Geological And Geochemical Characteristics Of Granite In Longling Of Western Yunnan And Their Mineralization

Longling region of Yunnan Province is located in the Sanjiang Orogenic Belt inthe Southwest of China which includes the Gaoligong Mountains. Because of itscomplex orogenic and evolutionary processes, it has become one of the forefronts andhot spots of study both at home and abroad. A large number of granite is distributed inWestern Yunnan. The granite is closely connected to the formation and developmentof many mineral deposits such as tin, niobium, tantalum, copper and iron.Furthermore, granitic rocks are also "probes" and "windows" for disclosingcontinental dynamics and geodynamical evolutionary processes.The following results and new insights are the conclusions of detailed studies onthe general geological features, petrology, and U-Pb ages of zircon and geochemistryof granitic rocks from the Longling region, Western Yunnan combining regional dataanalysis:1. A spatial distribution law of rockbodies within the study area was determined.Granitic rocks in the study area were divided into the Tengchong subzone and theLuxi subzone. The tengchong subzone can be classified into Triassic granite(Xinhuajie rockbody), Early Cretaceous granite (Menglian rockbody and Hetoujierockbody), Late Cretaceous granite (Longtangshan rockbody, Xiangguntangrockbody), Paleogene granite (Huaqiaohe rockbody and Donghuacun rockbody) etc.Cretaceous granite can be divided into normal granite and transformed metamorphicgranite. Luxi subzone can be roughly divided by its age into Cambrian(Xiaotangxiang rockbody)-Ordovician granite (deformed metamorphic Zhenanrockbody and non-deformed metamorphic Mengmao rockbody, Huangliangourockbody, Daxueshan rockbody), Cretaceous (Late Cretaceous) granite (Bangmiaorockbody) and Paleogene granite (Huataolin rockbody, Dapo rockbody and Mangaorockbody).2. The formation age of granitic rocks within the study area was determined. Theweighted average age of zircon206Pb/238U in the Xiangyangxiang rockbody is128.6±1.4Ma and belongs to Early Cretaceous. The weighted average age of zircon206Pb/238U in the Xiangguntang rockbody is70.05±1.1Ma and belongs to Late Cretaceous.The weighted average206Pb/238U age of the Xiaotangxiang rockbody in Luxi subzoneis504.1±2.9Ma and belongs to Cambrian. The206Pb/238U age of the Mengmaorockbody is454.1±1.5Ma and belongs to Ordovician. The weighted average206Pb/238U age of Bangmiao is90.7±1.3Ma. It belongs to Late Cretaceous.3.206Pb/238U ages of zircon in granitic rocks from the Longling region are mainlyconcentrated in3periods:(1)The first period is508~443Ma with magmatic activityrecords in the front of Caledonian movement.(2) The second period is266~205.8Ma with magmatic activity records of Hercynian-Indosinian movement in the region.(3)The third period is133~58.6Ma with a magmatic response of Yanshanian-Himalayanmovement in the region. It is speculated that granite of Longling region is closelyrelated to tectonic movements.4. Petrological and geochemical characteristics of granitic rocks formed indifferent ages in the area were studied:Triassic granite in the Tengchong subzone mainly featured an ash grey, medium-fine grained, adamellite. Its geochemical characteristics are rich in silicon and iron,high-alkaline and peraluminous. The total amount of REE has an average to high level.LREE is relatively enriched while HREE is relatively depleted. It has medium-strongnegative Europium anomaly and no significant Cerium anomaly and is rich in traceelements such as Rb, U, Th and K, while Nb, Ta, Sr and Ti etc. are depleted. Itbelongs to K-rich calc-alkaline granite sourced from the crust. Granite of EarlyCretaceous can be classified into normal granite and metamorphic transformed granite,including diorite, quartz diorite, black mica diorite and adamellite etc. Its geochemicalcharacteristics are as follows: rich in silicon, aluminum and alkali, and poor in iron. Italso belongs to the calc-alkaline series, whereas the transformed metamorphic granitebelongs to the high-K calc-alkaline-shoshonite series which is peraluminous. The totalamount of REEs is high with a wide variation range. In general, it has the trend ofenriched in LREE with right inclined REE patterns and depleted of HREE, medium-weak negative Eu anomaly and no significant cerium anomaly as well as rich in Rb,Th, U and depleted of Ba Sr, P, Ti. Granite of Late Cretaceous can also be divided intonormal granite and metamorphic transformed granite. Their main contents areadamellite and moyite and their main elements consist of silicon, stronglyperaluminous to peraluminous and are calc-alkaline. The total content of REEs is highand with a large variation range. In general, it has the distribution pattern inclined tothe right enriched with LREE and depleted of HREE. It has a medium negativeEuropium anomaly, but no Cerium anomaly. There is an enrichment of large ionlithophylic elements such as Rb and K and high field strength elements such as Thand U, while a depletion of large ion lithophylic elements such as Ba, Sr and K aswell as high field strength elements such as Nb, P and Ti in the trace elements.Paleogene granite is mainly adamellite. Its main elements consist of Fe2O3and K2Oetc., peraluminous and a high content of total REEs with a large variation range. REEdistribution pattern can be classified into two categories: one is the enrichment ofLREE and heavy depletion of rare earth model inclined to the right; the other is theflat HREE enrichment mode. There is a high enrichment of large ion lithophylicelements such as Rb and K as well as high field strength elements such as Tb, U, Ta,Nb, and a depletion of large ion lithophylic elements such as Ba and Sr as well as highfield strength elements such as P and Ti. The Cambrian-Ordovician granite of the Luxi subzone borderlines theHuangcaoba Fault. Most granite is transformed metamorphic granite in the north sideof the fault and normal adamellite in the south side of the fault. Main elements arerich in SiO2, K2O and P2O5etc., and they are peraluminous～strongly peraluminouswith high content of total REEs. The distribution pattern of REEs has rightward trendsand smooth LREE enrichment modes, negative Eu anomaly and weak negative Ceanomaly. Rb, K, Th, La, Nd and Y etc. are enriched in the trace elements, while Ba, Sr,P and Ti etc. are depleted. Late Cretaceous granite mainly includes biotite adamellite,granodiorite, moyite etc. SiO2content in the granite is high and it is rich in Fe2O3,FeO, MgO, CaO and P2O5but depleted in K2O etc. It is peraluminous～stronglyperaluminous. Total content of REEs is relatively high. There is an enrichment ofLREE, while a depletion of HREE. There is obscure negative Europium anomaly butno Cerium anomaly. Paleogene granite is mainly adamellite, granodiorite andplagiogranite. Main elements are enriched in SiO2, K2O, P2O5etc. but depleted inFe2O3, FeO, TiO2, MgO, CaO etc. It belongs to strongly peraluminous. Total contentof REEs is relatively low, but the variation range is large. The distribution pattern ofREEs belongs to the right-inclined LREE enrichment mode with strong negative Euanomaly and non-apparent negative Ce anomaly.5. Studies were conducted on genetic type and tectonic setting of granitic rocksof the Longling Region, Western Yunnan.Most Triassic granite from the Tengchong subzone is A-type or S-type which areKCG of crustal sources. Its tectonic setting may be a continental collisionenvironment. Early Cretaceous granite is mostly I-type granite, belonging to K-richcalc-alkaline granites and it should be volcanic arc granite. Most Late Cretaceousgranite is A-type granite, belonging to MPG. It was formed in the collision-postorogenic environment. Paleogene granite is mainly A-type or S-type granite,belonging to K-rich calc-alkaline granites and it was formed in intraplate settings.Cambrian-Ordovician granite in the Luxi subzone is mainly of A-type K-richcalc-alkaline granites. Its tectonic setting is relatively complex, mostly formed in thesame collision-post collision setting. Late Cretaceous granite is mainly of I-type K-rich calc-alkaline granites, mostly belonging to the post orogenic environment, withcharacteristics of complex structures such as island arcs, continental arcs andcontinental collision areas. Paleogene granite is mainly A-type granite, includingwhite mica bearing peraluminous granites and K-rich calc-alkaline granite, which maybe formed in the post orogenic tectonic setting of the same collision.6. The magmatic evolution process in the Longling region, Western Yunnan canbe divided into3periods: intra-plate extensional magmatic activity period inCambrian—Ordovician. intra-plate extensional magmatic activity period inPermian—Triassic. intra-plate extensional stage magmatic activity period in Jurassic-Cretaceous. 7. The magmatic evolution of the Longling region, Western Yunnan is closelyrelated to mineralization：Most endogenous metal deposits are distributed throughoutthe boundary zones of granites and the mineralization period of most endogenousmetallic minerals is closely related to the forming period of the granite and chemicalcompositions of most endogenous metal deposits are correlated with granites.