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Global Geology
2018 Vol.21 Issue.2
Published 2018-06-25
Articles
Articles
77
Geochemistry, petrogenesis and tectonic implication of Early Cretaceous A-type rhyolites in Hailar Basin, NE China
ZHENG Han, SUN Xiaomeng
Early Cretaceous A-type rhyolites of the Shangkuli Formation in the Hailar Basin of NE China exhibit geochemical characteristics of high silicon,alkali,Fe/Mg,Ga/Al,Zr,Pb,HFSEs,and REE contents but low Ca,Ba,Sr and Eu,which meet the criteria of typical reduced A-type granite.The A-type rhyolites are most probably derived from magmatic underplating and partial melting of quartz-feldspathic lower crust,with the lithospheric mantle material involved,due to the extensional deformation of the Erguna-Hulun Fault.Although the A-type rhyolites show A1-type trace elements characteristics,they were formed in a post-orogenic extensional background together with the coeval widespread bimodal volcanic rocks,metamorphic core complexes,volcanic fault basins and metallogenic province in the Sino-Russia-Mongolia border tract.This extension event was related to the collapse of thickened region of the continental crust after the closure of the Mongol-Okhotsk Ocean.
2018 Vol. 21 (2): 77-90 [
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308
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91
Zircon U-Pb geochronology and Hf isotopic constraints on petrogenesis of Carnian Huanglonggou granodiorites inWulonggou area of Eastern Kunlun Orogen, NW China
SONG Kai, DING Qingfeng, ZHANG Qiang
The Wulonggou area located in the Eastern Kunlun Orogen in NW China is characterized by extensive granitoid magmatism,ductile faulting and orogenic gold mineralization.The Huanglonggou granodiorite is cut by an orogenic gold-bearing fault.This study investigated the major and trace-element compositions,zircon U-Pb dates and zircon Hf isotopic compositions of the Huanglonggou granodiorite.One Huanglonggou granodiorite sample yielded a weighted mean U-Pb zircon age of ~221 Ma (Carnian).The Carnian granodiorite is metaluminous,with high alkalis contents of 6.37%-8.86%,high Al
2
O
3
contents of 15.41%-16.19%,high Sr contents of (426-475)×10
-6
,relatively high Sr/Y ratios,high (La/Yb)
N
values and low HREE,suggesting an adakite type high-SiO
2
granite.The Huanglonggou granodiorite sample has zircon ε
Hf
(
t
) values ranging from-4.4 to+1.1.These Hf isotopic data suggest that the Carnian granodiorite was likely derived from the partial melting of subducted Paleo-Tethys oceanic slab.It is suggested that the Late Triassic granodiorite was emplaced during the northward subduction of Paleo-Tethys oceanic slab.Orogenic gold mineralization in the Wulonggou area formed after the emplacement of the Late Triassic intrusive rocks.
2018 Vol. 21 (2): 91-107 [
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313
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108
Edge detection of gravity anomaly with an improved 3D structure tensor
DAI Weiming, LI Tonglin, HUANG Danian, YUAN Yuan, LIU Kai, QIAO Zhongkun
Edge detection plays an important role in geological interpretation of potential field data,which can indicate the subsurface faults,contact,and other tectonic features.A variety of methods have been proposed to detect and enhance the edges.3D structure tensor can well delineate the edges of geological bodies,however,it is sensitive to noise and additional false edges need to be removed artificially.In order to overcome these disadvantages,this paper redefines the 3D structure tensor with a Gaussian envelop and proposes a new normalized edge detector,which can remove the additional false edges and reduce the influence of noise effectively,and balance the edges of different amplitude anomalies completely.This method has been tested on the synthetic and measured gravity data,showing that the new improved method achievesbetter results and reveals more details.
2018 Vol. 21 (2): 108-113 [
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326
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551
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114
3D fast inversion of gravity data based on GPU
WANG Xusheng, ZENG Zhaofa
Large-scale gravity 3D interpretation depends on efficient and high-resolution 3D inversion processing of massive data.The authors applied the conjugate gradient method with minimum support function and prior model constraint to reduce multi-solutions in gravity inversion.Based on the parallel programming and computing platform NVIDIA CUDA with C++language,we achieve fast 3D gravity inversion by adopting GPU parallel technique into the most time consuming part relating to sensitivity matrix.The results of theoretical model show that the abnormal body can be clearly located and the inversion speed is improved greatly.Comparing with inversion speed of the Matlab program,speed of inversion with GPU parallel technique has improved more than 100 times under the hardware condition of Geforce GTX 1060 graphics card.
2018 Vol. 21 (2): 114-119 [
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280
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120
Research on realization of Kirchhoff remigration
LI Jianhua, SUN Jianguo
Remigration is an imaging method that maps migrated image fields of different migration velocity fields to each other.It is mainly used for migration velocity analysis,wave mode transformation,and data regularization.Theoretically,this kind of mapping can be realized by differential operator,or by integral operator.Compared between the two,the integral operator achieves higher computational efficiency and has more adaptability to the irregularity of the input data.Given the fact,the authors worked out the depth domain remigration method based on the Kirchhoff integral theory with the basic theory and workflow of the Kirchhoff remigration.The calculation results on the gradient model and Marmousi model verify the effectiveness of this method.In addition,numerical experiments show that integral method is faster than the differential method.
2018 Vol. 21 (2): 120-126 [
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238
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127
Combined analysis of gravity and magnetic anomalies using normalized source strength
LI Lu, ZHANG Shuang
The traditional combined gravity and magnetic analysis uses the linear regression of the first order vertical derivative of the gravity anomaly and the reduction to the pole (RTP) magnetic anomaly,and provides the quantitative or semi-quantitative interpretation by calculating the correlation coefficient,slope,and intercept.In the calculation process,due to the remanent magnetization,the RTP anomaly still contains the effect of oblique magnetization,as a result,the homologous gravity and magnetic anomalies may display irrelevant results in the linear regression calculation.To solve this problem,we present a new combined analysis using normalized source strength (NSS).Based on the Poisson's relation,the gravity field can be transformed into the pseudomagnetic field of the direction of geomagnetic field magnetization under the homologous condition.The NSS of the pseudomagnetic field and that of the original magnetic field are calculated,which are insensitive to the remanence,and then the linear regression analysis is carried out.The approach is tested using synthetic model under complex magnetization,the results show that it can still identify the gravity and magnetic anomalies from same source under strong remanence,and can establish the Poisson's ratio.Finally,this approach is applied in Wudalianchi in China.The results demonstrated that this approach is feasible and can provide the reference for further data processing and interpretation.
2018 Vol. 21 (2): 127-133 [
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301
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134
Edge detection of potential field data based on image processing methods
TAN Xiaodi, ZHANG Dailei, MA Guoqing
The conventional methods of edge detection can roughly delineate edge position of geological bodies,but there are still some problems such as low detection accuracy and being susceptible to noise interference.In this paper,three image processing methods,Canny,LoG and Sobel operators are briefly introduced,and applied to edge detection to determine the edge of geological bodies.Furthermore,model data is built to analyze the edge detection ability of this image processing methods,and compare with conventional methods.Combined with gravity anomaly of Sichuan basin and magnetic anomaly of Zhurihe area,the detection effect of image processing methods is further verified in real data.The results show that image processing methods can be applied to effectively identify the edge of geological bodies.Moreover,when both positive and negative anomalies exist and noise is abundant,fake edge can be avoided and edge division is clearer,and satisfactory results of edge detection are obtained.
2018 Vol. 21 (2): 134-142 [
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303
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143
Azimuthal moveout response of seismic waves in two-phase anisotropic media
XU Liying, MO Xiuwen
Dispersion and attenuation occur while seismic wave travels through cracks filled with fluids,which lead to the anisotropism of seismic azimuthal travel time.Based on latest rock physics models,this study aims to simulate seismic azimuthal moveout responses (AMR) and analyze the factors affecting this attribute.By numerical modeling,it is found that the AMR is very sensitive to the parameters of the cracks,especially these related to fluid;therefore AMR has the potential to qualitatively or even quantitatively identify cracks.
2018 Vol. 21 (2): 143-150 [
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306
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