The method of bi-dimensional empirical mode decomposition (BEMD) and the combined methods of entropy weight-Technique for Order of Preference by Similarity to an Ideal Solution (TOPSIS) were used to decompose gravity-magnetic data and evaluate targets in the Luziyuan Pb-Zn-Fe polymetallic ore field and surrounding areas. Three meaningful bidimensional intrinsic mode function (BIMF) images were obtained by BEMD at different wavelengths, depicting different layers of geological architectures in the study area. The results are as follows. (1) The BIMF2 images depict the shallow local geological architecture and show positive gravity-magnetic anomalies of the skarn alteration and Pb-Zn-Fe mineralization distributed around concealed granites. (2) The BIMF3 images depict the medium-depth geological architecture, indicating that concealed granitic stocks, which are shallow extensions of a deeply concealed pluton, intruded along the NE-trending fault. (3) The BIMF4 images depict gravity-magnetic anomalies at greater depth, which likely reflect regional geological architectures, indicating the potential presence of a large, concealed intermediate-acid pluton in the negative anomaly zone. Three potential targets (A, B, and C) were delineated based on BEMD results of the original gravity-magnetic data. The entropy weight-TOPSIS evaluation results show that the ranking of the metallogenic potential of the delineated targets in the study area is B, A, and C, with relative proximity values of 0.4576, 0.3925, and 0.1499, respectively. The results of this study can be used to guide future exploration.
The Gejiu Anisian alkaline basalts (GAAB), distributed in the southern part of the Emeishan large igneous province (ELIP), are crucial to understand the tectonomagmatic activity during the Triassic. Geochronological, geochemical, and Sr-Nd-Pb isotopic analyses were systematically applied to explore the origin, petrogenesis, and tectonic setting of the GAAB, and how they relate to the ELIP. Zircon U-Pb dating set the eruption date at 244 Ma. Most of the samples belonged to alkaline basalts and had high TiO2 (2.14–3.23 wt.%) and MgO (4.43–19.58 wt.%) contents. Large ion lithophile elements (LILEs) were enriched relative to high field strength elements (HFSEs). The rare earth elements (REEs) and trace element signatures in the normalized diagrams were similar to oceanic island basalts (OIB) and Emeishan high-Ti basalts. These samples had consistent Sr-Nd isotope compositions: the initial 87Sr/86Sr values ranged from 0.7044 to 0.7048 and εNd(t) = 3.25–4.92. The Pb isotopes were more complex, the (206Pb/204Pb)t, (207Pb/204Pb)t, (208Pb/204Pb)t ratios were 17.493–18.197, 15.530–15.722, and 37.713–38.853, respectively. Our results indicate that the GAAB originated from the deeper enriched mantle with 5% to 15% partial melting of garnet lherzolite and a segregation depth of 2 to 4 GPa (60–120 km). During the formation of the GAAB, clinopyroxene and Ti-Fe oxides were fractionally crystallized with insignificant crustal contamination. The GAAB were formed in a extensional regime that was related to the Gejiu-Napo rift event in the Triassic.
Numerous granitoids that principally formed in the Paleozoic and Mesozoic were distributed along the GXAR and adjacent areas, which in some respects makes elucidating the tectonic evolution of this area difficult (Zhang et al., 2010;Meng et al., 2011;Wu et al., 2011;Deng et al., 2019). However, granitoids can provide vital information on the geodynamic setting in which they were generated. Extensive geochemical and geochronological research has been performed on intrusive and volcanic rocks over the past decade. The Paleo-Asian ocean was closed in the late Paleozoic and then the North China Craton was amalgamated with Mongolian terranes along the northern margin of the North China Craton, forming the eastern segment of the CAOB (Geng et al., 2009;Jiang et al., 2010). The GXAR subsequently entered the Mongol-Okhotsk and the Paleo-Pacific ocean tectonic regime (Wu et al., 2011).Over the past decades, many Mesozoic Pb-Zn-Ag deposits have been discovered in the GXAR, including the
The study of ultramafic rocks in Western Yunnan is of great significance for an understanding of the tectonic evolution of the Neo-Tethys Ocean. The zircon U–Pb data indicated that the Santaishan serpentinized pyroxene peridotite (SSPP) was formed 186–190 Ma, and the Yingjiang hornblende pyroxenite (YHP) was formed 182–183 Ma. The content of MgO in the SSPP is relatively high, but the SiO2, Al2O3, CaO and TiO2 content and ΣREE are low, while the YHP has opposite characteristics. The samples from the SSPP and YHP have similar distribution patterns of trace elements, both being enriched in large ion lithophile elements (LILEs) such as Rb, Ba and Th and depleted in high field strength elements (HFSEs) such as Ti, P and Nb. These characteristics are consistent with the supra-subduction zone (SSZ) type and mid-ocean ridge basalt (MORB) type of ophiolite in the Bangong–Nujiang suture zone. The SSPP rocks have relatively high (87Sr/86Sr)i ratios (0.7091–0.7131) and positive Hf(t) values (11.2–13.8), with εNd(t) values varying from −1.1 to 9.4. The YHP has relatively low εHf(t) values (3.5 to 6.9), with the Nd–Hf isotopic model ages ranging from 610 to 942 Ma. The signatures of Sr–Nd and Lu–Hf isotopes indicate that the SSPP and YHP were derived from the depleted mantle, and the crustal material in the magma source may have originated from the Neoproterozoic Rodinia supercontinent. In the early Middle Jurassic (190 Ma), the Tengchong Block was in the setting of an active continental margin induced by the subduction of the Bangong–Nujiang Ocean, where the SSZ-type SSPP with ophiolite characteristics was formed. With the continuous subduction of the Bangong–Nujiang Ocean, the slab retreated and induced mantle convection, which resulted in the gradual thinning of the continental crust. Meanwhile, the Yingjiang back-arc basin was formed 183 Ma. Under the influence of the upwelling of the asthenosphere and the mixture of crustal materials, the MORB-type YHP was formed.
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