Geochemical characteristics of Proterozoic granite magmatism from Southern Granulite Terrain,India: Implications for Gondwana

Granitoid intrusions occur widely in the Southern Granulite Terrain (SGT) of India, particularly within the Cauvery Suture Zone (CSZ), which is considered as the trace of the Neoproterozoic Mozambique ocean closure. Here we present the petrological and geochemical features of 19 granite plutons across the three major tectonic blocks of the terrain. Our data show a wide variation in the compositions of these intrusions from alkali feldspathic syenite to granite. The whole rock geochemistry of these intrusions displays higher concentrations of \(\hbox {SiO}_{2}\), FeO*, \(\hbox {K}_{2}\hbox {O}\), Ba, Zr, Th, LREE and low MgO, \(\hbox {Na}_{2}\hbox {O}\), Ti, P, Nb, Y and HREE’s. The granitoids are metaluminous to slightly peraluminous in nature revealing both I-type and A-type origin. In tectonic discrimination plots, the plutons dominantly show volcanic arc and syn-collisional as well as post-collisional affinity. Based on the available age data together with geochemical constrains, we demonstrate that the granitic magmatism in the centre and south of the terrain is mostly associated with the Neoproterozoic subduction–collision–accretion–orogeny, followed by extensional mechanism of Gondwana tectonics events. Similar widespread granitic activity has also been documented in the Arabian Nubian shield, Madagascar, Sri Lanka and Antarctica, providing similarities for the reconstruction of the crustal fragments of Gondwana supercontinent followed by Pan-African orogeny.     

山西五台地区系舟山逆冲推覆构造地质特征

系舟山逆冲推覆构造带位于中生代燕山造山带的西南端,分布于系舟山掀斜向斜的北西翼,形成于晚侏罗世晚期,空间上由一系列近平行排列的逆冲断裂组成,剖面上表现为侧幕展布的犁式逆冲断裂所构成的前陡、后缓的单冲式叠瓦状构造.主体由北西向南东方向逆冲,逆冲扩展方式为前展式,运移距离大于5.8 km.推覆构造中应力状态在横、纵向上呈现有规律的变化,根带以挤压为主的高角度逆冲断裂及复杂多级褶皱为主;中带以单剪为主,形成叠瓦状构造;锋带挤压作用增强,发育反冲断层和不对称褶皱.随着挤压应力的松弛减弱,山前形成规模较大的正断层.     

Provenance and Palaeoweathering Attributes of Proterozoic Rock Formations from the Son Valley,Central India

The characteristics of source rocks and weathering of Palaeoproterozoic phyllitic rock of Mahakoshal Group and Mesoproterozoic shales and siliciclastics of Vindhyan Supergroup exposed in Son Valley, Central India have been investigated by analyzing their chemical compositions. The investigations pertaining to the relationship between major-elements were carried out along Parshoi, Chitrangi, and Markundi areas of Son valley, Central, India. The studied rock strata have been classified into three categories namely phyllitic rocks, shales and sandstone.The A-CN-K ternary diagram, CIW, CIA, MIA, and ICV values indicate about the similar provenance or source rocks subjected to severe chemical weathering, under dry and hot-humid climates in a basic and acidic environment with changing lower to higher PCO₂ of continental flora. Various geochemical discriminantts diagrams, elemental ratios suggest that rocks are derived from post-Archaean-Proterozoic igneous source. The igneous source was mainly granite with a minor contribution of granodioritic rock, in a passive margin setting. The sediments responsible for formation of shale and sandstones were most likely deposited in the interglacial period in between the Mesoproterozoic and Neoproterozoic glacial times. Compositionally the sandstones is distinctive of cratonic environments with their passive continental margin setting. However, the phyllities of Mahakoshal Group suggests their formation under lower weathering conditions in dry climatic conditions which were operating on more intermediate to basic igneous rocks with abundance of mafic minerals.     

Early Cretaceous Thrust and Nappe Tectonics in North Qilian Shan,Northern Tibetan Plateau: Evidence from Field Mapping,Geochronology, and Deep Structural Analysis

The North Qilian Shan fold and thrust belt, located at the northern Tibetan Plateau and southern margin of the Hexi Corridor, is a key tectonic unit to decode the formation and expansion of the plateau. Previous studies emphasize the Cenozoic deformation due to the far-field response to the Indo–Asian collision, but the Mesozoic deformations are poorly constrained in this area. We conducted detailed field mapping, structural analysis, geochronology, and structural interpretation of deep seismic ...     

Pressure-Temperature-Time Constraints for the Seve Nappe of the Singis-Tjuoltajaure area Central Norrbotten Caledonides,Sweden : Implications for Early Caledonian Marginal Baltica

Abstract

This study provides some of the first integrated P-T-t constraints for the evolution of the Finnmarkian tectonothermal event within the northern Scandinavian Caledonides. Samples from units within the Seve Nappe of the Singis-Tjuoltajaure region contain assemblages which allow the application of well calibrateted thermoharometers. New results include : I ) Eclogite grade rocks of the Aurek Assemblage yield temperatures and pressures in excess of 12 kb and 730° С. 2) Temperatures and pressures obtained for seven samples from the Savotjåkka Assemblage range from 571766” С and 8.9-13.6 kb. These pressures correspond to burial depths of approximately 30-45 km during the Finn-markian (190 Ma) for the outer margin of Baltica. when combined with ⁴⁰Ar/³⁹Ar data, uplift rates of .2-.4 mm/yr during the early Finnmarkian arc obtained, and 3) Within the Vidja Assemblage a pressure and temperature of 7.3 ± 1.7 kb and 616 ± 60° is obtained. These conditions are consistent with the interpretation of a late Finnmarkian (450 Ma) Intra-Seve juxtaposition of the Vidja and Aurek Assemblages after approximately 20-30 km of slow uplift from peak pressures recorded during the early Finnmarkian.

This study provides new constraints which need to be considered in future tectonic models. These constraints include : I ) The Savopakte Assemblage records high pressures and high temperatures during the Finnmarkian and 2) Finnmarkian uplift rates of .2-.4 mm/yr were likely for marginal Baltica after peak metamorphism. Rates of this magnitude may be accounted for solely by slow erosion and do not require (but do not preclude) more complex tectonic interpretations. A tectonic model provided by the Late Cenozoic thrust belts of the Apennine system of the mediterranean region, may lead to new insights into the Early Paleozoic evolution of the Scandinavian Caledonides.     

Nature, origin and evolution of the granitoid-hosted early Proterozoic copper-molybdenum mineralization at Malanjkhand, Central India

At Malanjkhand, Central India, lode-type copper (-molybdenum) mineralization occurs within calcalkaline tonalite-granodiorite plutonic rocks of early Proterozoic age. The bulk of the mineralization occurs in sheeted quartz-sulfide veins, and K-silicate alteration assemblages, defined by alkali feldspar (K-feldspar ≫ albite) + dusty hematite in feldspar ± biotite ± muscovite, are prominent within the ore zone and the adjacent host rock. Weak propylitic alteration, defined by albite + biotite + epidote/zoisite, surrounds the K-silicate alteration zone. The mineralized zone is approximately 2 km in strike length, has a maximum thickness of 200 m and dips 65°–75°, along which low-grade mineralization has been traced up to a depth of about 1 km. The ore reserve has been conservatively estimated to be 92 million tonnes with an average Cu-content of 1.30%. Supergene oxidation, accompanied by limited copper enrichment, is observed down to a depth of 100m or more from the surface. Primary ores consist essentially of chalcopyrite and pyrite with minor magnetite and molybdenite. δ³⁴S (‰) values in pyrite and chalcopyrite (−0.38 to +2.90) fall within the range characteristic of granitoid-hosted copper deposits. δ¹⁸O (‰) values for vein quartz (+ 6.99 to +8.80) suggest exclusive involvement of juvenile water. Annealed fabrics are common in the ore. The sequence of events that led to the present state of hypogene mineralization is suggested to be as follows: fracturing of the host rock, emplacement of barren vein quartz, pronounced wall-rock alteration accompanied by disseminated mineralization and the ultimate stage of intense silicification accompanied by copper mineralization. Fragments of vein quartz and altered wall rocks and striae in the ore suggest post-mineralization deformation. The recrystallization fabric, particularly in chalcopyrite and sphalerite, is a product of dynamic recrystallization associated with the post-mineralization shearing. The petrology of the host rocks, hydrothermal alteration assemblages, ore mineral associations, fluid inclusions and the sulfur and oxygen isotopes of ores are comparable to those in Phanerozoic (and reported Precambrian) porphyry-copper systems, and the Malanjkhand deposit has important implications for both metallogenic models for, and mineral exploration in, Precambrian terrains.     

Tectonics and thermal structure of western Satpura,India

Increased seismicity and occurrences of hot springs having surface temperature of 36–58 °C are observed in the central part of India (74–81° E, 20–25° N), where the NE trending Middle Proterozoic Aravalli Mobile Belt meets the ENE trending Satpura Mobile Belt. Earlier Deep Seismic Sounding (DSS) studies along Thuadara-Sendhwa-Sindad profile in the area has showed Mesozoic Sediments up to around 4 km depth covered by Deccan Trap and the Moho depth with a boundary velocity (P_n) of 8.2 km/s. In the present study, surface heat flow of 48 ± 4 mW m^?2 has been estimated based on P_n velocity, which agrees with the value of heat flow of 52 ± 4 mW m^?2 based on Curie point isotherms estimates. The calculated temperature-depth profile shows temperature of 80–120 °C at the basement, which is equivalent to oil window temperature in Mesozoic sediments and around 570–635 °C at Moho depth of 38–43 km and the thermal lithosphere is about 110 km thick, which is comparatively higher than those of adjoining regions. The present study reveals the brittle–ductile transition zone at 14–41 km depth (temperature around 250–600 °C) where earthquake nucleation takes place.     

Palaeomagnetism of metadolerite dykes and sills from Proterozoic Shillong basin,NE India: Implications related to the age and magmatism

The basement gneiss of the Shillong plateau and the overlying rocks of the Shillong basin have been dissected by a number of mafic dyke swarms represented by older Proterozoic dolerite dykes and sills named as Khasi greenstone and the younger set of Cretaceous dykes. The older dolerites dykes of Shillong basin are partly metamorphosed and have undergone low-grade metamorphism compared to the fresh unmetamorphosed Cretaceous dykes dominantly exposed in the BGC of West Garo hills region. The Khasi greenstones are tholeiitic in composition and range from basalt to basaltic andesite. Most of the metamorphosed mafic dykes indicate continental nature with some amount of overlapping oceanic tectonic setting. Palaeomagnetic study of the metadolerite dykes show a direction of magnetization of Dm=29, Im=38 (α₉₅ = 28.84; k = 48.33; N = 2) with a palaeolatitude of 21.3° N to the Indian sub-continent that clearly support the Proterozoic dyke/dyke swarm emplacement in the region. The magnetic carrier as inferred from IRM studies is magnetite in the range of psuedosingle to multi domain (MD) states with minor contribution from hematite.     

Proterozoic Metamorphic Rock-Hosted ZR,Y, and HREE Mineralization in the Dabie Mountain Area,Central China

Zr, Y, and heavy rare earth element (HREE) mineralization hosted by late Proterozoic metamorphic rocks from the western Dabie Mountains, northern Hubei Province, central China was discovered in 1978 by the Hubei Regional Geological Party (Li et al., 1981) and is examined further in this paper. The area is located in the EW-trending fold belt between the Sino-Korean and Yangtze cratons. Highly metamorphosed hornblende-plagioclase gneiss, granitic gneiss, and migmatites of the late Archean Dabie Group constitute the basement, which is unconformably covered by the late Proterozoic Hongan Group—a sequence of phosphorus-bearing layer(s), dolomitic marbles, muscovite-albite schist, felsic schists, and a variety of greenschists.

The stratiform Zr, Y, and HREE mineralization is hosted by felsic and albite-muscovite schists of the lower part of the Hongan Group, which was highly stressed and deformed. The mineralized layer, about 10 m in thickness, occurs concordantly in the strata and extends discontinuously more than 15 km from northwest to southeast. The host felsic schists have a mineral assemblage of quartz + albite + microcline + muscovite + magnetite ± epidote ± garnet, and are estimated to have been subjected to a metamorphism of 3607–440 °C. Accessory minerals include zircon, gadolinite, fergusonite, xenotime, monazite, allanite, gahnite, and apatite. The rocks are very similar to metaluminous rhyolite in major component composition: they are moderately to highly enriched (ppm) in Zr (4980–6246), Y (745–1119), Nb (186–232), Ta (18–24), Th (132–171), U (22–28), Yb (83–109), Hf (183–204), Ga (35–50) and Sn (39–48) and have relatively high Rb/Sr ratios (6–17). These chemical features are, to a certain extent, similar to those of high-silica rhyolites from North America, and comparable to H?gtuva Be-REE-U-Sn-mineralization in Precambrian granitic gneiss in Norway, to a volcanic-hosted rare-metals deposit at Brockman, Western Australia, and to Zr, Nb, Y, and HREE mineralization related to alkaline and peralkaline granites from Canada, the Arabian Shield, Nigeria, and northern China. A submarine sedimentary-volcanic origin is proposed for interpretation of the genesis of this unusual HREE mineralization.     

江西南城地区中—新生代推滑覆构造研究

通过对江西南城地区侏罗系林山群与下伏前泥盆纪变质地层、白垩纪地层与前白垩纪地层接触关系调查和谢家构造窗、竺由新村构造窗、上乌石构造窗、望天石飞来峰的发现及钻孔验证,初步厘定了区内燕山期滑覆构造和喜马拉雅期推覆构造。燕山期滑覆构造发生在中侏罗世末,主滑面沿华力西—印支构造层(D—T2)、燕山构造层(T3—K1)与下伏地层间角度不整合面扩展而成,泥盆纪—早白垩世地层为滑覆体,滑覆在前泥盆纪变质地层之上,动力学来源为太平洋板块由南东向北西对欧亚板块俯冲。喜马拉雅期推覆构造规模大、动力学机制复杂(顺层和切层推覆),彻底打破了华南地区原有的地层沉积层序,形成了现存的横向分带、纵向分层的多层叠瓦、堆垛推覆构造格局。前白垩纪地质体为外来推覆体,由南东向北西推覆堆垛在白垩纪地层之上,白垩纪地层呈"构造窗"分布于前白垩纪地质体之下,动力学来源可能为印度板块与欧亚板块碰撞形成。     

Geochemistry and Origin of Early Proterozoic Dongargarh Rapakivi Granite Complex, Central India - An Example for Magma Mixing and Differentiation

Early Proterozoic Dongargarh granite complex of Central India, intruding the tonalitic to granodioritic Amgaon gneisses and the Nandgaon Group bimodal volcanic suite, comprises three different textural and compositional types, viz., porphyritic granodiorite (PG), coarse equigranular granite (EG) and microgranite (MG). Synplutonic mafic dykes are common in the granite complex. The PG is characterised by rapakivi texture and the EG is the dominant facies and exhibits sporadically developed rapakivi texture. Microgranular enclaves are common in the EG while they are rare in PG. Major and trace element geochemistry of PG shows marked I- type and some occasional A-type granite characters unusual for a rapakivi granite while the EG shows A-type granite signatures. The field, petrographic, chemical and isotopic data of these granites suggest their derivation by mixing of mantle derived basic magma with a crustal-derived partly crystalline granitic magma. Episodic mafic magma underplating caused the anatexis of the Archaean lower continental crust in a continental margin tectonic setting resulting first in the formation of the I-type granodiorite followed by A-type granite. The I-type granodiorite is mixed with the basic magma (synplutonic dykes) while the EG is formed by mingling of A- type granite magma and the intruding basic magma.     

北京房山地区太平山褶皱的变形特征和形成时代:华北克拉通早白垩世挤压构造的意义

北京西山地区广泛发育轴向近E-W向的褶皱,对理解华北克拉通东部构造演化至关重要,但目前对其形成时代和构造动力学背景却没有明确的认识.北京房山地区的太平山褶皱是轴向近E-W向褶皱的典型代表.通过精细的野外构造观察和构造年代学研究,确定太平山褶皱的空间展布特征和构造样式,厘定构造变形的时代,为该区乃至华北克拉通东部的构造演化提供重要依据.通过详细的野外构造观察和系统的β图解研究,表明太平山褶皱由直立倾伏背斜和斜歪倾伏向斜构成.通过对早期卷入褶皱变形的煌斑岩席和晚期切层侵入褶皱的闪长玢岩脉进行锆石U-Pb定年,得到煌斑岩席和闪长玢岩脉的形成时代分别为147.2±2.4Ma和129.0±3.2Ma,表明太平山褶皱的形成时代为燕山运动B幕.另外,北京西山地区还存在NW-SE向的伸展构造样式.代表本区NW-SE向伸展的房山穹窿(约136Ma)晚于N-S向挤压构造,侵入切割太平山褶皱,塑造了研究区现今主体构造格架,进一步限定太平山褶皱的形成时代为147～136 Ma.因此,研究区轴向近E-W向的褶皱为早白垩世近N-S向挤压构造的产物;N-S向挤压和NW-SE向伸展的构造样式,为华北克拉通中生代构造体制的转换和动力学背景提供关键依据.     

Triassic Nappe in the Central Part of the Southern Central Asian Orogenic Belt (Ejinaq, NW China): Evidence from Structural Analysis and Geothermochronology

The thrust nappe played an important role in the Mesozoic tectonic evolution of the middle part of the Central Asian Orogenic Belt(CAOB). However, the timing, structural style and kinematic processes of the thrust nappe remain controversial, particularly the detail of the thrust nappe in the Guaizihu region(110 km east of Ejinaq). In this study, we investigate new field mapping, seismic sections, geochronology and low-temperature thermochronometric dating to provide constraints on the history of...