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Petrogenesis Of Indosinian Granitoids And Volcanic Rocks In Songpan-Garze Fold Belt: Constrains For Deep Geologic Processes

Posted on:2011-04-08Degree:DoctorType:Dissertation
Country:ChinaCandidate:H M CaiFull Text:PDF
GTID:1100360308975225Subject:Institute of Geochemistry
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The Songpan-Garze fold belt, located at the northeastern margin of the Tibetan Plateau, is the largest tectonic jounction in China. It covers a huge triangular area (>200,000 km2) confined by the North China, the Yangtze and the Qiangtang blocks. To the north, the limit is marked by the A'nimaque-Mianlue suture zone. To the east the Longmenshan thrust-nappe belt separates the belt from the Yangtze block. To the southwest, the Songpan-Garze fold belt is bounded by the Jinshajiang suture zone, which is considered as a Late Paleozoic Paleo-Tethys oceanic subduction zone dipping either to the west or the east. The belt is mainly filled by the Triassic flysch sediments with a thickenss of 5-15 km, which had experienced strong folding deformation during the Indosinian compressional tectonism due to closure of the Paleo-Tethys ocean and subsequent convergence between the North China, the Yangtze and the Qiangtang continental blocks. No pre-Cambrian basement is exposed within the Songpan-Garze fold belt, except for the Sinian crystallization basement exposed in Danba area, southeastern part of the belt.Within the Songpan-Garze fold belt, granitoids are widespread and volcanic rocks are rare, both can provide important information on geodynamics. Previous studies demonstrated that those granitoids from eastern and southeastern belt have variety genetic types, such as adakitic, A-type, I-type and strongly peraluminous granitoids, and mainly derived form ancient continental crust. It is infered that the crust partial melting process was triggered by the flux of heat coming from the rising asthenospheric material. However, the magma directly from mantle (including lithospheric mantle and asthenospheric mantle) has been reported rarely in the belt. In the Songpan-Garze fold belt, an Indosinian lithospheric delamination model has been proposed based on previous investigation on widespread granitoids. This model lacks information from volcanic magmatism and mantle-derived magmas. During the Indosinian delamination in the Songpan-Garze fold belt, whether partial melting of lithospheric mantle taken place is noteworthy.This paper reports petrography, U-Pb zircon LA-ICP-MS ages, geochemical and Sr-Nd isotopic and zircon Hf isotopic compositions of the granitoids from the Bayankala area of the western belt and Dari area of the central belt, as well as volcanic rocks from Jiuzhi area of the central belt. We use these data to discuss their magma source and petrogenesis. Our study provides constraints on Late Triassic deep geologic process of the belt. The main research results are as follows:(1) The precise chronology data by LA-ICPMS zircon U-Pb method for the magmatic rocks from the western and central Songpan-Garze fold belt are obtained. Results show that the Zhaduo and Bayankala plutons from the Bayankala area in the western belt have magmatic crystallization ages of 219±2 Ma and 216±5 Ma, respectively. They are consistent with each other within error, and imply that these plutons were likely product of a common magma event. The magmatism of western belt mainly occurred during 216-219 Ma, synchronous with the massive emplacement of adakitic granitic plutons from eastern belt, implying that they have had the same geodynamic setting. The Sangrima, Jianshe and Moba plutons from Dari area have magmatic crystallization ages of 208±1Ma,205±2 Ma and 206±2 Ma, respectively. And the Aba and Waisai andesites have magmatic crystallization ages of 210±3 Ma and 205±1 Ma, respectively.Combined with previous geochronology results, it is indicated that the Indosinian magmatism in the Songpan-Garze fold belt mainly occurred during 243-185 Ma, with a highest peak at~208 Ma. And the magmatisms are divided into three stages:the first one is the 243-228 Ma weak magma activity; the second one is the 221-216 Ma thickened crust-derived calc-alkaline intrusives; the third one is 211-200 Ma magmas characterized by presence of mantle derived alkaline rocks, corresponding to lithosphere extension.(2) The Sangrima pluton is mainly alkline syenite, belonging to A1-type granitoid. They have intermediate SiO2 (56.92-61.80%), high K2O+Na2O (9.23-12.26%), TFeO/MgO (10.88-31.98), Rb (254-400 ppm), HFSE (Nb=61.23-133.74 ppm, Ta=3.87-8.18 ppm, Zr= 505.9-1379.6 ppm, Hf=8.98-23.79 ppm, Y=37.08-102.29 ppm) and low MgO (0.15-0.81%) contents. The REE compositions show moderately fractionated patterns with (La/Yb)N=7.1-8.8 and Eu/Eu*= 0.28-0.53. Sr-Nd-Hf isotopic data for the syenite show ISr=0.70723-0.70806,εNd(t) =-0.3-0.0,εHf(t)=+4.9-+11.1. The near-zeroεNd (t) and highεHf(t) values indicate involvement of a depleted mantle source. The syenites were formed by crystallization fractionation of lithospheric mantle-derived melt with involvement of depleted mantle and limited crustal assimilation. The occurrence of the Sangrima A1-type granitoid is probably related to environment transfer from post-collisional to intraplate extension. The syenites have highest zircon saturation temperatures among the Songpan-Garze granitoids, probably corresponding to maximum asthenospheric material and maximum continental extension.(3) Obtain the first detail geochemical report on volcanic rocks of Songpan-Garze fold belt. The Aba and Wasai volcanic rocks from Jiuzhi area are calc-alkaline andesites. Geochemical and Sr-Nd-Hf isotopic compositions indicate that the magma of the Aba andesites was dominantly originated from a crustal source, with minor mantle-derived component. The magma generation location is likely at the boundary between crust and mantle. The magma of the Wasai andesites resulted from partial melting of lithosphere mantle, which was probably metasomatized by amphibole-bearing fluid.
Keywords/Search Tags:Songpan-Garze
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