Geochronological And Geochemical Characteristics Of The Tianmenshan Pluton And Its Cooling History

A little study in Petrology and Geochemistry on the Tianmenshan pluton had been performed. However, there is no high-resolution dating data available at present, neither did the characteristics of magma sources and rock-forming process. So geochronological studies on the Tianmenshan crustal-derived pluton were undertaken using SHRIMP zircon U-Pb dating and Ar-Ar dating in biotite, muscovite and K-feldspar. Owing to the closure systems in different minerals, the cooling history of the pluton can be determined with an age and temperature diagram. Late hydrothermal event has been recognized which is related with the mineralization possibly. With comparative age petrologic records, we conclude some constrains on tectonic evolution and suggest that the formation of the Tianmenshan pluton is in a transition circumstance which in the background of orogeny but with extension locally. The temporal gap between granite-forming and wolfram mineralization could last～10Ma due to the slowly cooling ratio of the pluton and the temperature of wolfram crystallization.1. The main rock type of the Tianmenshan pluton is biotite monzonitic granite, which belongs to high-evolved S-Type peralumious granite, rich in K, cal-alkaline, sharing almost the same geochemical characteristics with the Piaotang pluton. The total amount of REE of the Tianmenshan pluton is relatively low,ΣREE = 107.20～180.41 ppm, averaging in 133.17;LREE/HREE= 2.03～4.37,averaging in 3.34;LaN/YbN = 1.13～4.39,averaging in 2.65, LREE is low and HREE is enriched apparently, composing the typical"M"pattern. There is strong Eu negative anomaly, which indicates that the Tianmenshan pluton underwent a relatively strong feldspar fractionation or feldspars left in the rest of the source. Rb-Sr and Sm- Nd trace the source of the magma. Average of (⁸⁷Sr/⁸⁶Sr)i is 0.7142,εNd(t) -0.93,T2DM 1701Ma, which shows that the Tianmenshan pluton is partial melted by Precambrian base rock which had evolved relatively highly. The emplacement age of the Tianmenshan pluton is 167Ma, in the Middle-Jurasic, Early-Yashanian. Collated with the comparative age petrologic records in a larger scale, the circumstance of the Tianmenshan is in the background of syn-orogeny with extension locally.2. Since closure temperature theory was proposed by Dodson MH (1973), it is widespread used of thermal chronology to obtain thermal history both in domestic and abroad. The analyses target of muscovite was a mix of magmatic muscovite and hydrothermal muscovite, which means the Ar-Ar age we got, is a mixed average value. The late hydrothermal events delayed the start of dating, resulting in the relatively younger age, and probablely contributed to the formation of bearing-wolfram quartz veins. Consequently, we recognize that the age of muscovite could imply the commence of tungsten mineralization strongly related with the late hydrothermal events.3. Baxiannao tungsten deposit is the production of W-Sn-Nb-Ta metallogenic stage of Middle-Yashannian. According to the cooling curve of the Tianmenshan pluton, we can arrive that, if the rapid cooling of the Tianmenshan pluton since its emplacement is due to a steep temperature gradient between the magma and the country rock, intensive regional tectonic activity has to be involved for magma emplacement with the generation of tensional faults around the contact zone in this period of time. As a result, the magmatic fluid or the water heated by magma would migrate along the faults near the contact zone. In the slow and cooling stage, chemical interaction would occur between the tungsten minerals carried in the hydrothermal fluid and the country rocks or other substances, resulting in the decomposition of the wolframite and the precipitation of bearing-tungsten veins.It appears that the rapid cooling of the plutons concurred with the activity of ore-bearing hydrothermal fluid along fractures and thus favored the migration of fluid into the ore-forming space, but could not result in mineralization. As long as the plutons evolved into the stage of slow cooling, thermal diffusion and heat conduction became slower and the tectonic activity was getting weaker. Consequently, the ore-bearing hydrothermal fluid evolved into a situation that allows the deposition of ore and gangue minerals in the fractures.