The crustal tongue melting model and the origin of massive anorthosites

Recent detailed field studies in several anorthosite complexes have shown that anorthosites are frequently associated with weakness zones in the crust which may have favoured their emplacement at mid-crust levels. Recent experimental data have shown that the parent magma compositions of various anorthosite massifs lie on thermal highs in the relevant phase diagrams at 10–13 kbar, indicating that these magmas cannot be derived by fractionation of peridotitic mantle melts but by melting of gabbronoritic sources in the lower crust at 40–50 km depths. In the Sveconorwegian Province terne boundaries have been traced in deep seismic profiles to Moho offsets or to tongues of lower crustal material underthrust to depths higher than 40 km. In Southern Norway, we suggest that a lithospheric-scale weakness zone (the Feda transition zone?) has channelled the Rogaland anorthosites through linear delamination, asthenospheric uprise and melting of a mafic lower crustal tongue.     

Geological interpretation of Bouguer gravity and aeromagnetic data from the Gobi-desert covered area,Eastern Tianshan,China: Implications for porphyry Cu-Mo polymetallic deposits exploration

In this study, Bouguer gravity and aeromagnetic data have been used to better understand the geology and mineral resources near the late Carboniferous-late Permian porphyry Cu-Mo polymetallic mineralization in the Chinese Eastern Tianshan belt, which is extensively covered by Gobi-desert. The reduced-to-pole (RTP) transformation of regional-scale aeromagnetic data shows that the porphyry Cu-Mo deposit is within a cluster of magnetic anomaly highs that overprint on a northeast trending magnetic gradient belt generally along the crustal-scale Kanguertag-Huangshan fault. The 10 km upward continuation transformation of both Bouguer gravity and aeromagnetic data indicates that the known porphyry Cu-Mo polymetallic deposits are located on the flanks of prominent gravity and magnetic anomaly highs. These anomalies are spatially correlated with the late Carboniferous-late Permian igneous rocks and in the Tuwu-Yandong mineralization district are centered over the granodiorite rocks genetically related to porphyry copper systems. In order to minimize interpretational ambiguities, a useful approach that is correlation analysis (CA) based on correlation coefficient (CC) given by gravity and magnetic data was employed to separate positively and negatively correlated anomalies features. The CA procedure is applied to 10 km upward continuation transformation of both Bouguer gravity and RTP transformed aeromagnetic data for mapping correlative magnetization and density contrast anomalies from deep sources, which may be associated with the porphyry Cu-Mo polymetallic mineralization. Five prominent CC positive anomalies have been found in the southern margin of Dananhu-Tousuquan arc. Those anomalies zones could be interpreted to reflect a late Carboniferous-late Permian magmatic belt that is favorable for additional discoveries of late Carboniferous to late Permian porphyry copper systems in north region of Eastern Tianshan.     

Mass and magnetic properties for 3D geological and geophysical modelling of the southern Agnew–Wiluna Greenstone Belt and Leinster nickel deposits,Western Australia

Physical property measurements provide a critical link between geological observations and geophysical measurements and modelling. To enhance the reliability of gravity and magnetic modelling in the Yilgarn Craton's Agnew–Wiluna Greenstone Belt, mass and magnetic properties were analysed on 157 new rock samples and combined with an existing corporate database of field measurements. The new samples include sulfide ore, serpentinised and olivine-bearing ultramafic host-rocks, granitoid, and felsic and mafic volcanic and volcaniclastic country rock. Synthesis of the data provides a useful resource for future geophysical modelling in the region. Several rock types in the region have sufficiently distinct physical properties that a discriminant diagram is proposed to facilitate a basic classification of rock types based on physical properties. However, the accumulation of emplacement, metamorphic, hydrothermal and structural processes has complicated the physical properties of the rocks by imposing duplicate and sometimes opposing physical property trends. The data confirm that massive sulfide and ultramafic rocks have the most distinctive mass and magnetic properties but with variability imposed by their complex history. Sulfide content imposes the strongest control on densities, but can only be identified when comprising >10 vol% of the rock. The pyrrhotite-rich Ni-sulfide assemblages generally have similar magnetic properties to the host ultramafic rocks, but can have much lower susceptibilities where the thermal history of the rocks has favoured development of hexagonal pyrrhotite over monoclinic pyrrhotite. In ultramafic rocks that contain <10 vol% sulfides, density and susceptibility are primarily controlled by serpentinisation, with olivine breaking down to serpentine and magnetite in the presence of water. Serpentinisation dramatically lowered densities and increased susceptibilities, but had limited influence on the intensity of remanent magnetisation. All ultramafic rocks contain multidomain magnetite, and most contain low coercivity grains prone to overprinting by in situ viscous remanent magnetisation or drilling-induced isothermal remanent magnetisation during extraction. Despite the low coercivities, Koenigsberger ratios of 1–20 are observed indicating that viscous remanent magnetisation aligned parallel to the present Earth field must be considered in any magnetic modelling. It is also noted that coarser-grained intrusive varieties of all rock types (e.g. granite, gabbro) show remanent magnetisation intensities 1–2 orders of magnitude greater than their extrusive equivalents (felsic and basaltic volcanics).     

Imaging downward granitic magma transport in the Rogaland Igneous Complex, SW Norway

Combining geological mapping and petrological, structural and geophysical (gravity and seismic) data already available for the late Proterozoic Rogaland Igneous Complex of Norway allows the 3D shape of the Bjerkreim–Sokndal layered intrusion to be modelled as a thick cumulate series capped by massive granitic rocks. Using the latter data, along with the spectacular convergent linear flow pattern that covers both the cumulates and the felsic rocks of this chamber, evidence is presented to show that the granitic material was down-dragged through the sinking of its high-density mafic floor into lower density anorthosites and granulitic gneisses. This example illustrates that downward gravity-driven flows of rocks were active, in addition to upward flows, in the building of the early crust up to late-Proterozoic times, helping to explain the geochemical and structural complexities of the old crust.     

Evidence for Proterozoic Collision from Airborne Magnetic and Gravity Studies in Southern Granulite Terrain, India and Signatures of Recent Tectonic Activity in the Palghat Gap

The composite airborne total intensity map of the Southern Granulite Terrain (SGT) at an average elevation of 7000' (≈ 2100 m) shows bands of bipolar regional magnetic anomalies parallel to the structural trends suggesting the distribution of mafic/ultramafic rocks that are controlled by regional structures/shear zones and thrusts in this region. The spectrum and the apparent susceptibility map computed from the observed airborne magnetic anomalies provide bands of high susceptibility zones in the upper crust associated with known shear zones/thrusts such as Transition Zone, Moyar-Bhavani and Palghat-Cauvery Shear Zones (MBSZ and PCSZ). The quantitative modelling of magnetic anomalies across Transition Zone, MBSZ and PCSZ suggest the presence of mafic rocks of susceptibility (1.5-4.0 × 10⁻³ CGS units) in upper crust from 8-10 km extending up to about 21-22 km, which may represent the level of Curie point geotherm as indicated by high upper mantle heat flow in this section.Two sets of paired gravity anomalies in SGT and their modelling with seismic constraints suggest gravity highs and lows to be caused by high density mafic rocks along Transition Zone and Cauvery Shear Zone (CSZ) in the upper crust at depth of 6-8 km and crustal thickening of 45-46 km south of them, respectively. High susceptibility and high density rocks (2.8 g/cm³) along these shear zones supported by high velocity, high conductivity and tectonic settings suggest lower crustal mafic/ultramafic granulite rocks thrusted along them. These signatures with lower crustal rocks of metamorphic ages of 2.6-2.5 Ga north of PCSZ and Neoproterozoic period (0.6-0.5 Ga) south of it suggest that the SGT represents mosaic of accreted crust due to compression and thrusting. These observations along with N-verging thrusts and dipping reflectors from Dharwar Craton to SGT suggest two stages of N-S directed compression: (i) between Dharwar Craton and northern block of SGT during 2.6-2.5 Ga with Transition Zone and Moyar Shear towards the west as thrust, and (ii) between northern and southern blocks of SGT with CSZ as collision zone and PCSZ as thrust during Neoproterozoic period (0.6-0.5 Ga). The latter event may even represent just a compressive phase without any collision related to Pan-African event. The proposed sutures in both these cases separate gravity highs and lows of paired gravity anomalies towards north and south, respectively. The magnetic anomalies and causative sources related to Moyar Shear, MBSZ and PCSZ join with those due to Transition Zone, Mettur and Gangavalli Shears in their eastern parts, respectively to form an arcuate-shaped diffused collision zone during 2.6-2.5 Ga.Most of the Proterozoic collision zones are highlands/plateaus but the CSZ also known as the Palghat Gap represents a low lying strip of 80-100 km width, which however, appears to be related to recent tectonic activities as indicated by high upper mantle heat flow and thin crust in this section. It is supported by low density, low velocity and high conductive layer under CSZ and seismic activity in this region as observed in case of passive rift valleys. They may be caused by asthenospheric upwarping along pre-existing faults/thrusts (MBSZ and PCSZ) due to plate tectonic forces after the collision of Indian and Eurasian plates since Miocene time.     

Aeromagnetic signatures over western Marie Byrd Land provide insight into magmatic arc basement, mafic magmatism and structure of the Eastern Ross Sea Rift flank

Aeromagnetic signatures over the Edward VII Peninsula (E7) provide new insight into the largely ice-covered and unexplored eastern flank of the Ross Sea Rift (RSR). Positive anomalies, 10–40 km in wavelength and with amplitudes ranging from 50 to 500 nT could reveal buried Late Devonian(?)–Early Carboniferous Ford Granodiorite plutons. This is suggested by similar magnetic signature over exposed, coeval Admiralty Intrusives of the Transantarctic Mountains (TAM). Geochemical data from mid-Cretaceous Byrd Coast Granite, contact metamorphic effects on Swanson Formation and hornblende-bearing granitoid dredge samples strengthen this magnetic interpretation, making alternative explanations less probable. These magnetic anomalies over formerly adjacent TAM and western Marie Byrd Land (wMBL) terranes resemble signatures typically observed over magnetite-rich magmatic arc plutons. Shorter wavelength (5 km) 150 nT anomalies could speculatively mark mid-Cretaceous mafic dikes of the E7, similar to those exposed over the adjacent Ford Ranges. Anomalies with amplitudes of 100–360 nT over the Sulzberger Bay and at the margin of the Sulzberger Ice Shelf likely reveal mafic Late Cenozoic(?) volcanic rocks emplaced along linear rift fabric trends. Buried volcanic rock at the margin of the interpreted half-graben-like “Sulzberger Ice Shelf Block” is modelled in the Kizer Island area. The volcanic rock is marked by a coincident positive Bouguer gravity anomaly. Late Cenozoic volcanic rocks over the TAM, in the RSR, and beneath the West Antarctic Ice Sheet exhibit comparable magnetic anomaly signature reflecting regional West Antarctic Rift fabric. Interpreted mafic magmatism of the E7 is likely related to mid-Cretaceous and Late Cenozoic regional crustal extension and possible mantle plume activity over wMBL. Magnetic lineaments of the E7 are enhanced in maximum horizontal gradient of pseudo-gravity, vertical derivative and 3D Euler Deconvolution maps. Apparent vertical offsets in magnetic basement at the location of the lineaments and spatially associated mafic dikes and volcanic rocks result from 2.5D magnetic modelling. A rift-related fault origin for the magnetic lineaments, segmenting the E7 region into horst and graben blocks, is proposed by comparison with offshore seismic reflection, marine gravity, on-land gravity, radio-echo sounding, apatite fission track data and structural geology. The NNW magnetic lineament, which we interpret to mark the eastern RSR shoulder, forms the western margin of the “Alexandra Mountains horst”. This fundamental aeromagnetic feature lies on strike with the Colbeck Trough, a prominent NNW half-graben linked to Late Cretaceous(?) and Cenozoic(?) faulting in the eastern RSR. East–west and north–north–east to NE magnetic trends are also imaged. Magnetic trends, if interpreted as reflecting the signature of rift-related normal faults, would imply N–S to NE crustal extension followed by later northwest–southeast directed extension. NW–SE extension would be compatible with Cenozoic(?) oblique RSR rifting. Previous structural data from the Ford Ranges have, however, been interpreted to indicate that both Cretaceous and Cenozoic extensions were N–S to NE–SW directed.     

Geophysical evidence for an extensive Pie de Palo Complex mafic–ultramafic belt,San Juan,Argentina

The recent completion of a high-resolution aeromagnetic survey over the Pie de Palo uplift of the western Sierras Pampeanas has revealed an area of large magnetic anomalies associated with the Pie de Palo Complex. The Las Pirquitas thrust, which has transported and uplifted the Pie de Palo Complex, is recognized for at least 30 km in a roughly NE direction along the western boundary of the Pie de Palo Complex, beyond its limited outcrop. The type of sediments of the Caucete Group in the footwall of the Las Pirquitas thrust, which are regarded as the leading edge of the Precordillera terrane, are associated with much less pronounced magnetic anomalies.In addition, a conspicuous, NNE trending, broad magnetic high stands out in the survey, several kilometers to the east of the main outcrops of the Pie de Palo Complex; this broad magnetic anomaly bisects the Pie de Palo basement block, and continues further south at least as far as 32°S, the southern boundary of the latest aeromagnetic survey. This magnetic anomaly is interpreted to represent a structure corresponding to the Grenvillian Precordillera–Pie de Palo tectonic boundary zone, and would comprise the buried largest part of the mafic–ultramafic belt.The geophysical model of the magnetic data indicates that the boundary zone dips to the east, possibly suggesting the existence of a set of synthetic east dipping, west-verging thrusts, of which only one major structure (Las Pirquitas thrust) is exposed; the possibility of other slivers of upthrust boundary zone material cannot be excluded. It is considered that the Pie de Palo Complex represents a small sliver upthrust from the unexposed boundary zone material (containing highly magnetic mafic–ultramafic rocks).The east-dipping, west verging structures associated with the Pie de Palo Complex are suggested to represent an Ordovician reactivation of a Grenvillian suture zone developed when the Precordillera basement and Pie de Palo terrane docked; this reactivation probably resulted from the collision of the Cuyania terrane onto the western margin of Gondwana.     

Rock magnetic study of basic intrusions and massive sulphides in the Hercynian central Jebilets Massif (Occidental-Meseta), Morocco

This paper describes a multidisciplinary study approach (petrography and rock magnetism) conducted on samples collected from the study area to characterise the magnetic mineralogy and to determine if the magnetisation of both lithologies were induced or retain a remnant component. Petrophysical, mineralogical and geochemical analyses confirm bimodal aspects, particularly in basic rocks; the two magnetic modes depend essentially on the mineralogical and geochemical characteristics of the samples. The ultramafic rocks comprise a highly altered primary mineralogy with chromite and magnetite as magnetic phases. The second type is of mafic composition with a less altered primary mineralogy and essentially magnetite and/or (hemo-ilmenite) as a carrier of magnetic mineralisation. Sulphides are characterised by high concentrations of Cu, Zn and Pb. The mineralogy is composed mainly of pyrrhotite (85% to 90%), sphalerite, galena, chalcopyrite, arsenopyrite and, occasionally, stannite. Monocline pyrrhotite seems to be the magnetic carrier of magnetisation in both Draa Sfar and Koudiat Aïcha. However, we suspect a different amount of hexagonal pyrrhotite as the cause of different magnetic behaviour. Paleomagnetic and thermomagnetic analyses reveal different. The calculated characteristic direction of natural remnant magnetisation for sulphides was used to model the magnetic anomaly of Draa Sfar. The proposed model match the geological features concluded from geological mapping and boreholes. Results from this work can be very useful for any modelling processes of magnetic anomalies suspected due to a sulphide mineralisation in an area with poor outcrops and no presence of boreholes information or of any geological or geochemical data.     

Reappraising the P–T evolution of the Rogaland–Vest Agder Sector,southwestern Norway

The Rogaland-Vest Agder Sector of southwestern Norway comprises high-grade metamorphic rocks intruded by voluminous plutonic bodies that include the ~1000 km~2 Rogaland Igneous Complex(RIC).New petrographic observations and thermodynamic phase equilibria modelling of three metapelitic samples collected at various distances(30 km,10 km and ~ 10 m) from one of the main bodies of RIC anorthosite were undertaken to assess two alternative P-T-t models for the metamorphic evolution of the area.The results are consistent with a revised two-phase evolution.Regional metamorphism followed a clockwise P-T path reaching peak conditions of ~ 850-950 ℃ and ~7-8 kbar at ~1035 Ma followed by high-temperature decompression to ~5 kbar at ~950 Ma,and resulted in extensive anatexis and melt loss to produce highly residual rocks.Subsequent emplacement of the RIC at ~930 Ma caused regional-scale contact metamorphism that affected country rocks 10 km or more from their contact with the anorthosite.This thermal overprint is expressed in the sample proximal to the anorthosite by replacement of sillimanite by coarse intergrowths of cordierite plus spinel and growth of a second generation of garnet,and in the intermediate(10 km) sample by replacement of sapphirine by coarse intergrowths of cordierite,spinel and biotite.The formation of late biotite in the intermediate sample may suggest the rocks retained small quantities of melt produced by regional metamorphism and remained at temperatures above the solidus for up to 100 Ma.Our results are more consistent with an accretionary rather than a collisional model for the Sveconorwegian Orogen.     

Age and significance of voluminous mafic–ultramafic magmatic events in the Murchison Domain,Yilgarn Craton

Mafic–ultramafic rocks in structurally dismembered layered intrusions comprise approximately 40% by volume of greenstones in the Murchison Domain of the Youanmi Terrane, Yilgarn Craton. Mafic–ultramafic rocks in the Murchison Domain may be divided into five components: (i) the ～2810 Ma Meeline Suite, which includes the large Windimurra Igneous Complex; (ii) the 2800 ± 6 Ma Boodanoo Suite, which includes the Narndee Igneous Complex; (iii) the 2792 ± 5 Ma Little Gap Suite; (iv) the ～2750 Ma Gnanagooragoo Igneous Complex; and (v) the 2735–2710 Ma Yalgowra Suite of layered gabbroic sills. The intrusions are typically layered, tabular bodies of gabbroic rock with ultramafic basal units which, in places, are more than 6 km thick and up to 2500 km² in areal extent. However, these are minimum dimensions as the intrusions have been dismembered by younger deformation. In the Windimurra and Narndee Igneous Complexes, discordant features and geochemical fractionation trends indicate multiple pulses of magma. These pulses produced several megacyclic units, each ～200 m thick. The suites are anhydrous except for the Boodanoo Suite, which contains a large volume of hornblende gabbro. They also host significant vanadium mineralisation, and at least minor Ni–Cu–PGE mineralisation. Collectively, the areal distribution, thickness and volume of mafic–ultramafic magma in these complexes is similar to that in the 2.06 Ga Bushveld Igneous Complex, and represents a major addition of mantle-derived magma to Murchison Domain crust over a 100 Ma period. All suites are demonstrably contemporaneous with packages of high-Mg tholeiitic lavas and/or felsic volcanic rocks in greenstone belts. The distribution, ages and compositions of the earlier mafic–ultramafic rocks are most consistent with genesis in a mantle plume setting.     

Baddeleyite and zircon U-Pb ages of the ultramafic rocks in Chigu Tso area,Southeastern Tibet and their constraints on the timing of Comei Large Igneous Province

A suite of ultramafic and mafic rocks developed in the Chigu Tso area, eastern Tethyan Himalaya. Baddeleyite and zircon U-Pb ages acquired by SIMS and LA-ICP-MS from olivine pyroxenite rocks in the Chigu Tso area are 138.9±3.0 Ma and 139.0±1.9 Ma, respectively. These two Early Cretaceous ages are similar with the ages of the more abundant mafic rocks in the eastern Tethyan Himalaya, indicating that this suite of ultramafic and mafic rocks in the Chigu Tso area should be included in the outcrop area of the Comei Large Igneous Province (LIP). These ultramafic rocks provide significant evidence that the involvement of mantle plume/hot spot activities in the formation of the Comei LIP. Baddeleyite U-Pb dating by SIMS is one reliable and convenient method to constrain the formation time of ultramafic rocks. The dating results of baddeleyite and zircon from the olivine pyroxenite samples in this paper are consistent with each other within analytical uncertainties, suggesting that baddeleyite and zircon were both formed during the same magmatic process. The consistency of baddeleyite U-Pb ages in the Chigu Tso area with zircon U-Pb ages for a large number of Early Cretaceous mafic rocks in the eastern Tethyan Himalaya further support that zircon grains from such mafic rocks yielding Early Cretaceous ages are also magmatic in origin.     

冀东孤山子航磁异常特征及其找矿前景

[摘 要]冀东孤山子航磁异常为冀东三大航磁异常之一,且位于冀东石人沟-板城铁矿集中区。通过对本区地质构造特征、岩矿石物性及重磁异常特征等进行综合研究,认为孤山子航磁异常并非完全由孤山子基性-超基性岩杂岩体和已控制的浅地表超贫磁铁矿体所引起,进而采用2. 5D 拟合技术进行了定量拟合计算和资源量估算,认为孤山子航磁异常具有较好的找铁矿前景。同时,通过对孤山子航磁异常的推断解释,总结了减少航磁异常解释多解性的思路与做法。     

东南极南查尔斯王子山条带状含铁建造(BIF)的基本特征及成矿潜力

东南极南查尔斯王子山条带状含铁建造(BIF)产于鲁克山古元古代鲁克群的底部,总厚400 m,矿体厚度30~70 m,铁矿平均品位33.5%。该条带状含铁建造形成过程可能与变质火山岩有联系,在成因分类上属于苏必利尔湖型含铁建造和阿尔戈马型含铁建造之间的过渡类型。高精度航磁测量在鲁克山圈定出宽约10 km的北、南两条磁异常条带,延长分别约为50 km和60 km。据此初步建立该地区沉积变质型铁矿预测模型,圈定了含铁建造的资源分布范围,最终估算出铁矿石可开采的资源量大于百亿吨。      

琼北新生代火山构造的航磁异常特征及其地质意义

琼北地区发育大量新生代火山岩,是了解东南亚新生代构造演化的重要窗口之一。本文利用琼北地区最新航磁资料,采用航磁ΔT原始数据进行化极、垂向一阶导数等频率域转换处理等方法,并结合琼北新生代火山岩研究的其它资料,对琼北新生代火山岩区域航磁特征以及典型地区航磁异常特征进行了分析。研究结果表明:琼北东部与西部区域磁场特征存在一定的差异,即其东部具有平缓变化的负磁场特征,而中—西部具有强烈升高正磁场特征,这可能与琼北平缓变化的负背景场上叠加的新生代火山岩磁性强弱以及在空间上分布的不均匀有关。琼北新生代火山构造航磁异常具有两种不同的类型,第一类其内环航磁异常表现为不规则椭圆状负磁异常,负异常周围被环形升高的串珠状正磁异常所包围,该类航磁异常可能由塌陷式盾状火山锥群引起;第二类其外环由多个圈闭不规则的串珠状正磁异常组成,环形构造内部总体为负异常,但中心分布两个强度较大的正异常,该类航磁异常可能与早期塌陷式盾状火山锥群并叠加晚期磁性玄武质岩浆活动有关。     

内蒙古西部银根-额济纳旗盆地航磁 异常特征及地质意义

在对银根-额济纳旗(银-额)盆地出露的岩体的磁性特征和航磁异常特征分析的基础上,结合局部重力异常资料和前人在区域地质方面的研究成果,讨论了航磁异常与岩浆岩之间的联系,定量拟合解释了盆地东部地区的一条重磁剖面,进而探讨了岩浆岩的分布规律。研究结果表明,银根-额济纳旗盆地各类岩体磁性分布不均匀,哨马营东南侧的中性岩、银根附近的中性和酸性岩磁性较强,而其它地区出露的岩体的磁性则为弱磁性或无磁性,并且基性岩总体表现为弱磁性;区域磁异常反映了磁性基底的特征,额济纳旗西侧、苏亥图—乌力吉—苏红图一带区域磁力高异常是古元古界深变质岩系的反映,额济纳旗—绿园—特罗西滩—因格井和银根—查干德勒苏一带区域磁力低异常是前二叠系中浅变质岩系的反映;额济纳旗、哨马营北—特罗西滩、苏亥图西—因格井—乌力吉—查干德勒苏一带局部磁力高异常主要由中酸性岩所引起,哨马营—陶勒特—特罗西滩东一带局部磁力高异常主要由中性岩体所引起,绿园—务桃亥—因格井一带局部磁力高异常主要由沉积盖层中磁性物质所引起;区域性大断裂控制着银-额盆地磁性侵入岩的发育。     

Silkeborg gravity high revisited: Horizontal extension of the source and its uniqueness

Silkeborg Gravity High is a dominant positive gravity anomaly in Denmark. It is associated with an igneous intrusion within the crust. A deep refraction seismic profile locates the top of the intrusion in depths between 11 km and 25 km. The present contribution should be read together with two other papers by the author (Strykowski, 1998; Strykowski, 1999) dealing with the modelling problems of the same area.Strykowski (1998) discusses an advanced method of geological stripping. The focus is on coupling various types of piecewise information (depth to the top/base of geological bodies/layers obtained from depth converted seismograms and interpolated to a horizontal grid, surface gravity data, and mass density information from boreholes). The objective is to model the surface gravity response of known sediments to a depth level of 10 km.Illustrated by the practical example (modelling of the source of Silkeborg Gravity High) Strykowski (1999) discusses methodological aspects of extracting information about the geometry of the source body (in 3D) from (geologically stripped) surface gravity data and from a cross-secting deep seismic profile. The average mass density contrast between the source body (the intrusion) and the surroundings is estimated. The used geometrical information from the seismogram is weak (only the depth interval). A remarkable result of this investigation is that the along profile cross section of the obtained (3D-)structure agrees with the geometrical information of the refraction seismic profile.The present paper is an attempt to extend this result to the rest of the sedimentary basin. Of particular interest is another positive gravity anomaly (another intrusion?) located to the north-west of the studied anomaly. A “final model” obtained here estimates the depth to the source body to 14 km.Nevertheless, the focus of the present paper is not on finding a particular “best model” of the subsurface, but on ambiguity considerations. Especially, on how the different assumptions alter the obtained model? The interesting aspect is whether the used assumptions are supported by the available information.     

航空重力局部异常地质成因分类及找矿意义

以我国首次试验的航空重力数据为基础,通过局部重力异常的突出方法处理和反演拟合,结合航磁、地震、钻井等资料,按照引起局部异常的地质体性质,将研究区的局部重力异常划分为6类,即与基岩凸起有关的局部异常、与基岩凹陷有关的局部异常、与侵入岩有关的局部异常、与喷出岩有关的局部异常、与中酸性岩浆岩中老地层有关的局部异常、与深部地幔柱有关的局部异常.每类局部异常又可根据具体的地质体属性划分为若干种局部异常,并构成某种矿产资源的反映.其中,基岩凸起异常、基岩凹陷异常、岩浆岩体异常分别构成油气田、煤田、金属矿床的重要找矿标志.正确认识局部重力异常及其成因,是航空重力地质资源调查的目标之一,具有重要的找矿意义.     

Lu–Hf and U–Pb isotope systematics of zircons from the Storgangen intrusion, Rogaland Intrusive Complex, SW Norway: implications for the composition and evolution of Precambrian lower crust in the Baltic Shield

The Storgangen orebody is a concordantly layered, sill-like body of ilmenite-rich norite, intruding anorthosites of the Rogaland Intrusive Complex (RIC), SW Norway. 17 zircon grains were separated from ca. 5 kg of sand-size flotation waste collected from the on-site repository from ilmenite mining. These zircons were analysed for major and trace elements by electron microprobe, and for U–Pb and Lu–Hf isotopes by laser ablation microprobe plasma source mass spectrometry. Eight of the zircons define a well-constrained (MSWD=0.37) concordant population with an age of 949±7 Ma, which is significantly older than the 920–930 Ma ages previously reported for zircon inclusions in orthopyroxene megacrysts from the RIC. The remaining zircons, interpreted as inherited grains, show a range of ²⁰⁷Pb/²⁰⁶Pb ages up to 1407±14 Ma, with an upper intercept age at ca. 1520 Ma. The concordant zircons have similar trace element patterns, and a mean initial Hf isotope composition of ¹⁷⁶Hf/¹⁷⁷Hf_{949 Ma}=0.28223±5 (_Hf=+2±2). This is similar to the Hf-isotope composition of zircons in a range of post-tectonic Sveconorwegian granites from South Norway, and slightly more radiogenic than expected for mid-Proterozoic juvenile crust. The older, inherited zircons show Lu–Hf crustal residence ages in the range 1.85–2.04 Ga. One (undated) zircon plots well within the field of Hf isotope evolution of Paleoproterozoic rocks of the Baltic Shield. These findings indicate the presence of Paleoproterozoic components in the deep crust of the Rogaland area, but do not demonstrate that such rocks, or a Sveconorwegian mantle-derived component, contributed significantly to the petrogenesis of the RIC. If the parent magma was derived from a homogeneous, lower crustal mafic granulite source, the lower crustal protolith must be at least 1.5 Ga old, and it must have an elevated Rb/Sr ratio. This component would be indistinguishable in Sr, Nd and Hf isotopes from some intermediate mixtures between Sveconorwegian mantle and Paleoprotoerzoic felsic crust, but it cannot account for the initial ¹⁴³Nd/¹⁴⁴Nd of the most primitive, late Sveconorwegian granite in the region, without the addition of mantle-derived material.     

Airborne gamma-ray and magnetic anomaly signatures of serpentinite in relation to soil geochemistry, northern California

Serpentinized ultramafic rocks and associated soils in northern California are characterized by high concentrations of Cr and Ni, low levels of radioelements (K, Th, and U) and high amounts of ferrimagnetic minerals (primarily magnetite). Geophysical attributes over ultramafic rocks, which include airborne gamma-ray and magnetic anomaly data, are quantified and provide indirect measurements on the relative abundance of radioelements and magnetic minerals, respectively. Attributes are defined through a statistical modeling approach and the results are portrayed as probabilities in chart and map form. Two predictive models are presented, including one derived from the aeromagnetic anomaly data and one from a combination of the airborne K, Th and U gamma-ray data. Both models distinguish preferential values within the aerogeophysical data that coincide with mapped and potentially unmapped ultramafic rocks. The magnetic predictive model shows positive probabilities associated with magnetic anomaly highs and, to a lesser degree, anomaly lows, which accurately locate many known ultramafic outcrops, but more interestingly, locate potentially unmapped ultramafic rocks, possible extensions of ultramafic bodies that dip into the shallow subsurface, as well as prospective buried ultramafic rocks. The airborne radiometric model shows positive probabilities in association with anomalously low gamma radiation measurements over ultramafic rock, which is similar to that produced by gabbro, metavolcanic rock, and water bodies. All of these features share the characteristic of being depleted in K, Th and U. Gabbro is the only rock type in the study area that shares similar magnetic properties with the ultramafic rock. The aerogeophysical model results are compared to the distribution of ultramafic outcrops and to Cr, Ni, K, Th and U concentrations and magnetic susceptibility measurements from soil samples. Analysis of the soil data indicates high positive correlation between magnetic susceptibilities and concentration of Cr and Ni. Although the study focused on characterizing the geophysical properties of ultramafic rocks and associated soils, it has also yielded information on other rock types in addition to ultramafic rocks, which can also locally host naturally-occurring asbestos; specifically, gabbro and metavolcanic rocks.     

Gravity and magnetic 3D inversion of Morro do Engenho complex, Central Brazil

In the Goiás Alkaline Province, in central Brazil, some mafic–ultramafic alkaline bodies crop out, such as Morro do Engenho Complex (ME), whereas other intrusive bodies are completely covered by Quaternary sediments, such as A2 body, and can only be defined by their magnetic and gravity responses. The bodies give rise to strong gravity and magnetic anomalies. We inverted the gravity and magnetic data to estimate 3D density and magnetic intensity contrasts. The gravity inversion reveals that ME and A2 intrusions, totally emplaced in the upper crust, have maximum depths of about 10 and 6 km, respectively. The estimated density contrast distribution indicates a NE–SW preferential direction for ME intrusive body. Since the remnant magnetization is very important in the area, we inverted the magnetic data, testing two hypotheses: only induced field and induced plus remnant magnetizations. The results of the inversion using only induced field could not explain the observed magnetic field, which was about 1000 nT larger than the calculated one. Induced plus remnant field inferred from wandering polar path fitted the observations. 3D magnetic inversions when remnant magnetization was included recover deeper and thinner bodies compared with gravity inversion. The inverted bodies have spherical shapes which are more consistent with the model of magmatic chambers instead of plug intrusions.