. The Permian granitoids were likely the product of extensive fractional crystallization of H2O-rich parental magmas, which were possibly generated in a hydrated mantle wedge. The Yeongdeok adakite-like granitoids were derived from a normal H2O-rich arc magma by amphibole ± garnet fractionation. The juvenile Sr–Nd–Pb–Hf isotope compositions are comparable with those of coeval igneous rocks in the Central Asian Orogenic Belt, rather than intrusive counterparts in the North and South China cratons. We propose two possibilities that Permian–Early Triassic arc-related magmatism in the southeastern Korean Peninsula might have been associated with (1) northward subduction of the Paleo-Asian oceanic plate; or (2) westward subduction of the Paleo-Pacific plate at the eastern margin of the North China Craton.

Using characteristics of pressure-temperature induced permeability variation of typical carbonate rock to determine its performance as reservoir or cap rock

The current research on carbonate rock often focuses on the storage space for oil and gas, and exploration of carbonate rock in petroliferous basins also mainly searches for high-quality reservoir rock for hydrocarbons. However, the importance of effective carbonate reservoir-cap rock configuration is increasingly highlighted, as hydrocarbon exploration advances towards deeper and older strata. Exploration practice shows that the favorable facies belt of carbonate rock, though deeply buried, still develops highly desirable reservoir rock with high porosity and permeability. Therefore, the presence of an effective carbonate cap rock collaborating with this reservoir rock becomes key to hydrocarbon exploration in deep carbonate rock. This research, for the first time, systematically and thoroughly carries out permeability measurement through the whole triaxial compression process of typical carbonate rock at varied temperatures, and captures the permeability evolution during deformation and failure, affected by temperatures and confining pressures. Test results show that typical dolomite and limestone present varied permeability variation characteristics in a triaxial loading state. Dolomite appears to be more of reservoir rock. It presents higher critical confining pressure for brittle–ductile transition, and is more prone to fracturing. Its permeability grows with temperature elevation. As for limestone, it has better performance as cap rock, which is embodied as the lower critical confining pressure for brittle–ductile transition, high sealing capacity, and permeability reduction with elevated temperatures. To sum up, vertical stacking of dolomite and limestone can form an effective reservoir-cap rock assemblage for hydrocarbon accumulation.

Structural Evolution of the Rio das Velhas Greenstone Belt, Quadrilátero Ferrífero, Brazil: Influence of Proterozoic Orogenies on Its Western Archean Gold Deposits

The Quadrilátero Ferrífero region is located in the extreme southeast of the Brasiliano São Francisco craton, Minas Gerais state, Brazil. It is composed of (i) Archean TTG granite-gneaissic terranes; (ii) the Archean Rio das Velhas greenstone belt; (iii) the Proterozoic metasedimentary and metavolcano-sedimentary covers. The Rio das Velhas rocks were deposited in the synformal NW–SE-directed Nova Lima basin. The Archean deformation converted the Nova Lima basin into an ample synclinorium with an eastern inverted flank. Archean orogenic gold mineralization within the Rio das Velhas greenstone belt rocks is controlled by NNW–SSE-directed, Archean regional shear zones subparallel to the strata of the Nova Lima synclinorium borders. Transamazonian and Brasiliano orogenies are superposed onto the Archean structures that control gold mineralization. In the eastern domain, Brasiliano fold-and-fault belts prevail, whereas in the western domain Archean and Transamazonian structures abound. The present study focus mainly is the western domain where the Cuiabá, Morro Velho, Raposos, Lamego and Faria deposits are located. Gold orebodies plunge to the E–NE and are tectonically controlled by the Archean D1–D2 deformation. The D3 Transamazonian compression—Which had a SE–NW vector sub-parallel to the regional mineralized Archean foliation/bedding—Buckled these structures, resulting in commonly open, synformal and antiformal regional folds. These are well documented near the gold deposits, with NE–SW axial traces and fold axes plunging to E–NE. Such folds are normal to inverted, NW-verging, with an axial planar foliation dipping moderately to the SE. The Transamazonian compression has only been responsible for the reorientation of the mineralized Archean gold ores, due to coaxial refolding characterized by an opposite tectonic transport. It has therefore not caused any other significant changes. Thrust shear zones, sub-parallel to the strong Transamazonian foliation, have given rise to localized metric segmentation and to the dislocation of gold orebodies. Throughout the region, along the towns of Nova Lima to Sabará, structures pertaining to the Brasiliano Araçuaí orogeny are represented only by gentle folding and by a discrete, non-pervasive crenulation cleavage. Thrust-shear zones and small-scale normal faults have caused, at most, metric dislocations along N–S-oriented planes.

Geochemical Characteristics of Amphibolites in Parts of Iseyin-Oyan Schist Belt, Southwestern Nigeria

Amphibolite is an important lithologic unit of the Nigerian basement complex that are commonly intercalated within metasedimentary sequences. Disseminated grains of sulphide minerals and base metal deposits are reported to occur in amphibolites and some other lithologies in other schist belts of Nigeria apart from the Iseyin-Oyan schist belt. Detailed geochemical study of amphibolitic rocks in this schist belt is scarce in literature, whereas none exist for the area under study, thus, this study is aimed at delineating the amphibolites within the Iseyin-Oyan schist belt around Itasa area with detail appraisal of their petrography and geochemical characteristics. A geologic field mapping was undertaken, and fresh representative amphibolite samples were obtained. Petrographic and geochemical studies of the samples were carried out with a view to understand their nature and petrogenetic characteristics. Geochemical data were elucidated using diverse geochemical discrimination diagrams. The amphibolites occur as lenses of small to large rocky boulders trending NNE-SSW and are essentially basal to quartz mica schist. Modal compositions revealed that they are dominantly composed of hornblende and plagioclase. XRD and mineral chemistry revealed that the plagioclase ranged from andesine to anorthite while hornblende is mainly magnesiohornblende. Geochemical data and discrimination diagrams revealed that the amphibolites are formed from basaltic to basaltic-andesite protolith that are of tholeiitic nature and had suffered crustal contamination. Tectonic discrimination diagrams indicated that the rocks were emplaced in the Mid Ocean Ridge but had been affected by collision due to Pan African Orogenic tectonic event.

Quantitative finite strain and kinematic analyses of Dahomeyide orogenic belt (Benino-Nigerian shield) around Igarra, southwestern Nigeria

The Igarra area (including Sebe-Ogbe and Otuo) in Igarra schist belt, southwestern Nigeria exposes a good section of the Dahomeyide orogenic belt (Benino-Nigerian shield). The area provides an opportunity to investigate quantitative finite strain, types of strain and strain ellipsoid, kinematic vorticity number (Wk) and proportions of pure and simple shear components from metaconglomerate outcrops. Measurements of the long, short and intermediate axes of deformed clasts in the metaconglomerates were carried out. The angular relationships between a constant reference line and the long axes of some of the clasts were determined. Combined strain in two dimensions (2-D) and three dimensions (3-D) analyses involving the Rf/ɸ (Phi), centre to centre (Fry) and Flinn methods were performed on the data with the aid of Ellipsefit software to generate Flinn, the tear-drop shaped diagrams and finite strain ellipsoids. The kinematic vorticity numbers were estimated in order to quantify the amount of shear. The results indicated that the rocks in the Dahomeyide orogenic belt around Igarra have ellipticity in initial state (Ri) and true strain (Rs) of 3.6 and 4.4, respectively. The Ri and Rs for Otuo are 2.43 and 3.35, respectively while the values for Sebe-Ogbe are 1.96 and 3.16, respectively. The results of centre to centre method showed strain ellipses with different orientations from near horizontal to vertical, while the Flinn diagram showed that the rocks in the three areas have both constrictional and flattening strains corresponding to prolate and oblate strain ellipsoids, respectively which are characteristic of L-S tectonites. The kinematic vorticity number (WK or Wm) for the locations are 0.97 (Igarra), 0.79 (Sebe-Ogbe) and 0.97 (Otuo). These probably suggest homogeneous subsimple shear transpressional deformation involving approximately 42% pure shear and 58% simple shear for Sebe-Ogbe and 17% pure shear and 83% simple shear for both Igarra and Otuo. The state of strain of the rocks at Igarra, Otuo and Sebe-Ogbe was relatively high and increased towards the south from Igarra (north) to Sebe-Ogbe (south) where there is highest strain probably due to clast support nature of the metaconglomerates where the clasts acted as single rigid body, and influence of contact metamorphism. The implication of this is that the rocks at Igarra have undergone less deformation than those at Otuo which in turn have undergone less deformation than those at Sebe-Ogbe.

The Itabuna-Salvador-Curaçá Orogen revisited, São Francisco Craton, Brazil: New zircon U–Pb ages and Hf data support evolution from archean continental arc to paleoproterozoic crustal reworking during block collision

Precambrian orogenic belts are key areas to understand how the continental crust formed and evolved with time, and to constrain when plate tectonics began on planet Earth. The Itabuna-Salvador-Curaçá Orogen, in the São Francisco Craton, is an orogenic belt over 800 km long that is interpreted as the result of collision of four blocks during the Paleoproterozoic. However, its geological history before the final continental collision and the time span of collision are not well constrained. Here, we present new field relationships, in situ zircon U–Pb ages and Hf isotope data for gneisses and granites from the orogen and one of the blocks (Gavião block) that collided during the Paleoproterozoic. Most of the samples were collected along a geological traverse across the northern area of the orogen. The geochronological results indicate ages of 3335 Ma and 3307 Ma for gneisses of the Mairi Complex (Gavião block), 2637 Ma, 2620 Ma and 2581 Ma for gneisses of the Caraíba Complex within the orogen, and ages between 2100 Ma and 2058 Ma for the collisional granitoids from the Itabuna-Salvador-Curaçá Orogen. The slightly negative εHf(t) values (−1.9 and −4.5) for samples of the Mairi Complex suggest contribution from older continental crust in the genesis of the gneisses protoliths. For the Caraíba Complex, positive to negative εHf(t) values (0.9 to −3.7) for the orthogneisses, coupled with similar whole-rock εNd(t) signature from the literature, suggest a continental arc setting for emplacement of the Caraíba Complex orthogneisses. The εHf(t) values (−12.1 and −13.4) for one collisional granitoid indicate it can have originated from the partial melting (reworking) of the Caraíba Complex crust. From the tectonic point of view, the ages of the Caraíba Complex become younger from west to east and this age variation can be interpreted in terms of a continental arc that formed by trench/slab retreat on the edge of the Gavião block. This model is supported also by the occurrence in the Gavião block of coeval (2595 ± 21 Ma) volcanic rocks in the Mundo Novo greenstone belt (interpreted in the literature as formed in a back-arc basin) and some metamorphosed clastic sedimentary rocks of the Saúde Complex with provenance solely from sources older than 2500 Ma. On the other hand, the ages of the collisional granites can be used to constrain the timing and duration of the orogen's evolution. Accordingly, the early collisional granites with ages between 2100 Ma and 2094 Ma set a minimum age for the beginning of collision. The syn-to late-transcurrent granites were emplaced from 2084 Ma to 2060 Ma, and the post-collisional granites from about 2058 Ma until 2027 Ma. Therefore, block collision to form the Itabuna-Salvador-Curaçá Orogen has lasted at least 70 million years.

Early Proterozoic Basic Magmatism in the South Siberian Postcollisional Magmatic Belt (by the Example of the Ust’-Ignok Massif in the Urik–Iya Graben)

Abstract —We performed geological, geochronological, geochemical, and isotope-geochemical studies of igneous rocks of the Ust’-Ignok gabbrodiorite massif in the Urik–Iya graben of the Siberian craton and summarized the obtained and published data on early Proterozoic mafic igneous rocks in the South Siberian postcollisional magmatic belt. It has been established that the Ust’-Ignok massif is composed of rocks of the continuous series from biotite gabbro via gabbrodiorites and diorites to quartz diorites. U–Pb zircon dating of quartz diorites of the Ust’-Ignok massif yielded an age of 1836 ± 10 Ma, i.e., the massif rocks might have originated at the final stage of the formation of the South Siberian postcollisional magmatic belt. The rocks of the Ust’-Ignok massif are of normal and medium alkalinity. All igneous rocks from gabbro to quartz diorites show distinct negative anomalies of Nb–Ta and Ti in their multielement patterns, and their εNd(T) values vary from +0.3 to –0.9. The geochemical indicator ratios in the gabbroids point to insignificant contamination of their source with continental-crust material and to their formation through the melting of an enriched lithospheric-mantle source. Gabbrodiorites–quartz diorites of the Ust’-Ignok massif resulted, most likely, from the fractional crystallization of gabbroids. Analysis of the geochemical and isotope characteristics of mafic igneous rocks of the South Siberian postcollisional magmatic belt shows that most of them resulted from the melting of the subcontinental lithospheric mantle with suprasubductional geochemical features. This mantle might have formed during subduction processes preceding the formation of the Siberian craton.

Geophysical Study of Gold Mineralized Zones in the Carolina Terrane of South Carolina

Abstract Large pyrite-dominated gold deposits are hosted in hydrothermally altered metamorphic rocks in the Carolina slate belt of South Carolina and are partly covered by Coastal Plain sedimentary rocks. In this study, we investigate the utility of geophysical data including aeromagnetic, electromagnetic (EM), and land gravity surveys as exploration tools. Observed geophysical anomalies are correlated with rock properties such as resistivity, susceptibility, and density. Mineral concentrations were measured for 40 samples from 16 new drill holes, as well as densities and pyrite concentrations for 49,183 samples from 448 drill holes in the Haile ore zone. Regional positive gravity anomalies are observed over the Haile, Ridgeway, and Barite Hill mine areas, and residual high-pass filtered positive gravity anomalies are observed over all mine areas. New measurements for drill cores in the Haile mine area directly confirm high rock densities and high pyrite concentrations in the ore zone. In the Haile and Brewer mine areas, high-conductivity EM anomalies are observed over the ore zones as well as over nearby metasedimentary rocks. The high concentration of pyrite in the metasedimentary units and in the ore zones may explain the high conductivities observed. All gold deposits in the Carolina slate belt are hosted in similar geologic settings near the contact between the Persimmon Fork metamorphosed volcaniclastic rocks and Richtex metamorphosed sedimentary rocks. Density and conductivity contrasts between the Persimmon Fork and Richtex Formation rocks may permit mapping of the contact zone between the two units. The magnetic anomalies do not correlate with the mineralized zones but rather with granite and gabbro plutons and diabase dikes. A prominent east-northeast linear magnetic anomaly correlates with the Modoc shear zone that separates low-grade metamorphic rocks of the Carolina terrane from higher-grade metamorphic rocks of the Kiokee belt. We use the Modoc linear magnetic anomaly to predict the southeastern boundary of the Carolina slate belt where it is covered by Coastal Plain sedimentary rocks.