Research Of Re-Os Isotope System In Intermediate-acidic Rocks And The Application In Tibet Plateau

The porphyry deposits in Gangdese porphyry copper belt occurs under the continent-continent collision environment which is different from the typical porphyry deposits occur in arc environment. This area experienced multi-phase tectonic events, such as island arc orogeny, continental marginal arc accretion and collision orogeny. The origin of the metallogenic magma is very complex. The oceanic crust, the metasomatic mantle wedge by the subducting plate material, the asthenosphere mantle, the juvenile lower crust and the old crust are all the potential sources. However, because of the low resolution of Sr-Nd-Pb isotopic system to these sources, the origin and evlolution of the metallogenic magma is the hotpot and difficulty in researching continental porphyry deposits. Re-Os are siderophile and sulphophile elements, and have significantly different partition behavior during the crust-mantle differentiation, thus can provide unique prospective for judging the source of the intermediate-acidic magma. But Re and Os concentration in the intermediate-acidic rocks is very low, the capability of Re-Os chemical procedure and mass-spectrometric (MS) measurement limit the use of Re-Os system in this field. In this dissertation, The Re-Os chemical procedure are improved which can basically meet the demand of analyzing Re-Os in intermediate-acidic rocks; Os isotopic composition of several important geological end members in Tibet are determined; the Re-Os isotopic studies are conducted tentatively on Yaja intrusion and Qulong granodiorite and dioritic enclaves.In this dissertation, the device used for direct distilling Os from the Carius tube is improved, and the direct ditillation parameters for different samples and MS instruments are optimized, which reduce the volume of the device and raise the efficiency of the chemical procedure. This method combined with digestging sample with aqua regia in sealed Carius tube, extracting the Re from the residual solution using acetone and measuring the Re and Os on HR-ICP-MS and NTIMS, which established by previous work, can be used to study the Re-Os system in intermediate-acidic rocks. The procedure blank of Os is below3pg, the blank of the Natural abundance Os and187Os are0.2pg and below0.1pg respectively. The long time storage experiment of Os-bearing water absorption liquid indicates that both OsO4loss and being reduced are reasons for the MS count attenuation.The Os isotopic compositions of several important geological end members in Tibet are determined to provide the baseline for the researching on the metallogenic porphyry. The (187Os/188Os)i; of Dazi mafic subvolcanic rock, which are used to represent the asthenosphere mantle, is0.2964-0.4070:the187Os/188Os of amphibolite enclaves in Yardoi gneiss dome is0.1511-0.2200; the (187Os/188Os)i of Anduo gneiss, which are used to represent the upper crust, is2.1130-9.1495; the Qushui granitoid batholith which are used to represent the juvenile mafic lower crust and the MME in the batholiths which are used to represent the basaltic magma derived from the metasomatic mantle wedge have the similar (187Os/188Os)i (1.1244-2.2918and1.7352-2.1326, respectively).Comparing with Miocene metallogenic porphyries in Gangdese magmatic belt, Yaja intrusion has enriched Sr-Nd-Pb isotopic composition and contains LREE-and U-Th-bearing minerals, which demonstrate that the addition of enrichment components into the source. The average (187Os/188Os)i of Yaja intrusion is0.3620. The bivariate mixture model indicates that the mixture of primitive mantle (Os=15pg/g.187Os/188Os=0.1270) and enrichment end member (Os=2pg/g,187Os/188Os=1) can generate this Os isotopic composition.There two type dioritic enclaves in Qulong granodiorite. Cu-rich enclaves have significantly radiogenic Os composition (0.5089-1.9257); normal enclaves have relative depleted Os composition (0.1783-0.6383); granodiorite has the mixing Os composition of these enclaves. The correlation of Ba/Ta ratio, Cu concentrion and Sr-Nd-Os isotopic composition with the (Sr/P)N in Cu-rich enclaves indicated that they are derived from the metasomatic mantle wedge. The correlation of Os isotopic composition with Sr-Nd-Pb isotopic composition indicates that the metasomatic component is oceanic crust melt or fluid. The normal dioritic enclaves has relative depleted Os isotopic composition and similar concentration of compatible element and high Mg#with Dazi mafic subvolcanic rock, which indicates that the asthenosphere mantle it the potential source.