The Gangdese batholith, more than 2500 km in length, is composed mainly of JurassicMiocene igneous rocks. This batholith is one of the most important constituents of the Tibetan orogenesis and provides an ideal place for study of Neo-Tethyan ocean geodynamic evolution and plateau uplift. Recent studies on the Gangdese Jurassic felsic magmatism highlight its juvenile source. However, important aspects concerning the genesis of the juvenile magmatism and related deep geodynamic evolution are still unclear. Here, we report detailed petrological, geochronological, geochemical, whole-rock Sr-Nd isotopic, and in situ Sr-Hf isotopic data for a recently identified hornblende gabbro in the Dongga area, southern Lhasa sub-block. This hornblende gabbro is dominated by hornblende and plagioclase, dated at Early Jurassic(ca. 180–190 Ma), and characterized by a narrow compositional range in SiO_2(49.38wt%–52.27wt%), MgO(4.08wt%–7.00wt%), FeO(10.43wt%–11.77wt%), Na_2O(2.58wt%–3.51wt%), and K_2O(0.48wt%–1.53wt%). It has depleted isotopic signatures, with whole-rock(~(87)Sr/~(86)Sr)i ratios of 0.7033–0.7043, ε_(Nd)(t) values of +4.90 to +6.99, in situ plagioclase(~(87)Sr/~(86)Sr)i ratios of 0.7034–0.7042, and zircon ε_(Hf)(t) of +12.2 to +16.8. Our results integrated with published data suggest a model of Gangdese juvenile crustal growth by a subduction-related water-enriched mantle wedge. The hydrous partial melting of the lithosphere mantle was triggered by the dehydration of a Neo-Tethyan oceanic slab. This mafic magmatism emplaced in the middle-lower crust of intraoceanic arcs or active continental margins, leading to Jurassic juvenile crustal growth in southern Tibet. 相似文献
Most porphyry Cu–Mo–Au deposits are found in magmatic arcs worldwide, and are associated with hydrous, high-fO2, calc-alkaline magmas, derived from a mantle wedge that was metasomatized by the fluids from a subducted oceanic slab. Recently, such deposits have been documented as occurring widely in collisional settings, where they are associated with potassic magmas generated during the collisional process, but the genesis of the fertile magmas and the mechanism of metallic enrichment remain controversial. Here we present new geochemical and Sr–Nd–Hf isotopic data from the post-collisional fertile and barren porphyries of the Miocene Gangdese porphyry belt in the Tibetan orogen, an orogen formed by the collision of India and Asia in the early Cenozoic. Both types of porphyry are characterized by high K2O contents, and have geochemical affinities with adakite, but the fertile magmas were most likely derived from the melting of a thickened juvenile mafic lower-crust, formed by the underplating of earlier asthenospheric melts at the base of crust, whereas the derivation of the barren magmas involved variable amounts of old lower-crust in Tibet. The melting of sulfide-bearing phases in the juvenile mantle components of the Tibetan lower-crust probably provided Cu, Au, and S to the fertile magmas. The breakdown of amphibole during melting at the source released the fluids necessary for the formation of the porphyry Cu deposits in Tibet. The thickened crust (up to 70–80 km), due to collision, is thought to be responsible for a decrease in the fO2 of the fertile magmas during their ascent to the upper crust, thus preventing the generation of more porphyry Cu–Au and epithermal Au deposits in this collisional zone. 相似文献
China's urban expansion, food security, and energy transition are in a critical situation. One solution is to tap into the green production potential of the built urban environment and explore new ways to save land space and alleviate ecological pressure through food and solar energy production. This paper differs from previous ecological compensation studies, which mostly focus on key ecological functional areas or fiscal compensation mechanisms, in that it innovatively establishes an ecological compensation mechanism within the urban space. In this paper, we propose the "green productive area" of cities as a way to measure the ecological carrying capacity improvement potential of cities from the perspective of urban ecology, and it is based on converting the green resource income of cities into the ecological footprint area they could save under the same conditions. First, a typological approach was used to establish a compensation strategy for green production. Second, a spatial inventory was taken of all elements of the built environment and an analysis of their green production potential was carried out. Finally, it was necessary to establish a unified accounting standard for the ecological land saving benefits of different green production options, which could be converted into green productive land area indicators. In the case of Xuefu Street in Nankai District, Tianjin, the available rooftops and idle land were used for green production, which supplemented the ecological carrying capacity provided by the natural land occupied by 12% of the buildings in the district. 相似文献
The Beiya porphyry-skarn Au deposit is one of the largest gold deposits in China, temporally and spatially associated with Eocene intrusions in a post-collisional setting in western Yunnan, China. In this study, we report new whole-rock geochemistry, Sr-Nd isotope, zircon U-Pb geochronology and in situ zircon Hf-O isotopes of quartz-monzonite and biotite-monzonite porphyries from the Beiya deposit. The porphyry-skarn mineralization at the Beiya deposit is mainly associated with the quartz monzonite porphyry (35.8 ± 0.6 Ma), while the biotite-monzonite porphyry (34.3 ± 0.5 Ma) represents a post-mineralization intrusion crosscutting the main orebodies and the quartz-monzonite porphyry. Both intrusions have high-K and adakitic composition and are characterized by high Sr/Y ratios, high SiO2 and Al2O3 concentrations (SiO2 = 69.80–73.86 wt%; Al2O3 = 14.11–15.19 wt%), and low MgO, Cr, and Ni concentrations (MgO = 0.2–1.0 wt%; Cr = 1.76–11.13 ppm; Ni = 2.52–11.72 ppm). Their Sr-Nd isotope compositions (87Sr/86Sr = 0.7066–0.7077; εNd(t) = ?5.3 to ?1.5) are consistent with the lower crustal-derived amphibolite xenoliths (87Sr/86Sr = 0.7060–0.7100; εNd(t) = ?10.0 to 0.0), indicating that they might be derived from a thickened juvenile lower crust beneath the Yangtze Craton. The biotite-monzonite porphyry has lower zircon δ18O values of +5.3‰ to +6.8‰ and higher εHf(t) values of ?2.3 to +5.5 than those of the quartz-monzonite porphyry with δ18O values of +7.1‰ to +8.2‰ and εHf(t) values of ?3.8 to +1.5, implying that they were derived from different parts of the lower crust. High Ba/La and Pb/Ce ratios suggest that the quartz-monzonite porphyry is derived from a volatiles-rich reservoir. Relatively higher La/Yb, Sm/Yb and Dy/Yb ratios of the biotite-monzonite porphyry indicate residual garnet in the source, indicating a deeper source than that of the quartz-monzonite porphyry. The hydrous components should be represented by the amphibole-rich lithologies, which has relatively shallower depth than that of the garnet-bearing mafic thickened lower crust. Our data suggest that the mineralized quartz-monzonite porphyry at the Beiya deposit is derived from partial melting of amphibole-rich lithologies in the upper part of the thickened juvenile lower crust beneath the Yangtze Craton, while the post-mineralization biotite-monzonite porphyry is derived from the basal, and volatiles-poor, part of the juvenile lower crust. 相似文献
The Zhaxikang Pb-Zn-Ag-Sb deposit, the largest polymetallic deposit known in the Himalayan Orogen of southern Tibet, is characterized by vein-type mineralization that hosts multiple mineral assemblages and complicated metal associations. The deposit consists of at least six steeply dipping veintype orebodies that are hosted by Early Jurassic black carbonaceous slates and are controlled by a Cenozoic N–S-striking normal fault system. This deposit records multiple stages of mineralization that include an early period(A) of massive coarse-grained galena–sphalerite deposition and a later period(B) of Sb-bearing vein-type mineralization. Period A is only associated with galena–sphalerite mineralization, whereas period B can be subdivided into ferrous rhodochrosite–sphalerite–pyrite, quartz–sulfosalt–sphalerite, calcite–pyrite, quartz–stibnite, and quartz-only stages of mineralization. The formation of brecciated galena and sphalerite ores during period A implies reworking of pre-existing Pb–Zn sulfides by Cenozoic tectonic deformation, whereas period B mineralization records extensive openspace filling during ore formation. Fluid inclusion microthermometric data indicate that both periods A and B were associated with low–medium temperature(187–267°C) and low salinity(4.00–10.18% wt. Na Cl equivalent) ore-forming fluids, although variations in the physical–chemical nature of the period B fluids suggest that this phase of mineralization was characterized by variable water/rock ratios. Microprobe analyses indicate that Fe concentrations in sphalerite decrease from period A to period B, and can be divided into three groups with Fe S concentrations of 8.999–9.577, 7.125–9.109, 5.438–1.460 mol.%. The concentrations of Zn, Sb, Pb, and Ag within orebodies in the study area are normally distributed in both lateral and vertical directions, and Pb, Sb, and/or Ag concentrations are positive correlation within the central part of these orebodies, but negatively correlate in the margins. Sulfide S isotope compositions are highly variable(4‰–13‰), varying from 4‰ to 11‰ in period A and 10‰ to 13‰ in period B. The Pb isotope within these samples is highly radiogenic and defines linear trends in 206 Pb/204 Pb vs. 207 Pb/204 Pb and 206 Pb/204 Pb vs. 208 Pb/204 Pb diagrams, respectively. The S and Pb isotopic characteristics indicate that the period B orebodies formed by mixing of Pb–Zn sulfides and regional Sbbearing fluids. These features are indicative of overprinting and remobilization of pre-existing Pb–Zn sulfides by Sb-bearing ore-forming fluids during a post-collisional period of the Himalayan Orogeny. The presence of similar ore types in the north Rhenish Massif that formed after the Variscan Orogeny suggests that Zhaxikang-style mineralization may be present in other orogenic belts, suggesting that this deposit may guide Pb–Zn exploration in these areas. 相似文献
Saindak is one of the typical porphyry Cu deposits (PCDs) in the Chagai magmatic arc in Pakistan. Ore-forming porphyries at Saindak PCD are mainly composed of tonalite. Here, we use geochemistry of apatite enclosed in plagioclase phenocrysts from the ore-forming tonalite to constrain the releasing and recharging processes of S and Cl in the underlying parental magma chamber during PCD mineralization. Although apatite inclusions have homogeneous intra-grain S and Cl compositions, there is significant inter-grain S and Cl variations in apatite inclusions located from core to rim in the hosting plagioclase. Such inter-grain S and Cl variation in apatites are coupled with the core-to-rim trends of An, FeO and Mg contents of the hosting plagioclase phenocryst. It indicates that the Saindak PCD likely formed by episodic injection of primitive magmas during the growth of an underlying magma chamber, rather than by one major injection or by addition of mafic melt derived from different source region. Each primitive melt injection introduced essential ore-forming materials such as S and Cl, which were rapidly and effectively released to the coexisting fluids, causing mineralization. Once primitive melt injection stops, signaling the end of growth of underlying magma chamber, mineralization will cease quickly although the hydrothermal system can still survive for a long time. However, the later released fluids are relatively depleted in ore-forming materials, and thus have lower capability to generate mineralization. Accordingly, predominant porphyry-type mineralizations occurred during the growth rather than waning stage of a magmatic system.
The Nianzha gold deposit,located in the central section of the Indus-Yarlung Tsangpo suture(IYS) zone in southern Tibet,is a large gold deposit(Au reserves of 25 tons with average grade of 3.08 g/t) controlled by a E-W striking fault that developed during the main stage of Indo-Asian collision(~65-41 Ma).The main orebody is 1760 m long and 5.15 m thick,and occurs in a fracture zone bordered by Cretaceous diorite in the hanging wall to the north and the Renbu tectonic melange in the footwall to the south.High-grade mineralization occurs in a fracture zone between diorite and ultramafic rock in the Renbu tectonic melange.The wall-rock alteration is characterized by silicification in the fracture zone,serpentinization and the formation of talc and magnesite in the ultramafic unit,and chloritization and the formation of epidote and calcite in diorite.Quartz veins associated with Au mineralization can be divided into three stages.Fluid inclusion data indicate that the deposit formed from H_2O-NaCl-organic gas fluids that homogenize at temperatures of 203℃-347℃ and have salinities of 0.35wt%-17.17wt%NaCl equivalent.The quartz veins yield δ~(18)O_(fluid) values of 0.15‰-10.45‰,low δD_(V-SMOW)values(-173‰ to-96‰),and the δ~(13)C values of-17.6‰ to-4.7‰,indicating the ore-forming fluids were a mix of metamorphic and sedimentary orogenic fluids with the addition of some meteoric and mantle-derived fluids.The pyrite within the diorite has δ~(34)S_(V-CDT) values of-2.9‰-1.9‰(average-1.1‰),~(206)Pb/~(204)Pb values of 18.47-18.64,~(207)Pb/~(204)Pb values of 15.64-15.74,and ~(208)Pb/~(204)Pb values of 38.71-39.27,all of which are indicative of the derivation of S and other ore-forming elements from deep in the mantle.The presence of the Nianzha,Bangbu,and Mayum gold deposits within the IYS zone indicates that this area is highly prospective for large orogenic gold deposits.We identified three types of mineralization within the IYS,namely Bangbu-type accretionary,Mayum-type microcontinent,and Nianzha-type ophiolite-associated orogenic Au deposits.The three types formed at different depths in an accretionary orogenic tectonic setting.The Bangbu type was formed at the deepest level and the Nianzha type at the shallowest. 相似文献