Evolution And Genesis Of The Himalayan Magma In Baoxingchang Cu-Mo Deposit, Yunnan Province

Baoxingchang porphyry Cu-Mo polymetallic deposit is one part of the significant Cu-Mo polymetallic deposit of the Cenozoic porphyry metallogenetic zone along Jinshajiang-Red River fault, which is controlled by the Himilayan intensly tectonic magmatic activity and widely exposes multi-stage and multi-type alkaline-rich intrusions. This paper describes geology, geochronology, whole-rock geochemistry, Sr-Nd-Hf-Pb isotope, and zircon Hf isotope compositions of intrusions in Baoxingchang area to reveal the magmatic evolution.(1) Based on detailed field interspersed relationships and precision dating of intrusions, the magma evolution can be classified into four stages:I syenite porphyry or quartz syenite porphyry�' II porphyritic granite+early lamprophyre�' III granite porphyry+late lamprophyre �' IValkali-feldspar granite-porphyry, which suggests that the multi-stage magmas are homologous and act with pulsation rising over the same period. And the II stage and III stage magma activity as main intrusion period with different mafic magma.(2) Combined with previously reliable age data and LA-ICP-MS U-Pb dating from the fresh and weakly altered porphyry, we determined the Himalayan rock age from 38-33Ma, which is consistent with the Cenozoic alkali-rich magmatic activity (45-30 Ma) in west Yunnan, and the peak period of magmatism slightly lags with the peak age with 40 Ma from the North. Regionally, the Himalayan tectonics magmatism-metallogenic event taking place in Jinshajiang-Red River shear zone regularly:Petrogenetic and related metallogenic age became younger from the northern to southern gradually, which suggests that deep material of the Tibetan plateau hinterland escapes to the southeast part.(3) All the substances formed gradually towards rich silicon and rich alkaline, especially to rich potassium. The geochemistry results from the felsic rocks in study area show that the SiO2 composition of rocks becomes more acidic from the I stage to the IVstage, and especially for differentiation index (DI). However, Al2O3、 CaCO、 Fe2O3、 MgO and P2O5 composition was decreased gradually from the I stage to the IV stage. Meanwhile, the evolution process of the composition of mafic rocks, such as enclaves, early lamprophyre and late lamprophyre, is very obvious. Particularly, there are same or similar characteristics between enclaves and the early lamprophyre, all of which SiO2 content vary from 42.53%to 54.98%. The SiO2, MgO, K2O and (K2O+Na2O) content was increased gradually from enclaves and early lamprophyre to late lamprophyre, but Al2O3 content was decreased gradually from that. Along with the rise of the DI for mafic rocks, the SI was decreased gradually with 40-50, which suggests weaker fractional crystallization taking place.(4) Both felsic rocks and mafic rocks have the similar trace element behavior and REE patterns. Transition element spidergrams show roughly same "W" type with relatively depleted Cr and Ni component. Samples are rich in large-ionlithophile elements of Rb, Ba, U, Th, Sr and LREE, but anormaly low high field strength elements of Nb, Ta, Ti and HREE. All the characteristics as noted above indicate that the felsic rocks and mafic rocks are comagmatic. These rocks are characterized by weakly negative Eu anomaly and without Ce anomaly, which have no significant difference among four-stage felsic rocks. While the sum of DREE of enclaves and early lamprophyre is higher than the late lamprophyre and felsic rocks.(5) The felsic rocks have Sr-Nd-Pb-Hf isotopes similar to coeval lamprophyres and enclaves and are characterized by high initial 87Sr/86Sr ratios and low d(t) values and suggesting some its familiarities with the EMII-mantle source. But the εNd(t) values of mafic rocks are obtained higher than felsic rocks, implying the former have more mantle-derived components than the latter. The εHf(t) values of felsic inrtusions are positive, but the εHf(t) values of lamprophyres and enclaves hosted by felsic intrusions are negtive. This relatively higher εHf(t) and low εHf(t) values indicates that refractory ancient lithospheric mantle metasomatism caused by subducted crustal materials. The Baoxingchang felsic inrtusions and mafic rocks have the same Pb isotopes which are closely with the magma mixing action from lower crust and upper mantle. Meanwhile, typomorphic peculiatiries of zircon reveal felsic intrusions were formed in a low-temperature and peraalkaline environment characterized by rapid crystallization, and form crust-derived magma mixed with mantle-derived magma, this magma will become a favorable ore-forming magma system with rich water, rich alkali and rich ore-bearing materials. The zircon ΣREE of felsic rocks are high and characterized by left-dipping REE patterns, depleted in LREE and strongly enriched in HREE with a significant positive Ce-anomaly and middle-weak negative Eu-anomaly, displaying that magmatic fluid are crust origin or crust-mantle origin. All felsic rocks have good similar Hf isotope, which reveal they experienced a significant crust-mantle mixing process. The rocks have the characteristics by EM II which was induced by metasomatism of fluids from dehydration of subducted slab, and the mafic rocks shouid be originated mainiy from the partiai meiting of the enriched mantie in continental lithospheric mantle, but felsic rocks are originated mainiy from Crust-mantle transition zone.(6) Combined with the regional geological features of this area, the author considers that the Himalayan alkaline-rich intrusions might have been in the post-collision intraplate setting. Tectonic-magmatic activity experienced a special warming decompression process changed from compression to extension, which encourage to evolution from mafic magma to acidic-acid magma and beneficial to generate large-scale magma-fluid-mineralization. The alkaline-rich intrusions may be involved partial melting of mantle rocks, crust-mantle mixing and fractional crystallization during the magma generation. Our research suggests that felsic rocks and late lamprophyre were controlled by partial melting of EM II mantle, melting degree are respectively 15% and 10%. Both enclaves and early lamprophyre were contributed by partial melting of EM II mantle are respectively~3%. Besides, the intrusion show significant crust-mantle mixing that the proportion of crust and mantle contribution to original magma was between 0.44 and 0.60 (the peak is 0.50), which has obviously higher contaminated by continent crust in felsic rocks that lamprophyres. There is relatively insignificant fractional crystallization, but appears increasing tendency as follow:felsic inrtusions�'enclaves�'early lamprophyre�'late lamprophyre.(7) On the basis of analysis above for temporal and spatial evolution of magma, source characteristics and magmatic genesis, a model of the four-stage magma with fluid pulsatilly aroused was built in this paper. It indicates that mantle heat flow from Eastern Margin of Tibetan Plateau underplated to the bottom of the mantle lithosphere to constitute a magmatic circulatory system with low pressure and heterogeneous temperature, which occurred endless crust-mantle mixing. The crust-mantle mixation zone provides primary magma for alkaline-rich intrusions which experiences partial melting, crust-mantle mixing as well as fractional crystallization during the ascend processes of magma along a dilation center between Red River fault and Chenghai fault to emplace multi-stage and multi-type intrusions. In this evolution process, the I stage syenite porphyry rocks are higher depth of magma source, higher partial melting degree, lower SiO2 components and weaker crystallization differentiation. The II stage granite rocks, as the main intrusion, have shallower depth and weaker partial melting degree of magma source, but larger scale outcrop, stronger crystallization differentiation and more crustal contamination than the I stage rocks, which consists of porphyritic granite and granite porphyry mixing by the old crustal materials with 0.7~1.0Ga. The IV stage alkali-feldspar granite-porphyry have similar depth of magma source and crystallization differentiation, while it is smallest outcrop and lowest partial melting degree. Fathermore, the early lamprophyre and late lamprophyre, with the deepest magma source and reduced scale, intruded following porphyritic granite and granite porphyry, respectively.