Gridino melange zone of the Belomorian eclogite province: Succession of tectonic events and structural position of mafic dyke swarms

Based on relationships between Paleoproterozoic mafic dykes, lithotectonic complexes, and tectonic structures of the Gridino Zone in the Belomorian eclogite province of the Fennoscandian Shield, deformations have been divided into groups differing in age and the succession of tectonic events has been reconstructed. The formation of Neoarchean eclogite-bearing melange was related to disintegration of large eclogite sheets in the course of near-horizontal ductile flow accompanied by syntectonic granitoid magmatism, multiple migmatization, and granulite-to amphibolite-facies metamorphism. The exotic blocks, including eclogites, were incorporated into TTG gneisses as sheets and lenses up to a few hundreds of meters in thickness and oriented conformably with gneissic banding. As a result of ductile flow, the lithotectonic complexes were transported at the level of discrete brittle-ductile deformations expressed as strike-slip faults and associated folds. Under conditions of a relatively rigid medium, individual structural elements underwent rotation approximately through 90° in plan view. Under the extension regime in the Early Paleoproterozoic, several swarms of mafic dykes were injected into the already cold framework rocks, as is evident from dyke morphology. The dykes crosscut all predated structures, included turned blocks, and are therefore important reference points for subdivision of Neoarchean and Paleoproterozoic processes. The Svecofennian postdyke tectonic activity was accompanied by local shearing and boudinage of metabasic rocks, development of quartz and pegmatite veins along tension cracks, disharmonic folding, and discrete retrograde metamorphism up to amphibolite-facies conditions. The postdyke deformations did not exert a substantial effect on the previously formed regional structure.     

Changing mantle sources in a suture zone in the heart of Pangea: implications for collisional tectonics during the waning stages of ocean closure

The formation and emplacement of syn-collisional mafic dykes that intrude suture zones and their association with orogenic processes are enigmatic. Southern Iberia records the Late Paleozoic amalgamation of Pangea and exposes today a fragment of Laurussia (South Portuguese Zone), which is spatially juxtaposed with autochthonous Gondwana. Fault-bounded oceanic metasedimentary rocks, mélanges and ophiolite complexes characterize the suture zone and are in turn crosscut by intrusive granitoid rocks and mafic dykes. The generation and emplacement of these mafic dykes and their relationship to the suture zone are undetermined. Field evidence shows the dykes were emplaced at high angles to pre-existing orogenic fabrics in the mélange, granitoid and metasedimentary rocks. Geochemical analyses (major, trace, rare earth elements) indicate the dykes exhibit a mid-ocean ridge basalt signature. U/Pb zircon geochronology reveals the crystallization age of the dykes is ca. 316 Ma and Sm–Nd isotopic analysis suggests a deep mantle source. Taken together, these data support existing temporal constraints on events leading up to the amalgamation of Pangea, and suggest progressive lower crustal delamination during the waning stages of continent–continent collision.     

Thoughts on the Jiaodong Gold Province of China:Towards a Tectonic Model

The Jiaodong gold province is situated in the eastern Sino-Korean Platform within the so-calledJiaoliao Uplift. The basement rocks are Archaean and Proterozoic metamorphic rocks. Mesozoic sedimentary andvolcanic cover occur within extensional basins. Intrusive rocks are dominated by Mesozoic granitoid, with interme-diate-acid and basic dyke swarms. The structures form an E-W-trending anticlinorium in the basement complex, andlarge-scale NE-SW-and NNE-SSW-trending fault zones of Mesozoic age. The gold mineralization is associated withthe Mesozoic faults and related secondary fractures in the granites or granite-basement contacts. The mineralizationtypes are quartz-vein type and wall-rock alteration type. Wall-rock alteration is very well developed around the orezones. Alteration minerals include quartz, sericite (and fuchsite), pyrite, calcite, chlorite, hematite, rutile and graph-ite. The ore assemblage is uniform in all deposits, including pyrite, chalcopyrite, galena, sphalerite, arsenopyrite,pyrrhotite, gold, electrum, hessite, petzite, magnetite, molybdenite, tetrahedrite and wolframite. Mesozoic collisionand subduction between the South China and North China continental blocks contributed to formation of the Meso-zoic granitoid intrusions. The granitic magma is considered to be derived from partial melting of the crust throughunderplating processes. Gold was remobilised from basement rocks and deposited in fracture zones by the high-temperature fluids associated with these processes.     

Geochemistry and petrogenesis of late Ediacaran (605–580 Ma) post-collisional alkaline rocks from the Katherina ring complex,south Sinai,Egypt

The Katherina ring complex (KRC) in the central part of south Sinai, Egypt, is a typical ring complex of late Neoproterozoic age (605–580 Ma). It was developed during the final tectono-magmatic stage of the north Arabian–Nubian Shield (ANS) during evolution of the Pan-African crust. The KRC includes Katherina volcanics, subvolcanic bodies, ring dykes and Katherina granitic pluton. The Katherina volcanics represent the earliest stage of the KRC, which was subsequently followed by emplacement of the subvolcanic bodies and ring dykes. The Katherina granitic pluton depicts as the latest evolution stage of the KRC that intruded all the early formed rock units in the concerned area. The Katherina volcanics are essentially composed of rhyolites, ignimbrite, volcanic breccia and tuffs. Mineralogically, the peralkaline rhyolites contain sodic amphiboles and aegirine. The rhyolite whole rock chemistry has acmite-normative character. The subvolcanic bodies of the KRC are represented by peralkaline microgranite and porphyritic quartz syenite. The ring dykes are semicircular in shape and consist mainly of quartz syenite, quartz trachyte and trachybasalt rock types. The Katherina subvolcanic rocks, volcanic rocks as well as the ring dykes are alkaline or/and peralkaline in nature. The alkaline granitic pluton forms the inner core of the KRC, including the high mountainous areas of G. Abbas Pasha, G. Bab, G. Katherina and G. Musa. These mountains are made up of alkaline syenogranite and alkali feldspar granite. The mantle signature recorded in the KRC indicates a juvenile ANS crust partial melting process for the generation of this system. The evolution of the KRC rocks is mainly dominated by crystal fractionation and crustal contamination. Mineral geothermometry points to the high temperature character of the KRC, up to 700–1100 °C.     

Lower carboniferous zones and correlations based on faunas from the Gresford‐Dungog district,New South Wales

The Jindabyne Thrust has been mapped south of Lake Eucumbene, along the eastern side of Lake Jindabyne and thence southwards to the gorge of the Snowy River in Byadbo Lands. It is marked by a crush zone and a west‐facing scarp. Structure contours on the Thrust where it enters the gorge of the Snowy River in the Byadbo region indicate an easterly dip of about 20°.

The north‐south erosional valley now occupied by Lake Jindabyne is controlled by the Thrust and the gorge below the Jindabyne Dam has been rejuvenated by recent movement.

The nature of the Jindabyne Thrust and other faults in the Jindabyne‐Berridale region can be deduced from their effects on the Silurian granitoid plutons. Where a pluton, circular or elliptical in plan and with vertical walls, is transected by a thrust, a semi‐elliptical or semi‐circular shape results; granitoid rock types cannot be matched across the fault. Wrench faults in the region either curve into or are transected by the thrusts, depending upon the geometrical relationships of both.

It is suggested that the north‐south dividing line between granitoids derived from igneous rocks (I‐types) to the east and granitoids derived from metasedimentary rocks (S‐types) to the west is a major tectonic feature of eastern Australia. The line coincides with a transition from a regime where wrench faulting predominates to one dominated by thrust faulting. These changes in both tectonics and granitoid lithology suggest that the I‐S line marks the eastern boundary of crystalline basement, possibly of Precambrian age.     

Mechanics of low-angle normal faulting: An example from Roosevelt Hot Springs geothermal area, Utah

Roosevelt Hot Springs geothermal area is located in the Mineral Mountains of southern Utah. The geothermal reservoir is formed by systems of faults and joints in Cenozoic plutonic and Precambrian metamorphic rocks. Low-angle denudation faults, dipping between 5° and 35° to the west, form an important component of the reservoir's structure. These faults developed simultaneously with steeply dipping faults that dissect the low-angle fault plates and merge into denudation faults at depth.Gently westward dipping joints provided planes of weakness along which the denudation faults nucleated. The average coefficient of sliding friction along the faults was less than 0.5 and probably ranged between 0.15 and 0.4. The maximum depth for formation of the denudation faults was estimated as 5 km. Hydrothermally altered cataclasite preserved in the fault zones indicates that faulting occurred under brittle conditions in the presence of chemically reactive fluids. The hydrothermal alteration may have significantly reduced friction in the fault zones.Hydrothermal alteration along fractures in the present geothermal reservoir is similar to that observed in the exhumed denudation faults, indicating that the frictional resistance along faults and joints in the reservoir could be significantly lower than along similar structures in unaltered granitic rock. Studies of the structural stability of the reservoir as a consequence of fluid withdrawal and reinjection should consider possible mechanical effects of this hydrothermal alteration.     

Spatial variations in damage zone width along strike-slip faults: An example from active faults in southwest Japan

Field investigations reveal spatial variations in fault zone width along strike-slip active faults of the Arima–Takatsuki Tectonic Line (ATTL) and the Rokko–Awaji Fault Zone (RAFZ) of southwest Japan, which together form a left-stepping geometric pattern. The fault zones are composed of damage zones dominated by fractured host rocks, non-foliated and foliated cataclasites, and a fault core zone that consists of cataclastic rocks including fault gouge and fault breccia. The fault damage zones of the ATTL are characterized by subsidiary faults and fractures that are asymmetrically developed on each side of the main fault. The width of the damage zone varies along faults developed within granitic rocks of the ATTL and RAFZ, from ∼50 to ∼1000 m. In contrast, the width of the damage zone within rhyolitic tuff on the northwestern side of the ATTL varies from ∼30 to ∼100 m. The fault core zone is generally concentrated in a narrow zone of ∼0.5–∼5 m in width, consisting mainly of pulverized cataclastic rocks that lack the primary cohesion of the host rocks, including a narrow zone of fault gouge (<0.5 m) and fault-breccia zones either side of the fault. The present results indicate that spatial variations in the width of damage zone and the asymmetric distribution of damage zones across the studied strike-slip faults are mainly caused by local concentrations in compressive stress within an overstep area between left-stepping strike-slip faults of the ATTL and RAFZ. The findings demonstrate that fault zone structures and the spatial distribution in the width of damage zone are strongly affected by the geometric patterns of strike-slip faults.     

The Granny Smith gold deposit: the role of heterogeneous stress distribution at an irregular granitoid contact in a greenschist facies terrane

The Granny Smith gold deposits formed late in the structural history of the Yilgarn Block at a high crustal level in a largely brittle structural régime. Gold mineralisation is located along a N-S striking fault which wraps around the contact of a small granitoid intrusion. In different sections of the fault, mineralisation may be developed in the granitoid, in the adjacent sedimentary sequence and/or along the contact between them. In the granitoid, gold mineralisation is in conjugate networks of thin carbonate-quartz veins and their alteration halos. Small displacements along veins are common. In contrast, veins and faults in the sedimentary rocks are subparallel to bedding. Spatial variations in the conjugate vein orientations indicate that the local stress field was heterogeneous and controlled by the shape of the granitoid contact. The greatest variations in vein and implied stress orientations occur in zones where the contact is most irregular. These are also the areas of richest mineralisation. Fluid flow was thus focused in a regional-scale low mean-stress region created by the geometry of the granitoid intrusion. Its irregular contact caused deposit scale variations in fluid flow and resulted in heterogeneous gold grades along the contact zone.     

Metabasic Dyke Swarms in a High-Grade Metamorphic Terrane——A Case Study in the Taipingzhai-Jinchangyu Area,Eastern Hebei Province

Geological and petrological studies indicate that three phases of metabasic dykes are present in theTaipingzhai-Jinchangyu area within the high-grade metamorphic terrane of eastern Hebei. Garnet andhornblende in metabasic dykes of the second and third phases occur separately, forming two mineral aggregateareas gl+cpx+pl±hy and hb+cpx+pl-hy. P_(H_2O) in the rocks appears to be the main factor controlling theformation of the two aggregate areas. Both were formed simultaneously at the same metamorphic temperature. The second-phase basic dykes underwent metamorphism of pyroxene-granulite facies at a temperature ofsome 825C: later the dykes, together with the third-phase basic dykes experienced metamorphism ofamphibole-granulite facies at a temperature of about 750C under pressure of 0.9GPa.     

Controls on porphyry Cu mineralization around Hanza Mountain,south-east of Iran: An analysis of structural evolution from remote sensing,geophysical, geochemical and geological data

Hanza Mountain in Urmia–Dokhtar Magmatic Arc, southeast of Iran, consists of monocline of Eocene volcanic rocks into which the Oligocene granitoid rocks have been intruded. This area has excellent potential for economic porphyry copper deposits with Bondar Hanza, Daralu, and Sarmesk deposits among them. Hanza Mountain is located between NW–SE horsetail thrust faults derived from the Gowk and Sabzevaran strike-slip faults. The analysis of the kinematics of these strike-slip faults shows that they were not the cause of the formation of the pull-apart basin; thus they have not directly played any effective role in localizing the final emplacement of porphyries responsible for the formation of these copper deposits, but the Cu mineralization occurred mainly within a set of normal and thrust faults in the region. The alteration types and faults in Bondar Hanza were distinguished using detailed local geology, including distribution of known mineralization, supported by remote sensing (ASTER), airborne geophysics, and topography; the relationship between mineralization and faults was examined using Rose diagrams and Fry Analysis. This investigation of Bondar Hanza deposit has revealed that the trend of faults and dykes, as well as the distribution of copper analyses within drill cores, is aligned with the main trend of mineralization. The NW–SE trending faults in the Urmia–Dokhtar Magmatic Arc are effective in localizing the emplacement of porphyry copper ore deposits and those that trend between N125°–N145° are key to further exploration.     

Repeated seismic slips recorded in ultracataclastic veins along active faults of the Arima–Takatsuki Tectonic Line,southwest Japan

Field investigations, combined with meso- and microstructural analyses, reveal that numerous ultracataclastic veins are widely developed within a fault zone (<150 m wide) as simple veins, complex lenses, and networks, along active faults of the Arima–Takatsuki Tectonic Line, southwest Japan. These veins comprise mainly pseudotachylyte-like vein and weakly consolidated to unconsolidated fault gouge that is black, dark-brown, brown, gray, and brownish-red in color. Meso- and microstructural features show that these pseudotachylyte-like and fault gouge veins and networks formed during multiple stages, as earlier veins are generally cut and overprinted by younger veins, indicating that the vein-forming events occurred repeatedly and that ultracataclastic material was injected into networks of faults and fractures in the fault zone. The pseudotachylyte-like and fault gouge veins are characterized by an ultrafine- to fine-grained matrix and angular to subangular fragments of host granitic rocks of various sizes, ranging from submicron to millimeters. SEM–EDS (Scanning Electronic Microscope-Energy Dispersive X-ray) and powder X-ray diffraction analyses show that all the ultracataclastic veins are characterized by crystalline materials composed mainly of quartz and feldspar, similar to the host granitic rocks.The present results support the existing hypothesis that ultrafine- to fine-grained materials formed by comminution can be fluidized and injected rapidly into fracture networks located far from the source fault plane in a solid–fluid–gas system during seismic slip; therefore, such materials provide a record of paleoseismic faulting events that occurred repeatedly within the seismogenic fault zone.     

S–C fabrics developed in cataclastic rocks from the Nojima fault zone,Japan and their implications for tectonic history

S–C fabrics similar to those found in mylonites are observed in foliated cataclastic granitic rocks from the Nojima fault zone, southwest Japan. The foliated cataclastic rocks comprise cataclasite, fault breccia, gouge, and crushing-originated pseudotachylyte. The S–C fabrics observed in these cataclastic rocks involve S-surfaces defined by shape preferred orientation of biotite fragments or aggregates of quartz and feldspar fragments, and C-and C′-surfaces defined by microshears and shear bands, respectively, where fine-grained material is concentrated. Striations on the main fault plane are oriented parallel to the cataclasite lineations. A significant microstructural difference between the foliated cataclastic rocks and S–C mylonites is the absence of dynamically recrystallized grains in the foliated cataclasites. The striations, cataclastic lineations, and the S–C fabrics in the cataclastic rocks formed from the late Tertiary to the late Holocene indicate that the Nojima fault zone has moved as a dextral strike-slip fault, with a minor reverse component since it formed. S–C fabrics in cataclastic rocks provide important information on the tectonic history and are reliable kinematic indicators of the shear sense in brittle shear zones or faults.     

Structural control of weathering processes within exhumed granitoids: Compartmentalisation of geophysical properties by faults and fractures

In the latter stages of exhumation processes, rocks undergo weathering. Weathering halos have been described in the vicinity of structures such as faults, veins or dykes, with a lateral size gradually narrowing with depth, symmetrically around the structures.In this paper, we describe the geophysical characterisation of such alteration patterns on two granitoid outcrops of the Catalan Coastal Ranges (Spain), each of which is affected by one major fault, as well as minor faults and fractures. Seismic, electric and ground penetrating radar surveys were carried out to map the spatial distribution of P-wave velocity, electrical resistivity and to identify reflectors of electromagnetic waves. The analysis of this multi-method and complementary dataset revealed that, at shallow depth, geophysical properties of the materials are compartmentalised and asymmetric with respect to major and subsidiary faults affecting the rock mass. This compartmentalisation and asymmetry both tend to attenuate with depth, whereas the effect of weathering is more symmetric with respect to the major structure of the outcrops. We interpret such compartmentalisation as resulting from the role of hydraulic and mechanical boundaries played by subsidiary faults, which tend to govern both the chemical and physical alterations involved in weathering. Thus, the smoothly narrowing halo model is not always accurate, as weathering halos can be strongly asymmetrical and present highly irregular contours delimiting sharp contrasts of geophysical properties.These results should be considered when investigating and modelling fluid storage and transfer in top crystalline rock settings for groundwater applications, hydrocarbon or geothermal reservoirs, as well as mineral deposits.     

Geological observations in high‐grade mid‐Proterozoic rocks from Else Platform,northern Prince Charles mountains region,east Antarctica

Granulite facies rocks on Else Platform in the northern Prince Charles Mountains, east Antarctica, consist of metasedimentary gneiss extensively intruded by granitic rocks. The dominant rock type is a layered garnetbiotite‐bearing gneiss intercalated with minor garnet‐cordierite‐sillimanite gneiss and calc‐silicate. Voluminous megacrystic granite intruded early during a mid‐Proterozoic (ca 1000 Ma) granulite event, M₁, widely recognized in east Antarctica. Peak metamorphic conditions for M₁ are in the range of 650–750 MPa at ～800°C and were associated with the development of a gneissic foliation, S₁ and steep east‐plunging lineation, L₁. Strain partitioning during progressive non‐coaxial deformation formed large D₂ granulite facies south‐dipping thrusts, with a steep, east‐plunging lineation. In areas of lower D₂ strain, large‐scale upright, steep east‐plunging fold structures formed synchronously with the D₂ high‐strain zones. Voluminous garnet‐bearing leucogneiss intruded at 940 ±20 Ma and was deformed in the D₂ high‐strain zones. Textural relationships in pelitic rocks show that peak‐M2 assemblages formed during increasing temperatures via reactions such as biotite + sillimanite + quartz ± plagioclase = spinel + cordierite + ilmenite + K‐feldspar + melt. In biotite‐absent rocks, re‐equilibration of deformed M₁ garnet‐sillimanite‐ilmenite assemblages occurred through decompressive reactions of the form, garnet + sillimanite + ilmenite = cordierite + spinel + quartz. Pressure/temperature estimates indicate that peak‐M₂ conditions were 500–600 MPa and 700±50°C. At about 500 Ma, north‐trending granitic dykes intruded and were deformed during D₃‐M₃ at probable upper amphibolite facies conditions. Cooling from peak D₃‐M₃ conditions was associated with the formation of narrow greenschist facies shear zones, and the intrusion of pegmatite. Cross‐cutting all features are abundant north‐south trending alkaline mafic dykes that were emplaced over the interval ca 310–145 Ma, reflecting prolonged intrusive activity. Some of the dykes are associated with steeply dipping faults that may be related to basin formation during Permian times and later extension, synchronous with the formation of the Lambert Graben in the Cretaceous.     

Relative timing of igneous intrusion in the Eucalyptus area,northeastern Yilgarn Block,Western Australia

Field studies in the Eucalyptus area, northeastern Yilgarn Block have shown intrusive and extrusive rocks in an Archaean greenstone sequence to be comagma‐tic, and have suggested the sequence of subsequent granitoid intrusion and gold mineralisation. Andesitic volcanic rocks and related subvolcanic granodiorite porphyry and epiclastic sediments were followed by tholeiitic basalt with gabbro/dolerite sills and dykes, which were in turn succeeded by high‐Mg basalt with associated peridotite intrusions. Large, irregular gabbro and peridotite intrusions, which are inferred to represent subvolcanic magma chambers, occur in lower stratigraphic levels, whereas comformable subvolcanic sills occur in higher stratigraphic levels. Granodiorite plutons were followed by adamellite plutons; at least some gold mineralisation was contemporaneous with granitoid emplacement.     

衡山变质核杂岩体西缘剥离断层及其对铀成矿的控制作用

衡山变质核杂岩体是由元古界深变质岩和燕山期复式花岗岩体组成，其西缘的界版剥离断层是区域深断裂的一部分，在断裂下盘山现糜棱岩－混合岩带具韧性变形特征，沿断裂已发现一些有意义的铀矿化，根据剥离断层控矿模式，提出在断裂上盘王冲－谢马冲地段寻找陡倾正断层与离断层交切处的隐伏铀矿的设想。     

Arc related Jurassic igneous and meta-igneous rocks in the Coastal Cordillera of northern Chile/Region Antofagasta

A deep section of the Jurassic, 200-150 Ma old magmatic arc is exposed in the Coastal Cordillera south of Antofagasta in northern Chile. The chemical compositions of metabasic and plutonic rocks from the deep level are compared with those of Jurassic volcanic rocks and ≈ 150 Ma old dykes. The metabasites, most of the plutonic rocks, and the dykes have calc-alkaline characteristics. However, small postmetamorphic gabbro plutons are tholeiitic. The composition of the volcanic rocks is not related to the plutonic rocks, metabasites and dykes. All igneous and meta-igneous rocks of the arc are derived from a similar source in the upper mantle and evolved without major crustal contamination.

The general tectonic setting was dominated by extension, and Pre-Jurassic crust is extremely thinned or absent in the area. Details of the tectonic, magmatic and metamorphic development remain still a matter of speculation.     

SWIR ASTER band ratios for lithological mapping and mineral exploration: a case study from El Hudi area, southeastern desert, Egypt

This study aims to discriminate and to map the basement rocks as well as the barite mineralization exposed at El Hudi area, Southeastern Desert, Egypt using the processed short-wave infrared bands of advanced space-borne thermal emission and reflection radiometer (ASTER) in collaboration with the field verification and petrographic analysis. El Hudi area is covered dominantly by the Late Precambrian high-grade metamorphic complex of metasedimentary rocks (gneisses, schists, migmatites, and minor amphibolites) which are intruded by the younger granitoids. Nubian sandstones unconformably overlie the basement outcrops and occur as a remnant caps. The metasedimentary rocks cover the area of interest forming a belt of biotite gneisses and migmatites intercalated with hornblende biotite schists and minor amphibolites. Their exposures exhibit well-foliated and banded structures. The metasedimentary rocks have gray and dark gray image signatures on the ASTER band ratio image 8/5, which correspond to biotite gneiss, migmatites, and hornblende biotite schists, respectively. Presence of absorption feature near band 8 (2.295 – 2.365 μm) for the chlorite alteration product is probably responsible for the lowering of the 8/5 band ratio value and the dark gray image signature exhibited by hornblende biotite schists. The granitoid rocks in El Hudi area are late to postorogenic younger granitoids including three main rock types, Abu Aggag granites, El Hudi garnetiferous muscovite granites, and coarse-grained biotite granites. The acidic dykes are cutting across the granitoids and the gneisses and they form a highly elevated ridges and peaks showing sharp contact with the invaded rocks. Abu Aggag granites are highly dissected by great number of both strike- and dip-slip faults as well as joints trending in NNW–SSE, NNE–SSW, N–S, ENE–WSW, and WNW–ESE directions. On 7/8 band ratio image, Abu Aggag granites have dark gray image signature whereas postgranitic dykes have white image signature. Under the microscope, Abu Aggag granites are homogenous medium to coarse-grained rocks composed mainly of quartz, plagioclase, microcline, and biotite. Zircon, apatite, and opaques are accessories, while chlorite, kaolinite, and epidote are secondary minerals. Presence of absorption feature around band 7 (2.235–2.285 μm) for the kaolinite mineral may be responsible for the dark gray image signature exhibited by Abu Aggag granites. El Hudi garnetiferous muscovite granites are hosting El Hudi barite veins which extend mainly in NNW–SSE and NW–SE. Garnetiferous muscovite granites have gray image signature on 5/4 band ratio image whereas pegmatites and postgranitic dykes have black image signature. Barite veins can be distinguished within garnetiferous muscovite granites by their dark gray image signature on 5/4 band ratio image. The spectral reflectance curve of barite exhibits absorption feature around 2.1 μm (band 5), which leads to lower the ratio value and yields the dark image signature to barite veins. The above-described ASTER band ratio images were integrated into one false-color composite image (8/5:R; 5/4G; and 7/8B) which was used to produce 1:100,000 geological map for El Hudi area and to locate the barite mineralization.     

Growth, linkage, and termination processes of a 10-km-long strike-slip fault in jointed granite: the Gemini fault zone, Sierra Nevada, California

Field-based structural analysis of an exhumed, 10-km-long strike-slip fault zone elucidates processes of growth, linkage, and termination along moderately sized strike-slip fault zones in granitic rocks. The Gemini fault zone is a 9.3-km-long, left-lateral fault system that was active at depths of 8–11 km within the transpressive Late-Cretaceous Sierran magmatic arc. The fault zone cuts four granitic plutons and is composed of three steeply dipping northeast- and southwest-striking noncoplanar segments that nucleated and grew along preexisting cooling joints. The fault core is bounded by subparallel fault planes that separate highly fractured epidote-, chlorite-, and quartz-breccias from undeformed protolith. The slip profile along the Gemini fault zone shows that the fault zone consists of three 2–3-km-long segments separated by two ‘zones’ of local slip minima. Slip is highest (131 m) on the western third of the fault zone and tapers to zero at the eastern termination. Slip vectors plunge shallowly west-southwest and show significant variability along strike and across segment boundaries. Four types of microstructures reflect compositional changes in protolith along strike and show that deformation was concentrated on narrow slip surfaces at, or below, greenschist facies conditions. Taken together, we interpret the fault zone to be a segmented, linked fault zone in which geometrical complexities of the faults and compositional variations of protolith and fault rock resulted in nonuniform slip orientations, complex fault-segment interactions, and asymmetric slip-distance profiles.     

四川省冕宁县泸沽—喜德县冕山地区铁锡矿成矿规律分析

文章简述了四川冕宁县泸沽—喜德县冕山地区猴子崖铁锡矿、铁矿山铁矿、大顶山铁锡矿、下山堡—黑林子锡矿和拉克铁矿等矿床的成矿地质特征,分析了铁锡成矿与岩浆岩、地层及构造的时空关系,对铁锡矿的成矿地质背景、成矿作用和成矿过程进行了总结。壳源泥质岩类重熔形成的A型花岗岩体为成矿提供了Sn、W等成矿元素,泸沽花岗岩和中酸性火山岩的热能和挥发组分大量逸出形成成矿热液,并使幔源基性岩墙（脉）产生强烈蚀变,释放出大量的Fe质进入成矿热液,为铁锡矿的形成提供了主要的物质组分;中-新元古界登相营群的Fe元素丰度较高,在构造-岩浆活动中部分Fe元素被热液萃取,为成矿提供出部分成矿元素;区域性NNE向断裂、岩体构造、局部性褶皱（曲）构造、沿层和切层的滑动断裂（裂隙）带均可以控制矿床（体）的产出部位、形态特征和矿化类型;本区铁锡矿床包括矽卡岩型铁锡矿和云英岩型锡矿2种类型,分别受大理岩层与变质砂岩（或千枚岩）的岩性转换界面（钙-硅质岩界面）和变质砂岩（千枚岩）岩性界面（隔水界面）的控制。该区铁锡矿床为与新元古代复合岩浆热液作用有关的矽卡岩型-云英岩型铁锡矿床,具有成矿物质多源、成矿岩体多个、控矿构造多样、矿化多型（式）的特点。