| The Paleogene volcanic rocks in Linzizong Group are widely distributed in the Gangdese tectonic-magmatic belt, it is deemed to represent the result of Yarlung Zangbo Neo-tethys from its subduction to the India-Asia collision. The volcanic rocks were the important ore-bearing horizon of Pb-Zn-Cu polymetallic ore. Previous research are mostly concentrated in the southern, but the central and northern are litter attention in the Gangdese tectonic-magmatic belt. The Zexue areas of Tibet lie in the central of the Gangdese tectonic-magmatic belt, is located in the Gangdese Ag-Pb-Zn polymetallic metallogenic belt, belonging to the western of the Long Geer-Nyainqentanglha Pb, Zn, Ag, Fe, Cu mineralization subzones.Based on the exhaustive field geological survey, combining previous research results and indoor analysis work, the paper carry out a systematic analysis research to the volcanic rocks of Linzizong Group, determine the rock type of the study areas, obtain the LA-ICP-MS zircon U-Pb isotopic age of the Dianzhong formation and Nianbo formation volcanic rocks, redefine the age of the Linzizong volcanic rocks. Based on the systematic research on the lithogeochemical characteristics, discuss the tectonic setting ang petrogenesis, preliminary investigate a constraint of the Linzizong volcanic rocks to Pb-Zn polymetallic ore. The main conclusions in this paper are as follows:The volcanic rocks of Linzizong Group are relatively complete in the study areas. Dianzhong formation distribute in the southwest of Zexu sheet and the southern of Pudang sheet, with a outcrop area of about25km2; In the study areas, Nianbo formation is most widely distributed rock series in the volcanic rocks of Linzizong Group, mainly distribute in the southern of Zexu sheet, southern of Pudang sheet, southwestern and southeastern of Niangre sheet, with a outcrop area of about176km2; Pana formation is only distributed in the western and northeastern of Rendui sheet, eastern of Niangre sheet, with a outcrop area of about20.0km2.The volcanic rocks of Linzizong Group have various types of rocks, The volcanic rocks of Dianzhong formation consist of dacite, altered dacite, andesite, pyroxene andesite, altered andesite, dacitic vitric crystal tuff, dacitic crystal lithic tuffs, andesitic crystal crumbs glass tuff; The volcanic rocks of Nianbo formation consist of altered andesite, trachyandesites, pyroxene andesite, trachyte, dacite, brecciated dacite quality lithic crystal tuff, dacitic-andesitic volcanic breccia, altered andesitic crystal fragment tuff, rhyolite, volcanic breccia; The volcanic rocks of Pana formation consist of rhyolite, dacite, rhyolitic melt volcanic breccia lithic tuff, andesitic crystal tuff clip a small amount of volcanic rock.The LA-ICP-MS zircon U-Pb concordia age of the volcanic rocks in Dianzhong formation were62.90±4.7～63.78±0.46Ma that they formed in the Paleocene (E1); The LA-ICP-MS zircon U-Pb concordia age of the volcanic rocks in Nianbo formation were52.5±2.7～56.18±0.60Ma that they formed in the Eocene (E2); the whole rock K-Ar age of the biotite andesite in Pana formation were47.86±8.8Ma that they formed in the Eocene (E2).In terms of major elements, the w(SiO2) contents rank from51.88%to74.60%, ALK contents rank from5.73%to9.88%in the Dianzhong formation; the w(SiO2) contents rank from50.48%to68.74%, ALK contents rank from4.38%to8.45%in the Nianbo formation; the w(SiO2) contents rank from75.63%to76.71%, ALK contents rank from7.45%to8.14%in the Pana formation. we find that most volcanic rocks of Linzizong Group belong to metaluminous-peraluminous calc-alkaline to high-K calc-alkaline volcanic rocks, the rocks are mainly a combination of potassium (most of K2O> Na2O), and a small amount of Shoshonite. In the volcanic rocks of Dianzhong formation, REE patterns show LREE enrichment of the rightist-type [(La/Yb)N=6.68～15.94], showed moderate negative Eu anomalies(δEu=0.50～1.01, with an average of0.68) and nothing Ce anomalies (δCe=0.96～1.05, with an average of0.99); Trace elements are generally characterized by enrichment of lithophile element Rb, K, radioactive element Th,U and Nb, Ta, P, Ti elements depleted. In the volcanic rocks of Nianbo formation, REE patterns show LREE enrichment of the rightist-type [(La/Yb)N=6.89～14.29], HREE patterns are close to the level, showed moderate negative Eu anomalies(δEu=0.35～0.87, with an average of0.69) and nothing Ce anomalies (δCe=0.61～1.09, with an average of0.92); Trace elements are generally characterized by enrichment of lithophile element Rb, K, radioactive element Th,U and Nb, Ta, P, Ti elements depleted. In the volcanic rocks of Pana formation, REE patterns show LREE enrichment of the rightist-type [(La/Yb)N=6.59～8.62], HREE patterns are close to the level, showed strong negative Eu anomalies(δEu=0.20～0.21, with an average of0.21) and nothing Ce anomalies (δCe=0.99～1.00, with an average of0.99); Trace elements are generally characterized by enrichment of lithophile element Rb, K, radioactive element Th,U and Nb, Ta, P, Ti elements depleted.In the study area, Nianbo volcanic rocks of Sr-Nd-Pb isotopic composition shows high86Sr/87Sr ratio (ranging from0.706084to0.710826, with an average of0.70834), low143Nd/144Nd ratios (ranging from0.512148to0.512517, with an average of0.512331), εNd (t) varies from-9.0to-1.8,208and the average of208Pb/204Pb,207Pb/204Pb,206Pb/204Pb. ratios on39.0545,15.6928,18.6057in the Linzizong group. Volcanic source region is closely related to EM II-type enriched mantle, and exsit the mantle magma and crustal material interaction in Nianbo formation; the formation of rocks experienced subduction subduction to continental collision process. Compared to the Nianbo formation in the Linzhou basin, the study areas were more obvious controlled by the continental basement materials in Gangdese belt.The early Dianzhong formation mostly display the characteristics of the arc volcanic rocks, The mid-Nianbo formation display multiple characteristics of island arc, continental margin arc, collision and within-plate environment, however, the late Pana formation formed in a continental collision and within-plate environment. The volcanic rocks of Linzizog group formed in the transient process which were from volcanic arc to syn-collision, post-collision and within-plate.The magma source of Dianzhong formation were the island-arc type wedge-shaped mantle of the metasomatic enriched. Compared with the Dianzhong formation, Nianbo formation were more obvious controlled by the continental basement materials in Gangdese belt, promptly with a certain degree of crustal material occurred mix in the magma chamber. The magma source of Pana formation had more crustal material, which occurred uniform mixing between wedge mantle magma and crust-derived magma.The volcanic rocks of Linzizong group is an important ore-bearing strata in the Zexue areas, Tibet. For one thing, The volcanic rocks of Nianbo formation discover many Pb-Zn mineralization which have complete volcanic apparatus. The volcanic apparatus provided ample heat source and good fluid passage for the mineralization, and was good for finding Narusongduo type cryptoexplosive breccia-type lead-zinc mineralization; The volcanic rocks of Linzizong group discover many Pb-Zn mineralization which develop many NW structure. The NW structure provided enough space for the precipitation of metallogenic materials, and was good for finding Dexin and Rendui type hydrothermal vein type lead-zinc mineralization; Near the contact zone of Dianzhong formation and Xiala formation discover so many hydrothermal metasomatic lead-zinc mineralization. For another thing, in the outcrop areas of the Linzizong group, The geochemical anomalies register is very good, There exists obvious concentration zoning in anomalies of such main ore-forming elements as Pb, Zn and Ag, high peak. Combined regional metallogenic characteristics, geological and geochemical anomalies prospecting information advanced comprehensive analysis, I think the volcanic rocks of Linzizong group have a prospecting potential in the study areas, Tibet. The study area is the favorable area which is looking for the volcanic cryptoexplosive breccia type, hydrothermal-metasomatic and hydrothermal vein-type lead-zinc polymetallic ore favorable area. These provides new evidence and ideas. |
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